Refinement of MgB2 grains and the improvement of flux pinning in MgB2 superconductor through nano-Ni addition

Polycrystalline MgB2 samples with 4 at.% and 6 at.% Ni addition were prepared by an in-situ reaction process. Coarse (40 μm) and nano-size (40 nm) Ni powders were used for synthesis of the bulk samples. The phase formation, microstructure, and transport critical current density of the sintered samples were investigated as a function of the Ni particles size. A higher amount of nano-Ni addition (6 at.%) was found to dramatically decrease the reaction temperature of Mg and B, forming MgB2 fine grains with more defects and inferior crystallinity under the solid stage. Combined with the differential thermal analysis (DTA) and scanning electron microscopy (SEM) analysis, it was found that the nano-Ni particles is prone to form Mg-Ni eutectic liquid with Mg at 506 °C, which accelerates the formation of MgB2 at a significantly lower temperature with improved grain connectivity. The unique microstructure, with refined MgB2 grains, more grain boundaries and nano-inclusion inside matrix, is proposed to be responsible for the enhancement of flux pinning in the MgB2 bulk superconductor with nano-Ni doping. However, the positive effect of Ni was completely eliminated by doping of coarse-Ni particle due to the concentration of the inclusions with weak pinning and thus a strong reduction of Jc.