B-site La-doped BaFe_(0.95-x)La_xZr_(0.05)O_(3-δ) perovskite-type membranes for oxygen separation

摘要

Partial La-substitution for Fe on the B-site of the perovskite BaFe_(0.95-x)La_xZr_(0.05)O_(3-δ) (BFLZ) was achieved by applying a sol-gel synthesis method. The highest La content in BFLZ for the formation of a pure cubic perovskite structure without any detectable impurities is about x = 0.04. It is found for the first time that the introduction of La on the B-site of a mixed oxide stabilizes the cubic structure. Furthermore, the formation of the cubic structure of BFLZ increases significantly the oxygen permeability. The maximum oxygen permeation flux is found for a La-content of x = 0.04 with the largest volume of the cubic unit cell, reaching 0.63 and 1.24 cm3 (STP) min~(-1) cm~(-2) for a 1.1 mm thick membrane at 750 and 950 °C, respectively. This finding is in complete agreement with the XRD structure analysis, showing that the highest B-site La-substitution of BFLZ under conservation of the pure cubic perovskite phase without forming any foreign phase was about x — 0.04. For BFLZ with x >0.04, the secondary phase Ba6La2Fe4O_(15) forms increasingly and the oxygen permeation flux decreases. The influence of the sweep gas flow rates on the oxygen permeation flux and the oxygen ionic conductivity were found to be in good agreement with the Wagner theory, indicating the oxygen ion bulk diffusion as a rate-limiting step of oxygen transport. Stable oxygen permeation fluxes were obtained during the long-term oxygen permeation operation of the BFLZ (x = 0.04) membrane over 170 h at 750 and 950 °C, respectively.