Abstract
The allotropes of carbon make its solid phases amongst the most diverse of any element. It can occur naturally as graphite and diamond, which have very different properties that make them suitable for a wide range of technological and commercial purposes. Recent developments in synthetic carbon include Highly Oriented Pyrolytic Graphite (HOPG) and nano-carbons, such as fullerenes, nanotubes and graphene. The main industrial application of bulk graphite is as an electrode material in steel production, but in purified nuclear graphite form, it is also used as a moderator in Advanced Gas-cooled Reactors across the United Kingdom. Both graphene and graphite are damaged over time when subjected to bombardment by electrons, neutrons or ions, and these have a wide range of effects on their physical and electrical properties, depending on the radiation flux and temperature. This research focuses on intrinsic defects in graphene and dimensional change in nuclear graphite. The method used here is computational chemistry, which complements physical experiments. Techniques used comprise of density functional theory (DFT) and molecular dynamics (MD), which are discussed in chapter 2 and chapter 3, respectively. The succeeding chapters describe the results of simulations performed to model defects in graphene and graphite. Chapter 4 presents the results of ab initio DFT calculations performed to investigate vacancy complexes that are formed in AA stacked bilayer graphene. In AB stacking, carbon atoms surrounding the lattice vacancies can form interlayer structures with sp2 bonding that are lower in energy compared to in-plane reconstructions. From the investigation of AA stacking, sp2 interlayer bonding of adjacent multivacancy defects in registry creates a type of stable sp2 bonded wormhole between the layers. Also, a new class of mezzanine structure characterised by sp3 interlayer bonding, resembling a prismatic vacancy loop has also been identified. The mezzanine, which is a V6 hexavacancy variant, where six sp3 carbon atoms sit midway between two carbon layers and bond to both, is substantially more stable than any other vacancy aggregate in AA stacked layers. Chapter 5 presents the results of ab initio DFT calculations performed to investigate the wormhole and mezzanine defect that were identified in chapter 4 and the ramp defect discovered by Trevethan et al.. DFT calculations were performed on these defects in twisted bilayer graphene. From the investigation of vacancy complexes in twisted bilayer graphene, it is found that vacancy complexes are unstable in the twisted region and are more favourable in formation energy when the stacking arrangement is close to AA or AB stacking. It has also been discovered that the ramp defect is more stable in the twisted bilayer graphene compared to the mezzanine defect. Chapter 6 presents the results of ab initio DFT calculations performed to investigate a form of extending defect, prismatic edge dislocation. Suarez-Martinez et al.’s research suggest the armchair core is disconnected from any other layer, whilst the zigzag core is connected. In the investigation here, the curvature of the mezzanine defect allows it to swing between the armchair, zigzag and Klein in the AA stacking. For the AB stacking configuration, the armchair and zigzag core are connected from any other layer. Chapter 7 present results of MD simulations using the adaptive intermolecular reactive empirical bond order (AIREBO) potential to investigate the dimensional change of graphite due to the formation of vacancies present in a single crystal. It has been identified that there is an expansion along the c-axis, whilst a contraction along the a- and b- axes due to the coalescence of vacancy forming in-plane and between the layers. The results here are in good agreement with experimental studies of low temperature irradiation. The final chapter gives conclusions to this work.