Improvement of Jc and Hc2 in MgB2 superconductor with citric acid addition

This paper reports on the fabrication and characterization of citric acid (CA)-C6H8O7 added MgB2 superconductor. The relationships between microstructures, critical current density (Jc), critical temperature (Tc), upper critical field (Hc2), irreversibility field (Hirr), and normal state resistivity for 10 wt% C6H8O7 added MgB2 samples sintered at temperatures from 650C to 950C were systematically studied. A reduction in Tc and in lattice parameter a due to the C substitution and possible oxygen (O) effects occurs wtih C6H8O7 addition. Jc, Hc2, and Hirr are significantly enhanced, however, with the addition of C6H8O7. All the samples exhibit Jc above 104 A/cm2 at 5 K and 8 T. This value is higher than for the un-doped MgB2 by a factor of 9. The significant improvement in the superconducting properties is attributed to the lattice distortion due to the C and possible oxygen (O) substitution for boron, with the C and O coming from the decomposition of C6H8O7.>