<p> An investigation is reported on the oxidation properties of alpha-zirconium at 850°C and beta-zirconium at 950°C in oxygen for periods extending to 400 hr. and 100 hr. , respectively. Nitriding kinetics of zirconium in the range of 750° to 1000°C up to 200 hr. were investigated. The kinetics wen determined by volumetric and gravimetric techniques and may be represented by' a parabolic relationship after a period of more rapid oxidation. The uptakes of oxygen or nitrogen were consistent with the mathematical evaluations based on multi-phase diffusion models. Two diffusion models were advanced; one based upon differential and the other upon integral solutions of diffusion equations. It was possible to separate quantatively the oxygen or nitrogen partitions in the scale, alpha and beta phases of zirconium. </p> <p> The diffusivity of nitrogen in alpha-zirconium was determined by using transverse microhardness measurements. The diffusivity is: D = 0.15 exp (-54100/RT)cm/sec^2 for the temperature range of 750°- l000°C. </p> <p> The influence of oxygen-nitrogen atmospheres on the scaling rate of alpha-zirconium at 850°C was investigated. Small additions of either gas to the other increased the sealing rate. A definite breakaway point was observed in the scaling kinetics and the time interval to the transition point varied with the relative amounts of nitrogen to oxygen. </p> <p> Scaling rates of zirconium at 850° and 950°C in the oxygen-water vapor atmospheres initially obeyed to a good approximation a parabolic relationship which was followed by a much faster scaling rate. </p> / Thesis / Doctor of Philosophy (PhD)

The automobile industry has set the demand regarding Powder Metallurgy (PM) parts for decades, since this near-net shape technology is a cost-effective manufacturing process allying good mechanical properties with dimensional and geometrical precision. Aiming at the future of the electric automobiles high production and demand, many changes are on the way to guarantee the competitiveness of PM against other manufacturing process. The high costs of alloying elements such as Ni and Cu, the changes in health and safety regulations as well as light weighting of components are the topics of major importance in the field of PM and focus of main R&amp;D around the globe. The use of high temperature sintering and different alloying elements are possible solutions to overcome properties obtained by using Ni as an alloying element sintered at conventional temperatures. Materials with Cr, Mo and Si were investigated using high temperature sintering (1180°C and 1250°) in comparison to traditionally high Ni materials sintered at conventional temperature (1120°C). The dimensional stability, geometrical precision, density, and microstructure of ring-shaped specimens were studied by using a coordinate measuring machine (CMM) and the effect of HTS on the mechanical properties were estimated through the fraction of the load bearing section. The effect of HTS on the dimensional precision and geometrical stability was later investigated in real parts manufactured by industrial partners through an EPMA Club Project. The 4%Ni material sintered at 1120°C was also compared to Ni-less/Ni-free materials sintered at 1250°C using tensile testing, impact testing, and hardness. The use of HTS to improve the mechanical properties without impairing the dimensional and geometrical stability was confirmed in parts with both low and high complexity designs. This project sets the blueprint for future material developments using HTS as manufacturing process.

The purpose of this thesis is to examine the radiation from high-temperature superconducting mesas of Bi2Sr2CaCu2O8+ (BSCCO). This is motivated by the need for coherent sources of continuous wave terahertz (THz) emission capable of radiating practically in the THz frequency band. As BSCCO has been shown to be tunable from 0.5–2.4 THz (i.e., through the entire socalled terahertz gap centered about 1 THz), and has a higher peak operating temperature near 1 THz than most alternative sources, it is a good candidate for imaging and spectroscopy device applications [1]. When a static DC voltage is applied to a BSCCO mesa, the stack of Josephson junctions intrinsic to this type-II layered superconductor synchronously radiate. Adjustment of the bath temperature and applied voltage allows for the high degree of tunability observed for such an emitter [2]. To determine the angular dependence of radiation from BSCCO mesas, the dual source model from antenna theory is employed, and Love’s equivalence principle is used to simplify this framework. The total emission power obtained in this manner for the pie-shaped wedge is then fit to experimental results for a thin isosceles triangular mesa using the method of least squares, resulting in a standard deviation of = 0:4657. Additionally, symmetry is shown to play a significant role in the emissions for the transverse magnetic (TM) cavity modes of the equilateral triangular mesa. When the full group symmetry is imposed, the density of allowed modes is heavily diminished, and the original first excited even mode becomes the C3v symmetric ground state. These results for the equilateral triangle suggest, along with earlier experiments on the regular pentagonal mesa [3], that symmetry breaking effects can be used for purposes of tuning the characteristic frequency and angular dependence of the power radiated from BSCCO mesas with a high degree of symmetry.

Demand for capable and reliable semiconductor and fabrication technology for high temperature and power electronics applications has been increasing in recent years. Silicon Carbide (SiC), as a wide bandgap compound semiconductor, demonstrates superior characteristics such as high thermal conductivity, high breakdown voltage, and long-lasting reliable operation at elevated temperature. SiC-based circuits and systems are capable to offer significant performance enhancements to various applications. Integrated power management units and conversion modules in HEVs, integrated sensors for aircraft engines, development of small-sized portable power generators are among many applications that require reliable circuits with long-lasting functional lifetime. Nevertheless, there are numerous challenges associated with the design and fabrication of SiC-based circuits. The aim of this research is to practically design and implement novel operational amplifiers (opamps) based on Vertical Channel 4H-SiC JFET (SiC JFET) that can be utilized as sub-circuits of integrated SiC JFET-based circuits and systems. Recently, SiC power JFET-based power management units were developed that deploy non-SiC JFET-based circuits for analog signal processing, driving, and control, because all SiC JFET-based circuits were not available for full integration. However, utilizing SiC JFET for analog design (in order to close the mentioned gap) exhibits significant design challenges, even at room temperature. These fundamental challenges are low intrinsic gain, the requirement to limit the gate to source voltage range, and restrictions on utilizing channel length as a design parameter due to fabrication complexity. These challenges must be successfully overcome at room temperature, before moving towards high temperature SiC JFET-based analog design. The main objective of this dissertation is to establish a design base, overcome the challenges, demonstrate the feasibility, and present all SiC JFET-based opamps that are designed for gain, CMRR, and overall performance. Before attempting to design, both Enhancement and Depletion Mode SiC JFETs are characterized, analyzed, and modeled for simulation. Unique and reliable opamp configurations are presented that take design requirements into account, use threshold voltage instead of channel length as a design parameter, and employ gain enhancement techniques while obtaining maximum possible bandwidth. The final opamps are fabricated and tested and the results show that the objective is accomplished.

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High-temperature wet oxidation resistance of Type 310S stainless steel arises from the formation of a thin, adherent and compact external Cr2O3 scale that is more protective compared with the other oxides. Stability of the Cr2O3 scale is often at risk in the presence of water vapour at higher temperatures, which promotes the loss of oxidized Cr to volatilization. Continuous volatilization of the Cr2O3 scale accelerates the rate of oxidation and increases the risk of non-protective Fe-rich oxide formation that immensely contributes to the oxide thickening (breakaway oxidation). In this study, the possibility of surrogating high-pressure supercritical water with an ambient pressure air-10% H2O mixture is studied at temperatures associated with the predicted coolant outlet conditions in the current GEN IV design concepts. Factors influencing structure and composition of the Cr2O3 scale during wet oxidation are then examined in the wet environments. An increase in the total gas pressure, water vapour partial pressure and temperature is shown to accelerate the Fe-rich oxide formation by increasing the rate of oxidized Cr loss. A more complete physical description of the oxidation kinetics in terms of the evolution of the oxide scale structure and composition at the various exposure conditions is also reported. Presence of small amount of Mn in the alloy is shown herein to be beneficial as it assists the formation of a MnCr2O4 layer on top of the Cr2O3 scale, which serves to reduce the volatilization rate. It is shown however that the MnCr2O4 layer itself is only temporary protective and becomes prone to volatilization (loss of oxidized Cr) at relatively high temperatures, pressures and exposure times. The formation of a MnCr2O4 cap is therefore, only a temporary solution for delaying the onset of accelerated Fe-rich oxide formation. Addition of Si is proposed to be a more promising way of controlling the onset of the Fe-rich oxide formation. Increase in the Si content to ~6 wt.% results in the formation of a continuous SiO2 barrier layer under the Cr2O3 scale as well as Cr-rich silicide intermetallic phases in the starting microstructure that serve as effective Cr reservoirs in helping to maintain the structure and composition of the compact protective Cr2O3 scale despite the continued loss of oxidized Cr to volatilization. / Thesis / Doctor of Philosophy (PhD) / The effects of total gas pressure, water vapour partial pressure, temperature and minor alloying elements (Mn and Si in particular) on the wet oxidation performance of Type 310S stainless steel were examined within the context of the stability of the protective chromia (Cr2O3) scale formed. Focus was placed on examining factors that influence the onset of accelerated linear oxidation associated with the Fe-rich oxide formation and oxide thickening. Links between changes in structure and composition of the protective Cr2O3 scale and breakaway oxidation as exhibited in water vapour-containing environments are made using advanced electron microscopy techniques. Si addition up to ~6 wt.% assists the formation of a continuous silica (SiO2) barrier layer under the Cr2O3 scale as well as Cr-rich silicide intermetallic phases that act as reservoirs to supply for the oxidized Cr lost to volatilization. Si addition is proposed to be a promising way of prohibiting Fe-rich oxide formation albeit modifications to the morphology of the silicide intermetallic phases are required.