ARTICLE pubs.acs.org/JPCC
Comparative XRD, Raman, and TEM Study on Graphitization of PBO-Derived Carbon Fibers M. Beatriz Vazquez-Santos,*,† Erik Geissler,‡ Krisztina Laszlo,§ Jean-No€el Rouzaud,|| Amelia Martínez-Alonso,† and Juan M.D. Tascon† †
Instituto Nacional del Carbon, INCAR-CSIC, Apartado 73, 33080 Oviedo, Spain Laboratoire Interdisciplinaire de Physique UMR 5588, Universite J. Fourier de Grenoble, BP87, 38402 St Martin d’Heres Cedex, France § Department of Physical Chemistry and Materials Science, Budapest University of Technology and Economics, H-1521 Budapest, Hungary Laboratoire de Geologie de l’Ecole Normale Superieure, UMR CNRS 8538, 24 rue Lhomond, 75321 Paris Cedex 5, France
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ABSTRACT: Carbon fibers, obtained by carbonizing poly(p-phenylene benzobisoxazole) (PBO) fibers at 900 °C, graphitize extensively upon heat treatment at higher temperatures (2700 °C). In this work, XRD, Raman spectroscopy, and HRTEM are used to monitor the structural and nanostructural transformations of the carbon material under heat-treatment at several temperatures in the interval 9002800 °C. These different techniques provide complementary information, especially regarding the spatial resolution they achieve. They highlight a specific nonconventional mode of graphitization for this unexpectedly graphitizable precursor. The reliability in the determination of La crystallite sizes from these three techniques is compared and discussed. The existence of four steps in the graphitization of PBO-derived carbon fibers is inferred.
Heat treatment above 1000 °C removes most of the heteroatoms, thereby yielding cokes. Heat treatment up to 3000 °C promotes the growth of the layers within a given MO and improves their stacking by eliminating structural defects between the parallel preoriented BSUs. Triperiodic order (“true” graphitization stage with hkl reflections in the diffraction patterns) develops above 2000 °C within the large graphene layer stacks.6 The above mechanism is the standard process used for industrial scale production of graphite from graphitizable carbons such as petroleum cokes, pitch cokes, or both. The key requirement to form graphite is to have precursors with long-range (.1 μm) parallel prealignment of the BSUs. Note, however, that thin carbon films (thickness
