dimethylsulfoxide, or zinc chloride and water.6 Depending on the molecular weight, solvent, and the copolymer composition, the polymer concentration in solution is typically 5 to 25 wt percent. After spinning, fibers are successively drawn at several different temperatures (typically between room temperature and 175°C). Drawn fibers are oxidized under tension typically between 200°C and 350°C for approximately 2 hours. Oxidized fibers are then carbonized under tension in stages, similar to the carbonization of pitch-based fiber. Fibers with the highest tensile strength are typically obtained at about 1300°C to 1500°C.
CARBON NANOTUBE FIBERS
Carbon nanotube (CNT) fibers to date have been processed primarily by one of the following two techniques: (1) CNT smoke drawn directly from the chemical vapor deposition reactor in the form of aerogel fibers7 and (2) fiber processed from aqueous8 or acidic9 dispersions of CNTs. In both cases, it is important that the CNTs be as long as possible and as perfect as possible, and they should be free of catalyst and other foreign impurities, including amorphous carbon. The tube-to-tube diameter variation should be minimized and the diameter should be relatively small. Nanotube orientation also plays a critical role with respect to mechanical properties.10 Multiwall CNTs tend to undergo telescoping, with the individual tubular shells slipping past one another, whereas single-wall CNTs are essentially the ultimate for a high-strength polymer molecule, having a theoretical strength as high as 150 GPa and modulus values as high as 1,050 GPa, respectively. The theoretical modulus of carbon nanotubes is dependent on their diameter since their central portion is empty; however, their specific theoretical modulus is 469 N/tex irrespective of the diameter.
ALUMINA, BORON, SILICON CARBIDE, GLASS, AND ALUMINA BOROSILICATE CERAMIC FIBERS
Boron fiber is processed using chemical vapor deposition on substrates such as tungsten or carbon, whereas silicon carbide fibers can be processed either by chemical vapor deposition or by a precursor method similar to the processing of carbon fibers. Alumina and alumina borosilicate fibers are typically processed using a sol-gel precursor followed by sintering. Nextel fibers (from 3M Company) are ceramic oxide fibers that belong to the category of alumina-boro-silicate. Compared to polymeric and carbon fibers, these fibers retain their mechanical properties to much higher temperatures. Although the tensile strength of these fibers is not quite as high as that of some of the polymeric fibers, their compressive strength can be comparable to or higher than that of carbon fiber having the best compressive strength. Owing to ionic-covalent bonds in all directions, these fibers are much more isotropic than are carbon and polymer fibers, which exhibit a very high degree of anisotropy.11,12,13
Glass is melt-extruded and drawn into fibers typically at 1000°C to 1200°C. Fiber tensile strength is limited by defects, residual stresses, and structural inhomogeneities in the fibers.
6Gupta, V., and V. Kothari. 1997. Manufactured Fibre Technology. New York, N.Y.:Chapman and Hall.
7Koziol, K., J. Vilatela, A. Moisala, M. Motta, P. Cunniff, M. Sennett, and A. Windle. 2007. High-performance carbon nanotube fiber. Science 318(5858): 1892-1895.
8Vigolo, B., A. Penicaud, C. Coulon, C. Sauder, R. Pailler, C. Journet, P. Bernier, and P. Poulin. 2000. Macroscopic fibers and ribbons of oriented carbon nanotubes. Science 290(5495): 1331-1334.
9Ericson, L., H. Fan, H. Peng, V. Davis, W. Zhou, J. Sulpizio, Y. Wang, R. Booker, J. Vavro, C. Guthy, A. Parra-Vasquez, M. Kim, S. Ramesh, R. Saini, C. Kittrell, G. Lavin, H. Schmidt, W. Adams, W. Billups, M. Pasquali, W-F. Hwang, R. Hauge, J. Fisher, and R. Smalley. 2004. Macroscopic, neat, single-walled carbon nanotube fibers. Science 305(5689): 1447-1450.
10Liu, T., and S. Kumar. 2003. Effect of orientation on the modulus of SWNT films and fibers. Nano Letters 3 (5): 647-650.
11Chawla, K. 1998. Fibrous Materials. Cambridge, U.K.: Cambridge University Press.
12Elices, M., and J. Llorca. 2002. Fiber Fracture. Oxford, U.K.: Elsevier Science.
13Watt, W., and B. Perov, eds. 1986. Strong Fibers. North-Holland: Elsevier Science.