Chemically controlled condensation

By using a similar procedure for the preparation of hybrids of silica, hybrids materials consisting of other metal oxides were also prepared by the group of Wilkes [15]. For example, titania was incorporated into organic polymers by using the chemically controlled condensation (CCC) method for the preparation of poly(tetramethylene oxide)-silica or poly(dimethylsiloxane)-silica hybrids. Especially, in the case of the hybrid with poly (tetramethylene oxide), the modulus or ultimate strength of the hybrid increased in the presence of titania component, as shown in Table 3. This phenomenon was explained by the catalytic ability of... 

Overall, these features mean that chemical control standards are necessarily high. For example, supplementation of the water treatment plant by condensate polishing plant and periodic chemical cleaning are particularly important. In addition, before each period of operation, a clean-up of the cycle is applied to remove crud. Stringent attention must be paid to the feed-water conditioning. 

In general, the structure of sol gel materials evolves sequentially as the product of successive and/or simultaneous hydrolysis and condensation and their reverse reactions (esterification and depolymerization). Thus, in principle, by chemical control of the mechanisms and kinetics of these reactions, namely the catalytic conditions, it is possible to tailor the structure (and properties) of the gels over a wide range. For example, stable silica xerogels of tailored particle dimensions, pore morphology, density and porosity, from relatively... 

The CF-CVC (combustion flame-chemical vapor condensation) process developed by Kear and co-workers (Skandan et al., 1996 Tompa et al., 1999) is a continuous process using the equipment shown in Fig. 1. The starting materials are metal complexes that can be vaporized and fed into a flat flame, which immediately converts the compounds to nanostructured metal oxides. The particle dilution is controlled to prevent agglomeration in a hot state... 

Quantitative evaporation experiments using solar system and solar system-like materials were pioneered by Hashimoto (1983, 1990) in terms of chemical fractionations and by Davis et al. (1990) in terms of the isotopic fractionation of the evaporation residues. Quantitative condensation experiments are much more difficult, and there are no well-controlled condensation experiments available on chondritic or CAl compositions. The compositions that have been most extensively smdied in evaporation experiments intended for application to the solar system are pure forsterite (Mg2Si04), a type B CAI-like liquid consisting of CaO, MgO, AI2O3, and Si02, and a liquid in which major oxides including those of iron are initially in solar proportions. 

Control of ceramic properties through control of parameters. Variation of the reaction conditions affects product morphologies and bulk properties. Variation of pH, temperature, concentration, and chemical control of the rates of hydrolysis and condensation dramatically affect the final product. The complexity of the reactions involved often precludes a complete mechanistic understanding but the use of computer simulation and mathematical models to predict the behavior of the precursors under different conditions allows an insight into how the reaction might proceed and thus how the nature of the final product can be controlled. 

Most metal alkoxides are very reactive toward hydrolysis and condensation. They must be stabilized to avoid precipitation. The chemical control of these reactions is currently performed by adding complexing reagents that react with metal alkoxides at a molecular level, giving rise to new molecular precursors of different structure, reactivity, and functionality. Chemical modification is usually performed with hydroxylated nucleophilic ligands, such as carboxylic acids or P-diketones. In most cases complexation by XOH species can be described as a nucleophilic substimtion, as follows ... 

This chapter deals with the synthesis of nanoscale particles from supersaturated vapors generated by laser vaporization under controlled condensation conditions. The synthesis of nanoscale particles has received considerable attention in view of the potential for new materials with novel properties and the design of catalysts with specific dimensions and compositions (1-10). The reduced size and dimensionality of these atomically engineered materials are responsible for their unique electronic, magnetic, chemical and mechanical properties. The characterization of these properties can ultimately lead to identifying many potential uses, particularly in the field of catalysis. For example, particles of metal oxides and mixed oxides such as Si02/Ti02 exhibit unusual acidic properties and can be used as acidic catalysts (11). 

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