The role of heteroatoms during graphitisation: first principle calculations

Adjizian, Jean-Joseph (2013) The role of heteroatoms during graphitisation: first principle calculations. Doctoral thesis (PhD), University of Sussex.

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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.

Item Type: Thesis (Doctoral)
Schools and Departments: School of Life Sciences > Chemistry
Subjects: Q Science > QD Chemistry > QD0241 Organic chemistry
T Technology > TP Chemical technology > TP0155 Chemical engineering
Depositing User: Library Cataloguing
Date Deposited: 27 Jun 2013 14:30
Last Modified: 10 Sep 2015 15:19

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