Significantly enhanced critical current density in Ag-sheathed (Bi,Pb)2Sr2Ca2Cu3Ox composite conductors prepared by overpressure processing in final heat treatment

Abstract

Overpressure ͑OP͒ processing otherwise fully treated, commercial Ag-sheathed multifilament (Bi,Pb) 2 Sr 2 Ca 2 Cu 3 O x ͑2223͒ composite conductors increased the critical current density J c ͑0.1 T, 77 K͒ by 37% to 30.8 kA/cm 2 and the self-field J c ͑SF, 77 K͒ to 69.6 kA/cm 2 . These improvements were obtained on full-size high current conductors such that critical current I c ͑0.1 T, 77 K͒ reached 80.6 A and I c ͑SF, 77 K͒ 181.7 A, even though there was a very strong self-field suppression of I c . Estimated values for the non-self-field-limited I c and J c ͑0 T, 77 K͒ reached 235 A and 90 kA/cm 2 . Scanning electron microscopy and superconducting quantum inference device measurement revealed that OP processing effectively suppressed cracks, porosity, and the volume fraction of the Bi 2 Sr 2 Ca 1 Cu 2 O y ͑2212͒ phase, which are all major current-limiting mechanisms in present 2223 conductors.

A Shell Model for the Filament Structure of Bi-2223 Conductors

“…It appears that the overall structure (shell and core) and the phase content (conversion efficiency, secondary phases, porosity, Bi-2212 intergrowth structures) are largely determined in the initial stages of conversion. Perturbations to the heat treatment such as variable oxygen partial pressures, thermal slide heat treatments, or overpressure processing can change the relative phase assemblage, but not the overall structure [2], [16], [17]. To change the overall structure, it appears that substantial changes would have to be made in the initial stages of the tape manufacture and initial heat treatment.…”

“…In this region, many of the Bi-2223grainsaremisalignedrelativetothesilverinterface.Overpressureprocessinghasbeenshowntobeeffectiveforreducingthe porosity in the core [15]. Changes in the phase assemblage and/or reductionsinthesecondaryphasecontenthavebeenobtainedwith variations in the heat treatments [2], [11], [16], [17]. However, the best Bi-2223 tapes that are currently produced still have greater than 10 vol.% residual secondary phases, which includes Bi-2212 grains and intergrowth structures [11].…”

“…Critical current densities ( 's) have reached 69. 6 (self-field, 77 K) in experimental Ag-sheathed Bi-2223 tapes [2]. However, significantly higher values approaching 300 have been observed for parts of these tapes by current reconstructions of magneto-optical images [3].…”

“…High-, monocore and multifilamentary tapes were obtained from the University of Wisconsin where over-pressure processing was incorporated into the heat treatments [2].…”

Superconducting materials for large scale applications

“…The second is due to unreacted streaks of 2212 phase that occur either as intergrowths within the 2223 phase or as individual grains within a larger colony of 10 2223 grains which share a common axis. Porosity and cracking have been remarkably reduced by processing at hydrostatic pressures of up to 150 bar, [63]- [66] advancing at the appropriate benchmark of 0.1 T at 77 K to 304 A/mm , as shown in Fig. 9.…”