Competition of the nano-inclusions and the effects on critical current density in Ni and SiC co-doped MgB₂ bulks

Samples of (MgB₂)_0.96Ni_0.04 + x wt% SiC (with x = 0, 2, 5 and 10) were sintered at 750°C for 30 min, and the phase composition, microstructures, and superconducting properties were investigated. The highest critical current density (J_c) was obtained in the sample with 2 wt% SiC doping, owing to the precipitation of the nanoscale MgNi_2.5B₂ and Mg₂Si on the edge of regular grains. The generation of the two impurities displayed a competition, and consequently the MgNi_2.5B₂ is refined to be effective pinning centre, which is in contrast to its influence in the Ni-doped sample. Results from differential thermal analysis and modified Flynn-Wall-Ozawa method also supported our explanation, and the best fitting mechanism function of solid-solid reaction in this system was ascertained. We conclude that 'Avrami-Erofeev, n = 2' is the best fitting mechanism function, indicating random nucleation followed by an instantaneous growth of nuclei. The function of the described mechanism, G(á), is 2[-ln(1 - α)]^1/2, (α is the product fraction), which is different from that of pure MgB₂.