Yibing Huang

Bubble formation within filaments of melt-processed Bi2212 wires and its strongly negative effect on the critical current density

Most studies of Bi2Sr2CaCu2Ox (Bi2212) show that the critical current density Jc is limited by the connectivity of the filaments, but what determines the connectivity is still elusive. Here we report on the role played by filament porosity in limiting Jc. By a microstructural investigation of wires quenched from the melt state, we find that porosity in the unreacted wire agglomerates into bubbles that segment the Bi2212 melt within the filaments into discrete sections. These bubbles do not disappear during subsequent processing because they are only partially filled by Bi2212 grains as the Bi2212 forms on cooling. Correlating the microstructure of quenched wires to their final, fully processed Jc values shows an inverse relation between Jc and bubble density. Bubbles are variable between conductors and perhaps from sample to sample, but they occur frequently and almost completely fill the filament diameter, so they exert a strongly variable but always negative effect on Jc. Bubbles reduce the continuous Bi2212 path within each filament and force supercurrent to flow through Bi2212 grains that span the bubbles or through a thin Bi2212 layer at the interface between the bubble and the Ag matrix. Eliminating bubbles appears to be a promising new path to raise the Jc of Bi2212 round wires.

Experimental study of loss mechanisms of AgAu/PbBi-2223 tapes with twisted filaments under perpendicular AC magnetic fields at power frequencies

AC losses under perpendicular AC fields have been measured at 77 K and power frequencies for multifilamentary AgAu (10 wt.%)/Bi-2223 tapes with filaments twisted at different pitches. Using simultaneous measurements of the first and higher harmonics of the voltage induced in the pick-up coil, the main loss contributions (superconductor and coupling current losses) have been obtained separately. At power frequencies, twisting produces the desired uncoupling of the filaments at fields lower than the coupling field, which has also been determined experimentally. In the uncoupled-filament regime, the superconductor losses are reduced strongly with respect to the untwisted tapes. The reduction of the total loss with twisting is also observed. However, due to the important contribution of the coupling current losses for this field orientation, a very small pitch (<5 mm) is necessary for a considerably lower loss than that of untwisted tapes. The dependence of the coupling field and coupling current losses on the twist pitch has been analysed and compared with the theoretical predictions.

Reduction of Gas Bubbles and Improved Critical Current Density in Bi-2212 Round Wire by Swaging

Bi-2212 round wire is made by the powder-in-tube technique. An unavoidable property of powder-in-tube conductors is that there is about 30% void space in the as-drawn wire. We have recently shown that the gas present in the as-drawn Bi-2212 wire agglomerates into large bubbles and that they are presently the most deleterious current-limiting mechanism. By densifying short 2212 wires before reaction through cold isostatic pressing, the void space was almost removed and the gas bubble density was reduced significantly, resulting in a doubled engineering critical current density (JE) of 810 A/mm2 at 5 T, 4.2 K. Here we report on densifying Bi-2212 wire by swaging, which increased JE (4.2 K, 5 T) from 486 A/mm2 for as-drawn wire to 808 A/mm2 for swaged wire. This result further confirms that enhancing the filament packing density is of great importance for making major JE improvements in this round-wire magnet conductor.

Internal Tin

The critical current density (J_c) of Nb₃Sn strand has been significantly improved over the last several years. For most magnet applications, high J_c internal tin has displaced bronze process strand. The highest J_c values are obtained from distributed barrier strands. We have continued development of strands made with Nb-47 wt%Ti rods to supply the dopant, and have achieved J_c values of 3000 A/mm² (12 T, 4.2 K). Such wires have very good higher field performance as well, reaching 1700 A/mm² at 15 T. To reduce the effective filament diameter in these high J_c strands, the number of subelement rods incorporated into the final restack billet has been increased to 127 in routine production, and results are presented on experimental 217 stacks. A new re-extrusion technique for improving the monofilament shape is also described. For fusion applications such as ITER, we have developed single-barrier internal tin strands having non-Cu J_c values over 1100 A/mm² (12 T, 4.2 K) with hysteresis losses less than 700 mJ/cm³ over non-Cu volume. The J_c-strain behavior of such composites is also presented.

{\hbox {Nb}}_{3}{\hbox {Sn}}

The critical current density (J_c) of Nb₃Sn strand has been significantly improved over the last several years. For most magnet applications, high J_c internal tin has displaced bronze process strand. The highest J_c values are obtained from distributed barrier strands. We have continued development of strands made with Nb-47 wt%Ti rods to supply the dopant, and have achieved J_c values of 3000 A/mm² (12 T, 4.2 K). Such wires have very good higher field performance as well, reaching 1700 A/mm² at 15 T. To reduce the effective filament diameter in these high J_c strands, the number of subelement rods incorporated into the final restack billet has been increased to 127 in routine production, and results are presented on experimental 217 stacks. A new re-extrusion technique for improving the monofilament shape is also described. For fusion applications such as ITER, we have developed single-barrier internal tin strands having non-Cu J_c values over 1100 A/mm² (12 T, 4.2 K) with hysteresis losses less than 700 mJ/cm³ over non-Cu volume. The J_c-strain behavior of such composites is also presented.

Conductors Engineered for Fusion and Particle Accelerator Applications

Wind & react Bi-2212 inserts have been manufactured and tested inside a wide-bore NbTi - Nb3Sn magnet providing a background field up to 20 T at 4.2 K. A pair of six-layer concentric coils both achieved critical currents of 350 A (JE = 200 A/mm2) in a 20 T background field. A thicker 14-layer insert made from 119 m of round wire had a critical quench current IQ of 287 A (JE = 162 A/mm2) at the same field and contributed to a combined central field of 22.5 T. This is a record for a fully superconducting magnet at 4.2 K. The 14-layer coil, equipped with an external protective shunt, was used for an extensive series of quench measurements and endured > 150 quenches without damage. Minimum quench energies were found to be in the range of 200-500 mJ in background fields of 15-20 T when the coil carried 70-95% of its critical quench current.

Internal Tin Nb3Sn Conductors Engineered for Fusion and Particle Accelerator Applications

Magneto-optical current reconstruction has been used for detailed analysis of the local critical current density (J/sub c/) variation in monocore Bi-2223 tapes. We find, even in high quality tapes with bulk transport j/sub c//spl sim/40 kA/cm/sup 2/ (77 K, 0 T), that there exist local regions which possess current densities of more than 200 kA/cm/sup 2/. Overpressure processing at 148 bar significantly improved J/sub c/ to 48 kA/cm/sup 2/ by improving the connectivity. For the overpressure-processed sample we find that the current distribution is more uniform and that the maximum local current density at 77 K is increased almost to 300 kA/cm/sup 2/.

The Development of High Field Magnets Utilizing Bi-2212 Wind & React Insert Coils

There has been sustained interest in the development of Bi-2212/Ag round wire because of its unique potential for application in ultra-high-field magnets (>; 25 T). Our development activity with this material has been focused on improving the engineering current density. Filament densification by swaging and isostatic pressing processes have been evaluated, as has further optimization of the melt heat treatment conditions. These improvements lead to an increased mass density of the filament in the final wire, and are essential to reduce filament porosity and obtain high performance over long wire length. Engineering current density values exceeding 480 A/mm2 at 4.2 K, 15 T have been achieved on 1-m-long barrel samples.

Local measurement of current density by magneto-optical current reconstruction in normally and overpressure processed Bi-2223 tapes

Oxford Superconducting Technology continuously improves Bi-2212 round wire performance because this product has a unique property for ultra high-field magnet ( 25 T) applications. Our recent results on improving the engineering current density by filament densification and reducing trace carbon and hydrogen contamination are presented. The swaging, isostatic pressing, and over-pressure heat treatment processes have been demonstrated to effectively increase the Bi-2212 filament mass density in the final wire and results in high performance over long wire length. Engineering current density values exceeding 550 A/mm2 at 4.2 K, 15 T have been achieved on 1-m-long barrel samples. Several configurations have been developed to meet different operating current requirements with optimum filament size ( ~ 15 micron) on critical current density, Jc. The round wire has also been twisted down to 12 mm in twist pitch length without performance degradation.

Recent Advances in Bi-2212 Round Wire Performance for High Field Applications

Overpressure (OP) processing influences the microstructure and critical current density (J/sub c/) of Ag sheathed Bi-2223 tapes. SEM and mass density measurements show higher core density and fewer micro-cracks in OP tape than in 1 atm tape. The self-field critical current density, J/sub c/ (0 T, 77 K) in multifilamentary tapes was increased from 33.5 kA/cm/sup 2/ with 1 atm processing (1 atm IR) to 48 kA/cm/sup 2/ with OP processing (OP pressure = 148 atm) after the first heat treatment (OP HT1), and to 58.7 kA/cm/sup 2/ with OP processing after intermediate rolling (OP IR). The corresponding values for J/sub c/ (0.1 T, 77 K) are 12.3 kA/cm/sup 2/ (1 atm IR) to 18.2 kA/cm/sup 2/ for OP HT1 and to 22.4 kA/cm/sup 2/ for OP IR.