Mechanism of crystallization

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],... 

Nucleation The mechanism of crystal nucleation from solution has been studied by many scientists, and recent work suggests that—in commercial crystallization equipment, at least—the nucleation rate is the sum of contributions by (1) homogeneous nucleation and (2) nucleation due to contaci between crystals and a) other crystals, h) the walls of the container, and (c) the pump impeller. If is the net number of new crystals formed in a unit volume of solution per unit of time. 

Strickland-Constable, R.F. (1968) Kinetics and Mechanism of Crystallization. Acad. Press. 

In the world of molecular simulation, it would be more conventional to consider that the present model is a coarse grained model of real polymers, where the real time-scale is much longer than that of the present MD simulation time-scale. However, we did not intend to make a coarse grained model. The crystallization of polymers was shown to be rather universal. Various kinds of polymers, either fast crystallizing or slow crystallizing, were known to follow the same scheme with respect to the molecular mechanism of crystallization. So we studied this simple model expecting that the present model would also follow the same crystallization scheme and show the general molecular mechanisms of polymer crystallization. 

But there are two important unresolved questions regarding the mechanism of crystallization the first question is are the primary nuclei actually formed from the earliest stage ( induction period ) of crystallization and the second is what is the role of the topological nature of polymers in the polymer crystallization mechanism ... 

Figure 2.48 Possible cooperative mechanism of crystal-crystal transition from (a) trans planar form III of sPP into (c) isochiral helical form II. Transition occurs through formation of (b) intermediate conformationally disordered modifications containing kink bands, characterized by helical sequences having same chirality (R = right-handed helix). Formation of helical sequences of opposite chirality (right- and left-handed) produces (if) steric interactions between neighboring chains.

Aluminium distribution in A and B1 intermediate and final phases Various compositional zonings have been reported in silica-rich ZSM-5 zeolites prepared under particular conditions (11-13,19,26, 27,31,32,59,68) while other studies provided evidence for homogeneous A1 distribution throughout the crystallites (69-71). Obviously, the distribution of aluminium in ZSM-5 must depend on its mechanism of crystallization. 

Melting Mechanisms of Crystals (Ubbelohde). Metal-Ammonia Systems, Structure and Properties of... 

The properties of deposits may be controlled by changing the kinetics of the deposition and the mechanism of crystallization. One way to achieve this is by complexing the depositing ions, as stated above. 

For solution on the other hand no such difficulties occur. It has already been noted that the edges of the planes are already stripped under conditions of equilibrium a small decrease in the concentration of the solution simply extends the edges further towards the centre of the plane until it is stripped entirely. The mechanism of crystal growth thus presupposes a finite degree of super-saturation, this no doubt varies with the surface energies of the crystal faces and will probably be greatest for the surfaces for which <7,1 is least,... 

Terkeltaub RA. Gout and mechanisms of crystal-induced inflammation. Curr Opin Rheumatol 1993 5 510-516. 

Crystal form is the direct result of crystal growth, and we will therefore develop our arguments based on the mechanism of crystal growth. The book consists of two parts Part I presents the fundamental concepts, and Part II deals with the application of these concepts to complicated and complex systems (by looking at case studies). 

Kossel [8] and Stranski [9] were the first to focus attention on the interface structure, after considering the experimental results obtained by Volmer [10], who demonstrated the existence of surface diffusion. It thus became possible to discuss the mechanism of crystal growth at an atomic level, starting from these analyses. 

I. Sunagawa and A. Urano, Beryl crystals from pegmatites morphology and mechanism of crystal growth,/. Gemmol., 26,1999, 521-33... 

I. Sunagawa, Mechanism of crystal growth, etching and twin formation of hematite. Mineral. J 3,1960, 50-89... 

As well known, the crystalline structure of polymers is generally characterized by a comparatively high proportion of defects compared with the case of low molecular weight substances these defects may be due either to chemical faults or may be simply attributed to the mechanism of crystallization. As a consequence, this means that most polymers can contain a small proportion of extraneous units in the crystal state. However, we will not consider as real cases of macromolecular isomorphism those having a concentration of either component below 5%. 

As part of the overall scale-inhibiting threshold effect, the mechanism of crystal growth retardation is complemented by crystal distortion, whereby the crystal fails to successfully agglomerate into a larger crystalline scale. 

We also know very little about the chemical and physical interaction of the plasma with solid surfaces. Special attention must be paid to the problems of nuclea-tion — heterogeneous as well as homogeneous — and to the mechanism of crystal growth under plasma conditions. 

W. F. Hosford. The Mechanics of Crystals and Textured Polycrystals. Oxford, U.K. Oxford Science, 1993. 

An analysis of the mechanism of crystallization can best be carried out at a constant temperature. A sample is rapidly cooled down from the melt to a temperature below Tm and kept constant at that level. The crystallization process can then be studied by measuring the volume as a function of time in a dilatometer. The result is a curve as shown in Figure 4.9. The volume approaches to an equilibrium value at which the maximum possible crystallization is reached, and the rate of crystallization can, for example, be expressed as l/to.5, in which fo.5 is the time in which half of the route is... 

In order to test the effect of the shape of crystals, a sodium chloride crystal with (111) and (-1-1-1) faces, together with 100 faces, has been examined. In the crystal 28 cations and 28 anions are included, and the (111) face consists of only sodium ions, and the (-1-1-1) face of chloride ions. No significant difference has in fact been found in the dissolution mechanism of crystals with different faces. 

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