The preparation of powders

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.