Mechanism for Self-Diffusion in Graphite

The mechanism for self-diffusion in single crystals of graphite has been examined theoretically. Calculations for vacancy and direct interchange mechanisms are based on atomic interactions within the graphite hexagonal layers. These interactions are obtained from the known systematic change of carbon-carbon bond strength with interatomic distance. In order to be able to calculate the energy of formation of an interstitial carbon atom, a potential function has been devised to account for interaction between the planes. This potential function consists of two terms, a van der Waals' attraction and an exponential type repulsion. The adjustable constants have been evaluated from known physical data. The interplanar cohesive energy of graphite is calculated to be 4.36 kcal/mole. Comparison of the total activation energies for self-diffusion via vacancies, direct interchange, and interstitial atoms indicates that direct interchange is the preferred mechanism. The activation energy for self-diffusion by means of direct interchange is estimated to be 90 kcal/mole. The present theoretical treatment is not applicable to diffusion along grain boundaries or pores.