Ceramic fiber
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A ceramic fiber ( CEF ) or ceramic fiber is a fiber made of inorganic , non-metallic material. Originally only polycrystalline inorganic materials were referred to as ceramic. In the meantime, however, there are amorphous fibers produced by pyrolysis from various polymers, so-called precursors, which are called ceramic fibers due to their properties. The distinction to likewise amorphous glass fibersthat are not counted among the ceramic fibers, is best made possible by the manufacturing process (glass fibers from glass melt, amorphous ceramic fibers from polymeric precursors by pyrolysis). The ceramic fibers are divided into oxide and non-oxide. In the following only the polycrystalline and amorphous ceramic fibers are presented. Single crystalline fibers, so-called whiskers , are described elsewhere.
Manufacturing principle
Oxidic fibers
In terms of oxide ceramic fibers, there are basically only fibers on the market based on aluminum oxide and silicon dioxide in different proportions and in some cases with additional boron oxide or zirconium oxide. Mixed oxide fibers made from 85% Al 2 O 3 and 15% SiO 2 are also referred to as mullite fibers . All of these fibers are polycrystalline .
Spinning masses in which organic polymers such as polyvinyl alcohols or polyethylene oxides ensure spinnability are used as starting materials . Usually salts or colloidally dissolved inorganic components ( brine ) dissolved in water , sometimes supplemented by the addition of very fine powders, become what are known as green fibers after the spun fibers have dried. Like so-called green bodies in normal ceramics , these are converted into the finished oxide ceramic fiber in a sintering process .
Non-oxide fibers
In the case of non-oxidic, industrially produced fibers (apart from the carbon fibers), only different types of silicon carbide fibers are available so far .
The starting polymers are almost exclusively so-called polycarbosilanes . In principle, these are polymers made from hydrocarbons in which individual carbon atoms have been replaced by silicon atoms or silanes in which individual silicon atoms have been replaced by carbon atoms. The polymers are crosslinked by additives in a hardening process so that they do not simply evaporate after the spinning process during pyrolysis, but - as in the production of carbon fibers - are converted into an amorphous, mostly non- stoichiometric SiC ceramic fiber that still contains free carbon. Using special processes, the production of very finely crystalline and pure SiC fibers with significantly improved high temperature properties has also been successful.
Oxygen, titanium, zirconium and aluminum are mentioned as additional elements in the various fiber types available. Fibers that are made on the basis of polysilazanes contain high proportions of nitrogen and are then referred to as SiNC fibers. Boron has also been used as a further additional component in SiBNC fiber developments.
The wide range of possibilities in organic chemistry has kept the development of further variants of ceramic fiber types in motion in the last and probably also in the coming years.
The production of carbon fibers is described under carbon fiber .
properties
Depending on the manufacturing process and components, the ceramic fibers show different properties. The structure and the various additives ensure that all ceramic fibers have high tensile strength and ductility compared to normal ceramics.
| Fiber type | Density (g / cm³) | Diameter (µm) | Tensile strength (GPa) | Modulus of elasticity (GPa) | Elongation at break (%) |
|---|---|---|---|---|---|
| Alumina | 2.7-4.1 (*) | 10 | 1.7-2.9 | 150-370 | 0.6-1.1 |
| Silicon carbide | 2.5-3.1 | 8-15 | 2.6-3.4 | 170-420 | 0.6 (**) - 1.9 |
(*): the high density is due to significant zirconium oxide components. (**): the low elongation at break belongs to crystalline SiC fibers.
The table lists some properties of ceramic fibers and the range of data for the various types. For some applications, the properties at temperatures above approx. 500 ° C are important. In the presence of oxygen, carbon fibers are noticeably oxidized from this temperature, and the amorphous silicon carbide fibers also lose their strength due to oxidation of the residues of free carbon in the fibers. The available aluminum oxide fibers are resistant to oxidation at high temperatures, but above approx. 1000 ° C they show creep behavior under load , which makes many applications of these fibers in ceramic fiber composite materials difficult or impossible at high temperatures.
In the case of amorphous silicon carbide fibers, a recrystallization process begins at temperatures from around 1200 ° C, even in a vacuum , which deteriorates the strength properties. The development of crystalline SiC fibers, which can withstand high temperatures, has not yet been completed and has already yielded initial successes.
The electrical, thermal and corrosion properties of the fibers are similar to those of normal technical ceramics . Only those with an amorphous structure have poorer corrosion resistance and reduced thermal and electrical conductivity.
The properties of carbon fibers can be found under carbon fiber .
Applications
Aluminum oxide fibers are used, among other things, for thermal insulation and as flexible fabrics in heat-insulating protective suits and have also been used in oxide fiber-reinforced ceramics .
Aluminum silicate wool is a product for thermal insulation in industrial areas at working temperatures> 1000 ° C. The main focus of their use are wide areas of furnace, furnace and heating construction, automobile construction in the hot-end area and as storage mats for catalytic converters and soot particle filters. (See also high temperature wool .)
The main focus of the use of SiC fibers is also the production of fiber-reinforced ceramics.
Applications of carbon fiber are reported under carbon fiber .
Health risk
Ceramic fibers are rated as carcinogenic in animal experiments . They were therefore included in a list of proposals for binding occupational exposure limits published by the European Commission in May 2016 .
Some manufacturer names
- Carbon fiber: see carbon fiber
- Alumina fibers : Ibiden , Mitsui , Sumitomo Chemical , Denka / Nitivy, 3M Nextel, Unifrax , Rath Incorporated
- Silicon carbide fibers: Nippon Carbon, Ube Industries
literature
- German Ceramic Society, J. Kriegesmann (Ed.): Technical ceramic materials. German Wirtschaftsdienst, 1989, ISBN 978-3-87156-091-0 , HvB-Verlag, Ellerau 2005, ISBN 978-3-938595-00-8 , chap. 3.2, loose-leaf work with folder.
- B. Clauß: Fibers and preform techniques for the production of ceramic composites. from W. Krenkel (ed.): Ceramic composite materials. WILEY-VCH, Weinheim, 2003, ISBN 3-527-30529-7 , p. 23.
- AVK - Industrial Association Reinforced Ku: Handbook fiber composite plastics / composites. 4th edition, Springer, 2014, ISBN 978-3-658-02755-1 , pp. 162-165.
Web links
- Fraunhofer Center for High Temperature Lightweight Construction HTL: Research and development of Ceramic Matrix Composites (CMC), ceramic fibers and high-performance ceramics
Individual evidence
- ↑ Entry on ceramic mineral fibers in the GESTIS material database of the IFA , accessed on December 14, 2016 (JavaScript required)
- ↑ Entry on refractory ceramic fibers, fibers for special purposes, unless specifically listed in this appendix [Artificially produced non-directional glass-like (silicate) fibers with a proportion of alkali and alkaline earth metal oxides (Na 2 O + K 2 O + CaO + MgO + BaO ) of up to 18% by weight] in the Classification and Labeling Inventory of the European Chemicals Agency (ECHA), accessed on December 14, 2016. Manufacturers or distributors can expand the harmonized classification and labeling .
- ↑ H.-P. Baldus et al. a .: Properties of Amorphous SiBNC-Ceramic Fibers. In: Key Engineering Materials. Volumes 127-131, 1997, pp. 177-184, doi: 10.4028 / www.scientific.net / KEM.127-131.177 .
- ↑ Ulrich Welzbacher: Safe handling of ceramic fibers. In: Hazardous substances - cleanliness. Air . 62, No. 9, 2002, ISSN 0949-8036 , pp. 365-368.
- ↑ Eberhard Nies: Europe is targeting carcinogenic substances. In: Hazardous substances - cleanliness. Air. 76, No. 7/8, 2016, ISSN 0949-8036 , pp. 265-266.