Preferred Orientation of Carbon Fibres within Basal Planes

Abstract

CARBON fibres are generally accepted as being composed of a number of undulating, intermittently connected ribbons running approximately parallel to the fibre axis and separated over the major portion of their length by elongated micro-voids^1–6. The ribbons have been thought to be composed of stacks of graphite basal planes with their c axes parallel but rotated and translated randomly with respect to one another (turbostratic graphite). Ergun^7,8 has recently proposed that, for stress graphitized carbon fibres produced from polyacrylonitrile (PAN), this picture is incorrect in that there is some ordering of the basal plane a axes parallel to the fibre axis. But stress graphitized fibres are not typical of commercial carbon fibres made from PAN: their c axes are far better aligned, they contain appreciable areas of three dimensional graphite and their crystallite dimensions are much larger^6,8,9. Also, Ergun's proposed a axis alignment was based on the observation of anomalous intensities for (h0) and (hk) reflexions, and it is by no means certain that these anomalies could not-have arisen from the presence of areas of three dimensional order. No evidence for a axis alignment was obtained by Ergun when the commercial high modulus fibre ‘Thornel 50’ was investigated⁸. We report here the detection of preferred a axis orientation in three representative commercial carbon fibres and indicate how one may arrive at a quantitative estimate of the degree of this orientation.