Crystal Growth

CRYSTAL GROWTH

If nucleation and crystal growth are independent, the latter can be studied by seeding a melt with small crystals in conditions of little or no natural nucleation. The rate of growth can then be measured. The relation between growth rate and temperature, shown in Figure 9.1B, also exhibits an optimum degree of supercooling, although the maximum growth temperature is normally higher than the temperature of maximum nucleation. The form of the crystal growth curve is again explained by the kinetics of the molecules. At temperatures just below the melting point, molecules have too much energy to remain in the crystal lattice. As the temperature falls, more molecules are retained and the growth rate increases. Ultimately, however, diffusion to and orientation at the crystal surface are depressed.

For crystal growth in a single-component melt, the molecules at the crystal surface must reach the correct position at the lattice and become suitably ori-entated, losing kinetic energy. These energy changes appear as heat of crystal-lization, and this must be transferred from the surface to the bulk of the melt. The rate of crystal growth is influenced by both the rate of heat transfer and the changes taking place at the surface. Agitation of the system will increase heat transfer by reducing the thermal resistance of the liquid layers adjacent to the crystal until the changes at the crystal face become the controlling effect.

In multicomponent melts and solutions, deposition of material at the crystal face depletes the adjacent liquid layers, and a concentration gradient is set up with saturation at the face and supersaturation in the liquid. Diffusion of molecules to the crystal face is discussed in the next section.

The account above describes the behavior of certain carefully prepared melts from which all extraneous matter is rigidly excluded. Dust and other insoluble matter may increase the nucleation rate by acting as centers of crys-tallization. Soluble impurities may increase or decrease both rates of nucleation and crystal growth. The latter is probably due to adsorption of the impurities on the crystal face. Impurities may also affect the form in which the material crystallizes.