Crystals formation

When highly doped oxide such as BPSG is used as a substrate, beautiful looking crystals may develop on the surface after the CMP. It takes only a few days for these soft crystals to grow to a few micrometers in size. These crystals are water soluble. With regular TEOS, it is not likely to see crystal formation. 

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The cloud point, usually between 0 and -10°C, is determined visually (as in NF T 07-105). It is equal to the temperature at which paraffin crystals normally dissolved in the solution of all other components, begin to separate and affect the product clarity. The cloud point can be determined more accurately by differential calorimetry since crystal formation is an exothermic phenomenon, but as of 1993 the methods had not been standardized. 

Jones R, Tredgold R H, Hoorfar A, Allen R A and Hodge P 1985 Crystal-formation and growth in Langmuir-Blodgett multilayers of azobenzene derivatives—optical and structural studies Thin Solid Films 134 57-66... 

By cooling the solution in a freezing mixture (ice and salt, ice and calcium chloride, or solid carbon dioxide and ether). It must be borne in mind that the rate of crystal formation is inversely proportional to the temperature cooling to very low temperatures may render the mass... 

This procedure can be applied to most P2P mixes but is especially effective on the methods to follow. However, in super clean methods, such as the PdCl2 below, where lots of isosafrole is produced, the iso byproduct can interfere with crystal formation. Someone-Who-ls-Not-Strike once found that when an appreciable amount of isosafrole was formed to the detriment of MD-P2P, the oil screwed up the crystal matrix disallowing it to form. Confused, the chemist tried to rescue the uncrystallized oil from the aqueous solution by extracting out the oil to try other things. But when the solvent hit the solution, the P2P crystallized out. Go figure The... 

Footnote 2 - The Methylamine solutions in all steps should be cooled rapidly to promote smaller crystal formation. 

Stretching a polymer sample tends to orient chain segments and thereby facilitate crystallization. The incorporation of different polymer chains into small patches of crystallinity is equivalent to additional crosslinking and changes the modulus accordingly. Likewise, the presence of finely subdivided solid particles, such as carbon black in rubber, reinforces the polymer in a way that imitates the effect of crystallites. Spontaneous crystal formation and reinforcement... 

For large deformations or for networks with strong interactions—say, hydrogen bonds instead of London forces—the condition for an ideal elastomer may not be satisfied. There is certainly a heat effect associated with crystallization, so (3H/9L) t. would not apply if stretching induced crystal formation. The compounds and conditions we described in the last section correspond to the kind of system for which ideality is a reasonable approximation. 

Ice formation is both beneficial and detrimental. Benefits, which include the strengthening of food stmctures and the removal of free moisture, are often outweighed by deleterious effects that ice crystal formation may have on plant cell walls in fmits and vegetable products preserved by freezing. Ice crystal formation can result in partial dehydration of the tissue surrounding the ice crystal and the freeze concentration of potential reactants. Ice crystals mechanically dismpt cell stmctures and increase the concentration of cell electrolytes which can result in the chemical denaturation of proteins. Other quaHty losses can also occur (12). 

Equipment for food freezing is designed to maximize the rate at which foods are cooled to —18° C to ensure as brief a time as possible in the temperature zone of maximum ice crystal formation (12,13). This rapid cooling favors the formation of small ice crystals which minimize the dismption of ceUs and may reduce the effects of solute concentration damage. Rapid freezing requires equipment that can deHver large temperature differences and/or high heat-transfer rates. 

Freedom of rotation about the double methylene bridge in the compound (7) (dimethyl 4,4 -(l,2-ethanediyl)bisben2oate [797-21-7]) destroys the rod shape of the molecule and prevents Hquid crystal formation. The stilbene derivative (8) (dimethyl 4,4 -(l,2-ethenediyl)bisben2oate [10374-80-8]) however, is essentially linear and more favorable for Hquid crystal formation. 

Bulky, even if highly polari2able, functional groups or atoms that are attached anywhere but on the end of a rod-shaped molecule usually are less favorable for Hquid crystal formation. Enhanced intermolecular attractions are more than countered as the molecule deviates from the required linearity. For example, the inclusion of the bromine atom at position three of 4-decyloxy-3-bromoben2oic acid [5519-23-3] (9) prevents mesomorphic behavior. In other cases the Hquid crystal phases do not disappear, but their ranges are narrower. 

Because of the rotation of the N—N bond, X-500 is considerably more flexible than the polyamides discussed above. A higher polymer volume fraction is required for an anisotropic phase to appear. In solution, the X-500 polymer is not anisotropic at rest but becomes so when sheared. The characteristic viscosity anomaly which occurs at the onset of Hquid crystal formation appears only at higher shear rates for X-500. The critical volume fraction ( ) shifts to lower polymer concentrations under conditions of greater shear (32). The mechanical orientation that is necessary for Hquid crystal formation must occur during the spinning process which enhances the alignment of the macromolecules. 

Cloud Seeding. In 1947, it was demonstrated that silver iodide could initiate ice crystal formation because, in the [ -crystalline form, it is isomorphic with ice crystals. As a result, cloud seeding with silver iodide has been used in weather modifications attempts such as increases and decreases in precipitation (rain or snow) and the dissipation of fog. Optimum conditions for cloud seeding are present when precipitation is possible but the nuclei for the crystalliza tion of water are lacking. 

Secondary nucleation is crystal formation through a mechanism involving the solute crystals crystals of the solute must be present for secondary nucleation to occur. Thorough reviews have been given (8,9). 

Crystal Formation There are obviously two steps involved in the preparation of ciystal matter from a solution. The ciystals must first Form and then grow. The formation of a new sohd phase either on an inert particle in the solution or in the solution itself is called nucle-ation. The increase in size of this nucleus with a layer-by-layer addition of solute is called growth. Both nucleation and ciystal growth have supersaturation as a common driving force. Unless a solution is supersaturated, ciystals can neither form nor grow. Supersaturation refers to the quantity of solute present in solution compared with the quantity which would be present if the solution were kept for a veiy long period of time with solid phase in contac t with the solution. The latter value is the equilibrium solubility at the temperature and pressure under consideration. The supersaturation coefficient can be expressed... 

A reduction in the magma density will generally increase nucleation and decrease the particle size. This technique has the disadvantage that crystal formation on the equipment surfaces increases because lower shiny densities create higher levels of supersaturation within the equipment, particularly at the critical boiling surface in a vaporization-type ciystaUizer. 

Most investigators have focused their attention on a differential segment of the zone between the feed injection and the crystal melter. Analysis of crystal formation and growth in the recoveiy section has received scant attention. Table 22-4 summarizes the scope of the literature treatment for center-fed columns for both solid-solution and eutectic forming systems. 

A dense-bed center-fed column (Fig. 22-li) having provision for internal crystal formation and variable reflux was tested by Moyers et al. (op. cit.). In the theoretical development (ibid.) a nonadiabatic, plug-flow axial-dispersion model was employed to describe the performance of the entire column. Terms describing interphase transport of impurity between adhering and free liquid are not considered. 

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