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

• Graphene-added dense MgB2 was prepared by ex-situ spark plasma sintering.
• There is a limited interaction between graphene and MgB2.
• Addition of graphene (G) shows a small enhancement of Jc and μ0Hirr.
• G is one of the least effective C-containing additions.

Graphene nanopowder (G) with average thickness particle size of about 6–8 nm 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 25 K 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.

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