The structural evolution of multi-layer graphene stacks in carbon fibers under load at high temperature – A synchrotron radiation study

Single carbon fibers were directly heated and tensile tested in vacuum in a specially designed in-situ X-ray testing device. We followed the structural evolution by diffraction of synchrotron radiation X-rays and determined the effect of time, temperature and applied stress on the growth of nanocrystallites as well as their orientational change with respect to the fiber axis. As the nanocrystallites are stacks of graphene sheets, this is a direct measurement of the development of multi-layer graphene with temperature. In the same experiment, size and orientation of pores within the material were obtained from simultaneous in-situ small-angle X-ray scattering. It turned out that the final temperature is decisive for structural evolution, significantly more than time or load. The multi-layer graphene network structure in fibers after heat treatment without stress is stable, whereas this is not the case in fibers without heat treatment, no matter if the carbon fibers were produced from polyacrylonitrile or from mesophase pitch precursor.