Carbosilanes

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Carbosilanes are chemical compounds in which silane and hydrocarbon groups are linked. The chains made up of silicon atoms in silanes and those made up of carbon atoms in hydrocarbons consist of chains in which silicon and carbon atoms are alternating or mixed. Only the polymers of this material group, the polycarbosilanes, have achieved practical importance . Due to the large number of possibilities to incorporate radicals of different lengths and structures here , (poly) carbosilanes conceal a whole class of substances, mostly liquid or resinous.

The development that began in the 1970s was initially aimed at obtaining primary materials, so-called precursors, for the production of ceramic silicon carbide fibers . In the meantime, however, polycarbosilanes have also been developed as primary materials for coatings and for the production of the silicon carbide matrix of ceramic fiber composites .

properties

Many polycarbosilanes are changed in air by reacting with oxygen and water vapor. They therefore often only have a limited shelf life, can be stored deep-frozen and, when used, can be processed quickly or under protective gas.

use

Carbosilanes and special polycarbosilanes obtained from them by polymerisation are the starting material for the production of ceramic fibers from silicon carbide . In addition, they have become more important in the production of ceramic fiber composite materials as a starting material for the ceramic SiC matrix of this material class. The SiC matrix is ​​created by pyrolysis of the crosslinked hardened polycarbosilanes. In contrast to technical SiC ceramics, it is initially amorphous. As a rule, non- stoichiometric SiC is also present. A crystal formation process does not begin until the temperature lasts longer than about 1300 ° C.

In these applications of polycarbosilanes, properties such as viscosity (important for spinnability in fine nozzles), the effort required for hardening, and the yield and properties of the “silicon carbide” produced play a major role. Arise in empirical development and test series

  • by varying the radicals in the carbosilanes,
  • by controlling the formation of the polycarbosilanes and
  • through special additives that allow hardening,

Polymers adapted to the respective application.

Examples

A polycarbosilane that gives a high yield of silicon carbide after pyrolysis is allylhydridopolycarbosilane. It consists of a chain of alternating C and Si atoms of the form

-SiH 2 -CH 2 -SiH R -CH 2 -SiH 2 -,

the allyl group —CH 2 —CH = CH 2 being used as the radical R in this case .

The length of the chain and the frequency of the allyl groups determine the viscosity of the resin. Through the double bond in the allyl group, all molecules are irreversibly crosslinked with each other during curing of the polymer (a temperature treatment of around 150 to 200 ° C, during which solvent components also evaporate). In a subsequent pyrolysis process above 1000 ° C with argon as the protective gas, an amorphous, almost stoichiometric silicon carbide with a slight excess of carbon is produced. The volume shrinks, partly due to the outgassing of organic residues, partly due to the increase in density.

literature

  • W. Schäfer, WD Vogel: Fiber-reinforced ceramics produced by polymer infiltration . from: W. Krenkel (Hrsg.): Ceramic composite materials . Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, 2003, ISBN 3-527-30529-7 , p. 76.
  • J. Kriegesmann (Ed.): DKG - Technical Ceramic Materials . Fachverlag Deutscher Wirtschaftsdienst, Cologne, ISBN 3-87156-091-X .
  • LV Interrante, CW Whitmarsh, W. Sherwood: Fabrication of SiC Matrix Composites by Liquid Phase Infiltration with a Polymeric Precursor . Material Research Society Symposium 1995, Proceedings of the Material Research Society, Volume 365, 1995.