Different types of faceted grain boundaries in commercial poly-crystalline Altoz were examined. Three distinct cases, namely the basal twin, the rhombohedra1 twin, and other special highangle grain boundaries, are discussed, comparing theoretical models and experimental observations. It is shown that the faceting of the boundaries can be associated with their migration. A mechanism for the separation of a 2 = 13 grain houndary from a pinning void, small grain, or second-phase particle is illustrated. It is also shown experimentally that faceting of high-angle boundaries can occur on a scale of -5 nm, and that, theoretically, faceting most likely also occurs on an atomistic scale. Throughout the theoretical analysis the basal plane is predicted to be a favored facet plane, which is confirmed by experimental observation.
The growth of a-A1203 from a planar specimen of thermally grown y-alumina on a molybdenum transmission electron microscope grid was studied. The a-A1203 grows into the transition alumina matrix and then thickens via a ledge growth mechanism. Faceted M o crystallites cause pinning of a-A1203 ledges and are larger on a-A1203 than on the transition alumina matrix.
14853NDERSTANDING the surface of a!-AIZO3 uis important in studying sintering, catalysis, and solid-state reactions because factors such as surface diffusion and the structure and migration of grain boundaries and phase boundaries each depend on the crystallography of the surface. It has been shown, for example, that grain boundaries tend to facet so that the boundary plane lies parallel to the basal plane in one grain or the other.' This observation can be explained by the basal-plane facet being of lower energy or by noting that, because of the near close-packing of the oxygen ions, an interface will move more slowly in the direction normal to the basal plane than in a direction which is inclined to this plane.Similarly, the y-and 7-aluminas having a high surface area are used widely as adsorbents, catalysts, and catalyst supports. Transformation through 6 , 0-, and eventually to a!-A1203 is accompanied by a catastrophic loss of surface area' and changes in the surface chemistry3 that can
The occurrence of faceted grain boundaries in commercially produced Al2O3 is quite common. This type of boundary has been noted previously, but little structural information has been reported. The different types of faceted boundary include twin boundaries, special high-angle boundaries, and lowangle grain boundaries. Faceted interfaces between Al2O3 grains and secondphase particles have been briefly discussed elsewhere. The present work is concerned mainly with the crystallography of special high-angle boundaries and twins.The planes of the facets can be understood using the concepts of the O-lattice and the CSL, which have been successfully used to study metals and semiconductors. Wagner, Tan, and Balluffi have proposed the generalized CSL, in which the coincident lattice sites in the two grains need neither be identical nor be occupied by atoms. It is in this sense that the CSL model will be used in this work.
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