Enhancement of critical current density in MgB2 bulks burying sintered with commercial MgB2 powder
Pellets of Mg and B mixture was buried by commercial MgB2 and one-step sintered at 800 °C, two-step sintered at 750 °C followed by 900 °C, and two-step sintered at 800 and 600 °C, respectively. Although an increasing amount of MgO was found in the burying sintered samples, the particles were considerably refined and embedded in the MgB2 grains, as the MgO is likely to be from the absorbed O2 on the commercial MgB2. Commercial MgB2 served as nucleation sites for newly formed MgB2 grains, which mostly generated at solid–solid reaction stage following Ostwald ripening mechanism. Apart from low crystallinity, such low-temperature synthesis of MgB2 induced defects including grain boundaries and second-phase particles as effective pinning centers. As a result, the critical current density is enhanced at high field in the burying sintered samples, in contrast with the one without burying.
The authors are grateful to the China Postdoctoral Science Foundation Grant (Grant No. 2017M621429), the National High Technology Research and Development Program of China (Grant No. 2015AA042504) and the National Natural Science Foundation of China (Grant No. 51474156) for grant and financial support.