Analysis of the co-doping effect of graphene and nano-Ni on grain connectivity and critical current density in MgB2 superconductors

Abstract

MgB2 bulks codoped with graphene and nickel with a composition of Mg1−xNixB1.9G0.1 (x = 0.02, 0.04 and 0.06) have been synthesized by an in situ reaction method. The combination effect of graphene and Ni codoping on microstructure, critical current density (Jc) and irreversibility field (Hirr) of MgB2 has been studied. Graphene doping is more efficient than other forms of carbon doping for modifying the MgB2 microstructures due to the two-dimensional structures. The doping of Ni is confirmed to eliminate the porosity present in the graphene-doped sample by the assistance of a Mg–Ni eutectic liquid at low temperature. The results clearly indicate that graphene and Ni codoping are cooperative in improving the Jc of MgB2 bulks in the zero fields due to the superior grain connectivity and high density. The flux pinning by carbon substitution and MgNi2.5B2 nano-inclusions is expected to enhance the Jc value at high magnetic fields. However, the positive action has been extremely offset by the aggregation of impurity structures such as MgNi2.5B2, MgO and over-doped graphene. Consequently, a delicate balance between graphene and Ni, homogeneous dispersion of nano-inclusions in MgB2 grains and more lattice defects are further required to increase the flux pinning centers.