Systematic study of a MgB2 + C4H6O5 superconductor prepared by the chemical solution route
We evaluated the additive effects of malic acid (C4H6O5), from 0 to 30 wt% of the total MgB2, on the lattice parameters, lattice strain, amount of carbon (C) substitution, microstructures, weight fraction of MgO, critical temperature (Tc), critical current density (Jc), and irreversiblity field (Hirr) of a MgB2 superconductor. The calculated lattice parameters show a large decrease in the a-axis lattice parameter for MgB2 +C4H6O5 samples from 3.0861(6) to 3.0736(1) A, with even a 10 wt% addition. This is an indication of C substitution into boron sites, with the C coming from C4H6O5, resulting in enhancement of Jc and Hirr. Specifically, the Hirr of the MgB2 + C4H6O5 samples prepared by the chemical solution route reached around 7 T at 20 K, with a Tc reduction of only 1.5 K. In addition, the self-field Jc of the MgB2 +C4H6O5 samples was only slightly reduced at an additive level as high as 30 wt%. However, residual oxygen after evaporation processing contributed to a lrage amound of MgO in our MgB2 + 30 wt% C4H6O5 samples. These problems can be further controlled by the amound of C4H6O5 additive or different evaporation temperatures.