Other Factors Affecting Crystal Growth

In addition to the bulk diffusion and surface integration processes, several other factors may affect the growth of the sparingly soluble precipitates. 

Particle Crowding Effect. The growth of a crystal in a suspension can be influenced by the presence of other crystals when the crystal number concentration (number/volume) exceeds a certain level so that the interparticle distance becomes smaller than 20 particle diameters. For most precipitation systems, the crystal number concentration is typically higher than lO per cm, and the interparticle distances are normally less than 15 times the crystal size. In such a crowded system, the diffusion fields around the individual crystals begin to influence one another and one may expect some interparticle effects on crystal growth. 

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In summary, the factors that affect crystal growth in inorganic solutions are (i) level of supersaturation, (ii) mechanistic processes (screw-dislocations, surface nucleation, surface reactions, bulk diffusion, and dehydration), (iii) concentration of other (coprecipitating) ions and molecules in the system, (iv) pH and temperature. 

The nature of the deposit and the rate of nucleation at the very beginning of the deposition are affected, among other factors, by the nature of the substrate. A specific case is that of epitaxy where the structure of the substrate essentially controls the structure of the deposit.Plb lP ] Epitaxy can be defined as the growth of a crystalline film on a crystalline substrate, with the substrate acting as a seed crystal. When both substrate and deposit are of the same material (for instance silicon on silicon) or when their crystalline structures (lattice parameters) are identical or close, the phenomena is known as homoepitaxy. When the lattice parameters are different, it is heteroepitaxy. Epitaxial growth cannot occur if these stmctural differences are too great. 

It should be noted that there may be other possible factors affecting the normal growth rate of crystal faces. 

The supersaturation profile in a batch crystallizer has a profound effect on the nucleation and growth processes and the resulting CSD. It can also affect other factors (e.g., batch cycle time) related to the batch crystallization operation. Figure 10.13 shows schematically a supersaturation profile in a batch crystallization experiment (Nyvlt et al. 1985). At / = 0, the batch crystallizer is filled with a just-saturated solution that contains crystals with a negligible surface area. The solution begins to be supersaturated at a constant rate, and the supersaturation increases until it reaches the limit of the metastable zone (Acmoi)- At this point, nucleation... 

Crystallization Is one of the oldest separation operations In chemical industries. It serves not only to separate and purify substances, but also to produce crystals with a required shape. Both of these aspects are closely connected with the presence of admixtures In the solution. Among the many factors affecting the process of crystallization 1172.226], (e.g.. temperature, supersaturation, agitation), admixtures often exhibit the most pronounced effect. Even traces of admixtures can Influence the nucleatlon, crystal growth, shape and size of product crystals, and also other properties (caking, hygroscoplclty, etc.). On the other hand, they may be entrained into crystals and lower their purity. 

Let us look at the growth sites more closely and consider factors which affect their concentration on the metal electrode the composition of the solution and the potential. Like growth sites of any other crystal, those on electrodes are found at the grain boundaries and at crystal dislocations and imperfections. These depend on the history of the electrode, whether it was stretched, hammered or annealed and thus will also depend on the history of the electrode. If the solution contains species that adsorb on the electrode at the crystal growth sites, will be smaller than if the solution was free from such species. This alone demands the exercise of great care in handling experiments and experimental data on electrocrystallization. 

The number of nucleation sites can be increased by use of nucleation agents that not only shortens the crystallization time but also causes the elevation of the effective temperature of nonisothermal crystallization during cooling, especially in polyolefins. Addition of 0.2-0.5 wt% of bis(3,4-dimethylbenzylidene) sorbitol to iPP increased its crystallization peak temperature by approximately 18°C [40]. Polymer molecular weight, polymorphism, and application of pressure are other factors that affect nucleation and growth and, hence, the overall crystallization kinetics. 

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