Oxygen ionic transport in SrFe1−yAlyO3−δ and Sr1−xCaxFe0.5Al0.5O3−δ ceramics

The oxygen permeability of mixed-conducting Sr1−xCaxFe1−yAlyO3−δ (x=0-1.0; y=0.3-0.5) ceramics at 850-1000 °C, with an apparent activation energy of 120-206 kJ/mol, is mainly limited by the bulk ionic conduction. When the membrane thickness is 1.0 mm, the oxygen permeation fluxes under pO2 gradient of 0.21/0.021 atm vary from 3.7×10−10 mol s−1 cm−2 to 1.5×10−7 mol s−1 cm−2 at 950 °C. The maximum solubility of Al3+ cations in the perovskite lattice of SrFe1−yAlyO3−δ is approximately 40%, whilst the brownmillerite-type solid solution formation range in Sr1−xCaxFe0.5Al0.5O3−δ system corresponds to x>0.75. The oxygen ionic conductivity of SrFeO3-based perovskites decreases moderately on Al doping, but is 100-300 times higher than that of brownmillerites derived from CaFe0.5Al0.5O2.5+δ. Temperature-activated character and relatively low values of hole mobility in SrFe0.7Al0.3O3−δ, estimated from the total conductivity and Seebeck coefficient data, suggest a small-polaron mechanism of p-type electronic conduction under oxidising conditions. Reducing oxygen partial pressure results in increasing ionic conductivity and in the transition from dominant p- to n-type electronic transport, followed by decomposition. The low-pO2 stability limits of Sr1−xCaxFe1−yAlyO3−δ seem essentially independent of composition, varying between that of LaFeO3−δ and the Fe/Fe1−γO boundary. Thermal expansion coefficients of Sr1−xCaxFe1−yAlyO3−δ ceramics in air are 9×10−6 K−1 to 16×10−6 K−1 at 100-650 °C and 12×10−6 K−1 to 24×10−6 K−1 at 650-950 °C. Doping of SrFe1−yAlyO3−δ with aluminum decreases thermal expansion due to decreasing oxygen nonstoichiometry variations.