Theoretical Investigation On The Transport Properties Of Environment-friendly Thermoelectric Silicides

So-called thermoelectric material that converts heat into electricity thermoelectric material is one type of functional materials which enable the direct conversion between thermal and electrical power. Solid state generator, or thermoelectric cooler offer several advantages over conventional systems including: high reliability, low maintenance, and it's eco-friendly properties. With the growing public interest in environmental and energy problems in recent years, thermoelectric power generation as one of novel and promising technologies of rational energy use is recognized. High performance thermoelectric modules will address many applications in such as refrigeration, electronics, industrial and automotive. However, low conversion efficiencies of thermoelectric module limits large-scale commercial applications. So researchers are working hard to improve performance of thermoelectric materials using different ways, and now doping and nanocomposite approach both are regarded as promising and effective way to improve thermoelectric properties.With the rapid development of relevant theory and computer hardware, first principles calculation which is based on density functional theory (DFT) has become an important means of theoretical studies in condensed matter physics, quantum chemistry and materials science. From first principles calculation, we can draw the energy band structure, the density of states, phonon spectrum and other microscopic properties of the material, and may also make a good explanation on the nature of the thermoelectric micro-formation mechanism. Since the transport properties of thermoelectric materials are closely related to their electronic and phonon structure, investigating the electronic and phonon structure of material is helpful to predict the thermoelectric properties and explain the nature of thermoelectric. In this thesis, some physical properties of thermoelectric materials at atomic and electronic level are investigated using first principles method based on density functional theory.In this thesis, the environment-friendly silicide and oxide thermoelectric materials are mainly investigated including their electron and phonon properties. It is because that the silicide thermoelectric materials offer many advantages including: good thermal stability, a wide source of non-toxic and abundant raw materials, have attracted much attention for their thermoelectric performances. Here, all-electronic method is used to calculate the electronic characteristics of different systems, combined with the Boltzmann transport equation to calculate the thermoelectric transport properties. The lattice vibration and the behavior of phonon thermal conductivity are analyzed using density functional perturbation theory. The main contents of this thesis are as follows.1. Full-potential Linearized Augmented Plane Wave method (FLAPW) and Boltzmann transport properties are used to investigate the crystal structure and electronic structure of Mg2Si. Energy band structure shows that Mg2Si is an indirect semiconductor with energy band gap about0.20eV. Transport properties versus the doping level have been calculated for the n type and p type doped materials, result shows that the value of p type Mg2Si is larger than that of n type material at300K, while the condition is opposite at700K. The optimal carrier concentration corresponding to the maxima of power factor are obtained. Maximum ZT value of0.93is estimated in combination with experimental data of thermal conductivity.2. By analysis of the Eliashberg spectral function, the impact of electron-phonon interaction strength on the thermoelectric conversion in Mg2Si is discussed. Result shows that high-frequency optical phonons make a significant contribution on the electron-phonon interaction. Besides, through comparison between the phonon density of states and spectral function, the vibrations of Si atoms are found to play an important role in the electron-phonon interaction. That is might be the reason why investigators usually synthetize the Si site solid solution to optimise the thermoelectric properties of the system. Thermal properties of Mg2Si1-xSnx at different Sn content are investigated. Result shows that phonon group velocities reduce with the increase of Sn, and this can help to reduce the phonon thermal conductivity. Phonon spectrum tends to soften with increasing Sn content, which is mainly due to two reasons:heavier atomic mass of Sn and relatively weakened Sn-Mg chemical bonds. When the Sn content at least greater than0.75, the phonon band gap shows up in the phonon spectrum. Free energy calculations show that the thermodynamic stability of the solid solution increases with the increase of solid solution of Sn.3. The lattice vibration characteristics of BaSi2and BaGe2, and electronic transport properties of BaSi2are studied, and the reasons of BaSi2exhibits such low thermal conductivity is explored. Phonon dispersion relations show that both BaSi2and BaGe2have flat optical phonon bands and weak dispersive acoustic phonon branch. By analyzing the vibrational mode of the lowest optical phonon, we find that the low-lying optical phonon branch at Brillouin zone center corresponds to the rigid-unit vibration of Si or Ge tetrahedron. The rigid-unit vibrational mode confines the acoustic phonon modes and scatters the heat carrying acoustic modes, leading to the low lattice thermal conductivity. Free energy calculation demonstrates that BaGe2is thermodynamically more stable than BaSi2. The electronic structure shows that BaSi2is an indirect band gap semiconductor. The transport coefficients show strong anisotropy. For the n-type BaSi2,the maximum ZT value can reach to about0.7along the y-axis direction at carrier concentration1.0×1019cm-3of. And in p-doped system, ZT can reach the maximum value along the x direction when the hole concentration of3.2×1018cm-3. Therefore, in experiment adjustment composition can be combined with texture to improve the thermoelectric properties of BaSi2.4. The characteristics of phonon, thermodynamic properties, electronic structure and transport properties of β-FeSi2and OsSi2are investigated using density functional theory. Result shows that the coexistence of covalent and ionic bond has significant impact to the lattice vibrational spectra. Although the two materials have the same structure, the phonon spectrums show a larger difference. In OsSi2, phonon band gap is shown in the low-frequency optical. This is mainly caused by different bond type in two materials due to the difference in electronegativity of the elements. The electronic structures of the two materials are indirect band gap semiconductor, with the direct band gap tens of milli-electron volts larger than the indirect band gap. The bottom of the conduction band of β-FeSi2has heavier effective mass than that of OsSi2. By analyzing the transport properties, we find that for OsSi2, doped with low valence of the acceptor impurity is more conducive to the improvement of the thermoelectric properties of β-FeSi2doped with high valence state of the owner impurities5. The lanthanides doping-induced change of electronic structures of SrTiO3and the thermoelectric properties have been analyzed by ab initio band calculations and single band model. Result shows that except for the case of La, Lu, localized bands of states appear below the conduct band minimum, in the band gap. The impurity states are mainly contributed to the4f states of lanthanides hybridized some Ti3d states. The dispersion characteristic of the impurity band is very weak, so the contribution to the electronic conduction is very small. However, A-site doping with Ln doping is responsible for the donor-like features close-above the Fermi energy. Concurrently, impurity band can increase the density of states at the Fermi level and hence enlarge the Seebeck coefficient. Besides, at the same doping concentration, the Tb, Yb and Dy doped SrTiO3exhibit lager Seebeck coefficient than doped with other elements.