Thermochemical expansion of mixed-conducting (Ba,Sr)Co0.8Fe0.2O3−δ ceramics

The thermal and chemical expansion of Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) and SrCo0.8Fe0.2O3−δ (SCF) mixed ionic-electronic conductors were studied in combination with oxygen nonstoichiometry (δ) at 298–1223 K and p(O2) = 10−4 to 1.00 atm. In order to minimize the effects of phase separation or oxygen-vacancy ordering processes, the data were collected in dynamic cooling mode using dense ceramic samples. The procedure was justified by a very fast equilibration at given p(O2) in high-temperature range demonstrated for ceramics samples with different specific surface area. The difference in nonstoichiometry of BSCF and SCF at temperatures ≥973 K was found to be ≤0.03 oxygen atoms per formula unit. BSCF demonstrates favorably smaller chemical expansion compared to SCF and many other mixed conductors, originating from smaller δ variations and larger unit cell less sensitive to temperature and nonstoichiometry changes. Excessive thermochemical expansion impedes however the use of BSCF in single-phase fuel cell cathodes and planar mixed-conducting membranes.