There has been an increase in the interest in fluoride materials over the last decade. This interest has focused on multiferroic materials and kagome lattices, to name but a few areas. This thesis focuses on the synthesis and crystallographic characterisation of selected transition metal fluorides and oxyfluorides. Work is presented on the tetragonal tungsten bronze solid solutions of KₓFeF₃, where x = 0.58 and x ≈ 0.5, and the copper analogue, K₃Cu₃Fe₂F₁₅; the kagome structure of Cs₂ZrCu₃F₁₂; and hydrothermal reactions using vanadium, manganese, or molybdenum as the transition metals in the formation of new fluorides and oxyfluorides. The tetragonal tungsten bronze compounds KₓFeF₃ (x = 0.58 and x ≈ 0.5) are both tetragonal at 500 K. In the variant with the lower K-content, there is a clear phase separation into two tetragonal phases even at this temperature. The K₀.₅₈FeF₃ sample separates into two distinct phases below 340 K to possess one tetragonal and one orthorhombic phase. Then at roughly 300 K, both samples undergo a phase transition where the tetragonal phase in the P4/mbm space group in K₀.₅₈FeF₃ changes to an orthorhombic phase with a larger unit cell; and the tetragonal phase in P4₂bc for the K₀.₅FeF₃ sample changes to the same orthorhombic model, whilst the P4/mbm model remains unchanged. The evolution of the lattice parameters and phase fractions is studied in detail using synchrotron powder X-ray diffraction (sPXRD). The kagome structure investigated, Cs₂ZrCu₃F₁₂, possesses the “ideal” kagome lattice at room temperature, but previous work has suggested that there is a phase transition at 225 K. The two structures are determined by single crystal X-ray diffraction at 300 K and 125 K. Variable temperature sPXRD studies are performed between these two temperature ranges to determine the phase evolution as a function of temperature. The structure changes from a rhombohedral to a monoclinic phase at low temperature. This is the result of the buckling of the kagome layers at the phase transition. The Zr⁴⁺ ion changes from 6 to 7 coordinate and this is seen as the main driving force for the distortion of the kagome layer from its “ideal” planar arrangement. ii The phase transition is first-order as seen from the electrical impedance measurements. The hydrothermal reactions presented reveal seven new materials and their crystal structures. Sr₂V₂F₁₀·H₂O is new and found to be isostructural to Sr₂Fe₂F₁₀·H₂O. BaVO₂F₃ is a cubic material that is potentially piezoelectric. Two hybrid organic inorganic manganese compounds are reported. The ladder structure (C₃N₂H₅)[Mn₂F₆(H₂O)₂] crystallises in a polar space group and shows promise as a candidate for multiferroic studies. The second hybrid material, (C₇NH₁₆)₂[MnF₅(H₂O)]·2H₂O, crystallises in a centrosymmetric space group. The Mo hybrid materials are all centrosymmetric and possess isolated molybdenum-centred monomeric or dimeric octahedral units.

M. Tech. Metallurgical Engineering. / Studies the tribological and electrochemical behaviour of various WC-Co, WC-Co-Cr and CrC-NiCr based thermal sprayed coatings in synthetic mine water environment. The research aims to achieve the following objectives. 1. Characterize commercially available cermet powders used during thermal spray process. 2. Explore and understand the relationship between feedstock powder parameters, the phases and microstructures generated during thermal spray process. Study the interrelationship of feedstock powder chemistry and method of powder synthesis on wear and corrosion behavior of thermally sprayed coatings. Assess the joint actions of wear and corrosion behaviour of the coatings in synthetic mine water environment.

The first part of this dissertation is an introduction describing a brief historical background of carbon nanotubes (CNTs) and their pseudo 1D structure responsible for many exotic electronic properties. The second part describes our experimental setup. The third part is about the growing of Multi-Walled Carbon Nanotubes (MWCNTs) by the chemical vapor deposition (CVD) method. Then the fourth part demonstrates a simple but reliable method to make firm contact junctions between MWCNTs and metals such as tungsten (W). The novel point of our method consists, after making a mechanical preliminary contact at a selected MWCNT, in applying a series of voltage pulses across the contact. Thin oxide layers that may form between the MWCNT and the W wire, are removed in steps by the resistive heating and electron impact during the application of each voltage pulse. Furthermore, this simple process of contact welding in steps does not bring about any permanent change in the electronic transport properties of the MWCNTs. The fifth part discusses our bending experiments. We apply a uniform and continuous bending to a selected MWCNT at room and liquid nitrogen temperatures to study the strain effect on the electrical transport in the MWCNT. There are a few published experimental works related to the bending deformation; however, this is the first study of electronic transport properties in continuous bending and releasing deformations. We observed a saturation behavior with the MWCNT and also found the bending deformation causing an anomalous change in the saturation behavior. In the sixth part we depict some interesting phenomena due to the stretching deformation of MWCNT, where we were able to propose a simple model for electron localization induced by the deformation. The last part deals with the formation of the "X-junction" between two MWCNTs. A strong X-junction can be formed simply by means of the e-beam inside the Scanning Electron Microscope (SEM). The X-junctions may form the basic elements of nano-electronic circuits such as various metal-insulator junctions, quantum dots, and similar devices. / text

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There has been much work on electron emission. It has lead to the concept of the photon and new electron sources for imaging such as electron microscopes and the rst formulation of holographic reconstructions [1-6]. Analytical derivations are important to gain physical insight into the problem of developing better electron sources. However, to date, such formulations have su ered by a number of approximations that have masked important physics. In this thesis, a new approach is provided that solves the Schrodinger wave equation for photoemission from a single atom tungsten tip barrier or more generally, for photoemission from a Schottky triangular barrier potential, with or without image potential e ects. We describe the system, then introduce the mathematical derivation. We conclude with the applications of the theory.

There has been much work on electron emission. It has lead to the concept of the photon and new electron sources for imaging such as electron microscopes and the rst formulation of holographic reconstructions [1-6]. Analytical derivations are important to gain physical insight into the problem of developing better electron sources. However, to date, such formulations have su ered by a number of approximations that have masked important physics. In this thesis, a new approach is provided that solves the Schrodinger wave equation for photoemission from a single atom tungsten tip barrier or more generally, for photoemission from a Schottky triangular barrier potential, with or without image potential e ects. We describe the system, then introduce the mathematical derivation. We conclude with the applications of the theory.

Diffractive zone-plate lenses are widely used as optics in high-resolution x-ray microscopes. The achievable resolution in such microscopes is presently not limited by the x-ray wavelength but by limitations in zone-plate nanofabrication. Thus, for the advance of high-resolution x-ray microscopy, progress in zone-plate nanofabrication methods are needed.   This Thesis describes the development of new nanofabrication processes for improved x-ray zone-plate optics. Cold development of the electron-beam resist ZEP7000 is applied to improve the resolution of soft x-ray Ni zone plates. The influence of developer temperature on resist contrast, resolution, and pattern quality is investigated. With an optimized process, Ni zone plates with outermost zone widths down to 13 nm are demonstrated. To enhance the diffraction efficiency of Ni zone plates, the concept of Ni-Ge zone plates is introduced. The applicability of Ni-Ge zone plates is first demonstrated in a proof-of-principle experiment, and then extended to cold-developed Ni zone plates with outermost zone widths down to 13 nm. For 15-nm Ni-Ge zone plates a diffraction efficiency of 4.3% at a wavelength of 2.88 nm is achieved, which is about twice the efficiency of state-of-the-art 15-nm Ni zone plates. To further increase both resolution and diffraction efficiency of soft x-ray zone plates, a novel fabrication process for W zone plates is developed. High resolution is provided by salty development of the inorganic electron-beam resist HSQ, and cryogenic RIE in a SF6 plasma is investigated for high-aspect-ratio W structuring. We demonstrate W zone plates with 12-nm outermost zone width and a W height of 90 nm, resulting in a 30% increase in theoretical diffraction efficiency compared to 13-nm efficiency-enhanced Ni-Ge zone plates. In addition to soft x-ray zone plates, some lenses for hard x-ray free-electron-laser applications were also fabricated during this Thesis work. Fabrication processes for the materials W, diamond, and Pt were developed. We demonstrate Pt and W-diamond zone plates with 100-nm outermost zone width and respective diffraction efficiencies of 8.2% and 14.5% at a photon energy of 8 keV. / QC 20111114

Thesis (MScEng (Industrial Engineering))--University of Stellenbosch, 2009. / The titanium alloys have found wide application in the aerospace, biomedical and automotive industries. Soaring fuel prices and environmental concerns are the fundamental drivers that intensify the demand situation for titanium. From a machining viewpoint, one of the challenges companies face, is achieving high material removal rates while maintaining the form and function of the part. The ultimate aim for a machining business remains to make parts quickly. Conventional cutting speeds range from 30 to 100 m/min in the machining of Ti-6Al-4V. Milling this alloy faster however is challenging. Although titanium is becoming a material of choice, many of the same qualities that enhance titanium‟s appeal for most applications also contribute to its being one of the most difficult materials to machine. The author explored the potential for Polycrystalline diamond (PCD) inserts in high speed milling of Ti-6Al-4V, by trying to understand the fundamental causes of tool failure. The objective was to achieve an order of magnitude increase in tool life, while machining at high speed, simply by reducing some of the failure mechanisms through different cutting strategies. Tool wear is described as a thermo-mechanical high-cycle fatigue phenomenon. The capability of a higher material removal per tool life is achieved in the case of PCD inserts compared to Tungsten carbide (WC). The average surface roughness produced was relatively low. The collected chips were also analyzed. The work demonstrated progress over the performance reported in current literature. The work confirms that there is a region where a sufficiently high temperature in the cutting zone may contribute to extended tool life, provided that the tool material can withstand these extreme conditions.

In this work, new reconstruction and analysis methods were developed for 3D field ion microscopy (FIM) data, motivated by the goal of atomic scale characterisation of radiation damage for fusion applications. A comparative FIM/ atom probe tomography (APT) study of radiation damage in self-implanted tungsten revealed FIM advantages in atomistic crystallographic characterisation, able to identify dislocations, large vacancy clusters, and single vacancies. While the latter is beyond the detection capabilities of APT, larger damage features were observed indirectly in APT data via trajectory aberrations and solute segregation. An automated 3DFIM reconstruction approach was developed to maintain reliable, atomistic, 3D insights into the atomic arrangements and vacancies distribution in ion-implanted tungsten. The new method was utilized for the automated ‘atom-by-atom' reconstruction of thousands of tungsten atoms yielding highly accurate reconstructions of atomically resolved poles but also applied to larger microstructural features such as carbides and a grain boundary, extending across larger portions of the sample. Additional tools were developed to overcome reconstruction challenges arising from the presence of crystal defects and the intrinsic distortion of FIM data. Those were employed for the automated 3D mapping of vacancies in ion-implanted tungsten, analysing their distribution in a volume extending across 50nm into the depth of the sample. The new FIM reconstruction also opened the door for more advanced analyses on FIM data. It was applied to the preliminary studies of the distortion of the reconstructed planes, found to depend on crystallographic orientation, with an increased variance in atomic positions measured in a radial direction to the centre of the poles. Additional analyses followed the subtle displacements in atomic coordinates on consecutive FIM images, to find them affected by the evaporation of atoms from the same plane. The displacements were found to increase with size as the distance to the evaporated atom decreased, and are likely to be the result of a convolution between image gas effects, surface atoms relaxation, and charge re-distribution. These measurements show potential to probe the dynamic nature of the FIM experiment and possibly resolve contributions from the different processes effecting the final image. Finally, APT characterisation was performed on bulk and pre-sharpened needles to determine the effect of sample's geometry on the resulting implantation profiles, and the extent to which pre-sharpened needles could be employed in radiation damage studies. While the ions depth profiles in needles were not found within a good match to SRIM simulations, the damage profiles exhibited closer agreement. Further, the concentration of implanted ions in bulk samples was found significantly higher than in the respective needle implanted samples, with excessive loss found for the light ion implantation.