Improvement of the critical current density and upper critical field in C-doped MgB₂ wire using cellulose acetate

We fabricated carbon-doped MgB₂ wires by an in-situ reaction route using cellulose acetate (CA) as an organic dopant in order to investigate the correlation between the critical properties and the CA doping level based on the phase formation and lattice changes. The starting B powder was uniformly mixed with 0 wt.% to 7 wt.% CA powders by a liquid mixing technique. The precursor powders were mixed with Mg powder and loaded into Fe tubes, which were drawn into wires and then sintered at 900 °C for 1 h under an Ar atmosphere. The CA doping reduced the critical temperature (T_c), but improved the critical current density (J_c) in the magnetic field, upper critical field (Hc₂), and irreversibility field (Hirr). These effects were attributed to the CA doping-induced substitution of C for the B sites which consequently increased the flux pinning force and the charge-carrier scattering. The J_c value of the MgB₂ wire doped with 7 wt.% CA was improved by a factor of 31 (3.46 kA/cm²) as compared with that of the pure wire (0.11 kA/cm²) at 5 K and 6.6 T. This increase suggests that the CA doping via the liquid mixing is an effective way to improve the J_c(B) and H_c2 of MgB₂ wire.