Chemistry of Powder Production

A powder consists of a solid in a discontinuous state, that is, there are no material bonds between the individual pieces. The word ‘powder’ distinguishes an assembly of particles, in contrast to the integrity of the bulk solid. The particle size is generally considered to extend from sizes as large as 1 mm to sizes of the order of nanometres, that is, close to molecular size.

Studies on powders still present many difficulties depending on the particular properties of the powder, as described in Chapter 1, and the results of these studies have not yet been systematically organized. However, the knowledge gained from these studies certainly makes important contributions to the advance of the technology. Industry is widely concerned with powders, including grains, both as process materials and as products, and the application of powder technology in industrial processes is often very important.

A powder is an assembly of fine solid particles. It has, therefore, in general a very large surface area per unit weight (specific surface area) and a high surface energy resulting from numerous lattice defects existing on the surface. These characteristics could contribute to the increase of chemical reactivity of the solid surface and apply, for instance, to the preparation of ceramics. Further, knowledge of the surface activity of fine particles is also very useful in studies of adsorption and the catalytic action of solids. On the other hand, in connection with unit operations in chemical engineering, the bulk properties of powders have been investigated in relation to rheological phenomena such as compressibility, flowability, mixing and so on, which depend on interactions such as friction and aggregation between particles. However, quantitative relationships between the micro-view of particles derived from physicochemistry and the macro-view of particles from powder technology have still not been established at the present time.

The properties of powders are strongly influenced by the size, shape and state of packing of their particles. Different methods of preparation will give powders with different properties. When materials are crushed, the shapes of the product particles will be determined by the cracks that originate at grain boundaries and from cleavage planes within single crystals, whereas grinding will lead to a more uniform or spheroidal shape as the sharp corners are worn away. Within the particles themselves, the lattice structure may manifest considerable strain, an effect due to the mechanical work done. When solids are deposited from the liquid phase, the shape is determined by the way in which fresh layers of the material are laid onto an already existing crystal structure. In the initial stage of this process, fine particles deposited as nuclei are likely to form secondary particles by agglomeration. When particles are formed by deposition from the vapour phase at elevated temperatures just below their melting point, it is easy to obtain ultra-fine, single-crystalline particles with a narrow size distribution, spherical in shape and of high purity. To prepare powders consisting of particles having a particular size and shape, careful selection of the preparation technique is necessary. Grinding, the thermal decomposition of solids and the deposition of solids from the liquid or vapour phase are the commonest techniques used for the preparation of powders.

Information on composition, texture, crystalline structure, reactivity and other properties of powders is usually obtained by X-ray diffraction, thermal analysis, infrared absorption spectroscopy, electron microscopy and other methods that are generally used for the characterization of solid material. In this chapter we shall consider general methods used only for the characterization of powdered material. It may be said that the commonest characteristic of powder is the statistical element, which is always necessarily included in the properties of the powder itself because a powder is an assembly or an aggregation of innumerable particles.