Formation of calcium chromate hydroxylapatite on the surface of a calcium-doped lanthanum chromite sintered body

Abstract

Sintered bodies of highly sinterable calcium-doped lanthanum chromites with chromium deficiency have been found to deteriorate and form calcium chromate hydroxyl apatite on their surfaces when exposed to high-temperature water-containing atmospheres. Microstructure analysis, composition measurement, and phase identification were performed on the surface of the samples after annealing in various conditions. The mechanism of the surface deterioration is thus deduced: traces of liquid phase which are formed in the sintering process migrate to the external surface and then react with water vapour to yield an apatite-type compound. Electrical conductivity has also been measured. The poorly conductive nature of the surface will cause some problems when the subject materials are used as high-temperature electronic conductors. © 1994 Chapman & Hall.