Updated on 2022/01/28

写真a

 
OHKI, Yoshimichi
 
Affiliation
Faculty of Science and Engineering, Kagami Memorial Research Institute for Materials Science and Technology
Job title
Senior Research Professor

Education

  • 1975.04
    -
    1978.03

    Waseda University   Graduate School of Science and Engineering  

  • 1973.04
    -
    1975.03

    Waseda University   Graduate School of Science and Engineering  

  • 1969.04
    -
    1973.03

    Waseda University   School of Science and Engineering  

Degree

  • Waseda University   Doctor of Engineering

  • Waseda University   Master of Engineering

Research Experience

  • 2012.04
    -
    2013.03

    Nagoya University

  • 2011.02
    -
     

    西安交通大学名誉教授

  • 2010.09
    -
     

    西安交通大学電気設備電気絶縁国家重点実験室国際学術委員会委員

  • 2008.04
    -
     

    Shibaura Institute of Technology

  • 2006.05
    -
    2008.03

    (独)科学技術振興機構 研究開発戦略センター シニアフェロー

  • 2001
    -
    2005

    理化学研究所共同研究員

  • 1985
    -
     

    Waseda University, Professor

  • 1980
    -
    1985

    Waseda University, Associate Professor

  • 1982
    -
    1984

    MIT Visiting Scientist of Electrical Engineering

  • 1981
    -
    1983

    MIT Visiting Scientist of Electrical Engineering

  • 1978
    -
    1980

    Waseda University, Assistant Professor

  • 1976
    -
    1978

    Waseda University, Research Assistant

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Professional Memberships

  •  
     
     

    静電気学会

  •  
     
     

    応用物理学会

  •  
     
     

    米国電気電子学会

  •  
     
     

    電子情報通信学会

  •  
     
     

    電気学会

  •  
     
     

    放電学会

  •  
     
     

    日本原子力学会

  •  
     
     

    Institute of Electrical and Electronics Engineers

▼display all

 

Research Areas

  • Electric and electronic materials

  • Power engineering

Research Interests

  • 電子デバイス・機器工学

  • 光ファイバ

  • 電気絶縁材料

  • 誘電体

  • optical fiber

  • electrical insulating materials

  • dielectrics

▼display all

Papers

  • News From Japan: Development of a 6,912-Fiber Optical Fiber Cable with a 29-mm Outer Diameter

    Yoshimichi Ohki

    IEEE Electrical Insulation Magazine   37 ( 3 ) 42 - 44  2021.05

    Authorship:Lead author

    DOI

  • News From Japan: Ninth ISEIM Held in Tokyo

    Yoshimichi Ohki

    IEEE Electrical Insulation Magazine   37 ( 2 ) 40 - 43  2021.03

    Authorship:Lead author

    DOI

  • Effects of gamma irradiation on the degradation of silicone rubber by steam exposure

    Seitaro Ito, Yu Miyazaki, Naoshi Hirai, Yoshimichi Ohki

    Journal of Nuclear Science and Technology   58 ( 2 ) 166 - 172  2021.02  [Refereed]

    Authorship:Last author

    DOI

  • Aging Behavior of Flame-retardant Cross-linked Polyolefin under Thermal and Radiation Stresses

    Haolong Zhou, Wakana Hanafusa, Keigo Udo, Naoshi Hirai, Yoshimichi Ohki

    IEEE Transactions on Dielectrics and Electrical Insulation   28 ( 1 ) 303 - 309  2021.02  [Refereed]

    Authorship:Last author

    DOI

  • News From Japan: Development of Antenna-Type Partial- Discharge Testers for Coils

    Yoshimichi Ohki

    IEEE Electrical Insulation Magazine   37 ( 1 ) 40 - 42  2021.01

    Authorship:Lead author

    DOI

  • Filler‐dependent changes in thermal, dielectric, and mechanical properties of epoxy resin nanocomposites

    Emiri Nagase, Tomonori Iizuka, Kohei Tatsumi, Naoshi Hirai, Yoshimichi Ohki, Shigeyoshi Yoshida, Takahiro Umemoto, Hirotaka Muto

    IEEJ Transactions on Electrical and Electronic Engineering   16 ( 1 ) 15 - 20  2021.01  [Refereed]

    Authorship:Corresponding author

    DOI

  • Comparison of degradation behavior between soft and hard epoxy resins

    Hiroyuki Ishii, Naoshi Hirai, Yoshimichi Ohki

    Journal of Nuclear Science and Technology     1 - 9  2020.12  [Refereed]

    Authorship:Last author

    DOI

  • Effects of Heat and Gamma-rays on Mechanical and Dielectric Properties of Cross-linked Polyethylene

    Yu Miyazaki, Naoshi Hirai, Yoshimichi Ohki

    IEEE Transactions on Dielectrics and Electrical Insulation   27 ( 6 ) 1998 - 2006  2020.12  [Refereed]

    Authorship:Last author, Corresponding author

    DOI

  • News from Japan: Development of a braided piezoelectric cord for wearable sensors

    Yoshimichi Ohki

    IEEE Electrical Insulation Magazine   36 ( 6 ) 59 - 61  2020.11

    Authorship:Lead author

    DOI

  • Degradation of flame‐retardant ethylene‐propylene‐diene rubber by radiation and steam

    Jiayi You, Hikaru Yamaguchi, Hiroyuki Ishii, Naoshi Hirai, Yoshimichi Ohki

    IEEJ Transactions on Electrical and Electronic Engineering   15 ( 11 ) 1572 - 1579  2020.11  [Refereed]

    Authorship:Last author

    DOI

  • News from Japan Development of a Submarine Cable for an Offshore Wind Farm

    Yoshimichi Ohki

    IEEE Electrical Insulation Magazine   36 ( 5 ) 52 - 54  2020.09

    Authorship:Lead author

    DOI

  • Changes in Chemical Structure and Mechanical Properties of Cross-linked Polyethylene Aged under Simulated Severe Accident Conditions of a Nuclear Power Plant

    Yu Miyazaki, Naoshi Hirai, Yoshimichi Ohki

    IEEJ Transactions on Fundamentals and Materials   140 ( 9 ) 445 - 450  2020.09  [Refereed]

    Authorship:Last author

    DOI

  • Degradation of Cables Insulated with Silicone Rubber for Nuclear Power Plants by an Aqueous Solution of NaOH Sprayed during a Severe Accident

    Takefumi Minakawa, Masaaki Ikeda, Aiki Watanabe, Naoshi Hirai, Yoshimichi Ohki

    IEEJ Transactions on Fundamentals and Materials   140 ( 9 ) 457 - 463  2020.09  [Refereed]

    Authorship:Last author

    DOI

  • News from Japan Development of a New On-Line Partial Discharge Monitoring System

    Yoshimichi Ohki

    IEEE Electrical Insulation Magazine   36 ( 4 ) 67 - 70  2020.07

    Authorship:Lead author

    DOI

  • Changes in mechanical and dielectric properties of silicone rubber induced by severe aging

    S. Ito, N. Hirai, Y. Ohki

    IEEE Transactions on Dielectrics and Electrical Insulation   27 ( 3 ) 722 - 730  2020.06  [Refereed]

    Authorship:Last author

    DOI

  • News from Japan: Ocean thermal energy conversion development center in Okinawa

    Yoshimichi Ohki

    IEEE Electrical Insulation Magazine   36 ( 3 ) 47 - 50  2020.05

    Authorship:Lead author

    DOI

  • Numerical evaluation of complex permittivity of silicone rubber based on Jonscher's law

    Zhenyu Yang, Seitaro Ito, Naoshi Hirai, Yoshimichi Ohki

    IEEJ Transactions on Electrical and Electronic Engineering   15 ( 5 ) 658 - 662  2020.05  [Refereed]

    Authorship:Last author

    DOI

  • Possible cause of chemiluminescence in silicone rubber

    Yoshimichi Ohki, Naoshi Hirai

    Japanese Journal of Applied Physics   59 ( 4 ) 041005 - 041005  2020.04  [Refereed]

    Authorship:Lead author

    DOI

  • Dielectric properties of nanocomposites based on epoxy resin and HBP/plasma modified nanosilica

    Guoqing Yang, Jiaxin Li, Yoshimichi Ohki, Deyi Wang, Geng Liu, Yang Liu, Kai Tao

    AIP Advances   10 ( 4 ) 045015 - 045015  2020.04  [Refereed]

    DOI

  • Degradation of Mechanical and Dielectric Properties of Flame‐Retardant Ethylene Propylene Rubber by Thermal Aging

    Hikaru Yamaguchi, Hiroyuki Ishii, Naoshi Hirai, Yoshimichi Ohki

    IEEJ Transactions on Electrical and Electronic Engineering   15 ( 4 ) 488 - 495  2020.04  [Refereed]

    Authorship:Last author

    DOI

  • News from Japan World’s First DC 400-kV XLPE Cable System

    Yoshimichi Ohki

    IEEE Electrical Insulation Magazine   36 ( 2 ) 50 - 52  2020.03

    Authorship:Lead author

    DOI

  • Facile Synthesis of Isotactic Polyacrylonitrile via Template Polymerization in Interlayer Space for Dielectric Energy Storage

    Yu Wang, Ryota Nakamura, Takeo Suga, Shengtao Li, Yoshimichi Ohki, Hiroyuki Nishide, Kenichi Oyaizu

    ACS APPLIED POLYMER MATERIALS   2 ( 2 ) 775 - 781  2020.02

     View Summary

    Isotactic polyacrylonitrile (iso-PAN) is synthesized via template polymerization using the two-dimensional layered space of a CoCl2 crystal as a template. Isotacticity of polyacrylonitrile reaches a meso/meso triad = 93% (mm = 0.93), which indicates a superior packing of polymer chains, leading to a high breakdown strength. Tacticity enhancement is correlated with the interatomic (metal-metal) distance of template -layered crystals. Due to a large dipole moment (4.3 D) of nitrile group and a high stereoregularity, iso-PAN demonstrates a high discharge energy density of 6.7 J/cm(3) at 660 MV/m, nearly six times that of atactic polyacrylonitrile (ata-PAN) and biaxially oriented polypropylene (BOPP), a commercially available dielectric polymer film. Moreover, it exhibits a charge-discharge efficiency of ca. 90% in a wide applied electric field range, which is higher than the efficiencies of ata-PAN and BOPP. These results suggest that iso-PAN, exhibiting a large dipole moment and high stereoregularity, is a promising candidate for high energy density and low loss capacitor dielectrics.

    DOI

  • News from Japan Japan’s Electrical Insulation Conference Celebrates Its 50th Anniversary

    Yoshimichi Ohki

    IEEE Electrical Insulation Magazine   36 ( 1 ) 41 - 43  2020.01

    Authorship:Lead author

    DOI

  • Comparison of the effects of heat and gamma irradiation on the degradation of cross‐linked polyethylene

    Ziyang Liu, Yu Miyazaki, Naoshi Hirai, Yoshimichi Ohki

    IEEJ Transactions on Electrical and Electronic Engineering   15 ( 1 ) 24 - 29  2020.01  [Refereed]

    Authorship:Last author

    DOI

  • Quantum chemical verification of identification of three phenol-type antioxidants by far-infrared absorption spectroscopy

    Tomofumi Seki, Koji Endo, Naoshi Hirai, Yoshimichi Ohki

    Japanese Journal of Applied Physics   59 ( 1 ) 012002 - 012002  2020.01  [Refereed]

    Authorship:Last author

    DOI

  • Effects of metal content on electrical and physical properties in solution-processed IGZO thin films

    Morimoto, T., Yang, Y., Ochiai, Y., Fukuda, N., Ohki, Y.

    Applied Physics A: Materials Science and Processing   126 ( 5 )  2020  [Refereed]

    Authorship:Last author

    DOI

  • News from Japan A New 250-kV HVDC XLPE Cable System in Japan

    Yoshimichi Ohki

    IEEE Electrical Insulation Magazine   35 ( 6 ) 43 - 45  2019.11

    Authorship:Lead author

    DOI

  • Terahertz absorption spectra of several polymer nanocomposites indicating polymer-filler interactions

    Keigo Mori, Tomofumi Seki, Naoshi Hirai, Yoshimichi Ohki

    AIP Advances   9   105109-1 - 105109-4  2019.10  [Refereed]

    DOI

  • Degradation mechanisms of silicone rubber under different aging conditions

    Takuya Kaneko, Seitaro Ito, Takefumi Minakawa, Naoshi Hirai, Yoshimichi Ohki

    Polymer Degradation and Stability   2019 ( 168 ) 108936(1) - 108936(10)  2019.10  [Refereed]

    DOI

  • 重大事故模擬環境に暴露したエチレンプロピレンジエンゴム絶縁ケーブルの劣化状態分析

    皆川武史, 池田雅昭, 平井直志, 大木義路

    電気学会論文誌A   139 ( 9 ) 380 - 386  2019.09  [Refereed]

    DOI

  • Location Attempt of a Coaxial Cable with Multiple Abnormalities by Frequency Domain Reflectometry

    Naoshi Hirai, Yoshimichi Ohki

    IEEJ Trans. FM   139 ( 9 ) 367 - 372  2019.09  [Refereed]

    Authorship:Last author

    DOI

  • News from Japan Online Insulation Monitoring System for Rotating Machines

    Yoshimichi Ohki

    IEEE Electrical Insulation Magazine   35 ( 5 ) 47 - 49  2019.09

    Authorship:Lead author

    DOI

  • Aging Behavior of Flame-Retardant Cross-Linked Polyethylene in Nuclear Power Plant Environments

    Zhenyu Yang, Takuya Kaneko, Naoshi Hirai, Yoshimichi Ohki

    IEEJ Transactions on Electrical and Electronic Engineering   14 ( 8 ) 1133 - 1138  2019.08  [Refereed]

  • Broadband FIR absorption spectra of lowdensity polyethylene sheets containing sixdifferent antioxidants and estimation of their contents by chemometric analysis

    Takaaki Ogishima, Chiaki Kuroda, Naoshi Hirai, Yoshimichi Ohki

    High Voltage   4 ( 3 ) 161 - 166  2019.08  [Refereed]

    DOI

  • News from Japan Development of a Highly Durable SiC Power Device for Electric Vehicles

    Yoshimichi Ohki

    IEEE Electrical Insulation Magazine   35 ( 4 ) 41 - 43  2019.07

    Authorship:Lead author

    DOI

  • Identification of vibrational modes in thymine bases by broadband far-infrared absorption spectroscopy

    Yutaka Abe, Tomofumi Seki, Yoshimichi Ohki, Maya Mizuno

    Japanese Journal of Applied Physics   58 ( 5 ) 052002(1) - 052002(6)  2019.05  [Refereed]

    DOI

  • News from Japan Development of an Ultrahigh Speed Camera with Multichannel Imaging

    Yoshimichi Ohki

    IEEE Electrical Insulation Magazine   35 ( 3 ) 41 - 43  2019.05

    Authorship:Lead author

    DOI

  • Effects of Interaction between Filler and Resin on the Glass Transition and Dielectric Properties of Epoxy Resin Nanocomposites

    Keigo Mori, Naoshi Hirai, Yoshimichi Ohki, Yasutomo Otake, Takahiro Umemoto, Hirotaka Muto

    IET Nanodielectrics    2019.04  [Refereed]

    DOI

  • Rejuvenation of retired power cables by heat treatment

    Yue Xie, Gang Liu, Yifeng Zhao, Licheng Li, Yoshimichi Ohki

    IEEE Transactions on Dielectrics and Electrical Insulation   26 ( 2 ) 668 - 670  2019.03  [Refereed]

    DOI

  • News from Japan Development of a Clay Coating on Metal as a New Electrical Insulating Substrate

    Yoshimichi Ohki

    IEEE Electrical Insulation Magazine   35 ( 2 ) 56 - 57  2019.03

    Authorship:Lead author

    DOI

  • Identification and quantification of phenol-type antioxidants in low-density polyethylene by broadband far-infrared spectroscopy

    Takaaki Ogishima, Chiaki Kuroda, Naoshi Hirai, Yoshimichi Ohki

    Polymer Testing   79   10 - 19  2019.02  [Refereed]

    DOI

  • Insulation Performance of Safety-related Cables for Nuclear Power Plants under Simulated Severe Accident Conditions

    Takefumi Minakawa, Masaaki Ikeda, Naoshi Hirai, Yoshimichi Ohki

    IEEJ Transactions on Fundamentals and Materials   139 ( 2 ) 54 - 59  2019.02  [Refereed]

    DOI

  • First magnetic resonance imaging under three tesla magnetic field [News From Japan]

    Yoshimichi Ohki

    IEEE Electrical Insulation Magazine   35 ( 1 ) 55 - 58  2019.01

    Authorship:Lead author

    DOI

  • Detection of Degradation Occurring Over the Whole Cable Length by Frequency Domain Reflectometry

    Yoshimichi Ohki, Naoshi Hirai

    IEEE Transactions on Dielectrics and Electrical Insulation   25 ( 6 ) 2467 - 2469  2018.12  [Refereed]

    DOI

  • Location Attempt of a Degraded Portion in a Long Polymer-insulated Cable

    Yoshimichi Ohki, Naoshi Hirai

    IEEE Transactions on Dielectrics and Electrical Insulation   25 ( 6 ) 2461 - 2466  2018.12  [Refereed]

    DOI

  • Proposal of a chip capable of simultaneous excitation of waveguide-mode resonance and surface plasmon resonance

    Chiaki Kuroda, Ryota Iizuka, Midori Nakai, Hiroki Ashiba, Makoto Fujimaki, Yoshimichi Ohki

    Japanese Journal of Applied Physics   57 ( 12 ) 122002(1) - 122002(5)  2018.11  [Refereed]

    DOI

  • Effects of ultraviolet photon irradiation and subsequent thermal treatments on solution-processed amorphous indium gallium zinc oxide thin films

    Yuki Takamori, Takaaki Morimoto, Nobuko Fukuda, Yoshimichi Ohki

    AIP ADVANCES   ( 8 ) 115304(1) - 115304(12)  2018.11  [Refereed]

    DOI

  • How to Install TEMPO in Dielectric Polymers‐Their Rational Design toward Energy‐Storable Materials

    Yang Feng, Takeo Suga, Hiroyuki Nishide, Yoshimichi Ohki

    Macromolecular Rapid Communications   40 ( 4 ) 1800734(1) - 1800734(6)  2018.11  [Refereed]

     View Summary

    © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Polar groups and the charge-transport capability play significant roles in the dielectric properties of organic polymers, and thus influence the electric energy density upon application as a capacitor material. Here, the dielectric properties and electric conductivity of a series of polymers containing 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) radicals are investigated. The neat radical polymer poly(TEMPO methacrylate) (PTMA) has a high dielectric constant but poor breakdown strength. Poly(methyl methacrylate) (PMMA) is introduced as an insulating polymer with high resistivity on breakdown, along with molecular design of PTMA. Copolymers of TEMPO methacrylate and methyl methacrylate, P(TMA-r-MMA), exhibit high breakdown strengths but low dielectric constants. PMMA blended with TEMPO exhibits the highest electric energy density of 7.4 J cm −3 (that of PTMA is 0.48 J cm −3 as a control), with both a high dielectric constant (≈6.8) and a high breakdown strength (≈500 MV m −1 ). It benefits from long-range but not bulk charge transport in the blends, which is different from the bulk charge transport in PTMA and the short-range charge transport in P(TMA-r-MMA). These results indicate that the TEMPO moiety located in the high breakdown matrix leads to a high energy-storage density in the capacitor.

    DOI PubMed

  • Tanaka's committees awarded by IEEJ successively (News from Japan)

    Y. Ohki

    IEEE Electrical Insulation Magazine   34 ( 6 ) 44 - 46  2018.10  [Refereed]

    DOI

  • Development of an innovative repetitive impulse voltage generator (News from Japan)

    Y. Ohki

    IEEE Electrical Insulation Magazine   34 ( 5 ) 50 - 53  2018.08  [Refereed]

    DOI

  • Development of an XLPE-insulated cable for LCC-HVDC power lines (News from Japan)

    Y. Ohki

    IEEE Electrical Insulation Magazine   34 ( 4 ) 62 - 65  2018.08  [Refereed]

    DOI

  • Dielectric and relaxation properties of composites of epoxy resin and hyperbranched-polyestertreated nanosilica

    Guoqing Yang, Junda Cui, Yoshimichi Ohki, Deyi Wang, Yang Lia, Kai Tao

    RSC Advances   2018 ( 8 ) 30669 - 30677  2018.08  [Refereed]

    DOI

  • Terahertz spectroscopic estimation of crystallinity of poly(phenylene sulfide)

    Chisato Azeyanagi, Yoshimichi Ohki

    Journal of Applied Polymer Science   135 ( 27 ) 46427(1) - 46427(10)  2018.07  [Refereed]

     View Summary

    As a crystalline polymer, various properties of poly(phenylene sulfide) (PPS) depend on its crystallinity. Evaluation of crystallinity of PPS is examined by terahertz (THz) absorption spectroscopy in this research. For sheets of PPS thermally annealed differently, sharp absorption peaks appearing at 3.2 and 4.2 THz are well proportional to the crystallinity estimated by X-ray diffraction, differential scanning calorimetry, and Fourier transform infrared (FTIR) absorption spectroscopy. Therefore, THz absorption spectroscopy can be a good tool for estimating the crystallinity of PPS. Especially, it is useful for thick PPS sheets, for which FTIR spectroscopy in transmission mode is difficult to apply due to the saturation of absorbance. Since THz spectroscopy and FTIR spectroscopy are complementary in the vibrations that the two methods can detect, using the two methods should be effective to improve the accuracy of estimation of crystallinity. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46427.

    DOI

  • Fluorescence imaging of Escherichia coli on a rotating optical disk

    Takayuki Shima, Hideyuki Shiramizu, Makoto Fujimaki, Chiaki Kuroda, Yoshimichi Ohki

    Japanese Journal of Applied Physics   57 ( 8 ) 088003(1) - 088003(3)  2018.07  [Refereed]

    DOI

  • Development of a dielectrophoresis-assisted surface plasmon resonance fluorescence biosensor for detection of bacteria

    Chiaki Kuroda, Ryota Iizuka, Yoshimichi Ohki, Makoto Fujimaki

    Japanese Journal of Applied Physics   57 ( 5 ) 057001(1) - 057001(5)  2018.05  [Refereed]

     View Summary

    To detect biological substances such as bacteria speedily and accurately, a dielectrophoresis-assisted surface plasmon resonance (SPR) fluorescence biosensor is being developed. Using Escherichia coli as a target organism, an appropriate voltage frequency to collect E. coli cells on indium tin oxide quadrupole electrodes by dielectrophoresis is analyzed. Then, E. coli is stained with 4$,6-diamidino-2-phenylindole (DAPI). To clearly detect fluorescence signals from DAPI-stained E. coli cells, the sensor is optimized so that we can excite SPR on Al electrodes by illuminating 405 nm photons. As a result, the number of fluorescence signals is increased on the electrodes by the application of a low-frequency voltage. This indicates that E. coli cells with a lower permittivity than the surrounding water are collected by negative dielectrophoresis onto the electrodes where the electric field strength is lowest.

    DOI

  • Terahertz spectroscopic analysis of crystal orientation in polymers

    Chisato Azeyanagi, Takuya Kaneko, Yoshimichi Ohki

    Japanese Journal of Applied Physics   57 ( 5 ) 050302(1) - 050302(4)  2018.05  [Refereed]

     View Summary

    Terahertz time-domain spectroscopy (THz-TDS) is attracting keen attention as a new spectroscopic tool for characterizing various materials. In this research, the possibility of analyzing the crystal orientation in a crystalline polymer by THz-TDS is investigated by measuring angle-resolved THz absorption spectra for sheets of poly(ethylene terephthalate), poly(ethylene naphthalate), and poly(phenylene sulfide). The resultant angle dependence of the absorption intensity of each polymer is similar to that of the crystal orientation examined using pole figures of X-ray diffraction. More specifically, THz-TDS can indicate the alignment of molecules in polymers.

    DOI

  • Development of a compact on-site power source (News from Japan)

    Y. Ohki

    IEEE Electrical Insulation Magazine   34 ( 3 ) 44 - 46  2018.04  [Refereed]

    Authorship:Lead author

    DOI

  • Development of a TiO2/SiO2 waveguide-mode chip for an ultraviolet near-field fluorescence sensor

    Chiaki Kuroda, Midori Nakai, Makoto Fujimaki, Yoshimichi Ohki

    Optics Express   26 ( 6 ) 6796 - 6805  2018.03  [Refereed]

     View Summary

    Aimed at detecting fluorescent-labeled biological substances sensitively, a sensor that utilizes near-field light has attracted much attention. According to our calculations, a planar structure composed of two dielectric layers can enhance the electric field of UV nearfield light effectively by inducing waveguide-mode (WM) resonance. The fluorescence intensity obtainable by a WM chip with an optimized structure is 5.5 times that obtainable by an optimized surface plasmon resonance chip. We confirmed the above by making a WM chip consisting of TiO2 and SiO2 layers on a silica glass substrate and by measuring the fluorescence intensity of a solution of quantum dots dropped on the chip.

    DOI

  • Selective detection of Escherichia coli by imaging of the light intensity transmitted through an optical disk

    Hideyuki Shiramizu, Chiaki Kuroda, Yoshimichi Ohki, Takayuki Shima, Xiaomin Wang, Makoto Fujimaki

    Applied Physics Express   11 ( 3 ) 037001(1) - 037001(4)  2018.03  [Refereed]

     View Summary

    We have developed an optical disk system for imaging transmitted light from Escherichia coli dispersed on an optical disk. When E. coli was stained using Bismarck brown, the transmittance was found to decrease in images obtained at λ = 405 nm. The results indicate that transmittance imaging is suitable for finding the difference in light intensity between stained and unstained E. coli, whereas the reflectance images were scarcely changed by staining. Therefore, E. coli can be selectively discriminated from abiotic contaminants using transmittance imaging.

    DOI

  • Development of top-runner distribution transformers using palm fatty acid ester insulating oil (News from Japan)

    Y. Ohki

    IEEE Electrical Insulation Magazine   34 ( 2 ) 62 - 64  2018.02  [Refereed]

    DOI

  • Similarity in degradation behavior by heat and irradiation between ethylene-propylene-diene rubber and silicone rubber

    Yoshimichi Ohki, Hiromichi Kotani, Daichi Fujishima, Naoshi Hirai

    Annual Report - Conference on Electrical Insulation and Dielectric Phenomena, CEIDP   2017-   157 - 160  2018.01  [Refereed]

     View Summary

    Flame retardant ethylene-propylene-diene rubber (FR-EPR) and silicone rubber (SiR) are two major materials for safety-related cables in nuclear power plants. For such cables, degradation of their insulation is a matter of serious concern. Regarding the above, sheets of FR-EPR and SiR were aged either thermally or concurrently by heat and gamma radiation and the resultant changes in various properties were systematically compared and analysed from various aspects by FT-IR, differential scanning calorimetry (DSC), thermogravimetry/differential thermal analysis (TG/DTA), and scanning probe microscopy (SPM), and by measuring indenter modulus, elongation at break (EAB), tensile strength (TS), and complex permittivity. As a result of the experiments, both FR-EPR and SiR become hard in a similar manner in both cases where the degradation was given by heat only or concurrently by heat and radiation. In both polymers, degradation induced cross-linking and breaking of chemical bonds were observed by FT-IR, DSC, TG/DTA. Furthermore, a contrastive difference in dielectric behavior between SiR and FR-EPR was also observed in complex permittivity.

    DOI

  • Terahertz absorption spectra of polyphenylene sulfide sheets with different degrees of crystallinity

    Chisato Azeyanagi, Yoshimichi Ohki

    Annual Report - Conference on Electrical Insulation and Dielectric Phenomena, CEIDP   2017-   429 - 432  2018.01  [Refereed]

     View Summary

    Polyphenylene sulfide (PPS), one of the superengineering plastics, is used for various purposes. Since PPS is a crystalline polymer, its various properties depend on its crystallinity. In this paper, the possibility of evaluating the crystallinity of PPS by terahertz (THz) absorption spectroscopy is examined. In addition to THz absorption spectra, X-ray diffraction patterns (XRD) were measured on sheets of PPS thermally annealed differently. As a result, it has become clear that sharp absorption peaks appearing at 3.2 and 4.2 THz are well proportional to the crystallinity estimated by XRD. Therefore, THz absorption spectroscopy can be a tool to estimate the crystallinity of PPS.

    DOI

  • Dielectrophoresis-Assisted SPRF illumination biosensor for selective detection of biological substances

    Chiaki Kuroda, Yoshimichi Ohki, Makoto Fujimaki

    Annual Report - Conference on Electrical Insulation and Dielectric Phenomena, CEIDP   2017-   253 - 256  2018.01  [Refereed]

     View Summary

    Aiming at detecting biological substances accurately and speedy, we are developing a dielectrophoresis-Assisted surface plasmon resonance fluorescence (SPRF) illumination biosensor. A sensing chip of the sensor has Al layers, which are used for electrodes and SPR excitation layers. In this study, we used Escherichia coli (E. coli) stained with 4', 6-diamidino-2-phenylindole (DAPI) as a target substance. First, the structure of the sensing chip was optimized to excite SPR on the Al electrodes by 405-nm light. Then, an emulsion with DAPI-stained E. coli was dropped on the sensing chip and illuminated by a 405-nm laser via a prism. When ac voltages were applied to the Al electrodes, the number of bright spots, due to fluorescence from DAPI-stained E. coli, increased on the electrodes. This result indicates that E. coli, which has a lower permittivity than the emulsion, was collected onto the electrodes where the electric field strength is lowest by negative dielectrophoresis. Therefore, we have succeeded in collecting target biological substances on the surface of the sensing chip and detecting them.

    DOI

  • Development of a high-performance indirectly hydrogen-cooled turbine generator (News from Japan)

    Y. Ohki

    IEEE Electrical Insulation Magazine   34 ( 1 ) 61 - 63  2018.01  [Refereed]

    DOI

  • Identification of antioxidants in polymeric insulating materials by terahertz absorption spectroscopy

    Takuya Kozai, Takuya Kaneko, Naoshi Hirai, Yoshimichi Ohki

    Polymer Degradation and Stability   147 ( 147 ) 284 - 290  2018.01  [Refereed]

     View Summary

    For the purpose of using organic polymeric materials for electrical insulation, various additives such as antioxidants are added to prevent degradation or oxidative decomposition of the polymers. Therefore, it is desirable that we can identify antioxidants added in polymers by instrument analyses. In this research, terahertz absorption spectroscopy was conducted for nine kinds of antioxidants. The spectroscopy was also conducted for sheets of low-density polyethylene, to which each antioxidant had been added with different contents. As a result, it has become clear that each antioxidant has its own specific spectrum. In addition, for most antioxidants, the absorption intensity is proportional to the content of antioxidant added in LDPE. However, several absorption peaks change their spectral shapes when the antioxidant is in LDPE.

    DOI

  • Degradation Mechanisms of Silicone Rubber under Different Aging Conditions

    Takuya Kozai, Takuya Kaneko, Naoshi Hirai, Yoshimichi Ohki

    Polymer Degradation and Stability   147   284 - 290  2018.01  [Refereed]

    DOI

  • Successful detection of insulation degradation in cables by frequency domain reflectometry

    Yoshimichi Ohki, Naoshi Hirai

    Minerals, Metals and Materials Series   11   77 - 85  2018  [Refereed]

     View Summary

    We have succeeded in detecting the degradation of cable’s polymeric insulation well before its continual use becomes risky. Degradation of organic polymers is mainly caused by oxidation if the ambience around the cable contains oxygen. When organic polymers are oxidized, polar carbonyl groups are formed, by which the permittivity is increased. This in turn decreases the characteristic impedance of a polymer-insulated cable. If we inject electromagnetic waves in a very wide frequency range into the cable and measure the ratio of reflected power to injected power, the information on the effects of the characteristic impedance changes is included in the frequency spectra of the ratio. If we do inverse Fourier transform, we can convert the data to a time domain. Therefore, we can know the degraded portion by multiplying the velocity of electromagnetic waves in the cable.

    DOI

  • Degradation of silicone rubber analyzed by instrumental analyses and dielectric spectroscopy

    Yoshimichi Ohki, Naoshi Hirai, Daomin Min, Liuqing Yang, Shengtao Li

    Minerals, Metals and Materials Series   11   107 - 116  2018  [Refereed]

     View Summary

    Silicone rubber (SiR) was gamma irradiated at 125, 145 and 185 °C or thermally aged at 220, 250 and 280 °C and the resultant changes in performance were evaluated. It has become clear from instrumental analyses that crosslinking via oxidation of silicon atoms and chain scission are induced by gamma rays. Furthermore, from the temperature dependence of real relative permittivity at high frequencies, the thermal expansion coefficient was found to become smaller with the increase in dose. These results can be understood well by the chemical and structural changes in SiR induced by the degradation.

    DOI

  • Development of a low sag carbon fiber reinforced aluminum conductor for transmission lines (News from Japan)

    Y. Ohki

    IEEE Electrical Insulation Magazine   33 ( 6 ) 54 - 57  2017.12  [Refereed]

    DOI

  • Effects of addition of MgO fillers with various sizes and co-addition of nano-sized SiO2 fillers on the dielectric properties of epoxy resin

    Ryosuke Yanashima, Naoshi Hirai, Yoshimichi Ohki

    Proceedings of the International Symposium on Electrical Insulating Materials   2   650 - 653  2017.12  [Refereed]

     View Summary

    Complex permittivity (ϵr' and ϵr"), conductivity (σ), and glass transition temperature (Tg) were measured for composites of epoxy resin and MgO fillers with various sizes in a wide temperature and frequency range. The effect of coaddition of nano-sized SiO2 was also examined. It has become clean that Tg decreases by the addition of MgO fillers, and its decrement becomes more with a decrease in filler size. Moreover, Tg decreases if the silica nanofillers are co-added. However, all the three important parameters for electrical insulation ability of polymers, namely ϵr', ϵr", and σ, decrease with the decrease in size of MgO fillers or by the addition of silica nanofillers at high temperatures and low frequencies. This is a very astonishing result, since it seems reasonable that a polymer with a high Tg should have a better insulating ability. Probably, the above-mentioned results indicate that the charge transport becomes more difficult, presumably resulting from the suppression of molecular motion with the decrease in size of MgO fillers and by the co-addition of SiO2 nanofillers. This indicates that the suppression of molecular motion becomes very strong if the filler size is small, making the carrier transport difficult even in polymers with low Tg.

    DOI

  • Terahertz absorption spectroscopy of poly(ether ether ketone)

    Takuya Kaneko, Naoshi Hirai, Yoshimichi Ohki

    Proceedings of the International Symposium on Electrical Insulating Materials   2   539 - 542  2017.12  [Refereed]

     View Summary

    The crystallinity of sheets of poly(ether ether ketone) (PEEK) was changed by thermal treatments conducted at 295 oC for various periods and their terahertz (THz) and Fouriertransform infrared absorption (FT-IR) spectra were obtained. As a result, it has been confirmed that an absorption peak appears at around 3.5 THz in the sheet samples that exhibit clear X-ray diffraction patterns at 20= 18.8, 20.8, 22.8, and 28.8o. In contrast, no obvious sample-dependent changes were observed in FT-IR spectra. The absorption at around 3.5 THz shows a shift in frequency to a degree proportional to the crystallinity. Its maximum intensity is also in proportion to the crystallinity. These results indicate that the absorption at 3.62 THz has a close relation with the crystallinity of PEEK This in turn means that the THz absorption spectroscopy can be used for estimating the crystallinity of PEEK.

    DOI

  • Correlation between indenter modulus and elongation-at-break observed for four electrical insulating polymers

    Naoshi Hirai, Yoshimichi Ohki

    Proceedings of the International Symposium on Electrical Insulating Materials   2   753 - 756  2017.12  [Refereed]

     View Summary

    The correlation between degradation-induced changes in two important indicators of mechanical properties, namely elongation at break (EAB) and indenter modulus (IM), is examined for flame-retardant ethylene propylene rubber (FR-EPR), silicone rubber (SiR), flame-retardant crosslinked polyethylene (FR-XLPE), and XLPE. The samples were degraded by heat or concurrently by heat and radiation. In the case that the polymers become hard after they were degraded, the two indicators show a good correlation. However, if they become brittle, their correlation is not good. Regarding this, their correlation is good in SiR, while it is not good in FR-XLPE and XLPE. In FR-EPR, the correlativity changes, depending on the aging condition and presumably its additives.

    DOI

  • Insulation performance of safety-related cables for nuclear power plants under simulated severe accident conditions

    Takefumi Minakawa, Masaaki Ikeda, Naoshi Hirai, Yoshimichi Ohki

    Proceedings of the International Symposium on Electrical Insulating Materials   2   716 - 719  2017.12  [Refereed]

     View Summary

    To examine electrical insulation behavior of cables under severe accident environmental conditions, safety-related low voltage cables used as severe accident management equipment were subjected to two tests that simulated two environments: 1) at 155 oC, which is the simulated highest assumed temperature when proper accident management measures are taken, 2) at 200 oC, which is the maximum durable temperature of a primary containment vessel. In addition, electrical insulation resistance was measured for the cables during steam exposure in the two tests. As a result, all the cables passed the acceptance criteria of the tests. The minimum volume resistivity of the cables during the test at 155 oC was around 108 Ωm, confirming that the cables have a sufficient insulation performance. On the other hand, the minimum volume resistivity during the severer test at 200 oC was around 105 Ωm due to the increase in leak current.

    DOI

  • Enhanced conductivity of polyaniline in the presence of nonionic amphiphilic polymers and their diverse morphologies

    Liuqing Yang, Wenling Wu, Yoshimichi Ohki, Yang Feng, Shengtao Li

    JOURNAL OF APPLIED POLYMER SCIENCE   134 ( 47 ) 45547(1) - 45547(10)  2017.12  [Refereed]

     View Summary

    Poly(ethylene oxide) (PEO) and its copolymers have excellent affinity for protons and contribute to proton transfer. In the present study, PEO and its copolymers, poly[(ethylene oxide)(20)-(propylene oxide)(70)-(ethylene oxide)(20)] (EO20PO70EO20, P123) and poly[(ethylene oxide)(106)-(propylene oxide)(70)-(ethylene oxide)(106)] (EO106PO70EO106, F127), have been found to significantly enhance the conductivity of polyaniline (PANI). After introducing these polymers, the conductivity of PANI is markedly promoted more than two orders of magnitude compared to that of PANI without additives, from 5.2 to 667 S/m. The molecular weight of PEO affects the conductivity of PANI/PEO. The mechanism by which these amphiphilic polymers are beneficial to the conductivity of PANI is studied experimentally and theoretically. The PANI/P123 prepared in the presence of PEO block copolymer shows gradually varying morphologies containing leaflike sheets, rodlike particles, and uniform chestnutlike sphere particles. This is similar to the morphology change of micelles with surfactant concentration. PEO, P123, and F127 are further found to have a positive effect on PANI as a material for sensors or supercapacitors, since high specific capacity and fast response rate are desired qualities in sensors and supercapacitors. (c) 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 45547.

    DOI

  • Far-Infrared Absorption of Deoxyribonucleic Acid with Thymine

    Ayano Kitamura, Marina Komatsu, Maya Mizuno, Hiroaki Kojima, Yoshimichi Ohki

    ELECTRONICS AND COMMUNICATIONS IN JAPAN   100 ( 12 ) 53 - 60  2017.12  [Refereed]

     View Summary

    Recently, attempts to use deoxyribonucleic acid (DNA) as an industrially important substance have been emerging. For that purpose, easy and convenient determination of the high-order structure of DNA is desirable. In this research, we measured far-infrared absorption spectra of various synthetic single-stranded DNAs, each with an identical base in its nucleotides, and found that DNAs with different bases exhibit different spectra. We also demonstrated that the number of thymine bases on a single-stranded DNA can be estimated from the spectra.

    DOI

  • ジブチルヒドロキシトルエンにおける遠赤外吸収 ~測定と量子化学計算によるスペクトル同定~

    遠藤滉士, 香西拓哉, 吉川武司, 中井浩巳, 大木義路

    電気学会誘電・絶縁材料研究会資料   DEI ( 17 ) 23 - 28  2017.12  [Refereed]

  • Dielectric absorption behavior of YAlO3 at terahertz frequencies

    Takaaki Morimoto, Yasuhiro Kuroda, Yoshimichi Ohki

    JAPANESE JOURNAL OF APPLIED PHYSICS   56 ( 10 ) 102601(1) - 102601(5)  2017.10  [Refereed]

     View Summary

    Using THz time-domain spectroscopy, optical or dielectric absorption spectra were obtained for YAlO3(100) single crystals. A sharp absorption peak appears at around 3.4 THz only when the THz electric field is parallel to the sample's [011] axis. This peak should be due to a normal mode of vibration, to which Y largely contributes. (C) 2017 The Japan Society of Applied Physics

    DOI

  • Optimization of a waveguide-mode sensing chip for an ultraviolet near-field illumination biosensor

    Chiaki Kuroda, Yoshimichi Ohki, Makoto Fujimaki

    OPTICS EXPRESS   25 ( 21 ) 26011 - 26019  2017.10  [Refereed]

     View Summary

    A waveguide-mode sensor with a planar sensing chip, consisting of two waveguiding layers and a glass substrate, is a promising candidate for a near-field illumination biosensor. Aiming at using fluorescent labeling induced by ultraviolet light, we optimize the structure of a waveguide-mode sensing chip, based on the mechanism for enhancing ultraviolet near-field light revealed by numerical calculations. Candidates of optimal materials are also presented. The chip optimized as above should be able to enhance the intensity of ultraviolet near-field light 25 times as high as an A1 surface plasmon resonance sensing chip. (C) 2017 Optical Society of America

    DOI

  • Dielectric and Carrier Transport Properties of Silicone Rubber Degraded by Gamma Irradiation

    Daomin Min, Chenyu Yan, Yin Huang, Shengtao Li, Yoshimichi Ohki

    POLYMERS   9 ( 10 ) 533(1) - 533(15)  2017.10  [Refereed]

     View Summary

    Silicone rubber (SiR) is used as an insulating material for cables installed in a nuclear power plant. Gamma rays irradiated SiR sheets for various periods at temperatures of 145 and 185 degrees C, and the resultant changes were analyzed by examining complex permittivity spectra and surface potential decay characteristics. Three different processes, namely, instantaneous polarization, electrode polarization due to the accumulation of ions to form double charge layers at dielectric/electrode interfaces, and DC conduction caused by directional hopping of ions, contribute to the complex permittivity. By fitting the spectra to theoretical equations, we can obtain the dielectric constant at high frequencies, concentration and diffusion coefficient of ions and DC conductivity for the pristine and degraded samples. The instantaneous polarization becomes active with an increase of dose and ageing temperature. The thermal expansion coefficient estimated from the temperature dependence of dielectric constant at high frequencies becomes smaller with an increase in dose, which is in good agreement with the experimental results of the swelling ratio. Additionally, trap distributions are calculated from surface potential decay measurements and analyzed to explain the variation in conductivity. Trap energy increases firstly, and then decreases with an increase in dose, leading to a similar change in DC conductivity. It is concluded that generations of both oxidative products and mobile ions, as well as the occurrence of chain scission and crosslinking are simultaneously induced by gamma rays.

    DOI

  • Development of a 1 kW direct methanol fuel cell system

    Y. Ohki

    IEEE Electrical Insulation Magazine   33 ( 5 ) 52 - 55  2017.08  [Refereed]

    DOI

  • Development of an Insulated Metal Substrate with High Thermal Conductivity (News from Japan)

    Y. Ohki

    IEEE Electrical Insulation Magazine   33 ( 4 ) 67 - 69  2017.08  [Refereed]

    DOI

  • Aging of poly(ether ether ketone) by heat and gamma rays - Its degradation mechanism and effects on mechanical, dielectric and thermal properties

    Liuqing Yang, Yoshimichi Ohki, Naoshi Hirai, Shugo Hanada

    POLYMER DEGRADATION AND STABILITY   142 ( 8 ) 117 - 128  2017.08  [Refereed]

     View Summary

    In radiation research facility, where heat and radiation are two crucial factors inducing material degradation, highly stable materials are required. In the present work, various properties of poly(ether ether ketone) (PEEK) exposed to heat and gamma rays in air are instrumentally investigated. If PEEK is aged by heat and gamma irradiation in air, chemical bond scission, oxidation, crosslinking, and char formation occur as competing mechanisms. The temperature plays a decisive role in degradation, while the irradiation accelerates it. The mechanical properties deteriorate much earlier than the dielectric properties with the progress of aging. This means that the mechanical properties can be an important condition monitoring factor compared to the dielectric properties for PEEK-insulated apparatus or cables. If PEEK is aged severely, its electrical conductivity increases dramatically, whereas both real and imaginary parts of complex dielectric permittivity decrease, showing a marked contrast to many industrially important polymeric insulating materials. (C) 2017 Elsevier Ltd. All rights reserved.

    DOI

  • Detection of norovirus virus-like particles using a surface plasmon resonance-assisted fluoroimmunosensor optimized for quantum dot fluorescent labels

    Hiroki Ashiba, Yuki Sugiyama, Xiaomin Wang, Haruko Shirato, Kyoko Higo-Moriguchi, Koki Taniguchi, Yoshimichi Ohki, Makoto Fujimaki

    BIOSENSORS & BIOELECTRONICS   93 ( 93 ) 260 - 266  2017.07  [Refereed]

     View Summary

    A highly sensitive biosensor to detect norovirus in environment is desired to prevent the spread of infection. In this study, we investigated a design of surface plasmon resonance (SPR)-assisted fluoroimmunosensor to increase its sensitivity and performed detection of norovirus virus-like particles (VLPs). A quantum dot fluorescent dye was employed because of its large Stokes shift. The sensor design was optimized for the CdSe-ZnS-based quantum dots. The optimal design was applied to a simple SPR-assisted fluoroimmunosensor that uses a sensor chip equipped with a V-shaped trench. Excitation efficiency of the quantum dots, degree of electric field enhancement by SPR, and intensity of auto fluorescence of a substrate of the sensor chip were theoretically and experimentally evaluated to maximize the signal-to-noise ratio. As the result, an excitation wavelength of 390 nm was selected to excite SPR on an Al film of the sensor chip. The sandwich assay of norovirus VLPs was performed using the designed sensor. Minimum detectable concentration of 0.01 ng/mL, which corresponds to 100 virus-like particles included in the detection region of the V-trench, was demonstrated. (C) 2016 The Authors. Published by Elsevier B.V.

    DOI

  • Terahertz spectroscopic analysis of crystal growth in poly(ethylene naphthalate)

    Daisuke Odaka, Yoshimichi Ohki

    JAPANESE JOURNAL OF APPLIED PHYSICS   56 ( 7 ) 072401(1) - 072401(7)  2017.07  [Refereed]

     View Summary

    Terahertz absorption spectra and X-ray diffraction patterns were measured for amorphous poly(ethylene naphthalate) (PEN) while the sample temperature was elevated from 25 to 230 degrees C then lowered back to 25 degrees C. Before the elevation of the temperature, PEN exhibited broad absorption with a maximum at around 2.6 THz. This absorption seems to originate from amorphous regions. As the sample temperature increases, PEN becomes crystallized in the form of alpha crystal. With this change in crystallinity, the 2.6 THz absorption becomes smaller, while another absorption peak at 2.05 THz, originating from crystalline regions, becomes larger. Furthermore, in the cooling process to 25 degrees C, the 2.05 THz absorption shifts to 2.15 THz and the lattice constants associated with this absorption become smaller. Therefore, intermolecular vibrations closely related to the crystal growth in PEN at high temperatures seem to be responsible for the THz absorption. (C) 2017 The Japan Society of Applied Physics

    DOI

  • Comparison between poly(ethylene naphthalate) and poly(ethylene terephthalate) in terms of gamma-ray irradiation on their dielectric properties

    Maki Miyamoto, Yoshimichi Ohki

    JAPANESE JOURNAL OF APPLIED PHYSICS   56 ( 6 ) 061601(1) - 061601(8)  2017.06  [Refereed]

     View Summary

    The effects of gamma-rays on the complex permittivity (epsilon,' and epsilon,") and electrical conductivity were compared between poly(ethylene naphthalate) (PEN) and poly(ethylene terephthalate) (PET). Although both epsilon,' and epsilon," increase in PET with an increase in the total dose of gamma irradiation, such increases are hardly observed in PEN. The conductivity is always smaller in PEN than in PET. Therefore, it has been confirmed that charge transport is less activated by gamma irradiation in PEN than in PET. Together with experimental results obtained by ultraviolet-visible absorption spectroscopy and thermogravimetric analysis, it can be concluded that PEN has a superior anti-gamma-ray dielectric property to PET. (C) 2017 The Japan Society of Applied Physics

    DOI

  • Blue-laser scanned imaging system using positioning marks formed on an optical disk substrate

    Takayuki Shima, Xiaomin Wang, Hideyuki Shiramizu, Makoto Fujimaki, Koichi Awazu, Yoshimichi Ohki

    JAPANESE JOURNAL OF APPLIED PHYSICS   56 ( 5 ) 058003(1) - 058003(3)  2017.05  [Refereed]

     View Summary

    We improved the optical disk-based imaging system to make more precise images at low cost for safety enhancement applications to observe and detect micrometer-order hazardous substances collected on a large-area surface. An optical disk substrate with positioning marks is prepared, and a signal processor is developed to automatically construct an image using the marks. We successfully obtained an image of Escherichia coli dispersed on an optical disk sample surface with a laser-scanning interval of 0.40 mu m in the optical disk's radial direction; this interval is reduced from the 0.74 and 1.36 mu m intervals demonstrated in our previous papers. (C) 2017 The Japan Society of Applied Physic

    DOI

  • Development of epoxy flux for lead-free solder (News from Japan)

    Y. Ohki

    IEEE Electrical Insulation Magazine   33 ( 3 ) 39 - 41  2017.04  [Refereed]

    DOI

  • Design of a sedimentation hole in a microfluidic channel to remove blood cells from diluted whole blood

    Chiaki Kuroda, Yoshimichi Ohki, Hiroki Ashiba, Makoto Fujimaki, Koichi Awazu, Makoto Makishima

    JAPANESE JOURNAL OF APPLIED PHYSICS   56 ( 3 ) 037201(1) - 037201(6)  2017.03  [Refereed]

     View Summary

    With the aim of developing a sensor for rapidly detecting viruses in a drop of blood, in this study, we analyze the shape of a hole in a microfluidic channel in relation to the efficiency of sedimentation of blood cells. The efficiency of sedimentation is examined on the basis of our calculation and experimental results for two types of sedimentation hole, cylindrical and truncated conical holes, focusing on the Boycott effect, which can promote the sedimentation of blood cells from a downward-facing wall. As a result, we demonstrated that blood cells can be eliminated with an efficiency of 99% or higher by retaining a diluted blood sample of about 30 mu L in the conical hole for only 2 min. Moreover, we succeeded in detecting the antihepatitis B surface antigen antibody in blood using a waveguide-mode sensor equipped with a microfluidic channel having the conical sedimentation hole. (C) 2017 The Japan Society of Applied Physics.

    DOI

  • Crystallinity of poly(ethylene naphthalate) and its relation to terahertz absorption

    Daisuke Odaka, Yoshimichi Ohki, Maya Mizuno

    Japanese Journal of Applied Physics   56 ( 3 ) 032402(1) - 032402(6)  2017.03  [Refereed]

     View Summary

    The crystallinity of poly(ethylene naphthalate) (PEN) was changed by thermal annealing and its effects on terahertz absorption spectra and X-ray diffraction patterns were examined. As a result, the intensity of a relatively sharp absorption peak at around 2.2 THz was found to be proportional to the crystallinity. As a plausible cause of this absorption, a lattice vibration, a torsional vibration, or their combination mode associated with (010) planes in crystal α of PEN is proposed. Therefore, the crystallinity of PEN can be estimated from the intensity of this absorption.

    DOI

  • Complex Permittivity Spectra of Various Insulating Polymers at Ultrawide-Band Frequencies

    Yuka Hasegawa, Yoshimichi Ohki, Kaori Fukunaga, Maya Mizuno, Kensuke Sasaki

    ELECTRICAL ENGINEERING IN JAPAN   198 ( 3 ) 11 - 18  2017.02  [Refereed]

     View Summary

    Nowadays, trials to use even higher frequencies are pursued to realize high-speed wireless communications. In order to respond to such a trend, dielectric properties of insulating polymers at high frequencies have to be clarified, although the importance of clarifying low-frequency properties is still very high, especially for use as high-voltage insulators. In this regard, complex permittivity spectra from 10 mHz to 4.0 THz were obtained for various insulating polymers. The spectra obtained in the very wide frequency range are important, because they can be a valuable database and they provide various pieces of important information on dielectric characteristics. (C) 2016 Wiley Periodicals, Inc.

    DOI

  • 透過光像取得型光ディスク装置を用いた大腸菌の選択検出

    白水 秀幸, 黒田 千愛, 大木 義路, 島 隆之, 王 暁民, 藤巻 真

    電気学会研究会資料   LAV-17-005/IM-17-005   27 - 31  2017.02

  • Experimental investigation of the degradation mechanism of silicone rubber exposed to heat and gamma rays

    Shugo Hanada, Maki Miyamoto, Naoshi Hirai, Liuqing Yang, Yoshimichi Ohki

    High Voltage   2 ( 2 ) 92 - 101  2017  [Refereed]  [Invited]

    DOI

  • Terahertz imaging of iron powder in low density polyethylene sheets

    Takuya Kozai, Daisuke Odaka, Yoshimichi Ohki, Takashi Saitou, Takanori Yamazaki

    IEEJ Transactions on Fundamentals and Materials   137 ( 5 ) 311 - 312  2017  [Refereed]

     View Summary

    If powder of iron is included in the insulation of a polymer-insulated cable during its manufacturing process, breakdown or a water tree would be triggered from there. In this study, we mixed powder of iron into a sheet of low-density polyethylene and measured spatial distributions of absorption at terahertz (THz) frequencies. As a result, it has become clear that the absorption increases in the whole range from 0.5 to 5.0 THz at the places where the powder exists, even if the sheet is covered with an opaque carbon-black-loaded polyethylene sheet.

    DOI

  • Comparison between far-IR and IR absorption spectroscopy for estimating vinyl acetate content in ethylene-vinyl acetate copolymer

    Daisuke Odaka, Tomoyuki Izutsu, Marina Komatsu, Yoshimichi Ohki, Maya Mizuno

    IEEJ Transactions on Fundamentals and Materials   137 ( 3 ) 171 - 177  2017  [Refereed]

     View Summary

    Far-IR absorption spectra at frequencies from 0.5 to 21.0 THz were obtained for sheets of ethylene-vinyl acetate copolymer (EVA) containing different contents of vinyl acetate (VA) from 12 to 60% with different thicknesses from 1.0 to 3.0 mm. As a result, EVA exhibits absorption at 2.5, 6.7, 10.5, 11-17, and 17-20 THz. Especially, the 2.5-THz intensity with a good proportionality up to 60% of VA content exhibits comparable accuracy to the IR method and is applicable for estimating the VA content in EVA sheets 2.0 or 3.0 mm thick, for which IR method is difficult to be applied.

    DOI

  • 高分子電気絶縁材料の酸化検出法としてのケミルミネッセンスの最近の進歩

    大木義路, 平井直志, 池野理沙

    マテリアルライフ学会誌   29 ( 1 ) 1 - 5  2017  [Invited]

  • Far-infrared absorption of deoxyribonucleic acid with thymine

    Ayano Kitamura, Marina Komatsu, Maya Mizuno, Hiroaki Kojima, Yoshimichi Ohki

    IEEJ Transactions on Fundamentals and Materials   137 ( 1 ) 88 - 94  2017  [Refereed]

     View Summary

    Recently, attempts to use deoxyribonucleic acid (DNA) as an industrially important substance have been emerging. For that purpose, easy and convenient determination of the higher-order structure of DNA is desirable. In this research, we measured far-infrared absorption spectra of various synthetic single stranded DNAs, each with an identical base in its nucleotides, and found that DNAs with different bases exhibit different spectra. We also demonstrated that the number of thymine bases on a single stranded DNA can be estimated from the spectra.

    DOI

  • Failure aspects of electronic watt-hour meters due to direct lightning strikes to distribution lines - Electromagnetic disturbance caused by lightning current flowing through watt-hour meters and its countermeasure-

    Kazuyuki Ishimoto, Ryota Mori, Akira Asakawa, Yoshimichi Ohki

    IEEJ Transactions on Power and Energy   137 ( 12 ) 766 - 776  2017  [Refereed]

     View Summary

    Recently, advanced "smart meters" have been introduced widely to electric power distribution lines. Since smart meters consist of many low-voltage electronic devices, their vulnerability to disturbances such as lightning surges is a matter of concern. Therefore, to establish an effective lightning protection countermeasure against these disturbances, we have been investigating the lightning failure aspects of electronic watt-hour meters (WHMs), which have a function similar to that of a watt-meter section in smart meters. In this paper, we explored countermeasures for improving the performance of WHMs against the lightning transient magnetic fields. At first, we examined the performance of WHMs by lightning impulse current tests. As a result, it has become clear that the increase in distance between a bus conductor and a processing unit and magnetic shield of a processing unit are effective in reducing the probability of failures caused by the transient magnetic fields. Next, we proposed a calculation method of the lightning failure rate of WHMs based on a lightning surge analysis. The calculation results indicate that the failure occurs more frequently by subsequent strokes than by first strokes or if the grounding resistance of a pole transformer is higher.

    DOI

  • Commencement of Operation of the World's Largest Storage Battery Facility

    Y. Ohki

    IEEE ELECTRICAL INSULATION MAGAZINE   33 ( 1 ) 59 - 61  2017.01  [Refereed]

    DOI

  • Space charge distributions in epoxy/MgO nanocomposites at various temperatures

    Q. Xie, Y. H. Cheng, N. Hirai, Y. Ohki

    ICHVE 2016 - 2016 IEEE International Conference on High Voltage Engineering and Application    2016.12  [Refereed]

     View Summary

    The effect of addition of nano-sized magnesia fillers to epoxy resin on the distribution of space charges is examined at various temperatures from 40 to 200 °C. Although the spatial distribution of space charges in the sample varies depending on temperature and filler content, magnesia nanocomposites exhibit a smaller amount of homocharge near the cathode than the unfilled epoxy resin at 40 and 80 °C. At 120 and 140 °C, although the polarity of space charge in the vicinity of the cathode becomes positive or hetero, its amount becomes smaller with the increase in content of magnesia nanofillers. This finding agrees well with the fact that magnesia nanofillers effectively suppress the formation of space charge in low-density or crosslinked polyethylene. As another distinctive feature, it has become clear that the addition of magnesia nanofillers effectively suppresses carrier transport, especially at high temperatures.

    DOI

  • Dielectric Spectroscopic Analysis of Degradation in Ethylene-Propylene-Diene Copolymer

    Daomin Min, Shengtao Li, Naoshi Hirai, Yoshimichi Ohki

    IEEE TRANSACTIONS ON DIELECTRICS AND ELECTRICAL INSULATION   23 ( 6 ) 3620 - 3630  2016.12  [Refereed]

     View Summary

    Gamma-ray induced changes occurring in ethylene-propylene-diene copolymer (EPDM) were analyzed through its complex permittivity spectra. First, four different processes, namely, instantaneous polarization, hopping of ions, segmental relaxation, and dc conduction are found to contribute to the complex permittivity. By fitting the spectra to theoretical equations, it has become clear that the instantaneous polarization and ionic hopping become active by the irradiation, indicating that generation of oxidative products and mobile ions and occurrence of chain scission are induced by gamma rays. Furthermore, since both the thermal expansion and the contribution of the segmental relaxation to the permittivity become less with an increase in dose, it is assumed that crosslinking is induced by the gamma irradiation.

    DOI

  • Superior High-temperature Dielectric Properties of Dicyclopentadiene Resin

    Y. Masuzaki, Y. Suzuki, Y. Ohki

    IEEE TRANSACTIONS ON DIELECTRICS AND ELECTRICAL INSULATION   23 ( 5 ) 3078 - 3085  2016.10  [Refereed]

     View Summary

    Complex permittivity (epsilon(r)' and epsilon(r)") and conductivity were measured in a wide temperature range for dicyclopentadiene (DCP) resin and epoxy resin, which show glass transition at a similar temperature around 150 degrees C. Furthermore, space charge distributions remaining in the two resins that had been polarized in the same wide temperature range were measured at room temperature. As a result, it was found that epsilon(r)' epsilon(r)", and conductivity are much lower in DCP resin than in epoxy resin at almost all the temperatures and frequencies. A further analysis using complex electric modulus, which is the inverse of the complex permittivity, indicates that charge transport is much more difficult in DCP resin. Furthermore, while similar small amounts of homocharges appear in DCP resin at any polarization temperatures, a significant accumulation of heterocharges, most likely due to ions, is induced in epoxy resin in the vicinity of the cathode/sample interface at polarization temperatures above the glass transition temperature. These results indicate that DCP resin possesses superior stable dielectric behavior, especially at high temperatures.

    DOI

  • Electronic excitation and relaxation processes of oxygen vacancies in YSZ and their involvement in photoluminescence

    Takaaki Morimoto, Yasuhiro Kuroda, Yoshimichi Ohki

    APPLIED PHYSICS A-MATERIALS SCIENCE & PROCESSING   122 ( 9 ) 790-1 - 790-8  2016.09  [Refereed]

     View Summary

    Yttria-stabilized zirconia (YSZ) consists of zirconia and yttria and oxygen vacancies appear in accordance with the ratio of yttria. The oxygen vacancy would sometimes give annoyance, but it would be beneficial on other occasions, depending on its applications. Photoluminescence (PL) due to oxygen vacancies induced by photons with energies around 5.5 eV exhibits two decay time constants. As a possible reason for this, an oxygen vacancy changes its charging state from neutral to positive monovalent by losing an electron when YSZ is irradiated by ultraviolet photons. The PL decays either in a ms range or in a ns range, depending on whether the oxygen vacancies are neutral or positive monovalent.

    DOI

  • A New Sensor for Detecting a Short-Circuit Fault in a Rotating Electric Machine (News from Japan)

    Y. Ohki

    IEEE Electrical Insulation Magazine   32 ( 5 ) 56 - 58  2016.08  [Refereed]

    DOI

  • Observation of crystal growth in polyethylene naphthalate by terahertz spectroscopy

    D. Odaka, M. Komatsu, Y. Ohki

    Proceedings of the 2016 IEEE International Conference on Dielectrics, ICD 2016   1   593 - 596  2016.08  [Refereed]

     View Summary

    Various properties of polyethylene naphthalate (PEN) are affected by its crystallinity. In this paper, the possibility of using terahertz (THz) spectroscopy as a tool to examine the crystallinity of PEN is studied. When THz absorption is measured while the sample temperature is being raised, an absorption peak suddenly appears at around 2.10 THz at 180 °C. By analyzing the THz spectra together with XRD patterns, we found that the 2.10-THz absorption has a close relation with the crystal form α of PEN and its intensity increases with the progress of crystallization. Therefore, THz absorption spectroscopy can be used for monitoring the crystallization of PEN.

    DOI

  • Construction of a new phase-shifting transformer (News from Japan)

    Y. Ohki

    IEEE Electrical Insulation Magazine   32 ( 3 ) 44 - 47  2016.08  [Refereed]

    DOI

  • Observation of lightning strikes at Tokyo skytree (News from Japan)

    Y. Ohki

    IEEE Electrical Insulation Magazine   32 ( 4 ) 57 - 59  2016.08  [Refereed]

    DOI

  • Comparison of Gamma-ray Resistance between Dicyclopentadiene Resin and Epoxy Resin

    M. Miyamoto, N. Tomite, Y. Ohki

    IEEE TRANSACTIONS ON DIELECTRICS AND ELECTRICAL INSULATION   23 ( 4 ) 2270 - 2277  2016.08  [Refereed]

     View Summary

    Radiation resistance was compared between dicyclopentadiene (DCP) resin (P) and epoxy resin (E) by analyzing mainly their complex permittivity (epsilon(r)' and epsilon(r '')) and electrical conductivity. While both epsilon(r)' and epsilon(r)'' increase significantly in E with an increase in total dose of the gamma irradiation, no such increases are observed in P. The conductivity is always smaller in P than in E and the difference becomes larger with the increase in total dose. This difference becomes very obvious if we pay attention to electric modulus spectra. These results indicate clearly that P has a better gamma resistance than E.

    DOI

  • Estimation of Gel Fraction of Polyethylene Cross-linked with Silane by Far-infrared Absorption Spectroscopy

    Junya Takihana, Marina Komatsu, Yoshimichi Ohki

    IEEE TRANSACTIONS ON DIELECTRICS AND ELECTRICAL INSULATION   23 ( 3 ) 1500 - 1505  2016.06  [Refereed]

     View Summary

    Far-infrared absorption spectra were measured for low density polyethylene in a frequency range from 3.0 to 120 THz. When the polyethylene was cross-linked using vinyl silane, the absorption was found to increase in proportion to the cross-linking degree at 13.2, 30.9, and 32.7 THz, where absorption attributable to Si-O bonds appears. Therefore, the degree of cross-linking of polyethylene by vinyl silane can be estimated by far-infrared absorption spectroscopy.

    DOI

  • A New Method for Estimating the Content of Vinyl Acetate in Ethylene-Vinyl Acetate Copolymer

    Tomoyuki Izutsu, Daisuke Odaka, Marina Komatsu, Yoshimichi Ohki, Maya Mizuno, Yoshiaki Nakamura, Naofumi Chiwata

    IEEE TRANSACTIONS ON DIELECTRICS AND ELECTRICAL INSULATION   23 ( 3 ) 1260 - 1265  2016.06  [Refereed]

     View Summary

    A new method for estimating the content of vinyl acetate (VA) in ethylene-vinyl acetate copolymer (EVA) is proposed. It is a kind of optical absorption spectroscopy in a frequency range from 0.5 to 21.0 THz. Comparison of absorption spectra among sheets of EVA with differing contents of VA from 12 to 60% by weight and a sheet of low density polyethylene containing no VA revealed that the EVA sheets exhibit relatively sharp absorption at 2.5, 6.7, and 10.5 THz and rather broad absorption at 11 to 17 THz and 17 to 20 THz. All these absorption intensities are in proportion to the content of VA, up to certain contents, which enables us to estimate the content of VA in EVA quantitatively. Among them, the absorption at 2.5 THz measured by THz time-domain spectroscopy shows the best accuracy.

    DOI

  • World’s Largest-Class Battery Energy Storage System (News from Japan)

    Y. Ohki

    IEEE Electrical Insulation Magazine   32 ( 2 ) 64 - 66  2016.02  [Refereed]

    DOI

  • Effects of the Structure and Insulation Material of a Cable on the Ability of a Location Method by FDR

    Yoshimichi Ohki, Naoshi Hirai

    IEEE TRANSACTIONS ON DIELECTRICS AND ELECTRICAL INSULATION   23 ( 1 ) 77 - 84  2016.02  [Refereed]

     View Summary

    The authors have demonstrated that the estimation of the precise locations of points in cables aged thermally or mechanically or by the irradiation of gamma rays is possible by a combination of frequency domain reflectometry and inverse fast Fourier transform. This paper examines how this ability of fault location depends on the type, structure, and insulation material of the cable, using several kinds of polymer insulated cables such as a triple core cable insulated with flame-retardant ethylene propylene diene copolymer, a dual core cable insulated with silicone rubber, a dual core cable insulated with crosslinked polyolefin (XLPO), a coaxial cylindrical cable insulated with low density polyethylene, and a flat cord insulated by polyvinyl chloride (PVC). As a result, for cables with lengths from 16 to 72 m, the maximum sensitivity is attained when the highest frequency of the inputted electromagnetic waves is 1.0 GHz or higher for coaxial cables, but around 300 to 800 MHz for dual or triple core cables. For the location attempt of the position heated by a heater, cables insulated with PVC give higher signal intensities than those insulated with XLPO.

    DOI

  • Numerical Simulation on Molecular Displacement and DC Breakdown of LDPE

    Daomin Min, Shengtao Li, Yoshimichi Ohki

    IEEE TRANSACTIONS ON DIELECTRICS AND ELECTRICAL INSULATION   23 ( 1 ) 507 - 516  2016.02  [Refereed]

     View Summary

    It is generally known that the dc breakdown strength of low density polyethylene (LDPE) decreases with as the thickness and temperature of the sample increase. The breakdown strength is influenced by the charge transport and electric field distortion, and is also related to the molecular chain displacement and fracture. This paper investigates mutual relations among the charge transport, molecular chain displacement, and thickness dependent dc breakdown of LDPE. A model that combines the dynamics of charge transport and molecular displacement (CTMD) is used to calculate the space charge accumulation, molecular chain displacement, and dc breakdown properties of LDPE with various thicknesses at various constant voltage ramping rates. It is assumed that breakdown occurs when the molecular chain displacement reaches a critical value. The simulation results show that the breakdown field as a function of sample thickness satisfies an inverse power law with a power index of about 0.43 for various voltage ramping rates. This is consistent with experimental results. The CTMD model considers both the distortion of electric field and the displacement kinetics of molecular chains, resulting in a power index closer to the experiment than that calculated only from the electric field distortion. Adopted a Williams-Landel-Ferry type molecular chain mobility in the CTMD model, the simulation results are consistent with the results calculated by applying experimental results on polyisobutylene and polymethyl methacrylate to the free volume breakdown theory. It is also found that the CTMD model with temperature-dependent molecular chain mobility controlled by piecewise Arrhenius equations can explain well the temperature dependent breakdown experimental results of LDPE.

    DOI

  • Modeling of Oxidation Process and Property Changes of Ethylene-Propylene-Diene Copolymer

    Daomin Min, Shengtao Li, Naoshi Hirai, Yoshimichi Ohki

    IEEE TRANSACTIONS ON DIELECTRICS AND ELECTRICAL INSULATION   23 ( 1 ) 537 - 546  2016.02  [Refereed]

     View Summary

    The oxidation process induced in ethylene-propylene-diene (EPDM) copolymer by gamma irradiation was simulated by solving equations on chemical reactions and gas diffusion rates. As a result, distributions of oxidative products and gases and changes in material properties were clarified. All the oxidative products such as ketones, alcohols, and carboxylic acids, crosslinks between molecular chains, and chain scissions in EPDM increase with irradiation and they show concave spatial distributions inside the sample sheets. The simulation results demonstrates that EPDM becomes hard when it was irradiated by gamma rays and the increase in hardness is more significant at the surface of the sample sheet than its inside. Moreover, it was found that a low diffusion coefficient of oxygen in EPDM leads to the appearance of a clearer diffusion-limited regime of degradation. These simulation results are in good agreement with experimental results.

    DOI

  • Rapid detection of hemagglutination using restrictive microfluidic channels equipped with waveguide-mode sensors

    Hiroki Ashiba, Makoto Fujimaki, Koichi Awazu, Mengying Fu, Yoshimichi Ohki, Torahiko Tanaka, Makoto Makishima

    JAPANESE JOURNAL OF APPLIED PHYSICS   55 ( 2 ) 027002(1) - 027002(7)  2016.02  [Refereed]

     View Summary

    Hemagglutination is utilized for various immunological assays, including blood typing and virus detection. Herein, we describe a method of rapid hemagglutination detection based on a microfluidic channel installed on an optical waveguide-mode sensor. Human blood samples mixed with hemagglutinating antibodies associated with different blood groups were injected into the microfluidic channel, and reflectance spectra of the samples were measured after stopping the flow. The agglutinated and nonagglutinated samples were distinguishable by the alterations in their reflectance spectra with time; the microfluidic channels worked as spatial restraints for agglutinated red blood cells. The demonstrated system allowed rapid hemagglutination detection within 1 min. The suitable height of the channels was also discussed. (C) 2016 The Japan Society of Applied Physics

    DOI

  • Diagnosis of surface degradation of flame-retardant ethylene propylene diene copolymer by scanning probe microscopy

    Takayuki Niki, Naoshi Hirai, Yoshimichi Ohki

    JOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY   53 ( 1 ) 82 - 88  2016.01  [Refereed]

     View Summary

    Most electric cables installed in nuclear power plants use organic polymers for their electrical insulation. Regarding this, degradation of the polymers could lead to fatal accidents. However, no truly reliable diagnostic methods that can detect the degradation of polymer insulation in electric cables have been established. Therefore, development of a reliable diagnostic method is very important. The present research shows that scanning probe microscopy can be a good tool to evaluate microscopic changes induced on the surface of flame-retardant ethylene propylene diene copolymer by its degradation.

    DOI

  • 近接場光学センサーを用いた硫酸銅メッキ液中の添加剤管理方法

    藤巻真, 佐藤洸太朗, 大木義路

    表面技術   67 ( 11 ) 575 - 580  2016  [Refereed]

    DOI

  • 局所熱・放射線同時劣化シリコーンゴム絶縁ケーブルの周波数領域反射測定法と線路共振解析法による劣化位置標定

    平井直志, 大木義路

    電気学会論文誌A   136 ( 9 ) 553 - 560  2016  [Refereed]

    DOI

  • Effects of ion implantation on oxygen vacancies in YSZ

    Yasuhiro Kuroda, Takaaki Morimoto, Yoshimichi Ohki

    IEEJ Transactions on Fundamentals and Materials   136 ( 9 ) 592 - 593  2016  [Refereed]

     View Summary

    Photoluminescence (PL) due to oxygen vacancies appearing in yttria-stabilized zirconia (YSZ) decreases clearly by ion implantation in both bulky single crystal plates and thin multicrystal films. This indicates that segregation into Y2O3 and ZrO2 and change in charging state of oxygen vacancy occur similarly in the two substances despite big differences in their thicknesses and morphologies.

    DOI

  • Detection of overheated parts in low-density polyethylene by terahertz absorption spectroscopy and imaging

    Tomoyuki Izutsu, Daisuke Odaka, Takuya Kozai, Marina Komatsu, Yoshimichi Ohki, Takashi Saitou, Takanori Yamazaki

    IEEJ Transactions on Fundamentals and Materials   136 ( 9 ) 603 - 608  2016  [Refereed]

     View Summary

    Overheating during the manufacturing process of a cross-linked polyethylene-insulated cable often induces brownish resin, which may become a starting point of breakdown and water trees. In this study, an XLPE sheet was overheated and cut to many pieces of squares with various sizes from 0.5×0.5 to 10×10 mm2. Then, each piece was sandwiched between two sheets of low-density polyethylene and its absorption intensity was measured at terahertz (THz) frequencies. Furthermore, the spatial distribution of absorption was measured in the form of a two-dimensional image by scanning the sample. As a result, it has become clear that the whole absorption increases in a range from 0.5 to 5.0 THz when the volume of overheated part becomes bigger. Moreover, the distribution of overheated parts can be visualized by the THz absorption imaging.

    DOI

  • Electronic transition process of fluorescence appearing in various organic polymers

    Seiya Nishikawa, Masashi Tonoi, Takeshi Yoshikawa, Naoshi Hirai, Hiromi Nakai, Yoshimichi Ohki

    IEEJ Transactions on Fundamentals and Materials   136 ( 4 ) 205 - 211  2016  [Refereed]

     View Summary

    Low density polyethylene (LDPE) and polypropylene (PP) exhibit a photoluminescence (PL) band, which emits photons with an energy of around 4.3 eV when excited by photons with energies around 6.4 eV. The origin of this PL band has been assigned to α, β -unsaturated carbonyl. In this paper, the appearance of PL was examined for four kinds of polyolefin and four kinds of biodegradable polymers. As a result, it has become clear that all the polymers show a PL band with a PL excitation spectrum and a PL spectrum similar to those of the above-mentioned PL band in LDPE and PP. The decay profiles observed for these PL bands indicate that they are fluorescence. Furthermore, the intensity of the PL becomes weak for all the four polyolefin samples and the polylatic acid sample if ultraviolet photons were irradiated to the sample. Quantum chemical calculations carried out by assuming a model of α, β -unsaturated carbonyl have revealed that the PL originates in the π∗to π transition of electrons in the carbon-carbon double bond.

    DOI

  • Estimation of talc contents in ethylene-propylene-diene copolymer by terahertz absorption spectroscopy

    Tomoyuki Izutsu, Marina Komatsu, Yoshimichi Ohki, Maya Mizuno, Yoshiaki Nakamura, Naofumi Chiwata

    IEEJ Transactions on Fundamentals and Materials   136 ( 2 ) 81 - 85  2016  [Refereed]

     View Summary

    The authors examined the possibility of terahertz (THz) absorption spectroscopy whether it can be used as a tool to estimate the content of talc in ethylene-propylene-diene copolymer (EPDM), an important factor determining the electrical insulation properties of EPDM-insulated power cables. For this purpose, absorption spectra were measured in a frequency range from 0.3 to 10.0 THz for EPDM sheets containing different contents of talc, a disk-shaped sample made of talc powder, and a similar disk-shaped sample made of a mixture of talc and high-density polyethylene. As a result, the whole absorption in the frequency range from 0.3 to 10.0 THz is increased by the addition of talc into EPDM sheets. More specifically, it has become clear that talc in EPDM induces absorption peaks at 3.05, 4.8, 5.1, 5.3, 6.9, and 7.8 THz. The integrated absorption coefficients calculated for these absorption peaks show a good linearity with respect to the content of talc. Therefore, by watching the increments of these absorption peaks, the content of talc can be quantitatively estimated.

    DOI

  • Structural change induced in LaAlO3 by ion implantation

    Masayuki Harima, Takaaki Morimoto, Yoshimichi Ohki

    IEEJ TRANSACTIONS ON ELECTRICAL AND ELECTRONIC ENGINEERING   11 ( 1 ) 5 - 9  2016.01  [Refereed]

     View Summary

    Ion implantation is used for various purposes in manufacturing semiconductor devices such as MOS-FETs. In the present study, the effects of implantation of P+ or B+ ions on the structural change of single-crystal LaAlO3 were examined. The optical absorption edge located at approximate to 5.6 eV, which seems to correspond to the bandgap energy of LaAlO3, is hardly affected by the ion implantation. The X-ray diffraction peak intensity at 2=23.5 degrees is decreased by the ion implantation. The intensities of three sharp photoluminescence (PL) peaks detected at 1.62, 1.65, and 1.69 eV, which appear only when the samples are crystalline, become smaller by ion implantation. However, the intensity of a broad PL peak at approximate to 2.8 eV due to the oxygen vacancy, which is detectable in both amorphous and crystalline samples, hardly changes after the ion implantation. These results indicate that the ion implantation degrades the crystallinity of LaAlO3. (c) 2015 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

    DOI

  • Involvement of crystallinity in various luminescent bands in yttrium aluminate

    Takaaki Morimoto, Masayuki Harima, Yosuke Horii, Yoshimichi Ohki

    NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS   366   198 - 205  2016.01  [Refereed]

     View Summary

    When single crystal YAlO3 was implanted with P+ or B+ ions, optical absorption increases significantly at energies slightly lower than the band gap energy, indicating that localized electronic states were induced. Furthermore, the ion implantation decreases the intensity of an X-ray diffraction peak and changes its position randomly, which indicates that the crystalline structure of the sample was deformed. The inten-sities of photoluminescence (PL) bands due to impurities of Cr3+ and Er3+ and those originating in self trapped excitons and antisites become smaller or disappear after the ion implantation. On the other hand, the intensity of the PL due to oxygen vacancies does not change. Such contrasting effects of the sample's crystallinity on the luminescence intensity are explained by the different manners of involvement of the crystal structure in the luminescence mechanism among these PLs in YAlO3. (C) 2015 Elsevier B.V. All rights reserved.

    DOI

  • Chemiluminescence Characteristics of FR-EPDM and SiR Aged by Concurrently-Given Heat and Radiation

    Risa Ikeno, Naoshi Hirai, Yoshimichi Ohki

    2016 IEEE CONFERENCE ON ELECTRICAL INSULATION AND DIELECTRIC PHENOMENA (IEEE CEIDP)     853 - 856  2016  [Refereed]

     View Summary

    Flame-retardant ethylene propylene diene copolymer (FR-EPDM) and silicone rubber (SiR) were aged by heat with or without concurrent irradiation of gamma rays. For these samples, chemiluminescence (CL) and infrared absorption spectra were measured. Although the CL intensity decreases with an increase in total dose of gamma rays both in FR-EPDM and SiR, this negative dose-dependence is weaker in FR-EPDM than in SiR. This is reasonable, since CL appears when a substance is oxidized. Namely, the CL intensity becomes weak if the sample was already oxidized. While FR-EPDM is an organic polymer that can be oxidized easily, SiR is an inorganic polymer with siloxane bonds. Moreover, carbonyl groups appear when FR-EPDM is degraded, while SiR forms mainly a crosslinked structure by bridging two silicons by one oxygen when it is irradiated by gamma rays. Such differences in degradation mechanism between two insulating polymers are reflected on their CL properties.

  • Comparison of Location Abilities of Degradation in a Polymer-insulated Cable between Frequency Domain Reflectometry and Line Resonance Analysis

    Yoshimichi Ohki, Naoshi Hirai

    2016 IEEE INTERNATIONAL CONFERENCE ON HIGH VOLTAGE ENGINEERING AND APPLICATION (ICHVE)    2016  [Refereed]

     View Summary

    The ability of locating a degraded portion in a double-core cable insulated with silicone rubber (SiR) or in a coaxial cable insulated with low-density polyethylene (LDPE) was compared between a system based on frequency domain reflectometry (FDR) being developed by the authors and a commercially available system called line resonance analysis (LIRA). Both the FDR and the LIRA methods cannot detect the degradation of SiR before its further use becomes uncommendable in a nuclear power plant. However, for location trials of the degraded portion in the LDPE coaxial cable, the FDR method showed a higher sensitivity and a better resolution than LIRA.

  • Non-destructive Diagnosis of Degradation of Silicone Rubber by Indenter Modulus and Scanning Probe Microscopy

    Shugo Hanada, Daisuke Odaka, Liuqing Yang, Naoshi Hirai, Yoshimichi Ohki

    2016 INTERNATIONAL CONFERENCE ON CONDITION MONITORING AND DIAGNOSIS (CMD)     123 - 126  2016  [Refereed]

     View Summary

    We aged silicone rubber, which is widely used for electrical insulation of cables in nuclear power plants under various conditions. The surface of each sample was measured by an indenter and a scanning probe microscope (SPM) for non-destructive diagnosis. As a result, it has become clear that the surface of silicone rubber becomes hard with the progress of degradation in both cases of thermal aging and simultaneous aging by heat and radiation. The indenter modulus shows a good correlation with the elongation at break (EAB). On the other hand, the delay of phase angle measured by SPM exhibits a rather low correlation with EAB. Through the results of thermal analyses, it seems that the thermal aging without radiation induces cross-linking first, which is followed by bond break. However, bond breakage seems predominant over cross-linking in the simultaneous aging with heat and radiation.

  • Terahertz Spectroscopic Diagnosis of Degradation of Ethylene-Propylene-Diene Copolymer

    Yoshimichi Ohki, Takuya Kozai, Marina Komatsu, Naoshi Hirai

    2016 IEEE INTERNATIONAL CONFERENCE ON DIELECTRICS (ICD), VOLS 1-2     646 - 649  2016  [Refereed]

     View Summary

    Flame-retardant ethylene-propylene-diene copolymer (FR-EPDM) with the same additives as those used in nuclear power plants was analyzed by terahertz (THz) and infrared (IR) absorption spectroscopy, before and after the irradiation by Trays. As a result, it has become clear that THz spectroscopy can indicate the degradation of the FR-EPDM samples better than IR spectroscopy. Namely, the THz spectroscopy can indicate the oxidation of the samples, which is difficult by IR spectroscopy.

  • Fault Location in a Cable for a Nuclear Power Plant by Frequency Domain Reflectometry

    Yoshimichi Ohki, Naoshi Hirai

    2016 INTERNATIONAL CONFERENCE ON CONDITION MONITORING AND DIAGNOSIS (CMD)     36 - 39  2016  [Refereed]

     View Summary

    Cables installed in nuclear power plants are subjected to thermal burden and radioactive rays. Regarding this, if we want to monitor the integrity of a cable in a nuclear power plant continuously, we have to do measurements while the cable is heated and irradiated. In this research, we obtained frequency domain reflectometry signals for a 50-m long cable insulated with flame-retardant ethylene propylene diene copolymer (FR-EPDM) installed in an environment where heat and radiation were given simultaneously. Then, we separated signals originating in the degradation of the insulation from those due to heat and radiation. This result indicates that the separation of the signal arising from the degradation of insulation from other signals due to heat and radiation is in principle possible at least as far as FR-EPDM insulated cables are concerned.

  • Aging Mechanism of Silicone Rubber by Heat and Gamma-rays

    Yoshimichi Ohki, Shugo Hanada, Maki Miyamoto, Naoshi Hirai, Liuqing Yang

    2016 IEEE CONFERENCE ON ELECTRICAL INSULATION AND DIELECTRIC PHENOMENA (IEEE CEIDP)     869 - 872  2016  [Refereed]

     View Summary

    The aging mechanism of silicone rubber, which is important for electrical insulation of cables, is examined from many aspects such as instrumental, chemical, electrical, and mechanical analyses. As a result, it has become clear that silicone rubber degrades by forming crosslinked structures via the formation of abundant siloxane bonds.

  • Turbine Generator for Downwind Power Generation With Improved Antiseismic Design (News from Japan)

    Y. Ohki

    IEEE Electrical Insulation Magazine   32 ( 1 ) 50 - 51  2015.12  [Refereed]

    DOI

  • Dielectric Properties of Three Liquid Crystal Polymers

    Peng Yang, Yoshimichi Ohki

    IEEJ TRANSACTIONS ON ELECTRICAL AND ELECTRONIC ENGINEERING   10 ( 6 ) 609 - 613  2015.11  [Refereed]

     View Summary

    Dielectric properties of three kinds of liquid crystal polymers (LCPs) were investigated by measuring their thermal properties, thermally stimulated polarization and depolarization currents, and complex dielectric permittivity. To make clear the differences among the three LCP samples, complex electric modulus was introduced, which was found to be effective to suppress the effects of large conduction currents at high temperatures. Moreover, the slopes of the frequency dependence of [(r)' and ](r)'' are different below and above the phase transition temperature, indicating that the accumulation of charges occurs in front of the electrodes. The experimental results indicate that the phase transition has a significant influence on both the electrical conduction and dielectric relaxation for all LCPs. (c) 2015 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

    DOI

  • Some R&D Activity on Polymer Composites in Japan (News from Japan)

    Y. Ohki

    IEEE Electrical Insulation Magazine   31 ( 6 ) 64 - 65  2015.10  [Refereed]

    DOI

  • IEEJ Awards Historical Heritages (News from Japan)

    Y. Ohki

    IEEE Electrical Insulation Magazine   31 ( 5 ) 50 - 51  2015.08  [Refereed]

    DOI

  • Japan’s First In-Grid Operation of a 200-MVA Superconducting Cable System (News from Japan)

    Y. Ohki

    IEEE Electrical Insulation Magazine   31 ( 4 ) 61 - 63  2015.06  [Refereed]

    DOI

  • Cause of the appearance of oxygen vacancies in yttria-stabilized zirconia and its relation to 2.8 eV photoluminescence

    Shoji Kaneko, Takaaki Morimoto, Yoshimichi Ohki

    JAPANESE JOURNAL OF APPLIED PHYSICS   54 ( 6 ) 06GC03(1)-06GC03(8) - 06GC03(8)  2015.06  [Refereed]

     View Summary

    When we implanted P+ or B+ ions into yttria-stabilized zirconia (YSZ), its crystallinity was degraded. Concurrently, the photoluminescence at around 2.8 eV originating from two types of oxygen vacancies with one or two captured electrons became weak, indicating a decrease in the number of oxygen vacancies. Oxygen vacancies appear in YSZ as a result of the replacement of Zr4+ by Y3+ in ZrO2. Therefore, it seems that the separation of YSZ into ZrO2 and Y2O3 induced by the ion implantation is responsible for the decrease in the number of oxygen vacancies. Moreover, the intensity of the 2.8 eV photoluminescence returns to the value before the ion implantation if the sample is annealed thermally after the implantation at temperatures higher than the crystallization temperature of YSZ. The reaction opposite to the above seems to be induced by the thermal annealing. (C) 2015 The Japan Society of Applied Physics

    DOI

  • Experimental observations on the crystalline structures of YAlO3 single crystal at high temperatures

    Takahiro Inoue, Takaaki Morimoto, Yoshimichi Ohki

    APPLIED PHYSICS A-MATERIALS SCIENCE & PROCESSING   119 ( 4 ) 1423 - 1429  2015.06  [Refereed]

     View Summary

    To understand the effects of thermal annealing on a high-permittivity gate insulating material YAlO3, its single crystal was annealed at various high temperatures ranging from 900 to 1300 A degrees C and was examined by various instrumental analyses such as X-ray diffraction, infrared absorption spectroscopy, laser confocal microscopy, and atomic force microscopy. As a result, it was clarified that the crystalline YAlO3 takes at least three structures in a temperature range up to 1300 A degrees C. Namely, it is perovskite below 1160 A degrees C, at which the garnet structure starts to appear, while an unknown structure appears at 1250 A degrees C. On the other hand, the sample surface also shows drastic changes if the temperature exceeds 1160 A degrees C. Furthermore, many elliptical spots are induced after the annealing at 1200 or 1250 A degrees C. Moreover, hydroxyl groups are generated at 1300 A degrees C. It is desirable to take these structural changes into account in determining the annealing temperature of YAlO3 for various industrial purposes such as the manufacture of semiconductor devices.

    DOI

  • Development of a plasma separation system for a portable blood test device

    Chiaki Kuroda, Yoshimichi Ohki, Hiroki Ashiba, Makoto Fujimaki, Koichi Awazu, Torahiko Tanaka, Makoto Makishima

    IEEJ Transactions on Sensors and Micromachines   135 ( 5 ) 152 - 157  2015.05  [Refereed]

     View Summary

    Aiming at developing a portable and speedy blood test device, a microfluidic system is designed and developed. Blood is injected into a hole with a diameter of 3.0 mm and a height of 3.0 mm. When the blood sample is transported from the bottom to the top of the hole, blood cells sediment at the bottom and plasma is separated at the upper side. Using this system, approximately 1 μL pure plasma can be obtained when blood is injected at a flow rate of 0.70 μL/min for 15 minutes. Moreover, by retaining the blood in the hole for seven minutes, pure plasma can be obtained 11 minutes later.

    DOI

  • Parallel-incidence-type waveguide-mode sensor with spectral-readout setup

    Makoto Fujimaki, Xiaomin Wang, Takafumi Kato, Koichi Awazu, Yoshimichi Ohki

    OPTICS EXPRESS   23 ( 9 ) 10925 - 10937  2015.05  [Refereed]

     View Summary

    A waveguide-mode sensor of the spectral-readout type can be used to detect changes in the complex refractive index in the vicinity of the surface of a sensing plate by observing the change in the spectrum of light reflected on the surface. The sensor's configuration can be simplified by adopting a parallel-incidence-type optical setup. To obtain a high sensitivity, the optimization of the sensing-plate structure, incidence angle, and detection wavelength band is essential for the sensor. In the present report, the results predicted by simulations are compared with experimental results in order to evaluate their validity. A discussion of the optimal design for the parallel-incidence-type sensor is also presented, according to the results obtained. (C)2015 Optical Society of America

    DOI

  • Space Charges Remaining in Polymers after Electron Beam Irradiation and the Role of Conductivity in Their Decay Profiles

    Naoto Tomite, Yukitaka Arai, Yoshimichi Ohki, Toru Hinata, Masakazu Washio

    IEEJ TRANSACTIONS ON ELECTRICAL AND ELECTRONIC ENGINEERING   10 ( 3 ) 237 - 241  2015.05  [Refereed]

     View Summary

    Five typical insulating polymers and a carbon-loaded polyethylene were irradiated with 200-keV electrons, and the space charge distributions remaining in the polymers were measured. When the polymer film is short-circuited after irradiation, the amount of space charge gradually decreases monotonically. By analyzing such decay profiles, it becomes clear that the decay of the charges remaining after the irradiation is governed by the conductivity of the polymer. (c) 2015 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

    DOI

  • Development of a ThreePhase Superconducting Power Transformer (News from Japan)

    Y. Ohki

    IEEE Electrical Insulation Magazine   31 ( 3 ) 46 - 48  2015.04  [Refereed]

    DOI

  • Terahertz spectral change associated with glass transition of poly-epsilon-caprolactone

    Marina Komatsu, Maya Mizuno, Shingo Saito, Kaori Fukunaga, Yoshimichi Ohki

    JOURNAL OF APPLIED PHYSICS   117 ( 13 ) 133102(1)-133102(7) - 133102(7)  2015.04  [Refereed]

     View Summary

    We measured absorption spectra of unidirectionally stretched poly-epsilon-caprolactone (PCL) film in a range from 0.3 to 3.6 THz at temperatures from 10 to 300K. Several absorption peaks were observed, when the electric field of THz waves was set in directions parallel and perpendicular to the stretching direction. The absorption bandwidths became significantly broad at around 200K and above at least in two specific peaks. This temperature is close to the glass transition temperature of PCL. Further, it is shown by quantum chemical calculations that all the peaks obtained experimentally originate in skeletal vibrations of PCL. Therefore, it has become clear that a specific feature appears in the THz absorption spectrum of PCL associated with its glass transition. (C) 2015 AIP Publishing LLC.

    DOI

  • Hemagglutination detection for blood typing based on waveguide-mode sensors

    Hiroki Ashiba, Makoto Fujimaki, Koichi Awazu, Mengying Fu, Yoshimichi Ohki, Torahiko Tanaka, Makoto Makishima

    Sensing and Bio-Sensing Research   3 ( 3 ) 59 - 64  2015.03  [Refereed]

     View Summary

    ABO and Rh(D) blood typing is one of the most important tests performed prior to blood transfusion. Although on-site blood testing is desirable for expedient blood transfusion procedure, most conventional methods and instruments lack the required usability or portability. Here, we describe a novel method, based on the detection of hemagglutination using an optical waveguide-mode sensor, for on-site use. The reflectance spectrum of blood alone and that of blood mixed with antibody reagents was measured using the waveguide-mode sensor. Differences in reflectance by agglutinated and non-agglutinated blood samples were observed at the bottom of the spectral dips
    due to differences in the manner in which red blood cells interacted with the surface of the sensor chip. Following the addition of the antibody, blood types A, B, O, and AB were clearly distinguishable and Rh(D) typing was also possible using the waveguide-mode sensor. Furthermore, the waveguide-mode-based measurement exhibited the potential to detect weak agglutination, which is difficult for human eyes to distinguish. Thus, this method holds great promise for application in novel on-site test instruments.

    DOI

  • Complex permittivity spectra of various insulating polymers at ultrawide-band frequencies

    Yuka Hasegawa, Yoshimichi Ohki, Kaori Fukunaga, Maya Mizuno, Kensuke Sasaki

    IEEJ Transactions on Fundamentals and Materials   135 ( 2 ) 63 - 68  2015.02  [Refereed]

     View Summary

    Nowadays, trials to use even higher frequencies are pursued to realize high-speed wireless communications. In order to respond to such a trend, dielectric properties of insulating polymers at high frequencies have to be clarified, although the importance of clarifying low-frequency properties is still very high, especially for use as high voltage insulators. In this regard, complex permittivity spectra from 10 mHz to 4.0 THz were obtained for various insulating polymers. The spectra obtained in the very wide frequency range are important, because they can be a valuable database and they provide various pieces of important information on dielectric characteristics.

    DOI

  • Comparison of dielectric properties of olefin thermosetting polydicyclopentadiene and epoxy resin

    Yuki Masuzaki, Yoshimichi Ohki, Fuqiang Tian, Masahiro Kozako, Nobuhito Kamei

    IEEJ Transactions on Fundamentals and Materials   135 ( 2 ) 82 - 87  2015.02  [Refereed]

     View Summary

    Complex permittivity spectra were obtained in a wide temperature and frequency range from 20 to 200°C and from 10-2 to 105 Hz for polydicyclopentadiene resin (PDCPD). For comparison, two epoxy resins, one with a similar glass transition temperature (Tg) to PDCPD and the other with a lower Tg, were used. As a result, PDCPD shows much smaller values of εr′ and εr″ than the two epoxy resins in nearly the whole temperature and frequency range. The differences of εr′ and εr″ between PDCPD and the epoxy resin with a similar Tg are also significant at low frequencies and at high temperatures. The increase in εr′ is attributable to the accumulation of hetero space charges in front of the electrodes. On the other hand, the increase in εr″ is an indication of the increase in Joule heat caused by the enhanced charge transport. Therefore, the experiental results indicate that PDCPD possesses better electrical insulating properties than epoxy resin. Furthermore, analysis using complex electric modulus, which is the inverse of the complex permittivity, has revealed very clearly that both charge transport and dipolar orientation are much more difficult in PDCPD than the two epoxy resins.

    DOI

  • Space charge formation and charge transport in epoxy resin at varied temperatures

    Xiuting Li, Yuki Masuzaki, Fuqiang Tian, Yoshimichi Ohki

    IEEJ Transactions on Fundamentals and Materials   135 ( 2 ) 88 - 93  2015.02  [Refereed]

     View Summary

    In order to investigate space charge behavior and charge transport characteristics of epoxy resin, complex dielectric permittivity and conduction current were measured at various temperatures from 20 to 200°C. Space charge distributions polarized at these temperatures were measured at room temperature. At temperatures above 100°C, significant accumulation of heteropolar space charges, most likely due to ions, is seen around the cathode/sample interface. Furthermore, both the real and imaginary parts of complex permittivity increase to remarkably high values at these high temperatures. It seems that such increase in permittivity is ascribable to the electrode polarization due to the hetero-charge accumulation. These results indicate that the dielectric behavior in epoxy resin is mainly governed by the electronic charge injection at relatively low temperatures below 100°C and by the transport of ionic carriers at high temperatures above 100°C.

    DOI

  • Development of Low Loss Magnetodielectric Nanocomposites of Epoxy Resin and Iron Nanoparticles

    Yuichi Hirose, Daiji Hasegawa, Yoshimichi Ohki

    ELECTRICAL ENGINEERING IN JAPAN   190 ( 2 ) 17 - 23  2015.01  [Refereed]

     View Summary

    To achieve the aim of developing a new insulating substrate that can exhibit both high permittivity and high permeability, ferromagnetic iron nanoparticles were dispersed uniformly in epoxy resin. Measurements of the electrical conductivity, permittivity, and permeability indicate that the composite can be a good candidate for an insulating substrate with negligibly small eddy-current loss and sufficiently high permittivity.

    DOI

  • Terahertz and Far-infrared Spectroscopic Estimation of Vinyl Acetate Content in Ethylene-Vinyl Acetate Copolymer

    Tomoyuki Izutsu, Daisuke Odaka, Marina Komatsu, Yoshimichi Ohki, Maya Mizuno, Yoshiaki Nakamura, Naofumi Chiwata

    2015 IEEE CONFERENCE ON ELECTRICAL INSULATION AND DIELECTRIC PHENOMENA (CEIDP)     39 - 42  2015  [Refereed]

     View Summary

    For the purpose of estimating the content of vinyl acetate (VA) in ethylene-vinyl acetate copolymer (EVA), optical absorption spectra were measured in a frequency range from 0.5 to 21.0 THz for sheets of EVA containing different contents of VA from 12 to 60 % by weight and a sheet of low-density polyethylene containing no VA. As a result, it has become clear that EVA exhibits relatively sharp absorption at 2.5, 6.7, and 10.5 THz and rather broad absorption at 11 to 17 THz and 17 to 20 THz. All these absorption peaks and bands are in proportion to the content of VA, up to certain frequency-dependent contents. Using these proportionalities, the content of VA in EVA can be estimated quantitatively. Especially, the absorption at 2.5 THz shows a good accuracy similar to the method recommended by an international standard.

  • Influence of Cable Structure on the Fault Location by Frequency Domain Reflectometry

    Yoshimichi Ohki, Naoshi Hirai

    2015 IEEE CONFERENCE ON ELECTRICAL INSULATION AND DIELECTRIC PHENOMENA (CEIDP)     274 - 277  2015  [Refereed]

     View Summary

    The authors have demonstrated that the estimation of the precise locations of points in cables aged thermally or mechanically or by the irradiation of gamma rays is possible by a combination of frequency domain reflectometry (FDR) and inverse fast Fourier transform (IFFT). This paper examines how this ability of fault location depends on the type and structure of the cable, using several kinds of polymer insulated cables such as triple core cables insulated with flame-retardant ethylene propylene diene copolymer (FR-EPDM), dual core cables insulated with silicone rubber (SiR) or crosslinked polyolefin (XLPO), and coaxial cylindrical cables insulated with low density polyethylene (LDPE). As a result, for cables with lengths of 16 to 72 m, the maximum sensitivity is attained when the highest frequency of the inputted electromagnetic waves is 1.0 GHz or higher for coaxial cables, but around 600 to 800 MHz for dual or triple core cables.

  • Dielectric Relaxation Phenomena of Several Insulating Polymers Analyzed by Electric Modulus Spectra

    Yoshimichi Ohki

    2015 IEEE 11TH INTERNATIONAL CONFERENCE ON THE PROPERTIES AND APPLICATIONS OF DIELECTRIC MATERIALS     192 - 195  2015  [Refereed]

     View Summary

    Complex electric modulus is defined as the inverse of complex relative permittivity. In this paper, it is clearly shown by referring to the author's experimental results that electric modulus can be a significantly powerful tool for the analysis of dielectric behavior of various insulating polymers. The modulus is especially useful for the evaluation of dielectric relaxation and carrier transport processes at high temperatures in a relatively low frequency range.

  • Effects of Gamma Irradiation on the Dielectric Properties of Dicyclopentadiene Resin and Epoxy Resin

    Maki Miyamoto, Naoto Tomite, Yoshimichi Ohki

    2015 IEEE 11TH INTERNATIONAL CONFERENCE ON THE PROPERTIES AND APPLICATIONS OF DIELECTRIC MATERIALS     488 - 491  2015  [Refereed]

     View Summary

    Radiation resistance was compared between dicyclopentadiene resin (P) and epoxy resin (E) mainly by analyzing their dielectric properties. When the resins are irradiated by gamma-rays, their band gap energies decrease. This decrease is more significant in E than in P. Although electrical conductivity and both the real and imaginary parts of complex permittivity are increased very dramatically in E, no such increases are observed in P. These results indicate that P has superior radiation resistance to E.

  • Terahertz Spectroscopic Observation of Oxidation of Ethylene-Propylene-Diene-Copolymer

    Marina Komatsu, Tomoyuki Izutsu, Yoshimichi Ohki, Maya Mizuno, Kaori Fukunaga

    2015 IEEE 11TH INTERNATIONAL CONFERENCE ON THE PROPERTIES AND APPLICATIONS OF DIELECTRIC MATERIALS     544 - 547  2015  [Refereed]

     View Summary

    Flame-retardant ethylene-propylene-diene copolymer, which is the polymer insulating material most abundantly used for safety-related electric cables in nuclear power plants, was irradiated with gamma-rays to certain designated doses from 500 to 1500 kGy at room temperature or 100 degrees C in air, and absorption spectra were measured at terahertz frequencies. A broad absorption is induced by the irradiation in a range from 0.5 to 3.1 THz. It is confirmed that this absorption is induced by oxidation, by comparing its frequency spectrum with spectra of polyethylene obtained numerically by doing quantum chemical calculations and experimentally by measuring its absorption at THz frequencies. Therefore, the degree of oxidation of this copolymer can be estimated by the intensity of the broad absorption at these THz frequencies.

  • Estimation of the Number of Nucleotides with Thymine in Deoxyribonucleic Acid by Far-infrared Absorption

    Ayano Kitamura, Marina Komatsu, Yoshimichi Ohki, Maya Mizuno, Miki Hirabayashi, Hiroaki Kojima

    2015 IEEE CONFERENCE ON ELECTRICAL INSULATION AND DIELECTRIC PHENOMENA (CEIDP)     617 - 620  2015  [Refereed]

     View Summary

    A new research trend that regards deoxyribonucleic acid (DNA) as a new dielectric polymeric material has been emerging in recent years. Especially for such application purposes, a simple easy-to-use method for determining the higher-order structure of synthetic DNA is desirable. From this viewpoint, we demonstrated in this research that far-infrared absorption spectroscopy can be a tool to determine the DNA higher-order structure and we calculated experimentally the integrated molar absorption coefficients per nucleotide with thymine at various frequencies. Therefore, using these coefficients, the number of thymine base on a single stranded DNA can be estimated by far-infrared spectroscopy.

  • Analysis on the Thickness and Temperature Dependent DC Breakdown of Low Density Polyethylene

    Daomin Min, Shengtao Li, Yoshimichi Ohki

    2015 IEEE 11TH INTERNATIONAL CONFERENCE ON THE PROPERTIES AND APPLICATIONS OF DIELECTRIC MATERIALS     368 - 371  2015  [Refereed]

     View Summary

    The dc breakdown strength of low density polyethylene (LDPE) decreases with an increase in thickness and temperature of the sample. The breakdown strength is influenced by the charge transport and electric field distortion, and is also related to the molecular chain displacement and fracture. This paper investigates mutual relations among the charge transport, molecular chain displacement, and thickness dependent dc breakdown of LDPE. A model that combines the dynamics of charge transport and molecular displacement is used to calculate the space charge accumulation, molecular chain displacement, and dc breakdown properties of LDPE with various thicknesses at various constant voltage ramping rates. The model includes processes of charge injection, charge migration via hopping between shallow traps, charge trapping at and detrapping from deep traps, charge recombination, and molecular chain displacement. The simulation results show that the breakdown field as a function of sample thickness satisfies an inverse power law for various voltage ramping rates. This is consistent with experimental results. The model can also explain the result that the breakdown field increases with an increase in voltage ramping rate. It also demonstrates the negative correlation between the breakdown field and temperature, which is also in good agreement with experimental results.

  • THz Absorption of Adenine Base in Single-stranded Deoxyribonucleic Acid

    Marina Komatsu, Yoshimichi Ohki, Maya Mizuno

    2015 40TH INTERNATIONAL CONFERENCE ON INFRARED, MILLIMETER AND TERAHERTZ WAVES (IRMMW-THZ)    2015  [Refereed]

     View Summary

    Absorption spectra were measured in a frequency range from 3 to 18 THz for various single-stranded deoxyribonucleic acids (DNAs) containing adenine as their bases. Nine absorption components in total, most of which can be assigned to either phosphoric acid or adenine, appear in the range observed. From the dependence of each absorption on the number of nucleotides in one DNA, the integrated molar absorption coefficient per nucleotide with adenine was estimated.

  • Development of a 204-kV/40-kA Vacuum Circuit Breaker (News from Japan)

    Y. Ohki

    IEEE Electrical Insulation Magazine   31 ( 1 ) 44 - 46  2014.12  [Refereed]

    DOI

  • Analyses of various insulating polymers by broadband dielectric spectroscopy

    Y. Ohki, Y. Hasegawa, J. Takihana, K. Fukunaga, M. Mizuno, K. Sasaki

    2014 IEEE Conference on Electrical Insulation and Dielectric Phenomena, CEIDP 2014     582 - 585  2014.12  [Refereed]

     View Summary

    Nowadays, trials to use even higher frequencies are pursued to realize high-speed wireless communications. In order to respond to such a trend, dielectric properties of insulating polymers at high frequencies have to be clarified, although the importance of clarifying low-frequency properties is still very high, especially for use as high voltage insulators. In this regard, complex permittivity spectra in an ultrawide frequency range were obtained for various insulating polymers. These spectra are important, because they can be a valuable database that provide important information on dielectric characteristics. Furthermore, by analyzing the temperature dependence of permittivity in a relatively low frequency range, thermal expansion coefficients of polymers were estimated.

    DOI

  • Microfluidic sedimentation system for separation of plasma from whole blood

    Chiaki Kuroda, Yoshimichi Ohki, Hiroki Ashiba, Makoto Fujimaki, Koichi Awazu, Torahiko Tanaka, Makoto Makishima

    Proceedings of IEEE Sensors   2014- ( December ) 1854 - 1857  2014.12  [Refereed]

     View Summary

    A microfluidic system for a portable and speedy blood test device has been developed. In the system, blood is injected into a cylindrical channel with a diameter of 3.0 mm and a height of 3.0 mm from the bottom to the top. When the blood ascends the channel, blood cells sediment at the bottom and plasma is separated at the top. This system can be manufactured easily at a low cost by simply bonding two identical polydimethylsiloxane chips. Pure plasma can be obtained successfully from a very tiny amount of blood, less than a drop (9 μL).

    DOI

  • Redox Flow Battery System for the Stable Supply of Renewable Energy

    Y. Ohki

    IEEE ELECTRICAL INSULATION MAGAZINE   30 ( 6 ) 48 - 50  2014.11  [Refereed]

    DOI

  • Dielectric Properties of Poly(ethylene terephthalate) and Poly(ethylene 2,6-naphthalate)

    Peng Yang, Fuqiang Tian, Yoshimichi Ohki

    IEEE TRANSACTIONS ON DIELECTRICS AND ELECTRICAL INSULATION   21 ( 5 ) 2310 - 2317  2014.10  [Refereed]

     View Summary

    Dielectric behavior was compared experimentally between polyethylene terephthalate (PET) and poly(ethylene 2,6-naphthalate) (PEN). Due to their similar chemical structures, the two polymers exhibit many parallel dielectric properties. While the two polymers exhibit fairly similar thermally stimulated polarization and depolarization currents (TSPC and TSDC), the temperature at which TSPC or TSDC starts to increase rapidly is about 20 degrees C higher in PEN than in PET, mostly likely reflecting the difference in their glass transition temperatures (T-g's). At temperatures about 30 degrees C lower than T-g, the two polymers show a hump in their first-run TSPC spectra, probably originating from impurity or moisture. Both the real and imaginary parts of complex permittivity, epsilon(r)' and epsilon r '', increase significantly at temperatures above their T-g's for both PEN and PET, since molecular motion becomes active. Shoulders and plateaux clearly appear in epsilon(r)' and epsilon r '' spectra of PEN, which move toward higher frequencies with an increase in temperature. To further analyze them, complex electric modulus M* was introduced. As a result, it has become clear that electric conduction dominates the dielectric behavior of PET and PEN at temperatures above T-g, especially at low frequencies.

    DOI

  • Terahertz absorption spectra of oxidized polyethylene and their analysis by quantum chemical calculations

    Marina Komatsu, Masashi Hosobuchi, Xiaojun Xie, Yonghong Cheng, Yukio Furukawa, Maya Mizuno, Kaori Fukunaga, Yoshimichi Ohki

    JAPANESE JOURNAL OF APPLIED PHYSICS   53 ( 9 ) 092402(1)-092402(9) - 092402(9)  2014.09  [Refereed]

     View Summary

    Low-density polyethylene, either cross-linked or not, was oxidized and its absorption spectra were measured in the terahertz (THz) range and infrared range. The absorption was increased by the oxidation in the whole THz range. In accord with this, infrared absorption due to carbonyl groups appears. Although these results indicate that the increase in absorption is induced by oxidation, its attribution to resonance or relaxation is unclear. To clarify this point, the vibrational frequencies of three-dimensional polyethylene models with and without carbonyl groups were quantum chemically calculated. As a result, it was clarified that optically inactive skeletal vibrations in polyethylene become active upon oxidation. Furthermore, several absorption peaks due to vibrational resonances are induced by oxidation at wavenumbers from 20 to 100 cm(-1). If these absorption peaks are broadened and are superimposed on each other, the absorption spectrum observed experimentally can be reproduced. Therefore, the absorption is ascribable to resonance. (C) 2014 The Japan Society of Applied Physics

    DOI

  • Development of a RE-Ba-Cu-O Superconducting Current Lead

    Y. Ohki

    IEEE ELECTRICAL INSULATION MAGAZINE   30 ( 5 ) 43 - 45  2014.09  [Refereed]

    DOI

  • Estimation of thermal expansion coefficients of polymeric insulating films from temperature dependence of dielectric permittivity

    Yuka Hasegawa, Junya Takihana, Yoshimichi Ohki

    JAPANESE JOURNAL OF APPLIED PHYSICS   53 ( 7 ) 071501(1)-071501(4) - 071501(4)  2014.07  [Refereed]

     View Summary

    Dielectric permittivity (epsilon(r)') was measured for films of low-density polyethylene (LDPE), poly(methyl pentene) (PMP), syndiotactic polystyrene (SPS), and poly(ethylene terephthalate) (PET) in the frequency range from 10(-2) to 3 x 10(5) Hz. It was found that, in all the polymers, epsilon(r)' increases with an increase in temperature at frequencies below about 1.0 Hz when the temperature exceeds a certain polymer-dependent temperature. This is due to space charge polarization. At frequencies above about 1.0 Hz, epsilon(r)' was found to decrease with an increase in temperature for LDPE, PMP, and SPS. The reason for the decrease in epsilon(r)' is the thermal expansion of the film. Therefore, by analyzing the temperature dependence of epsilon(r)', the thermal expansion coefficients of the three polymer films were estimated. This method can be useful for estimating the thermal expansion coefficient of polymers, especially when the direct measurement of volume expansion is difficult. (C) 2014 The Japan Society of Applied Physics

    DOI

  • Optical Characterization and Computational Chemical Evaluation of Electronic Localized States in Polyolefin

    Tomoyuki Arai, Masashi Hosobuchi, Norikazu Fuse, Kyozaburo Takeda, Yoshimichi Ohki

    ELECTRICAL ENGINEERING IN JAPAN   188 ( 1 ) 1 - 8  2014.07  [Refereed]

     View Summary

    Optical absorption spectra and photoluminescence spectra were obtained for eight kinds of polyolefin sheet samples using photons in a range from visible to vacuum ultraviolet (UV). Almost all samples were found to exhibit an absorption peak at around 6.5 eV and a luminescence band at around 4.3 eV. The luminescence was found to be induced by the absorption. Furthermore, it was found that successive absorption of UV photons weakened the luminescence intensity. It is assumed from these results that ,-unsaturated carbonyls are luminous and that the carbonyls are decomposed through the Norrish type II reaction by absorbing UV photons. Quantum chemical calculations were carried out using polyethylene models with and without an unsaturated carbonyl to verify the above-mentioned assumption. As a result, the model with an unsaturated carbonyl was found to have localized electronic states in the forbidden band. One of the differential energies between the states is close to the photon energy, by which the luminescence is induced. The bond length of a double bond, which is next to the carbonyl, was found to be longer at the excited singlet state than at the ground state. These results obtained by computation support the above-mentioned assumption of the luminescence center and its decomposition.

    DOI

  • Development of directly molded bushings and cable sealing ends (News from Japan)

    Y. Ohki

    IEEE Electrical Insulation Magazine   30 ( 4 ) 55 - 56  2014.06  [Refereed]

    DOI

  • Electric Modulus Powerful Tool for Analyzing Dielectric Behavior

    Fuqiang Tian, Yoshimichi Ohki

    IEEE TRANSACTIONS ON DIELECTRICS AND ELECTRICAL INSULATION   21 ( 3 ) 929 - 931  2014.06  [Refereed]

     View Summary

    Complex electric modulus, defined as the inverse of complex relative permittivity, can be a significantly powerful tool for analyzing dielectric behavior of a polymeric insulating material, especially at relatively high temperatures, where complex permittivity usually becomes very high due to electrode polarization and carrier transport. In this paper, a typical example of the above is clearly shown by referring to an experimental result obtained for epoxy resin.

    DOI

  • Development of a Short-Term Power Interruption Compensator Using Lithium-Ion Capacitors (News from Japan)

    Y. Ohki

    IEEE Electrical Insulation Magazine   30 ( 3 ) 54 - 57  2014.04  [Refereed]

    DOI

  • Development of a Microdischarge Locator Using Analogue Signal Processing (News from Japan)

    Y. Ohki

    IEEE Electrical Insulation Magazine   30 ( 2 ) 59 - 60  2014.02  [Refereed]

    DOI

  • Charge transport and electrode polarization in epoxy resin at high temperatures

    Fuqiang Tian, Yoshimichi Ohki

    JOURNAL OF PHYSICS D-APPLIED PHYSICS   47 ( 4 ) 045311 (1)-045311 (9) - 045311 (9)  2014.01  [Refereed]

     View Summary

    Complex permittivity spectra were measured for epoxy resin at various temperatures and numerically fitted to theories of electrode polarization (EP) and ac conduction. Complex permittivity at high temperatures fits the Cole-Cole relation for EP well, with the shape parameter relating to the electrode blockage coefficient for ions. The Debye length, ion density and mobility, hopping rate and distance, and other parameters controlling the charge transport can be obtained. Both the density and mobility of ions are thermally activated, and the sum of their activation energies is nearly the same as the activation energy of ac conductivity. It is indicated that the theory of power-law frequency response as a result of short-range hopping of ions is an extension of the EP theory above its peak frequency. On the other hand, the relaxation of ac conduction revealed by electric modulus and that of EP appearing at low frequencies are due to long-range hopping of ions. It is demonstrated that data fitting of complex permittivity to the theories of EP and ac conduction in combination with the electric modulus analysis is effective to estimate various characteristic parameters of charge transport.

    DOI

  • 半導体高誘電率材料LaAlO3 とYAlO3 中の点欠陥 〜その種類と構造、光・熱・イオンの影響〜

    森本貴明, 堀井陽介, 井上貴博, 金子昇司, 針間正幸, 大木義路

    放電学会誌   57 ( 2 ) 3 - 12  2014  [Refereed]

  • 特集 高分子絶縁体の劣化 [解説]有機高分子の絶縁劣化とその検出

    大木義路

    マテリアルライフ学会誌   26 ( 3 ) 36 - 41  2014  [Invited]

  • Crystalline structures of YAlO3 single crystal at high temperatures

    Takahiro Inoue, Takaaki Morimoto, Shoji Kaneko, Yosuke Horii, Yoshimichi Ohki

    Proceedings of the International Symposium on Electrical Insulating Materials     208 - 211  2014  [Refereed]

     View Summary

    In order to clarify the influence of high temperature annealing on a next-generation gate insulating material YAlO3, single crystal YAlO3 samples were annealed at various temperatures from 900 to 1300°C in air for about 12 hours. The crystalline structure was examined by X-ray diffractometry, while the surface profile was examined by atomic force microscopy and a surface profilometer. Furthermore, infrared absorption spectroscopy and optical microscopy were used. As a result, the following serial changes in structure were estimated. The perovskite structure of crystalline YAlO3 starts to collapse and Y3Al5O 12 (YAG) is formed by the annealing at 1160 °C. Then, it segregates to Al2O3 andYAlO3 after the annealing at 1300 °C. © 2014 The Institute of Electrical Engineers, Japan.

    DOI

  • Chemiluminescence Due to Oxidation of Ethylene Propylene Diene Copolymers

    Yoshimichi Ohki, Naoshi Hirai

    2014 IEEE CONFERENCE ON ELECTRICAL INSULATION AND DIELECTRIC PHENOMENA (CEIDP)     425 - 428  2014  [Refereed]

     View Summary

    Chemiluminescence analysis was applied to evaluate oxidation of ethylene propylene diene copolymers (EPDMs), and the results were analyzed through comparisons with those of low density polyethylene (LDPE) and cross-linked polyethylene (XLPE) with or without antioxidant. If LDPE or XLPE is being oxidized, CL appears abruptly in a frequency range from 300 to 850 nm after a certain incubation time. This incubation time corresponds to oxidation induction time (OIT). In contrast to LDPE or XLPE, when EPDM samples are heated in air, a strong CL intensity is observable from its initial stage, even if the amount of antioxidant in the sample is high enough. However, after the CL intensity decays once, it increases gradually to a certain peak and then it decays again. We can regard the time at which CL shows the peak as OIT, because it shows a positive correlation with the weight content of antioxidant in the sample. Furthermore, after the EPDM samples were aged in different conditions, namely by heat alone, by gamma-rays alone, and by simultaneously given heat and gamma-rays, the dependence of OIT on the aging period was examined for each condition. The resultant dependence of OIT was found to agree with the dependence observed by measuring the elongation-at-break and also with the one measured by differential scanning calorimetry.

  • Structural Change Induced in LaAlO3 by Ion Implantation

    Masayuki Harima, Yosuke Horii, Takaaki Morimoto, Yoshimichi Ohki

    PROCEEDINGS OF 2014 INTERNATIONAL SYMPOSIUM ON ELECTRICAL INSULATING MATERIALS (ISEIM 2014)     180 - 183  2014  [Refereed]

     View Summary

    Ion implantation is used for various purposes in manufacturing semiconductor devices such as MOS-FETs. In the present study, effects of implantation of p(+) or B+ ions on the structural change of single crystal LaAlO3 were examined. The optical absorption edge located at about 5.6 eV, which corresponds to the band gap energy of LaAlO3, is scarcely affected by the ion implantation. The X-ray diffraction peak intensity at 2 theta = 23.5 degrees is decreased by the ion implantation. The intensities of three sharp photoluminescence (PL) peaks detected at 1.62, 1.65, and 1.69 eV, which appear only when the samples are crystalline, become smaller by ion implantation. However, the intensity of a broad PL peak at around 2.8 eV due to the oxygen vacancy, which is also detectable in amorphous samples, scarcely changes after the ion implantation. These results indicate that the ion implantation degrades the crystalline LaAlO3 However, as compared with YAlO3 with a similar perovskite structure, LaAlO3 is more resistant to ion implantation.

  • Analysis on Thermally Stimulated Currents in Polyethylene-terephthalate and Polyethylene-naphthalate

    Peng Yang, Yoshimichi Ohki, Fuqiang Tian

    PROCEEDINGS OF 2014 INTERNATIONAL SYMPOSIUM ON ELECTRICAL INSULATING MATERIALS (ISEIM 2014)     401 - 404  2014  [Refereed]

     View Summary

    Through analysis of thermally stimulated polarization and depolarization currents (TSPC and TSDC), dielectric properties of polyethylene terephthalate (PET) and polyethylene naphthalate (PEN) are compared. Since the two polymers have very similar chemical structures, their dielectric properties are also similar. However, the temperature at which TSPC or TSDC starts to increase rapidly is about 20 degrees C higher in PEN than in PET. This difference is mostly due to the fact that the glass transition temperature T-g is higher in PEN. This means that thermal properties including the glass transition affect significantly the dielectric behavior of PET and PEN. At temperatures about 30 degrees C lower than T-g, the two polymers show a hump in their first-run TSPC spectra, probably originating from impurity or moisture. It is also clearly demonstrated that both PET and PEN can be reliable electrical insulating materials, especially at temperatures below their T-g's.

  • Terahertz Spectroscopic Analysis of Ethylene-Propylene-Diene Copolymer

    Marina Komatsu, Tomoyuki Izutsu, Yoshimichi Ohki, Maya Mizuno, Kaori FUkunaga, Oshiaki Nakamura, Naofumi Chiwata

    PROCEEDINGS OF 2014 INTERNATIONAL SYMPOSIUM ON ELECTRICAL INSULATING MATERIALS (ISEIM 2014)     338 - 341  2014  [Refereed]

     View Summary

    Inorganic fillers are very often added to ethylene-propylene-diene copolymer (EPDM), which is a typical rubber material used for electrical insulation, for the purpose of improving its various properties. Regarding this, although the content and dispersion of inorganic fillers exert a significant influence on the properties of EPDM, detailed research has been done scarcely. In the present research, talc and magnesium hydroxide were added to EPDM, and detection of the content and dispersion of fillers was tried by terahertz absorption spectroscopy. As a result, it has become clear that the content of talc can be estimated by the increase in absorption at 3.05 THz. The possibility of estimating the talc content from incremental values of three absorptions in a far-infrared range has also been confirmed, in addition to the 3.05-THz absorption. Furthermore, the dispersion of magnesium hydroxide fillers is likely to correspond to the scattering of absorption spectra at around 1.0 to 1.5 THz.

  • Highly Sensitive Detection of Distorted Points in a Cable by Frequency Domain Reflectometry

    Naoshi Hirai, Yoshimichi Ohki

    PROCEEDINGS OF 2014 INTERNATIONAL SYMPOSIUM ON ELECTRICAL INSULATING MATERIALS (ISEIM 2014)     144 - 147  2014  [Refereed]

     View Summary

    This paper demonstrates that the location in an electric cable where some mechanical distortion appears can be estimated very precisely by a method based on frequency domain reflectometry and inverse fast Fourier transform. A coaxial communication cable insulated by low density polyethylene or a triplex cable insulated with flame-retardant ethylene propylene diene copolymer was distorted at different positions, and electromagnetic waves in a frequency range from one to several hundred MHz or 1.5 GHz were transmitted through the cable. The ratio between the incident power and the reflected power was measured using a network analyzer. As a result, the position in each cable where the distortion was given can be located with a spatial resolution as short as 1.0 cm, which indicates high sensitivity of this method.

  • Comparison of Dielectric Properties among Polydicyclopentadiene Resin, Epoxy Resin and Their Composites with Microsized SiO2 Fillers

    Yuki Masuzaki, Yoshimichi Ohki, Masahiro Kozako

    PROCEEDINGS OF 2014 INTERNATIONAL SYMPOSIUM ON ELECTRICAL INSULATING MATERIALS (ISEIM 2014)     374 - 377  2014  [Refereed]

     View Summary

    Complex permittivity spectra of polydicyclopentadiene resin and epoxy resin, and their composites with SiO2 microfiller were obtained in a wide temperature and frequency range. Both epsilon(r)' and epsilon(r)" at high temperatures above their glass transition temperatures (T-g 's) are much lower in polydicyclopentadiene resin than in epoxy resin, even though the two Tg(')s are almost the same. Furthermore, analysis using electric modulus has revealed that addition of the microfiller in epoxy resin suppresses dipolar orientation and charge transport However, both the dipolar orientation and charge transport seem to be difficult in polydicyclopentadien resin and its composite compared to epoxy resin and its composite.

  • Development of a New Curing Agent for Thermosetting Resin Using Natural Polyphenol

    Y. Ohki

    IEEE ELECTRICAL INSULATION MAGAZINE   30 ( 1 ) 57 - 58  2014.01  [Refereed]

    DOI

  • Precise Location of the Excessive Temperature Points in Polymer Insulated Cables

    Yoshimichi Ohki, Takayuki Yamada, Naoshi Hirai

    IEEE TRANSACTIONS ON DIELECTRICS AND ELECTRICAL INSULATION   20 ( 6 ) 2099 - 2106  2013.12  [Refereed]

     View Summary

    This paper demonstrates that it is possible to locate very precisely a point showing a high temperature in a polymer insulated cable. The method is based on frequency domain reflectometry and inverse fast Fourier transform. The cables tested are a coaxial communication cable with a length of 32 m insulated by low density polyethylene and a flat in-house electric cord with a length of 21 m insulated by polyvinyl chloride. The cable or cord was heated at different positions for different lengths. The ratio between the powers of electromagnetic waves incident to and reflected from the cable was measured using a network analyzer in a frequency range from one to several hundred MHz or 1.5 GHz. The spectra obtained by the measurements were then analyzed by inverse Fourier transform. As a result, the position exhibiting a temperature higher than the adjacent points can be located with a spatial resolution as short as 2.5 cm. It was also confirmed that the sensitivity or spatial resolution can be improved by an increase of the highest measurement frequency.

    DOI

  • Development of New Nanocomposite-Based Enameled Wire for Inverter-Fed Motor Coils (News from Japan)

    Y. Ohki

    IEEE Electrical Insulation Magazine   29 ( 6 ) 71 - 74  2013.10  [Refereed]

    DOI

  • Development of XLPE-Insulated Cable for High-Voltage DC Submarine Transmission Line (2) (News from Japan)

    Y. Ohki

    IEEE Electrical Insulation Magazine   29 ( 5 ) 85 - 87  2013.08  [Refereed]

    DOI

  • Development of XLPE-insulated cable for high-voltage dc ubmarine transmission line (1) (News from Japan)

    Y. Ohki

    IEEE Electrical Insulation Magazine   29 ( 4 ) 65 - 67  2013.06  [Refereed]

    DOI

  • A Layer-Short Tester for Pole-Type Distribution Transformers (News from Japan)

    Y. Ohki

    IEEE Electrical Insulation Magazine   29 ( 3 ) 62 - 63  2013.04  [Refereed]

    DOI

  • Location feasibility of degradation in cable through Fourier transform analysis of broadband impedance spectra

    Yoshimichi Ohki, Naoshi Hirai

    ELECTRICAL ENGINEERING IN JAPAN   183 ( 1 ) 1 - 8  2013.04  [Refereed]

     View Summary

    Since electric cables play important roles such as power supply and information transmission, their degradation may cause serious problems. We have been trying to monitor the degradation of cable insulation by measuring the magnitude and phase angle of the impedance as a function of frequency in a very wide frequency range. The cables tested were insulated with ?ame-retardant ethylene propylene rubber or special heat-resistant polyvinyl chloride. They were partially damaged by peeling off their insulation layers, or partially aged by heat and gamma-rays. The impedance and phase angle were measured from a terminal of the cable. The difference in impedance between the damaged and sound cables is made clear by fast Fourier transform analyses, from which the damaged portion can be located. It can be clearly shown that this method has the potential ability to detect the degradation of cable insulation induced by physical damage, gamma-ray irradiation, and thermal aging. (c) 2013 Wiley Periodicals, Inc. Electr Eng Jpn, 183(1): 18, 2013; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/eej.22384

    DOI

  • Development of a Circuit Breaker Condition Monitor (News from Japan)

    Y. Ohki

    IEEE Electrical Insulation Magazine   29 ( 2 ) 79 - 81  2013.02  [Refereed]

    DOI

  • Control of refractive index of fluorinated polyimide by proton beam irradiation

    Yukitaka Arai, Yoshimichi Ohki, Keisuke Saito, Hiroyuki Nishikawa

    Japanese Journal of Applied Physics   52 ( 1 ) 012601(1)-012601(5) - 012601(5)  2013.01  [Refereed]

     View Summary

    To clarify the feasibility of controlling the refractive index of a polymer by proton beam irradiation, we irradiated 1.0 MeV protons to a fluorinated polyimide film. Before and after the proton irradiation at a fluence between 1 × 1014 and 7 × 1016 cm-2, the film surface was scanned by a profilometer. It was found that the depth of a dent, which increases with fluence, was induced by the irradiation. The refractive index of the ionirradiated region was calculated using the Lorentz-Lorenz equation, substituting the depth of the dent and the projected range of the protons. When the fluorinated polyimide was irradiated at a fluence of 7 × 1016 cm-2, the refractive index increased by about 3.3%, which agrees with the increment in refractive index measured by spectroscopic ellipsometry. The increment in refractive index (0.21%) induced by the irradiation of protons at the fluence of 1 × 1015 cm -2 is comparable to the value (0.35%) observed when protons were irradiated to SiO2 glass at a similar fluence. Therefore, it is reasonable to assume that the ion irradiation to a polymer can be a good method for fabricating a high-performance polymer-based optical waveguide. © 2013 The Japan Society of Applied Physics.

    DOI

  • Effects of nanofiller materials on the dielectric properties of epoxy nanocomposites

    J. Katayama, Y. Ohki, N. Fuse, M. Kozako, T. Tanaka

    IEEE Transactions on Dielectrics and Electrical Insulation   20 ( 1 ) 157 - 165  2013  [Refereed]

     View Summary

    Effects of nanofiller materials on typical dielectric properties of bisphenol-A epoxy resin were compared among three kinds of nanofillers, i. e. boehmite alumina, titania, and silica. In all the samples, the complex permittivity εr' and εr become significantly higher with an increase in temperature beyond 120 °C, especially at low frequencies. Such increase iinεr' and εr indicates the abundance of mobile charge carriers, which was verified by the space charge distribution measurements. All the samples show two peaks at 110 and 180 °C in the thermally stimulated depolarization current spectra. The lower-temperature peak is apparently caused by depolarization of dipoles induced by the glass transition, while the higher-temperature peak is due to the release of space charge. These characteristic properties appear more significantly in the nanocomposites with boehmite alumina and titania and less significantly in the nanocomposite with silica than in the neat epoxy esin. This indicates that the silica nanofillers give the best results as far as these properties cconcerned. The suppression of molecular motion by the addition of nanofillers seems to work effectively in the case of silica, while the nanofiller addition is likely to aaccelerate the motion of ionic carriers and/or that of dipoles in the case of boehmite alumina and titania. © 1994-2012 IEEE.

    DOI

  • Comparisons of partial discharge resistance and dielectric properties after water absorption between polyetherimide and aramid insulating papers

    Jun Katayama, Yoshimichi Ohki

    IEEJ Transactions on Fundamentals and Materials   133 ( 2 ) 51 - 56  2013  [Refereed]

     View Summary

    Partial discharge resistance was compared between newly developed polyetherimide insulating paper and conventional aramid insulating paper. Effects of water absorption on dielectric properties were also compared. The time to breakdown via partial discharges observed for the polyetherimide paper by applying a constant voltage was found to be about twice as long as that observed for the aramid paper, while the partial discharge charge amount per unit time is similar in the two papers. Moreover, the average area of degraded portions induced by partial discharges is smaller in the polyetherimide paper than in the aramid paper. The polyetherimide paper absorbs less water than does the aramid paper, when they are immersed in water at 27 and 80?C for the same period. Because of this, complex permittivity and conductivity increase more significantly in the aramid paper than in the polyetherimide paper. Therefore, especially in a humid condition, the polyetherimide paper is superior to the aramid paper. © 2013 The Institute of Electrical Engineers of Japan.

    DOI

  • Detection of influenza viruses attached to an optical disk

    Takayuki Shima, Makoto Fujimaki, Akihiro Yoshida, Subash C. B. Gopinath, Masashi Kuwahara, Yoshimichi Ohki, Koichi Awazu

    Journal of Biomaterials and Nanobiotechnology   4 ( 2 ) 145 - 150  2013  [Refereed]

    DOI

  • 放電学会年次大会 特別講演 テラヘルツ分光とイメージングの放電・絶縁分野への応用

    大木義路

    放電学会誌 放電研究   56 ( 1 ) 7 - 16  2013  [Invited]

  • Roles of point defects in thermally enhanced generation and transfer of electrons and holes in LaAlO3

    Daiki Yamasaka, Yosuke Horii, Takaaki Morimoto, Yoshimichi Ohki

    Japanese Journal of Applied Physics   52 ( 7 ) 071501(1)-071501(5) - 071501(5)  2013.01  [Refereed]

     View Summary

    Thermal annealing was given to single crystal LaAlO3 and its effects were examined by measuring electron spin resonance (ESR) and optical absorption. When LaAlO3 was annealed at temperatures above 500°C in an oxidizing atmosphere, the intensities of ESR signals due to transition metal, likely ascribable to Fe3+, decreased. Concurrently with this, two optical absorption bands at 2.7 and 3.5 eV, attributable to a combination of a hole and a La3+ (or Al3+) vacancy, increased. These results indicate that thermal electron-hole generation is induced by oxidizing annealing and that the generated electrons and holes are then captured by Fe3+ ions and La3+ or Al3+ vacancies, respectively. It is also assumed that captured electrons and holes are released and recombine with each other by reducing annealing. © 2013 The Japan Society of Applied Physics.

    DOI

  • Development of low loss magneto-dielectric nanocomposites of epoxy resin and iron nanoparticles

    Yuichi Hirose, Daiji Hasegawa, Yoshimichi Ohki

    IEEJ Transactions on Fundamentals and Materials   133 ( 12 ) 10 - 673  2013  [Refereed]

     View Summary

    To achieve the aim of developing a new insulating substrate that can exhibit both high permittivity and high permeability, ferromagnetic iron nanoparticles were dispersed uniformly in epoxy resin. Measurements of the electrical conductivity, permittivity, and permeability indicate that the composite can be a good candidate for an insulating substrate with negligibly small eddy-current loss and sufficiently high permittivity. © 2013 The Institute of Electrical Engineers of Japan.

    DOI

  • Charge transport characteristics in epoxy resin at high temperatures based on electrode polarization analysis

    Fuqiang Tian, Yoshimichi Ohki

    Annual Report - Conference on Electrical Insulation and Dielectric Phenomena, CEIDP     1318 - 1321  2013  [Refereed]

     View Summary

    Taking epoxy resin as an example, this research demonstrates that characteristic parameters of charge transport and accumulation are obtainable from complex permittivity spectra by carrying out theoretical analysis based on the electrode polarization theory. The real and imaginary permittivity of the electrode polarization satisfy the Cole-Cole relations well at and above 160°C. The shape parameter of the Cole-Cole arc is found to be an indication of ion blockage at the electrodes. Both the density and mobility of ions are thermally activated and the sum of the activation energies of these two thermal processes is nearly equal to the activation energy of ac conductivity in the same temperature range. This very reasonable result indicates the adequacy of electrode polarization analysis. © 2013 IEEE.

    DOI

  • Detection of polymer degradation and metal corrosion by terahertz imaging using a quantum cascade laser and a THz camera

    Y. Ohki, M. Adachi, M. Komatsu, M. Mizuno, K. Fukunaga

    Proceedings of IEEE International Conference on Solid Dielectrics, ICSD     505 - 508  2013  [Refereed]

     View Summary

    Recently, THz waves with frequencies from about 0.1 to 10 THz have emerged as a new scientific tool. In this paper, feasibility of locating degraded portions in polymer-insulated power cables by THz waves has been examined, using a compact system consisting of a quantum cascade laser and a THz camera with uncooled microbolometers. The degradation we examined is oxidation of cross-linked polyethylene and rust on copper. As a result, we found that the compact system is capable of detecting polymer oxidation. © 2013 IEEE.

    DOI

  • Experimental and numerical analyses on terahertz spectra of oxidized cross-linked polyethylene

    M. Komatsu, M. Hosobuchi, Y. Ohki, M. Mizuno, K. Fukunaga, X. Xie, Y. Cheng

    Proceedings of IEEE International Conference on Solid Dielectrics, ICSD     501 - 504  2013  [Refereed]

     View Summary

    Chemically cross-linked polyethylene irradiated by gamma-rays to certain designated doses from 100 to 800 kGy at 100 °C in air was analyzed by terahertz spectroscopy. A peak appears at around 1.3 THz when the sample was irradiated by gamma-rays. By referring to the infrared absorption spectra obtained for the same samples, the peak at 1.3 THz was found to be due to an oxidation-related dipole. It was found that this peak can be fitted to both the Debye and the Lorentz functions similarly. In addition, numerical analyses were carried out using the density functional theory. As a result, it was found that the 1.3-THz peak is likely to be ascribable to resonance. © 2013 IEEE.

    DOI

  • Highly sensitive location method of an abnormal temperature point in a cable by frequency domain reflectometry

    Yoshimichi Ohki, Naoshi Hirai, Takayuki Yamada, Susumu Kumagai

    Proceedings of IEEE International Conference on Solid Dielectrics, ICSD     117 - 120  2013  [Refereed]

     View Summary

    This paper demonstrates that it is possible to locate very precisely a point showing a high temperature in a polymer insulated cable. A coaxial communication cable with a length of 32 m insulated by low density polyethylene or a flat inhouse electric cord with a length of 21 m insulated by polyvinyl chloride was heated at different positions for different lengths. The ratio between the incident and reflected powers of electromagnetic waves was measured using a network analyzer in a wide frequency range and the obtained spectra were analyzed by inverse Fourier transform. As a result, the position exhibiting a temperature higher than the adjacent points can be located with a spatial resolution as short as 2.5 cm. © 2013 IEEE.

    DOI

  • Dielectric Properties and Thermal Conductivity of Epoxy/AlN Composites with Several Kinds of Filler-Matrix Interfaces

    T. Tanaka, T. Iizuka, Y. Ohki, X. Huang, P. Jiang

    PROCEEDINGS OF THE 2013 IEEE INTERNATIONAL CONFERENCE ON SOLID DIELECTRICS (ICSD 2013), VOLS 1 AND 2     377 - 380  2013  [Refereed]

     View Summary

    Epoxy/AlN composites were prepared using high purity AlN particles with an average size of 1.1 mu m. AlN particles were surface-treated with three kinds of silane coupling agents and three kinds of surface-modifiers. Dielectric properties and thermal conductivity were evaluated. As a result, it was found that silane coupling agents with epoxy, mercapto, and amino groups create higher thermal conductivity than surface modifiers such as GO, POSS and hyper-branched polymer. A silane coupling agent with mercapto group and a graphene oxide surface modifier are promising at high and low loading levels, respectively. From the dielectric spectroscopy study, it can be said that interfacial conditions should be improved by finding out proper methods.

  • Charge Behavior of Epoxy/Silica Nanocomposites under AC High Fields

    Yutaro Watanabe, Kazuyuki Tohyama, Masahiro Kozako, Shohei Kawano, Koji Ito, Hiroaki Miyake, Yasuhiro Tanaka, Yuichi Hirose, Yoshimichi Ohki

    PROCEEDINGS OF THE 2013 IEEE INTERNATIONAL CONFERENCE ON SOLID DIELECTRICS (ICSD 2013), VOLS 1 AND 2     892 - 895  2013  [Refereed]

     View Summary

    If a power semiconductor module becomes compact to enhance its wiring density, the electric field at its insulation part should become high. This in turn causes the increase in heat generation per unit volume. Therefore, epoxy resin with high thermal conductivity, a low thermal expansion rate and good anti-aging performance is required In this regard we made nano-micro composites (NMCs), namely epoxy resin samples containing both nano-silica filler having high insulating performance and micro-alumina filler having a high thermal conductivity, and observed electroluminescence (EL) and dissipation current waveforms under AC high electric fields for these NMCs simultaneously in order to examine the effects of addition of fillers with different sizes. As a result, it was found that both dissipation current and the number of EL pulses depend on the sample thickness. It was also found that the addition of alumina-micro filler alone into epoxy resin increases the number of EL pulses compared to the neat epoxy and that the NMC shows improved insulating properties. It was also confirmed that the relative permittivity is around 4 for the neat resin and the NMCs, independent of the thickness and the type of fillers.

  • Experimental Study on the Dielectric Properties of a Liquid Crystal Polymer

    Peng Yang, Yoshimichi Ohki

    2013 IEEE CONFERENCE ON ELECTRICAL INSULATION AND DIELECTRIC PHENOMENA (CEIDP)     575 - 578  2013  [Refereed]

     View Summary

    Dielectric behavior of a typical liquid crystal polymer Vecstar was studied by measuring its dielectric permittivity, conductivity and other electrical properties. Dielectric spectra are analyzed combined with the thermal properties and thermally stimulated current spectra, and space charge distributions. Complex electrical modulus is introduced in the analysis of the permittivity spectra, which proves to be effective to suppress the effects of large conduction currents at high temperatures. Moreover, the slope of the frequency dependence of er' is different below and above 160 degrees C, which indicates that the Maxwell/Wagner polarization occurs inside the sample. The experimental results indicate that both the dielectric relaxation and electrical conduction are closely related with molecular motion and phase transition of the polymer. It is also shown that the polymer is an excellent insulating material in a wide temperature and frequency range.

  • Measurements of THz Absorption Peaks in Photo-degraded Polyethylene and Their Assignment by Quantum Chemical Calculations

    M. Hosobuchi, M. Komatsu, X. Xie, Y. Cheng, Y. Furukawa, Y. Ohki, M. Mizuno, K. Fukunaga

    2013 IEEE CONFERENCE ON ELECTRICAL INSULATION AND DIELECTRIC PHENOMENA (CEIDP)     1046 - 1049  2013

     View Summary

    Electrical insulation properties of low density polyethylene (LDPE) are still unclear, especially when it is degraded. In this regard, ultraviolet (UV) photons were irradiated to LDPE sheets at room temperature and 85 degrees C and their absorption spectra were measured at frequencies from 0.5 to 3.0 THz and from 18 to 105 THz. A very broad absorption band with its peak at around 1.6 THz appears by the UV irradiation. In accord with this appearance, the infrared absorption peak due to a carbonyl structure increases. Quantum chemical calculations were carried out using 3-dimensional models with and without carbonyl groups. As a result, the model with carbonyl groups was found to have optically active modes at about 1.7 THz. From these results, the increase in absorption at 1.6 THz is assumed to be induced by oxidation.

    DOI

  • Advanced Directly Molded Type Polymer-Housed Surge Arresters

    Y. Ohki

    IEEE ELECTRICAL INSULATION MAGAZINE   29 ( 1 ) 63 - 64  2013.01  [Refereed]

    DOI

  • Feasibility Study on Terahertz Imaging of Corrosion on a Cable Metal Shield

    Marina Komatsu, Ryo Sato, Maya Mizuno, Kaori Fukunaga, Yoshimichi Ohki

    JAPANESE JOURNAL OF APPLIED PHYSICS   51 ( 12 ) 122405(1)-122405(5) - 122405(5)  2012.12  [Refereed]

     View Summary

    The possibility of nondestructive diagnosis of corrosion on a metallic shielding layer in a power cable by time-domain imaging and spectroscopic measurements using terahertz waves was examined experimentally. First, absorbance was measured in a THz frequency range for copper oxide, basic copper carbonate, and copper chloride. Next, two copper plates, uncorroded and corroded, were placed in contact with each other horizontally, and a terahertz wave was scanned over them in order to get two-dimensional images. When the plate is corroded to form either basic copper carbonate or copper chloride, the intensity of reflected wave becomes lower than when the plate is uncorroded. It seems that the terahertz wave was scattered more on the surface of the corroded copper plate and was absorbed by the corrosion. Therefore, this research indicates the feasibility of nondestructive diagnosis to detect corrosion on a shield metal in a power cable. (C) 2012 The Japan Society of Applied Physics

    DOI

  • Development of High-Heat-Resistant Hybrid Electric Vehicle/Electric Vehicle Cables

    Y. Ohki, T. Fukai, T. Watanabe, K. Naoe

    IEEE ELECTRICAL INSULATION MAGAZINE   28 ( 6 ) 68 - 69  2012.11  [Refereed]

    DOI

  • Correlation between mechanical and dielectric relaxation processes in epoxy resin composites with nano- and micro-fillers

    Mayumi Hyuga, Toshikatsu Tanaka, Yoshimichi Ohki, Takahiro Imai, Miyuki Harada, Mitsukazu Ochi

    ELECTRONICS AND COMMUNICATIONS IN JAPAN   95 ( 9 ) 1 - 9  2012.09  [Refereed]

     View Summary

    Effects of addition of nano-sized and micro-sized fillers into epoxy resin on its mechanical and dielectric relaxation phenomena were examined. The glass transition temperature (Tg) decreased when a small content of nanoclay modified by octadecylamine was added, but the decrease in Tg was suppressed when the nanoclay was modified by dimethyldodecylamine. On the other hand, Tg increased when an abundant amount of microsilica was added. At temperatures above Tg, both mechanical and dielectric relaxations were accelerated in samples with octadecylamine-modified nanoclay, while the acceleration did not occur in samples with nanoclay modified by dimethyldodecylamine. Both relaxations were restricted in composites with abundant microsilica. Therefore, co-addition of dimethyldodecylamine-modified nanoclay and abundant microsilica is adequate in order to make an epoxy resin composite with a high Tg and low dielectric loss. (c) 2012 Wiley Periodicals, Inc. Electron Comm Jpn, 95(9): 19, 2012; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/ecj.11420

    DOI

  • Development of a high- performance transfer mold power module (News from Japan)

    Y. Ohki

    IEEE Electrical Insulation Magazine   28 ( 5 ) 66 - 67  2012.08  [Refereed]

    DOI

  • Frequency Dependence of Breakdown Performance of XLPE with Different Artificial Defects

    Weiwei Li, Jianying Li, Guilai Yin, Shengtao Li, Jiankang Zhao, Benhong Ouyang, Yoshimichi Ohki

    IEEE TRANSACTIONS ON DIELECTRICS AND ELECTRICAL INSULATION   19 ( 4 ) 1351 - 1359  2012.08  [Refereed]

     View Summary

    In this paper, the effect of the applied field frequency on the breakdown performances of cross-linked polyethylene (XLPE) insulation was investigated. Three kinds of artificial defects, namely an inserted needle, a needle void and a knife void, were introduced into a 10 kV commercial XLPE cable, to simulate real defects that may be introduced during on-site service. Breakdown tests were conducted in transformer oil at room temperature using a frequency-tuned resonant test system in a frequency range from 20 to 300 Hz. It was found that the breakdown voltage of specimens with the inserted needle was sensitive to the applied field frequency, and the breakdown voltage showed a maximum value at about 240 Hz. However, for other two kinds of specimens with the needle void and knife void, the breakdown behavior shows a weak relation with the applied field frequency. The breakdown voltages of the specimens with the needle and void are 25 kV and 20 kV respectively. It was further found that side channels appeared on the flank of the main channel of the breakdown path in the specimen with the inserted needle, and the frequency dependence of fractal dimension D of the side channel in the breakdown path are similar to that of electrical trees before breakdown, which may consequently lead to the maximum value in the experimental results.

    DOI

  • Development of a LargeScale Direct Methanol Fuel Cell (News from Japan)

    Y. Ohki

    IEEE Electrical Insulation Magazine   28 ( 4 ) 47 - 49  2012.06  [Refereed]

    DOI

  • Role of Interface on the Thermal Conductivity of Highly Filled Dielectric Epoxy/AlN Composites

    Xingyi Huang, Tomonori Iizuka, Pingkai Jiang, Yoshimichi Ohki, Toshikatsu Tanaka

    JOURNAL OF PHYSICAL CHEMISTRY C   116 ( 25 ) 13629 - 13639  2012.06  [Refereed]

     View Summary

    The interface between filler and matrix has long been a critical problem that affects the thermal conductivity of polymer composites. The effects of the interface on the thermal conductivity of the composite with low filler loading are well documented, whereas the role of the interface in highly filled polymer composites is not clear. Here we report on a systematic study of the effects of interface on the thermal conductivity of highly filled epoxy composites. Six kinds of surface treated and as received AlN particles are used as fillers. Three kinds of treated AlN are functionalized by silanes, i.e., amino, epoxy, and mercapto group terminated silanes. Others are functionalized by three kinds of materials, i.e., polyhedral oligomeric silsesquioxane (POSS), hyperbranched polymer, and graphene oxide (GO). An intensive study was made to clarify how the variation of the modifier would affect the microstructure, density, interfacial adhesion, and thus the final thermal conductivity of the composites. It was found that the thermal conductivity enhancement of the composites is not only dependent on the type and physicochemical nature of the modifiers but also dependent on the filler loading. In addition, some unexpected results were found in the composites with particle loading higher than the percolation threshold. For instance, the composites with AlN treated by the silane uncapable of reading with the epoxy resin show the most effective enhancement of the thermal conductivity. Finally, dielectric spectroscopy was used to evaluate the insulating properties of the composites. This work sets the way toward the choice of a proper modifier for enhancing the thermal conductivity of highly filled dielectric polymer composites.

    DOI

  • Development of a multifunctional electric power storage device (News from Japan)

    Y. Ohki

    IEEE Electrical Insulation Magazine   28 ( 3 ) 50 - 51  2012.05  [Refereed]

    DOI

  • First-Principles Study on Intrinsic Point Defects in Rhombohedral LaAlO3 and Their Effects on Electrical Properties

    Xiaojun Xie, Yonghong Cheng, Bing Xiao, Yoshimichi Ohki

    JAPANESE JOURNAL OF APPLIED PHYSICS   51 ( 4 ) 041103(1)-041103(9) - 041103(9)  2012.04  [Refereed]

     View Summary

    Formation energies and energy levels of various neutral and charged defects in bulk rhombohedral LaAlO3 are calculated numerically in different equilibrium conditions under different oxygen partial pressures, using first-principles plane-wave calculations. The roles of such defects in electrical properties of LaAlO3 are also discussed. The results obtained by calculations show that the formation energy becomes low for oxygen interstitial if LaAlO3 is in an oxygen-rich condition. In contrast, an oxygen vacancy is easily formed in a condition where oxygen is deficient and metal elements are relatively rich, even if the oxygen partial pressure is high. These results explain well the concentration change of constituent elements in LaAlO3 under different treatment atmospheres. Furthermore, the calculations can interpret a large tunnelling current that is often observed in a metal-oxide-silicon structure using LaAlO3 as the oxide layer, since the O vacancy has an energy level near the valence band maximum of Si. (C) 2012 The Japan Society of Applied Physics

    DOI

  • Dielectric properties of poly(phenylene sulfide) as a function of temperature and frequency

    Saki Hikosaka, Yoshimichi Ohki

    IEEJ TRANSACTIONS ON ELECTRICAL AND ELECTRONIC ENGINEERING   7 ( 2 ) 116 - 120  2012.03  [Refereed]

     View Summary

    Dielectric properties of poly(phenylene sulfide) (PPS), a kind of super engineering plastic, were investigated over a wide temperature range by measuring its frequency spectra of complex permittivity and thermally stimulated polarization and depolarization currents (TSPC and TSDC). Experimental results indicate that its permittivity and dielectric loss factor remain stable and maintain low values up to around 140 degrees C, despite the fact that its glass transition appears at a much lower temperature of 88 degrees C. As for TSPC and TSDC, a very small peak associated with the glass transition appears at about 110 degrees C. Furthermore, a much clearer TSPC peak due to orientation of dipole moments associated with the ether linkage induced by the cross-linking reaction appears at about 150-170 degrees C. The corresponding TSDC peak due to randomization of these dipoles is very small compared to the TSPC peak, since PPS was cross-linked during the high-temperature process in the TSPC measurement, which made rotation of the dipoles difficult. At temperatures higher than 150 degrees C, both the real and imaginary parts of the complex permittivity show significant increases with increase in temperature and decrease in frequency. This is attributable to the transport of the charge carriers. (C) 2011 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

    DOI

  • Development of a RE-Ba-Cu-O superconducting strong magnet (News from Japan)

    Y. Ohki

    IEEE Electrical Insulation Magazine   28 ( 2 ) 60 - 61  2012.03  [Refereed]

    DOI

  • A study of the critical factor determining the size of etched latent tracks formed on SiO2 glass by swift-Cl-ion irradiation

    Ken-ichi Nomura, Yoshimichi Ohki, Makoto Fujimaki, Xiaomin Wang, Koichi Awazu, Tetsuro Komatsubara

    NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS   272   1 - 4  2012.02  [Refereed]

     View Summary

    When a Cl ion with energy of the order of megaelectronvolts collides with SiO2 glass, it penetrates the glass along a straight line. The region through which the ion passes and its vicinity, called the latent track, can be easily etched by hydrofluoric acid, resulting in the formation of a nanopore. With increasing ion energy, the nanopore radius first increases, reaches a maximum, and then decreases. In order to analyze this strange phenomenon, we investigated the radius of the region that melted upon ion irradiation, as one of the possible approaches. We calculated its radius using heat diffusion equations and compared it with the radius of nanopores. We found that both the radii depend on the ion energy in a similar manner. (C) 2011 Elsevier B.V. All rights reserved.

    DOI

  • 高誘電率ゲート絶縁膜材料LaAlO3およびYAlO3における点欠陥の原因

    山坂大樹, 中原和也, 堀井陽介, 大木義路

    放電学会誌   55 ( 1 ) 27 - 31  2012  [Refereed]

  • Location feasibility of degradation in cable through Fourier transform analysis of broadband impedance spectra

    Yoshimichi Ohki, Naoshi Hirai

    IEEJ Transactions on Fundamentals and Materials   132 ( 2 ) 122 - 128  2012  [Refereed]

     View Summary

    Since electric cables play important roles such as power supply and information transmission, their degradation may cause a serious problem. We have been trying to monitor the degradation of cable insulation by measuring the magnitude and phase angle of impedance as a function of frequency in a very wide frequency range. The cables tested were insulated with flame-retardant ethylene propylene rubber or special heat-resistant polyvinyl chloride. They were damaged partially by peeling off their insulation layers, or aged partially by heat and γ-rays. Impedance and phase angle were measured from a terminal of the cable. The difference in impedance between the damaged and sound cables becomes clear by fast Fourier transform analyses, from which the damaged portion can be located. It can be clearly shown that this method has a potential ability to detect the degradation of cable insulation induced by physical damage, γ-ray irradiation, and thermal aging. © 2012 The Institute of Electrical Engineers of Japan.

    DOI

  • Terahertz imaging of water trees generated in low-density polyethylene

    Ryo Sato, Marina Komatsu, Norikazu Fuse, Yoshinobu Nakamichi, Maya Mizuno, Kaori Fukunaga, Yoshimichi Ohki

    IEEJ Transactions on Fundamentals and Materials   132 ( 2 ) 148 - 149  2012  [Refereed]

     View Summary

    Research was carried out whether it is possible to detect water trees in a low density polyethylene sheet by terahertz light. The attenuation of the reflected intensity of THz light was observed on several points in the sample under a polyvinylchloride sheet. The intensity distribution of the reflected light agrees with the distribution of water trees in the sample. Therefore, the distribution of water trees would be detectable even if the trees are invisible to naked eyes. © 2012 The Institute of Electrical Engineers of Japan.

    DOI

  • Optical characterization and computational chemical evaluation of electronic localized states in polyolefin

    Tomoyuki Arai, Masashi Hosobuchi, Norikazu Fuse, Kyozaburo Takeda, Yoshimichi Ohki

    IEEJ Transactions on Fundamentals and Materials   132 ( 9 ) 760 - 766  2012  [Refereed]

     View Summary

    Optical absorption spectra and photoluminescence spectra were obtained for eight kinds of polyolefin sheet samples using photons in a range from visible to vacuum ultraviolet. Almost all samples were found to exhibit an absorption peak at around 6.5 eV and a luminescence band at around 4.3 eV. The luminescence was found to be induced by the absorption. Furthermore, it was found that successive absorption of ultraviolet photons weakened the luminescence intensity. It is assumed from these results that α ,β -unsaturated carbonyls are luminous and that the carbonyls are decomposed through the Norrish type II reaction by absorbing ultraviolet photons. Quantum chemical calculations were carried out using polyethylene models with and without an unsaturated carbonyl to verify the above-mentioned assumption. As a result, the model with an unsaturated carbonyl was found to have localized electronic states in the forbidden band. One of the differential energies between the states is close to the photon energy, by which the luminescence is induced. The bond length of a double bond, which is next to the carbonyl, was found to be longer at the excited singlet state than at the ground state. These results obtained by computation support the above-mentioned assumption of the luminescence center and its decomposition. © 2012 The Institute of Electrical Engineers of Japan.

    DOI

  • Experimental Study on the Factors Determining the Partial Discharge Resistance of Polymers

    Yuka Hasegawa, Ikeda Isamu, Yoshimichi Ohki

    2012 IEEE 10TH INTERNATIONAL CONFERENCE ON THE PROPERTIES AND APPLICATIONS OF DIELECTRIC MATERIALS (ICPADM)   133 ( 3 ) 75 - 80  2012  [Refereed]

     View Summary

    The practical lifetime of a polymeric insulating material is determined in most cases by its partial discharge resistance at an operating temperature. Therefore, in this research, the partial discharge resistance was examined for eight kinds of polymer samples. An ac 500-Hz voltage with a designated value was applied to a sample sheet sandwiched between a pair of IEC (b) electrodes at 353K and 30% relative humidity for a designated period. The sheet surface degraded by partial discharges was then observed by a mechanical surface profilometer to measure the shape, from which the whole eroded volume was estimated. As a result, it has been clarified that polymers can exhibit superior partial discharge resistance when their melting point, molecular weight of chain repeating unit, density, weight ratio of benzene rings, and crystallinity are high.

    DOI

  • Mechanism and application of refractive index increase induced in fluorinated polyimide by ion irradiation

    Y. Arai, Y. Ohki, K. Saito, H. Nishikawa

    Annual Report - Conference on Electrical Insulation and Dielectric Phenomena, CEIDP     705 - 708  2012  [Refereed]

     View Summary

    This paper reports a result carried out in order to clarify the applicability of ion irradiation effects to polymer materials used for optical waveguides. We irradiated 950-keV He+ ions or 1.0-MeV H+ ions to a fluorinated polyimide film to a fluence between 1 × 10 14 and 7 × 1016 cm-2, and the film surface was scanned by a profilometer. The depth of a dent induced by the irradiation increases with an increase in the fluence. From the depth of the dent, the projected range of the He+ ions, and the Lorentz-Lorenz equation, the refractive index of the ion-irradiated region was found to increase by 2.9 %. This value agrees with the increment in refractive index measured by spectroscopic ellipsometry, which also increases as the fluence increases. Furthermore, the increment in refractive index, 0.21 %, induced by the irradiation of H+ ions to the fluence of 1 × 10 15 cm-2 is comparable to the value, 0.35 %, observed when H+ ions of a similar fluence were irradiated to SiO2 glass. Therefore, it is natural to assume that the ion irradiation to the polymer can be a good tool to fabricate a high-performance polymer-based optical waveguide. © 2012 IEEE.

    DOI

  • Two methods for improving electrochemical migration resistance of printed wiring boards

    Y. Ohki, Y. Hirose, G. Wada, H. Asakawa, T. Tanaka, T. Maeno, K. Okamoto

    ICHVE 2012 - 2012 International Conference on High Voltage Engineering and Application     548 - 552  2012  [Refereed]

     View Summary

    Experiments were carried out using space charge distribution measurements as a major measurement method in order to find effective measures to suppress electrochemical migration (ECM) along the thickness direction of a printed wiring board (PWB). As a result, the following valuable knowledge was obtained. First, in a PWB consisting of paper/ phenol-resin composite and a copper electrode, the electrical resistivity of epoxy resin used as an adhesive layer to connect the composite and the electrode is usually higher than that of the composite. In such a case, ECM is easily grown in the PWB. The resistance against ECM was found to increase significantly if we use an adhesive layer with a similar electrical resistivity to the insulating material in a PWB. Another important fact is that the growth of ECM can be suppressed if silica nanofillers were added to epoxy resin in the case of an epoxy-based PWB. © 2012 IEEE.

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  • Terahertz Spectroscopic Observation of Cross-linked Polyethylene Aged Simultaneously by Heat and Gamma-rays

    Marina Komatsu, Yoshimichi Ohki, Maya Mizuno, Kaori Fukunaga

    2012 IEEE 10TH INTERNATIONAL CONFERENCE ON THE PROPERTIES AND APPLICATIONS OF DIELECTRIC MATERIALS (ICPADM)    2012  [Refereed]

     View Summary

    Chemically cross-linked polyethylene irradiated by gamma-rays to certain designated doses from 100 to 800 kGy at 100 degrees C in air was analyzed by X-ray diffraction, infrared spectroscopy and terahertz spectroscopy. It was found by infrared spectroscopy that the degree of oxidation increased when the sample was irradiated. By the terahertz spectroscopy, a peak appears at around 44 cm(-1) (similar or equal to 1.3 THz) when the sample was irradiated by gamma-rays. By carrying out the Cole-Cole analysis on the complex permittivity spectra associated with the peak at 44 cm(-1), the dipolar relaxation was found to exhibit a nearly single relaxation process. This suggests that the terahertz spectroscopy has a potential to detect the oxidative degradation of cross-linked polyethylene.

  • Comparison of Broadband Impedance Spectroscopy and Time Domain Reflectometry for Locating Cable Degradation

    Naoshi Hirai, Takayuki Yamada, Yoshimichi Ohki

    PROCEEDINGS OF 2012 IEEE INTERNATIONAL CONFERENCE ON CONDITION MONITORING AND DIAGNOSIS (IEEE CMD 2012)     229 - 232  2012  [Refereed]

     View Summary

    A new method called the broadband impedance spectroscopy (BIS), which is a kind of frequency domain reflectometry, is being developed as a reliable method for locating a degraded portion of a low voltage cable. The sensitivity of the BIS method is compared to that of the time domain reflectometry (TDR). In the first case, the sheath and insulation of a 50-m-long polyvinyl chloride insulated cable were partially peeled off. In the second case, cables insulated with silicone rubber, flame-retardant ethylene propylene rubber, or crosslinked polyethylene, each about 25 m long, were aged simultaneously by heat and gamma-irradiation in air. In both two cases, a clear peak indicating the damaged or aged portion appears in the BIS measurement, while no such a peak appears in the TDR measurement. Therefore, the BIS method is much superior to the TDR method for locating a degraded portion in a cable.

  • Chemiluminescence as a Clear Diagnostic Tool of Polymer Oxidation

    Yoshimichi Ohki, Naoshi Hirai

    2012 IEEE 10TH INTERNATIONAL CONFERENCE ON THE PROPERTIES AND APPLICATIONS OF DIELECTRIC MATERIALS (ICPADM)    2012  [Refereed]

     View Summary

    Chemiluminescence (CL) was applied to evaluate the oxidation of low density polyethylene and cross-linked polyethylene. The CL measurements done at a constant temperature revealed that the CL spectra always exhibit a clear incubation period or oxidation induction time during which no CL appears. The CL appearing in an early stage of oxidation seems to be due to excited carbonyls, while the CL due to singlet oxygen is only observable in the end stage. Skewness and kurtosis are effective parameters to analyze the change in CL intensity spectrum as a function of time.

  • Improvement in Sensitivity of Broadband Impedance Spectroscopy for Locating Degradation in Cable Insulation by Ascending the Measurement Frequency

    Takayuki Yamada, Naoshi Hirai, Yoshimichi Ohki

    PROCEEDINGS OF 2012 IEEE INTERNATIONAL CONFERENCE ON CONDITION MONITORING AND DIAGNOSIS (IEEE CMD 2012)     677 - 680  2012  [Refereed]

     View Summary

    In order to develop a truly reliable method for locating a degraded portion in a cable, we are developing a method based on the frequency domain reflectometry and inverse fast Fourier transform (IFFT). The locating ability of this method was examined for three types of degradation given to three kinds of cables, each with a length ranging from 24.4 to 29.0 m. Namely, a vented water tree was simulated in a coaxial cable insulated with cross-linked polyethylene (XLPE) by making a tiny hole filled with a NaCl solution, and gamma-ray-induced aging was given for a length of 80 cm of an XLPE-insulated triplex cable, while the mutual distance between the two conductors was expanded for 10cm in a cord insulated with polyvinyl chloride. Analyses by IFFT using the data obtained from 1 MHz to 100 or 110 MHz can locate all three types of degradation, but the resolution is poor. If the highest measurement frequency is ascended to 200 to 400 MHz, the resolution becomes better. Moreover, the spectrum indicating the degraded portion shows quite a good reproducibility unless the cable is moved. This means that this method can be a very sensitive degradation locating tool usable for real cable systems.

  • Terahertz Spectroscopy as a Novel Method for Diagnosing the Integrity of Polymer Insulated Cables

    Yoshimichi Ohki, Marina Komatsu, Maya Mizuno, Kaori Fukunaga

    2012 ANNUAL REPORT CONFERENCE ON ELECTRICAL INSULATION AND DIELECTRIC PHENOMENA (CEIDP)     203 - 206  2012  [Refereed]

     View Summary

    Through examinations of the applicability of novel terahertz spectroscopy to the diagnosis of integrity of polymeric insulated cables, the following important examples have been obtained. First, the attenuation of the reflected intensity of THz light was scanned over a low density polyethylene sheet with water trees under a polyvinylchloride sheet. As a result, its intensity distribution agrees with the distribution of water trees in the sample. Therefore, water trees would be visualizable even if they are invisible to naked eyes. Secondly, chemically crosslinked polyethylene sheets were irradiated by gamma-rays to certain designated doses. A peak appears at around 44 cm(-1) (= 1.3 THz) when the dose is higher than 200 kGy. Combining the results obtained by instrumental analyses, the 44-cm(-1) peak is likely to be due to oxidation-related dipoles. Thirdly, the reflected THz light intensities were compared among an uncorroded copper plate and three corroded plates with copper oxide, basic copper carbonate, and copper chloride. The last two corroded plates were found to exhibit absorption strong enough for the THz light to give clear images of corrosion.

  • Advanced Optimum Reactive Power Control of Wind Power Generation Systems

    Y. Ohki

    IEEE ELECTRICAL INSULATION MAGAZINE   28 ( 1 ) 51 - 52  2012.01  [Refereed]

    DOI

  • Development of Epoxy/BN Composites with High Thermal Conductivity and Sufficient Dielectric Breakdown Strength Part I -Sample Preparations and Thermal Conductivity

    Zengbin Wang, Tomonori Iizuka, Masahiro Kozako, Yoshimichi Ohki, Toshikatsu Tanaka

    IEEE TRANSACTIONS ON DIELECTRICS AND ELECTRICAL INSULATION   18 ( 6 ) 1963 - 1972  2011.12  [Refereed]

     View Summary

    The aim of this research is to find a way to achieve the epoxy composites with both high thermal conductivity and acceptable dielectric breakdown (BD) strength. As high thermal conductivity, low permittivity and low thermal expansion coefficient of filler can endow composite with higher thermal conductivity, higher BD strength and lower thermal expansion coefficient respectively, BN (boron nitride) with high thermal conductivity, low permittivity and low thermal expansion coefficient was adopted as main filler in the research. Thermal conductivity was investigated in this part. The BD strength of samples will be discussed in Part II. Neat epoxy and other 25 kinds of epoxy/BN composites were prepared by a hot press method. Most of BN fillers were surface modified with silane coupling agent through ethanol/water reflux method to improve thermal conductivity. The values of 2.91 W/m.K, 3.95 W/m.K and 10.1 W/m.K as thermal conductivity were obtained for the composites that was single-loaded with h-BN(hexagonal boron nitride), c-BN (cubic boron nitride) or conglomerated h-BN, respectively. They were further improved to 5.26 W/m.K, 5.94 W/m.K and 12.3 W/m.K, respectively, by adding extra smaller AlN (aluminum nitride) to fill the voids in sample. Thermal conductivity of samples changes with the ratio of cBN and h-BN when c-BN and h-BN were co-loaded. A value of 5.74 W/m.K as maximum was obtained at their ratio of 1 to 1 when total filler content is 80 wt%. A much higher value of 7.69 W/m.K was obtained by adding extra AlN. From the experiment data, it is concluded that the filler orientation in vertical direction of sample surface and the decrease of voids in sample are very important to obtain high thermal conductivity, and that the filler surface modification is also necessary to improve thermal conductivity especially for epoxy/c-BN composites, and addition of nano silica in small amount can also increase thermal conductivity if sample is prepared appropriately.

    DOI

  • Development of Epoxy/BN Composites with High Thermal Conductivity and Sufficient Dielectric Breakdown Strength Part II - Breakdown Strength

    Zengbin Wang, Tomonori Iizuka, Masahiro Kozako, Yoshimichi Ohki, Toshikatsu Tanaka

    IEEE TRANSACTIONS ON DIELECTRICS AND ELECTRICAL INSULATION   18 ( 6 ) 1973 - 1983  2011.12  [Refereed]

     View Summary

    The aim of this research is to find a way to achieve the epoxy composites with high thermal conductivity and acceptable dielectric breakdown (BD) strength. A value 12.3 W/m.K is the highest thermal conductivity obtained for epoxy composite in Part I. Dielectric breakdown performances such as short-time dielectric breakdown strength (BD strength), partial discharge (PD) resistance and BD time for composites were investigated in the Part II. In general, micro filler inclusion will increase thermal conductivity and decrease dielectric breakdown performance. Influencing factors are considered to be the orientation of filler, the content of void space, the content ratio in the case of co-mixing, the addition of nano filler, and filler surface modification. Twenty six kinds of composites were prepared in consideration of the above influencing factors. There are two options for most appropriate ones among the composites evaluated in the research. One is an epoxy/conglomerated h-BN composite with co-loaded nano SiO2 and micro AlN filler. It has 12.3 W/m.K in thermal conductivity, 75.1 kV(peak)/mm in BD strength and 260 % of BD time for neat epoxy. It is most suitable when low BD strength and high thermal conductivity is needed. The other one is an epoxy/h-BN composite with co-loaded nano silica and AlN filler for requirement of very high BD strength but lower thermal conductivity. Optimum thermal conductivity is obtained if flaky h-BN filler is oriented in parallel to heat flow. Since it is difficult to realize full orientation, the use of conglomerated hBN filler is a suitable option. Optimum BD performance is obtained if void space is reduced by certain methods such as co-dispersion of different size fillers and addition of nano filler.

    DOI

  • Development of Fiber Lasers Capable of Emitting New Wavelengths

    Y. Ohki

    IEEE ELECTRICAL INSULATION MAGAZINE   27 ( 6 ) 46 - 47  2011.11  [Refereed]

    DOI

  • Optimal design of a spectral readout type planar waveguide-mode sensor with a monolithic structure

    Xiaomin Wang, Makoto Fujimaki, Takafumi Kato, Ken-Ichi Nomura, Koichi Awazu, Yoshimichi Ohki

    Optics Express   19 ( 21 ) 20205 - 20213  2011.10  [Refereed]

     View Summary

    Optical planar waveguide-mode sensor is a promising candidate for highly sensitive biosensing techniques in fields such as protein adsorption, receptor-ligand interaction and surface bacteria adhesion. To make the waveguide-mode sensor system more realistic, a spectral readout type waveguide sensor is proposed to take advantage of its high speed, compactness and low cost. Based on our previously proposed monolithic waveguide-mode sensor composed of a SiO2 waveguide layer and a single crystalline Si layer [1], the mechanism for achieving high sensitivity is revealed by numerical simulations. The optimal achievable sensitivities for a series of waveguide structures are summarized in a contour map, and they are found to be better than those of previously reported angle-scan type waveguide sensors. © 2011 Optical Society of America.

    DOI PubMed

  • Effects of ultraviolet photon irradiation on the transition metal impurities in LaAlO3

    Daiki Yamasaka, Kenta Tamagawa, Yoshimichi Ohki

    JOURNAL OF APPLIED PHYSICS   110 ( 7 ) 74103 - 074103(5)  2011.10  [Refereed]

     View Summary

    By measuring electron spin resonance (ESR) spectra at the X-band frequency and absorption spectra from the visible to ultraviolet region at room temperature, it was confirmed that perovskite single crystal LaAlO3 contains Cr and Fe as impurities. When LaAlO3 is exposed to photons with energies higher than 4.5 eV, the intensities of ESR signals due to Cr3+ and Fe3+ decrease, which indicates that electrons released by the photon irradiation are captured by Cr3+ and Fe3+ . Concurrently with this, a broad optical absorption band at about 3.0 eV and two new broad and weak ESR signals appear. The former is attributable to a combination of a hole and a La3+ (or Al3+) vacancy, and the two weak ESR signals are assigned to the O- center and F+ center. (C) 2011 American Institute of Physics. [doi:10.1063/1.3641974]

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  • Observation of Water Trees Using Terahertz Spectroscopy and Time-domain Imaging

    Ryo Sato, Marina Komatsu, Yoshimichi Ohki, Norikazu Fuse, Yoshinobu Nakamichi, Maya Mizuno, Kaori Fukunaga

    IEEE TRANSACTIONS ON DIELECTRICS AND ELECTRICAL INSULATION   18 ( 5 ) 1570 - 1577  2011.10  [Refereed]

     View Summary

    Terahertz measurements were carried out to detect water trees grown in low-density polyethylene sheets. Water absorbs light at terahertz frequencies, fairly strongly at about 5.0 THz and rather weakly from 0.1 to 1.0 THz. Using the absorption at these frequencies, observation of water trees was tried according to the following procedures. First, we made a model sample, consisting of a polyethylene sheet, a water layer, and a copper plate, and terahertz light was irradiated to this sample vertically. The waveform and intensity of electric field of the terahertz light reflected by the sample clearly pointed out the presence of water layer beneath the polyethylene sheet by the reflection peak appearance time and the phase of reflected electric field. Secondly, water trees were grown in a polyethylene sheet, and terahertz light was scanned over the sheet. As a result, the intensity distribution of terahertz light reflected by the sample was in good agreement with the shape of the water trees. Observation of terahertz image was also carried out using the same polyethylene sheet with water trees over which a polyvinyl chloride sheet or a carbon-loaded polyethylene sheet was put to simulate the structure of a real cable. An image of water trees was also successfully observed. These results indicate that the terahertz spectroscopy can be a new characterization tool to observe the presence of water trees in a test sample taken from an aged cable.

    DOI

  • Dielectric Properties of XLPE/SiO2 Nanocomposites Based on CIGRE WG D1.24 Cooperative Test Results

    Toshikatsu Tanaka, Alexander Bulinski, Jerome Castellon, Michel Frechette, Stanislaw Gubanski, Josef Kindersberger, Gian Carlo Montanari, Masayuki Nagao, Peter Morshuis, Yasuhiro Tanaka, Serge Pelissou, Alun Vaughan, Yoshimichi Ohki, Clive W. Reed, Simon Sutton, Suh Joon Han

    IEEE TRANSACTIONS ON DIELECTRICS AND ELECTRICAL INSULATION   18 ( 5 ) 1484 - 1517  2011.10  [Refereed]

     View Summary

    A comprehensive experimental investigation of XLPE and its nanocomposite with fumed silica (SiO2) has been performed by CIGRE Working Group D1.24, in cooperative tests conducted by a number of members; covering materials characterization, real and imaginary permittivity, dc conductivity, space charge formation, dielectric breakdown strength, and partial discharge resistance. The research is unique, since all test samples were prepared by one source, and then evaluated by several expert members and their research organizations. The XLPE used for preparation of the nanocomposites was a standard commercial material used for extruded power cables. The improved XLPE samples, based on nanocomposite formulations with fumed silica, were prepared specifically for this study. Results of the different investigations are summarized in each section; conclusions are given. Overall, several important improvements over unfilled XLPE are confirmed, which augur well for future potential application in the field of extruded HV and EHV cables. Some differences/discrepancies in the data of participants are thought to be the result of instrumental and individual experimental technique differences.

    DOI

  • Development of Low-Wind-Load Overhead Transmission Wires

    Y. Ohki

    IEEE ELECTRICAL INSULATION MAGAZINE   27 ( 5 ) 65 - 66  2011.09  [Refereed]

    DOI

  • Thermally Stimulated Current in Low-Density Polyethylene/MgO Nanocomposite. On the Mechanism of Its Superior Dielectric Properties

    Kazuyuki Ishimoto, Toshikatsu Tanaka, Yoshimichi Ohki, Yoitsu Sekiguchi, Yoshinao Murata

    ELECTRICAL ENGINEERING IN JAPAN   176 ( 3 ) 1 - 7  2011.08  [Refereed]

     View Summary

    Polymer nanocomposites are attracting attention as emerging insulating materials. We measured the thermally stimulated depolarization current (TSDC) in low-density polyethylene (LDPE)/MgO nanocomposites while varying measuring such parameters as the temperature increase rate and the intensity of the applied electric field. A TSDC peak that spreads over a wide temperature range from 0 degrees C to 60 degrees C was observed in all samples. As the amount of MgO nanofiller increases, the TSDC peak height decreases. Furthermore, by adopting a partial heating method, the wide TSDC peak was resolved into several component peaks. Among them, the peak at about 12 degrees C was found not to appear in the base LDPE. Analysis of its initial rising portion indicated that the 12 degrees C peak had a fairly large energy depth of about 2 eV. These results indicate that charge carriers tend to be captured more strongly after the addition of MgO nanofillers. If these captured charge carriers induce homocharge layers in the vicinity of the electrodes, further space charge formation would be suppressed. This seems to explain the fact that the amount of space charge is smaller in nanocomposites with a proper addition of MgO than in the base LDPE. (C) 2011 Wiley Periodicals, Inc. Electr Eng Jpn, 176(3): 1-7, 2011; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/eej.21136

    DOI

  • Development of High-Performance Environmentally Friendly Palm Fatty Acid Ester Insulating Oil for Transformers (News from Japan)

    Y. Ohki

    IEEE Electrical Insulation Magazine   27 ( 3 ) 55 - 57  2011.07  [Refereed]

    DOI

  • Effect of Tacticity on the Dielectric Properties of Polystyrene

    Saki Hikosaka, Yoshimichi Ohki

    IEEJ TRANSACTIONS ON ELECTRICAL AND ELECTRONIC ENGINEERING   6 ( 4 ) 299 - 303  2011.07  [Refereed]

     View Summary

    Dielectric properties of polystyrene (PS) were investigated by measuring electrical conduction current and frequency spectra of complex permittivity at temperatures from 20 to 199 degrees C. To clarify the effects of tacticity, the properties were compared among two kinds of syndiotactic PS (SPS) with different crystallinity values and one atactic PS (APS). Experimental results indicate that the two SPS samples show a high melting point of about 250 degrees C. The conductivity is lowest in SPS with the low crystallinity in the whole temperature range from 20 to 199 degrees C, while APS had a breakdown at 140 degrees C. Moreover, the low-crystallinity SPS shows the smallest permittivity values and the smallest increase in dielectric loss factor toward low frequencies among the three samples. The high-crystallinity SPS has alpha crystals. Since the density of alpha crystals is lower than the amorphous region, the charge transport and polarization are easier in the high-crystallinity SPS than in the low-crystallinity SPS. (C) 2011 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

    DOI

  • Effects of Crystallinity on Dielectric Properties of Poly(L-lactide)

    Saki Hikosaka, Hiroto Ishikawa, Yoshimichi Ohki

    ELECTRONICS AND COMMUNICATIONS IN JAPAN   94 ( 7 ) 1 - 8  2011.07  [Refereed]

     View Summary

    Poly(L-lactide) is attracting much attention as a biodegradable polymer. In this paper, effects of crystallinity on dielectric properties of PLLA are discussed. At 80 $C, which is above the glass transition temperature (= about 60 degrees C), the conductivity increases with a decrease in crystallinity. A thermally stimulated polarization current (TSPC) peak, which seems to be due to segmental-mode relaxation, appears in all the samples at temperatures around 65 to 70 degrees C, and it becomes smaller as the crystallinity increases. All the samples show two thermally stimulated depolarization current (TSDC) peaks at around 65 degrees C and around 90 to 100 degrees C. The lower-temperature peak seems to be due to the segmental-mode relaxation, and the other due to the normal-mode relaxation. Moreover, all the samples show a drastic increase in the real part of the complex permittivity (epsilon(r)') and a peak in its imaginary part (epsilon(r).) at frequencies from 10(-1) to 10(4) Hz, depending on the measurement temperature. The crystallinity dependencies of the two parts are similar to those of the TSPC and TSDC peaks at around 65 to 70 degrees C. Therefore, the increase in epsilon(r)' and the peak in epsilon(r)'' are ascribable to the segmental-mode relaxation. (C) 2011 Wiley Periodicals, Inc. Electron Comm Jpn, 94(7): 1-8, 2011; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/ecj.10348

    DOI

  • Development of an Advanced Solid Oxide Fuel Cell in Japan

    Y. Ohki

    IEEE ELECTRICAL INSULATION MAGAZINE   27 ( 4 ) 55 - 57  2011.07  [Refereed]

    DOI

  • The Role of Nano and Micro Particles on Partial Discharge and Breakdown Strength in Epoxy Composites

    Zhe Li, Kenji Okamoto, Yoshimichi Ohki, Toshikatsu Tanaka

    IEEE TRANSACTIONS ON DIELECTRICS AND ELECTRICAL INSULATION   18 ( 3 ) 675 - 681  2011.06  [Refereed]

     View Summary

    Epoxy composite is expected to become the substrate insulation of gate bipolar transistors (IGBTs) that can replace ceramics if it has high thermal conductivity. Fillers with high thermal conductivity were added to the epoxy resin, while this composite often suffers from lowering in breakdown (BD) strength. In order to reveal the above phenomena and to clarify the breakdown (BD) mechanism, we carried out experiments using several electrode structures, i.e. an MB-PWB (metal-base printed wiring board) insulation simulated structure, a rod-to-plane electrode for PD erosion resistance, and a sphere-to-sphere electrode for BD strength. As a result, it was clarified that the IGBT insulation breaks down after successive partial discharges (PDs), and the BD strength of microcomposite decreases with increasing the content of micro-fillers, while PD resistance of microcomposite is improved with increasing the content of micro-fillers. A trial was made to raise the once-lowered BD strength by adding nano-Al(2)O(3) fillers. Above experiments were carried out for nanocomposites and nano-micro-composites. It was clarified that the nano-fillers can improve the PD resistance of composites obviously, while nano-fillers improve the BD strength of composite slightly. Such a phenomenon was found that the BD strength of composites was increased with the dispersion state of nano-fillers.

    DOI

  • Terahertz Spectroscopy as a New Tool for Insulating Material Analysis and Condition Monitoring

    Norikazu Fuse, Tsuguhiro Takahashi, Yoshimichi Ohki, Ryo Sato, Maya Mizuno, Kaori Fukunaga

    IEEE ELECTRICAL INSULATION MAGAZINE   27 ( 3 ) 26 - 35  2011.05  [Refereed]

    DOI

  • Generation Mechanism of Electrochemical Migration in Printed Wiring Board Insulation

    Masashi Natsui, Hiroki Asakawa, Toshikatsu Tanaka, Yoshimichi Ohki, Takashi Maeno, Kenji Okamoto

    IEEJ TRANSACTIONS ON ELECTRICAL AND ELECTRONIC ENGINEERING   6 ( 3 ) 200 - 206  2011.05  [Refereed]

     View Summary

    In order to study the generation mechanism of electrochemical migration in a printed wiring board, paper/phenol-resin composites with and without an adhesive layer, epoxy resin, and a laminate of epoxy resin and a paper/phenol-resin composite were aged at 85 degrees C and 85% relative humidity (RH) with and without application of a DC voltage, and the space charge distributions in the samples were observed by the pulsed electroacoustic method. In the case of the composite with the adhesive layer, a large amount of negative charge is induced at the interface between the composite and the adhesive layer on the anode. It was found that such formation of space charge enhances the electric field intensity at the anode. The reason for this is because the conductivity is much lower in the adhesive layer than in the composite. Electrochemical migration does not occur when the same electric field is applied to the composite without the adhesive layer, whereas it does occur if the adhesive layer was replaced by epoxy resin with a similar conductivity to the adhesive layer. These results clearly indicate that electrochemical migration generates because the electric field at the anode surface is enhanced due to the low conductivity of the adhesive layer. (C) 2011 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

    DOI

  • Shape-sensitive reflectance by nanostructured metal attached on an optical waveguide-mode sensor

    Kazuki Sato, Yoshimichi Ohki, Ken-ichi Nomura, Makoto Fujimaki, Koichi Awazu

    NANOTECHNOLOGY   22 ( 24 ) 245503(1)-245503(6) - 245503(6)  2011.05  [Refereed]

     View Summary

    The optical reflectance of He-Ne laser light on a waveguide-mode sensor was measured as a function of light incident angle, in the case of either a metal (Au, Cr or Pt) film or nanoparticles being attached to the waveguide surface of the sensor. A dip appears in the reflectance spectrum as a function of incident angle at the angle where waveguide-mode excitation is induced. It is found that the dip moves toward a lower angle in the case that the attached metal is of a film shape, while it shifts toward a higher angle when the metal is an ensemble of nanoparticles. This difference in the direction of shift can be explained well by theoretical calculations using average refractive indices of the metal-containing layers. The present result indicates that one can estimate whether a metal nanostructure is film-like or an ensemble of spherical nanoparticles by the sensor.

    DOI

  • World's Largest Capacity Hydrogen-Cooled Turbine Generator

    Y. Ohki

    IEEE ELECTRICAL INSULATION MAGAZINE   27 ( 2 ) 61 - 62  2011.03  [Refereed]

    DOI

  • Effect of Annealing on Optical Absorption of LaAlO3 at Terahertz Frequencies

    Ken-ichi Nomura, Shimpei Okami, Xiaojun Xie, Maya Mizuno, Kaori Fukunaga, Yoshimichi Ohki

    JAPANESE JOURNAL OF APPLIED PHYSICS   50 ( 2 ) 021502(1)-021502(3) - 021502(3)  2011.02  [Refereed]

     View Summary

    Absorption spectra of LaAlO3(100) single crystals were measured at frequencies from 1.0 to 3.5 THz. A new absorption peak appears at similar to 3.0 THz after annealing at temperatures above 500 degrees C. Simultaneously, striped domains with widths ranging from about 20 to about 300 mu m in the original samples completely disappeared. Since oxygen vacancies are assumed to play an important role in the formation of domains, the absorption peak at similar to 3.0 THz seems to be related to oxygen vacancies. (C) 2011 The Japan Society of Applied Physics

    DOI

  • Correlation between mechanical and dielectric relaxation processes in epoxy resin composites with nano-and micro-fillers

    Mayumi Hyuga, Toshikatsu Tanaka, Yoshimichi Ohki, Takahiro Imai, Miyuki Harada, Mitsukazu Ochi

    IEEJ Transactions on Fundamentals and Materials   131 ( 12 ) 1041 - 1047  2011  [Refereed]

     View Summary

    Effects of addition of nano-sized and micro-sized fillers into epoxy resin on its mechanical and dielectric relaxation phenomena were examined. The glass transition temperature (T g) decreases when a small content of nanoclay modified by octadecylamine was added, while the decrease in T g is suppressed when the nanoclay was modified by dimethyldodecylamine. On the other hand, T g increases when microsilica was added abundantly. At temperatures above T g, both mechanical and dielectric relaxations are accelerated in samples with octadecylamine-modified nanoclay, while the acceleration does not occur in samples with nanoclay modified by dimethyldodecylamine. Both relaxations are restricted in composites with abundant microsilica. Therefore, co-addition of dimethyldodecylamine-modified nanoclay and abundant microsilica is adequate in order to make an epoxy resin composite with a high T g and low dielectric loss. © 2011 The Institute of Electrical Engineers of Japan.

    DOI

  • Detection of electrochemical migration grown in a two-layered dielectric by the pulsed electroacoustic method and numerical analysis of the signals

    Hiroki Asakawa, Masashi Natsui, Toshikatsu Tanaka, Yoshimichi Ohki, Takashi Maeno, Kenji Okamoto

    IEEJ Transactions on Fundamentals and Materials   131 ( 9 ) 771 - 777  2011  [Refereed]

     View Summary

    Electrochemical migration growth along the thickness direction in a two-layered dielectric, consisting of an Al cathode, paper/phenol-resin composite, epoxy resin, and a Cu anode that simulates a laminated structure of a printed wiring board, was detected by the pulsed electroacoustic (PEA) method. The PEA signals obtained were analyzed using a numerical model on the generation and propagation of acoustic waves. Accumulation of positive charge appears in front of the Cu anode of the sample, in which the growth of electrochemical migration was observed by SEM-EDS. At the same time, the signal due to negative charge at the interface between the resin and the composite decreases significantly. This is ascribable to the progress of electrochemical migration in the thickness direction. The numerical analysis done by assuming that the conductivity of the epoxy resin layer near the anode was increased gives a good agreement with the PEA spectra. As for the charge on the anode, the PEA signal appears at the correct position when the anode material is semi-conductive polymer, while it appears as if the anode were inside the real anode when the material is Cu. In order to solve this puzzle, numerical calculations were carried out by the finite difference method. As a result, it has become clear that the discontinuity in the acoustic impedance between the epoxy resin and Cu electrode is responsible. © 2011 The Institute of Electrical Engineers of Japan.

    DOI

  • 特集 ケミルミネッセンスの最近の進歩 [総説]原子力発電所における絶縁劣化診断の重要性およびケミルミネセンスによる酸化状態の把握

    大木義路, 平井直志

    マテリアルライフ学会誌   23 ( 1 ) 1 - 9  2011  [Invited]

  • Feasibility study on the detection of corrosion of a cable metal shield using terahertz imaging

    R. Sato, M. Komatsu, M. Mizuno, K. Fukunaga, Y. Ohki

    Annual Report - Conference on Electrical Insulation and Dielectric Phenomena, CEIDP     72 - 75  2011  [Refereed]

     View Summary

    Time-domain imaging and spectroscopic measurements were carried out using terahertz (THz) light in order to examine the feasibility of non-destructive corrosion diagnosis of a metallic shielding layer in a power cable. At first, the absorbance of the THz light was measured for various corroded copper, namely copper oxide, basic copper carbonate, and copper chloride. As a result, both basic copper carbonate and copper chloride were found to exhibit absorption clear and strong enough for the imaging. Next, a non-corroded copper plate and one of the corroded plates were placed on a metal plate without making an open interspace, and the THz light was irradiated and scanned in order to get two-dimensional images. When the plate is corroded to form either basic copper carbonate or copper chloride, the intensity of reflected light becomes small compared with the non-corroded copper plate. The reason for this seems that the THz light was scattered more at the surface of the corroded copper plate and was absorbed more by the corrosion. These phenomena indicate that non-destructive diagnosis to detect corrosion of a shield metal is possible. © 2011 IEEE.

    DOI

  • Superiority of Syndiotactic Polystyrene as an Electrical Insulating Polymer

    Isamu Ikeda, Saki Hikosaka, Yoshimichi Ohki

    PROCEEDINGS OF 2011 INTERNATIONAL CONFERENCE ON ELECTRICAL INSULATING MATERIALS (ISEIM)     309 - 312  2011  [Refereed]

     View Summary

    It is known that syndiotactic polystyrene (SPS) exhibits good thermal stability in various mechanical properties due to its syndiotactic stereoregularity, while it maintains the versatility of atactic polystyrene (APS). The present paper reports experimental results conducted for the purpose of revealing various dielectric properties of SPS in comparison with other engineering and general-purpose polymers including APS. By examining the partial discharge (PD) resistance, electrical conductivity, and complex permittivity, it was found that SPS has a stronger PD resistance than low density polyethylene, polyimide, polyethylene terephthalate and polypropylene, and that the conductivity, permittivity, and dielectric loss factor are lower in SPS than in APS. By taking all the above-mentioned results into account, it can be concluded that SPS is an engineering polymer with superior electrical insulating properties.

  • High Thermal Conductivity Epoxy/BN Composites with Sufficient Dielectric Breakdown Strength

    Toshikatsu Tanaka, Zengbin Wang, Tomonori Iizuka, Masahiro Kozako, Yoshimichi Ohki

    2011 ANNUAL REPORT CONFERENCE ON ELECTRICAL INSULATION AND DIELECTRIC PHENOMENA, VOLS 1 AND 2     691 - 694  2011  [Refereed]

     View Summary

    Various types of epoxy/BN composites were prepared to optimize material conditions for high thermal conductivity and high breakdown strength. The best composite was found to be epoxy/conglomerated h-BN/nano silica nano micro composite with thermal conductivity 12 W/m center dot K and BD strength 15 kV(peak)/0.2 mm. One of the most important factors to obtain high values of the two performances is to reduce void content. Surface treatment of fillers and nano filler addition are also useful.

  • Comparison of the Effects of Nanofiller Materials on the Dielectric Properties of Epoxy Nanocomposites

    J. Katayama, N. Fuse, M. Kozako, T. Tanaka, Y. Ohki

    2011 ANNUAL REPORT CONFERENCE ON ELECTRICAL INSULATION AND DIELECTRIC PHENOMENA, VOLS 1 AND 2     318 - 321  2011  [Refereed]

     View Summary

    Composites of organic polymers and well- dispersed nm- sized inorganic fillers, called polymer nanocomposites (NCs), have been attracting much attention as new electrical insulating materials. In this paper, experimental results on the typical dielectric properties, namely, complex permittivity (epsilon(r)'and epsilon(r) "), conductivity, and thermally stimulated depolarization current (TSDC) are evaluated for bisphenol- A epoxy resin and its NCs with boehmite alumina, titania, and silica. The increase in epsilon(r) ' and epsilon(r) " and the magnitudes of the TSDC peaks as well as the increase in conductivity appear more significantly in the NCs with boehmite alumina and titania and less significantly in the NCs with silica than in epoxy resin. This indicates that the silica nanofillers give the best results as far as these properties are concerned. The suppression of molecular motion by the addition of nanofillers seems to work effectively in the case of silica, while the nanofiller addition is likely to accelerate the motion of ionic carriers and/ or that of dipoles in the case of boehmite alumina and titania.

  • Experimental and Numerical Analyses of Molecular Vibrations in Poly-epsilon-caprolactone at Terahertz Frequencies

    Marina Komatsu, Ryo Sato, Yoshimichi Ohki, Maya Mizuno, Kaori Fukunaga, Shingo Saito

    PROCEEDINGS OF 2011 INTERNATIONAL CONFERENCE ON ELECTRICAL INSULATING MATERIALS (ISEIM)     253 - 256  2011  [Refereed]

     View Summary

    We observed absorption spectra of poly-epsilon-caprolactone in a frequency range from 0.3 to 3.6 THz at temperatures from 10 to 300 K by terahertz time-domain spectroscopy and analyzed the spectra using density functional theory calculations based on the B3LYP functional. In the absorption measurements, the electric field direction of the THz wave was set in either parallel or perpendicular to the stretched direction of the sample. The spectra observed at 10 K differ significantly between the parallel and perpendicular THz waves, while such a difference is hardly seen at 300 K. The numerically analyzed results agree to some extent with the spectra. It is shown by the analysis of the vector components of the molecular vibrations obtained numerically that all the spectra observed in the THz region are assumed to be due to skeleton vibrations with vibrational period of either one or two monomer units.

  • Diagnosis of Cable Aging by Broadband Impedance Spectroscopy

    Yoshimichi Ohki, Takayuki Yamada, Naoshi Hirai

    2011 ANNUAL REPORT CONFERENCE ON ELECTRICAL INSULATION AND DIELECTRIC PHENOMENA, VOLS 1 AND 2     24 - 27  2011  [Refereed]

     View Summary

    Since electric wires and cables play very important roles of supplying electric power and transmitting information and control signals, their degradation may cause a serious problem. In the present research, we examined the applicability of the broadband impedance spectroscopy (BIS) to monitor the degradation of cable insulation. The BIS method is to measure the magnitude and phase angle of impedance as a function of frequency in a very wide frequency range. First, cables were aged partially by heat and gamma-rays. Further, a hole was drilled from the outer to inner conductors in some cables, and was filled with a NaCl solution in order to simulate a vented water tree. The difference in magnitude or phase angle of impedance before and after the aging becomes clearer by fast Fourier transform analyses, regardless of whether the aging was given by the gamma-rays or water trees. The location of the degradation was also possible without referring to the data before the aging. These results clearly indicate that the BIS method has a potential ability to locate the degradation of cable insulation induced by various kinds of damages.

  • Various Ion-induced Phenomena Appearing in Dielectric Materials and Their Applications to Optical Devices and Biosensors

    Y. Ohki, Y. Arai, S. J. Yu, K. Nomura, M. Fujimaki

    PROCEEDINGS OF 2011 INTERNATIONAL CONFERENCE ON ELECTRICAL INSULATING MATERIALS (ISEIM)     39 - 42  2011  [Refereed]

     View Summary

    Various interesting phenomena are induced in dielectric materials by swift-ion irradiation. This paper reports effects of swift-light-and swift-heavy-ion irradiation on SiO2 glass, rutile TiO2 monocrystal, and fluorinated polyimide, which are typical dielectric materials. We also demonstrate that ion-induced phenomena can be applied to fabrication of advanced waveguide-type optical devices such as a planar-lightwave- circuit-type directional coupler and a highly sensitive waveguide-mode biosensor.

  • 2-MW Downwind Turbine Designed for Japanese Conditions

    Y. Ohki

    IEEE ELECTRICAL INSULATION MAGAZINE   27 ( 1 ) 63 - 64  2011.01  [Refereed]

    DOI

  • IEE Japan Executive Members Elected (News from Japan)

    Y. Ohki

    IEEE Electrical Insulation Magazine   26 ( 5 ) 70 - 70  2010.12  [Refereed]

    DOI

  • Effect of Water Absorption Temperature on Space Charge Profiles in Paper/Phenol-Resin Composites for Printed Circuit Boards

    Yasutsune Echigo, Masashi Natsui, Takashi Maeno, Yoshimichi Ohki

    ELECTRICAL ENGINEERING IN JAPAN   173 ( 2 ) 1 - 7  2010.11  [Refereed]

     View Summary

    More and more electronics are intended for use in adverse environments at high temperatures with high humidity. The spatial distributions of internal charge carriers, mainly due to ionic impurities that appear in hot and humid environments, are considered to affect the reliability of bulk insulation. Therefore, the authors examined space charge behavior inside paper/phenol-resin composites for printed circuit boards under DC voltages, focusing on the effect of the water absorption temperature. Both the sample weight and thickness were increased monotonically by immersion in water with an increase in the water temperature from 24 degrees C to 85 degrees C, indicating that the water absorption by the sample was temperature dependent. In the early periods of water absorption (up to 10 hours), the electric field decreased near the two electrodes and increased in the other regions. Furthermore, heterocharge formation was observed near the cathode as the water absorption progressed, becoming more significant at higher water temperatures. Ion chromatography analyses detected numerous ions such as Na(+), NH(4)(+), and Cl(-) from the water, in which the sample had been immersed for 100 hours at various temperatures. It is highly possible that these ions are responsible for the heterocharge formation. (C) 2010 Wiley Periodicals, Inc. Electr Eng Jpn, 173(2): 1-7, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20992

    DOI

  • Development of a High-Performance Casting Method for Solid-Insulated Switchgears

    Y. Ohki

    IEEE ELECTRICAL INSULATION MAGAZINE   26 ( 6 ) 63 - 65  2010.11  [Refereed]

    DOI

  • Tree Initiation Phenomena in Nanostructured Epoxy Composites

    Yu Chen, Takahiro Imai, Yoshimichi Ohki, Toshikatsu Tanaka

    IEEE TRANSACTIONS ON DIELECTRICS AND ELECTRICAL INSULATION   17 ( 5 ) 1509 - 1515  2010.10  [Refereed]

     View Summary

    Tree initiation time was evaluated by a pulse detection system to detect a small PD (partial discharge) signal that might take place at first just after a tree was formed. Six kinds of materials were prepared for evaluation, i.e. base epoxy resin, microcomposite, two kinds of nanocomposites, and two kinds of nano-micro-mixed-composites. Lengths of trees detected were around 100 mu m in our experiments. As a result, it was clarified that the tree initiation time is prolonged by the nanofiller inclusion, but shortened by the microfiller filling. Furthermore, the co-addition of nanofillers to the microcomposite increases the tree initiation time again in comparison to the once-shortened time in the microcomposite. As for the PD erosion evaluated for the same materials, a good correlation was confirmed between the tree initiation time and PD resistance. This correlation indicates that the tree formation process includes PD erosion. Simulation results show that trees should propagate between fillers.

    DOI

  • Detection of colored nanomaterials using evanescent field-based waveguide sensors

    Makoto Fujimaki, Ken-ichi Nomura, Kazuki Sato, Takafumi Kato, Subash C. B. Gopinath, Xiaomin Wang, Koichi Awazu, Yoshimichi Ohki

    OPTICS EXPRESS   18 ( 15 ) 15732 - 15740  2010.07  [Refereed]

     View Summary

    We have developed an optical system designed for detecting colored nanomaterials in aqueous solutions, using the concept of evanescent-field-coupled waveguide-mode sensors. In this study, we found that the waveguide modes induced in the sensor are intrinsically sensitive to a change in optical absorption, or a 'change in color'. The system detects less than one gold nanoparticle (diameter: 20 nm) adsorbed per square micrometer. It is also demonstrated that significant signal enhancement due to adsorption of molecules is achieved using a dye. The developed sensor rarely suffers from a drawback of impurity adsorption. The system is expected to be applied as an effective sensing tool for metal colloids, nanoparticles, and colored biomolecules in solution. (C) 2010 Optical Society of America

    DOI

  • Reduction of Transformer Inrush Currents Using a Controlled Switching System for Circuit Breakers With Ganged Three-Phase Operation

    Y. Ohki

    IEEE ELECTRICAL INSULATION MAGAZINE   26 ( 4 ) 52 - 54  2010.07  [Refereed]

    DOI

  • Advanced Site Assembly (ASA) Technologies for UHV Transformers (News from Japan)

    Y. Ohki

    IEEE Electrical Insulation Magazine   26 ( 2 ) 55 - 57  2010.06  [Refereed]

    DOI

  • Comparison of Nano-structuration Effects in Polypropylene among Four Typical Dielectric Properties

    Norikazu Fuse, Yoshimichi Ohki, Toshikatsu Tanaka

    IEEE TRANSACTIONS ON DIELECTRICS AND ELECTRICAL INSULATION   17 ( 3 ) 671 - 677  2010.06  [Refereed]

     View Summary

    Effects of nanofiller addition on four typical dielectric properties, namely permittivity epsilon(r)', dielectric loss factor epsilon(r)", space charge accumulation, and partial discharge (PD) resistance were evaluated for polypropylene (PP) and its nanocomposites (NCs) with nanoclay. While epsilon(r)' and epsilon(r)" are almost independent of temperature and frequency in the base unfilled PP, they are highly dependent on the two parameters in the two NCs. Namely, epsilon(r)' increases significantly at temperatures above 20 degrees C and the frequency spectrum of epsilon(r)" shows at least one temperature-dependent peak. Furthermore, space charge appears abundantly in the two NCs compared to the base PP. These results indicate that plenty of mobile carriers and/or dipoles, probably resulted from the manufacturing process, remain in the two NCs. Notwithstanding the above-mentioned 'inferior' insulating properties, the two NCs have an improved PD resistance compared with the base PP. Namely, the erosion depth on the surface induced by PDs is the smallest in the NC with the largest filler content, while it is the largest in the base PP. Such differences in the effects of nanofillers on different insulating properties are attributable to the fact that nanofillers can improve the PD resistance simply by their presence, while the chemicals needed for uniform dispersion of nanofillers may sometimes increase the permittivity and abundance of charge carriers.

    DOI

  • Effects of Nano-filler Addition on Partial Discharge Resistance and Dielectric Breakdown Strength of Micro-Al2O3/Epoxy Composite

    Zhe Li, Kenji Okamoto, Yoshimichi Ohki, Toshikatsu Tanaka

    IEEE TRANSACTIONS ON DIELECTRICS AND ELECTRICAL INSULATION   17 ( 3 ) 653 - 661  2010.06  [Refereed]

     View Summary

    It is often observed that the insulation structure for an insulated gate bipolar transistor (IGBT) suffers from dielectric failure, when the insulation is made of epoxy resin to which micro fillers with a high thermal conductivity were added. In order to reveal the above phenomena and to clarify the breakdown (BD) mechanism, we have carried out experiments using an MB-PWB (metal-base printed wiring board) insulation simulated structure. As a result, it was clarified that the IGBT insulation breaks down after successive partial discharges (PDs). It was also elucidated that BD strength becomes lower, when epoxy resin was loaded with high content of micro-fillers. A trial was made to raise the once-lowered BD strength by adding nano-Al2O3 fillers. Three kinds of experiments were carried out, i.e. an MB-PWB insulation simulated structure for dielectric failure, a rod-to-plane electrode for PD erosion resistance, and a sphere-to-sphere electrode for BD strength for four kinds of insulation samples, i.e. neat epoxy, 5-wt% nano-Al2O3/epoxy composite, 60-wt% micro-Al2O3/epoxy composite, and combined 2-wt% nano- and 60-wt% micro-Al2O3/epoxy composite. It was clarified that the nano- micro-composite is higher in both BD strength and PD resistance than the micro-composite. It should be noted that the addition of nano- fillers would provide an excellent approach that can increase the dielectric BD strength and time of micro-filled epoxy composites.

    DOI

  • Effects of Temperature and Crystallinity on Partial Discharge Resistance of Poly-L-lactic Acid

    Yoshimichi Ohki, Kazuhiro Miyata

    IEEJ TRANSACTIONS ON ELECTRICAL AND ELECTRONIC ENGINEERING   5 ( 3 ) 323 - 327  2010.05  [Refereed]

     View Summary

    Partial discharge (PD) resistance of brodegradable poly-L-lactic acid (PELA) was examined at temperatures between room temperature and 70 degrees C namely at around the glass transition temperature of 57 C The PD activity becomes more pronounced at temperatures above 60 degrees C. reflecting that the permittivity increases in the rubber state Furthermore. we have changed the crystallinity of PLEA by drawing or annealing thermally, and have examined its effect on the PD resistance Although the depth eroded by a fixed per rod of PD degradation becomes shallower when the sample was crystallized by the drawing cm the heat treatment. its reduction ratio is almost equal to the reduction ratio of PD activities due to the decrease in permittivity Therefore, it is hard to assume that crystallizatron of PLEA improves its PD resistance to a unit amount of charge (C) 2010 Institute of Electrical Engineers of Japan Published by John Wiley & Sons. Inc

    DOI

  • Recent Progress in Energy Storage by a NAS Battery System

    Y. Ohki

    IEEE ELECTRICAL INSULATION MAGAZINE   26 ( 3 ) 55 - 56  2010.05  [Refereed]

    DOI

  • Electrical Conduction and Dielectric Relaxation in Polyethylene Terephthalate Succinate

    Fukutaro Kato, Yoshimichi Ohki

    ELECTRICAL ENGINEERING IN JAPAN   170 ( 4 ) 1 - 8  2010.03  [Refereed]

     View Summary

    Electrical conduction and complex permittivity are examined in polyethylene terephthalate succinate, focusing on their relations to dielectric relaxation processes. Both the real and imaginary parts of the complex permittivity, namely, the dielectric constant epsilon(r)' and the dielectric loss factor epsilon(r)'' increase with a decrease in frequency, especially at high temperatures. They are both ascribed to the transport of ionic mobile carriers. Namely, the carrier transport forms a conduction current that should contribute to epsilon(r)''. On this occasion, if charge exchange does not occur at the two electrodes, heterocharge layers should be formed before the electrodes. This should increase the charge density on the electrodes, thus contributing to epsilon(r)'. In addition to the increase in epsilon(r)' and epsilon(r)'' due to mobile ions, two relaxation processes, one due to micro-Brownian motion of dipoles and the other due to orientation and magnitude change of the dipole moment induced by two end groups in the polymer main chain, are observed. Corresponding to these two relaxation processes, two thermally stimulated discharge current (TSDC) peaks appear. The two TSDC peaks as well as the increments in epsilon(r)' and epsilon(r)'' become larger when the crystallinity of the samples decreases. (c) 2009 Wiley Periodicals, Inc. Electr Eng Jpn, 170(4): 1-8, 2010; Published online in Wiley InterScience (www.interscience. wiley.com). DOI 10.1002/eej.20956

    DOI

  • Comparison of Dielectric Properties between Epoxy Composites with Nanosized Clay Fillers Modified by Primary Amine and Tertiary Amine

    N. Tagami, M. Hyuga, Y. Ohki, T. Tanaka, T. Imai, M. Harada, M. Ochi

    IEEE TRANSACTIONS ON DIELECTRICS AND ELECTRICAL INSULATION   17 ( 1 ) 214 - 220  2010.02  [Refereed]

     View Summary

    Epoxy-based nanocomposites (NCs) were prepared using clay modified by either primary amine or tertiary amine, and the effect of the difference in modifier on the thermal and dielectric properties of the NCs were discussed. The NC with clay fillers modified by the primary amine, 1C, shows a glass transition end temperature (T(eg)) at a temperature 20 C lower than the neat epoxy (N). This indicates that the resin of 1C is less crosslinked than that of N. On the other hand, the sample 3C, in which the clay was modified by the tertiary amine, shows a DSC spectrum close to that of N. Namely, 3C has a high crosslinking density similar to N. While the three samples show a relaxation peak in their dielectric loss spectra, the peak appears at high frequencies in 1C compared to N and 3C. Moreover, ionic conduction current flows more at high temperatures in 1C than in N or 3C. These facts are ascribable to the difference in their crosslinking densities.

    DOI

  • Observation and Analysis of Molecular Vibration Modes in Polylactide at Terahertz Frequencies

    Norikazu Fuse, Ryo Sato, Maya Mizuno, Kaori Fukunaga, Koichi Itoh, Yoshimichi Ohki

    JAPANESE JOURNAL OF APPLIED PHYSICS   49 ( 10 ) 102402(1)-102402(8) - 102402(8)  2010  [Refereed]

     View Summary

    Terahertz (THz) spectra were obtained for polylactide, and they were analyzed by numerical calculations based on density functional theory. An absorption band was observed at approximately 50 cm(-1), which becomes larger when the THz electric field is parallel to the sample drawing direction. An absorption peak with a similar directional dependence appears at approximately 40 cm(-1) in the simulation, which is due to normal-mode molecular vibrations parallel to the molecular helix. Since the 50 cm(-1) absorption decreases with an increase in crystallinity and the permittivity shows a step-like decrease, the molecular vibrations responsible for the 50 cm(-1) band seem to be subjected to strong damping in amorphous regions. On the other hand, another absorption peak at 65 cm(-1) with a positive absorption dependence on the sample's crystallinity is likely to be due to lattice vibration, since it does not appear in the calculations, which only deal with intramolecular vibration. (C) 2010 The Japan Society of Applied Physics

    DOI

  • Cr3+ Impurities and Photoluminescence in LaAlO3

    Eiji Hirata, Kenta Tamagawa, Yoshimichi Ohki

    JAPANESE JOURNAL OF APPLIED PHYSICS   49 ( 9 ) 091102(1)-091102(6) - 091102(6)  2010  [Refereed]

     View Summary

    Photoluminescence (PL) spectra induced in LaAlO3 by ultraviolet (UV) photons were measured for two types of samples, i.e., single-crystal plates and thin films. The films, which were amorphous when deposited, became polycrystalline after being annealed at 800 degrees C or higher. When the excitation photon energy exceeded the band-gap energy of LaAlO3, three PL peaks appeared at approximately 1.60 eV in the single crystals and polycrystalline films deposited and annealed on a Si single-crystal substrate. When the films were deposited and annealed on a CaF2 single-crystal substrate, the in-plane lattice parameters became smaller and the energies of the three PL peaks shifted to higher values. This indicates that the three PL peaks are susceptible to a crystal field. Furthermore, the intensities of the three PL peaks did not depend on the measurement temperature. Taking these results together with their spectral shapes into consideration, the three PL peaks are due to the R-line luminescence resulting from Cr3+ impurities in LaAlO3. The detection of Cr3+ ions by inductively coupled plasma optical emission spectrometry failed, indicating that the Cr3+ content of the present samples is below 1 ppm. The fact that the PL peaks appeared in both the single-crystal plates and thin films obtained from different raw chemicals with different processes indicates that there is a strong possibility that the Cr3+ ions are present in Al ore. Another PL peak appearing at 2.8 eV in the single crystals was assumed to originate from oxygen vacancies. (C) 2010 The Japan Society of Applied Physics

    DOI

  • Contribution to Green Power Technology Through Electrical Insulating Materials Research

    Yoshimichi Ohki

    Transactions on Electrical and Electronic Materials   11 ( 4 ) 149 - 154  2010  [Refereed]

    DOI

  • 2層誘電体構成の空間電荷分布が示す/MgOナノコンポジットの良好な絶縁性

    鐘ヶ江越, 石本和之, 田中祀捷, 大木義路, 関口洋逸, 村田義直

    電気学会論文誌A   130 ( 4 ) 349 - 354  2010  [Refereed]

    DOI

  • Experimental and Numerical Analyses of Molecular Vibrations in Amorphous and Crystalline Polylactide at Terahertz Frequencies

    N. Fuse, R. Sato, K. Itoh, Y. Ohki, M. Mizuno, K. Fukunaga

    PROCEEDINGS OF THE 2010 IEEE INTERNATIONAL CONFERENCE ON SOLID DIELECTRICS (ICSD 2010)    2010  [Refereed]

     View Summary

    Terahertz spectra were obtained for polylactide, and analyzed by numerical calculations based on the density functional theory. An absorption band was observed at around 50 cm(-1), which becomes larger when the THz electric field is parallel to the sample drawing direction. An absorption peak with a similar directional dependence appears at around 40 cm(-1) in the simulation, which is due to normal-mode molecular vibrations parallel to the molecular helix. Since the 50-cm(-1) absorption decreases with an increase in crystallinity and the permittivity shows a step-like downward change, the molecular vibrations responsible for the 50 cm(-1) band seems to be subjected to strong damping in amorphous regions. On the other hand, another absorption peak at 65 cm(-1) with a positive dependence on the sample's crystallinity is likely to be due to some lattice vibration, since it does not appear in the calculations that only deal with intramolecular vibration.

  • Detection of Electrochemical Migration Growth along the Thickness Direction in a Paper/phenol-resin Composite

    Hiroki Asakawa, Masashi Natsui, Yoshimichi Ohki, Toshikatsu Tanaka, Takashi Maeno, Kenji Okamoto

    PROCEEDINGS OF THE 2010 IEEE INTERNATIONAL CONFERENCE ON SOLID DIELECTRICS (ICSD 2010)    2010  [Refereed]

     View Summary

    In order to study the growth of electrochemical migration along the thickness direction, paper/phenol-resin composite on printed wiring boards was aged at 85 degrees C and 85 %RH by applying of a dc voltage. Then, the sample was observed by a scanning electron microscope with a function of energy dispersive spectroscopy (SEM-EDS) analysis. Space charge distributions in the sample were also measured by the pulsed electroacoustic method. Signals of SEM-EDS showing the presence of Cu were observed on both the anode and cathode. This fact indicates that ionized copper moves toward the cathode inside the composite when an electric field is applied at high temperatures in a highly humid atmosphere.

  • Band Gap Energies and Localized States in Several Insulating Polymers Estimated by Optical Measurements

    Yoshimichi Ohki, Norikazu Fuse, Tomoyuki Arai

    2010 ANNUAL REPORT CONFERENCE ON ELECTRICAL INSULATION AND DIELECTIC PHENOMENA    2010  [Refereed]

     View Summary

    The band gap energy was estimated experimentally by measuring the change in optical absorption intensity as a function of photon energy in a wide range from visible to vacuum-ultraviolet wavelengths for commercially available 11 kinds of insulating polymers. The absorption spectra as a function of photon energy measured by synchrotron radiation and by a spectrometer agree with each other at the photon energies where the two measurements were available. The band gap energy is quite high in linear polyolefin polymers. Namely, it is 6.9 and 7.0 eV for polyethylene and polypropylene, respectively. In these two polymers, the relative permittivity at 1 kHz is quite low in both polymers. On the other hand, the polymers with aromatic rings in their structures have low band gap energies and large permittivities. Namely, a negative relationship between the band gap energy and permittivity, which is well known for inorganic solids, is also seen in insulating polymers. Furthermore, to investigate the possibility of appearance of midgap localized states by the addition of nanofillers, absorption and luminescence spectra were examined for polymer nanocomposites based on polyethylene and polyamide. As a result, the nanofiller addition was found to give no influences on their band gaps.

  • Space Charge Behavior in Multi-layered Dielectrics with LDPE and LDPE/MgO Nanocomposites

    Etsu Kanegae, Yoshimichi Ohki, Toshikatsu Tanaka, Yoitsu Sekiguchi

    PROCEEDINGS OF THE 2010 IEEE INTERNATIONAL CONFERENCE ON SOLID DIELECTRICS (ICSD 2010)    2010  [Refereed]

     View Summary

    Space charge distributions were investigated in multilayered dielectric films consisting of combinations of low-density polyethylene ( LDPE) and its nanocomposite (NC) with MgO nano-fillers during application of a dc electric field of 100 kV/mm at room temperature and 50 degrees C. At 100 kV/mm, a packet-like charge appears in LDPE, but not in NC. Similar behavior is also observed in multi-layered samples. These phenomena are explained by assuming that the nano-fillers effectively suppress the increase in conductivity induced by the high electric field, which agrees well with related work done previously by the authors.

  • Development of Epoxy Resin Composites With High Thermal Conductivity

    Y. Ohki

    IEEE ELECTRICAL INSULATION MAGAZINE   26 ( 1 ) 48 - 49  2010.01  [Refereed]

    DOI

  • Superiority of Dielectric Properties of LDPE/MgO Nanocomposites over Microcomposites

    Kazuyuki Ishimoto, Etsu Kanegae, Yoshimichi Ohki, Toshikatsu Tanaka, Yoitsu Sekiguchi, Yoshinao Murata, C. C. Reddy

    IEEE TRANSACTIONS ON DIELECTRICS AND ELECTRICAL INSULATION   16 ( 6 ) 1735 - 1742  2009.12  [Refereed]

     View Summary

    Dielectric properties were compared between two kinds of low-density polyethylene (LDPE) composites prepared by adding near spherical MgO fillers with different diameters of around several ten nm and several mu m. In the whole range of temperature from 0 degrees C to 90 degrees C, the conductivity is decreased and the permittivity is increased by the addition of fillers, irrespective of their sizes. However, the decrement in the conductivity is more significant and permittivity increment is suppressed more in the case of the nm fillers. In addition, a drastic increase in dielectric loss factor, observed at low frequencies, is suppressed more significantly by the addition of the nm fillers. While the formation of packet-like charge, observed in LDPE, is suppressed by both fillers, the effect is far more significant in the nanocomposites. This means that the nano-fillers suppress the conductivity enhancement by the voltage application more effectively.

    DOI

  • Plasmonic activity on gold nanoparticles embedded in nanopores formed in a surface layer of silica glass by swift-heavy-ion irradiation

    Ken-ichi Nomura, Yoshimichi Ohki, Makoto Fujimaki, Xiaomin Wang, Koichi Awazu, Tetsuro Komatsubara

    NANOTECHNOLOGY   20 ( 47 ) 475306(1)-475306(7) - 475306(7)  2009.11  [Refereed]

     View Summary

    Silica glass was irradiated by swift heavy ions by selecting the ion species and its energy in order to induce the largest damaged regions. These regions were then selectively etched by hydrofluoric acid vapour to form nanopores on the glass surface. Subsequently, gold nanoparticles were embedded into the nanopores by vacuum evaporation, followed by thermal treatment. In the new plasmonic structure obtained with these procedures, the localized surface plasmon excitation wavelength induced around the gold nanoparticles was found to show a redshift, which agreed well with the theoretical calculation, when water was introduced into the nanopores. This indicates that the fabricated structure can be used as a sensing element to detect the adhesion of substances such as biomolecules to the nanoparticles by measuring the redshift.

    DOI

  • Isolated observation of high voltage signals from DC to 500 MHz using an optical fiber cable

    Y. Ohki

    IEEE ELECTRICAL INSULATION MAGAZINE   25 ( 6 ) 50 - 51  2009.11  [Refereed]

    DOI

  • Control of Coupling Ratio by Proton Implantation for a Directional Coupler of Planar-Lightwave-Circuit Type

    Seung-Jun Yu, Masahiro Suzuki, Yoshimichi Ohki, Makoto Fujimaki, Koichi Awazu

    JAPANESE JOURNAL OF APPLIED PHYSICS   48 ( 10 ) 102405(1)-102405(5) - 102405(5)  2009.10  [Refereed]

     View Summary

    By implanting protons into a cladding in the coupling region of a directional coupler of the planar-lightwave-circuit type at an acceleration energy that enables the protons to reach the center of the core, the coupling ratio of the coupler is changed periodically and continuously by increasing proton fluence. In the case of two optical waveguides formed in a planar lightwave circuit with a mutual spacing too large for optical coupling, proton implantation into the cladding between the waveguides induces optical coupling between them. These results indicate that the coupling ratio of a directional coupler can be controlled by ion implantation. (C) 2009 The Japan Society of Applied Physics

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  • Tree Initiation Characteristics of Epoxy Resin and Epoxy/Clay Nanocomoposite

    Stephanie Raetzke, Yoshimichi Ohki, Takahiro Imai, Toshikatsu Tanaka, Josef Kindersberger

    IEEE TRANSACTIONS ON DIELECTRICS AND ELECTRICAL INSULATION   16 ( 5 ) 1473 - 1480  2009.10  [Refereed]

     View Summary

    Tree initiation behavior of an epoxy nanocomposite with 5 wt % nanoclay (layered silicate) was investigated for ac voltage in comparison to neat epoxy resin without fillers. To shorten the time for experiments, 600 Hz was used instead of 60 Hz, as acceleration for tree initiation had been confirmed at 10 kVrms and 14 kVrms between the two frequencies. V-t characteristics for tree initiation rather than tree growth to bridge the electrodes were obtained for conventional type of treeing specimens with an embedded steel needle subjected to voltages from 2 kVrms to 14 kVrms. As a result, it was clarified that tree initiation V-t characteristics were improved by approximately one order of magnitude for the epoxy/nanoclay composite compared to the neat epoxy resin. Initial formation of trees is generally considered to be directly related to the fatigue of matter stressed by electric ac fields at comparatively low field strength. Experimentally obtained prolongation of tree initiation time especially at low electric field can be ascribed to the suppression of such a fatigue through an interaction of injected electrons with nanoscale filler particles or interfaces between nanoscale filler particles and their surrounding polymer matrices. Mechanisms for improvement of time to tree initiation are discussed on the basis of the above concept including a multi-core model that some of the authors have proposed.

    DOI

  • Proposal of a Grating-Based Optical Reflection Switch using Phase Change Materials

    Xiaomin Wang, Masashi Kuwahara, Koichi Awazu, Paul Fons, Junji Tominaga, Yoshimichi Ohki

    OPTICS EXPRESS   17 ( 19 ) 16947 - 16956  2009.09  [Refereed]

     View Summary

    We have proposed a novel grating-based optical reflection switch using a phase change material (PCM). The device switches on/off light or shifts the light propagation direction by switching the PCM grating between its amorphous and crystalline states. Thus, the switching status is non-volatile and the device is promising for realizing low power consumption. The device structure was designed and optimized by numerical simulations to obtain high switching efficiency. It is shown that there exists a parameter window where high efficiency is achievable. The static switching characteristics were confirmed by finite-difference time-domain (FDTD) simulations. The design scheme can also be applied to other planar dielectric gratings. (C) 2009 Optical Society of America

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  • New Electrical Insulation Testing Laboratory at CRIEPI

    Y. Ohki

    IEEE ELECTRICAL INSULATION MAGAZINE   25 ( 5 ) 43 - 44  2009.09  [Refereed]

    DOI

  • Development of an Innovative Monitoring System for Vacuum Circuit Breakers (News from Japan)

    Y. Ohki

    IEEE Electrical Insulation Magazine   25 ( 4 ) 44 - 45  2009.08  [Refereed]

    DOI

  • Partial Discharge Degradation of Several Biodegradable Polymers

    Norikazu Fuse, Shinjiro Fujita, Naoshi Hirai, Toshikatsu Tanaka, Masahiro Kozako, Masanori Kohtoh, Shigemitsu Okabe, Yoshimichi Ohki

    ELECTRICAL ENGINEERING IN JAPAN   168 ( 2 ) 1 - 10  2009.07  [Refereed]

     View Summary

    Partial discharge (PD) resistance was examined by applying a constant voltage to four different biodegradable polymers-poly-L-lactic acid (PLLA), polyethylene terephthalate succinate (PETS), poly e-caprolactone butylene succinate (PCL-BS), and polybutylene succinate (PBS)-and the results were compared with those of low-density polyethylene (LDPE) and crosslinked low-density polyethylene (XLPE). The PD resistance is determined by the erosion depth and the surface roughness caused by PDs, and is ranked as LDPE approximate to XLPE > PLLA approximate to PETS > PBS > PCL-BS. This means that the sample with a lower permittivity has better PD resistance. Furthermore, examination of the sample surfaces by a polarization microscope and a laser confocal microscope reveals that the PD resistance of crystalline regions with spherulites is higher than that of amorphous regions. Therefore, good PD resistance can be achieved by the sample with a high crystallinity and a low permittivity. (C) 2009 Wiley Periodicals, Inc. Electr Eng Jpn, 168(2): 1-10,2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20807

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  • Solution Conductivity as a Key Factor for Thin Silica Coating on Colloidal Silver

    Ken-ichi Nomura, Shinji Fujii, Yoshimichi Ohki, Koichi Awazu, Makoto Fujimaki, Nobuko Fukuda, Tsutomu Hirakawa

    JAPANESE JOURNAL OF APPLIED PHYSICS   48 ( 6 ) 06FE04 - 06FE04-4  2009.06  [Refereed]

     View Summary

    In order to utilize silver nanoparticles for various applications such as photocatalysts while maintaining their chemical stability, enhanced electric fields on the nanoparticle surfaces induced by localized surface plasmon excitation must be used effectively. For satisfying these requirements, an ultrathin silica coating with a thickness of only a few nm was formed around silver nanoparticles by a chemical reaction in a solution, while parameters such as the amount of sodium silicate and the number of dialysis procedures performed were changed. As a result, it was found that a key factor for obtaining stable thin-silica-coated silver nanoparticles is the conductivity of the solution. Using a solution with an appropriate conductivity above 2.7 mS/m, silica films can be coated on silver nanoparticles without causing deterioration of the plasmonic activity resulting from the aggregation of silver. (C) 2009 The Japan Society of Applied Physics

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  • Monitoring surface-assisted biomolecular assembly by means of evanescent-field-coupled waveguide-mode nanobiosensors

    Subash C. B. Gopinath, Koichi Awazu, Makoto Fujimaki, Katsuaki Sugimoto, Yoshimichi Ohki, Tetsuro Komatsubara, Junji Tominaga, Penmetcha K. R. Kumar

    ANALYTICAL AND BIOANALYTICAL CHEMISTRY   394 ( 2 ) 481 - 488  2009.05  [Refereed]

     View Summary

    Biological self-assembly is a natural process that involves various biomolecules, and finding the missing partner in these interactions is crucial for a specific biological function. Previously, we showed that evanescent-field-coupled waveguide-mode sensor in conjunction with a SiO(2) waveguide, the surfaces which contain cylindrical nanometric holes produced by atomic bombardment, allowed us to detect efficiently the biomolecular interactions. In the present studies, we showed that the assembly of biomolecules can be monitored using the evanescent-field-coupled waveguide-mode biosensor and thus provide a methodology in monitoring assembly process in macromolecular machines while they are assembling.

    DOI

  • News From Japan

    Y. Ohki

    IEEE ELECTRICAL INSULATION MAGAZINE   25 ( 3 ) 60 - 61  2009.05  [Refereed]

    DOI

  • Effects of Nanofiller Loading on the Molecular Motion and Carrier Transport in Polyamide

    Norikazu Fuse, Hiroki Sato, Yoshimichi Ohki, Toshikatsu Tanaka

    IEEE TRANSACTIONS ON DIELECTRICS AND ELECTRICAL INSULATION   16 ( 2 ) 524 - 530  2009.04  [Refereed]

     View Summary

    Effects of nanofiller loading on the molecular motion and carrier transport in polyamide-6/mica nanocomposites were discussed by analyzing their complex permittivity spectra. As a result, the following four dielectric polarization processes were observed; space charge polarization, interfacial polarization at crystalline/ amorphous boundaries, alpha relaxation due to dipolar orientation, and beta relaxation due to rotation of amide groups bonded with water molecules. The enhancement of dielectric constant and that of loss factor due to abundance of mobile charge carriers are suppressed by the nanofiller loading. Furthermore, it was found that the relaxation time becomes longer by the mica nanofiller loading both for the dipolar orientation and for the rotation of amide groups. These results suggest that molecular motion that assists carrier transport is suppressed in nanocomposites, which indicates a strong interaction between the resin and filler.

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  • Mechanism of elongation of gold or silver nanoparticles in silica by irradiation with swift heavy ions

    Koichi Awazu, Xiamin Wang, Makoto Fujimaki, Junji Tominaga, Shinji Fujii, Hirohiko Aiba, Yoshimichi Ohki, Tetsuro Komatsubara

    NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS   267 ( 6 ) 941 - 943  2009.03  [Refereed]

     View Summary

    It has been reported that elongated Au nanoparticles oriented parallel to one another can be synthesized in SiO(2) by ion irradiation. Our aim was to elucidate the mechanism of this elongation. We prepared Au and Ag nanoparticles with a diameter of 20 nm in an SiO(2) matrix. It was found that Au nanoparticles showed greater elongated with a higher flux of ion beam and with thicker SiO(2) films. In contrast, Ag nanoparticles split into two OF more shorter nanorods aligned end to end in the direction parallel to the ion beam. These experimental results are discussed in the framework of a thermal spike model of Au and Ag nanorods embedded in SiO(2). The lattice temperature exceeds the melting temperatures of SiO(2), Au and Ag for 100 ns after one 110 MeV Br(10+) ion has passed through the middle of an Au or Ag nanorod. Crown Copyright (C) 2009 Published by Elsevier B.V. All rights reserved.

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  • Optical Fiber Depolarizer Using Birefringence Induced by Proton Implantation

    Seung-Jun Yu, Yoshimichi Ohki, Makoto Fujimaki, Koichi Awazu, Junji Tominaga

    JAPANESE JOURNAL OF APPLIED PHYSICS   48 ( 3 ) 032404(1)-032404(3) - 032404(3)  2009.03  [Refereed]

     View Summary

    An optical fiber depolarizer was developed utilizing birefringence that was induced by ion implantation into a silica-core optical fiber. The implanted ions were protons, and their acceleration energy was chosen such that the protons reach only the center of the optical fiber core. To evaluate the depolarization effect, the degree of polarization was measured for various transmitted polarized lights with different polarization states. Several proton-implanted fibers were spliced, holding their slow axes at a mutual angle at which the degree of polarization was lowest. As a result, an optical fiber depolarizer that can reduce the degree of polarization of incident

    DOI

  • XLPE Recycling Technology in Japan

    Y. Ohki

    IEEE ELECTRICAL INSULATION MAGAZINE   25 ( 2 ) 48 - 49  2009.03  [Refereed]

    DOI

  • Development of 72-kV Low-Pressurized Dry Air Insulated Switchgear (News from Japan)

    Y. Ohki

    IEEE Electrical Insulation Magazine   25 ( 1 ) 57 - 58  2009.02  [Refereed]

    DOI

  • Effects of crystallinity on dielectric properties of poly (1-lactide)

    Saki Hikosaka, Hiroto Ishikawa, Yoshimichi Ohki

    IEEJ Transactions on Fundamentals and Materials   129 ( 4 ) 8 - 222  2009  [Refereed]

     View Summary

    Poly (L-lactide) (PLLA) is attracting much attention as a biodegradable polymer. In this paper, effects of crystallinity on dielectric properties of PLLA are studied. At 80 °C, which is above the glass transition temperature (about 60 °C), the conductivity increases with a decrease in crystallinity. A thermally stimulated polarization current (TSPC) peak, which seems to be due to segmental-mode relaxation, appears in all the samples at temperatures around 65 to 70°C, and it becomes smaller as the crystallinity increases. All the samples show two thermally stimulated depolarization current (TSDC) peaks at around 65 °C and around 90 to 100°C. The lower-temperature peak seems to be due to the segmental-mode relaxation, while the other due to the normal-mode relaxation. Moreover, all the samples show a drastic increase in the real part of complex permittivity (ε r') and a peak in its imaginary part (ε r,") at frequencies from 10 -1 to 10 4 Hz, depending on the measurement temperature. The crystallinity dependencies of the two parts are similar to those of the TSPC and TSDC peaks around 65 to 70 °C. Therefore, the increase in ε r and the peak in ε r are ascribable to the segmental-mode relaxation. © 2009 The Institute of Electrical Engineers of Japan.

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  • Thermally stimulated current in low-density polyethylene/MgO nanocomposite - On the mechanism of its superior dielectric properties

    Kazuyuki Ishimoto, Toshikatsu Tanaka, Yoshimichi Ohki, Yoitsu Sekiguchi, Yoshinao Murata

    IEEJ Transactions on Fundamentals and Materials   129 ( 2 ) 2 - 102  2009  [Refereed]

     View Summary

    Recently polymer nanocomposite has been attracting much attention as an emerging insulating material. In this paper, we measured thermally stimulated depolarization current (TSDC) in low-density polyethylene (LDPE)/MgO nanocomposite by changing measuring parameters such as the temperature increasing rate and the intensity of electric field applied. As a result, a TSDC peak that spreads over a wide temperature range from 0 °C to 60 °C was observed in all the samples. As the amount of MgO nano-filler increases, the TSDC peak height decreases. Furthermore, by adopting a partial heating method, the wide TSDC peak was resolved into several component peaks. Among them, the peak at around 12 °C was found not to appear in the base LDPE. By analyzing its initial rising portion, the 12-°C peak was assumed to have a fairly deep energy depth of about 2 eV. These results indicate that charge carriers tend to be captured more strongly by the addition of MgO nano-fillers. If these captured charge carriers induce homocharge layers in the vicinity of the electrodes further formation of space charge would be suppressed. This seems to explain the fact that the amount of space charge is smaller in the nanocomposite with a proper addition of MgO than in the base LDPE. © 2009 The Institute of Electrical Engineers of Japan.

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  • Possibility of non-destructive detection of electrochemical migration in epoxy resin by the pulsed electroacoustic method

    M. Natsui, H. Asakawa, T. Tanaka, Y. Ohki, T. Maeno, K. Okamoto

    Annual Report - Conference on Electrical Insulation and Dielectric Phenomena, CEIDP     104 - 107  2009  [Refereed]

     View Summary

    Pulsed electroacoustic (PEA) method was applied as a nondestructive method to detect electrochemical migration in a sheet of epoxy resin and a sheet of laminate consisting of the same resin and a paper/phenol-resin composite. A significant decrease in the amount of negative charge was observed at the interface between the resin and the composite when the sample was the laminate. This is assumed to be due to the progress of electrochemical migration in the thickness direction. On the other hand, such a change in charge density was hardly observed when the sample was a sheet of epoxy resin. The electric field intensity in the epoxy resin layer in the laminate is assumed to be significantly enhanced, since the resistivity is much higher in the epoxy resin than in the composite. This seems to be the main reason why the electrochemical migration was induced only in the epoxy layer in the laminate. This means that observation of electrochemical migration in epoxy resin by the PEA method becomes possible by forming a laminate structure using the epoxy resin and paper/phenol-resin composite. © 2009 IEEE.

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  • Effect of crystallinity on the partial discharge resistance of poly-L-lactide

    Y. Ohki, K. Miyata

    Annual Report - Conference on Electrical Insulation and Dielectric Phenomena, CEIDP     539 - 542  2009  [Refereed]

     View Summary

    We have changed the crystallinity of biodegradable poly-L-lactide (PLLA) by drawing or by annealing thermally, and have examined its effect on the partial discharge (PD) resistance. Although the depth eroded by a fixed period of PD degradation becomes shallower when the sample was crystallized by the drawing or by the heat treatment, its reduction ratio is almost equal to the reduction ratio of PD activities due to the decrease in permittivity. Therefore, it is hard to assume that crystallization of PLLA improves its PD resistance to a unit amount of charge. © 2009 IEEE.

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  • Role of Nano-Filler on Partial Discharge Resistance and Dielectric Breakdown Strength of Micro-Al2O3/Epoxy Composites

    Zhe Li, Kenji Okamoto, Yoshimichi Ohki, Toshikatsu Tanaka

    ICPADM 2009: PROCEEDINGS OF THE 9TH INTERNATIONAL CONFERENCE ON PROPERTIES AND APPLICATIONS OF DIELECTRIC MATERIALS, VOLS 1-3     753 - +  2009  [Refereed]

     View Summary

    Epoxy resin often suffers lower dielectric breakdown strength if micro-fillers are loaded. It might be possible to raise the once lowered breakdown strength, if nano-fillers are added. In this paper, such performances were investigated on model specimens consisting of alumina (Al2O3). First of all, PD (partial discharge) behavior on the surfaces of samples was clarified in the insulation system consisting of a flat copper electrode attached on an epoxy substrate. Then, on the basis of finding, the breakdown strengths were investigated by using a sphere to sphere electrode structure with a flat sample inserted. Several materials consist of neat epoxy, 5%wt nano-Al2O3/epoxy composite, 60%wt micro-Al2O3/epoxy composite and 2%wt nano- 60% wt micro-Al2O3/epoxy composite. It was found that nano-micro- Al2O3/epoxy composite is higher in both dielectric strength and PD resistance than micro-Al2O3/epoxy composite.

  • Suppression of Packet-like Space Charge Formation in LDPE by the Addition of Magnesia Nanofillers

    Yoshimichi Ohki, Kazuyuki Ishimoto, Etsu Kanegae, Toshikatsu Tanaka, Yoitsu Sekiguchi, Yoshinao Murata, C. C. Reddy

    ICPADM 2009: PROCEEDINGS OF THE 9TH INTERNATIONAL CONFERENCE ON PROPERTIES AND APPLICATIONS OF DIELECTRIC MATERIALS, VOLS 1-3     9 - +  2009  [Refereed]

     View Summary

    Space charge distributions were investigated in two-layer dielectric films consisting of certain combinations of low-density polyethylene (LDPE) and its nanocomposite with magnesia nanofillers at dc electric fields of 25 kV/mm and 100 kV/mm at room temperature. Space charge is formed at the interface of two different materials to maintain the continuity of current when the electric field is 25 kV/mm, while packet-like charge appears only in the LDPE at 100 kV/mm. These phenomena are explained by assuming that the nanofillers effectively suppress the increase in conductivity induced by the high electric field. Related work done previously by the authors is also briefly reviewed.

  • Tree Initiation Time Evaluation of Epoxy/Silica Composites by Partial Discharge Detection

    Yu Chen, Takahiro Imai, Yoshimichi Ohki, Toshikatsu Tanaka

    ICPADM 2009: PROCEEDINGS OF THE 9TH INTERNATIONAL CONFERENCE ON PROPERTIES AND APPLICATIONS OF DIELECTRIC MATERIALS, VOLS 1-3     404 - +  2009  [Refereed]

     View Summary

    Tree initiation time was evaluated by a pulse detection system developed by the authors to detect a small PD (partial discharge) signal that might take place at first just after a tree was formed. Six kinds of material were prepared for evaluation, i.e. base epoxy resin, microcomposite, two kinds of nanocomposites, and two kinds of nano-micro-mixed-composites. Lengths of trees detected were around 100 mu m in our experiments. As a result, it was clarified that tree initiation time is prolonged by nano filler inclusion, but shortened by micro filler filling, and furthermore that nano fillers would help once-shortened time increase for micro filled epoxy specimens. PD erosion was evaluated for the same materials. It was confirmed that a good correlation exists between tree initiation time and PD resistance that is inverse to PD erosion. This correlation indicates that tree formation includes a PD erosion process. Detection of trees smaller than 100 mu m is required to clarify how a tree takes place at first.

  • Nano-clay and Micro-silica Mixed Composites for Insulating Materials for Environmentally-conscious Switchgear

    Tamon Ozaki, Gen Komiya, Kiyoko Murayama, Takahiro Imai, Fumio Sawa, Toshio Shimizu, Miyuki Harada, Mitsukazu Ochi, Yoshimichi Ohki, Toshikatsu Tanaka

    ICPADM 2009: PROCEEDINGS OF THE 9TH INTERNATIONAL CONFERENCE ON PROPERTIES AND APPLICATIONS OF DIELECTRIC MATERIALS, VOLS 1-3     864 - +  2009  [Refereed]

     View Summary

    Nano-clay and micro-silica mixed composites were made for use as insulating materials for switchgear. Observation of nano-clay dispersion and measurements of insulation breakdown properties under a homogeneous electric field were conducted of these composites. Moreover, simulations were carried out to estimate the curing reaction and rheological behavior of the composite in an automatic pressure gelation system. Experimental results demonstrated that these composites have suitable material characteristics for practical use in switchgear.

  • Improvement in High-temperature Dielectric Properties of Epoxy Resin by Abundant Addition of Micro-silica

    Mayumi Hyuga, Naoki Tagami, Toshikatsu Tanaka, Yoshimichi Ohki, Takahiro Imai, Miyuki Harada, Mitsukazu Ochi

    CEIDP: 2009 ANNUAL REPORT CONFERENCE ON ELECTRICAL INSULATION AND DIELECTRIC PHENOMENA     237 - +  2009  [Refereed]

     View Summary

    Effects of addition of micro-sized silica fillers and/or nano-sized clay fillers into epoxy resin on its dielectric properties were examined. The glass transition intermediate temperature (T-g) was found to show a decrease by the addition of a small amount of nanofillers, while it is increased by the addition of abundant microfillers with and without co-addition of a small amount of nanofillers. At temperatures above T-g, the samples with abundant microfillers were added have low conductivity, low dielectric constant, and low dielectric loss factor regardless of the co-addition of nanofillers.

  • Evaluation of Dielectric Properties in Polypropylene/clay Nanocomposites

    N. Fuse, T. Tanaka, Y. Ohki

    CEIDP: 2009 ANNUAL REPORT CONFERENCE ON ELECTRICAL INSULATION AND DIELECTRIC PHENOMENA     35 - +  2009  [Refereed]

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    Effects of nanofiller addition on four typical dielectric properties, namely permittivity epsilon(r)', dielectric loss factor epsilon(r)", space charge accumulation, and partial discharge (PD) resistance were evaluated for polypropylene (PP) and its nanocomposites (NCs) with nanoclay. While epsilon(r)' and epsilon(r)" are almost independent of temperature and frequency in the base unfilled PP, they are highly dependent on the two parameters in the two NCs. Namely, epsilon(r)' increases significantly at temperatures above 20 degrees C and the frequency spectrum of er" shows at least one temperature-dependent peak. Furthermore, space charge appears abundantly in the two NCs compared to the base PP. These results indicate that plenty of mobile carriers and/or dipoles, probably resulted from the manufacturing process, remain in the two NCs. Notwithstanding the above-mentioned 'inferior' insulating properties, the two NCs have an improved PD resistance compared with the base PP. Namely, the erosion depth on the surface induced by PDs is the smallest in the NC with the largest filler content, while it is the largest in the base PP. Such differences in the effects of nanofillers on different insulating properties are attributable to the fact that nanofillers can improve the PD resistance simply by their presence, while the chemicals needed for uniform dispersion of nanofillers may sometimes increase the permittivity and abundance of charge carriers.

  • Tree Initiation and Growth in LDPE/MgO Nanocomposites and Roles of Nano Fillers

    Toshikatsu Tanaka, Tomonori Iizuka, Yoitsu Sekiguchi, Yoshinao Murata, Yoshimichi Ohki

    CEIDP: 2009 ANNUAL REPORT CONFERENCE ON ELECTRICAL INSULATION AND DIELECTRIC PHENOMENA     67 - +  2009  [Refereed]

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    Processes of tree initiation and growth in LDPE/MgO nanocomposites were investigated by using optical and partial discharge (PD) detection methods. They can be divided into three stages depending on tree length. The first stage corresponds to the initial growth of about 10 mu m and might be subtly affected by the filler content up to 1 phr. This might be influenced by Coulombic field formed around nano fillers. The second stage is interpreted as tree growth around 100 mu m, and is suppressed by nano fillers up to 2 phr, where trees grow by colliding with nano fillers. The third stage leads to final breakdown at which tree lengths are of 1 mm order, and are retarded by nano fillers up to 10 phr, where PD erosion processes are involved.

  • Complex Permittivity Spectra of Several Insulating Polymers at Electrical and THz Frequencies

    Y. Ohki, N. Fuse, S. Hikosaka, Y. Takemura, M. Mizuno, K. Fukunaga

    CEIDP: 2009 ANNUAL REPORT CONFERENCE ON ELECTRICAL INSULATION AND DIELECTRIC PHENOMENA     63 - +  2009  [Refereed]

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    Frequency and temperature dependencies of the real part (epsilon(r)') and imaginary part (epsilon(r)") of relative complex permittivity were measured at electrical frequencies from 10(-2) to 10(5) Hz at temperatures from -80 to 200 degrees C for various kinds of organic insulating polymer films. Furthermore, frequency spectra were obtained at 30 degrees C in a frequency range from 0.4 to 4.0 THz for both parts. As a result, it was found that there are significant differences in both the temperature and frequency dependencies of epsilon(r)' and epsilon(r)", depending mainly on the polymer being polar or nonpolar.
    By comparing terahertz (THz) time-domain spectra obtained for various polymeric insulating films such as polyamide and poly(L-lactide), it was found that the polar polymers have one or two distinct dielectric loss peaks around 2.0 and 3.0 THz. More specifically, among the polymers examined, only polyamide has both two peaks, while poly(L-lactide) and polyethylene terephthalate have only one peak. Nonpolar polymers show no peaks. Furthermore, it has become clear that er' approaches almost the same value of 2.3 at 3.8 THz in most polymers including polar ones. This agrees with the fact that only electronic polarization and atomic polarization associated with very light atoms such as hydrogen can participate in polarization at such high frequencies.

  • Japan ae power systems develops environmentally friendly fire retardant transformers with low- viscosity silicone oil (News from Japan)

    Y. Ohki

    IEEE Electrical Insulation Magazine   24 ( 6 ) 48 - 49  2008.12  [Refereed]

    DOI

  • Terahertz Time-Domain Spectroscopic Analysis of Molecular Behavior in Polyamide Nanocomposites

    Yoshimichi Ohki, Masahide Okada, Norikazu Fuse, Kentaro Iwai, Maya Mizuno, Kaori Fukunaga

    APPLIED PHYSICS EXPRESS   1 ( 12 ) 122401(1)-122401(3) - 122401(3)  2008.12  [Refereed]

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    A terahertz time-domain spectroscopic study was carried out as an effective measure to analyze the mechanism that induces superior properties in polyamide-6 nanocomposites. The peak frequency of dielectric loss appearing at about 3.0 THz shifts to a lower frequency by the presence of clay nanofillers, while it is independent of the sample's mechanical drawing. Comparison of such behavior with X-ray diffraction spectra revealed that restriction of molecular motion is induced by the addition of nanofillers, which should play an important role in the superior properties. (C) 2008 The Japan Society of Applied Physics

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  • Fabrication of Inert Silver Nanoparticles with a Thin Silica Coating

    Ken-ichi Nomura, Shinji Fuju, Yoshimichi Ohki, Koichi Awazu, Makoto Fujimaki, Junji Tominaga, Nobuko Fukuda, Tsutomu Hirakawa, Carsten Rockstuhl

    JAPANESE JOURNAL OF APPLIED PHYSICS   47 ( 11 ) 8641 - 8643  2008.11  [Refereed]

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    We present a genuine experimental technique to fabricate silver nanoparticles with all ultrathin silica coating, thus making the nanoparticles chemically inert. The impact of the coating on plasmonic properties is experimentally quantified and compared with theoretical predictions. Furthermore, numerical simulations of the near-field enhancing properties of the nanoparticles are conducted. It is found that the coatings fabricated are sufficiently thin to make the plasmonic resonance wavelength shift negligible and for observing a significant field enhancement on the surface of the silica shell at the resonance wavelength. Application of such inert nanoparticles to sensitize the absorption of near-ultraviolet light is discussed.

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  • Reduction in polarization dependent loss of a planar lightwave circuit by ion-implantation-induced birefringence

    Seung-Jun Yu, Yoshimichi Ohki, Makoto Fujimaki, Koichi Awazu, Junji Tominaga, Kimikazu Sasa, Tetsuro Komatsubara

    NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS   266 ( 21 ) 4762 - 4765  2008.11  [Refereed]

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    Reduction in polarization dependent loss of a planar lightwave circuit was achieved by asymmetric birefringence formed by ion implantation, in which oxygen ions were implanted along a diagonal of a cross-section of the planar lightwave circuit. The induced birefringence has a slow axis along the line perpendicular to the diagonal. In the present research, a decrease in polarization dependent loss of up to 3.7 dB was obtained, indicating that the method is effective for reducing polarization dependent loss. (C) 2008 Elsevier B.V. All rights reserved.

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  • Photoluminescence Analysis of Czochralski-Grown Lanthanum Aluminate Single Crystals

    Kotaro Kanai, Eiji Hirata, Yoshimichi Ohki

    JAPANESE JOURNAL OF APPLIED PHYSICS   47 ( 10 ) 7980 - 7982  2008.10  [Refereed]

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    Photolurninescence (PL) spectra induced by ultraviolet photons were measured for Czochralski-grown lanthanum aluminate single crystals. Five PL peaks were observed at approximately 1.60, 1.64, 1.675, 2.0, and 2.8eV at 10K, and the 2.8eV PL disappears at 300 K. From PL excitation and vacuum ultraviolet absorption measurements. it is found that the 2.8 eV PL arises owing to the excitation of electrons to localized states in the band gap, and that PLs at approximately 1.60. 1.64, and 1.675 eV arise owing to the excitation of electrons to the conduction band. The 2.0 eV PL with a decay profile similar to that of the 2.8 eV PL is assumed to be caused by a process similar to that in the case of the 2.8 eV PL. [DOI: 10.1143/JJAP.47.7980]

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  • Photoluminescence in Polyamide/mica and Polyethylene/MgO Nanocomposites Induced by Ultraviolet Photons

    Norikazu Fuse, Masahide Okada, Yoshimichi Ohki, Yoshinao Murata, Yoitsu Sekiguchi, Toshikatsu Tanaka

    IEEE TRANSACTIONS ON DIELECTRICS AND ELECTRICAL INSULATION   15 ( 5 ) 1215 - 1223  2008.10  [Refereed]

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    Nature of electronic states in the energy band is investigated by means of optical absorption and photoluminescence induced by vacuum ultraviolet photons for polyamide-6/mica and low-density polyethylene/MgO nanocomposites. The nanofiller loading does not affect the absorption spectra of polyamide samples. Contrary to this, two absorption bands are induced at around 5.0 and 6.2 eV by the nanofiller loading in polyethylene samples, but they are due to absorption by the fillers. A luminescence band is observed at around 3.0 eV in polyamide samples, whereas three luminescence bands are observed at around 4.3, 3.7, and 2.9 eV in polyethylene samples. However, for all the luminescence bands, neither emission nor excitation energies change by the addition of nanofillers. Decay profiles of all the luminescence bands are essentially unchanged by the nanofiller loading. Moreover, no new PL bands are induced in the observed wavelength range. These results indicate that localized states, at least as far as the ones that can emit luminescence photons with intensities more than the sensitivity of the present research, are not induced by the nanofiller loading.

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  • Aiming at a More Rigorous Understanding in Electrical Insulating Materials Research

    Yoshimichi Ohki

    IEEE TRANSACTIONS ON DIELECTRICS AND ELECTRICAL INSULATION   15 ( 5 ) 1201 - 1214  2008.10  [Refereed]

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    Professors Inuishi, Ieda and Yahagi were pioneers of research in dielectrics in Japan. Their philosophy was that in order to understand a certain property of a material or to clarify the mechanism of a phenomenon, research must be done with scientific rigor and a scholarly approach. Here, the author wishes to illustrate their philosophy through a number of examples of his own research on both inorganic and organic materials and on topics ranging from vacuum discharge, through defect centers in inorganic glasses, to the water tree degradation phenomena in insulating polymers. Finally he will make some comments upon the way in which fundamental research can be useful in an engineering context.

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  • Influence of nanometric holes on the sensitivity of a waveguide-mode sensor: Label-free nanosensor for the analysis of RNA aptamer-ligand interactions

    Subash C. B. Gopinath, Koichi Awazu, Makoto Fujimaki, Katsuaki Sugimoto, Yoshimichi Ohki, Tetsuro Komatsubara, Junji Tominaga, Kailash C. Gupta, Penmetcha K. R. Kumar

    ANALYTICAL CHEMISTRY   80 ( 17 ) 6602 - 6609  2008.09  [Refereed]

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    Evanescent-field-coupled (EFC) waveguide-mode sensors can be used to detect nucleic acids or proteins from the changes in the local index of refraction upon adsorption of the target molecule on a waveguide surface. We recently described an EFC waveguide-mode sensor in which nanometric holes on a waveguide film resulted in an improved sensitivity in the analysis of the interactions of biomolecules. In the present study, we have shown that sensitivity depends upon the diameter of the holes, where increase in diameter of holes increases spectral shift resulting in an improved sensitivity. Using this improved EFC waveguide-mode sensor, we could detect interactions between RNA and a small ligand, cyanocobalamin (vitamin 1312), and between RNA and a protein (human coagulation factor IXa). These two interactions were monitored on surfaces modified with biotin-streptavidin-biotin and N-(2-trifluoroethanesulfonatoethyl)-N-(methyl)triethoxysilylpropyl-3-amine, respectively.

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  • Development of a fault location system utilizing optical fiber surge current sensors

    Y. Ohki

    IEEE ELECTRICAL INSULATION MAGAZINE   24 ( 5 ) 44 - 45  2008.09  [Refereed]

    DOI

  • New Diagnostic Method for Estimating the Remaining Service Life of Insulators in Power Distibution Equipment (News from Japan)

    Y. Ohki

    IEEE Electrical Insulation Magazine   24 ( 3 ) 56 - 57  2008.08  [Refereed]

    DOI

  • Elongation of gold nanoparticles in silica glass by irradiation with swift heavy ions (vol 78, art no 054102, 2008)

    Koichi Awazu, Xiaomin Wang, Makoto Fujimaki, Junji Tominaga, Hirohiko Aiba, Yoshimichi Ohki, Tetsuro Komatsubara

    PHYSICAL REVIEW B   78 ( 5 ) 54102 - 054102(8)  2008.08  [Refereed]

    DOI

  • Defects in Yttria-Stabilized Zirconia Induced by Irradiation of Ultraviolet Photons

    Takaaki Morimoto, Masayuki Takase, Toshihide Ito, Hiromitsu Kato, Yoshimichi Ohki

    JAPANESE JOURNAL OF APPLIED PHYSICS   47 ( 8 ) 6858 - 6862  2008.08  [Refereed]

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    When single-crystal yttria-stabilized zirconia samples are exposed to ultraviolet photons with an energy higher than 4.0eV, three paramagnetic centers are observed at g = 2.006, 1.97-1.95, and 1.91-1.86. With irradiation by ultraviolet photons, an absorption band and a photoluminescence (PL) band are also induced, showing respective peak energies at around 3.3 and 2.8 eV. The onset of the PL-excitation spectrum occurs around 4.0 eV. The induced absorption band and paramagnetic centers disappear with thermal annealing. By doing a numerical analysis on the experimentally obtained angular dependence of the signal at g = 1.91-1.86, it is determined that signals at g = 2.006 and 1.91-1.86 are due to the F(+) center and the T center, respectively, and that the signal at g = 1.97-1.95 has some origin other than the two ESR centers. Since two ESR signals at g = 2.006 and 1.97-1.95, the 3.3-eV absorption, and the 2.8-eV PL have the same onset energy at around 4.0eV, the reactions that induce them are triggered by electrons excited into the conduction band tail.

    DOI

  • Elongation of gold nanoparticles in silica glass by irradiation with swift heavy ions

    Koichi Awazu, Xiaomin Wang, Makoto Fujimaki, Junji Tominaga, Hirohiko Aiba, Yoshimichi Ohki, Tetsuro Komatsubara

    PHYSICAL REVIEW B   78 ( 5 )  2008.08  [Refereed]

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    We examined the mechanism whereby nanoparticles of gold embedded in silica become elongated and oriented parallel to each other on ion irradiation. Elongation occurred for gold particles with radii smaller than 25 nm. The process was simulated by using a thermal spike model. For small-radius nanoparticles, ion irradiation raises the temperature above the melting points of both gold and silica, whereas for larger nanoparticles neither the gold nanoparticle nor the surrounding silica matrix is melted.

    DOI

  • Development of radiation-resistant cables

    Y. Ohki

    IEEE ELECTRICAL INSULATION MAGAZINE   24 ( 4 ) 52 - 53  2008.07  [Refereed]

    DOI

  • Silica-based monolithic sensing plates for waveguide-mode sensors

    Makoto Fujimaki, Carsten Rockstuhl, Xiaomin Wang, Koichi Awazu, Junji Tominaga, Yuuki Koganezawa, Yoshimichi Ohki, Tetsuro Komatsubara

    OPTICS EXPRESS   16 ( 9 ) 6408 - 6416  2008.04  [Refereed]

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    We developed a monolithic sensing plate for a waveguide-mode sensor. The plate consists of a SiO2 glass substrate and a thin silicon layer the surface of which is thermally oxidized to form a SiO2 glass waveguide. We confirmed that the sensing plate is suitable for high-sensitivity detection of molecular adsorption at the waveguide surface. In addition, a significant enhancement of the sensitivity of the sensor was achieved by perforating the waveguide with holes with diameters of a few tens of nanometers by selective etching of latent tracks created by swift heavy-ion irradiation. Possible strategies for optimizing the plate are discussed. (c) 2008 Optical Society of America.

    DOI

  • The design of evanescent-field-coupled waveguide-mode sensors

    Makoto Fujimaki, Carsten Rockstuhl, Xiaomin Wang, Koichi Awazu, Junji Tominaga, Nobuko Fukuda, Yuuki Koganezawa, Yoshimichi Ohki

    NANOTECHNOLOGY   19 ( 9 ) 95503(1)-95503(7) - 95503(7)  2008.03  [Refereed]

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    An evanescent-field-coupled waveguide-mode sensor with a multilayer structure consisting of a dielectric waveguide, a thin reflecting layer, and a glass substrate illuminated under the Kretschmann configuration operates as a sensor that is capable of detecting modifications in the dielectric environment near the waveguide surface with superior sensitivity by measuring the change in reflectivity. The sensitivity of the sensor is strongly dependent on the optical constants of the reflecting layer. Numerical simulations show that a sensor having a reflecting layer with a small value of the real part of the complex refractive index shows a good sensitivity for both S- and P-polarized light. Materials with values of the real and imaginary parts of the complex refractive index of > 4 and similar to 0.5, respectively, are suitable for use as reflecting layers when S- polarized light excites only the lowest order waveguide mode. The simulations were experimentally confirmed using sensors with Au, Cu, Cr, W, a-Si, or Ge reflecting layers deposited by radiofrequency magnetron sputtering by observation of specific adsorption of streptavidin on biotinyl groups using an S-polarized laser beam with a wavelength of 632.8 nm. From the results, guidelines are given for the fabrication of preferred sensor configurations.

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  • Development of radiation-resistant, single-mode optical fibers for the CERN LHC

    Y. Ohki

    IEEE ELECTRICAL INSULATION MAGAZINE   24 ( 2 ) 52 - 53  2008.03  [Refereed]

    DOI

  • Activity of the IEEJ Investigation Committee on Generation Characteristics and Prevention Methods of Ionic Migration (News from Japan)

    Y. Ohki

    IEEE Electrical Insulation Magazine   24 ( 1 ) 52 - 53  2008.02  [Refereed]

    DOI

  • Possible mechanisms of superior resistance of polyamide nanocomposites to partial discharges and plasmas

    Norikazu Fuse, Yoshimichi Ohki, Masahiro Kozako, Toshikatsu Tanaka

    IEEE TRANSACTIONS ON DIELECTRICS AND ELECTRICAL INSULATION   15 ( 1 ) 161 - 169  2008.02  [Refereed]

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    Degradation profiles induced by partial discharges and those induced by oxygen plasmas are compared for polyamide/mica nanocomposites. Both the resistances to partial discharges and to plasmas improve with an increase in nanofiller content. On the other hand, the partial discharge resistance is not improved if gm-sized glass fibers are added to polyamide. In order to investigate these phenomena, the superior resistance mechanism of nanocomposites is discussed, focusing on the effects of the nanofillers on the bulk and surface structures of the resin. It was revealed from X-ray diffraction and permittivity measurements that the nanofiller loading increases crystallinity of the resin and restricts the molecular motion. This should enhance the resistance to degradation. Furthermore, observation results by scanning electron microscopy, energy dispersive X-ray spectroscopy, and X-ray diffraction revealed that the nanofillers had piled up themselves to form a layered structure on the sample surface in an early stage of degradation. Such a structure acts as a barrier against impact of charged particles and diffusion of gases such as oxygen, which should contribute to the improvement of resistance to degradation as its direct effect and also as its indirect effect by suppressing the oxidation of resin. Moreover, it was also revealed from scanning electron microscopy that the nanofillers impede the growth of surface cavities by partial discharges drastically.

    DOI

  • Dielectric properties of epoxy/clay nanocomposites - Effects of curing agent and clay dispersion method

    N. Tagami, M. Okada, N. Hirai, Y. Ohki, T. Tanaka, T. Imai, M. Harada, M. Ochi

    IEEE TRANSACTIONS ON DIELECTRICS AND ELECTRICAL INSULATION   15 ( 1 ) 24 - 32  2008.02  [Refereed]

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    Effects of the differences in the curing agent and filler dispersion method on the dielectric properties were examined for epoxy/clay nanocomposites. Irrespective of the clay dispersion method, relative permittivity and electrical conductivity are higher in the samples cured with the amine. Moreover, negative heterocharge accumulates in the vicinity of the anode in the amine-cured samples, whereas positive homocharge accumulates in the acid anhydride-cured samples. From the results of UV photon absorption and PL measurement, the bandgap or the energy at which the photon absorption increases drastically is smaller in the amine-cured samples than in the acid anhydride-cured samples. Ion migration can occur easily in the amine-cured samples whose electrical conductivity and relative permittivity are higher than the acid anhydride-cured samples. The curing agent gives the strongest effect, while the existence of clay affects secondly and the filler dispersion method has the weakest effect.

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  • Enhanced partial discharge resistance of epoxy/clay nanocomposite prepared by newly developed organic modification and solubilization methods

    Toshikatsu Tanaka, Yoshimichi Ohki, Mitsukazu Ochi, Miyuki Harada, Takahiro Imai

    IEEE TRANSACTIONS ON DIELECTRICS AND ELECTRICAL INSULATION   15 ( 1 ) 81 - 89  2008.02  [Refereed]

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    Frequency accelerated partial discharge (PD) aging of epoxy nanocomposite with 5 wt % additions of clay was investigated in comparison with that of epoxy without clay in terms of ID erosion depth. It was found that the change in the erosion depth is far smaller in specimens with clay than those without clay. The newly developed organic modification and solubilization methods give comparable PD resistance characteristics. The latter would be more resistant to PD's than the former, if specimens were prepared properly. It was clarified that nano-micro mixed composites were superior to the single nanocomposite. Nano segmentation with some interaction zone effect is proposed as a mechanism of improvement in PD resistance.

    DOI

  • Biomolecular sensors utilizing waveguide modes excited by evanescent fields

    M. Fujimaki, C. Rockstuhl, X. Wang, K. Awazu, J. Tominaga, T. Ikeda, Y. Koganezawa, Y. Ohki

    JOURNAL OF MICROSCOPY   229 ( 2 ) 320 - 326  2008.02  [Refereed]

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    Properties of evanescent-field-coupled waveguide-mode sensors consisting of a multi-layer structure made of a SiO2 waveguide, a thin metal layer (Au, Cu, W or Ti), and a high refractive index glass substrate illuminated under the Kretschmann configuration have been theoretically and experimentally investigated. In all cases, reflectivity changes attributed to streptavidin combining to biotinyl groups were observed in close spectral vicinity of the waveguide resonances. The sensors with the Au and the Cu layers show superior sensitivity as compared to those with the W and the Ti layers, whereas the W and Ti layers show better thermal and chemical stability. The results indicate that the materials of thin metal layers should be chosen in accordance with the purpose of sensors and/or environment in which the sensors are used.

    DOI

  • A plasmonic photocatalyst consisting of sliver nanoparticles embedded in titanium dioxide

    Koichi Awazu, Makoto Fujimaki, Carsten Rockstuhl, Junji Tominaga, Hirotaka Murakami, Yoshimichi Ohki, Naoya Yoshida, Toshiya Watanabe

    JOURNAL OF THE AMERICAN CHEMICAL SOCIETY   130 ( 5 ) 1676 - 1680  2008.02  [Refereed]

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    Titanium dioxide (TiO2) displays photocatalytic behavior under near-ultraviolet (UV) illumination. In another scientific field, it is well understood that the excitation of localized plasmon polaritons on the surface of silver (Ag) nanoparticles (NPs) causes a tremendous increase of the near-field amplitude at well-defined wavelengths in the near UV. The exact resonance wavelength depends on the shape and the dielectric environment of the NIPS. We expected that the photocatalytic behavior of TiO2 would be greatly boosted if it gets assisted by the enhanced near-field amplitudes of localized surface plasmon (LSP). Here we show that this is true indeed. We named this new phenomenon "plasmonic photocatalysis". The key to enable plasmonic photocatalysis is to deposit TiO2 on a NP comprising an Ag core covered with a silica (SiO2) shell to prevent oxidation of Ag by direct contact with TiO2. The most appropriate diameter for Ag NIPS and thickness for the SiO2 shell giving rise to LSP in the near UV were estimated from Mie scattering theory. Upon implementing a device that took these design considerations into account, the measured photocatalytic activity under near UV illumination of such a plasmonic photocatalyst, monitored by decomposition of methylene blue, was enhanced by a factor of 7. The enhancement of the photocatalytic activity increases with a decreased thickness of the SiO2 shell. The plasmonic photocatalysis will be of use as a high performance photocatalyst in nearly all current applications but will be of particular importance for applications in locations of minimal light exposure.

    DOI

  • Suppression of charge injection into LDPE by addition of MgO nanofillers

    Kazuyuki Ishimoto, Toshikatsu Tanaka, Yoshimichi Ohki, Yoitsu Sekiguchi, Yoshinao Murata

    IEEJ Transactions on Fundamentals and Materials   128 ( 12 ) 742 - 743  2008  [Refereed]

     View Summary

    Recently, polymer nanocomposite has been attracting much attention as an emerging insulating material. In this paper, we measured space charge distribution in low-density polyethylene (LDPE)/MgO nanocomposite at room temperature and 90 °C. At room temperature, while a huge amount of packet-like charge moving from the anode to the cathode is observed in the base LDPE, such packet-like charge is not observed in the MgO-added nanocomposites. On the other hand, at 90 °C, while injection of negative charge from the cathode is dominant in the base LDPE, the filler addition significantly suppresses the injection. As a consequence space charge is barely formed in the 2-phr MgO-added sample at the two temperatures. © 2008 The Institute of Electrical Engineers of Japan.

    DOI

  • Temperature dependence of complex permittivity in biodegradable polybutylene succinate

    Hiroto Ishikawa, Yoshimichi Ohki

    IEEJ Transactions on Fundamentals and Materials   128 ( 10 ) 9 - 651  2008  [Refereed]

     View Summary

    Polybutylene succinate, a kind of biodegradable polymer, shows thermally stimulated polarization current (TSPC) peaks at around -35 °C and 50 °C. The lower-temperature TSPC peak can be well explained by dipolar polarization. As for the higher-temperature TSPC peak, the permittivity increases more significantly with a decrease in frequency, and the increment in permittivity estimated from the TSPC area agrees with the difference in permittivity at a sufficiently low frequency and at a sufficiently high frequency. It is assumed that a hetero charge layer is formed in front of the two electrodes and that such space charge is responsible for both the higher-temperature TSPC peak and the permittivity increase. The dielectric loss factor also increases with a decrease in frequency, and the increment is in good agreement with the assumption that the dielectric loss is ascribed to conduction loss due to high conductivity at high temperatures. All the results indicate that mobile ions are dominant carrier species in polybutylene succinate. © 2008 The Institute of Electrical Engineers of Japan.

    DOI

  • Effect of water absorption temperature on space charge profiles in paper/phenol-resin composites for printed circuit boards

    Yasutsune Echigo, Masasbi Natsui, Takashi Maeno, Yoshimichi Ohki

    IEEJ Transactions on Fundamentals and Materials   128 ( 9 ) 2 - 590  2008  [Refereed]

     View Summary

    More and more electronics are to be used under various adverse environments at high temperatures with high humidity. Spatial distributions of internal charge carriers mainly due to ionic impurities that appear in hot and humid environments are considered to affect the reliability of bulk insulation. Therefore, the authors examined space charge behavior inside paper/phenol-resin composites for printed circuit boards under dc voltages, focusing on the effect of water absorption temperature. Both the sample weight and thickness are increased monotonically by the immersion in water with an increase in water temperature from 24 °C to 85 °C, indicating that the water absorption by the sample is temperature dependent. In early periods of water absorption up to 10 hours, the electric field decreases near the two electrodes and increases in the other regions. Furthermore, heterocharge formation is observed near the cathode as the water absorption progresses, which becomes more significant at higher water temperatures. Ion chromatography analyses detected a lot of ions such as Na+, NH4+ and CI" from the water, in which the sample had been immersed for 100 hours at various temperatures. It is highly possible that these ions are responsible for the heterocharge formation. © 2008 The Institute of Electrical Engineers of Japan.

    DOI

  • Electrical conduction and dielectric relaxation in polyethylene terephthalate succinate

    Fukutaro Kato, Yoshimichi Ohki

    IEEJ Transactions on Fundamentals and Materials   128 ( 7 ) 4 - 496  2008  [Refereed]

     View Summary

    Electrical conduction and complex permittivity are examined in polyethylene terephthalate succinate, focusing on their relations to dielectric relaxation processes. Both the real and imaginary parts of complex permittivity, namely dielectric constant σrand dielectric loss factor σr", increase with a decrease in frequency, especially at high temperatures. They are both ascribed to the transport of ionic mobile carriers. Namely, the carrier transport forms conduction current that should contribute to σr". On this occasion, if charge exchange does not occur at the two electrodes, heterocharge layers should be formed before the electrodes. This should increase the charge density on the electrodes, thus contributing to σr' In addition to the increase in σr' and σr' due to mobile ions, two relaxation processes, one due to micro-Brownian motion of dipoles and the other due to orientation and magnitude change of the dipole moment induced by two end groups in the polymer main chain, are observed. Corresponding to these two relaxation processes, two thermally stimulated discharge current (TSDC) peaks appear. The two TSDC peaks as well as the increment in σr' and σr" become larger when the crystallinity of the sample decreases. © 2008 The Institute of Electrical Engineers of Japan.

    DOI

  • Effects of crystallinity on thermally stimulated current and complex permittivity of poly(L-lactide)

    S. Hikosaka, H. Ishikawa, Y. Ohki

    Annual Report - Conference on Electrical Insulation and Dielectric Phenomena, CEIDP     497 - 500  2008  [Refereed]

     View Summary

    Recently poly(L-lactide) synthesized from starch is attracting much attention as a biodegradable polymer. In this paper, effects of crystallinity on dielectric properties of PLLA were studied by preparing four kinds of PLLA samples with different crystallinities. A thermally stimulated polarization current (TSPC) peak was found to appear in all the samples at temperatures around 65 to 70 °C, which is due to the glass transition. The magnitude of the TSPC peak was found to be in the reverse order of crystallinity. This is presumably because the segmental-mo de relaxation, which brings about the TSPC, is restricted more as the crystallinity increases. All the samples show two thermally stimulated depolarization current (TSDC) peaks at around 65 °C and around 90 to 100 °C. The lower-temperature peak seems to be due to the segmental-mode relaxation, while the other due to the normal-mode relaxation. Moreover, all the samples show a drastic increase in the real part of complex permittivity and have a peak in its imaginary part at frequencies from 10 -1 to 104 Hz, depending on the measurement temperature. The crystallinity dependencies of the two parts are similar to those of the TSPC and TSDC peaks around 65 to 70 °C. Therefore, the increase in the real part and the peak in the imaginary part are ascribable to the segmental-mode relaxation. © 2008 IEEE.

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  • Terahertz dielectric relaxation spectroscopy in polyamide nanocomposite

    K. Iwai, M. Okada, N. Fuse, Y. Ohki, K. Fukunaga, M. Mizuno

    Proceedings of the International Symposium on Electrical Insulating Materials     593 - 596  2008  [Refereed]

     View Summary

    Terahertz time-domain spectroscopy was measured as an effective procedure to analyze the mechanism that induces superior properties in polymer nanocomposites. The peak frequency of dielectric absorption appearing at frequencies 2.7 - 3.1 THz shifts to a lower frequency by the presence of nanofillers, which seems to be ascribable to the restriction of molecular motion at the interfaces between the polymer matrix and nanofillers.

    DOI

  • Improving epoxy-based insulating materials with nano-fillers toward practical application

    Takahiro Imai, Gen Komiya, Kiyoko Murayama, Tamon Ozaki, Fumio Sawa, Toshio Shimizu, Miyuki Harada, Mitsukazu Ochi, Yoshimichi Ohki, Toshikatsu Tanaka

    Conference Record of IEEE International Symposium on Electrical Insulation     201 - 204  2008  [Refereed]

     View Summary

    A primary concern in recent nanocomposite research is practical application. In this study, various kinds of epoxy-based nanocomposites were made and their properties evaluated to determine their applicability as insulating materials for heavy electric apparatuses. Experimental results demonstrated that nano-fillers enhance insulation breakdown properties in nanocomposites. Moreover, nano- and micro-filler combinations were adopted as an approach toward practical application of nanocomposite insulating materials. These nano- and micro-filler mixed composites had the same low thermal expansion as aluminum, and insulation breakdown properties superior to those of conventional insulating materials. Consequently, an aluminum conductor and a vacuum interrupter were molded by the nano- and micro-filler mixed composites for the first time in nanocomposite research. ©2008 IEEE.

    DOI

  • High Field Light Emission in LDPE/MgO Nanocomposite

    Toshikatsu Tanaka, Kanan Yokoyama, Yoshimichi Ohki, Yoshinao Murata, Yoitsu Sekiguchi, Manabu Goshowaki

    ISEIM 2008: PROCEEDINGS OF 2008 INTERNATIONAL SYMPOSIUM ON ELECTRICAL INSULATING     152 - 152  2008  [Refereed]

  • THz Dielectric Relaxation Spectroscopy in Polyamide Nanocomposite

    Kentaro Iwai, Masahide Okada, Norikazu Fuse, Yoshimichi Ohki, Kaori Fukunaga, Maya Mizuno

    ISEIM 2008: PROCEEDINGS OF 2008 INTERNATIONAL SYMPOSIUM ON ELECTRICAL INSULATING     180 - 180  2008  [Refereed]

  • Nano- and Micro-filler Combination Enabling Practical Use of Nanocomposite Insulating Materials

    Takahiro Imai, Gen Komiya, Kiyoko Murayama, Tamon Ozaki, Fumio Sawa, Toshio Shimizu, Miyuki Harada, Mitsukazu Ochi, Yoshimichi Ohki, Toshikatsu Tanaka

    ISEIM 2008: PROCEEDINGS OF 2008 INTERNATIONAL SYMPOSIUM ON ELECTRICAL INSULATING     93 - 93  2008  [Refereed]

  • Comparison of Dielectric Properties of Low-density Polyethylene/MgO Composites with Different Size Fillers

    K. Ishimoto, T. Tanaka, Y. Ohki, Y. Sekiguchi, Y. Murata, M. Gosyowaki

    2008 IEEE CONFERENCE ON ELECTRICAL INSULATION AND DIELECTRIC PHENOMENA     223 - +  2008  [Refereed]

     View Summary

    Two kinds of low-density polyethylene (LDPE) composites were prepared by adding semi-spherical MgO fillers different in diameter, one being around several ten nm and the other being several pro, and their dielectric properties were compared. At all the measurement temperatures, the conductivity is decreased and the permittivity is increased by the addition of fillers, irrespective of their sizes. However, the conductivity decrease is more significant and the permittivity increase is suppressed more in the case of the nm fillers. In addition, a drastic increase in dielectric loss factor at low frequencies is suppressed more significantly by the addition of the nm fillers. While the formation of packet-like charge observed in LDPE, which is assumed to appear at the front of the high-conductivity region that proceeds from one electrode toward the counter one, is suppressed significantly by both fillers, the effect is more significant in the nanocomposites. This means that the nanofillers suppress the increase in conductivity induced by the voltage application more effectively.

  • Detection of Ion Migration in Composites for Printed Circuit Boards by the Pulsed Electroacoustic Method

    Masashi Natsui, Yasutsune Echigo, Toshikatsu Tanaka, Yoshimichi Ohki, Takashi Maeno

    ISEIM 2008: PROCEEDINGS OF 2008 INTERNATIONAL SYMPOSIUM ON ELECTRICAL INSULATING     118 - 118  2008  [Refereed]

  • Effects of Blending Various Biodegradable Polymer Plasticizers on the Conduction Current and AC Breakdown Strength in Starch Ester

    Yuki Nemoto, Fukutaro Kato, Naoshi Hirai, Yoshimichi Ohki, Shuhei Kaneko, Shigemitsu Okabe

    ISEIM 2008: PROCEEDINGS OF 2008 INTERNATIONAL SYMPOSIUM ON ELECTRICAL INSULATING     163 - 163  2008  [Refereed]

  • Terahertz Spectroscopy as a New Analyzing Tool for Dielectric Properties of Various Insulating Materials

    Y. Ohki, M. Okada, N. Fuse, K. Iwai, M. Mizuno, K. Fukunaga

    2008 IEEE CONFERENCE ON ELECTRICAL INSULATION AND DIELECTRIC PHENOMENA     199 - +  2008  [Refereed]

     View Summary

    Several examples of terahertz; (THz) spectroscopic measurement results carried out for dielectric solids are reviewed, including THz time-domain spectra of polyamide-6 composites with nm-sized clay fillers obtained by the authors. For polyamide nanocomposites, it has become clear that the dielectric loss factor shows two peaks at around 1.9 THz and 2.7 to 3.1 THz. By referring to X-ray diffraction spectra, it is assumed that the origin of the former peak is the a-phase crystal in polyamide, and that the nanofiller loading suppresses a-phase crystallization. The latter peak shifts toward a lower frequency in the nanocomposites, which indicates that the presence of strong ionic bonds between the nanofiller and the polymer matrix restrains molecular motion and increases the relaxation time. This is believed to be the main reason of superior properties of the polymer nanocomposites.

  • Effects of Mica Nanofillers on the Complex Permittivity of Polyamide Nanocomposites

    N. Fuse, H. Sato, T. Tanaka, Y. Ohki

    2008 IEEE CONFERENCE ON ELECTRICAL INSULATION AND DIELECTRIC PHENOMENA     85 - +  2008  [Refereed]

     View Summary

    Effects of nanofiller loading on the carrier transport and molecular motion in polyamide-6/mica nanocomposites were discussed by analyzing their complex permittivity spectra. As a result, the following four dielectric polarization processes were observed; space charge polarization, alpha relaxation due to dipolar orientation, and beta relaxation due to rotation of amide groups bonded with water molecules, in addition to interfacial polarization at crystalline/amorphous boundaries. The enhancement of dielectric constant and that of loss factor due to abundance of mobile charge carriers are suppressed by the nanofiller loading. These results suggest that carrier transport is suppressed by the nanofillers. Furthermore, it was found that relaxation time becomes longer by the mica nanofiller loading both for the dipolar orientation and for the rotation of amide groups. Molecular motion is considered to be restricted in nanocomposites, which indicates a strong interaction between the resin and filler.

  • Effects of Coupling Agent and Filler Dispersion on V-t Characteristics of Epoxy/Silica Nanocomposites

    Tomonori Iizuka, Yoshimichi Ohki, Toshikatsu Tanaka

    ISEIM 2008: PROCEEDINGS OF 2008 INTERNATIONAL SYMPOSIUM ON ELECTRICAL INSULATING     29 - 29  2008  [Refereed]

  • Improving epoxy-based insulating materials with nano-fillers toward practical application

    Takahiro Imai, Gen Komiya, Kiyoko Murayama, Tarnon Ozaki, Fumio Sawa, Toshio Shimizu, Miyuki Harada, Mitsukazu Ochi, Yoshimichi Ohki, Toshikatsu Tanaka

    CONFERENCE RECORD OF THE 2008 IEEE INTERNATIONAL SYMPOSIUM ON ELECTRICAL INSULATION, VOLS 1 AND 2     201 - +  2008  [Refereed]

     View Summary

    A primary concern in recent nanocomposite research is practical application. In this study, various kinds of epoxy-based nanocomposites were made and their properties evaluated to determine their applicability as insulating materials for heavy electric apparatuses. Experimental results demonstrated that nano-fillers enhance insulation breakdown properties in nanocomposites. Moreover, nano- and micro-filler combinations were adopted as an approach toward practical application of nanocomposite insulating materials. These nano- and micro-filler mixed composites had the same low thermal expansion as aluminum, and insulation breakdown properties superior to those of conventional insulating materials. Consequently, an aluminum conductor and a vacuum interrupter were molded by the nano- and micro.-filler mixed composites for the first time in nanocomposite research.

  • Enhanced Performance of Tree Initiation V-t Characteristics of Epoxy/Clay Nanocomposite in Comparison with Neat Epoxy Resin

    S. Raetzke, Y. Ohki, T. Imai, J. Kindersberger, T. Tanaka

    2008 IEEE CONFERENCE ON ELECTRICAL INSULATION AND DIELECTRIC PHENOMENA     159 - +  2008  [Refereed]

     View Summary

    Tree initiation behavior of an epoxy nanocomposite with 5 wt % nanoclay (layered silicate) was investigated in comparison to neat epoxy resin without fillers. To shorten the time for experiments, 600 Hz was used instead of 60 Hz, as acceleration for tree initiation had been confirmed at 10 kV(rms) and 14 kV(rms) between the two frequencies. V-t characteristics for tree initiation rather than tree growth to bridge the electrodes were obtained for conventional type of treeing specimens with an embedded steel needle subjected to voltages from 2 kV(rms) to 14 kV(rms) As a result, the n value in V-n proportional to t characteristics was confirmed to be 5.5 for neat epoxy and larger than 7 for nanocomposite. It was clarified that tree initiation V-t characteristics were improved by approximately one order of magnitude for the epoxy/nanoclay composite compared to the neat epoxy resin. To be precise, such an enhancement factor is one order at high field but even two orders at low field. Formed trees are field dependent. They are rather thick and short in shape at low field, but thin and long at high field. It is concluded from the analysis on the basis of interfacial models and other studies that initial trees are formed due to a PD erosion process at low field during a long time, but due to dielectric breakdown including charge trapping at high field for a short time.

  • Nanocomposite Insulating Materials for Environmental-conscious Heavy Electric Apparatuses

    Takahiro Imai, Tamon Ozaki, Fumio Sawa, Toshio Shimizu, Miyuki Harada, Mitsukazu Ochi, Naoki Tagami, Yoshimichi Ohki, Toshikatsu Tanaka

    ISEIM 2008: PROCEEDINGS OF 2008 INTERNATIONAL SYMPOSIUM ON ELECTRICAL INSULATING     210 - 210  2008  [Refereed]

  • Electrical conduction and breakdown properties of several biodegradable polymers

    Y. Ohki, N. Hirai

    IEEE TRANSACTIONS ON DIELECTRICS AND ELECTRICAL INSULATION   14 ( 6 ) 1559 - 1566  2007.12  [Refereed]

     View Summary

    In order to respond to soaring public concern about environmental protection, various biodegradable polymers have been developed. The present paper reports the electrical conduction and breakdown properties of various biodegradable polymers such as poly-L-lactic acid (PLLA), polyethylene terephthalate succinate (PETS), polycaprolactone butylene succinate (PCL-BS), polybutylene succinate (PBS), polybutylene succinate adipate (PBSA), and polyhydroxybutyrate/valerate (PHB/V) in comparison to those of low-density polyethylene (LDPE). While the permittivity and conductivity of PLLA and PETS are comparable to LDPE, those of PCL-BS and PBS are much higher. The conductivity is also higher in PBSA. This is because PLLA and PETS are in the glass state at room temperature, while PCL-BS, PBS, and PBSA are in the rubber state. Furthermore, PLLA and PETS show a strong temperature dependence of the conductivity, which is divided into two or three regions, and they also show thermally stimulated polarization or depolarization current around their respective glass transition temperatures. In contrast to the large difference in conductivity among different kinds of samples, all the polymers tested have almost similar impulse breakdown strength at room temperature. As for dc or ac breakdown strength, PLLA and PETS show a relatively higher strength than PCL-BS and PBS.

    DOI

  • Birefringence in optical fibers formed by proton implantation

    Seung Jun Yu, Masahiro Suzuki, Yoshimichi Ohki, Makoto Fujimaki, Kouichi Awazu, Eisuke Yamaguchi, Satoshi Okude

    NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS   265 ( 2 ) 490 - 494  2007.12  [Refereed]

     View Summary

    Birefringence can be induced in silica-based optical fibers by ion implantation. In the present research, protons were implanted in single-mode optical fibers with two different energies, one being the energy with which the protons can just reach the center of the optical fiber core and the other being a slightly lower energy. The degree of birefringence was evaluated by measuring reflection spectra of Bragg gratings formed at the proton-implanted region of the optical fibers. The results confirmed that birefringence is induced by unidirectional densification along the projected range of protons formed in the fiber core and by densification of the fiber cladding. The induced birefringence reached three to ten times higher than that of a conventional birefringent fiber. The birefringence caused by ion implantation can be a versatile tool for manufacturing various optical fiber devices. (C) 2007 Elsevier B.V. All rights reserved.

    DOI

  • Comparison of partial discharge resistance among several biodegradable polymers

    K. Miyata, S. Fujita, Y. Ohki, T. Tanaka

    IEEE TRANSACTIONS ON DIELECTRICS AND ELECTRICAL INSULATION   14 ( 6 ) 1474 - 1476  2007.12  [Refereed]

     View Summary

    The partial discharge (PD) resistance of three kinds of biodegradable polymers, namely poly-L-lactic acid (PLLA), polybutylene suceinate (PBS), and poly 6-caprolactone butylene succinate (PCL-BS), was studied in comparison to that of low-density polyethylene (LDPE). The polymers were exposed to PDs under the IEC (b) electrode system, and the PD resistance was judged by the surface erosion depth or the time to breakdown induced by PDs. As a result, the 'practical' PD resistance, which is the resistance to PDs under a constant voltage application, is ranked as LDPE > PLLA >> PBS >= PCL-BS, while the 'true' PD resistance, which is the resistance to PDs with a constant charge quantity, is ranked as PLLA congruent to LDPE > PBS >> PCL-BS.

  • Control of the properties of directional couplers using proton irradiation

    Makoto Fujimaki, Keita Kawabe, Masahiro Suzuki, Koichi Awazu, Yoshimichi Ohki, Junji Tominaga

    NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS   264 ( 2 ) 267 - 271  2007.11  [Refereed]

     View Summary

    The control of the coupling properties of a directional coupler formed in a planar lightwave circuit by proton irradiation was examined. Irradiation of the cladding between the two cores that constitute the coupling portion causes an increase in the coupling coefficient, whereas subsequent irradiation of the entire coupling portion decreases the coupling coefficient. The change in the coupling coefficient results in changes to the coupling ratios, which depend on the irradiated area and fluence. By adjusting these, the desired coupling property can be obtained. This method can be applied in rectifying fabrication errors in any kind of optical device formed in a planar lightwave circuit. (c) 2007 Elsevier B.V. All rights reserved.

    DOI

  • One-week IEC WG/MT meetings on coil insulation held in Tokyo

    Y. Ohki

    IEEE ELECTRICAL INSULATION MAGAZINE   23 ( 6 ) 42 - 43  2007.11  [Refereed]

    DOI

  • Prototype PLLA-insulated Cable Developed (News from Japan)

    Y. Ohki

    IEEE Electrical Insulation Magazine   23 ( 5 ) 52 - 52  2007.09  [Refereed]

    DOI

  • Modification of structural and optical properties of silica glass induced by ion microbeam

    H. Nishikawa, M. Murai, T. Nakamura, Y. Ohki, M. Oikawa, T. Sato, T. Sakai, Y. Ishii, M. Fukuda

    SURFACE & COATINGS TECHNOLOGY   201 ( 19-20 ) 8185 - 8189  2007.08  [Refereed]

     View Summary

    Structural and optical properties of silica glass induced by ion microbeam irradiation were studied using micro-photoluminescence (mu-PL) spectroscopy and atomic force microscopy (AFM). Ion microbeam irradiation was performed using microbeam lines of 3-MV single-ended or tandem accelerators with various ion species including H (+) He+, B3+, C4+, N4+, O4+, and Si5+ at energies of 1.7 to 18 MeV. The beam was focused to about 1 mu m and was scanned over the surface of high-purity silica glass with fluences of 10(13) to 10(18) ions/cm(2). The mu-PL spectrum in microbeam-irradiated silica shows two peaks at 540 and 650 nm. The mapping of the two PL bands reveals the distribution of defects induced along the track of ions. The compaction was observed in the form of groove at the surface of silica glass by AFM. The depth of the surface groove increases with increasing ion fluence and saturates at about several hundreds to 900 nm, depending on irradiated ion species. The mechanisms of structural and optical modifications of silica glass are can be understood in terms of energy loss due to electronic stopping and nuclear stopping powers. Refractive index changes with an order of 10(-4) to 10(-2) were estimated by a Lorentz-Lorenz relationship from the compaction. Technological implications of these results are also to be discussed. (c) 2007 Published by Elsevier B.V.

    DOI

  • The Institute of Electrical Engineers of Japan holds their annual conference

    Y. Ohki

    IEEE ELECTRICAL INSULATION MAGAZINE   23 ( 4 ) 61 - 61  2007.07  [Refereed]

    DOI

  • Investigation Committee on "Economical Assessment of Diagnosis for Power Apparatus" (News from Japan)

    Y. Ohki

    IEEE Electrical Insulation Magazine   23 ( 3 ) 39 - 40  2007.06  [Refereed]

    DOI

  • Effect of crystallinity on electrical conduction characteristics of Poly(L-lactic acid)

    Shingo Omori, Makoto Matsushita, Fukutaro Kato, Yoshimichi Ohki

    JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS BRIEF COMMUNICATIONS & REVIEW PAPERS   46 ( 6A ) 3501 - 3503  2007.06  [Refereed]

     View Summary

    The crystallinity of poly(L-lactic acid) was changed by thermal annealing and its effects on electrical conduction characteristics were studied. Low-crystallinity samples have a higher conductivity and a higher remaining charge quantity than high-crystallinity samples. The sample shows two thermally stimulated current (TSC) peaks at around T-g and about 20 degrees C higher than T-g when its crystallinity is low, while it shows only the former TSC peak when its crystallinity is high. The TSC peak at around T-g is attributable to the micro-Brownian motion that enables the orientation of dipoles in a direction perpendicular to the polymer main chain, whereas the high-temperature peak is attributable to the orientation and magnitude change of the end-to-end vector in the polymer main chain.

    DOI

  • Effect of endothermic reaction associated with glass transition on the breakdown strength of biodegradable polymer films

    F. Kato, M. Matsushita, S. Omori, Y. Ohki

    IEEE TRANSACTIONS ON DIELECTRICS AND ELECTRICAL INSULATION   14 ( 3 ) 653 - 655  2007.06  [Refereed]

     View Summary

    Effects of endothermic reaction associated with glass transition on the impulse, dc, and ac breakdown strengths of poly-L-lactic acid (PLLA) and polyethylene terephthalate succinate (PETS) were studied experimentally. Impulse breakdown strength at first decreases monotonically with an increase in temperature for both PLLA and PETS. However, it increases and shows a hump when the temperature approaches a certain temperature that is slightly lower than the glass transition temperature. Then it decreases again. Moreover, it is shown that the degree of increment in impulse breakdown strength at this hump depends upon the crystallinity in PLLA. In the case of dc and ac breakdown, a similar local maximum as seen in impulse breakdown is not observed. Two possibilities, namely energy absorption and structural change both due to glass transition are discussed to explain the experimental results.

    DOI

  • Investigation Committee on "Evaluation Technology for Interaction Between Radiation and Polymeric Materials" (News from Japan)

    Y. Ohki

    IEEE Electrical Insulation Magazine   23 ( 2 ) 40 - 41  2007.05  [Refereed]

    DOI

  • Nanoscale pore fabrication for high sensitivity waveguide-mode biosensors

    Makoto Fujimaki, Carsten Rockstuhl, Xiaomin Wang, Koichi Awazu, Junji Tominaga, Takahiro Ikeda, Yoshimichi Ohki, Tetsuro Komatsubara

    MICROELECTRONIC ENGINEERING   84 ( 5-8 ) 1685 - 1689  2007.05  [Refereed]

     View Summary

    Swift heavy ion irradiation of dielectric materials forms latent tracks and selective etching of them results in the formation of well defined nanopores in the material. Irradiation of Au ions accelerated with 137 MeV followed by etching with vapor of hydrofluoric acid successfully formed nanopores with diameter of 30 nm and depth of 400 nm in a waveguide made of amorphous SiO2. In the present research, the nanopores were applied to improve sensitivity of an evanescent-field-coupled waveguide mode sensor that detects adsorption and/or adhesion of molecules at the surface of the waveguide and in the nanopores. Increase in the sensitivity was confirmed by examining the shift in the angular position necessary to excite the waveguide mode before and after adsorption of streptavidin to biotinyl groups. (c) 2007 Elsevier B.V. All rights reserved.

    DOI

  • Micro-photoluminescence study on defects induced by ion microbeam in silica glass

    Masato Murai, Hiroyuki Nishikawa, Tomoharu Nakamura, Hirohiko Aiba, Yoshimichi Ohki, Masakazu Oikawa, Takahiro Sato, Tomihiro Kamiya

    JOURNAL OF NON-CRYSTALLINE SOLIDS   353 ( 5-7 ) 537 - 541  2007.04  [Refereed]

     View Summary

    We have evaluated the irradiation effects by ion microbeam on silica glass for various ion species by means of a micro-photoluminescence technique. Defect generation and refractive index change were observed for silica at the area of 10 mu m x 50 mu m scanned by ion microbeam of H+, He+, N4+ C4+, O4+, and Si5+ With energy from 1.7 to 18 MeV. The mu-PL spectroscopy measurements were performed along the side surface perpendicular to the microbeam irradiated surface. Based on the comparison with a result of SRIM (stopping and range of ions in matter) simulation, the defect generation mechanism was discussed in terms of the energy deposition processes due to electronic and nuclear stopping powers. We conclude that the electronic stopping power is responsible for the defect generation at the track of ions. The effect of the nuclear stopping power is also not negligibly small at the end of range. (c) 2007 Elsevier B.V. All rights reserved.

    DOI

  • High sensitivity sensors made of perforated waveguides

    Koichi Awazu, Carsten Rockstuhl, Makoto Fujimaki, Nobuko Fukuda, Junji Tominaga, Tetsuro Komatsubara, Takahiro Ikeda, Yoshimichi Ohki

    OPTICS EXPRESS   15 ( 5 ) 2592 - 2597  2007.03  [Refereed]

     View Summary

    Sensors based on surface plasmons or waveguide modes are at the focus of interest for applications in biological or environmental chemistry. Waveguide-mode spectra of 1 mu m-thick pure and perforated silica films comprising isolated nanometric holes with great aspect ratio were measured before and after adhesion of streptavidin at concentrations of 500 nM. The shift of the angular position for guided modes was nine times higher in perforated films than in bulk films. Capturing of streptavidin in the nanoholes is at the origin of that largely enhanced shift in the angular position as the amplitude of the guided mode in the waveguide perfectly overlaps with the perturbation caused by the molecules. Hence, the device allows for strongly confined modes and their strong perturbation to enable ultra-sensitive sensor applications. (c) 2007 Optical Society of America.

    DOI

  • Space charge distributions in glass fibre/epoxy resin composites under dc 10kV mm(-1) electric field

    Hidesato Tanaka, Yoshimichi Ohki, Kaori Fukunaga, Takashi Maeno, Kenji Okamoto

    JOURNAL OF PHYSICS D-APPLIED PHYSICS   40 ( 5 ) 1489 - 1496  2007.03  [Refereed]

     View Summary

    In this paper, the authors discuss one- and three-dimensional space charge distributions in glass fibre/epoxy resin composites. By the conventional pulsed electroacoustic (PEA) method, only a one- dimensional distribution of the average charge over a whole area parallel to the two electrodes can be observed. Therefore, the authors have developed a new PEA system capable of measuring a three-dimensional space charge distribution. Using this system, they measured the charge distribution in glass fibre/epoxy resin composites made of lattice-woven glass fibre and epoxy resin. It has become clear that spatial variation in signal intensity observed depends on the internal structure of the composite. There appear repetitious positions where a high charge density is observed on the same lateral cross section along the vertical direction in the composite. Such positions are consistent with the intersections of the glass fibres. Accumulation of mobile charge carriers or appearance of polarization charge due to mismatch of the ratio of the conductivity and permittivity between the glass fibre and the epoxy resin is thought to be responsible for the PEA signals.

    DOI

  • Comparison of Partial Discharge Resistance among Several Biodegradable Polymers

    Shinjiro Fujita, Kazuhiro Miyata, Toshikatsu Tanaka, Yoshimichi Ohki

    2007 IEEE INTERNATIONAL CONFERENCE ON SOLID DIELECTRICS, VOLS 1 AND 2   14 ( 6 ) 556 - +  2007  [Refereed]

     View Summary

    The partial discharge (PD) resistance of three kinds of biodegradable polymers; poly-L-lactic acid (PLLA), polybutylene succinate (PBS), and poly E caprolactone butylene succinate (PCL-BS), was studied in comparison to that of low density polyethylene (LDPE). The polymers were exposed to PDs under the IEC (b) electrode system, and the PD resistance was judged by the surface erosion depth or time to breakdown induced by PDs. As a result, the 'practical' PD resistance, which is the resistance to PDs under a constant voltage application, is ranked as LDPE > PLLA >> PBS > PCL-BS, while the 'true' PD resistance, which is the resistance to PDs with a constant charge quantity, is ranked as PLLA congruent to LDPE > PBS >>PCL-BS.

    DOI

  • プリント配線板用絶縁材料の空間電荷挙動に対する湿度と温度の影響

    越後易恒, 田中秀郷, 福永香, 岡本健次, 前野恭, 大木義路

    エレクトロニクス実装学会誌   10 ( 2 ) 148 - 151  2007  [Refereed]

    DOI

  • いくつかの生分解性高分子の部分放電劣化

    布施則一, 藤田慎二郎, 平井直志, 田中祀捷, 小迫雅裕, 向當政典, 岡部成光, 大木義路

    電気学会論文誌A   127 ( 8 ) 459 - 466  2007  [Refereed]

    DOI

  • 生分解性高分子の誘電特性におよぼす紫外光照射の影響

    山口雄也, 内堀直, 大木義路

    電気学会論文誌A   127 ( 2 ) 115 - 120  2007  [Refereed]

    DOI

  • Role of the voltage zero-crossing in the growth of water trees - Effect of superposition method of a high-frequency voltage and a low-frequency voltage

    Y. Ohki, H. Ishikawa, G. Morita, T. Konishi, Y. Nakamichi, M. Tanimoto

    Proceedings of 2008 International Conference on Condition Monitoring and Diagnosis, CMD 2008     328 - 331  2007  [Refereed]

     View Summary

    The authors have examined the effects of superposition of a low-frequency ac voltage and a high-frequency ac voltage on the growth of water trees in polyethylene, and have made it clear that the number of voltage zero-crossings is a decisive factor of water tree growth. In the present research, a short-duration high-frequency (2 kHz) voltage was superposed repeatedly onto a continuously applied low-frequency (0.5, 1.0, 3.0 or 5.0 Hz) voltage in synchronous with the low-frequency voltage either at its positive and negative maxima or at each zero-crossing point. By applying such superposed voltages to a low-density polyethylene sheet with a water needle electrode and a counter plane electrode, the length of a grown water tree was evaluated. As a result, it has become clear that the water tree length is longer when the high-frequency voltage is superposed at each zero-crossing point of the low-frequency voltage. This result supports the above-mentioned assumption that the number of voltage zero-crossings plays a significant role in the growth of water trees. ©2007 IEEE.

    DOI

  • Aiming at a more rigorous understanding in electrical insulating materials research

    Yoshimichi Ohki

    Annual Report - Conference on Electrical Insulation and Dielectric Phenomena, CEIDP     1 - 12  2007  [Refereed]

     View Summary

    In order to fully understand a certain property of a material or to clarify the mechanism of a phenomenon, research must be done with scientific rigor and a scholarly approach. The first three pioneers who introduced the rigorous scholarly approach into the field of electrical insulation research in Japan are Profs. Inuishi, Ieda, and Yahagi. Researchers and students have been inspired by their work and have been encouraged by this rich heritage by following in their footsteps. On this occasion of being awarded the Whitehead Memorial Lecture Award, the author is honored to introduce several examples of research done in his laboratory at Waseda University. ©2007 IEEE.

    DOI

  • Effects of Humidity and Temperature on Space Charge Distribution Profiles in Printed Circuit Board Insulations

    Yasutsune Echigo, Hidesato Tanaka, Yoshimichi Ohki, Kaori Fukunaga, Takashi Maeno, Kenji Okamoto

    2007 IEEE INTERNATIONAL CONFERENCE ON SOLID DIELECTRICS, VOLS 1 AND 2     458 - +  2007  [Refereed]

     View Summary

    The properties of bulk insulation in printed circuit boards (PCBs) have become even more important, especially for those with a multilayered or embedded structure. In particular, the spatial distribution of internal charge carriers, mainly due to ionic impurities, is thought to affect the reliability of bulk insulation. Therefore, the effects of humidity and temperature on space charge distribution profiles in a five-layered composite of aramid paper and epoxy resin are studied in this paper. More charge carriers are induced at higher temperatures with humidity. A relative humidity of 55% is high enough to induce a saturated amount of charge carriers in the present samples at 40 degrees C.

  • Effects of curing and filler dispersion methods on dielectric properties of epoxy nanocomposites

    N. Tagami, M. Okada, N. Hirai, T. Tanaka, Y. Ohki, T. Imai, M. Harada, M. Ochi

    2007 ANNUAL REPORT CONFERENCE ON ELECTRICAL INSULATION AND DIELECTRIC PHENOMENA     232 - +  2007  [Refereed]

     View Summary

    Effects of the differences in the curing agent and filler dispersion method on the dielectric properties were examined for epoxy/clay nanocomposites. Measurements of permittivity, conductivity, and space charge distribution suggest that the curing agent gives a strong effect and the presence of nanofillers affects secondly, while the filler dispersion method gives a weak effect.

  • Electrical conduction properties of several biodegradable polymers

    Naoshi Hirai, Hiroto Ishikawa, Yoshimichi Ohki

    2007 ANNUAL REPORT CONFERENCE ON ELECTRICAL INSULATION AND DIELECTRIC PHENOMENA     592 - 595  2007  [Refereed]

     View Summary

    Electrical conduction properties of five biodegradable polymers, polycaprolactone, polybutylene succinate adipate, polybutylene succinate, polybutylene adipate terephthalate, and poly-L-lactic acid, are discussed in relation to their measured values of glass transition temperature, complex permittivity, and breakdown strength. Conductivity is high in the polymers with low glass transition temperatures, and is in good proportion to the dielectric loss factor of each sample. Breakdown strength is low in such polymers. These results are reasonable, since molecular motion is active, and dipolar orientation and carrier transport are activated, when the polymer is in the rubber state.

  • Dielectric Properties of Polybutylene Succinate and Polybutylene Succinate Adipate

    Hiroto Ishikawa, Shingo Omori, Yoshimichi Ohki

    2007 IEEE INTERNATIONAL CONFERENCE ON SOLID DIELECTRICS, VOLS 1 AND 2     635 - 638  2007  [Refereed]

     View Summary

    Differential scanning calorimetry, direct current, thermally stimulated polarization current, and complex permittivity were measured for inflation formed polybutylene succinate, tubular formed polybutylene succinate and tubular formed polybutylene succinate adipate, which are available on the market. Polybutylene succinate formed by the inflation method has the highest conductivity at all the temperatures. Furthermore, only this polymer shows a thermally stimulated polarization current peak around 50 degrees C, which is probably due to ionic impurities. In all the samples, both the permittivity and dielectric loss factor increase significantly with a decrease in frequency. This seems attributable to the motion of ions in the samples.

  • Frequency Accelerated Partial Discharge Resistance of Epoxy/Clay Nanocomposite Prepared by Newly Developed Organic Modification and Solubilization Methods

    Toshikatsu Tanaka, Tatsuya Yazawa, Yoshimichi Ohki, Mitsukazu Ochi, Miyuki Harada, Takahiro Imai

    2007 IEEE INTERNATIONAL CONFERENCE ON SOLID DIELECTRICS, VOLS 1 AND 2     337 - +  2007  [Refereed]

     View Summary

    Frequency accelerated partial discharge degradation of epoxy nanocomposite with 5 wt % additions of clay (layered silicate) was evaluated by a rod-to-plane electrode system, and investigated in comparison with that of epoxy without clay. Comparisons were made as to erosion depth using a laser surface profilometer. It was found that the change in the erosion depth is far smaller in specimens with clay than those without clay. It was also found that the solubilization method using acid anhydride curing agent seems to give PD resistance characteristics superior to the organic modification method using amine and acid anhydride curing agents and even to the solubilization method using amine curing agent.

  • Need for condition monitoring and diagnosis of electric wires and cables used in nuclear power plants

    Yoshimichi Ohki, Naoshi Hirai, Toshio Yamamoto

    PROCEEDINGS OF 2008 INTERNATIONAL CONFERENCE ON CONDITION MONITORING AND DIAGNOSIS     1207 - +  2007  [Refereed]

     View Summary

    Electric cables in nuclear power plants have to function even after serious events such as a loss-of-coolant accident (LOCA). In most cables, organic polymers that are very sensitive to radiation are used. Nevertheless, much attention was not paid until recently. Under these circumstances, the Japanese government started an extensive research project to examine the degradation behavior of safety-related cables at three different temperatures with three different dose rates. An interim report published recently revealed that some cables would be unable to function after the LOCA test. In this paper, a brief outline of the interim report is described. Categorization of all the causes of malfunctions occurred in electric apparatus in all the Japanese nuclear power plants for recent about 40 years, revealed that there has been only one case of a slight malfunction of cable caused by degradation of polymer insulation, indicating that we don't have to worry so much. It is still very important to establish a proper condition monitoring method in order to secure much higher reliability.

  • Japan AE power systems develops high-voltage, vacuum circuit breakers

    Y. Ohki

    IEEE ELECTRICAL INSULATION MAGAZINE   23 ( 1 ) 48 - 49  2007.01  [Refereed]

    DOI

  • Tokyo Electric Power develops a new compact and flexible optical fiber current sensor (News from Japan)

    Y. Ohki

    IEEE Electrical Insulation Magazine   22 ( 6 ) 46 - 47  2006.12  [Refereed]

    DOI

  • IEE Japan Executive Members Elected (News from Japan)

    Y. Ohki

    IEEE Electrical Insulation Magazine   22 ( 5 ) 46 - 47  2006.10  [Refereed]

    DOI

  • Dielectric properties of BaTiO3 thin films prepared by laser ablation

    Junya Fujita, Keigo Suzuki, Nobuyuki Wada, Yukio Sakabe, Kazuo Takeuchi, Yoshimichi Ohki

    JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS BRIEF COMMUNICATIONS & REVIEW PAPERS   45 ( 10A ) 7806 - 7812  2006.10  [Refereed]

     View Summary

    To study the minimum particle size for a ferroelectric material at which ferroelectricity is retained, thin films composed of stoichiometric BaTiO3 nanoparticles were prepared by laser ablation of a Ba-Ti-O ceramic target using a differential mobility analyzer. Highly densified films of various uniform grain sizes were obtained after post-annealing at 600, 700, 800, and 900 degrees C in an oxygen atmosphere. The dielectric constants of the post-annealed films varied from 100 to 300, depending on the grain size from 65 to 136 nm. These values are comparable to those of BaTiO3 nano-granular films fabricated by metallorganic decomposition or the sol-crystal method. From the capacitance vs bias-voltage relationship and its hysteresis observed for the post-annealed films, it became clear that a phase transformation from paraelectric to ferroelectric occurs at a grain size of approximately 76 nm. It was also clarified from piezoresponse images of the post-annealed films obtained using a scanning probe microscope that ferroelectricity emerges in BaTiO3 nano-size grains at a grain size of approximately 55 nm.

    DOI

  • Core/shell ZrTiO4/LiAlSi2O6 nanocrystals: A synchrotron X-ray diffraction study of high-pressure compression

    Kristina E. Lipinska-Kalita, Michael Pravica, Gino Mariotto, Patricia E. Kalita, Yoshimichi Ohki

    JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS   67 ( 9-10 ) 2072 - 2076  2006.09  [Refereed]

     View Summary

    In situ synchrotron X-ray diffraction studies have been performed on an optically transparent lithium-aluminosilicate glass-ceramic composite with nanometer-sized LiAlSi2O6 crystals embedded in a host matrix. The pressure-induced evolution of X-ray diffraction patterns was followed in compression up to 50 GPa and in subsequent decompression to ambient conditions. In the low-pressure range, the diffraction patterns illustrated a progressive shift and broadening of the diffraction lines consistent with a gradual densification of the LiAlSi2O6 phase. The unit cell volume of the LiAlSi2O6 nanocrystalline phase calculated for the compression sequence between ambient pressure and 12.5 GPa decreased by about 13.5%. At higher pressures, the diffraction patterns displayed considerable line broadenings indicating a partial amorphization of the nanocrystalline phase. Additionally, the patterns revealed the increasing presence of the ZrTiO4 phase which was nucleated in the host matrix prior to the crystallization of the LiAlSi2O6 main nanocrystalline phase. The diffraction pattern of the composite quenched from 50 GPa to ambient pressure conditions did not show full reversibility of pressure-induced changes. Despite the dominating presence of the broad diffraction bands, the diffraction pattern of the pressure-quenched material suggested that the decompressed structure carries at least a partial signature of the initial ambient LiAlSi2O6 phase. (c) 2006 Elsevier Ltd. All rights reserved.

    DOI

  • Recent Progress in Superconducting Materials in Japan (III) (News from Japan)

    Y. Ohki

    IEEE Electrical Insulation Magazine   22 ( 4 ) 59 - 60  2006.08  [Refereed]

    DOI

  • Structure of latent tracks in rutile single crystal of titanium dioxide induced by swift heavy ions

    Koichi Awazu, Xiaomin Wang, Makoto Fujimaki, Tetsuo Komatsubara, Takahiro Ikeda, Yoshimichi Ohki

    JOURNAL OF APPLIED PHYSICS   100 ( 4 ) 44308(1)-44308(5) - 44308(5)  2006.08  [Refereed]

     View Summary

    The structurally damaged zone in titanium dioxide rutile single crystal induced by MeV-order heavy ions was observed using high resolution electronic microscopy (HREM). Stressed regions as well as amorphous regions were identified in the damaged areas. Both stressed and amorphous regions were etched with hydrofluoric acid. The thermal spike model was used to calculate the track radii variation versus electron stopping power. When the calculated lattice temperature did not exceed the melting point of rutile titanium dioxide (2130 K), no structural change introduced by ions, such as 90 MeV Cl, was observed by HREM. It was found that the radius of the lattice temperature over the melting point corresponded closely to the radius of the stressed region. It was concluded that both stressed and amorphous regions are the result of quenching by molten titanium dioxide. (c) 2006 American Institute of Physics.

    DOI

  • Improvement in electrical properties of hafnium and zirconium silicates by postnitriding

    T Ito, H Kato, T Nango, Y Ohki

    JOURNAL OF PHYSICS-CONDENSED MATTER   18 ( 26 ) 6009 - 6016  2006.07  [Refereed]

     View Summary

    Hafnium and zirconium silicate films were deposited on a silicon substrate and the effects of postannealing on their electrical properties were investigated. When the films are postannealed in nitrogen monoxide (NO), the leakage current becomes lower by more than one order of magnitude as compared with that of the as-deposited films. The capacitance-voltage (C-V) hysteresis width is also decreased drastically by the NO postannealing. From electron spin resonance spectroscopy, it is indicated that paramagnetic defects at the interface between the film and the substrate are responsible for the leakage current and the C-V hysteresis. It is also indicated by x-ray photoelectron spectroscopy that the postnitridation effectively terminates these interface defects and contributes to the improvement in electrical properties.

    DOI

  • Recent Progress in Superconducting Materials in Japan (II) (News from Japan)

    Y. Ohki

    IEEE Electrical Insulation Magazine   22 ( 3 ) 42 - 43  2006.06  [Refereed]

    DOI

  • Mechanisms of several photoluminescence bands in hafnium and zirconium silicates induced by ultraviolet photons

    T Ito, H Kato, Y Ohki

    JOURNAL OF APPLIED PHYSICS   99 ( 9 ) 94106(1)-94106(9) - 94106(9)  2006.05  [Refereed]

     View Summary

    Two photoluminescence (PL) components with peaks around 2.8-3.0 and 3.8 eV were induced in hafnium silicates by the irradiation of synchrotron radiation photons at 8.0 eV, while two similar ones were induced in zirconium silicates around 2.7-3.0 and 3.8 eV. By examining PL excitation spectra, PL decay characteristics, and vacuum-ultraviolet absorption spectra, it is assumed that the origin of the PL component around 2.7(2.8)-3.0 eV is the same as that of the PL component around 2.7-2.9 eV observed in hafnia and zirconia. In the band gaps of hafnium silicates, zirconium silicates, hafnia, and zirconia, luminescent centers responsible for the PL components around 2.7(2.8)-2.9(3.0) eV have their respective upper and lower states with a certain constant energy difference that does not change by the hafnium or zirconium content. Electrons (or holes) excited by ultraviolet photons to tail states at the band edges first relax to the upper state of the luminescent centers, and then they are deexcited to the lower state, which induces the PL components. (C) 2006 American Institute of Physics.

    DOI

  • Spectroscopic properties of Cr3+ ions in nanocrystalline glass-ceramic composites

    KE Lipinska-Kalita, PE Kalita, DM Krol, RJ Hemley, CL Gobin, Y Ohki

    JOURNAL OF NON-CRYSTALLINE SOLIDS   352 ( 6-7 ) 524 - 527  2006.05  [Refereed]

     View Summary

    Luminescence properties of Cr3+ ions in a silica-based precursor glass, and in fabricated optically transparent glass-based nanocrystalline composites, have been investigated. The luminescence spectra of the precursor glass revealed a wide range of crystal fields and showed the T-4(2)-(4)A(2), broadband emission of Cr3+ ions in a weak crystal field, combined with E-2-(4)A(2), emission characteristic for Cr3+ ions in a strong crystal field. Glass-ceramic nanocomposites, with gallium oxide nanocrystals nucleated in a host glass matrix, indicated the prevailing contribution of the crystal-like E-2-(4)A(2), emissions (R-lines) of Cr3+ ions in a strong crystal field. The low-temperature studies demonstrated that the fluorescence of Cr3+ ions could be altered from sharp R-lines of the E-2-(4)A(2), transition, below 70 K, to a combination of R-lines and their sidebands, above 70 K. Our results indicate that, in the developed glass-ceramic nanocomposites, most of the Cr3+ ions have migrated from the host glass matrix to the nucleated gallium oxide nanocrystalline phase. (c) 2006 Elsevier B.V. Ail rights reserved.

    DOI

  • Study Meeting on Polymer Nanocomposites Held in Tokyo (News from Japan)

    Y. Ohki

    IEEE Electrical Insulation Magazine   22 ( 1 ) 48 - 49  2006.04  [Refereed]

    DOI

  • Recent Progress in Applications of High Tc Superconducting Materials in Japan (News from Japan)

    Y. Ohki

    IEEE Electrical Insulation Magazine   22 ( 2 ) 45 - 46  2006.04  [Refereed]

    DOI

  • Enhanced crystallization of strontium bismuth tantalate thin films by irradiation of elongated pulses of KrF Excimer laser

    M Tsubuku, KS Seol, IH Choi, Y Ohki

    JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS BRIEF COMMUNICATIONS & REVIEW PAPERS   45 ( 3A ) 1689 - 1693  2006.03  [Refereed]

     View Summary

    The pulse width of a KrF excimer laser was elongated from 40 to 60 ns by Coupling dually Split laser beams with different optical paths. By the irradiation of such elongated laser Pulses Up to 10(3) times, strontium bismuth tantalate thin films can be crystallized efficiently to perovskite at a Substrate temperature of 650 degrees C, while the irradiation of nonelongated pulses fails to crystallize the films. The crystallized films have high remanent polarization and coercive field values. The presence of the energy threshold and the critical thickness for crystallization to perovskite is suggested.

    DOI

  • Preparation and various characteristics of epoxy/alumina nanocomposites

    Masahiro Kozako, Yoshimichi Ohki, Masanori Kohtoh, Shigemitsu Okabe, Toshikatsu Tanaka

    IEEJ Transactions on Fundamentals and Materials   126 ( 11 ) 1121 - 1127  2006  [Refereed]

     View Summary

    Epoxy/ alumina nanocomposites were newly prepared by dispersing 3, 5, 7, and 10 weight (wt) % boehmite alumina nanofillers in a bisphenol-A epoxy resin using a special two-stage direct mixing method. It was confirmed by scanning electron microscopy imaging that the nanofillers were homogeneously dispersed in the epoxy matrix. Dielectric, mechanical, and thermal properties were investigated. It was elucidated that nanofillers affects various characteristics of epoxy resins, when they are nanostructrued. Such nano-effects we obtained are summarized as follows. Partial discharge resistance increases as the filler content increases
    e.g. 7 wt% nanofiller content creates a 60 % decrease in depth of PD-caused erosion. Weibull analysis shows that short-time electrical treeing breakdown time is prolonged to 265 % by 5 wt% addition of nanofillers. But there was more data scatter in nanocomposites than in pure epoxy. Permittivity tends to increase from 3.7 to 4.0 by 5 wt% nanofiller addition as opposed to what was newly found in the recent past. Glass transition temperature remains unchanged as 109°C. Mechanical properties such as flexural strength and flexural modulus increase
    e.g. flexural strength and flexural modulus are improved by 5 % and 8 % with 5 wt% content, respectively. Excess addition causes a reverse effect. It is concluded from permittivity and glass transition temperature characteristics that interfacial bonding seems to be more or less weak in the nanocomposite specimens prepared this time, even though mechanical strengths increase. There is a possibility that the nanocomposites specimens will be improved in interfacial quality.

    DOI

  • Treeing phenomena in epoxy/alumina nanocomposite and interpretation by a multi-core model

    Toshikatsu Tanaka, Akira Matsunawa, Yoshimichi Ohki, Masahiro Kozako, Masanori Kohtoh, Shigemitsu Okabe

    IEEJ Transactions on Fundamentals and Materials   126 ( 11 ) 1128 - 1135  2006  [Refereed]

     View Summary

    Epoxy/alumina nanocomposites were prepared in laboratory to be shaped into specimens suitable for treeing experiments. It was elucidated that treeing breakdown strength would increase, if epoxy resins were nanostructured with nano-fillers such as nano boehmite alumina fillers with their loading from 5 to 10 wt%. Tree initiation time is prolonged, as the filler content increases. Tree morphology is different between base resin and its nanocomposite specimens. A new crossover phenomenon is recognized, i.e. thin channels extend more quickly to reach the opposite electrode in nanocomposites than in pure epoxy above ac 20 kV (1000 kV/mm in the tip of the needle), while they show slower growth in nanocomposites than in pure epoxy below ac 15 kV (750 kV/mm). It should be noted that the reach of such thin channels to the grounds does not mean breakdown, but such formed trees grow in width at later stage to cause final breakdown. Treeing phenomena obtained are analyzed and interpreted by a multi-core model to cover various findings of treeing phenomena in nanocomposites.

    DOI

  • Filler-content dependence of dielectric properties of low-density polyethylene/MgO nanocomposites

    Toshiaki Kikuma, Norikazu Fuse, Toshikatsu Tanaka, Yoshinao Murata, Yoshimichi Ohki

    IEEJ Transactions on Fundamentals and Materials   126 ( 11 ) 1072 - 1077  2006  [Refereed]

     View Summary

    This paper reports measurement results of instrumental analyses, permittivity, conduction current, and space charge distribution profiles observed in low-density polyethylene/MgO nanocomposites with various filler contents, done with the intention to clarify the filler-content dependence of dielectric properties. The permittivity shows the lowest value in the sample with 1-phr fillers and then increases monotonically with an increase in filler content. The conductivity measured under dc electric field lowers by the filler addition up to 5 phr at temperatures from 5 to 90 °C. In all the samples, positive homocharge is formed in the vicinity of the anode, and its amount is largest at 1 phr. The most adequate filler content for the electrical insulation purpose is around 1 to 5 phr.

    DOI

  • Effect of water treatment temperature on space charge profiles in printed circuit board insulations

    Hidesato Tanaka, Kaori Fukunaga, Takashi Maeno, Kenji Okamoto, Yoshimichi Ohki

    IEEJ Transactions on Fundamentals and Materials   126 ( 7 ) 709 - 715  2006  [Refereed]

     View Summary

    The reliability of bulk insulation is particularly important in multilayered or embedded printed circuit boards (PCBs). Moreover, PCBs are mounted in quite a lot of devices, which are often used at high temperatures and in high humidity. We observed internal space charge behavior in two typical types of epoxy composites for PCBs, namely those with aramid and with glass, under dc voltage to investigate the effect of water temperature. It was observed that the space charge profiles showed spatial oscillation with 3 or 5 iterative units in agreement with the number of composite layers in samples. In the case of the aramid/epoxy composite, homocharge is formed near the two electrodes in the sample at 40 °C, while hetrocharge is accumulated at 85 °C. The ion chromatography analysis showed that the contents of several kinds of cations were much higher especially in the aramid/epoxy composite treated in water at 85 °C than in the one treated at 40 °C. This indicates that the water treatment enhances dissociation of ions. In the case of the glass/epoxy composite, homocharge is formed near the electrodes in the sample under dc voltage if it was treated in water at 85 °C. Water seems to enhance the charge accumulation in both samples.

    DOI

  • プラズマ化学気相堆積により成膜したhigh-k膜のX線光電子分光

    及川圭太, 伊藤俊秀, 高瀬雅之, 大木義路, 加藤宙光

    放電研究   49 ( 1 ) 39 - 42  2006  [Refereed]

  • 高分解能三次元空間電荷分布測定装置

    前田和哉, 大木義路, 西方敦博, 前野恭

    電気学会論文誌A   126 ( 3 ) 185 - 190  2006  [Refereed]

    DOI

  • Spatial distribution of irradiation effects on silica glass induced by 15-MeV oxygen ion microbeam

    H Nishikawa, K Fukagawa, T Nakamura, Y Ohki, M Oikawa, T Kamiya, K Arakawa

    NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS   242 ( 1-2 ) 437 - 440  2006.01  [Refereed]

     View Summary

    High-purity silica glass was irradiated by a focused 15-MeV O4+ microbeam with diameter of 1 mu m up to a fluence of 1.0 x 10(14) ions/cm(2). Spatial distribution of irradiation effects by the O4+ microbeam on silica glass was investigated by optical microscopy, microphotoluminescence (PL)/Raman spectroscopy and atomic force microscopy (AFM). Distribution of refractive index change and defect formation was visualized by optical microscopy and PL mapping, indicating the structural changes of silica glass along the ion track up to the depth of 10 mu m. In addition, we observed deformed side surface with a groove by AFM along the track suggesting the internal compaction in silica glass. This is accompanied by increased threefold rings Of SiO2 network detected by Raman scattering. We also discuss technological implications of these results on the applications of microbeam irradiation effects to the fabrication of microoptical elements. (c) 2005 Elsevier B.V. All rights reserved.

    DOI

  • Dielectric properties of low-density polyethylene/MgO nanocomposites

    Toshiaki Kikuma, Norikazu Fuse, Toshikatsu Tanaka, Yoshinao Murata, Yoshimichi Ohki

    ICPASM 2005: PROCEEDINGS OF THE 8TH INTERNATIONAL CONFERENCE ON PROPERTIES AND APPLICATIONS OF DIELECTRIC MATERIALS, VOLS 1 AND 2     323 - +  2006  [Refereed]

     View Summary

    Polymer nanocomposite has been attracting much attention as a new insulating material, since homogeneous dispersion of only few nm-sized inorganic fillers can improve various properties significantly. In the present article, we report measurement results of permittivity, conduction current, and space charge distribution profiles observed in low-density polyethylene/MgO nanocomposites with various filler contents. The permittivity shows the lowest value at 1-phr addition of fillers and then increases monotonically with an increase in the filler content. The volume resistivity measured under a dc electric field becomes highest at 5 phr in the temperature range between 278 and 363 K. There is a possibility that molecular motions, which help electronic or ionic charge carriers escape from traps by hopping, are restricted by the addition of a proper quantity of nano-fillers. In all the samples, homocharge is found to be formed before the anode.

  • Effect of crystallinity on electrical conduction characteristics of poly-L-lactic acid

    F. Kato, S. Omori, M. Matsushita, Y. Ohki

    2006 ANNUAL REPORT CONFERENCE ON ELECTRICAL INSULATION AND DIELECTRIC PHENOMENA     15 - 18  2006  [Refereed]

     View Summary

    Effect of crystallinity change on electrical conduction characteristics of poly-L-lactic acid was studied experimentally. The crystallinity was changed by thermal annealing. The lower crystallinity samples have more remaining charge than the higher crystallinity samples. Moreover, at 80 degrees C that is above their glass transition temperatures (T-g : 55 - 60 degrees C), the conductivity is much higher in the lower crystallinity sample than in the higher crystallinity sample. The low crystallinity sample shows two thermally stimulated current (TSC) peaks-one around Tg and the other at a temperature about 20 T higher than Tg, while the higher crystallinity sample shows only the lower-temperature TSC peak. The TSC peak around T. is attributable to the micro-Brownian motion that enables orientation of dipoles having the direction perpendicular to the polymer main chains, while the higher-temperature TSC peak seems attributable to the orientation and magnitude change of the end-to-end vector in the polymer main chain.

  • <bold>Effect of Endothermic Reaction Associated with Glass Transition on the Breakdown Strength of Biodegradable Polymer Films</bold>

    F. Kato, M. Matsushita, S. Omori, Y. Ohki

    2006 ANNUAL REPORT CONFERENCE ON ELECTRICAL INSULATION AND DIELECTRIC PHENOMENA     241 - 244  2006  [Refereed]

     View Summary

    Effect of endothermic reaction associated with glass transition on the dc and impulse breakdown strength of poly-L-lactic acid (PLLA) and polyethylene tereplithalate succinate (PETS) was studied experimentally. Impulse breakdown strength decreases monotonically with an increase in temperature for both PLLA and PETS. However, it increases and shows a hump when the temperature approaches a certain temperature that is slightly lower than the glass transition temperature. Then it decreases again. Moreover, the degree of increment in impulse breakdown strength at this hump depends on the crystallinity of PLLA. In the case of dc breakdown, a similar local maximum as seen in impulse breakdown is not observed. Three possibilities, namely energy absorption and structural change both due to glass transition, and space charge formation, are discussed to explain the experimental results.

  • Comparison of dielectric properties among various poly(lactide) films

    S. Omori, M. Matsushita, Y. Ohki

    ICPASM 2005: PROCEEDINGS OF THE 8TH INTERNATIONAL CONFERENCE ON PROPERTIES AND APPLICATIONS OF DIELECTRIC MATERIALS, VOLS 1 AND 2     936 - +  2006  [Refereed]

     View Summary

    Poly(lactide) (PLA) synthesized from starch is attracting much attention, since it is biodegradable and is able to attain a good mechanical property by a proper process such as stretching. It is known that the two PLA isomers, PLLA and PDLA, strongly interact with each other. Therefore, if the two isomers are blended, they form racemic crystals (stereocomplex). Since little is known about the effect of forming the stereocomplex on the dielectric properties, permittivity and thermally stimulated polarization current are compared among two kinds of PLLA films and a stereocomplex film. As a result, both the permittivity and current were found to become larger with a decrease in the crystallinity, irrespective of the crystal structure of the sample.

  • Three-dimensional space charge distribution in glass fiber/epoxy composites

    Hidesato Tanaka, Kaori Fukunaga, Takashi Maeno, Yoshimichi Ohki

    ICPASM 2005: PROCEEDINGS OF THE 8TH INTERNATIONAL CONFERENCE ON PROPERTIES AND APPLICATIONS OF DIELECTRIC MATERIALS, VOLS 1 AND 2     69 - +  2006  [Refereed]

     View Summary

    By the conventional pulsed electroacoustic (PEA) method, only one-dimensional distribution of the average charge over a whole area parallel to the two electrodes can be observed. Therefore, we have developed a new PEA system capable of measuring three-dimensional space charge distribution. Using this system, we measured the charge distribution in glass/epoxy composites made of lattice woven glass fibers and epoxy resin. It has become clear that the space charge accumulation depends on the internal structure of the composite. There appear repetitious positions where a high charge density is observed on the same lateral cross section along the vertical direction in the composite. Such positions agree with the intersections of the glass fibers. These results clearly show that mismatch of the ratio of the conductivity and permittivity between the two composite materials is responsible for the observed space charge.

  • Dielectric properties of biodegradable polymers

    Y. Ohki, N. Hirai

    2006 ANNUAL REPORT CONFERENCE ON ELECTRICAL INSULATION AND DIELECTRIC PHENOMENA     668 - 671  2006  [Refereed]

     View Summary

    In order to respond to soaring public concern about environmental protection, various biodegradable polymers have been developed. The present paper reports the electrical conduction and breakdown properties of various biodegradable polymers such as poly-L-lactic acid (PLLA), polyethylene terephthalate succinate (PETS), polycaprolactone butylene succinate (PCL-BS), polybutylene succinate (PBS), polybutylene succinate adipate (PBSA), and polyhydroxybutyrate/valerate (PHB/V) in comparison to those of low-density polyethylene (LDPE).

  • PD resistance evaluation of LDPE/MgO nanocomposite by a rod-to-plane electrode system

    T. Tanaka, A. Nose, Y. Ohki, Y. Murata

    ICPASM 2005: PROCEEDINGS OF THE 8TH INTERNATIONAL CONFERENCE ON PROPERTIES AND APPLICATIONS OF DIELECTRIC MATERIALS, VOLS 1 AND 2     319 - +  2006  [Refereed]

     View Summary

    Low density polyethylene (LDPE) was melt-compounded with MgO nano-fillers to produce LDPE/MgO nanocomposite. Its partial discharge (PD) resistance was evaluated by using a rod-to-plane electrode system. Erosion depth was measured as an index of PD resistance. Permittivity and tan 6 were also measured to investigate interfacial morphological change due to nanostructuration. It was found that LDPE exhibits the increase in its partial PD resistance, if it is filled with the nanofillers by 1 to 10 phr (parts per hundred parts of resins). It should be noted that marked improvement appears in their content even as small as 1 phr. This tendency also appears for the permittivity of the same nanocomposite, i.e. the permittivity seems to show a minimum point, when nanofillers are added by 1 phr. PD resistance and dielectric properties of LDPE/MgO nanocomposite are discussed in terms of a multi-core model.

  • Photonic crystals of titanium dioxide fabricated by swift heavy ions

    K Awazu, M Fujimaki, Y Ohki, T Komatsubara

    RADIATION MEASUREMENTS   40 ( 2-6 ) 722 - 729  2005.11  [Refereed]

     View Summary

    We have developed a method of fabricating nano-micro-structures in a rutile TiO2 single crystal using swift heavy-ion irradiation that takes advantage of the good etching selectivity induced by ion irradiation. The areas into which ions heavier than Cl and accelerated with MeV-order high energy had been irradiated were readily etched by 20% hydrofluoric acid. By comparison, etching was not observed in pristine TiO2 single crystals. We discovered that the irradiated area could be etched to a depth at which the electronic stopping power of the ion decayed to a value of 6.2 keV/nm. In other words, etching was not observed in TiO2 single crystals possessing electron stopping power below a threshold value of 6.2 keV/nm. We also found that the value of the electronic stopping power first increased, and then decreased with depth in TiO2 single crystals when irradiated with, for example, 84.5 MeV Ca ion. Using this type of beam, the inside of the TiO2 single crystal was selectively etched with 20% hydrofluoric acid, while the top surface of the TiO2 single crystal subjected to irradiation was not etched. It initially appeared that an air gap was created in the region 4-8 mu m from the top surface subjected to irradiation by 84.5 MeV Ca ion at an accumulated dosage of 3 x 10(14) cm(-2) followed by etching. The roughness of the new surface created in the single crystal was within 7 nm as measured by atomic force microscopy. X-ray diffraction and high-resolution electron microscope analyses indicated that the irradiated area was composed of amorphous and stressed rutile phases. Both phases were highly soluble in 20% hydrofluoric acid. (c) 2005 Elsevier Ltd. All rights reserved.

    DOI

  • X-ray-assisted location of partial discharge defects in high-voltage power module packages

    Y Ohki

    IEEE ELECTRICAL INSULATION MAGAZINE   21 ( 6 ) 38 - 40  2005.11  [Refereed]

    DOI

  • Conference on spacecraft charging technology (News from Japan)

    Y. Ohki

    IEEE Electrical Insulation Magazine   21 ( 5 ) 46 - 47  2005.10  [Refereed]

    DOI

  • Synthesis of size- and structure- controlled Ge2Sb2Te5 nanoparticles

    HS Choi, KS Seol, K Takeuchi, J Fujita, Y Ohki

    JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS BRIEF COMMUNICATIONS & REVIEW PAPERS   44 ( 10 ) 7720 - 7722  2005.10  [Refereed]

     View Summary

    Pulsed laser ablation of Ge2Sb2Te5 target materials at approximately 400 Pa of ambient argon gas produces amorphous nanoparticles with a size distribution of from 4 to 30 nm. Thermal treatment of the nanoparticles in their aerosol states crystallizes the particles to both a hexagonal structure and a face-centered cubic structure at 300 degrees C, while only a face-centered cubic structure results at 400 degrees C. The crystallized nanoparticles were then size-classified by a differential mobility analyzer to produce size- and structure-controlled Ge2Sb2Te5 nanoparticles. The particles are revealed to consist of germanium, antimony and tellurium by composition analysis using energy dispersive X-ray spectroscopy.

    DOI

  • Properties of polyethylene blend tested as a non-cross-linked insulating material for power cable

    H Tamura, H Ono, M Ikeda, Y Yamamoto, Y Ohki

    ELECTRICAL ENGINEERING IN JAPAN   153 ( 1 ) 1 - 8  2005.10  [Refereed]

     View Summary

    From viewpoints of environmental protection and reduction in manufacturing Costs, it is desirable to use non-cross-linked polyethylene as an insulating material for power cables. A new type of polymerized polyethylene Using a single-site catalyst has all advantage of its high inching point, but its processability is very poor because of its high viscosity. Therefore, blends of this polyethylene and conventional low-density polyethylene with different blending ratios are examined in this research. We found that the blend with a blending ratio of 1:1 has an appropriate melt tension and adequate viscosity at a shear rate Suitable for cable manufacturing. Furthermore, the blend has a superior thermal property and a high dielectric strength for both DC and impulse voltages. (c) 2005 Wiley Periodicals, Inc.

    DOI

  • Temperature effects on luminescence properties of Cr3+ ions in alkali gallium silicate nanostructured media

    KE Lipinska-Kalita, DM Krol, RJ Hemley, PE Kalita, CL Gobin, Y Ohki

    JOURNAL OF APPLIED PHYSICS   98 ( 5 ) 54302(1)-54302(6) - 54302(6)  2005.09  [Refereed]

     View Summary

    We have investigated the optical properties of Cr3+ ions in an alkali gallium silicate glass system and in two glass-based nanocomposites with nucleated beta-Ga2O3 nanocrystals. The nucleation and growth of the nanocrystalline phase in the host glass matrix were monitored by Raman scattering spectroscopy and angle-dispersive x-ray diffraction. A broadband luminescence, associated with the T-4(2)-(4)A(2) transition from the weak crystal field of octahedral Cr3+ sites, dominated the emission of the precursor as-quenched glass. The luminescence spectra of the synthesized glass-ceramic nanocomposites revealed a crystal-like E-2-(4)A(2) strong emission and indicated that the major fraction of Cr3+ ions was located within the nanocrystalline environment. The variable-temperature studies of the nanocomposites demonstrated that the fluorescence of Cr3+ ions can be transformed from sharp R lines of the E-2-(4)A(2) transition to a combination of R lines and of the broad band of the T-4(2)-(4)A(2) transition. We propose a simple distribution model where the major part of Cr3+ ions is located in the nanocrystalline phase of the glass-ceramic composites in the octahedral environment, substituting the gallium atoms in the beta-Ga2O3 crystal structure. The developed nanocrystalline glass-ceramics are a promising class of Cr3+-doped oxide glass-based optically active composite materials.(c) 2005 American Institute of Physics.

    DOI

  • Synthesis and characterization of metal-dielectric composites with copper nanoparticles embedded in a glass matrix: A multitechnique approach

    KE Lipinska-Kalita, DM Krol, RJ Hemley, G Mariotto, PE Kalita, Y Ohki

    JOURNAL OF APPLIED PHYSICS   98 ( 5 ) 54301(1)-54301(6) - 54301(6)  2005.09  [Refereed]

     View Summary

    The precipitation and growth of copper nanoparticles in an optically transparent aluminosilicate glass matrix was investigated. The size of particles in this heterophase glass-based composite was modified in a controlled manner by isothermal heat treatments. A multi-technique approach, consisting of Raman scattering spectroscopy, high-resolution transmission electron microscopy, x-ray diffraction technique, and optical absorption spectroscopy, has been used to study the nucleation and crystallization processes. Optical absorption spectroscopy revealed the presence of intense absorption bands attributed to oscillations of free electrons, known as the surface-plasmon resonance band of copper particles, and confirmed a gradual increase of the particles' mean size and density with annealing time. The Raman scattering on acoustical phonons from Cu quantum dots in the glass matrix measured for off-resonance conditions demonstrated the presence of intense, inhomogeneously broadened peaks that have been assigned to the confined acoustic eigenmodes of copper nanoparticles. The particle-size dependence of the acoustic peak energies and the relation between the size distribution and bandwidths of these peaks were derived. High-resolution transmission electron microscopy was used to monitor the nucleation of the nanoparticles and to estimate their mean size. (c) 2005 American Institute of Physics.

    DOI

  • Effects of high pressure on stability of the nanocrystalline LiAlSi2O6 phase of a glass-ceramic composite: A synchrotron X-ray diffraction study

    KE Lipinska-Kalita, G Mariotto, PE Kalita, Y Ohki

    PHYSICA B-CONDENSED MATTER   365 ( 1-4 ) 155 - 162  2005.08  [Refereed]

     View Summary

    Synchrotron X-ray diffraction studies, under pressures up to 50 GPa, have been performed on a lithium-aluminosilicate glass-ceramic composite with nanometer-sized LiAlSi2O6 crystals embedded in a host matrix. The pressure-induced evolution of X-ray diffraction patterns was followed in a diamond anvil cell on compression and decompression cycles with the aim of probing the effect of high-pressure compression on the nanocomposite structure. On the compression cycle from ambient pressure up to 20 GPa the unit cell volume of the LiAlSi2O6 phase decreased by about 22%. The diffraction patterns also revealed the presence, at high pressures, of the ZrTiO4 phase that was nucleated in the matrix prior to the crystallization of the main LiAlSi2O6 phase. After quenching from 50 GPa to close to ambient conditions the diffraction pattern indicated that the high-pressure phase was retained to some extent although the decompressed structure still carried the signature of the initial ambient LiAlSi2O6 phase. A Birch-Murnaghan fit of the unit cell volume as a function of pressure yielded a zero pressure bulk modulus K-0 71 +/- 2 GPa and its pressure derivative K-0' = 4.4 +/- 0.6 GPa for the nanocrystalline phase. (c) 2005 Elsevier B.V. All rights reserved.

    DOI

  • Proposal of a multi-core model for polymer nanocomposite dielectrics

    T Tanaka, M Kozako, N Fuse, Y Ohki

    IEEE TRANSACTIONS ON DIELECTRICS AND ELECTRICAL INSULATION   12 ( 4 ) 669 - 681  2005.08  [Refereed]

     View Summary

    A multi-core model, i.e. a simplified term of a multi-layered core model, is proposed as a working hypothesis to understand various properties and phenomena that polymer nanocomposites exhibit as dielectrics and electrical insulation. It gives fine structures to what are called "interaction zones". An interfacial layer of several tens nm is multi-layered, which consists of a bonded layer, a bound layer, and a loose layer. In addition, the Gouy-Chapman diffuse layer with the Debye shielding length of several tens to 100 nm is superimposed in the interfacial layer to cause a far-field effect. Nano-particles may interact electrically with the nearest neighbors each other due to this effect, resulting in possible collaborative effect. Such a multi-core model with the far-field effect is discussed, for example, to explain partial discharge (PD) resistance of polyamide layered silicate nanocomposites, and is verified to demonstrate its effectiveness.

    DOI

  • Breakdown strength at the interface between epoxy resin and silicone rubber - A basic study for the development of all solid insulation

    T Takahashi, T Okamoto, Y Ohki, K Shibata

    IEEE TRANSACTIONS ON DIELECTRICS AND ELECTRICAL INSULATION   12 ( 4 ) 719 - 724  2005.08  [Refereed]

     View Summary

    As a key component for a future power substation system entirely consisting of solid insulating materials, a special connection system between different pieces of power equipment has to be developed. A soft dielectric such as silicone rubber and a hard one such as epoxy resin are to be used for this connection. This paper describes the interfacial breakdown strength between silicone rubber and epoxy resin using two types of model samples on which the electric field can be applied in parallel or perpendicularly to their interface. Partial discharge characteristics at the interface and the effect of air penetration on the breakdown voltage along the interface are discussed.

    DOI

  • Tokyo electric power operates a historical museum (News from Japan)

    Y. Ohki

    IEEE Electrical Insulation Magazine   21 ( 4 ) 52 - 53  2005.08  [Refereed]

    DOI

  • 3D Nanofabrication of Rutile TiO2 Single Crystals with Swift Heavy-Ions

    Koichi Awazu, Ken-ichi Nomura, Makoto Fujimaki, Yoshimichi Ohki

    The Nano-Micro Interface: Bridging the Micro and Nano Worlds     207 - 223  2005.07  [Refereed]

    DOI

  • Study on dielectric properties of LDPE-based nanocomposites by J-power systems (News form Japan)

    Y. Ohki

    IEEE Electrical Insulation Magazine   21 ( 3 ) 55 - 56  2005.06  [Refereed]

    DOI

  • Fabrication of two- and three-dimensional photonic crystals of titania with submicrometer resolution by deep x-ray lithography

    K Awazu, XM Wang, M Fujimaki, T Kuriyama, A Sai, Y Ohki, H Imai

    JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B   23 ( 3 ) 934 - 939  2005.05  [Refereed]

     View Summary

    Two-dimensional photonic crystals of titanium dioxide are predicted to have many advantages over semiconductor photonic crystals, e.g., silicon and GaAs: in particular, low optical loss in the near infrared region used for optical communication, low thermal expansion, and a refractive index which is close to that of optical fibers. However, it is difficult to create micronanostructures in titanium dioxide, since semiconductor microfabrication techniques cannot be applied to titanium dioxide. As the first step, we calculated the photonic band gap of titanium dioxide rod slab on SiO2. Band gap percent against thickness of the rod slab was also examined. Finally, we confirmed the most suitable structure for two-dimensional (213) photonic crystals. A deep x-ray lithography technique was employed to create a very deep and precise template. Liquid-phase deposition was then used to faithfully deposit a tightly packed layer of titanium oxide onto the template. Finally, the template was selectively removed to obtain a photonic nanostructure. We also calculated the photonic band gap for the 3D structure of TiO2. A template for the most appropriate structure was fabricated using the method proposed by Yablonovitch. By employing the same method, we successfully obtained the 3D structure of TiO2. The refractive index of the obtained TiO2 followed by heating at 700 degrees C was determined as being 2.5, which is close to that of the anatase phase. (c) 2005 American Vacuum Society.

    DOI

  • Similarities in photoluminescence in hafnia and zirconia induced by ultraviolet photons

    T Ito, M Maeda, K Nakamura, H Kato, Y Ohki

    JOURNAL OF APPLIED PHYSICS   97 ( 5 ) 54104(1)-54104(7) - 54104(7)  2005.03  [Refereed]

     View Summary

    Photoluminescence (PL) spectra induced by ultraviolet photons were measured for amorphous hafnia and zirconia deposited by plasma-enhanced chemical-vapor deposition (PECVD), amorphous hafnia deposited by pulse laser deposition, and crystalline yttria-stabilized zirconia. Two kinds of samples were prepared for both hafnia and zirconia deposited by PECVD using different source alkoxides in different deposition chambers. A PL peak was observed around 2.8 eV similarly in all hafnia and zirconia samples, irrespective of the difference in crystallinity, oxygen deficiency, source alkoxide, deposition method, or the substrate material. The decay profile of this PL is also similar in all the samples. These facts clearly show that neither impurities, oxygen vacancy, nor defects at the interface between the sample and the substrate are responsible for the PL. It is a luminescence inherent in hafnia and zirconia and is most likely due to radiative recombination between localized states at the band tails. When the samples were annealed in oxygen, a new PL peak appeared around 4.2 eV in all the amorphous samples. Its decay profile is also in common with these samples. Vacuum-ultraviolet absorption measurements and PL excitation measurements indicate that the 4.2-eV PL is excited due to the interband absorption. (C) 2005 American Institute of Physics.

    DOI

  • Symposium held in Tokyo (News from Japan)

    Y. Ohki

    IEEE Electrical Insulation Magazine   21 ( 1 ) 57 - 58  2005.02  [Refereed]

    DOI

  • Effects of crosslinking byproducts on space charge formation in crosslinked polyethylene

    Y Maeno, N Hirai, Y Ohki, T Tanaka, M Okashita, T Maeno

    IEEE TRANSACTIONS ON DIELECTRICS AND ELECTRICAL INSULATION   12 ( 1 ) 90 - 97  2005.02  [Refereed]

     View Summary

    Space charge is formed in cables insulated with crosslinked polyethylene. It has not been clear whether the crosslinking byproducts or the crosslinked polymer morphology is responsible for the space charge formation. In order to clarify this point additive-free non-crosslinked low-density polyethylene, additive-free crosslinked polyethylene, and degassed crosslinked polyethylene were soaked in the crosslinking byproducts and the space charge distribution was measured after dc voltage application. Samples tested are divided into two categories. The first category is a soaked single-layered sheet and the second category is a two-layered specimen consisting of a soaked sheet and a non-soaked sheet. As a result, the following conclusions were obtained. (1) Cumyl alcohol is responsible for homo-charge layers in front of the electrodes in both low-density and crosslinked polyethylene sheets. (2) Acetophenone is responsible for hetero-charge formation in crosslinked polyethylene, presumably as a synergistic effect with water. (3) alpha-methylstyrene has no effect on space charge formation in low-density polyethylene, while it assists charge trapping in crosslinked polyethylene. (4) Charge trapping occurs easier in degassed crosslinked polyethylene than in low-density polyethylene, probably because of carbonyl groups induced by crosslinking.

    DOI

  • Electrical Engineering Education in Waseda University

    Y. Ohki

    The Proceedings of The Korean Institute of Electrical Engineering   54 ( 10 ) 48 - 51  2005  [Invited]

  • Effect of Glass Transition on Electrical Conduction Characteristics of Poly-L-lactic Acid

    Yasumasa Maeno, Yuya Yamaguchi, Naoshi Hirai, Yoshimichi Ohki, Toshikatsu Tanaka, Yoshiro Tajitsu, Masanori Kohtoh, Shigemitsu Okabe

    IEEJ Transactions on Fundamentals and Materials   125 ( 3 ) 254 - 260  2005  [Refereed]

     View Summary

    The temperature dependence of electrical conduction current in additive-free poly-L-lactic acid was measured from 20°C to 90°C under application of dc 2.5 kV. Although the current obeys the Arrhenius formula, a hump appears in some conditions, depending on whether the temperature is being ascended or descended, and also on whether the sample was poled or short-circuited prior to the measurements. Even under short-circuit condition, a similar hump alone appears if the sample was poled by a dc voltage beforehand. From these results, it is assumed that orientational polarization or depolarization occurs around 74°C by the glass transition, and that its resultant current and the conduction current overlap with each other. The fact that polar groups, mainly carbonyl groups, in the sample really changed their directions was confirmed by Fourier-transform infrared spectroscopy. The permittivity also increases around 74°C. These facts indicate that the number of dipoles, which become possible to rotate toward the direction of electric field, is increased by the glass transition. © 2005, The Institute of Electrical Engineers of Japan. All rights reserved.

    DOI

  • Voltage Zero-crossing as a Factor Inducing Water Trees

    Tomoya Maeda, Daisaku Kaneko, Yoshimichi Ohki, Takeshi Konishi, Yoshinobu Nakamichi, Minoru Okashita

    The transactions of the Institute of Electrical Engineers of Japan.A   125 ( 1 ) 51 - 56  2005  [Refereed]

     View Summary

    We have been examining the effects of superposition of a high-frequency voltage to various voltages such as dc, low-frequency (0.1 to 5 Hz), and power-frequency voltages on the development of water trees in polyethylene. We have made clear that the number of voltage zero-crossings is a decisive factor in the length of water trees. In the present research, the water tree shapes grown under the various superposed voltages were carefully observed. As a result, the water tree tends to become a hand-like shape if the frequency of the lower-frequency component is between 0 and 0.5 Hz, while it becomes spherical if the frequency is higher than 1.0 Hz. This result is explained by assuming that the water tree shape is governed by the number of consecutive voltage zero-crossings. By combining the results reported in our former papers, it can be concluded that the voltage zero-crossing is a decisive factor for the formation of water trees. Frequent mechanical oscillation at the tree tip due to the Maxwell stress should play a significant role. © 2005, The Institute of Electrical Engineers of Japan. All rights reserved.

    DOI

  • Development of a Sub-micron Processing Method with Ion Implantation

    Seung Jun Yu, Keita Kawabe, Hideaki Ohkubo, Yoshimichi Ohki, Makoto Fujimaki, Makoto Fujimaki, Masaharu Hattori, Mikiko Saito, Yasuo Wada

    The transactions of the Institute of Electrical Engineers of Japan.A   125 ( 1 ) 69 - 70  2005  [Refereed]

     View Summary

    We have developed a precise sub-µm processing method by combining ion implantation and nano-plating. In order to show the applicability of this method, we have fabricated a mask for a Bragg grating, which has a striped gold structure with a height of 1.2 µm and a period of 1.59 µm on a 2-µm thick SiN membrane. We have also confirmed that the mask pattern can be transferred inside a silica glass by ion implantation. It is expected that this result can be utilized for the fabrication of optical communication devices. © 2005, The Institute of Electrical Engineers of Japan. All rights reserved.

    DOI

  • In situ space charge measurement of PCB insulations during the ageing test

    K. Fukunaga, H. Tanaka, Y. Ohki, K. Okamoto, T. Maeno

    Annual Report - Conference on Electrical Insulation and Dielectric Phenomena, CEIDP   2005   585 - 588  2005  [Refereed]

     View Summary

    Influence of water absorption on internal space charge behaviour of an aramid/epoxy composite was firstly discussed based on the experiments using specimens after water absorption pre-treatment. More hetero charges were observed in the specimen treated at higher temperature. Chemical analyses suggested that the increase of hetero charges was due to the increase of ion impurities. Since insulating materials for printed circuit boards are generally tested at high temperature and high humidity, we have developed an in-situ space charge measurement system in an environmental chamber using the pulsed electroacoustic method. Hetero charges observed in the aramid/epoxy resin increased significantly at 40°C and 90 % RH. This insitu space charge measurement should contribute to understand the ionic migration phenomena as well as to evaluate the test conditions of printed circuit boards. © 2005 IEEE.

    DOI

  • Two and Three-dimensional photonic crystals of titanium dioxide and optical properties

    Koichi Awazu, Xiaomin Wang, Makoto Fujimaki, Hiroaki Imai, Taketo Kuriyama, Yoshimichi Ohki

    Proceedings of SPIE - The International Society for Optical Engineering   5931   1 - 8  2005  [Refereed]

     View Summary

    Two-dimensional photonic crystals of titanium dioxide are predicted to have many advantages over semiconductor photonic crystals, e.g., silicon and GaAs: in particular, low optical loss in the near infrared region used for optical communication, low thermal expansion, and a refractive index which is close to that of optical fibers. However, it is difficult to create micro-nano structures in titanium dioxide, since semiconductor micro-fabrication techniques cannot be applied to titanium dioxide. As the first step, we calculated the photonic band gap of titanium dioxide rod-slab on SiO2. Band gap percent against thickness of the rod-slab was also examined. Finally, we confirmed the most suitable structure for 2D photonic crystals. A deep X-ray lithography technique was employed to create a very deep and precise template. Liquid-phase deposition was then used to faithfully deposit a tightly packed layer of titanium oxide onto the template. Finally, the template was selectively removed to obtain a photonic nano-structure. A template for the most appropriate three dimensional structure was also fabricated using the method proposed by Yablonovitch. By employing the same method, we successfully obtained the 3D structure of TiO 2.

    DOI

  • Thermally stimulated polarization and depolarization currents in polyethylene terephthalate succinate

    Y Ohki, Y Maeno, T Tanaka, M Kohtoh, S Okabe

    2005 Annual Report Conference on Electrical Insulation and Dielectric Phenomena     503 - 506  2005  [Refereed]

     View Summary

    When the temperature dependence of conduction current in biodegradable polyethylene tereplithalate succinate (PETS) was measured, a hump appeared around 330 K under some conditions depending on whether the temperature was being ascended or descended and on whether the sample had been poled or short-circuited prior to the measurements. It seems that orientational polarization occurs by the glass transition and that its resultant current overlaps with the conduction current. Moreover, another current far larger than the polarization or depolarization current flows in PETS above 353 K, probably due to relaxation of polarized ionic species.

  • Dipolar polarization and depolarization currents in biodegradable polymers

    Y Maeno, H Kino, S Omori, N Hirai, T Tanaka, Y Ohki, Y Tajitsu, M Kohtoh, S Okabe

    Proceedings of the 2005 International Symposium on Electrical Insulating Materials, Vols, 1-3     417 - 420  2005  [Refereed]

  • Preparation and preliminary characteristic evaluation of epoxy/alumina nanocomposites

    M Kozako, S Yamano, R Kido, Y Ohki, M Kohtoh, S Okabe, T Tanaka

    Proceedings of the 2005 International Symposium on Electrical Insulating Materials, Vols, 1-3     231 - 234  2005  [Refereed]

     View Summary

    Epoxy/alumina nanocomposites were newly prepared by dispersing 3, 5, and 7 weight % nano-scale boehmite alumina particles in a bisphenol-A epoxy resin. Dispersion of alumina particles in the nanocomposite specimen was observed by scanning electron microscopy (SEM). Flexural properties, dynamic viscoelasticity, permittivity, partial discharge (PD) resistance, and electrical breakdown time were investigated for the nanocomposite specimens in comparison with those of an epoxy resin without nanofillers. The following results were obtained. The nanocomposite specimens keep high transparency similar to the pure resin. Alumina particles with a size below about 50 rim. are homogeneously dispersed in the epoxy matrix. Due to dispersion of the nanofillers in the specimens, flexural properties, PD resistance, and electrical breakdown time are improved. The nanocomposites exhibit almost no change in glass transition temperature and permittivity.

  • Irradiation effects on silica glass by ion microbeam for fabrication of optical elements

    M Murai, K Fukagawa, H Nishikawa, T Nakamura, Y Ohki, M Oikawa, T Sato, K Arakawa

    Proceedings of the 2005 International Symposium on Electrical Insulating Materials, Vols, 1-3     210 - 213  2005  [Refereed]

     View Summary

    Irradiation of ion microbeam can induce a local refractive index change in silica glass. We have evaluated the changes in electronic structures of microbeam-irradiated silica glass by means of micro-photoluminescence and phase microscope measurements. Attention should be paid to the energy deposition processes by both electronic and nuclear stopping powers to control the depth distribution of refractive index changes. Also, a chemical effect induced by ion species such as protons at the projected range was also discussed.

  • Influence of metallic defects on the partial discharges at the interface between epoxy resin and silicone rubber

    M Onogi, Y Ohki, T Takahashi, T Takahashi, T Okamoto

    Proceedings of the 2005 International Symposium on Electrical Insulating Materials, Vols, 1-3     560 - 563  2005  [Refereed]

     View Summary

    We have proposed a concept of an all solid insulated substation system as a next generation substation from a viewpoint of environmental protection. In order to make the system compact and its layout flexible, a compact connection system with a soft insulating material and a hard one has to be developed. There is a high possibility that the dielectric strength of such a connection system is decreased when a metallic defect such as a wire exists at its interface. Thus, we prepared samples that imitate the interface of the connection system and measured partial discharge inception voltages (PDIV) in both cases that a metal wire was present or not at the interface. When we put the metal wire along the direction of electric field, the PDIV decreases significantly with an increase in the length of the wire, while it decreases a little when the wire is along the equipotential line. The presence of air at the interface also reduces the PDIV.

  • Effects of ultraviolet photon irradiation on the space charge accumulation properties of several biodegradable polymers

    Y Yamaguchi, N Hirai, T Tanaka, Y Ohki, Y Tajitsu, M Kohtoh, S Okabe

    Proceedings of the 2005 International Symposium on Electrical Insulating Materials, Vols, 1-3     421 - 424  2005  [Refereed]

     View Summary

    Five kinds of biodegradable polymers were irradiated for 30 minutes by ultraviolet photons from a KrCl excimer lamp. Space charge distribution profiles observed by the pulsed electroacoustic method were compared between the non-irradiated and irradiated samples. The absorption spectra as a function of wavelength obtained for the samples indicate intensive appearance of conjugated double bonds in the vicinity of the irradiated surface in all the biodegradable polymers. Molecular weight composition profiles observed by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry show that the samples were decomposed by the irradiation. This decomposition seems to be responsible for the higher conductivity of the irradiated samples, which in turn results in the change of space charge distribution profiles.

  • Effects of water temperature on degradation of several biodegradable polymers immersed in water

    N Hirai, T Tanaka, Y Ohki, Y Tajitsu, M Kohtoh, S Okabe

    Proceedings of the 2005 International Symposium on Electrical Insulating Materials, Vols, 1-3     771 - 774  2005  [Refereed]

     View Summary

    Four kinds of biodegradable polymers were immersed in water around 301 K and 351 K for fixed periods from 1 to 720 hours, and their degradation was studied by observing their shapes, weights, molecular weights, and space charge distributions. Samples tested were poly-L-lactic acid (PLLA), polycaprolactone butylene succinate (PCLBS), polyethylene terephthalate succinate (PETS), and polybutylene succinate (PBS). From space charge observation, it is assumed that water accelerates ionization of substances and extraction of ionizable species. The durability against water becomes much worse when the water is 351 K compared to 301 K for all the samples. For example, PLLA crumbles easily at 351 K, while it shows good durability at 301 K.

  • Effects of glass transition on the dielectric breakdown and electrical conduction in several biodegradable polymers

    M Matsushita, Y Maeno, H Kino, S Fujita, N Hirai, T Tanaka, Y Ohki

    Proceedings of the 2005 International Symposium on Electrical Insulating Materials, Vols, 1-3     413 - 416  2005  [Refereed]

     View Summary

    Results of the conductivity and breakdown strength measured for several biodegradable polymers are reported. At room temperature, poly-L-lactic acid (PLLA) and polyethylene terephthalate succinate (PETS) have relatively low conductivity values that are comparable to low density polyethylene (LDPE). However, when the fact that PLLA and PETS are in the glass state is taken into consideration, the conductivity values of PLLA and PETS should be considered to be relatively high. Moreover, PLLA and PETS show rapid increase in the conduction current around their glass transition temperatures. The other three biodegradable polymers that are in the rubber state at room temperature, namely epsilon-polycaprolactone butylene succinate (PCL-BS), polybutylene succinate (PBS), and polybutylene succinate adipate (PBSA), have much higher conductivities. Although the impulse breakdown strength is relatively similar in all the samples, PLLA and PETS have higher breakdown strengths in temperature regions around their glass transition temperatures. This apparently anomalous behavior is explainable by assuming that part of the energy supplied by the applied impulse voltage was consumed due to the glass transition. As for do or ac breakdown strength at room temperature, PLLA and PETS show a relatively higher strength than PCL-BS and PBS.

  • Tests of a 500-M High-Temperature Superconducting Cable (News from Japan)

    Y. Ohki

    IEEE Electrical Insulation Magazine   20 ( 6 ) 60 - 61  2004.12  [Refereed]

    DOI

  • Energy band profile of hafnium silicates estimated by X-ray photoelectron spectroscopy

    T Ito, H Kato, T Nango, Y Ohki

    JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS SHORT NOTES & REVIEW PAPERS   43 ( 12 ) 8199 - 8202  2004.12  [Refereed]

     View Summary

    Amorphous hafnium silicate films with several composition ratios were deposited on Si substrates by plasma-enhanced chemical vapor deposition, and their energy band profiles were studied by X-ray photoelectron spectroscopy. The band gap energy estimated from the energy loss spectrum of 0 Is electrons decreases monotonically and then approaches a constant value with an increase in hafnium content. The valence band offset and the conduction band offset were estimated using the sample sandwiched between an evaporated An electrode and the Si substrate. Although the two offsets decrease until they become almost constant with an increase in hafnium content at both silicate/Si and silicate/Au interfaces, they hold values higher than that necessary for a high-k dielectric material to maintain a good insulating performance for all the deposited silicates.

    DOI

  • Cable Aging Research for Life Extension of Nuclear Power Plant (News from Japan)

    Y. Ohki

    IEEE Electrical Insulation Magazine   20 ( 5 ) 41 - 41  2004.10  [Refereed]

    DOI

  • Surface degradation of polyamide nanocomposites caused by partial discharges using IEC(b) electrodes

    M Kozako, N Fuse, Y Ohki, T Okamoto, T Tanaka

    IEEE TRANSACTIONS ON DIELECTRICS AND ELECTRICAL INSULATION   11 ( 5 ) 833 - 839  2004.10  [Refereed]

     View Summary

    Partial discharge (PD) degradation of polyamide both without nanoscale fillers (nanofillers) and with 2, 4 and 5 wt% additions of nanorillers was investigated. Such materials were subjected to PDs using the IEC (b) electrodes for evaluation. Comparisons were made as to the surface roughness observed by scanning electron microscopy and atomic force microscopy. It was found that the change in the surface roughness is far smaller in specimens with nanorillers than those without nanofillers, and that the 2 wt% addition is sufficient for improvement of the surface roughness. Furthermore, it was elucidated that the difference of surface roughness of the degraded area due to PDs among the specimens originates from the difference in their crystalline structures. These results indicate that polyamide nanocomposite is more resistance to PDs than polyamide without nanorillers.

    DOI

  • Ferroelectricity of single-crystalline, monodisperse lead zirconate titanate nanoparticles of 9 nm in diameter

    KS Seol, K Takeuchi, Y Ohki

    APPLIED PHYSICS LETTERS   85 ( 12 ) 2325 - 2327  2004.09  [Refereed]

     View Summary

    Ferroelectric hysteresis at approximately 20 degreesC is measured in a lead zirconate titanate film prepared by stacking monodisperse and perovskite particles 9 nm in diameter. The particle-stacked film possesses a remanent polarization of 4.5 muC/cm(2) and an apparent coercive field of 250 kV/cm. This result demonstrates that particulate lead zirconate titanate of 9 nm in diameter retains ferroelectricity even at room temperature. (C) 2004 American Institute of Physics.

    DOI

  • Recent Activities of Japan Electric Cable Technology Center (News from Japan)

    Y. Ohki

    IEEE Electrical Insulation Magazine   20 ( 4 ) 41 - 42  2004.08  [Refereed]

    DOI

  • Role of number of consecutive voltage zero-crossings in propagation of water trees in polyethylene

    D Kaneko, T Maeda, T Ito, Y Ohki, T Konishi, Y Nakamichi, M Okashita

    IEEE TRANSACTIONS ON DIELECTRICS AND ELECTRICAL INSULATION   11 ( 4 ) 708 - 714  2004.08  [Refereed]

     View Summary

    Effects of superposition of a low-frequency (0.1 to 50 Hz) voltage and a high-frequency 2 kHz voltage on the growth of water trees in polyethylene are examined. It has become clear that both the number of total voltage zero-crossings during the whole voltage application period and the number of consecutive voltage zero-crossings in the instant of the polarity reversal of the low-frequency voltage play important roles in the length and shape of water trees. Namely, the water tree length becomes longer as the number of total zero-crossings increases. Furthermore, among the voltages with the same number of total zero-crossings, the length becomes even longer with a prolonged shape in the direction of electric field as the number of consecutive zero-crossings increases. Effects of superposition of a high-frequency voltage onto a dc voltage are also examined. In this case, the dc voltage seems not to have any influence to the tree growth. By assuming the presence of space charge around the water-tree tip, the effective zero-crossings become only sensitive to the high-frequency component. Therefore, this result also indicates that the number of voltage reversals at the tree tip plays a crucial role in the growth of water trees.

    DOI

  • Crystallization of monodisperse lead zirconate titanate nanoparticles produced by laser ablation

    T Katagiri, KS Seol, K Takeuchi, Y Ohki

    JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS SHORT NOTES & REVIEW PAPERS   43 ( 7A ) 4419 - 4423  2004.07  [Refereed]

     View Summary

    Monodisperse lead zirconate titanate (PZT) nanoparticles with diameters of 9 nm were produced by laser ablation followed by a gas-phase thermal treatment in combination with a size-classification technique using a very low-pressure differential mobility analyzer. The particles prior to the thermal treatment are amorphous. When the laser power density for ablation increases from 0.5 GW/cm(2) to 12 GW/cm(2), the production yield of the particles increases regardless of the laser photon energy. When the laser photon energy is 3.5 eV, the nanoparticles generated with laser power densities as high as 7 GW/cm(2) are crystallized to perovskite by thermal treatment at 900degreesC, while those generated with even higher power densities become pyrochlore. In contrast, when the laser photon energy is 2.3 eV, perovskite nanoparticles are obtained only at a power density of 0.5 GW/cm(2). These results reveal that the crystalline structure of PZT nanoparticles can be adjusted to perovskite or pyrochlore by changing thermal treatment temperature, laser power density, and photon energy. Furthermore, X-ray photoelectron spectroscopy indicates that the crystallization to pyrochlore is caused by lead deficiency.

    DOI

  • Activities of Investigation Committees, IEE Japan (News from Japan)

    Y. Ohki

    IEEE Electrical Insulation Magazine   20 ( 2 ) 48 - 48  2004.06  [Refereed]

    DOI

  • A Novel Water-Tree Detection Method for XLPE Power Cable Stepwise ac Residual Charge Method (News from Japan)

    Y. Ohki

    IEEE Electrical Insulation Magazine   20 ( 3 ) 54 - 55  2004.06  [Refereed]

    DOI

  • 固体絶縁材料研究の最近の課題と展開 (その1)本特集解説のねらいと国際会議に見る絶縁材料研究の最近の動向

    大木義路

    JIEED Japan   47 ( 2 ) 2 - 11  2004  [Invited]

  • Three-dimensional lithography for rutile TiO2 single crystals using swift heavy ions

    K Awazu, M Fujimaki, Y Ohki, T Komatsubara

    ENGINEERED POROSITY FOR MICROPHOTONICS AND PLASMONICS   797   69 - 74  2004  [Refereed]

     View Summary

    We have developed a nano-micro structure fabrication method in rutile TiO2 single crystal by use of swift heavy-ion irradiation. The area where ions heavier than Cl ion accelerated with MeV-order high energy were irradiated was well etched by hydrofluoric acid, by comparison etching was not observed in the pristine TiO2 single crystal. Noticed that the irradiated area could be etched to a depth at which the electronic stopping power of the ion decayed to a value of 6.2keV/nm. We also found that the value of the electronic stopping power was increased, eventually decreased against depth in TiO2 single crystal with, e.g. 84.5MeV Ca ion. Using such a beam, inside of TiO2 single crystal was selectively etched with 20% hydrofluoric acid, while the top surface of TiO2 single crystal subjected to irradiation was not etched. Roughness of the new surface created in the single crystal was within 7nm with the atomic forth microscopy measurement.

  • 固体絶縁材料研究の最近の課題と展開(その2)〜環境に優しい材料の模索(1)〜生分解性高分子の電気機器絶縁への適用可能性

    平井直志, 大木義路

    放電研究   47 ( 3 ) 2 - 8  2004  [Refereed]

  • Properties of Polyethylene Blend as a Non-crosslinked Insulating Material for Power Cable

    Hiroki Tamura, Hironaga Ono, Yoshimichi Ohki, Masaaki Ikeda, Yoshimi Yamamoto

    IEEJ Transactions on Fundamentals and Materials   124 ( 9 ) 817 - 822  2004  [Refereed]

     View Summary

    From viewpoints of environmental protection and reduction in manufacturing cost, it is desirable to use non-crosslinked polyethylene as an insulating material for power cables. A new type polymerized polyethylene using a single-site catalyst has an advantage of its high melting point, but its processability is very poor because of its high viscosity. Therefore, blends of this polyethylene and conventional low-density polyethylene with different blending ratios are examined in this research. As a result, it has become clear that the blend with a blending ratio of 1:1 has an appropriate melting stress and an adequate viscosity at a shear velocity suitable for cable manufacturing. Furthermore, the blend has a superior thermal property and a high dielectric strength for both dc and impulse voltages. © 2004, The Institute of Electrical Engineers of Japan. All rights reserved.

    DOI

  • 絶縁体中の電荷移動とデトラッピングに関する基礎電磁気学

    平井直志, 大木義路

    静電気学会誌   28 ( 4 ) 204 - 207  2004  [Refereed]

  • Recent Development of Different Mobility Analyzers for Size-Classification of Nanoparticles and Their Applications to Nanotechnologies

    K. S. Seol, Y. Ohki, K. Takeuchi

    KIEE International Transactions on EA   4-C ( 2 ) 39 - 44  2004  [Refereed]

  • Status Quo and Trends of Insulation Monitoring and Diagnosis Methods for Electric Power Apparatus in Japan

    Naoshi Hirai, Yoshimichi Ohki, Toshikatsu Tanaka, Takatoshi Shindo

    IEEJ Transactions on Power and Energy   124 ( 3 ) 496 - 503  2004  [Refereed]

     View Summary

    As a research aiming at obtaining basic information useful for the development of maintenance-free insulation, the status quo and trends of insulation monitoring and diagnosis methods for electric power apparatus that have been done in Japan are analyzed by reviewing about 8800 reports published for the last 50 years. It is found that the accuracy of monitoring and diagnosis has been improved well and unmanned monitoring and diagnosis have been developed significantly with the introduction of measurement systems using sophisticated computer-aided analyzing and fiber-optic monitoring techniques. Further implementation of such sophisticated methods in the whole area of insulation monitoring and diagnosis should even increase the total reliability of power apparatus, which will undoubtedly reduce the necessary personnel and costs to maintain the apparatus. © 2004, The Institute of Electrical Engineers of Japan. All rights reserved.

    DOI

  • Technology 2004: Reviews and Forecasts

    Yoshimichi Ohki

    The transactions of the Institute of Electrical Engineers of Japan.A   124 ( 1 ) 2 - 2  2004  [Invited]

     View Summary

    The Fundamentals and Materials Society of the Institute of Electrical Engineers of Japan consists of twelve Technical Committees (TC's) that cover various fields from education to materials. Aiming at introducing significant technological progress made in the year 2003 in the fields relating to each TC, the Editing Planning Committee of the Society asked each TC to write a review. This series of reviews was written in reply to this request by experts in the respective fields. © 2004, The Institute of Electrical Engineers of Japan. All rights reserved.

    DOI

  • Effects of Ultraviolet Photon Irradiation on the Dielectric Properties of Polyimide

    Akira Yamaguchi, Yoshimichi Ohki

    The transactions of the Institute of Electrical Engineers of Japan.A   124 ( 1 ) 98 - 103  2004  [Refereed]

     View Summary

    Ultraviolet photons were irradiated to the surface of all-aromatic polyimide films using a KrF excimer laser or a KrCl excimer lamp, and electrical properties such as permittivity, dielectric loss factor, conductivity, and space charge distribution were measured. In the case of the intense laser irradiation, the photo-induced change is drastic. The irradiated side of the sample is carbonized and becomes conductive. The apparent thickness of the remaining insulating region estimated by the space charge distribution measurements agrees with that observed by an optical microscope. The permittivity of the remaining apparently insulating region decreases as the irradiation proceeds, which is due to a decrease in the number of carbonyl groups. The dielectric loss factor and conductivity in this region increase since the leakage current increases even in this apparently insulating region. The activation energy of conduction also increases. In the case of the weak lamp irradiation, the permittivity decreases and the conductivity increases, although clear surface change is hardly observable. It is indicated from space charge distribution measurements that carrier traps are formed by the irradiation. © 2004, The Institute of Electrical Engineers of Japan. All rights reserved.

    DOI

  • Effects of Ion Microbeam Irradiation on Silica Glass

    H. Nishikawa, K. Fukagawa, T. Yanagi, Y. Ohki, E. Watanabe, M. Oikawa, T. Kamiya, K. Arakawa

    Trans. Materials Research Society of Japan   29 ( 2 ) 603 - 606  2004  [Refereed]

  • Difference in surface degradation due to partial discharges between polyamide nanocomposite and microcomposite

    M Kozako, R Kido, N Fuse, Y Ohki, T Okamoto, T Tanaka

    2004 ANNUAL REPORT CONFERENCE ON ELECTRICAL INSULATION AND DIELECTRIC PHENOMENA     398 - 401  2004  [Refereed]

     View Summary

    Partial discharge (PD) degradation was investigated to compare polyamide nanocomposites with two kinds of polyamide microcomposites. Such materials were exposed to Pds under the IEC(b) electrode configuration for evaluation of PD resistance. Comparisons were made as to the surface roughness using a scanning electron microscope, an atomic force microscope, and a mechanical surface profilometer. It is concluded that polyamide nanocomposite is more resistant to PDs than microcomposites, and that nanoeffects would work against PD degradation, which include filler-matrix bonding, inter-filler space, morphology, and mesoscopic interaction.

  • Effect of the applied voltage frequency on the water tree shape in polyethylene

    T Maeda, D Kaneko, Y Ohki, T Konishi, Y Nakamichi, M Okashita

    PROCEEDINGS OF THE 2004 IEEE INTERNATIONAL CONFERENCE ON SOLID DIELECTRICS, VOLS 1 AND 2     276 - 279  2004  [Refereed]

     View Summary

    We have been examining the effects of superposition of a high-frequency voltage to various voltages such as dc, low-frequency (0.1 to 5 Hz), and power-frequency voltages on the development of water trees in polyethylene. We have made clear that the number of voltage zero-crossings is a decisive factor in the length of water trees. In the present research, the water tree shapes grown under the various superposed voltages were carefully observed. As a result, we have confirmed that the water tree tends to become a hand-like shape if the frequency of the lower-frequency component is between 0 and 0.5 Hz, and become spherical if the frequency is higher than 1.0 Hz. This result is explained by assuming that the growth of water trees is governed by the effective number of consecutive voltage zero-crossings at the tip of the water tree.

  • Effect of glass transition on conduction current in biodegradable poly-l-lactic acid

    Y Maeno, Y Yamaguchi, N Hirai, T Tanaka, Y Ohki, Y Tajitsu, M Kohtoh, S Okabe

    2004 ANNUAL REPORT CONFERENCE ON ELECTRICAL INSULATION AND DIELECTRIC PHENOMENA     49 - 52  2004  [Refereed]

     View Summary

    The temperature dependence of electrical conduction current in additive-free poly-L-lactic acid was measured. Although the current obeys the Arrhenius formula, a hump appears under some conditions, depending on whether the temperature is being ascended or descended, and also on whether the sample was poled or short-circuited prior to the measurements. That polar groups consisting mainly of carbonyl groups in the sample changed their directions was confirmed by Fourier-transform infrared spectroscopy. The permittivity increases around 74 degreesC. From these results, it is confirmed that orientational polarization or depolarization occurs around 74 degreesC by the glass transition, and that its resultant current and the conduction current overlap with each other.

  • Possible mechanism of superior partial-discharge resistance of polyamide nanocomposites

    N Fuse, M Kozako, T Tanaka, S Murase, Y Ohki

    2004 ANNUAL REPORT CONFERENCE ON ELECTRICAL INSULATION AND DIELECTRIC PHENOMENA     322 - 325  2004  [Refereed]

     View Summary

    The dielectric properties of polyamide nanocomposites were examined, focusing especially on their resistance against partial discharges. The present paper compares the surface roughness caused by partial discharges and that caused by exposure to plasmas, between polyamide with and without inorganic nanofillers. From X-ray diffraction spectroscopy, it was confirmed that the nanofillers are rearranged in parallel with their surfaces with a mutual distance around 1 nm after the nanocomposite was subjected to partial discharges. Such ordered arrangement of nanofillers should contribute to the high durability against partial discharges in polyamide nanocomposites, in addition to the effects of the presence of mica, high crystallinity, and strong ionic interaction at mica/resin interfaces. Furthermore, the result of a preliminary attempt done to detect the structural change through observation of photoluminescence spectra is also reported.

  • Effects of Additives, Photodegradation, and Water-tree Degradation on the Photoluminescence in Polyethylene and Polypropylene

    Toshihide Ito, Norikazu Fuse, Yoshimichi Ohki

    The transactions of the Institute of Electrical Engineers of Japan.A   124 ( 7 ) 624 - 630  2004  [Refereed]

     View Summary

    Photoluminescence (PL) spectra induced by irradiation of ultraviolet photons are compared among low-density polyethylene (LDPE), crosslinked polyethylene (XLPE), and polypropylene (PP). Three PL bands appear around 4.2, 3.6, and 3.1 eV in LDPE and XLPE, while similar three PL bands are observed at similar energies in PP. The PL spectra and their decay profiles are independent of the presence of additives and are also independent of whether the samples were crosslinked or not. These results indicate that neither the additives nor the crosslinking has any significant effects on the respective three PLs in PE and PP. When the sample was pre-irradiated by the ultraviolet photons under different atmospheres (air, O2, and vacuum), all the PL intensities decrease with the progress of the pre-irradiation regardless of whether the sample is PE or PP. Therefore, all the PLs are considered to result from impurities. In all the pre-irradiated samples, a new PL band appears at 2.9 eV, of which intensity is stronger when the oxygen partial pressure during the pre-irradiation was lower. This PL is considered to be due to photo-induced conjugated double bonds. It has also been confirmed that water-tree degradation in LDPE or in XLPE does not contribute to PL. © 2004, The Institute of Electrical Engineers of Japan. All rights reserved.

    DOI

  • Fabrication of two-dimensional photonic structure of titanium dioxide with sub-micrometer resolution by deep x-ray lithography

    K Awazu, M Fujimaki, XM Wang, A Sai, Y Ohki

    NANOENGINEERED ASSEMBLIES AND ADVANCED MICRO/NANOSYSTEMS   820   295 - 300  2004  [Refereed]

     View Summary

    Two dimensional photonic crystals of titanium dioxide is expected to have many advantage compared with photonic crystals of semiconductors, e.g., silicon and GaAs. For example, low optical loss in the near infrared region used for optical communication, low thermal expansion, and its refractive index which is close to that for optical fiber are attractive advantages. However, it is difficult to create micro-nano structure in titanium dioxide because micro-fabrication technique for semiconductor is not available for titanium dioxide. As the first step we calculated photonic band gap of titanium dioxide rod-slab on SiO2. Also, band gap percent against thickness of the rod-slab was examined. Finally, we confirmed the most suitable structure of 2D photonic crystals. Deep x-ray lithography technique was employed for create a very deep and precise template of PMMA. Then, liquid-phase deposition was used to faithfully deposit a tightly packed layer of titanium oxide onto the template. Finally, the template is selectively removed to obtain a photonic nano-structure. We also calculate photonic band gap on the 3D-structure of TiO2. A template for the most appropriate structure was fabricated by the method proposed by Yablonovitch. By using of the same method, it was successful to obtain 3D structure of TiO2. Refractive index of obtained TiO2 followed by heating at 700degreesC was determined to 2.5 which is close to that for anatase phase.

  • Characterization of refractive index changes of silica glass induced by ion microbeam

    M Hattori, Y Ohki, M Fujimaki, T Souno, H Nishikawa, E Watanabe, M Oikawa, T Kamiya, K Arakawa

    NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS   210   272 - 276  2003.09  [Refereed]

     View Summary

    Distributions of structural and refractive index changes of silica irradiated by H+ microbeam were studied by optical and atomic force microscopes (AFM). The AFM measurements on the microbeam irradiated area show the formation of a groove on the surface. In addition, a cross sectional observation on the surface parallel to the incident plane reveals surface deformation along the ion tracks, which is deepest at the projected range of ions. Taking into account the possible structural changes of silica induced by energy deposition calculated by TRIM, the measured topological changes at the front and side surfaces result from internal compaction of silica glass. Refractive index changes were estimated from the Lorentz-Lorenz relationship using the distribution of the internal compaction estimated by the AFM measurements. (C) 2003 Elsevier B.V. All rights reserved.

    DOI

  • Characterization of ion-implanted silica glass by micro-photoluminescence and Raman spectroscopy

    T Souno, H Nishikawa, M Hattori, Y Ohki, E Watanabe, M Oikawa, T Kamiya, K Arakawa

    NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS   210   277 - 280  2003.09  [Refereed]

     View Summary

    We evaluated structural changes in silica glass induced by ion microbeam using microscopic photoluminescence (PL) and Raman scattering measurements. Microbeams (1.7 MeV H+) were scanned over the sharp right-edges of the silica substrates with a fluence of I x 10(17) cm(-2), then two PL bands of silica at 540 and 650 nm were observed at the irradiated region. The PL bands show different lateral and depth distributions. The distribution of the 540 nm PL is in good agreement with that of the refractive index changed region. The lateral distribution of the 650 nm band is broader by 1.5 times than those of the 540 nm PL and the refractive index changed region. The microscopic Raman scattering measurements show an increased intensity of 606 cm(-1) peak associated with compaction at the microbeam irradiated regions. (C) 2003 Elsevier B.V. All rights reserved.

    DOI

  • Structural change induced in TiO2 by swift heavy ions and its application to three-dimensional lithography

    K Nomura, T Nakanishi, Y Nagasawa, Y Ohki, K Awazu, M Fujimaki, N Kobayashi, S Ishii, K Shima

    PHYSICAL REVIEW B   68 ( 6 ) 64106 - 64106  2003.08  [Refereed]

     View Summary

    A rutile TiO2 single crystal was irradiated by heavy ions with a high energy of the order of several tens of MeV. A good etching selectivity, where only the irradiated surface is well etched by hydrofluoric acid is induced by the irradiation. Through x-ray diffraction and high-resolution electron microscopy, it became clear that the irradiated region lost crystallization. It is considered that this amorphous region and the surrounding region are dissolved in hydrofluoric acid. Through the calculation of the ion energy, it was found that the etching always stopped at the depth where the electronic stopping power of the ion decayed to a critical value of 6.2 keV/nm, regardless of the ion species in the case of I, Br, Cu, and Ti ions. However, in the case of Ca ions with energies higher than about 72 MeV or Cl ions with energies higher than about 77 MeV, the irradiated top surface was not etched with hydrofluoric acid, but the inside surface several mum deep from the irradiated surface was etched. A calculation shows that the critical factor which determines whether the irradiated surface can be etched or not is the lateral energy density on the surface deposited by ions. The etched surface observed by atomic force microscopy is very smooth with a roughness of the order of nm. Therefore, a combination of ion irradiation and etching can be used as a novel fabrication method of nanostructures in rutile.

    DOI

  • High-pressure x-ray diffraction studies of the nanostructured transparent vitroceramic medium K2O-SiO2-Ga2O3

    E. Lipinska-Kalita, B. Chen, B. Kruger, Y. Ohki, J. Murowchick, P. Gogol

    Physical Review B - Condensed Matter and Materials Physics   68 ( 11 )  2003.07  [Refereed]

     View Summary

    Synchrotron-radiation-based, energy-dispersive x-ray-diffraction studies have been performed on a composite containing nanometer-size aggregates embedded in an amorphous matrix, in the pressure range from ambient up to 15 GPa. The optically transparent material containing β-Ga2O3 nanocrystals was developed by the controlled crystallization of a silicon oxide-based amorphous precursor. Transmission electron microscopy and conventional x-ray-diffraction techniques allowed estimating the mean size of a single-crystalline phase to be 14.8±1.9 nm, distributed homogeneously in an amorphous medium. The pressure-driven evolution of x-ray-diffraction patterns indicated a progressive densification of the nanocrystalline phase. A structural modification corresponding to a pressure-induced coordination change of the gallium atoms was evidenced by the appearance of new diffraction peaks. The overall changes of x-ray-diffraction patterns indicated a β-Ga2O3 to α-Ga2O3 phase transformation. The low- to high-density phase transition was initiated at around 6 GPa and not completed in the pressure range investigated. A Birch-Murnaghan fit of the unit-cell volume change as a function of pressure yielded a zero-pressure bulk modulus, K0, for the nanocrystalline phase of 191±4.9 GPa and its pressure derivative, K0′=8.3±0.9. © 2003 The American Physical Society.

    DOI

  • High-pressure x-ray diffraction studies of the nanostructured transparent vitroceramic medium K2O-SiO2-Ga2O3

    KE Lipinska-Kalita, B Chen, MB Kruger, Y Ohki, J Murowchick, EP Gogol

    PHYSICAL REVIEW B   68 ( 3 ) 352091 - 352098  2003.07  [Refereed]

     View Summary

    Synchrotron-radiation-based, energy-dispersive x-ray-diffraction studies have been performed on a composite containing nanometer-size aggregates embedded in an amorphous matrix, in the pressure range from ambient up to 15 GPa. The optically transparent material containing beta-Ga2O3 nanocrystals was developed by the controlled crystallization of a silicon oxide-based amorphous precursor. Transmission electron microscopy and conventional x-ray-diffraction techniques allowed estimating the mean size of a single-crystalline phase to be 14.8 +/- 1.9 nm, distributed homogeneously in an amorphous medium. The pressure-driven evolution of x-ray-diffraction patterns indicated a progressive densification of the nanocrystalline phase. A structural modification corresponding to a pressure-induced coordination change of the gallium atoms was evidenced by the appearance of new diffraction peaks. The overall changes of x-ray-diffraction patterns indicated a beta-Ga2O3 to alpha-Ga2O3 phase transformation. The low- to high-density phase transition was initiated at around 6 GPa and not completed in the pressure range investigated. A Birch-Mumaghan fit of the unit-cell volume change as a function of pressure yielded a zero-pressure bulk modulus, K-0, for the nanocrystalline phase of 191 +/- 4.9 GPa and its pressure derivative, K-o' = 8.3 +/- 0.9.

    DOI

  • Vanadium redox-flow battery for various applications

    Y Ohki

    IEEE ELECTRICAL INSULATION MAGAZINE   19 ( 3 ) 38 - 39  2003.05  [Refereed]

  • Ultraviolet photon-induced absorption bands and paramagnetic centers in Ge and Sn co-doped SiO2 glass

    T Nakanishi, M Fujimaki, S Tokuhiro, K Nomura, Y Ohki, K Imamura

    JOURNAL OF NON-CRYSTALLINE SOLIDS   318 ( 1-2 ) 87 - 94  2003.04  [Refereed]

     View Summary

    Germanium and Sn co-doped SiO2 glass, a material for photosensitive optical fiber core that can be used for fabrication of optical fiber gratings, was exposed to photons from a KrF excimer laser (5.0 eV) or a XeCl lamp (4.0 eV). The photo-induced paramagnetic centers and optical properties of the glass have been examined with electron spin resonance and absorption measurements. Precursors of the paramagnetic centers are also discussed. (C) 2002 Elsevier Science B.V. All rights reserved.

    DOI

  • Chemical group in crosslinking byproducts responsible for charge trapping in polyethylene

    N Hirai, R Minami, T Tanaka, Y Ohki, M Okashita, T Maeno

    IEEE TRANSACTIONS ON DIELECTRICS AND ELECTRICAL INSULATION   10 ( 2 ) 320 - 330  2003.04  [Refereed]

     View Summary

    It is known that space charge is formed in cables insulated with crosslinked polyethylene. It is probable that the crosslinking byproduct or the crosslinked polymer morphology is responsible for the space charge. In order to examine the former possibility, an additive-free low-density polyethylene sheet was soaked in various liquid chemicals including the crosslinking byproducts and space charge distribution inside the sheet was measured under dc voltage application. As a result, space charge appears in the vicinity of the injecting electrode only in the case that the sheet was soaked in the liquids with at least one hydroxyl group in their structures. Therefore, it is concluded that cumyl alcohol, which is the only byproduct with a hydroxyl group in its structure, is responsible for the formation of space charge in polyethylene on the assumptions that chemical functions of a given molecule act separately from each other and that other contributions such as the crosslinked polymer morphology are negligible.

    DOI

  • Similarities in the electrical conduction processes in hydrogenated amorphous silicon oxynitride and silicon nitride

    H Kato, H Sato, Y Ohki, KS Seol, T Noma

    JOURNAL OF PHYSICS-CONDENSED MATTER   15 ( 13 ) 2197 - 2205  2003.04  [Refereed]

     View Summary

    Electrical conduction at high fields was examined in a series of hydrogenated amorphous silicon oxynitride and silicon nitride films with different nitrogen contents deposited by plasma-enhanced chemical vapour deposition. It was shown that the conduction is attributable to the Poole-Frenkel (PF) emission in the two materials. The energy depths of the PF sites and the dependences on the sample's chemical composition are quite similar for the two samples. It is considered that the PF sites in the two materials are identical.

    DOI

  • 電力機器の絶縁劣化診断法の変遷と現状

    平井直志, 田中たつ捷, 大木義路, 新藤孝敏, 穂積直裕, 坂田博

    電気学会全国大会講演論文集   2003 ( 2 ) 99 - 100  2003.03

    J-GLOBAL

  • Band-tail photoluminescence in hydrogenated amorphous silicon oxynitride and silicon nitride films

    H Kato, N Kashio, Y Ohki, KS Seol, T Noma

    JOURNAL OF APPLIED PHYSICS   93 ( 1 ) 239 - 244  2003.01  [Refereed]

     View Summary

    Photoluminescence (PL) measurements were performed on a series of hydrogenated amorphous silicon oxynitride and silicon nitride films with different nitrogen contents deposited by plasma-enhanced chemical-vapor deposition. From the PL and PL excitation spectra, the Urbach energy of the sample is found to be proportional to its PL half-maximum width, regardless of whether the sample is silicon oxynitride or silicon nitride. Time-resolved PL measurements showed that PL peak energy varies with time after the excitation, showing a systematic dependence on the chemical composition in the two materials. That the PLs observed in the two materials have very similar characteristics regardless of the presence of oxygen strongly indicates that the PLs result from the same chemical structure, more specifically Si-N bonds, and that the two materials have similar band-tail states associated with the static disorder. In the two materials, it is found that the electrons and holes photoexcited into such band-tail states recombine first through an excitonlike recombination process and then through a radiative tunneling recombination process. (C) 2003 American Institute of Physics.

    DOI

  • Structura defects in amorphous silicon oxynitride and silicon nitride

    H Kato, Y Ohki

    DEFECTS AND DIFFUSION IN CERAMICS: ANNUAL RETROSPECTIVE V   218   39 - 49  2003  [Refereed]

     View Summary

    Amorphous silicon oxynitride and silicon nitride have been gaining each important position as a material used in electronic and optoelectronic devices. The present paper gives a review of structural defects in these materials, focusing on their characterization by photoluminescence and electron spin resonance. Some typical effects induced in these materials by the presence of hydrogen, the irradiation of ultraviolet photons and application of a high electric field are also discussed.

  • Gas-phase production of monodisperse ferroelectric nanoparticles (< 10 nm in diameter)

    KS Seol, S Tomita, K Takeuchi, T Katagiri, T Miyagawa, Y Ohki

    NANOTECH 2003, VOL 3     94 - 97  2003  [Refereed]

     View Summary

    Laser ablative technology was used to prepare monodisperse nanoparticles (<10 nm in diameter) of lead zirconate titanate (PZT) and strontium bismuth tantalate (SBT). Laser ablation of a ceramic target in oxygen ambience produced amorphous and irregularly-shaped nanoparticles of either PZT or SBT. A subsequent on-line thermal treatment performed on the nanoparticles dispersed in the gas phase brought about compaction and crystallization of the nanoparticles without additional particle growth. It was found that the amorphous PZT nanoparticles began to crystallize above 600 degreesC, and they became a perovskite structure at 900 degreesC. In contrast, the SBT nanoparticles began to crystallize at around 800 degreesC. The crystallized nanoparticles were then size-classified by a differential mobility analyzer to yield monodisperse, highly pure, and single-crystalline nanoparticles; of PZT and SBT.

  • Role of the number of consecutive voltage zero-crossings on the water tree growth in polyethylene

    D Kaneko, T Maeda, T Ito, Y Ohki, T Konishi, Y Nakamichi, M Okashita

    PROCEEDINGS OF THE 7TH INTERNATIONAL CONFERENCE ON PROPERTIES AND APPLICATIONS OF DIELECTRIC MATERIALS, VOLS 1-3     932 - 935  2003  [Refereed]

     View Summary

    With the progress in power electronics, ac voltages with various harmonic components are often applied to insulating materials of power apparatus. Water treeing is the most-observed degradation pattern in polyethylene-insuIated power cables, and much work ha's been done. Nevertheless little is known about their dielectric properties under above-mentioned particular ac voltages. In this paper, the authors examine the influence of superposition of a low-frequency ac voltage (0.1 to 5 Hz) and a high-frequency 2-kHz ac voltage on the growth of water trees in polyethylene. It has become clear that the number of consecutive voltage zero-crossings in the instant of the polarity reversal of the low-frequency voltage plays an important role in the length and shape of water trees.,

  • Roles of cumyl alcohol and crosslinked structure in homo-charge trapping in crosslinked polyethylene

    N Hirai, Y Maeno, T Tanaka, Y Ohki, M Okashita, T Maeno

    2003 ANNUAL REPORT CONFERENCE ON ELECTRICAL INSULATION AND DIELECTRIC PHENOMENA     213 - 216  2003  [Refereed]

     View Summary

    Space charge is formed in cables insulated with crosslinked polyethylene. It has not been clear whether the crosslinking byproducts or the crosslinked polymer morphology is responsible for the space charge formation. In order to clarify this point, additive-free low-density polyethylene, crosslinked polyethylene, and degassed crosslinked polyethylene were soaked in cumyl alcohol and the space charge distribution was measured under do voltage application. Specimens tested are divided into two categories. The first category is a soaked single-layered sheet and the second category is a two-layered specimen consisting of a soaked sheet and a non-soaked sheet. As a result, the following conclusions were obtained. (1) The crosslinked structure has no effect on homo-charge layers. (2) Cumyl alcohol is responsible for traps in both cases of low-density and crosslinked polyethylene. Due to these traps, a homo-charge layer appears in front of each electrode in the single-layered sheet or on the surface of the soaked sheet facing the opposite electrode in the two-layered sample. (3) More space charge is trapped in crosslinked polyethylene than low-density polyethylene, probably because of the presence of carbonyl groups.

  • Effect of an air layer on the insulation performance at the interface between epoxy resin and silicone rubber

    K Shibata, Y Ohki, T Takahashi, T Okamoto

    PROCEEDINGS OF THE 7TH INTERNATIONAL CONFERENCE ON PROPERTIES AND APPLICATIONS OF DIELECTRIC MATERIALS, VOLS 1-3     622 - 625  2003  [Refereed]

     View Summary

    We have proposed a concept of an environment-conscious all-solid-insulalted substation system as a next generation substation. In order to make the system compact and its layout flexible, it is required to develop a compact connection system with solid insulating materials. A hard and a soft insulating material are to be used for this connection system. Therefore, there is a concern about air penetration into their interface at the time of connection, which reduces the insulation perormance. In order to simulate this situation, a model connection part was made using an epoxy resin board as the hard material and a silicone rubber-board as the soft material with a spacer inserted into their interface. The present paper reports experimental results on the breakdown strength under ac and impulse voltages. It is confirmed that the insulation performance decreases with an increase in the thickness of the air layer between the two boards.

  • Absorption changes induced by UV-photon irradiation in Ge-doped SiO2 thin films fabricated by plasma CVD method from tetraethoxysilane

    T Yanagi, Y Ohki, H Nishikawa

    PROCEEDINGS OF THE 7TH INTERNATIONAL CONFERENCE ON PROPERTIES AND APPLICATIONS OF DIELECTRIC MATERIALS, VOLS 1-3     1088 - 1091  2003  [Refereed]

     View Summary

    Optical absorption changes induced by the irradiation of ultraviolet photons in Ge-doped SiO2 thin films fabricated, by plasma-enhanced chemical vapor deposition from tetraethoxysilane are studied. It is confirmed that the sample has a higher photo-sensitivity compared with bulk Ge-doped SiO2 fabricated by conventional vapor-phase axial deposition. This is ascribed to the fact that the Ge E' center, a typical paramagnetic defect center in Ge-doped SiO2 with a large optical absorption, is induced by the photon irradiation. Namely, the high photo-sensitivity originates from large concentrations of two oxygen-deficient precursors of the Ge E' center, the germanium lone pair center and the neutral oxygen vacancy. It is suggested that residual hydrogen from tetraethoxysilane also contributes to the induced optical absorption.

  • Effect of crosslinking on space charge formation in crosslinked polyethylene

    N Hirai, Y Maeno, T Tanaka, Y Ohki, M Okashita, T Maeno

    PROCEEDINGS OF THE 7TH INTERNATIONAL CONFERENCE ON PROPERTIES AND APPLICATIONS OF DIELECTRIC MATERIALS, VOLS 1-3     917 - 920  2003  [Refereed]

     View Summary

    It is known that space charge is formed in cables insulated with crosslinked polyethylene. It is assumed that the,crosslinking byproducts or the crosslinked polymer morphology is responsible for the space charge. In order to examine these possibilities, low-density polyethylene, crosslinked polyethylene, and degassed crosslinked polyethylene were soaked, in crosslinking byproducts and their space charge distributions were measured under dc voltage application. As a result, it is concluded that cumyl alcohol is responsible for homo-charge layers in front of the electrodes in both low-density and crosslinked polyethylene and that the crosslinked structure has no effect.

  • Surface change of polyamide nanocomposite caused by partial discharges

    M Kozako, N Fuse, K Shibata, N Hirai, Y Ohki, T Okamoto, T Tanaka

    2003 ANNUAL REPORT CONFERENCE ON ELECTRICAL INSULATION AND DIELECTRIC PHENOMENA     75 - 78  2003  [Refereed]

     View Summary

    We have investigated partial discharge (PD) degradation for conventional polyamide-6 without nanoscale fillers (nanofillers) and polyamide-6 nanocomposites with 2 weight (wt) %, 4 wt% and 5 wt% addition. Such materials were subjected to partial discharge under the IEC (b) electrode configuration for evaluation. Comparisons were made as to the surface roughness observed by scanning electron microscopy and atomic force microscopy. It was found that the change in the surface roughness was far smaller in specimens with nanofillers than those without nanofillers, and that the 2 wt% addition was sufficient for improvement. This result indicates that polyamide-6 nanocomposite is more resistive to PDs than polyamide-6 without nanofillers.

  • Effect of post-nitriding on electrical properties of high-permittivity hafnium and zirconium silicate films

    H Kato, T Nango, T Ito, K Nakamura, M Maeda, Y Ohki

    PROCEEDINGS OF THE 7TH INTERNATIONAL CONFERENCE ON PROPERTIES AND APPLICATIONS OF DIELECTRIC MATERIALS, VOLS 1-3     765 - 768  2003  [Refereed]

     View Summary

    The effect of post-annealing in nitrogen monoxide (NO) on the electrical properties of hafnium and zirconium silicate films prepared by plasma enhanced chemical vapor deposition was investigated. The leakage current measured in the post-annealed film decreases by two orders of magnitude as compared to the as-deposited film. From FT-IR, ESR, and XPS analyses, it is found that the post-annealing makes the structural disorder smaller and reduces the interface states by nitridation. The C-V hysteresis width, which reflects the number of interface states, also decreases. These tendencies are observed in both the hafnium and zirconium silicate films. It is concluded that the NO post-annealing can effectively improve the electrical properties of the two films.

  • Activities of Investigation Committees, IEE Japan Committee for Electrical Insulation News in Asia (News from Japan)

    Y. Ohki, S. Yasufuku

    IEEE Electrical Insulation Magazine   19 ( 5 ) 67 - 69  2003  [Refereed]

  • Characteristics of nanoparticles formed during pulsed laser ablation of SrBi2Ta2O9

    KS Seol, K Takeuchi, T Miyagawa, Y Ohki

    JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS SHORT NOTES & REVIEW PAPERS   41 ( 9 ) 5654 - 5658  2002.09  [Refereed]

     View Summary

    Particle formation was investigated during laser ablation of strontium bismuth tantalate (SBT) with a background of pure oxygen (02). For an 0, pressure, P-O2, varying front 210 to 1300Pa, the mean particle size ranged from 9 to 22nm. and the particle concentration ranged from about 10(12) to 10(8) cm(-3). For P-O2 = 210Pa, the particle size ranged from 5 to 30 nm. and for P-O2 = 1300Pa, the particle size ranged from 10 to 80nm. Transmission electron microscopy measurements of particles size-classified at 10 and 20nm showed that some particles were spherical and others were irregularly shaped. Electron diffraction of the particles showed that the spherical particles were partially crystallized whereas the irregulary shaped particles were amorphous. The formation of these two different morphologies is attributed to different particle cooling rates stemming front spatially nonuniform gas temperatures induced by laser ablation. Regardless of their shape. the particles were composed of strontium, bismuth, tantalum, and oxygen.

    DOI

  • Gas-phase production of monodisperse lead zirconate titanate nanoparticles

    KS Seol, S Tomita, K Takeuchi, T Miyagawa, T Katagiri, Y Ohki

    APPLIED PHYSICS LETTERS   81 ( 10 ) 1893 - 1895  2002.09  [Refereed]

     View Summary

    Laser ablative technology was used to prepare monodisperse nanoparticles (4-20 nm in diameter) of lead zirconate titanate (PZT). Laser ablation of a PZT ceramic target in oxygen ambience produced amorphous and irregularly shaped PZT nanoparticles. A subsequent on-line thermal treatment performed on the PZT nanoparticles dispersed in the gas phase brought about compaction and crystallization of the nanoparticles without additional particle growth. It was found that the amorphous nanoparticles began to crystallize above 600 degreesC, and they became a perovskite structure at 900 degreesC. The crystallized nanoparticles were then size classified by a differential mobility analyzer to yield monodisperse, highly pure, and single-crystalline PZT nanoparticles. (C) 2002 American Institute of Physics.

    DOI

  • The World's first long-distance 500 kV-XLPE cable line. 2. Underground apparatus (News from Japan)

    Y. Ohki, S. Yasufuku

    IEEE Electrical Insulation Magazine   18 ( 3 ) 57 - 58  2002.08  [Refereed]

    DOI

  • Factors influencing the dielectric properties of linear low-density polyethylene

    H Ono, M Araoka, N Hirai, Y Ohki

    ELECTRICAL ENGINEERING IN JAPAN   140 ( 3 ) 1 - 7  2002.08  [Refereed]

     View Summary

    We investigated the factors that influence dielectric properties such as the breakdown strength and conduction current of linear low-density polyethylene. Films with a lower melt-flow-rate show a higher dielectric strength. Furthermore, the catalyst used to polymerize the film affects the dielectric strength, although additives in the film scarcely affect its electrical properties. The effect of crosslinking on dielectric breakdown in linear low-density polyethylene was found to be similar to that in low-density polyethylene. (C) 2002 Wiley Periodicals, Inc.

    DOI

  • Plasma-enhanced chemical vapor deposition and characterization of high-permittivity hafnium and zirconium silicate films

    H Kato, T Nango, T Miyagawa, T Katagiri, KS Seol, Y Ohki

    JOURNAL OF APPLIED PHYSICS   92 ( 2 ) 1106 - 1111  2002.07  [Refereed]

     View Summary

    Deposition of hafnium silicate films with various hafnium contents was tried by plasma-enhanced chemical vapor deposition using tetraethoxysilane and a hafnium alkoxide. From x-ray photoelectron spectroscopy, the deposited films are confirmed to be silicate with Hf-O-Si bonds but without any Hf-Si bonds. The permittivity calculated from the capacitance of the accumulation layer increases monotonically with an increase in the hafnium content, whereas the optical band gap energy estimated from vacuum ultraviolet absorption spectra decreases. Similar results were obtained from zirconium silicate films deposited using tetraethoxysilane and a zirconium alkoxide. If we compare the films with the same hafnium or zirconium content, the hafnium silicate exhibits a higher permittivity and a larger band gap energy than the zirconium silicate. (C) 2002 American Institute of Physics.

    DOI

  • Photo-induced refractive index change in hydrogenated amorphous silicon oxynitride

    H Kato, M Fujimaki, T Noma, Y Ohki

    JOURNAL OF APPLIED PHYSICS   91 ( 10 ) 6350 - 6353  2002.05  [Refereed]

     View Summary

    Refractive index change is shown to be induced by the irradiation of ultraviolet photons in hydrogenated amorphous silicon oxynitride films prepared by plasma-enhanced chemical vapor deposition. The mechanism of the index change and its dependence on the nitrogen content were investigated by electron spin resonance and scanning electron microscopy. It is concluded that the index change is due mainly to densification, and that the contribution of the formation of paramagnetic defects is only slight. To demonstrate the versatility of this refractive index change, a planar diffraction grating was fabricated. (C) 2002 American Institute of Physics.

    DOI

  • Evaluation of silica glasses implanted by high-energy ions using a UV-excited microspectroscopy

    T Yamaguchi, E Watanabe, T Souno, H Nishikawa, M Hattori, Y Ohki, T Kamiya, K Arakawa

    NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS   191   371 - 374  2002.05  [Refereed]

     View Summary

    In this study, defects induced by ion implantation were investigated by a photoluminescence (PL) microspectroscopy. When H+ or He2+ with high energy (similar toMeV) were implanted into silica, two PL bands at 290 and 650 nm were observed under 244 nm excitation. The PL bands at 290 and 650 nm were ascribed to oxygen deficient centers and nonbridging oxygen hole centers, respectively. The PL depth profile analysis shows that these PL centers are distributed throughout the tracks of ions, while they are quenched in the proximity of their projected ranges. Based on the observation on the PL profiles, the formation of the PL centers was ascribed to the energy deposition by electronic stopping power. The role of nuclear stopping power on the defect formation is also discussed. (C) 2002 Elsevier Science B.V. All rights reserved.

    DOI

  • Characterization of ion-implanted silica glass by vacuum ultraviolet absorption spectroscopy

    M Hattori, Y Nishihara, Y Ohki, M Fujimaki, T Souno, H Nishikawa, T Yamaguchi, E Watanabe, M Oikawa, T Kamiya, K Arakawa

    NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS   191   362 - 365  2002.05  [Refereed]

     View Summary

    We investigate the mechanisms of defect formation and optical absorption induced by ion implantation, for fabrication of optical devices by radiation effects. High-purity silica implanted by H+ or He2+ was characterized using vacuum-ultraviolet spectroscopy and electron-spin-resonance measurement. Defect formation is suppressed by OH groups, possibly by the release of atomic hydrogen. The E' center and nonbridging oxygen hole center were created through pair generation from the normal Si-O-Si bond. The peroxy radical was generated through the reaction of the E' center with interstitial oxygen, which is a Frenkel-defect pair with an oxygen vacancy. By the Kramers-Kronig analysis on the MeV-ion implantation-induced defects, a refractive index increase of the order of 10(-4) was estimated. (C) 2002 Elsevier Science B.V. All rights reserved.

    DOI

  • Radiation effects and surface deformation of silica by ion microbeam

    H Nishikawa, T Souno, M Hattori, Y Nishihara, Y Ohki, E Watanabe, M Oikawa, T Kamiya, K Arakawa

    NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS   191   342 - 345  2002.05  [Refereed]

     View Summary

    Radiation effects induced by ion microbeam were studied by a confocal microspectroscopy and an atomic force microscopy (AFM). We investigate two significant radiation effects, defect generation and compaction, which were ascribed to electronic (E-elec) and nuclear stopping powers respectively. Photoluminescence mapping of nonbridging oxygen hole centers at 650 run reveals the defect formation along the tracks of ions. The surface deformation measured by AFM depends on the width of irradiated by microbeam. Confinement effects from the interface of irradiated and nonirradiated regions are taken into account for the understanding of the correlation between the surface deformations and internal compactions. (C) 2002 Elsevier Science B.V. All rights reserved.

    DOI

  • 絶縁性高分子中の電荷分布計測における電磁気学

    大木義路

    静電気学会誌   26 ( 5 ) 204 - 208  2002  [Invited]

  • 水トリー劣化架橋ポリエチレンの残留電荷の測定と解釈

    平井直志, 小林啓一, 大木義路, 武田敏尚, 岡本達希

    電気学会論文誌A   122-A ( 6 ) 597 - 603  2002  [Refereed]

    DOI

  • ポリエチレン中の空間電荷分布形成に与える架橋剤分解残渣の影響

    平井直志, 南亮太郎, 大木義路, 岡下稔, 前野恭

    電気学会論文誌A   122 ( 3 ) 308 - 314  2002  [Refereed]

    DOI

  • Fabrication of nano/microstructures in SiO2 and TiO2 by swift ions

    K. Awazu, M. Fujimaki, S. Ishii, K. Shima, K. Nomura, Y. Ohki

    2002 International Microprocesses and Nanotechnology Conference, MNC 2002     68 - 69  2002  [Refereed]

     View Summary

    The authors introduce work on 2D and 3D nano/microfabrication with MeV ion beam irradiation on a-SiO2 and rutile single crystal TiO2. The electronic stopping of the ions is responsible for the track formation. The latent tracks atomic structure and enhanced etch rate are identified.

    DOI

  • Chemical group in crosslinking byproducts responsible for charge trapping in polyethylene

    N Hirai, R Minami, T Tanaka, Y Ohki, M Okashita, T Maeno

    2002 ANNUAL REPORT CONFERENCE ON ELECTRICAL INSULATION AND DIELECTRIC PHENOMENA     626 - 630  2002  [Refereed]

     View Summary

    It is known that space charge is formed in cables insulated with crosslinked polyethylene. It is probable that the crosslinking byproduct or the crosslinked polymer morphology is responsible for the space charge. In order to examine the former possibility, an additive-free low-density polyethylene sheet was soaked in vaxious liquid chemicals including the crosslinking byproducts and space charge distribution inside the sheet was measured under dc voltage application. As a result, space charge appears in the vicinity of the injecting electrode only in the case that the sheet was soaked in the liquids with at least one hydroxyl group in their structures. Therefore, it is concluded that cumyl alcohol, which is the only byproduct with a hydroxyl group in its structure, is responsible for the formation of space charge in polyethylene.

  • Origins of photoluminescence bands induced by ultraviolet photons in polyethylene

    T Ito, D Kaneko, Y Ohki

    2002 ANNUAL REPORT CONFERENCE ON ELECTRICAL INSULATION AND DIELECTRIC PHENOMENA     856 - 860  2002  [Refereed]

     View Summary

    Photoluminescence (PL) spectra in low-density polyethylene (LDPE) and crosslinked polyethylene (XLPE) induced by irradiation of ultraviolet photons are examined. The samples were pre-irradiated by the ultraviolet photons under different atmospheres (air, O-2, and vacuum) in order to induce photochemical reactions. Two PL bands appear around 3.6 and 42 eV in LDPE. Their intensities decrease with the progress of the pre-irradiation regardless of the irradiation atmosphere. The two PLs are considered to be due to conjugated double bonds present in the antioxidant and impurities. A new PL band appears at 2.9 eV in LDPE if the sample was pre-irradiated in vacuum. This PL is considered to be due to photoinduced conjugated double bonds. In XLPE, a PL band different from the ones observed in LDPE appears at 3.1 eV. It is considered that the crosslinking byproducts are responsible for this band.

  • The World's first long-distance 500 kV-XLPE cable line. 3. Underground apparatus (News from Japan)

    Y. Ohki, S. Yasufuku

    IEEE Electrical Insulation Magazine   18 ( 5 ) 45 - 46  2002  [Refereed]

  • Nucleation of Ga2O3 nanocrystals in the K2O-Ga2O3-SiO2 glass system

    R Ceccato, R Dal Maschio, S Gialanella, G Mariotto, M Montagna, F Rossi, M Ferrari, KE Lipinska-Kalita, Y Ohki

    JOURNAL OF APPLIED PHYSICS   90 ( 5 ) 2522 - 2527  2001.09  [Refereed]

     View Summary

    A multitechnique approach, consisting of x-ray diffraction, differential thermal analysis, low frequency Raman scattering from the acoustic vibrations of nanoclusters, and transmission electron microscopy associated with selected area diffraction, has been used to study the nucleation and crystallization processes in SiO2-Ga2O3-K2O glasses. The specific aim was to determine the structure and the size distribution of nanoparticles embedded in the glass matrix. It has been found that nearly spherical nanocrystals of beta -Ga2O3, with a size of similar to2-3 nm, nucleate during thermal treatments at 900 degreesC. Crystallization was observed after annealing at higher temperature. The amount of the crystalline phase and the mean size of the nanocrystals increased with heat treatment, time and temperature. beta -Ga2O3 was the only crystalline phase to appear in all glass samples. (C) 2001 American Institute of Physics.

    DOI

  • Visible electroluminescence in hydrogenated amorphous silicon oxynitride

    H Kato, A Masuzawa, H Sato, T Noma, KS Seol, M Fujimaki, Y Ohki

    JOURNAL OF APPLIED PHYSICS   90 ( 5 ) 2216 - 2220  2001.09  [Refereed]

     View Summary

    The mechanism of electroluminescence in hydrogenated amorphous silicon oxynitride was investigated. The luminescence can be observed only in the samples with high nitrogen content and annealed at high temperatures. It depends on the direction of the applied electric field, and its peak photon energy decreases from 2.3 to 1.8 eV as the nitrogen content increases. From the measurements of conduction current and Fourier transform infrared absorption spectroscopy, it was found that the electrical conduction in the electric field region where the luminescence was observed is governed by the Poole-Frenkel process at the defect centers induced by the high temperature annealing. The electroluminescence is considered to be caused by electronic transition between the band-tail states, at least one of which is related to N or Si-N bonds. (C) 2001 American Institute of Physics.

    DOI

  • Origin of photoluminescence around 2.6-2.9 eV in silicon oxynitride

    T Noma, KS Seol, H Kato, M Fujimaki, Y Ohki

    APPLIED PHYSICS LETTERS   79 ( 13 ) 1995 - 1997  2001.09  [Refereed]

     View Summary

    A broad photoluminescence (PL) around 2.6-2.9 eV is known to appear in hydrogenated silicon oxynitride. Although its origin was reported to be Si-N bonds, it is not so clear since the material contains hydrogen. In the present research, we have confirmed that the same PL appears in silicon oxynitride grown by nitriding of silicon dioxide. The depth profile of the PL intensity agrees with that of the nitrogen concentration. Furthermore, the emission spectrum, excitation spectrum, and decay constant of this PL agree with those of the PL observed in silicon nitride. Based on these results and theoretical discussion, the origin of the 2.6-2.9 eV PL is estimated to be Si-N bonds. (C) 2001 American Institute of Physics.

    DOI

  • Low-wavenumber Raman scattering spectroscopy in studies of new gallium-doped silica glass-based transparent vitroceramic medium

    R Ceccato, R Dal Maschio, M Ferrari, S Gialanella, KE Lipinska-Kalita, G Mariotto, M Montagna, Y Ohki, F Rossi

    JOURNAL OF RAMAN SPECTROSCOPY   32 ( 8 ) 643 - 647  2001.08  [Refereed]

     View Summary

    Optically transparent, silica glass-based vitroceramic material doped with gallium oxide was obtained by standard glass fusion and appropriate thermal treatments. Low-wavenumber Raman scattering spectroscopy allowed the determination of the microstructure and in particular the size distribution of nanoparticles embedded in the amorphous matrix The nanocrystal growth was accurately controlled by heat treatment conditions. Transmission electron microscopy and X-ray diffraction measurements revealed the presence of a single crystalline phase, spherical-shaped nanoaggregates, homogeneously distributed in the transparent composite medium. Copyright (C) 2001 John Wiley & Sons, Ltd.

    DOI

  • Thermally induced photoluminescence quenching centre in hydrogenated amorphous silicon oxynitride

    H Kato, A Masuzawa, T Noma, KS Seol, Y Ohki

    JOURNAL OF PHYSICS-CONDENSED MATTER   13 ( 30 ) 6541 - 6549  2001.07  [Refereed]

     View Summary

    The effect of thermal annealing on the photoluminescence around 2.2-2.9 eV in hydrogenated amorphous silicon oxynitride films was investigated. The luminescence intensity increases monotonically with an increase in the annealing temperature for the samples with lower nitrogen contents (N/O = 0.06, 0.10 and 0.12). It shows a similar increase up to 500 degreesC, while it decreases abruptly above 500 degreesC for the samples with higher nitrogen contents (N/O = 0.14 and 0.18). The density of silicon dangling bonds depends on the annealing temperature in a manner opposite to that of the luminescence intensity in all the temperature region. Based on this correlation, it is thought that the silicon dangling bonds act as the quenching Centre. Infrared absorption spectroscopy indicated that the precursor of silicon dangling bonds was the Si-H bond. Hydrogen was released at temperatures above 500 degreesC from Si-H bonds, resulting in a large number of silicon dangling bonds that quench the luminescence.

    DOI

  • Low-temperature crystallization induced by excimer laser irradiation of SrBi2Ta2O9 films

    KS Seol, H Hiramatsu, Y Ohki, IH Choi, YT Kim

    JOURNAL OF MATERIALS RESEARCH   16 ( 7 ) 1883 - 1886  2001.07  [Refereed]

     View Summary

    Transition of a SrBi2Ta2O9 precursor film from amorphous to crystalline was induced by excimer laser irradiation. Both fluorite and perovskite crystalline structures in such films were obtained by excimer laser irradiation at substrate temperatures between 200 and 500 degreesC, Either an addition of excess bismuth in the precursor film or an increase in the substrate temperature enhanced the formation of the perovskite structure in the excimer laser-induced annealing process, resulting in the perovskite crystalline phase at a relatively lower temperature of 500 degreesC, Such a low temperature is preferred when SrBi2Ta2O9 is used in ferroelectric devices. The mechanism involved in this laser-induced crystallization is also discussed.

    DOI

  • Thyristor valves and GIS in Kii Channel HVDC link

    Y Ohki

    IEEE ELECTRICAL INSULATION MAGAZINE   17 ( 3 ) 78 - 79  2001.05  [Refereed]

    DOI

  • Correlation between space charge distribution and water-tree location in aged XLPE cable

    K Suzuki, Y Tanaka, T Takada, Y Ohki, C Takeya

    IEEE TRANSACTIONS ON DIELECTRICS AND ELECTRICAL INSULATION   8 ( 1 ) 78 - 81  2001.02  [Refereed]

     View Summary

    The correlation between charge distribution in an aged crosslinked polyethylene (XLPE) cable and the location of water trees has been studied. The space charge radial distributions at different angle in the insulating wall of a service-aged XLPE coaxial cable, induced by application of a de stress, were measured using the pulsed electroacoustic (PEA) method. Furthermore, a cross section of sliced insulating wall was observed with an optical microscope to compare with the radial space charge distribution. The cable under investigation was electrically aged in service at 6.6 kV ac for 25 years in wet conditions. Water trees were found at the position in which space charge was observed. On the other hand, no water tree was found at the position in which the space charge was not observed. Judging from these results, the space charge measurement seems to be useful for the diagnosis of water trees in aged cables.

    DOI

  • Estimation of conductivity and permittivity of water trees in PE from space charge distribution measurements

    T Toyoda, S Mukai, Y Ohki, Y Li, T Maeno

    IEEE TRANSACTIONS ON DIELECTRICS AND ELECTRICAL INSULATION   8 ( 1 ) 111 - 116  2001.02  [Refereed]

     View Summary

    Using an improved pulsed electroacoustic (PEA) system which is capable of measuring space charge distribution at minimum intervals of 25 mus, the charge distribution inside water-treed polyethylene (PE) was examined. From the results, the frequency dependencies of conductivity and permittivity were examined for water trees grown in PE. The conductivity of the water-treed region is found to be >10(10)x higher than that of non-treed regions, while the permittivity increase is only slight.

    DOI

  • ポリイミドの耐水・耐アルカリ性―残留電圧等電気的測定による評価―

    山口晃, Wei Wei, 平井直志, 大木義路, 財満英一, 岡部成光

    電気学会論文誌A   121-A ( 12 ) 1117 - 1124  2001  [Refereed]

    DOI

  • ポリプロピレンの紫外光励起フォトルミネセンス

    伊藤俊秀, 豊田毅彦, 平井直志, 大木義路

    電気学会論文誌A   121-A ( 9 ) 865 - 871  2001  [Refereed]

    DOI

  • 直鎖状低密度ポリエチレンの絶縁特性に影響を与える因子

    小野裕永, 荒岡誠, 平井直志, 大木義路

    電気学会論文誌A   121-A ( 9 ) 854 - 859  2001  [Refereed]

    DOI

  • シリカ系フォトニクスガラスの高機能化

    西川宏之, 大木義路

    電気学会論文誌A   121-A ( 8 ) 721 - 724  2001  [Refereed]

    DOI

  • プラズマ化学気相堆積法によるハフニウムシリケート膜の堆積

    加藤宙光, 宮川武, 南向智広, 大木義路, 薛光洙, 滝山真功

    放電研究   44 ( 1, 2 ) 69 - 73  2001  [Refereed]

  • Characterization of hafnium and zirconium silicate films fabricated by plasma-enhanced chemical vapor deposition

    H. Kato, T. Nango, T. Miyagawa, T. Katagiri, Y. Ohki

    Extended Abstracts of International Workshop on Gate Insulator, IWGI 2001     166 - 169  2001  [Refereed]

     View Summary

    Recently, hafnium, and zirconium silicates have been considered to be attractive for new materials with high permittivity We have tried to deposit these silicate films by plasma-enhanced chemical vapor deposition (PECVD). In this report, we discuss the electrical properties and chemical structure of the deposited films.

    DOI

  • Evaluation of degradation in polyimide by recovery voltage measurement

    A. Yamaguchi, Wei Wei, N. Hirai, Y. Ohki, E. Zaima, S. Okabe

    Conference on Electrical Insulation and Dielectric Phenomena (CEIDP), Annual Report     216 - 220  2001  [Refereed]

     View Summary

    Polyimide (PI) has been widely used for insulation in motors, integrated circuits, electric wire, cable, and so on, due to its high mechanical strength, thermal stability, chemical stability, and radiation resistance. We have examined the degradation of API (all-aromatic polyimide) and TPI (thermoplastic polyimide) soaked in H2O or KOH by measuring recovery voltage. The peak value of the recovery voltage was found to depend on the soaking time. We have developed a theoretical equation of recovery voltage, and have found that the peak value of recovery voltage is a function of the dielectric constant. By measuring the recovery voltage, it is possible to diagnose the degradation of API and TPI.

    DOI

  • Effect of byproducts of dicumyl peroxide on space charge formation in low-density polyethylene

    N Hirai, R Minami, K Shibata, Y Ohki, M Okashita, T Maeno

    2001 ANNUAL REPORT CONFERENCE ON ELECTRICAL INSULATION AND DIELECTRIC PHENOMENA     478 - 483  2001  [Refereed]

     View Summary

    For dc transmission, crosslinked polyethylene (XLPE) cables axe still under feasibility study due to severe concern about space charge. In the bulk of XLPE insulated cables, there remain liquid chemicals such as acetophenone, alpha -methylstyrene, and cumylalcohol. We have examined the effects of these remaining chemicals on the space charge evolution in low-density polyethylene (LDPE). The most important conclusion obtained is that acetophenone and alpha -methylstyrene in LDPE assist the carrier transport, while cumylalcohol traps the carriers.

  • Effects of byproducts of dicumyl peroxide on space charge formation in polyethylene

    N Hirai, R Minami, Y Ohki, M Okashita, T Maeno

    ICSD '01: PROCEEDINGS OF THE 2001 IEEE 7TH INTERNATIONAL CONFERENCE ON SOLID DIELECTRICS     450 - 455  2001  [Refereed]

  • Photoluminescence in polymeric insulating materials (low density polyethylene and polypropylene) induced by ultraviolet photons

    T Ito, T Toyoda, N Hirai, Y Ohki

    ICSD '01: PROCEEDINGS OF THE 2001 IEEE 7TH INTERNATIONAL CONFERENCE ON SOLID DIELECTRICS     319 - 323  2001  [Refereed]

  • Photoluminescence analysis of plasma-deposited oxygen-rich silicon oxynitride films

    T Noma, KS Seol, M Fujimaki, H Kato, T Watanabe, Y Ohki

    JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS SHORT NOTES & REVIEW PAPERS   39 ( 12A ) 6587 - 6593  2000.12  [Refereed]

     View Summary

    Photoluminescence spectra were observed for hydrogenated oxygen-rich silicon oxynitride films with different N/O ratios, deposited by plasma-enhanced chemical vapor deposition. In the sample with a small ratio of N/O, two luminescence bands at 4.4 and 2.7 eV originated from silicon homobonds in SiO2 were observed, while a luminescence band at 2.6-2.9 eV which has very similar properties to the one observed in SiNx was observed in the sample with a large ratio of N/O. The results of Fourier-transform infrared spectroscopy, electron spin resonance, X-ray photoelectron spectroscopy and scanning electron microscopy indicated that the luminescence band at 2.6-2.9 eV results from Si-N bonds in the films and that the present films have regions where SiN bonds gathered.

    DOI

  • Ion-implantation-induced densification in silica-based glass for fabrication of optical fiber gratings

    M Fujimaki, Y Nishihara, Y Ohki, JL Brebner, S Roorda

    JOURNAL OF APPLIED PHYSICS   88 ( 10 ) 5534 - 5537  2000.11  [Refereed]

     View Summary

    Ion implantation induces a refractive index increase in silica-based glass, which is mainly due to densification of the glass. The refractive index increase can be used to fabricate optical fiber gratings that are formed with periodic refractive index modulation in the core of an optical fiber. In this article, the generation mechanism of the densification has been investigated through measurements of thickness changes of silica glass induced by proton and He2+ ion implantation. Furthermore, fabrication of the optical fiber grating using the refractive index increase has been demonstrated. From the result, ideal implantation conditions to fabricate the gratings are discussed. (C) 2000 American Institute of Physics. [S0021-8979(00)03922-0].

    DOI

  • Time-resolved photoluminescence study of hydrogenated amorphous silicon nitride

    KS Seol, T Watanabe, M Fujimaki, H Kato, Y Ohki, M Takiyama

    PHYSICAL REVIEW B   62 ( 3 ) 1532 - 1535  2000.07  [Refereed]

     View Summary

    Time-resolved measurements of photoluminescence were carried out on a hydrogenated amorphous silicon nitride film prepared by low-pressure chemical vapor deposition. When excited with 5.0-eV photons, photoluminescence occurs over a broad spectrum ranging from 1.8 to 3.6 eV. The peak energy of this photoluminescence varies with time from several nanoseconds to nearly 1 ms. These results are explained by a combination of an excitonlike recombination process and a radiative tunneling recombination process of photogenerated carriers within the band-tail states, which are affected by the contributions of thermalization, the Coulombic interaction, and the extent of localization.

  • Application of infrared attenuated total reflection spectroscopy to in situ analysis of atheromatous plaques in aorta

    A Nakamura, T Koga, M Fujimaki, Y Ohki, T Sota, K Lipinska-Kalita, T Nagae, S Ishimaru, K Aizawa

    JAPANESE JOURNAL OF APPLIED PHYSICS PART 2-LETTERS   39 ( 6A ) L490 - L492  2000.06  [Refereed]

     View Summary

    Ex vivo infrared attenuated total reflection (ATR) spectroscopic studies have been carried out to observe the accumulation of cholesterol ester in atheromatous plaques. It has been found that ATR spectra can be successfully obtained from the intima side of aorta and give the same information as the transmission spectra. This means, the ATR spectra include information about the chemical composition of atheromatous plaques together with the degree of eventual lesions. We have demonstrated that ATR microspectroscopic measurements can give images of atheromatous plaques distribution and enable in situ observation of the relative amount of accumulated cholesterol ester. The results clearly indicate that the ATR spectroscopy enables spectroscopic diagnoses in vivo as well as in vitro.

    DOI

  • Structures and optical properties of defects correlated with photo-induced refractive index changes in Ge-doped SiO2 glass

    M Fujimaki, Y Ohki

    DEFECTS AND DIFFUSION IN CERAMICS: AN ANNUAL RETROSPECTIVE II   177-1   43 - 50  2000  [Invited]

     View Summary

    Photosensitivity of Ge-doped SiO2 glass is closely related to defect formation with ultraviolet photon irradiation. In the present paper, optical properties of Ge oxygen deficient centers, which are involved in the defect formation, are reviewed. Furthermore, structures and generation mechanisms of the photo-induced defects are discussed.

    DOI

  • ポリエチレン中の水トリーの導電率と誘電率

    豊田毅彦, 向井茂雄, 大木義路, 李英, 前野恭

    電気学会論文誌A   121-A ( 7 ) 750 - 754  2000  [Refereed]

    DOI

  • Time-resolved photoluminescence study of hydrogenated amorphous silicon nitride

    Kwang Soo Seol, Takashi Watanabe, Makoto Fujimaki, Hiromitsu Kato, Yoshimichi Ohki, Makoto Takiyama

    Phys. Rev. B   62 ( 3 ) 1532 - 1535  2000  [Refereed]

    DOI

  • 熱可塑性ポリイミドの電気伝導

    武井精一, 田邊泰広, 大木義路

    電気学会論文誌A   120-A ( 4 ) 478 - 483  2000  [Refereed]

    DOI

  • Effects of internal postoxidation on the oxygen deficiency and dielectric strength of buried oxide formed by the separation-by-implanted oxygen (SIMOX) process

    KS Seol, H Koike, T Futami, Y Ohki

    ELECTRICAL ENGINEERING IN JAPAN   130 ( 1 ) 15 - 20  2000.01  [Refereed]

     View Summary

    Effects of internal postoxidation on buried silicon dioxide have been studied. The dioxide examined was the buried insulator in a silicon-on-insulator (SOI) structure fabricated by implantation of oxygen ions into Si, or the SIMOX process. Internal postoxidation is an oxidation process applied to the SOI structure after its fabrication. It was observed that the photoluminescence intensity due to neutral oxygen vacancies (O-3=Si-Si=O-3, "=" denotes three separate bonds to oxygens) increased after the internal oxidation. The oxide thickness and the number of EC centers (O-3=Si ., "." denotes an unpaired electron) were also found to increase similarly. The measurements repeatedly done by changing the oxide thickness revealed that the increased part of oxide by the internal oxidation contains the vacancies with a similar density to the original part. It is concluded that the internal oxidation scarcely affects the oxygen deficiency of the oxide. It was also observed that the number of breakdowns at low electric fields remarkably decreased after the internal oxidation, indicating that electrically weak spots such as silicon pipes were effectively reduced. (C) 1999 Scripta Technica.

    DOI

  • Fabrication of long-period optical fiber gratings by use of ion implantation

    M Fujimaki, Y Ohki, JL Brebner, S Roorda

    OPTICS LETTERS   25 ( 2 ) 88 - 89  2000.01  [Refereed]

     View Summary

    We report the fabrication of long-period optical fiber gratings by use of a refractive-index increase induced by ion implantation. Helium ions were implanted in an optical fiber core through a metal mask that had a 170-mu m-pitch grating with spacing of 60 mu m. We obtained a wavelength-dependent effective transmission loss by use of the grating. (C) 2000 Optical Society of America. OCIS codes: 060.0060, 220.4000, 230.1480, 160.6030, 230.0230, 350.2770.

    DOI

  • Effects of byproducts of crosslinking agent on space charge formation in polyethylene - Comparison between acetophenone and alpha-methylstyrene

    Y Ohki, N Hirai, K Kobayashi, R Minami, M Okashita, T Maeno

    2000 ANNUAL REPORT CONFERENCE ON ELECTRICAL INSULATION AND DIELECTRIC PHENOMENA, VOLS. I & II     535 - 538  2000  [Refereed]

     View Summary

    We have examined the effects of acetophenone and a-methylstyrene on the space charge evolution in PE. The samples tested are additive-free LDPE sheets. The important conclusions obtained are as follows: (1) Positive charges are easy to migrate from the byproduct-existing region to the non-existing region, while negative charges are difficult. (2) Positive charge migration is easier in the LDPE sheet soaked into alpha -methylstyrene than in the one soaked into acetophenone, which is probably due to the difference in permeation velocity of the two byproducts. (3) The values of mobility and conductivity calculated from the space charge profiles agree with the assumption that ionic carriers are responsible for the conduction in the byproduct-existing region.

  • Electroluminescence in silicon oxynitride

    Kato H, Masuzawa A, Sato H, Ohki Y, Fujimaki M, Seol K. S, Noma T, Iee

    Proceedings of the 6th International Conference on Properties and Applications of Dielectric Materials, Vols 1 & 2     402 - 406  2000  [Refereed]

  • Effect of ozone annealing on the charge trapping property of Ta2O5-Si3N4-p-Si capacitor grown by low-pressure chemical vapor deposition

    H Kato, KS Seol, M Fujimaki, T Toyoda, Y Ohki, M Takiyama

    JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS SHORT NOTES & REVIEW PAPERS   38 ( 12A ) 6791 - 6796  1999.12  [Refereed]

     View Summary

    The effect of ozone annealing on the charge trapping property of Ta2O5/Si3N4/p-Si capacitors was examined by measuring high-frequency capacitance-voltage and thermally stimulated current characteristics. The results suggest that two types of electron traps exist in the Ta2O5 layer and that the ozone annealing efficiently eliminates them.

    DOI

  • Experimental investigation on the cause of harmfulness of the blue water tree to XLPE cable insulation

    G Katsuta, A Toya, Y Li, M Okashita, F Aida, Y Ebinuma, Y Ohki

    IEEE TRANSACTIONS ON DIELECTRICS AND ELECTRICAL INSULATION   6 ( 6 ) 887 - 891  1999.12  [Refereed]

     View Summary

    Through investigations made on crosslinked polyethylene (XLPE) insulated power cables that had experienced failures, blue-colored water trees were observed in addition to ordinary white water trees. The blue water tree was found to be much more harmful than the white water tree, since more electrical trees started at the blue trees. Elemental analysis revealed that iron and sulfur exist in the blue tree. In the case of this blue water tree, space charge exists abundantly but only at the tree tip, while no charge signal is observed on the electrode from which the tree starts. This indicates that the conductivity of the blue tree is very high. This is considered to be the main reason of the harmfulness of this tree.

    DOI

  • Effect of annealing on Ge-doped SiO2 thin films

    M Fujimaki, S Shimoto, N Miyazaki, Y Ohki, KS Seol, K Imamura

    JOURNAL OF APPLIED PHYSICS   86 ( 9 ) 5270 - 5273  1999.11  [Refereed]

     View Summary

    Thermal annealing effects on optical and structural properties of Ge-doped SiO2 thin films prepared by the chemical vapor deposition and flame hydrolysis deposition methods were investigated. The thin film prepared by the former method showed inhomogeneous Ge distribution, and Ge oxygen-deficient centers were observed. When it was thermally annealed at temperatures higher than 800 degrees C, the Ge distribution became uniform. The concentration of oxygen deficient centers was found to decrease with the thermal annealing in an O-2 atmosphere, while it increased with the thermal annealing at 1000 degrees C in N-2. This suggests that improvement of the film quality can be achieved by thermal annealing. On the other hand, neither inhomogeneity of Ge distribution nor the appearance of oxygen deficient centers was observed in the film prepared by the latter method, and its film quality was scarcely affected by the thermal annealing. (C) 1999 American Institute of Physics. [S0021-8979(99)02621-3].

    DOI

  • Insulation characteristics of XLPE cables for high frequency voltage with a PWM inverter

    A Seki, S Miyamoto, Y Ohki, T Hisatsune, T Ono, S Sakuma

    ELECTRICAL ENGINEERING IN JAPAN   129 ( 2 ) 31 - 39  1999.11  [Refereed]

     View Summary

    The development of feeder cables is being advanced for use in supplying power to propulsion coils to realize the Yamanashi experimental lines of a superconducting magnetic levitation railway (Maglev). A step voltage waveform is applied to the XLPE insulation of feeder cables by an inverter in which high frequencies are superimposed on the basic commercial frequency. The authors devised electrical testing equipment that can output such specific waveforms, and they investigated the effect of such waveforms on the insulation properties of XLPE cable, the breakdown characteristics of the model cables, and their long-term characteristics. As a result, it was learned that the insulation breakdown depends on the voltage waveform peak value and that the long-term characteristics are determined by the basic frequency. This knowledge will be useful for the insulation design of the feeder cable system of the Yamanashi experimental line. (C) 1994 Scripta Technica, Elects Eng Jpn, 129(2): 31-39, 1999.

    DOI

  • Structural changes induced by KrF excimer laser photons in H-2-loaded Ge-doped SiO2 glass

    M Fujimaki, T Kasahara, S Shimoto, N Miyazaki, S Tokuhiro, KS Seol, Y Ohki

    PHYSICAL REVIEW B   60 ( 7 ) 4682 - 4687  1999.08  [Refereed]

     View Summary

    Photochemical reactions related to the Ge lone-pair center (GLPC) that an induced by KrF excimer laser photons in H-2-loaded Ge-doped SiO2 glass have been investigated. Without the H-2 loading, the Ge electron center (GEC) and the positively charged GLPC were induced by the laser irradiation. In the H-2-loaded sample, the GEC, the Ge E' center, and the germyl radical (GR) were induced by the irradiation, while the positively charged GLPC was not observed after the irradiation. If the H-2-loaded sample was thermally annealed after the photon irradiation, the concentration of the photo-induced GEC decreased monotonically with an increase in the annealing temperature. On the other hand, the concentration of the GR increased up to the annealing temperature of 160 degrees C, and it decreased at higher temperatures. Without the pre-irradiation, the induction of the GR was not observed even in the H-2-loaded sample. From these results, it is concluded that the positively charged GLPC is terminated with a hydrogen atom in the H-2-loaded sample and then becomes the GR by trapping an electron thermally released from the GEC. [S0163-1829(99)05631-3].

  • Characteristic red photoluminescence band in oxygen-deficient silica glass

    Y Sakurai, K Nagasawa, H Nishikawa, Y Ohki

    JOURNAL OF APPLIED PHYSICS   86 ( 1 ) 370 - 373  1999.07  [Refereed]

     View Summary

    We studied a red photoluminescence (PL) band at about 1.8 eV with full width at half maximum of 0.2-0.4 eV in a series of oxygen deficient-type silicas before and after gamma irradiation. The decay lifetime of the PL was estimated to be about 200 ns. The PL excitation peak was found to be located at 2.1 eV. The intensity of the 1.8 eV band was enhanced by about 100 times after gamma irradiation up to a dose of 10 MGy. These results suggest that the 1.8 eV PL is associated with oxygen deficient states in silica glass, which were introduced during manufacture and were enhanced further by the gamma irradiation. Comparison of the PL properties was made with other luminescent Si-based materials in terms of the peak energy, lifetime, and temperature dependence. (C) 1999 American Institute of Physics. [S0021-8979(99)08013-5].

    DOI

  • Direct deposition of a blanket tungsten layer on SiO2 by preexposure of helium plasma

    T Noma, KS Seol, M Fujimaki, Y Ohki

    JOURNAL OF APPLIED PHYSICS   85 ( 12 ) 8423 - 8426  1999.06  [Refereed]

     View Summary

    By utilizing preexposure of helium plasma, a method of depositing a blanket tungsten layer directly on SiO2 has been developed. When SiO2 films are exposed to a helium plasma, oxygen atoms are knocked on by active species in the plasma and the surface becomes covered with reduced silicon. It is assumed that this silicon reacts with WF6 to form tungsten nuclei, and that tungsten layers grow through the reaction of H-2 and WF6 at these nuclei. The deposited tungsten layer is stable with a low resistivity in the form of the alpha-W crystal and shows strong adhesion to the SiO2 film due to the anchoring effect of tungsten. (C) 1999 American Institute of Physics. [S0021-8979(99)03912-2].

    DOI

  • Dielectric breakdown of new type polymerized polyethylene using a single-site catalyst

    M Araoka, H Yoneda, Y Ohki

    IEEE TRANSACTIONS ON DIELECTRICS AND ELECTRICAL INSULATION   6 ( 3 ) 326 - 330  1999.06  [Refereed]

     View Summary

    We investigated the dielectric properties of a new type of polyethylene (PE) polymerized with a single-she catalyst. The new PE has much higher dielectric strength and volume resistivity than both conventional low-density PE and linear low-density PE. The increase in the density is considered to be responsible for the increase in dielectric strength. The type of the comonomer and melt flow rate (MFR) scarcely affect the dielectric properties.

    DOI

  • Effects of ion implantation and thermal annealing on the photoluminescence in amorphous silicon nitride

    KS Seol, T Futami, T Watanabe, Y Ohki, M Takiyama

    JOURNAL OF APPLIED PHYSICS   85 ( 9 ) 6746 - 6750  1999.05  [Refereed]

     View Summary

    When amorphous silicon nitride films are irradiated by a KrF excimer laser, they exhibit broad photoluminescence (PL) centered around 2.4 eV. The PL intensity gradually decreases and the PL peak energy shifts to a lower energy with an increase of the implanted dose of Ar+ ions. This means that the PL consists of two components with peak energies at 2.66 and 2.15 eV and that implantation-induced defects such as vacancies are not the PL centers. The PL intensity is found to decrease if the film was thermally annealed, while the decreased PL intensity of the ion-implanted film recovers by the thermal annealing. Based on these results, it is concluded that the defects generated by hydrogen release or bond breaking act as nonradiative recombination centers that quench the PL. (C) 1999 American Institute of Physics. [S0021-8979(99)03609-9].

  • Paramagnetic centres induced in Ge-doped SiO2 glass with UV irradiation

    M Fujimaki, T Katoh, T Kasahara, N Miyazaki, Y Ohki

    JOURNAL OF PHYSICS-CONDENSED MATTER   11 ( 12 ) 2589 - 2594  1999.03  [Refereed]

     View Summary

    Changes in concentrations of the photo-induced paramagnetic centres, Ge E' centre, Ge electron centre (GEC) and positively charged Ge lone-pair centre ((GLPC)(+)) in four Ge-doped SiO2 glasses with Ge contents of 1.0, 1.4, 6.9 and 9.2 mol% were investigated, using a KrCl excimer lamp (5.6 eV, 7.0 mW cm(-2)) and a KrF excimer laser (5.0 eV, 4 MW cm(-2)) as photon sources. When the glasses were irradiated with photons from the lamp, the Ge E' centre and the GEC were induced in all the glasses. However, the (GLPC)(+) was observed only in the sample with Ge content of 1.4 mol% where the concentration of the induced GeE' centre was smaller than that of the induced GEC. The irradiation of photons from the laser induced the GEC and (GLPC)(+) in all the glasses. When the photon irradiation from the laser was continued onto the glasses, the Ge E' centres were induced and the concentration of the (GLPC)(+) was found to decrease with an increase in the concentration of the Ge E' centres. From these phenomena, it was concluded that the generation of the Ge E' centres diminishes the concentration of the (GLPC)(+).

    DOI

  • アセトフェノンがポリエチレンの空間電荷形成に与える影響

    小林啓一, 大原尚, 大木義路, 前野恭

    電気学会論文誌A   119-A ( 12 ) 1465 - 1471  1999  [Refereed]

    DOI

  • Effect of High Harmonic Voltage on Long-Term Dielectric Properties of Crosslinked Polyethylene Insulated Cable-Fundamental Dielectric Properties of Feeder Cable for Superconducting Magnetic Levitation System

    Yoshinobu Nakamichi, Hiroyasu Toda, Shinobu Ikeuchi, Junichi Shinagawa, Yoshimichi Ohki, Waseda University

    IEEJ Transactions on Industry Applications   119 ( 11 ) 1327 - 1336  1999  [Refereed]

     View Summary

    In the superconducting MAGLEV system using a linear synchronous motor (LSM), the power converter for driving LSM is installed above ground. As one useful power converter, a PWM (pulse width modulation) inverter has been developed. The output voltage of the PWM inverter contains much harmonics caused by the PWM controller. Crosslinked polyethylene insulated cable (CV cable) has been adopted as a feeder cable for the MAGLEV power supply system, and accordingly the voltage with much harmonics, which is produced by the PWM inverter, loads on the CV cable. The dielectric strength of CV cable under the voltage with much harmonics, which has never been reported so far, has come to be recognized as a very important issue in the development of MAGLEV. The authors have examined the output voltage waveform of PWM inverter and have found that it can be simulated by an AC voltage of commercial frequency upon which high frequency AC voltage is superimposed. Using a special generator capable of generating this particular voltage, the harmonic-superimposed voltage for long-term aging test has applied to CV cables and the dielectric properties of the cables have been investigated. © 1999, The Institute of Electrical Engineers of Japan. All rights reserved.

    DOI

  • Structural changes induced by KrF excimer laser photons in H2-loaded Ge-doped SiO2 glass

    Makoto Fujimaki, Toshiaki Kasahara, Shigeyuki Shimoto, Nahoko Miyazaki, Shin-ichi Tokuhiro, Kwang Soo Seol, Yoshimichi Ohki

    Phys. Rev. B   60 ( 7 ) 4682 - 4687  1999  [Refereed]

    DOI

  • プラズマCVD堆積SiO2薄膜の絶縁破壊電界におよぼすフッ素添加の効果

    加藤宙光, 高見明宏, 酒井真吾, 石井啓介, 大木義路

    電気学会論文誌A   119 ( 5 ) 658 - 664  1999  [Refereed]

    DOI

  • 実験結果より考察したインバータ電圧に対する電力ケーブルの絶縁設計へのいくつかの指針

    向井茂雄, 大木義路, 中道好信, 安喰浩司

    電気学会論文誌B   119 ( 1 ) 117 - 122  1999  [Refereed]

    DOI

  • プラズマCVD法による希土類添加シリカ薄膜―成膜法の開発と発光の物性研究への応用

    四元茂之, 酒井真吾, 大木義路

    電気学会論文誌A   119 ( 1 ) 113 - 117  1999  [Refereed]

    DOI

  • 鉄蒸気の混入した高温アルゴンの熱力学特性と粘性率

    天川正士, 足立和郎, 大木義路, 稲葉次紀

    電気学会論文誌A   119 ( 1 ) 75 - 79  1999  [Refereed]

    DOI

  • Temperature dependence of the lifetime of 4.3 eV photoluminescence in oxygen-deficient amorphous SiO2

    Kwang Soo Seol, Makoto Fujimaki, Yoshimichi Ohki, Hiroyuki Nishikawa

    Phys. Rev. B   59 ( 3 ) 1590 - 1593  1999  [Refereed]

    DOI

  • Low temperature crystallization of SrBi2Ta2O9 film by excimer laser irradiation

    KS Seol, H Hiramatsu, Y Ohki, DS Shin, IH Choi, YT Kim

    FERROELECTRIC THIN FILMS VII   541   293 - 298  1999  [Refereed]

     View Summary

    Using a KrF or ArF excimer laser as an irradiation photon source, we have succeeded in crystallizing amorphous SrBi2Ta2O9 films at 200-290 degrees C, quite lower temperatures than the conventional crystallization temperature. The crystallization is enhanced when the substrate temperature, irradiation time, or the laser power density is higher.

    DOI

  • Temperature dependence of the lifetime of 4.3-eV photoluminescence in oxygen-deficient amorphous SiO2

    KS Seol, M Fujimaki, Y Ohki, H Nishikawa

    PHYSICAL REVIEW B   59 ( 3 ) 1590 - 1593  1999.01  [Refereed]

     View Summary

    The temperature dependence of the lifetime of 4.3-eV photoluminescence (PL) excited by three PL excitation bands in oxygen-deficient amorphous silica has been investigated in the range of 13-280 K. When the PL is excited at the 5.0- or 6.7-eV band, it decays single exponentially with a constant Lifetime of about 4 ns irrespective of temperature. On the other hand, PL excited at 7.3 eV decays nonexponentially, and its effective lifetime monotonically decreases with an increase in temperature. Such a temperature dependence of the PL lifetime is explained by assuming an energy diagram involving two configurations of the oxygen-deficient defect. [S0163-1829(99)06604-7].

  • Space charge formation in water-treed insulation

    Y Ohki, Y Ebinuma, S Katakai

    IEEE TRANSACTIONS ON DIELECTRICS AND ELECTRICAL INSULATION   5 ( 5 ) 707 - 712  1998.10  [Refereed]

     View Summary

    Observations of the space charge distribution around water trees are reviewed, and the nature of water trees analyzed from those results is discussed. It was concluded that at least two mechanisms are responsible for the formation of space charges. One is interfacial polarization due to the high conductivity of a tree channel, and the other is carrier injection from the tree tip at high electric fields.

    DOI

  • Water-tree characteristics in low-density PE under simulated inverter voltages

    H Suzuki, S Mukai, Y Ohki, Y Nakamichi, K Ajiki

    IEEE TRANSACTIONS ON DIELECTRICS AND ELECTRICAL INSULATION   5 ( 2 ) 256 - 260  1998.04  [Refereed]

     View Summary

    Water tree characteristics have been investigated for low-density polyethylene by applying power-frequency ac voltages with high-frequency components, which are simulated output voltages of a pulse-width-modulation inverter. If we compare the water-tree length among single-frequency voltages, the water tree grows faster if the frequency is higher. However, if rye superpose a power-frequency voltage on the high-frequency voltage, tree growth is suppressed. The tree length becomes shorter if the superposed power-frequency voltage is higher. This surprising result is explainable by considering that the zero-crossing of the applied voltage plays an important role in tree propagation.

    DOI

  • Structures and generation mechanisms of paramagnetic centers and absorption bands responsible for Ge-doped SiO2 optical-fiber gratings

    M Fujimaki, T Watanabe, T Katoh, T Kasahara, N Miyazaki, Y Ohki, H Nishikawa

    PHYSICAL REVIEW B   57 ( 7 ) 3920 - 3926  1998.02  [Refereed]

     View Summary

    Paramagnetic centers and absorption bands induced by ultraviolet photons in Ge-doped SiO2 glass are investigated. Four kinds of samples with different Ge contents were exposed to ultraviolet photons from a KrF excimer laser (5.0 eV), a XeCl excimer lamp (4.0 eV), and a KrCl excimer lamp (5.6 eV). Irradiation with the KrF excimer laser induces two paramagnetic centers, named Ge(1) and Ge(2), in proportion with a decrease in the absorption at 5.1 eV and with an increase in absorption at 4.5 and 5.8 eV. The total density of the induced paramagnetic centers is linearly proportional to each induced change of the three absorption components and their proportionality constants are independent of the Ge content of the samples. The 4.0-eV photons from the XeCl excimer lamp induce only a Ge E' center, while the 5.6-eV photons from the KrCl excimer lamp induce a Ge(1) besides a Ge E' center. From these results, Ge(1) and Ge(2) are, respectively, assigned to the Ge electron center (GEC) and the positively charged Ge oxygen-deficient center (GODC)(+), which donated an electron to the GEC. The oscillator strength of the GODC for the absorption at 5.1 eV was found to be 0.1. From this, it is considered that the GODC that acts as the electron donor is the Ge lone pair center (GLPC). Thermally stimulated luminescence (TSL) is also examined in Ge-doped SiO2 glass that was exposed to photons from the KrF excimer laser. The TSL spectrum is very similar to the photoluminescence spectrum that is known to be due to the GLPC's. It was found that the absorption, which was induced by the KrF excimer laser photons, decreases during the TSL measurement and that this decrement of the absorption is proportional to the TSL intensity. As mentioned above, the electrons that are to be trapped to generate the GEC's are released from the GLPC's during the photon irradiation. Then, in its reverse reaction, these electrons are thermally detrapped from the GEC's to regenerate the GLPC's, and the TSL is caused by an electronic de-excitation in such formed GLPC's. To conclude, the TSL phenomenon further validates the assumption that the GLPC is the electron donor to generate the GEC's. [S0163-1829(98)05407-1].

  • Concentration of neutral oxygen vacancies in buried oxide formed by implantation of oxygen

    KS Seol, T Futami, Y Ohki

    JOURNAL OF APPLIED PHYSICS   83 ( 4 ) 2357 - 2359  1998.02  [Refereed]

     View Summary

    Using synchrotron radiation as a photon source, photoluminescence spectra were obtained for buried oxide formed by implantation of oxygen. From the spectra, the oxide was known to have relaxed and unrelaxed neutral oxygen vacancies with respective concentrations of 1.4 x 10(20) and 1.0 x 10(17) cm(-3). (C) 1998 American Institute of Physics.

  • γ線照射されたポリエーテルスルホンの誘電特性に与える熱処理の影響

    加藤学, 斉尾奈穂子, 大木義路

    電気学会論文誌A   118 ( 9 ) 1053 - 1054  1998  [Refereed]

    DOI

  • 内部酸化がSIMOX埋め込み酸化膜の酸素欠乏性と絶縁破壊電界におよぼす影響

    薛光洙, 小池英巳, 二見毅, 大木義路

    電気学会論文誌A   118 ( 7, 8 ) 826 - 831  1998  [Refereed]

    DOI

  • 固体絶縁材料中の電荷と電気伝導

    大木義路

    静電気学会誌   22 ( 3 ) 132 - 136  1998  [Invited]

  • Structures and generation mechanisms of paramagnetic centers and absorption bands responsible for Ge-doped SiO2 optical-fiber gratings

    Makoto Fujimaki, Tomofumi Watanabe, Tetsuya Katoh, Toshiaki Kasahara, Nahoko Miyazaki, Yoshimichi Ohki, Hiroyuki Nishikawa

    Phys. Rev. B   57 ( 7 ) 3920 - 3926  1998  [Refereed]

    DOI

  • Photoluminescence and electron-spin-resonance studies of defects in amorphous SiO2 films

    H Nishikawa, H Fukui, E Watanabe, D Ito, KS Seol, Y Ohki

    1998 INTERNATIONAL SYMPOSIUM ON ELECTRICAL INSULATING MATERIALS, PROCEEDINGS     59 - 62  1998  [Refereed]

  • Dielectric breakdown in F-doped SiO2 films formed by plasma-enhanced chemical vapor deposition

    H Kato, S Sakai, A Takami, Y Ohki, K Ishii

    PROCEEDINGS OF THE 1998 IEEE INTERNATIONAL CONFERENCE ON CONDUCTION AND BREAKDOWN IN SOLID DIELECTRICS - ICSD '98     368 - 371  1998  [Refereed]

     View Summary

    Fluorine-doped thin silicon dioxide films were synthesized by plasma-enhanced chemical vapor deposition of tetraethoxysilane and CF4, and the dielectric strength was measured with a self-healing breakdown technique by applying short duration voltage pulses. As a result, the film containing a higher amount of fluorine has a higher dielectric strength. The reason for this increase is discussed from various aspects, and two persuasive mechanisms are presented.

  • Effects of ion implantation and thermal annealing on the photoluminescence in amorphous silicon nitride films

    KS Seol, T Futami, T Watanabe, Y Ohki, M Takiyama

    1998 INTERNATIONAL SYMPOSIUM ON ELECTRICAL INSULATING MATERIALS, PROCEEDINGS     127 - 130  1998  [Refereed]

  • Effect of ozone annealing on the charge trapping property of Ta2O5-Si3N4-p-Si capacitor grown by low pressure chemical vapor deposition

    H Kato, KS Seol, T Toyoda, Y Ohki

    1998 INTERNATIONAL SYMPOSIUM ON ELECTRICAL INSULATING MATERIALS, PROCEEDINGS     131 - 134  1998  [Refereed]

  • Structural changes in Ge-doped SiO2 planar waveguides induced by ultraviolet photons

    Shimoto S, Fujimaki M, Miyazaki N, Nishihara Y, Ohki Y, Imamura K, Terasawa K, Ieee Deis

    1998 International Symposium on Electrical Insulating Materials, Proceedings     47 - 50  1998  [Refereed]

    DOI

  • Effect of gamma-ray irradiation on the TSC in polyethersulfone

    EJ Kim, G Kato, T Ohara, Y Ohki, ST Li

    IEEE TRANSACTIONS ON DIELECTRICS AND ELECTRICAL INSULATION   4 ( 6 ) 732 - 737  1997.12  [Refereed]

     View Summary

    When polyethersulfone was irradiated by gamma rays and then poled at 37 degrees C, a new TSC (thermally stimulated current) peak appeared at similar to 100 degrees C. This new peak and other peaks near 40 and 150 degrees C increase with an increase in dose of similar to 0.6 kGy, and this is considered to be due to trapped charges and other ionic carriers becoming mobile when rearrangement of scissioned chains occurs. However, if the dose exceeds 0.6 kGy, the peak magnitude decreases. Furthermore, if the TSC was poled at 120 degrees C, only the peak near 150 degrees C appeared and this peak monotonically decreased with an increase in the irradiation dose. A similar decreasing tendency was also observed in the conduction current and in the residual voltage. These decreasing features are considered to be caused by the radiation-induced change that rearrangement of scissioned chains decreases free volume for ionic transport.

    DOI

  • Photoluminescence of oxygen-deficient-type defects in a-SiO2

    N Nishikawa, Y Miyake, E Watanabe, D Ito, KS Seol, Y Ohki, K Ishii, Y Sakurai, K Nagasawa

    JOURNAL OF NON-CRYSTALLINE SOLIDS   222   221 - 227  1997.12  [Refereed]

     View Summary

    Oxygen-deficient-type defects in a-SiO2 were studied by means of photoluminescence (PL) measurements. Various properties of the 4.4-eV PL such as the decay lifetime and the temperature dependence in oxygen-deficient-type a-SiO2 can be explained in terms of an energy diagram involving two configurations of the oxygen-deficient-type defect. The 4.4-eV PL observed from the ion-implanted thermal oxides and the oxides prepared by the plasma-enhanced CVD method, has a stretched-exponential decay, suggesting a large structural distribution in the local network structures. A PL band at similar to 1.8 eV associated with highly oxygen-deficit states is also observed in oxygen-deficient-type a-SiO2 after high-dose gamma-irradiation (dose: 10 MGy). (C) 1997 Elsevier Science B.V.

    DOI

  • Electrical conduction in highly resistive polyolefin films modified by polar groups

    Y Ohki, T Asada, Y Umeshima, M Ikeda

    ELECTRICAL ENGINEERING IN JAPAN   120 ( 3 ) 9 - 16  1997.08  [Refereed]

     View Summary

    Electrical conduction and space charge distribution in oxidized low-density polyethylene (samples A1-A6) and copolymers of ethylene and carbon monoxide (samples B1-B3) with different carbonyl contents have been studied. A V-shaped dependence of conduction current on carbonyl content was observed for samples A1-A6, while the current decreases monotonically with an increase in carbonyl content for samples B1-B3. The effect for polar groups on electrical conduction is discussed, focusing on their bonding forms in polymer molecules. (C) 1997 Scripta Technica, Inc.

    DOI

  • Rare-earth-doped SiO2 films prepared by plasma-enhanced chemical vapour deposition

    M Yoshihara, A Sekiya, T Morita, K Ishii, S Shimoto, S Sakai, Y Ohki

    JOURNAL OF PHYSICS D-APPLIED PHYSICS   30 ( 13 ) 1908 - 1912  1997.07  [Refereed]

     View Summary

    Rare-earth-doped thin SiO2 films were made by plasma-enhanced chemical vapour deposition using a complex containing chelating ligands and tetraethoxysilane. By this means of deposition, the film was successfully doped with terbium or erbium and the luminescence properties were investigated. In the case of Tb3+-doped films, strong luminescence peaks from the D-5(4) level were observed during excitation by a KrF excimer laser. Upon thermal treatment at 800 or 900 degrees C, luminescence peaks from the D-5(3) level appear. Under DC voltages, electroluminescence from the D-5(4) level was also observed.

    DOI

  • Thermal annealing behavior of defects induced by ion implantation in thermally grown SiO2 films

    KS Seol, T Karasawa, Y Ohki, H Nishikawa, M Takiyama

    MICROELECTRONIC ENGINEERING   36 ( 1-4 ) 193 - 195  1997.06  [Refereed]

     View Summary

    When ion-implanted thermal SiO2 films are irradiated by photons at 5.0 eV, photoluminescence (PL) appears at 4.3, 2.7, and 1.9 eV. These PLs are related with defects in the SiO2 films. When the SiO2 films are annealed at high temperatures, both the 4.3 and 2.7 eV PL intensities decrease gradually with an increase in the annealing temperature and disappear at 500 degrees C, while the 1.9 eV PL increases or stays constant up to 400 degrees C and suddenly disappears at 500 degrees C. Such annealing behavior is not influenced by the annealing atmosphere. In the case of the films degassed in high vacuum before implantation, it is observed that the PL bands survive the annealing at 500 degrees C. This result suggests that dissolved gases in the film play an importa nt role in the annihilation of defects induced by ion implantation.

    DOI

  • Evaluation of microscopic structural randomness in SiO2 by analysis of photoluminescence decay profiles

    K Ishii, KS Seol, Y Ohki, H Nishikawa

    ELECTRICAL ENGINEERING IN JAPAN   119 ( 3 ) 1 - 6  1997.05  [Refereed]

     View Summary

    Microscopic structural randomness in SiO2, a typical electrical insulating material, was evaluated by observing the decay profile of the photoluminescence due to oxygen vacancies (equivalent to Si-Si equivalent to). As samples with different degrees of randomness, an ion-implanted thermal SiO2 film, SiO2 films formed by plasma-enhanced chemical vapor deposition of tetraethoxysilane with and without doped fluorine, a buried oxide film prepared by SIMOX (separation by ion-implanted oxygen), and a bulk silica glass prepared by the soot-remelting method were tested. By analyzing the decay profile with a stretched exponential function, it was found that the deviation of the decay profile from a single exponential function is larger in the samples whose infrared absorption properties and HF etch rate suggest greater structural randomness. (C) 1997 Scripta Technica, Inc.

    DOI

  • Dielectric breakdown of low-density polyethylene under simulated inverter voltages

    H Suzuki, S Mukai, Y Ohki, Y Nakamichi, K Ajiki

    IEEE TRANSACTIONS ON DIELECTRICS AND ELECTRICAL INSULATION   4 ( 2 ) 238 - 240  1997.04  [Refereed]

     View Summary

    Breakdown characteristics have been investigated for low-density polyethylene immersed in silicone oil by applying power-frequency ac voltages with high-frequency components, which are simulated output voltages of a pulse-width-modulation inverter. It was found that the breakdown voltage decreases as the high-frequency component increases. However, the number of zero-crossings of the applied voltage, which is very influential in water-tree growth, does not seem to play an important role in the dielectric breakdown process.

    DOI

  • Structural stabilization induced by oxygen plasma post-exposure of SiO2 films deposited from tetraethoxysilane

    T Noma, T Sugiura, K Ishii, Y Ohki, Y Hama

    JOURNAL OF PHYSICS D-APPLIED PHYSICS   30 ( 6 ) 937 - 943  1997.03  [Refereed]

     View Summary

    Structural changes induced by post-exposure to oxygen plasma were studied for SiO2 films deposited at low temperatures (200-600 degrees C) by plasma-enhanced chemical vapour deposition from tetraethoxysilane. Carbon- and water-related impurities remaining in the film are decomposed by the oxygen plasma and then disappear. This brings about the disappearance of micropores and structural consolidation and stabilization, through which the degree of waterproofing improves, the relative dielectric constant decreases and the absorption edge shifts towards a higher energy.

    DOI

  • Excited-state absorption measurement in Ge-doped SiO2 glass

    M Fujimaki, KS Seol, Y Ohki

    JOURNAL OF APPLIED PHYSICS   81 ( 6 ) 2913 - 2915  1997.03  [Refereed]

     View Summary

    Optical absorption change in the microsecond order in oxygen-deficient Ge-doped SiO2 glass was measured as a function of time just after photon irradiation from a KrF excimer laser. The absorption above 3 eV was found to decay with a similar time constant as that of the luminescence at 3.1 eV. From this, it is confirmed that the observed absorption change is due to the excitation of electrons from the lowest excited triplet state to an upper state. By taking account of the energy range of the absorption, there is a high possibility that the upper state is the conduction band. (C) 1997 American Institute of Physics.

    DOI

  • Effects of fluorine addition on the structure and optical properties of SiO2 films formed by plasma-enhanced chemical vapor deposition

    K Ishii, A Takami, Y Ohki

    JOURNAL OF APPLIED PHYSICS   81 ( 3 ) 1470 - 1474  1997.02  [Refereed]

     View Summary

    In order to elucidate the effects of fluorine addition on the structure and optical properties of SiO2 films formed by plasma-enhanced chemical vapor deposition of tetraethoxysilane, photoluminescence and optical absorption characteristics were studied using synchrotron radiation as an excitation photon source. Luminescence appears at 4.4 eV in both fluorine-doped and nondoped films when excited by 7.6 eV photons, indicating that the oxygen vacancies exist in both films. The decay profile of the 4.4 eV luminescence deviates from a single-exponential curve, indicating that the microscopic structure is not uniform in the sample. This deviation becomes smaller by the fluorine addition. Infrared absorption spectra indicate that the width of the distribution of bond angle angle SiOSi becomes smaller with an increase in the fluorine content. From these results, it is considered that the distribution of the bond angle decreases by the addition of fluorine. The optical gap energy was found to increase with the increase in the fluorine content. Presumably, this is also due to the homogeneous structure with a uniform bond angle caused by the fluorine addition. (C) 1997 American Institute of Physics.

    DOI

  • Space charge behavior under ac voltage in water-treed PE observed by the PEA method

    Y Li, J Kawai, Y Ebinuma, Y Fujiwara, Y Ohki, Y Tanaka, T Takada

    IEEE TRANSACTIONS ON DIELECTRICS AND ELECTRICAL INSULATION   4 ( 1 ) 52 - 57  1997.02  [Refereed]

     View Summary

    The pulsed electroacoustic method (PEA) has been applied to observe space charge formation under ac application (7 kv(pk), 50, 0.1 and 0.001 Hz) in water-treed polyethylene samples in order to understand the degradation mechanism of water trees. A system with phase-resolving capability has been developed to measure the space charge distribution at any phase angle of the applied ac voltage. By comparing the observed charge distributions with theoretically predicted ones, it has been shown that the interfacial polarization due to the differencein conductivity between the treed and the regions without trees dominate the space charge behavior It has become clear that the PEA method can be a powerful nondestructive method for evaluating water tree length.

    DOI

  • Energy states of Ge-doped SiO2 glass estimated through absorption and photoluminescence

    M Fujimaki, Y Ohki, H Nishikawa

    JOURNAL OF APPLIED PHYSICS   81 ( 3 ) 1042 - 1046  1997.02  [Refereed]

     View Summary

    The energy states of oxygen-deficient type defects in the vacuum ultraviolet region are discussed based on the experimental results of vacuum ultraviolet absorption, temperature dependence of the photoluminescence (PL) intensities at 4.3 and 3.1 eV, and lifetimes of the PLs. It was found that the oxygen-deficient type glass has a large absorption tail above 6 eV in addition to an absorption band around 5 eV and that the 3.1 eV PL intensity scarcely depends on temperature when excited above 6 eV. It was also found that the lifetime of the 4.3 eV PL is 9 ns and that of the 3.1 eV PL is 113 mu s irrespective of the excitation photon energy. The obtained results are explainable by assuming that electrons excited into the conduction band by photons above 6 eV contribute to the PLs. (C) 1997 American Institute of Physics.

    DOI

  • PWMインバータ出力波形に対する架橋ポリエチレンケーブルの絶縁特性

    関秋生, 宮本茂樹, 大木義路, 久恒豊一, 小野努, 佐久間進

    電気学会論文誌B   117 ( 12 ) 1553 - 1560  1997  [Refereed]

    DOI

  • Interfacial charge between ZnO varistor ceramics and inorganic insulating materials and its role

    Li Shengtao, Liu Fuyi, Li Bo, Ohki Yoshimichi

    Chinese Journal of Materials Research   11 ( 5 ) 479 - 482  1997  [Refereed]

  • ポリメチルペンテンの熱刺激電流

    鈴木英範, 黒田博文, 朝井壮太郎, 大木義路

    電気学会論文誌A   117 ( 3 ) 269 - 275  1997  [Refereed]

    DOI

  • Luminescence properties of sol-gel synthesized silica glass induced by an ArF excimer laser

    K Mukasa, M Ono, R Wakabayashi, K Ishii, Y Ohki, H Nishikawa

    JOURNAL OF PHYSICS D-APPLIED PHYSICS   30 ( 2 ) 283 - 285  1997.01  [Refereed]

     View Summary

    When sol-gel synthesized silica glass containing about 260 ppm of OH groups is irradiated with an ArF excimer laser, photoluminescence appears at 1.9 eV (650 nm). From measurements of luminescence lifetime, the absorption spectrum and electron-spin-resonance spectroscopy, the luminescence is thought to be due to a non-bridging oxygen hole centre, a kind of point defect in silica glass.

    DOI

  • Effect of post-oxidation on the oxygen deficiency of buried oxides

    KS Seol, T Karasawa, H Koike, Y Ohki, M Tachimori

    AMORPHOUS AND CRYSTALLINE INSULATING THIN FILMS - 1996   446   219 - 223  1997  [Refereed]

     View Summary

    The effect of post-oxidation of the silicon-on-insulator structure produced by implantation of oxygen (SIMOX) upon neutral oxygen vacancies (O-3=Si-Si=03, ''='' denotes bonds with three separate oxygens) in the buried oxide has been investigated. It is observed that the photoluminescence intensity from the vacancies and the thickness of buried oxide increase after the post-oxidation. An Ar plasma was used to expose the buried oxide in order to convert the vacancies to E'gamma, (O-3=Si-., ''(.)'' denotes an unpaired electron) centers which are detectable by electron-spin-resonance. The total number of E'gamma, centers was found to increase, which should suggest a similar increase in the number of the vacancies in the buried oxide. Similar measurements repeatedly done on the buried oxide by changing its thickness by etching revealed that the increased part of oxide by the post-oxidation contains the vacancies with a similar density to the original part of oxide, and that the post-oxidation scarcely affects the oxygen deficiency of the original part.

    DOI

  • Formation mechanisms of paramagnetic defect centers induced by excimer lasers, gamma rays, and mechanical fracturing in amorphous SiO2

    Hiroyuki Nishikawa, Eiki Watanabe, Daisuke Ito, Yoshimichi Ohki

    Electrical Engineering in Japan (English translation of Denki Gakkai Ronbunshi)   121 ( 3 ) 9 - 19  1997  [Refereed]

     View Summary

    The formation mechanisms of paramagnetic defect centers in amorphous SiO2 induced by excimer lasers, 60Co γ rays, and mechanical fracturing were investigated. Correlation between the paramagnetic defect centers and their precursors introduced during manufacture is discussed for the cases of excimer lasers and γ rays. For the case of mechanical fracturing, formation of strained Si-O-Si bonds as well as paramagnetic defects is examined. The mechanism of laser- or γ-ray-induced paramagnetic defect centers is compared with that of fracture-induced centers. © 1998 Scripta Technica, Inc.

    DOI

  • Role of voltage zero-crossing in propagation of water trees

    S Mukai, H Suzuki, Y Ohki, Y Nakamichi, K Ajiki, Y Li

    IEEE 1997 ANNUAL REPORT - CONFERENCE ON ELECTRICAL INSULATION AND DIELECTRIC PHENOMENA, VOLS I AND II     300 - 303  1997  [Refereed]

     View Summary

    Water tree characteristics have been investigated for low-density polyethylene by applying power-frequency ac voltages with high-frequency components, which are simulated output voltages of a pulse-width-modulation inverter, If we compare the water-tree length among single-frequency voltages, the water tree grows faster if the frequency is higher. However, if we superpose a power-frequency voltage onto the high-frequency voltage, the tree growth is suppressed. The tree length becomes shorter if the superposed power-frequency voltage is higher. This surprising result is explainable by considering that the zero-crossing of the applied voltage plays an important role for the tree propagation. In case that the high-frequency voltage is superposed onto a de voltage, the de voltage seems not to have any influence to the tree growth. This result can be also explained by considering the number of voltage reversals at the tree tip.

  • Effect of implanted ion species on the decay kinetics of 2.7 eV photoluminescence in thermal SiO2 films

    KS Seol, Y Ohki, H Nishikawa, M Takiyama, Y Hama

    JOURNAL OF APPLIED PHYSICS   80 ( 11 ) 6444 - 6447  1996.12  [Refereed]

     View Summary

    Decay kinetics of photoluminescence (PL) existing around 2.7 eV has been studied in various ion-implanted thermal SiO2 films as a function of implantation conditions. The PL observed in many samples shows decay constants shorter than 10 ms, which is a well-observed decay constant for silica glass. The change in the decay constant and that in the PL intensity have been found to be systematically related with the mass and the dose of the implanted ions. Therefore, despite the shea decay constant, the present 2.7 eV PL is attributable to a tripler-to-singlet transition of oxygen deficient centers, as in the case of silica glass. The rapid decay is interpreted as the increase in spin-orbit coupling interaction due to structural deformations by ion implantation such as the formation of paramagnetic defects and/or densification. (C) 1996 American Institute of Physics.

    DOI

  • Visible photoluminescence from Si clusters in gamma-irradiated amorphous SiO2

    H Nishikawa, E Watanabe, D Ito, Y Sakurai, K Nagasawa, Y Ohki

    JOURNAL OF APPLIED PHYSICS   80 ( 6 ) 3513 - 3517  1996.09  [Refereed]

     View Summary

    Visible photoluminescence (PL) bands around 2 eV were studied in Co-60 gamma-irradiated (dose<1 MGy) oxygen-deficient-type amorphous SiO2 (a-SiO2) excited by 2-4 eV photons. In addition to the well-known 1.9 eV PL band due to nonbridging oxygen hole centers, another PL band was observed at 2.2 eV when excited by 3.8 eV photons. The intensity of the 2.2 eV band increases with decreasing oxygen partial pressure during the sample preparation. Electron-spin-resonance measurements show that the intensity of the 2.2 eV band is correlated with the concentration of the E'(delta) center, a paramagnetic state of a cluster of silicons. After much higher gamma irradiation with a dose up to 10 MGy, a new PL band was induced at 1.75 eV under excitation by 2.5 eV photons, as well as the 1.9 and 2.2 eV PL bands. By comparing its spectral shape and excitation energy with known PL band in Si-implanted a-SiO2, it is suggested that the 1.75 eV band is associated with Si nanocrystals formed from Si clusters in a-SiO2 by the high-dose gamma irradiation. (C) 1996 American Institute of Physics.

    DOI

  • Origins of thermally stimulated current in polyethersulfone

    EJ Kim, T Takeda, Y Ohki

    IEEE TRANSACTIONS ON DIELECTRICS AND ELECTRICAL INSULATION   3 ( 3 ) 386 - 391  1996.06  [Refereed]

     View Summary

    In order to understand the dominant carrier species in electrical conduction in polyethersulfone (PES), thermally stimulated current (TSC) measurements were carried out under various conditions. It was found that PES has two TSC peaks in the temperature range from 20 to 220 degrees C, consisting of an cr peak at similar to 210 degrees C and a beta peak whose peak temperature moves towards a higher temperature with an increasing poling temperature, Even when the sample was not poled, PES shows a spontaneous current with its peak at similar to 210 degrees C, Both the spontaneous current peak and the alpha peak were found to disappear when the sample had been heated to 230 degrees C, keeping the external circuit closed. From such similarity of appearance and disappearance between the two peaks, it is concluded that they have the same origin, From the change in the spatial distribution of space charges inside the sample measured simultaneously with the TSC measurements, positive charges, probably due to K+ ions, existed nonuniformly in the sample from the beginning, and are thought to be responsible for the two peaks. From a similar study, the beta peak is considered to be due to polarization of ionic space charges during the poling process.

    DOI

  • Laser-power dependence of absorption changes in Ge-doped SiO2 glass induced by a KrF excimer laser

    M Fujimaki, K Yagi, Y Ohki, H Nishikawa, K Awazu

    PHYSICAL REVIEW B   53 ( 15 ) 9859 - 9862  1996.04  [Refereed]

     View Summary

    The generation mechanism of the absorption changes, which cause a photorefractive change through the Kramers-Kronig relation in Ge-doped SiO2 glass, has not been clarified yet. In the present paper, we examined the laser-power dependence of the absorption changes around 5 eV, induced by a KrF excimer laser. The induced absorption around 5 eV is composed of three different components, centering at 4.50, 5.08, and 5.80 eV. The increasing behavior of each absorption component depends strongly on the energy density. The three absorption components reach different saturation levels, depending on the energy density. Furthermore, the absorption induced by a high-power KrF excimer laser is bleached by a laser, the energy density of which is about one-twentieth of the inducing laser. Combining the results of mathematical analysis, it was found that a two-photon process and a one-photon process are, respectively, involved with the induction and the bleach of each absorption. It was also found that the precursor defect, which causes the absorption change, is of an oxygen-deficient type.

  • エキシマレーザ、ガンマ線、および機械的応力による非晶質SiO2中の常磁性中心の生成機構

    西川宏之, 渡辺英紀, 伊藤大佐, 大木義路

    電気学会論文誌A   116 ( 12 ) 1129 - 1137  1996  [Refereed]

    DOI

  • 極性基が導入された高抵抗性オレフィン系高分子フィルムの電気伝導

    浅田規, 大木義路, 梅島由香, 池田雅昭

    電気学会論文誌A   116 ( 12 ) 1113 - 1120  1996  [Refereed]

    DOI

  • Structural Origin of Dimensional Effect on Dielectric Breakdown Strength of ZnO Vraistors

    Shengtao Li, Fuyi Liu, Guangping Jia, Yoshimichi Ohki

    T. IEE Japan   116 ( 12 ) 1146 - 1147  1996  [Refereed]

    DOI

  • 発光減衰曲線の解析によるSiO2の微視的構造の乱雑さの評価

    石井啓介, 薛光洙, 大木義路, 西川宏之

    電気学会論文誌A   116 ( 10 ) 881 - 885  1996  [Refereed]

    DOI

  • 水トリー劣化した架橋ポリエチレンにおける空間電荷の挙動

    李英, 川井二郎, 海老沼康光, 藤原靖隆, 大木義路

    電気学会論文誌A   116 ( 9 ) 818 - 824  1996  [Refereed]

    DOI

  • ポリメチルペンテンにおける異常放電電流

    大木義路, 朝井壮太郎

    電気学会論文誌A   116 ( 6 ) 545 - 551  1996  [Refereed]

    DOI

  • シリコン酸化膜中の点欠陥の新しい検出法

    大木義路, 石井啓介, 薛光洙, 西川宏之

    電気学会論文誌A   116 ( 5 ) 387 - 391  1996  [Refereed]

    DOI

  • ポリエーテルスルホンの熱刺激電流と電気伝導

    大木義路, 金恩住, 武田敏尚

    電気学会論文誌A   116 ( 4 ) 357 - 363  1996  [Refereed]

    DOI

  • Laser-power dependence of absorption changes in Ge-doped SiO2 glass induced by a KrF excimer laser

    Makoto Fujimaki, Kanta Yagi, Yoshimichi Ohki, Hiroyuki Nishikawa, Koichi Awazu

    Phys. Rev. B   53 ( 15 ) 9859 - 9862  1996  [Refereed]

    DOI

  • Photoluminescence study on point defects in buried SiO2 film formed by implantation of oxygen

    KS Seol, A Leki, Y Ohki, H Nishikawa, M Tachimori

    JOURNAL OF APPLIED PHYSICS   79 ( 1 ) 412 - 416  1996.01  [Refereed]

     View Summary

    Defects in buried SiO2 films in Si formed by implantation of oxygen ions were characterized by photoluminescence (PL) excited by KrF (5.0 eV) excimer laser and synchrotron radiation. Two PL bands were observed at 4.3 and 2.7 eV. The 4.3 eV band has two PL excitation bands at 5.0 and 7.4 eV, and its decay time is 4.0 ns for the 5.0 eV excitation and 2.4 ns for the 7.4 eV excitation. The decay time of the 2.7 eV PL band is found to be 9.7 ms. These results are very similar to those for the 4.3 eV and the 2.7 eV PL bands, which are observed in bulk silica glass of an oxygen-deficient type and attributed to the oxygen vacancy. Through the change in the PL intensity with the film thickness, the buried SiO2 film is considered to contain the oxygen vacancy defects in a high amount throughout the oxide. (C) 1996 American Institute of Physics.

    DOI

  • Experimental observation and numerical calculation of space charge formation in gamma-irradiated polyethylene

    T Takada, H Kitajima, Y Tanaka, Y Ohki

    JOINT CONFERENCE OF 96' AICDEI / 4T-JCCEID     105 - 108  1996  [Refereed]

     View Summary

    In this paper, experimental results and their simulation on the residual space charge formation in gamma-irradiated low density polyethylene (LDPE) are summarized. To measure the space charge distribution, the pulsed electroacoustic (PEA) method was employed. From the measurement result, it became clear that a large amount of positive charges exist near the cathode in gamma-irradiated LDPE with de voltage application, and that these positive charges enhance the electric field at the interface between the cathode and LDPE. On the other hand, almost no space charge was observed in non-irradiated LDPE. It is considered that the mobile carriers are electrons, and that such moved electrons and the remaining positive ions form the observed space charge distribution. The observed space charge distribution was simulated numerically based on a model where the conduction is governed by hopping of electrons. A good agreement between the simulation and the observation was obtained.

  • Photoluminescence and electron-spin-resonance studies of defects in amorphous SiO2 films

    H Nishikawa, H Fukui, E Watanabe, D Ito, KS Seol, K Ishii, Y Ohki, M Takiyama, M Tachimori

    PHYSICS AND CHEMISTRY OF SIO(2) AND THE SI-SIO(2) INTERFACE-3, 1996   96 ( 1 ) 418 - 427  1996  [Refereed]

     View Summary

    Photoluminescence (PL) and electron-spin-resonance (ESR) studies were carried out on ion-implanted thermal a-SiO2 and as-grown a-SiO2 samples prepared by the plasma-enhanced chemical vapor deposition and by the separation by implantation of oxygen techniques, including bulk a-SiO2. Two PL bands at 4.3 eV and 2.6 eV were observed for these samples. Based on a comparison with PL bands observed in oxygen-deficient-type bulk a-SiO2, the two PL bands are ascribed to oxygen vacancies. The decay of the 4.3 eV PL band observed for the ion-implanted sample follows a power law or a stretched exponential function, suggesting the distribution, of PL lifetime, The ESR spectrum of the paramagnetic E' centers in the ion-implanted sample was found to be broadened by the dipolar interactions between the closely spaced defects. The PL decay kinetics at 4.3 eV for various a-SiO2 samples is also compared.

  • Water tree characteristics in low-density polyethylene under power-frequency voltages with high-frequency components

    H Suzuki, Y Ohki, Y Nakamichi, F Ajiki

    IEEE 1996 ANNUAL REPORT - CONFERENCE ON ELECTRICAL INSULATION AND DIELECTRIC PHENOMENA, VOLS I & II     742 - 745  1996  [Refereed]

     View Summary

    For magnetic levitation railway systems using linear synchronous motors, pulse-width-modulation inverters and polymer insulated cables are used for driving cars and feeding electricity, respectively. This implies that ac voltages tilth many harmonics induced by inverters are applied to cables. However, little is known about dielectric properties of polymers under such ac voltages. Therefore, water-tree characteristics have been investigated for low-density polyethylene using pouter-frequency ac voltages with high-frequency components. It was found that water tree grows very fast under high-frequency voltage. However, in the case of the superposed voltage, water tree does not grow so fast if the high-frequency component is not large. It is considered that the zero-crossing of the applied voltage plays an important role for the tree growth.

  • Effect of fluorine-doping on the dielectric strength of thin SiO2 films formed by plasma-enhanced chemical vapor deposition

    K Ishii, A Takami, Y Ohki

    IEEE 1996 ANNUAL REPORT - CONFERENCE ON ELECTRICAL INSULATION AND DIELECTRIC PHENOMENA, VOLS I & II     675 - 678  1996  [Refereed]

     View Summary

    Fluorine-doped thin SiO2 films were formed by plasma-enhanced chemical vapor deposition of tetraethoxysilane and CF4, and the intrinsic dielectric strength was measured with a self-healing breakdown technique by applying short-duration voltage pulses. The vacuum-ultraviolet absorption and photoluminescence were observed using synchrotron radiation as a photon source. From the decay profile of luminescence, the microscopic structure of the film was estimated. In the case of the film containing a higher amount of fluorine, the randomness in microscopic structure is smaller and the dielectric strength is higher. From this, it is considered that the higher dielectric strength comes from the relaxation of the structural distortion in the film.

  • Treeing and water treeing resistance of ethylene styrene copolymers

    K Akiyama, Y Ohki, Y Umeshima, M Ikeda

    JOINT CONFERENCE OF 96' AICDEI / 4T-JCCEID     61 - 64  1996  [Refereed]

     View Summary

    This paper reports the results of the investigation on the treeing and water treeing resistance of copolymers of ethylene and styrene. The samples are copolymer of ethylene and styrene at 0.42 percent by weight (referred to as S) low-density polyethylene (referred to as P) and laminated layers of the two polymers. As a result, the sample which has S on the needle electrode side has a better treeing resistance than the one which has P on the needle electrode side irrespective of the fact that the test piece is laminated or not. Similar tendency is also seen for the water-treeing resistance.

  • Dielectric breakdown properties of low-density polyethylene under power-frequency voltages with high-frequency components

    H Suzuki, Y Ohki, Y Nakamichi, K Ajiki

    JOINT CONFERENCE OF 96' AICDEI / 4T-JCCEID     203 - 206  1996  [Refereed]

     View Summary

    For magnetic levitation railway systems using linear synchronous motors, pulse-width-modulation inverters and polymer insulated cables are used for driving cars and feeding electricity, respectively. This implies that ac voltages with many harmonics induced by inverters are applied to cables. However, little is known about dielectric properties of polymers under such ac voltages. Therefore, breakdown characteristics have been investigated for low-density polyethylene using power-frequency ac voltages with high-frequency components. It was found that breakdown voltage decreases as the frequency component increases. However, the number of zero-crossings of the applied voltage, which is very influential in the water-tree length, does not seem to play an important role in the dielectric breakdown.

  • PHOTOLUMINESCENCE STUDY OF DEFECTS IN ION-IMPLANTED THERMAL SIO2-FILMS

    H NISHIKAWA, E WATANABE, D ITO, M TAKIYAMA, A LEKI, Y OHKI

    JOURNAL OF APPLIED PHYSICS   78 ( 2 ) 842 - 846  1995.07  [Refereed]

    DOI

  • NATURE OF PHOTOLUMINESCENCE INVOLVING TRANSITIONS FROM THE GROUND TO 4F(N-1) 5D(1) STATES IN RARE-EARTH-DOPED GLASSES

    T YOKOKAWA, H INOKUMA, Y OHKI, H NISHIKAWA, Y HAMA

    JOURNAL OF APPLIED PHYSICS   77 ( 8 ) 4013 - 4017  1995.04  [Refereed]

    DOI

  • IONIC BEHAVIOR OF DC CONDUCTION IN POLYETHERETHERKETONE

    EJ KIM, Y OHKI

    IEEE TRANSACTIONS ON DIELECTRICS AND ELECTRICAL INSULATION   2 ( 1 ) 74 - 83  1995.02  [Refereed]

     View Summary

    The conduction process in polyetheretherketone (PEEK) is discussed mainly from the results of de conduction, thermal pulse current (TPC), thermally stimulated current (TSC), and pulsed electroacoustic (PEA) measurements. It was found that de conduction currents in PEEK are consistent with the values predicted by a theory based on the conventional thermally activated ionic hopping process. The ionic hopping distance was calculated to change from 2.5 to 3.9 nm when the temperature crosses the glass transition temperature. The results of TPC and PEA measurements show the existence of heterocharges in the vicinity of both electrodes. In TSC spectra, a peak appears at a temperature of similar to 5 to 20 degrees C above the biasing temperature. The peak position approaches to the constant temperature of 155 degrees C when the biasing temperature is > 150 degrees C. The peak height is maximum when the biasing temperature is 150 degrees C. It is considered that this peak is related to the glass transition and caused by the depolarization of ionic space charge. From these results, it is concluded that ionic carriers which may come from byproducts of synthesis play an important role in the de conduction process in PEEK.

    DOI

  • Paramagnetic Defect Centers Induced by Excimer Lasers, γ-rays and Mechanical Fracturing in Amorphous SiO2

    H. Nishikawa, Y. Ohki

    Defect and Diffusion Forum/Scitec Publication   123-124 ( Part A ) 123 - 124  1995  [Invited]

  • Photoluminescence and electron-spin-resonance studies of defects in ion-implanted thermal SiO2 films

    H Nishikawa, H Fukui, E Watanabe, D Ito, M Takiyama, A Ieki, Y Ohki

    ICDS-18 - PROCEEDINGS OF THE 18TH INTERNATIONAL CONFERENCE ON DEFECTS IN SEMICONDUCTORS, PTS 1-4   196- ( Part 1 ) 97 - 101  1995  [Invited]

     View Summary

    Photoluminescence (PL) and electron-spin-resonance (ESR) studies were carried out on thermal SiO2 films after B+ or P+ implantation. Two PL bands at 4.3 eV and 2.6 eV were observed. For the 4.3-eV bands, two PL excitation bands were observed at 5.0 eV and 7.4 eV. Based on the comparison with those observed in oxygen-deficient-type bulk SiO2, the 4.3 eV and 2.5 eV PL bands are ascribed to oxygen vacancies induced by ion implantation. The decay of the 4.3-eV band in ion-implanted thermal SiO2 films follows a power low or stretched exponential, suggesting the distribution of PL lifetime. The ESR signal of the paramagnetic E' centers in ion-implanted thermal SiO2 films were found to be broadened by dipole-dipole interactions between the closely spaced defects. The PL and ESR results suggest that the oxygen vacancies induced by ion implantation in thermal SiO2 films are perturbed by the local network structures.

  • Photoluminescence study on point defects in SIMOX buried SiO2 film

    KS Seol, A Ieki, Y Ohki, H Nishikawa, M Tachimori

    ICDS-18 - PROCEEDINGS OF THE 18TH INTERNATIONAL CONFERENCE ON DEFECTS IN SEMICONDUCTORS, PTS 1-4   196- ( Part 4 ) 1909 - 1913  1995  [Invited]

     View Summary

    Defects in buried SiO2 films in Si formed by implantation of oxygen ions (SIMOX) were characterized by photoluminescence (PL) excited by KrF excimer laser (5.0 eV) and synchrotron radiation. Two PL bands were observed at 4.3 eV and 2.7 eV. The 4.3 eV band has two PL excitation bands at 5.0 eV and 7.4 eV, and its decay time is 4.0 ns for the 5.0 eV excitation and 2.4 ns for the 7.4 eV excitation. The decay time of the 2.7 eV PL band is found to be 9.7 ms. These results are very similar to those for the 4.3 eV and the 2.7 eV PL bands, which are observed in bulk silica glass of an oxygen-deficient type and attributed to the oxygen vacancy.

  • ROLE OF POINT-DEFECTS IN DIELECTRIC-BREAKDOWN OF SIO2 FORMED BY PLASMA-ENHANCED CHEMICAL-VAPOR-DEPOSITION OF TETRAETHOXYSILANE

    K ISHII, D ISSHIKI, Y OHKI, H NISHIKAWA, M TAKIYAMA

    JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS SHORT NOTES & REVIEW PAPERS   34 ( 1 ) 205 - 211  1995.01  [Refereed]

     View Summary

    The intrinsic breakdown strength of thin SiO2 films formed by plasma-enhanced chemical vapor deposition of tetraethoxysilane was measured using a self-healing technique by applying short-duration voltage pulses. Point defects in the films and other film qualities were examined by instrumental analyses and by optical absorption and luminescence using synchrotron radiation. The intrinsic breakdown strength decreases when the deposition temperature becomes higher than 400 degrees C, in spite of the fact that the optical gap and the density increase and the impurity content decreases. Based on the fact that the film deposited at high temperatures shows absorption at 7.6-eV and luminescence at 4.4-eV, the existence of oxygen vacancies (=Si-Si=) is suggested. The intrinsic breakdown strength is increased by decreasing oxygen vacancies through oxygen treatment. From these results, it is considered that oxygen vacancies play a crucial role in lowering the breakdown strength. Three possible mechanisms are discussed.

    DOI

  • パルス静電応力法による空間電荷分布の計測と熱刺激電流により考察したポリエーテルスルホンの自発電流

    金恩住, 武田敏尚, 大木義路

    電気学会論文誌A   115 ( 5 ) 437 - 444  1995  [Refereed]

    DOI

  • 直流電圧印加によりガンマ線照射低密度ポリエチレン中に形成される空間電荷分布

    小嶋雅之, 田中康寛, 高田達雄, 大木義路

    電気学会論文誌   115-A ( 2 ) 93 - 98  1995  [Refereed]

    DOI

  • Luminescence properties of defects in P+- or B+-implanted thermally grown silicon dioxide

    K. S. Seol, A. Ieki, Y. Ohki, H. Nishikawa, M. Takiyama

    Proceedings of the Symposium on Electrical Insulating Materials     85 - 88  1995

     View Summary

    The photoluminescence (PL) spectra from P - or B -implanted thermally grown film obtained by synchrotron radiation and excimer laser are studied and the nature of the defects induced by the ion implantation is discussed. Results show that under the excitation of KrF excimer laser, three PL bands at 4.3 eV, 2.7 eV, and 1.9 eV are observed. As for the PL bands at 4.3 eV and 1.9 eV, the intensity increases with a decrease in the temperature, while it decreases for the PL at 2.7 eV. From the similarities of the temperature dependence and the decay profile between the present ion-implanted SiO film and the well-identified bulk silica glass, the two PL bands at 4.3 eV and 2.7 eV are attributable to the oxygen vacancy, while the PL at 1.9 eV is from NBOHC. + + 2

  • Dielectric breakdown of SiO2 formed by plasma-enhanced chemical vapor deposition of tetraethoxysilane

    K ISHII, D ISSHIKI, T KARASAWA, Y OHKI

    ICSD '95 - PROCEEDINGS OF THE 1995 IEEE 5TH INTERNATIONAL CONFERENCE ON CONDUCTION AND BREAKDOWN IN SOLID DIELECTRICS     656 - 660  1995  [Refereed]

  • Observation and numerical calculation of space charge behavior in gamma-irradiated low density polyethylene

    T TAKADA, M KAJIMA, Y TANAKA, Y OHKI

    ICSD '95 - PROCEEDINGS OF THE 1995 IEEE 5TH INTERNATIONAL CONFERENCE ON CONDUCTION AND BREAKDOWN IN SOLID DIELECTRICS     274 - 278  1995  [Refereed]

  • New Design Criterion for XLPE Cable (News from Japan)

    Y. Ohki, Sachio Yasufuku

    IEEE Electrical Insulation Magazine   11 ( 6 ) 49 - 50  1995  [Refereed]

  • OPTICAL CHARACTERISTICS OF SIO2 FORMED BY PLASMA-ENHANCED CHEMICAL-VAPOR-DEPOSITION OF TETRAETHOXYSILANE

    K ISHII, Y OHKI, H NISHIKAWA

    JOURNAL OF APPLIED PHYSICS   76 ( 9 ) 5418 - 5422  1994.11  [Refereed]

     View Summary

    Optical characteristics of thin SiO2 films deposited from tetraethoxysilane by the plasma-enhanced chemical-vapor-deposition method were studied using synchrotron radiation. When the deposition temperature is 600 degrees C, 7.6 eV absorption and 4.4 eV luminescence with a small decay constant (<3 ns) due to the oxygen vacancy (drop Si-Si drop) are observed. The decay curve of the 4.4 eV luminescence has the appearance of a stretched-exponential function. This indicates that the decay constant consists of widely distributed components, reflecting structural distortion of the film.

    DOI

  • KINETICS OF ENHANCED PHOTOGENERATION OF E' CENTERS IN OXYGEN-DEFICIENT SILICA

    H NISHIKAWA, E WATANABE, D ITO, Y OHKI

    JOURNAL OF NON-CRYSTALLINE SOLIDS   179   179 - 184  1994.11  [Refereed]

     View Summary

    The kinetics of the generation of E' centers induced by a 6.4 eV excimer laser were investigated. Enhanced defect generation was observed in OH-containing oxygen-deficient silicas. The increased E' centers were found to be correlated with an absorption band at 5.7 eV. The intensity of the 5.7 eV band alpha, as a function of the laser fluence, F, follows a simple formula of alpha(F)= alpha(s)[1- exp(-DF)], where the values of alpha(s) and the decay coefficient, D, depend on the concentrations of Si-H and Si-OH bonds.

    DOI

  • Role of Point Defects in Dielectric Breakdown of SiO2 Films Deposited from Tetraethoxysilane by Plasma CVD Method

    K. Ishii, D. Isshiki※, Y. Ohki※

    Proceedings of 1994 International Joint Conference: 26th Symposium on Electrical Insulating Materials     33 - 37  1994.09  [Refereed]

     View Summary

    TEOSより堆積されたプラズマCVD法によるSiO2膜の絶縁破壊強度を調べた。堆積温度が100℃から400℃の間では,堆積温度の上昇に伴い堆積されたSiO2膜の絶縁破壊強度が上昇した。一方,400℃を超えると,緻密度や不純物が減少にも関わらず,堆積温度上昇とともに絶縁破壊が減少した。この原因を確かめるため,高温堆積された試料に酸素熱処理を施した。その結果,試料内の酸素空孔の減少とともに高温堆積膜の破壊強度の上昇が観測された。

  • GAMMA-RAY-INDUCED LOSS OF ER3+-DOPED SILICA-CORE OPTICAL-FIBER

    T KOYAMA, N DOHGUCHI, Y OHKI, H NISHIKAWA, Y KUSAMA, T SEGUCHI

    JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS SHORT NOTES & REVIEW PAPERS   33 ( 7A ) 3937 - 3941  1994.07  [Refereed]

     View Summary

    Erbium-doped silica optical fibers were irradiated by gamma-rays, and their loss-increasing characteristics were analyzed. The loss is mainly induced by Er2+ created by the reduction of Er3+. It does not show a simple dependence on the Er3+ concentration. The loss increase is described by the sum of several saturating exponential terms and one linear term, which can be derived from the yield equation of E'gamma centers or other species induced from precursors by hole trapping. It is considered that hole trapping supplies electrons to induce the reduction, and plays an important role in the loss increase.

    DOI

  • PREPARATION OF CD1-XYXSB2O6 THIN-FILM ON GLASS SUBSTRATE BY RADIO-FREQUENCY SPUTTERING

    K YANAGAWA, Y OHKI, T OMATA, H HOSONO, N UEDA, H KAWAZOE

    APPLIED PHYSICS LETTERS   65 ( 4 ) 406 - 408  1994.07  [Refereed]

     View Summary

    Cd1-xYxSb2O6 thin films were deposited onto a silica glass substrate by rf sputtering method. The highest conductivity observed for Cd0.95Y0.05Sb2O6 thin films was 4.1 x 10 S cm-1, with a carrier concentration of 1.3 X 10(20) cm-3 and a Hall mobility of 1.9 cm2 V-1S-1. No distinct optical absorption band was observed from the visible to the infrared region. These observations show that Cd1-xYxSb2O6 thin films have characteristics of a transparent conductor, whose transparent wavelength region covers the near infrared.

    DOI

  • LIGHTNING ARRESTERS DEVELOPED FOR 500 KV TRANSMISSION-LINES

    Y OHKI, S YASUFUKU

    IEEE ELECTRICAL INSULATION MAGAZINE   10 ( 4 ) 61 - 62  1994.07  [Refereed]

  • MITSUBISHI 275 KV XLPE CABLES IN LIBYA OPERATE SUCCESSFULLY FOR 8 YEARS

    Y OHKI, S YASUFUKU

    IEEE ELECTRICAL INSULATION MAGAZINE   10 ( 3 ) 46 - 46  1994.05  [Refereed]

  • DECAY KINETICS OF THE 4.4 EV PHOTOLUMINESCENCE ASSOCIATED WITH THE 2 STATES OF OXYGEN-DEFICIENT-TYPE DEFECT IN AMORPHOUS SIO2

    H NISHIKAWA, E WATANABE, D ITO, Y OHKI

    PHYSICAL REVIEW LETTERS   72 ( 13 ) 2101 - 2104  1994.03  [Refereed]

     View Summary

    We present the first observation of 4.4 eV photoluminescence (PL) decay in an oxygen-deficient-type silica excited with ultraviolet and vacuum ultraviolet photons from synchrotron radiation. The lifetime of the 4.4 eV PL is 4.2, 4.3, and 2.1 ns for the 5.0, 6.9, and 7.6 eV excitations, respectively, indicating the presence of multiple decay channels. This can be explained by an energy diagram involving the interconversion between two states of the oxygen-deficient-type defect.

    DOI

  • NEW OXIDE PHASE CD2(1-X)Y2XSB2O7 PYROCHLORE WITH A WIDE-BAND GAP AND HIGH ELECTRICAL-CONDUCTIVITY

    K YANAGAWA, Y OHKI, T OMATA, H HOSONO, N UEDA, H KAWAZOE

    JAPANESE JOURNAL OF APPLIED PHYSICS PART 2-LETTERS   33 ( 2B ) L238 - L240  1994.02  [Refereed]

     View Summary

    It was found for the first time that the electrical conductivity of Y-substituted Cd2Sb2O7, Cd2(1-x)Y2xSb2O7, with the pyrochlore structure, is larger than at least 3.3 x 10(0) Scm-1 and the optical band gap of the sample is wider than 3.5 eV. These values indicate that Cd2(1-x)Y2xSb2O7 pyrochlore has a characteristic of a transparent conductor.

    DOI

  • POINT-DEFECTS IN HIGH-PURITY SILICA-INDUCED BY HIGH-DOSE GAMMA-IRRADIATION

    Y SAKURAI, K NAGASAWA, H NISHIKAWA, Y OHKI

    JOURNAL OF APPLIED PHYSICS   75 ( 3 ) 1372 - 1377  1994.02  [Refereed]

     View Summary

    The defects induced by high-dose (10 MGy) gamma irradiation (Co-60) are studied in various types of high-purity silica glasses [including synthetic crystal (alpha-quartz)]. While the defects induced by gamma irradiation of up to 1 MGy have been reported to be generated through the bond breaking of manufacturing-method-dependent point defect sites (precursors), such precursor dependency disappears or at least weakens in the defects induced by 10 MGy gamma irradiation. Electron spin resonance, optical absorption, and luminescence investigations suggest that at high-dose irradiation the defects are created mainly by radiolysis or bond breaking, and associated oxygen diffusion occurred at silicon-oxygen bonds other than at point defect sites. Crystalline alpha-quartz shows much higher radiation resistivity than amorphous silica glasses, suggesting that strained silicon-oxygen bonds are the breaking sites.

    DOI

  • 絶縁材料の期待される方向

    大木義路

    静電気学会誌   18 ( 3 ) 204 - 205  1994  [Invited]

  • Dielectric Properties of Fluorine-containing Plasma Polymer Films

    Keisuke Ishii, Yoshimichi Ohki, Toshiki Nakano

    J. Photopolym. Sci. Technol.   7 ( 2 ) 353 - 360  1994  [Refereed]

    DOI

  • Electrical breakdown characteristics of liquid-crystal polymer

    K AKIYAMA, Y OHKI, Y UMESHIMA, M IKEDA

    PROCEEDINGS OF THE 4TH INTERNATIONAL CONFERENCE ON PROPERTIES AND APPLICATIONS OF DIELECTRIC MATERIALS, VOLS 1 AND 2     79 - 82  1994  [Refereed]

  • Measurement of residual space charge distribution in gamma-irradiated polyethylene

    M KOJIMA, Y TANAKA, T TAKADA, Y OHKI

    PROCEEDINGS OF THE 4TH INTERNATIONAL CONFERENCE ON PROPERTIES AND APPLICATIONS OF DIELECTRIC MATERIALS, VOLS 1 AND 2     479 - 482  1994  [Refereed]

  • Generation of Oxygen-Deficient Defects in TEOS-SiO2 Films by High Temperature Deposition

    K. Ishii, Y. Ohki※

    Proceedings of 2nd International Conference on Reactive Plasmas and 11th Symposium on Plasma Processing     669 - 972  1994.01

     View Summary

    TEOSを主原料としプラズマCVD法により作成されたSiO2膜の光学特性をシンクロトロン放射光を用い調べた。高温で堆積されたSiO2膜には7.6eVの光吸収帯が観測された。酸素空孔による吸収帯と思われる。また,1070cm-1近傍に現れる,Si-O-Si伸縮振動に起因する赤外吸収帯を解析した結果,プラズマCVD法により作成されたSiO2膜は,シリコン熱酸化膜に比べ,Si-O-Si結合角が小さく,より歪んだ構造を多く含んでいる可能性が示唆された。

  • CORRELATION OF PREEXISTING DIAMAGNETIC DEFECT CENTERS WITH INDUCED PARAMAGNETIC DEFECT CENTERS BY ULTRAVIOLET OR VACUUM-ULTRAVIOLET PHOTONS IN HIGH-PURITY SILICA GLASSES

    H NISHIKAWA, R NAKAMURA, Y OHKI, Y HAMA

    PHYSICAL REVIEW B   48 ( 21 ) 15584 - 15594  1993.12  [Refereed]

     View Summary

    Electron-spin-resonance (ESR) and vacuum-ultraviolet (vuv) absorption measurements were performed on a series of high-purity silica glasses exposed to 6.4-eV photons, 7.9-eV photons from excimer lasers, and to gamma rays. The concentration of defect centers varies from 10(14) to 10(16) CM-3 depending on the method of material fabrication and photon energy of the irradiating lasers. Variation of the defect species with both the incident-photon energy and manufacturing condition is observed by ESR measurements. E' centers (=Si.) are observed in all types of silicas. Nonbridging-oxygen hole centers (NBOHC's =Si-O.) in high-OH silica ([OH] almost-equal-to 1000 ppm) and peroxy radicals (PR's, =Si-O-O.) in oxygen-surplus silica ([OH] < 1 ppm) are observed, only when the samples are exposed to 7.9 eV photons. The defect centers observed in gamma-irradiated silica are qualitatively in agreement with those in 7.9 eV laser-irradiated silica. The variation of defect species with manufacturing methods indicates that the observed paramagnetic centers are created from preexisting defects. Concentration of the defects induced by either 6.4 or 7.9 eV laser photons is proportional to the square of the pulse energy, indicating that two-photon absorption process dominates in the defect formation. The defect formation process can be understood in terms of the creation of an electron-hole (e-h) pair by two-photon excitation and the subsequent hole trapping or decay of an e-h pair at the site of preexisting defects. Dependence of the induced-defect species on incident-photon energy can be explained by the variation in the energy level of preexisting defects or in the defect formation energy. Results of vuv-absorption measurements reveal the conversion of diamagnetic precursor defects (e.g., =Si-Si=, =Si-O-O-Si=, and =Si-OH) introduced during the manufacturing process, into paramagnetic defect centers, the E' centers, NBOHC's, and PR's.

    DOI

  • NEW OXIDE PHASE CD1-XYXSB2O6 WITH A WIDE-BAND GAP AND HIGH ELECTRICAL-CONDUCTIVITY

    K YANAGAWA, Y OHKI, N UEDA, T OMATA, T HASHIMOTO, H KAWAZOE

    APPLIED PHYSICS LETTERS   63 ( 24 ) 3335 - 3337  1993.12  [Refereed]

     View Summary

    It was initially found that CdSb2O6, in which a small part of the Cd ions was substituted with Y ions, has characteristics of a transparent conductor. The electrical conductivity at room temperature of the Y-substituted CdSb2O6 ceramic was larger, by at least 1 X 100 S cm-1, while that of the nonsubstituted CdSb2O6 Was smaller than 10(-5) S cm-1. The optical band gap of CdSb2O6 was found to be larger than 4.1 eV by the measurements of diffuse reflectance spectra.

    DOI

  • STUDY OF DEFECTS IN SILICON DIOXIDE FILMS ON SI(100) BY A VARIABLE-ENERGY POSITRON BEAM

    M FUJINAMI, NB CHILTON, K ISHII, Y OHKI

    JOURNAL OF APPLIED PHYSICS   74 ( 9 ) 5406 - 5409  1993.11  [Refereed]

     View Summary

    Amorphous SiO2 films grown by both thermal oxidation and plasma chemical vapor deposition (CVD) in a tetraethylorthosilicate and O2 atmosphere were studied using variable-energy positron annihilation spectroscopy and infrared (IR) spectroscopy. For thermally grown SiO2 layers with growth temperatures of over 1000-degrees-C it was found that the measured Doppler broadening parameter was identical for both wet and dry growth atmospheres. The measured Doppler broadening parameter in the case of SiO2 Prepared by plasma CVD was found to be strongly influenced by the substrate temperature during deposition. For a substrate growth temperature of 600-degrees-C, the data are essentially identical to those of the thermally grown oxide layers. IR analysis revealed that the concentration of Si-OH in the SiO2 layer is affected by the substrate temperature during growth. The level of the Doppler broadening parameter in the SiO2 film exhibited changes that can be correlated with this Si-OH concentration. We thus show that the concentration of Si-OH in amorphous SiO2 film is a factor that may affect the Doppler broadening parameter.

    DOI

  • A POSITRON BEAM STUDY OF DEFECTS IN SIO2

    M FUJINAMI, NB CHILTON, K ISHII, Y OHKI

    JOURNAL DE PHYSIQUE IV   3 ( C4 ) 169 - 175  1993.09  [Refereed]

     View Summary

    Defects in crystalline and amorphous SiO2 were studied by variable-energy positron annihilation spectroscopy in an attempt to clarify what defects in SiO2 are observable in positron annihilation studies. The level of the S parameter in amorphous SiO2 films grown by plasma chemical vapor deposition (CVD) in a tetraethylorthosilicate (TEOS) and O2 atmosphere was found to be correlated to the concentration of Si-OH in the silicon dioxide layer. The difference in S parameter between crystalline and amorphous SiO2 is assumed to be due to the effects of para-Ps self-annihilation in free volumes in the amorphous sample. The S parameters in crystalline and thermally grown silicon dioxide were both found to decrease after C ion implantation (1x10(14) cm-2, 140 keV) or alternatively, ArF excimer laser (6.4eV) irradiation. Both methods are expected to produce numerous types of Frenkel defects of which, it is most likely that negatively charged species such as =Si-O- are the trapping sites. In the case of ion implantation into amorphous SiO2 a greatly lowered S parameter is observed, this is explained as a combination of defect trapping and reduction in para Ps production after ion implantation.

    DOI

  • VARIOUS BONDING FORMS OF OH GROUPS IN HYDROGEN-TREATED SILICA

    M KITAGAWA, H NISHIKAWA, Y OHKI, Y HAMA

    JOURNAL OF APPLIED PHYSICS   74 ( 4 ) 2378 - 2380  1993.08  [Refereed]

     View Summary

    The bonding state of OH groups in silica glasses is studied by examining infrared absorption spectra and thermal annealing characteristics of gamma-induced paramagnetic detect centers. When an oxygen-rich silica is treated by hydrogen, OH groups exist in the form of those hydrogen-bonded with nonbridging oxygen (=Si-O- --- -H-O-Si=) besides the forms of H2O and =Si-O-H.

    DOI

  • ENHANCED PHOTOGENERATION OF E' CENTERS FROM NEUTRAL OXYGEN VACANCIES IN THE PRESENCE OF HYDROGEN IN HIGH-PURITY SILICA GLASS

    H NISHIKAWA, R NAKAMURA, Y OHKI, Y HAMA

    PHYSICAL REVIEW B   48 ( 5 ) 2968 - 2973  1993.08  [Refereed]

     View Summary

    Electron-spin-resonance (ESR) and ultraviolet-vacuum-ultraviolet (uv-vuv) spectra were measured on two types of oxygen-deficient silicas, i.e., low-OH ([OH] < 1 ppm) and high-OH ([OH] almost-equal-to 20 ppm) silicas, and a high-OH ([OH] almost-equal-to 1000 ppm) stoichiometric silica. The results of ESR show that the E' concentration induced in an OH-containing oxygen-deficient silica is one or two orders of magnitude higher than in low-OH oxygen-deficient and high-OH stoichiometric silicas when exposed to 6.4-eV or 7.9-eV photons. It is shown from the vuv spectra that the conversion of an oxygen vacancy into an E' center is enhanced in the OH-containing oxygen-deficient silica compared with the low-OH oxygen-deficient silica. The conversion into an E' center is considered to be mediated by diffusion of atomic hydrogen released from the Si-H bond. A slight peak shift of the 5.8-eV absorption band due to the E' center suggests that the formation of an E(beta)'-like center is involved in the enhanced E' creation. A possible correlation of the enhanced E' creation with the appearance of a 7.4-mT doublet in the ESR spectrum is also discussed.

  • Improvement of Dielectric Properties by Plasma Co-polymerization

    T. Nakano, K. Ishii, Y. Ohki※

    Proceedings of 6th Symposium on Plasma Science for Materials     113 - 118  1993.06

     View Summary

    窒素およびフッ素を含有させることでプラズマ重合エチレン膜の電気的特性の向上を図った。窒素を含有させた膜では,局在順位の存在に伴う光学ギャップの減少が観測され,高電界伝導電流が増加した。高電界伝導電流の時間波形には,電子雪崩の発生をうかがわせる雑音成分が現れた。一方,フッ素含有膜では,電気陰性度の高いフッ素による電子散乱ために高速電子の平均自由行程が減少し,電子雪崩の発生が抑制された。その結果,絶縁破壊強度の増加が観測された。

  • 低密度ポリエチレン中の不純物が空間電荷形成挙動に与える影響

    田中康寛, 田中貞嗣, 大木義路

    電気学会論文誌A   113 ( 8 ) 598 - 599  1993  [Refereed]

    DOI

  • Enhanced photogeneration of E'centers from neutral oxygen vacancies in the presence of hydrogen in high-purity silica glass

    Hiroyuki Nishikawa, Ryuta Nakamura, Yoshimichi Ohki, Yoshimasa Hama

    Phys. Rev. B   48 ( 5 ) 2968 - 2973  1993  [Refereed]

     View Summary

    Electron-spin-resonance (ESR) and ultraviolet-vacuum-ultraviolet (uv-vuv) spectra were measured on two types of oxygen-deficient silicas, i.e., low-OH ([OH]<1 ppm) and high-OH ([OH]20 ppm) silicas, and a high-OH ([OH]1000 ppm) stoichiometric silica. The results of ESR show that the E′ concentration induced in an OH-containing oxygen-deficient silica is one or two orders of magnitude higher than in low-OH oxygen-deficient and high-OH stoichiometric silicas when exposed to 6.4-eV or 7.9-eV photons. It is shown from the vuv spectra that the conversion of an oxygen vacancy into an E′ center is enhanced in the OH-containing oxygen-deficient silica compared with the low-OH oxygen-deficient silica. The conversion into an E′ center is considered to be mediated by diffusion of atomic hydrogen released from the Si-H bond. A slight peak shift of the 5.8-eV absorption band due to the E′ center suggests that the formation of an Eβ′-like center is involved in the enhanced E′ creation. A possible correlation of the enhanced E′ creation with the appearance of a 7.4-mT doublet in the ESR spectrum is also discussed. © 1993 The American Physical Society.

    DOI

  • 液晶ポリマの電気伝導特性

    平井知生, 宇田茂晴, 田中康寛, 大木義路, 池田雅昭

    電気学会論文誌A   113 ( 1 ) 32 - 38  1993  [Refereed]

    DOI

  • High-efficiency Water Pipe Cooling System for Underground Transmission Lines

    Y. Ohki

    IEEE Electrical Insulation Magazine   9 ( 1 ) 39 - 40  1993  [Refereed]

    DOI

  • BREAKDOWN CHARACTERISTICS OF THIN SIO2-FILMS DEPOSITED FROM TEOS USING PLASMA CVD METHOD

    K ISHII, T MORITA, D ISSHIKI, Y OHKI

    IEEE 1993 ANNUAL REPORT     355 - 360  1993  [Refereed]

  • EFFECTS OF ADDITIVES ON SPACE-CHARGE FORMATION IN POLYETHYLENE

    Y TANAKA, S TANAKA, Y OHKI

    SIXTH INTERNATIONAL CONFERENCE ON DIELECTRIC MATERIALS, MEASUREMENTS AND APPLICATIONS   363   135 - 137  1993  [Refereed]

  • High-efficiency water pipe cooling system for underground transmission lines (News from Japan)

    Yoshimichi Ohki

    IEEE Electrical Insulation Magazine   39 ( 40 ) 9 - 1  1993  [Refereed]

    DOI

  • CHARACTERIZATION OF CLOX RADICALS IN VACUUM-ULTRAVIOLET-IRRADIATED HIGH-PURITY SILICA GLASS

    H NISHIKAWA, R NAKAMURA, Y OHKI, K NAGASAWA, Y HAMA

    PHYSICAL REVIEW B   46 ( 13 ) 8073 - 8079  1992.10  [Refereed]

     View Summary

    An electron-spin-resonance (ESR) study was made on the defect centers induced by vacuum-ultraviolet (vuv) laser (7.9-eV) irradiation in high-purity silica glasses. We observed two types of ESR spectra, each consisting of a concentric pair of four-line peaks, in low-OH oxygen-surplus silica produced with plasma methods. Spin-Hamiltonian parameters determined by computer simulation of the experimental ESR line shapes are consistent with oxy-radicals of chlorine. The observed ClO(x) (x = 2,3) radicals are considered to be due to the reaction between chlorine and oxygen, which were both produced by vuv-laser irradiation.

    DOI

  • MORPHOLOGY OF ETHYLENE-STYRENE COPOLYMERS AND ITS EFFECT ON DIELECTRIC STRENGTH

    Y TANAKA, Y OHKI, M IKEDA

    IEEE TRANSACTIONS ON ELECTRICAL INSULATION   27 ( 3 ) 432 - 439  1992.06  [Refereed]

     View Summary

    The relation between morphological change, induced by copolymerized benzene rings or by kneading, and the dielectric strength of ethylene-styrene random copolymer was studied. Random distribution of styrene in the copolymer was confirmed by C-13 Fourier-transform nuclear magnetic resonance. With increasing the styrene content of the unkneaded copolymer, the crystallinity, the size of spherulite and the lamellar thickness decrease, while the lattice constant increases. However, in kneaded samples, the lamellar thickness and the lattice constant remain unchanged up to the styrene content of 3.10 wt%. Styrene content of the sample showing the highest dielectric strength increases as the temperature decreases. It is considered that the effect of benzene rings, which hinder the acceleration of electrons, becomes pronounced in the low temperature region where the breakdown process is electronic. Kneaded samples, especially the kneaded sample with the styrene content of 3.10 wt%, have higher dielectric strength than unkneaded samples of the same styrene content. It is considered that benzene rings are moved to the interfacial region between lamellar and amorphous parts by kneading and hinder the acceleration of electrons along the lamellae more effectively.

    DOI

  • プラズマ重合エチレン薄膜の光伝導特性

    石井啓介, 大木義路

    電気学会論文誌A   112 ( 9 ) 789 - 796  1992  [Refereed]

    DOI

  • プラズマ重合エチレンおよびフッ素含有エチレン薄膜の絶縁破壊機構

    石井啓介, 大木義路

    電気学会論文誌A   112 ( 3 ) 188 - 195  1992  [Refereed]

    DOI

  • PHOTOLUMINESCENCE FROM DEFECT CENTERS IN HIGH-PURITY SILICA GLASSES OBSERVED UNDER 7.9-EV EXCITATION

    H NISHIKAWA, T SHIROYAMA, R NAKAMURA, Y OHKI, K NAGASAWA, Y HAMA

    PHYSICAL REVIEW B   45 ( 2 ) 586 - 591  1992.01  [Refereed]

     View Summary

    Photoluminescence under 7.9-eV excimer-laser excitation was measured at room temperature on various types of high-purity silica glasses, which were classified based on the oxygen stoichiometry and OH concentrations. Several luminescence bands with different peak energies from 1.9 to 4.3 eV and decay constants were observed in different types of silicas. The 1.9-eV band was observed in low-OH oxygen-surplus and high-OH silicas. The 2.7-eV band was observed only in low-OH oxygen-deficient silicas. The 3.1- and the 4.2-eV bands were observed in a particular type of silica, which has a characteristic absorption band at 5.1 eV (referred to as the B2beta band). The 4.3-eV band was observed in all types of silicas but the oxygen-surplus type. These results indicate that defects responsible for these luminescence bands are diamagnetic defects introduced during preparation or paramagnetic species induced during excitation.

    DOI

  • ELECTRICAL-CONDUCTION IN LIQUID-CRYSTAL POLYMER

    Y OHKI, T HIRAI, S UDA, Y TANAKA, M IKEDA

    PROCEEDINGS OF THE 4TH INTERNATIONAL CONFERENCE ON CONDUCTION AND BREAKDOWN IN SOLID DIELECTRICS     22 - 26  1992  [Refereed]

  • CHLORINE-RELATED AND OXYGEN-RELATED PARAMAGNETIC CENTERS IN VUV-IRRADIATED HIGH-PURITY SILICAS

    H NISHIKAWA, R NAKAMURA, K NAGASAWA, Y OHKI, Y HAMA

    PHYSICS OF NON-CRYSTALLINE SOLIDS     494 - 498  1992  [Refereed]

  • ANALYSIS OF BONDING STATE IN PURE SILICA GLASS FROM PARAMAGNETIC DEFECTS INDUCED BY MECHANICAL FRACTURE

    Y OHKI, S MUNEKUNI, N DOHGUCHI, H NISHIKAWA, K NAGASAWA, Y HAMA

    PHYSICS OF NON-CRYSTALLINE SOLIDS     684 - 688  1992  [Refereed]

  • High field properties of polyether-ether-ketone

    Kenji Ohishi, Tomoo Hirai, Yasuhiro Tanaka, Yoshimichi Ohki

        1018 - 1020  1991.12

     View Summary

    The mechanism of electrical conduction in PEEK (polyether-ether-ketone) was studied from the results of DC conduction and thermal pulse current (TPC) measurements. The DC conduction current in PEEK films was measured at various electric fields at room temperature. From the results of electric-field dependence, it is thought that the conduction mechanism in PEEK is due to hopping. The TPC spectra, which were obtained under short-circuit conditions after the application of various voltages, show the existence of heterospace charges near the two electrodes. The TPC spectra increase with an increase in the electric field applied. From these results, it is thought that the electrical conduction in PEEK is due to carrier hopping. The carriers reached near the counterelectrode are not easily neutralized and begin to form heterocharges.

  • SI-O-SI STRAINED BOND AND PARAMAGNETIC DEFECT CENTERS INDUCED BY MECHANICAL FRACTURING IN AMORPHOUS SIO2

    S MUNEKUNI, N DOHGUCHI, H NISHIKAWA, Y OHKI, K NAGASAWA, Y HAMA

    JOURNAL OF APPLIED PHYSICS   70 ( 9 ) 5054 - 5062  1991.11  [Refereed]

     View Summary

    Mechanical stress is applied to high-purity amorphous SiO2 samples by means of fracturing. The electron-spin-resonance spectra suggest that the formation of a majority of paramagnetic defects is from cleavage of Si-O-Si bridges in the glass network, but there are some sample-to-sample variances in fracture-induced paramagnetic defects, suggesting cleavage of differing chemical bonding states in the samples. Nonstoichiometric bonds, = Si-Si = and = Si-O-O-Si =, are assumed to be one reason for the sample dependency. Formation of Si-O-Si strained bonds from mechanical fracturing is confirmed from sequential gamma-ray irradiation and heat annealing experiments. The Si-O-Si strained bond is approximately annealed at about 300-degrees-C. By comparing the fracture-induced defects for glass preforms and optical fibers, the change in chemical bonding state can be analyzed. Analysis of mechanical-fracture-induced defects is a strong technique for elucidation of the chemical bonding state of silica glass.

    DOI

  • EFFECT OF HIGH-TEMPERATURE TREATMENT ON OPTICAL-ABSORPTION BANDS IN AMORPHOUS SIO2

    N DOHGUCHI, S MUNEKUNI, H NISHIKAWA, Y OHKI, K NAGASAWA

    JOURNAL OF APPLIED PHYSICS   70 ( 5 ) 2788 - 2790  1991.09  [Refereed]

     View Summary

    The intensity change in optical absorption at 3.8, 5.0, and 5.8 eV was measured for thermal treatment up to 1000-degrees-C done on silica glass samples manufactured by various methods. It was found that these absorption bands change differently with heat, depending on the synthesis process of the sample. The 3.8-eV absorption band due to peroxy linkage is found to increase with high-temperature treatment when the sample contains dissolved oxygen molecules within the sample. For the sample with no oxygen molecules, high-temperaturetreatment does not alter the absorption intensity at 3.8 eV. The 5.0-eV absorption band is found to decrease by high-temperature treatment. In the case of an oxygen-containingsample, the reaction of oxygen molecules with oxygen vacancy sites is a cause of this decrease. The 5.1-eV band, which was though to have no temperature dependence, is found to be annealed at a temperature region around 950-degrees-C.

    DOI

  • EFFECTS OF CRYSTALLINITY AND ELECTRON MEAN-FREE-PATH ON DIELECTRIC STRENGTH OF LOW-DENSITY POLYETHYLENE

    Y TANAKA, N OHNUMA, K KATSUNAMI, Y OHKI

    IEEE TRANSACTIONS ON ELECTRICAL INSULATION   26 ( 2 ) 258 - 265  1991.04  [Refereed]

     View Summary

    The relation between crystallinity and the mean free path of high-energy electrons in low-density polyethylene and their effects on dielectric strength was studied. Thin LDPE films obtained by vacuum evaporation were annealed at temperatures between room temperature and 100-degrees-C. Crystallinity was measured by X-ray diffraction. Electrons were injected into the film by photo-emission and the energy of electrons emitted into vacuum through the film was analyzed to obtaine their energy distribution. The mean free path was then calculated from the energy distribution. It was found that the crystallinity increases and the mean free path becomes longer as the annealing temperature increases. Dielectric strength was measured by applying an impulse voltage of 1.2/50-mu-s at room temperature and -50-degrees-C. It was found that the breakdown field decreases with an increase in annealing temperature at both measurement temperatures. Thus, increased crystallinity makes electron transport easier and the mean-free-path longer, resulting in a lower dielectric strength.

    DOI

  • 水素プラズマによるYBa2Cu3O7の劣化

    沓掛孝夫, 大木義路

    電気学会論文誌A   111 ( 7 ) 684 - 685  1991  [Refereed]

    DOI

  • シリカガラスの光吸収・放出と構造欠陥

    大木義路

    電気学会論文誌A   111 ( 3 ) 223 - 226  1991  [Refereed]

    DOI

  • 高純度シリカガラスの放射線・レーザ誘起欠陥

    大木義路, 長沢可也

    応用物理   60 ( 7 ) 708 - 711  1991  [Refereed]

    DOI

  • Effect of plasma surface modification on electrical conduction in polyethylene

    Hisaaki Hayashi, Toshiki Nakano, Yoshimichi Ohki

    Electrical Engineering in Japan   111 ( 6 ) 17 - 24  1991  [Refereed]

     View Summary

    High‐density polyethylene films 50 μm thick were exposed to the downstream plasma flow of RF glow discharges in argon, hydrogen, or oxygen. It was found that many transvinylene and unreacted radicals remain after the argon or hydrogen plasma treatment, and that carbonyl groups are formed in the oxygen plasma treatment. The conduction current was smaller in the hydrogen‐plasma treated samples and the samples exposed to oxygen or ozone after hydrogen‐plasma treatment than in the untreated samples. Thermal pulse current measurements indicate that the treated surface layer traps electrons or holes and reduces the field strength at the electrode surface, thus suppressing further injection of electrons or holes. Copyright © 1991 Wiley Periodicals, Inc., A Wiley Company

    DOI

  • HIGH-FIELD CONDUCTION AND SPACE-CHARGE FORMATION IN ETHYLENE STYRENE COPOLYMERS

    Y TANAKA, Y MITA, K OHISHI, T HIRAI, Y OHKI, M IKEDA

    7TH INTERNATIONAL SYMPOSIUM ON ELECTRETS ( ISE 7 ) : PROCEEDINGS     159 - 164  1991  [Refereed]

  • DIELECTRIC-PROPERTIES OF POLYMER BLEND OF POLYPROPYLENE AND POLYETHYLENE

    K KATSUNAMI, K ISHII, Y TANAKA, Y OHKI

    PROCEEDINGS OF THE 3RD INTERNATIONAL CONFERENCE ON PROPERTIES AND APPLICATIONS OF DIELECTRIC MATERIALS, VOLS 1 AND 2     999 - 1002  1991

     View Summary

    直流電気伝導,熱パルス電流,熱刺激電流測定結果を基に,ポリエチレン,ポリプロピレンおよびそれらの共重合膜における空間電荷形成の特性を調べた。熱パルス電流波形を解析することにより,10分間の高電圧印加後には,ポリエチレンでは正の,ポリプロピレンでは負の空間電荷が形成されることが示された。共重合膜では,エチレンとプロピレンの割合に応じて正と負の電荷形成がなされた。また,球晶界面の機械的強度が増加するため,供重合により室温での絶縁破壊強度は増加した。

  • RELATION BETWEEN MORPHOLOGY CHANGE DUE TO MECHANICAL KNEADING AND DIELECTRIC-BREAKDOWN CHARACTERISTICS IN ETHYLENE-STYRENE COPOLYMERS

    Y TANAKA, T OTSUBO, Y OHKI, M IKEDA, F YAZAKI

    PROCEEDINGS OF THE 3RD INTERNATIONAL CONFERENCE ON PROPERTIES AND APPLICATIONS OF DIELECTRIC MATERIALS, VOLS 1 AND 2     1169 - 1172  1991

     View Summary

    The relation between morphological change, induced by copolymerized benzene rings or by kneading, and the dielectric strength of ethylene-styrene copolymer was studied. In unkneaded samples, the lamellar thickness decreases and the lattice constant along the a-axis increases with increasing styrene content. However, in kneaded samples, the lamellar thickness and the lattice constant remain constant up to a styrene content of 3.10 wt%. Spherulite is observed with a polarizing microscope in unkneaded samples, but it disappears in kneaded samples. Kneaded samples, especially the kneaded sample with a styrene content of 3.10 wt%, have higher breakdown strength than unkneaded samples. The styrene content of the sample showing the highest breakdown strength increases as the temperature decreases. At -50°C or -80°C, the sample with a styrene content of 15.40 wt% has the highest breakdown strength of all the samples. It is considered that benzene rings are moved to the interfacial region between lamellar and amorphous parts by kneading and hinder the acceleration of electrons along the lamellae. This effect of benzene rings becomes pronounced in the low-temperature region where the breakdown process is electronic.

  • PHOTOLUMINESCENCE LIFETIME IN SILICA GLASSES EXCITED BY VACUUM ULTRA-VIOLET LASER IRRADIATION

    H NISHIKAWA, R NAKAMURA, K NAGASAWA, Y OHKI, Y HAMA

    PROCEEDINGS OF THE 3RD INTERNATIONAL CONFERENCE ON PROPERTIES AND APPLICATIONS OF DIELECTRIC MATERIALS, VOLS 1 AND 2     1032 - 1035  1991  [Refereed]

     View Summary

    Photoluminescence bands in silica glasses excited by a VUV (vacuum ultraviolet) laser (7.9 eV) are reported. Luminescence spectra of various types of silicas, prepared under different conditions, exhibit sample-to-sample variations. Several luminescence bands (1.9-eV, 2.7-eV, 3.1-eV, 4.2-eV, 4.3-eV, and 2-4-eV bands) were observed at room temperature. The 2.7-eV band, previously ascribed to a triplet-to-singlet transition of neutral oxygen vacancy, shows a slow decay rate of τ ≅ 10 ms. Relatively slow decay rates of τ ≅ 15 μs and τ ≅ 110 μs were obtained for the 1.9- and 3.1-eV bands, respectively.

  • Technical report: extrusion-molded joints for world's first long-distance XLPE cable line (News from Japan)

    Yoshimichi Ohki

    IEEE Electrical Insulation Magazine   7 ( 3 ) 31 - 33  1991  [Refereed]

    DOI

  • EFFECT OF POLAR GROUPS ON THE ELECTRICAL BREAKDOWN STRENGTH OF PLASMA-POLYMERIZED FILMS

    T NAKANO, M FUKUYAMA, H HAYASHI, K ISHII, Y OHKI

    IEEE TRANSACTIONS ON ELECTRICAL INSULATION   25 ( 6 ) 1085 - 1091  1990.12  [Refereed]

     View Summary

    Electrical breakdown of plasma polymer films was studied by applying rectangular voltage pulses. The breakdown field increases after incorporation of fluorine atoms into the film when the pulse width is &lt; 5-mu-s. This is due to the scattering of electronic carriers caused by C-F bonds.

    DOI

  • SPACE CHARGES IN ETHYLENE STYRENE COPOLYMERS IRRADIATED BY LOW-DOSE GAMMA-RAYS

    Y TANAKA, Y MITA, Y OHKI, H YOSHIOKA, M IKEDA, F YAZAKI

    JOURNAL OF PHYSICS D-APPLIED PHYSICS   23 ( 12 ) 1491 - 1496  1990.12  [Refereed]

    DOI

  • VARIOUS TYPES OF NONBRIDGING OXYGEN HOLE CENTER IN HIGH-PURITY SILICA GLASS

    S MUNEKUNI, T YAMANAKA, Y SHIMOGAICHI, R TOHMON, Y OHKI, K NAGASAWA, Y HAMA

    JOURNAL OF APPLIED PHYSICS   68 ( 3 ) 1212 - 1217  1990.08  [Refereed]

    DOI

  • ELECTRICAL-PROPERTIES OF NITROGEN-CONTAINING PLASMA POLYMER-FILMS

    T NAKANO, S KOIKE, Y OHKI

    JOURNAL OF PHYSICS D-APPLIED PHYSICS   23 ( 6 ) 711 - 718  1990.06  [Refereed]

    DOI

  • GENERATION MECHANISM OF PHOTOINDUCED PARAMAGNETIC CENTERS FROM PREEXISTING PRECURSORS IN HIGH-PURITY SILICAS

    H NISHIKAWA, R NAKAMURA, R TOHMON, Y OHKI, Y SAKURAI, K NAGASAWA, Y HAMA

    PHYSICAL REVIEW B   41 ( 11 ) 7828 - 7834  1990.04  [Refereed]

    DOI

  • TRIPLET-STATE DEFECT IN HIGH-PURITY SILICA GLASS

    R TOHMON, Y SHIMOGAICHI, Y TSUTA, S MUNEKUNI, Y OHKI, Y HAMA, K NAGASAWA

    PHYSICAL REVIEW B   41 ( 10 ) 7258 - 7260  1990.04  [Refereed]

    DOI

  • SPATIAL-DISTRIBUTION OF DEFECTS IN HIGH-PURITY SILICA GLASSES

    R TOHMON, A IKEDA, Y SHIMOGAICHI, S MUNEKUNI, Y OHKI, K NAGASAWA, Y HAMA

    JOURNAL OF APPLIED PHYSICS   67 ( 3 ) 1302 - 1306  1990.02  [Refereed]

    DOI

  • エチレンースチレン共重合体の機械的練込みによる高次構造の変化と絶縁破壊特性

    田中康寛, 尾坪利信, 大木義路, 池田雅昭, 矢崎文彦

    電気学会論文誌A   110 ( 12 ) 919 - 926  1990  [Refereed]

    DOI

  • プラズマによるポリエチレンの表面構造の変化と電気伝導

    林央晶, 中野俊樹, 大木義路

    電気学会論文誌A   110 ( 12 ) 913 - 918  1990  [Refereed]

    DOI

  • PARAMAGNETIC CENTERS INDUCED BY ARF EXCIMER LASER IRRADIATION IN HIGH-PURITY SILICA GLASSES

    H NISHIKAWA, R NAKAMURA, R TOHMON, Y OHKI, Y HAMA, Y SAKURAI, K NAGASAWA

    PROPERTIES AND CHARACTERISTICS OF OPTICAL GLASS II   1327   69 - 78  1990  [Refereed]

  • DIELECTRIC-BREAKDOWN DUE TO HOLE AVALANCHE IN PLASMA POLYMER-FILMS

    K ISHII, Y OHKI, T NAKANO

    CONFERENCE RECORD OF THE 1990 IEEE INTERNATIONAL SYMPOSIUM ON ELECTRICAL INSULATION     76 - 79  1990

     View Summary

    プラズマ重合エチレン(PPE)膜およびフッ素含有プラズマ重合エチレン(PPEF)膜について,その高電界伝導特性と絶縁破壊特性を調べた。破壊前駆電流から計算したファウラーノードハイムトンネル注入障壁は金電極の時よりアルミニウム電極の方が大きかった。破壊遅れ時間は,アリミニウム電極の方が長かった。破壊前駆電流の時間波形から,正孔なだれによる絶縁破壊が起きておりPPEFの方がPPEに比べ絶縁破壊強度が大きいことが分かった。

  • GAMMA-IRRADIATION EFFECT ON HIGH-TC SUPERCONDUCTOR YBA2CU3O7-X

    T KUTSUKAKE, H SOMEI, Y OHKI, K NAGASAWA, F KANEKO

    JAPANESE JOURNAL OF APPLIED PHYSICS PART 2-LETTERS & EXPRESS LETTERS   28 ( 8 ) L1393 - L1394  1989.08  [Refereed]

    DOI

  • DEFECTS AND OPTICAL-ABSORPTION BANDS INDUCED BY SURPLUS OXYGEN IN HIGH-PURITY SYNTHETIC SILICA

    H NISHIKAWA, R TOHMON, Y OHKI, K NAGASAWA, Y HAMA

    JOURNAL OF APPLIED PHYSICS   65 ( 12 ) 4672 - 4678  1989.06  [Refereed]

    DOI

  • RELATION BETWEEN THE 1.9 EV LUMINESCENCE AND 4.8 EV ABSORPTION-BANDS IN HIGH-PURITY SILICA GLASS

    R TOHMON, Y SHIMOGAICHI, S MUNEKUNI, Y OHKI, Y HAMA, K NAGASAWA

    APPLIED PHYSICS LETTERS   54 ( 17 ) 1650 - 1652  1989.04  [Refereed]

    DOI

  • 2.7-EV LUMINESCENCE IN AS-MANUFACTURED HIGH-PURITY SILICA GLASS

    R TOHMON, Y SHIMOGAICHI, H MIZUNO, Y OHKI, K NAGASAWA, Y HAMA

    PHYSICAL REVIEW LETTERS   62 ( 12 ) 1388 - 1391  1989.03  [Refereed]

    DOI

  • エチレン−スチレン共重合体の耐水トリー性

    秋山敬一郎, 大木義路, 矢崎文彦, 池田雅昭

    電気学会論文誌A   109 ( 10 ) 464  1989  [Refereed]

    DOI

  • Effect of introduced fluorine on electrical breakdown characteristics of plasma polymer films

    Toshiki Nakano, Masao Fukuyama, Yoshimichi Ohki

    IEEJ Transactions on Fundamentals and Materials   109 ( 8 ) 343 - 350  1989  [Refereed]

    DOI

  • Carrier Traps in Ethylene-Styrene Copolymer.

    Yasuhiro Tanaka, Yoshimichi Ohki, Masaaki Ikeda

    IEEJ Transactions on Fundamentals and Materials   109 ( 7 ) 295 - 302  1989  [Refereed]

    DOI

  • CORRELATION OF THE 5.0-EV AND 7.6-EV ABSORPTION-BANDS IN SIO2 WITH OXYGEN VACANCY

    R TOHMON, H MIZUNO, Y OHKI, K SASAGANE, K NAGASAWA, Y HAMA

    PHYSICAL REVIEW B   39 ( 2 ) 1337 - 1345  1989.01  [Refereed]

    DOI

  • 325-nm absorption band by peroxy linkage in pure silica core fiber

    H. Nishikawa, R. Tohmon, Y. Ohki, Y. Hama, K. Nagasawa

        158 - 159  1989

     View Summary

    Experimental results showing that peroxy linkage, a form of excess oxygen, gives rise to an optical absorption band at 325 nm are presented. Moreover, understanding the nature of excess oxygen is important for fibers used in optical communication because excess oxygen is the cause of the 1.52-μm absorption band. The characteristics of the three principal fibers and a specially synthesized SA4 fiber used in the experiment are shown and discussed.

  • TRIPLET-STATE IN HIGH-PURITY SILICA GLASSES

    R TOHMON, Y SHIMOGAICHI, Y OHKI, K NAGASAWA, Y HAMA

    GLASSES FOR OPTOELECTRONICS   1128   228 - 235  1989  [Refereed]

  • ELECTRONIC BREAKDOWN PROCESS OF PLASMA POLYMER-FILMS

    T NAKANO, M FUKUYAMA, H HAYASHI, K ISHII, Y OHKI

    PROCEEDINGS OF THE 3RD INTERNATIONAL CONFERENCE ON CONDUCTION AND BREAKDOWN IN SOLID DIELECTRICS     82 - 86  1989  [Refereed]

  • ROLE OF NONSTOICHIOMETRY ON UV ABSORPTION AND LUMINESCENCE IN HIGH-PURITY SILICA

    H NISHIKAWA, R TOHMON, K NAGASAWA, Y OHKI, Y HAMA

    GLASSES FOR OPTOELECTRONICS   1128   281 - 289  1989  [Refereed]

  • RED LUMINESCENCE IN PURE SILICA GLASS

    R TOHMON, Y SHIMOGAICHI, S MUNEKUNI, Y OHKI, K NAGASAWA, Y SAKURAI, Y HAMA

    GLASSES FOR OPTOELECTRONICS   1128   198 - 204  1989  [Refereed]

  • CARRIER TRAPS IN ETHYLENE-STYRENE COPOLYMER

    Y OHKI, Y TANAKA, M IKEDA

    IEEE 1989 ANNUAL REPORT     278 - 283  1989  [Refereed]

  • EFFECT OF PLASMA POLYMER COATING ON THE DIELECTRIC STRENGTH OF POLYETHYLENE

    T NAKANO, T KIHIRA, Y OHKI

    PROCEEDINGS OF THE 3RD INTERNATIONAL CONFERENCE ON CONDUCTION AND BREAKDOWN IN SOLID DIELECTRICS     374 - 376  1989  [Refereed]

  • 500-kV long-distance cable completed in Japan (News from Japan)

    Yoshimichi Ohki

    IEEE Electrical Insulation Magazine   5 ( 2 ) 28 - 32  1989  [Refereed]

    DOI

  • Development of a Closed AC 2,000-kV Testing Facility

    Y. Ohki

    IEEE Electrical Insulation Magazine   5 ( 4 ) 20 - 21  1989  [Refereed]

    DOI

  • DIELECTRIC-PROPERTIES OF PLASMA-POLYMERIZED TETRAFLUOROETHYLENE FILMS

    T NAKANO, M FUKUYAMA, Y OHKI

    JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS SHORT NOTES & REVIEW PAPERS   27 ( 6 ) 1042 - 1046  1988.06  [Refereed]

    DOI

  • RELATION BETWEEN GE(2) CENTER AND 11.9 MT HYPERFINE-STRUCTURE OF ELECTRON-SPIN-RESONANCE SPECTRA IN GE-DOPED SILICA FIBERS

    K NAGASAWA, T FUJII, Y OHKI, Y HAMA

    JAPANESE JOURNAL OF APPLIED PHYSICS PART 2-LETTERS   27 ( 2 ) L240 - L243  1988.02  [Refereed]

    DOI

  • Radiation Resistance of Ethylene-Styrene Copolymers

    Kaoru Matsumoto, Masaaki Ikeda, Yoshimichi Ohki, Yasuo Kusama, Masahiro Harashige, Fumihiko Yazahi

    IEEJ Transactions on Fundamentals and Materials   108 ( 8 ) 335 - 342  1988  [Refereed]

    DOI

  • Influence of morphology on electrical breakdown characteristics of ethylene-styrene copolymers.

    Ikeda Masaaki, Matsuo Katsuya, Ohki Yoshimichi, Hozumi Naohiro, Harashige Masahiro

    IEEJ Transactions on Fundamentals and Materials   108 ( 8 ) 321 - 328  1988  [Refereed]

    DOI CiNii

  • Electrical Breakdown Characteristics of Plasma-Copolymerized Thin Films from Allylamine and Ethylene.

    Toshiki Nakano, Osama Nakamura, Naoki Shibayama, Yoshimichi Okki

    IEEJ Transactions on Fundamentals and Materials   108 ( 3 ) 103 - 110  1988  [Refereed]

    DOI

  • Electrical breakdown characteristics of copolymers of ethylene and various aromatic monomers

    Masaaki Ikeda, Yoshimichi Ohki

    Electrical Engineering in Japan   108 ( 2 ) 1 - 15  1988  [Refereed]

    DOI

  • Plasma polymer surface layer for suppression of charge injection into polyethylene

    Toshiki Nakano, Tohru Kihira, Yoshimichi Ohki

    Electrical Engineering in Japan   108 ( 2 ) 24 - 30  1988  [Refereed]

    DOI

  • HYDROGEN-BOND OF OH-GROUPS IN SILICA GLASS AND ITS RELATION TO THE 1.39 MU-M ABSORPTION

    Y YOKOMACHI, R TOHMON, K NAGASAWA, Y OHKI

    JOURNAL OF NON-CRYSTALLINE SOLIDS   95-6 ( PART2 ) 663 - 670  1987.12  [Refereed]

    DOI

  • CAUSE OF THE 5.0 EV ABSORPTION-BAND IN PURE SILICA GLASS

    R TOHMON, Y YAMASAKA, K NAGASAWA, Y OHKI, Y HAMA

    JOURNAL OF NON-CRYSTALLINE SOLIDS   95-6   671 - 678  1987.12  [Refereed]

    DOI

  • GAMMA-RAY INDUCED 2-EV OPTICAL-ABSORPTION BAND IN PURE-SILICA CORE FIBERS

    K NAGASAWA, Y OHKI, Y HAMA

    JAPANESE JOURNAL OF APPLIED PHYSICS PART 2-LETTERS & EXPRESS LETTERS   26 ( 6 ) L1009 - L1011  1987.06  [Refereed]

    DOI

  • EFFECT OF OXYGEN-CONTENT ON DEFECT FORMATION IN PURE-SILICA CORE FIBERS

    K NAGASAWA, Y HOSHI, Y OHKI

    JAPANESE JOURNAL OF APPLIED PHYSICS PART 2-LETTERS   26 ( 5 ) L554 - L557  1987.05  [Refereed]

    DOI

  • PLASMA-POLYMERIZED FILMS AS ORIENTATING LAYERS FOR LIQUID-CRYSTALS

    R WATANABE, T NAKANO, T SATOH, H HATOH, Y OHKI

    JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS SHORT NOTES & REVIEW PAPERS   26 ( 3 ) 373 - 376  1987.03  [Refereed]

    DOI

  • HIGH-ENERGY ELECTRON-TRANSPORT IN HYDROCARBONS

    RACZ, I, Y OHKI

    JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS SHORT NOTES & REVIEW PAPERS   26 ( 2 ) 209 - 215  1987.02  [Refereed]

    DOI

  • Plasma Polymer Surface Layer for Suppression of Charge Injection into Polyethylene

    Toshiki Nakano, Tohru Kihira, Yoshimichi Ohki

    IEEJ Transactions on Fundamentals and Materials   107 ( 11 ) 511 - 516  1987  [Refereed]

    DOI

  • Electrical Breakdown Characteristics of Copolymers of Ethylene and Various Aromatic Monomers

    Masaaki Ikeda, Yoshimichi Ohki, Hideo Kawabata

    IEEJ Transactions on Fundamentals and Materials   107 ( 9 ) 415 - 422  1987  [Refereed]

    DOI

  • Possibility of Ethylene-styrene Copolymer Applying to Power Cable Insulator

    Ikeda Masaaki, Hozumi Naohiro, Ohki Yoshimichi

    IEEJ Transactions on Fundamentals and Materials   107 ( 8 ) 404 - 404  1987  [Refereed]

    DOI CiNii

  • CAUSE OF THE HIGHER BREAKDOWN STRENGTH IN ETHYLENE-STYRENE COPOLYMER, A NEW INSULATING MATERIAL

    RACZ, I, Y OHKI

    JAPANESE JOURNAL OF APPLIED PHYSICS PART 2-LETTERS   26 ( 1 ) L32 - L34  1987.01  [Refereed]

    DOI

  • EFFECT OF CLADDING MATERIAL ON 2-EV OPTICAL-ABSORPTION IN PURE-SILICA CORE FIBERS AND METHOD TO SUPPRESS THE ABSORPTION

    K NAGASAWA, R TOHMON, Y OHKI

    JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS SHORT NOTES & REVIEW PAPERS   26 ( 1 ) 148 - 151  1987.01  [Refereed]

    DOI

  • RADIATION EFFECTS ON POLYMERS

    Y OHKI

    IEEE TRANSACTIONS ON ELECTRICAL INSULATION   21 ( 6 ) 919 - 922  1986.12  [Refereed]

    DOI

  • THE 1.52-MU-M ABSORPTION-BAND IN OPTICAL FIBERS INDUCED BY HYDROGEN TREATMENT

    K NAGASAWA, T TODORIKI, T FUJII, Y OHKI, Y HAMA

    JAPANESE JOURNAL OF APPLIED PHYSICS PART 2-LETTERS   25 ( 10 ) L853 - L855  1986.10  [Refereed]

    DOI

  • DIELECTRIC-PROPERTIES OF WATER AND WATER ETHYLENE-GLYCOL MIXTURES FOR USE IN PULSED POWER-SYSTEM DESIGN

    M ZAHN, Y OHKI, DB FENNEMAN, RJ GRIPSHOVER, VH GEHMAN

    PROCEEDINGS OF THE IEEE   74 ( 9 ) 1182 - 1221  1986.09  [Refereed]  [Invited]

  • STRUCTURAL INHOMOGENEITY IN GE-DOPED SILICA CORE OF OPTICAL FIBERS MADE BY THE VAD METHOD

    K NAGASAWA, Y OHKI

    JAPANESE JOURNAL OF APPLIED PHYSICS PART 2-LETTERS   25 ( 8 ) L682 - L683  1986.08  [Refereed]

    DOI

  • RADIATION EFFECTS ON PURE SILICA CORE OPTICAL FIBERS BY GAMMA-RAYS - RELATION BETWEEN 2 EV BAND AND NON-BRIDGING OXYGEN HOLE-CENTERS

    K NAGASAWA, Y HOSHI, Y OHKI, K YAHAGI

    JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS SHORT NOTES & REVIEW PAPERS   25 ( 3 ) 464 - 468  1986.03  [Refereed]

    DOI

  • Electrical Breakdown Characteristics of Ethylene-Styrene Copolymers

    Masaaki Ikeda, Yoshimichi Ohki

    IEEJ Transactions on Fundamentals and Materials   106 ( 10 ) 473 - 479  1986  [Refereed]

    DOI

  • 原子力施設における光ファイバ利用の現状と今後の展望

    矢作吉之助編, 大木義路他

    日本原子力学会誌   27 ( 9 ) 768 - 803  1985  [Invited]

    DOI

  • IMPROVEMENT OF RADIATION-RESISTANCE OF PURE SILICA CORE FIBERS BY HYDROGEN TREATMENT

    K NAGASAWA, Y HOSHI, Y OHKI, K YAHAGI

    JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS SHORT NOTES & REVIEW PAPERS   24 ( 9 ) 1224 - 1228  1985  [Refereed]

    DOI

  • ELECTRO-OPTIC CHARGE INJECTION AND TRANSPORT MEASUREMENTS IN HIGHLY PURIFIED WATER AND WATER ETHYLENE-GLYCOL MIXTURES

    M ZAHN, Y OHKI, K RHOADS, M LAGASSE, H MATSUZAWA

    IEEE TRANSACTIONS ON ELECTRICAL INSULATION   20 ( 2 ) 199 - 211  1985  [Refereed]

    DOI

  • EFFECT OF CARRIER GAS ON STRUCTURE OF PLASMA POLYMERIZED TETRAFLUOROETHYLENE FILMS

    T NAKANO, Y KASAMA, Y OHKI, K YAHAGI

    JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS SHORT NOTES & REVIEW PAPERS   24 ( 1 ) 83 - 86  1985  [Refereed]

    DOI

  • EFFECT OF ATMOSPHERE ON DIELECTRIC-PROPERTIES OF PLASMA POLYMERIZED ETHYLENE

    N HOZUMI, T TAKAO, Y KASAMA, Y OHKI

    JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS SHORT NOTES & REVIEW PAPERS   22 ( 4 ) 636 - 639  1983  [Refereed]

    DOI

  • DIELECTRIC LOSS MEASUREMENT OF PLASMA POLYMERIZED ETHYLENE FILMS IN VACUO AND EFFECT OF HYDROGEN TREATMENT

    N HOZUMI, T TAKAO, Y OHKI

    JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS SHORT NOTES & REVIEW PAPERS   21 ( 1 ) 195 - 196  1982  [Refereed]

    DOI

  • Deformation of Nematic-Positive Liquid Crystals by an Electric Field between Two Co-Axial Cylindrical Electrodes

    Norio Aoki, Yoshimichi Ohki, Kichinosuke Yahagi

    Japanese Journal of Applied Physics   18 ( 3 ) 523 - 526  1979  [Refereed]

    DOI

  • Impulse Surface Flashover along a Polycarbonate Plate with a Back Electrode in Vacuum

    Yoshimichi Ohki, Michio Sakai, Kichinosuke Yahagi

    Japanese Journal of Applied Physics   17 ( 10 ) 1739 - 1743  1978  [Refereed]

    DOI

  • A Modified Charge Superposition Method for the Calculation of Electric Field

    Yoshimichi Ohki, Kichinosuke Yahagi

    Japanese Journal of Applied Physics   16 ( 2 ) 355 - 358  1977  [Refereed]

    DOI

  • On the Conditioning Phenomena of Surface Flashover of Epoxy Resins in Vacuum

    Yoshimichi Ohki, Kichinosuke Yahagi

    Japanese Journal of Applied Physics   15 ( 1 ) 177 - 178  1976  [Refereed]

    DOI

  • Temperature Dependence of Surface Flashover Voltage of Polyethylene in Vacuum

    Y. Ohki, K. Yahagi

    Journal of Applied Physics   46 ( 8 ) 3695 - 3696  1975  [Refereed]

    DOI

  • 真空中における固体誘電体のインパルス沿面フラッシオーバについて

    大木義路, 大塚重信, 矢作吉之助

    電気学会論文誌A   95 ( 1 ) 41 - 47  1975  [Refereed]

    DOI

  • On the Surface-Flashover Phenomena in Polymer Insulators in Vacuum

    M. Akahane, Y. Ohki, D. Ito, K. Yahagi

    Electrical Engineering in Japan   94 ( 1 ) 1 - 8  1974  [Refereed]

    DOI

  • 真空中における高分子誘電体の直流沿面放電について

    赤羽正雄, 大木義路, 伊藤大佐, 矢作吉之助

    電気学会論文誌   94-A ( 1 ) 1 - 8  1974  [Refereed]

    DOI

▼display all

Books and Other Publications

  • 誘電体現象論(改訂版)

    大木 義路, 大久保 仁, 鈴置 保雄, 穂積 直裕(第1章全体。第2章大半、第3章大半など全体の約1/2)

    電気学会,オーム社(発売)  2021.03 ISBN: 9784886863157

  • 高分子材料の絶縁破壊・劣化メカニズムとその対策

    技術情報協会(第1章 高分子絶縁破壊のメカニズム, 第1節 高分子絶縁材料における電気絶縁性と破壊メカニズム)

    技術情報協会  2021.01 ISBN: 9784861048210

  • 原子力保全ハンドブック

    一般社団法人 日本保全学会( Part: Contributor, 36頁~39頁、71頁~73頁、967頁~968頁)

    株式会社ERC出版  2020.02

  • ナノテク材料 ~ポリマーナノコンポジット絶縁材料の世界~

    電気学会  2014 ISBN: 9784886862945

  • コンポジット材料の混練・コンパウンド技術と分散・界面制御

    技術情報協会  2013 ISBN: 9784861044779

  • 電気化学/インピーダンス測定のノウハウと正しいデータ解釈

    技術情報協会  2013 ISBN: 9784861044823

  • 先端エレクトロニクス分野における封止・シーリングの 材料設計とプロセス技術

    技術情報協会  2013 ISBN: 9784861044861

  • 高分子材料の耐久性・信頼性試験ノウハウ集

    技術情報協会  2012 ISBN: 9784861044199

  • 高電界現象論 基礎と応用

    オーム社  2011 ISBN: 9784274209642

  • 教えて?わかった!電磁気学

    オーム社  2011 ISBN: 9784274210082

  • 高分子絶縁材料技術とその実例・評価

    サイエンス&テクノロジー  2010 ISBN: 9784903413815

  • 太陽電池に用いられるフィルム、樹脂の高機能化とその応用

    技術情報協会  2010 ISBN: 9784861043130

  • フィルムの機能性向上と成形加工・分析・評価技術

    AndTech  2010 ISBN: 9784990538804

  • 電子/電気製品の発火・不良原因の究明技術と安全対策 第3章第1節[2]

    技術情報協会  2009 ISBN: 9784861042812

  • 最新 ポリ乳酸の改質・高機能化と成形加工技術 第5節 電気特性

    技術情報協会  2007 ISBN: 9784861041358

  • EEText 電磁気学

    オーム社  2007 ISBN: 9784274204845

  • 電気電子材料ー基礎から試験法までー (電気学会大学講座)

    電気学会  2006 ISBN: 4886862527

  • 3D Nanofabrication of Rutile TiO2 Single Crystals with Swift Heavy-Ions (a part of The Nano-Micro Interface)

    WILEY-VCH  2005 ISBN: 9783527309788

  • 3D Nanofabrication of Rutile TiO2 Single Crystals with Swift Heavy-Ions (a part of The Nano-Micro Interface)

    WILEY-VCH  2005 ISBN: 9783527309788

  • 平成13年度「原子力プラントのケーブル経年変化評価技術調査に関する事業報告書

    (財)発電設備技術検査協会  2002

  • セラミック工学ハンドブック(第2版)

    (社)日本セラミックス協会  2002

  • 誘電体物性 (電気・電子・情報工学系テキストシリーズ11)

    培風館  2002 ISBN: 4563036919

  • 誘電・絶縁材料の空間電荷分布計測法と標準化

    電気学会技術報告  2001

  • 平成12年度高経年化対策関連技術開発事業報告書

    発電設備技術検査協会(経産省原子力安全・保安院委託)  2001

  • 電気絶縁材料の誘電率および誘電正接試験方法通則

    電気学会  2001

  • 平成12年度電気設備技術基準機能性化適合調査(電気設備)報告書

    日本電気協会(資源エネルギー庁委託)  2001

  • 平成11年度電気施設技術基準機能性化適合調査(電気設備)報告書

    日本電気協会  2000

  • 平成10年度電気施設技術基準機能性化適合調査(電気設備)報告書

    日本電気協会  1999

  • 非晶質シリカ材料応用ハンドブック

    株式会社 リアライズ社  1999

  • 科学・技術大百科事典

    朝倉書店  1999

  • 特別高圧(11 kV~275 kV)架橋ポリエチレンケーブルおよび接続部の高電圧試験法

    電気学会電気規格調査会標準規格  1998

  • 平成9年度電気施設技術基準機能性化適合調査(電気設備)報告書

    電気協同研究  1998

  • 放電ハンドブック

    電気学会  1998 ISBN: 4886863086

  • 電気工学教育の歴史

    電気学会技術報告  1997

  • 誘電・絶縁材料の分析法の新展開

    電気学会技術報告  1996

  • CVケーブルおよび接続部の高電圧試験法

    電気協同研究 /電気協同研究会  1995

  • 電磁気学演習

    産業図書  1982 ISBN: 4782855095

  • 基礎 電磁気学

    産業図書  1981

▼display all

Misc

  • Modeling of Oxidation Process and Property Changes of Ethylene-Propylene-Diene Copolymer (vol 23, pg 537, 2016)

    Daomin Min, Shengtao Li, Naoshi Hirai, Yoshimichi Ohki

    IEEE TRANSACTIONS ON DIELECTRICS AND ELECTRICAL INSULATION   23 ( 2 ) 1234 - 1234  2016.04

    Other  

    DOI

  • 集束陽子線描画による三次元柔構造デバイスの創出,統合および超実装工学の推進

    西川宏之, 松村一成, 大石知司, 小池義和, 吉見靖男, 山口正樹, 長谷川忠大, 神谷富裕, 石井保行, 前川康成, 成沢忠, 大木義路, 内田諭, 渡辺徹, 堀口常雄, 林秀臣

    芝浦工業大学特別教育・研究報告集(CD-ROM)   2014   330 - 335  2015.06

    J-GLOBAL

  • 平成27年度SIT総合研究所研究成果報告書 集束陽子線描画による三次元柔構造デバイスの創出、統合および超実装工学の推進

    西川 宏之, 松村 一成, 大石 知司, 小池 義和, 吉見 靖男, 山口 正樹, 長谷川 忠大, 神谷 富裕, 石井 保行, 前川 康成, 成沢 忠, 大木 義路, 内田 諭, 渡辺 徹, 堀口 常雄, 林 秀臣

    特別教育・研究報告集     388 - 393  2015

    CiNii

  • 集束陽子線描画による三次元柔構造デバイスの創出、統合および超実装工学の推進

    西川 宏之, 松村 一成, 大石 知司, 小池 義和, 吉見 靖男, 山口 正樹, 長谷川 忠大, 神谷 富裕, 石井 保行, 前川 康成, 成沢 忠, 大木 義路, 内田 諭, 渡辺 徹, 堀口 常雄, 林 秀臣

    芝浦工業大学SIT総合研究所(先端工学研究機構)研究成果報告 Research report of S.I.T. Research Laboratories and Research Organization for Advanced Engineering, S.I.T.     2 - 7  2015

    CiNii

  • 集束陽子線描画による三次元柔構造デバイスの創出,統合および超実装工学の推進

    西川宏之, 松村一成, 大石知司, 小池義和, 吉見靖男, 山口正樹, 長谷川忠大, 神谷富裕, 石井保行, 前川康成, 成沢忠, 大木義路, 内田諭, 渡辺徹, 堀口常雄, 林秀臣

    芝浦工業大学特別教育・研究報告集(CD-ROM)   2013   348 - 359  2014.06

    CiNii J-GLOBAL

  • 集束陽子線描画による三次元柔構造デバイスの創出,統合および超実装工学の推進

    西川宏之, 松村一成, 大石知司, 小池義和, 吉見靖男, 山口正樹, 長谷川忠大, 神谷富裕, 石井保行, 前川康成, 成沢忠, 大木義路, 内田諭, 渡辺徹, 堀口常雄, 林秀臣

    芝浦工業大学特別教育・研究報告集(CD-ROM)   2012   429 - 436  2013.06

    J-GLOBAL

  • Comparison of Behavior of Alumina Plate-like Particles in an Uncured Epoxy Resin among Several Conditions of Applied Electric Field

    KINOSHITA Satoshi, KOZAKO Masahiro, HIKITA Masayuki, OHKI Yoshimichi

      2013 ( 11 ) 37 - 42  2013.01

    CiNii

  • Space charges induced in polymers by electron beam irradiation and their decay profiles

    Naoto Tomite, Yukitaka Arai, Yoshimichi Ohki, Hirotaka Nakmura, Toru Hinata, Masakazu Washio

    Proceedings of IEEE International Conference on Solid Dielectrics, ICSD     736 - 739  2013

     View Summary

    Electrons were irradiated to low-density polyethylene (LDPE), polyimide (PI), polyethylene terephthalate (PET), polyethylene naphthalate (PEN) and syndiotactic polystyrene (SPS). Many electrons were back scattered and large numbers of electrons and holes were generated in the film by ionization. Since the amount of negative charge observed under short-circuit after the irradiation is significantly smaller than the electron fluence, most electrons and holes generated by the irradiation were either recombined or transported to the electrodes. The amount of negative charge gradually decreases monotonically, showing the order of the decay speed, PI &gt
    LDPE &gt
    PET &gt
    SPS &gt
    PEN. The conductivity also shows the same order. When a dc voltage is applied to the sample, in which electrons and holes were generated abundantly by ionization, the electric field is enhanced significantly at the two electrodes due to the presence of positive charge near the cathode and negative charge near the anode. © 2013 IEEE.

    DOI

  • 集束陽子線描画による三次元柔構造を利用したフレキシブルデバイスの創出,統合および超実装工学の推進(フレキシブル実装工学研究センター)

    西川宏之, 長谷川忠大, 吉見靖男, 大石知司, 小池義和, 山口正樹, 松村一成, 堀口常雄, 神谷富裕, 石井保行, 前川康成, 成沢忠, 大木義路, 内田諭, 渡辺徹, 林秀臣

    芝浦工業大学特別教育・研究報告集(CD-ROM)   2011   336 - 339  2012.12

    J-GLOBAL

  • 超音波顕微鏡によるエポキシ硬化過程の定量観察

    山崎智司, 梶間翔太, 村上義信, 長尾雅行, 穂積直裕, 大木義路

    電気関係学会東海支部連合大会講演論文集(CD-ROM)   2012   ROMBUNNO.G4-2  2012.09

    J-GLOBAL

  • The History, the Present State and the Future Prospect of IEEJ Technical Committee on Dielectrics and Electrical Insulation : Anniversary of 200th Technical Committee Meeting on DEI

    NAGAO Masayuki, HOZUMI Naohiro, OKAMOTO Tatsuki, KIMURA Ken, OHKI Yoshimichi, KOSAKI Masamiysu, MIZUTANI Teruyoshi, TAKADA Tatsuo, TANAKA Toshikatsu

    電気学会研究会資料. DEI, 誘電・絶縁材料研究会   2011 ( 42 ) 1 - 6  2011.03

    CiNii

  • Space charge accumulation in two-layer dielectrics with LDPE and LDPE/MgO nanocomposite

    鐘ケ江越, 石本和之, 田中祀捷, 大木義路, 関口洋逸, 村田義直, REDDY C. C.

    電気学会誘電・絶縁材料研究会資料   DEI-09 ( 14-25 )  2009

    J-GLOBAL

  • LDPEとLDPE/MgOナノコンポジットの2層誘電体界面に低電界下で現れる空間電荷の原因

    鐘ケ江越, 石本和之, 田中祀捷, 大木義路, 関口洋逸, 村田義直, REDDY C.C.

    電気学会全国大会講演論文集   2009 ( 2 )  2009

    J-GLOBAL

  • LDPEとLDPE/MgOナノコンポジットの2層誘電体における空間電荷形成の原因

    鐘ケ江越, 石本和之, 田中祀捷, 大木義路, 関口洋逸, 村田義直, REDDY Chandupatla Chakradhar

    電気学会基礎・材料・共通部門大会講演論文集(CD-ROM)   2009  2009

    J-GLOBAL

  • Material challenge of MgO/LDPE nanocomposite for high field electrical insulation

    Masayuki Nagao, Yoshinobu Murakami, Yoshinao Murata, Yasuhiro Tanaka, Yoshimichi Ohki, Toshikatsu Tanaka

    42nd International Conference on Large High Voltage Electric Systems 2008, CIGRE 2008    2008.12

     View Summary

    Polymeric electrical insulating materials are widely used in various power equipments and cables. Additives and fillers are often adopted to polymeric materials for improving insulating and mechanical properties. Recently, nano-fillers are attracting attentions of many researchers and engineers, since they seem to bring higher potentials for advancement of electrical insulating properties as nanocomposites. This paper reports evaluation results of various insulating properties of a MgO/LDPE nanocomposite, jointly carried out by several organizations, aiming at development of an electrical insulating material for higher DC electric field. The DC breakdown strength and the volume resistivity increased with the addition of MgO nanofillers. The thermally stimulated depolarization current (TSDC) peak that spreads over a wide temperature range was observed in all the MgO/LDPE nanocomposites. Furthermore, by adopting a partial heating method, the wide TSDC peak was resolved into several component peaks and it turned out that one of the peaks was attributed to MgO nano-filler, from which it is suggested that a new trap is generated in the nanocomposites. And it was observed from space charge measurement that addition of MgO nano-fillers suppressed packet-like charges in the nanocomposite. The relative permittivity once decreased slightly by addition of MgO nano-fillers and then increased by more addition of them. The electrical treeing inception voltage increased with increasing MgO nano-filler content and it is also recognized that the tree propagation in the MgO/LDPE nanocomposite was suppressed significantly by the addition of MgO nano-fillers. The partial discharge erosion depth of the MgO/LDPE nanocomposite also becomes smaller than that of base LDPE. It is summarized that addition of a small amount of nano-fillers brought excellent insulating performances of the MgO/LDPE nanocomposite. The MgO/LDPE nanocomposite exhibits promising electrical insulating properties to cope with severe requirements and is expected as a new generation of electrical insulating materials for higher electric field application, especially for DC voltages.

  • Birefringence in optical fibers formed by proton implantation (vol 265, pg 490, 2007)

    Seung Jun Yu, Masahiro Suzuki, Yoshimichi Ohki, Makoto Fujimaki, Koichi Awazu, Eisuke Yamaguchi, Satoshi Okude

    NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS   266 ( 7 ) 1144 - 1144  2008.04

    Other  

    DOI

  • Effect of Addition of MgO Nano-filler on Thermally Stimulated Current in Low-density Polyethylene

    石本和之, 田中祀捷, 大木義路, 関口洋逸, 村田義直

    電気学会放電研究会資料   ED-08 ( 19-36 )  2008

    J-GLOBAL

  • MgO粒子の粒径がLDPEの誘電特性に与える影響

    石本和之, 田中祀捷, 大木義路, 関口洋逸, 村田義直, 御所脇学

    電気学会全国大会講演論文集   2008 ( 2 )  2008

    J-GLOBAL

  • Dielectric Properties of LDPE with MgO Fillers Different in Diameter

    石本和之, 田中祀捷, 大木義路, 関口洋逸, 村田義直, 御所脇学

    電気学会誘電・絶縁材料研究会資料   DEI-08 ( 65-72 )  2008

    J-GLOBAL

  • 低密度ポリエチレン/MgOナノコンポジットにおける電気伝導

    石本和之, 菊間俊明, 田中祀捷, 関口洋逸, 村田義直, 大木義路

    電学論A   128   pp. 7  2008

  • Photoluminescence in Polymer Nannocomposites Induced by Ultraviolet Photons – Effects of Nanofiller Loading on their Electronic Localized States —

    N. Fuse, M. Okada, Y. Ohki, Y. Murata, Y. Sekiguchi, T. Tanaka

    IEEE Trans. Dielectr. Electr. Insul.   15 ( 1 ) pp. 8  2008

  • LDPE/MgOナノコンポジットにおける熱刺激電流

    石本和之, 菊間俊明, 田中祀捷, 村田義直, 関口洋逸, 大木義路

    電気学会全国大会講演論文集   2007 ( 2 )  2007

    J-GLOBAL

  • Effects of Measuring Parameters on Thermally Stimulated Current in Low-Density Polyethylene/MgO Nanocomposite

    石本和之, 田中祀捷, 関口洋逸, 村田義直, 大木義路

    電気電子絶縁材料システムシンポジウム予稿集   38th  2007

    J-GLOBAL

  • Effect of the Sheet Formation Method on the Electric Conduction Characteristics in Low-Density Polyethylene/MgO Nanocomposites

    石本和之, 菊間俊明, 田中祀捷, 関口洋逸, 村田義直, 大木義路

    電気学会誘電・絶縁材料研究会資料   DEI-07 ( 55-67 )  2007

    J-GLOBAL

  • Search for Adequate Biodegradable Polymer as an Eco-friendly Electrical Insulating Materials

    Y. Ohki, N. Hirai, N. Fuse, T. Tanaka, M. Kohtoh, S. Okabe

    Proc. Internat’l Symp. On EcoTopia Science 2007 (ISEET07)     1 - 5  2007

  • Treeing Phenomena in Epoxy /Alumina Nanocomposite

    T. Tanaka, Y. Ohki, S. Okabe

    Proc. CIGRE SC A1&D1 Joint Colloquium     199 - 206  2007

  • Structural changes and irradiation effects on silica glass by ion microbeam

    村井将人, 西川宏之, 相場洋彦, 大木義路, 及川将一, 佐藤隆博, 神谷富裕

    電気電子絶縁材料システムシンポジウム予稿集   37th  2006

    J-GLOBAL

  • シリカガラスに生じたイオンマイクロビーム誘起変形の熱的安定性

    相場洋彦, 中村知晴, 大木義路, 村井将人, 西川宏之, 及川将一, 佐藤隆博, 荒川和夫

    応用物理学関係連合講演会講演予稿集   53rd ( 2 )  2006

    J-GLOBAL

  • Comparison of Treeing Breakdown Channel between Base Epoxy and Epoxy/Alumina Nanocomposite

    M. Kozako, T. Kusunoki, Y. Ohki, M. Kohtoh, S. Okabe, T. Tanaka

    Proc. IEEJ A Society Japan-Korea Symposium   ( JK-4-6 ) 64  2006

  • Nano-filler Dispersion and Tree Growth Lengths in Commercially Available Epoxy/Silica Nanocompsites

    T. Iizuka, M. Kozako, Y. Ohki, M. Kohtoh, S. Okabe, T. Tanaka

    Proc. IEEJ A Society Japan-Korea Symposium   ( JK4-5 ) 63  2006

  • Treeing phenomena in epoxy/alumina nanocomposite

    T. Tanaka, Y. Ohki, M. Kozako, M. Kohtoh, S. Okabe

    Proc. IEEJ A Society Japan-Korea Symposium   ( JK4-4 ) 57 - 62  2006

  • Photoluminescence in Low-Density Polyethelene/MgO Nanocomposites Induced by Ultraviolet Photons at 10 k

    M. Okada, N. Fuse, T. Tanaka, Y. Murata, Y. Ohki

    Proc. IEEJ A Society Japan-Korea Symposium   ( JK4-1 ) 50  2006

  • Space Charge Distribution in Low-density Polyethylene/MgO Nanocomposites under Low Electric Field

    T. Kikuma, Y. Murata, T. Tanaka, Y. Ohki

    Proc. IEEJ A Society Japan-Korea Symposium   ( JK3-9 ) 48  2006

  • Formation of Mica Layers on the Surface of Polyamide-6 Nanocomposite due to Oxygen Plasma Exposure”, Proc. IEEJ FM-Society National Convention

    N. Fuse, M. Okada, M. Kozako, T. Tanaka, Y. Ohki

    Proc. IEEJ A Society Japan-Korea Symposium   ( JK3-4 ) 43  2006

  • Superiority in Partial Discharge Resistance of Several Polymer Nanocomposites

    T. Tanaka, Y. Ohki, T. Shimizu, S. Okabe

    CIGRE Paper D1-303, pp.8, 2006.   D1 ( 303 ) 1 - 8  2006

  • Densification and Refractive Index Increase in Microscopic Area of Silica Glass Induced by High-Energy Ion Microbeam Irradiation

    中村知晴, 相場洋彦, 大木義路, 村井将人, 西川宏之, 及川将一, 佐藤隆博, 荒川和夫

    宇宙航空研究開発機構特別資料 JAXA-SP-   ( 05-024 ) 115 - 119  2005.12

    J-GLOBAL

  • Partial Discharge Degradation of Several Biodegradable Polymers

    FUSE Norikazu, FUJITA Shinjiro, HIRAI Naoshi, KOZAKO Masahiro, TANAKA Toshikatsu, OHKI Yoshimichi, KOHTOH Masanori, OKABE Shigemitsu

      2005 ( 15 ) 19 - 25  2005.01

    CiNii

  • Partial Discharge Degradation Properties of Polyamide Nanocomposite : Evaluation by SEM and the surface profilometer

    KIDO Ryoichi, KOZAKO Masahiro, FUSE Norikazu, OHKI Yoshimichi, TANAKA Toshikatsu

      2005 ( 1 ) 23 - 26  2005.01

    CiNii

  • シリカガラスへのイオンマイクロビーム照射効果のイオン種依存性

    村井将人, 深川一成, 西川宏之, 中村知晴, 大木義路, 佐藤隆博, 及川将一, 荒川和夫

    応用物理学関係連合講演会講演予稿集   52nd ( 2 )  2005

    J-GLOBAL

  • Effects of mica fillers on dielectric properties of polyamide nanocomposites

    N. Fuse, M. Kozako, T. Tanaka, Y. Ohki

    Annual Report - Conference on Electrical Insulation and Dielectric Phenomena, CEIDP   2005   148 - 151  2005

     View Summary

    Complex permittivity, dielectric strength, and electrical conductivity of polyamide-6/mica nanocomposites were measured. By analyzing the permittivity spectra, it was found that the distribution of relaxation times becomes smaller with the addition of nanofillers. This indicates restriction of molecular motion and strong ionic bonds at the resin/mica interfaces. The conduction current was found to decrease with the addition of nanofillers. The barrier effect, formation of nanometric spherulites, and carrier trapping are considered to be responsible, in addition to the above-mentioned two effects at the interfaces. The breakdown strength is independent of the nanofiller content. © 2005 IEEE.

    DOI

  • Various Characteristics of Epoxy/ Alumina Nanocomposite Processed in Laboratory Scale

    T. Tanaka, M. Kozako, Y. Ohki, M. Kohtoh, S. Okabe

    Proc. ICEE, pp.477-480, 2005.     477 - 480  2005

  • イオンマイクロビーム照射によるシリカガラスの微細構造変化

    中村知晴, 柳崇, 大木義路, 深川一成, 西川宏之, 渡辺英紀, 及川将一, 神谷富裕, 荒川和夫

    応用物理学関係連合講演会講演予稿集   51st ( 2 )  2004

    J-GLOBAL

  • シリカガラスへのイオンマイクロビーム照射効果のドーズ依存性 (II)

    深川一成, 西川宏之, 中村知晴, 大木義路, 及川将一, 神谷富裕, 荒川和夫

    応用物理学会学術講演会講演予稿集   65th ( 2 )  2004

    J-GLOBAL

  • シリカガラスへのイオンマイクロビーム照射効果のドーズ依存性

    深川一成, 西川宏之, 渡辺英紀, 中村知晴, 柳崇, 大木義路, 及川将一, 神谷富裕, 荒川和夫

    応用物理学関係連合講演会講演予稿集   51st ( 2 )  2004

    J-GLOBAL

  • Modification of Silica Glass for Optical Devices by Ion Microbeam Irradiation-Evaluation of the Microscopic Structural Change Including Densification by AFM-

    中村知晴, 柳崇, 大木義路, 深川一成, 西川宏之, 及川将一, 佐藤隆博, 荒川和夫

    電気電子絶縁材料システムシンポジウム予稿集   35th  2004

    J-GLOBAL

  • Activities of investigation committees, IEE japan

    Y Ohki, S Yasufuku

    IEEE ELECTRICAL INSULATION MAGAZINE   19 ( 5 ) 67 - 68  2003.09

    Article, review, commentary, editorial, etc. (trade magazine, newspaper, online media)  

  • A new carbon-fiber reinforced aluminum conductor

    Y Ohki, S Yasufuku

    IEEE ELECTRICAL INSULATION MAGAZINE   19 ( 4 ) 67 - 68  2003.07

    Article, review, commentary, editorial, etc. (trade magazine, newspaper, online media)  

  • Japan's three new cable companies, part 3: Exsym corporation

    Y Ohki, S Yasufuku

    IEEE ELECTRICAL INSULATION MAGAZINE   19 ( 2 ) 43 - 44  2003.03

    Article, review, commentary, editorial, etc. (trade magazine, newspaper, online media)  

  • シリカガラスへの重イオンマイクロビーム照射効果 (II)

    深川一成, 西川宏之, 渡辺英紀, 柳崇, 大木義路, 及川将一, 神谷富裕, 荒川和夫

    応用物理学会学術講演会講演予稿集   64th ( 2 )  2003

    J-GLOBAL

  • イオンマイクロビーム照射によるシリカガラスの屈折率変化分布

    服部雅晴, 柳崇, 大木義路, 惣野崇, 深川一成, 西川宏之, 渡辺英紀, 及川将一, 荒川和夫

    応用物理学関係連合講演会講演予稿集   50th ( 2 )  2003

    J-GLOBAL

  • シリカガラスへの重イオンマイクロビーム照射効果

    深川一成, 惣野崇, 西川宏之, 渡辺英紀, 柳崇, 服部雅晴, 大木義路, 及川将一, 荒川和夫

    応用物理学関係連合講演会講演予稿集   50th ( 2 )  2003

    J-GLOBAL

  • プラズマCVD法によるGeO2-SiO2薄膜の紫外感光性に及ぼす熱処理の影響

    服部智三, 西川宏之, 柳崇, 大木義路

    応用物理学会学術講演会講演予稿集   64th ( 2 )  2003

    J-GLOBAL

  • Ge添加SiO2薄膜中の紫外光誘起GeE′中心の熱緩和

    柳崇, 大北正毅, 大木義路, 西川宏之

    応用物理学関係連合講演会講演予稿集   50th ( 2 )  2003

    J-GLOBAL

  • Japan's three new cable companies, part 2: VISCAS Corporation

    Y Ohki, S Yasufuku

    IEEE ELECTRICAL INSULATION MAGAZINE   19 ( 1 ) 45 - 46  2003.01

    Article, review, commentary, editorial, etc. (trade magazine, newspaper, online media)  

  • Fundamental Study on Self-healing Insulation Performance of Silicone Rubber Affected by Electrical Tree

    穂積直裕, 西岡幸治, 村本裕二, 長尾雅行, 中村道昭, 新藤孝敏, 平井直志, 大木義路

    電気電子絶縁材料システムシンポジウム予稿集   34th   37 - 40  2002.11

    J-GLOBAL

  • The world's first long-distance 500kV-XLPE cable line, part 2: Joints and after-installation test

    Y Ohki, S Yasufuku

    IEEE ELECTRICAL INSULATION MAGAZINE   18 ( 3 ) 57 - 58  2002.05

    Article, review, commentary, editorial, etc. (trade magazine, newspaper, online media)  

  • The world's first long-distance 500-kV XLPE cable line, Part 1: Cable and cable installation

    Y Ohki, S Yasufuku

    IEEE ELECTRICAL INSULATION MAGAZINE   18 ( 2 ) 65 - 66  2002.03

    Article, review, commentary, editorial, etc. (trade magazine, newspaper, online media)  

  • イオンマイクロビーム照射したシリカガラスの顕微分光計測のよる評価

    惣野崇, 西川宏之, 渡辺英紀, 服部雅晴, 大木義路, 及川将一, 神谷富裕, 荒川和夫

    応用物理学会学術講演会講演予稿集   63rd ( 2 )  2002

    J-GLOBAL

  • イオンマイクロビーム照射したシリカガラスにおける欠陥分布と屈折率変化

    西川宏之, 惣野崇, 山口喬之, 渡辺英紀, 服部雅晴, 大木義路, 及川将一, 神谷富裕, 荒川和夫

    応用物理学関係連合講演会講演予稿集   49th ( 2 )  2002

    J-GLOBAL

  • 高エネルギーイオン注入したシリカガラスの紫外光励起顕微分光による評価 (II)

    山口喬之, 渡辺英紀, 惣野崇, 西川宏之, 服部雅晴, 大木義路, 及川将一, 神谷富裕, 荒川和夫

    応用物理学関係連合講演会講演予稿集   49th ( 2 )  2002

    J-GLOBAL

  • TEOS-CVD法によるGeO2-SiO2薄膜の紫外光感度に及ぼす不純物の影響

    柳崇, 中西哲也, 服部雅晴, 大木義路, 西川宏之

    応用物理学関係連合講演会講演予稿集   49th ( 2 )  2002

    J-GLOBAL

  • イオンマイクロビーム照射したシリカガラスの欠陥分布と屈折率変化 (II)

    惣野崇, 西川宏之, 渡辺英紀, 服部雅晴, 大木義路, 及川将一, 神谷富裕, 荒川和夫

    応用物理学会学術講演会講演予稿集   63rd ( 2 )  2002

    J-GLOBAL

  • News from Japan

    Yoshimichi Ohki

    IEEE Electrical Insulation Magazine   18 ( 1 )  2002

  • News from Japan

    Yoshimichi Ohki

    IEEE Electrical Insulation Magazine   18 ( 2 )  2002

  • イオンマイクロビーム照射によるシリカガラスの高密度化

    服部雅晴, 西原義孝, 藤巻真, 大木義路, 惣野崇, 西川宏之, 山口喬之, 渡辺英紀, 及川将一, 神谷富裕, 荒川和夫

    応用物理学関係連合講演会講演予稿集   48th ( 2 )  2001

    J-GLOBAL

  • 高エネルギーイオン注入したシリカガラスの紫外光励起顕微分光による評価

    山口喬之, 渡辺英紀, 惣野崇, 西川宏之, 西原義孝, 服部雅晴, 藤巻真, 大木義路, 及川将, 神谷富裕, 荒川和夫

    応用物理学関係連合講演会講演予稿集   48th ( 2 )  2001

    J-GLOBAL

  • イオンマイクロビーム照射したシリカガラスの顕微分光評価

    惣野崇, 西川宏之, 山口喬之, 渡辺英紀, 西原義孝, 服部雅晴, 藤巻真, 大木義路, 及川将一, 神谷富裕, 荒川和夫

    応用物理学関係連合講演会講演予稿集   48th ( 2 )  2001

    J-GLOBAL

  • TEOS-CVD法による光導波路の紫外線照射誘起吸収変化

    柳崇, 服部雅晴, 中西哲也, 大木義路, 西川宏之

    応用物理学会学術講演会講演予稿集   62nd ( 2 )  2001

    J-GLOBAL

  • TEOS-CVD法により作製した光導波路薄膜の顕微ラマン分光による評価

    西川宏之, 梁川圭介, 惣野崇, 山口喬之, 渡辺英紀, 大木義路

    応用物理学会学術講演会講演予稿集   62nd ( 2 )  2001

    J-GLOBAL

  • イオンマイクロビーム照射したシリカガラスの顕微分光評価(II)

    西川宏之, 惣野崇, 山口喬之, 渡辺英紀, 服部雅晴, 大木義路, 及川将一, 神谷富裕, 荒川和夫

    応用物理学会学術講演会講演予稿集   62nd ( 2 )  2001

    J-GLOBAL

  • 電子機器・デバイス絶縁の実績と将来展望

    西川宏之, 津久井勤, 岩本光正, 大木義路

    電気学会全国大会講演論文集   2001 ( 2 )  2001

    J-GLOBAL

  • News from Japan

    Yoshimichi Ohki

    IEEE Electrical Insulation Magazine   17 ( 1 )  2001

  • News from Japan

    Yoshimichi Ohki

    IEEE Electrical Insulation Magazine   17 ( 2 )  2001

  • News from Japan

    Yoshimichi Ohki

    IEEE Electrical Insulation Magazine   17 ( 3 )  2001

  • News from Japan

    Yoshimichi Ohki

    IEEE Electrical Insulation Magazine   17 ( 4 )  2001

  • News from Japan

    Yoshimichi Ohki

    IEEE Electrical Insulation Magazine   17 ( 5 )  2001

  • News from Japan

    Yoshimichi Ohki

    IEEE Electrical Insulation Magazine   17 ( 6 )  2001

  • Microscopic Characterization of Ion-Implanted Silica Glass.

    惣野崇, 西川宏之, 山口喬之, 渡辺英紀, 西原義孝, 服部雅晴, 大木義路, 及川将一, 荒川和夫

    電気電子絶縁材料システムシンポジウム予稿集   32nd  2000

    J-GLOBAL

  • 高エネルギーイオン注入したシリカガラスの顕微分光による評価(II)

    山口喬之, 渡邉英紀, 惣野崇, 西川宏之, 藤巻真, 大木義路

    応用物理学会学術講演会講演予稿集   61st ( 2 )  2000

    J-GLOBAL

  • 高エネルギーイオン注入したシリカガラスの誘起欠陥の熱処理特性

    惣野崇, 西川宏之, 山口喬之, 渡辺英紀, 西原義孝, 藤巻真, 大木義路

    応用物理学会学術講演会講演予稿集   61st ( 2 )  2000

    J-GLOBAL

  • Ge,Sn共添加SiO2ガラスにおける常磁性欠陥のブリーチ現象

    徳廣真一郎, 中西哲也, 野村健一, 大木義路, 藤巻真, 西川宏之, 今村一雄

    応用物理学会学術講演会講演予稿集   61st ( 2 )  2000

    J-GLOBAL

  • Ge,Sn共添加SiO2ガラスにおける反磁性欠陥

    中西哲也, 徳廣真一郎, 高橋正典, 大木義路, 藤巻真, 西川宏之, 今村一雄

    応用物理学会学術講演会講演予稿集   61st ( 2 )  2000

    J-GLOBAL

  • News from Japan

    Yoshimichi Ohki

    IEEE Electrical Insulation Magazine   16 ( 1 )  2000

  • News from Japan

    Yoshimichi Ohki

    IEEE Electrical Insulation Magazine   16 ( 2 )  2000

  • News from Japan

    Yoshimichi Ohki

    IEEE Electrical Insulation Magazine   16 ( 3 )  2000

  • News from Japan

    Yoshimichi Ohki

    IEEE Electrical Insulation Magazine   16 ( 4 )  2000

  • News from Japan

    Yoshimichi Ohki

    IEEE Electrical Insulation Magazine   16 ( 5 )  2000

  • News from Japan

    Yoshimichi Ohki

    IEEE Electrical Insulation Magazine   16 ( 6 )  2000

  • 1999 Annual Conference of Fundamentals and Materials Society

    OHKI Yoshimichi

      119 ( 12 ) 1381 - 1382  1999.12

    CiNii

  • Prof. Ieda's farewell party

    Y Ohki, S Yasufuku

    IEEE ELECTRICAL INSULATION MAGAZINE   15 ( 4 ) 60 - 60  1999.07

    Article, review, commentary, editorial, etc. (trade magazine, newspaper, online media)  

  • 12(th) EMC 1999 exhibition held near Tokyo

    Y Ohki, S Yasufuku

    IEEE ELECTRICAL INSULATION MAGAZINE   15 ( 4 ) 60 - 61  1999.07

    Article, review, commentary, editorial, etc. (trade magazine, newspaper, online media)  

  • News from Japan

    Yoshimichi Ohki

    IEEE Electrical Insulation Magazine   15 ( 1 )  1999

  • News from Japan

    Yoshimichi Ohki

    IEEE Electrical Insulation Magazine   15 ( 2 )  1999

  • News from Japan

    Yoshimichi Ohki

    IEEE Electrical Insulation Magazine   15 ( 3 )  1999

  • News from Japan

    Yoshimichi Ohki

    IEEE Electrical Insulation Magazine   15 ( 4 )  1999

  • News from Japan

    Yoshimichi Ohki

    IEEE Electrical Insulation Magazine   15 ( 5 )  1999

  • News from Japan

    Yoshimichi Ohki

    IEEE Electrical Insulation Magazine   15 ( 6 )  1999

  • Development of electric power storage battery technology in Japan

    Y Ohki, S Yasufuku

    IEEE ELECTRICAL INSULATION MAGAZINE   14 ( 3 ) 48 - 49  1998.05

    Article, review, commentary, editorial, etc. (trade magazine, newspaper, online media)  

  • News from Japan

    Yoshimichi Ohki

    IEEE Electrical Insulation Magazine   14 ( 1 )  1998

  • News from Japan

    Yoshimichi Ohki

    IEEE Electrical Insulation Magazine   14 ( 2 )  1998

  • News from Japan

    Yoshimichi Ohki

    IEEE Electrical Insulation Magazine   14 ( 3 )  1998

  • News from Japan

    Yoshimichi Ohki

    IEEE Electrical Insulation Magazine   14 ( 4 )  1998

  • News from Japan

    Yoshimichi Ohki

    IEEE Electrical Insulation Magazine   14 ( 5 )  1998

  • News from Japan

    Yoshimichi Ohki

    IEEE Electrical Insulation Magazine   14 ( 6 )  1998

  • Hitachi cable builds a new plant

    Y Ohki, S Yasufuku

    IEEE ELECTRICAL INSULATION MAGAZINE   13 ( 5 ) 60 - 60  1997.09

    Article, review, commentary, editorial, etc. (trade magazine, newspaper, online media)  

  • Development of UHV PPLP OF cable

    Y Ohki, S Yasufuku

    IEEE ELECTRICAL INSULATION MAGAZINE   13 ( 4 ) 56 - 59  1997.07

    Article, review, commentary, editorial, etc. (trade magazine, newspaper, online media)  

  • News from Japan

    Yoshimichi Ohki

    IEEE Electrical Insulation Magazine   13 ( 1月4日 )  1997

  • News from Japan

    Yoshimichi Ohki

    IEEE Electrical Insulation Magazine   13 ( 1 )  1997

  • News from Japan

    Yoshimichi Ohki

    IEEE Electrical Insulation Magazine   13 ( 2 )  1997

  • News from Japan

    Yoshimichi Ohki

    IEEE Electrical Insulation Magazine   13 ( 3 )  1997

  • News from Japan

    Yoshimichi Ohki

    IEEE Electrical Insulation Magazine   13 ( 4 )  1997

  • News from Japan

    Yoshimichi Ohki

    IEEE Electrical Insulation Magazine   13 ( 5 )  1997

  • News from Japan

    Yoshimichi Ohki

    IEEE Electrical Insulation Magazine   13 ( 6 )  1997

  • 発光減衰曲線の解析によるSiO2の微視的構造の乱雑さの評価 (特集:いま若手は何をめざしているのか?)

    石井 啓介, 大木 義路

    放電研究   ( 151 ) 11 - 19  1996.06

    CiNii

  • News from Japan

    Yoshimichi Ohki

    IEEE Electrical Insulation Magazine   12 ( 1 )  1996

  • News from Japan

    Yoshimichi Ohki

    IEEE Electrical Insulation Magazine   12 ( 2 )  1996

  • News from Japan

    Yoshimichi Ohki

    IEEE Electrical Insulation Magazine   12 ( 3 )  1996

  • News from Japan

    Yoshimichi Ohki

    IEEE Electrical Insulation Magazine   12 ( 4 )  1996

  • News from Japan

    Yoshimichi Ohki

    IEEE Electrical Insulation Magazine   12 ( 5 )  1996

  • News from Japan

    Yoshimichi Ohki

    IEEE Electrical Insulation Magazine   12 ( 6 )  1996

  • News from Japan

    Yoshimichi Ohki

    IEEE Electrical Insulation Magazine   11 ( 3 )  1995

  • News from Japan

    Yoshimichi Ohki

    IEEE Electrical Insulation Magazine   11 ( 4 )  1995

  • News from Japan

    Yoshimichi Ohki

    IEEE Electrical Insulation Magazine   11 ( 5 )  1995

  • News from Japan

    Yoshimichi Ohki

    IEEE Electrical Insulation Magazine   11 ( 6 )  1995

  • Possibility of ethylene-styrene copolymer as cable insulating material.

    大木義路, 田中康寛, 松尾克也, 松本薫, 秋山敬一郎, 穂積直裕, 池田雅昭, 小島慎二

    電気学会絶縁材料研究会資料   EIM-89 ( 66-72 ) 41 - 48  1989.07

    J-GLOBAL

  • Higher order structure and dielectric breakdown of the ethylene-styrene copolymer.

    大木義路, 松尾克也, 尾坪利信, 穂積直裕, 原重雅浩, 池田雅昭

    電気学会絶縁材料研究会資料   EIM-88 ( 30-40 ) 23 - 32  1988.05

    J-GLOBAL

  • Dielectric breakdown characteristics of chemically crosslinked LDPE and ethylene-aromatic monomer copolymer.

    池田雅昭, 大木義路, 穂積直裕

    電気学会絶縁材料研究会資料   EIM-86 ( 93-103 ) 77 - 86  1986.10

    J-GLOBAL

  • プラズマ重合エチレン膜の電気特性

    笠間靖裕, 穂積直裕, 大木義路, 矢作吉之助

    電気学会絶縁材料研究会資料   EIM-83 ( 107-114 ) 45 - 54  1983.11

    J-GLOBAL

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Industrial Property Rights

  • 高分子の異物検査方法、および電力ケーブルの製造方法

    6238245

    大木 義路, 小松 麻里奈, 井筒 智之, 小? 大祐

    Patent

  • 絶縁電線又はケーブルの歪み量変化箇所の位置標定方法

    大木 義路, 平井 直志

    Patent

  • 絶縁電線又はケーブルの温度変化箇所の位置標定方法

    5763594

    平井 直志, 山田 貴之, 大木 義路

    Patent

  • 金属ベースプリント配線板

    6239510

    大木 義路, 田中 祀捷, 廣瀬 雄一, 和田 玄太

    Patent

  • プリント配線板

    5982710

    大木 義路, 浅川 洋貴, 難波 裕, 田中 祀捷

    Patent

  • プリント配線板とその製造方法

    5807225

    大木 義路, 夏井 正嗣, 浅川 洋貴

    Patent

  • 絶縁信頼性を高めた固体絶縁スイッチギヤ

    大木 義路, 田中 祀捷, 田上 直紀

    Patent

  • 電気絶縁材料及びそれを用いた絶縁電線、通信用ケーブル、電気部品

    平井 直志, 大木 義路

    Patent

  • 荷電粒子加速器のメンテナンス方法

    4674898

    藤巻 真, 大木 義路

    Patent

  • 光ファイバデポラライザの製造方法

    藤巻 真, 山口 英祐, 奥出 聡, 劉 昇, 河辺 柱太, 大木 義路

    Patent

  • 結晶TiO2の微細加工方法及び微細加工された結晶TiO2

    4025886

    大木 義路, 中西 哲也, 野村 健一, 島 邦博, 石井 聡

    Patent

     View Summary

    特開2003-279766

  • 光導波路形成方法及び光導波路

    大木 義路, 加藤 宙光, 藤巻 真, 野間 崇

    Patent

     View Summary

    特開2003-075659

  • 酸化物強誘電体素子の作製方法

    大木 義路, 薛 光洙, 平松 大直, 伊藤 貴俊

    Patent

     View Summary

    特開2002-151503

  • 耐熱処理装置及び耐熱処理方法並びにグレーティング

    大木 義路, 徳廣 真一郎, 藤巻 真, 野村 健一

    Patent

     View Summary

    特開2002-071974

  • 透過光特性調整方法及び透過光特性調整装置並びに長周期グレーティング

    大木 義路, 藤巻 真, 宮崎 奈穂子, 内藤 要

    Patent

     View Summary

    特開2001-272553

  • グレーティング並びにグレーティング形成方法及び装置

    3456927

    大木 義路

    Patent

     View Summary

    特許第3456927

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Awards

  • Waseda Research Award 2020

    2021.03   Waseda University  

  • Best Paper Award

    2020.10   Eighth International Conference on Condition Monitoring and Diagnosis   Experimental Verification of Good Spatial Resolution of Fault Location in a Cable by Frequency Domain Reflectometry

  • Outstanding Achievement Award

    2020.05   The Institute of Electrical Engineers of Japan   Contribution to industrial development and strengthening of in