Chapter 4 - Low-temperature compaction of nanosize powders

Abstract

This chapter explores the various techniques of low-temperature compaction of nanosize powders. One of the more important challenges in the fabrication of nanostructured materials is how to achieve full densification of the powder while simultaneously retaining a nanoscale microstructure. High-pressure compaction of nanosize particles has been used in an attempt to achieve the desired nanoscale microstructure and high density in the green body. Typical compaction pressures usually exceed 1.0 GPa and, therefore, require specialized equipment. There are two different types of low-temperature high pressure powder compaction: diamond anvil pressure cell and high-pressure compaction with the piston-cylinder device. 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. Obtaining a high green density is not the ultimate accomplishment in the processing of nanosize powders. The reason for this is that, in many cases, ceramic powders exist as metastable phases when they are prepared as very fine powders. These materials are metastable and may transform, typically involving significant volume changes, during high-temperature sintering.