Design & Characterization of Thermoelectrics
Thermoelectric materials realize the interconversion between thermal energy and electricity. They are considered promising candidates to alleviate the energy and environmental crisis. The performance of thermoelectrics is determined by the dimensionless figure-of-merit zT. The Seebeck coefficient and electrical conductivity are interrelated by the carrier concentration and effective mass with an opposing trend, both depending on the intrinsic energy band structures. The lattice thermal conductivity depends on the propagation of phonons, which can be lowered on one side by softening the lattice vibration, and on the other side by introducing structural defects to scatter phonons. Yet, these defects could also impede the transport of charge carriers, which reduces the electrical conductivity. In short, the improvement of zT depends on the space dispersion relationship of electrons and phonons as well as the complex manipulation of structural defects.
The intrinsic properties of materials are determined by their constitutional elements and the chemical bonds connecting these atoms. It has been revealed that many main-group chalcogenides possess a unique combination of properties including large electronic and chemical bond polarizabilities and a strong lattice anharmonicity, etc., as well as abnormal bond-rupture behavior[1,2]. This special portfolio of properties is attributed to an unconventional chemical bonding mechanism, called metavalent bonding (MVB). These properties inherited from MVB are naturally beneficial to the improved zT values. Therefore, the discovery of MVB materials and the transformation from other bonding mechanisms to MVB are efficient methods for developing intrinsically high-zT thermoelectrics.
Besides chemical bonding, structural defects provide another way to tune the transport of electrons and phonons and thus the zT value. Given the different mean free paths of electrons and phonons in the same material, judiciously designed defects might selectively enhance the phonon scattering while remaining the transmission of electrons. Even though the zT values can be enhanced by introducing structural defects, the exact mechanisms underpinning this improvement are often obscure due to the complex microstructures and compositions of defects. Precisely characterizing the atomic configurations and chemical compositions of defects is of critical significance for unraveling the relationship between structures and properties. In turn, the thermoelectric properties can be tuned by controlling the structural defects.
Our group aims to design high-performance thermoelectrics by characterizing their chemical bonding mechanisms and structural defects using advanced characterization methods, e.g., mainly atom probe tomography (APT) in combination with electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM). The “treasure map” for chemical bonding and materials properties developed by Wuttig et al. will be used as a guide to tailor chemical bonds and to design structural defects such as the solubility of dopants.
Specific research topics are summarized in the figure and listed below:
1. Developing high-performance chalcogenide thermoelectrics by transforming covalently bonded materials into metavalently bonded alloys.
2. Enhancing the degree of band convergence and point defect phonon scattering by improving the solubility of dopants enabled by mixing metavalently bonded systems.
3. Strengthening the phonon scattering while remaining the electron transmission at grain boundaries by tuning the grain boundary complexions.
4. Revealing the origin of charge carrier scattering at grain boundaries by measuring the transport properties of a micro-scale bi-crystal and the structural and chemical information of the same grain boundary.
 Wuttig et al., Incipient Metals: Functional Materials with a Unique Bonding Mechanism. Adv. Mater. 2018, 30, 1803777.
 Zhu et al., Unique bond breaking in crystalline phase change materials and the quest for metavalent bonding. Adv. Mater. 2018, 30, 1706735.
 Yu et al., Chalcogenide thermoelectrics empowered by an unconventional bonding mechanism. Adv. Funct. Mater. 2020, 30, 1904862.
 Raty et al., A quantum-mechanical map for bonding and properties in solids. Adv. Mater. 2019, 31, 1806280.
 Liu et al., Improved Solubility in Metavalently Bonded Solid Leads to Band Alignment, Ultralow Thermal Conductivity, and High Thermoelectric Performance in SnTe. Adv. Funct. Mater. 2022, 32, 2209980.