Carbon Fibers preferred orientation

The structure of CBCF is shown in the SEM micrograph in Fig. 4. The crenellated surface of the rayon derived carbon fibers is clearly visible, as is the phenolic derived carbon binder. The preferred orientation of the fibers (resulting from the slurry molding operation) is obvious in Fig. 4, and imparts considerable anisotropy to the material. The molding direction is perpendicular to the plane of the carbon fibers in Fig. 4. 

The carbon fibers present a filamentary shape with a diameter of few tens of nanometers (nanofibers) to few tens of micrometers. The structure is that of short graphitic segments. The preferred orientation of the graphitic planes is parallel to the fiber axis. The morphology can be radial, cross-sectional, or circumferential [25]. The carbon fibers present excellent mechanical and thermal properties. The carbon fibers are produced by two ways [26] ... 

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. 

Carbon fibers can have extremely anisotropic mechanical properties, depending on the intensity of the preferred orientation of the graphitic layers in the direction parallel with the fiber axis. This orientation can be seen in the TEM micrographs of Figure 17. 

The overwhelming success of PAN-based carbon fibers over rayon and pitch can be attributed to several key aspects.f Structurally, PAN has a faster rate of pyrolysis without much disturbance to its basic structure and to the preferred orientation of the molecular chains along the fiber axis present in the original fiber. By contrast, carbon fibers from rayon suffer from extremely low carbon yield (20-25%) due to chain fragmentation, which eliminates the orientation of the precursor fiber. While improved properties can be achieved by stretch graphitization, this process is expensive and does not compensate for the low yields. 

Much less ordered than PAN-based high-strength CFs are the isotropic CFs. They are produced by the carbonization of isotropic pitch fibers (or other fibrous precursors such as phenolic resins or cellulose, including rayon), without any attempt to obtain a preferred orientation of the polyaromatic molecules in the fiber direction. Consequently, they have a random nanotexture and belong to the low modulus class of CFs [16]. Rather than being used for high-performance reinforcement purposes, they find their application as thermal insulators for furnaces or as reinforcements for cement [1]. Another important use of isotropic CFs is as a feedstock for the production of activated carbon fibers, a material dealt with in Section 2.4.4. 

Carbon fibers, which utilize the preferred orientation of the graphene layers, show not only high modulus and high strength but also high thermal conduction and low thermal expansion along the fiber axis. On the basis of these properties, fiber reinforced materials present the opportunity to design the thermal properties into materials. 

The forced flow-thermal gradient CVI process (FCVI) has been shown to permit the rapid consolidation of SiC matrix composites. Recently, the FCVI process has been extended to the fabrication of carbon fiber-carbon matrix composites. Using 2D carbon cloth preforms, composite disks 0.8 cm thick have been fabricated in less than three hours a small fraction of the time required for either the resin/pitch or conventional CVI processing. Further, the FCVI process facilitates the incorporation of oxidation inhibitors within the carbon matrix and may permit obtaining a preferred crystallographic orientation that yields the high thermal conductivity required for thermal management applications. 

As a result, the density of PAN based carbon fibers, ranging from 1.76 g/cm for HT fibers to 1.87 g/cm for HM fibers, is lower than that of mesopitch (MP) based carbon fibers, i.e., 2.0-2.20 glonf. As a result, the density of MP based carbon fibers approaches that of the ideal graphite single crystal, i.e., 2.278 g/cm [8]. Finally, a more preferred orientation of the coherent domains is achieved with MP based carbon fibers than with PAN based carbon fibers and it is improved for MP based fibers by raising the heat treatment temperature (HTT) and thereby reaching Z-values as low as 5°. 

Figure 13. Young s modulus and preferred orientation parameter of PITCH and PAN based carbon fibers reproduced with permission from Elsevier Sclence-NL Amsterdam and Chapman Hall.

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