R. Caton

Rugged low‐resistance contacts to YBa2Cu3Ox

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.

Low‐resistance noble metal contacts to high‐temperature superconductors

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.

High Tc leads for remote sensing applications

Abstract Several NASA programmes designed to monitor the earths atmosphere from space utilize infrared detectors which operate at or below 4.2 K for optimum performance. At present, the detectors are maintained at cryogenic temperatures by a stored volume of liquid helium. These detectors must be electrically linked to amplification electronics and data storage instruments maintained at 80 K. The electrical connections over the temperature gradient account for ≈20% of the total heat load on the Dewar for some systems, accelerating the boil-off of liquid helium cryogen and reducing the operational lifetime of the space-borne instruments. The recent discovery of high temperature superconductors has provided an opportunity to develop electrically conductive, thermally insulating links to bridge this thermal gradient. This paper describes the modelling of the thermal transport properties of thick film, high T c electrical bridges across a 4.2–80 K temperature gradient and the impact of such devices on a space-borne remote sensing system.

High-Tc thermal bridges for space-borne cryogenic infrared detectors

Abstract 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 alloys 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.

Environmental Testing of High Tc Superconductive Thermal Isolators for Space-Borne Cryogenic Detector Systems

Thick films of superconductive material on low thermal conductivity substrates (e.g., yttria-stabilized zirconia and fused silica) are considered as a replacement for the existing electrical connections between the detector array and data acquisition and storage electronics in the cryogenic detector systems being developed by NASA. The paper describes some of the design constraints on the superconducting device and presents results of a preliminary analysis of the effects of vibration, gamma irradiation, and long-term exposure to high vacuum and liquid nitrogen encountered in operating such a device in space.

The effect of electron irradiation on YBa2Cu3Ox with gold bead contacts

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 YBa2C...

The effect of electron irradiation on YBa sub 2 Cu sub 3 O sub x with gold bead contacts

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 YBa2C...