high degree of molecular disorder in polymer chains. Most processes of carbon fiber fabrication involve the following steps:(1)a stabilizing treatment(essentially an oxidation process)that enhances the thermal stability of the fibers and prevents the fiber from melting in the subsequen high-temperature treatment, and (2)a thermal treatment at 1000-1500 C called carbonization that removes noncarbonelements(e. g, N2 and H2). An optional thermal treatment called graphi- tization may be done at 3000C to further improve the mechanical properties of the carbon fiber by enabling the hexagonal crystalline sheets of graphite to increase their ordering To pro- duce high-modulus fiber, the orientation of the graphitic crystals or lamellae is improved by graphitization which consists of thermal and stretching treatments under rigorously controlled conditions. Besides the PAN and cellulosic(e.g, rayon precursors, pitch is also used as a raw material to grow carbon fibers. Commercial pitches are mixtures of various organic compounds with an average molecular weight between 400 and 600. There are various sources of pitch; the three most commonly used are polyvinyl chloride(PVC), petroleum asphalt, and coal tar. The same processing steps(stabilization, carbonization, and optional graphitization) are involved in converting the pitch-based precursor into carbon fiber. Pitch-based raw materials are generally cheap, and the carbon fiber yield from pitch-based precursors is relatively high A recent innovation in carbon-based materials has been carbon nanotubes (CNt). Carb nanotubes are relatively new materials-discovered in 1991--as a minor by-product of the carbon-arc process that is used to synthesize carbons fullerene molecules. They present exciting possibilities for research and use. CNTs are a variant of their predecessor, fullerene carbon (with a geodesic dome arrangement of 60, 70, or even a few hundred C atoms in a molecule) Figure 6-2 shows a photograph of CNT. Single-walled CNT have been grown to an aspect ratio of 10, with a length of about 100 um, and therefore, from a composite mechanics standpoint, they can be considered as long, continuous fibers. The multiwalled CNT has an onion- like"layered structure and is under extremely high internal stress, as evident from FIGURE 6-2 Photograph of carbon nanotubes and polyhedral nanoparticles during fullerene pro- duction(R, Malhotra, R, S. Ruoff and D. C. Lorents, "Fullerene Materials, " Advanced materials rocesses, April 1995 p. 30). Reprinted with permission from ASM International, Materials Park, Oh(www.asminternational.org Composite Materials 401high degree of molecular disorder in polymer chains. Most processes of carbon fiber fabrication involve the following steps: (1) a stabilizing treatment (essentially an oxidation process) that enhances the thermal stability of the fibers and prevents the fiber from melting in the subsequent high-temperature treatment, and (2) a thermal treatment at 1000-1500~ called carbonization that removes noncarbon elements (e.g., N2 and H2). An optional thermal treatment called graphi￾tization may be done at ~3000~ to further improve the mechanical properties of the carbon fiber by enabling the hexagonal crystalline sheets of graphite to increase their ordering. To pro￾duce high-modulus fiber, the orientation of the graphitic crystals or lamellae is improved by graphitization which consists of thermal and stretching treatments under rigorously controlled conditions. Besides the PAN and cellulosic (e.g., rayon) precursors, pitch is also used as a raw material to grow carbon fibers. Commercial pitches are mixtures of various organic compounds with an average molecular weight between 400 and 600. There are various sources of pitch; the three most commonly used are polyvinyl chloride (PVC), petroleum asphalt, and coal tar. The same processing steps (stabilization, carbonization, and optional graphitization) are involved in converting the pitch-based precursor into carbon fiber. Pitch-based raw materials are generally cheap, and the carbon fiber yield from pitch-based precursors is relatively high. A recent innovation in carbon-based materials has been carbon nanotubes (CNT). Carbon nanotubes are relatively new materials--discovered in 1991--as a minor by-product of the carbon-arc process that is used to synthesize carbon's fullerene molecules. They present exciting possibilities for research and use. CNTs are a variant of their predecessor, fullerene carbon (with a geodesic dome arrangement of 60, 70, or even a few hundred C atoms in a molecule). Figure 6-2 shows a photograph of CNT. Single-walled CNT have been grown to an aspect ratio of ~105, with a length of about 100 Ixm, and therefore, from a composite mechanics standpoint, they can be considered as long, continuous fibers. The multiwalled CNT has an "onion-like" layered structure and is under extremely high internal stress, as evident from FIGURE 6-2 Photograph of carbon nanotubes and polyhedral nanoparticles during fullerene pro￾duction (R. Malhotra, R. S. Ruoff and D. C. Lorents, "Fullerene Materials," Advanced Materials & Processes, April 1995 p. 30). Reprinted with permission from ASM International, Materials Park, OH (www.asminternational.org). Composite Materials 401