High Temperature Ultrasonic Characterization Of Ag Clad Superconductor Tapes

Prereacted Pb-doped Bi-Sr-Ca-Cu-0 powders were packed into Ag tubes, drawn, rolled, and given intermediate heat treatment to prepare long lengths of Ag-clad Bi-2223 superconductor tapes. With the use of improved process conditions, transport critical current density (J{sub c}) values exceeding 105 A/cm2 at 4.2 K and 27 K, and greater than 4 x 104 A/cm2 at 77 K, were obtained in zero applied field with short tape samples. Detailed microstructural analysis and J{sub c} measurements in applied fields up to 20 T are reported. Rolled tapes were cut into lengths of up to 10 m, and several such tapes were used in parallel to fabricate small superconducting pancake coils by the ''wind-and-react'' technique. Recently, we fabricated a test magnet with six pancake coils, each containing three 10-m lengths of rolled tapes and tested at 4.2, 27, and 77 K as a function of applied fields up to 14.5 T.A maximum generated field of 1.25 T was measured in zero applied field at 4.2 T. Detailed measurements made on the pancake coils and test magnets are reported in this paper.

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The powder-in-tube (PIT) technique was used to fabricate multifilament (Bi, Pb)2Sr2Ca2Cu3O{sub y} (Bi-2223) superconducting tapes. Transport current properties of these tapes were enhanced by increasing the packing density of the precursor powder and improving the mechanical deformation condition. A critical current (I{sub c}) of> 35 A in long lengths (> 200 m) tapes has been achieved. In measuring the dependence of critical current density on magnetic field and temperature for the optimally processed tapes, we found a J{sub c} of> 104 A/cm2 at 20 K in magnetic fields up to 3 T and parallel to the c-axis, which is of interest for use in refrigerator-cooled magnets. I{sub c} declined exponentially when an external field was applied perpendicular to the tape surface at 77 K. Mechanical stability was tested for tapes sheathed with pure Ag and Ag-Mg alloy. Tapes made with pure Ag sheathing can withstand a tensile stress of (almost equal to)20 MPa with no detrimental effect on I{sub c} values. Mechanical performance was improved by using Ag-Mg alloy sheathing: values of transport critical current began to decrease at the tensile stress of (almost equal to) 100 MPa. Transport current measurements on tapes wound on a mandrel of 3.81 cm (1.5 in.) diameter at 30° to the longitudinal axis, showed a reduction of (almost equal to) 10% in I{sub c} values for pure Ag-sheathed tapes and 5% reduction in I{sub c} values for Ag-Mg sheathed tapes, compared with the I{sub c} values of as-coiled tapes.

The phenomena of superconductivity has enormous industrial potential, but the penetration of superconductor-based products has, thus far, been slow. An indisputable fact is that there are prizes, and profits, for the teams that solve the critical problems in understanding and processing these complex materials. The continuous processing of superconducting tapes and wires is a necessary stage that will open up the industrial market to the designers of a diverse range of time and energy saving devices, particularly in ultra-fast computing and lossless power transmission. The aim of the research reported herein was to investigate the fabrication of superconducting tapes and films via two different processes, namely: the established PIT technique, and a less well known electrochemical route. The PIT research yielded partially textured (a characteristic not found in previously reported thallium-based PIT tapes) and high purity tapes with promising transport properties. The main subject of this work is the electrodeposition of superconducting films with particular emphasis on the optimisation of the deposition process, as opposed to the optimisation of the post-deposition heat treatment. This is not a well researched area. Therefore the research herein deals with the mechanisms of the deposition process in order to achieve a high standard of reproducibility in producing a variety of superconductor precursor films. The heat-treatment of the as-deposited films was determined, and results are presented concerning the characteristics of the resulting superconducting films. An attempt was made to fabricate, for the first time via electrodeposition, (Tl, Pb)-1223 films. Superconductivity was successfully observed in both Bismuth- and Thallium-based films. Extensive further work is required, but the impact upon society from superconducting devices draws ever nearer.