Sol gel
From Wikipedia, the free encyclopedia
Sol gel is a colloidal suspension that can be gelled to form a solid. The resulting porous gel then is chemically purified and fired at high temperatures into high purity oxide materials. The gel can be modified with a number of dopants to produce unique properties in the resultant glass unattainable by other means. 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, including a form superior to teflon. Sol-gel derived materials have diverse applications in optics, electronics, energy, space, sensors and separation technology.
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[edit] The sol-gel process
The sol-gel process is a process for making glass/ceramic materials. The sol-gel process involves the transition of a system from a liquid (the colloidal “sol") into a solid (the "gel") phase. The sol-gel process allows the fabrication of materials with a large variety of properties: ultra-fine powders, monolithic ceramics and glasses, ceramic fibers, inorganic membranes, thin film coatings and aerogels. Solgel chemistry is a remarkably versatile approach for fabricating materials. Scientists have used it to produce the world’s lightest materials and some of its toughest ceramics.
The sol is made of solid particles of a diameter of few hundred nm, usually inorganic metal salts, suspended in a liquid phase. In a typical sol-gel process, the precursor is subjected to a series of hydrolysis and polymerization reactions to form a colloidal suspension, then the particles condense in a new phase, the gel, in which a solid macromolecule is immersed in a solvent.
The beginnings of sol-gel processing can be traced back over 70 years to research done by Geffcken at . In 1939, he patented for Schott glass company in Germany the first application by coating large panes of windows. Sol-gel research grew to be so important that in the 1990s more than 50,000 papers were published worldwide on the process.
[edit] Applications
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 submicron 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.
[edit] References
- 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
