Effect of Zr⁴⁺ doping on the oxygen stoichiometry and phase stability of SrCo_0.8Fe_0.2O3 - δ oxygen separation membrane

The effect of Zr doping in SrCo_0.8Fe_0.2O _{3 - δ} (SCF) membranes was investigated. The lattice parameter of SCF increased with the Zr content until 4 at.%, and then leveled off with further increase in the Zr content, revealing that the Zr solubility limit lies at ∼ 4%. The Zr doping resulted in an increase in oxygen stoichiometry (3 - δ) of SCF membranes especially at low oxygen partial pressure. The brownmillerite to perovskite phase transition was investigated by dilatometry in nitrogen (pO₂ = 2 × 10^{- 3} atm). The phase transition temperature was lowered by Zr doping: 770 °C for undoped SCF and 710 °C for SCF doped with 4 at.% of Zr. The change in volume associated to the phase transition also became less pronounced for the Zr-doped samples. Compared to the undoped membrane, the Zr-doped SCF exhibited a somewhat lower oxygen permeability, but it remained well-permeable to oxygen at lower operation temperatures due to the stabilization of the perovskite structure.