Fabrication, microstructure and persistent current measurement of an intermediate grown superconducting (iGS) joint between REBCO-coated conductors

A superconducting joint technology used for high-temperature superconductors (HTS) is the key for enabling persistent operation of HTS magnets. In the present work, we have succeeded in developing a superconducting joint between REBCO-coated conductors (CCs) using a joint strap with a microcrystalline GdBCO precursor intermediate layer. Heat treatment and oxygen annealing, with a total processing time of less than 1 d, grows a biaxially-textured intermediate layer to connect the GdBCO layers in the CCs. Microstructure observation of a part of the joint cross-section with SEM and TEM showed that the intermediate layer and the GdBCO layers in the conductors were atomically connected. An electron backscatter diffraction result showed that both the c - and a -axis misorientations among the GdBCO layers of the joined conductor and the GdBCO layer of the joint strap were about less than 5°. This intermediate grown superconducting joint gives a critical current of >100 A at 77 K in a self-field. A critical current of a joint at 4.2 K in a self-field is seven times higher than that at 77 K. The persistent field decay of a small double pancake coil, terminated with this joint, showed a joint resistance in the range of <3 × 10⁻¹² Ω to <5 × 10⁻¹³ Ω at 77 K in a self-field over three days, with an operating current of ∼10 A (∼14% of the calculated coil critical current). The results show a promising prospect of the joint to be used for persistent magnets such as NMR and MRI. (paper)