Structure of PAN-based carbon fibers

Analytical Techniques. Analytical techniques to determine the structure of carbon fibers include wide-angle and small-angle x-ray diffraction, electron diffraction, neutron scattering, Raman spectroscopy, electron microscopy, and optical microscopy. Detailed reviews of these techniques are found in the literature.  

Structure. The structure of PAN-based carbon fibers is still conjectural to some degree. Yet, thanks to the recent advances in analytical techniques just mentioned, an accurate picture is beginning to emerge. 

Unlike the well-ordered parallel planes of pyrolytic graphite which closely match the structure of the graphite crystal, the structure of PAN-based carbon fibers is essentially turbostratic and is composed of small two-dimensional fibrils or ribbons. These are already present in the precursor and are preferentially aligned parallel to the axis of the fiber. The structure may also include lamellas (small, flat plates) and is probably a combination of both fibrils and lamellas.  

The straightening of the fibrils occurs preferentially, the outer fibrils being more oriented (straightened) than the inner ones as shown in Fig. 8.8.1 1 This has an important and favorable consequence, that is, most of the load-beetring capacity is now transferred to the outer portion or skin of the fiber. 

Sp and Sp Bonding. Another important structural characteristic of PAN-based fibers is the probable existence of sp hybrid bonding as indicated by Raman spectroscopy and shown in Fig. 8.10. In this figure, the pitch-based graphitized fiber (PI 00) is the only one to exhibit a strong sp line. All others show structural disorders which may be caused by some sp bonding.t l The fibers listed in Fig. 8.10 are identified in Secs. 6.3 and 6.4 below. 

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Figure 8.8 (c) three-dimensional structure of PAN-based carbon fiber (after Bennett et al, 1983). 

Figure 12.7 Structure of carbon fiber, (a) A schematic iiiustralion of tree trunk or onion skin structure (ieft) and radial structure (right), (b) A typicai opticai micrograph of carbon fiber cross sections under polarized light in crossed nicols condition showing maltose cross patterns. Polarizer and analyzers are parallel to picture edges. Source Reprinted from Nyo H, Heckler AJ, Hoemschemeyer DL, Characterizing the structures of PAN based carbon fibers, 24 Nat Symposium, San Francisco, 179, 51-60, May 8-10.

To summeirize, small ciystallite size, high interlayer spacing, and general structural disorder are the factors that contribute to the unique and stable turbostratic structure of PAN-based carbon fibers (which is likely to include both sp and sp hybrid bonds), and explain their inability to form a graphitic structure even after high-temperature heat-treatment (i.e., 3000°C). 

Johnson, D. J., Structural studies of PAN-based carbon fibers. In Chemistry and Physics of Carbon, Vol. 20, ed. P. L. Walker. Marcel Dekker, New York, 1987, pp. I 58. 

In DC discharge plasmas, the sudden decrease of contact angle after 15 min treatment has been assigned by S. Okasaki et al. to a structural change of the material surface from crystallised graphite to an amorphous state [93]. It has been shown that the fluorination of PAN-based carbon fibers is more effective in the case of CF4-He plasmas than in 5% F2—He plasmas [94]. The study of the thermal stability of these F-treated fibers has shown however that a 70% loss of fluorine occurred when the samples were heated at 293°C for 10 min. 

Structural Studies of PAN-Based Carbon Fibers, David J. Johnson The Electronic Structure of Graphite and Its Basic Origins, Marie-France Char-lier and Alphonse Charlier... 

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. 

FINE STRUCTURE AND TEXTURE OF PAN BASED CARBON FIBERS... 

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.

The effect of N2 on the structure and properties of PAN based carbon fibers has been studied by Tsai [116]. 

ABSTRACT. The paper focuses on carbon fibers including carbon fiber preparation, the physical properties of carbon fibers, functional groups present on carbon fiber surfaces and the relationship of surface chemistry to composite properties. Specific topics include thermal treatment of PAN-based carbon fibers, carbon fiber structure, tensile breaking strength and modulus, surface area and surface energy, XPS analysis, and chemical derivatization. 

Structural Studies of PAN-Based Carbon Fibers, D. J. Johnson... 

Johnson DF. Structural studies of PAN-based carbon fibers. Chem Phys Carbon 1987 20 1-58. 

Because of their unique blend of properties, composites reinforced with high performance carbon fibers find use in many structural applications. However, it is possible to produce carbon fibers with very different properties, depending on the precursor used and processing conditions employed. Commercially, continuous high performance carbon fibers currently are formed from two precursor fibers, polyacrylonitrile (PAN) and mesophase pitch. The PAN-based carbon fiber dominates the ultra-high strength, high temperature fiber market (and represents about 90% of the total carbon fiber production), while the mesophase pitch fibers can achieve stiffnesses and thermal conductivities unsurpassed by any other continuous fiber. This chapter compares the processes, structures, and properties of these two classes of fibers. 

Since PAN-based carbon fibers tend to be fibrillar in texture, they are unable to develop any extended graphitic structure. Hence, the modulus of a PAN-based fiber is considerably less than the theoretical value (a limit which is nearly achieved by mesophase fibers), as shown in Fig. 9. On the other hand, most commercial PAN-based fibers exhibit higher tensile strengths than mesophase-based fibers. This can be attributed to the fact that the tensile strength of a brittle material is eontrolled by struetural flaws [58]. Their extended graphitic structure makes mesophase fibers more prone to this type of flaw. The impure nature of the pitch preciusor also contributes to their lower strengths. 

The surface properties of carbon fibers are intimately related to the internal structure of the fiber itself, which needs to be understood if the surface properties are to be modified for specific end applications. Carbon fibers have been made from a number of different precursors, including polyacrylonitrile (PAN), rayon (cellulose) and mesophase pitch. The majority of commercial carbon fibers currently produced are based on PAN, while those based on rayon and pitch are produced in very limited quantities for special applications. Therefore, the discussion of fiber surface treatments in this section is mostly related to PAN-based carbon fibers, unless otherwise specified. 

Recall from Section 1.4.5.1 that there are two primary types of carbon fibers polyacrylonitrile (PAN)-based and pitch-based. There are also different structural forms of these fibers, such as amorphous carbon and crystalline (graphite) fibers. Typically, PAN-based carbon fibers are 93-95% carbon, whereas graphite fibers are usually 99+%, although the terms carbon and graphite are often used interchangeably. We will not try to burden ourselves with too many distinctions here, since the point is to simply introduce the relative benefits of continuous-fiber composites over other types of composites, and not to investigate the minute differences between the various types of carbon-fiber-based composites. The interested reader is referred to the abundance of literature on carbon-fiber-reinforced composites to discern these differences. 

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