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.
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.
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