Research Projects

Thermoelectric (TE) materials are crucial in the conversion of waste heat into usable electricity, offering a significant potential in future energy management. The efficiency of converting waste heat into electrical energy is defined by the dimensionless figure of merit, zT= S²σT/(κe+κlat), where S, σ, T, κe, and κlat represent the Seebeck coefficient, electrical conductivity, absolute temperature, electronic conductivity, and lattice thermal conductivity, respectively [1, 2]. The strong interdependence among S, σ, and κe makes it highly challenging to increase the zT value. As κlat is the only independent parameter, efficient TE performance necessitates minimizing κlat. Superionic materials are promising candidates for thermoelectric applications due to their “phonon-liquid electron-crystal (PLEC)” type of behavior suitable for decoupling electrical and thermal transport. The liquid-like behavior of the cationic sublattice impedes the transfer of heat via phonons, whereas their rigid anionic sublattice maintains a crystalline structure that facilitates electrical conduction. In addition to the superionic nature, the high anharmonicity associated with disordered cationic constituents can reduce the mean free path of phonons. This combination leads to a decrease in κlat while preserving high σ, hence, significantly enhancing zT. In this project, we aim to design and engineer superionic silver-based chalcogenides, namely Frank-Kasper type AgyTe6(S, Se) phase [3], to achieve ultra-low κlat, potentially exhibiting glass-like behavior while maintaining suitable electrical transport. In these compounds, highly disordered Ag sublattices are prone to exhibit superionic-type liquid-like behavior with high anharmonicity, which serves as a key mechanism for intrinsic phonon scattering. We specifically focus on disordered systems that inherently undergo superionic phase transitions with high anharmonicity, a distinctive hallmark of these silver-based Frank-Kasper materials, rather than relying on conventional extrinsic strategies like point defect engineering, alloying, or nanostructuring to significantly reduce phonon transport. By employing advanced synthesis and precise structural and thermal characterization, we want to elucidate the correlation between lattice thermal transport, chemical bonding, and crystal symmetry. Furthermore, we will optimize the electronic transport of these compounds by tuning the carrier concentration to enhance their TE performance. Our target zT is ≥ 2.0 at 900 K. Our thorough investigation will not only shed light on the fundamental physics governing phonon dynamics in disordered systems but also establish a way for predictive design principles for next-generation thermoelectric materials with intrinsically low κlat. References: 1. Q. Yan et al., Nat. Mater. 21, 503 (2022). 2. S. Roychowdhury et al., Science 371, 722 (2021). 3. H. Mikus et al., Z. Anorg. Allg. Chem. 631, 12331236 (2005).