Investigating nanostructures in carbon fibres using Raman spectroscopy

Nanostructures in the PAN-based carbon fibres were investigated by means of the Raman spectroscopy which was performed using a series of different excitation wavelengths. UV-Raman measurements showed the predominance of sp2 carbon for the carbon fibres with Young's moduli ranging from 55 to 440 GPa. The excitation wavelength dependence of the spectral line shape implied the presence of the graphitic (sp2 carbon layers) as well as the amorphous carbon-like component (sp2 carbon clusters) in these fibres. An analytical model based upon the Tuinstra-Koenig relation and the rule-of-mixtures concept was proposed to account for the observed behaviour of the peak intensity ratio ID/IG. It was suggested that the carbonisation heat treatment led to the sp2 carbon layers extending rapidly throughout the nanostructure, resulting in the complete loss of the sp2 carbon clusters for the higher-modulus carbon fibres. These new findings together with the proposed analytical model are believed to be essential keys towards the development of new generation carbon fibres.