Surface and interface properties of alumina via model studies of microdesigned interfaces

The ability to produce controlled-geometry, controlled-crystallography inte rnal voids in ceramics has made possible several new model experiments for studying the high-temperature properties of surfaces and interfaces in cera mics. Recent advances have enabled the production of more complex microdesi gned internal defect structures, and have exploited new means of examining them, thus, broadening the range of problems that can be addressed. A parti cular topic of concern is the effect of surface energy anisotropy on both t he driving force for and the mechanism of shape changes. This paper reviews and previews recent research focussing on improving our understanding of s urface diffusion in ceramics. Rayleigh instabilities provide one means of e xamining morphological evolution. The modelling of Rayleigh instabilities i n materials with surface energy anisotropy is reviewed, and the results of experiments utilizing microdesigned pore arrays in sapphire are summarized. In a material with anisotropic surface energy and a facetted Wulff shape, the driving force for shape changes hinges on both the absolute and relativ e surface energies. Microdesigned pore structures have been used to determi ne the stable surfaces in both undoped and doped sapphire and to provide th e relative values of the energies of these stable surfaces; Nonequilibrium shape, controlled-crystallography cavities have been introduced into undope d sapphire, and the effect of crystallographic orientation on their morphol ogical evolution has been studied. Comparisons of the results with predicti ons of models of surface-diffusion-controlled evolution indicate that surfa ce-attachment-limited kinetics (SALK) play an important role. (C) 1999 Else vier Science Ltd. All rights reserved.