Inmaterials science, thesol–gel process is a method for producing solid materials from small molecules. The method is used for thefabrication ofmetal oxides, especially the oxides ofsilicon (Si) andtitanium (Ti). The process involves conversion of monomers in solution into a colloidal solution (sol) that acts as the precursor for an integrated network (orgel) of either discrete particles or networkpolymers. Typicalprecursors aremetal alkoxides. Sol–gel process is used to produceceramic nanoparticles.
In this chemical procedure, a "sol" (a colloidal solution) is formed that then gradually evolves towards the formation of a gel-like diphasic system containing both aliquid phase andsolid phase whose morphologies range from discrete particles to continuous polymer networks. In the case of thecolloid, the volume fraction of particles (or particle density) may be so low that a significant amount of fluid may need to be removed initially for the gel-like properties to be recognized. This can be accomplished in any number of ways. The simplest method is to allow time forsedimentation to occur, and then pour off the remaining liquid.Centrifugation can also be used to accelerate the process ofphase separation.
Removal of the remaining liquid (solvent) phase requires adrying process, which is typically accompanied by a significant amount ofshrinkage and densification. The rate at which the solvent can be removed is ultimately determined by the distribution ofporosity in the gel. The ultimatemicrostructure of the final component will clearly be strongly influenced by changes imposed upon the structural template during this phase of processing.
Afterwards, a thermal treatment, orfiring process, is often necessary in order to favor further polycondensation and enhance mechanical properties and structural stability via finalsintering, densification, andgrain growth. One of the distinct advantages of using this methodology as opposed to the more traditional processing techniques is that densification is often achieved at a much lower temperature.
Theprecursor sol can be either deposited on asubstrate to form a film (e.g., bydip-coating orspin coating),cast into a suitable container with the desired shape (e.g., to obtain monolithicceramics,glasses,fibers,membranes,aerogels), or used to synthesize powders (e.g.,microspheres,nanospheres).[1] The sol–gel approach is a cheap and low-temperature technique that allows the fine control of the product's chemical composition. Even small quantities of dopants, such asorganic dyes andrare-earth elements, can be introduced in the sol and end up uniformly dispersed in the final product. It can be used inceramics processing and manufacturing as aninvestment casting material, or as a means of producing verythin films of metaloxides for various purposes. Sol–gel derived materials have diverse applications inoptics,electronics,energy,space, (bio)sensors,medicine (e.g.,controlled drug release),reactive material, and separation (e.g.,chromatography) technology.
The interest in sol–gel processing can be traced back in the mid-1800s with the observation that the hydrolysis oftetraethyl orthosilicate (TEOS) under acidic conditions led to the formation ofSiO2 in the form of fibers and monoliths. Sol–gel research grew to be so important that in the 1990s more than 35,000 papers were published worldwide on the process.[2][3][4]
The sol–gel process is a wet-chemical technique used for the fabrication of both glassy and ceramic materials. In this process, the sol (or solution) evolves gradually towards the formation of a gel-like network containing both a liquid phase and a solid phase. Typical precursors are metal alkoxides and metal chlorides, which undergo hydrolysis and polycondensation reactions to form a colloid. The basic structure or morphology of the solid phase can range anywhere from discrete colloidal particles to continuous chain-like polymer networks.[5][6]
The termcolloid is used primarily to describe a broad range of solid-liquid (and/or liquid-liquid) mixtures, all of which contain distinct solid (and/or liquid) particles which are dispersed to various degrees in a liquid medium. The term is specific to the size of the individual particles, which are larger than atomic dimensions but small enough to exhibitBrownian motion. If the particles are large enough, then their dynamic behavior in any given period of time in suspension would be governed by forces ofgravity andsedimentation. But if they are small enough to be colloids, then their irregular motion in suspension can be attributed to the collective bombardment of a myriad of thermally agitated molecules in the liquid suspending medium, as described originally byAlbert Einstein in hisdissertation. Einstein concluded that this erratic behavior could adequately be described using the theory ofBrownian motion, with sedimentation being a possible long-term result. This critical size range (or particle diameter) typically ranges from tens ofangstroms (10−10 m) to a fewmicrometres (10−6 m).[7]
In either case (discrete particles or continuous polymer network) thesol 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.
In both cases (discrete particles or continuous polymer network), the drying process serves to remove the liquid phase from the gel, yielding a micro-porousamorphous glass or micro-crystalline ceramic. Subsequent thermal treatment (firing) may be performed in order to favor further polycondensation and enhance mechanical properties.
With the viscosity of a sol adjusted into a proper range, both optical qualityglass fiber and refractory ceramic fiber can be drawn which are used for fiber optic sensors andthermal insulation, respectively. In addition, uniform ceramic powders of a wide range of chemical composition can be formed byprecipitation.
TheStöber process is a well-studied example of polymerization of an alkoxide, specificallyTEOS. The chemical formula for TEOS is given by Si(OC2H5)4, or Si(OR)4, where thealkyl group R =C2H5.Alkoxides are ideal chemical precursors for sol–gel synthesis because they react readily with water. The reaction is called hydrolysis, because ahydroxyl ion becomes attached to the silicon atom as follows:
Depending on the amount of water and catalyst present, hydrolysis may proceed to completion to silica:
Completehydrolysis often requires an excess of water and/or the use of a hydrolysiscatalyst such asacetic acid orhydrochloric acid. Intermediate species including [(OR)2−Si−(OH)2] or [(OR)3−Si−(OH)] may result as products of partialhydrolysis reactions.[1] Early intermediates result from two partiallyhydrolyzedmonomers linked with asiloxane [Si−O−Si] bond:
or
Thus,polymerization is associated with the formation of a 1-, 2-, or 3-dimensional network ofsiloxane [Si−O−Si] bonds accompanied by the production of H−O−H and R−O−H species.
By definition, condensation liberates a small molecule, such as water oralcohol. This type of reaction can continue to build larger and larger silicon-containing molecules by the process of polymerization. Thus, a polymer is a huge molecule (ormacromolecule) formed from hundreds or thousands of units calledmonomers. The number of bonds that a monomer can form is called its functionality. Polymerization ofsilicon alkoxide, for instance, can lead to complexbranching of the polymer, because a fully hydrolyzed monomer Si(OH)4 is tetrafunctional (can branch or bond in 4 different directions). Alternatively, under certain conditions (e.g., low water concentration) fewer than 4 of the OR or OH groups (ligands) will be capable of condensation, so relatively little branching will occur. The mechanisms of hydrolysis and condensation, and the factors that bias the structure toward linear or branched structures are the most critical issues of sol–gel science and technology. This reaction is favored in both basic and acidic conditions.
Sonication is an efficient tool for the synthesis of polymers. Thecavitationalshear forces, which stretch out and break the chain in a non-random process, result in a lowering of themolecular weight and poly-dispersity. Furthermore, multi-phase systems are very efficient dispersed andemulsified, so that very fine mixtures are provided. This means thatultrasound increases the rate ofpolymerisation over conventional stirring and results in higher molecular weights with lower polydispersities.Ormosils (organically modified silicate) are obtained whensilane is added to gel-derivedsilica during sol–gel process. The product is a molecular-scale composite with improved mechanical properties. Sono-Ormosils are characterized by a higherdensity than classic gels as well as an improved thermal stability. An explanation therefore might be the increased degree of polymerization.[11]
For single cation systems like SiO2 and TiO2, hydrolysis and condensation processes naturally give rise to homogenous compositions. For systems involving multiple cations, such asstrontium titanate, SrTiO3 and otherperovskite systems, the concept of steric immobilisation becomes relevant. To avoid the formation of multiple phases of binary oxides as the result of differing hydrolysis and condensation rates, the entrapment of cations in a polymer network is an effective approach, generally termed thePechini process.[12] In this process, achelating agent is used, most often citric acid, to surround aqueous cations and sterically entrap them. Subsequently, a polymer network is formed to immobilize the chelated cations in a gel or resin. This is most often achieved by poly-esterification usingethylene glycol. The resulting polymer is then combusted under oxidising conditions to remove organic content and yield a product oxide with homogeneously dispersed cations.[13]
If the liquid in a wet gel is removed under asupercritical 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 calledxerogels. In addition, a sol–gel process was developed in the 1950s for the production ofradioactive powders ofUO2 andThO2 fornuclear fuels, without generation of large quantities of dust.
Differential stresses that develop as a result of non-uniform drying shrinkage are directly related to the rate at which thesolvent can be removed, and thus highly dependent upon the distribution ofporosity. Such stresses have been associated with a plastic-to-brittle transition in consolidated bodies,[15] and can yield tocrack propagation in the unfired body if not relieved.
In addition, any fluctuations in packing density in the compact as it is prepared for thekiln are often amplified during thesintering process, yielding heterogeneous densification.Some pores and other structural defects associated with density variations have been shown to play a detrimental role in the sintering process by growing and thus limiting end-point densities. Differential stresses arising from heterogeneous densification have also been shown to result in the propagation of internal cracks, thus becoming the strength-controlling flaws.[16][17][18][19][20]
It would therefore appear desirable to process a material in such a way that it is physically uniform with regard to the distribution of components and porosity, rather than using particle size distributions which will maximize the green density. The containment of a uniformly dispersed assembly of strongly interacting particles in suspension requires total control over particle-particle interactions.Monodisperse colloids provide this potential.[8][9][21]
Monodisperse powders ofcolloidal silica, for example, may therefore be stabilized sufficiently to ensure a high degree of order in thecolloidal crystal orpolycrystalline colloidal solid which results from aggregation. The degree of order appears to be limited by the time and space allowed for longer-range correlations to be established. Such defective polycrystalline structures would appear to be the basic elements of nanoscale materials science, and, therefore, provide the first step in developing a more rigorous understanding of the mechanisms involved in microstructural evolution in inorganic systems such as sintered ceramicnanomaterials.[22][23]
Ultra-fine and uniform ceramic powders can be formed by precipitation. These powders of single and multiple component compositions can be produced at a nanoscale particle size for dental,biomedical,agrochemical, orcatalytic applications. Powderabrasives, 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 outzeolite synthesis. Other elements (metals, metal oxides) can be easily incorporated into the final product and the silicate sol formed by this method is very stable. Semi-stable metal complexes can be used to produce sub-2 nm oxide particles without thermal treatment. During base-catalyzed synthesis, hydroxo (M-OH) bonds may be avoided in favor of oxo (M-O-M) using aligand which is strong enough to prevent reaction in the hydroxo regime but weak enough to allow reaction in the oxo regime (seePourbaix diagram).[24]
The applications for sol gel-derived products are numerous.[25][26][27][28][29][30] For example, scientists have used it to produce the world's lightest materials and also some of itstoughest ceramics.
One of the largest application areas is thin films, which can be produced on a piece of substrate byspin coating or dip-coating. 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.[citation needed] Other coating methods include spraying,electrophoresis,inkjet[31][32] printing, or roll coating.
With theviscosity of a sol adjusted into a proper range, bothoptical andrefractory ceramic fibers can be drawn which are used for fiber optic sensors and thermal insulation, respectively. Thus, many ceramic materials, bothglassy and crystalline, have found use in various forms from bulk solid-state components to high surface area forms such as thin films, coatings and fibers.[10][33] Also, thin films have found their application in the electronic field[34] and can be used as sensitive components of a resistive gas sensors.[35]
Sol-gel technology has been applied for controlled release of fragrances and drugs.[36]
Macroscopicoptical elements and active optical components as well as large areahot mirrors,cold mirrors,lenses, andbeam splitters can be made by the sol–gel route. In the processing of high performance ceramic nanomaterials with superior opto-mechanical properties under adverse conditions, the size of the crystalline grains is determined largely by the size of the crystalline particles present in the raw material during the synthesis or formation of the object. Thus a reduction of the original particle size well below the wavelength of visible light (~500 nm) eliminates much of thelight scattering, resulting in a translucent or eventransparent material.
Furthermore, microscopic pores in sintered ceramic nanomaterials, mainly trapped at the junctions of microcrystalline grains, cause light to scatter and prevented true transparency. The total volume fraction of these nanoscale pores (both intergranular and intragranular porosity) must be less than 1% for high-quality optical transmission, i.e. the density has to be 99.99% of the theoretical crystalline density.[37][38]
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