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 I_D/I_G. 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.