Development of Superconducting Soldered Joints Between Bi-2212/Ag Wires

Bi-2212/Ag superconducting wires are being considered as key conductor candidates for the development of high-field magnets (up to 25 T), but it is recognized that a reliable joining process between these wires is a critical challenge for most high-field applications. This work focuses on the design and testing of jointing processes between multifilamentary Bi-2212 wires to make persistent mode joints using soldering techniques. The ability of several different superconducting solder alloys from the PbBi and SnInBi systems to remove and replace the Ag matrix in fully reacted Bi-2212 wires has been explored. The SnInBi lead-free solder was found to be relatively effective at removing the Ag matrix, but does not make good contact with the Bi-2212 filaments. PbBi solder wets the Bi-2212 filaments more effectively, but is slower at removing the Ag matrix. Therefore, a two-stage process has been developed using molten Sn to first remove the Ag matrix followed by PbBi to replace the nonsuperconducting Sn. Joints made using this two-stage process have higher critical currents (133 A in self-field at 4 K) than joints made using PbBi alone (120 A in self-field at 4 K).