Ion Transport and Thermomechanical Properties of SrFe ( Al ) O3 − δ – SrAl2O4 Composite Membranes

Moderate additions of monoclinic SrAl 2 O 4 to perovskite-type SrFe(Al)O 3-δ mixed conductors improve the sinterability and thermomechanical properties, including the thermal shock resistance, Vickers hardness, and fracture toughness, and decrease thermal expansion. The iron solubility in SrAl 2 O 4 , a mixed ionic and n-type electronic conductor with insulating properties, is lower than 5%. The total conductivity of SrAl 2 O 4 ceramics in air varies in the range 10 -7 -10 -5 S/cm at 973-1223 K. The transport properties and phase stability of dual-phase (SrFe) 1-x (SrAl 2 ) x O 2 (x = 0.3-0.7) composite membranes, where the partial dissolution of strontium aluminate in the ferrite phase leads to formation of A-site-deficient Sr 1-y Fe 1-2y Al 2y O 3-δ (y ≈ 0.08-0.12), are determined by the perovskite component. The total conductivity and Seebeck coefficient oxygen partial pressure dependencies exhibit general trends typical for SrFeO 3 -based solid solutions. Although the conductivity and oxygen permeability of (SrFe) 1-x (SrAl 2 ) x O z composites decrease with increasing x, the permeation fluxes through (SrFe)o.7(SrAl 2 ) 0.3 O z ceramics are comparable to those through single-phase SrFe 0.7 Al 0.3 O 3-δ . Under high p O2 gradients such as air/(H 2 -H 2 O), the oxygen transport is limited by surface-related processes, enabling stable operation of (SrFe) 0.7 (SrAl 2 ) 0.3 O 2 membranes. This composition was selected for fabrication of tubular membranes by the cold isostatic pressing. Surface modification of (SrFe) 0.7 (SrAl 2 ) 0.3 O z in order to enhance the exchange kinetics was found inappropriate from a stability point of view.