Graphene addition to MgB2 superconductor obtained by ex-situ spark plasma sintering technique

Abstract

Graphene nanopowder (G) with average thickness particle size of about 6–8nm was added to MgB2 commercial powder. Starting composition was (MgB2)(1−x)(G)x, x=0.0125, 0.025, 0.05. Processing was performed by Spark Plasma Sintering (SPS) technique. All added samples have high density (above 95%). The critical temperature (Tc) and the lattice parameter a (c-axis lattice parameter is constant) show a small variation suggesting that carbon substitution for boron is low. TEM observations show the presence of un-reacted graphene plates supporting the Tc and structural results. It also indicates that G-addition does not modify the MgB2 microstructure. Despite this, there is an optimum doped sample (MgB2)0.9875(G)0.0125 for which the critical current density at temperatures below 25K is slightly higher at high magnetic fields than for the pristine sample. The addition of G is found as one of the least effective C-source additions enhancing Jc. We discuss results as being strongly related to variation of the residual stress.