Slow crystal growth

We have seen that the deposition of crystals from the vapor is much too slow to model by MD techniques. Most laboratory equipment for producing thin films involves relatively slow crystal growth processes, and is not suitable for direct simulation. Information on the stability and properties of thin films can be obtained by similar modeling techniques, however. We describe below some of our results that provide necessary data to find the equilibrium configuration of thin films at low temperatures. 

Reaction Time. The rate of formation of to at reflux temperature is illustrated in Figure 2. The rate was estimated by taking samples from the reaction mixture at various times and examining them by x-ray diffraction. The crystallization is characterized by an unusually long induction period of about 13 hr, which is followed by a relatively slow crystal growth. No alteration of the zeolite was observed during an extra 50 hr reaction time after crystallization had been completed. 

Certain ocean precipitates incorporate the Mg2+ ion in their makeup. In calcite (CaC03), it has been shown that the magnesium content enhances calcite solubility, which, in turn, slows crystal growth. This raises concerns about increasing the magnesium content of seawater (as from oil production), which can have adverse implications for calcareous marine organisms such as plankton. 

The general recommendation is to keep supersaturation levels in the low to medium range to prevent additional nucleation events. This strategy has an additional beneficial effect on purity owing to slow crystal growth, the purity of the final crystals is usually higher compared to that in the case of fast growth rates. Finally, the addition of seeds efficiently avoids the formation of amorphous material, which can later recrystallize in an uncontrollable fashion. 

In the ideal case, very slow, indefinitely slow, crystal growth leads to perfect crystals, which also means perfect purity of the crystals. However, the indefinite slow growth is not possible in industrial applications. Faster growth leads from the perfect, flat, planar crystals to a more rough interface between melt and crystal or melt and crystal layers. 

Fig. 3.12 An apparatus for electrocrystallisation. Depending upon the polarity, the crystals grow from a suitable solution when a voltage is applied to one of the two electrodes. A sintered-glass filter in the lower part of the apparatus prevents contamination from the reference electrode during the slow crystal-growth process.

In practice, all crystals have imperfections. If a substance crystallizes rapidly, it is likely to have many more imperfections, because crystal growth starts at many sites almost simultaneously. Each small crystallite grows until it runs into its neighbors the boundaries between these small crystallites are called grain boundaries, which can be seen on microscopic examination of a polished surface. Slow crystal growth reduces the number of grain boundaries, because crystal growth starts from a smaller number of sites. However, even if a crystal appears to be perfect, it will likely have imperfections on an atomic level caused by impurities in the material or by dislocations within the lattice. 

Migration regime with Vp Vp Uj. 0 assumes very slow crystal growth. If the diffusion proceeds quicker than the growth, solution of these stationary conditions leads [47,141] to the logarithmic distribution law. 

This technique has a high operating safety due to the simple assembly (no moving parts) and no additional solid/liquid separation. However, the space-time yield of this technique is relatively small. The reason is that the heat and mass transfer are forced just by conduction and diffusion, respectively, and are supported only by natural convection. Hence, only very slow crystal growth rates lead to high purities. 

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