Surfaces and Colloids

Abstract

Ceramic fabrication processes would often benefit from using powders with very small particles. However, such powders are notoriously difficult to handle when dry. They get airborne very easily, making everything look messy very quickly, and often pose significant health hazards. They readily form fluffy aggregates that make it difficult to pour the powder into dies and molds. They stick strongly to die walls and trap large amounts of air when pressed in dies. These processing problems led to the belief that there was an optimum particle size of about 1 µm. The optimum was supposedly determined by a trade-off between the benefit of improving sintering rates and the detriment of increasing particle handling problems with decreasing particle size. However, handling of fine particles as suspensions in liquids (or colloids) rather than as dry powders can facilitate the ceramic forming processes and thus extend downward the range of practically useful particle sizes significantly. Actually, particle suspensions have traditionally been used in tape casting and slip casting, as described in Chapter 12. More recently, success in producing submicrometer, nearly monodispersed powders (consisting of particles of all the same size and shape) has made possible vast improvements in the perfection of the ceramic bodies obtained by colloidal consolidation methods. One could think of colloidal consolidation methods as a glorified name for what are really just variants of the old mudpie baking. The practical aspects of these methods are described in Chapter 12.