Comparison of oxygen permeability and stability of perovskite type La_0.2A_0.8Co_0.2Fe_0.8O(3-δ) (A = Sr, Ba, Ca) membranes

The oxygen permeation and stability of La_0.2A_0.8Co_0.2Fe_0.8O(3-δ) (A = Sr, Ba, Ca) perovskite-type membranes were studied at high temperatures and low oxygen partial pressures. The oxygen vacancy diffusivity and concentration gradient calculated from the unsteady-state and steady-state oxygen data for the three membranes decrease in the order of Sr > Ba > Ca. The activation energies for oxygen permeation increase in the order of Sr < Ba < Ca. The oxygen permeation data of the three membranes can be explained by the average bond energy, free volume, and critical radius of the materials. After exposure to air at 1173 K for 10 h, part of La_0.2Sr_0.8Co_0.2Fe_0.8O(3-δ) transforms to La₂O₃, SrO, CoO, and Fe and the crystallite size of the perovskite phase decreases. These changes were not observed for La_0.2Ba_0.8Co_0.2Fe_0.8O(3-δ) under the same conditions, indicating that La_0.2Ba_0.8Co_0.2Fe_0.8O(3-δ) membrane is much more stable than La_0.2Sr_0.8Co_0.2Fe_0.8O(3-δ) at high temperatures and low oxygen partial pressures.