Crystallization mechanism regimes

During the initial crystallization, the reaction is exothermic however, at some point, it switches to an endothermic regime. As an increase in pH is directly related to the exo-endothermic switch, the second crystallization mechanism is likely to be related to coalescence of small crystals into larger particles, leading to a lower surface area and thus a higher surface charge density. 

Several features of secondary nucleation make it more important than primary nucleation in industrial crystallizers. First, continuous crystallizers and seeded batch crystallizers have crystals in the magma that can participate in secondary nucleation mechanisms. Second, the requirements for the mechanisms of secondary nucleation to be operative are fulfilled easily in most industrial crystallizers. Finally, low supersaturation can support secondary nucleation but not primary nucleation, and most crystallizers are operated in a low supersaturation regime that improves yield and enhances product purity and crystal morphology. 

At very low temperatures, Holstein predicted that the small polaron would move in delocalized levels, the so-called small polaron band. In that case, mobility is expected to increase when temperature decreases. The transition between the hopping and band regimes would occur at a critical temperature T, 0.40. We note, however, that the polaron bandwidth is predicted to be very narrow ( IO Viojo, or lO 4 eV for a typical phonon frequency of 1000 cm-1). It is therefore expected that this band transport mechanism would be easily disturbed by crystal defects. 

Measurements of the lifetimes of NIESST states by time-resolved MES [51] and of LIESST states by time-resolved optical spectroscopy [52] on the very same system (a single crystal of [FC , Mni , (bpy)3] (bpy = bipyridine)) gave similar results. This supports the suggestion that the mechanisms for LIESST and NIESST relaxation are very similar, at least for the low-energy regime. The NIESST effect was also studied in Co(ll) SCO compounds, viz. [ Co/Co(terpy)2]X2 H20 (X = [CIOJ, n = V2,X = Cl , n = 5), where terpy is the tridentate ligand terpyridine... 

The crystallization kinetics of commercial polyolefins is to a large extent determined by the chain microstructure [58-60]. The kinetics and the regime [60] of the crystallization process determine not only the crystalline content, but also the structure of the interfaces of the polymer crystals (see also Chapter 7). This has a direct bearing on the mechanical properties like the modulus, toughness, and other end use properties of the polymer in fabricated items such as impact resistance and tear resistance. Such structure-property relationships are particularly important for polymers with high commercial importance in terms of the shear tonnage of polymer produced globally, like polyethylene and polyethylene-based copolymers. It is seen that in the case of LLDPE, which is... 

Using secondary nucleation analysis, Huang and Chang [51] found PTT to go through a transition in the multiple nucleation mechanisms from regimes II to III at around 194 °C. However, Lee et al. [50] found only regime II crystallization between 180 and 200 °C. 

Dynamic atomic-resolution ETEM and diffraction studies provide fundamental insights into the catalyst precursor transformation mechanism. The studies reveal that the temperature regimes are critical to the formation of active catalysts. They show that the nature of the VHPO -> VPO transformation is topotac-tic. Topotaxy is defined as the conversion of a single crystal to a pseudomorph... 

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