The role of heteroatoms during graphitisation: first principle calculations

Graphite is widely used in modern industry, particularly in nuclear power generation in the UK. Understanding its formation is important for economical and safety reasons. The process to turn carbon materials into graphite by heat treatment is called the graphitisation process. It is the transformation of amorphous carbon, through a 2D turbostratic carbon intermediate, into 3D ordered layers of graphite. While many manufacturing processes have been established and many authors have contributed to understanding the important stages of graphitisation, the chemistry involved is not fully understood. It appears that impurities found in precursors can have a direct impact on the final graphite obtained. The following work is an investigation of the role played by these heteroatoms during the graphitisation process. Using density functional theory (DFT), calculations on possible mechanisms involved in the graphitisation process are investigated. However, the initial stages contain complex and poorly defined chemistry, so we have chosen to avoid this area, even though factors such as the C:H:O ratios are clearly important. Instead, this work is focussed on the latter stages of graphitisation in order to better understand the ordering processes to obtain graphite (and their inverse disordering, insofar as it is relevant to radiation damage). In this way it is still possible to invoke standard concepts in the physics and chemistry of defects in crystals. If there is too much disorder, and the system is close to amorphous in nature, complexity would overwhelm the project. The descriptions of an amorphous material with a little extra order would be much more difficult than the descriptions of a crystal with some disorder. For this reason, we have focussed on the heteroatoms which endure until the later stages of graphitisation, boron and sulphur, and also on turbostratic graphite, where calculations of interlayer separation as a function of relative rotation of a layer and of its neighbours are described. We find for sulphur that it can open up folds in graphite, forming very stable sulphur decorated edges. In dislocation terms, this could be the beginning of the dissociation of a perfect prismatic edge dislocation. An edge dislocation is described as an added half plane. If the plane is a bilayer graphene terminating in a fold, the dislocation is perfect. If the plane is a single graphene the dislocation is ‘partial’. Importantly two partial dislocations have lower elastic energy than the perfect, so dissociation is important in stabilising the structure. For boron, we show how it can pin twist boundaries, preventing slip and suggest that radiation damage can achieve the same effect through vacancies. The mechanism does not appear to involve cross-linking bonds and provides a good explanation for the variations in C44 between different graphites and different methods of measurement. Furthermore, we show that B can aid in the removal of twist boundaries by pushing up their formation energy with respect to AB graphite.