Processing, microstructure, and critical current density of Silver-sheathed Bi2Sr2CaCu2Ox multifilamentary round wire

Ag-sheathed multifilamentary Bi2Sr2CaCu2Ox round wire is one of the leading hightemperature superconductors that can generate a magnetic field exceeding the maximum of ~23 T available in present Nb-based low-temperature superconducting magnet technology. However, the magnet fabrication of power-in-tube (PIT) Ag-Bi2Sr2CaCu 2Ox multifilamentary round wire to develop critical current density Jc > 105 A/cm2 in magnetic fields up to 45 T is difficult, due to complicated material processing, as-yet incompletely understood microstructure, and the problem that Jc is sensitive to high-temperature reactions. This thesis analyzed the critical steps of melt processing PIT Bi2Sr2CaCu2Ox multifilamentary wires, systematically investigating the relationships between processing, microstructure, and conductor & magnet performance.;The phase transformation and microstructure development during the melt processing of Bi2Sr2CaCu2Ox wires were thoroughly examined using a brine-quench technique that preserves the high-temperature microstructures. On heating to the maximum temperature (~890 °C), Bi2Sr2CaCu2Ox powder melts incongruently, producing a mixture of liquid and secondary solid phases. On subsequent cooling, the liquid reacts with the solid phases and Bi 2Sr2CaCu2Ox reforms. The phase reaction to Bi2Sr2CaCu2Ox is often incomplete, leaving remnant nonsuperconducting phases from the melt and the Bi2Sr 2CaCu2Ox phase and intergrowth in the superconducting matrix, all of which become current limiting mechanisms (CLMs) and block current flow. Moreover, the gas between precursor powder grains accumulates into large pores upon melting, which divide the filament into segments. The consequence of having large pores in the melt is that the pore regions may become bottlenecks for current flow in fully reacted wires. The high population of CLMs strongly indicates that the fraction of oxide filament area that is effectively used for carrying current is low and increasing the connectivity is the key to improving Jc of Bi2Sr2CaCu2Ox wires.