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Masters Degrees (Physics)

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    Wigner functionals and ghost imaging.
    (2023) Durgapersadh, Akshay.; Konrad, Thomas.; Roux , FiIippus Stefanus.
    This dissertation discusses Wigner functionals and an application, namely ghost imaging. Wigner functionals aim to provide more accurate measurement results due to the inclusion of all the degrees of freedom of light. The main concepts discussed are spontaneous parametric down-conversion (SPDC), the evolution equation for light through the SPDC crystal, the probability distribution for the ghost image, conditional probability distribution, and the point spread function. The ghost imaging calculation is done for the rational thin crystal and extreme thin crystal limits. It is shown that the probability distribution produced is the same for the rational thin crystal and extreme thin crystal limits, and consequently, the point spread function, and conditional probability distributions are the same.
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    Synthesis and characterization of magnetic iron oxide nanoparticles.
    (2022) Ndlovu, Nkanyiso Linda.; Masina, Colani John.
    Three high-purity cubic spinel-type crystalline magnetic iron oxides i.e. Fe3O4, CoFe2O4, and NiFe2O4 nanoparticles were successfully synthesized by co-precipitation method. X-ray diffraction (XRD) showed the formation of stoichiometric phases with average particle size of 11.7 nm, 23.6 nm, and 16.4 nm for the as-prepared Fe3O4, CoFe2O4, and NiFe2O4 nanoparticles, respectively. Transmission electron microscopy (TEM) observation for all three samples revealed spherical morphology with single magnetic domain structure. From high resolution TEM (HR-TEM) imaging, lattice fringes with d-spacing of 0.473 nm and 0.248 nm corresponding to (111) and (311) reflections planes, were observed for both the Co-doped and Ni-doped samples. Energy-dispersive x-ray spectroscopy (EDX) analysis showed the presence and homogeneous distribution of main elements Fe, O, Co, and Ni in the samples. Quantitative EDX results confirmed the formation of stoichiometric CoFe2O4 and NiFe2O4 phases with the experimentally measured weight wt% of the samples closely equal to the theoretical calculated wt% values i.e. Fe = 46.35 wt%, O = 26.79 wt%, and Co = 26.87 wt% for CoFe2O4, and Fe = 47.02 wt%, O = 27.27 wt%, and Ni = 24.75 wt% for NiFe2O4. The magnetic properties of these nanoparticles were investigated by 57-Fe Mossbauer spectroscopy (MS) and Vibrating Sample Magnetometer (VSM) techniques. Room temperature MS spectrum for the pure Fe3O4 phase consist of two superimposed sextets with isomer shifts (0.321, 0.463) mm/s and hyperfine field (57.3, 43.4) T attributed to tetrahedral (A-sites) and octahedral (B-sites). The CoFe2O4 and NiFe2O4 samples both showed room temperature MS spectra consisting of two sextets and a single central paramagnetic doublet. The two sextets in each sample had almost equal isomer shifts for both A- and B-sites i.e. 0.2956 & 0.3247 mm/s and 0.3784 & 0.2761 mm/s for each of the sites of the CoFe2O4 and NiFe2O4 sample, respectively. The paramagnetic doublet was fitted with isomer shift of 0.3272 mm/s for the CoFe2O4 sample and 0.3249 mm/s for the NiFe2O4 sample. Temperature dependence M-T magnetization curves measured at H = 500 Oe inthe zero-field-cooled (ZFC) and field-cooled (FC) conditions showed the superparamagnetic nature of all three particles. The MZFC magnetization curve showed a maximum (cusp) at 225 K, 300 K, and 228 K corresponding to blocking temperature (TB), for Fe3O4, CoFe2O4, and NiFe2O4, respectively. For the CoFe2O4 sample the irreversibility temperature (Tirr) was equal to the blocking temperature (TB). While measured Tirr for Fe3O4 and NiFe2O4 was 300 K for both samples. The M-H magnetization curves at 300 K for all three samples revealed the coexistence of ferrimagnetic and superparamagnetic behaviour of the nanoparticles. At 300 K all three samples exhibit symmetrical and almost "closed" hysteresis loops with coercivity approximately 36, 70, and 117 Oe and remanence magnetization of approximately 5, 3, and 4 emu/g, for Fe3O4, NFe2O4, and CoFe2O4, respectively. Furthermore, M-H measurements at 300 K showed a high saturation magnetization of 89 emu/g for the Fe3O4 sample compared to 37 emu/g and 26 emu/g for the CoFe2O4, and NiFe2O4, respectively. M-H measurements recorded at low temperatures showed rather "opened" hysteresis loops compared to loops measured at 300 K. In contrast to saturated magnetization M-H curves for the Fe3O4 and NiFe2O4 nanoparticles, unsaturated M-H loops were observed for CoFe2O4 sample in the temperature range 10 - 100 K. A significant increase in coercivity to 102 Oe, 391 Oe, and 2.4 kOe was observed for Fe3O4, NiFe2O4, and CoFe2O4, respectively, when the temperature was reduced from 300 K to 10 K. For the CoFe2O4 sample, a highest coercivity of 2.7 kOe was measured at 100 K. And finally, M-H data at 10 K showed high saturation magnetization of 100 emu/g, 51 emu/g, and 31 emu/g, for the pure magnetite, CoFe2O4, and NiFe2O4 samples, respectively.
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    Hyperpolarizability contributions to the second Kerr-effect virial coefficients of non-dipolar molecules.
    (2020) Mhlongo, Mbukeni Mzwandile.; Couling, Vincent William.
    The molecular theory of the second Kerr-effect virial coeffcient, BK, describing the effects of interacting pairs of molecules on the molecular Kerr constant for molecules with non-linear symmetry is reviewed, and then extended to include second hyperpolarizability contributions in the molecular interactions. The classical long-range dipole{induced-dipole model is used to describe the interactions between pairs of molecules. This investigation has been limited to non-dipolar species, where the permanent electric quadrupole moment is the leading multipole moment, since for dipolar species, the hyperpolarizability contributions will likely be masked by the generally much-larger contributions arising from the permanent electric dipole moment. The resulting expressions for contributions to BK are evaluated numerically (using Gaussian quadrature) for nitrogen (N2), carbon dioxide (CO2) and ethene (C2H4), these molecules having measured data against which to assess the theoretical predictions. N2 and CO2 are axially-symmetric molecules, while C2H4 is of lower symmetry, belonging to the D2h point group. Previous attempts to approximate the molecular properties of C2H4 to axial symmetry in calculations of BK have produced theoretical results which signifi cantly underestimate the measured data. Inclusion of the full molecular symmetry has been shown to be essential if the molecular-tensor theory is to yield reasonable agreement with experimental data. For CO2 the quadrupole{induced-dipole contribution dominates, and the interaction induced hyperpolarizability contribution to BK is only 0.3% at 200 K rising to 1.5% at 500 K. For the N2 and C2H4 molecules, the collision-induced hyperpolarizability contributes just under 2% at 200 K, rising to 4% at 500 K for N2, and 5.5% for C2H4. These contributions are non-negligible, and are hence worth refi ning in future work through full ab initio quantum mechanical computation of the interaction-induced hyperpolarizability contribution where dispersion force and electron cloud overlap effects can be included.
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    Non-reversal open quantum walks.
    (2015) Goolam Hossen, Yashine Hazmatally; Petruccione, Francesco.; Sinayskiy, Ilya.
    In this thesis, a new model of non-reversal quantum walk is proposed. In such a walk, the walker cannot go back to previously visited sites but it can stay static or move to a new site. The process is set up on a line using the formalism of Open Quantum Walks (OQWs). Afterwards, non-reversal quantum trajectories are launched on a 2-D lattice to which a memory is associated to record visited sites. The “quantum coins” are procured from a randomly generated unitary matrix. The radius of spread of the non-reversal OQW varies with di↵erent unitary matrices. The statistical results have meaningful interpretations in polymer physics. The number of steps of the trajectories is equivalent to the degree of polymerization, N. The root-mean-square of the radii determines the end-to-end distance, R of a polymer. These two values being typically related by R ⇠ N⌫, the critical exponent, ⌫, is obtained for N  400. It is found to be closely equal to the Flory exponent. However, for larger N, the relationship does not hold anymore. Hence, a di↵erent relationship between R and N is suggested. ii
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    Study on extreme geomagnetic events and ionospheric response.
    (2018) Tire, Adila Wamisho.; Venkataraman, Sivakumar.; Ogunjobi, Olakunle.
    Extreme geomagnetic events are geophysical phenomena that result from the violent eruptive nature of the Sun. One type Geospace event is magnetic cloud (MC), which is an attendant of coronal mass ejection (CME). MC-triggered storms can cause injection of particles into the ionosphere. This can result in an enhance ionization and conductivity of upper and middle atmosphere. MC can be identi ed based on geomagnetic parameters and solar wind conditions which show high magnetic eld magnitudes, low ratio of plasma to magnetic pressure, low proton temperature, and smooth rotation of the magnetic eld vector. MC events that occurred on 29 April 2014, 17 March 2015, 31 December 2015 and 13 October 2016 were selected for the study. The hourly average of particle dropouts, precipitation, local ionospheric response and magnetometer variations in the region over South Africa (33:3oS, 26:5oE) are examined during geomagnetic storms triggered by MC. The Geostationary Operational Environmental Satellites (GOES) were inspected for radiation belt particle dropouts during MC events. Energetic particle precipitation associated with MC events are obtained from National Oceanic and Atmospheric Administration (NOAA) Polar Orbiting Environmental Satellites (POES). Results show that particle dropouts and precipitation vary with the arrival of MC. A closer look of the ground based magnetometer and the time history of available daytime E-layer critical frequency from ionosonde indicate that lower ionosphere respond to MC-driven storm.
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    Electrical and magnetic properties of NI-TI substituted perovskites.
    (2018) Dlamini, Sanele Tobias.; Moyo, Thomas.
    All perovskites samples studied in this work: (La,Bi,Sr)Ni0.25Ti0.25Fe0.5O3 were synthesized by a combination of high energy ball milling (HEBM) on a Retsch PM 400 instrument and heat treatment on a Sentro Tech (type: STT-1600C-3-24) high temperature tube furnace. These samples were selected in anticipation of a future study of gas sensing properties not undertaken in this work because of equipment constraints. The additional Ni0:5Ti0:5Fe2O4 spinel ferrite was synthesized by HEBM. The compounds were characterized by X-ray di raction (XRD), Fourier transform infrared spectroscopy (FTIR), high resolution transmission electron microscopy (HRTEM), M ossbauer spectroscopy, vibrating sample magnetometer (VSM) at temperatures between 10 K and 300 K, resistivity four probe method, Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BHJ) for surface area measurements. The X-ray di raction data for LaNi0.25Ti0.25Fe0.5O3 (LNTFO) showed the formation of single phase compound with orthorhombic structure which is similar to that of reported LaFeO3. The average particle size obtained from HRTEM was 91 5 nm. The cosubstitution of Ni2+ and Ti3+ at the Fe3+ site indicated enhanced magnetic properties. Magnetization measurements showed soft ferromangetic behavior with saturation magnetisation MS in the range 2.25 0.2 emu/g (at 300 K) to 2.61 0.2 emu/g (at 10 K). The coercivity increased from 0.36 0.02 kOe to 0.41 0.02 kOe with decreasing temperatures. M ossbauer data revealed the sample was magnetically ordered at room temperature. The isomer shift values indicated possible strong covalent bonds between metal and the oxygen ions and existence of only Fe3+ ions in the structure. Furthermore, the milled material showed semiconducting behavior with activation energy of 3.4 0.1 eV whilst the milled and annealed sample has lower activation energy of 0.9 0.06 eV. A BiNi0.25Ti0.25Fe0.5O3 (BNTFO) perovskite compound has been successfully synthesized by high energy ball milling and annealing. The crystallites size were obtained to be in the range 10 nm to 70 nm. The grains were observed to be semi-spherical with good surface coverage. We found improved saturation magnetization relative to the LNTFO sample with MS in the range 6.5 0.2 emu/g (at 300 K) to 7. 3 0.2 emu/g (10 K) and the coercivity in the range 0.49 0.02 kOe (300 K) to 0.62 0.02 kOe (10 K). M ossbauerspectrum reveals magnetic ordering at room temperature. Isomer shift values indicated only the presence of Fe3+ ions. Magnetic hyper ne values at 300 K were obtained to be 518 5 kOe and 510 4 kOe for the A and B sites respectively. A low activation energy of 0.66 0.02 eV was obtained for this sample. SrNi0.25Ti0.25Fe0.5O3 (SNTFO) belonging to an orthorhombic crystal system showed paramagnetic behavior at room temperature whilst at lower temperatures it was superparamagnetic. The magnetization MS was relatively low ranging from 14.14 0.23 emu/g to 0.0032 0.0001 emu/g and the coercive eld increased with decreasing temperature from 0.34 0.06 kOe to 1.14 0.06 kOe. The M ossbauer spectrum indicated the presence of both Fe4+ and Fe3+ iron ions which is consistent to that reported for SrFeO3 compounds. Particle size obtained from FETEM averaged 127 12 nm and the surface morphology was indicative of a rough absorber surface with semi-spherical grains of di erent sizes. An activation energy of 0.37 0.03 eV for the annealed SNTFO indicated good electronic conductivity at relatively higher temperature. An additional Ni0.5Ti0.5Fe2O4 compound was successfully synthesized by HEBM. The sample was characterized by quick phase formation. Prolonged milling destroyed the phase. From the structural analysis it was evident that starting precursors for a chemical reaction are of vital importance as they have great in uence on the reaction product. The mean particle size was obtained to be 45 9 nm. Particle size reduced with milling time whilst the strain increased. The coercivity and saturation magnetization appeared to follow the Stoner{Wohlfarth model in two distinct regions at high temperatures (300 K to 100 K) and low temperatures (50 K to 10 K) with approximately equal anisotropies in each temperature range. Saturation magnetization was obtained to be between 38.73 0.03 emu/g to 38.84 0.03 emu/g and the coercivity was between 820 32 Oe to 407 32 Oe. Room temperature M ossbauer spectrum revealed hyper ne elds of 446 1 kOe and 480 1 kOe for A and B sites respectively. Isomer shift values indicated co-existence of both Fe3+ and Fe2+.
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    Some topics in modelling South African COVID-19 epidemiological data.
    (2023) Nene, Sinoxolo Moyomuhle.; Bucher, Martin.; Petruccione, Francesco.
    The ongoing COVID-19 epidemic produces a wide variety of quality data, which can be analyzed using simple mathematical models inspired by statistical physics. We explore some underlying methods and apply them to the simulated data using parameters extracted from the previously analyzed data by Pulliam et al. to determine whether the Omicron variant reinfects at a higher rate compared to other previous Variants of Concern. First, using simple prototype models, we investigate whether simple dynamical systems of low dimensions are inherently predictable. The concept of a strange attractor is defined and numerically explored using the Duffing oscillator as an example. The statistical theory of linear parametric models is then investigated mathematically and applied to some standard datasets with the R statistical programming language. We also study the Markov Chain Monte Carlo technique, exploring complicated models and presenting mathematical theory, numerical implementation, optimization, and convergence diagnostics. Finally, we apply the MCMC techniques to estimate the parameters of our model using simulated data for reinfections. Based on the analysis of the mock data, we found that the Omicron variant does not have a higher reinfection rate, as expected since our simulated data for reinfections had no difference in the reinfection rate. Although Pulliam et al. claimed to make all their data available, the required data for analyzing relative reinfection rates was not made publicly available, supposedly on account of privacy concerns. This is why we were forced to use mock data, which in any case would need to be generated to test and validate the code.
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    Avoidance of trappings in continual collapse of spherical stars.
    (2019) Govender, Terricia.; Goswami, Rituparno.
    In this dissertation we study the physical process of a spherically symmetric perfect uid experiencing a continuous gravitational collapse in concurrence with continuous radiating energy in an outward spacetime. Trapped surfaces are avoided and the nal fate of the collapse is a at spacetime. In addition, the collapsing matter conforms to the weak and dominant energy conditions at all epochs. Our investigation clearly unveils the purpose of the equation of state and reveals the bounds on the thermodynamic potentials the equation of state admits for such a model. We a rm that these models are generic without any of the issues and paradoxes attached to horizons and singularities, because the system of Einstein eld equations accepts such a theoretical account for an open set of initial data and the equation of state function in their respective functional spaces. High resolution radio telescopes of today, should ideally detect the existence of these compact bodies in the sky.
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    Instrumentation and operation of a new fatigue tester.
    (1993) Bird, Jeanette.; Jackson, Paul J.
    Abstract available in PDF.
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    Pulsed laser ablation synthesis of silver nanoparticles with rapid ionophore derivatives for antivirus activities.
    (2022) Gaza, Luke.; Moodley, Mathew Kisten.
    The world woke up to a deadly pandemic SARS Covid-19 that broke out of Wuhan, China in December 2019. Many drugs were proposed to help in treating this disease. However, at the time, few drugs showed potential effectiveness in curing covid-19. This research proposed the Covid-19 use of ionophores conjugated silver nanoparticle as a potential drug that can be effective because of its antiviral activities. Ionophores allow metal ions to enter the selective cell membrane of a cell. Once silver ions enter the cell, they start disrupting the replication of the virus DNA. Hence, silver plays an important role as a potential ingredient in drugs that can stop the spread of viruses. Pulsed laser ablation synthesis of a silver target placed in a solution was the method employed in this research to develop relatively smaller sized, spherical nanoparticles. In general, pulsed laser ablation synthesis in solution is a low environmental impact technique which does not need metal precursors and reductants and produces colloids of a relatively high purity as compared to chemical methods. The silver nanoparticles were produced by focusing a pulsed laser beam directed on a silver metal target in a liquid medium. As the laser beam hit the silver target a plasma plume is generated, as the evaporated material expands it then cools and condenses, forming nanoparticles. This approach employs the use of a Q-Switched Nd: YAG dual pulsed laser operating at fundamental 1064 nm and second harmonic 532 nm with energy densities in the range of 34.8 J.cm-2 to 37.3 J.cm-2. The lens focuses the pulsed laser which irradiate a silver target surface that is immersed in deionized water. Silver nanoparticles that were produced had comparable sizes, spherical in shape and similar composition. Their nanoparticle surface chemistry allowed them to be easily grafted with doxycycline. The period of irradiation was varied from 5 minutes to 25 minutes per sample to get different qualities ofAgNPs. The average diameter of the silver nanoparticles was 10,6 nm. The AgNPs were prepared and used to synthesise doxycycline silver nanoparticles. The samples of silver nanoparticles formed were characterised using high-resolution transmission electron microscopy (HR-TEM). The identification of the elements in the nano-material composition was done using Energy dispersive X-ray spectroscopy (EDS or EDX), Raman spectroscopy, and UV-VIS was used to confirm the absorption band gap of AgNPs. The silver nanoparticles that were produced was of high quality, free from toxic reagents and could be used for further medical (in-vivo tests).
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    The synthesis and characterization of magnetic α-Fe₂O₃ and ϵ-Fe₂O₃ nanoparticles.
    (2021) Chithwayo, Aphile Eustice.; Masina, Colani John.
    Abstract available in PDF.
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    Biochemical-physical mechanisms of light-tissue interactions.
    (2021) Buthelezi, Musawenkosi Doctor.; Chetty, Naven.; Adeleye, Oluwabamise.
    Optical tissue phantom samples simulating the optical properties of the human prostates and brain tissues were fabricated. The experimental set-up was designed to be cost-effective but reliable, allowing for convenience in its usage and replication, making it ideal for biomedical optical measurements. Gel agar was the base material, and aluminum oxide (Al 2 O3 ) with black ink was employed as the scatter and absorber, respectively. The latter were mixed in various amounts into the gel agar to simulate the desired phantom tissues. Argon red laser and He-Ne green laser light, with wavelengths of 630 nm and 532 nm were incident on varying thicknesses of the phantom samples. The transmitted and incident light powers were measured to determine the scattering and absorption coefficients, from which the attenuation coefficients, penetration depth, and optical albedo were estimated. The optical penetration depths were found to be 0.30 for brain and 0.15 for prostate tissue phantoms. The fabricated tissues successfully mimicked the brain and prostate tissues, with µ a = 0.69 cm−1and µ a = 0.24 cm−1 absorption coefficients as well as 𝜇𝜇𝑠𝑠 = 1.73 cm−1 and µ s = 5.48 cm−1 scattering coefficients at 532 nm and 630 nm wavelengths, respectively. The optical albedo for brain phantom was found to be a = 0.71 and a = 0.96 for prostate phantom tissue. The results verify the reliability of the experimental technique and suitability of the fabricated tissues for use in biomedical, going forward, thus allowing for future work without the need for experimentally complex and expensive setups.
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    Development of a mathematical model to enable optimal efficiency of the indabuko lithium-ion battery.
    (2020) Mphaka, Johannes.; Chetty, Naven.; Maphanga, Regina R.
    Cathode materials are the foremost primary challenge for the vast scale application of lithium-ion batteries in electric vehicles and the stockpiles of power. Foreseeing the properties of cathode materials is one of the central issues in energy storage. In the recent past, density functional theory (DFT) calculations aimed at materials property predictions offered the best trade-off between computational cost and accuracy compared to experiments. However, these calculations are still excessive and costly, limiting the acceleration of new materials discovery. Now the results from different computational materials science codes are made available in databases, which permit quick inquiry and screening of various materials by their properties. Such gigantic materials databases allow a dominant data-driven methodology in materials discovery, which should quicken advancements in the field. This study was aimed at applying machine learning algorithms on existing computations to make precise predictions of physical properties. Thus, the dissertation primary goal was build best ML models that are capable of predicting DFT calculated properties such as, formation energy, energy band-gap and classify materials as stable or unstable based on their thermodynamic stability. It was established that the algorithms only require the chemical formula as input when predicting materials properties. The theoretical aspect of this work describes the current machine learning algorithms and presents "descriptors"-representations of materials in a dataset that plays a significant role in prediction accuracy. Also, the dissertation examined how various descriptors and algorithms influence learning model. The Catboost Regressor was found to be the best algorithm for determining all the properties that were selected in this study. Results indicated that with appropriate descriptors and ML algorithms it is feasible to foresee formation energy with coefficient of determination (R2) of 0.95, mean absolute error (MAE) of 0.11 eV and classify materials into stable and unstable with 86% of accuracy and area under the ROC Curve (AUC) of 89%. Lastly, we build a web application that allow users to predict material properties easily.
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    Investigation of IBMQ quantum device hardware calibration with Markovian master equation.
    (2022) Brand, Dean Charles.; Sinayskiy, Ilya.; Petruccione, Francesco.
    In the design of quantum technology, it is crucial to account for the quantum system interacting with its environment to understand the influence of thermal processes and design the devices to avoid e↵ects of relaxation and decoherence of quantum states deteriorating the system beyond use. To accomplish this, a broadening of ideal isolated quantum mechanics is required, namely the theory of open quantum systems. This is most prevalent in the research of quantum error correction, which ensures that the initial quantum state remains intact when it is received and doesn’t decay into a di↵erent state which would change the information carried by the qubit. To investigate the intersection of all these phenomena, open-access cloud-computing services o↵er the ideal experimental environment. One such test-bed is o↵ered by IBM in their Quantum Experience platform which allows for remote access to quantum devices. The IBMQ quantum processors, which make use of superconducting qubit technology, are openly accessible through a cloud service. As such, they have been the focus of a lot of research into the evolution of quantum states while interacting with the environment. In the study of open quantum systems, an assumption is often made that the system and environment share no memory of the interaction of individual quantum states, which simplifies the analysis of the system’s evolution while also being e↵ectively true for large enough systems. Systems that obey this assumption are known as Markovian. New research has devised methods of error correction and tomography of quantum processors when this assumption no longer holds. Additionally, the calibration of the IBMQ processors performed by IBM to provide hardware parameters is performed through a set of techniques that are not guaranteed to yield cohesive results. These primary factors, among others, give rise to the research discussed in this dissertation, and pose the question of how accurate the hardware calibrations are when compared to results obtained through experiments performed on the devices. Furthermore, the approach uses the theory of open quantum systems to assess the hardware calibration while also testing whether the Markovian assumption of a memoryless system holds for the IBMQ quantum devices. This gives insight into the current state of superconducting quantum computers while providing a possible new avenue for quantum error correction from the perspective of the theory of open quantum systems.
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    The influence of metal oxide transport layer and annealing temperature on perovskite solar cells (PSCs)
    (2021) Lekesi, Lehlohonolo Petrus.; Malevu, Thembinkosi Donald.; Motaung, Tshwafo Elias.; Gumbi, Bhekumuzi Prince.
    Organic-inorganic halide Perovskite Solar Cells (PSCs) are the leading third-generation solar cells with the possibility to provide a fraction of green and affordable energy in the next technological era of solar energy. This type of photovoltaic cell is still new and has recently gained interest due to its low production cost, easy fabrication and rapidly improving power conversion efficiency (PCE). The performance of PSCs depends on the metal oxide/perovskite interface. This work focuses on improving the intermediate contact between the light-active perovskite layer and the electron transport layer (ETL) in a fully ambient PSC by annealing titanium dioxide (TiO2) ETL at extreme temperatures. The TiO2 semiconducting material was successfully synthesized using the hydrothermal method due to the method’s ability to produce pure and crystalline nanoparticles at low temperatures. With the future application of TiO2 projected to flexible conductive substrates, the as-synthesized TiO2 nanopowders were preheated from 200 to 1200 ℃ temperature range prior to deposition to avoid substrate deformation. To investigate the effect of annealing on the synthesized TiO2 nanopowders X-ray diffraction (XRD), transmission electron microscopy (TEM), Scanning electron microscopy (SEM), Energy dispersive x-ray (EDX) spectroscopy, Fourier transform infrared spectroscopy (FT-IR), and Ultraviolet-visible spectroscopy (UV-Vis) techniques were employed to study the structural, morphological, and opto-chemical property changes according to the temperature range described. From the crystal and morphology analysis, the as-synthesized TiO2 nanomaterial appears to be a crystalline multiphase material showing coexistence of anatase and rutile phases. Annealing increases the metal oxide crystallinity, porosity, and particle dispersion. The optical analysis of TiO2 material reveals successful bandgap tuning of the metal oxide wide-bandgap structure. The perovskite active layer was formed through the two-step spin coating of lead iodide (PbI2) and methylammonium iodide (CH3NH3I) respectively. To investigate the morphological structure, thermal and optical properties of TiO2/CH3NH3PbI3 SEM, Thermogravimetric analysis (TGA), Photoluminescence spectroscopy (PL), and UV-Vis techniques were used. Finally, perovskite solar cells of device structure ITO/c-TiO2/m-TiO2/MAPbI3/Spiro-MeoTAD/Conductive Ag ink/ITO were fabricated and their performance was evaluated using the Keithley solar simulator.
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    Development of near real time and cumulative GIC proxy indices.
    (2021) Essop, Abu-Bakr.; Mtumela, Zolile.; Lotz, Stefan.
    Geomagnetic storms are phenomena which can give rise to geomagnetically induced currents (GICs), which have an adverse effect on technology in that they can cause anomalous low frequency currents that damage critical infrastructure. The problems with quantifying the damage in the absence of accurate GIC data (which can show the level of damage) are twofold, namely, for near real-time applications and the other for long-term applications respectively. Since GIC data is not easily available due to power utilities either not having measuring devices or not allowing its dissemination readily, other methods of quantifying damage as unambiguously as possible using data from more attainable sources such as local magnetometer stations, are necessary improvements that can be made. Attempts are made in this work, using an algorithm similar to that of Wintoft et al. [1], to address these problems via the creation of two GIC proxies to, in the case of near-real time applications, track damage, and in the long-term case, by combining ideas from Yu and Ridley [2] as well as Lotz and Danskin [3], to indicate damage incurred during storms. Using these algorithms, results are acquired by making use of Pearson’s correlation and graphical methods, although the data set is too small to draw statistically significant conclusions. The results from the short-term index show that the index works well with the best indicators of shortterm behaviour available as well as GIC data from power stations in South Africa. The results from the long-term index corroborates with the literature, in that damage done in long, yet less intense events can can be as significant as damage done by short-term, yet highly intense events, as reported by Lotz and Danskin [3].
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    Plastic shear in a model amorphous solid.
    (1992) Moji, Nthobane Cable.; Jackson, Paul J.
    Abstract available in PDF.
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    Shape invariance in supersymmetric quantum mechanics.
    (1992) Welter, Allard.; De Lange, Owen Leon.
    Abstract available in PDF.
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    An interferometric investigation of the quadratic electro-optic effect in KDP.
    (1995) Gunning, Mark Julian.; Raab, Roger Edouard.
    Abstract available in PDF.
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    Phase transitions in induced lattice gauge models.
    (1995) Moodley, Mervlyn.; IIchev, Assen.
    The present research is based on the study of the phase structure of lattice models incorporating selfinteracting scalars and gauge background fields otherwise known as induced gauge models. Emphasis is placed on the effect the choice of the integration measure over the radial modes of the scalar fields have on the phase structure of these models. Both numerical simulations and analytical results based on the mean field approximations are presented. In Chapter 1 an introduction to quantum field theory is given leading to the for­mulation of Euclidean quantum field theory. In Chapter 2 global and local gauge invariance together with the mechanism of spontaneous symmetry breaking are discussed. In Chapter 3 the formulation of quantum field theory on the lattice is introduced. The lattice regularization entails discretizing space and time and presents an elegant approach to studying certain phenomena of the continuum theory which are beyond the reach of standard perturbative analysis. In Chapter 4 the Monte Carlo methods for evaluating the Euclidean Feynman path integral as applied to lattice gauge theory are discussed. In Chapter 5 numerical studies of some lattice gauge models are presented. Both pure lattice gauge models and gauge-Higgs models are examined. In Chapter 6 the Kazakov-Migdal model which presents an interesting approach to inducing QCD is discussed. In Chapter 7 the mixed fundamental-adjoint induced model is introduced. This model succeeds in breaking the local ZN symmetry of the Kazakov-Migdal model by adding to it scalar fields in the fundamental representation of the gauge group. The effect of the choice of the radial integration measure on the phase structure of a class of Abelian induced models is studied.