Influence of Ni addition on the process of phase formation in MgB2 bulk

Nickel is an important sheath material for the fabrication of MgB₂ wires. However, the effects of Ni doping on the phase formation and superconducting properties of MgB₂ remain controversial. In this work, Ni powder is selected for doping in MgB₂ bulk in order to examine the corresponding changes. Combining with the DSC analysis and in-situ XRD results, we find indications that the Ni powder reacted with Mg and B, forming MgNi_2.5B₂ at 600°C. The ternary compound began to decompose at a temperature above 800°C. The reactive phase, MgNi_2.5B₂, acted as an obstacle to the supercurrent flow, creating weak links among the MgB₂ grain boundaries. However, it is found that the added Ni formed a eutectic liquid phase with Mg at 506°C. The liquid phase helps the formation of MgB₂ at low temperature, which not only increases the density of the sample, but also improves the grain connectivity. Consequently, the presence of Ni in the MgB₂ sample is not necessarily a disadvantage; it depends on the desired application.