Chapter 3 - Low-temperature compaction of nanosize powders

Abstract

The driving force for densification of nanosize ceramic powders is very high because of their small particle size. However, in many situations, their relatively low green-body densities require longer sintering times or higher temperatures than expected to achieve full densification. The longer sintering times and higher temperatures required promote excessive grain growth and/or irregular grain growth, resulting in a microstructure that does not exhibit the desired nanosize grain structure. The homogeneity and density of the green compact is critical in many situations, especially because the cold compaction density of most ceramic nanosize powders is extremely low (<45% of theoretical). The high-pressure compaction can be a means of increasing the green-body density and structural integrity of the compact. Green densities in excess of 70% can be achieved with nanosize amorphous silicon nitride powders. Liquid nitrogen appears to be an excellent lubricant for nanosize particles. Care must be taken when using LN2 to compact nanosize particles, because it can experience a series of phase transformations.