An MgB₂ Superconducting Joint with its Own Heat-Treatment Schedule

We suggested an MgB₂ joint process with its own heat-treatment schedule to apply it for our 1.5-T MgB₂ "finger" MRI magnet. In fabricating the MgB₂ magnet, the optimal heat-treatment schedule to attain a reproducible and high critical current is different in a joint and a coil. To solve this problem, we introduced an additional heating system, which is composed of a cartridge heater and a thermocouple connected with a copper block, into a box-type furnace. Then, we carried out heat-treatments with exclusively increasing the joint-part temperature above the Mg melting point of 645 °C-the joint was actually heated up to 700 °C. We evaluated a critical current and a crystal structure of the obtained MgB₂ joint. From experimental results, we found that the joint heated with the own heat-treatment schedule, which is 700 °C for 1 h + 600 °C for 11 h, showed a good I _c of over 450 A at 15K under self-field. The joint resistance was estimated by the coil operation for 18 days, and it was expected to be less than 10^-12 Ω.