Uniformity of Coronene-Added MgB 2 Superconducting Wires Fabricated Using an Internal Mg Diffusion Process

We fabricated 100-m-long class-MgB₂ superconducting wires using an internal Mg diffusion (IMD) process. Coronene was added as a carbon source to enhance the in-field critical current properties. We prepared a coil with a 4.8-cm diameter with 12 turns from a 2.6-m wire cut from the long wire. We obtained uniform critical currents (the critical current (I_c), the critical current density (J_c), and engineering J_c(J_e)) at different magnetic fields and temperatures, wire structures, compositions, and microstructures of the MgB₂ layer in ten short wires cut from the middle ten turns of the coil. We also took seven pieces of a monofilament wire from the IMD long wire to fabricate a seven-filament IMD MgB₂ wire, and we found no obvious decrease in J_c and J_e when the diameter of the wires decreased from 1.06 to 0.88 mm. These results suggest that the IMD process could be used to fabricate MgB₂ wires with a uniform structure. Coronene addition is useful for obtaining uniform composition and microstructure in the MgB₂ layer because the B powder is uniformly coated with coronene for the whole wire during the annealing steps in the drawing process and the final heat treatment process. Therefore, the IMD process, together with coronene addition, is expected to fabricate MgB₂ superconducting wires with uniform in-field critical current properties for practical large-scale applications.