X-Ray Diffraction Studies of Structural Dimensionality in Carbon Fibers
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JAMES STAMATOFF, HARRIS GOLDBERG, AND ILMAR KALNIN Celanese Research Company, 86 Morris Ave., Summit, N.J. 07901, USA
ABSTRACT Diffraction patterns of carbon fibers have been recorded photographically using monochromatized MoKa radiation. and reflections are observed for fibers pyrolyzed at lower temperatures. The absence of reflections suggests that the graphite planes are turbostratic. A three-dimensional structural transition is manifest in the appearance of reflections. The results suggest that the degree of three dimensionality increases progressively through the sample as a function of increasing temperature of pyrolysis. There is no evidence for coexistence of three dimensional and tubostratic phases. It is further demonstrated that turbostratic carbon fibers may be intercalated to stage two with AsF 5 . INTRODUCTION Researchers exploring the intercalation of graphite normally specify the origin of graphite used, because it is well known that the microstructure and the degree of perfection of the host graphite affects not only the intercalation kinetics, but also the equilibria and the establishment of well defined stages. Even so, samples of a given type of graphite can show variability in the structural perfection which can conceivably affect the intercalation. Thus, the large spread in the electrical conductivity observed in HOPG (Highly Oriented Pyrolytic Graphite) graphite intercalated to stage 2 with AsF 5 has been attributed, at least partly, to variation in the defect density and crystallite size of the host samples [1]. Even greater variations in the properties may be caused by the structural features of the commercially available carbon graphite fiber types. Thus, the availability of such fibers offers an excellent opportunity to determine the effect of these features on the intercalation process. Both the "carbon" and "graphite" fiber structures have been investigated by x-ray diffraction and electron microscopy [2-8]. We have systematically studied fibers formed by the pyrolysis of polyacrylonitrile polymer (PAN) and pitch using x-ray diffraction methods. We examine the full range of available pyrolysis products (i.e., fibers having Young's moduli varying from 30 to 100 million psi(Msi). We also report the intercalation behavior of these fibers with AsF5 and relate this behavior to the structure of the host fiber. The most detailed results will be given for PAN fibers. However, similar results have been obtained for pitch based fibers. EXPERIMENTAL For this study all x-ray diffraction measurements were performed photographCarbon fibers were mounted within a glass capillically using MoKa radiation. Measurements ary which was removed very near the incident x-ray beam. Scans were of the patterns were made using a Joyce-Loebl microdensitometer. transmitted to a computer for analysis.
Mat. Res. Soc. Symp. Proc. Vol. 20 (1983) OQEsevier Science Publishing Co.,
Inc.
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RESULTS Figure 1 shows diffraction patterns obtained from four different PAN based carbon fibers with Young's moduli of approximat
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