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

MgB 2 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 MgB 2 has been studied, and hence, the correlation between the critical current density, porosity, and density of the MgB 2 samples has been established. The excess magnesium in the starting composition expands the synthesis temperature range over which the MgB 2 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 MgB 2 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 MgB 2 samples.