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.