In recent years there has been a strong interest in the processing of nanosize ceramic powders because of the potential of sintering them at low temperatures and also because ceramic pieces made of nanosize gram structures may exhibit superior mechanical properties. However, nanosize powders are very difficult to press into high density green body compacts that can withstand handling before high temperature sintering. High green densities have also been suggested as a requirement for low temperature and short time sintering steps. This chapter reviews some techniques of compacting nanosize powders using high pressures and liquid nitrogen as a lubricating fluid to achieve incremental increases in the green body density. The first experiments using liquid nitrogen were conducted in the diamond anvil cell to achieve the high pressures (>1 Gpa) necessary for the compaction of nanosize powders. The reports cited in this chapter present clear evidence that high densities can be achieved with nanosize silicon nitride and y-A12O^3^. These experiments also have shown that samples compacted under liquid nitrogen can achieve higher green densities than samples compacted under dry conditions. A number of parallel experiments are also reviewed wherein a piston/cylinder (WC/Co) die and liquid nitrogen are used to compact nanosize powders, and confirm the benefits of using liquid nitrogen to achieve high densities during compaction. These reports speculate that liquid nitrogen is effective because it reduces interparticle friction permitting easier sliding for particle rearrangement. However, specific details that describe the actual lubricating mechanism of liquid nitrogen have never been studied. The pressure-temperature phase diagram of nitrogen is also reviewed with emphasis on the important solid and liquid phases that are produced under compaction pressures at low temperatures. The effects these phases have on the final density of the green body compact are discussed. An explanation of the effects of liquid nitrogen cannot be found by analyzing current compaction equations because they tend to be for the most part empirical in nature.
Low Temperature Compaction of Nanosize Powders
Handbook of Nanostructured Materials and Nanotechnology ,