Measurements of O self-diffusion were performed on unintentionally doped ceramics and oriented single crystals. The 18O tracer was introduced by means of solid/gas isotopic exchange, and the diffusion profiles were determined by using secondary ion mass spectrometry. In the case of the monocrystals, evidence of short-circuit effects led to the consideration of a diffusion contribution from dislocation walls in the samples. The bulk and sub-boundary diffusion coefficients were determined, and were found to be characterized by activation enthalpies of 636 and 896kJ/mol, respectively. Results for polycrystalline material (obtained by sintering alumina powder from the same source as the monocrystals) indicated an activation enthalpy, for grain boundary diffusion, of 921kJ/mol. This unusual result was explained in terms of the effect of impurity segregation upon the grain boundary self-diffusion.

Oxygen Grain Boundary Diffusion in Alumina, Impurity Effects. D.Prot, M.Miloche, C.Monty: Materials Science Forum, 1993, 126-128, 403-6