Doctor of Philosophy
Institute for Superconducting and Electronic Materials - Faculty of Engineering
Li, Wenxian, Explorations of Superconductivity Mechanism and High Critical Current Performance of MgB2, Doctor of Philosophy thesis, Institute for Superconducting and Electronic Materials - Faculty of Engineering, University of Wollongong, 2010. http://ro.uow.edu.au/theses/3155
The discovery of superconductivity in magnesium diboride (MgB2, with a critical temperature of 39 K, observed in January 2001) has generated enormous interest and excitement in the superconductivity community and the world in general, especially for those involved in researching superconductivity in non-oxides and boron related compounds. MgB2 possesses an AlB2-type hexagonal structure (space group P6/mmm) with alternating boron honeycomb planes and magnesium triangular planes. Each Mg atom is located at the center of a hexagon formed by boron atoms, and it donates its electron to the boron planes; hence, the B-B bonding is strongly anisotropic. The unit cell parameters are a = 0.3086 nm and c = 0.3524 nm at room temperature. These values for the lattice parameters of MgB2 are in the middle of the values of lattice parameters of AlB2-type compounds. Owing to the simple hexagonal structure with space group P6/mmm, four optical modes at the Г point of the Brillouin zone are predicted for MgB2: a silent B1g mode (at 87.1 meV, ~700 cm-1), the E2g Raman mode (at 74.5 meV, ~600 cm-1), and the infrared active E1u (at 40.7 meV, ~330 cm-1) and A2u (at 49.8 meV, ~400 cm-1) modes. The E2g mode is responsible for the high critical temperature, Tc, in MgB2. Further studies based on a number of experimental techniques, such as angle resolved photoemission spectroscopy (ARPES), the de Haas-van Alphen effect, and the Hall resistivity, have found that MgB2 exhibits a rich multiple-band structure. These results are in agreement with band structure calculations and reveal stronglytwo-dimensional spxpy(σ) bands, as well as three-dimensional pz(π) bands. The identification of MgB2 as a two gap superconductor has resulted in much research associated with the spectroscopy of this material. It has become generally accepted that the larger gap is associated with the 2D σ bands arising from the boron planes and has a value of Δσ ≅ 7.069 meV, while the 3D π bands have a gap of Δπ ≅ 2.700 meV. MgB2 has been fabricated in bulks, single crystals, thin films, tapes, and wires for different application. In addition to the relatively high Tc and simple crystal structure, MgB2 possesses a large coherence length, high critical current density, and transparency of grain boundaries to current flow. The in situ route seems to be the most promising method to improve the upper critical field, Hc2, and the critical current density, Jc, performances of MgB2. MgB2 is a promising superconductor for high-magnetic field applications because of its high Jc. The grain boundaries in MgB2 do not significantly degrade Jc and even serve as pinning centers, which is different from the weak-link effects in high-Tc superconductors. Chemical or compound doping changes the reaction kinetics and therefore influences the grain growth, the formation of secondary phases, the density, or the stoichiometry. Doping or alloying with carbon or SiC has been shown to significantly enhance Hc2 and the pinning force. Wires and tapes of MgB2 have been made using the powder-in-tube (PIT) technique with encouraging results. There are two main focuses of this work: first, the constraints and mechanism of improvement of Tc of MgB2 are discussed based on the estimation of the electronphonon coupling (EPC) in the Mg-B system; second, the contributions of connectivity and distortion to Jc and Hc2 are explored to improve the application potential of MgB2 based superconductors. In the first part, the influence of sintering temperature on the critical transition temperature, Tc, for MgB2 superconductor is investigated systematically with the aid of Raman scattering measurements and Raman spectral fit analysis. The superconducting transition properties of MgB2 should be partly attributed to the phonon frequencies and linewidths, especially those of the E2g mode deduced from the Raman spectral fit analysis. The strength of the EPC, which is related to the E2g mode, is the dominant factor that will determine the Tc of MgB2. Sampling of the partial density of states (PDOS) causes two additional peaks to appear in the Raman spectrum of MgB2. The presence of the high frequency PDOS peak, ω3, in the low Tc samples indicates weakening of the EPC strength, while this peak becomes weak in the high Tc samples. At the same time, the low frequency PDOS peak, ω1, is weak in the low Tc samples, but becomes strong for the high Tc samples. The enhanced ω1 blocks improvement in the EPC intensity, which is necessary to obtain further high Tc in MgB2. Then, the influences of the phonon frequency and the unit cell volume on the superconductivity of element-doped MgB2 are discussed with reference to a Raman study on SiC, C, Mn, and Al-Ag doped Mg-B materials. A phenomenon has been discovered in the doped samples, in that the phonon frequency changes to counteract the crystal lattice variation to keep the system stable within a Grüneisen parameter of 2.0−4.0. The chemical doping effects on phonon frequency and unit cell volume can be explained by the harmonicity-anharmonicity competition in the compounds. A decreased electronic density of states is responsible for the depression of superconductivity that is seen in doped MgB2. The possibility of a high critical temperature, Tc, in the Mg-B system exists if the material can possess both a highphonon frequency and a big unit cell volume at the same time, as indicated by the isotope effect and hydrogenation experiments. The last topic of the first part is the magnetic scattering effects of Fe, Co, and Ni dopants in MgB2. The superconductivity is sensitive to the spin-flip pair-breaking scattering due to the exchange interaction between conduction electrons and magnetic moments of the magnetic dopant ions. The observation of superconductivity in ternary-iron silicides and Fe-As based superconductors shows the importance of investigation into the relationship between superconductivity and magnetism caused by the magnetic components. It is found that weak magnetic scattering effects are partly responsible for the Tc depression in Fe and Ni doped MgB2. The phonon behavior is mostly responsible for the slight decrease in the coupling strength in Mg1-xCoxB2, as in the case of non-magnetic impurities. The ferromagnetic nature of Fe, Co, and Ni does not induce strong pair-breaking in MgB2 compared with Mn. The superconductivity of the two-band superconductor MgB2 is independent of the magnetism of the individual component elements and of any particular impurity. In the second part, the influence of sintering temperature on the critical current density, Jc, for MgB2 superconductor was investigated systematically with observations of Raman scattering and flux pinning force, Fp, analysis. The enhanced E2g mode in Raman spectra with increasing in situ sintering temperature shows gradual strengthening of the electron-phonon coupling in MgB2, which means that the crystals become more homogeneous after higher temperature sintering. However, the crystallinity is degraded for samples sintered at even higher temperature, due to Mg deficiency. A possible explanation for the Jc(H) performance, which is in accordance with the Raman spectroscopy observation and Fp analysis, is the cooperation between the electron-phonon coupling in the E2g mode and the flux pinning centers, mainly originating from the lattice distortion due to the different sintering temperatures. Then, the field dependence of the superconducting properties and mechanism of nano-SiC doped Mg1.15B2/Fe wires was investigated systematically to explore the combined effects of connectivity and distortion on Jc. The connectivity is responsible for the high Jc performance in the single-vortex regime, while strong disorder is responsible for the promising Jc in the small-bundle pinning regime. The promising Jc values in Mg1.15B2+10wt%SiC are the result of optimized connectivity and disorder, which are reflected in the Raman spectrum, with both a strong E2g mode and a strong PDOS. The Raman scattering measurements imply that excess Mg is effective in improvement of the connectivity of MgB2 grains, while nano-SiC is responsible for the great lattice distortion in the SiC-doped samples. The superconductivity transition is advanced in Mg excess samples according to the decreased ΔTc. The EPC analysis shows that the excess Mg can also improve the electron-E2g coupling. However, the impurity phases depress the total EPC strengths.
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