Carbon based fibers

The advent of high-strength carbon-based fibers has led to a number of structural applications in medical treatment. The specifications for implanted materials required that they offer acceptable long-term mechanical properties and surfaces that are biocompatible [126]. Surface compatibility affects immediate acceptance, while the long-term mechanical performance is determined by the bulk properties of the implant. 

Carbon itself has been successfully used as a biomaterial. Carbon based fibers used in composites are known to be inert in aqueous (even seawater) environments, however they do not have a track record in the biomaterials setting. In vitro studies by Kovacs [1993] disclose substantial electrochemical activity of carbon fiber composites in an aqueous environment. If such composites are placed near a metaUic implant, galvanic corrosion is a possibility. Composite materials with a polymer matrix absorb water when placed in a hydrated environment such as the body. Moisture acts as a plasticizer of the matrix and shifts the glass transition temperature towards lower values [Delasi and Whiteside, 1978], hence a reduction in stiffness and an increase in mechanical damping. Water immersion of a graphite epoxy... 

Similar to the Types la and lb fibers, the Type Ic fibers consist of a thin, conductive metal layer electrodeposited upon a carbon base fiber (see Figure 5). Their manufacture is described by Morin(27) and by HaU and Ando(25). The paper by Hall and Ando provides a good overview of their properties and characteristics. Nickel plated exPAN carbon fiber are typical of the Type Ic fibers that are readily commercially available. General uses for Type Ic fibers are in ESI shielding, conductive adhesives and paints, conductive fabrics, and high performance electric contacts(2P). They are included in the Type 1 category because their conductivity is characteristically metallic. Thus, by this convention they appear in the Type 1 classification while the various other carbon fibers fall into the Type 2 category. 

It has been more than 50 years that carbon fibers have been under development from rayon, polyacrylonitrile (PAN), isotropic and mesophase pitches. PAN-based technologies are most commercial production of carbon fibers and rayon-based carbon fibers are no longer in production. Pitch-based carbon fibers currently account for niche markets. Isotropic pitch-based carbon fibers have modest level of strength and modulus and are the least expensive carbon fibers. PAN and mesophase-based carbon fibers heat treated to improve modulus. Both PAN and mesophase pitch-based carbon fibers are not subject to creep or fatigue failure and set them apart from other material which is critical for application such as tension leg platforms for deep sea oil production [201]. The new generation of carbon based fibers is carbon nanotube (single and multi-walled). There are different synthesis methods for carbon... 

Lastly, synthetic varieties of textiles are present primarily in apparel and are either petroleum-based or blends with natural fibers. Polyester fibers, aramid fibers, acrylics, nylon, polyurethane, olefins (hydrophobic), polylactide (hydrophilic), milk protein-based fibers, and carbonization-based fibers all constitute synthetics which require some level of surface-modification. This includes nonwovens, structures bonded together by entangling fiber or filaments mechanically, thermally, or chemically. 

Fibers produced from pitch precursors can be manufactured by heat treating isotropic pitch at 400 to 450°C in an inert environment to transform it into a hquid crystalline state. The pitch is then spun into fibers and allowed to thermoset at 300°C for short periods of time. The fibers are subsequendy carbonized and graphitized at temperatures similar to those used in the manufacture of PAN-based fibers. The isotropic pitch precursor has not proved attractive to industry. However, a process based on anisotropic mesophase pitch (30), in which commercial pitch is spun and polymerized to form the mesophase, which is then melt spun, stabilized in air at about 300°C, carbonized at 1300°C, and graphitized at 3000°C, produces ultrahigh modulus (UHM) carbon fibers. In this process tension is not requited in the stabilization and graphitization stages. 

Pitch-based fibers generally have higher moduh but lower strengths than theh PAN-based counterparts. The specific properties of the various types of carbon fibers are compared in Figure 4. Pitch-based fibers also have higher electrical conductivity, which can be an important consideration in certain circumstances, for example, for use in electromagnetic inductance (EMI) shielding. 

Since PAN-based carbon fibers tend to be fibrillar in texture, they are unable to develop any extended graphitic structure. Hence, the modulus of a PAN-based fiber is considerably less than the theoretical value (a limit which is nearly achieved by mesophase fibers), as shown in Fig. 9. On the other hand, most commercial PAN-based fibers exhibit higher tensile strengths than mesophase-based fibers. This can be attributed to the fact that the tensile strength of a brittle material is eontrolled by struetural flaws [58]. Their extended graphitic structure makes mesophase fibers more prone to this type of flaw. The impure nature of the pitch preciusor also contributes to their lower strengths. 

In contrast, there is also current interest in investigating PAN-based fibers in low thermal conductivity composites [62], Such fibers are carbonized at low temperature and offer a substitute to rayon-based carbon fibers in composites designed for solid rocket motor nozzles and exit cones. 

Carbon dioxide is another clinically important analyte, but also of highest interest in marine sciences and in context with the greenhouse effect. While sensable , in principle, by IR absorptiometry, this is difficult in case of fluid samples. An indicator-based fiber optic device for CO2 was described... 

Piston type aerosol system, 1 784 PIT (powder-in-tube) conductors, critical current density in, 23 833-834. See also PIT technique Pitch-based carbon fibers, 26 733-735 asbestos substitute, 3 314t compressive strength versus tensile modulus for, 26 742-743 Pitch-based fibers, 26 760 Pitch binders, 12 724-726 Pitchblende, 25 396-397 Pitch control, 10 304 Pitched blade turbine (PBT), 16 672 Pitches... 

Chen T, Qiu L, Cai Z, Gong F, Yang Z, Wang Z, et al. Intertwined aligned carbon nanotube fiber based dye-sensitized solar cells. Nano Lett. 2012 Apr 13 12(5) 2568-72. 

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