Dissipation in high temperature superconducting tapes

In order to optimise the critical current density J_c of conductors based on high-temperature superconducting (HTSC) materials, we must understand the dissipation mechanisms dominating these systems under various conditions. We have used a pulsed current transport rig to investigate the critical current densities J_c's and dissipation in HTSC Ag-sheathed tapes incorporating the TI-1223 and BSCCO-2223 phases. We have also made magnetic studies of the critical current densities in these conductors using a vibrating sample magnetometer. We show how transport and magnetisation measurements complement each other in the identification and analysis of the dissipation mechanisms that limit J_c. Transport and magnetic data are presented on BSCCO-2223/Ag tapes prepared under identical conditions but with different superconducting core thickness. This analysis highlights both the non-uniform nature of current flow across the thickness of these conductors and the importance of the BSCCO/Ag interface in the formation of well-aligned single phase material necessary for high J_c's. Transport data are also presented on three Tl-1223/Ag tapes. Two were fabricated using in situ reaction and the third was prepared using a melt-processing technique. Although their low field J_c performance is limited by grain boundary 'weak-links', there is a plateau region in their J_c(B) characteristics at high fields. In this plateau region, the melt-processed tape shows a transport J_c that is enhanced significantly over that of the in situ processed tapes. We discuss possible explanations for this and address the nature of the current paths that survive in TI-based conductors at high magnetic fields. Encouragingly, magnetic studies of the in situ and melt-processed tapes show that the intragranular pinning is strong enough to suggest that any further improvements in tape microstructure will have a major impact on TI-1223 conductor technology. (author)Available from British Library Document Supply Centre-DSC:DXN027468 / BLDSC - British Library Document Supply CentreSIGLEGBUnited Kingdo