Study On Preparation, Superconductivity, Electronic Structure And Elastic Properties Of Novel Superconductors

Both the research on theories of superconductivity and the applications of superconductor materials have made tremendous progress since the discovery of superconductivity in 1911. In the latest decade, several kinds of novel superconductor, including intermetallic compounds MgB2 and MgCNi3, Fe-based superconductor materials LnFeAsO1-xFx(Ln=Rare Earth elements), Ba1-xKxFe2As2, LiFeAs (NaFeAs), FeSe(Te), Sr4V2O6Fe2As2, et al., have been discovered in turn. In this thesis, we systematically studied the preparation, crystal structure, appearance and superconductivity of MgB2 and MgCNi3 series samples by microwave direct synthesis (solid state reaction). We also studied the effects of doping on crystal lattice parameters, electronic structure and elastic properties of superconductor MgB2, MgCNi3, Fe-based superconducting parent materials LaFeAsO and BaFe2As2 by virtual crystal approximation (VCA) and the first principles based on density functional theory (DFT). The main contents and results are listed in the following.We prepared MgB2, nano SiC doped Mg(B1-2xSiCx)2 (x=0,0.05, 0.10) and nano CuO doped MgB2-xCuO(x=0,0.03,0.05) samples by microwave direct synthesis. And we systematically studied their crystal structure, phase composition, appearance and superconductivity via X-ray diffraction (XRD), scan electron microscope (SEM) and superconducting quantum interferometer device (SQUID). Results show that B atoms are partly substituted in SiC doped Mg(B1-2xSiCx)2 samples, while substitution not take place in CuO doping MgB2-xCuO samples. Owe to the advantages of microwave synthesis and nano materials doping, the cost of time on synthesizing the doped samples is less, the grains of the samples are finer, the densities of the samples are tight and the connection of the grains is better, and the Jc of the doped samples is also greatly increased in external magnetic field and temperature 20 K.We prepared MgCNi3 sample with lower temperature and in shorter time by hybrid microwave synthesis. The results indicate that the phases of the synthesized sample are MgCNi3 (major phase), a small amount of unreacted graphite and a little MgO. The MgCNi3 particle sizes are very fine. The onset superconducting transition temperature of the MgCNi3 sample was 6.9 K, and the superconducting transition width was about 0.8 K. We also synthesized MgCNi3 using carbon nanotubes (CNTs) as starting materials by the conventional powder metallurgy method. The results present that the phases of the synthesized samples are MgCNi3 (major phase) and traces of Ni (or C) and MgO. The MgCNi3 particle sizes range from several hundreds of nanometres to several micrometres. The onset superconducting transition temperature Tc of the optimal MgCNi3 sample is about 7.2 K. The critical current density Jc is about 3.44×104 A/cm2 at 5K and zero applied fields, which is higher than Jc of MgCNi3 sample synthesized by other type of carbon.We calculated the crystal structure, energy band structure, density of states and elastic constants of single crystal and polycrystal of C doped Mg(B1-xCx)2 (x=0,0.05,0.10,0.15,0.2) and C. Si co-doped Mg(B1-2xSixCx)2 (2x=0,0.5,0.1,0.15,0.2), Mg(Bo.9C0.1-xSix)2 (x=0,0.02, 0.03,0.04,0.05) crystal by VCA and the first principles based on DFT. Calculated results show that the theory value of lattice parameters for C doped Mg(B1-xCx)2 is consistent with experiment value, while lattice parameters of C and Si co-doped Mg(B1-2xSixCx)2 and Mg(B0.9C0.1-xSix)2 are increasing with dopant augmented. The energy band structures of Mg(B1-xCx)2 and Mg(B1-2xSixCx)2 near the Fermi Energy level are changed evidently. The change of the density of states near the Fermi Energy level is consistent with the change of superconductivity transformation temperature Tc, which is obeying the BCS theory. Elastic characters of single crystal and polycrystal are changed obviously. Passion ratio v and value of BHIGH indicate that Mg(B1-xCx)2, Mg(B1-2xSixCx)2 and Mg(B0.9C0.1-xSix)2 are brittle compounds. Compared with C doped Mg(B1-xCx)2, the brittleness of C and Si co-doped Mg(B1-2xSixCx)2 and Mg(B0.9C0.1-xSix)2are improved.We calculated the crystal structure, energy band structure, density of states and elastic characters of single crystal and polycrystal of B doped MgC1-xBxNi3 (x=0,0.05,0.10,0.15,0.2) and Zn doped Mg1-xZnxCNi3 (x=0,0.1,0.15,0.2,0.25,0.3) crystal. The calculated lattice parameters are in agreement with experiment results. The change of the density of states near the Fermi Energy level is consistent with the change of superconductivity transformation temperature Tc, which is obeying the BCS theory. The energy band structure near the Fermi Energy level and the elastic characters of single crystal and polycrystal of Zn doped Mg1-xZnxCNi3 are changed evidently, while that of B doped MgC1-xBxNi3 are changed less. Passion ratio v and value of BH/GH indicate that MgC1-xBxNi3 is ductile compounds, while Mg1-xZnxCNi3is in the border of brittleness and ductibility.We also calculated the crystal structure, energy band structure, density of states and elastic parameters of single crystal and polycrystal of F doped LaFeAsO1-xFx (x=0,0.05,0.08,0.11,0.15) and K doped Ba1-xKxFe2As2 (x=0,0.1,0.2,0.3,0.4,0.5) crystal. The calculated lattice parameters are quality consistent with experiment results. The energy band structures near the Fermi energy level are almost not changed. The change of the density of states near the Fermi Energy level is not consistent with the change of superconductivity transformation temperature Tc, and is not obeying the traditional BCS theory. Elastic characters of single crystal and polycrystal are changed obviously. Passion ratio v and value of BH/GH indicate that LaFeAsO1-xFxis in the border of brittleness and ductibility, while Ba1-xKxFe2As2 is brittle compounds.