Mesophase carbon fiber

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

In the previous symposium, we reviewed mesophase mechanisms involved in the formation of petroleum coke ( 2 ). Since 1975, two significant developments have been the use of hot-stage microscopy to observe the dynamic behavior of the carbonaceous mesophase in its fluid state (3-6), and the emergence of carbon fibers spun from mesophase pitch (7-9) as effective competitors in applications in which high elastic modulus or good graphiticity is important. This paper focuses on mesophase carbon fibers as an example of how the plastic mesophase can be manipulated to produce fibers with intense preferred orientations and elastic moduli that approach the theoretical limit for the graphite crystal in the a-direction. 

Figure 1 shows where mesophase carbon fibers fit in the rapidly developing field of structural carbon fibers. At present, fibers produced from polyacrylonitrile (PAN) constitute the bulk of the carbon fiber produced in 1981 all commercial PAN-based fibers fell below and to the left of the "1981 limit" for tensile strength and modulus. Since 1981, there have been major advances in the development of PAN-based fibers to achieve high strengths, particularly at the lower modulus levels (near 40 Mpsi, 276 GPa). 

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. 

The most common morphology observed in current mesophase carbon fibers of moderate modulus (55 to 75 Mpsi, 379 to 517 GPa) is a cylindrical filament with a random-structured core and a radial rim (12) Given the fracture section of Figure 3, with its scroll-like features, the core appears to be an array of +2ir and -ir disclinations. The radial rim of heavily wrinkled layers usually constitutes half or more of the cross section. 

Figure 2. Mesophase carbon fiber with radial structure and open-wedge shape. Polarized light.

Figure 3. Mesophase carbon fiber with random-core structure and round shape.

Figure 4. Tensile fracture surfaces for three structural types of high-modulus mesophase carbon fibers (E = 100 Mpsi ...

On the basis of this discussion, the mechanisms of mesophase carbon fiber formation are closely related to those of needle coke, the principal differences being the extent to which the deformation and relaxation mechanisms are able to act. Because delayed coking involves relatively gentle but random deformation processes by bubble percolation and the long dwell times in the coke drum afford opportunity for extensive disclination annihilation and micro-structural relaxation, the structure of needle coke can be well defined by polarized-light microscopy (2,36). 

Similarly, PZ pitch as precursor for HPCF was replaced by other mesophase pitches (12). At this point in time, as is well-known, Singer (13) and Lewis (14) of the Union Carbide Corporation developed similar methods. Mesophase carbon fiber progressed more rapidly in the USA than in Japan because Japanese defense and aerospace needs were less demanding. Recently, however, the drive toward higher-added-value products from the heavy fractions of coal and petroleum has intensified, and pitch-based carbon fibers, including HPCF, are now the subjects of extensive investigation in many Japanese laboratories. 

Figure 4.30 shows the effect of temperature on the structure and a pitch fiber, with an onion skin structure, is preferred to a radial type structure. Possible cross sectional microstmctures of mesophase carbon fibers are given in Figure 4.34 and modification of the flow profile during extrusion can produce a less flow-sensitive product and higher tensUe strength. The lines within each section depict carbon layers, which are at least preferentially parallel to the fiber axis. 

Figure 4.34 Possible cross-sectional microstructures of mesophase carbon fibers. Source. Reprinted with permission from Savage G, Carbon-Carbon Composites, Chapman and Hall, London, p. 63,1992. Copyright 1992, Springer.

Matsumoto and Mochida [33], using NMR with a hydrogenated coal tar pitch mesophase carbon fiber, showed that the initial attack by oxygen was on -CH3 and -CH2- groups, with the gradual formation of carboxyls, esters and aryl carbonyls. Fairly stable cross-links were formed via phenols, ethers and esters. The workers also found that slower heating rates (0.5°C/min instead of 2.0°C/min) produced better mechanical properties and that the final choice would be controlled by the desired target properties and economics. 

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