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The impact of grain size, A/B-cation ratio, and Y-doping on secondary phase formation in (Ba0.5Sr0.5)(Co0.8Fe0.2)O3−δ

Meffert, Matthias, Unger, Lana-Simone, Grünewald, Lukas, Störmer, Heike, Wagner, Stefan F., Ivers-Tiffée, Ellen, Gerthsen, Dagmar
Journal of materials science 2017 v.52 no.5 pp. 2705-2719
cations, electron microscopy, oxygen, temperature
The application of mixed ionic–electronic conducting (Ba₀.₅Sr₀.₅)(Co₀.₈Fe₀.₂)O₃₋δ (BSCF) as gas separation membrane is up to now hampered by secondary phase formation which impairs the excellent oxygen permeation properties of this material. In this work, we have studied the impact of grain size and A/B-cation ratio on secondary phase formation in BSCF and Y-doped (Ba₀.₅Sr₀.₅)(Co₀.₈Fe₀.₂)₀.₉Y₀.₁O₃₋δ (BSCF10Y) by electron microscopic techniques before and after long-term thermal exposure at an application-relevant temperature (~760 °C). A large content of secondary phases is found in samples with small grain sizes because grain boundaries provide nucleation sites for secondary phases. Higher sintering temperatures increase the grain sizes and substantially reduce the content of secondary phases. Variations of the A/B-cation ratio between (Ba₀.₅Sr₀.₅)₀.₉₅(Co₀.₈Fe₀.₂)O₃₋δ and (Ba₀.₅Sr₀.₅)₁.₀₅(Co₀.₈Fe₀.₂)O₃₋δ do not lead to a change of the composition of the cubic BSCF phase but changes the volume fraction of Co₃O₄ precipitates which are already formed during sintering. BSCF with an excess of A-site cations contains the smallest overall amount of secondary phases in undoped BSCF due to the minimization of Co₃O₄ precipitation during sintering and the reduction of nucleation sites for other secondary phases at application-relevant temperatures. Secondary phase formation in BSCF10Y can be almost completely suppressed due to the stabilization of the cubic BSCF phase by Y-doping and large grain sizes after high-temperature sintering.