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carbon fiber[′kär·bən ‚fī·bər]
a fiber consisting mainly of carbon. Carbon fibers are usually obtained by heat-treating synthetic chemical fibers or natural organic fibers in such a way that mainly carbon atoms remain in the fibrous material. If the treatment temperature is less than 900°C, the carbon fibers will contain 85–90 percent carbon; at 900°-1500°C the content will be 95–99 percent, and at 1500°-3000°C it will exceed 99 percent. In addition to ordinary organic fibers, which are most often viscose and polyacrylonitrile fibers, special fibers of phenolic resins, lignin, and coal-tar and petroleum pitches are used in producing carbon fibers.
Depending on the form of the starting material, carbon fibers can take the form of continuous or staple yarns, filaments, ribbons, felts, and fabrics. In addition, ordinary textile machinery can be used to make woven and unwoven materials from carbon fibers.
Carbon fibers have extremely high thermal stability. Upon heating to 1600°-2000°C in the absence of oxygen, the mechanical indexes of the fiber remain unchanged. This stability recommends carbon fibers for use in heat shields and as insulating material in high-temperature technology. The reinforced plastics made from carbon fibers are distinguished by their good ablation characteristics.
Carbon fibers, although resistant to corrosive chemical media, undergo oxidation upon heating in the presence of oxygen. The temperature ceiling for fibers used in an air medium is 300°-350°C. The application of a thin layer of carbides, especially SiC, or boron of nitride to the carbon fibers does much to correct this drawback. Owing to their high chemical stability, carbon fibers are used in the filtration of corrosive media, the purification of gases, and the manufacture of protective suits.
A change in the conditions of heat treatment can alter the electrophysical properties of the fibers; the volume electrical resistivity can range from 2 × 10–3 to 106 ohm-cm. Certain fibers can be used as electrical heating elements and in the production of thermocouples.
By the activation of carbon fibers, materials are obtained having larger active surfaces (300–1,000 m2/g). These materials are excellent sorbents. The application of catalysts to the fibers makes possible the creation of catalytic systems with highly developed surfaces.
Carbon fibers usually have strengths of the order of 0.5–1 giga-newton per sq m (GN/m2), equivalent to 50–100 kilograms-force per sq mm (kgf/mm2), and elastic moduli of 20–70 GN/m2(2,000–7,000 kgf/mm2). Fibers subjected to orientational elongation have strengths of 2.5–3.5 GN/m2 (250–350 kgf/mm2) and elastic moduli of 200–450 GN/m2 (20 × 103 –45 × 103 kgf/mm2). Owing to the low density (1.7–1.9 g/cm3), low, that is, in relation to the fibers’ strength and elastic modulus, the mechanical properties of carbon fibers are superior to all known heat-resistant fibrous materials. Structural carbon-fiber reinforced composites having polymeric binders are obtained from carbon fibers possessing high strength and high elastic moduli. Composite materials based on carbon fibers and ceramic binders, carbon fibers and a carbon matrix, and carbon fibers and metals have been developed that are capable of withstanding more rigorous temperature conditions than ordinary plastics.
REFERENCEKonkin, A. A. Uglerodnye i drugie zharostoikie voloknistye materialy. Moscow, 1974.
A. A. KONKIN