Secondary Phase Formation in Ba0.5Sr0.5Co0.8Fe0.2O3-d Studied by Electron Microscopy

Ba0.3Sr0.3Co0.8Fe0.2O3-d (BSCF) compacts were prepared and annealed under application-relevant temperatures between 700 and 1000 degrees C for 100 h. The microstructure and chemical composition was investigated by electron microscopic techniques and energy dispersive X-ray spectroscopy (EDXS) to obtain a detailed insight into the formation of secondary phases which are involved in the degradation of the ionic conductivity of BSCF. Secondary phases are precipitated from the cubic BSCF phase at temperatures <= 900 degrees C. In addition to the well-known hexagonal phase, another secondary phase was identified which has the same crystal structure as Ban+1ConO3n+3(Co8O8) with n >= 2 which is denoted as a BCO-type phase. Regions with plate-like morphology are formed which contain thin lamellae of the cubic, hexagonal, and BCO-type phase. The negative impact of the secondary phases on the material performance is discussed.