Modification of graphite surface layers by nitrogen ion irradiation

To study the modified surface layer of polycrystalline graphite MPG-8 (produced by NIIGraphite, Moscow, Russia) under high-fluence irradiation by 30 keV N-2(+) ions, electron paramagnetic resonance (EPR) spectra of surface layer and the dependence of the ion-electron emission yield gamma on the target temperature have been measured. The dependence of gamma(T) manifests a step-like behaviour at the temperature T-a, which is typical of the radiation-induced phase transition. The EPR analysis shows that below T-a, which corresponds to annealing of radiation damage created by ion bombardment, paramagnetic defects are typical of carbon as well as defects connected with bonding of the carbon atom with three N-14 nuclei. Above T-a, the defects are typical of graphite-like structures. The results for MPG-8 are compared with those for MPG-LT and POCO-AXF-5Q graphites published earlier. (C) 2003 Elsevier B.V. All rights reserved.