Enhancement of flux pinning and high critical current density in graphite doped MgB_2 superconductor

摘要

We report the synthesis and characterization of graphite (C) doped MgB_(2-x)C_x (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 J_c and H_(c2) 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 MgB_2 superconductor. In case of optimally doped (x=0.1) sample, the critical current density at 5 K corresponds to 1.1 × 10~6 and 5.3 × 10~4 A/cm~3for 3 and 5 T 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 behavior has improved significantly by doping. TEM micrographs show the graphite nanoparticles of size ～5 - 10 nm which are invariably present in MgB_2 grains. These nanoparticles act as flux pinning center and are responsible for enhancement of superconducting properties of MgB_2.