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Chapter II Solution Growth….
The transformation can be divided into two discrete steps that occur
one after the other.
(1) The formation of tiny particles of new phase, which is known as
nucleation, and
(2) The increased in size of the stable particle, which is called growth.
A distinction between the nucleation and growth stages of
transformation is important. When a particle in a new phase is formed a new
interface is created between the particle and the liquid. Like all surfaces this
interface has a positive energy which must be supplied during the
transformation process. The tiny particles have large surface area to volume
ratio and can therefore be unstable. For a spherical particle of radius r,
Surface Area
Volume
.
.
4r
4
r
3
2
3
.
96
.
3
r
(2.9)
As r tends to zero, this ratio becomes very large and the energy of the
surface can effectively prevent the initial formation of tiny particles. A particle
is said to be nucleated, when it becomes stable and will not disappear
because of thermal fluctuations. Once a particle achieves this stage, it grows
further with continuous decrease in energy. The surface energy is no longer a
dominant factor in the growth process.
2.19 Kinetic of Nucleation
The nucleation process is explained after considering the under-
cooled and supersaturated phases which refers to unstable conditions. When
the pressure of a vapor P is less than, equal to, or greater than the vapor