A.N. Iyer

THE POWDER-IN-TUBE PROCESSING AND PROPERTIES OF BI-2223

Substantial progress has been made in fabricating long lengths of flexible silver-clad high-critical-temperature superconductor tapes by the powder-in-tube method. Tapes with high critical current density (Jc) that are attractive for electrical power and high-field magnet applications have been produced. At liquid helium (4.2 K) and liquid neon (27 K) temperatures, tapes made by this improved processing method yielded Jc values greater than 100 kA/cm2 at zero field; at liquid nitrogen temperatures (77 K), the Jc values exceeded 40 kA/cm2. This article discusses the processing and properties of short-length tapes and of pancake coils made from long-length tapes via “wind and react.”

Superconducting joints for silver-clad BSCCO tapes

The advent of high-Tc superconducting tapes for a variety of applications has resulted in the need for superconducting joints. Although long-length tapes can be custom-made for some applications, interconnections between subcoils of a magnet or shorter-length conductors for coil winding and current leads, make the development of such joints imperative. Additionally, high-quality short-or medium-length conductors, which are easier to make, can be joined for improved performance. Employing a novel chemical etching technique, we have fabricated lap joints between short lengths of silver-clad BSCCO tapes. Each joint was formed by etching the silver away and bringing together the exposed superconductor cores of two tapes together. The joined tapes were then subjected to a series of thermomechanical treatments. Detailed microstructural and electrical characterization within and across the joint was performed. Critical currents of up to 37 A within the joint region and 10 A through the joint region(at 77K) have been achieved.

Fabricating superconducting joints between Ag-clad BSCCO conductors

Abstract Significant progress has been made in the development of Ag-clad BSCCO conductors, which makes them attractive for electrical power and high-field magnet applications. Long conductors of up to several hundred meters in length and with high critical current densities have been fabricated by the powder-in-tube technique. For efficient use of these conductors, it is imperative that they be linked by superconducting joints. With the use of a novel chemical-etching technique, the Ag sheath from on side of a tape was selectively etched to expose the underlying superconductor core. Joints were formed by bringing together the exposed cores of two such tapes and heat treating them at the required temperature. Critical current (Ic) within and through the joint was measured, and the microstructure analyzed in detail. Ic within and through the joint was typically 37 and 10 A, respectively. At present, effort is underway to improve current transport through the joint.

Recent issues in fabrication of Ag-clad BSCCO superconductors

Long lengths of mono-and multifilament Ag-clad BSCCO superconductors were fabricated by the powder-in-tube technique. Critical current density (Jc) up to 12,000 A/cm2 has been achieved in an 850 m long multicore conductor. Long length conductors were formed into pancake-shaped coils by the wind-and-react approach. Test magnets were then fabricated by stacking the pancake coils and connecting them in series. The magnets were characterized as a function of applied magnetic field at various temperatures. A test magnet, fabricated with ≈770 m of BSCCO tape, generated fields of ≈1 T at 4.2K and ≈ 0.6 T at 27K, both in an applied background field of 20 T. Additionally, the strain tolerance of both mono-and multifilament conductors at 77K in 0.5 T applied field has been studied. We observed that multifilament conductors have better strain tolerance than monofilament tapes, retaining more than 90% of the initial critical current (at 0.5 T) with strain ≥1%.

Advances in fabrication of mono- and multifilament Ag-clad BSCCO superconductors

Fabricating long lengths of robust and high-quality conductors is imperative for various applications of high-{Tc} superconductors. Long lengths of mono- and multifilament Ag-clad Bi-Sr-Ca-Cu-0 conductors have been fabricated by the powder-in-tube technique. High values for critical current density (J{sub c}) have been achieved in both short- and long-length conductors. J{sub c} values up to 12,000 A/cm{sup 2} have been achieved in an 850-m-long multifilament conductor. Pancake-shaped coils and test magnets fabricated from long-length conductors were characterized at various temperatures and applied magnetic fields. A magnet containing 770 m of high-{Tc} conductor generated a record high field of {approx} 1 T at 4.2 K in a background field of {approx} 20 T. In-situ tensile and bending characteristics of both mono- and multifilament conductors have also been studied. Multifilament conductors exhibited better axial strain tolerance ({var_epsilon} {approx} 1%) than that of monofilament conductor ({var_epsilon} {approx} 0.2%), while retaining 90% of their initial critical current. An analysis of the results is presented, along with effects of parameters such as thickness, superconductor/Ag ratio, and microstructural details.

Processing and properties of long-lengths of Ag-clad BSCCO superconductors and high-{Tc} magnets

Long lengths of Ag-clad mono and multicore BSCCO tapes were fabricated by the powder-in-tube technique. The critical current density (J{sub c}) of 125-m-long monocore tapes was {approx}12,000 A/cm{sup 2} (critical current, I{sub c} 20 A) at 77 K. A 230-m-long 37-filament tape carried an I{sub c} of 14 A (corresponding to a J{sub c} of {approx}10,000 A/cm{sup 2}). Pancake-shaped coils were formed from long-length conductors by the wind-and-react approach. High-T{sub c} magnets were then assembled by stacking the pancake coils and connecting them in series. The magnets were tested as a function of applied magnetic fields at 4.2, 27, 64, and 77 K. A magnet containing 480 m of high-{Tc} tape generated a record-high field of 2.6 T at 4.2 K. Another magnet assembled with {approx}770 m of tape generated a field of {approx}1 T at 4.2 K and {approx}0.6 T at 27 K, both in an applied background field of {approx}20 T. Strain tolerance of high-{Tc} tapes was evaluated by measuring J{sub c} retention as a function of applied strain in an 0.5 T applied field at 77 K.

Processing and fabrication of high-{Tc} superconductors for electric power applications

Recent developments in the powder-in-tube fabrication of (Bi,Pb){sub 2}Sr{sub 2}Ca{sub 2}Cu{sub 3}O{sub x} tapes include identification of high current transport regions of the superconductor core, optimization of conductor design and processing to take advantage of these high current regions, optimization of superconductor powders and heat treatments, and incorporation of flux pinning defects into the superconductor grains. These developments are briefly discussed and their implications are assessed.

Fabrication of superconducting joints for Ag-clad BSCCO conductors

Potential applications of high-T{sub c} superconductors include motors, generators, transmission cables, magnets, etc. At present, resistive connections are used to connect various high-T{sub c} components for such applications. However, to improve efficiency, it is imperative that the resistive connection be replaced by a true superconducting joint. Using a novel etching technique, we have fabricated superconducting lap and butt joints between Ag-clad BSCCO conductors. The Ag sheath from one side of the tape was selectively etched to expose the underlying superconductor core. Joints were formed by bringing the two tapes together and heat treating them. Detailed microstructural analysis and current transport measurements of the joints have been performed. Critical current (I{sub c}) through a monofilament lap- and butt-joint were 10 and 23 A, respectively. I{sub c} within the joint for mono- and multifilament conductors were 37 and 21 A, respectively. Additionally, effects of various joint configurations, processing techniques, and strain on the transport property of the joint are also being studied.

Fabrication and properties of silver and silver-sheathed BSCCO conductors

Significant progress has been made in the development of silver- sheathed BSCCO conductors for potential electric power and high-field magnet applications. High critical current density (J{sub c}) has been achieved in mono- and multifilament conductors fabricated by the powder-in-tube technique; J{sub c} up to 12,000 A/cm{sup 2} has been observed at 77 K, in an 1260-m-long multifilament conductor. A high- {Tc} magnet generated a self-field of {approx} 3.2 T at 4.2 K. A 0.25 KVA high-{Tc} transformer has been developed with the use of a racetrack-wound solenoid. Strain tolerance of the conductors was evaluated by in-situ tensile and bending tests. Tensile testing indicated that multifilament conductors have better strain tolerance than monofilament conductors and are able to retain 90% of their initial current (I{sub c}) at a strain of {ge}1%. Effect of superconducting/Ag ratio on bending characteristics of the conductors was also evaluated; preliminary results indicate that the irreversible strain limit of the monofilament conductor increases with decreasing superconductor/Ag ratio.

Fabrication and characteristics of tapes and test magnets made from Ag-clad Bi-2223 superconductors

Pb0.4Bi1.8Sr2Ca2.2Cu3Ox (Bi-2223) precursor powder was prepared by a solid-state reaction of carbonates and oxides of lead, bismuth, strontium, calcium, and copper, and the powder was then used to fabricate silver-clad tapes by the powder-in-tube technique. Transport critical current density (Jc) values>4×104 A/cm2 at 77K and 2×105 A/cm2 at 4.2 and 27K have been achieved in short tape samples. Long lengths of tape were tested by winding them into pancake coils. Recently, we fabricated a test magnet by stacking ten pancake coils, each containing three 16m lengths of rolled tape, and tested it at 4.2, 27 and 77K. A maximum generated field of 2.6 T was measured in zero applied field at 4.2K and the test magnet generated significant self-field in background fields up to 20 T. The results are discussed in this paper.