Sol-gel

The sol-gel process is a wet-chemical technique (Chemical Solution Deposition) for the fabrication of materials (typically a metal oxide) starting from a chemical solution that reacts to produce colloidal particles (sol). Typical precursors are metal alkoxides and metal chlorides, which undergo hydrolysis and polycondensation reactions to form a colloid, a system composed of solid particles (size ranging from 1 nm to 1 μm) dispersed in a solvent. The sol evolves then towards the formation of an inorganic network containing a liquid phase (gel). Formation of a metal oxide involves connecting the metal centers with oxo (M-O-M) or hydroxo (M-OH-M) bridges, therefore generating metal-oxo or metal-hydroxo polymers in solution. The drying process serves to remove the liquid phase from the gel thus forming a porous material, then a thermal treatment (firing) may be performed in order to favor further polycondensation and enhance mechanical properties.

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The precursor sol can be either deposited on a substrate to form a film (e.g. by dip-coating or spin-coating), cast into a suitable container with the desired shape (e.g. to obtain a monolithic ceramics, glasses, fibers, membranes, aerogels), or used to synthesize powders (e.g. microspheres, nanospheres). The sol-gel approach is interesting in that it is a cheap and low-temperature technique that allows for the fine control on the product’s chemical composition, as even small quantities of dopants, such as organic dyes and rare earth metals, can be introduced in the sol and end up in the final product finely dispersed. It can be used in ceramics manufacturing processes, as an investment casting material, or as a means of producing very thin films of metal oxides for various purposes. Sol-gel derived materials have diverse applications in optics, electronics, energy, space, (bio)sensors, medicine (e.g. controlled drug release) and separation (e.g. chromatography) technology.

The interest in sol-gel processing can be traced back in the mid-1880s with the observation that the hydrolysis of tetraethyl orthosilicate (TEOS) under acidic conditions led to the formation of SiO₂ in the form of fibers and monoliths.^[1] Sol-gel research grew to be so important that in the 1990s more than 50,000 papers were published worldwide on the process.^{[citation needed]}

Applications

Scientists have used it to produce the world’s lightest materials and some of its toughest ceramics.

The applications for sol gel-derived products are numerous. One of the largest application areas is thin films, which can be produced on a piece of substrate by spin-coating or dip-coating. Other methods include spraying, electrophoresis, inkjet printing or roll coating. Optical coatings, protective and decorative coatings, and electro-optic components can be applied to glass, metal and other types of substrates with these methods.

Cast into a mold, and with further drying and heat-treatment, dense ceramic or glass articles with novel properties can be formed that cannot be created by any other method. Macroscopic optical elements and active optical components as well as large area hot mirrors, cold mirrors, lenses and beam splitters all with optimal geometry can be made quickly and at low cost via the sol-gel route.

With the viscosity of a sol adjusted into a proper range, both optical and refractory ceramic fibers can be drawn which are used for fiber optic sensors and thermal insulation, respectively.

Ultra-fine and uniform ceramic powders can be formed by precipitation. These powders of single- and multicomponent compositions can be made in submicrometre particle size for dental and biomedical applications. Composite powders have been patented for use as agrochemicals and herbicides. Also powder abrasives, used in a variety of finishing operations, are made using a sol-gel type process.

One of the more important applications of sol-gel processing is to carry out zeolite synthesis. Other elements (metals, metal oxides) can be easily incorporated into the final product and the silicalite sol formed by this method is very stable.

Other products fabricated with this process include various ceramic membranes for microfiltration, ultrafiltration, nanofiltration, pervaporation and reverse osmosis.

If the liquid in a wet gel is removed under a supercritical condition, a highly porous and extremely low density material called aerogel is obtained. Drying the gel by means of low temperature treatments (25-100 C), it is possible to obtain porous solid matrices called xerogels.

Finally of historical note, a sol-gel process was developed in the 1950s for the production of radioactive powders of UO₂ and ThO₂ for nuclear fuels, without generation of large quantities of dust.

References

1. ^ L.L.Hench, J.K.West The Sol-Gel Process Chem. Rev. 1990, 90, 33-72

Further reading

• Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing by C. Jeffrey Brinker, George W. Scherer
• Sol-Gel Materials: Chemistry and Applications by John D. Wright, Nico A.J.M. Sommerdijk
• Sol-Gel Technologies for Glass Producers and Users by Michel A. Aegerter and M. Mennig
• German Patent 736411 (applied for on 28 May 1939 and granted on 6 May 1943) Verfahren zur Änderung des Reflexionsvermögens optischer Gläser - Process for changing the reflection capacity of optical glass, Drs Walter Geffcken and Edwin Berger of the Jenaer Glasswerk Schott