Microstructure and superconducting property research of Cu-Mg reinforcing graphene and grain refining MgB2 by carbon-contained substances doping

We report the C substitution effects in MgB2 bulks on the lattice parameters, FWHM, and critical current density (Jc) as a function of doping level. It is shown that using carbon-coated boron powder can effectively control the grain growth of MgB2 during the whole sintering process. Accompanied by the rapid cooling process, the final average grain size can be restrained within merely 50 nm, and the Jc value of the sample can be significantly improved at high field. On top of that, adding Cu is also considered a key factor in improving the current carrying property of the graphene-doped sample throughout the whole field, which can be attributed to the formation of the MgCu2 alloy phase.