Analysis of the precipitation process of secondary phase MgNi _2.5B₂ and superconducting properties in Ni-doped MgB ₂ bulk

A polycrystalline MgB₂ sample with 4 at.% Ni powder addition has been synthesized by a solid-state reaction at 750 °C. Two different Ni powders, raw (40 μm) and ball-milled (10 μm) particles were selected to clarify the influence of the Ni particle size on the secondary phase MgNi _2.5B₂ formation and the critical current density J _c of MgB₂ bulks. According to the investigation of the sintering, it was found that the smaller Ni particles may significantly decrease the formation temperature of the MgNi_2.5B₂ ternary compound to 550 °C. Combined with scanning electron microscopy (SEM) analysis and magnetic measurements, it was suggested that the addition of fine Ni particles led to MgNi_2.5B₂ inclusion precipitating along the spiral terraces of MgB₂ grains and with small particles sizes. The unique microstructure is responsible for the smaller reduction in J_c, which partly eliminates the effect of weak links compared with the coarse Ni particles. Besides, the formation mechanism of the microstructures and the relative location of MgNi_2.5B₂ varying from cluster at the MgB₂ grain boundaries to a screw arrangement around the MgB₂ grains with the decrease of the Ni grain sizes were also discussed in detail.