Preparation of coated alumina powders and their microstructure development during heating.

Abstract

A uniform coating of precursors of various metal oxides on individual alumina particles was achieved by controlled hydrolysis of metal alkoxides in a slurry of alumina. Heterogeneous deposition of the precursors on the surface of the alumina particles was attributed to the electronegative character of alkoxy groups of the metal alkoxides. The powder coating techniques provided superior microstructures with homogeneous size and spatial distribution of secondary phases. It also lowered the sintering temperature of alumina in certain systems. In order to characterize microstructure development of the coated alumina during heating, powder compacts were rapidly quenched from elevated temperatures into liquid nitrogen and their interfaces and microstructures were examined by analytical TEM and FE-SEM. The EM studies revealed that the alumina particle surfaces act as sites for heterogeneous nucleation. The final structure of the alumina simultaneously doped with precursors of cupric oxide and titania was reached in the presence of a liquid phase but a large shrinkage occurred before the liquid formed. This phenomenon was explained from the viewpoints of superplasticity of the precursors and of a solid state reaction during heating. This speculation was supported by the similar accelerated sintering behavior with an addition of bismuth oxide and titania. The sintering behavior of alumina coated with a precursor of titania or zirconia, oxides of group 4 elements, was very different. The solid solution between alumina and titania after the nucleation of rutile on the surface of alumina resulted in sintering rate enhancement, while the slow self-diffusion characteristics of zirconia resulted in "droplets" on the surface of alumina particles which impeded the grain boundary migration.