Orientation and dimensional changes in mesophase pitch-based carbon fibers

The dimensional changes and microstructure evolution of AR-mesophase pitch fibers are reported as a function of heat treatment temperatures (HTTs) that ranged from 300 to 3000 °C. The length measurements for AR fibers indicate that starting from an oxidized state, there is partial relaxation of orientation in the range of 600–900 °C, and the length of the fibers shrinks about 8% relative to that in the stabilized state. Above 900 °C, the fiber length does not decrease; instead, it increases slightly. Thus, fibers heat treated to 2400 °C on average have a length that is bigger than those heat-treated to 900 °C. The small increase in length of fibers heat treated to high temperatures (>900 °C) is likely a consequence of alignment of the graphene layer planes along the fiber axis that can result in an expansion along the longitudinal direction. The small minimum in the length profile at 900 °C corresponds to a small maximum in the misorientation angle of the graphene layer planes measured by wide-angle X-ray diffraction on the fiber bundles. Single filament orientation measurements, using monochromated synchrotron radiation, show a similar maximum misorientation at 600 °C. The fiber microstructure, characterized by scanning electron microscopy, also reveals that the first major change is observed at 900 °C, where a radial texture is observed. At higher HTTs, the development of radial texture is more pronounced and by 2400 °C the graphene-layer planes are seen very clearly.