Effect Of Grain Boundaries on Diffusion‐Controlled Processes in Aluminum Oxide

Abstract

Oxygen ion diffusion coefficients in single‐crystal Al 2 O 3 are several orders of magnitude less than alminum ion diffusion coefficients in polycrystalline Al 2 O 3 . In polycrystalline Al 2 O 3 , oxygen ion diffusion is enhanced by the presence of grain boundaries as in the chloride ion diffusion in the alkali halides. Creep and sintering of polycrystalline Al 2 O 3 occur at a faster rate than is possible through control by lattice diffusion of oxygen; the rates are in fair quantitative agreement with cation diffusion. It is tentatively concluded that enhanced oxygen diffusion in regions adjacent to boundaries allows aluminum ion bulk diffusion to be rate controlling for these processes. The electrical conductivity in Al 2 O 3 is too high to be related to either anion or cation diffusion and is predominantly electronic.