Growth of Single Crystals from the Melt

For the most part, single crystals are grown from a melt of the compound, provided that melting the compound does not cause it to decompose. The 

Measuring particle size and growing single crystals 

Thermal conductivity of both liquid and solid phases. 

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Single crystals of many materials occur naturally (e.g. quartz and diamond). For other materials such as metals and semi-conductors the growth of single crystals from the melt is now a routine matter. With polymers it is only recently that true single crystals have been prepared. This can only be done by using the process of solid-state polymerization outlined in Section 2.11. The most perfect crystals are obtained with certain substituted diacetylene monomers which polymerize topochemi-cally to give polymer crystals which can be 100% crystalline and are of macroscopic dimensions. 

Potentials or gradients in concentration or other driving forces determine the direction of the reactions. If the potentials are low and the mobility of the atoms is high enough, potentials affect rates. These effects have been well studied and dominate the formation of solids if it takes place under conditions not far removed from thermodynamic equilibrium, e.g., the growth of single crystals from a melt or from solution. 

Another concept in synthesis is epitaxy. Epitaxy is the continuation of the crystal orientation of the monocrystalline substrate in the deposited crystalline product, which may be the same compound as the substrate or a different solid that has the same crystal orientation as the monocrystalline solid. Epitaxial layers are essential for microlithography in the electronic industry carefully formed epitaxial layers do not have localized electronic interface states, which are deleterious for the functioning of the device. The process conditions for epitaxy by molecular beam epitaxy (MBE) are very low process pressure, comparatively high temperatures, and a low growth rate. MBE is a form of CVD, which was described in Chapter 6. Liquid phase epitaxy (LPE) is a form of growth of single crystals from a melt. 

While the growing of single crystals from the melt or the gas pha.se may sometimes be important for industrial purposes (as in the crucible pulling method used for semiconductors such as Si and Ge), in X-ray structure analysis such methods are secondary to crystal growth from solution, and they are employed only when other techniques are not possible. 

Real polymer processes involved in polymer crystallization are those at the crystal-melt or crystal-solution interfaces and inevitably 3D in nature. Before attacking our final target, the simulation of polymer crystallization from the melt, we studied crystallization of a single chain in a vacuum adsorption and folding at the growth front. The polymer molecule we considered was the same as described above a completely flexible chain composed of 500 or 1000 CH2 beads. We consider crystallization in a vacuum or in an extremely poor solvent condition. Here we took the detailed interaction between the chain molecule and the substrate atoms through Eqs. 8-10. 

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... 

B. G. Zaslavsky, Automated Pulling of Large-Diameter Alkali Halide Scintillation Single Crystals from the Melt, J. Cryst. Growth 200 (1999) 476-482. 

Some purification methods are connected with the growth of single crystals. On the whole, the growth of organic crystals is an independent process which must be carried out with great care. There are growth methods starting from the gas phase, from the melt and from liquid solutions, as well as electrocrystallisation. 

Symons NKJ (1963) Growth of single crystals of polytetrafluoroehylene from the melt. J Polym Sci 1 2843... 

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