Mesophase pitch carbon fiber structure

Figure 7.13 Structural transition from random to radial transverse texture in a mesophase pitch carbon fiber after heat treatment up to 2800°C. Source Reprinted from Edie DD, Research into pitch based carbon fibers. In Genisio M ed.. Recent Research into Carbon-Carbon Composites, Southern Illinois University, 16-41, 1987.

Mochida I, Zeng SM, Korai Y, Hino T, Toshima H, Structure and properties of mesophase pitch carbon fiber with a skin core structure carbonized under strain, J Mater Sci, 27(7), 1960-1968, 1992. 

Cross Section. The cross-section structure of mesophase-pitch carbon fibers is one of the four types shown in Fig. 8.12 and is determined by the spinning method, the temperature of stabilization, and the partial pressure of oxygen.The formation of a skin-core structure or skin effect is often observed. This structure is similar to that of the PAN-based carbon fiber shown In Fig. 8.8 above. 

Hamada, T., Nishida, T., Furuyama, M. and Tomioka, T., Transverse structure of pitch fiber from coal tar mesophase pitch. Carbon, 1988, 26(6), 837 841. 

Low density, carbon fiber-carbon binder composites are fabricated from a variety of carbon fibers, including fibers derived from rayon, polyacrylonitrile (PAN), isotropic pitch, and mesophase pitch. The manufacture, structure, and properties of carbon fibers have been thoroughly reviewed elsewhere [3] and. therefore, are... 

Because of their unique blend of properties, composites reinforced with high performance carbon fibers find use in many structural applications. However, it is possible to produce carbon fibers with very different properties, depending on the precursor used and processing conditions employed. Commercially, continuous high performance carbon fibers currently are formed from two precursor fibers, polyacrylonitrile (PAN) and mesophase pitch. The PAN-based carbon fiber dominates the ultra-high strength, high temperature fiber market (and represents about 90% of the total carbon fiber production), while the mesophase pitch fibers can achieve stiffnesses and thermal conductivities unsurpassed by any other continuous fiber. This chapter compares the processes, structures, and properties of these two classes of fibers. 

The properties of mesophase pitch-based carbon fibers can vary significantly with fiber texture. Inspection of the cross-section of a circular mesophase fiber usually shows that the graphitic structure converges toward the center of the fiber. This radial texture develops when flow is fully developed during extrusion through the spinnerette. Endo [48] has shown that this texture of mesophase pitch-based carbon fibers is a direct reflection of their underlying molecular structure. 

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. 

The surface properties of carbon fibers are intimately related to the internal structure of the fiber itself, which needs to be understood if the surface properties are to be modified for specific end applications. Carbon fibers have been made from a number of different precursors, including polyacrylonitrile (PAN), rayon (cellulose) and mesophase pitch. The majority of commercial carbon fibers currently produced are based on PAN, while those based on rayon and pitch are produced in very limited quantities for special applications. Therefore, the discussion of fiber surface treatments in this section is mostly related to PAN-based carbon fibers, unless otherwise specified. 

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