Nucleation, heterogeneous

Nucleation in solids is very similar to nucleation in liquids. Because solids usually contain high-energy defects (like dislocations, grain boundaries and surfaces) new phases usually nucleate heterogeneously homogeneous nucleation, which occurs in defect-free regions, is rare. Figure 7.5 summarises the various ways in which nucleation can take place in a typical polycrystalline solid and Problems 7.2 and 7.3 illustrate how nucleation theory can be applied to a solid-state situation. 

An alloy is cooled from a temperature at which it has a single-phase structure (a) to a temperature at which the equilibrium structure is two-phase (a -i- ji). During cooling, small precipitates of the P phase nucleate heterogeneously at a grain boundaries. The nuclei are lens-shaped as shown below. 

When the vapor phase is generated at a solid interface rather than in the bulk of the liquid, the process is known as heterogeneous nucleation. Heterogeneous nucleation theories on smooth surfaces yield similar expression to Eq. 8.6-1 for J, with modified groupings A and B that account for the contribution of geometry and energy of the solid surface (22). 

Modern experimental work on the nucleation of liquids was pioneered by Turnbull, who developed droplet dispersion methods during the early 1950s and applied them to a wide variety of metals. His approach was based on the idea that even highly purified liquid samples contain impurities that can catalyze nucleation heterogeneously in the bulk, so that samples of macro-... 

If, additionally, several different nucleating heterogeneities are present, the overall crystallization is a complicated superposition of the contributions of them all. Frequently, the crystallization that is caused by a certain heterogeneity is completed within time... 

Discussion/Colncidence of Crystallization Temperatures. Let us first consider the PVDF/PA-6 blend. In view of the nonaltered T of PVDF, we suppose that the PVDF crystallization induces the PA-6 crystallization rather than vice versa. Hence, the just created crystals of the PVDF matrix act as nucleating heterogeneity for the PA-6. The A y-value between PVDF crystals and PA-6 melt, obviously, is smaller than that of all other heterogeneities which are present in PA-6 to a sufficient extent except, possibly, the species "A". Its associated specific undercooling, moreover, must be so small that the PVDF crystals can induce the crystallization of the PA-6 from the instant of their own creation. 

The coincident crystallization of the PVDF matrix and dispersed PBTP particles in the 85/15 blend (z = 4) takes place at (142...148)°C, that is, above the T of pure PVDF. It is not clear whether the PVDF or the PBTp crystallizes first. In either case, the nucleation of the first crystallizing component may be induced either by a species of nucleating heterogeneities or by the molten second blend component. The newly created crystals of the one component, then, act immediately as nuclei for the crystallization of the other in the same manner as already described for the PVDF/PA-6 blends. 

As a consequence, contrary to homogeneous nucleation, heterogeneous nuclea-... 

The absence of nucleating heterogeneities that can act as nucleating agents, the presence of which can reduce the size of the critical nucleus and greatly enhance the nucleation kinetics. 

In crystallizable dispersed droplets, several different nucleating heterogeneities (type i) can be... 

PP/PS SBS The drop size of PP did not change, but T increased, owing to immiscibility of PP with polybutadiene block. SBS is poor compatibitizer, but capable to transfer nucleating heterogeneity s to PP. Santana and Muller, 1994... 

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