Yoshimichi OHKI
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Yoshimichi OHKITEL : 03-5286-3375 FAX : 03-3204-1258 e-mail :yohki@waseda.jp URL:http://www.eb.waseda.ac.jp/ohki/ Yoshimichi Ohki received the B. Eng., M. Eng., and Dr Eng. degrees in 1973, 1975, and 1978, respectively, all from the Waseda University, Japan. He joined the teaching staff of the Department of EE, Waseda University, in 1976 and is presently working as a Professor. He is also a Visiting Scientist at the Massachusetts Institute of Technology from 1982 to 1984 and an IEEE Fellow. He is a recipient of IEEE-DEIS Forster, Whitehead and Ieda Memorial Awards, two Best Paper Awards from IEE Japan, and other awards. He has published more than 310 papers in refereed journals, contributed to over 700 papers in Japanese conferences, and has contributed to several textbooks and handbooks. His research interests include organic and inorganic dielectric materials for optical fiber and power cable. |
Research
Dielectric materials are key materials for the following two primary reasons:
1) They are electrical insulators. Effective electrical insulation is essential for any electronic device or electrical apparatus.
2) Their band gap energy is large, making them transparent to light of short wavelengths and creating the potential to serve as effective light-transmitting and light-emitting devices.
Dr. Ohki's laboratory examines many aspects of the optical and electrical properties of solid dielectrics, ranging from fundamental aspects to applications. Typical examples include the following:
1) A basic understanding of the dielectric properties of polymer/nanofiller composites and applications to the compact design of electric and electronic apparatuses.
Polymer nanocomposites, or polymers to which very fine inorganic fillers smaller than 1 m are added, have drawn significant interest for their superior mechanical properties. We have confirmed that nanocomposites also exhibit superior insulating properties. For example, adding a mere few percent of nanofiller to polypropylene can dramatically improve partial discharge resistance. We are currently undertaking several joint research projects, including an international project called VAMAS and two university/industry projects supported by the Japanese government. The goal of these projects is to develop printed wiring boards with high thermal conductivity and high resistance to electrochemical migration; insulating materials for power devices with thermal expansion coefficients similar to metals and high resistance against various degradation failures such as partial discharge erosion and treeing; and insulating materials for DC power cables that resist the accumulation of space charge. Fig. 1 shows a typical result of the third project.
2) Evaluations of the applicability of biodegradable polymers to electrical insulation.
Biodegradable polymers that can be decomposed by microbes have drawn significant interest for their eco-sensitive characteristics, especially polymers synthesized from plantoriginated raw materials, since the CO2 emitted by such materials upon waste treatment is part of the CO2 previously absorbed. We are examining the basic properties of these materials with the goal of clarifying their applicability to practical electrical insulating materials.
3) In efforts related to dielectric materials research and development, we have shown that the implantation of ions into silica glass increases its refractive index (Fig. 2). We can harness this increase in the refractive index to fabricate various functional optical devices, including an optical fiber grating, an optical coupler, and a polarizer/depolarizer.
▲Fig. 1 space charge and implanted fiber |
 ▲Fig. 2 |
Representative publications
N. Fuse, Y. Ohki, and T. Tanaka, "Comparison of Nano-structuration Effects in Polypropylene among Four Typical Dielectric Properties", IEEE Transactions on Dielectrics and Electrical Insulation, (in press).
M. Natsui, H. Asakawa, T. Tanaka, Y. Ohki, T. Maeno, and K. Okamoto, "Generation Mechanism of Electrochemical Migration in Printed Wiring Board Insulation", IEEJ Transactions on Electrical and Electronic Engineering, (in press).
F. Kato and Y. Ohki, "Electrical Conduction and Dielectric Relaxation in Polyethylene Terephthalate Succinate", Electrical Engineering in Japan, Vol. 170, No. 4, pp. 1-8, 2010. 3.
N. Tagami, M. Hyuga, Y. Ohki, T. Tanaka, T. Imai, M. Harada, and M. Ochi, "Comparison of Dielectric Properties between Epoxy Composites with Nanosized Clay Fillers Modified by Primary Amine and Tertiary Amine", IEEE Transactions on Dielectrics and Electrical Insulation, Vol. 17, No. 1, pp. 214-220, 2010. 2.
K. Ishimoto, E. Kanegae, T. Tanaka, Y. Ohki, Y. Sekiguchi, Y. Murata, and C. C. Reddy, "Superiority of Dielectric Properties of LDPE/MgO Nanocomposites over Microcomposites", IEEE Transactions on Dielectrics and Electrical Insulation, Vol. 16, No. 6, pp. 1735-1742, 2009. 12.
S. J. Yu, M. Suzuki, Y. Ohki, M. Fujimaki, and K. Awazu, "Control of Coupling Ratio by Proton Implantation for a Directional Coupler of Planar-Lightwave-Circuit Type", Japanese Journal of Applied Physics, Vol. 48, pp. 102405(1)-102405(5), 2009. 10.
S. Raetzke, Y. Ohki, T. Imai, T. Tanaka, and J. Kindersberger, "Tree Initiation Characteristics of Epoxy Resin and Epoxy/Clay Nanocomoposite", IEEE Transactions on Dielectrics and Electrical Insulation, Vol. 16, No. 5, pp. 1473-1480, 2009. 10.
K. Nomura, Y. Ohki, M. Fujimaki, X. Wang, K. Awazu, and T. Komatsubara, "Plasmonic Activity on Gold Nanoparticles Embedded in Nanopores Formed in a Surface Layer of Silica Glass by Swift-heavy-ion Irradiation", Nanotechnology, Vol. 20, pp. 475306(1)-475306(7), 2009. 10.
X. Wang, M. Kuwahara, K. Awazu, P. Fons, J. Tominaga, and Y. Ohki, "Proposal and Design of a Grating-based Optical Reflection Switch using Phase Change Material", Optics Express, Vol. 17, No. 19, pp. 16947-16956, 2009. 9.
T. Iikura, E. Hirata and Y. Ohki, "Effect of Crystallization on the Photoluminescence in LaAlO3 Induced by Ultraviolet Photons", UVSOR ACTIVITY REPORT 2008, p. 81, 2009. 6.
K. Nomura, S. Fujii, Y. Ohki, K. Awazu, M. Fujimaki, N. Fukuda, and T. Hirakawa, "Solution Conductivity as a Key Factor for Thin Silica Coating on Colloidal Silver", Japanese Journal of Applied Physics, Vol. 48, pp. 06FE04(1)-06FE04(4), 2009. 6.
S. C. B. Gopinath, K. Awazu, M. Fujimaki, K. Sugimoto, Y. Ohki, T. Komatsubara, J. Tominaga, and P. K. R. Kumar, "Monitoring Surface-assisted Biomolecular Assembly by Means of Evanescent-field-coupled Waveguide-mode Nanobiosensors", Analytical and Bioanalytical Chemistry, Vol. 394, pp. 481-488, 2009. 5.
N. Fuse, H. Sato, Y. Ohki, and T. Tanaka, "Effects of Nanofiller Loading on the Molecular Motion and Carrier Transport in Polyamide", IEEE Transactions on Dielectrics and Electrical Insulation, Vol. 16, No. 2, pp. 524-530, 2009. 4.
S. Hikosaka, H. Ishikawa, and Y. Ohki, "Effects of Crystallinity on Dielectric Properties of Poly (L-lactide)", IEEJ Transactions on Fundamentals and Materials, Vol. 129, No. 4, pp. 217-222, 2009. 4.