A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Doctor of Philosophy.
April 2016. / Chemical vapour deposition was used to synthesise four carbon microspheres
(CMS) samples. Introduction of acetonitrile in different quantities produced
spheres of differing nitrogen concentration. The structure of the spheres was
investigated using Raman spectroscopy, scanning electron microscopy and
X-ray photoelectron spectroscopy techniques. The Raman investigation revealed
a decrease in average graphitic
flake size which forms the surface layers
of the spheres with nitrogen incorporation. XPS showed that increased nitrogen
doping caused a larger proportion of pyridinic nitrogen, which process
likely restricts the growth of the crystallite
flakes detected with the Raman
technique. Microscopy revealed spheres with differing morphologies which
did not correlated with the level of nitrogen doping. Electron paramagnetic
resonance techniques were employed to investigate the impact of nitrogen
doping on the spin system of the samples. Electrical transport and Hall effect
data were collected with an automated experiment station purpose built
for this work. Samples displayed semiconducting behaviour at low temperatures
which was ascribed to
fluctuation assisted tunnelling. At higher temperatures
all four samples display a transition to metallic behaviour. Models
for conduction, which were tested but ultimately rejected, include variable
range hopping in all its dimensional forms, Efros-Shklovskii VRH and weak
localisation. A comparison of the conduction results and the structural information
showed the conductivity to be more closely affected by the structure
of the spheres than the overall doping level. A case is made for the dominant
conduction mechanism being determined by the intersphere rather than
the intrasphere conduction. This research shows that creating carbon microspheres
with specific electrical properties requires control of the structure
induced during synthesis. Nitrogen doping alone does not determine the
final physical and electrical transport properties. / LG2017
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/21736 |
| Date | January 2016 |
| Creators | Marsicano, Vincent Derek |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | Online resource (158 pages), application/pdf |
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