Surface reaction kinetics in oxygen nonstoichiometry re-equilibration of BaTiO3−δ

We report the (bare) surface redox-reaction rate constant k̃ that was determined, along with the chemical diffusivity D̃, by a conductivity relaxation technique on Al-doped single crystal and undoped polycrystal BaTiO3−δ as a function of oxygen activity in its range of −16≤log aO2≤0 at elevated temperatures of 800–1100 °C. It takes a value in the range of −4<log(k̃/cm s−1)≤−1, which is even larger than that of the oxides that are considered best as oxygen membranes. It has been found that the surface reaction step grows more rate controlling as the electronic transference number gets smaller or the electronic stoichiometric composition (δ≈0) is approached. The oxygen potential drop due to the surface reaction was estimated by an oxygen concentration cell technique. The oxygen potential drop grows larger as the stoichiometric composition is approached, that is in accord with the variation of k̃ against oxygen activity.