Solution seeded, crystal growth

In the flux-growth method, crystals of the desired ceramic are precipitated from a melt containing the components of the product phase, often in addition to additives used to suppress the melting point of the flux. These additives remain in solution after crystal growth is complete. Crystals are precipitated onto seeds by slowly cooling the melt or the seed, or occasionally by evaporating volatile components of the melt such as alkaH haHdes, depressing the solubiHty of the product phase. 

Calcium phosphate precipitation may also be involved in the fixation of phosphate fertilizer in soils. Studies of the uptake of phosphate on calcium carbonate surfaces at low phosphate concentrations typical of those in soils, reveal that the threshold concentration for the precipitation of the calcium phosphate phases from solution is considerably increased in the pH range 8.5 -9.0 (3). It was concluded that the presence of carbonate ion from the calcite inhibits the nucleation of calcium phosphate phases under these conditions. A recent study of the seeded crystal growth of calcite from metastable supersaturated solutions of calcium carbonate, has shown that the presence of orthophosphate ion at a concentration as low as 10-6 mol L" and a pH of 8.5 has a remarkable inhibiting influence on the rate of crystallization (4). A seeded growth study of the influence of carbonate on hydroxyapatite crystallization has also shown an appreciable inhibiting influence of carbonate ion.(5). 

Scheme 3 The solid yielded by mechanical mixing of the reactants can be used to seed crystal growth from solution to obtain crystals for single-crystal X-ray diffraction experiments. This procedure allows one to compare the X-ray powder diffraction pattern measured on the mechanochemical sample with that calculated on the basis of the single-crystal experiment for the solids obtained by crystallization via seeding of a solution of the ground powder of the adduct...

Assume a solution supersaturated with one crystallizable solute. To part of the solution, a crystal growth inhibitor is added. It is established that in both cases the relation Lc = a(ln fi)2 holds, only the constant a being different. Assume now that both solutions have the same initial, rather weak, supersaturation and that crystallization is started by adding tiny seed crystals. Would the ratio of the overall crystallization rates d(p/dt now be the same as the ratio of both a values ... 

Concept used in sophisticated scaling models, whereby certain ions in aqueous solution are said to associate in pairs (e.g., CaS04, CaHC03-). These ion pairs are then deducted from the total analytical value, to provide an estimate of the free ion content available for seed crystal scaling or growth agglomeration and deposition. 

Carefully selected seed crystals are sometimes added to a crystalliser to control the final product crystal size. The rapid cooling of an unseeded solution is shown in Figure 15.20a in which the solution cools at constant concentration until the limit of the metastable zone is reached, where nucleation occurs. The temperature increases slightly due to the release of latent heat of crystallisation, but on cooling more nucleation occurs. The temperature and concentration subsequently fall and, in such a process, nucleation and growth cannot... 

The seed crystals of D-fructose were obtained by ball milling crystals produced by spontaneous nucleatlon from an aqueous ethanolIc solution of fructose and allowing them to stand at room temperature in slightly supersaturated ethanolIc solution until the desired crystal size (20-40 microns) was achieved. They were then stored at 30 C in saturated, anhydrous ethanol to prevent further crystal growth. A portion of this slurry was added to the crystallizer as the seeds. 

The crystallization of the D-enantiomer is therefore considered to be induced by crystal growth on the surface of the seed crystal and at the same time initial breeding may play a role that causes small crystals near the seed crystal. The propagation of nucleation in distance from the seed may be caused by convective flow of the solution due to density difference during the crystal growth. 

Figure 7. Schematic representation of growth behavior of seed crystals in stagnant solutions.

In this study, D-SCMC seed crystals were put in a racemic SCMC supersaturated solution in a batchwise agitated vessel and growth rates in longitudinal and lateral directions and the optical purity of D-SCMC crystals were measured. The growth rates and optical purity were discussed considering surface states of grown crystal observed by a microscope. The kinetics of crystal growth were measured and a model of inclusion of impurity was proposed. 

When a series of tests carried for the particular seed was over, some crystals obtained by the series of the growth tests, were put in the saturated solution of D-SCMC at 303 K together with the seed crystal of D- and L-SCMC as dipping test and the surface states of the crystal in the solution were observed through a microscope. 

To determine the rate of dissolution of hemlhydrate crystals, the same vessel was used as for the crystallization study. The vessel was filled with the sulphate-rich solution (zero Initial calcium concentration). An amount of sieved hemlhydrate seed crystals, about 10% In excess of that required to saturate the solution, was added. At very short time Intervals, samples were taken using a similar procedure to that for the gypsum growth Investigation. Samples were separated Into crystals for size analysis (with a 190pm orifice) and crystal content and solutions for analysis. Further details are given by Mukhopadhyay (17). 

The experimental procedure was as follows. One litre of supersaturated aqueous solution was prepared, of which 0.8 was poured Into the crystallizer. After a constant temperature (of 298.2 K) was attained, the solution concentration was determined, and a seed crystal (0.59-0.71 mm) was put Into the solution and was allowed to grow for either 600 or 900 s. The crystal was then taken out of the crystallizer and put Into a 50 ml beaker In which a small quantity of alcohol was placed. The crystal was removed from the beaker and dried In the air for a few mlnltes, then Its weight and surface area were measured. The same crystal was again placed Into the crystallizer and allowed to resume growth. 

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