Chapter 14 - Nucleation and Crystallization

Abstract

A supercooled liquid is any liquid cooled below its normal freezing point. Crystallization from a supercooled liquid is a two-step process, consisting of nucleation and crystal growth. Nucleation occurs as a result of random fluctuations in the liquid. A nucleus is a precursor to a crystal, lacking recognizable growth habit planes. Within the framework of classical nucleation theory, the stability of a nucleus is the result of a competition between the volume free energy of nucleation and the free energy associated with creating a new liquid-nucleus interface. Classical nucleation theory considers two barriers to nucleation: a thermodynamic barrier and a kinetic barrier. A nucleus is stable if it overcomes these barriers and becomes larger than the critical nucleus size. Nucleation can be either homogeneous or heterogeneous. Homogeneous nucleation has the same probability throughout the supercooled liquid. Heterogeneous nucleation promotes accelerated nucleation at the presence of a free surface or other interface, where the thermodynamic barrier to nucleation is lowered. Once a stable nucleus is formed, crystal growth proceeds by transporting atoms from the supercooled liquid and incorporating them into the crystal. The overall rate of crystallization is described by the Johnson-Mehl-Avrami equation, which combines nucleation and crystal growth rates at a particular temperature. The microstructure of the material can be tailored by controlling the temperature and time of the nucleation and crystal growth steps, for example, in the development of glass-ceramic materials.