Aluminum-Doped Perovskites As High-Performance Oxygen Permeation Materials

Previously unreleased compositions of (Ba 0.5 Sr 0.5 )(Fe 1− x Al x )O 3−δ perovskites in the range of 0≤ x ≤ 0.2 were synthesized and studied with respect to electronic and crystallographic structure, as well as oxygen permeation. The perovskite phase in all synthesized oxides was found to be cubic, without any impurities for aluminum fractions in the range x = 0.01−0.09. Electron energy-loss spectroscopy (EELS) revealed a significant amount of covalency by Fe-3d−O-2p hybridization and a mixed Fe 3+ /Fe 4+ valence state of iron for all synthesized perovskites, which was quantified by Mössbauer spectroscopy. Trivalent aluminum replaces a higher fraction of Fe 4+ than of Fe 3+ while both iron species are in high-spin state. The Mössbauer quadrupole splittings indicate a greater disorder around iron with increasing aluminum content and, together with the EELS result of an abatement of covalent character in the bonding of iron and oxygen, the observed lattice expansion can be understood. In situ XRD and TG/DTA measurements revealed high temperature stability of the materials up to 1350 °C. The oxygen permeation increases with rising aluminum content from 0 to 0.1, and the (Ba 0.5 Sr 0.5 )(Fe 0.9 Al 0.1 )O 3−δ membranes show a very high oxygen permeation (1.19 mL cm −2 min −1 at 950 °C) compared to known perovskite membranes. Even among the previously published iron and aluminum containing membranes, they exhibit the highest oxygen permeation.