Graphitic carbon fibers microstructure

Fig. 9.1 Top view on two variants of C3 materials. The carbon fibers (a) themselves exhibit a complex inner microstructure that needs carful optimization for strength and stability. The isotropic filler phase (b) should be free of pores and other weak points caused by uneven distribution in the composite body. The ordered graphitic BSU (c) can provide a very strong but still flexible anchoring of the fibers in the isotropic matrix.

Microstructure Formation in Mesophase Carbon Fibers and Other Graphitic Materials... 

This paper commences with evidence for lamelliform morphologies in mesophase carbon fiber, summarizes relevant information on disclination structures in the carbonaceous mesophase, and then reviews what we learn of disclination behavior from hot-stage observations and from deformation and carbonization experiments. The results indicate that disclination interactions that occur before the mesophase is fully hardened play an important role in determining the microstructures of mesophase carbon fibers, as well as those of cokes and graphites that form through the carbonaceous mesophase. 

Attempts to produce a viable SiC fiber by Reaction 1 have not been successful because the approximately 87 percent increase in volume that occurs upon conversion disrupts the microstructure and, therefore, compromises the mechanical properties of the fiber. Reaction 2 proceeds, theoretically, with approximately 6 percent shrinkage. However, recent attempts by MER Corporation (Kowbel, 1997) to convert entire filaments via Reaction 2 have been unsuccessful. Furthermore, earlier attempts to fully convert oriented carbon fiber were also unsuccessful. The interior of these carbon fibers generally remains unconverted graphite attempts by the New Oji Paper Company (Okada et al., 1995) using a porous, activated carbon fiber (ACF), have been successful... 

Figure 5.23 A schemalic microstructure of PAN based carbon fiber depicting combination of basic structural units into microdomains. A, Skin region B, Core region C, A hairpin defect D, A wedge disdination. Source Reprinted with permission from Bennett SC, Johnson DJ, Strength structure relationships in PAN-based carbon fibres, London International Carbon and Graphite Conference, Soc Chem Ind, Lend, 377,1978. Copyright 1978, The Society of Chemical Industry.

Kowbel W, Hippo E, Murdie N, Influence of graphitization environment of PAN based carbon fibers on microstructure. Carbon, 27(2), 219-226, 1989. 

The microscopy techniques described for the evaluation of glass fiber composites are widely used to determine the microstructure of carbon and graphite fiber composites. Microscopy of crack propagation in carbon fiber reinforced composites is also very important in understanding mechanical properties. Test specimens and actual composite products are often evaluated to determine the distribution of the fibers in the resin, typically epoxy, and the degree of resin wetting of the fibers. Voids in the composite can be the locus of failure, and their identification and cause are quite important to mechanical property evaluation. 

The microscopy techniques described for evaluation of glass fiber composites are widely used to determine the microstructure of carbon and graphite fiber composites. Microscopy of crack propagation in carbon fiber reinforced composites is also very important in understanding... 

Carbon materials can also be classified in other ways, such as stacking methods, crystallinity, and structural symmetry. From the viewpoint of crystallinity, carbons can be classified as crystalline or amorphous. From the point of view of stacking methods, carbon structures can be classified as graphite, glassy carbon, carbon fiber, and carbon black. From the point of view of symmetry of the microstructure, carbons can be classified as randomly orientated or having point symmetry, axial symmetry, or planar symmetry (Figure 7.3) [2]. 

At RT, thermal conductivities of carbon fiber composites with high tensile fibers differ from those of high modulus fibers and are much higher than the thermal conductivity of the epoxy matrix. Below 7 K, they become similar within 25% and lower than the thermal conductivity of the epoxy matrix (37,47). The similarity is owing to the fact that at low temperatures only long phonon wavelengths are activated they cannot resolve different graphite microstructures of different carbon fiber types which are dominant at RT (36,43). In most cases, the specific heat of composites is lower than that of the polymeric matrix. 

---