Annealing effects in PIT-processed MgB₂/SUS316 (stainless steel) composite wires

The MgB₂/SUS316 (stainless steel) wires were fabricated by powder-in-tube (PIT) method using commercially available superconducting MgB₂ powder at ambient pressure, and the annealing effects on T _c as well as J_c were investigated. The as-rolled MgB ₂/SUS316 wire showed a zero-resistance superconducting transition temperature (T_c) at 33.4 K and a critical current density (J _c^self-field) of 2.22 kAcm^-2 at 18.4 K. In order to check the annealing effect, the different pieces of the as-rolled wires were annealed at various annealing conditions under Ar atmosphere in vacuum (60 cm Hg) sealed silica-tube. As compared to the MgB₂/Fe wires, the annealing temperature and duration were lower and much shorter respectively, for the MgB₂/SUS316 (stainless steel) wires. On annealing at 873 K for ∼ 1 s, we obtained the best sample with a J_c^self-field of 12.1 kAcm^-2 at 16.9 K with a maximum T_c of 36.1 K. Longer annealing time (∼ 10 s) above 873 K reduced the T_c as well as the J_c, indicating a possible chemical reaction between the MgB₂ cores and the sheath material.