Sol-gel powder

Sol-Gel Techniques. Sol-gel powders (2,13,15,17) are produced as a suspension or sol of coUoidal particles or polymer molecules mixed with a Hquid that polymerizes to form a gel (see Colloids SoL-GELtechnology). Typically, formation of a sol is foUowed by hydrolysis, polymerization, nucleation, and growth. Drying, low temperature calciaation, and light milling are subsequently required to produce a powder. Sol-gel synthesis yields fine, reactive, pseudo-crystalline powders that can be siatered at temperatures hundreds of degrees below conventionally prepared, crystalline powders. 

Microamorphous sihca is made of particles with diameters less than 1pm. They have very high surface areas, usuady greater than 3m2/g. These microamorphous sihca are an aggregation of colloidal ultimate particles that broadly include sols, gels, powder, and porous glass. An important class of... 

Fine ceramic powders made by spray drying, by sol-gel powder synthesis, and sometimes by precipitation usually have very low packing densities. A possible reason for these low packing densities is that the basic packing units are porous agglomerates. In sol-gel s3mthesis. 

Similar investigations of the mechanochemical synthesis of zircon using anhydrous and sol-gel powders of zirconium and silicon oxides as initial reagents were reported in [43,44]. 

Fig. 3.3. Schematic apparatus for continuous sol-gel powder preparation. (Adapted from McColm Clark, 1988.)...

This list is by no means exclusive. Other available deposition techniques include sol-gel, powder pressing, etc. 

Microamorphous silica,- which includes sols, gels, powders, and porous glasses which generally consist of ultimate particles less than a micron in size or have a specific surface area greater than about 3 m g . (These are discussed in detail in Chapters 4 and S.)... 

De Souza Brito GE, Santilli CV, PulcineUi SH (1995) Evolution of the fractal stmcture during sintering of SnO compacted sol-gel powder. Colloid Surf A 97 217-225... 

Besides research on thin films obtained by sol-gel, powders can also be obtained by this method and they are of particular interest in the production of phosphors. In a study of LuBOs phosphors (Boyer, 2002), the evolution of the Lu site as a function of thermal treatment was carried out by the EXAFS technique, from the xerogel to the conpletely crystallized samples. In this study, EXAFS revealed, in agreement with crystallographic results, that a vaterite structure is formed, instead of the calcite structure observed in the conventional solid-state reaction method. Also, Eu-doped Y2SiOs phosphors have been studied (Tao, 1996), relating the Eu-O bond length variation upon thermal treatment with the luminescence properties. 

The stmctural evolution dining sintering of compacted SnOj sol-gel powder was investigated by Brito et al. (1995). The dried gel compacts prepared from aqueous SnCU-SHaO, were sintered under isothermal conditions at 200°C, 300°C, 400°C, 500°C, 600°C and 700° C for 10 min. The mean pore size of the samples exhibited a systematic increase with increase in the sintering temperature. The average pore size of the 100°C sintered sample was 1.1 nm and that increased to 16.1 nm for the 700°C sintered sample. For sintering temperatures higher than 400°C, a linear dependence of pore size on mean crystalline... 

Around 1970, three different groups in the field of inorganic materials published research results on preparation of glass and ceramics via solutions or sol-gel route. H. Dislich prepared a pyrex-type borosilicate glass lens by heating a compact of metal alkoxide derived powder at temperatures as low as 650°C. R. Roy prepared a milhmeter-size small piece of silica glass via sol-gel route at temperatures around 1000°C. Mazdiyasni et al. showed that well-sintered, dense ferroelectric ceramics can be obtained at temperatures as low as 900°C, when sol-gel powders prepared from solutions of metal alkoxides are employed for sintering. 

Sol-gel processing allows formation of thin films, which are preferable sensor materials. Additionally sol-gel powders andincreasingly monoliths are suitable for chromatographic and fixed bed catalytic applications. 

The powders obtained by the Pechini method [14] were assimilated to the sol-gel powders, and several papers concerning preparation of such powders as precursors for advanced ceramics were published [15-17]. Often, the Pechini method is called the sol-gel combustion process. 

In the ceramic industry, sol-gel powders find applications when high-performance materials are required. They are also especially used for ceramics that present difficulties in obtaining well-sintered bodies either evaporation of some components or poor sintering ability. Sintering time and temperature can be significantly reduced with powders having narrow particle size distribution. 

Brito, G.E.S., Santilli, C.V., and PulcinelU, S.H. (1995) Evolution of the fractal structure during sintering of Sn02 compacted sol-gel powder. Colloids Surf. 

Entrapment of organometallic catalysts in sol-gel matrices opens the way to work with these catalysts in solvents not tailored originally for the catalysts. A simple example was the entrapment of several chiral catalytic complexes (Ru-BINAP, Rh-DIOP, and Rh—BPPM) [25] in order to perform enantioselective hydrogenation of itaconic acid. The catalysts are hydrophobic, and itaconic acid is water soluble however, after entrapment the catalytic reaction was made possible in water, by the dispersion of the catalytically doped sol-gel powder in water. More sophisticated approaches also involved emulsions, and here are several examples. 

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