Synthesis and Properties of YBaCo2O5+δ for Solid Oxide Fuel Cell Cathodes
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The layered double perovskite YBaCo2O5+δ was synthesised and characterised. The synthesis was done by solid-state reaction with several calcination steps at temperatures between 1000°C and 1130°C. The structure was investigated at elevatedtemperatures, from room temperature to 800°C, in oxygen, air and nitrogen usingX-ray diffraction. The oxygen stoichiometry, 5+δ, was determined by thermogravimetricanalysis in the same atmospheres from room temperature and up to 1100 °C. The crystal structure of YBaCo2O5+δ at room temperature is orthorhombic (space group Pmmm) with ordered oxygen vacancies and alternating layers of yttrium and barium. Oxygen content, 5+δ, was determined to 5.42, thus showing large oxygen deficiency compared to the stoichiometric compound LnBaM2O6. Heating of the material induces a phase transition to a tetragonal (space group P4/mmm) oxygen disordered phase. Heating in both nitrogen and oxygen cause decomposition. Inoxygen decomposition to single perovskites occur above 600°C: YBaCo2O5+d -> YCoO3 + BaCoO3-d At approximately 925°C the reaction is reversed and YBaCo2O5+ forms again. Innitrogen, decomposition results in both primary and ternary oxides according to: YBaCo2O5+d -> YBaCo4O7 + Y2O3 + BaCoxY1-xO3-d. No sign of secondary phases was seen during high temperature XRD in air, butannealing at 800°C caused decomposition as seen in oxygen. Thermal expansion coefficient of YBaCo2O5+δ in oxygen, air and nitrogen was determined. The isotropic thermal expansion based on volumetric thermal expansion was determined to 20.0 /MK for all atmospheres.