Structuring of particles in concentrated pastes: A source of defects in powder processed materials

Pastes made by mixing particles into liquids are of great use in the materials and processing industries. The dependence of paste products on the composition and treatment of the particle dispersions has not beenfully understood. For example, defects seem toform during the preparation of a paste, making the end product far weaker than expected. Therefore, it should be possible to improve paste products significantly by understanding the genesis and removal of these defects, then optimising the proc;ess to remove flaws. Some flaws, such as bubbles or agglomerates, can be removed by adding surfactants and shearing to give remarkable improvements in structure and properties of cements or ceramics. But there may be other more fundamental sources of flaw genesis in pastes. One such mechanism of flaw formation is the disorder-order transition which is known to occur as particles are concentrated in a fluid. Another new possibility is that adhesion of particles within the paste causes the growth of large multiplets which act as nuclei for defects as the paste is compacted. The measurement and interpretation of such phenomena are discussed.