DISCUSSION OF DIFFUSION MECHANISM IN GRAPHITE WITH PARTICULAR REFERENCE TO IRRADIATED MATERIAL

Principally due to doubt concerning the activation energy of interstitial formation, Efi, it has been difficult to differentiate between an interstitial and vacancy mechanism for self-diffusion in graphite. However, the recent studies of Turnbull and Stagg, which have been verified by Thrower, yield a value of 8.3 ± 0.3 eV for the activation energy of diffusion occurring both in pyrolytic and naturally occurring graphites. This value was identified with Ef + 1 2(Ema + Emc). By considering the structural perfection of the graphites employed by the above authors, it has been concluded here that there is no necessity to assume a discrete value describing Emc. Rather the observed activation energy, E*, is interpreted as the summation of 1 2(Efi + Efv) + Emai, which implies that self diffusion occurs via a dynamic interchange mechanism. It is implicit for the operation of this mechanism that the matrix should contain sites of high vacancy concentrations, such as intercrystallite boundaries. If the value of Efv is assumed to be 7.0 ± 0.5 eV, the values of 8.8 ± 0.6 and 9.4 ± 0.6 eV are derived for Efi, when the experimental and theoretical values of Emai are substituted, respectively. The latter number is in fairly close agreement with both Coulson's theoretical value and that determined experimentally by Murty 9.6 ± 0.8 eV. © 1969.