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.