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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Magnox, a magnesium alloy, was specifically developed in early 1950s for use as a fuel cladding in the British first generation nuclear civilian reactors. Magnesium metal demonstrates outstanding properties for use as a nuclear fuel cladding; however, it has an intrinsic ability to undergo oxidation. This introduces significant limitations during aqueous storage required prior to reprocessing of the spent fuel. A possibility exists for a failure of the dated reprocessing facilities, and therefore this may require for the spent fuel to be kept in the aqueous storage for an extended period of time. In a most extreme case, the corrosion of the fuel cladding may lead to a contamination of the storage facilities with hazardous radioactive fission species and corrosion products. A comprehensive study of chemical behaviour of the Magnox alloy may allow a deeper understanding of the reactivity of the cladding and lead to improvements in management of storage of spent Magnox fuel, thus preventing corrosion induced leakage of hazardous products. The understanding of chemistry of the Magnox alloy during storage may be improved by the development of a novel approach to study corrosion reactions. Infrared spectroscopy and Raman spectroscopy are widely used to study properties of surfaces. In addition, electron microscopy provides with information on the structure and physical appearance of materials. The results show clear evidence for reactivity of the alloy to be greatly influenced by changes induced by nature of cooling processes upon simulated discharge of spent Magnox fuel from a reactor. It is evident that the fast cooling process using water introduces the most deleterious change to the properties of the material. It is understood that presence of water provides with favourable conditions for oxidation of the metal to take place. Opposite effect is observed with slow cooling under an atmosphere of carbon dioxide gas. Further studies using X-ray diffraction suggest that crystallinity of the alloy is increased during simulated reactor exposure and phase segregation takes place during cooling. The latter appears to be dependent on the nature of the cooling process, and thus as a result different strains are applied. Through the studies it is shown that the pond conditions also contribute to control of the behaviour of the fuel cladding. A series of experiments demonstrated that sodium carbonate offers paramount corrosion protection when compared to sodium hydroxide. Systematic investigations allowed for a complete corrosion mechanism of the Magnox cladding to be drawn. It is demonstrated that the effects of present as well past conditions should be assessed and taken into consideration when establishing chemical behaviour of a material.

Initial developments within group 2 chemistry led to the chemistry being described as ‘lanthanide mimetic’ however, over the last 10 years group 2 catalysis has emerged in its own right, making these comparisons unjustified. Development of this catalytic chemistry has, until now, largely focussed upon the use of protic reagents in order to achieve turnover. Reported in this thesis is the development of magnesium hydride chemistry for both stoichiometric and catalytic purposes. Reported in chapter 2 of this thesis, is the use of the pharmaceutically relevant magnesium dihydropyridide complexes and explores their use as hydride transfer reagents with respect to a representative ketone, benzophenone, whilst further study with various different isocyanate reagents with differing electronic and steric demands provides divergent reactivity. Extension of this chemistry with respect to carbodiimides provides a series of N-heterocyclic guanidinates in all but one case. The chemistry described in chapters 3-6 investigates the use of magnesium hydrides in catalysis. Using the commercially available hydridic pinacol borane (HBpin) reagent a series of catalytic reactions with respect to pyridines (chapter 3), nitriles (chapter 4), iso-nitriles (chapter 5) and heterocumulenes (chapter 6) are investigated. In each case, studies have sought to underpin the catalytic reactivity by examining the single steps of the proposed catalytic cycle via a series of stoichiometric reactions which has allowed for the isolation and characterisation of numerous potential catalytic intermediates. Monitoring of these catalytic reactions in situ with NMR spectroscopy, combined with kinetic analysis, has allowed for further information to be obtained with regards to the mechanism and calculation of the activation energy parameters associated with each reaction.

by Man-Ling Wong. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2002. / Includes bibliographical references. / Text in English; abstracts in English and Chinese. / by Man-Ling Wong. / Acknowledgments --- p.i / Abstract --- p.ii / 摘要 --- p.iv / Table of contents --- p.v / Chapter Chapter 1 --- Introduction --- p.1-1 / Chapter 1.1 --- Overview of metal matrix composites --- p.1 -1 / Chapter 1.1.1 --- Types of MMCs --- p.1-1 / Chapter 1.1.2 --- The matrices --- p.1 -2 / Chapter 1.1.2.1 --- Mg-based matrix --- p.1 -2 / Chapter 1.1.2.2 --- Al-based matrix --- p.1-2 / Chapter 1.1.2.3 --- Ti-based matrix --- p.1 -3 / Chapter 1.2 --- Fabrication methods of MMCs --- p.1 -3 / Chapter 1.2.1 --- Solid-liquid reaction --- p.1-4 / Chapter 1.2.2 --- Vapor-liquid-solid (VLS) reaction --- p.1 -4 / Chapter 1.2.3 --- Solid-Solid reaction --- p.1-5 / Chapter 1.2.4 --- Liquid-liquid reaction --- p.1-5 / Chapter 1.3 --- Applications of MMCs --- p.1 -6 / Chapter 1.4 --- Previous works versus our work --- p.1 -7 / Chapter 1.5 --- Layout of the thesis --- p.1 -8 / Figures --- p.1-9 / References --- p.1-10 / Chapter Chapter 2 --- Methodology and Instrumentation --- p.2-1 / Chapter 2.1 --- Introduction --- p.2-1 / Chapter 2.2 --- Powder metallurgy --- p.2-1 / Chapter 2.3 --- Sample preparation --- p.2-2 / Chapter 2.3.1 --- Cold pressing --- p.2-2 / Chapter 2.3.2 --- Sintering --- p.2-3 / Chapter 2.4 --- Characterization methods --- p.2-4 / Chapter 2.4.1 --- Differential Thermal Analyzer (DTA) for thermal analysis --- p.2-4 / Chapter 2.4.2 --- X-Ray powder Diffractometry (XRD) for phase determination --- p.2-5 / Chapter 2.4.3 --- Scanning Electron Microscopy (SEM) and Electron Dispersive X-ray analysis (EDX) for structural analysis --- p.2-6 / Chapter 2.4.4 --- Mechanical properties --- p.2-7 / Chapter 2.4.4.1 --- Relative density --- p.2-7 / Chapter 2.4.4.2 --- Porosity --- p.2-9 / Chapter 2.4.4.3 --- Tensile strength --- p.2-10 / Chapter 2.4.4.4 --- Hardness test --- p.2-10 / Figures --- p.2-12 / References --- p.2-18 / Chapter Chapter 3 --- Formation of the Mg-ZnO MMCs --- p.3-1 / Chapter 3.1 --- Thermal analysis on the reactions between Mg and ZnO --- p.3-1 / Chapter 3.1.1 --- Introduction --- p.3-1 / Chapter 3.1.2 --- Experiments --- p.3-1 / Chapter 3.1.3 --- Results and Discussions --- p.3-1 / Chapter 3.2 --- Characterization of the Mg-ZnO MMCs --- p.3-2 / Chapter 3.2.1 --- Introduction --- p.3-2 / Chapter 3.2.2 --- Experiments --- p.3-3 / Chapter 3.2.3 --- Results and Discussions --- p.3-3 / Chapter 3.2.3.1 --- Scanning electron microscopy (SEM) and Electron dispersive X-ray analysis (EDX) --- p.3-3 / Chapter 3.2.3.2 --- X-ray Diffraction (XRD) --- p.3-4 / Chapter 3.2.3.3 --- Mg-Zn intermetallics Phases --- p.3-5 / Chapter 3.2.4 --- Model of formation of Mg-ZnO MMCs --- p.3-5 / Chapter 3.2.4.1 --- Chemical reactions --- p.3-5 / Chapter 3.2.4.2. --- Order of priority of reactions --- p.3-6 / Chapter 3.2.4.3 --- Diffusion during sintering --- p.3-7 / Chapter 3.2.4.4 --- Reaction Model --- p.3-8 / Chapter 3.2.5 --- Conclusions --- p.3-8 / Figures --- p.3-10 / References --- p.3-18 / Chapter Chapter 4 --- Mechanical properties of the Mg-ZnO MMCs --- p.4-1 / Chapter 4.1 --- Introduction --- p.4-1 / Chapter 4.2 --- Experiments --- p.4-1 / Chapter 4.3 --- Results and Discussions --- p.4-2 / Chapter 4.3.1 --- Relative density --- p.4-2 / Chapter 4.3.2 --- Porosity --- p.4-3 / Chapter 4.3.3 --- Tensile strength --- p.4-4 / Chapter 4.3.4 --- Hardness --- p.4-6 / Chapter 4.4 --- Conclusions --- p.4-7 / Figures --- p.4-9 / References --- p.4-23 / Chapter Chapter 5 --- Reinforcement in Mg-ZnO MMCs --- p.5-1 / Chapter 5.1 --- Introduction --- p.5-1 / Chapter 5.2 --- Experiments --- p.5-1 / Chapter 5.3 --- Results and Discussions --- p.5-1 / Chapter 5.3.1 --- Microstructure of the Mg-ZnO MMCs --- p.5-2 / Chapter 5.3.2 --- Fracture of Mg-ZnO MMCs --- p.5-5 / Chapter 5.3.2.1 --- Fracture surface --- p.5-5 / Chapter 5.3.2.2 --- Fracture mode --- p.5-7 / Chapter 5.4 --- Conclusions --- p.5-8 / Figures --- p.5-9 / References --- p.5-18 / Chapter Chapter 6 --- Conclusions and Future Works --- p.6-1 / Chapter 6.1 --- Conclusions --- p.6-1 / Chapter 6.2 --- Future Works --- p.6-2 / References --- p.6-4

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