Crystallites stability

D. Crystallite stability theory E. Sequential phase transitions (first amorphous 242... 

Techniques to determine the interplay between random copol3rmer statistics and crystallite stability in a situation where two comonomers (hydroxybut5T ate... 

Camblor MA, Corma A, Martinez A, Mocholf FA, Perez Pariente J. Catalytic cracking of gasoil Benefits in activity and selectivity of small Y zeolite crystallites stabilized by a higher silicon-to-aluminium ratio by synthesis. Appl Catal 1989 55 65-74. 

Two nucleation processes important to many people (including some surface scientists ) occur in the formation of gallstones in human bile and kidney stones in urine. Cholesterol crystallization in bile causes the formation of gallstones. Cryotransmission microscopy (Chapter VIII) studies of human bile reveal vesicles, micelles, and potential early crystallites indicating that the cholesterol crystallization in bile is not cooperative and the true nucleation time may be much shorter than that found by standard clinical analysis by light microscopy [75]. Kidney stones often form from crystals of calcium oxalates in urine. Inhibitors can prevent nucleation and influence the solid phase and intercrystallite interactions [76, 77]. Citrate, for example, is an important physiological inhibitor to the formation of calcium renal stones. Electrokinetic studies (see Section V-6) have shown the effect of various inhibitors on the surface potential and colloidal stability of micrometer-sized dispersions of calcium oxalate crystals formed in synthetic urine [78, 79]. 

In general, the stability of titanium oxide surfaces in moist environments is less of a concern than it is for aluminum oxide surfaces. For example, an FPL or PAA oxide on aluminum would be completely converted to hydroxide in less than 5 min after exposure to boiling water, whereas even after 24 h only slight changes such as crystallite formation and reduction in density of the cell structure occur for... 

The occurrence of a restricted range within which most of the melting may be confined when conditions conducive to equilibration are adopted lends support to the concept of a crystalline phase, a subdivided one notwithstanding, having approximately uniform properties throughout. (It follows also that although the crystallites which melt under the conditions described may be small by ordinary standards, they are not so small as to cause their stabilities to be much dimin- —Specific volume-temperature... 

Most often, these disperse metal catalysts are supported by an electronically conducting substrate or carrier that should provide for uniform supply or withdrawal of electrons (current) to or from all catalyst crystallites. The substrate should also serve to stabilize the disperse state of the catalyst and retard any spontaneous coarsening of the catalyst crystallites. Two situations are to be distinguished (1) the disperse metal catalyst is applied to a substrate consisting of the same metal, and (2) it is applied to a chemically different substrate (a foreign substrate). Platinized platinum is a typical example of the former situation. 

Similar changes in the absorption spectrum were observed for colloidal ZnS. A colloid in methanol solution at —77 °C had a maximum at 228 nm. Upon aging, this maximum became weaker and a new one arose at 243 nm. Further aging at room temperature led to a spectrum with one maximum at 265 nm. It was concluded that the —77 °C synthesis created two very small types of crystallites (<2 nm) identified by the 228 and 243 nm peaks. Upon warming, those crystallites were lost and new crystallites of about 2 nm were formed which persisted at room temperature Absorption spectra with several maxima were also observed for ZnSe, CdSe and In Soj stabilized by polyphosphate... 

Moreover, it was found that incorporation of nanoparticles about 8 nm in diameter in a-Si H led to improved properties, the most important one being enhanced stability against light soaking and thermal annealing [387]. A later study revealed a typical crystallite size of 2-3 nm. with a hexagonal close-packed structure [388]. Diamond structures can also be observed [389]. Hence the name polymorphous silicon is justified. 

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