Fabrication of km-Class 30-Filament MgB ₂ Wires by the Internal Magnesium Diffusion Method

MgB<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> is one of the practical superconductors that has been widely studied over the past two decades. The internal magnesium diffusion (IMD) method serves as a promising method because it can enhance the grain connectivity of MgB<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> phase and thereby improve the superconducting current of MgB<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> wires. Here, we present the fabrication of 30-filament MgB2 wires using the IMD method. The wires have a diameter of 1.00 mm and length more than 1 km. The filling factor of MgB<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> is around 10% and the superconducting current <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I_c</i> (4.2K, 3T) reaches 650 A. The highest layer-superconducting current density <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J_e</i> (4.2K, 3T) is 8.9×103 A mm-2 and engineering superconducting current density <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J_e</i> (4.2K, 3T) is 8.3×102 A mm-2. Meanwhile, the percentage of copper in the MgB<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> wire is 28% and the residual resistivity ratio RRR(300K/40K) is 60. This can enhance the thermal stability for the low-temperature application. All of these indicate the great potential for the application of our MgB<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> wires in the future.