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.

The magnetic field angle dependence of the critical current density J c (H, θ) was measured in epitaxial YBa 2 Cu 3 O 7−δ (YBCO) thin films with strong flux pinning (J c > 25 GA m −2 at 77 K). The YBCO films were classified into two categories: (1) films that showed J c (θ ) peaks around H ab with the shape of a stratovolcano (i.e., like Mount Fuji) and ( 2) films that showed high, broad J c (θ ) peaks centered at H c in addition to less prominent H ab peaks. Transmission electron microscope observations revealed that the films in category 1 contained a high density of very small precipitates, most of which were less than 7 nm, and that the films in category 2 contained a high density of precipitates whose typical diameters ranged from 5 to 25 nm. The J c (H, θ) data were analyzed based on the angular-dependent coherence length ξ(θ) within an anisotropic Ginzburg-Landau approximation. The pinning of the films in category 1 can be described by a direct summation of the core pinning interaction that is due to small point defects whose diameters are less than 2ξ . The high, broad J c (θ ) peaks centered at H c in the films in category 2 were due to a high density of larger precipitates, and they can also be explained by a similar analysis for spherical pinning centers whose diameters are larger than 2ξ .

The magnetic-field angle dependence of the critical current density J c (H, θ) was measured in YBa 2 Cu 3 O 7−δ (YBCO) thin films with strong flux pinning (J c 2.5 MA cm −2 at 77.3 K) prepared by a fluorine-free (FF) metal organic deposition (MOD) method and by thermal co-evaporation. Steep J c (θ ) peaks around H ab were observed in FF-MOD films, and anisotropic scaling analysis showed that the pinning is mainly due to small random (point) pins and ab-plane-correlated pins. Few small precipitates with diameter less than 10 nm were observed by transmission electron microscopy (TEM); instead, a high density of stacking faults parallel to the ab-plane was observed in some areas in cross-sectional TEM images. We hypothesize that at 77 K most stacking faults are weak planar pinning centers by themselves and that (partial) dislocations formed at the boundary between stacking faults and the YBCO matrix become strong linear pinning centers parallel to the ab-plane. The linear pin acts as an ab-plane-correlated pin when it is perpendicular to the current direction, and acts as a small random pin in other cases, which well explains the observed J c (H, θ) of FF-MOD YBCO films.

This paper describes the first persistent-mode medium magnetic field (400 MHz; 9.39 T) nuclear magnetic resonance (NMR) magnet which uses superconducting joints between high-temperature superconductors (HTSs). As the ultimate goal, we aim to develop a high-resolution 1.3 GHz (30.5 T) NMR magnet operated in the persistent-mode. The magnet requires superconducting joints between HTSs and those between an HTS and a low-temperature superconductor (LTS). Towards this goal, we have been developing persistent-mode HTS inner coils to be operated in a 400 MHz (9.39 T) NMR magnet and here we present the first prototype inner coil wound with a single piece (RE = rare earth)Ba2Cu3O7−x (REBCO) conductor. The coil and a REBCO persistent current switch are connected with intermediate grown superconducting joints with high critical currents in external magnetic fields. To evaluate the performance of the joints in an ultimately stable and homogeneous magnetic field, the coil is operated in the persistent-mode, generating 0.1 T, in a 9.3 T background magnetic field of a persistent-mode LTS outer coil. The magnetic field drift over two years of the 400 MHz LTS/REBCO NMR magnet is as small as ∼1 ppm, giving high-resolution NMR spectra. The magnetic field drift rate over the second year was 0.03 × 10−3 ppm h−1, which is more than three orders of magnitude smaller than that required for an NMR magnet, demonstrating that the superconducting joints function satisfactorily in a high-resolution NMR system. The corresponding joint resistance is inferred to be <10−14 Ω.

Multilayered structures of YBa2Cu3O7−δ (YBCO) interlayered with thin layers of DyBa2Cu3O7-δ (DyBCO) were prepared on CeO2-buffered r-cut sapphire (Al2O3) substrates by pulsed laser deposition. Evaluation of the magnetic-field angular dependence of the critical current density [Jc(H,θ)] revealed that the flux pinning properties of multilayered YBCO/DyBCO films were significantly enhanced in comparison to single-layer YBCO films (monolayers) prepared using the same experimental parameters. The YBCO/DyBCO multilayers are highly anisotropic, i.e., the angular-dependent Jc exhibits a very prominent peak when the applied magnetic field (H) is oriented parallel to the ab-plane direction (H∥ab). Analysis of the Jc(H,θ) data revealed an enhanced random pinning for the multilayers for the entire range of field investigated. In the angular-dependent Jc data, correlated pinning along the c-axis crystallographic direction was also evidenced at low applied fields by a less prominent peak at H∥c. This result was further corroborated by the presence of defect microstructures comprised of linear and planar defects which were considered as strong sources of c-axis-correlated pinning. However, for higher applied fields the contribution of c-axis-correlated pinning is highly diminished and the ratio of Jc(H∥ab) to Jc(H∥c) is significantly enhanced. In addition to enhanced random pinning, it is considered that improved pinning along the H∥ab direction occurs due to ab-correlated pinning, arising from intrinsic pinning and possibly extended planar defects.