Sol-gel

The sol-gel process is a process for the preparation of non-metallic inorganic or hybrid polymeric materials of colloidal dispersions, so-called sols (derived from the English solution). The raw materials are also referred to as precursors. Out of them arise in solution in the first basic reactions finest particles. A special processing of the brine can be powder, produce fibers, films or aerogels. Because of the small size of the sol particles produced initially in the nanometer range can the sol- gel process understood as part of chemical nanotechnology.

  • 3.1 Drying of gels
  • 3.2 density moldings
  • 3.3 powder
  • 3.4 fibers
  • 3.5 Sol -gel layers

Precursors

The starting materials for sol synthesis is often alcoholates of metals or non-metals, the properties of Si- precursors are particularly well studied:

  • Silicon: tetramethylorthosilicate ( TMOS), tetraethylorthosilicate (TEOS), Tetraisopropylorthosilicat ( TPOS ).

Here, one or more of the alcoholate Si-OR replaced by a hydrocarbon radical R, the bond formed is stable to hydrolysis, ie the organic radicals remain firmly bound in the sol particles during hydrolysis and condensation reactions. Non-polar side chains, thus allowing the production of hydrophobic materials. Does the side chain functional groups that can participate in an organic polymerization reaction, hybrid polymers can be obtained.

Many other metals besides silicon and transition metals can be employed in the sol -gel process:

  • Aluminium: aluminum (2- propoxide ), aluminum (2- butoxide );
  • Zircon: Zirconiumpropylat;
  • Titan: Titanethylat, titanium -(2- propoxide ).

These compounds are more sensitive to hydrolysis than the silicon alkoxides. Complexation with 2,4- diketones ( β -diketones ) this reactivity can be significantly reduced, which allows the joint use of different metals or precursors improves the resistance of sols against humidity. Of transition metals and carboxylates such as acetates and propionates are used, the solubility of the corresponding compound plays an important role in the solvent.

Basic reactions

The hydrolysis of precursor molecules, and the condensation between the resulting reactive species are the essential reactions of the sol- gel process. The processes taking place there and the properties of the precursor molecules have a decisive influence on the resulting material properties.

Hydrolysis

From metal alkoxides and water arise with elimination of alcohol molecules MOH groups:

This equation describes the partial hydrolysis of a metal alkoxide. Similar reactions can be formulated for metal carboxylates or - diketonates, but these groups have a significantly higher hydrolytic stability.

Condensation

In reality, MOH groups are already partially hydrolyzed Prekursormoleküle condense with elimination of water with each other:

From the dimer arise in the way of an inorganic polycondensation reaction trimers, tetramers and other oligomers, until finally a particle has formed. Depending on the solvent, a distinction between alcoholic sols and hydrosols. Hydrolysis and condensation reactions are dynamically many interlocking equilibria that also catalysts (acids, bases) can be influenced. Sol particles can not hydrolyzed alkoxide, carboxylate or Diketonatgruppen considerable extent. Progressing end particle growth and aggregation of sol particles into secondary particles lead to an increase in viscosity. Such an " aging " of coating sols can be detrimental to industrial production.

Gelation

Once between the walls of the reaction vessel has a network formed from sol particles, it is called gelation. The flowing viscous sol was transferred into a viscoelastic solid. The gel consists of the gel structure and the trapped solvent from him, but all the pores are in communication ( " interpenetrating network " ) with each other. An accurate determination of gelation is difficult because the gel formation is also a function of vessel size. Also, can greatly affect the training of the network rheological measurements.

Further processing

Drying of gels

If dried gels at atmospheric pressure, is obtained xerogels. Here, the gel body shrinks strongly, because work by the solvent in the pores of the gel network high capillary forces. During the drying reactive groups condense to the juxtaposition of gel particles with each other, thereby changing the original microstructure of the gel. By increasing pressure and temperature of the pore fluid of gels can be brought to the supercritical state. Because the pore fluid has lost its surface tension, capillary forces act no more. After draining the supercritical medium aerogels remain. Theoretically represents an airgel in the resulting sol- gel transition material structure. It should however be borne in mind that the high temperature may have changed in the gel network of the supercritical drying process.

Density moldings

The production of non-porous ceramic molded body is problematic because of the processes involved in drying and sintering strong compression without cracks. Although it is possible in principle workpieces produce vitreous composition, sol- gel techniques but have not enforced here because of the high compared to classical procedural costs.

Powder

The simple drying of sols does not basically powdered products, because the loss of solvent aggregate the sol particles. As a result, precipitation or gel networks can form, which are to a xerogel by further drying. To obtain fine powder particles can unreactive functionalities on the particle surface - for example, in combination with a spray-drying - be protected.

Fibers

The sol -gel process allows for the preparation of inorganic and ceramic fibers. The starting materials are called dopes. Under certain conditions, sols can constrict to viscous masses in vacuum at elevated temperature without a gel network with covalent bonds formed between the sol particles. Upon cooling, the material solidifies vitreous, but can be remelted. Such remelted dopes may be forced through nozzles. The combination of shear stress during extrusion, the presence of moisture and evaporation of residual solvent to form covalent bonds. The resulting gel fiber is not meltable in contrast to the dope. There are no universally applicable rules for synthesis functioning dopes. Corresponding formulas are often complex and are based on empirical test series and experiences with the appropriate material systems. The gel fiber may contain a high proportion of organic residual components, which must be removed by drying and thermal pyrolysis. By sintering at higher temperatures, the fiber is crystallized and compacted. The material may not, during the spinning process, the drying, pyrolysis and sintering, for example, are not damaged by cracking, which makes the fiber development in a scientifically and technologically very demanding task. Next ceramic oxide fibers ( alumina, mullite, lead zirconate titanate, yttrium-aluminum- garnet), there are also non-oxide fibers ( silicon carbide ), or oxide, non- crystalline systems of silica gel.

Sol -gel layers

By wet chemical coating methods such as dip and spin coating and blade coating or spraying can be produced from brines coatings. Hybrid polymers can in this case apply also to thermally sensitive materials. The need for inorganic sol -gel materials heat treatment, however, only allows the coating of metals, ceramics and glass.

Commercial products

The sol -gel method is used for the production and finishing of very different products, so it is seldom perceived to be independent technology:

  • Porous silica layers are used for antireflection coating of solar collectors.
  • Of the anti -reflective coating of ophthalmic lenses, a scratch protection layer of hybrid polymer protects the plastic lens.
  • By alternating sol- gel coating with niedrigbrechendem silica and titania are highly refractive interference filter for optical applications produced for anti-reflection and for creating color effects in the lighting industry.

A set of commercial applications of the sol- gel coatings can be found in Aegerter et al., 2008.

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