Systematic study of a MgB2+C4H6O5superconductor prepared by the chemical solution route

We evaluated the additive effects of malic acid (C 4 H 6 O 5 ), from 0 to 30 wt% of the total MgB 2 , on the lattice parameters, lattice strain, amount of carbon (C) substitution, microstructures, weight fraction of MgO, critical temperature ( T c ), critical current density ( J c ), and irreversibility field ( H irr ) of a MgB 2 superconductor. The calculated lattice parameters show a large decrease in the a -axis lattice parameter for MgB 2 +C 4 H 6 O 5 samples from 3.0861(6) to 3.0736(1) Å, with even a 10 wt% addition. This is an indication of C substitution into boron sites, with the C coming from C 4 H 6 O 5 , resulting in enhancement of J c and H irr . Specifically, the H irr of the MgB 2 +C 4 H 6 O 5 samples prepared by the chemical solution route reached around 7 T at 20 K, with a T c reduction of only 1.5 K. In addition, the self-field J c of the MgB 2 +C 4 H 6 O 5 samples was only slightly reduced at an additive level as high as 30 wt%. However, residual oxygen after evaporation processing contributed to a large amount of MgO in our MgB 2 +30 wt% C 4 H 6 O 5 samples. These problems can be further controlled by the amount of C 4 H 6 O 5 additive or different evaporation temperatures.