The kinetics of O exchange between oxide ceramics and the surrounding gas phase was modelled by taking account of surface exchange reactions and fast grain boundary diffusion. Numerical Laplace inversion was used with respect to a spherical grain model, while numerical solutions for a square grain model were obtained by means of the finite element method. The time dependence of the total amount of exchanged O (relaxation curves) was predicted within Harrison's type-A kinetics. Appropriate relations for the effective diffusion coefficient and the effective surface exchange coefficient were given. If the grain boundary diffusion coefficient exceeds the bulk diffusion coefficient by many orders of magnitude, the new O activity was attained instantaneously at the surface of each grain (grain boundary) after a step-wise alteration of the O partial pressure. The rate-determining step for the O stoichiometry change was slow bulk diffusion from the grain boundaries into the grains. This mechanism allows the interpretation of the re-oxidation kinetics of donor doped barium titanate ceramics.

Modeling of Fast Diffusion Along Grain Boundaries in Oxide Ceramics. W.Preis, W.Sitte: Solid State Ionics, 2008, 179[21-26], 765-70