Nucleation, crystal growth

Polymers crystallize from the molten state by the two-step process of nucleation and crystal growth. Nucleation initiates crystallization, followed by the addition of linear chain segments to the crystal nucleus. 

Another important development which altered our view of crystallization processes was the realization of the importance of secondary nucleation due to contact between crystals and the impeller and vessel. Secondary nucleation of this type has been shown (2-6) to often have a dominant role in determining crystallizer performance. Our understanding of crystal growth, nucleation, fluid mechanics and mixing have all greatly improved. A number of review (2r 101 have appeared in recent years which describe the advances in these and... 

Figure 1-21 Relationships Among Crystal Growth, Nucleation, and Crystallization Rate between Melting Temperature (Tm) and Glass Temperature (Tg)...

Mechanism of crystal growth Nucleation sporadic in time (primary nucleation)... 

Keywords crystallization, crystal growth, nucleation, thermodynamics, nano-ciystals... 

One of the critical operating junctions in OFET devices is the source and drain contact to the channel. Any barrier between the contacts and the channel will appear in series and impede the flow of charge through the device. The source and drain electrode formation and structure can also influence the properties of the transistor channel itself. Crystal growth nucleated on the source and drain and the processes used to pattern the source and drain electrodes can have a significant effect on overall device performance. 

A mathematical model of Silicaiite crystallization from clear solution was developed and solved numerically. Crystallization was considered to be solution-mediated, and to involve three categories of silica species and three steps activation, nucleation, and crystal growth. Nucleation was represented by a condensation reaction between the hydroxyl groups present on the surface of 10 nm amorphous silica particles and the soluble silica species, producing activated complexes, which transform to crystalline nuclei. Both nucleation and crystal growth were considered to be reaction controlled. The model simulated well the in situ experimental data. This suggests that the hypothesized nucleation mechanism can be used to qualitatively describe the nucleation event during clear solution Silicaiite crystallization. 

The crystallization process consists of a nucleation phase followed by crystal growth. Nucleation is the process where, at random locations throughout the liquid. 

Crystallization is the competition between two processes nucleation and crystal growth. Nucleation is the formation of small sites (nuclei) from which crystallites can grow. Primary nucleation creates the initial nuclei. Crystallites develop aroimd these nuclei. Then in secondary nucleation, the surfaces of the crystallites are nucleated. More polymer chains diffuse to the crystallite surfaces and growth continues. 

Equation (6.29) predicts that nucleation occurs more readily at lower crystallization temperatures because of the lower critical nucleus size and the lower free energy barrier associated with the crystallization process. As with the case of simple crystal growth, nucleation can take a number of forms ... 

The crystallization process involves two basic steps, nucleation and crystal growth. Nucleation starts with small, nanometer-sized areas where, as a result of heat motion, some chains or their segments arrange parallel to one another. Those nuclei, or seeds , can either dissociate, if thermal motion destroys the molecular order, or grow further, if the nuc-... 

The crystallization process consists of two major events, nucleation and crystal growth. Nucleation is the step where the solute molecules dispersed in the solvent start to gather into clusters, on the nanometer scale (elevating solute concentration in a small region), that becomes stable under the current operating conditions. These stable clusters constitute the nuclei. However when the clusters are not stable, they redissolve. 

Dislocation theory as a portion of the subject of solid-state physics is somewhat beyond the scope of this book, but it is desirable to examine the subject briefly in terms of its implications in surface chemistry. Perhaps the most elementary type of defect is that of an extra or interstitial atom—Frenkel defect [110]—or a missing atom or vacancy—Schottky defect [111]. Such point defects play an important role in the treatment of diffusion and electrical conductivities in solids and the solubility of a salt in the host lattice of another or different valence type [112]. Point defects have a thermodynamic basis for their existence in terms of the energy and entropy of their formation, the situation is similar to the formation of isolated holes and erratic atoms on a surface. Dislocations, on the other hand, may be viewed as an organized concentration of point defects they are lattice defects and play an important role in the mechanism of the plastic deformation of solids. Lattice defects or dislocations are not thermodynamic in the sense of the point defects their formation is intimately connected with the mechanism of nucleation and crystal growth (see Section IX-4), and they constitute an important source of surface imperfection. 

Once nuclei form in a supersaturated solution, they begin to grow by accretion and, as a result, the concentration of the remaining material drops. There is thus a competition for material between the processes of nucleation and of crystal growth. The more rapid the nucleation, the larger the number of nuclei formed before relief of the supersaturation occurs and the smaller the final crystal size. This, qualitatively, is the basis of what is known as von Weimam s law [86] ... 

The mechanism of crystal growth has been a topic of considerable interest. In the case of a perfect crystal, the starting of a new layer involves a kind of nucleation since the first few atoms added must occupy energy-rich positions. Becker and Doring [4],... 

The kinetics of crystal growth has been much studied Refs. 98-102 are representative. Often there is a time lag before crystallization starts, whose parametric dependence may be indicative of the nucleation mechanism. The crystal growth that follows may be controlled by diffusion or by surface or solution chemistry (see also Section XVI-2C). 

In general, zeohte crystallization consists of three stages (/) formation of precursors, ie, building blocks that can generate nuclei (2) nucleation and (J) crystal growth. 

Over 50 acidic, basic, and neutral aluminum sulfate hydrates have been reported. Only a few of these are well characterized because the exact compositions depend on conditions of precipitation from solution. Variables such as supersaturation, nucleation and crystal growth rates, occlusion, nonequilihrium conditions, and hydrolysis can each play a role ia the final composition. Commercial dry alum is likely not a single crystalline hydrate, but rather it contains significant amounts of amorphous material. 

Physical properties of the acid and its anhydride are summarized in Table 1. Other references for more data on specific physical properties of succinic acid are as follows solubiUty in water at 278.15—338.15 K (12) water-enhanced solubiUty in organic solvents (13) dissociation constants in water—acetone (10 vol %) at 30—60°C (14), water—methanol mixtures (10—50 vol %) at 25°C (15,16), water—dioxane mixtures (10—50 vol %) at 25°C (15), and water—dioxane—methanol mixtures at 25°C (17) nucleation and crystal growth (18—20) calculation of the enthalpy of formation using semiempitical methods (21) enthalpy of solution (22,23) and enthalpy of dilution (23). For succinic anhydride, the enthalpies of combustion and sublimation have been reported (24). 

Scaling is not always related to temperature. Calcium carbonate and calcium sulfate scaling occur on unheated surfaces when their solubiUties are exceeded in the bulk water. Metallic surfaces are ideal sites for crystal nucleation because of their rough surfaces and the low velocities adjacent to the surface. Corrosion cells on the metal surface produce areas of high pH, which promote the precipitation of many cooling water salts. Once formed, scale deposits initiate additional nucleation, and crystal growth proceeds at an accelerated rate. 

Supersaturation has been observed to affect contact nucleation, but the mechanism by which this occurs is not clear. There are data (19) that infer a direct relationship between contact nucleation and crystal growth. This relationship has been explained by showing that the effect of supersaturation on contact nucleation must consider the reduction in interfacial supersaturation due to the resistance to diffusion or convective mass transfer (20). 

Although magma density is a function of the kinetic parameters fP and G, it often can be measured iadependentiy. In such cases, it should be used as a constraint ia evaluating nucleation and growth rates from measured crystal size distributions (62), especially if the system of iaterest exhibits the characteristics of anomalous crystal growth. 

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