Multifilamentary Nb3Sn wires are fabricated starting from Nb/Cu-Sn or Nb/Cu/Sn composites, which facilitates their mass production. Nb3Sn superconducting wires are now one of the key materials in modern science and technology, such as NMR, fusion and refrigerator cooled magnets. Present volume is the special issue on the recent status of mass productionscaled Nb3Sn conductors in Japan. Then this article describes aspects of Nb3Sn conductors, and overviews recent activities in foreign industries. Bronze process and internal tin process are common fabrication process of Nb3Sn multifilamentary wires, whose features are briefly described in this article. The former process is dominantly used in Japan and Europe including Russia, while the latter is rather frequently used in USA, Korea and China. Thus countries are divided into roughly two groups with respect to the major fabrication technology of Nb3Sn. Beside these, Nb tube process and powder-in-tube process are being proceeded in foreign industries. Nb3Sn conductors still have a possibility of improvement in their fabrication technologies, and will continue to contribute for the application of superconductivity.
We have developed superconducting wire about bronze route Nb3Sn and internal tin processed Nb3Sn. SH Copper Products Ltd. manufactures bronze route Nb3Sn superconducting strands for ITER-TF coils under a contract with the Japan Atomic Energy Agency (JAEA). The Nb3Sn strands which are composed of Nb-1 wt%Ta filaments and Cu-15.5 wt%-0.3 wt%Ti bronze have good workability and high performance. More than 850 A/mm2 of non Cu Jc at 12 T can be stably obtained using two-stage heat treatment technique. The internal-tin technique is an excellent method for fabricating Nb3Sn superconducting wire with a high critical current density. We have developed a new type of internal-tin Nb3Sn wire in which the multi-filamentary wire is fabricated by combining mono-filamentary Nb elements and mono-filamentary Sn elements. The simple fabrication process will enable the fabrication cost of the wires to be reduced.
The development of elemental technologies is required for the practical use of React-and-Wind (R&W)-processed Nb3Sn coil applications. The critical currents (Ic) of a Nb3Sn strand under several common strains are able to be estimated using both characteristics of the Ic-magnetic field-temperature-axial strain (Ic-B-T-ε) property and considerations of the bending strain effects. However, designing the Ic of a large-scale Nb3Sn conductor becomes much more difficult because the Nb3Sn wires are under extremely complicated strains. Recently, Nb-rod-method Cu-Nb reinforced Nb3Sn wires were successfully developed using the bronze process. The performance of the Cu-Nb/Nb3Sn Rutherford cables was improved by utilizing the pre-bending process. It is expected that pre-bent Cu-Nb/Nb3Sn wires will be very useful for R&W-processed Nb3Sn coil. This paper describes the progress and prospects for developing elemental technologies used in R&W Nb3Sn wires.
Nb3Sn wires are widely used for high-field superconducting magnets. For the improvement of superconducting properties, several producing processes have been developed such as bronze-route process, internal diffusion process and powderin-tube process. Nb3Sn wires of high critical current density (Jc) can contribute to advanced applications: nuclear magnetic resonance (NMR) spectrometers are the most successful case. This paper reviews the manufacturing processes of Nb3Sn in each case, reviewing the Jc improvement methodology first. Then, the research on high JcNb3Sn wires at Kobe Steel, Ltd. and Japan Superconductor Technology, Inc. is described. We have achieved high Jc Nb3Sn wires by focusing development on the amount of Nb3Sn phase and grain size composition. We have also developed Nb3Sn wire for the International Thermonuclear Experimental Reactor (ITER) project. These wires are utilized for high field superconducting magnet applications.
High tin bronze is used as a component of practical Nb3Sn superconducting wires. In particular, the addition of titanium to bronze is well known to improve superconducting properties as well as the workability of the bronze alloy. In the present study, change in the microstructure of bronze by adding titanium was investigated. The important results are summarized as follows. First, when the Cu-10.5 at%Sn-0.72 at%Ti alloy was held at 550oC for 1000 min, three phases of primary α, γ and CuSnTi (Ti111) were equilibrated. Second, after holding at 750oC for 1000 min, three phases of primary α, β and CuSn3Ti5 (Ti135) coexisted. Third, during heating from 560oC to 750oC, a remarkable phase change was observed near 675oC, below which the Ti111 phase was dominant, but above which the principal Ti-based compound phase was Ti135. Fourth, in the present region of alloy composition and heat treatment temperature, two invariant reactions of Ti135 + β = α + Ti111 and α + β = γ + Ti111 were recognized to take place in the Cu-Sn-Ti ternary system. Finally, characteristic features of workability at room temperature as well as at elevated temperatures could be well explained in conjunction with the behavior of titanium ternary compounds.
The degradation of transport current property due to the high mechanical strain on practical Nb3Sn wire is a serious problem for applying future fusion magnets operated under higher electromagnetic force environments such as DEMO. We have attempted to improve the mechanical strength of Nb3Sn bronze-processed wire by using a high-strength bronze matrix produced through a solid-solution strengthening process. In this study, we developed several Zn solid-solution Cu-Sn-(Ti) alloys with a high Sn content, from 10 to 13.5 mass% (Cu-Sn-Zn), in order to investigate the effect of adding Zn to the microstructure and the superconducting properties of bronze-processed Nb3Sn wire. We observed clearly that the amount of Zn in the matrix remained constant after Nb3Sn formation. Thicker Nb3Sn layers were formed as the result of adding Zn. No degradation of transition temperature due to adding Zn was observed, and the temperature attained was approximately 17.5 K. Finally, we succeeded to fabricate multi-filament wires (Nb filament: 7771 with a diameter of 3.4 μm) using various Cu-Sn-Zn matrices through the Cu restacking method.