High Energy Photoelectron Spectroscopic Study of Carbon Fibre Surfaces

Abstract

KNOWLEDGE of the internal structure of carbon fibres, which are derived from polyacrylonitrile and rayon by carbonization in special conditions^1–3, has increased significantly in the past few years. Investigations using small-angle and conventional X-ray diffraction^4–9, electron diffraction^6,10,11, intercalation with caesium¹², helium density determinations⁶, polarized light microscopy¹¹, and high-resolution electron microscopy^6,13, have established that the carbon layers form long (greater than 100 nm) ribbons with preferred orientation of the layers parallel to the fibre axis. A number of these parallel ribbons form a so-called microfibril⁶, the average thickness (or stack size) of which, in fibres that have been heat treated to 1,300 K or beyond, is in the range 3–10 nm. The microfibrils are believed to be wrinkled and bent^6,9 and they may even be twisted to form rather inaccessible voids. Within the layers, the C–C bond distance⁹ is 1.416±0.007 A, and for this, and other reasons⁶, the bonding is believed to involve only sp² hybridized carbon atoms. There is no evidence of cross-linking between the layers (which contain a few per cent of structural defects⁹) perpendicular to the axis of the microfibrils.