Sol–gel process

Source: Wikipedia, the free encyclopedia.

In

metal alkoxides. Sol–gel process is used to produce ceramic nanoparticles
.

Stages

Schematic representation of the different stages and routes of the sol–gel technology

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 a liquid phase and solid phase whose morphologies range from discrete particles to continuous polymer networks. In the case of the colloid, 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 for sedimentation to occur, and then pour off the remaining liquid. Centrifugation can also be used to accelerate the process of phase separation.

Removal of the remaining liquid (solvent) phase requires a drying process, which is typically accompanied by a significant amount of

shrinkage and densification. The rate at which the solvent can be removed is ultimately determined by the distribution of porosity in the gel. The ultimate microstructure
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, or firing process, is often necessary in order to favor further polycondensation and enhance mechanical properties and structural stability via final sintering, densification, and grain 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.

The

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 of tetraethyl orthosilicate (TEOS) under acidic conditions led to the formation of SiO2 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]

Particles and polymers

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 term

dissertation. Einstein concluded that this erratic behavior could adequately be described using the theory of Brownian motion, with sedimentation being a possible long-term result. This critical size range (or particle diameter) typically ranges from tens of angstroms (10−10 m) to a few micrometres (10−6 m).[7]

In either case (discrete particles or continuous polymer network) 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.

In both cases (discrete particles or continuous polymer network), the drying process serves to remove the liquid phase from the gel, yielding a micro-porous

amorphous
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 quality glass fiber and refractory ceramic fiber can be drawn which are used for fiber optic sensors and thermal insulation, respectively. In addition, uniform ceramic powders of a wide range of chemical composition can be formed by precipitation.

Polymerization

Simplified representation of the condensation induced by hydrolysis of TEOS

The

hydroxyl
ion becomes attached to the silicon atom as follows:

Si(OR)4 + H2O → HO−Si(OR)3 + R−OH

Depending on the amount of water and catalyst present, hydrolysis may proceed to completion to silica:

Si(OR)4 + 2 H2O → SiO2 + 4 R−OH

Complete

hydrolyzed monomers linked with a siloxane
[Si−O−Si] bond:

(OR)3−Si−OH + HO−Si−(OR)3 → [(OR)3Si−O−Si(OR)3] + H−O−H

or

(OR)3−Si−OR + HO−Si−(OR)3 → [(OR)3Si−O−Si(OR)3] + R−OH

Thus, polymerization is associated with the formation of a 1-, 2-, or 3-dimensional network of siloxane [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 or alcohol. 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 (or macromolecule) formed from hundreds or thousands of units called monomers. The number of bonds that a monomer can form is called its functionality. Polymerization of silicon alkoxide, for instance, can lead to complex branching 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.

Sono-Ormosil

silica during sol–gel process. The product is a molecular-scale composite with improved mechanical properties. Sono-Ormosils are characterized by a higher density than classic gels as well as an improved thermal stability. An explanation therefore might be the increased degree of polymerization.[11]

Pechini process

Main article: Pechini process

For single cation systems like SiO2 and TiO2, hydrolysis and condensation processes naturally give rise to homogenous compositions. For systems involving multiple cations, such as

chelating 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 using ethylene glycol. The resulting polymer is then combusted under oxidising conditions to remove organic content and yield a product oxide with homogeneously dispersed cations.[13]

Nanomaterials, aerogels, xerogels

Nanostructure of a resorcinol-formaldehyde gel reconstructed from small-angle X-ray scattering. This type of disordered morphology is typical of many sol–gel materials.[14]

If the liquid in a wet gel is removed under a

radioactive powders of UO2 and ThO2 for nuclear 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 the

crack propagation
in the unfired body if not relieved.

In addition, any fluctuations in packing density in the compact as it is prepared for the kiln are often amplified during the sintering 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 of

polycrystalline 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 ceramic nanomaterials.[22][23]

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 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 a ligand which is strong enough to prevent reaction in the hydroxo regime but weak enough to allow reaction in the oxo regime (see Pourbaix diagram).[24]

Applications

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 its toughest ceramics.

Protective coatings

One of the largest application areas is thin films, which can be produced on a piece of substrate by

inkjet[31][32]
printing, or roll coating.

Thin films and fibers

With the

refractory ceramic fibers can be drawn which are used for fiber optic sensors and thermal insulation, respectively. Thus, many ceramic materials, both glassy 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]

Controlled release

Sol-gel technology has been applied for controlled release of fragrances and drugs.[36]

Opto-mechanical

Macroscopic

transparent 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]

See also

References

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  4. ISBN 978-0-7923-9424-2
    .
  5. ^ Klein, L.C. and Garvey, G.J., "Kinetics of the Sol-Gel Transition" Journal of Non-Crystalline Solids, Vol. 38, p.45 (1980)
  6. ^ Brinker, C.J., et al., "Sol-Gel Transition in Simple Silicates", J. Non-Crystalline Solids, Vol.48, p.47 (1982)
  7. ^ Einstein, A., Ann. Phys., Vol. 19, p. 289 (1906), Vol. 34 p.591 (1911)
  8. ^ a b Allman III, R.M., Structural Variations in Colloidal Crystals, M.S. Thesis, UCLA (1983)
  9. ^ a b Allman III, R.M. and Onoda, G.Y., Jr. (Unpublished work, IBM T.J. Watson Research Center, 1984)
  10. ^ a b Sakka, S. et al., "The Sol-Gel Transition: Formation of Glass Fibers & Thin Films", J. Non-Crystalline Solids, Vol. 48, p.31 (1982)
  11. ^ Rosa-Fox, N. de la; Pinero, M.; Esquivias, L. (2002): Organic-Inorganic Hybrid Materials from Sonogels. 2002.
  12. ISBN 9781402079696
    .
  13. S2CID 44149390
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  14. ^ Gommes, C. J., Roberts A. (2008) Structure development of resorcinol-formaldehyde gels: microphase separation or colloid aggregation. Physical Review E, 77, 041409.
  15. doi:10.1111/j.1151-2916.1996.tb08091.x
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  16. doi:10.1080/14786436908228708
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  19. doi:10.1111/j.1151-2916.1983.tb10069.x
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  20. doi:10.1111/j.1151-2916.1982.tb10340.x
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  21. ^ Allman III, R. M. in Microstructural Control Through Colloidal Consolidation, Aksay, I. A., Adv. Ceram., Vol. 9, p. 94, Proc. Amer. Ceramic Soc. (Columbus, OH 1984).
  22. PMID 1962191
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  24. ^ Curran, Christopher D., et al. "Ambient temperature aqueous synthesis of ultrasmall copper doped ceria nanocrystals for the water gas shift and carbon monoxide oxidation reactions." Journal of Materials Chemistry A 6.1 (2018): 244-255.
  25. ^ Wright, J. D. and Sommerdijk, N. A. J. M., Sol-Gel Materials: Chemistry and Applications.
  26. ^ Aegerter, M. A. and Mennig, M., Sol-Gel Technologies for Glass Producers and Users.
  27. ^ Phalippou, J., Sol-Gel: A Low temperature Process for the Materials of the New Millennium, solgel.com (2000).
  28. ISBN 9780121349707
    .
  29. ^ German Patent 736411 (Granted 6 May 1943) Anti-Reflective Coating (W. Geffcken and E. Berger, Jenaer Glasswerk Schott).
  30. ^ Klein, L. C., Sol-Gel Optics: Processing and Applications, Springer Verlag (1994).
  31. PMID 26805775
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  33. ^ Patel, P.J., et al., (2000) "Transparent ceramics for armor and EM window applications", Proc. SPIE, Vol. 4102, p. 1, Inorganic Optical Materials II, Marker, A.J. and Arthurs, E.G., Eds.
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  37. S2CID 137347665
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  38. doi:10.1111/j.1151-2916.1983.tb11004.x
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Further reading

  • Colloidal Dispersions, Russel, W. B., et al., Eds.,
    Cambridge University
    Press (1989)
  • Glasses and the Vitreous State, Zarzycki. J., Cambridge University Press, 1991
  • The Sol to Gel Transition. Plinio Innocenzi. Springer Briefs in Materials. Springer. 2016.

External links
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