In certain single-component systems, such as piperidine, nucleation and crystal growth are independent and can be separately studied.
NUCLEATION
In
certain single-component systems, such as piperidine, nucleation and crystal
growth are independent and can be separately studied. The rate of nucleation as
a function of supercooling is studied by maintaining the melt for a certain
time at the given temperature and then quickly raising the temperature to the
metastable region where further nucleation is negligible but the already formed
nuclei can grow. Figure 9.1A describes the results of such an experiment. At
low degrees of supercooling, little or no nucleation takes place. With further
cooling, the rate of nucleation rises to a maximum and then falls. The
relation, therefore, indicates that excessive cooling may depress the rate of
crystallization by lim-iting the number of nuclei formed.
Spontaneous
nucleation is considered to occur when sufficient molecules of low kinetic
energy come together in such a way that the attraction between them is
sufficient to overcome their momentum. The growth of a nucleus probably takes
place over a very short period of time in a region of high local concentration.
Once a certain size is reached, the nucleus becomes stable in the prevailing
conditions. As the temperature falls, more molecules with low energy are
present and the rate of nucleation rises. The decrease in nucleation rate at
lower temperature is due to the increase in the viscosity of the melt.
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