Enhancement of flux pinning and high critical current density in graphite doped MgB2 superconductor

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.