Improved pinning effects in ball-milled MgB2 bulks containing glycine by introducing shedding impurities from agate balls
MgB2 samples containing 3 wt% glycine were prepared from milled B powders and were sintered at 800 ◦C for 30 min. Ball milling, through the interaction with the SiO2 milling media, led to the formation of Mg2Si pinning centers, which inhibited grain growth and thereby increased the grain boundary area density; MgO, which formed as an impurity both as a result of the glycine addition and the milling media, ripened into large particles, which probably served to decrease the intergranular connectivity in the samples. As a result, the sample using 80-h-milled B powders exhibited the highest irreversible critical field and the highest critical current density at 20 K under the field larger than 4.7 T. On the other hand, ball milling breaks the limited C-substitution level in glycine-doped MgB2 samples, and the favorable properties also benefited fromthe relatively high C-substitution level, low crystallinity, and small grain size caused by ball milling. The irreversible magnetic field increased up to 5.8 T after ball milling, indicating that the upper critical field at 20 K is increased, which correlates with the decrease in low-field critical current density.