Permeation properties and structural stability of oxygen permeable $La_{1-x}Sr_xB_{1-y}Fe_yO_3$ (B=Co,Ga) Perovskite membrane

Abstract

Perovskite-type oxygen permeable membrane, which can permeate oxygen molecule selectively from an oxygen-containing gas mixture, was investigated. The dense membrane, basically, should be a good ionic conductor to permselect an oxygen molecule. Because perovskite-type conductors have not only a higher ionic conductivity than that of well-known fluorite structure oxides, such as $ZrO_2$, but also have a good electronic conductivity (so called, mixed ionic-electronic conductivity, MIEC), there is no need to provide an electron path to keep the oxygen ionic conduction as long as a gradient of oxygen chemical potential is maintained. The viewpoints in the research for a perovskite-type oxygen permeable membrane can be classified into two categories: an enhancement in oxygen permeability and a stability of the membrane under a reducing environment. Especially, it has been pointed out that the low stability under a reducing atmosphere at high temperature is a serious limitation for industrial application of this material. We, therefore, have selected $La_{0.7}Sr_{0.3}Ga_{0.6}Fe_{0.4}O_3$ composition, which had been reported to have a good chemical, structural stability, but also to have limited oxygen permeability, and investigated the electrochemical and mechanical properties in comparison with $La_{0.6}Sr_{0.4}Co_{0.2}Fe_{0.8}O_3$ composition, which has been known to have good oxygen permeability. The overall oxygen permeability of the MIEC membrane could be determined by the bulk conduction of oxygen ions and the exchange rate at the membrane surface. We, thus, modified the surfaces of $La_{0.7}Sr_{0.3}Ga_{0.6}Fe_{0.4}O_3$ with a cation-substituted perovskite-type composition to improve the limited oxygen permeability and scrutinized the factors affecting the surface reactivity. On the other hand, the effects of microstructure and thickness of the membrane, which are associated with bulk conduction, on the oxygen permeability were investigated. As an application...