Effects of MgO evolution on the critical current density in bulk MgB2 containing histidine

Highlights: •Histidine is an effective dopant for improving the critical current density. •The dominating pinning effects are provided by nano-scale MgO pinning centers. •MgO performed stronger pinning effects than defects caused by C substitution. •We clarified how to select the amino acid for doping. -- Abstract: Glycine, an amino acid with the simplest composition, was introduced into MgB2 systems, making great contribution to the enhancement of critical current density in our previous study. Aiming at investigating the effects of histidine, another amino acid with more complicated structure and lower decomposition temperature than glycine, on the superconducting properties of MgB2, samples of MgB2 + x wt.% histidine (with x = 0, 2, 3, 5, and 8) were sintered at 800 °C after mechanical mixing. The best critical current density was obtained in the sample with 2 wt.% histidine addition, owing to the small-sized MgO and C substitution. The sample showed a significant increase in critical current density under high field compared with pure MgB2, and this property maintained at a relatively high level under low field as well. However, the growth and agglomeration of MgO with the increasing amount of histidine should be responsible for the tendency of the decrease in the connectivity and critical current density versus doping content. Finally, the conditions that the used amino acid should meet were investigated as a guide for effective amino acid doping