High T c composite silver/oxide superconductors
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High Tc Composite Silver/Oxide Superconductors P.A. CURRERI, P.N. PETERS, R.C. SISK, M.K. WU, and C.Y. HUANG A major issue concerning the application and understanding of high Tc superconductors is the low critical current densities, Jc, in the presence of magnetic fields. There is no doubt that the upper critical magnetic field of these oxides is more than adequate for use in high field. However, for the Cu-O-based systems, J~ decreases rapidly with a field applied perpendicular to the copper oxide planes. [~] This implies that highly oriented material is required for most applications. It is also reported that the critical current densities derived from the magnetization measurements of polycrystalline samples are 103 to 106 times higher than their transport critical P.A. CURRERI, Materials Scientist, P.N. PETERS, Physicist, and R.C. SISK, Physicist, are with the Space Science Laboratory, NASA Marshall Space Flight Center, Marshall Space Flight Center, AL 35812. M.K. WU, Professor, is with the Department of Applied Physics, School of Engineering and Applied Science, Columbia University, New York, NY 10027. C.Y. HUANG, Research Scientist, is with the Research and Development Division, Lockheed Missiles and Space Company, Inc., Palo Alto, CA 94304. Manuscript submitted February 23, 1989. METALLURGICAL TRANSACTIONS A
current densities. [2] These results suggest that these materials consist of regions of excellent superconductivity separated by extensive networks of weak links. Other experimental results, such as the evidence for intragrain Josephson junctions and the development of a tail in the resistive transition in the presence of even moderate applied fields, indicate that the phenomenology of the high T,. oxides is significantly different from that of the classical superconductors.t31 However, there are results confirming that high Jc's can be achieved (in zero field) in high-quality single-crystal films and highly textured bulk polycrystals. [4,5j More than a year ago, we explored the possibility of improving the critical current density in the bulk oxide superconducting material through the dispersion of fine metal oxide. Based on the discovery of a high Tc phase in Y-Sr-Cu-O (YSCO-111), [6] which was found to form only under a nonequilibrium condition, we have prepared the Y-123/AgO composites using a relatively hightemperature annealing condition. As a result of these studies, we discovered a novel effect in which a cooled superconductor can be suspended in a space below a permanent magnet, tT] indicating strong flux pinning in these materials. The suspension effect was first observed in samples of YBa2Cu3OT/AgO (Y-123/AgO) composites, tTi Magnetization measurements t8j of these samples show a much larger hysteresis, which corresponds to a large critical current density interpreted within the Bean critical state model. [9] These data also provide a quantitative explanation of the magnetic suspension. [8J We have also observed the same suspension effect in other RE-123/AGO composites, tl~ where RE are rare
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