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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 −14 Ω.

We have been studying REBa 2 Cu 3 O x (RE: rare earth elements) coated conductors on clad-type substrates. We developed coated conductors on the 10 mm wide clad-type tape, and succeeded in obtaining the maximum critical current of 380 A/cm with the 2.0 μm thick GdBa 2 Cu 3 O x superconducting film. At present, we are trying to widen the tape width from 10 mm to 30 mm in order to increase production throughput. We report our recent progress on scaling-up of the buffer layers and their properties. The buffer layers consisted of three layers; CeO 2 as the seed layer, yttrium-stabilized zirconia (YSZ) as the diffusion barrier layer and CeO 2 as the lattice matching layer. They were grown by the RF-sputtering method and the electron beam evaporation technique (EB). EB deposition has possibility of higher throughput compared with the RF-sputtering. The intensity ratio of (2 0 0) to (2 0 0) + (1 1 1) for the lattice matching layer showed as high as 98%. The uniform properties of 40 m long and 30 mm wide substrates were successfully produced by all sputtering method. Additionally, the tape travel speed of the seed layer can be increased from 7 m/h (RF-sputtering deposition) to 10 m/h (EB deposition) on short sample (0.3 m long).

Relatively thick REBa2Cu3O7?? (RE = rare?earth) films (thickness ~400?600?nm) with significantly improved surface morphology and critical current properties using a multilayered structure which alternates main layers of YBa2Cu3O7?? (YBCO) with intermediate DyBa2Cu3O7?? (DyBCO) layers on CeO2-buffered sapphire substrates were investigated. The DyBCO layer, which has a close lattice matching with YBCO, functions as a good starting template for the growth of high-quality YBCO layers. Critical current density (Jc) drastically increased up to a factor of 2 for YBCO/DyBCO multilayer films, compared to YBCO monolayer films in both the self-field and applied magnetic field. The significant improvement in Jc is attributed to the improvement of surface smoothness and enhanced flux pinning properties as revealed by the magnetic-field angular dependence of Jc.

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

The magnetic-field angle dependence of the critical current density Jc(H, θ) was measured in Y Ba2Cu3O7 − δ (YBCO) thin films with strong flux pinning (Jc ≥ 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 Jc(θ) peaks around 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 Jc(H, θ) of FF-MOD YBCO films.

The magnetic field angle dependence of the critical current density Jc(H,θ) was measured in epitaxial YBa2Cu3O7−δ (YBCO) thin films with strong flux pinning (Jc>25 GA m−2 at 77 K). The YBCO films were classified into two categories: (1) films that showed Jc(θ) peaks around with the shape of a stratovolcano (i.e., like Mount Fuji) and (2) films that showed high, broad Jc(θ) peaks centered at in addition to less prominent 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 Jc(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 Jc(θ) peaks centered at 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.

We investigated the effect of oxygen annealing on the magnetic-field angular-dependent critical current density (Jc (θ)) in YBa2Cu3O7−x films deposited by large-area pulsed laser deposition. In an as-grown film, Jc (θ) exhibits a large maximum centered at , and a small one centered at . After oxygen annealing at 425 °C, the peak grew and surpassed the peak. We analyzed the Jc (θ) curves using the anisotropic scaling theory, and the result shows that mostly the peak due to random pinning increased with oxygen annealing. It is shown that the pinning center due to random defects is more sensitive to the oxygen content of the film.