Analysis of the co-doping effect of graphene and nano-Ni on grain connectivity and critical current density in MgB 2 superconductors

MgB 2 bulks codoped with graphene and nickel with a composition of Mg 1−x Ni x B 1.9 G 0.1 (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 ( J c ) and irreversibility field ( H irr ) of MgB 2 has been studied. Graphene doping is more efficient than other forms of carbon doping for modifying the MgB 2 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 J c of MgB 2 bulks in the zero fields due to the superior grain connectivity and high density. The flux pinning by carbon substitution and MgNi 2.5 B 2 nano-inclusions is expected to enhance the J c value at high magnetic fields. However, the positive action has been extremely offset by the aggregation of impurity structures such as MgNi 2.5 B 2 , MgO and over-doped graphene. Consequently, a delicate balance between graphene and Ni, homogeneous dispersion of nano-inclusions in MgB 2 grains and more lattice defects are further required to increase the flux pinning centers.