Abstract
Sintering of translucent polycrystalline alumina involves grain-boundary diffusion, and the retardation of grain growth has been ascribed to drags due to solute and/or spinel second phase particles. Magnesia solubility in alumina was proposed as grain-size dependent. We will report that the solid solubility of magnesia in alumina is indeed a function of grain size. During grain growth and sintering of magnesia-doped alumina, the enriched dopant level at grain boundaries and the equilibrium dopant content in the lattice undergo dynamic evolution. Recent advancements have converted alumina to sapphire through abnormal grain growth, in which magnesia-doped alumina is heat treated to first achieve a state of equiaxed grain structure followed with out-diffusion of the dopant so as to give a high rate of grain growth to result in sapphire. Nucleation can start from the interior of the wall, which is ascribed to a gradient in the magnesia level of the green body.
