Temperature effect of electron-irradiation-induced structural modification in graphite

The structural change of graphite under high-energy electron irradiation at a wide range of temperatures was investigated by means of conventional transmission electron microscopy (CTEM), transmission electron diffraction (TED), high-resolution transmission electron microscopy (HRTEM) and electron energy-loss spectroscopy (EELS). The experimentally observed result at lower temperatures (<600 K) could be basically interpreted by the local formation of non-hexagonal atomic rings (loss of the local hexagonal symmetry) in the graphite planes associated with the change in chemical bonding states. At higher temperatures (>650 K), a graphite single crystal was polycrystallized into nano-size crystallites with the original hexagonal network locally retained within each crystallite. The apparent activation energy derived from the present data over the whole temperature range was estimated to be 0.014 eV, which is in agreement with a reported migration energy of a self-interstitial atom between the basal planes.