Browsing by Author "Lang, Candace"

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    The electrical properties of ruthenium-aluminium alloys
    (1995) Smith, Ernest Gregory; Lang, Candace
    The electrical properties of platinum, gold-palladium and a selection of alloys from the ruthenium-aluminium system have been studied at high temperatures (up to 1000°C). The majority of the ruthenium-aluminium alloy compositions studied lie near or in the ruthenium aluminide phase field. Ruthenium aluminide is a B2 structure intermetallic which is suited to high temperature applications because in addition to a high melting point (2060°C), oxidation resistance to 1200°C and high temperature strength, it is also relatively ductile at room temperature. The possibility of high temperature electrical applications required an investigation of the electrical properties of ruthenium-aluminium alloys as compared to platinum and gold-palladium. Two sets of apparatus, capable of measuring the resistivity and thermo-e.m.f to high temperatures, were constructed and used to obtain the first experimental results for the electrical properties of ruthenium-aluminium alloys. Chemical analysis of these alloys has been performed for the first time, and together with energy dispersive spectroscopy, has revealed a composition at which there is a resistivity minimum and a positive thermo-e.m.f maximum, which appears to be associated with the formation of the ordered ruthenium aluminide phase. The resistivity and the temperature dependence of resistivity of some ruthenium-aluminium alloys are similar to that of platinum, the least resistive of the three materials investigated.
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    Investigation of a computationally predicted structure in the Ag-Pt system
    (2013) Allies, Sorayo; Lang, Candace
    Computational modelling is fast becoming the chosen way to predict novel structures. These structures need to be validated in order to gain credibility and generate more confidence in computational predictions. This investigation of the equiatomic region of the Ag-Pt system shows that computational modelling can be used successfully as a precursor to experimental investigations. Various techniques including electron microscopy, hardness and Differential Scanning Calorimetry were used to investigate different properties of the alloy. These techniques have shown that an ordered phase may exist in the Ag-Pt system. This ordered phase has been shown to have an increased hardness and has produced extra reflections in electron diffraction patterns. Scanning Electron Microscope equipped with a Backscattered Electron detector has shown that a third phase is present in the alloy and the composition is close to 50:50; within experimental error. The alloys showed considerable inhomogeneity and it was not homogenised prior to or post cold rolling. This could be a reason for the third phase not reaching an equilibrium state after prolonged heat treatments. The final structure might be the L11 structure but the full transformation is slow and further investigation is required. It is recommended that future research be carried out taking into account the recommendations provided in Chapter 7.
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    The synthesis of novel Pt-based nanoparticles
    (2011) Leteba, Gerard; Lang, Candace; Topic, Mira
    The ultimate goal of this project was to explore alternative and various synthetic routes for the design and fabrication of V@Pt core-shell and bimetallic nanoparticles.
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    Synthesis of Ti₂AlC, Ti₃AlC₂ and Ti₃SiC₂ MAX phase ceramics; and their composites with c-BN
    (2011) Rampai, Tokoloho; Lang, Candace
    MAX phase ceramics are ternary ceramics with both metallic and ceramic properties. The existing backing materials in grinding wheels can be made of ceramics or metals. In these applications, ceramics have the disadvantage of low toughness, and most metals have the disadvantages of relatively high density and intolerance to some very high temperatures. The MAX phases have a combination of the main advantages of both metals and ceramics: they are soft and machinable yet also heat-tolerant, strong and lightweight. Cubic boron nitride (c-BN) is a widely used abrasive in grinding wheels, which is exceeded in hardness only by diamond. Composites of c-BN and selected MAX phases may result in materials of some interesting and useful properties for application in industry. Firstly MAX phases, Ti₃SiC₂; Ti₃AlC₂ and Ti₂AlC were synthesised, then reaction couples of MAX-cBN are made in order to investigate the best conditions for composite synthesis, and to analyse the interfacial phases which occur. Finally, the MAX-cBN composites were synthesised from the reaction couple studies. The following results were obtained: 1. Samples synthesised to obtain Ti₃AlC₂ were largely composed of the Ti₂AlC, and thus synthesis of the Ti₃AlC₂ MAX phase was deemed unsuccessful. 2. Nearly pure samples of Ti₂AlC and Ti₃SiC₂ were successfully synthesised with high densities, 99.16% and 98.21%, respectively, of the theoretical density. 3. Reaction couple studies revealed that the Ti₃SiC₂ /c-BN couple was successfully made at 1400°C, 10MPa pressure for 30 minutes, and Ti₂AlC/c-BN couple was successfully made at 1500°C, 10MPa pressure for 30 minutes. The interfacial phases characterised by XRD and SEM found here were TiN, TiC, TiB₂ and AlN for the latter and TiN, TiS₂ and TiB₂ for the former. 4. These conditions were used to successfully synthesise MAX/c-BN composites where both could react and still remain intact. The interfacial phases characterised by XRD and SEM found here were TiAl, TiC, TiB₂ and AlN for Ti₂AlC/c-BN and TiN, TiC, TiS₂ and TiB₂ for Ti₃SiC₂ /c-BN. From these results the following conclusion was drawn: Ti₂AlC and Ti₃SiC₂ are fully compatible with c-BN in order to synthesise a composite with notable properties such as the fracture toughness, suggested by the observed fracture mechanism seen from the fracture surface of these composites.
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    Synthesis, characterization and catalytic investigations of Pt-based binary (bimetallic) and ternary (trimetallic) nanoparticles
    (2016) Leteba, Gerard Malefane; Lang, Candace; Levecque, Pieter B J
    This work tests the hypothesis that nanoparticles of 75 at.% platinum (Pt) composition and anisotropic morphology, will outperform standard catalysts in (PEMFC) hydrogen fuel cells. A survey of the scientific literature on this topic is first presented. The synthetic strategies which were developed for the preparation of novel Pt-based binary (bimetallic) and ternary (trimetallic) nanoparticles, containing nickel (Ni), cobalt (Co) and/or vanadium (V), are then described. The synthesis protocols for solution-grown colloidal nanoparticles all required the heat-up of a chemical mixture (of metal precursors, surfactants as stabilizers, solvents and/or reductants) from room temperature to high temperatures (up to 310 °C), for thermal decomposition or thermal co-reduction. These protocols were successful in producing nanostructures of high quality, with exceptional solubility in polar solvents such as chloroform after repeated washing and drying. Detailed microstructural investigations of the synthesized nanoparticles were carried out using scanning transmission electron microscopy (STEM), TEM and X-ray diffraction (XRD). The nanoparticles were anisotropic with composition around 75 at.% Pt. Depending on the particular synthesis protocol, the as-prepared nanoparticles exhibited different morphologies, surface facets, size and structure (alloy or core-shell). To measure the oxygen reduction reaction (ORR) functionality of these nanoparticles, electrochemical measurements were conducted, including cyclic voltammetry (CV), carbon monoxide stripping voltammetry (CO-stripping) and rotating disk electrode measurements (RDE). These measurements determined (a) electrochemical surface area, (b) mass-specific activity and (c) area-specific activity; which were used to compare the performance of the synthesized nanoparticles with the performance of a standard catalyst. The synthesised nanoparticles, containing 75 at.% Pt and having anisotropic morphologies, exhibited better catalytic functionality than the standard catalysts currently in widespread use. The enhanced functionality of these alloy nanostructures is attributed to their anisotropic nature and structure (mixed or core-shell). It is shown accordingly that high surface area nanoparticles, with platinum composition around 75 at.%, are more effective than the best catalysts currently in use. Subsequently, electrochemical measurements were used to determine longevity: catalytic functionality was measured after cycling for considerably longer than the norm in nanoparticle research (5000 cycles). These measurements show a decay in catalytic activity after prolonged potential cycles, although the final value is similar to the initial value for commercial Pt catalyst. This decay is suggestive of alloying dissolution and surface facet deformation; further work is recommended.
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    The thermal conductivity of intermetallics
    (1996) Anderson, Stephen Ashcraft; Lang, Candace
    The thermal conductivity of titanium aluminide and several ruthenium-aluminium alloys has been studied from room temperature up to 500°C. Ruthenium aluminide is a B2-type intermetallic which is unusual and of special interest because of its toughness, specific strength and stiffness, oxidation resistance and low cost. The possible use of ruthenium aluminide in high temperature industrial applications required an investigation of the thermal properties of this compound. Apparatus, capable of measuring thermal conductivity at elevated temperatures has been designed and constructed. This study represents the first experimental results for the thermal conductivity of ruthenium aluminide alloys. The electrical resistivity of the intermetallic compounds has been measured using apparatus based on the Van der Pauw method. The Weidman-Franz ratio of the ruthenium aluminide alloys has been calculated and this indicates that the primary source of heat conduction in these alloys is by electronic movement and that the lattice contribution is minor. The electrical and thermal properties of ruthenium aluminide are shown to be similar to that of platinum and nickel aluminide. This has important implications for the use of these alloys in high temperature applications.