Correlation between the normal state resistivity and the critical current density of Ag sheathed Bi(2223) tapes
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Abstract : The causes of bubbles that form when heat treating Ag-sheathed Bi2Sr2CaCu2O8 (2212) tapes and wires, and methods to eliminate them were studied. H2O and CO2, which form bubbles, are present as condensed phases in the precursor oxide powder, and O2 is released from 2212 as it melts. Infrared spectroscopy of gases that evolved from the tape during heating showed the H2O and CO2 content in the tape could be significantly reduced by heating the tape at 700 deg C in vacuum for 24h then at 835 deg C for 48h in 1 atm O2 before melt processing the tape. No bubbles formed after the H2O and CO2 were removed, and the tape was heated at ^1 deg C/m through the 2212 melting temperature. Overpressure processing (hydrostatic pressing with a mixture of Ar and O2) up to 10 atm total pressure could prevent bubbling but it did not remove H2O and CO2 from the tape. Overpressure processing could also densify the 2212 core by compressing the Ag-sheath tape during the heat treatment, eliminating the packing porosity present in the as-rolled green tape.
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Cu-stabilized 18-filament MgB2 tapes with NbZr buffer have been fabricated by using powder-in-tube technique. The phase composition and microstructure were examined by X-ray diffraction and optical microscopy. Transport critical current measurements were performed by a standard four-probe technique at different magnetic fields and temperature. The sample shows a high transport critical current density of 80,000A/cm2 (10K,0T) and 13,600A/cm2(10K,1T)
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Analysis of temperature behavior of the critical current in YBa2Cu3O7−δ films is presented. We demonstrate that the overall current can be separated into two components caused by weak pinning on oxygen vacancies in CuO2 planes and strong pinning on defects in volume of the superconductor. Temperature dependences of the components are obtained and discussed.
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Superconducting YBa 2 Cu 3 O 7 wires and tapes were fabricated by cold drawing, rolling and extrusion processes. It was found that the critical current density, after O 2 equilibration, correlates both with density and Cu 3+ concentration. Full density was achieved by using a special heat treatment, but the critical current density was low owing to the low Cu 3+ concentration present in this heavily twinned material. The best critical current density results were obtained for material with density of 92–95% of the theoretical value. The low critical current density of the porous specimens is attributed not only to a poor connectivity between grains but also to a low Cu 3+ concentration due to the instability of Cu 3+ at crystallite surfaces which increase in area with specimen porosity.
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By optimizing the tape fabrication process, high critical current densities over long length (up to 25 m) of Bi,Pb(2223) Ag-sheathed tapes were reproducibly obtained. The critical current densities of multifilamentary tapes (up to 55 filaments) are typically in the range of 25-30 kA/cm/sup 2/. At 77 K, up to 35 kA/cm/sup 2/ were recently measured for a monofilamentary tape of 0.5 m length. Detailed measurements of the transport properties of these tapes as well as of test pancakes and currents leads will be presented.
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Over the last decade, several approaches for the fabrication of high-current cables from second-generation high-temperature superconductor (HTS) REBCO tapes were investigated, among them Roebel cable, conductor on round core, and several twisted stacked tape cable types. We introduced a novel type of stacked HTS tape arrangement, which is called HTS CrossConductor or HTS CroCo, aiming for high engineering critical current densities j e that can be scaled to application-relevant lengths of conductors. Measurements of the critical current of a triple-HTS CroCo superstrand at T = 77 K and self-field conditions and at T = 4.2 K, as well as in fields up to B = 12 T, show the expected current calculated from the individual tape critical currents. Twisting experiments of the triple-HTS CroCo superstrand indicate a critical bending strain of around 0.6%. Based on these results, a Rutherford cable design for a cable with a critical current of 80 kA is suggested.
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Ag-sheathed Bi2Sr2CaCu2Ox round wires having 37 and 259 filaments, with filament diameters ranging from 11 to 100 μm, were fabricated by the oxide-powder-in-tube method. They were heat treated using step-solidification melt processing in air and pure O2. The transport critical current density was highest in wires processed in pure O2. It increased with decreasing filament diameter and was highest [165 000 A/cm2 (4.2 K, O T)] in wires with 11 μm filaments processed in O2.
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