Mono-core Wires Research Articles

Fabrication and Transport Properties of<tex>$rm MgB_2$</tex>Mono-Core Wire and Solenoid Coil

Round 100 m long class MgB/sub 2/ mono-core wires with Fe/Cu composite sheaths were successfully fabricated using an in situ PIT method. The J/sub c/ of the round MgB/sub 2//(Fe/Cu) wires reached 380 A/mm/sup 2/ at 4.2 K and 6 T, and exceeded 2500 A/mm/sup 2/ at 3 T. High areal reduction of the MgB/sub 2/ core was found to be effective in improving the transport properties of the MgB/sub 2/ wires. Using a 58 m long MgB/sub 2//(Fe/Cu) wire, we made a solenoid coil with 495 turns and tested it in liquid helium. The dimensions of the MgB/sub 2/ coil were 30 mm in inner diameter, 48 mm in outer diameter, and 50 mm in height. The I/sub c/ of the coil reached 214 A, which corresponds to a J/sub coil/=238 A/mm/sup 2/ and a core-J/sub c/=1420 A/mm/sup 2/. The coil generated a magnetic field at a center of B/sub o/=2.05 T and a maximum magnetic field of B/sub m/=2.17 T.

Laser treatment of press-formed bismuth based superconductor powder: K. Hotta et al. (Nippon Inst. of Technology, Saitama, Japan.) J. Japan Soc. Powder/Powder Metall., Vol 48, No 12, 2001, 1156–1159. In Japanese

Although Ag sheathed Bi2223 tapes have the performances of Ic values over 100 A and length of 1000 m which are enough for prototype applications, their AC loss is still too high for practical applications. To reduce the AC loss, the introduction of high resistive barrier between filaments and the filament twisting are effective. In this study, the Bi2223 tapes with CuO barriers between filaments were fabricated through the in situ oxidation method. Mono-core wires covered with metal Cu were stacked and deformed to form 19-filamentary tapes. These tapes were sintered, and the metal Cu layer was oxidized to form CuO barrier layer. Although the CuO grains in barrier layer easily agglomerated during sintering, the continuity of CuO barrier could be kept well by decreasing total sintering time and by the use of thick barrier layer. However the conversion ratio from Bi2212 to Bi2223 became low by increasing the barrier thickness. In addition, XRD analysis revealed that the impurity phase of (Sr, Ca)14Cu24Ox phase easily formed in the thick barrier tape. The Jc value of over 1 × 104 A/cm2 at 77 K, 0T was achieved in CuO barrier tape with the initial thickness of 20 μm. The proper control of initial thickness and sintering conditions is important to achieve high Jc and good continuity of barrier.

Evaluations of Nb-25 wt.%Ti alloy wires

An arc-melted Nb-25 wt.%Ti alloy was fabricated into monocore wires with Cu matrix. The structure and superconducting properties of the wires were compared with those of Nb-46.5 wt.%Ti and Nb-36 wt.%Ti wires prepared by similar process. The formation of a thick Cu-Ti-Nb compound layer is observed at the Cu/Nb-46.5Ti interface after heat treatment at above 600/spl deg/C. A thin compound layer is also formed at the Cu/Nb-36Ti interface after heat treatment. Meanwhile, no compound layer is observed at the Cu/Nb-25Ti interface. This is a major advantage of Nb-25Ti wire, which may save the wrapping of Nb foil around the cores to prevent the reaction with the Cu matrix, when they are fabricated into fine filaments. The T/sub c/ of Nb-25Ti wire is about 0.7 K higher than that of Nb-46.5Ti wire. The T/sub c/ and B/sub c2/ of Nb-25Ti wire are less sensitive to the reduction in diameter and the heat treatment compared to the Nb-46.5Ti wire. The B/sub c2/(4.2 K) of Nb-25Ti as wire seems to be high enough for AC applications. The addition of boron enhances the T/sub c/ and B/sub c2/ of Nb-25Ti alloy.

Texture analysis of Bi-2212 and 2223 tapes and wires by neutron diffraction

The crystallographic texture of silver sheathed monocore tapes and wires, and multifilamentary tapes containing Bi 2Sr 2CaCu 2O x (Bi-2212) and Bi 2Sr 2Ca 2Cu 3O x (Bi-2223) superconducting oxides was determined by neutron diffraction, using a position sensitive detector. The orientation distribution (OD) was obtained from measurements of 4–6 pole figures. The oxide textures of these tapes and wires displayed axial symmetry, indicative of a grain-shape-controlled orientation mechanism. In monocore and multifilamentary tapes the c-axes of Bi-2212 and Bi-2223 are oriented perpendicular to the rolling plane, in wires they are perpendicular to the wire direction. The multifilamentary tape has the strongest texture with an OD maximum of 46 times that of a random sample. Monocore tapes are 26 and 35 mrd, and wires are 2.1 and 1.5 mrd. For comparison, a sinter-forged Bi-2223 barpowder was analyzed and exhibited c-axes parallel to the compression direction with a maximum of only 3.4 mrd. The texture of the silver sheaths depends on the processing conditions.

Axial and bending strain effects in Ag and AgNiMg/Bi-2212 wires

The effects of axial tensile and bending strains on the critical current densities of Ag and AgNiMg/Bi-2212 wires produced by the powder-in-tube method have been studied. At T = 4.2 K and B = 12 T the critical current densities of the investigated single and multicore wires exceeded 150 and 80 A mm −2 (2 μV cm −1 offset criterion), respectively. The observed degradation of the critical current density due to strain effects is irreversible and can be attributed to the formation of microcracks within the superconducting core. In the case of Ag/Bi-2212 wires the axial tensile strain values ε 0.8 leading to a 20% reduction of the critical current density at 4.2 K and 12 T are around 0.2% for both single and multicore wires with seven filaments, whereas the ε 0.8 values for AgNiMg/Bi-2212 single and multicore wires exceed 0.5 and 0.8%, respectively. The bending strain experiments reveal a reduced sensitivity to strain effects for Ag/Bi-2212 wires with a larger fraction of Ag in the conductor cross-section. Bending at elevated temperatures has been found to affect the critical current density of Ag/Bi-2212 monocore wires at lower strain levels than axial tensile strains applied at 4.2 K. The strain leading to an irreversible reduction of the critical current density seems to be determined mainly by the residual thermal strains. Finally, results of mechanical properties of the composite wires at 4.2 K are presented. The Young's modulus of the composite wires is nearly the same for the two sheath materials, whereas the yield strengths σ 0.2 are very different. For Ag and AgNiMg/Bi-2212 single core wires, the average σ 0.2 values at 4.2 K are 46 and 89 MPa, respectively.

Critical current density of Bi 2Sr 2Ca 1Cu 2O δ monocore and multifilamentary wires from 4.2 K up to Tc in high magnetic fields

The transport critical current density ( J c) of a mono, 19 and 37 filament Bi 2Sr 2Ca 1Cu 2O δ wire has been measured in high fields at temperatures of 4.2 K up to 80 K. Measurements were made with the field applied parallel and perpendicular to the wire axis. The low-temperature high-field J c values for the 19 and 37 filament wires are similar and markedly higher than the monocore wire. The data have been analyzed in terms of pinning-force functions and an exponential field function. In low fields, below 40 K, the volume pinning force is given by a power law of the form F p ∝ B 3 4 . In high fields, the functional form is given by J c( B, T) = α( T) exp[ − B/ β( T)]. The parameters α( T) and β( T) are measured for the three wires and compared with those of a monocore Bi 2− x Pb xSr 2Ca 2Cu 3O δ silver-shathed tape.

Diameter-dependent critical current densities for superconducting Bi-2212/Ag wires

Superconducting Bi-2212/Ag monocore wires of 0.65, 1.0, 1.2 and 1.5 mm diameter have been fabricated by the powder-in-tube method. The critical current densities increase with decreasing wire diameters. For short specimens of 0.65 mm diameter, maximum critical current densities of up to 120 000 A cm −2 ( B=D) and 40 000 A cm −2 ( B = 12 T) have been achieved at 4.2 K. For wires of 1.5 mm diameter the scatter in critical current data is ≈ 10% within groups of specimens heat treated together. For the thinner wires this scatter is more pronounced. The heat treatment conditions leading to the highest critical current densities have been found to depend on the wire diameter. An increasing thickness of the silver sheath may be responsible for a retarded release of oxygen from the core before the superconductor is partially molten. Evidence for grain alignment in a thin layer at the interface to the silver of typically 10–20 μm thickness has been found by scanning electron microscopy. A model which explains the observed scaling behaviour of the critical current density with the wire diameter has been developed. Using this model the critical current density in the interfacial region can be estimated.

Characteristics of superconducting multi- and monocore wires based on Bi 2Sr 2CaCu 2O x

Using the power-in-tube technique superconducting monocore and multicore wires (up to 2527 cores) have been produced with different diameters (down to 0.1 mm) and a length of several hundred metres. These wires show critical current densities of − 13 500 A cm −2 in zero magnetic field and 6000 A cm −2 in a field of 10 T at 4.2 K, respectively. The wires were characterized with the intention of producing a superconducting coil. For this purpose, the transition temperature, critical current density, volume pinning force and flux flow resistance of the wires were investigated.

Characteristics of high-temperature superconducting wires on the basis of YBa2Cu3Ox

Using the powder-in-tube technique, superconducting monocore and multicore wires have been produced with several outer diameters (the thinnest one was (1)/(10) mm) and a length of more than 1000 m. The procedure was optimized for achieving high-transport current density. Three parameters were varied, namely, wire diameter, sinter duration, and sinter temperature. The most powerful wire prepared had a critical transport current density of 750 A/cm2.