Growth rate impurity effects

Effects of Impurities nd Solvent. The presence of impurities usually decreases the growth rates of crystalline materials, and problems associated with the production of crystals smaller than desired are commonly attributed to contamination of feed solutions. Strict protocols should be followed in operating units upstream from a crystallizer to minimize the possibiUty of such occurrences. Equally important is monitoring the composition of recycle streams so as to detect possible accumulation of impurities. Furthermore, crystalliza tion kinetics used in scaleup should be obtained from experiments on solutions as similar as possible to those expected in the full-scale process. 

KlMURAH, H. J. Cryst. Growth 73 (1985) 53-62. Impurity effect on growth rates of CaCl2-6H20 crystals. 

The use of tailor made additives holds great promise in the area of crystal growth and morphology control. The routine selection and use of these type of additives will require a fundamental understanding of the mechanism which the additives work on a molecular basis. At the same time, the effect of solvent molecules on the crystal growth process is another related and important problem. In both instances, the relationship between internal aystal structure, aystal growth rate, solvent and impurities are needed to predict the habit of a crystal and thus allow seleaion of the proper conditions and components required to obtain a desired habit... 

In this equation a, b and e are constants depending on the physical properties of the material to be separated. In Figure 2 the effective distribution coefficient is plotted over the growth rate on the example of a binary mixture of dodecanole with 3,3% impurity of decanole. One curve represents the measured purities (l ff real) is compared to the curve of k ff for the ideal growth rate and... 

Since the edge free energies, y, are different for the vapor and solution phases, and particularly for solute-solvent interaction energies, the same crystal species will exhibit different Tracht and Habitus in different ambient phases and different solvents. If impurities are present in the system, this affects y and the advancing rates of steps. There are two opposite cases in impurity effects, and, depending on the interface state, some will promote growth, whereas others will suppress growth. 

Polyhedral (principally octahedral) crystals of Type I may have appeared due to selective adsorption of impurities to suppress the growth rate of 111, and therefore pure Type II grew without such an effect, thus resulting in irregular forms. 

Foreign substances even in minute amounts may have other kinds of effects on crystallization They may inhibit or accelerate growth rate or change the shape of crystals, say from rounded to needlelike, or otherwise. One of the problems sometimes encountered with translating laboratory experience to full scale operation is that the synthetic liquors used in the laboratory may not contain the actually occurring impurities, and thus give quite different performance. Substances that modify crystal formation are very important industrially and many such materials have been the subject of patents. 

Traditionally, CVD reaction data have been reported in terms of growth rates and their dependence on temperature. The data are often confounded by mass-transfer effects and are not suitable for reactor analysis and design. Moreover, CVD reaction data provide little insight, if any, into impurity incorporation pathways. Therefore, the replacement of traditional macroscopic deposition studies with detailed mechanistic investigations of CVD reactions is an area of considerable interest. A recent, excellent review of CVD mechanistic studies, particularly of Si CVD, is available (98), and the present discussion will be limited to highlighting mechanisms of Si CVD and of GaAs deposition by MOVCD as characteristic examples of the combined gas-phase and surface reaction mechanisms underlying CVD. 

The work of Yamamoto (43) with growth-active impurities such as Pb+2, Sn+2, and Mn+2 ions reveals that their presence in very small quantities decreases the probability of nucleation, thus extending the metastable region of aqueous solutions of the alkali halides. These ions of the transition elements because of their screening demands withdraw Cl- ions from solution and form complexes such as (PbCl6) 4 and (MnCl6)-4. Without changing the over-all composition of the solution, these ions lower the effective concentration of the Cl ions and thus decrease the nucleation rate of NaCl. 

Factors that influence growth of sucrose crystals have been listed by Smythe (1971). They include supersaturation of the solution, temperature, relative velocity of crystal and solution, nature and concentration of impurities, and nature of the crystal surface. Crystal growth of sucrose consists of two steps (1) the mass transfer of sucrose molecules to the surface of the crystal, which is a first-order process and (2) the incorporation of the molecules in the crystal surface, a second-order process. Under usual conditions, overall growth rate is a function of the rate of both processes, with neither being rate-controlling. The effect of impurities can be of two kinds. Viscosity can increase, thus reducing the rate of mass transfer, or impurities can involve adsorption on specific surfaces of the crystal, thereby reducing the rate of surface incorporation. 

Neither the dopant distribution coefficient nor the catalytic effect on growth rate is understood in detail. It is reasonable to expect that the dissociation rate of an impurity molecule might be higher than for SiH4, enhancing the concentration in the film. When the plasma contains molecular species a and and the dissociation rate is higher for p by a factor d, then a simple model predicts a distribution coefficient of... 

Whatever the details of the kinetic mechanism, impurities cause crystal habit modification. Buckley [65] has classified many impurity effects on different crystal habit modifications. In most cases, impurities decrease the growth rate of specific crystal faces, which lead to a change in the crystal habit because the slowest growing faces will dictate the crystal morphology. In some exceptional cases, impurities can increase the growth rate of a particular crystal face. For example, 1% Fe added... 

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