An Assessment of Energetic and Kinetic Properties of Surfaces and Interfaces in Alumina

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9617392 Glaeser An experimental technique for determining the equilibrium shape of pores in alumina has been developed and applied to both undoped and Ti-doped sapphire. Ti doping changes the Wulff shape of alumina. Concurrently, the use of pre- perturbed pore channels has elevated studies of the Rayleigh instabilities to a new level, allowing quantitative measurement of key thermodynamic and kinetic parameters; a framework for correcting for anisotropy effects in assessing the surface diffusivity has been developed. Ti doping affects the morphological stability, but appears to have only a mild effect on the surface diffusivity. Finally, Ti doping has been shown to promote highly anisotropic grain growth, leading to the formation of elongated grains, and to substantially change the grain boundary mobility. The proposed work builds on and extends the first two components of this prior work. Specifically, by using lithographically introduced pores and atomic force microscopy (AFM) to assess the equilibrium pore shape, we propose to determine the effect of Ti, Cr, Ca and Mg additions on the relative energies and thermodynamic stabilities of a range of low index surfaces in sapphire. In addition, the instability of lithographically introduced pore channels with pre-existent finite perturbations of known but systematically varied amplitude and wavelength will be examined. The independent effects of dopants and crystallographic orientation on the thermodynamic and kinetic characteristics of perturbation evolution will be determined. This method of determining the surface diffusivity, and more conventional scratch smoothing methods will be used in tandem to compare apparent surface diffusivities obtained under conditions where the amplitude of the surface perturbation is increasing with time, and decreasing with time, respectively. The diffusivities determined using these two complementary approaches will be compared to the diffusivities that can be obtaine d from an analysis of pore-boundary separation conditions. The comparison may help to identify which (if any) of the two approaches used to determine the surface diffusivity provides data that is useful in predicting and describing microstructural evolution in ceramics. %%% The proposed research will help improve our understanding of surface diffusion in ceramics. By studying the effect of dopants and crystallographic orientation, the energetics of surface diffusion can be determined, and thus, how the changes in energetics and kinetic properties impact microstructural evolution in ceramics. These processes are important in designing optimum microstructures for properties. ***