Chemical Diffusion in Polycrystalline Al2O3

Chemical diffusion in poly crystalline α‐Al2O3 doped with Ti or Fe was studied through the movement of a color front on oxidation of reduced samples. The diffusion is faster in polycrystal‐line than in single‐crystal material. The oxygen pressure dependence of the rate of diffusion indicates that neutral interstitial oxygen is the major mobile species at the grain boundaries. A theory is presented which accounts for the data and explains the difference in the rates of oxidation and reduction and their dependence on oxygen pressure. Grain boundaries with precipitates of FeAl2O4 or TiAl2O5 appear to transport oxygen faster than clean boundaries.