Processing and characterization of La0.5Sr0.5FeO3-supported Sr1−xFe(Al)O3–SrAl2O4 composite membranes

Perovskite-type La0.5Sr0.5FeO3−δ and composite Sr0.77Fe0.54Al0.46O2.54−δ exhibit relatively high oxygen permeation fluxes and similar thermal expansion coefficients, (12.4–13.1) × 10−6 K−1 at 350–950 K and (23.4–23.7) × 10−6 K−1 at 950–1300 K in air. Due to higher sinterability of Sr0.77Fe0.54Al0.46O2.54−δ consisting of strontium-deficient Sr(Fe, Al)O3−δ perovskite and SrAl2O4 phases, such properties enable to combine these materials in asymmetric membranes where dense Sr0.77Fe0.54Al0.46O2.54−δ layer is supported by porous lanthanum–strontium ferrite ceramics. In order to achieve a sufficient porosity and mechanical strength of the supports, various pore-forming agents were tested, including graphite, methylcellulose powder and an aqueous methylcellulose solution. The asymmetric membranes were fabricated adding 2 wt.% graphite into pre-synthesized La0.5Sr0.5FeO3−δ powder, with subsequent co-pressing and co-sintering at 1723 K. As oxygen permeation through Sr0.77Fe0.54Al0.46O2.54−δ ceramics is substantially affected by the oxygen exchange kinetics, the surface of the dense layer can be effectively activated applying a mixture of the composite, metallic Pt and/or praseodymium oxide. At 1023–1173 K the oxygen fluxes through a model asymmetric membrane, with dense layer thickness of 0.5 mm, varied in the range (1–3) × 10−7 mol s−1 cm−2 under the oxygen partial pressure gradient of 21/2.1 kPa.