  We report the synthesis and variation of superconductivity parameters such as
transition temperature Tc, upper critical field Hc, critical current density
Jc, irreversibility field Hirr and flux pinning parameter (Fp) for the MgB2-xCx
system with nano-Carbon doping up to x=0.20. Carbon substitutes successfully on
boron site and results in significant enhancement of Hirr and Jc(H).
Resistivity measurements reveal a continuous decrease in Tc under zero applied
field, while the same improves remarkably at higher fields with an increase in
nano-C content for MgB2-xCx system. The irreversibility field value (Hirr) is
7.6 & 6.6 Tesla at 5 and 10K respectively for the pristine sample, which is
enhanced to 13.4 and 11.0 Tesla for x = .08 sample at same temperatures.
Compared to undoped sample, critical current density (Jc) for the x=0.08
nano-Carbon doped sample is increased by a factor of 24 at 10K at 6 Tesla
field.
