Theses and Dissertations (Physics)
http://hdl.handle.net/10386/26
2015-10-06T14:50:15ZAtomistic simulation studies of nanostructural titanium dioxide and its lithiation
http://hdl.handle.net/10386/1124
Atomistic simulation studies of nanostructural titanium dioxide and its lithiation
Matshaba, Malili Gideon.
Titanium dioxide (TiO2) nanoparticles, nanowires, nanosheets and nanoporous are of
great interest in many applications. This is due to inexpensive, safety and rate capability
of the material. It has being considered as a replacement of graphite anode material in
rechargeable lithium batteries. Much experimental work on pure and lithiated
nanostructures of TiO2 has been reported, mostly with regards to their complex
microstructures. In this work we employ molecular dynamics (MD) simulation to
generate models of TiO2 nano-architectures including: nanosheet, nanoporous,
nanosphere and bulk. We have successfully recrystallised all four nanostructures from
amorphous precursors; calculated radial distribution functions (RDFs), were used to
confirm crystallinity. Configuration energies, calculated as a function of time, were used
to monitor the recrystallisation. Calculated X-Ray Diffraction (XRD) spectra, using the
model nanostructures, reveal that the nanostructures are polymorphic with TiO2 domains
of both rutile and brookite in accord with experiment.
Amorphisation and recrystallisation was successful in generating complex
microstructures. In particular, bulk and nanoporous structures show zigzag tunnels
(indicative of micro-twinning) while nanosphere and nanosheet shows zigzag and straight
tunnels in accord with experiment. All model nanostructures of TiO2 were lithiated with
different lithium content. RDFs, microstructures, configuration energies, calculated as a
function of time and XRDs of all lithiated structures are presented.
Thesis (P.hD (Physics)) --University of Limpopo, 2013
2013-01-01T00:00:00ZComparison of measured photon and electron beam dose distributions between 3D water phanton and profiler 2 scanning system, South Africa
http://hdl.handle.net/10386/1086
Comparison of measured photon and electron beam dose distributions between 3D water phanton and profiler 2 scanning system, South Africa
Moji, Kabelo McDonald
Background and Objectives: To establish whether the profiler 2 scanning system can be used
as a substitute for the 3D-water phantom, by comparing the percentage depth doses and beam
profiles for both the photons and electron beams, and validating the results using CMS XiO
treatment planning system.
Methods: Beam data (profiles, percentage depth doses and absolute dosimetry) were acquired
for the two systems: (3D-water phantom and profiler 2 scanning system) for beam energies
6 MV and 15 MV photon beams, and 4, 6, 8, 10, 12 and 15 MeV electron beams generated by
the Elekta Synergy linear accelerator (linac) for the field sizes of 6 × 6 cm2, 10 × 10 cm2, 14 × 14
cm2, 20 × 20 cm2, and 25 × 25 cm2 at depths of 0.5 cm, 1.0 cm, 2.0 cm, and 5.0 cm respectively.
These measurements were acquired using ionization chambers in water and diode detectors in
Perspex. The acquired data was sent to CMS XiO treatment planning system for validation.
Results: In general, the dose distributions for both systems compared very well with
uncertainties within recommended limits. The largest maximum difference in symmetry was
1.6 % for a 6 MV photon beam defined at 25 × 25 cm2 field size. The largest maximum
difference in flatness was 2.77 % for a 4 MeV electron beam defined at 10 × 10 cm2 applicator
size. The penumbra largest maximum difference was 1.708 cm for 8 MeV electron beam defined
at 25 × 25 cm2 applicator size, which was outside the recommended limit of 1.2 cm. The largest
maximum difference in field size was 2.388 cm for a 6 MeV electron beam defined at 20 × 20
cm2 applicator size, which was outside the recommended limit of 0.4 cm.
The largest maximum difference in percentage depth dose at 10 cm depth was 1.69 % for the
6 MV photon beam. The absolute dose output measurements showed a very good agreement
between the two systems to a maximum percentage difference and highest standard deviation of
-0.99 % and 0.69 % respectively for the 6 MV photon beam. Validation measurements showed
an agreement to less than 1 % and 2 mm for percentage depth doses and beam profiles
respectively.
Conclusion and recommendation: From the results obtained, it is evident that the profiler 2
scanning system can be used as a substitute for the 3D-water phantom beam data acquisitions
during linear accelerator commissioning. The future work based on this study could be to study
the limitations involved with the profiler 2 scanning system when used during measurements for
commissioning of a linear accelerator. Limitations like field size (maximum field size of
20 × 30 cm2 at SSD = 100 cm), number of Perspex slabs to be used on top of the profiler 2
scanning system and diagonal profile measurements.
Thesis ( MSc ( Physics) ) -- University of Limpopo, 2013.
2014-01-01T00:00:00ZStatistical modelling of the relationship between learner support intervention and matric pass rates in Letlhabile area, North-West Province of South Africa
http://hdl.handle.net/10386/1066
Statistical modelling of the relationship between learner support intervention and matric pass rates in Letlhabile area, North-West Province of South Africa
Matshego, Joseph Nkashe
This study used statistical methods to determine the relationship between matric pass rates and interventions in the schools in the Letlhabile area. Nine schools were identified in this area. Five years (2007 to 2011) were looked at. Regressions methods were tried in which various forms of relationships were compared. The methods were linear, curvilinear (quadratic to polynomial of 4th power), exponential and power regressions were used in the tentative models investigated. The measures of bias and precision were used to compare the
models. Multicollinearity was also investigated where it was possible. Time series analysis was used to illustrate the trend patterns of the pass rates in the various schools as well as the pattern of the numbers of interventions. In most of these schools the numbers of interventions increased over the five years and in only a few schools the number of interventions decreased over these years. . A highlight of this study is that more interventions enhanced the matric pass rates. It was also evident that in the years in which the interventions decreased, the pass rates also de.creased. The regression methods investigated were all showing to be applicable in the prediction of pass rates from the numbers of interventions. This was concluded from realising that thelneasures of bias, precision and quality all ratified them. The methods were compared in order to select the best one based on the measures. The linear regression in which the pass
rates.areregressed on the number of interventions came out as the leading model in terms of all the criteria used.
Thesis (MSc(Statistics)) -- University of Limpopo (Medunsa Campus), 2012.
2014-05-29T00:00:00ZComputational modeling studies of cobalt pentlandite (Co₉S₈)
http://hdl.handle.net/10386/984
Computational modeling studies of cobalt pentlandite (Co₉S₈)
Mehlape, Mofuti Amos
The intention of the current study is to investigate structure, ion transport and reactivity of various forms of the cobalt pentlandite, Co9S8, at different temperatures using atomistic simulation methods with the support of electronic structure calculations. The first interatomic potentials of Co9S8 were derived with input data as structure and elastic properties from experiment and electronic structure calculations respectively. The potentials were validated by running energy minimization and molecular dynamics calculations. Structure, elastic properties and phonon spectra were well reproduced, together with the complex high temperature transformations and melting of Co9S8 as deduced from crystal structure, radial distribution functions, density profiles and diffusion coefficients.
Amongst the high symmetry surfaces {111}, {101} and {101} atomistic surface energy calculations proposed the {111} surface of Co9S8 as the most stable in agreement with experimental morphologies, and water adsorption energies on the such surfaces which mostly agreed with those from electronic structure calculations. The structural and ion transport variations with temperature were investigated and predicted surface melting at lower temperatures than the bulk. The effects of hydration on the surfaces at low and high temperatures were also studied.
The structural and ion transport properties of Co9S8 nanoparticles of varying sizes, covered by high symmetry surfaces {111}, {101} and {100} were predicted using molecular dynamics method based on our derived interatomic potentials. The structural and ion transport properties of Co9S8 nanoparticles of varying sizes, covered by high symmetry surfaces {111}, {101} and {100} were predicted using molecular dynamics method based on our derived interatomic potentials. Generally for {111}, {101} nanoparticles, high temperature transitions were abrupt for smaller nanoparticles and these tended to disintegrate and form voids. However, for larger nanoparticles the transitions were more gradual. Transitions in the {100} bound nanoparticles were less dramatic for all sizes and the formation of voids was reduced at high temperatures. Generally, the melting temperatures of different sizes of nanoparticles increases with the particle size hence approach the bulk limit. The interaction of nanoparticles with water was investigated.
Thesis (Ph.D. (Physics)) --University of Limpopo, 2013
2013-01-01T00:00:00Z