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Optimisation par inclusion, alliage et dopage des matériaux thermoélectriques d'intérêt - application des méthodes ab initio et de dynamique moléculaire

Abstract : Thermoelectricity is considered a promising source of energy since it is able to directly convert heat into electricity. This makes it possible to recover dissipated heat without causing pollution. However, large-scale applicative options are still under restriction because of the dim thermoelectric conversion yield. Therefore, numerous research works are dedicated to improving thermoelectric performance of different materials, which is characterized by the dimensionless figure of merit ZT. A favorable ZT includes simultaneously a satisfying Seebeck coefficient, a high electrical conductivity and a low thermal conductivity. To seek a suitable material with a better thermoelectric performance is the objective of our analyses. With doping technics, different elements can be added into semi-conductors within different concentrations. The charge density could be thus modified in order to change thermoelectric properties. Due to hurdles related to materials synthesis, numerical simulations based on different methods, such as density functional theory (DFT), molecular dynamics (MD), are then implemented to estimate the most promising improvement approach. During this thesis, thermoelectric properties of several materials are investigated for applications in different situations, i.e. CsSnI₃ as a potential candidate with its high electronic conductivity, ZnO as a transparent thermoelectric material, Bi₂Te₃ as a traditional material with further improvements and cellulose as future organic semi-conductor. As DFT concerns only properties of electrons (Seebeck coefficient, electric conductivity, thermal conductivity due to electrons), lattice thermal conductivity is not included herein. Therefore, DFT with finite displacement and MD are used as a complementary method to establish thermal conductivity due to phonons. In this way, this thesis is divided into two parts. In the first part, theoretical backgrounds of DFT are introduced starting with Schrödinger equation. Results of classical DFT simulations are presented afterwards. By using atomic positions from experimental measurements, we launched crystal structure relaxation to ensure that every atom in the system is at its equilibrium position. Electronic band structures are also calculated to validate calculation configurations (cutoff energy, convergence conditions, etc.). A full mapping of Eigenvalues in reciprocal space is realized and thermoelectric properties are calculated by solving Boltzmann transport equations. In the second part, basic theories of phonons are mentioned, followed by introductions of DFT with finite displacements and MD methods. We implemented MD simulations to study the influence of aluminum doping on lattice thermal conductivity for ZnO. We also used DFT with finite displacements method to study lattice thermal conductivity variation of Bi₂Te₃₋ₓSeₓ alloy.
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Lantao Yu. Optimisation par inclusion, alliage et dopage des matériaux thermoélectriques d'intérêt - application des méthodes ab initio et de dynamique moléculaire. Science des matériaux [cond-mat.mtrl-sci]. Université Paris-Saclay, 2018. Français. ⟨NNT : 2018SACLS056⟩. ⟨tel-01865835⟩

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