Homogeneous Nucleation

5.7.2.1. Homogeneous Nucleation

It should be remembered that the crystallization of a homogeneous phase (liquid or glass) does not occur in a material in the entire mass. First, it starts at a point and then it extends progressively from the discrete centers, and distributed in the entire mass. That's how the whole process takes place.

When a liquid is cooled below its fusion point, the crystallization occurs by the growth of crystals at a ‘finite rate’ from a ‘finite number’ of nuclei. The glass formation can be attributed to a low rate (i.e. almost nil rate) of crystal growth, or low rate of nuclei formation, or a combination of both the factors.

Here comes the question of ‘stability’. The stability of a particle of the new phase in homogene- ous nucleation will depend on two contributions :

1. One from a difference in free energy between the two phases, and

2. The other from the interfacial energy. At the fusion point, the free energy of a given quantity of a material is the same in the crystalline

and in the liquid forms. At lower temperatures, the crystalline form will always have lower free energy and the liquid will crystallize, if nuclei of correct sizes are available in sufficient numbers for germina- tion.

In the nucleation state, very small particles (called embryos) become the starting point for the development of an ‘ordered region’. These small particles, which are formed due to the ‘structural fluctuations’ produced by the thermal excitation, have different sizes - which fluctuates or varies con- stantly.

It will be seen later that this size attains a value of a ‘critical size’ so that it can serve as a ‘starting point’, i.e. a germ or a nucleus, for the formation of a new crystalline phase. The germination or nuclea- tion across the system is called homogeneous - if all the elements of the parent phase is identical struc- turally, chemically and energetically with those of the nucleated volume. That’s the necessary condition for homogeneous nucleation.

Evidently, this is only possible if the entire volume of material is chemically homogeneous and is free from any type of structural imperfections. In practice, this is quite difficult, since the surface itself is already an inevitable source of imperfections. It can further aggravate from the presence of some foreign impurities. In this case, the energy required to form a ‘germ’ or a ‘nucleus’ is found to be reduced for such impurity-laden sites, and the nucleation will be produced preferentially at their contacts. In that case, the nucleation is called heterogeneous nucleation.

In practice, it is quite rare that this alternative route of nucleation can be avoided, and we some- times wonder whether the homogeneous nucleation is totally realizable. However, this case signifies an ideal situation.

The embryos becoming nucleus then increases in size by the successive addition of atoms taken out of the liquid phase, giving rise to the formation of a crystalline particle, which grows with a certain rate at the expense of the phase in its environment or the parent phase, i.e. the parent phase supplies the necessary atoms to the surface of the ‘already formed’ small embryos to help them grow further to complete the process of crystallization. This is called the “crystal growth stage”.

MAGNETIC PROPERTIES