First-principles study of rare-earth effects on grain growth and microstructure in {beta}-Si{sub 3}N{sub 4} ceramics
- Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6114 (United States)
- Computational Science Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6367 (United States)
- Universitaet Karlsruhe, Institut fuer Keramik im Maschinenbau-Zentrallaboratorium, D-76131 Karlsruhe (Germany)
Rare earth (RE) and group III oxide additions are frequently used to optimize densification during the processing of ceramics. Silicon nitride ceramics frequently serve as model cases, and in these systems the effects of rare earths are important. Additions often determine the morphology of {beta}-Si{sub 3}N{sub 4} crystallites that grow in the multiphase ceramic, thereby affecting the microstructure and mechanical toughness of the ceramic. The influence of different rare earths has recently been experimentally characterized in terms of their effects on grain growth aspect ratios. In the study reported here, a new energy parameter is introduced that provides a first-principles based understanding of these effects. Grain growth aspect ratios measured for various RE additions in silicon nitride correlate well with corresponding differential binding energies (DBE) calculated within the partial wave self-consistent field atomic cluster model. The DBE provides a second-difference measure of relative site stabilities of RE vs Si atoms in regions of variable O/N content. The physical mechanism that underlies anisotropic grain growth is found to originate from the site competition between REs and Si for bonding at {beta}-Si{sub 3}N{sub 4} interfaces and within the O-rich glass. The different segregation strengths exhibited by rare earth elements in oxynitride glasses are simply a reflection of their different local chemistries in O, N environments. Elements that segregate to the prism planes of the embedded {beta}-Si{sub 3}N{sub 4} grains impede the attachment of Si-based silicon nitride growth units, and the extent of this limitation leads to the observed grain growth anisotropy.
- OSTI ID:
- 20664911
- Journal Information:
- Physical Review. B, Condensed Matter and Materials Physics, Vol. 70, Issue 14; Other Information: DOI: 10.1103/PhysRevB.70.144108; (c) 2004 The American Physical Society; Country of input: International Atomic Energy Agency (IAEA); ISSN 1098-0121
- Country of Publication:
- United States
- Language:
- English
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