No description available.

<p> It is believed that Sr2RuO4 is a triplet superconductor that breaks time reversal symmetry, and it is expected to have spontaneous magnetization both at the sample edge, as well as at domain walls. Recent magnetic microscopy results place upper limits on the magnetic fields differing from previous theoretical calculations by 2 orders of magnitude. Using a Ginzburg-Landau formalism we investigate the effects of a rough surface as well as parameter choices which differ from the typical weak coupling parameters on the magnitudes of the spontaneous supercurrents and magnetic fields. The dependance on surface roughness is found to be small resulting in only a 20% reduction for the weak coupling parameters. Changing the parameters from weak coupling in addition to pair breaking surface effects is also found to affect the magnitudes of the spontaneous fields weakly, except in certain unphysical parameter regimes. The effects of the surface stabilizing another non-magnetic order parameter are considered, and give rise to field distributions with similar features to those present at domain walls.</p> / Thesis / Master of Science (MSc)

Infrared sensor satellites are used to monitor the conditions in the earth's upper atmosphere. In these systems, the electronic links connecting the cryogenically cooled infrared detectors to the significantly warmer amplification electronics act as thermal bridges and, consequently, the mission lifetimes of the satellites are limited due to cryogenic evaporation. High-temperature superconductor (HTS) materials have been proposed by researchers at the National Aeronautics and Space Administration Langley's Research Center (NASA-LaRC) as an alternative to the currently used manganin wires for electrical connection. The potential for using HTS films as thermal bridges has provided the motivation for the design and the analysis of a spaceflight experiment to evaluate the performance of this superconductive technology in the space environment The initial efforts were focused on the preliminary design of the experimental system which allows for the quantitative comparison of superconductive leads with manganin leads, and on the thermal conduction modeling of the proposed system (Lee, 1994). Most of the HTS materials were indicated to be potential replacements for the manganin wires. In the continuation of this multi-year research, the objectives of this study were to evaluate the sources of heat transfer on the thermal bridges that have been neglected in the preliminary conductive model and then to develop a methodology for the estimation of the thermal conductivities of the HTS thennal bridges in space. The Joule heating created by the electrical current through the manganin wires was incorporated as a volumetric heat source into the manganin conductive model. The radiative heat source on the HTS thermal bridges was determined by performing a separate radiant interchange analysis within a high-Tc superconductor housing area. Both heat sources indicated no significant contribution on the cryogenic heat load, which validates the results obtained in the preliminary conduction model. A methodology was presented for the estimation of the thermal conductivities of the individual HTS thermal bridge materials and the effective thermal conductivities of the composite HTS thermal bridges as functions of temperature. This methodology included a sensitivity analysis and the demonstration of the estimation procedure using simulated data with added random errors. The thermal conductivities could not be estimated as functions of temperature; thus the effective thermal conductivities of the HTS thermal bridges were analyzed as constants. / Master of Science

Infrared sensor satellites, which consist of cryogenic infrared sensor detectors, electrical instrumentation, and data acquisition systems, are used to monitor the conditions of the Earth's upper atmosphere in order to evaluate its present and future changes. Currently, the electrical instrumentation (connections), which act as thermal bridges between the cryogenic infrared sensor and the significantly warmer data acquisition unit of the sensor satellite system, constitute a significant portion of the heat load on the cryogen. As a part of extending the mission life of the sensor satellite system, the researchers at the National Aeronautics and Space Administration's Langley Research Center (NASA-LaRC) are evaluating the effectiveness of replacing the currently used manganin wires with high-temperature superconductive (HTS) materials as the electrical connections (thermal bridges). In conjunction with the study being conducted at NASALaRC, the proposed research is to design a space experiment to determine the thermal savings on a cryogenic subsystem when manganin leads are replaced by HTS leads -printed onto a substrate with a low thermal conductivity, and to determine the thermal conductivities of HTS materials. / Master of Science

Phase equilibria studies of two ternary systems, the vanadium rhodium-silicon system and the niobium-rhodium-silicon system, were made to obtain information about the formation of the Cr₃O-type structure in these systems. All samples had compositions in the . vicinity of the stoichiometric Cr₃O composition line and were prepared by powder metallurgy techniques. The compacts were then arc-melted, given a homogenization anneal, and examined by x-ray diffraction. Complete solid solubility of silicon for rhodium was found across the V-Rh-Si system in the Cr₃O-type structure. A large range of vanadium compositions for the formation of the Cr₃O-type structure in the V-Rh-Si system was also found. A minimum lattice parameter was found for samples closely approximating the stoichiometric Cr₃O composition in the V-Rh-si system. Either an increase or a decrease in vanadium composition from stoichiometry caused an increase in the lattice parameter of the Cr₃O-type structure formed. No solid solubility of silicon for rhodium was found in the niobium-rhodium-silicon system. Apparently the only Cr₃O-type structure which forms in the Nb-Rh-Si system is of' the stoichiometric composition Nb₃Rh. / Master of Science

With the recent discovery of High T_c ceramic superconducting materials, many potential applications which were considered impossible just a few years ago are now being realized. These new uses of superconductors will take place in electronics and energy systems where many different materials are used together. If these superconducting materials are to be used successfully, they must be characterized. Film quality was characterized by measuring the electrical properties under different conditions. Films were tested on alumina and magnesia substrates, to characterize the substrate interactions. Films were printed in different thicknesses and different widths, to determine the effect of film geometry on film quality. Finally these films were aged in a room ambient to determine the effect of atmospheric exposure. The results of these tests show that thick film Superconductors can be easily fabricated on magnesia substrates. Superconducting thick films can also be fabricated on alumina substrates, provided the film thickness is greater than 100μm and line width is greater than 50 mils. On either substrate material, critical currents of the films consistently decreased with thinner, narrower lines and environmental exposure. The maximum critical current on 2 magnesia substrates was 635 A/cm² for an unaged 77μm thick by 200 mil wide line, and the minimum was 345 A/cm² for a 25μm thick by 50 mil wide line aged for thirty days. Alumina substrates showed the same trend with a maximum of 75 A/cm² for an unaged 125μm thick by 200 mil wide line and a minimum of 47 A/cm² for a 100μm thick by 100 mil wide line aged for thirty days. / Master of Science

In 1993, the international trade groups predicted that the market for superconducting devices will grow 100-fold in the next three decades. ! Some of the growth areas for bulk superconductors are applications that rely on the ability of the superconductor to exclude magnetic fields or shape electromagnetic waves, for example, levitated vehicles, motors, and magnetic shields. Additional research, development, and manufacturing scale-up are needed to achieve full commercialization, especially with high temperature superconductors. Development of bulk superconducting applications relying on the Meissner effect or shielding capability are limited by the mechanical performance, processability, and environmental resistance of high temperature superconductors; however, these problems can be alleviated by making superconductor/polymer composites. One goal of this research is to process high temperature superconductor/polymer composites and to systematically characterize them. A second objective of this research 1s to determine if a superconductor/polymer composite can shield electromagnetic waves and to define the critical composite requirements for effective shielding. Three types of composite structures, 3-3, 0-3, and 2-3 superconductor/ polymer composites, were processed. The 3-3 composites have continuous ceramic and polymer phases. The 0-3 composites have ceramic particles in a polymer matrix, and the 2-3 composites have ceramic platelets in a polymer matrix. These composites were compared on the basis of mechanical performance, machinability, environmental resistance, diamagnetic strength, electrical resistance, and shielding effectiveness. If the superconductor/polymer 0-3 composites are processed well, they have improved mechanical performance, machinability, and environmental resistance compared to superconductor/polymer 3-3 composites; however, they do not have zero resistance nor provide any shielding capability, and they have a lower diamagnetic strength than superconductor/polymer 3-3 composites. The YBCO/polymer 3-3 composites have lower superconducting properties than 90% dense bulk YBCO, but they have improved machinability and mechanical performance. Unfortunately, the processability of superconductor/ polymer 3-3 composites is not improved compared to bulk superconductors. The superconductor/polymer 2-3 composites combine the formability of the 0-3 composites and the shielding effectiveness of 3-3 composites. This is the first time a discontinuous ceramic phase/polymer composite has demonstrated shielding capability. Superconducting shielding 1s determined to be a result of reflection, dependent on the Meissner effect, and absorption, dependent on the high conductivity, but it does not rely on zero sample resistivity. Based on these results, the critical composite material requirements for effective shielding are 1) that the superconducting phase be of sufficient size and critical current density to absorb electromagnetic waves and 2) that the nonnormal volumes of the superconducting phase overlap. Thus, a superconductor/polymer 0-3 composite can not provide any shielding, but a superconductor/polymer 2-3 composite shields at both low frequencies and microwave frequencies. / Ph. D.

A superconducting niobium triaxial cavity has been designed and fabricated to study residual surface resistance of planar superconducting materials. Unlike many other structures where the test samples are placed in strong magnetic field positions, the edge of a 25.4 mm or larger diameter sample in the triaxial cavity is located outside the strong field region. Therefore, the edge effects and possible losses between the thin film and the substrate have been minimized in this design, ensuring that the induced RF losses are intrinsic to the test material. The fundamental resonant frequency of the cavity is 1.5 GHz, the same as the working frequency of CEBAF cavities. The cavity has a compact size compared to its TE₀₁₁ counterpart, which makes it more sensitive to the sample's loss. / Ph. D.

Adaptive microwave systems can benefit from the use of low loss tunable microwave filters. Realizing these tunable filters that show low loss characteristics can be very challenging. The proper materials, tuning elements, and filter designs need to be considered when creating a low loss tunable filter. The integration of low loss microelectromechanical systems (MEMS) and superconducting circuits is one method of achieving these types of tunable filters. The thesis introduces new multi-layer low temperature superconducting (LTS) filters and diplexers and novel topologies for tunable filters and switched multiplexers. An efficient method of designing such filters is proposed. A fabrication process to monolithically integrate MEMS devices with high temperature superconducting (HTS) circuits is also investigated in this thesis. The reflected group delay method, usually used for filter tuning, is further developed for use in designing microwave filters. It is advantageous in the design of filters to have electromagnetic simulation results that will correlate well to the fabricated microwave filters. A correction factor is presented for use with the reflected group delay method so the group delay needs to be matched to the appropriate value at the center frequency of the filter and be symmetric about the center frequency of the filter. As demonstrated with an ideal lumped element filter, the group delay method can be implemented when a closed form expression for the circuit is not known. An 8-pole HTS filter design and an 8-pole multi-layer LTS filter design demonstrate the use of the reflected group delay method. Low temperature superconducting filters, couplers and diplexers are designed and fabricated using a multilayer niobium fabrication process traditionally used for superconducting digital microelectronics. The feasibility of realizing highly miniaturized microwave niobium devices allows for the integration of superconducting digital microelectronics circuits and analog microwave devices on a single chip. Microwave devices such as bandpass filters, lowpass filters, bandstop filters, quadrature hybrids, and resistive loads are all demonstrated experimentally. New tunable filter designs are presented that can make use of MEMS switches. A manifold-coupled switched multiplexer that allows for 2^N possible states is presented. The tunable multiplexer has N filters connected to two manifolds and has embedded switches, which detune certain resonators within the filters to switch between ON and OFF states for each channel. The new concept is demonstrated with a diplexer design and two 3-pole coplanar filters. The concept is further developed through test results of a fabricated HTS triplexer and electromagnetic simulations to demonstrate a superconducting manifold-coupled switched triplexer. Another filter design is presented that makes use of switches placed only on the resonators of the filters. This filter design has N possible states and the absolute bandwidth can be kept constant for all N states. Finally, the integration of HTS circuits and MEMS devices is investigated to realize low loss tunable microwave filters. The hybrid integration is first performed through the integration of an HTS microstrip filter and commercially available RF MEMS switches. A fabrication process to monolithically integrate MEMS devices and high temperature superconducting circuits is then investigated. The fabrication process includes a titanium tungsten layer, which acts as both a resistive layer and an adhesion for the dielectric layer, an amorphous silicon dielectric layer, a photoresist sacrificial layer, and the top gold layer. The fabrication process is built up on a wafer with a thin film of a high temperature superconducting material covered with a thin film of gold. Several processes are tested to ensure that the superconducting properties of the thin film are not affected during the MEMS fabrication process.

Adaptive microwave systems can benefit from the use of low loss tunable microwave filters. Realizing these tunable filters that show low loss characteristics can be very challenging. The proper materials, tuning elements, and filter designs need to be considered when creating a low loss tunable filter. The integration of low loss microelectromechanical systems (MEMS) and superconducting circuits is one method of achieving these types of tunable filters. The thesis introduces new multi-layer low temperature superconducting (LTS) filters and diplexers and novel topologies for tunable filters and switched multiplexers. An efficient method of designing such filters is proposed. A fabrication process to monolithically integrate MEMS devices with high temperature superconducting (HTS) circuits is also investigated in this thesis. The reflected group delay method, usually used for filter tuning, is further developed for use in designing microwave filters. It is advantageous in the design of filters to have electromagnetic simulation results that will correlate well to the fabricated microwave filters. A correction factor is presented for use with the reflected group delay method so the group delay needs to be matched to the appropriate value at the center frequency of the filter and be symmetric about the center frequency of the filter. As demonstrated with an ideal lumped element filter, the group delay method can be implemented when a closed form expression for the circuit is not known. An 8-pole HTS filter design and an 8-pole multi-layer LTS filter design demonstrate the use of the reflected group delay method. Low temperature superconducting filters, couplers and diplexers are designed and fabricated using a multilayer niobium fabrication process traditionally used for superconducting digital microelectronics. The feasibility of realizing highly miniaturized microwave niobium devices allows for the integration of superconducting digital microelectronics circuits and analog microwave devices on a single chip. Microwave devices such as bandpass filters, lowpass filters, bandstop filters, quadrature hybrids, and resistive loads are all demonstrated experimentally. New tunable filter designs are presented that can make use of MEMS switches. A manifold-coupled switched multiplexer that allows for 2^N possible states is presented. The tunable multiplexer has N filters connected to two manifolds and has embedded switches, which detune certain resonators within the filters to switch between ON and OFF states for each channel. The new concept is demonstrated with a diplexer design and two 3-pole coplanar filters. The concept is further developed through test results of a fabricated HTS triplexer and electromagnetic simulations to demonstrate a superconducting manifold-coupled switched triplexer. Another filter design is presented that makes use of switches placed only on the resonators of the filters. This filter design has N possible states and the absolute bandwidth can be kept constant for all N states. Finally, the integration of HTS circuits and MEMS devices is investigated to realize low loss tunable microwave filters. The hybrid integration is first performed through the integration of an HTS microstrip filter and commercially available RF MEMS switches. A fabrication process to monolithically integrate MEMS devices and high temperature superconducting circuits is then investigated. The fabrication process includes a titanium tungsten layer, which acts as both a resistive layer and an adhesion for the dielectric layer, an amorphous silicon dielectric layer, a photoresist sacrificial layer, and the top gold layer. The fabrication process is built up on a wafer with a thin film of a high temperature superconducting material covered with a thin film of gold. Several processes are tested to ensure that the superconducting properties of the thin film are not affected during the MEMS fabrication process.