【作者】 夏庆林；

【导师】 易健宏；

【作者基本信息】 中南大学 ， 材料学， 2011， 博士

【摘要】 自从1911年超导电性被发现以来,无论是超导电性理论研究还是超导材料的应用研究,都取得了巨大的发展。近10年来,又有几类重要的超导材料相继被发现,其中包括金属间化合物超导体MgB2和MgCNi3.铁基超导体LnFeAsO1-xFx(Ln=Rare Earth elements), Ba1-xKxFe2As2, LiFeAs(NaFeAs), FeSe(Te),Sr4V2O6Fe2As2等。本论文利用微波直接合成(固相反应)方法制备了MgB2和MgCNi3系列多晶块材样品,并研究了其晶体结构、相成分、形貌和超导电性；利用虚晶近似(VCA)和基于密度泛函理论(DFT)第一性原理的广义梯度近似(GGA)研究了掺杂对MgB2和MgCNi3、铁基超导母体材料LaFeAsO和BaFe2As2的晶格参数、电子结构和弹性性质的影响。论文主要研究内容和结果如下：利用微波直接合成方法制备了MgB2、纳米SiC掺杂Mg(B1-2x(SiC)x)2 (x=0,0.05,0.10)和纳米CuO掺杂MgB2-xCuO (x=0,0.03,0.05)多晶块材样品,通过X射线衍射(XRD)、扫描电子显微镜(SEM)以及超导量子干涉仪(SQUID)等对它们的晶体结构、相成分、形貌和超导电性进行了研究。结果证实纳米SiC掺杂的Mg(B1-2x(SiC)x)2样品中实现了B位原子被部分替代；而MgB2-xCuO超导样品中CuO并未进入MgB2晶格。纳米掺杂MgB2样品的合成时间更短,晶粒更细,致密度更高,晶粒间的连接性更好。在20K温度下,掺杂样品的临界电流密度J。在较高外场下都有较大幅度的提高,显示出了微波直接合成和纳米掺杂的优越性。利用杂化微波合成方法在短时间内和较低温度下制备了Mg1.1 C1.2Ni3超导样品。样品主相为MgCNi3,含有少量未反应的石墨和微量的MgO杂相,样品晶粒较细,超导转变温度约为6.9K,转变宽度约为0.8K。用固相反应法、以C纳米管为碳源制备了Mg1.1CxNi3 (x=1.35,1.45,1.55)超导样品。除主相MgCNi3外,还含有少量未反应的Ni(或C纳米管)和微量的MgO杂相,晶粒大小在几百纳米至几微米之间。最佳样品的转变温度Tc为7.2K,在自场、5K下其临界电流密度J。约为3.44×104A/cm2,比其它碳源制备的MgCNi3样品的要高。利用VCA和第一性原理GGA计算了C掺杂对Mg(B1-xCx)2 (x=0,0.05,0.10,0.15,0.2)晶体及C、Si共掺杂对Mg(B1-2xSixCx)2 (2x=0,0.5, 0.1,0.15,0.2)和Mg(Bo.9C0.1-xSix)2 (x=0,0.02,0.03,0.04,0.05)晶体的晶格参数、能带结构、态密度、单晶体和多晶体的弹性性质的影响。结果表明C掺杂Mg(B1-xCx)2晶体晶格参数变化与实验结果符合较好,而C、Si共掺杂Mg(B1-2xSixCx)2和Mg(B0.9C0.1-xSix)2晶体晶格参数随掺杂量增加而增大；Mg(B1-xCx)2和Mg(B1-2xSixCx)2费米能级附近的能带结构发生明显变化；费米能级处态密度的变化与超导转变温度的变化一致,符合BCS超导机制；单晶体和多晶体的弹性性质都发生较大变化；泊松比v和BH/GH的比值表明Mg(B1-xCx)2、Mg(B1-2xSixCx)2和Mg(B0.9C0.1-xSix)2晶体是脆性的化合物；与C单独掺杂Mg(B1-xCx)2晶体比较而言,C、Si共掺杂Mg(B1-2xSixCx)2和Mg(B0.9C0.1-xSix)2晶体的脆性有所改善。计算了B掺杂对MgC1-xBxNi3 (x=0,0.05,0.10,0.15,0.2)晶体和Zn掺杂对Mg1-xZnxCNi3 (x=0,0.1,0.15,0.2,0.25,0.3)晶体的晶格参数、能带结构、态密度、单晶体和多晶体的弹性性质的影响。结果显示掺杂晶体晶格参数变化与实验结果符合较好；费米能级处态密度的变化与超导转变温度的变化一致,符合BCS超导机制；Zn掺杂Mgl-xZnxCNi3晶体费米能级附近的能带结构、单晶体和多晶体的弹性性质都发生明显变化,而B掺杂MgC1-xBxNi3晶体的变化不明显；泊松比v和BH/GH的比值表明MgC1-xBxNi3晶体是延展性的化合物,而Mg1-xZnxCNi3晶体处于延性和脆性的边缘。计算了F掺杂对LaFeAsO1-xFx (x=0,0.05,0.08,0.11,0.15)晶体和K掺杂对Ba1-xKxFe2As2 (x=0,0.1,0.2,0.3,0.4,0.5)晶体的晶格参数、能带结构、态密度、单晶体和多晶体的弹性性质的影响。结果说明掺杂晶体晶格参数变化与实验结果定性一致；费米能级附近的能带结构变化不大；费米能级处态密度的变化与超导转变温度的变化不一致,不符合传统的BCS超导机制；单晶体和多晶体的弹性性质都发生较大变化；泊松比v和BH/GH的比值表明LaFeAsO1-xFx处于延性和脆性的边缘,而Ba1-xKxFe2As2晶体是脆性的化合物。更多还原

【Abstract】 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.更多还原