Crystallization from melts

We carried out thermodynamic studies on the crystallization from melts of flexible-chain polymers uniaxially stretched at various degrees of molecular orientation in the melt and studied the effect of the stretching stress on thermodynamic parameters such as degree of... 

Figure 2.30 X-ray powder diffraction patterns of sPP samples isothermally crystallized from melt at indicated temperatures.172 Indices of 211 and 020 reflections are given for unit cell of form I of Figure 2.29a.

Wintermantel, K. Chem. Ing. Tech. 58 (1986) 6, 498-499. Effective separation power in freezing out layers of crystals from melts and solutions - a unified presentation. 

Considerable effort has been expended in growing single crystals of compositions in the solid solution, with particular emphasis on the superconducting composition. A variety of methods have been employed, all flux type, as the solid solution members are not congruently melting. There have been a few detailed reports on growth of single crystals from melts in the... 

Fig. 8. Schematic representation of the folded chain model of crystalline domains in thermoplastics. a Lamella crystallized from dilute solution (multiple/adjacent re-entry) and b Lamella crystallized from melt (single entry/far distance re-entry)...

The most common methods of growing crystals involve solidification from the melt (in the case of one-component systems) or crystallization from solution. Some of the methods for growing crystals from melt are described schematically in Fig. 3.6. In the Czochralski method, commonly known as the pulling technique, the material is melted by induction or resistance heating in a suitable nonreactive crucible. The melt temperature is adjusted to slightly above the melting point and a seed crystal is dipped into the melt. After thermal equilibration is attained, the seed is slowly lifted from the... 

Another method for growing crystals from melt is the floating zone technique in which a section of the starting material, held vertically in the form of a rod, is melted by... 

Frequently, growth of crystals from melt involves more than one component, such as impurities, intentionally added dopants, etc., in addition to the major component. In these cases, it is essential to know the distribution of the second component between the growing crystal and the melt. This distribution occurs according to the phase diagram relating the equilibrium solubilities of the second component (impurity) in the liquid and the solid phases. 

Common igneous processes (partial melting and fractional crystallization) lead to element fractionations. Incompatible elements tend to be concentrated in melts and compatible elements in solids. Separation of partial melts from residual crystals as the melts ascend to higher levels, or accumulation of early-formed crystals from melts, ultimately produces rocks with compositions different from the starting materials. These processes account for the fractionations seen in differentiated meteorites and planetary samples. 

Fig. 15 Diffraction patterns of ethylene-1-octene copolymer (5.2 mol %) shown from 100 °C to 25 °C while cooling at 10 °C/min recorded during crystallization from melt at 3.8 kbar. The open-orthorhombic phase appears at 80 °C, intensity and position of this reflection remains unchanged. The open-orthorhombic phase is followed by the incoming of the (100) monoclinic reflection concomitant with a shift to higher angles and drop in the intensity of the (110) dense-orthorhombic reflection. The X-ray wavelength used for these experiments is 0.744 A...

Several other anhydrous calcium aluminosilicates are known, including grossular or garnet (C3AS3), which is a high-pressure phase, various dehydration products of zeolites, and various products formed metastably by crystallization from melts or glasses. Most are too acid in composition to be of clear relevance to cement chemistry, but some of the devitrification products, especially those with compositions near to CA and structures similar to those of nepheline (Na3KAl4Si40i6) or kalsilite (KAlSiOj (Y4), are of possible interest in relation to the formation of calcium aluminate cements. 

Temperature has also been studied as a variable in determining nucleation rate, particularly in crystallization from melts or from solutions maintained at constant supersaturation. Figure 10 shows a typical result, obtained with a piperine melt by Tammann (Tl) and interpreted by Coulson and Richardson (C5). Here, as temperature is lowered, the degree of supersaturation and driving force causing nucleation are increased, causing a steep rise in the curve. A maximum is attained, how-... 

The conventional methods of mullite preparation are the following crystallization from melt, high-temperature sintering of oxides, high-temperature annealing of kaolin [32]. Mechanical activation of hydrated oxides brings to decrease of mullite synthesis temperature [32-39]. 

Krot A. N., Huss G. R., and Hutcheon 1. D. (2001b) Corundum-hibonite refractory inclusions from Adelaide condensation or crystallization from melt Meteorit. Planet. Set 36, A105. 

In the oceanic setting, spinel Iherzolite xenoliths from Pali (Hawaii) have olivine 5 0 values of 5.09-5.12 per mil, typical of olivines from other oceanic and continental mantle rocks (Ducea et al., 2002). In contrast, olivines from plagioclase peridotites are enriched by 0.5 per mil. This is interpreted to be due to the formation of plagioclase by reaction with or crystallization from melts intruding the Pacific lithospheric mantle. 

In calculating the Avrami exponent from DSC data, it was observed that fractional Avrami exponents are obtained, and not whole numbers, as is observed for the application of the equation to metal melts for which the theory was developed or in the majority of cases for polymeric crystallization from melts. In certain instances the practice of rounding off the Avrami exponent value could lead to ambiguity, especially where rounding off intermediate values between two integers, e.g., where exponents of 2.45 or 2.48 are obtained. 

High temperature methods used in synthetic solid state chemistry [3] such as solid-solid reactions, crystallization from melts and vapour-phase transport are not appropriate for the preparation of compounds of low thermal stability. 

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