Effects of niobium doping site and concetration on the phase structure and oxygen permeability of Nb-substituted SrCoOx oxides
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Niobium doping effect on phase structure, phase stability and electrical conductivity of SrCoOx oxides and oxygen permeability of the corresponding membranes were systematically investigated. Niobium was successfully incorporated into A-site, B-site or simultaneously A and B double sites of SrCoOx oxide and stabilized the perovskite structure with a cubic symmetry in air down to room temperature at a proper doping amount. However, the A-site doping could not stabilize the cubic structure under a more reducing atmosphere of nitrogen, as to SrCo1−yNbyO3−δ, y ≥ 0.1 is necessary to sustain the cubic perovskite structure. Simultaneous doping of Nb at A and B sites is the most effective way to stabilize the perovskite structure under nitrogen atmosphere. Irrespective of doping site, the electrical conductivity decreased monotonously with Nb-doping amount. Both NbxSr1−xCoO3−δ and NbzSr1−zCo1−zNbzO3−δ envisaged a decrease in oxygen permeation flux with Nb-doping amount while SrCo1−yNbyO3−δ reached the maximum flux at y = 0.1. Among all the membranes, SrCo0.9Nb0.1O3−δ and Nb0.05Sr0.95Co0.95Nb0.05O3−δ show the highest oxygen fluxes of 3.5 and 2.7 ml cm−2 min−1 at 900 °C under an air/helium gradient, respectively.
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