Perspective on Kilometer-Scale MgB₂ Superconducting Wires With 127 Filaments

Since 2015, we have successfully developed magnesium diboride (MgB<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub>) wire at the 3 km-scale and achieved a high critical current density (<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i><sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub>) of 3.5 × 10<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> A/mm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 4.2 K and 3 T for various applications. To ensure mechanical strength and performance stability, it is necessary to design MgB<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> wire with a large number of internal filaments. In this study, we conducted a comparative analysis of the number/size of filaments and electrical properties of 19 (18 + 1Cu)-, 37 (30 + 7Cu)-, 61 (54 + 7Cu)-, 91 (72 + 19Cu and 78 + 13Cu)-, and 127 (114 + 13Cu)-filament MgB<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> wires. Increasing the number of filaments had a minor effect on the fill factor and did not significantly enhance the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i><sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub>. However, it was observed that the deformation of the internal filaments became more inhomogeneous. The mean filament size of the 19-filament architecture was approximately 80 µm, while that of the 127-filament architecture showed a significant reduction, effectively reaching less than 33 µm. The microstructure and characterization of the produced wires are discussed in detail.