An aging study was performed to determine the stability of YBa sub 2 Cu sub 3 O sub 7 - x ceramics in humid environments at 20 degrees C. In this study, fired ceramic specimens were exposed to humidity levels ranging from 30.5 to 100 percent for 2-, 4-, and 6-week time intervals. After storage under these conditions, the specimens were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and electrical resistance measurements. At every storage condition evaluated, the fired ceramics were found to interact with H sub 2 O present in the surrounding environment, resulting in the decomposition of the YBa sub 2 Cu sub 3 O sub 7 - x phase. XRD data showed that BaCO sub 3, CuO, and Y sub 2 BaCuO sub 5 were present after aging and that the peak intensities of these impurity phases increased both with increasing humidity level and with increasing time of exposure. Additionally, SEM analyses of the ceramic microstructures after aging revealed the development of needle-like crystallites along the surface of the test specimens after aging. Furthermore, the superconducting transition temperature T sub c was found to decrease both with increasing humidity level and with increasing time of exposure. All the specimens aged at 30.5, 66, and 81 percent relative humidity exhibited superconducting transitions above 80 K, although these values were reduced by the exposure to the test conditions. Conversely, the specimens stored in direct contact with water (100 percent relative humidity) exhibited no superconducting transitions.

The Materials In Devices As Superconductors (MIDAS) spaceflight experiment is a NASA payload which launched in September 1996 on the Shuttle Atlantis (STS-79), and was transferred to the Mir Space Station for several months of operation. MIDAS was developed and built at NASA Langley Research Center (LaRC). The primary objective of the experiment was to determine the effects of microgravity and spaceflight on the electrical properties of high-temperature superconductive (HTS) materials. Cooling was provided by a tactical cryocooler, which maintained the specimens at or below 80 K. The superconductive specimens and the coldfinger of the cryocooler were mounted in a vacuum chamber. The entire experiment was mounted for operation in a stowage locker inside Mir. Three separate cycles of the experiment were performed autonomously after circuit breaker activation by the astronaut. Issues discussed include some of the experiment historical background, such as the different spacecraft that were to be the carrier at different times, the selection of components such as the cryocooler and ion pump, and the entire development through design, testing and flight. Some of the many challenges faced by project personnel were maintaining the HTS samples at cryogenic temperatures and in a vacuum, preparation and bonding of the samples, meeting the mass and volume limits imposed by the Shuttle and Mir, and performing all necessary testing to meet required performance standards.

Screen-printed thick films of the YBa2Cu3O7-x high-temperature superconductor were integrated into hybrid microelectronics circuits and characterized on orbit as part of the Materials In Devices As Superconductors (MIDAS) space-flight experiment. The experiment operated autonomously for 90 days on the Mir space station and acquired electrical data on the superconductive films at temperatures ranging from 75 to 250 K. This report describes the on-orbit critical transition temperature, Tc, and critical current density, Jc, performance of the YBa2Cu3O7-x thick films and compares the flight data with those obtained during pre- and post-flight testing. The results of this investigation show no significant difference between the space-flight and ground data, indicating that no degradation occurred in the superconductive films due to either vibrational loads experienced during launch or continuous operation in a microgravity environment.

Pyrotechnics accomplish many functions on today''s spacecraft, possessing minimum volume/weight, providing instantaneous operation on demand, and requiring little input energy. However, functional shock, safety, and overall system cost issues, combined with emergence and availability of new technologies question their continued use of space missions. Upon request from the National Aeronautics and Space Administration''s (NASA) Program Management Council (PMC), Langley Research Center (LaRC) conducted a survey to identify and evaluate state-of-the-art nonexplosively actuated (NEA) alternatives to pyrotechnics, identify NEA devices planned for NASA use, and investigate potential interagency cooperative efforts. In this study, over 135 organizations were contacted, including NASA field centers, Department of Defence (DOD) and other government laboratories, universities, and American and European industrial sources resulting in further detailed discussions with over half, and 18 face-to-face briefings. Unlike their single use pyrotechnic predecessors, NEA mechanisms are typically reusable or refurbishable, allowing flight of actual tested units. NEAs surveyed include spool-based devices, thermal knife, Fast Acting Shockless Separation Nut (FASSN), paraffin actuators, and shape memory alloy (SMA) devices (e.g., Frangibolt). The electro-mechanical spool, paraffin actuator and thermal knife are mature, flight proven technologies, while SMA devices have a limited flight history. There is a relationship between shock, input energy requirements, and mechanism functioning rate. Some devices (e.g., Frangibolt and spool based mechanisms) produce significant levels of functional shock. Paraffin, thermal knife, and SMA devices can provide gentle, shock-free release but cannot perform critically timed, simultaneous functions. The FASSN flywheel-nut release device possesses significant potential for reducing functional shock while activating nearly instantaneously. Specific study recommendations include: (1) development of NEA standards, specifically in areas of material characterization, functioning rates, and test methods; (2) a systems level approach to assure successful NEA technology application; and (3) further investigations into user needs, along with industry/government system-level real spacecraft cost-benefit trade studies to determine NEA application foci and performance requirements. Additional survey observations reveal an industry and government desire to establish partnerships to investigate remaining unknowns and formulate NEA standards, specifically those driven by SMAs. Finally, there is increased interest and need to investigate alternative devices for such functions as stage/shroud separation and high pressure valving. This paper summarizes results of the NASA-LaRC survey of pyrotechnic alternatives. State-of-the-art devices with their associated weight and cost savings are presented. Additionally, a comparison of functional shock characteristi

This report describes the optimization of the firing process used in the production of YBa/sub 2/Cu/sub 3/O/sub 7-x/ thick films screen printed on yttria-stabilized zirconia substrates. The highest critical current density (J/sub c/) values were obtained by employing a double layer printing technique in which a single superconductive layer was printed onto a zirconia substrate and fired, followed by the subsequent deposition and firing of second superconductive layer. Using this procedure, thick film superconductors with a superconductive transition temperature (T/sub c/) of 85 K and a J/sub c/ of 130 A/cm/sup 2/ were obtained by sintering the printed films at 950/spl deg/C for 90 minutes, followed by a six hour oxygen annealing treatment at 600/spl deg/C. Specimens sintered for comparable periods of time at 940 and 960/spl deg/C did not exhibit superconductive behavior above 77 K due to either incomplete microstructural development or thermal decomposition of the superconductive phase respectively.< >

Several applications of high temperature superconductor technology have been identified for the National Aeronautics and Space Administration's (NASA) aerospace systems. However, validation of critical superconductive properties in the space environment is necessary before this technology can be inserted into satellite systems. Researchers at NASA's Langley Research Center have designed the Materials In Devices As Superconductors (MIDAS) experiment to evaluate the electrical characteristics of high temperature superconductive materials during extended spaceflight. The MIDAS experiment will evaluate four superconductive test circuits over a temperature range of 300 to 75 K. The MIDAS test circuit is produced by thick film printing and combines both superconductive and conventional electronics into a single, active microelectronics package designed to operate at cryogenic temperatures. All electrical measurements are performed directly on the test circuit, eliminating the need for intricate wiring and reducing thermal losses. This paper describes the design, fabrication, and testing of the primary subsystems of the MIDAS instrument.< >

An investment casting process has been developed to produce net-shape, superconducting ceramics. In this work, a factorial experiment was performed to determine the critical process parameters for producing cast YBa2Cu3O7 ceramics with optimum properties. An analysis of variance procedure indicated that the key variables in casting superconductive ceramics are the particle size distribution and sintering temperature. Additionally, the interactions between the sintering temperature and the other process parameters (e.g., particle size distribution and the use of silver dopants) were also found to influence the density, porosity, and critical current density of the fired ceramics.

Several space-borne infrared detectors require cryogenic temperatures for successful operation. As a result, mission durations are substantially limited due to cryogen evaporation. The electrical leads connecting the detectors to the amplification electronics comprise a significant portion of the heat load on the dewar (i.e., 20% for some systems). Currently, manganin wires are used for these connections, due to the alloy's low thermal conductivity at cryogenic temperatures. However, replacement of these leads within high Tc materials would result in a substantial reduction in thermal loss, translating into approximately 10–15% enhancement in mission lifetime. The potential for using the high-Tc materials as thermal bridges to replace the manganin connections is currently under investigation at NASA-LaRC.

A ceramic casting process based on the fundamentals of slip casting has been developed to form high Tc superconductive ceramics. In this process, YBa2Cu3O7−x powders (<20 μm) dispersed in acetone are cast into foundry molds prepared by a lost wax process. After casting, the mold is peeled away from the superconductor, yielding a ceramic monolith in the shape of the mold. This work describes the casting process, as well as the result of a preliminary investigation into the use of magnetic fields to orient the YBa2Cu3O7−x grains during the forming process.

High temperature superconductive materials can have significant impact in several space-based applications due to the improved electrical, magnetic, or thermal properties of the superconductive devices over existing components. However, for high temperature superconductors to be successfully applied in space-borne systems, several environmental considerations associated with spaceflight must first be addressed. The environmental factors encountered during spaceflight missions will be discussed, and a review of studies addressing the effects of these factors on the performance of the superconductive devices will be provided.