Critical Current Distributions of Recent Bi-2212 Round Wires

Bi ₂ Sr ₂ CaCu ₂ O $_{8+x} $ (Bi-2212) is the only high-field, high-temperature superconductor (HTS) capable of reaching a critical current density J _c (16 T, 4.2 K) of 6500 A·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 _c ) to depairing current density ( J _d ) ratio around 20 to 30 times lower than that of state-of-the-art Nb–Ti or REBCO. Previously, we have shown that recent improvements in Bi-2212 RW J _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 _c ) distribution, since the local I _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. Modeling the experimental V-I transition measured on a low resistance shunt as a complex sum of voltage contributions of individual filament and wire sub-sections allows a numerical extraction of the I _c distribution from the d²V/dI² treatment of the V-I curves. Here we compare ∼ 0.1 m length I _c distributions of Bi-2212 RWs with recent state-of-the-art very high- J _c Engi-Mat powder and lower J _c and older Nexans granulate powder. We do find that the I _c spread for Bi-2212 wires is about twice the relative standard of high- J _c Nb–Ti well below H _irr . We do not yet see any obvious contribution of the Bi-2212 anisotropy to the I _c distribution and are rather encouraged that these Bi-2212 round wires show relative I _c distributions not too far from high- J _c Nb–Ti wires.