文档介绍：3 Homogeneous Nucleation
Importance of Homogeneous Nucleation
Condensation in the supersaturated state was first observed during the last century
in the expansion of the superheated steam jet and the famous cloud chamber ex-
periments of Wilson (1897). Nucleation is important in many of areas of science
including atmospheric physics and technology. Nucleation can occur from a vapor
or liquid phase. Cloud formation has implications in the atmosphere as well as in
certain manufacturing processes. In material science, the properties of films and
casting may depend on the grain structure caused by the nucleation mechanism.
The safety of a nuclear reactor needs the understanding of nucleation problems
especially when the overheating of its reactor core occurs (Williams and Loyalka,
1991). Superconductors may be used as high current transmission lines; cooled by
liquid helium or liquid nitrogen. But, if overheating occurs due to some reason,
its cooling can be plished by heat transfer by boiling due to homogeneous
nucleation (Brodie et al., 1977; Sinha et al., 1987). In many cases, nucleation also
occurs around foreign particles: ions, large molecules, or on solid walls (hetero-
geneous nucleation) (Dillmann and Meier, 1989).
Irrespective of the importance of homogeneous nucleation, nucleation theory
has not been considered to be essful in all cases although it has recently be-
come to be promising.
Classical Nucleation Theory
Consider nucleation or condensation, at a constant temperature T, from the vapor
phase by forming a liquid droplet of radius r, containing n molecules. The droplet
will grow like a chemical reaction bining vapor molecules one by one.
However, we do not follow such details and treat the process thermodymamically.
The nucleation process is irreversible and does not occur in thermal equilibrium,
but we consider that the state of the system involved in the process is not far
from eqilibrium.
Classical Nucleation