Updated on 2022/05/26

写真a

 
HIRAI, Naoshi
 
Affiliation
Faculty of Science and Engineering, Kagami Memorial Research Institute for Materials Science and Technology
Job title
Senior Researcher(Professor)

Concurrent Post

  • Faculty of Science and Engineering   Graduate School of Advanced Science and Engineering

Education

  •  
    -
    1998

    Ehime University   Graduate School, Division of Engineering  

  •  
    -
    1994

    Konan University   Faculty of Science  

Degree

  • (BLANK)

  • 甲南大学   修士(理学)

  • (BLANK)

  • 愛媛大学   博士(工学)

Professional Memberships

  •  
     
     

    放電学会

  •  
     
     

    電気学会

 

Research Areas

  • Electric and electronic materials

Research Interests

  • Electronics and Electric Material

Papers

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

    Yoshimichi Ohki, Naoshi Hirai

    Minerals, Metals and Materials Series   11   77 - 85  2018

     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

     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

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

    Takuya Kozai, Takuya Kaneko, Naoshi Hirai, Yoshimichi Ohki

    Polymer Degradation and Stability   147   284 - 290  2018.01

     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

  • 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

     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

     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

     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

     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

  • Terahertz absorption spectra of antioxidants in insulating polymers

    Kozai, Takuya, Kaneko, Takuya, Hirai, Naoshi, Ohki, Yoshimichi, Ohki, Yoshimichi

    Proceedings of the International Symposium on Electrical Insulating Materials   1   61 - 64  2017.10

     View Summary

    Terahertz absorption spectra were measured for nine kinds of antioxidants. The spectra were also measured for lowdensity polyethylene sheets that contain each antioxidant with different contents. As a result, a fingerprint spectrum appears for each antioxidant, from which the antioxidants become distinguishable and their contents can be estimated.

  • 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   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

  • Improved high-temperature dielectric property of epoxy resin composites with nano- and micro-sized magnesia fillers

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

    2017 IEEE International Workshop on Integrated Power Packaging, IWIPP 2017    2017.05  [Refereed]

     View Summary

    © 2017 IEEE. Nano-sized magnesium oxide or magnesia has been reported to be useful for suppressing the space charge accumulation when it is added in epoxy resin. However, the addition of nano-magnesia to epoxy resin decreases the glass transition temperature (Tg) signficantly, which may limit industrial applications of magnesia/epoxy nanocomposites. In this work, nanomagnesia particles with an average size of 52 nm and micro-magnesia with an average size of 3.1 pm were mixed in a commercially available bisphenol-A epoxy resin. The weight ratio of nano- and micro-fillers in each composite was changed, while the total content of the two fillers was fixed at 5 weight%. As a result, it has become clear that the addition of micro-sized magnesia fillers to magnesia/epoxy nanocomposites recovers Tg back to its original high temperature. Then, a dc electric field of 30 kV/mm was applied to the samples for 20 minutes at various temperatures from 40 to 200 °C and space charge distributions were measured by the pulsed electro-Acoustic method. The results show that space charge accumulates in both vicinities of the cathode and the anode with the same polarities as those of the nearby electrodes when the temperature is 80 °C or below. However, space charges become hetero and are observed only in the vicinity of the cathode at 140 °C and above. These homocharges and heterocharges become very small when the weight content of nano-sized fillers increases, which is industrially very important. Both complex dielectric permittivity and electrical conductivity also decrease with the increase in the content of nanofillers.

    DOI

  • Comparison of location abilities of degradation in a polymer-insulated cable between frequency domain reflectometry and line resonance analysis

    Ohki, Yoshimichi, Ohki, Yoshimichi, Hirai, Naoshi

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

     View Summary

    © 2016 IEEE.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 uncommend-able 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.

    DOI

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

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

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

     View Summary

    © 2016 IEEE.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

  • Erratum: Modeling of oxidation process and property changes of ethylene-propylene-diene copolymer( IEEE Transactions on Dielectrics and Electrical Insulation (2016) 23:1 (537-546))

    Daomin Min, Shengtao Li, Naoshi Hirai, Yoshimichi Ohki

    IEEE Transactions on Dielectrics and Electrical Insulation   23 ( 2 ) 1234  2016.04

    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

  • 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

  • Location attempts of a heated and irradiated portion in a silicone rubber insulated cable by frequency domain reflectometry and line resonance analysis

    Naoshi Hirai, Yoshimichi Ohki

    IEEJ Transactions on Fundamentals and Materials   136 ( 9 ) 553 - 560  2016

     View Summary

    Electric cables play important roles for supplying power and for transmitting information and control signals. Therefore, their degradation may cause a serious problem. Silicone rubber (SiR) is widely used for electrical insulation in safety-related cables in nuclear power plants. In this paper, the ability of locating a degraded portion in a duplex cable insulated with SiR 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). Reflecting the fact that the FDR system can expand the measurement frequency range, FDR shows a higher sensitivity and a better resolution than LIRA. However, both methods cannot detect the degradation of SiR before its further use becomes uncommendable.

    DOI CiNii

  • Electronic Transition Process of Fluorescence Appearing in Various Organic Polymers

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

    The transactions of the Institute of Electrical Engineers of Japan.A   136 ( 4 ) 205 - 211  2016

     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.

    CiNii

  • 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

  • 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

  • Location attempts of a heated and irradiated portion in a silicone rubber insulated cable by frequency domain reflectometry and line resonance analysis

    Naoshi Hirai, Yoshimichi Ohki

    IEEJ Transactions on Fundamentals and Materials   136 ( 9 ) 553 - 560  2016

     View Summary

    Electric cables play important roles for supplying power and for transmitting information and control signals. Therefore, their degradation may cause a serious problem. Silicone rubber (SiR) is widely used for electrical insulation in safety-related cables in nuclear power plants. In this paper, the ability of locating a degraded portion in a duplex cable insulated with SiR 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). Reflecting the fact that the FDR system can expand the measurement frequency range, FDR shows a higher sensitivity and a better resolution than LIRA. However, both methods cannot detect the degradation of SiR before its further use becomes uncommendable.

    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

     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

  • 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   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.

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  • 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.

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  • 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.

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  • 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)   2016-December   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.

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  • 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)   2016-December   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.

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  • 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)   2015-December   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.

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  • 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.

  • 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

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  • Report on 2012 IEEE International Conference on Condition Monitoring and Diagnosis (CMD 2012)

    HIRAI Naoshi

      2013 ( 38 ) 5 - 8  2013.03

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  • Location of the Position with an Extraordinary Temperature in a Cable by Frequency Domain Reflectometry

    YAMADA Takayuki, HIRAI Naoshi, OHKI Yoshimichi, KUMAGAI Susumu

      2013 ( 1 ) 39 - 44  2013.02

    CiNii

  • Location of the Position with an Extraordinary Temperature in a Cable by Frequency Domain Reflectometry

    YAMADA Takayuki, HIRAI Naoshi, OHKI Yoshimichi, KUMAGAI Susumu

      2013 ( 29 ) 39 - 44  2013.02

    CiNii

  • Sensitivity of Broadband Impedance Spectroscopy as a Location Method of Cable Degradation

    YAMADA Takayuki, HIRAI Naoshi, OHKI Yoshimichi

      2012 ( 53 ) 11 - 16  2012.02

    CiNii

  • 研究グループ紹介:早稲田大学 理工学術院 大木義路研究室

    平井 直志

    電気学会論文誌. B   132 ( 5 ) NL5_2 - NL5_2  2012

    CiNii

  • 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

     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 CiNii

  • Importance of Insulation Diagnosis in Nuclear Power Plants and Evaluation of Oxidation by Chemiluminescence

    OHKI Yoshimichi, HIRAI Naoshi

    Journal of Materials Life Society   23 ( 1 ) 1 - 9  2011.02

    CiNii

  • L型ポリ乳酸の電気伝導特性に及ぼすガラス転移の影響

    2004年電気学会基礎・材料・共通部門大会講演論文集   pp. 130-135  2004.08

  • L型ポリ乳酸の絶縁破壊

    2004年電気学会基礎・材料・共通部門大会講演論文集   p. 455  2004.08

  • Dielectric properties of biodegradable polylactic acid and starch ester

    Proceedings of the 2004 IEEE International Conference on Solid Dielectrics   pp. 87-90  2004.07  [Refereed]

  • 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

    静電気学会誌   Vol. 28, No. 4, pp. 204-207  2004.07

  • 我国における電力機器絶縁監視・診断法の変遷と現状

    電気学会部門誌B   124-B, No. 3, pp. 496-503  2004.03

  • 生分解性エステル化澱粉におよぼす紫外光照射の影響

    平成16 年電気学会全国大会講演論文集   第2分冊、p. 129  2004.03

  • 生分解性高分子L型ポリ乳酸の伝導電流に及ぼすガラス転移の影響

    平成16 年電気学会全国大会講演論文集   第2分冊、pp. 132-133  2004.03

  • クミルアルコールが80℃の低密度ポリエチレン中の空間電荷形成に与える影響

    平成16 年電気学会全国大会講演論文集   第2分冊、p. 75  2004.03

  • L型ポリ乳酸の電荷トラップに影響を与える因子

    平成16 年電気学会全国大会講演論文集   第2分冊、pp. 130-131  2004.03

  • 生分解性・リサイクル材料の電力機器用絶縁材料としての評価

    平成16 年電気学会全国大会講演論文集   第2分冊、pp. 2-S7(11)-2-S7(14)  2004.03

  • Comparison of Surface Roughness Change due to Partial Discharges between Polyamide-6 and Nanocomposites

    Proceedings of the 2003 Japan-Korea Joint Symposium on Electrical Discharge and High Voltage Engineering   pp. 33-36  2003.11

  • Surface change of polyamide nanocomposite caused by partial discharges

    Annual Report of the 2003 Conference on Electrical Insulation and Dielectric Phenomena   pp. 75-78  2003.10  [Refereed]

  • Roles of cumyl alcohol and crosslinked structure in homo-charge trapping in crosslinked polyethylene

    Annual Report of the 2003 Conference on Electrical Insulation and Dielectric Phenomena   pp. 213-216  2003.10  [Refereed]

  • 架橋ポリエチレン中の空間電荷形成に与える架橋剤分解残渣の効果

    2003年電気学会基礎・材料・共通部門大会講演論文集   pp. 160-165  2003.08

  • 生分解性高分子ポリ乳酸および修飾澱粉の誘電特性

    2003年電気学会基礎・材料・共通部門大会講演論文集   p. 278  2003.08

  • Effect of Crosslinking on Space Charge Formation in Crosslinked Polyethylene

    Proceedings of the 7th International Conference on Properties and Applications of Dielectric Materials   pp. 917-920  2003.06  [Refereed]

  • Chemical Group in Crosslinking Byproducts Responsible for Charge Trapping in Polyethylene

    IEEE Trans. Dielectrics and Electrical Insulation   Vol. 10, No. 2, pp. 320-330  2003.04  [Refereed]

  • 架橋ポリエチレン中のホモ電荷トラップに及ぼすクミルアルコールの役割

    2003年電気学会全国大会講演論文集   第2分冊 p. 72  2003.03

  • 電力機器の絶縁劣化診断法の変遷と現状

    2003年電気学会全国大会講演論文集   第2分冊 pp. 99-100  2003.03

  • 架橋ポリエチレンの空間電荷形成に与える架橋構造の影響

    平成15年放電、誘電・絶縁材料合同研究会資料   pp. 51-56  2003.01

  • 脱気処理が架橋ポリエチレンの空間電荷分布形成に及ぼす影響

    第34回電気電子絶縁材料システムシンポジウム予稿集   pp. 197-200  2002.11

  • Interpretation of the Relation between Residual Charge and Water Trees in XLPE Cables

    Proceedings of IEEE/PES Transmission and Distribution Conference and Exhibition 2002   pp. 1094-1099  2002.10  [Refereed]

  • Chemical Group in Crosslinking Byproducts Responsible for Charge Trapping in Polyethylene

    Annual Report of the 2002 Conference on Electrical Insulation and Dielectric Phenomena   pp. 626-630  2002.10  [Refereed]

  • 架橋構造がポリエチレン中の空間電荷形成に与える影響

    2002年電気学会基礎・材料・共通部門大会講演論文集   p. 295  2002.09

  • 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

    電気学会部門誌A   122巻6号 pp. 597-603  2002.06

  • 低密度ポリエチレン中の空間電荷形成に与る架橋剤分解残渣の化学基

    平成14年誘電・絶縁材料、電線・ケーブル合同研究会資料   pp. 7-12  2002.06

  • 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

    電気学会部門誌A   122巻3号、pp. 308-314  2002.03

  • パルス静電応力法による紫外光照射ポリイミドの劣化評価

    平成14年電気学会全国大会講演論文集2   pp. 54-55  2002.03

  • ポリエチレン架橋剤分解残渣の電荷トラップ作用に与る化学構造

    平成14年電気学会全国大会講演論文集2   pp. 56-57  2002.03

  • クミルアルコールが架橋ポリエチレンの空間電荷形成に与える影響

    平成14年電気学会全国大会講演論文集2   pp. 68-69  2002.03

  • ポリエチレンの電気絶縁特性に及ぼすブレンド効果

    平成14年電気学会全国大会講演論文集2   pp. 85-86  2002.03

  • ポリエチレンにおける紫外光誘起フォトルミネセンス(その2)-発光帯の原因-

    平成14年電気学会全国大会講演論文集2   p. 91  2002.03

  • ポリエチレンにおける紫外光誘起フォトルミネセンス(その1)-LDPEとXLPEの比較-

    平成14年電気学会全国大会講演論文集2   pp. 93-94  2002.03

  • 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—

    電気学会論文誌A   121巻12号、pp. 1117-1124  2001.12

  • On the Mechanism of Recovery Voltage in a Dielectric Solid - Proposal of a Simple Equation-

    Proceedings of the 2001 International Symposium on Electrical Insulating Materials   pp. 67-70  2001.11

  • Effects of Liquid Chemicals on Space Charge Evolution in Low-density Polyethylene

    Proceedings of the 2001 International Symposium on Electrical Insulating Materials   pp. 87-90  2001.11

  • Effect of blending on the dielectric breakdown of polyethylene

    Proceedings of the 2001 International Symposium on Electrical Insulating Materials   pp. 574-577  2001.11

  • Effect of Byproducts of Dicumyl Peroxide on Space Charge Formation in Low-density Polyethylene

    Annual Report of the 2001 Conference on Electrical Insulation and Dielectric Phenomena   pp. 478-483  2001.10  [Refereed]

  • Evaluation of Degradation in Polyimide by Recovery Voltage Measurement

    Annual Report of the 2001 Conference on Electrical Insulation and Dielectric Phenomena   pp. 216-220  2001.10  [Refereed]

  • 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

    電気学会論文誌A   121巻9号, pp. 854-859  2001.09

  • ポリプロピレンの紫外光誘起フォトルミネセンス

    電気学会論文誌A   121巻9号, pp. 865-871  2001.09

  • ポリエチレンにおける紫外光誘起フォトルミネセンス

    2001年電気学会基礎・材料・共通部門大会講演論文集   p. 313  2001.09

  • 架橋剤分解残渣がLDPEの空間電荷挙動に与える影響 -類似の化学薬品との比較-

    2001年電気学会基礎・材料・共通部門大会講演論文集   p. 325  2001.09

  • 絶縁体における残留電圧発生機構の検討 -簡単化した計算式の提案-

    2001年電気学会基礎・材料・共通部門大会講演論文集   p. 335  2001.09

  • 非架橋ケーブル用高分子絶縁材料の電気特性

    2001年電気学会基礎・材料・共通部門大会講演論文集   p. 339  2001.09

  • 水トリー劣化架橋ポリエチレンの残留電荷

    平成13年誘電・絶縁材料研究会資料   DEI-01-92、pp. 25-30  2001.06

  • Photoluminescence in Polymeric Insulating Materials (Low Density Polyethylene and Polypropylene) induced by ultraviolet photons

    The Proceedings of 7th International Conference on Solid Dielectrics   pp. 319-323  2001.06  [Refereed]

  • Effects of Byproducts of Dicumyl Peroxide on Space Charge Formation in Polyethylene

    The Proceedings of 7th International Conference on Solid Dielectrics   pp. 450-455  2001.06  [Refereed]

  • 架橋剤分解残渣がポリエチレンの空間電荷分布形成に与える影響

    電気学会 放電/誘電・絶縁材料合同研究会資料   DEI-01-76、pp. 93-98  2001.03

  • 高分子絶縁体(LDPE,PP)の紫外光誘起フォトルミネセンス

    電気学会 放電/誘電・絶縁材料合同研究会資料   DEI-01-77、pp. 99-104  2001.03

  • 全芳香族ポリイミドと熱可塑性ポリイミドの劣化の評価

    電気学会 放電/誘電・絶縁材料合同研究会資料   DEI-01-78、pp. 105-110  2001.03

  • 残留電圧測定によるポリイミドの劣化の評価

    平成13年電気学会全国大会予稿集   p. 565  2001.03

  • クミルアルコールが低密度ポリエチレンの電荷挙動に与える影響

    平成13年電気学会全国大会予稿集   p. 580  2001.03

  • ポリエチレン中の空間電荷形成にクミルアルコールの与える影響

    第32回電気電子絶縁材料システムシンポジウム予稿集   pp. 25-28  2000.11

  • Effects of Byproducts of Crosslinking Agent on Space Charge Formation in Polyethylene.-comparison between acetophenone and α-methylstyrene-

    Annual Report of the 2000 Conference on Electrical Insulation and Dielectric Phenomena   pp. 535-538  2000.10  [Refereed]

  • Comparison of degradation between all-aromatic and thermoplastic polyimide films

    2000年電気学会基礎・材料・共通部門大会予稿集   p. 36  2000.09

  • クミルアルコールがポリエチレンの空間電荷形成に与える影響

    2000年電気学会基礎・材料・共通部門大会予稿集   p. 37  2000.09

  • 低密度ポリエチレンの紫外レーザー光誘起フォトルミネセンス

    2000年電気学会基礎・材料・共通部門大会予稿集   p. 40  2000.09

  • 架橋剤分解残渣がポリエチレンの空間電荷形成に与える影響 -アセトフェノンとαメチルスチレンの比較—

    平成12年誘電・絶縁材料研究会資料   DEI-00-55、pp. 19-26  2000.06

  • The Effect of Contaminant in Breakdown Time Lag of Uniform Electric Field Using Impulse Breakdown in Mineral Oil.

    1999 IEEE 13th International Conference on Conduction and Breakdown in Dielectric Liquids   pp. 207-210  1999.07

  • The Effect of Electrode Construction in Breakdown Time Lag of Impulse Breakdown in Mineral Oil.

    Japanese Journal of Applied Physics(JJAP)   Vol.37 pp.4884-4886  1998.09  [Refereed]

  • The Effect of Copper Powder and Aluminium Oxide on Breakdown Time Lag of Transformer Oil Under Uniform Field.

    the Fourth International Conference KSEEE’98    1998.08

▼display all

Industrial Property Rights

  • 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

    大木 義路, 平井 直志, 山田 貴之

    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

    大木 義路, 平井 直志

    Patent

Awards

  • 電気学会優秀論文発表賞

    2000  

  • 電気学会優秀論文発表賞

    1993  

Research Projects

  • Dielectric Properties and Insulation Diagnosis of Polymers by Ultra-wideband Dielectric and Absorption Spectroscopy

    Project Year :

    2012
    -
    2013
     

     View Summary

    The objective of this research is the understanding of dielectric properties of polymer films through measurements of permittivity and absorption spectra in an ultrawide-band frequency range.
    In FY 2012, permittivity spectra were obtained for seven kinds of polymers by the free-space method at GHz frequencies for the purpose of getting useful information on the future ultrafast wideband communication system. The essentials of this research were submitted as a refereed paper. In FY 2013, absorption spectra induced in cross-linked polyethylene by gamma-ray assisted oxidation were analyzed experimentally and numerically using quantum chemical calculations. As a result, it can be clarified that the absorption is due to resonance vibrations induced by the phenomenon that optically inactive lattice vibrations become active through oxidation-induced structural changes. In addition, possibilities of detecting the degradation of ethylene-propylene rubber by THz imaging were also examined.

  • Development of High Performance Insulating Materials Using Polymer Nanocomposites with Inorganic Fillers

    Project Year :

    2011
    -
    2013
     

     View Summary

    We have demonstrated that the resistance against electrochemical migration of epoxy resin used as a printed wiring board can be improved by adding a small amount of silica nanofillers. Three patents were submitted based on this result. Next, iron nanoparticles were filled into epoxy resin, which resulted in the development of insulating materials with low eddy-current loss, high permittivity, and high permeability.
    Complex electric modulus is the inverse of complex permittivity. Relaxation phenomena due to charge transport in polymer, which are dominant in a low frequency range, become evident by showing frequency-dependent electric modulus spectra. We have done this analysis for various polymers, and the results were summarized in papers, which are now under review. Time-domain spectroscopic measurements at THz frequencies were also done for several polymers, and the results were analyzed by quantum chemical calculations.

  • Evaluation of Oxidation for Biodegradable Polymers by the Measurement of Chemiluminescence

    Project Year :

    2011
    -
    2013
     

     View Summary

    Since the chemiluminescence (CL) behavior in biodegradable polymers, which have carbonyl group in its structure, indicates a similar behavior of polyethylene (PE), it is thought that the carbonyl group consisted in the polymer structure exhibits no CL. While a silane coupling agent or a thiobis-based antioxidant, which is added in low density PE, exhibits no proper CL, a sulfur-based antioxidant, a halogen-based antioxidant, a metal hydroxide, an antirad, or an ion-trapping agent, which is added in ethylene propylene rubber (EPR), exhibits CL. The CL behavior in EPR is different to the one of LDPE or biodegradable polymers because of flame retarder added in EPR. Therefore, CL measurement is a good tool to evaluate the resistivity against oxidation of polymers.

  • Dominant factors influencing dielectric properties in several biodegradable polymers

    Project Year :

    2008
    -
    2010
     

     View Summary

    For typical biodegradable polymers, namely, polyethylene terephthalate succinate, polybutylene succinate, polycaprolactone, polybutylene succinate adipate, polybutylene adipate terephthalate, starch ester, and poly-L-lactic acid, effects of the crystal structure on their dielectric properties were examined by conducting advanced measurements such as photoluminescence using synchrotron radiation, terahertz time domain spectroscopy, and numerical calculations using density functional theory. As a result, much knowledge was obtained on the dominant factors influencing the dielectric properties of biodegradable polymers. This will greatly contribute to the development of eco-friendly dielectric materials.

  • テラヘルツ分光によるポリマーナノコンポジットの優れた誘電特性出現機構の解明

    科学研究費助成事業(早稲田大学)  科学研究費助成事業(萌芽研究)

    Project Year :

    2007
    -
    2008
     

     View Summary

    ポリマーナノコンポジットの優れた誘電・絶縁特性はポリマーマトリクスとナノフィラーとの界面にあると考えられている。そこで、我々は分子の振動、あるいは分子間相互作用の解析に用いられているテラヘルツ領域に注目し、ポリマーナノコンポジットの良好な物性の発現機構を解明する一助として、ポリアミドナノコンポジットに対してテラヘルツ時間領域分光(THz-TDS)測定を行った。
    2THzと3THz付近において初めて誘電分散を観測した。2THz付近の誘電分散について、その誘電損率がポリアミドのアルファ型結晶に起因するX線回折強度と正に相関し、THz電界と試料延伸方向が平行であるとき最大となることから、この分散がアルファ型結晶内の分子鎖と平行方向の分子振動に由来すると推測した。また、3THz付近に誘電損率ピークを持つ誘電分散は、THz電界と試料延伸方向が垂直であるとき最大となることから、分子鎖と垂直方向の分子振動に起因すると推測した。さらに、この3THzの誘電損率のピークがNC化により低周波数側にシフトすることを見出し、これはナノフィラー添加によりポリマー/ナノフィラー間に生じる強い相互作用が分子振動を抑制するためであると推測した。
    THz分光は、最近可能となった斬新な計測法であり、これを高分子の誘電特性の解明に用いた研究例は極めて少ない。分光強度のTHz偏光角依存性等を用いた上記解析は、多くの高分子とそのNCに適用しうる重要で新しい手法と考えている。

  • Development of Functional Dielectrics for Future All-Solid Substations

    Project Year :

    2002
    -
    2004
     

     View Summary

    With future all-solid substations, functional dielectrics have been targeted as R & D. Polymer nanocomposites were chosen as such functional dielectrics. These materials were found to be much advantageous over unfilled polymers as well as conventional micro-filler filled polymers in thermal, mechanical and electrical properties. Materials investigated include polyamide/layered silicate nanocomposites, epoxy/layered silicate nanocomposites, epoxy/titania nanocomposites, and epoxy/alumina nanocomposites. Materials are characterized by SEM, AFM, XRD, Surface Roughness Meter, EDX and the like, while they are subjected to partial discharge and high local electric field to degrade.
    Some of the outstanding results are summarized as follows :
    1.Interfaces between nanofillers and surrounding polymer matrices are crucial to determine the properties of nanocomposites. Those form an interaction zone in mesoscopic scale.
    2.A multi-core model is proposed as fine structures of the interaction zone from theoretical consideration. Typically it consists of three layers, i.e. the first kyer, the second layer and the third layer from the contact point toward the matrices.
    3.It was found firstly that polyamide/layered silicate nanocomposites are far more resistant to partial discharges than unfilled polyamide and micro-filler filled polyamide. This is the first discovery of nanocomposite advantage to have initiated such studies worldwide.
    4.A unique manufacturing method has been developed to prepare nanocomposites in laboratory scale, especially epoxy nanocomposites.
    5.Epoxy nanocomposites exhibit not only excellent partial discharge resistance but also treeing resistance. These findings can be explained in term of the proposed multi-core model.
    6.Permittivity decreases due to nanostrucuration as our surprise, while it increase for micro-filler filled polymers. It clearly shows that interfaces are deeply involved to determine macroscopic parameters.

  • 高分子絶縁材料中の不純物が絶縁特性に与える影響の解明と直流ケーブル用材料の開発

     View Summary

    上記の研究課題において、今年度得られた研究実績は以下の通りである。(1)架橋ポリエチレン(XLPE)フィルムのみの空間電荷分布を測定したところ、低密度ポリエチレン(LDPE)中にアセトフェノンを浸透させた場合と同じ傾向を示した。このXLPEフィルムのフーリエ赤外吸収分光測定より、XIPE中には元々架橋剤分解残渣のうち少なくともアセトフェノンは残存しており、このアセトフェノンはXLPE中における空間電荷形成を支配していることがわかった。(2)XLPE中に元々存在する架橋剤分解残渣を除去するため、約80℃で1週間の真空脱気処理を行ったXLPEフィルムのフーリエ赤外吸収分光測定より、架橋剤分解残渣のうち少なくともアセトフェノンは、XLPEフィルム中から除去できたことを確認した。これより、XLPE中に元々存在するアセトフェノン除去のためには、本実験条件で行った真空脱気処理は有効であることがわかった。(3)未処理XLPEフィルムおよび脱気処理済みXLPEフィルム中にクミルアルコールが浸透した場合の空間電荷分布は、クミルアルコールが注入電荷をトラップすることによるホモ電荷の形成が見られたLDPEフィルムの場合と同じ傾向であった。これより、ポリエチレンの架橋構造はポリエチレン中におけるクミルアルコールの注入電荷トラップ作用に影響していないことがわかった。(4)XLPEフィルム中にアセトフェノンが浸透した場合の空間電荷分布では、ヘテロ電荷の蓄積が見られた。これは、LDPE中にアセトフェノンが存在した場合、電荷の移動が促進されるとした結果を指示するものである。また、アセトフェノンは水分と共存することによりイオン性の電荷を形成するとした報告より、このヘテロ電荷の起源はアセトフェノンと水分の共存によるものであることがわかった

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Specific Research

  • 高分子絶縁材料中の不純物が絶縁特性に与える影響の解明と直流ケーブル用材料の開発

    2001  

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     本研究は、架橋ポリエチレンケーブル中の空間電荷形成機構を空間電荷分布の実時間測定により解明し、架橋ポリエチレンケーブルが直流長距離送電に利用できる可能性を示すことを目的としている。有機高分子絶縁体の試料としてポリエチレンを使用し、架橋剤分解残渣である、アセトフェノン、αメチルスチレン、クミルアルコールそれぞれを浸漬させた試料を用いて、これらの残渣がポリエチレン中における空間電荷形成に及ぼす影響を測定した。 アセトフェノン、αメチルスチレンの場合は、浸漬試料と未浸漬試料で構成された2層誘電体では、浸漬試料―未浸漬試料の界面に浸漬試料が接している電極と同極性の電荷が蓄積した。クミルアルコールの場合は、前2者とは対照的に、浸漬試料―未浸漬試料の界面に浸漬試料が接している電極と逆極性の電荷が蓄積した。アセトフェノン、αメチルスチレンにおける電荷蓄積の原因は、伝導電流測定より(導電率/誘電率)の不連続性によるものであると考えられる。これより、アセトフェノン、αメチルスチレンは、電荷移動を促進する作用があると判断できる。一方、クミルアルコールの場合は、伝導電流測定より浸漬試料の導電率の増加は確認されたが、浸漬試料―未浸漬試料の界面に蓄積された電荷の極性が前2者とは違うことより、電荷蓄積の原因は、(導電率/誘電率)の不連続性によるものではなく、クミルアルコールは注入電荷をトラップする作用を持っていると考えられる。 そこで、注入電荷トラップの原因を明確にするため、化学薬品の化学構造とくにカルボニル基、水酸基、ベンゼン環に注目し、それらの化学基の少なくとも1つ以上を含む薬品を使用して空間電荷分布測定を行った。架橋剤分解残渣である3種類の薬品に加えて、ベンゼン環を持つドデシルベンゼン、水酸基とカルボニル基を持つグリセリンモノステアラート、ベンゼン環と水酸基を持つジブチルヒドロキシルトルエンを使用した。空間電荷分布測定の結果、浸漬試料―未浸漬試料の界面電荷は、ドデシルベンゼンの場合は、アセトフェノン、αメチルスチレンと同極性の電荷、グリセリンモノステアラート、ジブチルヒドロキシルトルエンはクミルアルコールと同極性の電荷の蓄積が得られた。この結果より明らかなことは、界面における蓄積電荷の極性と化学構造の関係において水酸基を持つ薬品がポリエチレン中に存在すれば注入電荷をトラップする作用があると考えられることである。 つまり、アセトフェノン、αメチルスチレンは、ポリエチレン中において電荷移動を促進する作用を持つのに対し、クミルアルコールは、電荷をトラップして空間電荷を蓄積させる作用があり、その電荷トラップの原因は水酸基であることを見いだした。

  • 有機高分子絶縁材料中の電荷分布の実時間測定

    2000  

     View Summary

     本研究は、架橋ポリエチレンケーブル中の空間電荷形成機構を空間電荷分布の実時間測定により解明し、架橋ポリエチレンケーブルが直流長距離送電に利用できる可能性を示すことを目的としている。有機高分子絶縁体の試料としてポリエチレンを使用し、架橋剤分解残渣である、アセトフェノン、αメチルスチレン、クミルアルコールそれぞれを浸漬させた試料を用いて、これらの残渣がポリエチレン中における空間電荷形成に及ぼす影響を測定した。 アセトフェノン、αメチルスチレンの場合は、浸漬試料が陽極に接触している場合は、浸漬、未浸漬試料の界面において浸漬試料から未浸漬試料中へ正電荷の移動が確認された。一方、浸漬試料が陰極に接触している場合は、浸漬、未浸漬試料の界面において浸漬試料から未浸漬試料中へ電荷の移動はなく、界面に負電荷が蓄積されるのみであった。1層の浸漬試料においては空間電荷は測定できなかった。これに対して、片面のみにアセトフェノン、αメチルスチレンを塗布した1層の試料では、塗布面が陽極側のときのみ正電荷の移動が確認された。これらの電荷は伝導電流測定より、導電率の増加によるものであることが分かった。これより、アセトフェノン、αメチルスチレンは、電荷移動を促進する作用があると判断できる。 クミルアルコールの場合には、前者の場合とは対称的に、浸漬試料が陽極に接触している場合は、電極、浸漬試料の界面に正電荷が蓄積し、浸漬、未浸漬試料の界面に負電荷が蓄積した。また、浸漬試料から未浸漬試料中へ負電荷の移動は確認されなかった。浸漬試料が陰極に接触している場合は、電極、浸漬試料の界面に負電荷が蓄積し、浸漬、未浸漬試料の界面において正電荷が蓄積した。また、浸漬試料から未浸漬試料中へ電荷の移動はなかった。伝導電流測定より浸漬試料の導電率の増加は確認されたが、界面に蓄積された電荷の極性の違いの原因は明確でない。また、1層の浸漬試料においては、電極と試料の界面に、接触している電極と同極性の電荷が蓄積されていることが測定できた。これは、クミルアルコールが存在すると電荷はその付近でトラップされることを示すものである。 つまり、アセトフェノン、αメチルスチレンは、ポリエチレン中において電荷移動を促進する作用を持つのに対し、クミルアルコールは、電荷をトラップして空間電荷を蓄積させる作用があることを見いだした。このことより、架橋ポリエチレン中よりその架橋剤分解残渣であるクミルアルコールのみを除去できれば、架橋ポリエチレンには空間電荷は形成されないことになり、架橋ポリエチレンケーブルが直流長距離送電に利用できる可能性を示した。