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One of the prominent features in sediment sequences formed around the AllerOd-Younger Dryas transition (c. 12.9-12.8 ka bp) in North America is a dark layer of organic-rich material, i.e. the black mat. The black mat sequences in southeast Arizona contain a thin sandy basal layer corresponding to the lower Younger Dryas boundary. Trace element concentrations in the lower Younger Dryas boundary sediments, in the black mat, in the host sediments, and in charcoal from Western Europe and southeast Arizona were studied using LA-ICP-MS. The black mat samples and samples of the underlying host sediments display compositions similar to the average continental crust, while the sediments from the lower Younger Dryas boundary are enriched in rare earth elements, Ni, and Co whereas Ta, Nb, Zr, and Hf are depleted relative to the rare earth elements. Such a difference in compositions between the lower Younger Dryas boundary sediments and other sediments points to a short enigmatic event, which changed conditions of sedimentation just before the onset of the Younger Dryas cooling. The presence of products of biomass burning of still unknown origin is suggested on the basis of trace element features of sediments from the lower Younger Dryas boundary.

Digitized by Kansas Correctional Industries

by Oi-Ming Cheng. / Thesis (M.Phil.)--Chinese University of Hong Kong, 1996. / Includes bibliographical references (leaves 104-108). / Chapter 1 --- INTRODUCTION / Chapter A) --- Sources of zinc and its functional role --- p.10 / Chapter B) --- Effects of excessive zinc intake --- p.11 / Chapter (a) --- Human --- p.11 / Chapter (b) --- Animal --- p.11 / Chapter (c) --- Fish --- p.12 / Chapter C) --- Techniques in zinc determination --- p.12 / Chapter 2 --- ANODIC STRIPPING VOLTAMMETRY / Chapter A) --- Basic principles --- p.16 / Chapter (a) --- Cell --- p.16 / Chapter (b) --- Electrodeposition and stripping --- p.20 / Chapter B) --- Major interferences in anodic stripping and the common solutions to these problems --- p.25 / Chapter (a) --- Intermetallic compound formation --- p.25 / Chapter (b) --- Overlapping peaks --- p.26 / Chapter (c) --- Organic compounds adsorbed at the electrode --- p.28 / Chapter 3 --- MUTUAL INTERFERENCE FROM ZINC AND COPPER / Chapter A) --- Introduction --- p.30 / Chapter B) --- Review of reported methods to remove copper interference --- p.31 / Chapter (a) --- Dual-working electrode approach --- p.31 / Chapter (b) --- Adjustment of the deposition potential --- p.31 / Chapter (c) --- Standard addition --- p.33 / Chapter (d) --- Addition of a third element --- p.33 / Chapter C) --- The proposed solution --- p.35 / Chapter 4 --- EXPERIMENTAL / Chapter A) --- Proposed method --- p.37 / Chapter (a) --- Apparatus --- p.37 / Chapter (b) --- Reagents --- p.38 / Chapter (c) --- Procedure --- p.40 / Chapter B) --- Reference method --- p.45 / Chapter (a) --- Apparatus --- p.45 / Chapter (b) --- Reagents --- p.45 / Chapter (c) --- Procedure --- p.47 / Chapter 5 --- RESULTS AND DISCUSSION / Chapter A) --- Optimization of instrumental parameters and working conditions --- p.49 / Chapter (a) --- Effect of plating potential on zinc peak current --- p.49 / Chapter (b) --- Effect of plating time on zinc peak current --- p.53 / Chapter (c) --- Effect of holding time and holding potential on zinc peak current --- p.56 / Chapter (d) --- Effect of sweep rate on zinc peak current --- p.58 / Chapter (e) --- Effect of final potential and strip time on zinc peak current --- p.61 / Chapter (f) --- Effect of pH on the peak current and recovery of zinc in the presence of copper after the sulphide treatment --- p.63 / Chapter (g) --- Effect of the concentration of buffer on the peak current --- p.66 / Chapter (h) --- Effect of the reaction time of sulphide with copper on the zinc recovery --- p.68 / Chapter B) --- "Calibration graph, precision and detection limit" --- p.71 / Chapter C) --- Effect of copper on the zinc determination by the proposed method --- p.78 / Chapter D) --- Interference studies --- p.80 / Chapter E) --- Recovery tests --- p.84 / Chapter F) --- Determination of zinc in real samples using the proposed method --- p.86 / Chapter (a) --- Clean water samples such as sea water and tap water --- p.86 / Chapter (b) --- Contaminated natural water and domestic wastewater samples --- p.91 / Chapter (c) --- Air samples --- p.93 / Chapter (d) --- Oyster tissue samples --- p.96 / Chapter (e) --- Sewage sludge and sediment samples --- p.99 / Chapter 6 --- CONCLUSION --- p.102 / REFERENCES --- p.104

A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of requirements of the requirements for the degree of Doctor of Philosophy. Johannesburg, 2014. / Boron nitride (BN) in its cubic form (cubic boron nitride (c-BN)) is one of the known superhard materials with superior mechanical, chemical and electronic properties. These properties have made it an excellent material in many modern industrial and electronic applications and as such, extensive research grounds have been developed for over half a decade now with the aim of finding alternative ways to synthesize it. The work presented in this thesis was inspired by the fact that defects introduced into the hexagonal form of boron nitride (h-BN) under certain conditions can lead to a change in its local structure and hence the formation of the cubic BN symmetry. The work focused on the introduction of different ions which included helium, lithium, boron, nitrogen and argon into h-BN, by the ion implantation process, in order to promote a defect-induced phase change to the cubic symmetry and possibly to other BN polymorphs. We introduced these ions at different fluences (number of ions per unit area) and energies so as to investigate the best conditions that will influence the lowest activation energy that will in turn favour the c-BN formation. The resulting thin hard layer could be an excellent sub-surface treatment. All the samples used were high quality polycrystalline and single crystal h-BN, obtained from various manufacturers. The fluence range used was from 1×1013 ions/cm2 to 5×1016 ions/cm2, with energy ranging from 40 keV to 150 keV. This energy and fluence choice was inspired by previous research that had been done at higher energies (MeV range) and recommended that low energy (keV range) and fluence could induce similar change. To investigate these effects, various analysis techniques were employed. The major techniques involved optical vibrational methods using Raman Spectroscopy ii iii (RS) and Fourier Transform Infrared Spectroscopy (FTIR) carried out on the samples before and after implantation. Other techniques used included Glancing Incidence X-ray Diffraction (GIXRD), Transmission Electron Microscopy (TEM), and Energy Dispersive X-ray Spectroscopy (EDS). Raman and FTIR measurements showed the introduction of new phonon and vibrational modes in the samples after implantation. The position, size and broadening suggested that they originated from a symmetry attributed to nano-structured cubic BN (nc-BN). The nature and extent of the nc-BN features was very dependent on the implantation parameters with different atomic mass ions each having an optimum fluence with regards to the intensities of the Raman and FTIR signal associated with them. Glancing incidence X-ray diffraction showed new diffraction patterns whose angles corresponded to the cubic and rhombohedral BN symmetries. The linewidths of these peaks were used to estimate the crystal size, which were in the nanoscale range, hence complementing the results obtained by optical spectroscopy. The High-Angle Annular Dark-field Scanning Transmission Electron microscopy (HAADF-STEM) analyses showed regions with low contrast within the implanted region, suggesting that there were regions within the implanted layer that contained dense structures which were attributed to the cubic BN symmetry. Computer simulations using the Stopping and Range of Ions in Matter (SRIM) programme were performed to understand the events that take place during the interaction of the ions with h-BN. Phonon confinement model calculations were also performed to understand the nature of peaks forming after implantation with an aim of support Raman measurements and to estimate the size of the nc-BN domains. With these complementary analyses, it was concluded that indeed implantation is an effective method of creating nanocrystalline c-BN under less extreme conditions of pressure and temperature.

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