Critical current densities in Ag-added bulk MgB₂

Highlights: • Bulk MgB₂ samples with Ag contents of 0–10 wt% were sintered at 775 °C for 3 h. • Magnetization measurements showed a sharp superconducting transition with T_c (onset) around 38.5–38.7 K. • Atomic force microscopy indicated that Ag-based MgB₂ particles are of nanometer size. • The sample with 4 wt% Ag addition exhibited the highest J_c of 400 kA/cm² at 10 K and self field. - Abstract: In previous studies, we found that bulk MgB₂ contained numerous voids in various shapes and sizes. With the aim of improving the critical current density as well as the mechanical performance of the disk-shaped MgB₂ bulk superconductors, we added Ag and optimized the processing conditions. The samples with varied Ag content from 0, 2, 4, 6, to 10 wt% were synthesized in pure Ar atmosphere. Microstructural observation by scanning electron microscopy confirmed that metallic Ag particles are embedded in the void regions. Furthermore, atomic force microscopy indicated that silver-based MgB₂ particles are of nanometer size. As a result, the critical current density (J_c) values were improved with Ag addition as compared to pure MgB₂ bulk. The sample with 4 wt% Ag addition exhibited the highest J_c of 293 kA/cm² at 20 K and self field. The respective J_c values at 10 K were 400 kA/cm², 300 kA/cm², and 100 kA/cm² in self field, 1 T and 2 T. These values are the highest record values so far reported in bulk MgB₂ materials.