The modification of fine powders by inorganic coatings

Abstract

This review describes the modification of fine particles by the formation of an inorganic coating. It shows how a technology, which has been developed in, but mainly confined to, the titanium dioxide pigment industry, has considerable potential in other areas. Inorganic coatings may be used to encapsulate a solid or modify its dispersion properties. They may also be used to impart desired properties, e.g. electrical conductivity or to obtain specific optical effects. In ceramics, inorganic coatings offer a way of distributing minor components, such as stabilisers for zirconia or toughening agents for alumina, in a way that does not depend on the vagaries of mixing. Examples of different coating procedures are presented, and for two of these – dense silica and silica-alumina – the mechanism is briefly described. The uses and limitations of gas adsorption, electron microscopy, electrophoresis, ESCA and SIMS for coatings characterisation are summarised. It is shown that each is useful for coating characterisation but none can unambiguously differentiate between uniform and patchy coating. Finally, specific examples of improved dispersion, improved barrier performance and beneficial powder properties are presented.