Yasuhiro Iijima

In‐plane aligned YBa2Cu3O7−x thin films deposited on polycrystalline metallic substrates

C‐axis oriented YBa2Cu3O7−x thin films are conventionally obtained on polycrystalline substrates, but a‐ and b‐axes are randomly distributed. Due to the weak links at the high‐angle grain boundaries in the a–b plane, the critical current density (Jc) are comparatively low, from 103 to 104 A/cm2 (77 K, 0 T), and the Jc decreases in magnetic field in a manner similar to bulk YBa2Cu3 O7−x samples. To reduce weak links at the high‐angle grain boundaries, biaxially oriented buffer layers of yttrium stabilized zirconia (YSZ) were formed on polycrystalline, Ni‐based alloy by ion‐beam assisted deposition (IBAD), and subsequently the a–b plane aligned YBa2Cu3 O7−x film was deposited by laser ablation. Jc of 2.5×105 A/cm2 (77 K, 0 T) and 2.2×104 A/cm2 (77 K, 8.0 T) were obtained. A new method to prevent intergranular weak links has been developed for potential applications using practical polycrystalline substrates.

Structural and transport properties of biaxially aligned YBa2Cu3O7−x films on polycrystalline Ni‐based alloy with ion‐beam‐modified buffer layers

Biaxially aligned YBa2Cu3O7−x (YBCO) thin films were produced on polycrystalline Ni‐based alloy, by using biaxial yttria‐stabilized‐zirconia (YSZ) intermediate layers formed by off‐normal ion‐beam‐assisted deposition. Most explicit in‐plane alignment was obtained when the YSZ layer formed with the beam‐incident angle of 55° from substrate normal. Jc‐B characteristics and angular dependence of Jc on the magnetic field were measured. 5.0×105 and 5.5×104 A/cm2 were obtained at 77 K with 0 and 8 T, respectively. The distribution of misorientation angles of in‐plane a and b axes between YBCO grains was evaluated by both x‐ray pole figure measurement and planar observations of transmission electron microscopy. 50% of the grains had the misorientation angles restricted within ±5°. From the image of dislocations, the elastic strains at grain boundaries were estimated to be relaxed with lower misorientation angle. The high‐Jc properties are understood to be obtained by the current paths through low‐angle grain bou...

High-temperature-superconductor coated conductors : technical progress in Japan

It is now a widespread view that a high-Jc superconducting wire with a high performance under strong magnetic fields at liquid-nitrogen temperature needs to be realized by the material Y-123. However, because of the weak linking nature of this material, the fabrication of the wire must be based on a coated-conductor process to achieve a highly textured Y-123 thin film on a long starting wire tape. Various attempts to obtain a textured substrate with sizable length have been made by various methods, with or without the aid of buffer layers. Approaches to the fabrication of an epitaxial thin film of Y-123 have also been made from vapour, liquid, or solid phases by different methods. Therefore, combinations of these proposed processes in each category have led to several promising on-going worldwide approaches to achieve the realization of practical coated superconducting wires in the near future. This review focuses on the most outstanding achievements in Japan.

Biaxial alignment control of YBa 2 Cu 3 O 7− x films on random Ni-based alloy with textured yttrium stabilized-zirconia films formed by ion-beam-assisted deposition

Biaxially aligned YBa 2 Cu 3 O 7− x (YBCO) films were fabricated on random Ni-based alloy tapes with yttrium stabilized-zirconia (YSZ) buffer layers deposited by ion-beam-assisted deposition (IBAD). Ar + ion bombardment was found to have two significant effects on the crystalline structure of the YSZ buffer layers: to align a [100] axis with the substrate normal and a [111] axis with the bombarding beam axis. The resulting YSZ films were biaxially aligned on the random polycrystalline tapes, and the azimuthal distribution of the a - and b -axes of YBCO films on the top of the YSZ films was restricted to 10° FWHM. A critical current density ( J c ) of 1.13 × 10 6 A/cm 2 (77 K, 0 T) was obtained, and 1.1 × 10 5 A/cm 2 was maintained at 5 T (77 K, B⊥c). The existence of both intrinsic and extrinsic pinning properties was clearly observed in the angular dependence of J c with B⊥I. The longitudinal field effect on J c was clearly observed, which indicated straight transport currents. This is evidence for strongly coupled current paths that demonstrate the bulk pinning properties of YBCO.

Biaxially aligned YBa2Cu3O7−x thin film tapes

Abstract We report the formation of biaxially oriented films of yttorium-stabilized ZrO2 (YSZ) on a polycrystalline, Ni-based alloy (Hastelloy c276) by Ion-Beam-Assisted Deposition (IBAD), and the subsequent a-b plane aligned YBa2Cu3O7−x (YBCO) films deposited by laser ablation. Jc of 6.0×104 A/cm2 (77 K, O T) and 1.4×104 A/cm2 (77 K, 0.6 T) were obtained. A new method to prevent intergranular-weak-links has been developed for application of oxide superconducting thin films, for tape-shaped cables, magnets, magnetic shields, microwave devices, etc.

Normal transition and propagation Characteristics of YBCO tape

YBCO tape is expected to be used in future HTS applications, because it has better J/sub c/ characteristic in high temperatures and in high applied magnetic fields. It is important to consider the stability in superconducting applications. To establish the stability criterion for coated conductors, the characteristics of the transition to the normal state and normal-zone propagation in YBCO sample tapes were examined experimentally and numerically. The YBCO sample tapes were produced by IBAD/PLD method. Measurements were performed as a function of both heat input and operating current for operating temperatures of 55 K, 60 K, and 65 K in a background magnetic field of 5 T. The experimental results were compared with a simulation using a newly developed computer program based on the finite element method (FEM). In the computer program, the I-V characteristics of the YBCO sample tapes were taken into account. The computer program could simulate the normal transition and propagation phenomena and its validity was confirmed. The influence of the thickness of a Ag-stabilizer on the transient thermal behavior of YBCO tapes was examined experimentally and numerically and the results were presented here.

Reel to reel continuous formation of Y-123 coated conductors by IBAD and PLD method

Continuous deposition of Y-123 tapes was performed by ion-beam-assisted deposition (IBAD) and pulsed laser deposition (PLD) method. YSZ template films were deposited by IBAD on Ni-based alloy tapes with tape shifting speeds of 0.1-0.3 m/h. They were uniformly coated with an in-plane mosaic spread of 12-13 degrees in 5 m length. In order to overcome the low deposition rate of the IBAD process, large area ion sources with inductively coupled discharge plasma were applied. Y-123 films were deposited by PLD on the YSZ templates with a tape shifting speed of 1.0 m/h. The degradation of Y-123 targets for the PLD process was effectively restricted by combined motion of rapid rotation and linear shifting. I/sub c/ of 84.0 A and J/sub c/ of 0.76MA/cm/sup 2/ were obtained at 77 K, 0 T in the length of 1.0 m when the in-plane mosaic spread for the Y-123 film was 7 degrees. I/sub c/ of 35.0 A and J/sub c/ of 0.25MA/cm/sup 2/ were obtained in a 4.6 m long sample at the length of 4.0 m, when the in-plane mosaic spread for the Y-123 film was 11 degrees.

In-plane texturing control of Y-Ba-Cu-O thin films on polycrystalline substrates by ion-beam-modified intermediate buffer layers

Biaxially aligned YBCO thin films were successfully formed on polycrystalline Ni-based alloy by using ion-beam-modified yttria-stabilized-zirconia (YSZ) intermediate layers. YSZ layers were deposited by ion-beam-assisted deposition (IBAD) with concurrent off-axis ion beam bombardment. The YSZ

Fabrication of long-Y123 coated conductors by combination of IBAD and PLD

Abstract Continuous deposition of Gd 2 Zr 2 O 7 films were carried out on 10 mm width Hastelloy tapes by ion-beam-assisted deposition (IBAD). They were uniformly deposited with the in-plane mosaic spread (Δ φ ) of 10–14° in the 50 m long and of 16–18° in the 60 m long, at a production rate of 0.5–1.0 m/h. The 6 cm sample with the I c value of 141 A and the J c value of 1.25 MA/cm 2 were obtained by 6 repeated depositions using pulsed laser deposition (PLD). Continuous deposition of Y-123 films were performed by PLD on long IBAD template tapes at a tape speed of 4.0 m/h. The I c value of 40 A and the J c value of 0.8 MA/cm 2 were obtained at the length of 30 m. Furthermore, the 46 m YBCO coated tape with the I c value of 74 A and the n value of 32 was formed uniformly.

GROWTH STRUCTURE OF YTTRIA-STABILIZED-ZIRCONIA FILMS DURING OFF-NORMAL ION-BEAM-ASSISTED DEPOSITION

Biaxially aligned YSZ thin films with strong [100] fiber texture were formed on a polycrystalline Ni-based alloy by off-normal ion-beam-assisted deposition. Growth structures were characterized by x-ray diffraction (XRD), transmission electron microscopy (TEM), atomic force microscopy (AFM), etc., and the alignment mechanism was discussed using a selective growth model. Peculiar structural evolution of the crystalline orientation was observed and its development was well described by an exponential equation. It was explained as a collaboration of an anisotropic growth condition and epitaxial crystallization. The [100] fiber texture was formed by columnar structures of diameter of 30–100 nm, which were composed of 5–10 nm diameter crystallites. Very smooth surfaces were observed by AFM imaging with a roughness of 2–3 nm and a peculiar ripple structure. The origin of the azimuthal alignment was discussed with emphasis on the surface structure of YSZ films produced using ion-beam-assisted deposition (IBAD) and the etching rate measurements of (100) surfaces of YSZ single crystals.