An electron spin resonance study of carbon fibres based on polyacrylonitrile

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Abstract
The electron spin resonance characteristics of fibres derived from a polyacrylonitrile based precursor heat treated to between 1000 and 2800°C have been examined. The resonance is in general similar to that observed in graphitizing carbons and has been analysed by methods developed for the latter materials. One striking feature of the results is that a g-value anisotropy is observed when the processing temperature exceeds approximately 1750°C. The appearance of this anisotropy is associated with discontinuities in several other parameters. The resonance intensity decreases rapidly up to 1750°C then much more slowly above this temperature, and it would appear that the localized component in the resonance is constant above this transition point. Various mechanisms for the appearance of the anisotropy are discussed including increased perfection of the layers in the crystallites and the existence of a separate three-dimensionally ordered graphite phase. It is suggested that the elimination of nitrogen from the carbon structure may play an important part in the changes which occur in this region. The g-anisotropy seems to be a reliable and sensitive measure of the degree of graphitization in these fibres and its magnitude is consistent with the limited degree of graphitic crystallite development indicated by x ray methods.