The compressive creep behaviour of a high purity silicon nitride ceramic with and without the addition of Ba was studied at 1400°C. Two distinct creep stages were observed during high temperature deformation of these materials. The stress exponents for creep of the two materials indicate that they have different creep mechanisms during the second stage of creep. Cavitation during creep was determined by measuring the density change before and after creep using a water­-displacement method. The Ba doped material exhibited an obvious density decrease, indicating cavitation during creep, whereas the undoped material exhibited no cavitation. This is consistent with TEM observations. The microstructure of the materials, especially the amorphous grain-boundary phase was investigated for both as-sintered and crept specimans by means of transmission electron microscopy (TEM). Statistical analysis of a number of grain-­boundary films indicates that the film thickness is confined to a narrow range (standard deviation less than 0.15 nm) in the as-sintered materials. The average film thickness depends on film chemistry, increasing from 1.0 nm to 1.4 nm when Ba is added. The standard deviation of the film thickness of a given material after creep, however, is considerably larger than before (0.30 nm ~ 0.59 nm). This suggests that the grain-boundary glass phase is redistributed during creep. Viscous flow of the glass phase is proposed as die mechanism responsible for the first stage of creep. The data are compared with a model for viscous creep, yielding good correlation. / Thesis / Master of Engineering (ME)

The purpose of this research was to assess the suitability of the thermistor based method for measuring thermal conductivity and diffusivity of moist food materials at high temperatures. Research focused on aspects of calibration, thermal contact in solid food materials, natural convection in liquid media and the performance in moist food materials at high temperatures. Thermistor probes were constructed in house and calibrated in three materials of known thermal conductivity and diffusivity, water, glycerol, and a heat transfer fluid, HTF 500. With few exceptions, the calibrated probe estimated thermal properties with an error of less than 5%, over the range of thermal properties spanned by the those of the calibration media. An alternate calibration using two media was also investigated. It was found to give better accuracy over a more limited range. Thermal contact in potato and lean beef was investigated through a comparative study that used a miniature line heat source probe as a reference method. The food materials were measured at 25, 50 and 100 °C. Good agreement was found between the measurements with the line heat source probe and the bead thermistor probe, indicating adequate thermal contact at the thermistor probe. The effect of fluid viscosity and the magnitude of the temperature step on the occurrence of natural convection was studied for aqueous solutions of a thickening agent. During a sample time of 30 seconds, convection was absent in solutions with a viscosity of 25 cp or greater, when measured with a temperature step of 1.5 and 2.5 °C, and in solutions with a viscosity of 50 cp or greater, when measured with a temperature step of 5.0 °C. A Rayleigh number was defined to study the notion of a critical Rayleigh number at the onset of convection. This study found that when the Rayleigh number was below 43, convection could not be demonstrated. For a Rayleigh number of 84 and higher, convection was observed. The performance at high temperatures in food materials was studied through tests in tomato concentrate and in a liquid food supplement. Tomato puree and tomato paste were sampled at 100, 130 and 150 °C. The thermal conductivity of tomato puree at 100, 130 and 150 °C was measured as 0.638, 0.645 and 0.647 W/m°C respectively. The thermal diffusivity was 1.63, 1.64 and 1.62 10^-7 m²/s respectively. For tomato paste at 100, 130 and 150 °C, a thermal conductivity was obtained of 0.590, 0.597 and 0.534 W/m°C respectively. The thermal diffusivity was 1.63, 1.84 and 2.36 10 ^-7 m²/s respectively. With some notable exceptions the results of this study agreed well with Choi and Okos (1983). A liquid food supplement was also studied at 95 and 150 °C. The thermal conductivity of the food supplement decreased with increasing solids content from 0.62 W/m°C at a solids level of 15% to 0.41 W/m°C at a solids level of 50%. The results of this study indicate that the thermistor based method was suitable for measuring thermal conductivity and diffusivity of moist food materials at high temperatures. However, the type of thermistor used in the research, a glass encapsulated thermistor, was too fragile for routine work. In particular the high temperature use of the glass thermistor was impacted by its susceptibility to fracture. / Ph. D.

Silicon carbide (SiC) is a wide bandgap material capable of higher voltage and higher temperature operation compared to its silicon (Si) counterparts due to its higher critical electric field (E-field) and higher thermal conductivity. Using SiC, MOSFETs with a theoretical high temperature operation and reliability is achievable. However, current bottlenecks in high temperature SiC MOSFETs lie within the limitations of standard packaging. Additionally, there are reliability issues relating to the gate oxide region of the MOSFET, which is exacerbated through high temperature conditions. In this thesis, high temperature effects on current-generation SiC MOSFETs are studied and analyzed. To achieve this, a high temperature package is created to achieve reliable operation of a SiC MOSFET at junction temperatures of 300 °C. The custom, high temperature package feasibility is verified through studying trends in SiC MOSFET behavior with increasing temperature up to 300 °C by static characterization. Additionally, the reliability of SiC MOSFETs at 300 °C is tested with accelerated lifetime bias tests. / M.S. / Electrical devices that are rated for high temperature applications demand a use of a material that is stable and reliable at the elevated temperatures. Silicon carbide (SiC) is such a material. Devices made from SiC are able to switch faster, have a superior efficiency, and are capable of operating at extreme temperatures much better than the currently widely used silicon (Si) devices. There are limitations on SiC certain structures of SiC devices, such as the metal oxide semiconductor field effect transistor (MOSFET), have inherent reliability issues related to the fabrication of the device. These reliability issues can get worse over higher temperature ranges. Therefore, studies must be made to determine the feasibility of SiC MOSFETs in high temperature applications. To do so, industry standard tests are conducted on newer generation SiC MOSFETs to ascertain their use for said conditions.

Feldspar minerals are framework aluminosilicates that comprise approximately 60 percent of the Earth's crust. The elastic and thermodynamic properties of this important mineral group are needed for the interpretation of seismic wave velocities, for understanding cation partioning patterns and for the determination of phase boundaries and reactions involving feldspars in the Earth's crust. Until recently, no systematic approach has been applied to describe the structural behavior of feldspars as a function of pressure, temperature and composition. In this thesis, high-pressure and high-temperature X-ray diffraction data were collected for feldspars over a range of compositions which has led to the development a structural model that allows one to predict the structural evolution of feldspars at depth in the Earth's crust. Specifically, the equations of state have been determined for two plagioclase feldspars (An20 and An78) with different states of Al/Si ordering using single-crystal X-ray diffraction. This study has shown that the introduction of Al,Si disorder into plagioclase structures at constant composition softens the structure by 4(1)% for An0, 2.5(9)% for An20 and is essentially zero for An78 compositions. The effect of pressure on the structure of an ordered An20 was also determined up to 9.15 GPa using single-crystal X-ray diffraction and it was found that the dominant compression mechanism involves tilting of the AlO4 and SiO4 tetrahedra. Similarly, high-temperature single-crystal X-ray diffraction data collected from an ordered An26 plagioclase and powder X-ray diffraction collected on a suite of Na-rich plagioclases that were refined using the Rietveld method indicate that the major structural response to increased temperature involves tilting of the tetrahedra. Building on ideas originally proposed by Dr. Helen Megaw, the changes in the conformation of the tetrahedral framework of feldspars can be described in terms of four distinct tilt systems of rigid tetrahedra. This model demonstrates that the fundamental reason for the observed anisotropy and volume change of feldspars lies in the topology of the tetrahedral framework with the greatest contribution attributed to tilt systems 2 and 3. / Ph. D.

Tam Ka Ming. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2002. / Includes bibliographical references (leaves 142-145). / Text in English; abstracts in English and Chinese. / Tam Ka Ming. / Acknowledgement --- p.ii / Chapter 1 --- Introduction --- p.1 / Chapter 1.1 --- Models for Many Body Systems --- p.1 / Chapter 1.2 --- High Temperature Superconductor --- p.5 / Chapter 1.3 --- Reasons of Study Double Layers --- p.9 / Chapter 1.4 --- "Incommensurate Magnetic Excitation, Order Parameters" --- p.10 / Chapter 1.5 --- Thesis Organization --- p.10 / Chapter 2 --- Monte Carlo Methods --- p.12 / Chapter 2.1 --- General Aspects of Monte Carlo Simulation --- p.12 / Chapter 2.2 --- "Markov Chain, Metropolis Algorithm, Heat-Bath Algorithm" --- p.18 / Chapter 2.3 --- Quantum Monte Carlo --- p.21 / Chapter 2.4 --- Variational Quantum Monte Carlo --- p.22 / Chapter 2.5 --- Green Function Monte Carlo --- p.27 / Chapter 2.6 --- Auxiliary Field Monte Carlo --- p.35 / Chapter 2.7 --- Summary of Quantum Monte Carlo --- p.38 / Chapter 3 --- Constrained Path Monte Carlo --- p.40 / Chapter 3.1 --- Various Formulas Used in Constrained Path Monte Carlo --- p.40 / Chapter 3.2 --- Constrained Path Monte Carlo --- p.47 / Chapter 3.3 --- Constrained Path Approximation --- p.51 / Chapter 3.4 --- Measurements --- p.52 / Chapter 3.5 --- Method of Avoiding Numerical Instability --- p.55 / Chapter 4 --- Parallelisation of CPMC Program --- p.57 / Chapter 4.1 --- Architecture of Parallel Machine --- p.57 / Chapter 4.2 --- Communication Methods in Parallel Program --- p.65 / Chapter 4.3 --- Comparison of Parallel and Serial Program --- p.74 / Chapter 4.4 --- Measure of Parallel Program --- p.85 / Chapter 4.5 --- Analysis of Serial CPMC Program --- p.89 / Chapter 4.6 --- Analysis of the Parallel CPMC Program --- p.93 / Chapter 5 --- Results from CPMC --- p.101 / Chapter 5.1 --- Hubbard Model with Next Nearest Neighbour Hopping --- p.101 / Chapter 5.2 --- Spin and Charge Correlations --- p.104 / Chapter 5.3 --- Vertex Contributions --- p.111 / Chapter 6 --- Mean Field Calculations of bilayer --- p.119 / Chapter 6.1 --- Objective --- p.119 / Chapter 6.2 --- Theoretical Model --- p.121 / Chapter 6.3 --- Method of Solving the Model --- p.122 / Chapter 6.4 --- Finding Susceptibility by RPA --- p.131 / Chapter 6.5 --- Results --- p.136 / Chapter 6.6 --- Summary --- p.140 / Chapter 7 --- Summary --- p.141 / Bibliography --- p.142

To assure nuclear power sustainability, ongoing efforts on advanced closed-fuel cycle options and adapted open cycles have led to investigations of various strategies involving utilization of Transuranic (TRU) nuclides in nuclear reactors. Due to favorable performance characteristics, multiple studies are focused on transmutation options using High Temperature Gas-cooled Reactors (HTGRs). Prismatic HTGRs allow for 3-Dimensional (3D) fuel shuffling and prior shuffling algorithms were based on experimental block movement and/or manual block shuffle patterns. In this dissertation, a physics based 3D multi-directional reloading algorithm for prismatic deep burn very high temperature reactors (DB-VHTRs) was developed and tested to meet DB-VHTR operation constraints utilizing a high fidelity neutronics model developed for this dissertation. The high fidelity automated neutronics model allows design flexibility and metric tracking in spatial and temporal dimensions. Reduction of TRUs in DB-VHTRs utilizing full vectors of TRUs from light water reactor spent nuclear fuel has been demonstrated for both a single and two-fuel composition cores. Performance of the beginning-of-life and end-of-life (EOL) domains for multi-dimensional permutations were evaluated. Utilizing a two-fuel assembly permutation within the two-fuel system domain for a Single-Fuel vector, the developed shuffling algorithm for this dissertation has successfully been tested to meet performance objectives and operation constraints.

Thesis (Ph. D.)--University of Missouri-Columbia, 2003. / Typescript. Vita. Includes bibliographical references (leaves 87-88). Also available on the Internet.

Thesis (Ph. D.)--University of Missouri-Columbia, 2003. / Typescript. Vita. Includes bibliographical references (leaves 87-88). Also available on the Internet.

The aim of this project was to investigate the potential of fused-slurry coatings for improving the oxidation resistance of wrought alloys. Slurry-aluminised coatings were deposited on Alloy 800H (Fe-33Ni-20Cr), Alloy HCM12A (Fe-12Cr-2W), Alloy 214 (Ni-16Cr-4Al-3Fe), Fe-27Cr-4Al and Fe-14Cr-4Al alloys. The slurry contained a cellulose-based binder in an aqueous carrier and spherical aluminium powder, with a particle size below 20 microns. The slurries were applied with a paint-brush, dried in air and heat treated in either hydrogen or argon at temperatures between 700 and 1150C. The slurries were characterised by thermogravimetry, differential scanning calorimetry and viscometry. The coatings were characterised by optical microscopy, scanning electron microscopy, energy-dispersive X-ray analysis, X-ray diffraction and Vickers hardness measurements. The oxidation resistance of selected slurry-coated specimens was assessed in air at 1000 and 1100C in tests lasting up to 1000 hours.Slurry-aluminising was found to be a simple, effective way of forming protective coatings that were similar in composition and microstructure to chemical vapour deposits. However, it was difficult to control the amount of slurry applied to the substrate and produce coatings of uniform thickness.The coatings on Alloy HCM12A and the Fe-Cr-Al alloys contained cracks in the brittle FeAl phase due to tensile stresses arising from differences in the thermal expansion coefficients of the substrates and the coatings. Rapid interdiffusion between the coatings and the ferritic substrates resulted in the appearance of Kirkendall voids.Coatings on Alloy 214 required a two-stage heat treatment to convert the brittle δ-Ni2Al3 to β-NiAl. Cracking along the coating/substrate interface was prevented by limiting the coating thickness to a maximum of 250 microns. During oxidation at 1100C, the β-NiAl was converted to γ'-Ni3Al. After 1000 h, the centre of the coating consisted chiefly of γ'-Ni3Al and bands of austenite (γ-Ni) were present at the inner and outer edges of the coating. The aluminium content at the coated surface was higher than the original aluminium content of the alloy, the protective alumina scale was improved and the oxidation life of the substrate was extended. An additional life of 1250 h at 1100C is estimated from a slurry coating before the aluminium content returns to that of the original alloy (4%), providing a potential improvement in oxidation resistance.Microstructural changes such as grain growth, sensitisation and formation of aluminium nitride particles near the coating/substrate interface, were detected in the alloy substrates after forming the slurry coatings. However, these microstructural changes did not detract from the good performance of the coatings during oxidation tests at 1100C.The work in this study has demonstrated a low-cost method of coating high-temperature alloys providing coatings with microstructures, densities and modes of degradation similar to those obtained by other coating methods. The coatings are potentially applicable to a wide range of high-temperature substrates.

This dissertation presents new solutions for turbine engines in need of wireless temperature sensors at temperatures up to 1300oC. Two important goals have been achieved in this dissertation. First, a novel method for precisely characterizing the dielectric properties of high temperature ceramic materials at high temperatures is presented for microwave frequencies. This technique is based on a high-quality (Q)-factor dielectrically-loaded cavity resonator, which allows for accurate characterization of both dielectric constant and loss tangent of the material. The dielectric properties of Silicon Carbonitride (SiCN) and Silicoboron Carbonitride (SiBCN) ceramics, developed at UCF Advanced Materials Processing and Analysis Center (AMPC) are characterized from 25 to 1300oC. It is observed that the dielectric constant and loss tangent of SiCN and SiBCN materials increase monotonously with temperature. This temperature dependency provides the valuable basis for development of wireless passive temperature sensors for high-temperature applications. Second, wireless temperature sensors are designed based on the aforementioned hightemperature ceramic materials. The dielectric constant of high-temperature ceramics increases monotonically with temperature and as a result changes the resonant frequency of the resonator. Therefore, the temperature can be extracted by measuring the change of the resonant frequency of the resonator. In order for the resonator to operate wirelessly, antennas need to be included in the design. Three different types of sensors, corresponding to different antenna configurations, are designed and the prototypes are fabricated and tested. All of the sensors successfully perform at temperatures over 1000oC. These wireless passive sensor designs will significantly benefit turbine engines in need of sensors operating at harsh environments