Papers by Author: Koichi Niihara

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Authors: Kazuki Tajima, Hae Jin Hwang, Mutsuo Sando, Koichi Niihara
Authors: Kazuki Tajima, Hae Jin Hwang, Mutsuo Sando, Koichi Niihara
Authors: Jun Inoue, Tadachika Nakayama, Tsuneo Suzuki, Hisayuki Suematsu, Wei Hua Jiang, Koichi Niihara
Abstract: Chromium oxynitride (Cr(N,O)) thin film have been successfully prepared by using pulsed laser deposition. The composition of the thin film was determined to be Cr0.50N0.23O0.28 by Ruthreford backscattering spectroscopy (RBS). The structural analysis was carried out by using X-ray diffraction (XRD), and out-of-plane and in-plane measurements were used to clarify the axial ratio (c/a) of the Cr(N,O) phase. The lattice constants of a and c axes in the Cr(N,O) phase were found to be 0.414 and 0.419 nm, respectively. From these results, the cubic to tetragonal phase change by substitution of the oxygen atoms for nitrogen atoms was confirmed for the crystal Cr(N,O) compounds.
Authors: Tatsuzo Nagai, Hae Jin Hwang, Masaki Yasuoka, Mutsuo Sando, Koichi Niihara
Authors: Y. Tokoi, Tadachika Nakayama, Hisayuki Suematsu, Wei Hua Jiang, K. Yatsui, Koichi Niihara
Abstract: Copper nanosized powders were prepared by pulsed wire discharge (PWD) in N2 gas at low pressure (130 Pa) with high-speed gas puff. The pressure around one of the electrodes was changed to investigate the effect of electrical discharge prevention by gas puff. The peak pressure was changed from 22 kPa to 10 kPa in the gas puff apparatus by changing the distance from an acrylic resin tube and a bottom plate. The energy deposition in the copper wire at 22, 15 and 10 kPa was about 40, 37 and 33 J, respectively. The energy deposition in the wire increased with the increase in pressure around the electrode.
Authors: S. Nishijima, Tatsuya Okada, Koichi Niihara
Authors: H. Takano, Takafumi Kusunose, Tohru Sekino, Rajagopalan Ramaseshan, Koichi Niihara
Abstract: 6061 aluminum alloy composite reinforced with α-alumina and aluminum borate is fabricated by casting method. The above mentioned composite has been compared to a similar composite without aluminum borate reinforcement for their machinability. The aluminum borate filler was controlled up to (2.5, 5.0 and 30vol %) on the surface of α-alumina. The reaction zone between alumina and the matrix in both the systems are analyzed with the help of TEM micrographs as well as X-ray diffraction profiles. At the reaction zone between alumina and the aluminum alloys, some spinel-like compounds (MgAl2O4) are identified. Finally, machinability was analyzed with the single point tool machining.
Authors: Takafumi Kusunose, Tohru Sekino, P.E.D. Mogan, Koichi Niihara
Abstract: The Si3N4/YSiO2N composite in which crystalline YSiO2N was formed as grain boundary phase was fabricated by hot-pressing the mixture of SiO2, Si3N4 and Y2O3. The fracture toughness of this composite was significantly improved, compared to the Si3N4 composites containing Y5Si3O12N or Y2Si3O3N4 as a grain boundary phases. To clarify the toughening mechanism, the microstructure and the crack propagation profiles were observed.
Authors: Li Wei Huang, Zheng Yi Fu, Jin Yong Zhang, Wei Min Wang, Hao Wang, Yu Cheng Wang, Koichi Niihara, Soo Wohn Lee
Abstract: Carbon nanotubes reinforced alumina was fabricated by spark plasma sintering method. When adding 0.2wt% nanotubes, the fracture toughness of the composites prepared increases 19% compared with the pure alumina ceramics. The effect of sintering schedule on microstructure and mechanical properties is investigated systematically. Microstructure studies reveal that at high sintering temperature, the nanotubes tend to gather in the gaps surrounded by three or more grains in a flocculent state, which leads to poor mechanical properties. Raman spectrum indicates that long sintering duration may cause serious nanotubes destruction and lower the mechanical properties.
Authors: M. Hussain, Atsushi Nakahira, S. Nishijima, Koichi Niihara
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