Mesophase pitch-based carbon fibers

To date, there has been relatively little work reported on the mesophase pitch rheology which takes into account its liquid crystalline nature. However, several researchers have performed classical viscometric studies on pitch samples during and after their transformation to mesophase. While these results provide no information pertaining to the development of texture in mesophase pitch-based carbon fibers, this information is of empirical value in comparing pitches and predicting their spinnability, as well as predicting the approximate temperature at which an untested pitch may be melt-spun. 

The above equations have been solved to predict the commonly observed radial and line-origin textures seen in circular and non-circular mesophase pitch-based carbon fibers [39]. 

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. 

Further improvements in the properties of PAN-based carbon fibers are likely to emerge through improved stabilization, that is, by creating the ideally cross-linked fiber. On the other hand, as purer pitch precursors become available, further improvements in mesophase pitch-based carbon fibers are likely to arise from optimized spinnerette designs and enhanced understanding of the relationship between pitch chemistry and its flow/orientation behavior. Of course, the development of new precursors offers the potential to form carbon fibers with a balance of properties ideal for a given application. 

Fig. 8. Observed textures of mesophase pitch-based carbon fibers (adapted from [55]).

Mesitylene, production from acetone, 1 164 Mesityl oxide, 14 589-590 characteristics of, 16 337 hydrogenation, 16 337-338 hydrogen peroxide treatment of, 16 338 Z-menthol from, 24 520 production of, 16 336-337 production from acetone, 1 164, 174 Mesogenic diols, 25 460 Mesogenic molecules, solids of, 15 82 Mesogens, 24 53, 54 Mesomixing, 16 683 Mesomorphic behavior, 24 53-54 Mesomorphic phase transitions, 15 102 Mesomorphism, 15 81. See also Liquid crystalline materials Mesophase pitch-based carbon fiber, 26 734-735... 

Mesophase pitch Mesophase-pitch-based carbon fibers... 

FIGURE 2.41 Scanning electron micrographs and structural parameters of mesophase-pitch-based carbon fibers with different cross-sectional nanotextures. 

Takami N, Satoh A, Hara M, Ohsaki T. Rechargeable Lithium-ion cells using graphitized mesophase -pitch-based carbon fiber anodes. J Electrochem Soc 1995 142 2564-2571. 

Nishimura Y, Yakahashi T, Tamaki T, Endo M, Dresselhaus MS. Anode performance of B-doped mesophase pitch-based carbon fibers in lithium ion secondary batteries. Tanso 1996 172 89-94 (in Japanese). 

Matthews MJ, Dresselhaus MS, Dresselhaus G, Endo M, Nishimura Y, Hiraoka T, Tamaki N. Magnetic alignment of mesophase pitch-based carbon fibers. Appl Phys Lett 1996 69 430-432. 

The technology of mesophase-pitch-based carbon fiber has stimulated the rapid development of the chemistry of mesophase behavior and preparation. The carbonization schemes and mechanisms leading to optical anisotropy via the mesophase, the control of carbonization with emphasis on the preparation of spinnable mesophase, and the mesophase transition and reactivity in relation to the structure of its constituent molecules are summarized in this paper. 

Figure 17. Transmission electron micrographs of PAN-based and mesophase-pitch-based carbon fibers, as delivered and after heat treatment (42,43). HT and HM refer to high strength and high modulus PAN-based fibers P55 and P100 refer to mesophase-pitch-based fibers, with tensile moduli of 55 and 100 Mpsi, respectively.

Mesophase-pitch-based carbon fibers can have up to three times the thermal conductivity of copper. This would make them an ideal material for thermal management applications, e.g. brake disks where heat dissipation is of prime consideration. The extremely high thermal conductivity is a direct result of the extremely high degree of crystallinity obtained during carbonization of the mesophase-pitch precursor fiber. 

Mochida, I., Yoon, S.-H., Takano, N., et al. (1996). Microstructure of mesophase pitch-based carbon fiber and its control. Carbon, 34, 941-56. 

Another structure for carbons is texture, the ways that the crystallites joined together. Texture is often characterized by the degree of orientation from random to systematic arrangement. If the crystalhte size is small enough and there is no specific orientation, the carbon appears to be amorphous. Texture control cannot change the properties of individual crystalhtes but can alter the properties of the agglomerates of these crystallites like electricity and active surface area The comparative study of mesophase-pitch-based carbon fibers with different textures showed that the radical texture is more favorable for Lb intercalation than the concentric texture, but the radical texture is more easily broken into pieces by solvent cointercalated Lb. 

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