Optimization of Mg Precursor Concentration to Obtain High J _c in MgB ₂ Synthesized with Ag Addition and Carbon Encapsulated Boron

In this work, we report an optimum amount of Mg precursor required for obtaining high critical current density (J _c ) of bulk MgB ₂ superconductor synthesized via solid state sintering. We add a slight excess of Mg precursor to balance its losses while formation of pinning phases and impurities such as Ag-Mg phases and MgO, respectively. Several amounts of Mg had been varied such as x = 1.05, 1.075, 1.1, 1.125, and 1.15 in Mg _x B ₂ . To obtain the best critical current density values, we used carbon encapsulated boron (1.5% carbon) and added 4 wt% Ag. The X-ray diffraction analysis showed the presence of AgMg and AgMg ₃ phases and a minute amount of MgO. The SQUID measurements indicated that highest J _c of 350 kA/cm ² at 20 K and self-field in sample with x = 1.075. All results analyzed explain the improved critical current performance based on nanometer-sized Ag-Mg particles in the final product.