Oxygen permeability, thermal expansion and stability of SrCo0.8Fe0.2O3−δ-SrAl2O4 composites

Additions of SrAl2O4 phase to mixed-conducting SrCo0.8Fe0.2O3−δ promote oxygen-vacancy ordering and brownmillerite formation at temperatures below 1050 K due to Al3+ incorporation, but also decrease thermal expansion coefficients (TECs) and improve thermal shock stability. The SrCo0.8Fe0.2O3−δ-SrAl2O4 composite membranes exhibit also a relatively high stability with respect to interaction with CO2 due to A-site deficiency of the perovskite-related phase, caused by partial SrAl2O4 dissolution. The oxygen permeability and electronic conductivity of (SrCo0.8Fe0.2O3−δ)1−x(SrAl2O4)x (x = 0.3-0.7) composites are determined by the perovskite component and decrease with increasing x. Despite minor diffusion of the transition metal cations into SrAl2O4, hexagonal above 940 K and monoclinic in the low-temperature range, this phase has insulating properties. Nonetheless, at x = 0.3 the oxygen permeation fluxes at 1073-1173 are similar to those through single-phase SrCo0.8Fe0.2O3−δ membranes. The average TECs of the composite materials, calculated from dilatometric data in air, vary in the ranges (10.0-11.3) × 10− 6 K− 1 at 300-900 K and (14.7-21.1) × 10− 6 K− 1 at 900-1100 K. The low-p(O2) stability limit and electronic transport properties of SrCo0.8Fe0.2O3−δ are briefly discussed.