Papers by Author: Masato Uehara

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Authors: T. Matsukawa, K. Sugawara, Satoshi Tanaka, Nozomu Uchida, Keizo Uematsu, Yoshikazu Shinohara, Masato Uehara
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Authors: Koji Matsui, N. Etoh, M. Ohgai, Masato Uehara, Naoya Enomoto, Junichi Hojo
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Authors: Junichi Hojo, Hironori Kugimoto, Masato Uehara, Naoya Enomoto
Abstract: Si3N4-based composite powders have been synthesized by vapor phase and liquid phase processes. Nano-sized Si3N4-TiN composite particles were formed by the vapor phase method, in which TiN nanocrystallites were included in amorphous Si3N4 particles. The composite powder was also formed by the liquid phase method, where Si and Ti complex imide powders were prepared in an organic solvent and decomposed by heating. Si3N4-TiN nanocomposites were fabricated by hot pressing of the composite powders. The Si3N4-TiN composites were also fabricated by in-situ process from amorphous Si3N4 and Y2O3-TiO2-AlN additive. In all processes, rod-like Si3N4 grain growth was stimulated by TiN inclusion.
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Authors: Koji Matsui, Takanori Yamakawa, Masato Uehara, Naoya Enomoto, Junichi Hojo
Abstract: Small amounts of Al2O3 were added to fine zirconia powder by different ways: powder mixing, hydrolysis of alkoxide, and homogeneous precipitation. During a constant rate heating process, the Al2O3 addition slightly raised the starting temperature of densification of powder compact, and the densification was remarkably stimulated by Al2O3 at temperatures above about 1100oC. According to an isothermal analysis of densification, the densification rate was retarded by Al2O3 addition just after the start of sintering and then the densification rate increased significantly during sintering compared to Al2O3-free powder. These results mean that Al2O3 particles pinned the shrinkage of zirconia powder compact at the initial stage, and diffuse toward zirconia surface to enhance the sintering. The sintering mechanism was explained by the grain-boundary diffusion for the Al2O3-free powder and the volume diffusion for Al2O3-added powder. When the Al2O3 was added to zirconia powder by homogeneous precipitation and alkoxide methods, the densification rate was more stimulated compared to powder mixing method. The sintering mechanism did not change by the way for Al2O3 addition. The Al2O3 addition by the chemical process tended to enhance the grain growth of zirconia, while the uniform microstructure was achieved because of homogeneous addition of Al2O3 by these chemical processes.
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