IEEE Transactions on Applied Superconductivity | 2021
Critical Current Distributions of Recent Bi-2212 Round Wires
Abstract
Bi<sub>2</sub>Sr<sub>2</sub>CaCu<sub>2</sub>O<inline-formula><tex-math notation= LaTeX >$_{8+x} $</tex-math></inline-formula> (Bi-2212) is the only high-field, high-temperature superconductor (HTS) capable of reaching a critical current density <italic>J</italic><sub>c</sub>(16 T, 4.2 K) of 6500 A·mm<sup>−2</sup> in the highly desirable round wire (RW) form. However, state-of-the-art Bi-2212 conductors still have a critical current density (<italic>J</italic><sub>c</sub>) to depairing current density (<italic>J</italic><sub>d</sub>) 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 <italic>J</italic><sub>c</sub> 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 (<italic>I</italic><sub>c</sub>) distribution, since the local <italic>I</italic><sub>c</sub> 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 <italic>V-I</italic> 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 <italic>I</italic><sub>c</sub> distribution from the <italic>d<sup>2</sup>V/dI<sup>2</sup></italic> treatment of the <italic>V-I</italic> curves. Here we compare ∼ 0.1 m length <italic>I</italic><sub>c</sub> distributions of Bi-2212 RWs with recent state-of-the-art very high-<italic>J</italic><sub>c</sub> Engi-Mat powder and lower <italic>J</italic><sub>c</sub> and older Nexans granulate powder. We do find that the <italic>I</italic><sub>c</sub> spread for Bi-2212 wires is about twice the relative standard of high-<italic>J</italic><sub>c</sub> Nb–Ti well below <italic>H</italic><sub>irr</sub>. We do not yet see any obvious contribution of the Bi-2212 anisotropy to the <italic>I</italic><sub>c</sub> distribution and are rather encouraged that these Bi-2212 round wires show relative <italic>I</italic><sub>c</sub> distributions not too far from high-<italic>J</italic><sub>c</sub> Nb–Ti wires.