Glass Pocket Evolution in Vitreous Bonded Sapphire
Jong S. Lee, M K. Kang, Sheldon M. Wiederhorn, B Hockey, J Blendell
This paper discusses the evolution of glass filled pockets formed by the growth of sapphire through polycrystalline aluminum oxide bonded by anorthite glass. During growth, excess glass is entrapped within the sapphire as channels, which, upon annealing evolve into isolated glass pockets. The pockets have a faceted morphology that appears to be kinetically stabilized in various non-equilibrium shapes. Nevertheless, a comparison of the pocket shapes with simulated crystal shapes and transmission electron microscope observations indicate that these facets are part of the equilibrium shape of sapphire. The C-plane (0001) and the R-planes } are formed throughout the evolution of the pockets, whereas the prismatic A-planes } do not form. Evidence for the S-planes } and the P planes } is weak, although these planes are part of the Wulff shape of pure aluminum oxide in contact with the vapor. Bubbles form within the glass filled pockets after extensive annealing. This observation is rationalized by the diffusion of the Ca2+ and Si4+ ions from the glass pockets to external surfaces or sinks in the bicrystal grain boundary resulting in the formation of a bubble within the glass pocket.