- Phonon anharmonicity of SnSe with Raman spectroscopy, with Academia Sinica, Taiwan, and Air Force Research Laboratory, WPAFB, OH
Thermoelectric materials have been of interest due to their ability to harvest waste heat energy. For efficient thermoelectric conversion, these materials must exhibit a low thermal conductivity (κ) and a high Seebeck coefficient and moderately high electrical conductivity. Recently, binary chalcogenides such as SnSe have emerged as attractive candidates for thermoelectric energy generation at moderately high temperatures due to their considerably low thermal conductivity. There is experimental and theoretical evidence for anharmonicity in SnSe, however, the effect of the entire phonon band structure on thermal transport, was not completely understood. Using combined temperature-dependent polarized Raman spectroscopic measurements (performed by Dr. Rahul Rao at the Air Force Research Laboratory in OH) and heat-capacity study on fully dense single-crystalline SnSe that revealed that the anharmonicity is driven by soft optical modes in the b−c plane, the details of which were published in Phys. Rev. B, 98 (2018) and Materials Horizon (2022). The high-quality single crystalline SnSe were synthesized by Prof. Y. Y. Chen’s group at Academia Sinica, Taiwan.
- Thermal properties and phonon anharmonicity of GeTe in collaboration with Academia Sinica, Taiwan
The efficiency of a thermoelectric material is quantified by a dimensionless figure-of-merit, , which is dependent on the absolute temperature , the Seebeck coefficient the electrical and thermal conductivities, and , respectively. Bulk GeTe exhibits a comparatively high total thermal conductivity κ but a surprisingly low lattice thermal conductivity κlat at high temperatures, which is further reduced via Sb-doping through a simultaneous reduction in κe and κlat. From the temperature dependence of κlat and analysis of Raman spectra of GeTe, we inferred that a three-phonon Umklapp scattering process is the dominant scattering mechanism in Sb-doped GeTe, in contrast to pristine GeTe where a four-phonon process is the dominant mechanism, the details of which were published in Adv. Sci (2020) and Materials Horizon (2022). The high-quality single crystalline GeTe and Sb-doped GeTe were synthesized by Prof. Y. Y. Chen’s group at Academia Sinica, Taiwan.
- Famatinite project (with Furman University, Greenville, SC)
Copper based sulfides materials, viz., famatinides (general formula, Cu3SbS4) are of interest for thermoelectric applications due to their Earth-abundance and non-toxicity. Our goal is to develop n-type famatinites by doping at the copper site in Cu3-xMxSbS4 (M=Zn, Fe, Ni, Mn). The famatinide nanoparticles are synthesized by Prof. Mary Elizabeth Anderson and her group at Furman University Greenville, SC, and their thermoelectric properties are characterized at Clemson Nanomaterials Institute.
- Peltier cooling with Prof. Goldsmid, University of New South Wales, Sydney, Australia.
Thermoelectric (TE) phenomena comprising both the Seebeck and the Peltier effects enable environmentally friendly power generation and refrigeration applications. Traditionally, the thermoelectric figure of merit or zT of a material is found indirectly by independently measuring the three temperature-dependent properties, , and . We, for the first time, used Peltier cooling to experimentally determine the thermoelectric figure-of-merit of a material, where the maximum temperature depression, Tmax, is simply related to of a thermocouple. However, for thermocouples comprising good thermoelectric materials with T , Tmax can be large ( 100 K), making it difficult to assign the measurement to a particular temperature. This problem was addressed by using a thermocouple consists of a semiconductor and a metal, whose T and Tmax will be much smaller. Thus, in collaboration with Prof. Goldsmid at the School of Physics, University of New South Wales, Sydney, Australia, we developed a procedure for measuring the dimensionless figure-of-merit of a semiconductor using semiconductor-metal thermocouples, the details of which are published in J. Appl. Phys. 2022 (Editor’s choice).