Surface diffusion of oxides (A review)

Abstract

The existing data for surface diffusion on oxides have been gathered by two types of measurements: mass transport (grain boundary grooving and scratch smoothing) and tracers. Mass transport methods yield an effective surface diffusion coefficient, without being able to determine directly whether metal ions or oxide ions control the rate of mass transport, while tracer studies measure the diffusion of a single ion. The analysis of the experimental data for both methods must take into account volume diffusion and evaporation-condensation in addition to surface diffusion.Mass transfer measurements on aluminum oxide, magnesium oxide, and uranium dioxide indicate that surface diffusion is dominant over volume diffusion at high temperatures for these oxides. The only tracer measurements of surface diffusion on an oxide to date are measurements on uranium dioxide. Reanalysis of the tracer results indicates the volume diffusion contribution to the tracer motion may be considerable.Small amounts of impurities can greatly affect the apparent surface diffusion coefficient. Departures from stoichiometry can affect the mechanism of material transport by altering the absolute value of the surface diffusion coefficient and its value relative to the volume diffusion coefficient. The validity of models of surface diffusion on oxides is examined in relation to the existing experimental data.