Chemical relaxation experiments were performed on sintered samples of Ca-doped material by suddenly changing the O partial pressure in the atmosphere and by monitoring the change in conductivity. The re-equilibration kinetics were analyzed by fitting the relaxation data to solutions of Fick's second law for suitable boundary conditions. The diffusion equation, ignoring the effect of surface reaction, failed to describe the transient behavior; especially in the initial stages. Equations which took account of surface effects provided a satisfactory interpretation of the overall relaxation process and permitted a precise determination to be made of 2 kinetic parameters: the O chemical diffusion coefficient and the surface reaction rate constant. The chemical diffusion coefficient increased with decreasing O partial pressure, due to a corresponding change in the concentration of the mobile species. The activation energy was similar to that for O vacancy diffusion in other monocrystalline perovskites; thus suggesting that the diffusion coefficients could be attributed to lattice diffusion. The surface reaction rate constant increased with decreasing O partial pressure and implied that the presence of O vacancies played an important role in the surface reaction kinetics.

I.Yasuda, T.Hikita: Journal of the Electrochemical Society, 1994, 141[5], 1268-73