Significant enhancement of Hc2 and Hirr in MgB 2+C4H6O5 bulks at a low sintering temperature of 600 C

We report the carbon (C) substitution effects of MgB2 + 10 wt% C4H6O5 (malic acid) on the lattice parameters, critical temperature (T-c), upper critical field (H-c2), and irreversibility field (H-irr) as a function of sintering temperature in the range from 600 to 900 degrees C. The additive C4H6O5 as the C source resulted in a small depression in T-c, but significantly increased the C substitution level, and hence improved the H-c2 and H-irr performance at a low sintering temperature of 600 degrees C in conjunction with a short sintering period of 4 h. In addition, the low-temperature sintering process resulted in small grain size and higher impurity scattering compared to a pure MgB2 superconductor. These effects promote flux pinning.