Chapter 5 - Particles and Particle Technology

Abstract

This chapter discusses various mechanisms of particle growth. Solids appear in one of two forms, either as crystals or powders. The difference is one of size, since many of the powders used are very fine crystals. This, of course, depends upon the manner in which the solid is prepared. Each particle has its own unique diameter and size. Additionally, their physical proportions can vary in shape from spheres to needles. For a given powder, all grains will be the same shape, but the particle shape and size can be altered by the method used to create them in the first place. Methods of particle formation include: precipitation, solid state reaction, and condensation. Embryo formation leads to nucleation, and that this nucleation preceeds any solid state reaction or change of state. In a like manner, nuclei must form in order for any precipitation process to proceed. Once formed, these nuclei then grow until impingement of the growing particles occurs. Impingement implies that all of the nutrient supplying the particle growth has been used up. This mechanism applies to both solid state reaction and precipitation processes to form product particles. Then a process known as Ostwald Ripening takes over. This process occurs even at room temperature. The sequences in particle growth are: embryo formation, nucleation, nuclei Growth, impingement, Ostwald Ripening (coarsening), sintering, and formation of Grain Boundaries.