Strong enhancement of high-field critical current properties and irreversibility field of MgB₂ superconducting wires by coronene active carbon source addition via the new B powder carbon-coating method

We report an effective carbon-containing additive, coronene (C₂₄H₁₂), for MgB₂ superconducting wires. We used B powder coated with C₂₄H₁₂ to fabricate MgB₂ wires using the powder-in-tube (PIT) and internal Mg diffusion (IMD) processes. The in-field critical current properties are strongly enhanced for both PIT- and IMD-processed MgB₂ wires. For PIT MgB₂ wires, a critical current density (J_c) value of 1.8 × 10⁴ A cm⁻² is obtained at 4.2 K and 10 T. For IMD MgB₂ wires, we obtained a J_c of 1.07 × 10⁵ A cm⁻² and an engineering J_c (J_e) of 1.12 × 10⁴ A cm⁻² at 4.2 K and 10 T. These J_c and J_e values are similar to the highest values reported for MgB₂ wires thus far. Furthermore, the irreversibility field, B_irr, determined with a current density criterion of 100 A cm⁻², is strongly enhanced to 25 T at 4.2 K, which is also the highest value reported for MgB₂ superconducting wires thus far. Coronene is an active carbon source for MgB₂ superconducting wires because (1) coronene has a high carbon content (96 wt%) with a small amount of hydrogen (impurity), (2) the decomposition temperature for coronene is near the reaction temperature between Mg and B, and (3) uniform dispersion of coronene on the B surface can be obtained due to the melting point of coronene being lower than the decomposition temperature. Carbon substitution for B caused by the coronene active carbon source is mainly responsible for the high field critical current properties and the high B_irr obtained in this work. (paper)