Process of making fine ceramic powders from aqueous suspensions

Abstract

The new preparation process for making fine high specific surface ceramic powders suitable as catalysts or precursors for ceramics uses lanthanum (or other rare earth lanthanide) oxide as one of the precursors. The oxide is mixed with water to form a liquid slurry, whereby it is transformed to the hydroxide by reaction with water. The resulting hydroxide slurry, which can be milled to reduce the particle size and to speed up the reaction, is then combined, while stirring vigorously to assure homogenous mixing, with a solution of required amount of remaining metal nitrate precursors, for example strontium and cobalt nitrates. The reaction between lanthanum hydroxide and transition metal nitrates produces a colored (color depending on the transition metal) slurry consisting of metal hydroxides suspended in aqueous nitrate solution with pH>2. This perovskite precursor slurry is spray-frozen and freeze dried. The freeze-dried material is transformed to perovskite by slow controlled calcination at temperatures above 550.degree. C. For example calcination 12 h at 585.degree. C. and subsequently 4 h at 620.degree. C. produces nearly phase pure perovskite having specific surface area of >10 m.sup.2 /g, depending on the composition. The perovskite precursor slurry can alternatively be processed first by spray-drying (instead of spray-freezing/freeze-drying) and subsequent calcination.