Factors enhancing crystallization rate

Let us now And out how the system works. Assume that it starts at a large reduced flow-rate (point A) and reduce the input slowly. Up to the point C, any deviation from the equilibrium curve will die out rapidly. At C, concentration fluctuations become unstable and the system evolves quite rapidly towards D (p is fixed) where it finds a stable steady-state. The system has become unstable because reducing the flow-rate enhances crystallization which through the kinetic factor enhances the rate of precipitation and thereby depletes the residual liquid. The system quenches. Upon reducing the flow-rate further, the stable evolution continues towards point E. 

Mirskii and Pirozhkov [66] reported on experiments in which seed crystals were added to normal batch zeoUte synthesis mixtures. In one set of experiments, different amounts of seed crystals of a desired phase (not specifically mentioned, but probably zeoUte NaA) were added to the synthesis mixture, and noted to ehminate the induction time and increase the rate of crystallization. Two additional factors were investigated and reported a) the rate of crystallization increased more with increased amounts of seed crystals added, and b) the rate of crystallization was enhanced more using the same mass of smaller seed crystals than with larger seed crystals. Both of these results were concluded to imply that the rate enhancement was due to the cumulative seed crystal surface area used to assimilate material from the solution. This point was illustrated further by adding seed crystals of one phase to a solution which nominally produced a different zeoHte phase. For example, zeoUte NaP seed crystals were added to a synthesis mixture, which was demonstrated to precipitate zeolite NaX, after about 30 % of the amorphous reagents had already crystalUzed. After two additional hours of crystallization, the absolute amoimt of zeoUte had... 

From the above results, it could be concluded that the addition of excess amount of sodium ions into the crystallization system has apparent effect on the particulate properties of the product. At low batch alkalinity, the additional sodium ions causes de-aggregation of the final products, rendering the particles with more uniform size distributions. At high batch alkalinity, the excess amount of sodium ions triggers the surface condensation reactions on the crystalline end products. However, the crystallization rate is not enhanced by the increase of batch sodium ion content, indicating that the determining factor of crystallization of ZSM-5 zeolites in SDA-free system is concentration of low molecular weight silicate species, determined by batch alkalinity. 

Further evidence for the influence of chain entanglement in the crystallization process is found in the crystallization of an isotactic poly(styrene) sample that was prepared from a freeze-dried dilute benzene solution.(38a) Entanglements in such a sample will be miiumal. The overall crystallization rate in such samples, in terms of ii/2, is enhanced by a factor of seven to eight relative to conventional crystallization from the pure melt.(38a) Experiments with isotactic poly(propylene), freeze-dried from n-octane, showed a similar enhancement in the crystallization rate.(38b) This type of experiment complements the overall crystallization rate of polymers from dilute solution. 

ZnO is, apparently, a very suitable support for the copper particles. Evidence exists, however, that its role does not have to be limited to that of a support only. Nakamura et al. have studied the influence of Zn on methanol synthesis on copper crystals by depositing Zn on the surface [J. Nakamura, I. Nakamura, T. Uchijima, Y. Kanai, T. Watanabe, M. Saito, and T. Fujitani, J. Catal. 160 (1996) 65]. They found that the rate was enhanced by a factor of six (see Fig. 8.14), suggesting that Zn atoms also act as a chemical promoter. Whether some of the ZnO in the real catalyst is actually reduced to such a degree that it can alloy into the copper particles and segregate to the surface, as suggested by Nakamura, is still a controversial topic. 

According to the equation, the diffusional creep rate of a polycrystal may be enhance by reducing the crystal size cl, and by increasing the boundary diffusivity I. Nanoceramics are therefore expected to exhibit enhanced diffusional creep for two reasons first, the reduction of the crystal size from about 10 pm to -10 nm enhances the creep rate by a factor of 109, and second, the enhanced boundary diffusivity may increase the creep rate by 103, so that the total enhancement is 1012. 

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