The high pressure behaviour of glassy carbon
Date
2019
Authors
Shiell, Thomas
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Abstract
Carbon is the fourth most abundant element in the universe,
commonly
found in stars, comets, and the atmosphere of most planets. On
Earth it is
found in the soil, oceans, atmosphere, planetary interior, and it
is the major
constituent of all organic matter. There are two well-known
naturally
occurring pure carbon structures, graphite and diamond. By
subjecting
graphite to high pressure and high temperature it is possible to
transform
it into a variety of other structures. These other pure carbon
structures,
which are dissimilar to either graphite or cubic-diamond, may
have useful
and applicable physical properties. Interestingly, it is known
that in
some elemental systems using disordered precursors can lower the
kinetic
energy barriers which act to prevent high pressure phase
transformations.
So by using a precursor that is pure carbon and highly
disordered, it may
be possible to induce phase transformation to new structures
using less
extreme pressures and temperatures, and also to access
transformation
pathways that are not possible from rigid crystal structures.
The work presented in this thesis is focused on exploring and
creating new
high pressure carbon phases. This is done by compressing a
disordered
graphitic precursor, called glassy carbon, at room temperature
using a
diamond anvil cell. Analysis of the composition and structure of
different
glassy carbons at ambient revealed that a glassy carbon produced
at
2500C was the most suitable for high pressure experiments. This
was
used as the precursor from this point onward. Ex situ analysis
techniques
such as Raman spectroscopy and electron microscopy are used to
analyse
the recovered samples following compression and decompression,
revealing
a pressure threshold that represents a permanent structural
transformation
between 35-45 GPa. When recovered from compression below 35
GPa the glassy carbon structure is retained. However, when
compressed
to 45 GPa and above nanocrystalline graphite is recovered. In
situ X-ray
diffraction measurements of glassy carbon under compression
reveal that
the material evolves continuously toward this threshold, with
regard to
both structure and bonding. On compression the graphitic layers
gradually
orient perpendicular to the compression axis of the diamond
anvil
cell. Beyond 35 GPa small isolated regions of sp3 bonded atoms
are
formed, which revert to sp2 bonds on decompression. It is further
shown
by annealing GC at a moderate temperature of 400C at 100 GPa
that
a different crystalline structure, hexagonal diamond, can be
recovered at
ambient. The recovered sample is nanocrystalline, and is shown to
have
the highest purity of hexagonal stacking of any hexagonal diamond
formed
under static compression.
Further detailed characterisation of recovered hexagonal-diamond
samples
using electron microscopy shows that shear strain (which is
present as a
result of the non-hydrostatic compression environment) is an
important
driver that promotes the pressure induced kinetic phase
transformation
from the disordered graphitic precursor. These results suggest
that shear
strain, which is usually minimised in high pressure experiments
by the
use of hydrostatic pressure media, is a useful and potentially
underutilised
tool. This knowledge can be applied to other materials to lower
energy
barriers which impede the formation of new structures that form
via kinetic
transformation pathways.
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Keywords
glassy carbon, diamond, graphite, high pressure
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