First-principles study of rare-earth effects on grain growth and microstructure in β − Si 3 N 4 ceramics

摘要: Rare earth (RE) and group III oxide additions are frequently used to optimize densification during the processing of ceramics. Silicon nitride ceramics serve as model cases, in these systems effects rare earths important. Additions often determine morphology {beta}-Si{sub 3}N{sub 4} crystallites that grow multiphase ceramic, thereby affecting microstructure mechanical toughness ceramic. The influence different has recently been experimentally characterized terms their on grain growth aspect ratios. In study reported here, a new energy parameter is introduced provides first-principles based understanding effects. Grain ratios measured for various RE silicon correlate well with corresponding differential binding energies (DBE) calculated within partial wave self-consistent field atomic cluster model. DBE second-difference measure relative site stabilities vs Si atoms regions variable O/N content. physical mechanism underlies anisotropic found originate from competition between REs bonding at interfaces O-rich glass. segregation strengths exhibited by elements oxynitride glassesmore » simply reflection local chemistries O, N environments. Elements segregate prism planes embedded grains impede attachment Si-based units, extent this limitation leads observed anisotropy.« less