Effect of preparation method on chemical; stability of sinterable calcium doped lanthanum chromites

The effect of preparation method on the chemical stability of sinterable (La, Ca) CrO_3-δ powder was investigated. The ceramic method (powder mixing method) resulted in inhomogeneous distributions of the calcium content (x) in the perovskite phase (La_1-xCa_xCrO_3-δ) after calcining at a temperature from 1223 to 1373 K. More homogeneous powder was obtained by the liquid mixing method. The distribution of calcium content caused a large amount of second phases such as calcium oxychromates (Ca_m(CrO₄)_n) in a powder prepared by the ceramic method, and an excess amount of the second phase remained as Ca₅(CrO₄)₃OH on the surface of the ceramics after sintering at 1573 K. During a treatment in an H₂+H₂O gaseous mixture at 1273 K for 627 h, Ca_m(CrO₄)_n decomposed to CaO and A-CaCr₂O₄, and the decomposition was enhanced by the existence of silicon containing impurities. The liquid mixing method minimized the amount of Ca_m (CrO₄)_n and resulted in sufficient chemical stability in H₂+H₂O.