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%0 Thesis
%A Lin, Nan
%T Thermoelectrics by design: improved properties of chalcogenides through metavalent bonding
%I Rheinisch-Westfälische Technische Hochschule Aachen
%V Dissertation
%C Aachen
%M RWTH-2024-09736
%P 1 Online-Ressource : Illustrationen
%D 2024
%Z Veröffentlicht auf dem Publikationsserver der RWTH Aachen University
%Z Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2024
%X Thermoelectric (TE) materials offer a solution to address energy consumption challenges by solid-state refrigeration and waste heat recovery. Chalcogenides are attracting research interest due to their diverse structures and high TE performance. Bi2Te3 (near-room temperature) and SnSe/Pb-based compounds (mid- to high-temperature) exemplify this potential. The dimensionless figure of merit (zT) quantifies TE performance: zT = S2σT/κtot, where S is the Seebeck coefficient, σ the electrical conductivity, κtot the total thermal conductivity, and T the absolute temperature. zT enhancement strategies target both electrical and thermal properties. Electrical properties are optimized by decoupling S and σ through energy band engineering (band convergence, density-of-states resonance, band anisotropy) and interface engineering (energy filtering, modulation doping). In terms of thermal properties, the minimization of the lattice thermal conductivity (κlat) is achieved by decreasing the phonon relaxation time or phonon group velocity, including the introduction of lattice defects and lattice softening. Besides, research on novel materials with intrinsically high TE properties is a burgeoning topic. Metavalent bonding (MVB), distinct from classical covalent, ionic, and metallic bonding, is crucial for high-performance TE chalcogenides. MVB exhibits a unique combination of properties: large Born effective charges, high optical dielectric constants, low Debye temperatures, and near-metallic electrical conductivity. Quantum-chemical calculations unveil MVB's unique nature, visualized via an electron transferred-electron shared (ET-ES) map. This map reveals MVB materials occupying a unique region with moderate electron transfer and nearly one electron shared between the nearest neighbors.Bi2Te3 and Sb2Te3, classic MVB compounds, are commonly alloyed for p-type TE materials at room temperature. However, the impact of alloying on MVB and its effect on TE properties remain unclear. To minimize the impact of defects on TE properties, high-quality BiₓSb2−xTe3 (x = 0.5, 0.6, and 0.7) single crystals were synthesized using the vertical Bridgman method. Characterization techniques (Fourier-transform infrared spectroscopy for optical properties and atom probe tomography for bond analysis) validated the MVB character of the alloys. Their favorable transport properties arise from the interplay between MVB and the electronic band structure, featuring high valley degeneracy, low band effective mass, and strong phonon anharmonicity – key factors for high TE performance. Building on the MVB-performance link in Bi-Te alloys, this study explores novel MVB-based materials for enhanced TE efficiency. SnSe, a material with abundant elements, shows high TE performance only in its high-symmetry phase, especially in polycrystalline forms. This work addresses this challenge by stabilizing the desired rock-salt SnSe phase at lower temperatures through AgVVI2 (V = Sb, Bi; VI = Se, Te) doping. Cubic SnSe exhibits MVB character, while the Pnma phase is covalently bonded. This work demonstrates that alloying-induced transition from covalent bonding to MVB stabilizes the desired cubic phase at lower temperatures. Consequently, zT near room temperature is significantly enhanced by over tenfold in Fm"3" ̅m SnSe alloys compared to pristine Pnma SnSe. The reported structural transformation in SnSe could also be potentially linked to the high-entropy effect. To elucidate the interplay between MVB bonding and the high-entropy effect, n-type polycrystalline BiPbAgQ (Q = S3, Se3, Te3, and TeSeS) alloys were fabricated. Atom probe tomography confirmed abnormal bond-breaking behavior, indicative of the MVB character of these alloys. Interestingly, the maximum optical absorption decreased (BiPbAgTe3 > BiPbAgSe3 > BiPbAgS3) with increasing charge transfer, suggesting a weakening of MVB. This correlates with a decline in TE performance as the chalcogen element changes from Te to S. These results emphasize the potential of controlled charge transfer in designing MVB-based high-entropy thermoelectrics. This study establishes the link between MVB and high zT in Bi-Te alloys. Then we leverage MVB principles to design cubic SnSe alloys with significantly improved average zT across a wide temperature range. Finally, we explore high-entropy MVB alloys, promoting solid solution formation and increasing configurational entropy. This approach offers a promising avenue for optimizing TE properties through controlled charge transfer within the high-entropy framework.
%F PUB:(DE-HGF)11
%9 Dissertation / PhD Thesis
%R 10.18154/RWTH-2024-09736
%U https://publications.rwth-aachen.de/record/995132