Understanding processing–microstructure–properties relationships in Bi₂Sr₂CaCu₂O_x/Ag round wires and enhanced transport through saw-tooth processing

Abstract

Superconducting magnets generating magnetic fields above 25 T are needed for many scientific applications. Due to fundamental limitations in NbTi and Nb 3 Sn, such high-field superconducting magnets require alternative high-field conductors. One candidate conductor is round wire composites of Bi 2 Sr 2 CaCu 2 O x sheathed in a Ag-alloy matrix (Bi2212/Ag). The performance of such wires is sensitive to the heat treatment, so improvements in the critical current density (J c ) require a thorough understanding of the processing-structure-properties relationships. Here we present a two-part study. In part I, a new heat treatment approach, saw-tooth processing (STP), is introduced based upon previous results showing that Bi2212 nucleation is site-saturation limited. The microstructural evolution of Bi2212 filaments during processing is discussed and results from STP are compared with those from other processes. STP is shown to increase J c by 120% and 70% relative to partial-melt processing at 5 T and self-field respectively, and by 65% and 34% relative to split-melt processing. Yet STP also complicates the heat treatment by introducing a number of new heat treatment variables that affect the grain morphology, phase assemblage and oxygen content of the Bi2212 filaments and thus the transport properties. In part II, the effects of STP heat treatment parameters on the microstructure and transport properties are discussed. It is shown that wires with the highest transport critical current densities primarily have filaments with two types of microstructures, one comprised primarily of highly textured Bi2212 grains, and another with a noticeable amount of Bi 2 Sr 2 CuO x with the Bi2212.

High strength oxide dispersion strengthened silver aluminum alloys optimized for Bi2Sr2CaCu2O8+xround wire

“…Ongoing advances in high energy physics depend on developing high field superconducting magnets capable of producing fields in excess of 20 T. Neither NbTi nor Nb 3 Sn can provide fields greater than 18 T in dipole magnets or 20-22 T in solenoids, so future devices require high temperature superconductors (HTS) capable of generating high magnetic field [1][2][3][4][5]. The potential candidates are YBa 2 Cu 3 O 7−δ (YBCO) coated conductor and Bi 2 Sr 2 CaCu 2 O 8+x (Bi2212) round wire (RW).…”

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

“…The relationship between microstructure and transport behavior in Bi-2212 wires is a major issue, which is addressed by an extensive amount of literature [ 7,8,9 ]. In this work, we propose to take into consideration a further aspect, i.e.…”

“…On the individual filament scale, secondary phases such as Bi-2201 grains obstruct the supercurrent transport, by reducing the Bi-2212 grain connectivity [ 7,8 ]. The effect of oxygen post annealing on secondary phase formation and microstructure has been studied in [ 9 ]. In YBa2Cu3O7− (YBCO) conductors, transparency of grain boundaries was improved by chemical (Calcium) doping [ 10 ], which heals their small carrier density and proximity to a parent, antiferromagnetic insulating state at the grain boundaries.…”

Statistical study of the void structure of Bi2212 multifilamentary superconducting wires and its effect on the critical current density

“…For practical Bi2212 multifilamentary wires with porous microstructure, usually, to improve the J c of wires, besides improving the number of Bi2212 filaments (can be implemented by means of increasing the number of filaments in bundles or/and the number of bundles), suppressing the void porosity of filaments is regarded as an effective method. So far, there have been many studies of making efforts on reducing the void porosity (for example, over-pressure processing) during heat treatment (Larbalestier et al, 2014;Jiang et al, 2011;Jiang et al, 2013;Naderi et al, 2013;Miao et al, 2014). Here, on the electrical model, for the wires with three practical conductor architectures, we predicted the porosity dependence of the J c of wires.…”