Sol-gel 1701055 225395204 2008-07-13T13:26:04Z Mion 724747 -rv spam The '''sol-gel''' process is a wet-chemical technique ([[Chemical Solution Deposition]]) for the fabrication of materials (typically a [[metal oxide]]) starting either from a chemical solution (''sol'' short for solution) or colloidal particles (''sol'' for nanoscale particle) to produce an integrated network (''gel''). 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 continuous 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. 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, [[aerogel]]s), 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 [[ceramic]]s manufacturing processes, as an [[investment casting]] material, or as a means of producing very [[thin film]]s of metal [[oxide]]s 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 SiO<sub>2</sub> in the form of fibers and monoliths.<ref>L.L.Hench, J.K.West ''The Sol-Gel Process'' Chem. Rev. '''1990''', 90, 33-72</ref> Sol-gel research grew to be so important that in the 1990s more than 35,000 papers were published worldwide on the process.<ref>ISI Web of Knowledge: 36,838 publications with topic "sol-gel" as of 26 May 2008</ref> == Applications == Scientists have used it to produce the world’s lightest materials and some of its [[toughness|toughest]] ceramics. The applications for sol gel-derived products are numerous. One of the largest application areas is [[thin film]]s, 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 coating]]s, 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 mirror]]s, [[cold mirror]]s, [[lens (optics)|lenses]] and [[beam splitter]]s 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 fiber|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 [[agrochemical]]s and [[herbicide]]s. 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 [[artificial membrane|membranes]] for [[microfiltration]], [[ultrafiltration]], [[nanofiltration]], [[pervaporation]] and [[reverse osmosis]]. If the liquid in a wet gel is removed under a [[critical point (chemistry)|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 [[xerogel]]s. Finally of historical note, a sol-gel process was developed in the 1950s for the production of [[radioactive]] powders of [[uranium dioxide|UO<sub>2</sub>]] and [[thorium dioxide|ThO<sub>2</sub>]] for [[nuclear fuel]]s, without generation of large quantities of dust. == References == <references/> == 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 == External links == *[http://www.isgs.org/ International Sol-Gel Society] *[http://www.springerlink.com/app/home/journal.asp?wasp=785385c13ec244eebc516625e4e8e65c&referrer=parent&backto=linkingpublicationresults,1:100301,1 Journal of Sol-Gel Science and Technology] *[http://www.solgel.com/index.asp The Sol-Gel Gateway] {{Glass science}} [[Category:Industrial processes]] [[Category:Gels]] [[Category:Thin film deposition]] [[Category:Glass engineering and science]] [[Category:Transparent materials]] [[cs:Sol gel]] [[de:Sol-Gel-Prozess]] [[fr:Procédé sol-gel]] [[it:Sol-gel]] [[nl:Sol-gel]] [[pt:Sol-gel]] [[ru:Золи]] [[sv:Sol-gel]] [[uk:Золі]] [[zh:溶膠凝膠]]