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The use of magnesium hydroxide (Mg(OH)2) as a flame retardant and smoke-suppressor in polymeric materials has been of great interest recently. Because it contains no halogens or heavy metals, it is more environmentally friendly than the flame retardants based on antimony metals or halogenated compounds. Mg(OH)2 can be produced by the hydration of magnesium oxide (MgO), which is usually produced industrially from the calcination of the mineral magnesite (MgCO3). The thermal treatment of the calcination process dramatically affects the reactivity of the MgO formed. Reactivity of MgO refers to the extent and the rate of hydration thereof to Mg(OH)2. The aim of this study was to investigate the effect of calcination time and temperature on the reactivity of MgO, by studying the extent of its hydration to Mg(OH)2, using water and magnesium acetate as hydrating agents. A thermogravimetric analysis (TGA) method was used to determine the degree of hydration of MgO to Mg(OH)2. The reactivity of MgO was determined by BET (Brunauer, Emmett and Teller) surface area analysis and a citric acid reactivity method. Other techniques used included XRD, XRF and particle size analysis by milling and sieving. The product obtained from the hydration of MgO in magnesium acetate solutions contains mainly Mg(OH)2, but also some unreacted magnesium acetate. Magnesium acetate decomposition reaction takes place in the same temperature range as magnesium hydroxide, which complicates the quantitative TG analysis of the hydrated samples. As a result, a thermogravimetric method was developed to quantitatively determine the amounts of Mg(OH)2 and Mg(CH3COO)2 in a mixture thereof. The extent to which different experimental parameters (concentration of magnesium acetate, solid to liquid ratio and hydration time) influence the degree of hydration of MgO were evaluated using magnesium acetate as a hydrating agent. Magnesium acetate was found to enhance the degree of MgO hydration when compared to water. By increasing the hydration time, an increase in the percentage of Mg(OH)2 formed was observed. In order to study the effect of calcining time and temperature on the hydration of the MgO, the MgO samples were then calcined at different time periods and at different temperatures. The results have shown that the calcination temperature is the main variable affecting the surface area and reactivity of MgO. Lastly, an attempt was made to investigate the time for maximum hydration of MgO calcined at 650, 1000 and 1200oC. From the amounts of Mg(OH)2 obtained in magnesium acetate, it seems that the same maximum degree of hydration is obtained after different hydration times. A levelling effect that was independent of the calcination temperature of MgO was obtained for the hydrations performed in magnesium acetate. Although there was an increase in the percentage of Mg(OH)2 obtained from hydration of MgO in water, the levelling effect observed in magnesium acetate was not observed in water as a hydrating agent, and it seemed that the extent of MgO hydration in water was still increasing. The results obtained in this study demonstrate that the calcination temperature can affect the reactivity of MgO considerably, and that by increasing the hydration time, the degree of hydration of MgO to Mg(OH)2 is enhanced dramatically. / Chemistry / M. Sc. (Chemistry)

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Tetrahydrobiopterin (BH4) is a limiting cofactor for nitric oxide synthase (NOS) catalysed conversion of L-arginine to nitric oxide and citrulline. Content of BH4 in mammalian cells is regulated at many levels, but most important is de novo biosynthesis from GTP. GTP cyclohydrolase (GTPCH) is the rate-limiting enzyme for the de novo synthesis of BH4. While various immunostimulants, hormones and growth factors have been reported to increase GTPCH mRNA levels and intracellular biopterin (BH4 degradation product), it is not known whether these factors act at the level of GTPCH gene transcription. To test this I utilised 1, 3 and 6 kb 5'upstream GTPCH gene sequence in a secreted alkaline phosphatase reporter vector (SEAP). These constructs were stably transfected in PC-12 cells and rat aortic smooth muscle cells, and the cells were treated with various immunostimulants and growth factors in order to determine whether these factors could enhance GTPCH gene transcription. Intracellular biopterin levels were also measured to confirm that the upregulation of the SEAP-reporter correlated with a rise in biopterin. Our investigations conclude that transcriptional regulation of the GTPCH gene is indeed a major site for control of intracellular BH4 levels. In further experiments, we have characterised the binding of [3H]BH4 to endothelial NOS (eNOS) and examined influences of the substrate, arginine, on the BH4 binding. In addition we selected tetrahydropterins (that support NOS catalysis) and dihydropterins (that are catalytically incompetent) to determine the extent to which modifications of BH4 alter pterin binding affinity to eNOS. Dihydropterins are unable to support NOS catalysis. Studies showed for the first time that dihydropterins, but not tetrahydropterins, support superoxide generation by eNOS. We also have determined that eNOS may be able to produce NO in the absence of BH4 cofactor from the reaction intermediate hydroxyarginine. We have characterised this reaction and are able to provide a plausible mechanism for the NOx generation from eNOS in the absence of BH4 cofactor.

This thesis demonstrates that the formation of titanium disilicide for gate level interconnects in silicon VLSI processes is possible, and is compatible with the processes considered. By using this new material the operating speed of fine geometry integrated circuits can be increased. The first two chapters consider the choice of titanium disilicide as a replacement for polysilicon. A process schedule is developed which enables the deposition and annealing of cosputtered films of titanium and silicon. By carefully controlling their deposition, cosputtered films have been annealed in both standard diffusion furnaces and also in rapid isothermal anneal (RIA) systems. This success in annealing titanium disilicide films in a RIA system is a world first. Next a process schedule for the deposition and anneal of titanium films over silicon is determined. The reaction of the film with the anneal ambient and the movement of impurities inevitably present in the titanium film is considered in some detail. This work was the first to highlight the benefits gained from the use of nitrogen as the anneal ambient. Self-aligned processes rely on the interaction between titanium and silicon dioxide being negligible. The silicide formation anneal is therefore optimised to minimise this. Finally, reaction of the silicide with common dopants and with both oxidising and nitriding ambients is presented. A novel method of forming a titanium nitride over silicide contact structure is developed. In summary, this thesis demonstrates how a titanium disilicide based metallisation can be implemented into an existing MOS process.

Transmission [57]Fe Mossbauer Spectroscopy has been used to investigate two standard iron oxides and two environmental corrosion products in the temperature range 10-300 Kelvin. This was achieved using a specially developed Air Products cryogenic refrigeration system using gaseous helium as the refrigerant - thereby removing the requirement of an expensive liquid helium facility normally required for low temperature work. Characteristic spectra and transitions were observed for both the standard oxides. The environmental corrosion products demonstrated the differences in observed spectra between a sample which exhibits only bulk properties and one in which the particle size distribution within the sample is such that relaxation phenomena become important. The interpretations-made from the variable temperature Mossbauer data were confirmed by the complementary technique of X-ray diffraction. Conversion electron Mossbauer spectroscopy (C.E.M.S.) and conversion X-ray Mossbauer spectroscopy (C.X.M.S.) backscatter techniques have also been developed and are now routinely available in our laboratory. The greater escape depth of the conversion X-ray allows the C.X.M.S. method to be used to record spectra from samples to which a protective layer such as a paint, varnish, grease or oil layer has been applied. To ensure a surface sensitive signal, the substrate surfaces had to be enriched in the Mossbauer isotope 57Fe. This was achieved by vacuum evaporation of 57Fe onto the substrate surface and subsequent diffusion of the 57Fe into the near surface region - this diffusion had to be accomplished without oxidising the surface. Samples thus prepared were subsequently exposed to aggressive atmospheres and their CXM spectra recorded. The limitation of the method is that only room temperature spectra can be recorded at present and difficulties are encountered in the assignment of spectra that contain only quadrupole doublets.

Cerium Oxide Nanoparticles (CeO2-NPs) are commonly used in polishing, engine enhancement agents and many other commercial products. Increased applications and accidental release have raised concerns on the potential impact of CeO2-NPs on the environment. Most previous studies focused on the short term effect of CeO2-NPs. Information is severely lacking on the long-term impact of CeO2-NPs at environmentally relevant concentrations. The main goal of the current dissertation was to investigate the chronic phytotoxicity of CeO2-NPs on two plant species, tomato (Solanum lycopersicum L.) and Turnip mustard (Brassica rapa L.) fast growing variety, and the physiological and biochemical responses of these two plant species to CeO2-NPs. Four specific objectives were established. The first objective was to investigate the effects of combined CeO2-NPs and TiO2-NPs exposure on tomato plant growth and oxidative stress. The second objective was to investigate the impact of a lifecycle exposure to CeO2-NPs on wild type tomato growth and fruit yield and to evaluate the transgenerational effects of CeO2-NPs exposure on plant growth and the oxidative stress of the second generation tomato seedlings. The third objective was to compare the influence of CeO2-NPs and bulk CeO2 particles on plant growth and oxidative stress of standard fast growing Brassica rapa. The final objective was to evaluate the risks of CeO2-NPs exposure over three generations on Brassica plant growth, oxidative stress and reproduction. The results suggested that firstly, CeO2-NPs pre-exposure at concentrations of 4 and 40 mg/kg dry soil followed by 1000 mg/L of TiO2-NPs post-treatment increased H2O2 content and antioxidant proteins activities compared with CeO2-NPs alone and TiO2-NPs alone, and reduced biomass of the tomato plants compared with CeO2-NPs alone. Secondly, irrigation of CeO2-NPs at concentrations up to 10 mg/L stimulated wild type tomato plant growth, but exposure to CeO2-NPs over a lifecycle harmed plant growth and induced higher H2O2 content in tomato seedlings of the second generation. Thirdly, bulk CeO2 exposure at irrigation concentrations of 10, and 100 mg/L were more beneficial for Brassica rapa plant growth than CeO2-NPs at equivalent concentrations. CeO2-NPs irrigation at 10 and 100 mg/L increased H2O2 content and antioxidant proteins activities than bulk CeO2 at equivalent concentrations. However, the mustard plants treated by CeO2-NPs or bulk CeO2 showed changes of H2O2 content in different growth stages, which illustrated that CeO2-NPs and the bulk counterpart induced the change of H2O2 content differently. Finally, irrigating mustard plants with 10 to 1000 mg/L CeO2-NPs over three generations resulted in an inhibited plant growth, stronger oxidative stress, less seed yield and poorer reproduction of offspring plants. To our knowledge, this is the first report on the chronic (multigenerational) effects of CeO2-NPs on plant growth and oxidative stress of tomato and mustard plants grown in soil.

Electrocatalytic reduction and oxidation of nitric oxide (NO) using cobalt phthalocyanine complexes have been studied and compared to vitamin B₁₂ and other metallophthalocyanine (MPc) complexes. Modifying a glassy carbon electrode with these complexes resulted in improved sensitivity of the electrode allowing detection of NO to 10⁻⁹ mol dm⁻³. The mechanisms of catalysis were studied. Electrocatalysis of NO involves coordination of NO to the MPc complex. Hence catalytic activity is affected by the nature of the metal center. However coordination of NO to the MPc complex has to be reversible to eliminate poisoning of the electrode. Though FePc gave the best sensitivity and lowered the reduction potential more than CoPc, the strong Fe-NO bond resulted in the poisoning of the electrode hence, rendering the electrode unstable. Rate constants for NO coordination to the MPc complexes were studied. These rates were smaller than the studied NO porphyrin coordination rates. Electrocatalytic reduction of NO using MPc complexes involves a transfer of an electron from the metal center to the NO ligand. Hence, substitution of electron-donating grohps on the cobalt pthalocyanine complex resulted in improved sensitivity and catalytic activity. A CoPc modified microelectrode (11μm) was used to monitor NO in human blood components and to detect NO in a rat brain. Detections of NO were also done in aqueous solutions in the presence of interfering species such as dopamine and serotonin. An interaction between NO and serotonin was observed.

L’étude présentée ici est consacrée à une approche expérimentale dans les verres silicatés d’alcalins lourds : (SiO2)1-x(Rb2O)x et (SiO2)1-x(Cs2O)x. La polymérisation du réseau est dépendante de la composition du verre, la concentration en cation modificateur ainsi que sa nature vont donc être des facteurs importants influençant la topologie du réseau silicaté. Afin de mieux comprendre les propriétés de ces verres, il est indispensable d'en connaître la structure. Dans ce travail nous nous sommes particulièrement intéressés à l’ordre à moyenne distance qui contrôle les propriétés mécaniques, et à l’ordre à courte distance déterminant pour les propriétés optiques. La spécificité de ce travail réside dans la volonté comprendre le rôle des cations lourds sur la structure et les propriétés du réseau silicaté, dans les binaires alcalins, à l’échelle macroscopique à l’aide de diffusion Brillouin et à l’échelle microscopique en utilisant les spectroscopies Raman et Infrarouge. / The study presented here is devoted to an experimental approach in heavy alkali silicate glasses (SiO2)1-x (Rb2O)x and (SiO2)1-x (Cs2O)x. The polymerization of the network is dependent on glass composition, the modifier cation concentration and its nature will be important factors influencing the topology of the silicate network. To better understand the properties of these glasses, it is essential to know the structure. In this work we are particularly interested in the medium-range order, which controls the mechanical properties, and short-range order for determining the optical properties. The specificity of this work lies in the will understand the role of heavy cations on the structure and properties of silicate network in the binary alkali at the macroscopic scale using Brillouin scattering and microscopic scale using Raman and Infrared spectroscopy.

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