This paper describes the microstructure to improve the magnetic field dependence of the critical current density, Jc, of Y(RE)Ba2Cu3O7−x [Y(RE)123, RE: Gd and Sm] coated conductors. A columnar microstructure 10nm in diameter has been obtained by using Y123 targets including yttrium-stabilized zirconium for the pulsed-laser deposition. This columnar structure, composed of BaZrO3 and Y123, continued from the substrate to the surface of the film 0.25μm in thickness. We have named it “the bamboo structure” from its morphology. The bamboo structure was effective for increasing Jc in a magnetic field especially parallel to the c-axis. We have also found stacking faults in RE123 effective to improve Jc.

In order to increase the critical current, I c , we have fabricated thick GdBa 2 Cu 3 O 7−δ (GdBCO) coated conductors (CCs) by the pulsed laser deposition (PLD) method on PLD-CeO 2 /ion-beam assisted deposition (IBAD)-Gd 2 Zr 2 O 7 (GZO)/hastelloy metal substrate tapes. The highest critical current value was 522 A cm −1 for a thickness of 3.6 µm in self-field at 77 K. It was found that a low volume fraction of a-axis orientated grains was obtained in the thick GdBCO CCs, compared to YBa 2 Cu 3 O 7−δ (YBCO) CCs. Consequently, the GdBCO CCs showed higher critical current density (J c ) than YBCO CCs in all thicknesses from 0.2 to 3.6 µm. Furthermore, we have succeeded in improving I c in a magnetic field by the introduction of artificial pinning centres using a 5 mol% ZrO 2 doped GdBCO target. In the measurement of the I c dependence on the magnetic field angle, θ , I c was much improved, especially at 0 • , i.e., with the magnetic field parallel to the c-axis. The I c value at 3 T was 59.5 A cm −1 at 0 • and it showed a minimum of 42.3 A cm −1 at 82 • for 2.28 µm thick CC. The minimum value in the angular dependence of I c at 3 T was about five times higher than that of YBCO CC and two times higher than that of pure GdBCO CC.

Intensive research work has been carried out in order to develop industrially available HTS REBCO-coated conductors under the NEDO project in Japan. Recently, several groups in the project succeeded in the development of high performance coated conductors. Their characteristic features have been evaluated in terms of mechanical properties and their influence on critical currents. The mechanical properties at RT and 77 K were analyzed on the basis of the rule of mixtures. The force-free strain (Aff) was analytically deduced, which indicates the strain at which the residual stress exerted on the superconducting layer becomes zero. Tensile strain dependence on critical currents could be divided into elastic and brittle regions. The reversible strain limit (Arev) was defined as a strain at which the critical current recovers elastically to the level of 99% Ico. Within the elastic region, the critical current showed a convex strain dependence, which is explained as Ekin’s intrinsic strain effect. The degradation beyond the reversible strain limit was attributed to a fracture of the superconducting layer. As a whole, the present study made clear quantitatively the tensile strain behavior of critical currents and proposed a reasonable definition for the reversible strain limit.

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