Phase equilibria and microstructure in Sr4Fe6-xCoxO13 0 <= x <= 4 mixed conductors

The densification, microstructure and phase evolution of Sr4Fe6-xCoxO13 (0 less than or equal to x less than or equal to 4) materials have been invest igated by powder X-ray diffraction, electron microscopy and thermal analysi s. Powders were prepared by the solid state reaction method or by the EDTA precursor method. Pure Sr4Fe6O13 is stable above 775 +/- 25 degreesC in air until it melts peritectically at 1220 +/- 5 degreesC. Below 775 degreesC, Sr4Fe6O13 is unstable with respect to the formation of Sr1-xFeO3-delta and SrFe12O19. Co substituted Sr4Fe6O13 is only stable in a narrow temperature region near 900 degreesC. At higher or lower temperature, the Co-content is reduced due to formation of the perovskite SrFe1-zCozO3-delta and the soli d solutions CO3-yFeyO4 (below 900 degreesC) or Co1-yFeyO (above 900 degrees C). A plate-like morphology of Sr4Fe6-xCoxO13 grains was observed both in c alcined powders and in sintered ceramics. Ball milling of the calcined powd ers was necessary prior to the sintering in order to achieve dense material s in the temperature region 1120-1170 degreesC. Only pure Sr4Fe6O13 appeare d as a single-phase material after sintering. Increasing amounts of the pha ses SrFe1-zCozO3-delta and Co1-yFeyO were observed with increasing sinterin g temperature and increasing Co-content due to the limited solubility of Co in Sr4Fe6-xCoxO13. The thermal expansion coefficient of the materials devi ates from linear behavior due to the decreasing oxidation state of iron wit h increasing temperature. The present investigation demonstrates that Sr4Fe 4Co2O13 materials with high oxygen permeability are not single-phase materi als when sintered at high temperature. (C) 2001 Elsevier Science B.V. All r ights reserved.