Crystallization stability

The compositions with the highest germanium content in this ternary have the highest glass transition temperature, Tg, and near the highest crystallization temperatures, giving them much greater 

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Sodium terbium borate is used in solid-state devices. The oxide has potential application as an activator for green phosphors used in color TV tubes. It can be used with Zr02 as a crystal stabilizer of fuel cells which operate at elevated temperature. Few other uses have been found. 

HTS catalyst consists mainly of magnetite crystals stabilized using chromium oxide. Phosphoms, arsenic, and sulfur are poisons to the catalyst. Low reformer steam to carbon ratios give rise to conditions favoring the formation of iron carbides which catalyze the synthesis of hydrocarbons by the Fisher-Tropsch reaction. Modified iron and iron-free HTS catalysts have been developed to avoid these problems (49,50) and allow operation at steam to carbon ratios as low as 2.7. Kinetic and equiUbrium data for the water gas shift reaction are available in reference 51. 

Liquid crystals stabilize in several ways. The lamellar stmcture leads to a strong reduction of the van der Waals forces during the coalescence step. The mathematical treatment of this problem is fairly complex (28). A diagram of the van der Waals potential (Fig. 15) illustrates the phenomenon (29). Without the Hquid crystalline phase, coalescence takes place over a thin Hquid film in a distance range, where the slope of the van der Waals potential is steep, ie, there is a large van der Waals force. With the Hquid crystal present, coalescence takes place over a thick film and the slope of the van der Waals potential is small. In addition, the Hquid crystal is highly viscous, and two droplets separated by a viscous film of Hquid crystal with only a small compressive force exhibit stabiHty against coalescence. Finally, the network of Hquid crystalline leaflets (30) hinders the free mobiHty of the emulsion droplets. 

Table 3.1-5 Melting points and heats of fusion for isomeric [BMIM][PFg] and [PMIM][PFs] ionic liquids, showing melting point and crystal stability increasing with the degree of branching in the alkyl substituent.

Solid solutions are more rare. Crystals are stable because of the regularity of the positioning of the atoms. A foreign atom interferes with this regularity and hence with the crystal stability. Therefore, as a crystal forms, it tends to exclude foreign atoms. That is why crystallization provides a good method for purification. 

Formal sequential addition of amino groups to 2,4,6-trinitroaniline gives 1,3-diamino-2,4,6-trinitrobenzene (DATB, 56) and 1,3,5-triamino-2,4,6-trinitrobenzene (TATB, 57). TATB is more stable than expected from the additivity calculation. The ability to have hydrogen bonding with three amino groups both intra- and inter-molecularly in the crystal stabilizes the molecule. The molecule that results is thermally stable and used as an explosive in situations where a very insensitive explosive is needed. 

A theoretical analysis of the stability of such colloidal crystals of spherical latex particles has been carried out by Marcel ja et al (28.). They employ the Lindemann criterion that a crystal will be stable if the rms thermal displacement of the particles about their equilibrium positions is a small fraction f of the lattice spacing R. Comparison with Monte Carlo simulations shows that f is about 0.1 for "hard crystals, and 0.08 for "soft crystals stabilized by long-ranged electrostatic forces. This latter criterion translates into a critical ratio... 

Figure 16.4 A plot showing likely crystal stability with increasing particle size based on the observation that for a given surface roughness (e.g. 10 A) the likely contact area is proportional to the particle radius R, whilst the dispersive force is proportional to (i.e. the particle mass). PPb = glycogen phosphorylase b, FMDV = foot-and-mouth disease virus PRDl = PRDl bacteriophage.

Certain factors are likely to influence future analyses of more complex viruses. Crystal stability is governed by packing interactions and, as can be seen from Fig. 16.4, is, to a first approximation, inversely proportional to the square of the virus radius, presumably underl)dng the problems with crystal stability for analyses such as that of PRDl. Even assuming that well-ordered, stable crystals can be formed, technical considerations will place an upper limit on the unit cell size from which useful data can be collected. Nevertheless, with some improvements in beam and detector technology, we expect that data collection from cells up to 2000 A should be feasible for even a primitive unit cell. 

It has been suggested that in the crown-ether complexes, crystal stability is correlated with an increase in the X—H acidity of the guest9). Bifurcated hydrogen bonds are also common in these host/guest complexes, and their occurrence has been correlated with an increase in the acidity of the X—H groups10>. 

If all the packing atoms are no longer neutral (e.gn half are cations and half are anions), the closest packed structures are no longer the most stable, as can be seen from the similar two-dimensional case (see above). However, these structures may still be useful when considered as limiting cases for certain ionic crystals. Consider lithium iodide, in which the iodide anions are so much larger than the lithium cations that they may be assumed to touch or nearly touch. They can be considered to provide the framework for the crystal. The much smaller lithium ions can then fit irto the small interstices between the anions. If they expand the lattice slightly to remove the anion-anion contact, the anionic repulsion will be reduced and the crystal stabilized, but the simple model based on a closest packed system of anions may still be taken as the limiting case and a useful approximation. 

Significant pigment attributes are tinctorial strength, durability (photochemical stability), biding power, transparency, and heat and solvent resistance. Other properties include brightness (saturation), gloss, rheology, crystal stability, bleed resistance, flocculation resistance, and other properties associated... 

Crystal stability is related to the amount of barium and water in the zeolite. Single crystal data by their number tend to indicate a decrease in the crystal quality of zeolite A at higher and lower H2O contents. Although hydrated Bag-A had... 

Function Coating agent crystal stabilizer. Used as a blend with other fats and oils. 

Thus we suggest that there are two factors for the stabilization of the diastereomeric crystals of the present system, hydrogen-bonding and van der Waals interactions. The difference in crystal stability between the less and more soluble diastereomeric salts, which are successfully separated upon crystallization, arises from the difference in magnitude of the interactions between them (Figure 4.5). Namely the crystals of the less soluble diastereomeric salts are stabilized by the two factors, while the crystals of the more soluble diastereomeric salts are stabilized by only one of the two factors. 

The experiments with N2 cryostats are usually run at temperatmes between 100 and 170K the desire for the lowest possible temperatme is balanced by the necessity to avoid icing. Such temperatmes are almost always sufficient to prevent both the chemical (or photochemical) decomposition of the sample and the solvent loss. The choice between rapid (flash) and slow cooling depends on the natme of the sample. When the thermal expansion coefficient is likely to be high, as is usually the case with conformationally flexible molecules, slow cooling is preferable if the crystal stability permits it. On the other hand, flash-cooling is the only technique possible for protein crystals, because slow cooling will allow the crystallization water to form ice crystals,... 

Dainippon (1994). 5-type and anthrone blue pigment—is a mixture containing titanium dioxide and is useful as car paint having good weatherability, crystal stability and dispersibility. Patent WO 9605255-Al. [262t]... 

Cochran W (1960) Crystal stability and the theory of ferroelectricity. Adv Phys 9 387... 

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