Naoki Uno

Fabrication of in-plane aligned YBCO films on polycrystalline Ni tapes buffered with surface-oxidized NiO layers

Strongly in-plane aligned, c-axis oriented YBCO films were successfully grown on polycrystalline nickel tapes buffered with [100]-oriented NiO layers. The in-plane and out-of-plane alignments of the NiO layer were achieved by a surface-oxidation epitaxy (SOE) technique using a [100]<001> cube-textured nickel tape. The in-plane textures of NiO layers fabricated so far with the full width half maximum (FWHM) values of 11-13 degrees are sufficient for the epitaxial growth of high-J/sub c/ YBCO films. Pulsed laser deposited YBCO layers on the NiO/Ni tapes were not only c-axis oriented with respect to the type surface, but also strongly in-plane aligned. The YBCO films on NiO/Ni tapes have a zero resistance T/sub c/=87 K and J/sub c/=10/sup 4/-10/sup 5/ A/cm/sup 2/ at 77 K.

Uniform current distribution conductor of HTS power cable with variable tape-winding pitches

In the multilayer conductor, the inner layers have higher impedance than the outer layers. As a result, the current concentrates in the outer layers. Our early study showed that its AC losses were reduced to one-tenth by making the current of each layer uniform. From such a point of view, a trial to realize the uniform current distribution was made by adjusting the winding pitches of high-temperature superconducting (HTS) tapes layer by layer. A 1 m long conductor was fabricated, where the inner layer had longer winding pitch than the outer layer. Experimental results showed that the currents flowing in individual layers were almost the same and that this conductor had lower AC losses than the nonuniform current distribution conductor.

High critical current density YBa2Cu3O7−δ tapes prepared by the surface-oxidation epitaxy method

Abstract YBa2Cu3O7−δ (YBCO) films with high critical current density (Jc) were successfully fabricated on nickel tapes buffered with epitaxial NiO. NiO was prepared on the textured nickel tape by the surface-oxidation epitaxy (SOE) method. We have reported so far a critical temperature (Tc) of 87 K and Jc=4–6×104 A/cm2 (77 K, 0 T) for the YBCO films on NiO/Ni tapes. To enhance the superconducting properties of the YBCO films on the SOE-grown NiO, depositions of thin oxide cap layers such as YSZ, CeO2, and MgO on NiO were investigated. These oxide cap layers were epitaxially grown on NiO and provided the template for the epitaxial growth of YBCO films. Substantially improved data of Tc=88 K and Jc=3×105 A/cm2 (77 K, 0 T) and 1×104 A/cm2 (77 K, H∥c, 4 T) were obtained for YBCO film on NiO, by using a MgO cap layer with a thickness of 50 nm. The method described in this paper is a simple way to produce long YBCO tape conductors with high-Jc values.

The High Field Magnetic Dependence of Critical Current Density at 4.2 K for Ag-Sheathed Bi2Sr2CaCu2Oy Superconducting Tape

Critical current density (Jc) at 4.2 K and its dependence on applied magnetic field was measured for Ag-sheathed Bi2Sr2CaCu2Oy superconducting tape. The specimen showed a strong grain alignment which could be attained by adjusting the conditions of heat treatment. The degree of crystal orientation (F) was evaluated to be up to 95% by XRD for this specimen. In the case of magnetic field applied in a direction parallel to the tape, the Jc initially decreased with the magnetic field from 1.2×105 A/cm2 at 1 T to 5×104 A/cm2 at 10T and then remained almost constant up to 23 T. As the F value decreased from 95%, Jc also decreased. The pinning force (Fp) of the 95%-F-value tape was estimated to be about 1×1010 N/m3 at 23 T. This value is larger than those of conventional low-Tc superconductors.

High critical current density YBa2Cu3O7−δ films on surface-oxidized metallic substrates

Abstract YBa 2 Cu 3 O 7−δ (YBCO) films with high critical current density ( J c ) were fabricated on nickel tapes buffered with bi-axially textured NiO prepared by surface-oxidation epitaxy (SOE). The effects of oxide cap layers, such as YSZ, CeO 2 and MgO, on the SOE-grown NiO were investigated to improve the superconducting properties of the YBCO films on NiO. By inserting a thin MgO cap layer between NiO layer and YBCO film, a J c of 3×10 5 A/cm 2 (77 K, 0 T) was achieved. This result indicates the potentiality of the SOE method. In this paper, the application of the NiO/Ni substrate to non-vacuum processings, such as metal-organic deposition (MOD) and liquid phase epitaxy (LPE) will be also introduced.

50-m long HTS conductor for power cable

The long conductor fabrication is one of key technologies to realize the High-T/sub c/ superconducting power cable. A 50-m long conductor was fabricated by helically winding the High-T/sub c/ superconducting tape onto a former with a winding machine. The conductor consisted of ten layers of Ag-sheathed Bi-2223 tape which had a high critical current density of 10,000 A/cm/sup 2/ (at 77 K). AC losses and layer-by-layer current distribution were measured, feeding AC current of 100 to 2,000 Arms to the conductor cooled by LN2. The results showed that most of the current flowed in the outer layers where the impedance was low, and that AC losses were remarkably reduced by making the current distribution uniform.

Design and testing of a pair of current leads using bismuth compound superconductor

The thermal behavior of current leads using an oxide superconductor for the low-temperature portion has been studied. Numerical calculations predict a reduction of the necessary coolant flow rate and refrigerator input power. A pair of current leads has been manufactured where the low-temperature portion consists of six sintered Bi compound cylindrical bars and the high-temperature portion consists of a Cu wire bundle. The lead, cooled by gaseous helium along its entire length, is 0.9 m long and designed to carry 1 kA. The leads have been tested in the same arrangement as practical applications. The helium flow rate necessary to hold thermal equilibrium was about 80% of that for conventional copper leads. The calculation shows that power consumption of the refrigerator needed to cool high-temperature superconductor current leads with an optimum cooling scheme will be about one-third of that for conventional current leads.<<ETX>>

Synthesis of Superconductive Oxides by Vacuum Calcination Method

The vacuum calcination method (VCM) was developed to synthesize Ln-Ba-Cu-O group superconductive oxides represented by YBa2Cu3O7-x through solid phase reaction. When Y2O3, BaCO3 and CuO are used as raw materials in the form of powder in this method, the carbon content of the resultant calcinated product will be 1/10 that of the normal methods or lower. VCM powder is a mixture of Y2BaCuO5 and BaCuxOy, an unknown phase, and has extremely high activity because of oxygen content that is lower than the stoichiometric composition of YBa2Cu3O7-x. For this reason, it absorbs oxygen during sintering under an oxidizing atmosphere causing transition to the perovskite phase of YBa2Cu3O7-x, thus indicating excellent sinterability and high Jc and magnetization.

Solenoid Coil Using Multilayered Composite Wire of Bi-Based Superconductor

Fabrication of long wires is one of the key technologies for electric power application of high-Tc superconducting materials. For this, we fabricated a long wire with a multilayer structure of the Jelly-Roll type using Bi-based superconductor. The wire has high critical current and no anisotropy of a critical current density to a direction of an applied magnetic field. As a practical application of the wire, solenoid coils were fabricated and their characteristics were evaluated. At 4.2K, the coils with a 10 m long wire and a 22 m long wire quenched at experienced maximum fields (Bm) of 1.08T and 1.65T, respectively. And long wires wound in the coils had excellent superconducting properties in higher fields above 10T, which is similar to that of a shorter wire. Therefore, the wire is suitable for a high field solenoid type magnet.

The Transport Critical Current Property of High Tc Superconducting Wires

The Ag-sheathed high Tc superconducting wires were prepared by the powder- in-tube method. The transport critical current density (Jc) was measured by the four-probe method with 1 μ v/cm criterion. And the magnetic field dependence of transport Jc at 77K and 4.2K were evaluated. In YBCO system, we obtained the Jc of 103 A/cm2 order, but we could not obtain the higher Jc because of the weak link problem. In case of BSCCO system, the Ag-sheathed tapes showed strong grain alignment which could be attained by adjusting the process parameters. So we obtained the maximum Jc of 3.5x104A/cm2 at 77K and 0T. But this Jc decreased drastically under high magnetic field over 0.1T, because of the weak pinning force and resulting flux creep. At 4.2 K, the Jc of BSCCO Ag-sheathed tape under 10T showed 1.7x 105 A/cm2.