In-situ analysis of chemical expansion and stability of SOFC cathodes
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In-situ analysis of chemical expansion and stability of SOFC cathodes Mirela Dragan1 and Scott Misture2 1
Alfred University, Center for High Temperature Characterization; 2 Pine Street, Binns-Merrill Hall, Alfred, NY 14802, USA 2
Alfred University, Inamori School of Engineering; 2 Pine Street, Binns-Merrill Hall, Alfred, NY 14802, USA
ABSTRACT In this work high-temperature X-ray diffraction has been used to investigate thermal and chemical expansion as well as overall phase stability for various cathode materials: Ba0.5Sr0.5Co0.8Fe0.2O3 (BSCF), La0.3Sr0.7CoO3 (LSC37), La0.6Sr0.4CoO3 (LSC64) and La0.6Sr0.4Fe0.8Co0.2O3 (LSCF), as a function of temperature in reducing conditions. When perovskites materials are under a low oxygen partial-pressure condition, the lattice parameter and overall dimension increase. Their chemical expansion has comparable values. From the viewpoint of the stability of these phases, the high-temperature X-ray diffraction results indicate no phase decomposition can be one of the reasons for material failure at the current experimental oxygen partial pressure. LSF is most stable, while LSC and LSCF form oxygen vacancy-ordered phases and then decompose when heated to 1000°C under atmospheres with pO2 as low as 10-5 atm. INTRODUCTION Among the mixed metal oxides, perovskite structure oxides continue to be significant for functional oxide materials. For industrial application of solid oxide fuel cell (SOFC) cathode materials must meet numerous requirements such as a high electronic conductivity, long term chemical and structural stability at working conditions, and mechanical reliability. In general, stability is controlled by the B-site cations by adjusting their valence state and their coordination number under reduced oxygen pressure [1]. Lattice expansion in the oxygen-deficient perovskites occurs through two mechanisms: thermal expansion and chemical expansion that occurs against a decrease in oxygen stoichiometry, accompanied by reduction of the B-site cations to maintain the charge neutrality in the solid with increasing temperature and decreasing oxygen partial. Chemical expansion is of particular interest for industrial application of perovskite structure oxides, as SOFC cathodes. EXPERIMENTAL DETAILS LSC powders were purchased from Praxair Ltd. and used as-received. LSCF and BSCF powders were prepared using a modified Pechini method. Each solution was kept on a hot plate at 100°C and stirred until gelation took place. After 24 hours, the gelled LSCF/BSCF samples
were baked in a drying oven at 200°C for 6hr. The as-produced powders were then calcined at 950°C for 7 hours in air. High temperature X-ray diffraction was carried out using a Siemens theta-theta D500 diffractometer equipped with a Braun position sensitive detector and high temperature unit, and using Co Kα radiation with an iron filter. Gas flows were controlled using two Omega FMA5400/5500 mass flow controllers, and oxygen concentrations measured using a Thermox CG1000 oxygen analyzer at the outlet side of the diffraction
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