Structural development of PAN-based carbon fibers studied by in situ X-ray scattering at high temperatures under load

PAN-based carbon fiber bundles were simultaneously loaded and heated in an X-ray transparent testing device, which made it possible to follow the structural development in situ. The stress–strain curves are recorded together with structural parameters such as the inter-layer spacing, the crystallite size Lc and the orientation of the graphene planes, obtained from the scattering intensity of the d002-reflection. Simultaneously, the size and orientation of pores were determined from small-angle X-ray scattering. Experiments were performed at temperatures up to 1700 °C and stresses up to 260 MPa. It was observed that the main part of the structural change occurs in the short time period of heating the fibers up to high temperatures. An increased orientation and growth of crystallites take place as a consequence of combined mechanical forces and high temperatures. This is followed by an additional but only smaller amount of structural change with time, which is due to dislocation movement, dislocation annealing and gliding of graphene planes.