Crystallization in a Temperature Gradient

Derivations of expressions for a are limited to crystallization in a uniaxial temperature gradient [10,11] and require additional simplifying assumptions, like linear dependence of G on a spatial coordinate and 

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Among the three known crystalline structures (a, j3, y) of isotactic polypropylene (PP), the -modification is certainly the most fascinating one. While the stable a-structure develops under standard process conditions, the occurrence of the /J-form has to be forced (i) by directional crystallization in a temperature gradient field [1-3] (ii) by shear-induced crystallization [4-12] or (iii) by the addition of specific nucleating agents [13-31]. This latter technique is preferred at the industrial scale. 

Lovinger AJ, Chua JO, Gryte CC. Studies on the a and p forms of isotactic polypropylene by crystallization in a temperature gradient J Polym Sci B 1977 15 641-656. 

By beginning with methane, the diamonds formed have only in them. These tiny diamonds may then be used as the carbon source to form large (5 mm) single crystals by growth from molten catalyst metal in a temperature gradient. The resulting nearly pure crystals have outstanding thermal conductivities suitable for special appHcations as windows and heat sinks (24). 

The crude product contains a small amount of the less volatile tungsten(VI) dibromide dioxide as an impurity, which can be separated by sublimation at 120°C. under dynamic vacuum (10-3 torr). A yield of approximtely 15 g. (96%) of pure tungsten(VI) tetrabromide oxide is obtained after sublimation. Large crystals can be obtained by sublimation of the purified product in another tube with 1 mg. of bromine per milliliter of tube volume in a temperature gradient of 220/160°C. Anal. Calcd. for WOBr4 W, 35.40 Br, 61.52 O, 3.08. Found W, 35.37 Br, 61.56 O, 3.08. 

Tairov et al [14] were the first to propose Sn and Ga melts as solvents for SiC low temperature LPE (LTLPE). They used the usual graphite sliding boat to deposit SiC layers at temperatures between 1100 and 1400°C. Growth in a temperature gradient of 10°Ccm has been employed. Layers had single crystal structure and thicknesses from 0.5 to 4 pm. More recently, Dmitriev et al [16] reported 6H-SiC p-n junction growth and SiC selective growth at 1100- 1200°C by LTLPE. This is the minimum temperature ever achieved for SiC... 

The reaction is carried out in a sealed reactor tube (Fig. 307, II) in a high vacuum. Aluminum foil (for example, 50 mg.) is introduced at b, while TaCls (4-5 g.) in an ampoule is at f both are introduced under anhydrous conditions. The TaClg is made to sublime (in high vacuum) toward tube section d, and the tube is sealed off at constrictions a and e. The sealed tube is heated for 70 hours in a temperature gradient (see Fig. 306) such that b is at 400°C and the remainder of the tube at 200°C. The TaCl deposits at c as large crystals. The tube is allowed to cool and only section c is reheated to 200°C to remove any TaClg present in it. 

The crystallisation from melt in a temperature gradient or in contact with a surface acting as a nucleating substrate may result in a transcrystalline structure with the growth a.xis of the crystals, the h-axis in the case of polyethylene, the hydrogen bonds in the case of nylon parallel to the gradient or perpendicular to the substrate. 

Abe Y, Hosono H., Hosoe M., Iwase J., and Kub Y, "Superconducting Glass-Ceramic Rods in BiCaSrCu20jj Prepared by Crystallization under a Temperature Gradient," AppL Phys. Lett., 53, 1341-42 (1988). 

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