Improvement of Critical Current in Superconducting Joint Between Multi-Filamentary Bi2223 Tapes

The superconducting joint between Bi2223 tapes has been developed with a method of joint by incongruent melting (JIM) in our Laboratory. The multi-junction method used to increase the contact area yielded a demonstrated increase in critical current. In previous studies, a simple boundary shape with lap-joint was chosen to connect several filaments in the tape, and this method is not suitable for connecting all about 100 filaments. Of course, more filaments are necessary to be joined with superconducting boundary, in order to increase the critical current of joint. However, due to the thin tape structure, there is difficult to use of butt joint between two cross-sections of tapes. In this study, we proposed a multi-junction method by using multiple boundaries with stepped structure. In experiments, joint samples with increasing of junction number to three from one were prepared, and the critical current for each unction number were measured at 77 K (self-magnetic field). In result, the critical current is increased with increasing of the junction number, and a highest critical current of 51.8 A was obtained in three-junction sample. in particular, the critical current at first junction (end of tapes) is highest, indicating that critical current at the end is important in total critical current of joint. Therefore, single junction at end of tapes was prepared and evaluated by using X-ray diffraction and critical current measurement. From X-ray diffraction patterns, the joint area is estimated to be about 1 cm. The critical current was 36.5 A at 77 K.