The discovery of a new class of ceramic superconductors with transition temperatures above the boiling point of liquid nitrogen has opened the doors for several space applications. One important space application is the fabrication of an electrically conducting and thermally isolating link to replace manganin wires used in connecting IR detectors to data acquisition electronics on remote sensing platforms like SAFIRE and SIRTF. These NASA platforms designed to monitor the earth's atmosphere from space use infrared detectors which operate at liquid helium temperature (4.2K) for optimum performance. The SAFIRE mission employs hybrid dewars which combine both mechanical and cryogenic liquid cooling. The lifetime of such a mission is limited by the heat conducted through sensor array leads that connect the electronics (at approximately 80 K) to the sensors (at approximately 4 K). Currently these remoter sensing systems use manganin wires to connect the IR detectors to the data acquisition electronics. This link between a detector operating at 4 K and electronics operating at 80 K must be made of material that has high electrical conductivity and high thermal resistance. The YBCO superconductor with a transition temperature, Tc, of 93 K can achieve these conflicting requirements. A link with these characteristics will improve the thermal isolation of IR detectors and will increase the lifetime of the cryogen. The fabrication of an electrically conducting and thermally isolating link that replaces the manganin wires is an important application that will improve thermal isolation of IR detectors and will increase the lifetime of the cryogen. The link is made by screen printing superconducting lines on a low thermal conductivity ceramic substrate. Developing, modeling, and testing this high temperature superconducting link is a collaborative effort among NASA-Langley Research Center, Christopher Newport University (CNU), Clemson University and the industrial companies that have joined the Commercialization of Space Program for the purpose of developing and testing this link. CNU's effort in the development of this superconducting link included the following major efforts: (1) Development of a thermal conductivity measurement system for high temperature superconductors and ceramic materials which are potential candidates for use as substrates in this link. (2) Development of a mathematical model for the superconducting link that studies the effect of materials and geometry on the heat load and life time of missions. (3) Characterization of high Tc materials and assemblies made for space applications. Properties studied include humidity effects and aging effects on high Tc materials. This report summarizes the results of the research studies that were completed. Copies of publications detailing these findings are attached to this report.

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.

Materials exhibiting superconductivity above liquid nitrogen temperatures (77 K) will enable new applications of this phenomena. One of the first commercial applications of this technology will be superconducting magnets for medical imaging. However, a large number of aerospace applications of the high temperature superconducting materials have also been identified. These include magnetic suspension and balance of models in wind tunnels and resistanceless leads to anemometers. The development of superconducting wires fabricated from the ceramic materials is critical for these applications. The progress in application of a patented fiber process developed by Clemson University for the fabrication of superconducting wires is reviewed. The effect of particle size and heat treatment on the quality of materials is discussed. Recent advances made at Christopher Newport College in the development of micro-ohm resistance electrical contacts which are capable of carrying the highest reported direct current to this material is presented.

Newly developed technique involving use of gold makes possible to fabricate low-resistance contacts with rugged connections to high-Tc superconductors. Gold diffused into specimen of superconducting material by melting gold beads onto surface of specimen, making strong mechanical contacts. Shear strength of gold bead contacts greater than epoxy or silver paste. Practical use in high-current-carrying applications of new high-Tc materials, including superconducting magnets, long-wavelength sensors, electrical ground planes at low temperatures, and efficient transmission of power.

The development and testing of a ceramic superconducting link for an infrared detector is summarized. Areas of study included the materials used, the electrical contacts, radiation and temperature cycling effects, aging, thermal conductivity, and computer models of an ideal link. Materials' samples were processed in a tube furnace at temperatures of 840 C to 865 C for periods up to 17 days and transition temperatures and critical current densities were recorded. The project achieved better quality high superconducting transition temperature material through improved processing and also achieved high quality electrical contacts. Studies on effects of electron irradiation, temperature cycling, and aging on superconducting properties indicate that the materials will be suitable for space applications. Various presentations and publications on the study's results are reported.

We have extended our studies of a melting technique for making low-resistance contacts to high-temperature superconductors. We have made contacts to both YBa2Cu3O7−x and Bi2BaSr2Cu2O8, and to related superconducting compounds by melting gold or silver pads onto the samples before the final oxygen treatment. Scanning electron microscope studies show that both gold and silver do not diffuse far from the contact area. The surface contact resistivity of the best contacts made by the melting technique has an upper limit value in the 10−8 Ω cm2 range at 77 K. This contact resistivity shows no significant change in its value over a period of 17 months. Furthermore, an electron radiation dose of 5.7×1017 electron/cm2 only doubled the contact resistivity. This method of making low-resistance contacts to high-Tc materials can be integrated into the final oxygen treatment of many prospective superconducting elements or devices.

We have investigated the effect of 1-MeV electron irradiation up to a total dose of 5.7×1017 electrons/cm2 at room temperature on YBa2Cu3Ox with gold bead contacts made by the melting technique. We measured the superconducting transition temperature Tc, the critical current density Jc at 77 K, the normal-state resistivity, and the contact resistance for gold bead contacts as a function of fluence on the same samples without disturbing the contacts. Tc remained constant at 91 K, and Jc at 77 K remained constant around 90 A/cm2. The normal-state resistivity increased systematically by about 15% for the total dose. Finally, the surface contact resistance at 77 K remained less than 4.2 μΩ cm2 throughout the radiations. These studies took place over an 8-month period, and subsequent measurements indicate that the results are definitely due to radiation effects and not aging effects. Since the total dose represents 120 years of electron exposure in geosynchronous orbit, we conclude that the superconductor YBa2Cu3Ox with gold bead contacts would perform well in a space environment of electron irradiation.

At temperatures near ${T}_{c}$, experimental measurements of temperature-dependent ultrasonic attenuation at three frequencies exhibit maxima, and velocity measurements display softening of the lattice. These attenuation maxima result from a relaxation process which occurs around the superconducting transition, and the softening of velocity around ${T}_{c}$ may evidence a structural instability of Y${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ at these temperatures.

We have made rugged low-resistance contacts to the high Tc superconductor YBa2Cu3Ox by melting gold beads onto the surface of the material. After retreating the samples in oxygen, we have measured contact resistance <50 μΩ. This allowed a direct current of ≊5 A to pass through the contacts without heating while the sample remained in the superconducting state at 20 K. In this letter we present results of scanning electron microscopy, and measurements of contact resistance, critical current, and shear strength on these contacts. Such contacts will be of practical use in high current carrying applications of the new high Tc superconductors.

Samples of Nb3Sn were prepared by sintering with concentrations of iron impurity of up to 10 at.%. Resistive measurements of the superconducting transition temperature Tc showed a depression of Tc as iron was added at a rate of about 0.4 K (nominal at.% Fe)−1. This rate is much lower than that in some other carefully studied A15 systems. The actual amount of iron entering the A15 phase, the magnetic nature of the iron impurities and the compositional differences are discussed as explanations for the low rate of depression of Tc in Nb3Sn. The important point of this work is that for Nb3Sn produced by sintering, a process similar to the commercial method for producing Nb3Sn tapes, Tc is not as sensitive to iron impurities as would be expected. Lower grade starting materials could be used resulting in considerable cost savings.

The direct current comparator resistance bridge, Model 9975, manufactured by Guildline Instruments, Inc., which permits electrical resistance measurements up to eight digits, has been fully automated. Details of this automation are described in this paper. The performance of the automated bridge is demonstrated with applications to electrical resistivity studies on some metallic alloys.

The available data on the effect of neutron and α-particle irradiation on the superconducting properties and the low-temperature specific heat capacity of single- and polycrystalline V3Si were analyzed in a unified manner. It was found that the electron-phonon renormalization parameter λ does not vary simply proportionally to the bare density of electron states at the Fermi level as had been earlier conjectured by Dynes and Varma, nor does it vary in exactly the same manner as found by Fradin and Williamson to hold for a variety of vanadium-based A15 structure compounds and pseudobinary alloys. When the behavior of V3Si is compared with that of Nb3Sn, Nb3Al, and Nb3Ge, it is seen that the response to disorder of parameters of importance in the superconductivity of these compounds does not follow any simple, universal behavior.

Electrical resistivity (ρ) and low-field magnetization (σ) have been investigated as a function of temperature (T) on Dy-Sm alloys containing 5, 10, 20, 40, and 80 at.% Sm. Anomalies in the ρ vs T and σ vs T curve have been used to determine the magnetic transitions in this system. A magnetic phase diagram for the Dy-rich alloys has been proposed. X-ray studies have been employed to establish the structural features of this system.

We have measured the heat capacity of irradiated and non-irradiated Mo3Ge and Mo5Ge3 in the temperature range ≈ 1.6 to 10 K. An irradiation of 2.2 × 1019 neutrons cm-2 results in an increase in the superconducting transition temperature Tc from < 1.6 K in the non-irradiated state to ∼ 4 K after irradiation for Mo3Ge and a corresponding change from < 1.6 K to ∼ 3 K for Mo5Ge3. Our analysis shows that this change in Tc is not accompanied by a change in the electronic density of states (within experimental error) but rather a decrease in the Debye temperature from 392 to 322 K for Mo3Ge and 377 to 320 K for Mo5Ge3.

The normal-state resistivity of the $A15$ superconductors ${\mathrm{V}}_{3}$Si, ${\mathrm{Nb}}_{3}$Pt, and ${\mathrm{Nb}}_{3}$Al has been studied as a function of neutron damage. Resistivity data have been taken from the superconducting transition temperature ${T}_{c}$ to room temperature on unirradiated samples and irradiated samples with degraded ${T}_{c}$'s ranging down to 2-3 K. The ${\mathrm{V}}_{3}$Si data are the most extensive and both a single-crystal and polycrystalline samples have been studied. The ${\mathrm{Nb}}_{3}$Pt and ${\mathrm{Nb}}_{3}$Al data were taken for comparison. The data are fitted to several theoretical expressions put forth to explain the normal-state resistivity in $A15$ superconductors at low temperatures, high temperatures, and the full range of temperatures. The results are discussed in light of these theories. The most striking feature of the ${\mathrm{V}}_{3}$Si data is that there is no observable change in the shape of the temperature-dependent contribution to the resistivity down to a fractional degradation of ${T}_{c}$ of \ensuremath{\sim}0.5. This does not appear to be the case for the Nb-based $A15$ superconductors. It is suggested that this difference in behavior may be related to the different sensitivity of ${T}_{c}$ to disorder in ${\mathrm{V}}_{3}$Si as opposed to the Nb-based $A15$ superconductors.

The crystallization of glassy ${\mathrm{Fe}}_{84}{\mathrm{B}}_{16\ensuremath{-}x}{\mathrm{C}}_{x}$ ($x=0 \mathrm{to} 10$) has been studied with differential-scanning-calorimetry, electrical-resistivity, low-field-magnetization, M\"ossbauer-spectroscopy, and transmission-electron-microscopy measurements. A crystallization process is suggested by these data, and it is shown that all the data are at least consistent with or explained by this process. It is noted that the carbon addition does not merely produce a compositional change, but a structural change as well.

We have studied the crystallization and the effects of neutron irradiation and annealing on glassy Ti50Be40Zr10 (Metglas 2204) using resistivity measurements. The resistivity was measured from 2–1000 K for as-received Metglas 2204. Jumps in the resistivity were observed at the various stages during the crystallization process in agreement with previously reported results. Further, the negative temperature dependence of resistivity is affected by neutron irradiation and annealing. In both cases, interpretation of the results in terms of the Ziman theory of liquid metals indicates that the structure factor has sharpened. In the neutron irradiation case the structural relaxation is most likely the result of radiation enhanced diffusion due to the formation of vacancies.