Reactive nanostructured carbon as an effective doping agent for MgB2

AMgB(2-x)C(x) 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 degrees 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 ( J(c)) increased by approximately one order of magnitude to J(c) = 1.7 x 10(4) A cm(-2) at 9 T and 4.2 K compared to the undoped MgB2.