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By manipulating the cooling rate from the final heat treatment, we have raised the 77 K, self-field critical current density (J c ) of multifilament (Bi,Pb) 2 Sr 2 Ca 2 Cu 3 O x ͑2223͒ tapes by a factor of 3, and the irreversibility field (H*) by more than 50%. The J c of samples cooled in 7.5% O 2 from their reaction temperature of 825°C increased from ϳ8 to ϳ24 kA/cm 2 and H*͑77 K͒ increased from ϳ120 to ϳ200 mT as the cooling rate was decreased from 5 to 0.016°C/min. The results unambiguously show that the flux pinning properties of 2223 tapes can be improved by simple changes in wire processing. © 1996 American Institute of Physics. ͓S0003-6951͑96͒00145-3͔Improving the J c of 2223 superconductors in an applied magnetic field provides the most direct path to broad application of high-temperature superconductor ͑HTS͒ wire. Agclad 2223 made using the powder-in-tube process is currently the most promising HTS wire technology because established processes can be used to manufacture long lengths of strongly linked material. However, in contrast to Y-and Tl-based superconductors, 2223 has relatively weak flux pinning properties above about 30 K. For this reason, use of 2223 wires in moderate to high applied fields is currently limited to low temperature.Three strategies have been used previously to improve flux pinning in Bi-based superconductors. One successful approach has been postprocessing irradiation. 1 However, although the in-field transport J c can be significantly enhanced, it is not clear to what extent this method can be applied to practical wire manufacturing. A second approach has focused on introducing defects associated with dopants, dislocations, and particles. [2][3][4] While this has achieved some success, enhancements to the transport J c of HTS wires in an applied field has not yet occurred. Finally, controlled manipulation of the oxide constitution to produce impurity phase particles in Bi-2212 5 has met with limited success. In this contribution, we show that simple modifications to the heat treatment can enhance both the zero field and the infield properties of 2223 tapes.Parrell et al. 6 have shown that the rate at which 2223 tapes are cooled after heat treatment can have a large effect on the transport J c , critical temperature (T c ) transition, and filament microstructure. They reported that slow cooling ͑Ͻ0.1°C/min͒ in 7.5% O 2 increased the J c ͑77 K, self-field͒ by as much as 50% over tapes cooled more quickly, and considerably sharpened the T c transition, despite decomposition of the 2223 phase at temperatures below the 2223 phase stability limit. This letter builds on this previous work of Parrell et al. by exploring in detail one contribution to the J c improvement. In particular, we have evaluated the flux pinning contribution by measuring the cooling rate dependence of the irreversibility field, H*.Ag-clad 85-filament samples of nominal composition Bi 1.8 Pb 0.3 Sr 1.9 Ca 2.0 Cu 3.1 O x were processed to develop a dense and aligned 2223 phase structure. The final heat treatment...

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.

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The connectivity and flux pinning components of the critical current density J c in a high-J c Pb) 2 Sr 2 Ca 2 Cu 3 O x (Bi-2223) composite throughout important steps in its thermomechanical treatment were investigated by mass density measurement, microstructural observation and extensive superconducting property characterization. The mechanical integrity of the filaments was greatest after the first heat treatment, because damage produced by intermediate rolling was not completely erased by subsequent heat treatment. The final relative filament mass density was less than 75%, even though J c more than tripled to reach ∼50 kA cm −2 (77 K, 0 T) after final heat treatment. Filament resistance and J c data showed that the electrical connectivity improved significantly throughout the process, although ultrasonic fracturing, magneto-optical images and density measurements all showed that the mechanical and physical connectivity degraded. Our study suggests that the attainable J c of Bi-2223 is determined by the electrical connectivity of each filament which is itself an uncertain compromise between minimizing the significant porosity produced by the Bi-2223 formation reaction and the extensive crack network that porosity reduction by intermediate rolling produces.