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
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