Nuclei of crystallization

The image is usually formed via photoelectrochemical deposition of metals (Pd, Ag, etc.). Metallic particles either directly form the image of a sufficient optical density (Goryachev et al, 1970, 1972) or serve as nuclei of crystallization in the course of development (Kelly and Vondeling, 1975). Photoelectrochemical reactions of colored organic compounds may also be employed for producing image (see, for example, Reichman et al, 1980). 

Since the degree of supersaturation and concentration of salt are largest in the cesium fluoride system, the largest clusters seem to be formed in solution, as expected. However, since the number of ions in solutions examined are too small, the clusters cannot grow to the size of nuclei of crystals. Nevertheless, it can be seen from Fig. 7 that small aggregates, which may be regarded as embryos for crystallization, become the cores of crystals, and have an irregular shape. 

The time passed from the beginning of crystallization to the formation of nuclei of crystals of each size can be calculated from the data represented by the curves in Figures 2 and 3. 

Such an increase in the nucleation rate can be explained by postulating that not only the aluminosilicate blocks formed in the liquid phase but also the similar blocks with ordered structure occurring in the gel skeleton can be the nuclei of crystals. The number of such blocks passing into solution and coming out at the surface of gel particles for a unit of time must increase with increasing dissolution rate of the gel skeleton during the autocatalytic stage of crystallization. 

Poly(ethylene glycol) alkylaryl ether phosphates and poly(ethylene glycol) alkyl ether phosphates have been used as inhibitors for the deposition of salts in the petroleum field. These reagents have high surface activity and are adsorbed on the nuclei of crystallization centres, thus preventing further crystallization and thus inhibiting the deposition of salts. 

The above remarks are also confirmed by the fact that these deviations become more evident with a lowering of the temperature of crystallization. The lowering is accompanied, in turn, by an increase in the number of nuclei of crystallization. 

The visible crystals that develop during a crystallization procedure are built up as a result of growth either on nuclei of the material itself or surfaces of foreign material serving the same purpose. Neglecting for the moment the matter of impurities, nucleation theory provides an explanation for certain qualitative observations in the case of solutions. 

Once nuclei form in a supersaturated solution, they begin to grow by accretion and, as a result, the concentration of the remaining material drops. There is thus a competition for material between the processes of nucleation and of crystal growth. The more rapid the nucleation, the larger the number of nuclei formed before relief of the supersaturation occurs and the smaller the final crystal size. This, qualitatively, is the basis of what is known as von Weimam s law [86] ... 

In this chapter we have shown that diffusive transformations can only take place if nuclei of the new phase can form to begin with. Nuclei form because random atomic vibrations are continually making tiny crystals of the new phase and if the temperature is low enough these tiny crystals are thermodynamically stable and will grow. In homogeneous nucleation the nuclei form as spheres within the bulk of the material. In... 

A number of authors have developed mechanistic descriptions of the processes causing secondary nucleation in agitated crystallizers (Ottens etal., 1972 Ottens and de Jong, 1973 Bennett etal., 1973 Evans etal., 1974 Garside and Jancic, 1979 Synowiec etal., 1993). The energy and frequency of crystal collisions are determined by the fluid mechanics of the crystallizer and crystal suspension. The numbers of nuclei formed by a given contact and those that proceed to survive can be represented by different functions. 

Growth of secondary nuclei, small crystals and attrition fragments... 

Garside, J. and Larson, M.A., 1978. Direct observation of secondary nuclei production. Journal of Crystal Growth, 43, 694. 

For the case of melt crystallization, the blending with PPO favors the obtainment of the P form. On the basis of several experimental results, it has been suggested that this behavior would be due to the interactions between PS and PPO chains in the melt, which would produce a more rapid disappearence of a memory (that is of possible nuclei) of the a form in the melt [104]. 

S(l) is the nucleation rate for non-interacting nuclei and is further interpreted as the probability distribution for a crystal to have thickness l. Notice that for 2xsJAF < 1, S([) is negative, which corresponds to the statement that a lamella of this thickness is unstable. The total flux, ST, in an ensemble of crystals is obtained by summing S(l) over all possible values of l ... 

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