Nucleation-driven crystallization

C). The crystal growth is then disturbed by the impingement of crystallites with each other (D). Our schematic model is consistent with the nucleation-driven crystallization process (Zaou et al., 2000). 

The essential feature of PC materials is the ultrafast phase transition between amorphous and crystalline structures that occurs on a nanosecond time scale. In the previous sections, we have discussed extensively the amorphous and crystalline structures of GST and their properties. These correspond to the starting and end points for the actual phase transition, which are crucial to understand the function of PC materials. We now present results for the nucleation-driven crystallization process of GST using DF calculations combined with MD [31], A sample of fl-GST with 460 atoms was studied at 500, 600, and 700 K, and a second sample of 648 atoms was simulated at 600 K. In all cases we used a fixed crystalline seed (58 atoms, 6 vacancies) in order to speed up the crystallization process. More recent experience has shown that the time scale for the crystallization is of the order of several nanoseconds for these system sizes in the absence of a fixed seed, while those here are of the order of 0.3-0.6ns. This means that we cannot discuss the onset of nucleation, but this is also true in the case of smaller systems (<200 atoms) discussed by other groups. In very small systems, periodic boundary conditions bias the process severely. Our larger samples reduce finite-size effects, and we show the effect of choosing different annealing temperatures. Simulations of this scale (up to 648 atoms over 1 ns) are near the limit of present day DF/MD calculations. 

Creation of new crystals (nucleation) can take place through a variety of mechanisms. Figure 4-2 summarizes the types of nucleation which can occur. Some of these are true nucleation (driven essentially only by free energy considerations) others are hcavi ly dependent on imposed conditions (mixing and others). 

The patentee employs a pressure driven crystallization to form crystals from material dissolved in a supercritical fluid. A seed crystal acts as the heterogenous nucleation site for the precipitating material when the pressure is reduced. We don t know, however, bow the patentee intends to prevent homogeneous nucleation during the pressure driven crystallization. ... 

Hu W, Cai T (2008) Regime transitions of polymer crystal growth rates molecular simulations and interpretation beyond Lauritzen-Hoffman model. Macromolecules 41(6) 2049-2061 Hu W, Frenkel D (2004) Effect of metastable liquid-liquid demixing on the morphology of nucleated polymer crystals. Macromolecules 37(12) 4336-4338 Hu W, Frenkel D (2005) Polymer crystallization driven by anisotropic interactions. Adv Polym Sci 191 1-35... 

The entropically driven disorder-order transition in hard-sphere fluids was originally discovered in computer simulations [58, 59]. The development of colloidal suspensions behaving as hard spheres (i.e., having negligible Hamaker constants, see Section VI-3) provided the means to experimentally verify the transition. Experimental data on the nucleation of hard-sphere colloidal crystals [60] allows one to extract the hard-sphere solid-liquid interfacial tension, 7 = 0.55 0.02k T/o, where a is the hard-sphere diameter [61]. This value agrees well with that found from density functional theory, 7 = 0.6 0.02k r/a 2 [21] (Section IX-2A). 

Periodic reactions of this kind have been mentioned before, for example, the Liese-gang type phenomena during internal oxidation. They take place in a solvent crystal by the interplay between transport in combination with supersaturation and nuclea-tion. The transport of two components, A and B, from different surfaces into the crystal eventually leads to the nucleation of a stable compound in the bulk after sufficient supersaturation. The collapse of this supersaturation subsequent to nucleation and the repeated build-up of a new supersaturation at the advancing reaction front is the characteristic feature of the Liesegang phenomenon. Its formal treatment is quite complicated, even under rather simplifying assumptions [C. Wagner (1950)]. Other non-monotonous reactions occur in driven systems, and some were mentioned in Section 10.4.2, where we discussed interface motion during phase transformations. 

The processes of crystal nucleation and growth are driven by the basic laws of thermodynamics in which a greater free energy must exist in the original solution phase than the resultant... 

As noted in previous chapters, the influence of seeding on crystallization is often critical to control of a process, and its importance cannot be overemphasized. Many or most organic solutes in crystallizations driven by cooling will nucleate spontaneously if the cooling is sufficiently rapid. However, control of a process, especially on scale-up, that depends primarily on spontaneous nucleation can be subject to extreme process variation. 

Crystal slurry is classified (crystal size classification) by the circulation amount of the outer cycle which is controlled by the suction nozzles. Small crystals from the solution are entrained from the crystal bed and recycled to the inner circulation driven by the propeller pump 3, where supersaturation is reduced due to degradation of crystal fines. Different crystal concentrations and sizes in inner and outer suspension cycles are caused by different circulation amounts in each cycle. With known crystallization behavior the cycle may be controlled in a way, that in the inner cycle at a low suspension concentration and a relatively high supersaturation nucleation occurs. Otherwise, in the outer cycle at a longer residence time and smaller supersaturation crystal growth is favored. Crystal sizes 0.8-4 mm No crystal fines below 0.3 mm... 

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