Practical Cuprate Superconductors

Abstract

AbstractSustained research at R&D centres in several countries yielded huge success in the production of three most popular cuprates, namely REBCO, Bi-2223 and Bi-2212. The best superconductor the so-called 2G REBCO-‘coated conductor’ is produced by depositing highly oriented thin film of (GdY)BCO on biaxially textured Hastelloy substrates buffered with a number of oxide layers. The tape conductor is suitably laminated with Ag-alloy and then a high conducting hardened copper for stability and increased mechanical strength. Critical current density, \(J_{\text{c}}\) = 20 MA/cm2 (30 K, 3 T), has been achieved in (GdY)BCO tape (0.1 mm thick) deposited on a 30 µm Hastelloy substrate and doped with 25 mol% Zr. The BZO nanocolumns introduced act as pining centres and enhance the \(J_{\text{c}}\) to high value. Engineering current density, \(J_{\text{e}}\) = 586 A/mm2 (4.2 K, 20 T), has been obtained in round wires of (GdY)BCO, fabricated by winding number of narrow tapes over a central Cu former under condition of 15 mm radius bending. Cables capable of carrying currents as high as 90–100 kA have been designed for future fusion machine EU-DEMO and future accelerator detector magnets. Another promising cuprate, Bi-2212 conductor is produced in the form of round wire by the PIT method using pure Ag matrix. The round Bi-2212 wire generates a dominant a-axis growth texture and aligns the c-axis of its polycrystals along the filament axis making Bi-2212 to display a macroscopic isotropic behaviour. In a landmark development, it was found that \(J_{\text{c}}\) in Bi-2212 is limited by the presence of bubbles in the filaments formed by the agglomeration of porosity in the PIT wires during the heat treatment. These bubbles hindered the current flow. The bubble problem was overcome by employing an over-pressure heat treatment which densifies the core material to nearly 100% and allowed Bi-2212 to regrow inside the bubbles. NHMFL developed PIT Bi-2212 flexible round wires under 100 bar over-pressure which carried a \(J_{\text{e}}\) = 900 A/mm2 (4.2 K, 5 T). The wire was used for winding a small insert magnet which produced a magnetic field of 2.6 T in a background field of 31 T, thus generating a record field of 33.6 T. Bi-2223 is a tape conductor with anisotropic properties, yet it has sufficiently high \(J_{\text{c}}\) in the ⊥ field direction. Grain texture and high density in Bi-2223 are achieved by rolling and sintering the tapes under pressure. The tapes are produced through the standard PIT technique involving multiple drawing and rolling followed by sintering under pressure. For high strength, Bi-2223 tapes are laminated with either SS, Cu-alloy or Ni-alloy. Sumitomo, the only manufacturer of Bi-2223 tape wires, produces a variety of Bi-2223 wires suitable for different types of applications. The maximum critical tensile stress is 400 MPa for the HT-NX wires. HT-G wire has Ag-Au matrix and used for current leads because of its very low thermal conductivity. HT-AC wires have very low AC losses. Bi-2223 wires find application in power transmission and other devices.