This study demonstrates the effect of addition of Fe particles of different sizes on the critical properties of the superconductor MgB₂. Bulk MgB₂ is synthesized by ball milling Mg and B powders with Fe particles at 900°C. When Fe particles with size less than 10 μm are added in MgB₂, they easily react with B and form the FeB phase, resulting in a reduction in the amount of the MgB₂ phase and deterioration of the crystallinity. Accordingly, both the critical temperature and the critical current density are significantly reduced. On the other hand, when larger Fe particles are added, the Fe₂B phase forms instead of FeB due to the lower reactivity of Fe toward B. Accordingly, negligible loss of B occurs, and the critical properties are found to be similar to those of the intact MgB₂.

MgB₂ bulk superconductors are synthesized by the solid state reaction of (MgB₄+xMg) precursors with excessive Mg compositions (x=1.0, 1.4, 2.0 and 2.4). The MgB₄ precursors are synthesized using (Mg+B) powders. The secondary phases (MgB₄ and MgO) present in the synthesized MgB₄ are removed by HNO₃ leaching. It is found that the formation reaction of MgB₂ is accelerated when Mg excessive compositions are used. The magnetization curves of Mg1+xB₂ samples show that the transition from the normal state to the superconducting state of the Mg excessive samples with x=0.5 and x=0.7 are sharper than that of MgB₂. The highest J_c-B curve at 5 K and 20 K is achieved for x=0.5. Further addition of Mg decreases the J_c owing to the formation of more pores in the MgB₂ matrix and smaller volume fraction of MgB₂.