RATE AND CRYSTAL SIZE

Crystal Size Distribution. Crystal size was briefly mentioned in Section 14.2.3 Figure 14.7 gives a rough indication of the effects of supersaturation on growth rate, nucleation rate, and crystal size. 

The procedure of Zhdanov and Samulevich enables the calculation of isothermal nucleation rate profiles from determinations of growth rate and crystal size distribution [16,82]. Originally implemented in analyses of zeolite Na-A [83] and Na-X [82] crystallisation, the method has subsequently been applied to other zeolite systems, including silicalite [84,85]. If it is supposed that all the crystals in a batch have the same (known) growth rate behaviour, the total growth time of each crystal can be calculated. Assuming also that the nuclcation point for each crystal can be obtained by linear extrapolation to zero time, the nucleation profile for the whole batch can be determined from their final sizes. 

Incorporation of carbonate and other impurities occurs during irreversible hydrolysis reactions of ceramic precursors towards precipitation of HA. These ions decrease the transformation rate and crystal size of precipitated HA (LeGeros 2008). 

Eig. 5. The Widmanstatten pattern ia this poHshed and etched section of the Gibbeon iron meteorite is composed of iatergrown crystals of kamacite and taenite, NiFe phases that differ ia crystal stmcture and Ni content. Ni concentration gradients at crystal boundaries ia this 3-cm-wide sample can be used to estimate the initial cooling rates and corresponding size of the asteroid from which the meteorite was derived. 

Geothite is produced by air oxidation of alkaline suspension of ferrous hydroxide (Sada etal., 1988). As a starting material for ferrous oxide, the preparation of fine particles with prescribed size, size distribution and shape is required in its application to magnetic materials for recording tapes and disks. With increasing oxidation rate, the crystal size decreases and the size distribution becomes sharper (Sada etal., 1988). The first step of the reaction. 

Growth rates and ultimate sizes of crystals are controlled by limiting the extent of supersaturation at any time. 

Fig. 29. Effect of heating rate and particle size on the dehydration of whevellite. A 8 °C/min, B 2 C/min. Solid line powdered material (150 mesh), dotted line single crystal...

Lopez, C., Bourgaux, C., Bernadou, S., Keller, G., Ollivon, M. 2002a. Thermal and structural behavior of milk fat. 3. Influence of cooling rate and droplet size on cream crystallization. 

Rollmann and Walsh (266) have recently shown that for a wide variety of zeolites there is a good correlation between shape-selective behavior, as measured by the relative rates of conversion of n-hexane and 3-methyl-pentane, and the rate of coke formation (see Fig. 24). This correlation was considered to provide good evidence that intracrystalline coking is itself a shape-selective reaction. Thus, the rather constrained ZSM-5 pore structure exhibits high shape selectivity, probably via a restricted transition-state mechanism (242b), and therefore has a low rate of coke formation. Zeolite composition and crystal size, although influencing coke formation, were found to be of secondary importance. This type of information is clearly... 

The size of the crystalites produced will depend on the nucleation rate and crystal growth rate at the solidification front. Both are controlled by the local supersaturation. The rate expressions for homogeneous nucleation (equation (6.15)) and heterogeneous nucleation (equation... 

Alkalinity also has an important influence on the crystallization rate of zeolites. A remarkable example is the crystallization of zeolite A (LTA) from the precursor gel with a batch composition of 5Na20 Al203 2Si02 (100-200) H20.[13] Figure 3.8 shows the effect of different alkalinity (H2O/Na2O=20, 30,40) on the crystallization rate (including induction period and growth rate) and particle size of the product. Clearly, with an increase of alkalinity the crystallization process is speeded up, the particle size is decreased, and the distribution of the particle size is narrowed due to an increased nucleation rate and an increased polymerization rate between polysilicate and aluminate anions. 

The critical mixing factors in a stirred tank at e impeller speed and type, as well as their influence on local turbulence and overall circulation. Since all aspects of these factors cannot be maintained constant on scale-up either locally or globally, the extent to which changes in the crystallizing environment will affect nucleation is extremely difficult to predict. To the mixing issue must be added the uncertainties caused by soluble and insoluble impurities that may be present in sufficiently different concentrations from batch to batch to cause variation in induction time, nucleation rate, and particle size. 

This helps to confirm that nucleation, crystallization rate, and spherulite size are strongly influenced by the presence of fillers. It is still uncertain what role a filler plays in the mechanism of nucleation. 

In molten state PETP consists of associates which may be the cores of formation of crystallization nuclei. Completion of crystallization process and crystal size depend very much on rates chains lay-up on the side of growing crystal and relaxation of large segments relation. 

BCMO can be polymerized in the solid state under the influence of ionizing radiation 23 25>. This is a handy method of preparing polymer directly from monomer, in the absence of solvent and initiators. Polymerizations proceed to limited conversions (maximum 15-20% was observed at temperatures just below the melting point Tm = 18.5 °C). The rate increases with increasing temperatures (up to the melting point), irradiation time, dose rate and the size of the monomer crystals. Molecular weights are relatively high, fn] = 0.5 dl/g (in cyclohexanone at 40 °C). 

Effect of Alkalinity on the Rate of Crystallization. The causes of the effect of alkalinity on the rate of crystallization have not been discussed in the literature. The authors (22) only reported that crystallization is catalyzed by excess alkali. Usually, an increase in alkali concentration in aluminosilica gels and aluminosilicate mixtures leads to a decrease in both the duration of crystallization and crystal sizes (32, 38). This can be explained by the growth of the rate of nucleation with increasing alkali concentration. Therefore, a greater number of nuclei should form during crystallization in a more alkaline medium. 

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