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.