The energetics and kinetics of pore shape changes in alumina

Microlithography and ion beam etching were used to introduce controlled size cavities into the surfaces of undoped sapphire substrates of controlled (and varied) surface orientation. These cavities were converted into internal intragranular pores by diffusion bonding of the etched substrate to an unetched substrate. Two studies were performed. In the first, arrays of cavities as small as approximate to 2 mu m x 2 mu m wide and approximate to 0.37 mu m deep were introduced, annealed for prolonged periods at 1600 degrees and 1800 degrees C, and the equilibrium cavity shape was determined using scanning electron and atomic force microscopy. By measuring the pore shape precisely, and indexing the facets that form, the relative surface energies of different low index surfaces were determined using the Wulff construction. In the second study, wider (approximate to 20 mu m x 20 mu m) but similarly shallow (approximate to 0.5 mu m) intragranular pores were produced, and pore shape changes during high temperature annealing (1900 degrees C) were examined. A model based on surface diffusion controlled evolution was developed, and the predictions of the model were compared to the experimentally observed pore morphology changes. Evolution rates were found to vary widely among the substrates, despite an initially identical pore geometry. The most dramatic differences appear to reflect the stability of the surface into which the pores are etched, i.e., the substrate orientation. A more detailed assessment of the results suggests that surface attachment limited kinetics (SALK) play an important role in determining the rate of surface mass transport on the examined low index surfaces of alumina.