REBCO coated conductors by IBAD/PLD process are suitable for varied field coil applications to have excellent and uniform in-field Jcproperties and robust mechanical strain strength, Many prototype magnets were wound by using commercial wires and recently an example of social implementation has achieved as the inner coils for high-field NMR systems of 1.2 GHz class. Such kind of magnetic resonance application strongly requires good Icuniformity in long piece length and reduction of joint numbers which can affect resolution of the signal.
Under the NEDO program of "Project to Promote Commercialization of High-Temperature Superconductivity Technology (2016-2020)," comprehensive magnet technologies using REBCO coated conductors were developed aiming for liquid helium free compact MRI. We contributed two kinds of elemental theme as development of high performance and productive REBCO wire and also low resistance joint of REBCO wire. In the former theme, we concentrated on the optimization of growth conditions for ~1km long uniform BMO-doped REBCO wires and in-field Jc properties of ~2-times bigger were obtained than non-doped ones in low temperature range below 40 K, at the growth rate over several 10s nm/sec without spoiling process throughput.
We also contributed to low resistance splicing joint development aiming for persistent current mode MRI operation. Superconducting joints of REBCO wires usually require direct epitaxial growth between REBCO surfaces that can only occur in high temperature procedure. On the other hand, at 4.2 K operation, even conventional solder spliced joints of coated conductors should have quite low joint resistances if low enough interfacial resistances are obtained, because resistances for both solder and Ag metal themselves should be negligible. But the typical interfacial resistances between REBCO and sputter deposited Ag protection layer are 10-11~-12 ohm*m2 at wide temperatures for commercial coated conductors.
We deposited deeply Ag-doped REBCO films of 0.5~2.0 μm thick by hot-wall pulsed laser deposition (PLD) method on 12mm wide and 50 μm thick Ni-Cr alloy tapes with IBAD templates. Ag protection metal films of 2.0 mm thick were deposited on them by sputtering. Lap or bridge Joints of those tapes were made just by soldering. Joint resistances were measured by DC 4 probe method at 77 K and by loop current attenuation at 4.2 K. Microscopic structure was evaluated by XRD and SEM/TEM observations. Metal Ag particles were initially grown on the top of REBCO surface and they dispersed in thicker REBCO films than 1 μm. Jc of those REBCO films ranged from 0.2~1.8 MA/cm2 at 77 K, 0 T. Joint resistance wasere measured as 0.9~-1.1x10-12 ohm*m2 at 77 K, and 1.6~-2.8x10-13 ohm*m2 at 4.2 K. Double layer structure of a thin Ag-doped REBCO film on a high-Jc 2.5 μm thick REBCO film had also low joint resistance without spoiling high-Jc properties. The results indicates a possibility to make low resistance joint of 10-12 ohm just by soldering of commercial REBCO coated conductors.
A part of this talk include results obtained from "Project to Promote Commercialization of High-Temperature Superconductivity Technology (2016-2020)," being consigned by the New Energy and Industrial Technology Development Organization (NEDO), MITI.
Keywords: Coated Conductors, Low Resistance Joint, BMO Doped REBCO, Hot-Wall PLD