Oxygen transport in ferrite-based ceramic membranes: Effects of alumina sintering aid

Abstract Moderate additions of Al 2 O 3 to strontium ferrite-based mixed conductors, such as SrFe 0.7 Al 0.3 O 3− δ and La 0.2 Sr 0.8 Fe 0.8 Ga 0.2 O 3− δ with the composition close to the solid solution formation limits, make it possible to improve ceramics sinterability, to increase oxygen permeability and to decrease thermal expansion. These effects are associated with the segregation of alumina-rich phases, primarily SrAl 2 O 4 , and the formation of A-site cation-deficient perovskite. The improved properties of the SrFe 0.7 Al 0.3 O 3 -based material were used to fabricate high-quality tubular membranes for methane conversion reactors. Similar enhancement in sinterability is also observed for another promising parent material of mixed-conducting membranes, La 0.5 Sr 0.5 FeO 3− δ . However, extensive dissolution of Al 3+ cations in the iron sublattice, creation of A-site vacancies and changing the La:Sr concentration ratio all lead to decreasing ionic transport in La 0.5 Sr 0.5 FeO 3− δ . As a result, additions of either Al 2 O 3 or SrAl 2 O 4 have a deteriorating influence on the oxygen permeation fluxes through La 0.5 Sr 0.5 FeO 3 -based ceramics.