Types of Nucleation

There are three commonly known nucleation processes homogeneous, heterogeneous, and secondary nucleation. 

Homogeneous nudeation is the most common of the three, where the formation of nuclei from solute atoms can occur in the absence of any outside stimulant, such as solid interface and contaminant The solute atoms are thermodynamically unstable when supersaturahon is reached, and form stable nuclei to reduce the overall system energy. The classical nucleahon theory can be used to depict a homogenous nucleahon process. 

The third type of nucleation-secondary nudeation-refers to the formation of nuclei induced by experimental conditions such as stirring, and can continue in the growth stage of particles. As yet, no complete theory has been developed to describe secondary nucleation. 

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Most of the kinetic theories to date have invoked nucleation as the primary barrier to growth. However, even within this area there are different types of nucleation... 

Because of its dependence on a, ae, 51, and AF its size can be expected to vary according to the type of nucleation model employed in analyzing a set of experimental data (see Refs. [106 and [107]). The size of a stable nucleus has consequences as far as, e.g., the number of molecules needed to form it and the minimum required substrate length are concerned. 

Crystallization is generally preceded by two types of nucleation. The primary nucleation occurs with the formation of clusters of molecules at the submicron level. When the concentration exceeds saturation to afford supersaturation, the clusters become nuclei. The secondary nucleation is caused by particles due to primary nucleation or seeds. There are many strategies to achieve supersaturation to initiate crystallization such as cooling, evaporation, and antisolvent addition. 

CD can occur either by initial homogeneous nucleation in solution or by het-eronucleation on a substrate, depending on the deposition mechanism (see Chapter 3). For this reason, we consider both types of nucleation. 

The results can be presented graphically, as in Fig. 19.14. The plot shows the kinetically dominant type of nucleation as a function of grain size (via Rb), AQB, and yaa/ 0lP. By setting the nucleation rate, J, at a fixed value, a curve such as abode can be plotted to indicate, for given value of L/S, the dominant modes of nucleation at the designated nucleation rate at various values of yao / fat3. 

The overall current-time relationships for the simultaneous nucleation and growth of nuclei are of the form iwhere f3 is a variable depending primarily on the model of nuclei (2D, 3D) and the type of nucleation (instantaneous, progressive). 

Applications for these foamed products are many. An example is meat trays that are normally produced from PS foamed sheet with a thickness of approximately 0.095 in. and a density of about 5 lb/ft3. These trays are manufactured with little skin. The meat should not stick to the plastic when frozen, nor should the meat juice penetrate into the tray, and the tray should not break when handled. Strength is achieved by correcdy orienting the sheet resulting in cells that are round from a plan view. The cell size, which is determined by the amount and type of nucleating agent, is kept fairly fine to give the trays a soft, glossy appearance. 

Table 17.2. Avrami exponent (n) values for different types of nucleation and dimensionality of growth.

Crystallization is generally preceded by two types of nucleation. The primary nucleation occurs with the formation of clusters of molecules at the submicron level. When the concentration exceeds saturation to... 

Values for the Avrami exponent n for various types of nucleation and growth are as shown in Table 1. 

Values of the Avrami Exponent n for the Various Types of Nucleation and Growth Mechanisms ... 

Crystal nucleation from the melt is but one type of nucleation in the condensed phase. Other processes of interest include the nucleation of salt crystals from aqueous solution, of one crystalline phase from another or from a glass, and of liquid crystal phases from one another or from the isotropic liquid. In this review we discuss only the nucleation of crystals from the melt. The major emphasis will be on single-component systems, although crystallization of alloys and binary mixtures will also be considered. 

Creation of new crystals (nucleation) can take place through a variety of mechanisms. Figure 4-2 summarizes the types of nucleation which can occur. Some of these are true nucleation (driven essentially only by free energy considerations) others are hcavi ly dependent on imposed conditions (mixing and others). 

Another type of nucleation, polycrystalline breeding, occurs at higher supersaturations from irregularly formed aggregates, but this is not a common problem. 

Nucleation and the distinctions between primary and secondary nucleation, and homogeneous versus heterogeneous nucleation, are discussed in Chapter 4. As noted in that chapter, more detailed discussion of these topics may be found in the excellent treatments of Myerson (2001), Mullin (2001), and Mersmann (2001). In the current discussion, the different types of nucleation are not separated because they often occur simultaneously and can result from more than one of these mechanisms. 

Fillers in these systems affect two types of nucleation nucleation of bubble formation and nucleation of crystallization. Nucleation of bubble formation affects the density of foam. Nucleation of crystallization affects the balance of gas formation and phase transition. The timing of both processes is critical. ... 

HOMOGENOUS NUCLEATION. In crystallization from solution, homogenous nucleation almost never happens, except perhaps in some precipitation reactions. The fundamentals of the phenomenon, however, are important in understanding the more useful types of nucleation. 

CONTACT NUCLEATION. It has been known for a long time that secondary nucleation is influenced by the intensity of agitation, but it was not until the 1970s that the phenomenon of contact nucleation was isolated and studied experimentally. It is the most common type of nucleation in industrial crystallizers, for it occurs at low supersaturations where the growth rate of the crystals is at an optimum for good quality. It is proportional to the first power of the supersaturation, instead of the twentieth or more power for primary nucleation, so that control is comparatively easy without unstable operation. 

However, due to the slow rate of crystallization, PET has not been used in injection molding applications until recently [Burke and Newcombe, 1982 Hecht and Ford, 1985]. Through the use of specific types of nucleators (e.g., sodium stearate, sodium ionomers) and other crystalliza-... 

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

In addition to these, one must consider intentional and accidental seeding as a surrogate form of nucleation, which is so often encountered industrially that it is convenient to consider it with other types of nucleation. 

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