Theoretical Studies On The Thermoelectric Properties Of Doped CoSb₃

Skutterudite CoSb3-based materials is a new kind environment friendly middle-temperature thermoelectric materials with great future prospects, however, the high thermal conductivity limits their widespread use. Nowadays, the main methods to reduce the thermal conductivity of CoSb3are filling and doping with some metal elements to it. But the optimization and microscopic physical mechanisms on the performance of thermoelectric materials remain unclear, which cause by the coupling relationship among those thermoelectric parameters. First principles calculation based on density functional theory (DFT) is an important theoretical research mean in condensed matter physics, computational physics and chemistry; therefore, it is also the most common method on researching and predicting the properties of thermoelectric materials. In this thesis, based on density functional theory, the effects of filling with In atoms or several kinds of double atoms and substituting in Co site on lattice structures and transport properties of CoSb3have been investigated at a microscopic level, and the electrical transport mechanisms in several CoSb3-based thermoelectric materials are analyzed. The main contents and conclusions are as follows:1. The effects of double-doping on electronic properties of CoSb3are studied.From the energy band diagrams, a flat impurity band appears at the bottom of conduction band in In atoms filled CoSb3, the locality of those electronics in this energy band is very strong and their effective mass is relatively larger; simultaneously, the hybridization of p and d electron orbits caused by In filling makes density of states increasing near fermi level. The above two points both improve the Seebeck coefficient of materials.After In filling, fermi level of CoSb3drills through the bottom of conduction band, which leads to the increase of electric conductivity.From the phonon dispersion, the low-frequency optical phonon mode of In scatters the acoustic branch caused by Sb atoms vibration, which makes the phonon branch flatting and leads to the decrease of thermal conductivity. Besides, the optical branch becomes flatter after In filling, which further reduces the thermal conductivity.Therefore, the combined effect of above three mechanisms makes electric conductivity and Seebeck coefficient increasing, as well as thermal conductivity decreasing, which largely improves the ZT value of In-filled CoSb3.2. The effects of double-doping on electrical transport properties of CoSb3are investigated. After double-doping, a flatter impurity band appears in CoSb3and the electron effective mass become larger, which improve the Seebeck coefficient of materials; simultaneously, the increase of density of states near fermi level also improves the Seebeck coefficient; besides, fermi level drills through the bottom of conduction band, which leads to the increase of electric conductivity. Therefore, the combined effect of above largely improves the power factor of CoSb3. Moreover, the results show that:(1) In-Ca, In-Sr or In-Ba atoms filling makes lattice expanding and bandgap value varying in CoSb3, and those change should be affected by ionic radius of alkaline earth atoms. Among those filling, the s electrons of Ca atoms are closer to fermi level than those of Sr and Ba atoms, and they have the most contribution to the bottom of conduction band, as well as the corresponding In atoms have the most conduction electron.(2) Lattice of CoSb3obviously expands when Ba-La, Ba-Eu or Ba-Yb atoms filling. Compared the results of μ and v before and after doping, it is found that the order degree of filled CoSb3lattice decreases. Among those, the anisotropy of band structure in the CoSb3filled with Ba-La atoms is more evident, which is helpful to improve the electrical conductivity and Seebeck coefficient of materials.3. The effects of Ni, Pd, and Pt atoms substitution in Co site on the electronic structure of CoSb3are calculated and analyzed. In CoSb3, lattice distorts as Co being substituted by Ni, Pd or Pt, and the cell volume increases; after substituting, bandgap value decreases, and band degeneracy significantly increases; fermi level goes up, and drills through the bottom of the conduction band, which leads to the increase of electric conductivity. Compared with In atoms, the extra d electrons in the outermost shell of Ni, Pd, or Pt atoms make carrier concentration and electrical conductivity increasing in CoSb3, which leads power factor improving and thermoelectric performance optimizing. The results of electron effective mass in Γ-Z direction show that the electron effective mass with Ni substituting is the largest.