The influence of the maximum heat treatment temperature on the performance of thin reinforced multifilament Bi-2212 wires for cables

For high field applications and applications operated at 20-30 K round Bi-2212 wires are still very interesting. For accelerator magnets or the future generation of fusion magnets operation currents up to 10 kA or even several 10 kA are required. For this purpose cabling of BSCCO wires in a low AC loss arrangement is necessary. With Bi-2212 wires optimized on Jc and Tc cabling to 3x3 and 7x7 strands was the goal, in order to demonstrate the feasibility of a cable-in-conduit concept. To reach the necessary mechanical stability for a cabling process a AgMg (2 at% Mg) outer sheath was used with a reinforcement twice of usual material. However, this sheath affects strongly the reaction kinetics, which leads to a decrease of the critical current density in comparison to a pure silver sheath. 7, 19 and 37 filament wires were fabricated by the powder-in-tube (PIT) and partial melting process, treated in pure oxygen. The current carrying capacity of the Bi-2212 wires essentially depends on the maximum heat treatment temperature Tmax and the filament diameter. The best average Jc of 90 kA/cm2 (4.2 K, self-field) was reached in 37 filament wires with filament sizes of 29 μm, treated at the maximum temperature between 8790C and 8840C. We realized stranded conductors and report first results of their superconducting properties.