Thesis (Ph. D.)--University of Hong Kong, 2006. / Title proper from title frame. Also available in printed format.

Thesis (Ph.D.)--Tufts University, 1989. / Submitted to the Dept. of Physics. Includes bibliographical references (leaves 192-204). Access restricted to members of the Tufts University community. Also available via the World Wide Web;

High temperature, solvent resistant materials which also display good mechanical properties are desired for use as aerospace structural adhesives and polymer matrix/carbon fiber composites. High molecular weight amorphous poly(arylene ether sulfone) thermoplastic materials display many of these desirable characteristics but are deficient in solvent resistance. Previous attempts to prepare poly(arylene ether) based thermosets to improve solvent resistance have been largely unsuccessful due to processiblity issues from the low curing temperature and high glass transition temperature of the thermoset precursor. Incorporation of a high temperature curable (* 350°C) endgroup such as 3-phenylethynylphenol in the synthesis of controlled molecular weight poly(arylene ether sulfone) oligomers has allowed for a large processing window prior to the exothermic cure that forms the desired networks. Control of oligomer molecular weight and backbone structure has allowed for further control of the processing, thermal transitions and adhesive properties of the thermosets. A systematic series of phenylethynyl terminated oligomers derived from either bisphenol A, or wholly aromatic hydroquinone or biphenol has been synthesized and characterized to determine the influence of backbone structure, molecular weight, and endgroup structure on thermoset properties. The features most affected by backbone structure included thermal stability (weight loss behavior) as well as transition temperatures (Tg, Tm), and processing characteristics. Increasing molecular weight of the oligomer produced a decrease in the glass transition temperature of the network and an increase in the adhesive properties of the thermoset. Comparison of the curing behavior of the 3-phenylethynylphenol endcapped materials with other related phenylethynyl terminated compounds led to the synthesis and systematic investigation of the curing behavior of phenylethynyl endcappers in which the electronic environment in relation to the reactive ethynyl carbons was systematically varied. Electron withdrawing groups, eg. sulfone, ketone, imide on the aryl ring para to the acetylene bond enhanced the rate of cure and also appear to improve the lap shear adhesion to suface treated titanium adherands. Discussion of the background, synthesis and characterization are described in this dissertation. / Ph. D.

Since the recent discovery of a new class of high critical temperature superconductors (HTS), much interest has been shown in their potential use as optical detectors. The purpose of this research was to test thin film samples of the HTS Y1Ba2Cu3O7-delta as detectors and investigate any response to optical radiation. A laboratory test facility was designed and built for this purpose. The experimental results exhibit a variety of optical responses that are dependent upon the physical characteristics of each HTS thin film. Polycrystalline films exhibited a different detection mode than did epitaxial films. This research demonstrates that HTS thin films are viable optical detectors and have the potential to become competitive high-performance detectors as the new technology continues to emerge.

Supercritical carbon dioxide was explored as a medium for nucleation and growth processes for ceramic oxide superconductors. The high temperature superconductor examined, YBa₂Cu₃O₇₋ₓ (YBC), was found to be stable upon limited exposure to CO₂. The solubility of the compound was studied in the region near the critical point using spectrophotometry. The species solubilized was indirectly identified by elemental analysis. Preliminary studies on the potential of the medium for metastable synthesis, comminution, and crystallization were conducted and showed promise for further development.

As an electromagnetic response detection technique, non-resonant microwave absorption (NRMA) has been without doubt, one of the fundamental tools in characterizing high temperature superconductors (HTSC). The technique can explicitly give factual information on flux pinning, granularity, magnetization, and detection of iota superconducting phases among many more. The emergence of iron pnictides superconductors has brought an enormous impact on HTSC field due to their relatively high 𝑇c, high critical fields 𝐵c2, huge critical current density and low anisotropy. Accordingly, they look appealing candidates in industrial applications more especially in high magnetic field applications. As of yet, its electromagnetic response particularly the low field microwave absorption (LFMA) or the non-resonant microwave absorption (NRMA) is relatively unknown. Consequently, in this work, systematic studies have been done on SmFeAs(O,F), superconductors to determine the low field sweep microwave absorption. Furthermore, effect of varying temperature, microwave power and field modulation amplitude on NRMA line shape have been addressed and the results obtained are compared with NRMA results of cuprates superconductors. Interestingly, the NRMA line shape has been found to evolve as a function of temperature, microwave power and field modulation amplitude. A structure i.e a broad peak 1 and a narrow peak 2 have been identified. Furthermore, the line shape shows a phase reversal at moderately high microwave power. This dissertation presents the theoretical background of superconductors, experimental techniques, working principles of the equipments, results, discussions and conclusions. As pertains to the future work, recommendations have been suggested in trying other forms of sample and also different sample materials of iron based superconductors to fully understand the NRMA and ensure a progressive and continuous work in this field. Also, we will carry out extensive studies on critical current density, fluxon dynamics and irreversibility fields on iron-based superconductors by means of NRMA technique. / Physics / M. Sc. (Physics)

published_or_final_version / Physics / Doctoral / Doctor of Philosophy

published_or_final_version / abstract / Physics / Doctoral / Doctor of Philosophy

The interfacial properties of a surfactant in a CO₂-aqueous system at a temperature above 100 °C, and how they influence foams are essentially unknown. A cationic surfactant, C₁₂₋₁₄N(EO)₂ in the protonated state below pH 5.5, was demonstrated to be soluble in an aqueous phase with up to 22% total dissolved salt (TDS) at 120 °C. Moreover, the strong solvation in brine (high cloud point) and simultaneous affinity for CO₂ led to significant adsorption of the surfactant at the CO₂-water interface. Given that the surfactant favored the brine phase over the CO₂ phase, the preferred curvature was a CO₂-in-water (C/W) macroemulsion (foam). The surfactant stabilized foam in the presence of crushed calcium carbonate at ~ pH 4 upon suppressing the dissolution of calcium carbonate upon addition of Ca²⁺ and Mg²⁺ according to the common ion effect. Cationic alkyltrimethylammonium surfactants with an alkyl tail of average carbon number less than 15 were soluble in 22% TDS brine up to 120 oC. The head group was properly balanced with a C₁₂₋₁₄ hydrocarbon tail for a sufficiently dense surfactant layer at the CO₂-water interface to reduce the interfacial tension. For C₁₂₋₁₄N(CH₃)₃Cl the solubility in brine and the surfactant adsorption were sufficient to stabilize C/W foam at 120 °C in both a crushed calcium carbonate packed bed (76 Darcy) and a capillary tube at the downstream of the bed. The stability of the foam at high temperature may be attributed to the high surfactant adsorption at the interface. The use of nonionic surfactants as a foam stabilizer is usually limited by their poor aqueous solubility at elevated temperatures, particularly at high salinity. A nonionic surfactant C₁₂₋₁₄(EO)₂₂ with high degree of ethoxylation gave higher salt tolerance at elevated temperature. The surfactant stabilize C/W foam at 80 °C in the presence of 90 g/L NaCl brine in a 30 Darcy sand pack, which has not yet been reported by a nonionic surfactant. Both the formation of strong foam in the porous media and the low of oil-brine partition coefficient suggest C₁₂₋₁₄(EO)₂₂ is a potential candidate for a CO₂ EOR field trial. / text

High alloy Fe-Cr-Ni-C austenitic stainless steels have become the principal alloys for use in steam-methane reforming furnaces within the petrochemical industry. Each furnace contains a large array of vertically oriented centrifugally cast tubes through which a mixture of methane and steam is flowed across a nickel-oxide catalyst in order to obtain a mixture of hydrogen, carbon monoxide and carbon dioxide and water commonly known as synthesis gas (or syngas). Generally, the tubes operate at temperatures between 850-1050°C, internal pressures between 1-3.5MPa and are expected to withstand service lives in excess of 100,000 hours. The combination of high temperatures and moderate stresses causes creep to be the dominant failure mechanism experienced by these tubes in service. The HP austenitic alloys are the latest in a series of heat resisting (H-series) stainless steels developed to provide high temperature strength, ductility, and corrosion resistance in the oxygen, carbon, and sulphur rich environments typical of these furnaces. Extensive work has been carried out to optimise HP alloys’ microstructure in order to maximise the alloy’s creep resistance. Strength increases have largely been realized through the use of niobium and/or titanium additions, which modify the primary precipitates (formed during solidification) and secondary precipitates (formed during exposure to the service temperatures). These strength increases have typically been observed during laboratory accelerated creep testing of the ‘modified’ HP alloys where the temperature and/or stress is increased to achieve failure of the material within reasonable time period (typically between 1000-2000 hours). However, since the duration of typical accelerated creep tests often represent less than 2% of the tubes’ actual service life, uncertainty surrounds the validity of using this testing method as the basis to predict the tubes actual service life. This uncertainty has largely arisen due to the significant microstructural evolution that occurs within these alloys during prolonged service exposure and is not captured within the typical accelerated testing time-frame. In the present work, the microstructures of HP alloys modified with a single addition of niobium (HP-Nb) and dual additions of niobium and titanium (HP-NbTi) have been characterized in the as-cast condition and after long-term (10,000 hours) isothermal laboratory ageing at 1000, 1050 and 1100°C. The main focus of this study is to provide further insight into the microstructural features that increase the HP-NbTi alloy’s creep resistance in comparison to the HP-Nb alloy when performing accelerated creep testing and determine if these microstructural features remain stable during long-term ageing. The microstructure and crystallography of the primary and secondary precipitates in each alloy have been studied in detail using light optical microscopy, high resolution scanning electron microscopy, transmission electron microscopy, various electron diffraction methods (EBSD, SAD and CBED), Powder X-ray Diffraction and energy dispersive X-ray spectroscopy. Specific attention has been paid to the niobium-rich and niobium-titanium-rich phases that form as a direct result of HP alloy’s modification with niobium and titanium. The current research is part of a wider project conducted in collaboration with Quest Integrity Group Ltd. (Wellington, New Zealand) that aims to characterize the microstructural and mechanical properties of the HP-Nb and HP-NbTi alloys during long-term service exposure. The microstructural characterization presented in this thesis will subsequently be used by Quest Integrity Group to build a comprehensive understanding of the relationship between HP-Nb and HP-NbTi alloy’s microstructure and creep properties. This understanding will allow Quest Integrity Group to more accurately predict the service life of HP-Nb and HP-NbTi alloy tubes within steam-methane reforming furnaces.