Crystallization volume

Chayen, N. E., Stewart, P. D. S. and Blow, D. M. (1992). Microbatch crystallization under oil - a new technique allowing many smaU-volume crystallization trials. J. Crystal Growth 122,176-180. 

Thus, a plot of ln[— ln(l — x)] versus In t yields a straight line of slope n. The value of n is important not only because it indicates the reaction order, but also because it has some physical interpretation, depending on whether the crystals are growing at the surface of a material or in the bulk (sometimes called volume crystallization). See Table 3.1 for a description of the dimension of the crystals for various values of n. The intercept of this plot gives the rate constant, k. Obtaining k at different... 

Many structures are included from Wyckoff s volumes, Crystal Structures, and Pearson s The Crystal Chemistry and Physics of Metals and Alloys. Refinements are available for some of these structures. The descriptions and figures of these and other some older sources are good for visualizing and understanding the structures. That is important and CrystalMaker has been a great aid in this regard. 

Figure 10.17(a) Comparison of the growth rates of potassium hydrogen tartrate (KHT) occurring in a fluidized bed and in a stirred vessel, as reported by Ratsimba and Laguerie (1991). Initial (10.42) crystal mean size Lj = 1.125 x 10 m, ethanol content = 10% volume, crystallization temperature T = 273 K. 

Wright, A. R., and Rees, S. A. (1998). Cardiac cell volume Crystal clear or murky waters A comparison with other cell types. Pharmacol. Then 80,89-121. 

This kaleidoscope of contemporary research interests reveals that another distinctive feature of supramolecular chemistry is its ability to unite areas with seemingly widely differing perceptions. In keeping with such a feature, structural chemists and crystallographers have had little difficulty in recognizing a molecular crystal as the ultimate example of a supermolecule. Consequently, supramolecular chemistry today encompasses the study of molecular crystals with all the applications and ramifications that such study implies in the fields of solid-state chemistry, crystal engineering and materials science. This then is the theme of this volume. Crystals constitute one end of the supramolecular continuum and may be viewed as hard supermolecules in contrast to the softer supramolecular aggregates which exist in solution. 

A crystal is a solid having a regularly repeating internal arrangement of atoms. Crystal structure is the mutual arrangement of atoms, molecules, or ions packed together on a lattice to fonn a crystal.In the context of this volume, crystals are ordered supramolecular systems, and crystallization is an impressive display of supramolecular self-assembly in a periodic arrangement. 

In constructing the ITT diagram (101), if it is assumed that the nucleation and growth rates are constant, which is applicable when the volume crystallized is small, the volume fraction of crystals produced by a given thermal history can be ealculated by Avrami s equation (102),... 

The mechanism of volume crystal nucleation is predominantly applied in the development of glass-ceramics. The development of the first glass-ceramic by Stookey (1959) demonstrated how crystals of uniform size can be precipitated in the glass matrix with controlled volume nucleation. 

By utilizing phase separation in the base glass, volume crystallization can be achieved at an earlier stage or delayed because of changing the composition of the matrix phase. Thus, surface crystallization or uncontrolled volume crystallization can be suppressed or avoided. 

The preferential crystallization mechanism is that of volume crystallization. However, surface reactions cannot be neglected when considering crystallization and nucleation in powder compacting and subsequent sintering and crystallization. In these processes, water has a special effect on the production of lithium disilicate glass-ceramics, as demonstrated by Helis and Shelby (1983) and Davis (1997). 

Heat treatment of glass powders results in the precipitation of leucite from the surface of the glass-ceramics and the volume crystallization of needlelike apatite. 

Kokubo et al. (1969) studied the crystallization mechanism of the above glass-ceramic in detail. They found that a metastable benitoite-type crystal, BaTiSi Og, was formed at 800°C, according to a mechanism of surface crystallization. Beginning at 950 C, the crystal was progressively transformed into the stable barium titanate and hexacelsian. The hexacelsian crystal grew anisotropically and parallel to the surface of the glass-ceramic. The preferred orientation of hexacelsian was attributed to the metastable benitoite-type crystals. Moreover, volume crystallization took place at the same time as surface crystallization. In volume crystaUization, however, barium titanate and hexacelsian were formed as primary crystals. 

The DTA method is also very usefiil to distinguish between surface and volume crystallization. Ray et al. (1996) demonstrated in different types of glasses that the height of the exothermic peak decreased with increasing particle size when surface crystallization is the dominant mechanism. But on the other hand, the peak intensity will increase with increasing particle size when volume crystallization of glasses is dominant. 

The main crystal phase of the Zerodur glass-ceramic is composed of P-quartz solid solution This phase is produced by volume crystallization. Special thermal properties devdop as a result of the formation of very small crystallites and the simultaneous high crystallite count per unit of glass volume. 

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