Diffusion in alumina

Experimental results on diffusion in alumina are summarized and critically discussed. The most reliable results are those on volume diffusion in single crystals (sapphire), and are available for a wide variety of substances, including the structural elements oxygen and alumina, water, monovalent cations, and trivalent cations. Experimental results on volume diffusion in polycrystalline alumina have also been reported for hydrogen, water, some actinides, and rare earths. Diffusion coefficients have been deduced from deep penetration tails on diffusion profiles, and have been interpreted to result from fast diffusion along dislocations or grain boundaries. However, there are questions about these interpretations that are discussed. Mechanisms of diffusion in alumina are uncertain; a variety of charged defects has been suggested to control diffusion in alumina, but no interpretation is widely accepted because of discrepancies with experimental results. The possibility of an AlO defect is put forward to stimulate exploration and discussions; together with a diffusion-reaction mechanism, it can explain many puzzling features of diffusion in alumina. The electrical conductivity of alumina results from the transport of H+(H3O+) ions if the OH concentration in the alumina is greater than about 3×10−7 OH groups per Al atom. At lower OH concentrations electronic conductivity becomes dominant.