From melts nucleation

Many authors studying the formation of ECC from melts and solutions suggested that preliminary unfolding and extension of macromolecules occurs. Keller and Maehin25 have shown that in all known cases (including such extreme variants as the crystallization of natural rubber under extension and a polyethylene melt under flow) the same initial process of linear nucleation occurs and fibrillar structures is formed by the macromolecu-lar chains oriented parallel to the fibrillar axes27. ... 

The central role of imperfections in mechanistic interpretations of decompositions of solids needs emphasizing. Apart from melting (which requires redistribution of all crystal-bonding forces, by a mechanism which has not yet been fully established) the decompositions of most solids involve the participation of atypical lattice constituents, structural distortions and/or surfaces. Such participants have, in particular instances, been identified with some certainty (e.g. excitons are important in the decompositions of some azides, dislocations are sites of nucleation in dissociations of a number of hydrates and carbonates). However, the... 

Bulk Ag-Al alloys, containing up to 12 a/o Al, were electrodeposited from melt containing benzene as a co-solvent. Examination by x-ray diffraction (XRD) indicated that the low-Al deposits were single-phase fee Ag solid solutions whereas those approaching 12 a/o were two-phase, fee Ag and hep i>-Ag2Al. The composition at which ti-Ag2Al first nucleates was not determined. The maximum solubility of aluminum in fee silver is about 20.4 a/o at 450 °C [20] and is reduced to about 7 a/o at room temperature. One would expect the lattice parameter of the fee phase to decrease only slightly when aluminum alloys substitutionally with silver because the... 

AIChESymp. Ser. (a) 65 (1969) no. 95, Crystallization from solutions and melts (b) 67 (1971) no. 110, Factors affecting size distribution (c) 68 (1972) no. 121, Crystallization from solutions Nucleation phenomena in growing crystal systems (d) 72 (1976) no. 153, Analysis and design of crystallisation processes (e) 76 (1980) no. 193, Design, control and analysis of crystallisation processes (f) 78 (1982) no. 215, Nucleation, growth and impurity effects in crystallisation process engineering (g) 80 (1984) no. 240, Advances in crystallisation from solutions. 

On a nucleation rate versus pressure diagram (Figure 4-2c), melt nucleation rate below the crystal-melt equilibrium pressure and crystal nucleation above the pressure are roughly S3mimetric. In Equation 4-9, only AG would vary with pressure or concentration. Hence, both melt nucleation rate and crystal nucleation rate increase monotonically with departure from equilibrium. There is no peak nucleation rate. 

Bubble Nucleation in a Liquid Phase The above classical nucleation theory can be easily extended to melt nucleation in another melt. It can also be extended to melt nucleation in a crystal but with one exception. Crystal grains are usually small with surfaces or grain boundaries. Melt nucleation in crystals most likely starts on the surface or grain boundaries, which is similar to heterogeneous nucleation discussed below. Homogeneous nucleation of bubbles in a melt can be treated similarly using the above procedures. Because of special property of gases, the equations are different from those for the nucleation of a condensed phase, and are hence summarized below for convenience. 

A similar expression is found for nucleation rate from melt systems, except the driving force is written in terms of the subcooling, as seen in Equation (4.5). 

Crystallization can occur from melts, solutions, or vapors. Since crystallization from aqueous solutions is most pertinent to chemical engineering, this aspect of the general topic is stressed in the following presentation. Historical and descriptive material are minimized. The fundamental principles underlying solubility, nucleation, and crystal growth are presented first, followed by a brief discussion of their application in modem practice, so that the reader may be apprised of recent significant advances in the design and operation of crystallization equipment. 

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-... 

Fia. 10. Nucleation of piperine from melt. After Coulson and Richardson (C5). 

Crystal Geometry Principles of Crystallization Equilibria and Yields Nucleation Crystal Growth Crystallization Equipment Applications of Principles to Design MSMPR Crystallizer Crystallization from Melts Symbols Problems References... 

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