High Strength Silver/Alumina Sheath for Bismuth Strontium Calcium Copper Oxide Conductor

Ongoing advances in high energy physics depend on developing high field superconducting (HTS) magnets capable of producing fields in excess of 20 T. These HTS magnets have the potential to be transformative for a variety of applications, including magnets for future high-energy physics accelerators. Neither NbTi nor Nb3Sn can provide fields greater than 18 T in dipole magnets or 20-22 T in solenoids, so future devices require HTS materials capable of generating high magnetic field. The potential candidates are YBa2Cu 3O7-delta (YBCO) coated conductor tapes and Bi2Sr 2CaCu2O8+x (Bi2212) round wire. YBCO tapes are highly anisotropic, constraining magnet design and adding complexity to its fabrication. Bi2212 is the only HTS material available as an isotropic multifilamentary round wire with high critical current density (Jc), which is preferred for magnets.;Ag/0.20wt%Mg is the most commonly used alloy for the outer sheath of Bi2212 wire. Despite the recent successes with Ag/0.20wt%Mg sheathed Bi2212, the use of Ag/0.20wt%Mg for the outer alloy sheath poses some challenges and limitations, including increased difficulty in conductor manufacturing. The alloy has low strength in the un-reacted condition and loses ductility rapidly as it is drawn, necessitating frequent anneals. Annealing restores ductility, but it must be done in an inert atmosphere or MgO will precipitate on the grain boundaries and embrittle the alloy, making further cold working impossible.;In this work, we report on dispersion-strengthened (DS) Ag/Al alloys with various compositions as potential candidates for sheathing Bi2212 wire. The fabrication of Ag/Al alloys by powder metallurgy, their internal oxidation heat treatment to form DS Ag/Al alloy, the relationship between microstructure, physical and mechanical properties of the DS Ag/Al alloys are studied in detail. Next, the Ag/Al alloys are used for making Bi2212 wire. Heat treatment, microstructure, mechanical and electrical properties of Bi2212/AgAl wires having various Al content are studied to improve the strain sensitivity of Bi2212 after partial melt processing (PMP). The properties of Ag/Al alloys and Bi2212/AgAl wires are compared with Ag/0.20wt%Mg alloy and Bi2212/AgMg wire.;The non-optimized DS Ag/0.50wt%Al alloy showed high yield strength and tensile strength in the annealed condition at both room temperature and 4.0 K. The DS Ag/0.50wt%Al alloy also showed significant ductility at 4.0 K, i.e., a temperature at which Ag and Ag/0.20wt%Mg have little measurable ductility. Transport measurements showed that the Bi2212/Ag0.50Al wires outperform Bi2212/Ag0.20Mg wires by nearly 50%, indicating that the Al2O3 precipitates in the sheath have neither any significant effect on oxygen solubility or oxygen diffusion through the sheath, nor any detrimental reactions with the Bi2212 filaments. Tensile studies showed that the as-drawn Bi2212/Ag0.50Al wire has very high strength. After PMP, the Bi2212/Ag0.50Al wire not only had yield and tensile strength that were slightly higher than those of Bi2212/Ag0.20Mg wire but also exhibited > 2% elongation, which was several times higher than that of Bi2212/Ag0.20Mg. The internal oxidation heat treatment was optimized to maximize the strength and modulus of DS Ag/Al alloy and it was found that the Ag/Al alloy oxidized at 675-700°C for 4 hours gave the highest tensile strength and hardness after PMP. In addition, this alloy could retain its fine grain size and strength during PMP in oxygen.;Scanning transmission electron microscope studies demonstrated the formation of nanosize MgO and Al2O3 precipitates via internal oxidation. Large spherical MgO precipitates were formed on the Ag grain boundaries of Ag/0.20wt%Mg alloy sheath, whereas the Al2O3 precipitates were distributed homogenously in the DS Ag/Al alloy. These large MgO precipitates made the Bi2212/Ag0.20Mg wire more brittle than Bi2212/AgAl wire. EDS elemental mappings demonstrated that less Cu diffused-out of the Bi2212 filament cores in the Bi2212/Ag0.75Al wire during PMP than that of Bi2212/Ag0.20Mg.