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

The oxygen permeability of mixed-conducting Sr1-xCaxFe1-yAlyO3-delta(x = 0-1.0; y = 0.3-0.5) ceramics at 850-1000 degreesC, 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 x 10(-10) mols s(-1) cm(-2) to 1.5 x 10(-7) mol s(-1) cm(-2) at 950 degreesC. The maximum solubility of Al3+ cations in the perovskite lattice of SrFe(1-y)A(y)O(3-delta) is approximately 40%, whilst the brownmillerite-type solid solution formation range in Sr1-xCax Fe0.5Al0.5O3-delta 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+delta. Temperature-activated character and relatively low values of hole mobility in SrFe0.7Al0.3O3-delta, 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-pO(2) stability limits of Sr1-xCaxFe1-yAlyO3-delta seem essentially independent of composition, varying between that of LaFeO3-delta and the Fe/Fe1-gammaO boundary. Thermal expansion coefficients of Sr1-xCaxFe1-yAlyO3-delta ceramics in air are 9 x 10(-6) K-1 to 16 x 10(-6) K-1 at 100-650 degreesC and 12 x 10(-6) K-1 to 24 x 10(-6) K-1 at 650-950 degreesC. Doping of SrFe1-yAlyO3-delta with aluminum decreases thermal expansion due to decreasing oxygen nonstoichiometry variations. (C) 2004 Elsevier Ltd. All rights reserved.