The chemical diffusion coefficient in Ce0.8Gd0.2O1.9-δ was estimated by analyzing the weight relaxation behavior after an abrupt change in the O partial pressure. The weight relaxation data were analyzed by fitting the data to solutions of Fick’s second law for appropriate boundary conditions. The diffusion equation, which ignored the effect of surface reaction, failed to describe the transient behavior; especially in the first stage. Upon taking the surface effect into account, the fitting gave a satisfactory interpretation of the overall relaxation process and permitted a precise determination of the chemical diffusion coefficient and surface-reaction rate-constant. The chemical diffusion coefficient was found to decrease with the decrease of the O partial pressure, while the surface reaction rate constant increased with the decrease of the O partial pressure. The increase of the surface reaction rate constant may be related to the increase of the O non-stoichiometry, implying that the O vacancies play an important role in the surface reaction kinetics. The ionic mobility was estimated from the obtained chemical diffusion coefficients, with the help of O non-stoichiometry and electrical conductivity data obtained before.

The Oxygen Transport in Gd-Doped Ceria. M.Katsuki, S.Wang, K.Yasumoto, M.Dokiya: Solid State Ionics, 2002, 154-155, 589-95