Comparison of carbon-doped MgB₂ bulks fabricated from pre-synthesized Mg/CNT and Mg/amorphous carbon composites

MgB₂ samples containing 2.5 wt% multi-walled carbon nanotubes (CNTs) and amorphous carbon (nano-C) were sintered at 800 C. The doped samples were prepared from Mg/CNT and Mg/nano-C composites, which were previously synthesized from coarse Mg and fine Mg by chemical vapor deposition, respectively. We made comparisons between the two samples on the C-doping level, the MgO contents, and the effects of the carbon source on critical current density (J _c) by investigating the phase composition, microstructure, and magnetic properties. Compared with pure MgB₂ (coarse Mg), the J _c performance at high field was improved in the CNT-doped sample (coarse Mg) due to the high C substitution level induced lattice defects as pinning centers. However, J _c values of the amorphous C-doped sample with sufficient C doping decreased over the entire field in contrast with the pure sample (fine Mg), which exhibited the highest J _c performance among the samples. This is attributed to the much more MgO impurities, mostly generated during the preparation of Mg/C composites, than the CNT-doped sample.