Nucleation/crystallization promoter

The crystallization behavior of milk fat (which contains minor lipids) and a pure triacylglycerol fraction of milk fat were compared by Herrera et al. (1999). The results suggested that minor lipids delay nucleation but promote crystal growth. Other workers who examined the effects of added phospholipids on palm oil, suggested that some phospholipids delayed nucleation while others had more significant effects on the rate of growth of fat crystals (Smith, 2000),... 

Tietz and Hartel (2000) studied the effects of removing or adding minor components naturally present in milk fat on the crystallization of milk fat-cocoa butter blends. They suggested that at low concentrations, minor lipids act as sites for nucleation and promote the rate of crystallisation and at higher concentrations inhibit crystallisation. They concluded that the presence of minor lipids, at the concentrations naturally occurring in milk fat, were sufficient to affect crystallisation rates, chocolate microstructure and fat bloom formation in chocolate. 

In addition to requiring nucleating agents to promote crystallization, glass ceramics usually must be heat-treated at the critical nucleating, crystallization, and annealing temperatures to restructure the glass (Fig. 1). These temperatures can be conveniently determined, to a first approximation, by differential thermal analysis (DTA)i (Fig. 2). 

The reaction of this solid [48] was the first [133] example of Smith-Topley behaviour recognized and studies of this rate process have continued. Flanagan and Kim [133] showed that irradiation decreased the induction period to dehydration and the rate of water evolution rapidly reached a maximum value which was maintained between 0 < nr < 0.4. Water evolution was more rapid than that found for unirradiated salt and the value of , was decreased. Irradiation damage to the crystal promoted nucleation and there was rapid initial establishment of a constant area of reaction interface (the contracting volume equation approximates to zero-order kinetics at low values of nr). There was also evidence [134] that preirradation aided recrystallization during vacuum dehydration. 

A few authors have observed coincident crystallization of both phases in crystalline/crystaUine immiscible blends. This phenomenon was reported for blends in which the minor phase exhibits a higher degree of undercooling for crystallization due to its fine dispersion (see Section 3.4.4.) and the matrix phase crystallizes at its bulk T that is lower than that of the minor phase. An additional factor that should be taken into account is that a heterogeneous nucleation is promoted on surfaces with a high interfacial tension [Helfand and Sapse, 1975] (i.e., a crystal-... 

These disadvantages are due to the crystalline properties of the crystalline polymer compounds. The solution is to form fine crystals very rapidly. In order to do this, the crystallization temperature may be increased, or crystal nucleating agents and crystallization promoters may be added (6). 

Scaling is not always related to temperature. Calcium carbonate and calcium sulfate scaling occur on unheated surfaces when their solubiUties are exceeded in the bulk water. Metallic surfaces are ideal sites for crystal nucleation because of their rough surfaces and the low velocities adjacent to the surface. Corrosion cells on the metal surface produce areas of high pH, which promote the precipitation of many cooling water salts. Once formed, scale deposits initiate additional nucleation, and crystal growth proceeds at an accelerated rate. 

Crystal Morphology. Size, shape, color, and impurities are dependent on the conditions of synthesis (14—17). Lower temperatures favor dark colored, less pure crystals higher temperatures promote paler, purer crystals. Low pressures (5 GPa) and temperatures favor the development of cube faces, whereas higher pressures and temperatures produce octahedral faces. Nucleation and growth rates increase rapidly as the process pressure is raised above the diamond—graphite equiUbrium pressure. 

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