Analysis of carbon fibers and carbon composites by asymmetric X-ray diffraction technique

The effect of annealing on the microstructure of three sets of carbon fibers and their composites with a phenol-formaldehyde matrix was investigated by X-ray diffraction. An asymmetric diffraction geometry with parallel beam optics was used to get more information in comparison with conventional diffraction experiments. It was found that the microstructure of the samples is improved after annealing up to 2800 degrees C in terms of the following parameters: the narrowing of the orientation distribution function of (001) planes by a factor between 2 and 3 was observed, and this effect of the annealing ordering is more pronounced for the composites than for the fibers alone. The degree of graphitization increases by annealing, which is reflected in the decrease of the interplanar distance of (002) planes from about 3.51 Angstrom to 3.40 Angstrom, without any significant difference between the fibers and composites. The line broadening due to microstrains and small size effects is also strongly reduced by the annealing but the asymmetric shape of 100 and 110 reflections suggests that the layer stacking is still turbostratic. (C) 1999 Elsevier Science Ltd. All rights reserved.