The impurities within a popular superconducting round wire have been found to influence its performance. Materials scientists based at North Carolina State University in the US found that the size of the impurities determines whether the wire’s superconducting properties are either hindered or improved.

Bismuth strontium calcium copper oxide, Bi2Sr2CaCu2Ox  better known as Bi2212, is the only high-temperature superconductor that can be molded into round wires. Combined with silver to make Bi2212/Ag multifilamentary round wires, it has found a range of uses including in high field dipoles and quadrupoles for high energy hadron colliders, and in NMR magnets.   

The impact sizable voids in the Bi2212 has on its superconducting capabilities is well studied, this team instead chose to assess how the presence of impurities such as Bi2201 in the material hurts or hinders its performance. “Here, we study the roles of intrinsic nanoscale defects in Bi2212 superconductiving behavior,” one of the authors Golsa Naderi told Materials Today.

Focused ion beam image of a Bi2212 sample. Credit: Golsa Naderi
Focused ion beam image of a Bi2212 sample. Credit: Golsa Naderi

The wires were made by heating Bi2112 and silver together to 900ºC, a method developed by the team in 2013. “We then used state-of-the-art FEI Titan 80-300 probe aberration corrected STEM with monochromator to characterize the defects in depth,” says Naderi.

The team’s experiments described in Applied Physics Letters [Naderi G. & Schwartz J., Appl. Phys. Lett. (2014) doi: 10.1063/1.4871805] showed that 1.2nm to 2.5nm sized impurities appear to improve the superconducting capabilities of Bi2212/Ag. In contrast larger-scale, micrometer-sized, impurities were shown to reduce its superconductivity.

“The nanoscale impurities serve as centers for ‘pinning’ magnetic flux in place,” explains Naderi. “Without those pinning centers, the magnetic vortices can move, creating resistivity and impeding superconductivity when a magnetic field is present. People want to use Bi2212 to create high magnetic fields using current, so pinning magnetic flux is essential.”

The larger Bi2201 impurities, however, act as barriers to current. This, the team explain, forces the electrons to change paths and therefore reduces the material’s superconducting capabilities.  

Next, the plan is to work towards a new way to make these round wires that minimizes the formation of the large impurities while ensuring a sufficient number of small impurities remain. “It’s not easy,” Naderi concedes. “There are many factors to consider when optimizing [the synthesis]. But now we have a better understanding of what we want to obtain, so it will become easier.”