An advanced concept high temperature reactor (HTR) design has been proposed - The ‘U-Battery’, which utilises a unique sealed coolant loop, and is intended to operate with minimal human oversight. In order to reduce the need for moving parts within the design, CO2 has been selected as a candidate coolant, potentially allowing a naturally circulated system. HTR fuel is held within a semi-graphitic fuel-matrix material, and this has not previously been tested within a CO2 environment. Graphite in CO2 is subject to two oxidation reactions, one thermally driven and one radiolytically. As such, the oxidation performance of fuel-matrix material has been tested within CO2 at both high temperatures and under ionising radiation within a sealed-system. Performance has been compared to that of the Gilsocarbon and NBG-18 nuclear graphite grades. Gilsocarbon is the primary graphite grade used within the currently operating AGR fleet within the UK, and as such is known to have acceptable oxidation performance under reactor conditions. NBG-18 is a modern graphite grade, and is a candidate material for use within the U-Battery. Virgin characterisation of all materials was performed, including measurements of bulk mass and volume, skeletal volumes and surface areas. High-resolution optical microscopy has also been performed and pore size distributions inferred from digital image analysis. All results were seen to agree well with literature values, and the variation between samples has been quanti- fied and found to be < 10% between samples of Gilsocarbon, and < 4% for samples of fuel-matrix and NBG-18. Thermal performance of fuel-matrix material was observed between 600 °C – 1200 °C and seen to be broadly comparable to that of the nuclear graphite grades tested. NBG-18 showed surprisingly poor performance at 600°C, with an oxidation rate of 3×10−4%/min, approximately ten times faster than Gilsocarbon in similar conditions, and three times faster than fuel-matrix material. The radiolytic oxidation performance of fuel-matrix material and NBG-18 has been observed by irradiating sealed quartz ampoules. Ampoules were pressurised with CO2 prior to irradiation, and the pressure after 30 days of irradiation was measured and seen to fall by 50%. Radiolytic oxidation, and the subsequent radiolysis of the reaction product, CO, was seen to cause significant carbonaceous deposition on the internal surfaces of the ampoule and throughout the samples. Due to the short irradiation times available in the present study, an investigation of the microporosity within irradiated samples has been carried out, using nitrogen adsorption and small-angle neutron scattering (SANS). Pore size distributions produced from SANS show the closure of microporosity within NBG-18, most likely as a result of low-temperature neutron irradiation.As a result of this work, CO2 is no longer a candidate coolant for use with the U-Battery design, due to the rapid deposition observed following irradiation.

No abstract available. / Dissertation (MSc (Metallurgy))--University of Pretoria, 2006. / Materials Science and Metallurgical Engineering / unrestricted

Please read the abstract in the section front of this document / Dissertation (MSc (Physics))--University of Pretoria, 2006. / Physics / unrestricted

The work described in this thesis consists of an investigation into the behaviour of the critical current density (Jc) of several different high temperature superconductors (HTSCs) as a function of temperature and applied magnetic field. The focus of this research has been to investigate the discrepancies which generally exist between magnetic and transport measurements on HTSCs. In order to do this a number of systems were selected with different weak link strengths, overall alignment and pinning characteristics. Systematic studies were carried out on these systems using both transport and magnetic techniques. The results obtained were compared to obtain a coherent picture of the relation between critical currents in HTSCs, their granularity and structure, and how this accounts for the differences in magnetic and transport measurements. The Jc of granular bulk sintered YBCO was measured as a function of magnetic field and orientation, and attempts made to fit the results obtained to theory. The results obtained indicate that hysteresis of Jc with field cannot be explained by flux trapping alone. Jc measurements were carried out on the 'hub-and-spoke' (H-S) grains of melt-processed YBCO thick films. These indicated that within each H-S grain the current is constrained to radial paths through the centre of the grain. This has major implications for the analysis of any measurements carried out on these samples, as well as for applications, as only a small fraction of the sample carries the applied current. Silver-clad tapes of Tl:1223 and Tl:2223 were measured in different temperatures, applied magnetic fields and orientations. The variation of Jc along the tape length was also measured to provide an estimate of their homogeneity. It was found that the processing route used in the production of these tapes had not aligned the superconductor within them, and also that the tape properties were not homogenous along their length. This probably arises from the crystal structure of the thallium materials. The Jcs of melt-processed thick films of Bi:2212 on silver substrates were measured as a function of temperature and applied magnetic field, and compared with results obtained from magnetic measurements obtained from a VSM. Attempts were made to fit these results to theory. The results obtained show clear scaling behaviour of Jc with T at constant B and imply that the variation in the form of the Jc versus T curves is due simply to the suppression of Tc with increasing applied field.

The excitement which followed the discovery of high-temperature superconductors in 1986 was short-lived, as it became clear that their current carrying capacity (the critical current density Jc) was limited by grain boundaries (GBs). In order to reduce their detrimental effects coated conductors have been developed, in which a superconducting thin film is deposited on a polycrystalline, textured substrate. Within certain temperature and magnetic field ranges, however, GBs still limit the overall Jc. This fact motivated the present thesis, for which the electrical properties of different types of coated conductors, and in particular their GBs, were investigated. Several GBs and a single grain were isolated in a tape produced by metal-organic deposition (MOD), using a novel approach based on electron backscatter diffraction and a focused ion beam microscope. Measurements of their critical current densities for fields swept in the film plane showed the expected decrease with increasing misorientation angle at low fields. At higher fields an angle dependent crossover was found, from a GB to grain limited Jc. In order to confirm this result and put it into broader perspective, the dependence of Jc on the width of polycrystalline tracks was studied, and then explained in terms of the behaviour of the single GBs. Investigations using low-temperature scanning laser microscopy rounded out the picture, which also showed GB dissipation at certain angles and grain limitation at others. In measurements on samples produced by metal-organic chemical vapour deposition (MOCVD) characteristic differences compared to the MOD film were found. While both conductors exhibited high values of Jc, the variation with in-plane angle was significantly stronger for the MOCVD conductor, which can be explained by its sharper texture. In a track patterned perpendicular to the tape direction the phenomenon of vicinal channelling was observed, which previously was known only from films on single crystal substrates. Finally, an isolated boundary showed very high values of Jc, consistent with its low misorientation. In order to better understand how the substrate influences the superconducting properties, measurements were carried out on otherwise identical samples grown on different substrates. A tape with grains elongated along its rolling direction showed particularly good properties at all examined field orientations. This extends the previously reported result that high aspect ratios are beneficial at fields applied perpendicular to the tape plane.

In this dissertation, the problem of calculating and measuring AC losses in superconducting coils is addressed, with a particular focus on the transport AC loss of coils for electric machines. In order to model the superconducting coil's electromagnetic properties and calculate the AC loss, an existing two dimensional (2D) finite element model that implements a set of equations known as the H formulation, which directly solves the magnetic field components in 2D, is extended to model a superconducting coil, where the cross-section of the coil is modelled as a 2D stack of superconducting coated conductors. The model is also modified to allow the nclusion of a magnetic substrate, which is present in some commercially available HTS wire. The analysis raises a number of interesting points regarding the use of superconductors with magnetic substrates. In particular, the presence of a magnetic substrate affects the penetration of the magnetic flux front within the coil and increases the magnetic flux density within the penetrated region, both of which can increase the AC loss significantly. In order to investigate these findings further, a comprehensive analysis on stacks of tapes with weak and strong magnetic substrates is carried out, using a symmetric model that requires only one quarter of the cross-section to be modelled. In order to validate the modelling results, an extensive experimental setup is designed and built to measure the transport AC loss of a superconducting coil using an electrical method based on inductive compensation by means of a variable mutual inductance. Measurements are carried out on the superconducting racetrack coil and it is found that the experimental results agree with the modelling results for low current, but some phase drift occurs for higher current, which affects the accuracy of the measurement. In order to overcome this problem, a number of improvements are made to the initial setup to improve the lock-in amplifier's phase setting and other aspects of the measurement technique. New measurements are carried out on a single, circular pancake coil and the discrepancies between the experimental and modelling results are described in terms of the assumptions made in the model and aspects of the coil that cannot be modelled. Using the original measured properties of the superconducting tape, there is an order of magnitude difference between the experiment and model. The properties of the superconductor can degrade during the winding and cooling processes, and a critical current measurement of the coil showed that the tape critical current reduced from nearly 300 A, down to around 100 A. Applying this finding to the model, the experimental and modelling results show good agreement, and the difference in the slope of the AC loss curve can be described in terms of the B-dependent critical current dependency Jc(B) used in the model. Finally, methods used to mitigate AC loss in superconducting wires and coils are summarised, and the use of weak and strong magnetic materials as a flux diverter is investigated as a technique to reduce AC loss in superconducting coils. This technique can achieve a significant reduction in AC loss and does not require modification to the conductor itself, which can be detrimental to the superconductor's properties.

Thermal Barrier Coatings (TBC) are used to shield hot sections of gas turbine engines, helping to prevent the melting of metallic surfaces. TBC is a sophisticated layered system that can be divided into top coat, bond coat, and the super-alloy substrate. The highly heterogeneous microstructure of the TBC consists of defects such as pores, voids, and cracks of different sizes, which determine the coating’s final thermal and mechanical properties. The service lives of the coatings are dependent on these parameters. These coatings act as a defensive shield to protect the substrate from oxidation and corrosion caused by elevated temperatures. Yttria Stabilized Zirconia (YSZ) is the preferred thermal barrier coating for gas turbine engine applications. There are a certain number of deposition techniques that are used to deposit the thermal coating layer on the substrate; commonly used techniques are Air Plasma Sprayed (APS) or Electron Beam Physical Vapour Deposition (EB-PVD). The objective of this thesis is to model an optimized TBC that can be used on next-generation turbine engines. Modelling is performed to calculate the effective thermal conductivity of the YSZ coating deposited by EB-PVD by considering the effect of defects, porosities, and cracks. Bruggeman’s asymmetrical model was chosen as it can be extended for various types of porosities present in the material. The model is used as an iterative approach of a two-phase model and is extended up to a five-phase model. The results offer important information about the influence of randomly oriented defects on the overall thermal conductivity. The modelled microstructure can be fabricated with similar composition to have an enhanced thermal insulation. The modelling results are subsequently compared with existing theories published in previous works and experiments. The modelling approach developed in this work could be used as a tool to design the porous microstructure of a coating.

LFC is cementatious material integrated with mechanically entrained foam in the mortar slurry which can produce a variety of densities ranging from 400 to 1600 kg/m3. The application of LFC has been primarily as a filler material in civil engineering works. This research explores the potential of using LFC in building construction, as non-load-bearing partitions of lightweight load-bearing structural members. Experimental and analytical studies will be undertaken to develop quantification models to obtain thermal and mechanical properties of LFC at ambient and elevated temperatures. In order to develop thermal property model, LFC is treated as a porous material and the effects of radiant heat transfer within the pores are included. The thermal conductivity model results are in very good agreement with the experimental results obtained from the guarded hot plate tests and with inverse analysis of LFC slabs heated from one side. Extensive compression and bending tests at elevated temperatures were performed for LFC densities of 650 and 1000 kg/m3 to obtain the mechanical properties of unstressed LFC. The test results indicate that the porosity of LFC is mainly a function of density and changes little at different temperatures. The reduction in strength and stiffness of LFC at high temperatures can be predicted using the mechanical property models for normal weight concrete provided that the LFC is based on ordinary Portland cement. Although LFC mechanical properties are low in comparison to normal weight concrete, LFC may be used as partition or light load-bearing walls in a low rise residential construction. To confirm this, structural tests were performed on a composite walling system consisting of two outer skins of profiled thin-walled steel sheeting with LFC core under axial compression, for steel sheeting thicknesses of 0.4mm and 0.8mm correspondingly. Using these test results, analytical models are developed to calculate the maximum load-bearing capacity of the composite walling, taking into consideration the local buckling effect of the steel sheeting and profiled shape of the LFC core. The results of a preliminary feasibility study indicate that LFC can achieve very good thermal insulation performance for fire resistance. A single layer of 650 kg/m3 density LFC panel of about 21 mm would be able to attain 30 minutes of standard fire resistance rating, which is comparable to gypsum plasterboard. The results of a feasibility study on structural performance of a composite walling system indicates that the proposed panel system, using 100mm LFC core and 0.4mm steel sheeting, has sufficient load carrying capacity to be used in low-rise residential construction up to four-storeys.

Indiana University-Purdue University Indianapolis (IUPUI) / Observations of heterogeneities in Earth’s mantle motivate studies of mantle phase assemblages with variable composition. As samples cannot be directly collected from these regions, synthetic glasses can act as analogues for mantle melt and starting materials for high-pressure synthesis of stable mantle minerals in experiments. Here, I develop a specific methodology to produce homogeneous glasses that accurately span the composition space from enstatite (MgSiO3) to forsterite (Mg2SiO4), as well as Fe-bearing enstatite ((Mg0.1Fe0.9)SiO3 and ((Mg0.95Fe0.05)(Si0.95Fe0.05)O3) with variable oxidation states. This study systematically tests and iterates upon glass synthesis methods using an aerodynamic levitation laser furnace, in which a spherical glass sample levitates on a gas stream flowing vertically through a conical nozzle, while being heated from above with a 400 W CO2 laser. With sample diameters of 0.6-2.0 mm, shutting off the laser results in supercooling of levitated spheres at rates between 350 and 1350 °C/s. Sample preparation begins with grinding and mixing pure oxide powders in an agate mortar and pestle, followed by heating in a high temperature oven to devolatilize the mixture. Powders (0.5-7 mg aliquots) are fused into spheres in a copper hearth plate. To tune Fe valency and vitrify each sphere, samples are then levitated on flows of Ar, O2, 5% CO in Ar, 5% CO2 in Ar, or combinations of two of these gases, while being heated with the laser to temperatures above the liquidus for each composition for ~10 s before quenching. After x-ray diffraction (XRD) analyses confirm vitrification, a dual polish is applied, exposing parallel flat polished surfaces for scanning electron microscope (SEM) and electron probe microanalyzer analyses (EPMA). Back-scattered electron images and energy-dispersive x-ray spectroscopy (EDS) analyses of the spheres are obtained first on the SEM to gauge compositional accuracy and homogeneity, then EPMA analyses determine quantitatively the samples’ compositions. Once fully characterized, these glasses can be used in diamond anvil cell experiments, where they can act as proxies for an otherwise inaccessible area of the Earth. In addition to the development of this methodology, two web applications produced during this research aid in visualization of both data logs and analytical results.

京都大学 / 0048 / 新制・課程博士 / 博士(農学) / 甲第17964号 / 農博第2032号 / 新制||農||1019(附属図書館) / 学位論文||H25||N4808(農学部図書室) / 30794 / 京都大学大学院農学研究科農学専攻 / (主査)教授 白岩 立彦, 教授 奥本 裕, 教授 稲村 達也 / 学位規則第4条第1項該当 / Doctor of Agricultural Science / Kyoto University / DFAM