O-free polyacrylonitrile doping to improve the J_c(B) and H _c2 of MgB₂ wires

We selected polyacrylonitrile (PAN, -[C₃H₃N]-) as an O-free organic dopant and fabricated C-doped MgB₂ wires by in situ and powder-in-tube techniques. 0-5 wt.% PAN powders were uniformly mixed with B powder using a liquid mixing method. The precursor powders were mixed with Mg powder, filled into Fe tubes, and then drawn into wires. Sintering was performed at 900 °C for 1 h in a flowing Ar gas. The PAN doping decreased the critical temperature (T_c) and a-axis lattice parameter, but significantly improved the critical current density (J_c) in high fields, upper critical field (H_c2), and irreversibility field (H _irr) performances. These results are attributed to the replacement of B sites with C by the PAN doping. Furthermore, as expected, the MgO amount did not increase as the doping content increased. The J_c of the PAN-doped MgB₂ wires was more than one order of magnitude higher than that of the undoped MgB₂ wire at 5 K and 6.6T (1.46-3.82 kA/cm² vs. 0.11 kA/cm²).