Study Of Electronic Structure Of Thermoelectric Materials-Effects Of Doping On Thermoelectric Properties Of Single Crystal Materials

Thermoelectric properties are related to the electronic structure of thermoelectric materials. Studying the electronic structure and the effects of doping on the electronic structure of thermoelectric materials can provide means to improve the thermoelectric properties of the current thermoelectric materials and ideas to develop the new and high-performanced thermoelectric materials. Doping is one of the significant means to improve the thermoelectric transport performances of thermoelectric materials. The crystal boundary and the melting defects such as impurity and hole are eliminated in single crystals. Thus, single crystal can be used to investigate the thermoelectric properties along the different crystallographic directions of thermoelectric materials. CrSi₂ is a candidate thermoelectric material used at intermediate or high temperature. Studying the effects of Si substitution by Al as well as Cr substitution by V on the thermoelectric transport performances and electronic structure of the CrSi₂ single crystal is an important means to improve the thermoelectric properties of CrSi₂.Damped Newton Dynamics Schemes were used to study the relaxation process of CoSi_1-xY_x (Y=Al,P) where Si was substituted by Al or P. Linearized Augmented Plane Wave (LAPW) based on the Density Functional Theory (DFT) of the first-principles was utilized to calculate the electronic structure of CoSi and CoSi_1-xY_x (Y=Al,P). Based on the calculated electronic structures of CoSi and CoSi_1-xAl_x as well as the existing experimental data on thermoelectric transport performances of the CoSi_1-xAl_x single crystal, the essence of the effects of Al doping on the thermoelectric properties of the CoSi_1-xAl_x single crystal were analyzed. The electronic structure of CoSi showed that the overlap between valence band and conduction one existed at its Fermi level, which meant that CoSi was a semi-metal. The Si substitution by Al made the electronic structure of CoSi_1-xAl_x complex. With the increase of Al, the Fermi level of CoSi_1-xAl_x shifted toward the low energy band. In contrast, the Si substitution by P made the Fermi level of CoSi_1-xP_x shift upward (toward high energy band). The changing trends of electrical resistivity and Seebeck coefficient of the CoSi_1-xAl_x single crystal were explained by the effects of the Si substitution by Al on the electronic structure.Doping is an effective method to increase the electrical performances and decrease the thermal conductivity. Two inequivalent sites of Fe as well as Si exist inβ-FeSi₂. The method of comparing total energy was used to determine the substitution sites of Al and Co, respectively. Damped Newton Dynamics Themes were used to relax the crystal structures of Fe(Si_0.96875Al_0.03125)₂ and Fe_0.9375Co_0.0625Si₂. LAPW was used to calculate the electronic structures ofβ-FeSi₂ and Fe(Si_0.96875Al_0.03125)₂ as well as Fe_0.9375Co_0.0625Si₂. The effects of doping by Co and Al on the thermoelectric transport performances ofβ-FeSi₂ were analyzed theoretically. The electronic structure ofβ-FeSi₂ showed that an energy gap existed at the Fermi level, which meant thatβ-FeSi₂ was a typical semiconductor. A direct energy gap of 0.74eV existed at H and an indirect energy gap of 0.71eV lies between H andΛ/3. The substitution sites of Co and Al were Feâ…¡and Siâ… , respectively. The Si substitution by Al made the Fermi level of Fe(Si_0.96875Al_0.03125)₂ shift toward its valence band. In comparison with that, doping by Co made the Fermi level of Fe_0.9375Co_0.0625Si₂ shift toward its conduction band. Doping could increase the Seebeck coefficient as well as the thermal coductivity greatly.La filling is an important method to reduce the thermal conductivity for CoSb3. The filling of La atom will result in the relaxation of geometrical structure of CoSb3. In this thesis, the effects of La filling on the geometrical structure of La0.5Co4Sb12 were investigated following the Damped Newton Dynamics Themes. LAPW was used to calculate the electronic structures and electron densities of CoSb3 as well as the relaxed La0.5Co4Sb12. The relaxation results showed that La filling had significant effects on the geometrical structure of CoSb3. In addition to that, La filling made the bond length, bond angle and charge density change. CoSb3 was a narrow-gap semiconductor. The energy band in the vicinity ofΓat the Fermi level was linear with the k vector. Ascribing to the La filling, the Fermi level of La0.5Co4Sb12 shifted toward the conduction band. La filling could increase the thermoelectric properties, which was attributed to the decrease of thermal conductivity while increasing the electrical transport performances.CrSi₂ is a thermoelectric materials utilized at the intermediate or high temperature. The electronic structures of CrSi₂, Cr(Si_1-xAl_x)₂ as well as Cr_1-xV_xSi₂ were calculated by LAPW. The effects of doping by Al and V on the electronic structures were analyzed. The range of solubility limits of Al and V in CrSi₂ were investigated with EPMA (Electronic Probe Microanalysis) and EDS (Energy Dispersive Spectroscopy), respectively. Based on the X-ray diffraction spectrum, the effects of doping by Al and V on the lattice parameters of Cr(Si_1-xAl_x)₂ and Cr_1-xV_xSi₂ were studied, respectively. Within the solubility limit, the single crystals of CrSi₂, Cr(Si_1-xAl_x)₂ and Cr_1-xV_xSi₂ were prepared by optical-heating floating zone melting method. The effects of doping by Al and V on the thermoelectric transport performances of Cr(Si_1-xAl_x)₂ and Cr_1-xV_xSi₂ single crystals were studied, respectively. Based on the calculated electronic structures and observed experimental data, the essence of the effects of doping by Al and V on the thermoelectric properties was analyzed. The electronic structure showed that CrSi₂ was a p-type semiconductor and hole carriers dominated its thermoelectric transport performances. An indirect energy gap of 0.35eV existed in the band structure of CrSi₂. The Fermi levels of Cr(Si_1-xAl_x)₂ and Cr_1-xV_xSi₂ shifted toward their valence bands resulting from the doping by Al and V, which had great effects on their thermoelectric properties. The solubility limit of Al in CrSi₂ was in the range of 0.20≤x<0.30, while V could be dissolved in CrSi₂ completely when the solubility x of V in CrSi₂ is less than or equal to 0.4. The change of the lattice parameters of CrSi₂ resulting from the doping by Al and V was in accord with the Vegard's law. The dopings by Al and V had significant effects on the thermoelectric transport performances and could increase the thermoelectric properties of the CrSi₂ single crystal greatly.