Irradiation-induced amorphization of graphite

A model is proposed to explain the irradiation-induced amorphization of graphite. This model assumes that the single-vacancy concentration saturates to CVS(T) at irradiation temperature T; that the single-vacancy contained regions gradually transform to disordered regions, of which accumulation leads to amorphization; and that the transformation rate is proportional to the single-vacancy concentration. Utilizing CVS(T) as a fitting parameter, this model explains the dose and the temperature dependencies of the Raman spectra of 25 keV He⁺-irradiated highly oriented pyrolytic graphite. Also, the Arrhenius plot of the obtained values of CVS^-1, which indicates the critical doses of amorphization, corresponds well to a previous TEM study, which showed two activation energies of 0.036 eV below {~}573 K and 0.25 eV above {~}573 K. By analyzing the chemical kinetics of the steady state, activation energies for single and di-interstitial migration of 0.14 and 0.86 eV, respectively, are obtained. The stored energy is attributed to a significant accumulation of di-interstitials below {~}573 K, which originated in the reduced annihilation with vacancies due to a barrier. By extending the theory to the quasisteady state with the collapsed line formation and the loop growth, the dimensional changes before amorphization can be also explained. This model should provide insight into the graphitization process and the formation mechanism of various carbon clusters.