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Figure 3. Schematic 3-D structural model of Fortafil 5-Y PAN-based, 345 GPa tensile modulus carbon fiber (ref. 14).
Based on these studies. PAN-based carbon fibers appear to contain extensively folded and interlinked turbostratic layers of carbon with an interlayer spacing considerably larger than that of true graphite. They show a low degree of graphitization, and the turbostratic layers are not highly oriented with the fiber axis.
Microstructure of Pitch-Based Carbon Fibers
Unlike PAN-based carbon fibers, pitch-based fibers show a variety of microstructures. Varying the spinning conditions of the liquid-crystalline precursor produces these various microstructures (shown schematically in fig. 6), which are apparent upon microscopically examining the fiber cross section. Commercial fibers usually exhibit either radial, flat-layer, or random microstructures, and these three formations appear to be the preferred microstructures of mesophase pitch. Endo (ref. 15) showed that Carbonic pitch-based fibers have a radial-folded microstructure; other investigators have produced pitch-based fibers with line-origin, onion skin, and quasi-onion microstructures in laboratory experiments. Each type of microstructure, except for random microstructure, is viewed as a collection of large flat plates extending down the fiber axis, arranged in some geometric order
across the fiber cross section. In the random microstructure, the plates are relatively small and have no long-range geometric ordering across the fiber. Nevertheless, they are still oriented almost parallel to the fiber axis.
Radial Onion skin Random Quasi-onion
Figure 6. Microstructures of pitch-based carbon fibers.
In his study, Endo (ref. 15) compared several grades of commercial pitch-based fibers, Thornel PI00 and PI20 (Amoco Performance Products, Incorporated) and Carbonic HM50, HM60, and HM80 (Kashima Oil Company), with the Torayca PAN-based fibers. Table 1 lists the tensile strength, Young's modulus, failure strain, interlayer spacing, and crystallite thickness of each fiber.
The X-ray diffraction results showed that only the Thornel fibers exhibited separation of the 100 and 101 peaks and the appearance of a 112 peak. This indicated that only the Thornel fibers had a high degree of 3-D ordering. Endo also found that, as the strength of Carbonic fibers increased, the X-ray diffraction profile began to resemble that of the Torayca PAN-based fibers.
In addition, the layer planes in the Carbonic fibers appeared to be oriented within 15° of the fiber axis, while the layer planes in the Thornel fibers were oriented almost perfectly with the fiber axis. This misorientation in the Carbonic fibers was more typical of PAN-based fibers. Further TEM studies indicated that the layer planes of the Carbonic fibers were a folded structure, while the layer planes of the Thornel fibers exhibited no folds. These findings led Endo (ref. 15) to conclude that (1) Thornel fibers consist of straight, well-oriented plates with a high degree of 3-D graphitization, and (2) Carbonic fibers consist of turbostratic, somewhat misoriented, graphite-like layer planes that fold back and forth as they extend radially outward from the center of the fiber. Endo's visualization of these structures is shown schematically in figure 7.
Hamada et al. (refs. 16 and 17) have produced pitch-based fibers with various microstructures using a laboratory-scale, melt-spinning system. To alter the preferred microstructure, they stirred the pitch above the spinnerette capillary. By using stirrers of different shapes, as shown in figure 8, they were able to alter the flow profile so that random, onion, and quasi-on ion microstructures resulted. Without any stirrer, radial microstructures resulted, showing that the microstructure of mesophase, pitch-based carbon fibers is locked-in during spinning.
Table 1. Properties of Pitch-Based and PAN-Based Carbon Fibers (ref. 15)
Stirring not only changed the microstructure of the fibers, but also it modified their crystalline characteristics; an example of this is shown in table 2. Hamada used X-ray diffraction to compare the degree of 3-D order present in the various fibers. For fibers spun without stirring (radial), he found separation of the 100 and 101 peaks and the appearance of a clear 112 peak. Fibers spun with stirring showed no separation of the 100 and 101 peaks and a very weak 112 peak.