Phase equilibria and microstructure in Sr4Fe6−xCoxO13 0≤x≤4 mixed conductors

The densification, microstructure and phase evolution of Sr4Fe6−xCoxO13 (0≤x≤4) materials have been investigated by powder X-ray diffraction, electron microscopy and thermal analysis. Powders were prepared by the solid state reaction method or by the EDTA precursor method. Pure Sr4Fe6O13 is stable above 775±25°C in air until it melts peritectically at 1220±5°C. Below 775°C, Sr4Fe6O13 is unstable with respect to the formation of Sr1−xFeO3−δ and SrFe12O19. Co substituted Sr4Fe6O13 is only stable in a narrow temperature region near 900°C. At higher or lower temperature, the Co-content is reduced due to formation of the perovskite SrFe1−zCozO3−δ and the solid solutions Co3−yFeyO4 (below 900°C) or Co1−yFeyO (above 900°C). A plate-like morphology of Sr4Fe6−xCoxO13 grains was observed both in calcined powders and in sintered ceramics. Ball milling of the calcined powders was necessary prior to the sintering in order to achieve dense materials in the temperature region 1120–1170°C. Only pure Sr4Fe6O13 appeared as a single-phase material after sintering. Increasing amounts of the phases SrFe1−zCozO3−δ and Co1−yFeyO were observed with increasing sintering temperature and increasing Co-content due to the limited solubility of Co in Sr4Fe6−xCoxO13. The thermal expansion coefficient of the materials deviates from linear behavior due to the decreasing oxidation state of iron with increasing temperature. The present investigation demonstrates that Sr4Fe4Co2O13 materials with high oxygen permeability are not single-phase materials when sintered at high temperature.