The Effect Of Ni-doping On The Formation Of MgB₂ Phase And Its Superconductivity

The effect of doping elements on the formation mechanism and the superconducting properties of MgB2 sample have become more and more important for guiding the preparation of high quality MgB2 superconductors. In the present paper, the appropriate experimental parameters were firstly defined by traditional solid-state sintering method. Based on the analysis, the in-situ sintering mechanism, microstructure and superconducting properties of Ni-doped MgB2 samples were systematically explored by means of metallographic analysis, differential thermal analysis, powders sintering theory, thermodynamic and kinetics analysis. Additionally, the MgB2 samples doped with carbon were successfully obtained by deposition of carbon on B matrix, which was expected to enhance the critical current density ( J c) by inducing Ni nanoparticles as pinning centers.A high-resolution Differential Thermal Analysis apparatus was employed to sinter the mixture of Mg and B powder and analyze the kinetics of MgB2 phase formation with the different heating rates (5, 10,20 and 30 K/min).The measured results of the critical current density suggested that the 5 K/min sample had the best performance in high-field because of the better connectivity in MgB2 grain formed at low temperature. However, more vacancy in the sample and the worse grain connectivity will be generated by the fast heating rate (30 K/min), which is the main reason for the degeneration of superconducting properties in magnetic field.The effect of Ni doping on the formation of MgB2 was investigated by the thermal analysis. It shows that the low-melting eutectic liquid firstly formed by Mg-Ni at 506°C in the Ni-doped MgB2 sample, which could provide a favorable area for the further diffusion between Mg and B atoms and promote the reaction between Mg and B at solid state. The sintering model of Mg-Ni-B system indicated that the precipitation of MgNi2.5B2 phase vary from grain boundaries to MgB2 intra-grains as the sintering temperature increases.The different grain sizes of Ni were adopted to detect the influence of the grain size on the formation process of MgB2 phase and the variety of the superconducting properties. It is found that the smaller grain size of Ni will promote the formation of the secondary MgNi2.5B2 phase at low temperature. The microstructure shows that the precipitation of the MgNi2.5B2 phase is from the MgB2 intra-grains, the morphology of secondary phase further reveal the screw dislocation mechanism of the formation of MgB2 crystalline. The value of J c have the strong correlation with the secondary phase, the large grain size may lead to the degeneration of J cas impurities.Ni nanoparticles were successfully introduced for preparing polycrystalline MgB2 samples by a novel reduction method. The microstructure and magnetization measurements of the precursor powders and MgB2 samples were explored. It was found that the obtained Ni nanoparticles with average grain size of 5 nm are distributed in the B matrix without agglomeration. The sample doped with Ni nanoparticles has better critical current density ( J c) with respect to the commercial Ni-doped sample because of the improvement of grain connectivity.The carbon onions and carbon nanotubes were prepared on the B matrix at 500°C using Chemical vapor deposition (CVD) method by decomposition of methane. The carbon doped MgB2 sample with high critical current density ( J c) was obtained by sintering the mixture of Mg and precursor powders.The content of the carbon on the B matrix will increase with the reaction time, but cease when the reaction time exceeding 1.5 hours. The Ni could not react with Mg/B forming the secondary MgNi2.5B2 phase because of the carbon wrap. However, the Ni nanoparticles covered by carbon can act as effective pinning center to enhance the J cat magnetic field.