Phase equilibria and microstructure in Sr 4Fe 6− xCo xO 13 0≤ x≤4 mixed conductors

Abstract

The densification, microstructure and phase evolution of Sr 4Fe 6− x Co x O 13 (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 Sr 4Fe 6O 13 is stable above 775±25°C in air until it melts peritectically at 1220±5°C. Below 775°C, Sr 4Fe 6O 13 is unstable with respect to the formation of Sr 1− x FeO 3− δ and SrFe 12O 19. Co substituted Sr 4Fe 6O 13 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 SrFe 1− z Co z O 3− δ and the solid solutions Co 3− y Fe y O 4 (below 900°C) or Co 1− y Fe y O (above 900°C). A plate-like morphology of Sr 4Fe 6− x Co x O 13 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 Sr 4Fe 6O 13 appeared as a single-phase material after sintering. Increasing amounts of the phases SrFe 1− z Co z O 3− δ and Co 1− y Fe y O were observed with increasing sintering temperature and increasing Co-content due to the limited solubility of Co in Sr 4Fe 6− x Co x O 13. 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 Sr 4Fe 4Co 2O 13 materials with high oxygen permeability are not single-phase materials when sintered at high temperature.