Papers by Author: Shoichi Kume

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Authors: Shoichi Kume, Saruhan Saklar, Koji Watari
Abstract: Attempts have been made to sinter hexagonal boron nitride (h-BN) by pressureless-sintering in air. In order to achieve this purpose, feldspar has been selected as a sintering aid to produce sintered compacts at a temperature above the melting point of feldspar. Even though h-BN was not wetted by the molten feldspar in nitrogen, the wettability was significantly improved in air. Through the heating process, the h-BN powder disappeared owing to the oxidation of the powder followed by sublimation of the boron oxide. The decomposition of BN was prevented effectively by the presence of molten feldspar in the h-BN/feldspar (30 vol.%) compact. It was shown that h-BN/feldspar composite can be sintered in air under normal pressure, although the bulk density of h-BN/feldspar composites (1.40 to 1.51 g/cm3) was not satisfactory enough.
Authors: Shoichi Kume, Masaki Yasuoka, Naoki Omura, Koji Watari
Abstract: The effects of sintering additives on dielectric loss tangent of AlN ceramics were explored. Different amounts of Y2O3 and TiO2 were respectively added as sintering additives to AlN powders and pressureless sintering was performed in a nitrogen flow atmosphere at 1850 °C or 1900 °C for 2 hours. The resulted AlN ceramics became denser when TiO2 addition was more than 0.5 mol%, and a dense sinter with a relative density of 0.987 could be obtained. Tan δ decreased when TiO2 content was more than 0.5 mol% and a tan δ value of as low as 1.0 x 10-3 could be attained.
Authors: Weiwu Chen, Shoichi Kume, Cihangir Duran, Koji Watari
Abstract: In the present work, effects of different salts (NaCl, 0.5NaCl-0.5KCl and KCl) on molten-salt-synthesized Sr0.5Ba0.5Nb2O6 (SBN50) powder were explored. A variety of particle morphology, compositions and lattice parameters of SBN50 were investigated. The results showed that SBN50 particles synthesized in NaCl salt have the finest diameter (0.1-0.2μm) and the highest aspect ratio (around 10), while the particles from 0.5NaCl-0.5KCl salt show the coarsest diameter (0.2-0.3μm) and the lowest aspect ratio (around 5). If only using KCl salt, trace Sr2Nb2O7 normally co-exists with SBN50 phase. In addition, Na+ and K+ cations from salts are easily incorporated with SBN structure and reach a very high content.
Authors: Shoichi Kume, Masaki Yasuoka, Koji Watari
Abstract: The effect of sintering additives on dielectric loss tangent and microstructure of AlN ceramics have been investigated. Different amounts of Y2O3 and La2O3 were added as sintering additives to AlN powder and pressureless sintering was performed at 1900 °C for 2 h in a flowing nitrogen atmosphere. Bulk densities of sintered AlN were found to be 3.27 and 3.32 Mg·m-3 when Y2O3 contents were 0.5 and 1 mol%, respectively. The bulk densities of AlN-Y2O3 increased with increasing La2O3 content. Densities of 3.41 and 3.42 Mg·m-3 were obtained for 0.5 mol%Y2O3-1 mol%La2O3-AlN and 1 mol%Y2O3-1 mol%La2O3-AlN ceramics respectively. A decrease in tan δ was observed with increase in La2O3 content and a minimum value of 1.3 x 10-3 was obtained for 0.5 mol%La2O3-1 mol%Y2O3-AlN ceramic in the present study.
Authors: K. Suzuki, Shoichi Kume, K. Nakano, T.W. Chou
Authors: N.F. Gao, Koji Watari, Shoichi Kume
Abstract: Industrial wastes have exerted high loads on environment in view of the difficulties to reserve landfill space and the risk of air pollution. In this study we proposed a method of simultaneous disposing of inorganic waste and common organic waste, in light of cascade recycling concept. Zeolite-carbon composite was synthesized by a combination of calcinations and hydrothermal treatment of coal fly ash and sawdust mixtures. The microstructure, phase compositions and electromagnetic wave absorption properties of the composite were evaluated. The results showed that the low cost composite has the potential to be used as high value-added humidity-controlling building material with the ability to absorb electromagnetic wave.
Authors: Ikuko Yamada, Shoichi Kume, Hiromi Nakano, Koji Watari
Abstract: Thermal properties of SiC at the micrometer-scale were measured quantitatively with a thermal microscope using thermo-reflectance and periodic heating techniques. In this study, SiC single crystal and polycrystal were investigated. The small values of standard deviation suggest that the SiC single crystal had constant thermal conductivities. For the single crystal, the average value of the thermal conductivity at the micrometer-scale was in good agreement with the macro-scale thermal conductivity value obtained by the laser flash technique. On the other hand, thermal conductivity of the polycrystal was heterogeneous at the micrometer-scale. An average thermal conductivity value of 257 Wm-1K-1 was obtained within an area of 50 m ×100 µm. The highest and lowest values of the thermal conductivity from the polycrystal were 300 and 220 Wm-1K-1, respectively.
Authors: Sang Kee Lee, Ikuko Yamada, Shoichi Kume, Hiromi Nakano, Koji Watari
Abstract: The thermal conductivity at the micrometer-scale of AlN ceramics with eliminated grain boundary phase was measured by the thermoreflectance technique with periodic heating. Thermal conductivities were ranged from 160 to 260 W/m•K and an average value of 201 W/m•K was obtained from a 22 m2 area. The variation in the thermal conductivities was related to the individual AlN grains and grain boundary characteristics.
Authors: Sang Kee Lee, Kazuo Nakamura, Shoichi Kume, Koji Watari
Abstract: Hexagonal boron nitride ceramics with sintering additives were synthesized by uniaxial hot-pressing. Hot pressing was carried out at 1800oC for 2 h under the uniaxial pressure of 30 MPa under a nitrogen atmosphere. Thermal conductivity perpendicular and parallel to the direction of the hot-pressing axis was measured by the laser-flush method.
Authors: Naoki Omura, Yuji Hotta, Yoshiaki Kinemuchi, Shoichi Kume, Koji Watari
Abstract: Wet jet milling is a new technology for the pulverization of powders, in which streams of slurry, higher than 100m/s, collide in the channel made of diamond. The effect of processing parameters was investigated in the system of BN/SiO2 mixed powders. The particle size after milling was evaluated by the measurement of BET surface area. The BET surface area of BN increased with increase in the number of milling, in the mixture ratio of SiO2/BN, in the particle size of SiO2 and in the processing pressure, whereas no significant change in the size of SiO2 particle was found. The surface area of BN increased by about 2.1 times by wet jet milling for only about 20 minutes (5 times).
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