Performance of zinc-doped perovskite-type membranes at intermediate temperatures for long-term oxygen permeation and under a carbon dioxide atmosphere

Zinc-doped (BaSr)FeO3-delta perovskite-type membranes were found to exhibit excellent long-term stability at high and intermediate temperatures under an oxygen partial pressure difference of p(1)/p(2) = 21/2.1 x 10(3) Pa at 950 degrees C and p(1)/p(2) = 21/0.8 x 10(3) Pa at 750 degrees C. After the change to carbon dioxide as the sweep gas, the oxygen permeation diminishes almost completely. By XRD, a phase mixture of hexagonal, tetragonal and cubic perovskite with a (Ba(0.4 +/- 0.1)Sro(0.6 +/- 0.1))CO3 layer and some zinc oxide was found. TEM showed that the dense perovskite membrane is decomposed into phases with different morphologies accompanied by pore and crack formation in a surface layer with a depth of approximately 5 mu m. After switching to helium as the sweep gas, the carbonate layer is decomposed, but a hexagonal and tetragonal perovskite phase in addition to the cubic phase are still present. A short heating to 950 degrees C regenerates the oxygen permeation flux due to the abatement of hexagonal perovskite for the benefit of the cubic and tetragonal perovskite phase. The appearance of the carbonate structure, even to a small amount of 8%, leads to a breakdown of the oxygen permeation ability, but also the hexagonal perovskite is disadvantageous. In contrast, the cubic and tetragonal perovskite modifications exhibit good oxygen permeation abilities. The effect of the carbon dioxide is totally reversible after a short high temperature treatment. (C) 2009 Elsevier B.V. All rights reserved.