Improved Pinning Effects in Ball-Milled \hbox Bulks Containing Glycine by Introducing Shedding Impurities From Agate Balls

MgB 2 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 SiO 2 milling media, led to the formation of Mg 2 Si 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 MgB 2 samples, and the favorable properties also benefited from the 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.