At the first stage of the sol-gel process the hydrolysis and polycondensation reactions lead to the formation of a colloidal solution, i. e. sol, of hydroxide particles whose size does not exceed several dozen nm. Increasing bulk concentration of the dispersed phase or other changes in external conditions (pH, solvent substitution) leads to the intense formation of contacts between particles and the formation of a monolithic gel, in which the solvent molecules are enclosed in a flexible, but fairly stable, three-dimensional grid formed by particles of hydroxides. Concentration of sols followed by gelation is carried out using dialysis, ultrafiltration, electrodialysis, evaporation at relatively low temperatures, or extraction.
A crucial role in the sol-gel process is played by the processes of solvent removal from the gel (drying). Depending on the method the synthesis can result in various products (xerogels, ambigels, cryogels, aerogels), whose properties are described in the corresponding sections. The common features of these products include the preservation of the nanosizes of the structural elements and sufficiently high values of specific surface area (hundreds of m2/g), although the bulk density can vary by hundreds of times. Most products of sol-gel synthesis are used as precursors in obtaining oxide nanopowders, thin films or ceramics. The sol-gel method is also effective for obtaining xerogels with a pronounced quasi-one-dimensional structure. For example, the V2O5·nH2O xerogel is the basis for the synthesis of vanadium oxide nanotubes.
According to some authors the sol-gel method also includes the polymer-gel process, in which the gel formation is achieved by introducing a water-soluble polymer into the initial solution, followed by evaporation, and the Pechini method (citrate gel). Freeze-drying or supercritical drying of polymer gels with subsequent heat treatment in an inert atmosphere is used for the production of carbon cryogels and aerogels.
- Gusev Alexander I.
- Shlyakhtin Oleg A.
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