Research in superconductivity is heading towards overcoming the limitations imposed by extreme conditions, and promising candidates in this pursuit are superconductors made from hydrides. Carbonaceous Sulfur Hydride (CSH) was reported in Nature 586, 373-377 (2020) as a room-temperature superconductor in the pressure range of 140-267 GPa; however, there is controversy in the literature regarding these results. Here, we use density functional theory to confirm the hypothesis of Nature 596, E9-E10 (2021) that a metallic path is the reason for the sharp drop in resistance interpreted in Nature 586, 373-377 (2020) as indicative of a weak type 2 superconductor. We find that the metallic behavior of CSH is dominated by sulfur p-orbitals, and not by metallization of hydrogen. If CSH would be a superconductor, the predicted Ginzburg Landau parameter would be 1356.9, reflecting an unusually strong type 2 superconductor and thus contradicting the interpretation of Nature 586, 373-377 (2020). The fact that we find no metallic states below 220 GPa casts doubts on the onset of superconductivity at 140 GPa reported in Nature 586, 373-377 (2020). Additionally, the small fraction of active hydrogen density of states at the Fermi level shows that CSH is not a high-temperature superconductor.

The objective of this research based on both the theory and experimentation was to prove that the total pressure difference is the primary driving force during the vacuum drying. The theoretical drying rates of diffusion, free water bulk flow and water vapor bulk flow were calculated and compared. The concept of equilibrium moisture content under the vacuum was developed. The theoretical maximum moisture content drop in one cycle was calculated using energy balance. The model was developed for the vacuum drying to understand the mechanism of the vacuum drying including the boiling front and its movement. To evaluate the effect of the sample size on the drying rate, four different thicknesses (1, 1.5, 2, 2.5 inches) and three different lengths (5, 10, 15 inches) were used. In the cyclic drying, the specimens were heated to the 60 C. The vacuum was pulled to about 18 mm Hg. The vacuum pump was kept running for 140 minutes. It was found that in cyclic vacuum drying, drying rate was not affected by the thickness. However, it was affected by the length. The cyclic drying curve consisted of two distinct parts. The fast drying period lasted about 10 to 20 minutes. The slow drying period occurred next when the pressure inside wood got close to the ambient pressure. In end grain vacuum drying, the specimens were coated with wax, wrapped in the plastic film and inserted into a rubber tube to prevent the moisture loss from the side surfaces during drying. The specimen size was 1×1×10 inches. Red oak and white oak were sealed and dried in both cyclic and continuous vacuum drying. The results showed that sealed specimens dried almost as fast as unsealed specimen. There was little moisture loss from the side surfaces. There was a moisture gradient along the length in both cyclic drying and continuous vacuum drying. Red oak specimens of 2.5×1.5×10 inches were used to study the boiling front in the vacuum drying. In order to detect the boiling phenomenon, the saturation pressures were calculated and were compared with the pressures at the same time and the same location. Boiling occurred during drying and the boiling front retreated to the center of wood as drying proceeded. The retreating speed depended on the heat supply and the permeability. Vacuum drying at room temperature was investigated. The specimens were dried at 20 C and pressure near 18 mm Hg. The results showed that wood can be vacuum dried at room temperature with little or no degrade at a reasonable drying rate. All experimental results support the objective of this study that the primary driving force is the total pressure difference. / Ph. D.

The present study focuses on the evolving microstructure of Mg alloy AZ31. The material is subjected to channel die compression at room temperature to simulate a reduction stage in the rolling process. Samples are annealed to provoke recovery, static recrystallization, and grain growth. Annealing is carried out at three temperatures for times ranging from 10s to 10,000s. The material’s response is exhibited through the use of data collection methods such as microhardness, optical microscopy, and electron backscatter diffraction (EBSD). Methodology behind experimentation and data collection techniques are documented in detail. Conclusions are made about the effects of the compression and annealing processes on the material’s microstructure. The Johnson-Mehl-Avrami-Kolmogorov (JMAK) model is introduced, and a simple recrystallization kinetics plot is attempted.

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Detection of explosive gas leaks such as hydrogen (H2) becomes key element in the wake of counter-terrorism threats, introduction of hydrogen powered vehicles and use of hydrogen as a fuel for space explorations. In recent years, a significant interest has developed on metal oxide nanostructured sensors for the detection of hydrogen gas. Gas sensors properties such as sensitivity, selectivity and response time can be enhanced by tailoring the size, the shape, the structure and the surface of the nanostructures. Sensor properties (sensitivity, selectivity and response time) are largely modulated by operating temperature of the device. Issues like instability of nanostructures at high temperature, risk of hydrogen explosion and high energy consumption are driving the research towards detection of hydrogen at low temperatures. At low temperatures adsorption of O2- species on the sensor surface instead of O- (since O- species reacts easily with hydrogen) result in need of higher activation energy for hydrogen and adsorbed species interaction. This makes hydrogen detection at room temperature a challenging task. Higher surface area to volume ratio (resulting higher reaction sites), enhanced electronic properties by varying size, shape and doping foreign impurities (by modulating space charge region) makes nanocrystalline materials ideal candidate for room temperature gas sensing applications. In the present work various morphologies of nanostructured tin oxide (SnO2) and indium (In) doped SnO2 and titanium oxide (titania, TiO2) were synthesized using sol-gel, hydrothermal, thermal evaporation techniques and successfully integrated with the micro-electromechanical devices H2 at ppm-level (as low as 100ppm) has been successfully detected at room temperature using the SnO2 nanoparticles, SnO2 (nanowires) and TiO2 (nanotubes) based MEMS sensors. While sensor based on indium doped tin oxide showed the highest sensitivity (S =Ra/Rg= 80000) and minimal response time (10sec.). Highly porous SnO2 nanoparticles thin film (synthesized using template assisted) showed response time of about 25 seconds and sensitivity 4. The one dimensional tin oxide nanostructures (nanowires) based sensor showed a sensitivity of 4 and response time of 20 sec. Effect of aspect ratio of the nanowires on diffusion of hydrogen molecules in the tin oxide nanowires, effect of catalyst adsorption on nanowire surface and corresponding effect on sensor properties has been studied in detail. Nanotubes of TiO2 prepared using hydrothermal synthesis showed a sensitivity 30 with response time as low as 20 seconds where as, TiO2 nanotubes synthesized using anodization showed poor sensitivity. The difference is mainly attributed to the issues related to integration of the anodized nanotubes with the MEMS devices. The effect of MEMS device architecture modulation, such as, finger spacing, number and length of fingers and electrode materials were studied. It has been found that faster sensor response (~ 10 sec) was observed for smaller finger spacing. A diffusion model is proposed for elucidating the effect of inter-electrode distance variation on conductance change of a nano-micro integrated hydrogen sensor for room temperature operation. Both theoretical and experimental results showed a faster response upon exposure to hydrogen when sensor electrode gap was smaller. Also, a linear increase in the sensor sensitivity from 500 to 80000 was observed on increasing the electrode spacing from 2 to 20 μm. The improvement in sensitivity is attributed to the higher reactive sites available for the gaseous species to react on the sensor surface. This phenomenon also correlated to surface adsorbed oxygen vacancies (O-) and the rate of change of surface adsorbed oxygen vacancies. This dissertation studied in detail dimensionality aspects of materials as well as device in detecting hydrogen at room temperature.

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