Ag-sheathed Bi2223 multi-filamentary tapes have been extensively used in various practical applications, such as power cables, superconducting magnets and current leads, in these 15 years. High Tc above 110 K, large electromagnetic anisotropy substantially originated in the thick blocking layer, large plate-like crystals with wide ab-plane and easy cleavage along the ab-plane are well-known characteristic features of Bi2223 superconductors. Since the large electromagnetic anisotropy decreases effective pinning volume and changes vortex state being almost two-dimensional, flux pinning properties of Bi2223 tapes at high temperatures are intrinsically poor, resulting in low Jc compared to the RE123 coated conductors. On the other hand, the porous microstructure spontaneously forms in Bi2223 polycrystalline bulks due to anisotropic crystal growth of Bi2223. Even by applying the rolling in the middle of the fabrication process of Bi2223 tapes, crystal growth easily occurs during sintering which generates pores in the Bi2223 filaments. The over-pressure sintering under controlled oxygen partial pressure (CT-OP) developed by Sumitomo Electric Industries solved this problem and developed DI-BSCCO tapes show high homogeneity in critical current properties because of the highly dense Bi2223 filaments, which enabled us to improve poor flux pinning properties through systematic controls of starting cation compositions and sintering conditions. In addition, enhancement of mechanical strength of the DI-BSCCO tapes achieved by the composite of high strength alloys enlarged their magnet applications to high fields. The latest products of DI-BSCCO, their various physical properties and the recent challenges to increase Jc in magnetic field by an increase in actual lead concentration in Bi2223 crystals of DI-BSCCO will be shown. Moreover, development of superconducting joints connecting DI-BSCCO tapes will be also demonstrated as a hot topic. In this technology, establishment of low angle polishing process less than 1°for DI-BSCCO tape to increase in number of joined filaments of DI-BSCCO tapes was the first milestone. Followed by densification by addition of intermediate pressing process and control of cation composition of intermediate Bi2223 thick film layer resulted in high Ic joint between DI-BSCCO tapes. Even for the superconducting joint connecting reinforced DI-BSCCO (type HT-NX) tapes, high Ic values, ~120 A at 77 K and 300 A at 4.2 K under 3 T, were achieved. Design of a superconducting magnet equipped with the persistent current circuit for the 1.3 GHz NMR system is undergoing under JST-MIRAI project (JPMJMI17A2). The superconducting joints connecting DI-BSCCO tapes will take an important role in the project.
 Y. Takeda et al., Appl. Phys. Express 12 (2019) 023003
Keywords: Bi2223, superconducting joint, DI-BSCCO