Sol-gel process

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The sol-gel process is a process for the production of non-metallic inorganic or hybrid polymeric materials from colloidal dispersions , the so-called sols (derived from the English solution ). The starting materials are also known as precursors . In the first basic reactions, very fine particles are formed from them in solution. Through special processing of the brine, powders, fibers, layers or aerogels can be produced. Due to the small size of the initially generated sol particles in the nanometer range, the sol-gel process can be understood as part of chemical nanotechnology .

Principle of the sol-gel process

Precursors

The starting materials for a sol synthesis are often alcoholates of metals or non-metals, with the properties of Si precursors being particularly well investigated:

If one or more of the alcoholate groups Si – OR are replaced by a hydrocarbon radical R, the bond formed becomes stable to hydrolysis, i. H. the organic residues remain firmly bound in the sol particle during hydrolysis and condensation reactions. Non-polar side chains allow the production of hydrophobic materials. If the side chain carries functional groups that can take part in an organic polymerization reaction, hybrid polymers can be obtained.

In addition to silicon, many other metals and transition metals can be used in the sol-gel process:

These compounds are more sensitive to hydrolysis than the silicon alkoxides. By complexing with 2,4-diketones (β-diketones), this reactivity can be reduced significantly, which enables the joint use of precursors of different metals or improves the resistance of brines to atmospheric moisture. Of transition metals, carboxylates such as acetates and propionates are also used, the solubility of the corresponding compound in the solvent used playing an important role.

Basic reactions

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

hydrolysis

MOH groups are formed from metal alcoholates and water with the elimination of alcohol molecules:

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

condensation

In reality, MOH groups of partially hydrolyzed precursor molecules will condense with one another with elimination of water:

Trimers, tetramers and other oligomers are formed from the dimer in an inorganic polycondensation reaction until a particle has finally formed. Depending on the solvent, a distinction is made between alcoholic brines and hydrosols. Hydrolysis and condensation are dynamic reactions of many interlocking equilibria, which are also influenced by catalysts (acids, bases). Sol particles can contain a significant amount of non-hydrolyzed alcoholate, carboxylate or diketonate groups. Progressive particle growth and the aggregation of sol particles to form secondary particles lead to an increase in viscosity. Such “aging” of coating sols can have a detrimental effect on industrial production. The particle size can be controlled by means of the Stöber synthesis .

Gel formation

Sol, gel, xerogel and airgel

As soon as a network of sol particles has formed between the walls of the reaction vessel, it is called gelation. The viscous flowing sol has changed into a viscoelastic solid . The gel consists of the gel structure and the solvents it encapsulates, although all pores are connected to one another (“interpenetrating network”). An exact determination of gel times is made more difficult because the gel formation time is also a function of the vessel size. In addition, rheological measurements can strongly influence the formation of the network.

Further processing

Drying of gels

If gels are dried at normal pressure, xerogels are obtained . The gel body shrinks strongly because of the high capillary forces acting in the pores of the gel network due to the solvent. During the drying process, reactive groups on touching gel particles condense with one another, which changes the original microstructure of the gel. By increasing the pressure and temperature, the pore fluid of gels can be brought into the supercritical state . Because the pore fluid has lost its surface tension, capillary forces are no longer effective. After draining the supercritical medium, aerogels remain . Theoretically, an airgel represents the material structure created during the sol-gel transition. However, it must be taken into account that the high temperature during supercritical drying may have changed the gel network.

Dense moldings

The production of non-porous ceramic molded bodies is problematic because of the strong compression that takes place during drying and sintering without cracks. In principle, workpieces with a vitreous composition can be produced, but sol-gel techniques have not caught on here because of the high costs compared to conventional processes.

powder

Simply drying brines does not generally lead to powdery products because the sol particles aggregate when the solvent is lost. As a result, precipitates or gel networks can form, which become a xerogel on further drying. In order to obtain fine powder particles, unreactive functionalities on the particle surface can be protected - for example in combination with spray drying  .

Fibers

Manufacture of inorganic fibers from sol-gel precursors

The sol-gel process offers the possibility of producing inorganic and ceramic fibers . The starting material are so-called spinning masses. Under certain conditions, brine can be concentrated to viscous masses in a vacuum at elevated temperature without a gel network with covalent bonds forming between the sol particles. When it cools down, the material solidifies like glass, but can be melted again. Such re-melted spinning masses can be pressed through nozzles. The combination of shear stress during extrusion , exposure to air humidity and evaporation of residual solvents creates covalent bonds. In contrast to the spinning mass, the resulting gel fiber cannot be melted. There are no universally applicable rules for the synthesis of functioning spinning masses. Corresponding recipes are often complex and are based on empirical test series and experience with the corresponding material systems. The gel fiber can contain a high proportion of residual organic components, which must be removed by drying and thermal pyrolysis steps . By sintering the fiber is crystallized at higher temperatures and compacted. The material must not be damaged during the spinning process, drying, pyrolysis and sintering, for example through the formation of cracks, which makes fiber development a scientifically and technologically very demanding task. In addition to ceramic oxide fibers ( aluminum oxide , mullite , lead zirconate titanate , yttrium aluminum garnet ) there are also non-oxidic fibers ( silicon carbide ) or oxidic, non-crystalline systems of silica gel fibers .

Sol-gel layers

Coatings can be produced from brines using wet chemical coating processes such as dip and rotary coating, as well as knife coating or spraying ( spray pyrolysis ). Hybrid polymers can also be applied to thermally sensitive materials. The temperature treatment required for inorganic sol-gel materials, on the other hand, only allows the coating of metals, ceramics and glass.

Commercial products

The sol-gel process is used to manufacture and refine very different products, which is why it is seldom perceived as an independent technology:

A compilation of commercial applications of sol-gel layers can be found in Aegerter et al. 2008.

literature

  • Ulrich Schubert , Nicola Hüsing : Synthesis of Inorganic Materials. 2. revised and updated edition. Wiley-VCH, Weinheim et al. 2005, ISBN 3-527-31037-1 (English).
  • C. Jeffrey Brinker, George W. Scherer (Eds.): Sol Gel Science. The Physics and Chemistry of Sol-Gel Processing. The Physics and Chemistry of Sol-gel Processing . Academic Press, Boston et al. 1990, ISBN 0-12-134970-5 .
  • HK Schmidt: The sol-gel process. In: Chemistry in Our Time. 35, No. 3, 2001, ISSN  0009-2851 , pp. 176-184.
  • G. Jonschker: Practice of Sol-Gel Technology . Vincentz Network, Hannover 2012. ISBN 978-3-86630-875-6

Individual evidence

  1. ^ Ralph K. Iler: The Chemistry of Silica: Solubility, Polymerization, Colloid and Surface Properties and Biochemistry of Silica: Solubility, Polymerization, Colloid and Surface Properties and Biochemistry . John Wiley & Sons, New York 1979, ISBN 0-471-02404-X .
  2. ↑ Sumio Sakka: Sol-gel technology as representative processing for nanomaterials: case studies on the starting solution . In: Journal of Sol-Gel Science and Technology . tape 46 , no. 3 , 2008, p. 241-249 , doi : 10.1007 / s10971-007-1651-6 .
  3. ↑ George W. Scherer: Theory of Drying . In: Journal of the American Ceramic Society . tape 73 , no. 1 , 1990, p. 3-14 , doi : 10.1111 / j.1151-2916.1990.tb05082.x .
  4. ↑ Fikret Kirkbir, Hideaki Murata, Douglas Meyers, S. Ray Chaudhuri, Arnab Sarkar: Drying and sintering of sol-gel derived large SiO 2 monoliths . In: Journal of Sol-Gel Science and Technology . tape 6 , no. 3 , 1996, p. 203-217 , doi : 10.1007 / BF00402691 .
  5. ↑ M. Aegerter, R. Almeida, A. Soutar, K. Tadanaga, H. Yang, T. Watanabe: Coatings made by sol-gel and chemical nanotechnology . In: Journal of Sol-Gel Science and Technology . tape 47 , no. 2 , 2008, p. 203-236 , doi : 10.1007 / s10971-008-1761-9 .