A superconducting joining method for high-temperature superconductors (HTSs) is one of the key technologies needed to realize persistent current operations in HTS magnets such as NMR and MRI. We have succeeded in developing a practical superconducting joining method between REBa2Cu3Oy (REBCO, RE: rare earth elements)-coated conductors with a microcrystalline REBCO precursor intermediate layer. The microcrystalline REBCO precursor intermediate layer is sandwiched between c-axis oriented REBCO layers and is joined with them. The crystal growth process during heat treatment has been investigated by observing cross-sectional views of the precursor layer. Cross-sectional TEM images showed that the precursor grew epitaxially from both the upper and lower surfaces of the c-axis-oriented REBCO layers. Then the entire precursor grew with both of the REBCO layers being joined with c-axis orientation. The iGS joint gives a critical current of >70 A at 77 K under self-magnetic fields. The total processing time, including joining heat treatment and oxygen annealing, is less than one day. This joining method allows us to operate a 400-MHz (9.4 T) HTS NMR magnet with a sufficiently stable magnetic field, and thus we have succeeded in obtaining NMR spectra.
