Kinetics mechanism on the efficiency of C substituting B in the MgB2 tape fabrication

Effective carbon doping is essential for MgB2 superconductors due to its enhancement on in-field critical current density. However, C-doped MgB2 samples prepared with different boron powders exhibit varying levels of C substitution efficiency, which directly affect the superconducting properties. In this article, to further investigate the factors influencing C substitution efficiency, a two-stage carbon doping process was established based on the in-situ X-ray diffraction (XRD) measurement. The process begins with the initial in-situ ternary reaction of Mg, B and C, followed by the subsequent diffusion of C into the MgB2 lattice. Through the analysis of thermodynamic equations, it can be inferred that the key factor in improving the C substitution efficiency for MgB2 is to align the activity of the B and C precursor powders, which enables a greater participation of carbon in the initial stage of the ternary reaction. According to the systematic study on nano-C doped MgB2 tapes, the C substitution efficiency in MgB2 tapes prepared using nano amorphous boron powder is notably lower compared to those prepared using micron crystalline boron powder. These findings suggest that high-activity amorphous B powder necessitates a C dopant with a lower activation energy to enhance its C substitution efficiency.