  We report the synthesis and characterization of graphite (C) doped MgB2-xCx
(x = 0.0, 0.1, 0.2 and 0.3) samples. The crystal structure and microstructural
characterization have been investigated by x-ray diffractometer and
transmission electron microscopic (TEM) analysis. The superconducting
properties especially Jc and Hc2 have been measured by employing physical
property measurement system. We found that the graphite doping affects the
lattice parameters as well as the microstructure of MgB2 superconductor. In
case of optimally doped (x=0.1) sample, the critical current density at 5K
corresponds to 1.1 x 10^6 and 5.3 x 10^4 A/cm^2 for 3T and 5T fields
respectively. The upper critical field has been enhanced nearly two times after
doping. The flux pinning behavior has been investigated by flux pinning force
density curve and it reveals that the flux pinning behaviour has improved
significantly by doping. TEM micrographs show the graphite nanoparticles of
size ~5-10 nm which are invariably present in MgB2 grains. These nanoparticles
act as flux pinning centre and are responsible for enhancement of
superconducting properties of MgB2.
