Systematic investigation on structure stability and oxygen permeability of Sr-doped BaCo0.7Fe0.2Nb0.1O3−δ ceramic membranes

Ba1-xSrxCo0.7Fe0.2Nb0.1O3-delta (BSCFN, x = 0.0-0.4) materials were synthesised by a solid-state reaction process for oxygen separation applications. The influence of strontium doping in BSCFN oxides on the phase structure stability, oxygen non-stoichiometry, electrical conductivity and oxygen permeation behaviour were investigated. Improved cubic structural stability under a reducing atmosphere and structure reversibility in alternative oxidising/reducing atmospheres were obtained by strontium doping. Strontium introduction effectively suppressed the phase transformation from cubic to hexagonal under high temperature and harsh atmosphere conditions. Strontium doping increased the electrical conductivity, but slightly decreased the oxygen permeability. Structural parameters played an important role in the charge compensation mechanism and oxygen permeability. Ba0.6Sr0.4Co0.7Fe0.2Nb0.1O3-delta membranes exhibited good structural stability and the sample with a thickness of 1 mm had a high level of oxygen permeation flux of 1.31 mL/cm2.min at 900 C under an Air/He gradient. The oxygen bulk diffusion was the rate-controlling step for oxygen permeation in BSCFN (x = 0.4) membranes when the thickness was > 1 mm, while the oxygen surface exchange process should be taken into consideration when the thickness was < 1 mm.