Nucleation processes, types

It is important to consider why H depends on the degree of order of the crystalline phase. Three different types of diffusion process act during the nucleation process. They are diffusion within the melt, within the interface between the melt and a nucleus (or crystal), and within the nucleus. It is obvious that the diffusion of chains within the melt can not be related to the dependence of H on the degree of order of the crystalline phase within the nucleus (or crystal). Therefore, the phase dependence of H should arise from... 

Since some effects, observed during the crystallization of zeolites from gels, as for instance, the autocatalytic nature of zeolite nucleation (2,3,12, Katovic, A. Subotic, B. mit, I. Despotovic, Lj. A. Zeolites, in press) cannot be readily explained only by the classical approaches to the nucleation processes in the liquid phase (2,6,10,12), the objective of this work is to analyse the crystallization kinetics of different types of zeolites by the kinetic equation derived on the basis of Zhdanov s idea on autocatalytic nucleation (2), in order to explain the influence of autocatalytic nucleation on zeolite crystallization processes. 

Source of Nuclei Type of Nucleation Process Prevention or Remedy... 

Many chemical reactions in the solid state follow rate laws that are based on the process of nucleation. The active sites have been observed microscopically in some cases, and the phenomenon of nucleation is well established. Although they will not be described in detail, several other processes have nucleation as an integral part of at least the early stages. For example, crystal growth has been successfully modeled by this type of rate law. Condensation of droplets is also a process that involves nucleation. Consequently, kinetics of a wide variety of transformations obey rate laws that have some dependence on a nucleation process. 

Having a set of a,t) data available, a graph was prepared to illustrate the type of plot that can be expected when a reaction follows an Avrami-Erofeev rate law. The result is shown in Figure 7.2, and the sigmoidal curve is characteristic of a reaction that follows a nucleation rate law. In Chapter 2, it was shown that a sigmoidal rate plot results from autocatalysis, but for reactions in the solid state such plots are more likely to indicate that the reaction is controlled by some type of nucleation process. 

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