Modelling defect evolution in irradiated graphite

The diffusion of point defects after irradiation events in graphite is considered using high temperature molecular dynamics and adaptive kinetic Monte Carlo. The system is modelled with a ReaxFF potential model. It is shown that monovacancies can diffuse both within the graphite layers and also between layers to form stable divacancy and trivacancy structures. Interstitials can also combine, first forming interlayer strings which transform to ring structures. Separated ring structures can also combine to form mobile platelets which can be the seed for new layer formation. When a defective lattice contains a local mixture of vacancies and interstitials, both recombination and larger defect clusters can form. The Dienes defect, cannot easily occur by direct transformation as originally proposed, because of high energy barriers but is shown to occur as an intermediate step in interstitial-defect recombination process. At high temperature the graphite layers bend which has the effect of enhancing defect motion and changing the relative stability of monovacancy structures. The consequences of this are discussed.