After the over-pressure sintering method and the pre-tensioned lamination technique were introduced, critical current and mechanical properties of Ag-sheath Bi-2223 wire have been drastically improved, though further enhancement of their properties is necessary for promoting practical applications. This paper reviews the recent understanding of the critical current properties and improvement of the mechanical properties, and introduces achievements of Ag-sheath Bi-2223 wire which have been used for current lead, cable, magnet and motor applications.

Further performance improvement in Nb₃Sn conductors is strongly demanded towards realization of upcoming high-field magnet applications such as the Future Circular Collider (FCC) and the DEMOnstration power plant (DEMO). However, we are facing the problem that the J_c performance of the Nb₃Sn strands are almost fully optimized in terms of Nb/Cu/Sn area ratio, cross-sectional design, Nb filament diameter, and so on. We thus need some breakthrough to overcome the problem. In this context, we have been studying the new microstructure control by the element addition into the Cu matrix of the internal tin processed Nb₃Sn conductors. In this paper, some results of the more fundamental study on the effect of the Zn addition are reported. In addition, results of the simultaneous addition of Zn and a small amount of Mg, and the influence of Ti-doping position on the microstructure and J_c performance are reported. It was found that significantly different diffusion behaviors happen in the Cu-Zn/Sn diffusion reaction. For instance, in the Cu-Zn/Sn diffusion, a solid ternary Cu-Sn-Zn phase widely forms at the outermost reaction front at 400℃ heat treatment, whereas the porous ε phase widely forms in the Cu/Sn diffusion. A small addition of Mg into the brass matrix resulted in finer grain size and better J_c performance. Ti doping to the Nb filaments but not Sn cores leads to elimination of Ti-rich layer at the boundary that forms in Ti doping to Sn cores. The absence of Ti-rich layer contributes significantly to improvement of Sn and Ti distribution across the cross-section.

In the longitudinal magnetic field state, critical current density (J_c) of a superconductor enhances compared with the J_c in self-field and it is called as J_c gain. We have reported J_c gain in BaHfO₃-doped-multilayered (ML) SmBa₂Cu₃O_y(Sm123) films on oxide single crystalline substrates. In this study, we aim for the J_c gain appearance in ML-Sm123 films on IBAD-MgO metal tapes for power cable application. We observed the J_c gain on metal tapes with good reproducibility. From viewpoints of the flux pinning and super-current flow, important factors for the J_c gain at 77 K were higher interface density, self-organized BHO nanorods with larger diameter, and the reduction of a ML-structure disturbances.

Improvement of critical current density (J_c) in magnetic fields is required in YBa₂Cu₃O₇ films, and process parameters should be optimized for controlling pinning centers. In the present study, a deposition temperature was varied in pulsed laser deposition of YBa₂Cu₃O₇+BaHfO₃ films to control the nanorod structure, and its influence on J_c was analyzed. The YBa₂Cu₃O₇+BaHfO₃ film deposited at 850℃ exhibited pinning force maximum (F_p,max) as high as 413 GN/m³ at 40 K, while the F_p,max for the deposition temperature of 850℃ at 77 K was smaller than that in the YBa₂Cu₃O₇+BaHfO₃ film deposited at 900℃. A critical temperature decreased and matching field increased with decreasing the deposition temperature. Increase in deposition temperature is effective in improving the F_p,max in high temperatures, since the critical temperature and matching field dependences of J_c value dominate the F_p,max. On the other hand, low deposition temperature improves the F_p,max in low temperatures since the F_p shift in accordance with matching field is dominant to the F_p,max. Thus, the deposition temperature should be set in pulsed laser deposition of YBa₂Cu₃O₇ films containing nanorods considering the J_c variation with critical temperature and matching field.

To adopt coated conductors to various superconducting devices, characteristics such as microstructure and critical current of commercially available coated conductors were investigated. The microstructure was analyzed using a transmission microscope with an energy dispersion spectroscopy. The critical current (I_C) were measured at 4.2 K and magnetic fields (B//c), and at 77 K and self-magnetic field. As a result of microstructure analyses, the magnetic field dependencies on I_C at 4.2 K are found to depend on the microstructure of the superconducting layer. Moreover, almost coated conductors were found to have a potential to improve their I_C at 77 K and self-magnetic field.

High-temperature superconducting REBa₂Cu₃O_y (RE: Rare earth elements, REBCO) films need both a high-speed film fabrication with a well-oriented crystalline texture and an introduction of artificial pinning centers (APCs) to improve critical current densities J_cs in magnetic fields．A vapor-liquid-solid (VLS) growth method enables the REBCO films to grow rapidly with a high quality but makes it difficult to introduce the APCs．In this study, Y₂BaCuO₅ (Y211) was doped into YBa₂Cu₃O_y (Y123) films fabricated via the VLS method with a liquid layer of Ba₃Cu₇O₁₀．A microstructure analysis revealed that nano-particle APCs with diameter less than 10 nm were successfully introduced into the YBCO films．The Y211-doped Y123 films show the highest critical temperature T_c and the J_c in magnetic fields with a chemical composition ratio close to the stoichiometry of Y123.

Magnesium Vapor Transport method (MVT method) has been developed, in which magnesium vapor is treated with boron to obtain MgB₂ superconducting polycrystalline bulks. The obtained disk-shaped MgB₂ bulk with 20 mm in diameter and 2 mm in thickness had high packing factor of 80％ and high phase purity. The critical current density J_c reached 800000 A/cm² at 20 K under self-field, which is about twice as high as a sample produced by the conventional in situ technique, due to the reduction of voids and impurities. Trapped field of 1 T was measured at 10 K at the surfaces of the thin disk-shaped bulk. Our results show that MVT method is one of the most effective techniques to extract superior trapped field performance of polycrystalline bulk MgB₂ magnets through enhancing circulating supercurrent density and bulk size both of which are essential for high field trapped field magnets.

The fabrication of Fe-based Ba-122 superconducting tapes was studied by ex-situ powder-in tube (PIT) process using double sheathing technique of stainless steel (SS) and Ag-Sn alloy. We found that the double sheathed SS/Ag-Sn/Ba-122 tapes show much more improved critical current density (J_c) compared to the previously reported single sheathed Ag/Ba-122 tapes. By optimizing the Sn content, heat treatment temperature, and tape thickness, we have achieved high J_c of well over 10⁵ A/cm² at 4.2 K and 10 T, which is the target value for practical application. Taking into account coil winding in the future, we fabricated a ～1 m long SS/Ag-Sn/Ba-122 tape and evaluated the uniformity of J_c along tape length and the J_c degradation by bending. The J_c distributed within 5.5-7.2 × 10⁴ A/cm² (4.2 K, 10 T) over the entire length and showed no degradation until 3 cm diameter bending.