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

Abstract

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 (CIPping), the void space was almost removed and the gas bubble density was reduced significantly, resulting in a doubled engineering critical current density (J E ) of 810 A/mm 2 at 5 T, 4.2 K. Here we report on densifying Bi-2212 wire by swaging, which increased J E (4.2 K, 5 T) from 486 A/mm 2 for asdrawn wire to 808 A/mm 2 for swaged wire. This result further confirms that enhancing the filament packing density is of great importance for making major J E improvement in this roundwire magnet conductor. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Abstract-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 asdrawn 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 (CIPping), the void space was almost removed and the gas bubble density was reduced significantly, resulting in a doubled engineering critical current density (J E ) of 810 A/mm 2 at 5 T, 4.2 K. Here we report on densifying Bi-2212 wire by swaging, which increased J E (4.2 K, 5 T) from 486 A/mm 2 for as-drawn wire to 808 A/mm 2 for swaged wire. This result further confirms that enhancing the filament packing density is of great importance for making major J E improvement in this round-wire magnet conductor.

Investigation of inter-grain critical current density in Bi2Sr2CaCu2O8+δsuperconducting wires and its relationship with the heat treatment protocol

“…In any case, regardless the particular choice of parameters values, the comparison of experimental curves with calculated ones unambiguously indicates that in the W_Q and W_Qbp samples either the parameter B or the parameter M, or else both of them, increase as compared to the reference W_O2 sample. Considering the expression (4) and (5) for B and M, this trend can be again explained with a decrease of N in the W_Q sample as compared to the W_O2 sample, related to more insulating character of grain boundaries. On the other hand, in the W_PA sample either the parameter B or the parameter M, or else both of them, decrease, as a consequence of the increase of N in the more metallic grain boundaries.…”

“…(3) in [ 30 ]), with suitable boundary conditions at the SN interface and at asymptotic distance from it (see eqs. (4)(5)(6)(7)(8) in [ 30 ]). In these equations and boundary conditions, two key parameters describe the junction, namely B and M.…”

“…In the following discussion we focus on the central role of the parameter N, upon which the tunneling current depends exponentially, as compared to other parameters such as RB and N in eq. (4) and (5), that appear as pre-factors in the expression of the tunneling current [ 29 ]. In Figure 8a), the computed normalized critical current is plotted versus the reduced temperature T/Tc for different values of the ratio dN/N from zero to 12, as calculated on the basis of Likharev's work [ 32,33 ].…”

“…Yet, to further maximize inter-grain current in Bi-2212 wires, the main issues to investigate and improve are the presence of voids due to bubbles grown during the melt stage and the formation of secondary and offstoichiometric phases within filaments (mainly either Bi2Sr2CuOx (Bi2201) grains or Bi-2201 intergrowths within Bi-2212 grains) and at the grain boundaries. The former problem, which is responsible for the degradation of the supercurrent over long lengths, can be addressed either by applying an over pressure during the heat treatment [ 1,4,5 ] or by the use of a suitable alternation of groove rolling and drawing to diminish the porosity prior to the heat treatment [ 6 ]. On the other hand, the latter problem requires fine tuning of fabrication protocol, and there may still exist an edge of improvement in this direction.…”

Optimization of intergrain connection in high-temperature superconductor Bi 2 Sr 2 CaCu 2 O x

“…[8] The introduction of pinning centers and modification of lattice structures can effectively enhance the pinning mechanism. [8][9][10][11][12] The other factor is the intergrain weak links due to the low texture degree, [12][13][14][15] high porosity, [16][17][18][19] and/or grain boundaries with secondary phases or amorphous layers. [20,21] Therefore, new techniques are needed in order to obtain Bi-2212 superconductors with aligned microstructures, higher density, cleaner grain boundaries, and stronger flux pinning properties for the practical applications.…”