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@PHDTHESIS{Lin:995132,
author = {Lin, Nan},
othercontributors = {Mayer, Joachim and Wuttig, Matthias},
title = {{T}hermoelectrics by design: improved properties of
chalcogenides through metavalent bonding},
school = {Rheinisch-Westfälische Technische Hochschule Aachen},
type = {Dissertation},
address = {Aachen},
publisher = {RWTH Aachen University},
reportid = {RWTH-2024-09736},
pages = {1 Online-Ressource : Illustrationen},
year = {2024},
note = {Veröffentlicht auf dem Publikationsserver der RWTH Aachen
University; Dissertation, Rheinisch-Westfälische Technische
Hochschule Aachen, 2024},
abstract = {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.},
cin = {025010 / 025000 / 520000 / 131110 / 080018 / 130000},
ddc = {620},
cid = {$I:(DE-82)025010_20140620$ / $I:(DE-82)025000_20140620$ /
$I:(DE-82)520000_20140620$ / $I:(DE-82)131110_20140620$ /
$I:(DE-82)080018_20160203$ / $I:(DE-82)130000_20140620$},
pnm = {BMBF 03ZU1106BA - NeuroSys: Skalierbare Photonische
Neuromorphe Schaltkreise (Projekt B) - A (03ZU1106BA) /
China scholarship council (201906050145)},
pid = {G:(BMBF)03ZU1106BA / G:(CSC)201906050145},
typ = {PUB:(DE-HGF)11},
doi = {10.18154/RWTH-2024-09736},
url = {https://publications.rwth-aachen.de/record/995132},
}
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