  The enhancement of the critical current density (Jc(H)) of carbon and
nano-SiC doped MgB2 is presented and compared. The upper critical field (Hc2)
being determined from resistivity under magnetic field experiments is though
improved for both C substitution and nano-SiC addition the same is more
pronounced for the former. In MgB2-xCx carbon is substituted for boron that
induces disorder in the boron network and acts as internal pinning centres. The
optimal Jc(H) values are obtained for x = 0.1 sample . In case of nano-SiC
doped in MgB2, the Jc(H) improves more profoundly and two simultaneous
mechanisms seems responsible to this enhancement. Highly reactive nano-SiC
releases free carbon atom, which gets easily incorporated into the MgB2 lattice
to act as intrinsic pinning centres. Further enhancement is observed for higher
nano-SiC concentrations, where the un-reacted components serve as additional
extrinsic pinning centres.
