A new concept for developing a compact joint structure for reducing joint resistance between high-temperature superconductors (HTS) and low-temperature superconductors (LTS)

Abstract

It is significantly difficult to develop a superconducting joint between REBCO and low-temperature superconductors (LTS) using a solder matrix replacement technique. This is because the REBCO superconducting layer is highly corrosive to Sn. In this work, TEM observations were first conducted on the reaction interface between the REBCO layer and Sn to reveal the reaction. Then, a new idea to create a compact low-resistance joint to reduce the joint resistance between REBCO and LTS was proposed. In this method, the REBCO tape is rolled in a metal boat, with the Cu stabilizer remaining around the tape. Then, an LTS wire, whose superconducting filament ends are coated with a superconducting solder, is set straight into a boat. Then, the boat is filled with a superconducting solder, so that the joint state between the LTS and superconducting solder matrix remains superconductive. However, the electrical joint between the REBCO tape and the solder matrix is resistive, even if the solder matrix is superconductive. Consequently, the overall joint resistance is determined by the boundary resistance between the REBCO tape and the superconducting solder matrix. However, to achieve a joint resistance below 10–10 Ω, a long joint length of more than 5 m, preferably more than 10 m, will be required. Considering the strain state of the REBCO layer when it is rolled in to a boat, the boat-type joint structure proposed in this work enables the joint size to be significantly compact, even if a length of more than 10 m is required. At present, a joint resistance of 0.7 nΩ was obtained in a field range of less than 0.4 T by using a boat with an inner size dimensions as follows: 50 mm length, 16 mm width, 7 mm height, radius of curvature of 8 mm, and tape length of 2 m.

Review of recent developments in ultra-high field (UHF) NMR magnets in the Asia region

“…In order to form a superconducting joint, superconducting solders via the sheath-dissolution method were employed. It was found, however, that Sn in the solder solution deteriorates the superconductor's surface, making joint resistance orders of 10 −10 Ω [55], however this is acceptable for the persistent-mode operation of the 1.3 GHz NMR magnet.…”

Review of the temporal stability of the magnetic field for ultra-high field superconducting magnets with a particular focus on superconducting joints between HTS conductors

“…This can be attributed to the chemical difficulty in forming superconducting joints between ceramic HTS and metallic superconducting solders. Banno et al [48] recently reported the challenges faced when developing superconducting joints for REBCO using a solder owing to the highly corrosive nature of the REBCO superconducting layer under Sn in the solder.…”

“…HTS conductors by soldering. The REBCO thin film of a CC is corrosive under exposure to Sn in the solder, it is difficult to achieve a superconducting joint between a REBCO CC and the solder including Sn [48]. In contrast, Mousavi et al [109] attempted to form the superconducting joint between reacted Bi-2212/Ag wires through soldering.…”

“…Banno et al reported the difficulties in forming the superconducting joint between the REBCO CC and NbTi conductor [48]. Consequently, the compact 'boat-type' joint structure with a reduced joint resistance between the REBCO CC and NbTi conductor was reported, as shown in figure 19.…”

“…Therefore, using the joining methods described before, a joint resistivity less than 10 nΩ cm 2 can be achieved, indicating that the joint length can be shortened to achieve a sufficiently low resistance compared to that shown in table 1 (∼40 nΩ cm 2 ). A practical joint showing substantially low resistance might be achievable using the aforementioned joining methods and a compact joint structure [34,48].…”