Oxygen transport and thermomechanical properties of SrFe(Al)O_3-δ –SrAl₂O₄ composites: microstructural effects

Abstract

Measurements of oxygen permeation through dense ${\hbox{Sr}}_{{1 - x}} {\left( {{\hbox{Fe,Al}}} \right)}{\hbox{O}}_{{3 - \delta }} - {\hbox{SrAl}}_{{\text{2}}} {\hbox{O}}_{4}$ composite membranes showed a considerable influence of processing conditions on the surface exchange kinetics, while the bulk ambipolar conductivity is almost unaffected by microstructural factors. Compared to the materials prepared via the glycine–nitrate process (GNP), the surface limitations to oxygen transport are significantly higher for dual-phase ${\left( {{\hbox{SrFe}}} \right)}_{{0.7}} {\left( {{\hbox{SrAl}}_{{{2}}} } \right)}_{{0.3}} {\hbox{O}}_{{3.3 - \delta }}$ made of a commercial powder synthesized by spray pyrolysis. This difference in behavior may be related to compositional inhomogeneities in the grains of A-site deficient perovskite phase and an enhanced surface concentration of grain boundaries in the case of GNP-synthesized composite, which has also smaller grain size, slightly higher thermal expansion and lower total conductivity. No essential effects on Vickers hardness, varying in the range 6.3–6.5 GPa, were found. The deposition of porous catalyst layers onto the composite surface exposed to reducing environment leads to membrane decomposition. For the fabrication of tubular membranes, the cold isostatic pressing technique was, hence, combined with mechanical treatment to increase the specific surface area without incorporation of catalytically active components.