The three-dimensional microstructure and macrostructure of
various PAN-based carbon fibres has been characterized by bright- and dark-field transmission electron microscopy, and quantitative electron-diffraction analysis.
A skin-core heterogeneity was observed for all fibres heat
treated to 2500°C, irrespective of type and time of stabilization cycle, the skin being variable in longitudinal extent and width, but typically, 0.1 vm in thickness - much smaller than the sheath zone attributable
to oxidation, which has been observed in optical microscopy. At 1000°C and 1500°C, only a few layer planes at the fibre surface can be considered as forming a skin structure, and it is proposed that progressive growth inwards of a well-oriented skin structure occurs at heat-treatment temperatures in excess of 1800°C. The nature of the fibre surface so formed would suggest that the number of edge sites suited to bonding with a resin matrix in untreated fibre is'inversely related to heat-treatment temperature. Examination of 'first-cut' sections suggests that the surfaces of the fibres are rippled, with,
at 2500°C, the c-axes of skin crystallites being predominantly normal to the fibre surface; this is confirmed by scanning electron microscopy and examination of transverse sections.
In cross section, the sheath and core zones, first observed
in optical microscopy, have been shown to possess no preferred transverse c-axis orientation, it being proposed that structural differences between the zones lie in a crosslinking mechanism during oxidative stabilization.
The origin of the intrinsic oriented structure of PAN-based
carbon fibres has been traced, by quantitative electron-diffraction analysis, through heat treatment at 2500°C, 1500°C and 1000°C to the important pyrolysis range 400 - 600°C. At this temperature it is proposed that a uniform angular spread of stacking size exists within the azimuthal spread of the (0O&) reflection. Heat treatment, particularly at 2500°C, causes the preferential growth of those crystallites aligned closest to the fibre axis, while the röle of smaller crystallites oriented at high angles to the fibre axis is thought to be one of interlinking.
Surface and internal flaws, involving large crystallite
misorientations, have been observed in type I fibres, and using the Reynolds - Sharp theory for fibre failure, estimates of mechanical properties for such fibres have been made which are encouragingly close to the observed values. In the absence of internal voids and the surface skin characteristic of these fibres, an intrinsic
strength of 7 GNm-2 and strain-to-failure of 2% is predicted. Enhanced crystallization effects have not been observed in fibres heat treated to 1000°C or 1500°C, and the fracture of these fibres at strains below 2% is thought to be due to the presence of gross internal and surface voids, flaws and irregularities.