Calculation of Grain-Boundary Bonding in Rare-Earth-Doped β-Si3N4 |
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Authors: | Tetsuo Nakayasu Tetsuo Yamada Isao Tanaka Hirohiko Adachi Seishi Goto |
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Affiliation: | Ube Research Laboratory, UBE Industries, Ltd., Ube, Yamaguchi 755, Japan;Department of Materials Science and Engineering, Kyoto University, Sakyo, Kyoto, 606-01, Japan;Faculty of Engineering, Yamaguchi University, Ube, Yamaguchi 755, Japan |
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Abstract: | First-principles molecular orbital calculations are performed by the discrete variational Xalpha method using model clusters of rare-earth-doped β-Si3N4 and the interface between prismatic planes of β-Si3N4 and intergranular glassy films. On the basis of the total overlap population of each cluster, the rare-earth ions are implied to be stable in the grain-boundary model, while they are not stable in the bulk model. These results are consistent with experimental observations showing significant segregation of Ln3+ ions at the grain boundary and no solubility of Ln3+ into bulk β-Si3N4. Grain-boundary bonding is weakened with an increase of the ionic radius of the rare-earth ions, which provides a reasonable explanation for the ionic size dependence of the crack propagation behaviors as well as the growth rate of the prismatic plane in the rare-earth-doped β-Si3N4 during liquid-phase sintering. |
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