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Author

Takahiro Imai

Other affiliations: Waseda University
Bio: Takahiro Imai is an academic researcher from Toshiba. The author has contributed to research in topics: Epoxy & Nanocomposite. The author has an hindex of 21, co-authored 115 publications receiving 1544 citations. Previous affiliations of Takahiro Imai include Waseda University.


Papers
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Journal ArticleDOI
TL;DR: In this paper, the electrical insulation properties of a newly prepared composite material by nano-and micro-filler mixture were investigated by measuring the thermal expansion coefficient and insulation breakdown properties by a needle-plate electrode method.
Abstract: This paper focuses on the electrical insulation properties of a newly prepared composite material by nano- and micro-filler mixture. Nano- and micro-filler mixture composites were made by dispersing nano-scale layered silicate fillers and micro-scale silica fillers in epoxy resin. To investigate the effects of nano- and micro-filler mixture, the thermal expansion coefficient and insulation breakdown properties by a needle-plate electrode method were measured for the filler mixture composite and the conventional filled epoxy. The filler mixture composite had almost the same thermal expansion coefficient as the conventional filled epoxy. In a continuous voltage rising test, the filler mixture composite had 7% higher insulation breakdown strength than the conventional filled epoxy. Moreover, under constant AC voltage (10 kV at 1 kHz), the filler mixture composite had an insulation breakdown time of more than 20,000 minutes whereas the conventional filled epoxy had a breakdown time of 830 minutes. Electron microscope observation showed that the area surrounded by dispersed micro-scale silica fillers were also filled with the nano-scale layered silicate fillers. Furthermore, the estimate of spacing between the fillers and the filler/epoxy interface area showed a more densely-packed structure of the filler mixture composite than the conventional filled epoxy. The morphological feature of the filler mixture composite seems to improve its insulation breakdown strength and time.

221 citations

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TL;DR: In this paper, the frequency accelerated partial discharge (PD) aging of epoxy nanocomposite with 5 wt % additions of clay was investigated in terms of PD erosion depth, and it was found that the change in the erosion depth is far smaller in specimens with clay than those without clay.
Abstract: 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 PD 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.

162 citations

Journal ArticleDOI
TL;DR: In this article, the authors investigated the influence of temperature on the mechanical and insulation properties of a newly developed epoxy-layered silicate nanocomposite, which has a higher thermal resistance with respect to mechanical properties than a base epoxy resin.
Abstract: The aim of this study is to investigate the influence of temperature on the mechanical and insulation properties of a newly developed epoxy-layered silicate nanocomposite. This nanocomposite has a higher thermal resistance with respect to mechanical properties than a base epoxy resin (epoxy resin without fillers). The volume resistivity of the nanocomposite gradually decreases with increasing temperature, and its relative permittivity gradually increases with increasing temperature. Its properties are more dependent on temperature than those of the base epoxy resin. Moreover, under a constant AC voltage, the insulation breakdown time of the nanocomposite was twice as long as that of the base epoxy resin at 20 /spl deg/C and six times as long at 80 /spl deg/C. In particular, at 145 /spl deg/C, the nanocomposite had a breakdown time of more than 20,000 minutes while the base epoxy resin had breakdown time of 280 minutes. This improvement in breakdown time resulted from electrical treeing shapes with many branches and smaller internal stress of the nanocomposite in comparison with the base epoxy resin.

104 citations

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TL;DR: In this article, the differences in the curing agent and filler dispersion method on the dielectric properties were examined for epoxy/clay nanocomposites, and the results of UV photon absorption and PL measurement showed that 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.
Abstract: 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.

65 citations

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TL;DR: In this article, the 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.
Abstract: 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 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, 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 multicore model that some of the authors have proposed.

65 citations


Cited by
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Journal ArticleDOI
TL;DR: This review first outlines the crucial issues in the nanodielectric field and then focuses on recent remarkable research developments in the fabrication of FNDMs with special constitutents, molecular structures, and microstructures.
Abstract: Study of flexible nanodielectric materials (FNDMs) with high permittivity is one of the most active academic research areas in advanced functional materials. FNDMs with excellent dielectric properties are demonstrated to show great promise as energy-storage dielectric layers in high-performance capacitors. These materials, in common, consist of nanoscale particles dispersed into a flexible polymer matrix so that both the physical/chemical characteristics of the nanoparticles and the interaction between the nanoparticles and the polymers have crucial effects on the microstructures and final properties. This review first outlines the crucial issues in the nanodielectric field and then focuses on recent remarkable research developments in the fabrication of FNDMs with special constitutents, molecular structures, and microstructures. Possible reasons for several persistent issues are analyzed and the general strategies to realize FNDMs with excellent integral properties are summarized. The review further highlights some exciting examples of these FNDMs for power-energy-storage applications.

1,131 citations

Journal ArticleDOI
TL;DR: In this paper, a multi-core model with the far-distance effect, which is closely related to an "interaction zones", has been proposed from consideration of mesoscopic analysis of electrical and chemical structures of an existing interface with finite thickness.
Abstract: Polymer nanocomposites possess promising high performances as engineering materials, if they are prepared and fabricated properly. Some work has been recently done on such polymer nanocomposites as dielectrics and electrical insulation. This was reviewed in 2004 based on the literatures published up to 2003. New significant findings have been added since then. Furthermore, a multi-core model with the far-distance effect, which is closely related to an "interaction zones", has been proposed from consideration of mesoscopic analysis of electrical and chemical structures of an existing interface with finite thickness. It is speculatively examined in the paper how the model works for various properties and phenomena already found in nanocomposites as dielectrics focusing on electrical characteristics, resistance to high voltage environment, and thermal properties.

903 citations

Journal ArticleDOI
TL;DR: In this article, the future of mesoscopic properties of nanocomposite polymers is discussed, and several interesting results to indicate the foreseeable future have been revealed, some of which are described on materials and processing, together with basic concepts and future direction.
Abstract: Polymer nanocomposites are defined as polymers in which small amounts of nanometer size fillers are homogeneously dispersed by only several weight percentages. Addition of just a few weight percent of the nanofillers has profound impact on the physical, chemical, mechanical and electrical properties of polymers. Such change is often favorable for engineering purpose. This nanocomposite technology has emerged from the field of engineering plastics, and potentially expanded its application to structural materials, coatings, and packaging to medical/biomedical products, and electronic and photonic devices. Recently these 'hi-tech' materials with excellent properties have begun to attract research people in the field of dielectrics and electrical insulation. Since new properties are brought about from the interactions of nanofillers with polymer matrices, mesoscopic properties are expected to come out, which would be interesting to both scientists and engineers. Improved characteristics are. expected as dielectrics and electrical insulation. Several interesting results to indicate the foreseeable future have been revealed, some of which are described on materials and processing in the paper together with basic concepts and future direction.

889 citations

Journal ArticleDOI
TL;DR: It is demonstrated that covalently bonded organic cages can assemble into crystalline microporous materials and design principles for responsive porous organic solids and for the modular construction of extended materials from prefabricated molecular pores are suggested.
Abstract: Porous materials are important in a wide range of applications including molecular separations and catalysis. We demonstrate that covalently bonded organic cages can assemble into crystalline microporous materials. The porosity is prefabricated and intrinsic to the molecular cage structure, as opposed to being formed by non-covalent self-assembly of non-porous sub-units. The three-dimensional connectivity between the cage windows is controlled by varying the chemical functionality such that either non-porous or permanently porous assemblies can be produced. Surface areas and gas uptakes for the latter exceed comparable molecular solids. One of the cages can be converted by recrystallization to produce either porous or non-porous polymorphs with apparent Brunauer–Emmett–Teller surface areas of 550 and 23 m2 g-1, respectively. These results suggest design principles for responsive porous organic solids and for the modular construction of extended materials from prefabricated molecular pores.

857 citations