  Polycrystalline MgB2-xCx samples with x=0.05, 0.1, 0.2, 0.3, 0.4
nano-particle carbon powder were prepared using an in-situ reaction method
under well controlled conditions to limit the extent of C substitution. The
phases, lattice parameters, microstructures, superconductivity and flux pinning
were characterized by XRD, TEM, and magnetic measurements. It was found that
both the a-axis lattice parameter and the Tc decreased monotonically with
increasing doping level. For the sample doped with the highest nominal
composition of x=0.4 the Tc dropped only 2.7K. The nano-C-doped samples showed
an improved field dependence of the Jc compared with the undoped sample over a
wide temperature range. The enhancement by C-doping is similar to that of
Si-doping but not as strong as for nano-SiC doped MgB2. X-ray diffraction
results indicate that C reacted with Mg to form nano-size Mg2C3 and MgB2C2
particles. Nano-particle inclusions and substitution, both observed by
transmission electron microscopy, are proposed to be responsible for the
enhancement of flux pinning in high fields.
