Philosophiae Doctor - PhD (Physics)

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Optical modeling of amorphous and metal induced crystallized silicon with an effective medium approximation
(University of the Western Cape, 2009) Muller, Theophillus Frederic George; Knoesen, Dirk; Arendse, Christopher; Dept. of Physics; Faculty of Science
In this thesis we report on the metal-mediated-thermally induced changes of the structural and optical properties of hydrogenated amorphous silicon deposited by hot-wire CVD, where aluminium and nickel were used to induce crystallization. The metal-coated amorphous silicon was subjected to a thermal annealing regime of between 150 and 520°C. The structural measurements, obtained by Raman spectroscopy, show partial crystallization occurring at 350 °C. At the higher annealing temperatures of 450°C and 520°C complete crystallization occurs. Reflection and transmission measurements in the UV-visible range were then used to extract the optical properties. By adopting the effective medium approximation a single optical model could be constructed that could successfully model material that was in different structural phases, irrespective of metal contamination. Changes in the absorption of the material in various stages of transition were confirmed with a directly measured absorption technique, and the modelled absorption closely followed the same trends This study forms part of the larger overall solar cell research project, of which the primary aim is to eventually develop a silicon solar panel that optimises the characteristics for best performance.
Proton conductivity stability studies by modelling
(University of the Western Cape, 2016) Square, Lynndle Caroline; Arendse, Christopher
In this thesis, some of the challenges experienced by high temperature polymer electrolyte membrane fuel cells are explored through material modelling techniques. A very important aspect for a fuel cell is that it should have high proton conductivity. As hydrogen enters a fuel cell it gets broken down into its constituents, protons and electrons. The electrons travel to an external load, whilst the protons travel through a diffusive layer, catalyst layer and membrane area, before recombining with oxygen to form water and leave the system. In this particular study, polytetrafluoroethylene and carbon form the diffusive layer, platinum the catalyst and poly(2,5-benzimidazole) doped with phosphoric acid the membrane area. The effects to proton conductivity are investigated as a result of the mixing of materials and adsorption of the phosphoric acid on the platinum active sites. A third study as an alternative avenue for proton conductivity improvements, is also explored. The results from these investigations promotes the idea that polytetrafluoroethylene, which is found in the ionomer layer, should be replaced as its mechanical properties decrease significantly with increase in temperature. Increasing pressure would further promote proton transfer over the doped polymer membrane region.
Spectrally selective AlXOY/Pt/AlXOY solar absorber coatings for high temprature solar-thermal applications
(University of Western Cape, 2014) Nuru, Zebib Yenus; Maaza, Malik; Arendse, Christopher
The limited supply of fossil hydrocarbon resources and the negative impact of CO2 emission on the global environment dictate the increasing usage of renewable energy sources. Concentrating solar power (CSP) systems are the most likely candidate for providing the majority of the renewable energy. For efficient photo-thermal conversion, these systems require spectrally selective solar absorber surfaces with high solar absorbance in the solar spectrum region and low thermal emittance in the infrared region. In this thesis, a spectrally selective AlxOy/Pt/AlxOy multilayer solar absorber was designed and deposited onto copper substrate using electron beam evaporation at room temperature. The employment of ellipsometric measurements and optical simulation was proposed as an effective method to optimize and deposit the multilayer solar absorber coatings. The optical constants measured using spectroscopic ellipsometry, showed that both AlxOy layers, which used in the coatings, were dielectric in nature and the Pt layer was semi-transparent. The optimized multilayer coatings exhibited high solar absorptance ~ 0.94±0.01 and low thermal emittance ~ 0.06 ± 0.01 at 82oC.The structural and optical properties of the coatings were investigated. It was found that the stratification of the coatings consists of a semitransparent middle Pt layer sandwiched between two layers of AlxOy. The top and bottom AlxOy layers were nonstoichiometric with no crystalline phases present. The Pt layer is in the fcc crystalline phase with a broad size distribution and spheroidal shape in and between the rims of AlxOy. The surface roughness of the stack was found to be comparable to the inter-particle distance. To study the thermal stability of the multilayer solar absorber coatings, the samples were annealed at different temperatures for different duration in air. The results showed changes in morphology, structure, composition, and optical properties depend on both temperature and duration of annealing. The XRD pattern showed that the intensity of Pt decreased with increasing annealing temperature and therefore, disappeared at high temperature. With increasing annealing temperature, an increase in the size of Pt particles was observed from SEM. The AlxOy/Pt/AlxOy multilayer solar absorber coatings deposited onto Cu substrate were found to be thermally stable up to 500oC in air for 2 h with good spectral selectivity of 0.951/0.09. At 600oC and 700oC, the spectral selectivity decreased to 0.92/0.10 and 0.846/0.11 respectively, which is attributed to the diffusion of Cu and formation of CuO and Cu2O phases. Long term thermal stability study showed that the coatings were thermally stable in air up to 450oC for 24 h. To elucidate the degradation mechanism beyond 500oC, HI-ERDA has been used to study depth-dependent atomic concentration profiles. These measurements revealed outward diffusion of the copper substrate towards the surface and therefore, the decrease in the constituents of the coating. Hence, to prevent copper from diffusing towards the coatings, a thin Tantalum (Ta) layer was deposited between the base AlxOy layer and the copper substrate.The effect of a thin Ta layer on the thermal stability of AlxOy/Pt/AlxOy multilayer solar absorber coatings was investigated. The Cu/Ta/AlxOy/Pt/AlxOy multilayer solar absorber coatings were found to be thermally stable up to 700oC in air for 2 h with good spectral selectivity of 0.937/0.10. At 800oC, the spectral selectivity decreased to 0.870/0.12, which is attributed to the diffusion of Cu and formation of CuO phase. The formation of CuO phase was confirmed by XRD, EDS and Raman spectroscopy. Long term thermal stability study showed that the coatings were thermally stable in air up to 550oC for 24 h. Therefore, the Cu/Ta/AlxOy/Pt/AlxOy spectrally selective solar absorber coatings can be used for high temperature solar-thermal applications.
Structural and gas sensing properties of TiO₂-based (Sn, Mg) nano-structures induced by mechanical milling and annealing
(University of the Western Cape, 2015) Bolokang, Amogelang Sylvester; Motaung, David E.; Arendse, Christopher; Muller, Theophillus F. G.
Titanium oxynitride has attracted research interest for the fact that it is a bioactive non-toxic material. It is suitable for surface coating of biomaterials and in other applications such as anti-reflective coatings, while oxygen-rich titanium oxynitride has been applied in thin film resistors and photocatalysis. Two common types of titanium oxynitrides are TiOₓNᵧ. and TiO₂-ₓNᵧ. In this work, titanium mixed metals oxynitrides (Ti-TiO₂, Mg-TiO₂ and Mg-Sn-TiO₂) were synthesized for the first time using ball milling (BM) and annealing processes. Their structural, morphological, surface, optical, and gas sensing properties were studied in detail. Structural analyses showed that upon milling a pure TiO₂ phase, tetragonal to orthorhombic phase transformation occurred. However, when milling TiO₂ mixed with Mg, Sn and Ti no evidence of the transformation was observed. Furthermore, scanning electron microscopy, transmission electron microscopy and atomic force microscopy showed that the milling process promotes particle refinement. The gas sensing analyses also demonstrated that the sensing response of the TiO₂, Mg-TiO₂ and Mg-Sn-TiO₂ materials improved upon milling. Moreover, the Mg-TiO₂ showed improved sensing compared to pure TiO₂ due to incorporation of Mg, which might have resulted in a decrease of charge carrier concentration. The Mg-TiO₂ sensing materials showed fast response-recovery time of ~32 s and ~48 s, respectively, as well as high selectivity to NH₃ gas compared to other gases (H₂, and CH₄). In addition, the improved response observed for the milled samples is due to increased surface area and pore diameter, providing more active sites for the target gas and allowing more gas adsorption with an increase in point defects related to oxygen vacancies (Vo), which are the most favorable adsorption sites for oxygen species and thus can enhance the possibility of interaction with gas molecules. A combination of photoluminescence, x-ray photoelectron spectroscopy, vibrating sample magnetometer and sensing analyses demonstrated that a direct relation exists between the magnetization, sensing and the relative occupancy of the Vo present on the surface of TiO₂ nanoparticles. Therefore, based on these finding we conclude that the milling process promotes particle refinement, resulting in an increased BET surface and partial breaking of Ti–O bonds on the TiO₂ surface layer, which results in the formation of oxygen vacancies in the TiO₂ lattice, therefore anticipating improved sensing response.
Structure property relationship and thermal stability of organic photovoltaic cells
(University of the Western Cape, 2010) Motaung, David Edmond; Malgas, Gerald F.; Arendse, Christopher; Dept. of Physics; Faculty of Science
In this thesis, regioregularpoly( 3-hexylthiophene) (rr-P3HT) polymer was used as a light absorption and electron donating material, while the C60 fullerene and its derivative [6,6]-phenyl C61-butyric acid methyl ester (PCBM) were used as electron acceptor materials. The effect of solvent to control the degree of mixing of the polymer and fullerene components, as well as the domain size and charge transport properties of the blends were investigated in detail using P3HT:C60 films. The photo-physical, structural and electrical transport properties of the polymer blends were carried out according to their ratios. A distinctive photoluminescence (PL) quenching effect was observed indicating a photo-induced electron transfer. In this thesis, the effect of solvents on the crystallization and interchain interaction of P3HT and C60 fullerene films were studied using XRD, UV-vis, PL, Raman and FTIR spectroscopy. The polymer blends formed with non-aromatic solvents exhibited an improved crystallinity and polymer morphology than that formed with aromatic solvents. An improved ordering was demonstrated in the polymer films spin coated from non-aromatic solvents. This indicates that the limited solubility of rr P3HT in a marginal solvent such as non-aromatic solvents can offer a strategy to obtain highly ordered crystal structures and lead directly to optimal morphologies on the films.
Synthesis of bimetallic immiscible alloy nanoparticles through green and gamma radiolysis approaches for environmental remediation applications
(University of the Western Cape, 2022) Noukelag, Sandrine Kamdoum; Arendse, Christopher
The synthesis of bimetallic immiscible alloy nanoparticles (NPs) using versatile routes, is a major concern since physio-chemical methods are not environmentally benign. Breaking down the immiscibility would generate NPs with remarkable properties and consequently more applications. As a result, it urges the development of one-step, eco-friendly, efficient, and reliable methods for getting more metastable bimetallic alloys from immiscible metals. To that aim, unconventional approaches such as green and gamma radiolysis were considered as the paths forward in this thesis. The wide immiscibility gaps of iron-silver (Fe-Ag), and iron-zinc (Fe-Zn) led to their selection.

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