Computational study of topological line defects in graphite

The major motivation of this study is the development of understanding of line defect behaviour such as dislocations and ripplocations in nuclear graphite. Synthetic graphite is a suitable material in the nuclear industry due to its desirable physical properties, such as low neutron absorption cross-section and high scattering cross-section. Its main use is to moderate highly energetic fission neutrons in nuclear reactors. Neutron flux in irradiated synthetic graphite results in the formation of topological line defects such as dislocations and causes severe dimensional changes. Mutual interactions and pile-ups of basal dislocations can give rise to the formation of bulk ripplocations, such as the ruck and tuck defects. Both dislocations and ripplocations constitute an excess of material to a perfect graphite crystal, where dislocations accommodate the strain energy in-plane by creating stacking faults, yet ripplocations release the strain energy out-of-plane by buckling and delamination. In this thesis, first principles and molecular dynamics methods have been employed to study the physical properties of these type of defects. Perfect basal dislocations and their corresponding dissociated partial dislocations have been studied in bilayer graphene ribbons. Dislocated supercells of 100s of nanometres in length along the zigzag and armchair directions have been optimised for both flat and buckled configurations. Formation energies and dislocation core widths have been extracted from the atomic disregistry and the Burgers vector distribution. Pile-up of basal partial edge dislocations illustrates a potential atomistic mechanism of the ruck and tuck defect formation. Transition energy barrier calculations have revealed that partial edge dislocations are highly mobile and readily could pile-up to form the ruck and tuck defect. Surface and bulk ripplocations have been studied in bilayer, multilayer graphite and several other layered materials. A classical analytical approach has been used to obtain the expressions for the energetic and structural scaling factors of surface ripplocations. Simulations of supercells containing surface ripplocations have demonstrated a good agreement between the two approaches. In bulk graphite, simulations of ripplocations show that the lowest energy structure of basal dislocations pile-up is the ruck and tuck defect. However, multilayer dislocations pile-ups exhibit buckling and delamination by creating voids. As a conclusion, a progressive basal dislocation pile-up and rippling of graphene layers is another important contributing mechanism for the strain induced dimensional change of synthetic graphite caused by neutron irradiation.