<p> In this thesis, we investigated a recent and interesting issue in magnetism; spin ice, which is a term used for systems where there is a analogy between their magnetic structure and the proton structure of water ice. Until now, only three spin ices, Dy2Ti2O7, Ho2Ti2O7, and Ho2Sn2O7, have been discovered. In 2002, Ho2Ru2O7 was proposed as a candidate spin ice by Bansal et al. Given the similar structure and experimental behaviors of Ho2Ru2O7 and known spin ice systems, it has been an issue whether Ho2Ru2O7 is the fourth spin ice.</p> <p> In order to determine whether the new candidate is spin ice or not, the magnetic characteristics of Ho2Ru2O7 have been investigated. The frustrated system Ho2Ru2O7 has a pyrochlore structure with magnetic spins located on lattice of corner sharing tetrahedra. The crystal field originated <1 1 1> anisotropy of this sites and ferromagnetic interaction of spins give the preference of a two spin in and two spin out to a center of the tetrahedra.</p> <p> High quality polycrystalline samples were prepared and crystal growth attempts were made, then various measurements have been conducted. DC susceptibility data were used to determine the effective magnetic moment and value of Weiss temperature(θ). Zero field cooled (ZFC) and field cooled (FC) susceptibility data show a small irreversibility below 95K, which indicate ruthenium antiferromagnetic ordering. AC susceptibility measurements show a strong frequency dependence of the susceptibility which is a feature characteristic of spin glass or superparamagnetic materials.</p> <p> Specific heat experiments were conducted to also confirm the existence of Ru magnetic ordering at 95 K. The Debye temperature is estimated to be ~441 K.</p> <p> μSR measurements were conducted at TRIUMF. The measurements of Ho2Ru2O7 show signatures of ruthenium ordering near 95K and holmium near 1.4K. In agreement with previous neutron scattering experiment we conclude that the ground state of Ho2Ru2O7 is antiferromagnetic rather than spin ice. Presumably the ruthenium ordering acts to preclude the holmium moments entering the spin ice state.</p> / Thesis / Master of Science (MSc)

<p> The work described in this thesis is devoted to the application of defect engineering in the development of silicon photonic devices. The thesis is divided into simulation and experimental portions, each focusing on a different form of defect engineered silicon: ion implantation induced amorphous silicon and solid-phase epitaxial regrowth suppressed polycrystalline silicon.</p> <p> The simulations are directed at silicon rib waveguide Raman laser applications. It is shown that a uniform, divacancy defect concentration will not enhance Raman gain. The excess optical loss and free carrier lifetime of rib waveguides with remote amorphous silicon volumes were simulated. Net gain was demonstrated depending on the geometry of the structure. For a waveguide structure with rib width, rib height and slab height of W = 1.5, H = 1.5 and h = 0.8 μm respectively, the optimal separation between the edge of the rib and the amorphous region is ~2.0 μm. Surface recombination velocity modification was shown to be an effective means to reduce free carrier lifetime.</p> <p> Experimental work was devoted to the characterization of a novel form of polycrystalline silicon created by amorphizing the entire silicon overlayer of a silicon-on-insulator wafer. Solid-phase epitaxial regrowth of the amorphous silicon is suppressed upon annealing due to the lack of a crystal seed and results in polycrystalline silicon. This material was characterized with ellipsometry, positron annihilation spectroscopy and x-ray diffraction. The material properties are shown to be heavily dependent on the annealing conditions. Ellipsometry showed that the refractive index at 1550 nm is comparable to crystalline silicon. Positron annihilation spectroscopy showed that the polycrystalline material exhibits a high concentration of vacancy-type defects while vertically regrown crystalline silicon does not. X-ray diffraction showed that the polycrystalline silicon is non-textured, strained in tension and is characterized by grain sizes less than 300 nm.</p> <p> Defect etching and optical measurements using a waveguide geometry were performed in order to characterize the lateral regrowth and the optical loss of the polycrystalline material. Lateral regrowth in the [011] direction was 1.53 and 0.96 μm for 10 minute anneals at 750 and 900 °C respectively, and at least 2.5 μm at 650 °C. Waveguide optical loss measurements with adjacent polycrystalline regions separated from the rib by at least 5.5 μm showed no separation dependence. The intrinsic optical loss of the polycrystalline material was estimated to be 1.05 and 1.57 dB/cm for TM and TE polarizations after a 900 °C anneal. Vertically regrown c-Si was shown to exhibit less than 3.0 dB/cm optical loss after annealing at 550 °C .</p> / Thesis / Master of Applied Science (MASc)

Diploma thesis is focused on the superhard cutting materials (polycrystalline diamond, polycrystalline boron nitride) and presents their physico-mechanical properties, production, efficient use, new trends. It assesses the range of cutting tool materials and individual front world producers in terms of optimum cutting conditions for turning operations and type of material to be machined. Further, these cutting materials are compared between different manufacturers to bring out thein differences in cutting conditions.

The experimental discovery of graphene in 2004 has opened up a new research field in the direction of atomically thin two-dimensional layered materials for exploration of many fundamental research problems and technological applications. The charge carriers in graphene are massless Dirac fermions due to which they exhibit absence of localization, thereby giving rise to huge intrinsic mobilities and ballistic transport even at room temperatures. But it was observed that the extrinsic disorder and intrinsic structural disorder can significantly influence the transport in graphene films. This thesis focuses on three different aspects of graphene -disorder, magnetism and proximity-induced superconductivity. We have reported conductance fluctuations-based transport studies to investigate these aspects as they provide more detailed information than what can be obtained from the standard transport measurements. Even though these conductivity fluctuations pose a serious bottleneck for various applications, they can also yield useful insights into the various scattering mechanisms and the symmetry properties of graphene. In the first half of the thesis, we describe the measurement of low frequency 1/f noise in large area polycrystalline graphene films to understand the role of grain boundaries in charge carrier transmission in graphene. TEM studies on the low and high angled GBs formed in these graphene samples showed that they form distinct disordered regions of varying widths depending on the tilt angle of the GBs. At low temperatures, the 1/f noise measurements indicated spontaneous breaking of time reversal symmetry across graphene grain boundaries which suggests the magnetic nature of these grain boundaries. In the second half of the thesis, we will concentrate on the universal conductance fluctuations (UCF) in graphene which is the manifestation of quantum interference phenomena at low temperatures. We find that the absolute magnitude of the UCF is directly related to various symmetry-breaking disorder present in graphene. We also discuss how the UCF can be used to study the nature of proximity-induced superconducting correlations in graphene. In the end, we have proposed new device schemes for the integration of ferromagnetic and superconducting materials with graphene.

Phase field models have played an important role in shaping our understanding of a variety of micro structural phenomena in materials. Their attractive features include (a) their ability to capture instabilities in microstructures, and (b) their ability to handle topological transitions { such as splitting or coalescence { gracefully. Therefore, we have chosen to use a phase field model in our study of instabilities in cylindrical pores in nanoporous membranes which eventually lead to their failure. Our study is motivated by recent studies on thermal stability of nanoporous membranes of alumina, titania and zirconia. The key feature in our model is its ability to incorporate surface discussion as the mechanism for mass transport. We first benchmark the model through a critical comparison of our results on early stages of surface evolution during Rayleigh instability and grain boundary grooving with those from linear theories of these phenomena. We have then used longer simulations (which go beyond early stages, and therefore, can incorporate non-lineare effects) to study instabilities in a hollow cylinder in three different systems: single crystal or amorphous solid (which fails through Rayleigh instability), a model sys-tem with parallel grain boundaries (which fails through grain boundary grooving), and a polycrystal (whose failure depends on a combination of grain growth and grooving). In all the cases, the surface energy is assumed to be isotropic, and the operative mechanism for mass transport is assumed to be surface discussion.

The master’s thesis is aimed at superhard cutting materials (polycrystalline diamond and polycrystalline cubic boron nitride) and their effective utilization. It describes the characteristics and production of these materials and evaluates product ranges of the most significant producers of tools and tool materials in given area. It further compares working conditions of selected world's producers which are related to the growing utilization of these materials in machining process. The thesis also deals with the cutting power of tools and provides a technical-economic assessment of the discussed issue.

Diploma thesis is focused on cutting superhard materials (polycrystalline diamond and polycrystalline cubic boron nitride). It deals with physical, mechanical and cutting properties, production and effective use. Evaluative product range of superhard cutting materials of the prominent world producers in terms of cutting conditions and type of materials machined in turning.

The ever increasing demand for a clean and renewable energy source has stimulated research for alternatives for the use of fossil fuels, which contribute significantly to global warming. The SO2 oxidation reaction was studied for production of hydrogen as a clean and renewable energy carrier. This reaction occurs at a lower standard electrode potential (0.158 V vs. SHE) than normal water electrolysis (1.23 V vs. SHE). This is a theoretical indication that the SO2 oxidation reaction has possible potential when compared to normal water electrolysis, since hydrogen production may occur at lower potentials and therefore lower cost. Rh was compared with Pt for the SO2 oxidation reaction since little research has been done on this catalyst and many studies exist in which Pt was used as catalyst. The oxygen reduction reaction and ethanol oxidation reaction were also included in this study to create a foundation for the catalysts studied, since the SO2 oxidation reaction is complicated by different adsorbed species that can form according to various mechanisms. The electrochemical techniques employed in this study to characterize the catalysts included cyclic voltammetry from which onset potentials and limiting current densities were determined, as well as from which some qualitative analysis was done. Linear polarization experiments were used during rotating disk electrode studies from which Levich and Koutecky-Levich analyses were done and the number of electrons transferred calculated and compared between the two catalysts. From the Koutecky-Levich analysis the kinetic current density was also obtained for use in Tafel analysis for further comparison between catalysts. It was found that Rh showed good behaviour for the oxygen reduction reaction when compared to Pt with similar onset potentials and limiting current densities. From Levich analysis it was concluded that both catalysts achieved diffusion limitation at high overpotentials. However, from the calculated number of electrons transferred it was evident that a difference in mechanism existed between catalysts and that the mechanism for both changed in the potential range studied, which is confirmed by the Tafel slopes. For the ethanol oxidation reaction it was shown that Rh exhibited very low catalytic activity in comparison with Pt. However, it was concluded from cyclic voltammetry and rotating disk electrode studies that more adsorbed species were present on the surface of Rh than on Pt. These results confirmed the possibility of using Rh as a co-catalyst together with Pt since it was shown from rotating disk electrode studies that low adsorption of ethanol and its oxidation products caused species to be transported away from the surface of the electrode during rotation. For the SO2 oxidation reaction it was found that Rh exhibited very poor catalytic activity together with being very susceptible to poisoning by adsorbed species. Pt showed very good behaviour, which corresponded well with what had been observed in literature. Levich analysis revealed that Pt did not exhibit diffusion limitation and Koutecky-Levich analysis revealed that a 2 electron reaction occurred on Pt, which corresponds with the SO2 oxidation reaction during which 2 electrons are transferred. It was, therefore, shown that Rh could exhibit good behaviour and act as a suitable catalyst in certain circumstances. However, for the SO2 oxidation reaction, which was the main focus of this study it was shown that Rh is not a suitable catalyst, either alone or as co-catalyst. / MSc (Chemistry), North-West University, Potchefstroom Campus, 2014

The ever increasing demand for a clean and renewable energy source has stimulated research for alternatives for the use of fossil fuels, which contribute significantly to global warming. The SO2 oxidation reaction was studied for production of hydrogen as a clean and renewable energy carrier. This reaction occurs at a lower standard electrode potential (0.158 V vs. SHE) than normal water electrolysis (1.23 V vs. SHE). This is a theoretical indication that the SO2 oxidation reaction has possible potential when compared to normal water electrolysis, since hydrogen production may occur at lower potentials and therefore lower cost. Rh was compared with Pt for the SO2 oxidation reaction since little research has been done on this catalyst and many studies exist in which Pt was used as catalyst. The oxygen reduction reaction and ethanol oxidation reaction were also included in this study to create a foundation for the catalysts studied, since the SO2 oxidation reaction is complicated by different adsorbed species that can form according to various mechanisms. The electrochemical techniques employed in this study to characterize the catalysts included cyclic voltammetry from which onset potentials and limiting current densities were determined, as well as from which some qualitative analysis was done. Linear polarization experiments were used during rotating disk electrode studies from which Levich and Koutecky-Levich analyses were done and the number of electrons transferred calculated and compared between the two catalysts. From the Koutecky-Levich analysis the kinetic current density was also obtained for use in Tafel analysis for further comparison between catalysts. It was found that Rh showed good behaviour for the oxygen reduction reaction when compared to Pt with similar onset potentials and limiting current densities. From Levich analysis it was concluded that both catalysts achieved diffusion limitation at high overpotentials. However, from the calculated number of electrons transferred it was evident that a difference in mechanism existed between catalysts and that the mechanism for both changed in the potential range studied, which is confirmed by the Tafel slopes. For the ethanol oxidation reaction it was shown that Rh exhibited very low catalytic activity in comparison with Pt. However, it was concluded from cyclic voltammetry and rotating disk electrode studies that more adsorbed species were present on the surface of Rh than on Pt. These results confirmed the possibility of using Rh as a co-catalyst together with Pt since it was shown from rotating disk electrode studies that low adsorption of ethanol and its oxidation products caused species to be transported away from the surface of the electrode during rotation. For the SO2 oxidation reaction it was found that Rh exhibited very poor catalytic activity together with being very susceptible to poisoning by adsorbed species. Pt showed very good behaviour, which corresponded well with what had been observed in literature. Levich analysis revealed that Pt did not exhibit diffusion limitation and Koutecky-Levich analysis revealed that a 2 electron reaction occurred on Pt, which corresponds with the SO2 oxidation reaction during which 2 electrons are transferred. It was, therefore, shown that Rh could exhibit good behaviour and act as a suitable catalyst in certain circumstances. However, for the SO2 oxidation reaction, which was the main focus of this study it was shown that Rh is not a suitable catalyst, either alone or as co-catalyst. / MSc (Chemistry), North-West University, Potchefstroom Campus, 2014

Thin films of Strontium Ruthenate have been grown on Strontium Titanate and Lanthanum Aluminate (100) substrates by pulsed laser deposition. X-ray diffraction results show that the films grown on the Strontium Titanate are amorphous and polycrystalline on the Lanthanum Aluminate. Resistances versus temperature measurements show that the films exhibit semiconducting characteristics. In addition to the growth of Strontium Ruthenate thin films, multilayer heterostructures of Terfenol-D thin films on polycrystalline Lead Titanate thin films were grown by pulsed laser deposition. By using a novel experimental technique called magnetic field assisted piezoelectric force microscopy it is possible to investigate the magnetoelectric coupling between the electrostrictive Lead Titanate and magnetostrictive Terfenol-D thin film. Upon examination of the produced thin films the phase and amplitude components of the piezoelectric signal experience changes in response to an applied in-plane magnetic field. These changes provide experimental evidence of a magnetoelectric coupling between the Terfenol-D and Lead Titanate layers.