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Research area 1 Superconductivity and superconductors

Research area 1 Superconductivity and superconductors

Research area 1 Superconductivity and

Reports from Research areas Research area 1 Co-ordinators: Prof. H. Eschrig Prof. J. Fink Prof. L. Schultz Superconductivity and superconductors The basic research in the area of high- T c superconductors was focused on theoretical and experimental electronic structure studies and on investigations of the influence of the grain boundary network on the critical currents in these materials. In the field of the transition metal borocarbides, most of the activities were related to the coexistence of magnetism and superconductivity. The analytical tools and the possibilities to prepare new compounds, single crystals and thin films of interesting new superconductors have been improved. Technology development were continued successfully in the field of high-T c superconductors. In particular, using the Rolling Assisted Biaxially Textured Substrated (RABITS) method superconducting tapes with exceptional high current densities and increased mechanical strength could be achieved on new substrated developed in the IFW Dresden. Using the powder in tube technique the overall critical current density of high-T c wires for cables and transformer could be improved. Finally, by chemical replacement or introduction of impurities in superconducting permanent magnet materials new record values for magnetic fields could be obtained. The use of these materials for friction-free superconducting bearings, for the development of a pump for liquid gases and the construction of a reluctance motor has been pushed forward. Torsten Fahr, Claus Fischer, Volker Haas, Wolfgang Häßler, Bernhard Holzapfel, Christian Rodig, Margitta Schubert, Hans-Peter Trinks Fig.: Critical current density j c of a 55 filament tape in dependence on temperature and O 2 partial pressure of the second annealing step Funded by: BMBF/Siemens AG Superconducting Bi 2 Sr 2 Ca 2 Cu 3 O x /Ag-tapes For improving the application possibilities of Bi 2 Sr 2 Ca 2 Cu 3 O x /Ag-tapes in cables or transformers it is necessary to improve the overall critical current density and to prepare special tapes with low AC-losses. For enhancing of the overall critical current j e density the filling factor of the tapes was increased from 25% to 34% and the thermo-mechanical treatment (TMT) process was optimized. Due to the great number of parameters of the TMTprocess (at least temperature, oxygen partial pressure and dwell time of three different treatment steps if the temperature ramps and the parameters of the intermediate rolling procedures are kept constant) the application of the method of design of experiments (DOS) can be very useful. The number of experiments can be reduced and the method gives additional informations about the process , especially the statistical significance of the process parameters and informations about interactions between different parameters. A response surface describing the dependence of the critical current density on the process parameters can be calculated (Fig.). As the result of these investigations j e could be enhanced to 10 kAcm -2 (I c =90A). On twisted tapes with a twist pitch of 10mm without additional ceramic barriers a reduced AC-loss of 0,6 mW/Am at a j e of 5 kAcm -2 was measured. In situ neutron and x-ray diffraction experiments on real tapes were carried out to investigate the reaction formation of the 2223 phase and the changes in the secondary phase assemblage under real reaction conditions. A phase formation mechanism with heterogeneous nucleation and growth of 2223 and 2212 template grains beginning at the Ag/superconductor interface is proposed. Precursor powders with different phase compositions have been prepared to investigate the influence of the precursor phase composition on the critical current. Cooperation: Siemens AG Erlangen, Vacuumschmelze Hanau, FZ Karlsruhe, Universität Erlangen, Nexans Superconductors GmbH Huerth, Universität Graz 53

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