The Sol Gel Process

The sol-gel process is a low-temperature synthesis technique for producing amorphous inorganic solids, i.e., glasses. A sol is generally defined as a colloidal solution with particles of <100 nm suspended in a liquid, whereas a gel is a semisolid colloid. One of the distinct advantages of the sol-gel process is that glasses can be prepared at room temperature with excellent homogeneity and high purity. The glasses fabricated using this process are highly porous, so that selected dopants can be incorporated into the porous glass matrix. Moreover, it is possible to make these sol-gel materials optically transparent, so that they can be used as sensors. There has long been considerable interest in making transparent glasses that contain various organic and inorganic dopants, and the ability to make glasses at low temperatures is extremely useful because many dopants undergo irreversible changes at high temperatures.

In the sol-gel process, a colloidal sol forms from the hydrolysis and poly-condensation of metalorganic precursors. The precursors are typically metal alkoxides, M(OR)„, in which OR is OCH3 (methoxy), OC2H5 (ethoxy), and so on. The most commonly used alkoxides for making silica-based materials are tetramethoxysilane (TMOS) (OR = OCH3) and tetraethoxysilane (OR = OC2H5), because hydrolysis and condensation can be well controlled. An over-

Fig. 1. Overview of sol-gel processing in fabrication of wet (aged) gels, aerogels, and xerogels. (Adapted from ref. 4.)

view of the sol-gel process can be divided into the following steps: sol formation, gelation, drying, and densification, as illustrated in Fig. 1. As can be seen, the method of drying (solvent evaporation or extraction) can greatly affect the final matrix structure, pore size, and pore volume. The resulting material can be in the form of a monolith, thin film, or fiber, depending on the fabrication method.

In the hydrolysis reaction, the precursor (metal alkoxide) is mixed with water in the presence of a catalyst. An alcohol can be added as a cosolvent since alkoxides and water are immiscible, but both are soluble in ethanol and methanol. The hydrolysis step leads to formation of silanol groups (Si-OH), which are intermediates, and alcohol as a byproduct. Hydrolysis is followed by condensation, as silanol groups condense to form siloxane groups (Si-O-Si), releasing alcohol or water as a byproduct. Note that the hydrolysis and condensation reactions occur concurrently. Both of these reactions are depicted in Fig. 2.

A variety of factors affect the hydrolysis and condensation reactions and final microstructure of the gel. These factors include the solution pH, temperature, nature of alkoxide, amount of alcohol, ratio of alkoxide to water to alcohol, and type of catalyst used. The relative rates of hydrolysis and condensation determine the final structure of the gel (1), and there have been extensive studies on the effect of these factors on the polymerization reactions and final microstructure (1-5). The final structure of the glass can be controlled to a large degree by controlling the hydrolysis and condensation reactions. A fast hydrolysis and slow condensation favor formation of more highly condensed silicate polymers, whereas slow hydrolysis and fast condensation result in less

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