Nucleation

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.