Optimisation des performances thermoélectriques des composées Skutterudites

Abstract

The theoretical studies have been fundamental in the development of new materials and new devices for diverse industrial applications. With advanced ab initio method, it is now feasible to access a database of crystal structure and use computer software to obtain interesting properties in the case in which experimental measurements are missing. For this raison we have attempt to determine the effect of filling or/and substituting elements on the structural, electronic properties and thermoelectric performance for CoSb3-based Skutterudite materials using calculations within the full-potential linearized augmented plane wave method (FP-LAPW) inside of the density functional theory (DFT) and semi classical Boltzmann transport theory which are implemented in the Wien 2K and Boltztrap codes. Firstly, we present theoretical study of structural, electronic and thermoelectric properties of binary skutterudite compounds MX3 (M= Co, Fe, Rh and Ir; X=Sb). The calculated structural parameters are in excellent agreement with the theoretical and experimental data. From the calculation of structural properties, we have found that our calculated structural parameters are in excellent agreement with the theoretical and experimental data. The calculated electronic band structures of the challenging compounds show that the CoSb3, IrSb3, CoAs3, IrAs3 and RhAs3 compounds are semiconductors materials with fundamentals narrowing directs band gaps, however, the CoP3, IrP3 and RhSb3 compounds exhibit indirects band gaps. Whereas the electronic band structures for the binary skutterudite RhP3 compound shows a metallic behavior. In the other hand, the spin up band structure in FeSb3 show a direct band gap semiconductor behavior while in the opposite case, it show metallic character and thus confirm the half metallicity of compound. From the investigation of the their thermoelectric properties, the CoSb3 compound has been found to have a large Seebeck coefficient, combined with high electrical conductivity and consequently resulting in high ZTe value than the other compounds, although it has a comparatively high thermal conductivity, which apparently makes it less attractive as an applied thermoelectric material. Also, we have study the role of filling in the structural voids of CoSb3 based skutterudites by species (AM= K, Na, Rb and Cs; EM= Ba and RE= Yb) or by doping at Co/Sb sites with Fe/(Ge, Te) atoms on the electronic properties and thermoelectric performance. From the theoretical analysis of the electronic properties we found that semiconductor behavior of compound is damaged by filling or doping CoSb3 due to the charge transfer from these elements; whereas the both seebeck coefficient and the electrical conductivity are improved in partially filled skutterudite compounds than the fully or doped and consequently, the power factor is enhanced. Finally, in this thesis, we analyze the effect of charge balance on the structural, electronic, and thermoelectric properties on partially filled CoSb3 (Ba0.25Co8Sb24-xSnx) where (x= 0, 1, 2, 3 and 4) and Fe/Te co-doped CoSb3 (Co4-xFexSb24-yTey). The theoretical analyses show that the charge balance in Ba0.25Co8Sb24-xSnx and Co4-xFexSb24-yTey compounds is found to be strongly dependent on the substitution configuration, where the Ba0.25Co8Sb23Sn composition found to be the most stable energetically than the other ones. Examining the SnFe/Te substitution effect on the electronic and thermoelectric properties, we point out that the electronic properties are moderate and the TE performance is enhanced with the estimated dimensionless electronic figure of merit (ZTe) value of about 0.67, 0.72 and 0.78 at T= 500K for Ba0.25Co8Sb23Sn and FeCo7Sb23Te and Fe2Co7Sb23Te2 compositions, respectively making these compounds as promising candidates for thermoelectric applications.