Critical Current Distributions of Recent Bi-2212 Round Wires

Bi-2212 is the only high-field, high-temperature superconductor (HTS) capable of reaching a critical current density \(J_{\text{c}}\)(16 T, 4.2 K) of 6500 \(\mathrm{A\cdot mm^{-2}}\) in the highly desirable round wire (RW) form. However, state-of-the-art Bi-2212 conductors still have a critical current density (\(J_{\text{c}}\)) to depairing current density (\(J_{\text{d}}\)) ratio around 20 to 30 times lower than that of state-of-the-art \(\mathrm{Nb-Ti}\) or REBCO. Previously, we have shown that recent improvements in Bi-2212 RW \(J_{\text{c}}\) are due to improved connectivity associated with optimization of the heat treatment process, and most recently due to a transition to a finer and more uniform powder manufactured by Engi-Mat. One quantitative measure of connectivity may be the critical current (\(I_{\text{c}}\)) distribution, since the local \(I_{\text{c}}\) in a wire can vary along the length due to variable vortex-microstructure interactions and to factors such as filament shape variations, grain-to-grain connectivity variations and blocking secondary phase distributions. Here we compare \(\sim\) 0.1 m length \(I_{\text{c}}\) distributions of Bi-2212 RWs with recent state-of-the-art very high-\(J_{\text{c}}\) Engi-Mat powder and lower \(J_{\text{c}}\) and older Nexans granulate powder. We do find that the \(I_{\text{c}}\) spread for Bi-2212 wires is about twice the relative standard of high-\(J_{\text{c}}\) \(\mathrm{Nb-Ti}\) well below \(H_{\text{irr}}\). We do not yet see any obvious contribution of the Bi-2212 anisotropy to the \(I_{\text{c}}\) distribution and are rather encouraged that these Bi-2212 round wires show relative \(I_{\text{c}}\) distributions not too far from high-\(J_{\text{c}}\) \(\mathrm{Nb-Ti}\) wires.