Critical Current Distributions of Recent Bi-2212 Round Wires

Bi2Sr2CaCu2O₈₊ₓ(Bi-2212) is the only high-field, high-temperature superconductor (HTS) capable of reaching a critical current densityJc(16 T, 4.2 K) of 6500 A·mm−2in the highly desirable round wire (RW) form. However, state-of-the-art Bi-2212 conductors still have a critical current density (Jc) to depairing current density (Jd) 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 RWJcare 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 (Ic) distribution, since the localIcin 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 experimentalV-Itransition 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 theIcdistribution from thed2V/dI2treatment of theV-Icurves. Here we compare ∼ 0.1 m lengthIcdistributions of Bi-2212 RWs with recent state-of-the-art very high-JcEngi-Mat powder and lowerJcand older Nexans granulate powder. We do find that theIcspread for Bi-2212 wires is about twice the relative standard of high-JcNb–Ti well belowHirr. We do not yet see any obvious contribution of the Bi-2212 anisotropy to theIcdistribution and are rather encouraged that these Bi-2212 round wires show relativeIcdistributions not too far from high-JcNb–Ti wires.