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.

Abstract not available

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Silicon nanocrystals embedded in silicon nitride have received attention as promising materials for optoelectronic applications. More specifically, band gap engineering of novel materials based on silicon nanocrystals has been proposed for possible application in an all-silicon tandem solar cell within the field of `third generation' photovoltaics. Such an application would require nanocrystals to exhibit quantum confinement whereby the optical and electrical properties of a film could be tuned by controlling the size of these `quantum dots'. This thesis investigates the correlation between the structural and optical properties of silicon nanocrystals grown in silicon nitride multilayer structures via solid phase crystallisation, as part of an experimental investigation into quantum confinement. A study of the relevant processing parameters for the solid phase crystallization of silicon nanocrystals in amorphous silicon nitride is presented and the effectiveness of the multilayer approach for controlling nanocrystal size is demonstrated. Structural characterisation using transmission electron microscopy and glancing incidence x-ray diffraction is complemented with a new application of Fourier transform infrared spectroscopy for the detection of silicon nanocrystals. A case study on the effects of annealing temperature on the photoluminescence from silicon nitride multilayers is presented. While a clear correlation between the structural, molecular and optical properties is demonstrated, evidence of quantum confinement remains ambiguous. The investigation into the limits of parameter space for the formation of silicon nanocrystals in silicon nitride multilayers also leads to the formation of a novel Si-Si3N4 nanocomposite material. A comprehensive study of the photoluminescence from silicon nanocrystals embedded in nitride is presented in the context of homogeneous and multilayer nitride films. Size dependent PL and absorption is demonstrated for silicon nitride multilayers with silicon-rich silicon nitride layer thicknesses varying from 1 to 4.5 nm, indicating the formation of quantum wells. These same structures are annealed to form arrays of silicon nanocrystals. Although the PL and absorption spectra suggest quantum effects, inherent ambiguities remain. The findings in this thesis provide greater insight into the nature of confinement and indicate the need for further research if the successful implementation of these structures into an all silicon tandem cell is to be achieved.

An 89 mm-diameter vertical tubular-flow reactor was used to study the kinetics of nano-sized silicon nitride powder synthesis via the animonolysis of SiO vapor at temperatures ranging from 1300°C to 1400°C. The SiO generation rate was controlled by adjusting the mass of SiO particles initially charged in the SiO generator, when the flow rate of carrier gas argon was maintained unchanged. The molar feed ratio of NH₃/SiO at the feeder outlets was maintained in large excess of the stoichiometric ratio ranging from about 100 to 1200 mol NH₃/mol SiO. The SiO-NH₃ reaction yielded two different morphologies of silicon nitride products at different locations in the reactor: nano-sized powder with an averaged particle size of about 17 nm and whiskers with a variety of shapes and diameters of a few micrometers. Nano-sized powder was the dominant product in the system and its mass fraction over the total product varied from 83% to 100%, depending on operating conditions. The contact pattern between SiO vapor and NH₃ inside the reacting zone was one of the most important parameters that affected Si₃N₄ formation kinetics. When a small single tube was employed for feeding NH₃ (flow condition J), a highest efficiency of SiO vapor utilization was achieved at a high level of SiO conversion. The SiO conversion increased from 72% to 91% with an increase in the residence time from 0.17 s to 0.69 s, indicating that the SiO-NH₃ reaction was not instantaneous but was relatively fast. When the molar feed rate of NH₃ was 2-3 orders of magnitude greater than that of SiO vapor, the rate of nano-sized powder synthesis was independent of NH₃ concentration and of first order with respect to the SiO concentration. A pseudo-first order rate expression was proposed, and the apparent activation energy was determined to be 180 kJ/mol. The gas flow in the reactor simulated with a computational fluid dynamic program revealed that whisker formed where the stagnation of gas flow formed. A power law rate expression for whisker formation was proposed based on measured rates of whisker formation and simulation-predicted reactant-gas concentrations. / Graduation date: 2004

Graduation date: 1996

The purpose of this work is to investigate the implementation of nitride fuels containing little or no uranium in a fast-spectrum nuclear reactor to reduce the amount of plutonium and minor actinides in spent nuclear fuel destined for the Yucca Mountain Repository. A two tier recycling strategy is proposed. Thermal spectrum transmutation systems converted from the existing LWR fleet were modeled for the first tier, and the Japanese fast reactor MONJU was used for the fast-spectrum transmutation. The modeling was performed with the Monteburns code. Transmutation performance was investigated as well as delayed neutron fraction, heat generation rates, and radioactivity of the spent material in the short and long term for the different transmutation fuel cycles. A two-tier recycling strategy incorporating fast and thermal transmutation with uranium-free nitride fuel was shown to reduce the long-term heat generation rates and radioactivity of the spent nuclear fuel inventory.

Abstract The behaviors of the TaNx barrier layer that placed between the Cu metal and GaAs have been studied by using X-ray diffraction, X-ray photoelectron spectroscopy and scanning electron microscopy. The TaNx and Cu films were deposited on GaAs sequentially with RF magnetron sputter. With a 250 nm thick TaNx barrier layer, the Cu metal can be impeded from reacting with GaAs substrate at 575¢Jannealed for one hour. Within an As or Ga overpressure environment condition, the failure temperature still occurred below 600¢J. The failure of TaNx diffusion barrier layer for preventing the reaction of the Cu and GaAs was originated for the dissociation of the GaAs itself at 580¢J. The outgoing As atoms increased the deterioration speed of the TaNx film and reduced its blocking ability.

Thesis (M. Phil.)--University of Hong Kong, 2010. / Includes bibliographical references. Also available in print.

Thesis (Ph. D.)--Ohio State University, 2001. / Title from first page of PDF file. Document formatted into pages; contains xiii, 165 p.; also contains graphics (some col.). Includes abstract and vita. Advisor: Jonathan P. Pelz, Dept. of Physics. Includes bibliographical references (p. 160-165).