Effects of an oxygen potential gradient and water vapor on mass transfer in polycrystalline alumina at high temperatures

Abstract The mass transfer mechanisms in polycrystalline alumina wafers serving as models for anti-oxidation films were investigated by evaluating oxygen permeability at 1873 K using 18O2 as a tracer. The grain boundary (GB) diffusion coefficients for oxygen in the vicinity of the higher oxygen partial pressure (PO2(hi)) surface were directly determined from the secondary ion mass spectrometry line profile for each GB, obtained from cross-sections of the exposed wafer. An especially interesting result was that, under dry conditions, the oxygen GB diffusivity obtained with an applied oxygen potential gradient (dμO) was significantly lower than that associated with oxygen self-diffusion in the absence of a dμO. Moreover, the contribution of electronic conductivity near the PO2(hi) surface was increased upon applying a dμO. The addition of H2O to the PO2(hi) environment in conjunction with a dμO accelerated oxygen GB diffusion, while the presence of D2O retarded the diffusivity compared to that observed under dry conditions. However, in the absence of a dμO, the oxygen GB diffusivity was essentially equal to that under dry conditions. The dissociative adsorption reactions of H2O or D2O over the entire PO2(hi) surface appear to greatly affect the diffusivity of oxygen and aluminum on the surface and along the GBs in the vicinity of the surface.