On the Superconductivity and Mg Outdiffusion in Vacuum-Synthesized $\hbox {MgB}_{2}$ Samples

MgB₂ samples have been synthesized over a wide temperature range and from the stoichiometric and excess magnesium composition. The role of magnesium overdoping on the superconducting characteristics of MgB₂ has been studied, and hence, the correlation between the critical current density, porosity, and density of the MgB₂ samples has been established. The excess magnesium in the starting composition expands the synthesis temperature range over which the MgB₂ phase can be synthesized and also enhances the superconducting properties in terms of Tc, ΔT, and residual resistivity ratio. The reduced weight loss due to Mg vaporization, and hence the lower porosity in the sample synthesized from the stoichiometric composition and at T_s = 900°C, reflected the higher J_c in the MgB₂ sample in this series. Whereas with the excess magnesium in the starting composition, the sample with the highest J_c has been synthesized at a lower synthesis temperature of T_s = 700°C. The correlation between weight loss during heat treatment, sample density, resistivity, and active area fraction has been established for vacuum-synthesized bulk MgB₂ samples.