Reactive nanostructured carbon as an effective doping agent for MgB2

A MgB2−xCx superconductor was prepared with reactive nanostructured carbon up to nominal x = 0.316 by high-energy ball milling. These products crystallize at temperatures below 700 °C, forming mainly MgB2−xCx with a particle size of about 20 nm and with lattice-dissolved carbon content up to about x = 0.13 under normal pressure conditions, and minor MgB4. The nominal higher addition of reactive nanostructured carbon does not have an influence on the a-axis of the MgB2−xCx structure. The superconducting behaviour reflects the optimum interplay of the lattice-dissolved carbon, which influences the carrier density, and the homogeneously distributed carbon, which probably acts as a pinning centre. For the sample with a nominal x = 0.221, the critical current density (Jc) increased by approximately one order of magnitude to Jc = 1.7 × 104 A cm−2 at 9 T and 4.2 K compared to the undoped MgB2.