Comparison of oxygen permeability and stability of perovskite type La(0.2)A(0.8)Co(0.2)Fe(0.8)O(3-delta) (A = Sr, Ba, Ca) membranes

The oxygen permeation and stability of La(0.2)A(0.8)Co(0.2)Fe(0.8)O(3-delta ) (A = Sr, Ba, Ca) perovskite-type membranes were studied at high temperatu res and low oxygen partial pressures. The oxygen vacancy diffusivity and co ncentration gradient calculated from the unsteady-state and steady-state ox ygen data for the three membranes decrease in the order of Sr > Ba > Ca. Th e activation energies for oxygen permeation increase in the order of Sr < B a < Ca. The oxygen permeation data of the three membranes can be explained by the average bond energy, free volume, and critical radius of the materia ls. After exposure to air at 1173 K for 10 h, part of La0.2Sr0.8Co0.2Fe0.8O 3-delta transforms to La2O3, SrO, CoO, and Fe and the crystallite size of t he perovskite phase decreases. These changes were not observed for La0.2Ba0 .8Co0.2Fe0.8O3-delta under the same conditions, indicating that La0.2Ba0.8C o0.2Fe0.8O3-delta membrane is much more stable than La0.2Sr0.8Co0.2Fe0.8O3- delta at high temperatures and low oxygen partial pressures.