The magnetic properties of some rare earth based Heusler alloys have been investigated. Rare earth Heusler alloys of the form Pd2Reln exhibit magnetic behaviour characterised by magnetic moments localised on the rare earth atoms. X-ray and neutron diffraction investigation show that all alloys crystallise in the cubic L21 Heusler structure with space group Fm3m.

Photoelectrochemical (PEC) water splitting, using sunlight and appropriate semiconductors to produce hydrogen (H2) fuel, is a promising route to solve both the production of clean H2 fuel and storage for solar energy. Owing to its various advantages, hematite (α-Fe2O3) has emerged as a promising photoanode material for PEC water splitting. However, its poor electrical conductivity, low carrier mobility, short-hole diffusion length, and fast recombination rates of the electron-hole pairs have greatly limited its full potential for PEC performance. One way to improve the PEC activity of α-Fe2O3 is by doping with other elements. In particular, surface doping is proved to be more beneficial than bulk doping because it reduces the distance moved by the charge carriers from inside the bulk to the surface where they are required for interfacial transfer. In this study first-principles calculations based on density functional theory (DFT) were carried out to investigate the influence of Cu, Zn, Ti and Zr on the {0001} and {01 2} hematite surfaces for enhanced PEC water splitting. Various surfaces of hematite were constructed and their thermodynamic stabilities were determined by calculating surface and formation energies. The {0001} and {01 2} surfaces were found to be the most stable. Besides, all the doped systems were found thermodynamically stable. Furthermore, it was found that Cu doped surface systems does not only decrease the bandgap but also leads to the correct conduction band alignment for spontaneous water splitting. In all calculations, the charge density difference plots and the Bader charge analysis showed accumulation of charge at the top outmost surface, implying the photogenerated charge carriers can efficiently diffuse to the surface for enhanced interfacial charge transfer to the adsorbates. Morever, it was found that even with mono doping of Zn on the topmost layer of the {0001} α-Fe2O3 surface, the bandgap can be decreased without impurity states in the band structure which normally acts as recombination centres. Furthermore, the energetic stability and electronic properties of bimetallic doped {0001} α-Fe2O3 surface with (Zn, Ti) and (Zn, Zr) pairs for enhanced PEC water splitting was also studied. Bimetallic doping is viewed as an important and executable way of not only increasing the conductivity of a semiconductor material but also reducing the quick recombination of the electron-hole pairs. The doped systems showed negative formation energies under both O-rich and Fe-rich conditions implying that they are thermodynamically stable and could be prepared experimentally. Additionally, bimetallic doping of (Zn, Ti) and (Zn, Zr) on the {0001} surface is expected to enhance the PEC performance of α-Fe2O3 because Ti or Zr is capable of increasing the conductivity of α-Fe2O3 due to the substitution of Fe3+ with Ti4+ or Zr4+, while Zn can foster the surface reaction and reduce quick recombination of the electron-hole pairs. We hope that our results provided here will be of great interest to both experimental and theoretical researchers. / Thesis (PhD (Physics))--Univesity of Pretoria, 2020. / Ministry of Higher Education, Copperbelt University, Zambia / The University of Pretoria, Department of Physics / Centre for High-Performance Computer (CHPC), Cape Town / Physics / PhD (Physics) / Restricted

In this dissertation, α-Fe2O3 thin film deposition techniques were first evaluated to understand their effects on the structural, optical and photoelectrochemical (PEC) properties of the films. α-Fe2O3 films were deposited by dip, spin and combined dip/spin coating techniques on fluorine-doped tin oxide (FTO) substrates at an annealing temperature of 500°C. Structural properties suggest better crystallinity for films prepared by dip and combined dip/spin coating techniques as compared to spin coated films. Field emission scanning electron microscopy showed spherical nanoparticles with some agglomeration into small larvae-shape nanostructures for all the films. All films absorb in the visible region due to their bandgap of 1.98 ± 0.03 eV. Maximum photocurrent densities of 34.6, 7.8, and 13.5 µA/cm2 were obtained at 1.23 V vs reversible hydrogen electrode (RHE) for dip, spin and combined dip/spin coated films with the thickness of 740-800 ± 30 nm respectively. Improved crystallization, low charge transfer resistance at the solid/electrolyte junction, high surface states capacitance, and a more negative flat band potential values obtained for dip coated films using electrochemical techniques, have been associated to their improved photocurrent response. Furthermore, the annealing approach for preparing multi-layered α-Fe2O3 films using the dip coating technique was modified to enhanced their PEC performance. The first three layers of the films were annealed at 500°C and the fourth layer at 500, 600, 700, 750 and 800°C respectively. Films annealed at 750°C recorded the best performance, producing 0.19 mA/cm2 photocurrent at 1.23 V vs RHE; 5.3 times more than what was recorded for films sintered at 500°C, and the onset potential yielded a cathodic shift of 300 mV. The enhanced performance was linked to improved crystallization and absorption coefficient, lowered flat band potential, increased charge carrier density, decreased charge transfer resistance at the solid/liquid interface and increased surface states capacitance for films annealed at 750°C. Also, nanostructured heterojunction of α-Fe2O3 and porous copper (II) oxide (CuO) composites represented as α-Fe2O3/CuO was prepared for the enhancement of PEC water splitting. Structural studies confirmed the high purity of α-Fe2O3/CuO heterostructures produced. Enhanced photocurrent density of 0.53 mA/cm2 at 1.0 V vs RHE was achieved for α-Fe2O3/CuO photoanodes, representing a 19-fold increase compared to the value recorded for α-Fe2O3. The formation of a heterojunction coupled with the porous surface morphology of α-Fe2O3/CuO facilitated charge separation of photogenerated electron-hole pairs, reduced the bandgap and increased the charge carrier density of the heterostructure, enhancing PEC water splitting. / Thesis (PhD (Physics))--University of Pretoria, 2021. / National Research Foundation - The World Academy of Sciences (NRF) grant #110814 and South African Research Chairs Initiative (SARCHI) grant #115463. / Physics / PhD (Physics) / Restricted

published_or_final_version / Economics and Finance / Doctoral / Doctor of Philosophy

An infrared active polyatomic molecule has several vibrational modes, each of which has a characteristic frequency. If the molecule is trapped in a matrix of perturbing atoms, those vibrational frequencies will shift, and if the vibrational mode is degenerate, the perturbation may lift the degeneracy. Such shifts and splitting are due to the dependence of the chromophore/matrix-atom interaction potential on the internal vibrational motion of the chromophore. Applying a previously-developed model for the shifting and splitting of the triply degenerate <em>&nu;</em>₃ mode of SF₆ perturbed by a rare gas atom, we use Monte Carlo simulations to sample the accessible equilibrium configurations of the system and to predict the associated thermally averaged perturbed IR spectra. Since the experimental spectrum has 10 peaks while the triply degenerate <em>&nu;</em>₃ mode of SF₆ in a particular environment could have at most 3 peaks, the observed spectrum must be a combination of spectra for SF₆ trapped in different types of lattice sites. A fit to experiment of simulated spectra generated from a family of lattice sites is then used to identify the peaks in the experimental spectrum, determine the relative importance of the various lattice sites, and semi-quantitatively reproduce the experimental spectrum.

Résumé indisponible. / Résumé indisponible.

Doctor of Philosophy / The aim of the studies reported in this project was to examine factors associated with the prevention and management of trapeziometacarpal osteoarthritis, both in musculoskeletal physiotherapists and the general patient population. Two studies were undertaken to investigate factors associated with the aggravation of thumb pain in musculoskeletal physiotherapists. Study 1 was a survey of the prevalence of thumb pain, and allowed determination of the most aggravating spinal manipulative therapy technique. It identified that 83% of respondents complained of an aggravation of thumb pain due to the performance of spinal manipulative therapy techniques, with 85-87% of the painful respondents complaining of thumb pain aggravated by unilateral and central PA glides. Study 2 was conducted to determine whether the alignment of the joints of the thumb during performance of these glides was associated with thumb pain. This observational study of 129 musculoskeletal physiotherapists performing a PA glide identified that aligning the metacarpophalangeal and interphalangeal joints in extension was associated with a lower prevalence of work-related thumb pain. Therefore, it is suggested that musculoskeletal physiotherapists be taught to perform these techniques with the joints of their thumb in extension in an effort to reduce the development of work-related thumb pain. Furthermore, it is suggested that those who are unable to maintain this alignment voluntarily be provided with a thermoplastic thumb splint to maintain the extended alignment. Two studies were undertaken to investigate the conservative and surgical management of patients with trapeziometacarpal osteoarthritis. Study 3 was a randomised controlled trial conducted to compare the efficacy of a new thumb strap splint and an abduction exercise regimen against the standard approach to conservative management of trapeziometacarpal osteoarthritis, namely a short opponens splint and pinch exercise regimen. While there was no additional benefit of one approach over the other, all participants improved in the outcomes of pain, strength and hand function over the six-week period of intervention. Nevertheless, some people find that symptom relief from conservative intervention is inadequate and short-lived, requesting surgery for the treatment of disabling and persistent pain from trapeziometacarpal osteoarthritis. Study 4 was a systematic review, conducted to determine evidence of efficacy of one surgical procedure over another. This review identified six randomised controlled trials of surgery for trapeziometacarpal osteoarthritis. While there was evidence of no difference in the reduction in weakness between the procedures, there was insufficient evidence to confirm that there was no difference in the outcomes of pain, contracture, hand function, or patient global assessment. Furthermore, there was sufficient evidence to conclude that trapeziectomy had significantly fewer adverse effects, and trapeziectomy with ligament reconstruction and tendon interposition (LRTI) had significantly more, when compared with the other procedures analysed in this review. It is suggested that the decision as to which intervention is most appropriate for a given patient be based upon the individual patient’s requirements, the extent of disease, and the demands placed upon the joint by domestic duties, work, leisure and recreational activities. The studies presented in this project assist in formulating preventative and management strategies for people with trapeziometacarpal osteoarthritis.

An infrared active polyatomic molecule has several vibrational modes, each of which has a characteristic frequency. If the molecule is trapped in a matrix of perturbing atoms, those vibrational frequencies will shift, and if the vibrational mode is degenerate, the perturbation may lift the degeneracy. Such shifts and splitting are due to the dependence of the chromophore/matrix-atom interaction potential on the internal vibrational motion of the chromophore. Applying a previously-developed model for the shifting and splitting of the triply degenerate <em>&nu;</em>₃ mode of SF₆ perturbed by a rare gas atom, we use Monte Carlo simulations to sample the accessible equilibrium configurations of the system and to predict the associated thermally averaged perturbed IR spectra. Since the experimental spectrum has 10 peaks while the triply degenerate <em>&nu;</em>₃ mode of SF₆ in a particular environment could have at most 3 peaks, the observed spectrum must be a combination of spectra for SF₆ trapped in different types of lattice sites. A fit to experiment of simulated spectra generated from a family of lattice sites is then used to identify the peaks in the experimental spectrum, determine the relative importance of the various lattice sites, and semi-quantitatively reproduce the experimental spectrum.

Five cation exchange membranes and four anion exchange membranes were tested in a three-compartment, two-membrane, electrolysis salt-splitting cell for the recycle of sodium sulfate into sodium hydroxide and sulfuric acid. The cell is further examined using DuPont Nafion 324 cation exchange membrane and Sybron Ionac MA-7500 anion exchange membrane to determine the maximum concentration of sodium hydroxide that can be produced by electrolysis salt-splitting as well as to determine the chief source of inefficiency. The discussion includes recommendations for future electrolysis salt-splitting cells and a mathematical model of the cell is created to determine optimum operating conditions.

The spin-splitting energy in wurtzite structure semiconductors had been investigated by linear combination of atomic orbital method (LCAO), atomic bond orbital method and two-band k¡Ep method. In order to explain the large zero field spin splitting in wurtzite GaN, a different mechanism (£GC1¡V£GC3 coupling) was proposed, which originated from the intrinsic wurtzite effects (band folding and wurtzite bulk inversion asymmetry). The band-folding effect generates two conduction bands (£GC1 and £GC3), in which p-wave probability has tremendous change when kz approaches the anticrossing zone. The spin-splitting energy induced by the£GC1¡V£GC3 coupling and wurtzite bulk inversion asymmetry is much larger than theory calculation of Kane model. When we apply the coupling to GaN/AlN quantum wells, we find that the spin-splitting energy is sensitively controllable by an electric field. It is also found that ideal wurtzite bulk inversion asymmetry yields not only a spin-degenerate line (along the kz axis; time reversal axis) but also a minimum-spin-splitting surface, which can be regarded as a spin-degenerate surface in the form of bkz2- k//2=0 (b≈4) near the £F point. This phenomenon is referred to as the Dresselhaus effect (defined as the cubic-in-k term) in bulk wurtzite materials because it generates a term £^wz(bkz2- k//2)(£mxky-£mykx)=0 in the two-band k¡Ep Hamiltonian. And it is also demonstrated that in the k.p scheme, the spin splitting vanishes to cubic order in k. Consequently, the D¡¦yakonov-Perel¡¦ (DP) spin relaxation mechanism can be effectively suppressed for all spin components in [001] wurtzite quantum wells (QWs) at a resonance condition through device design with appropriate strain, gate voltage or optical illumination. In conclusion: (1) the spin-splitting energy is enhance by wurtzite bulk inversion asymmetry; (2) the spin-splitting energy in wurtzite quantum well is sensitively controllable by electric field; (3) there exist a spin degenerate surface for wurtzite materials in k¡Ep scheme. Therefore, wurtzite QWs (e.g., InGaN/AlGaN and InN/AlInN) are potential candidates for spintronic devices such as the resonant spin lifetime transistor.