Faculty Publications

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Complementary effect of co-doping aliovalent elements Bi and Sb in self-compensated SnTe-based thermoelectric materials
(Royal Society of Chemistry, 2021) Kihoi, S.K.; Shenoy, U.S.; Bhat, D.K.; Lee, H.S.
Research on Pb-free thermoelectric materials as a potential eco-friendly and solid-state source of energy has continuously advanced over time, with SnTe-based materials having shown utmost promising properties owing to their tunable electronic structure and scalable thermal conductivity. In this study, we self-compensate Sn to reduce inherent Sn vacancies, and further tune the carrier concentration by doping with Bi. Sb is further alloyed to incorporate nanostructures that significantly reduce the thermal conductivity. Multiple aliovalent dopants result in a continually decreased carrier concentration and subsequent significantly decreased electrical conductivity. The Seebeck values are seen to increase with temperature, where a maximum value of ?171 ?V K?1is reported with a maximum power factor of ?22.7 ?W cm?1K?2. We show through first principles DFT calculations the synergistic effect of Bi and Sb to introduce resonance states and an additional valence band convergence effect with increasing Sb that contribute to improved electronic properties. A decreased phonon frequency with co-doping is also reported. A maximumZTof ?0.8 at 823 K is reported in the Sn0.90Bi0.03Sb0.10Te composition, showing good potential in Sb co-doped SnTe-based materials. © The Royal Society of Chemistry 2021.
Asymmetric Thermoelectric Performance Tuning in Low-Cost ZrFexNi1-xSb Double Half-Heusler Materials
(American Chemical Society, 2023) Kahiu, J.N.; Kihoi, S.K.; Kim, H.; Shenoy, U.S.; Bhat, D.K.; Lee, H.S.
The new paradigm for increasing the commercial viability of thermoelectric materials in the energy sector is the theoretical prediction and subsequent experimental validation and optimization of cheaper and inherently more efficient compositions. Herein, the experimental validation of the recently theoretically predicted ZrFe0.50Ni0.50Sb double half-Heusler and the ability to intrinsically tune this system to optimized p- or n-type materials by varying the Fe/Ni ratio in the synthesized ZrFexNi1-xSb (x = 0.35-0.65) samples are demonstrated. The samples are synthesized by arc melting, hot pressing, and annealing. Subsequent microstructural analysis confirms the crystallization of the ZrFexNi1-xSb into the half-Heusler structure and reveals that the variation of the Fe/Ni ratio favors the Ni-rich side. Consequently, the best p-type x = 0.55 and n-type x = 0.35 samples exhibit higher power factor values stemming from an increased carrier concentration, higher density of state effective mass, and suppressed bipolar conduction, as indicated by the Hall data analysis and density functional theory simulations. The additional lattice disorders introduced by varying the Fe/Ni ratio suppress the thermal conductivity and increase the microhardness of the n-type samples. The ZrFe0.35Ni0.65Sb and ZrFe0.55Ni0.45Sb samples achieve maximum zTs of ∼0.43 and 0.06, respectively, which is a great improvement over the ∼0.001 value of the ZrFe0.50Ni0.50Sb sample. These results highlight the viability of tuning the performance of double half-Heuslers on the doubly doped site. They will be instrumental in demonstrating the feasibility of developing low-cost double half-Heusler materials with better intrinsic and highly tunable properties. © 2023 American Chemical Society.
Enhanced Electrical, Thermal, and Mechanical Properties of SnTe through Equimolar Multication Alloying for Suitable Device Applications
(American Chemical Society, 2024) Kihoi, S.K.; Shenoy, U.S.; Kim, H.; Kahiu, J.N.; Kim, C.M.; Park, K.-I.; Bhat, D.K.; Lee, H.S.
With the ever-growing demand for eco-friendly energy sources to mitigate the global rising temperatures, the universal insatiable need for sustainable and efficient energy sources are earnestly being intensively sought after. The ubiquitous heat within, if successfully tapped, is an utterly promising source of energy. To achieve this, a thermoelectric device (TED) is needed. To enhance the conversion efficiency from heat to useful electrical power, we developed a strategy to improve the thermoelectric performance of the materials involved. In this work, equimolar multication alloying (EMMCA) is proposed for the first time and employed to enhance the performance of SnTe-based thermoelectric materials. Beyond the cation’s solubility limit, in situ compositing is observed with an increasing doping ratio, whereby distinct CuInTe2 ternary second phases are dispersed within the SnTe matrix. The electronic properties of the ensuing alloy are significantly enhanced by the resulting carrier concentration modulation and the unique electronic band engineering. A decrease in the thermal transport properties is likewise reported, benefiting from enhanced phonon scattering and diminished electronic contribution. The mechanical properties are also shown to increase with increased alloying. As a result, single-leg TED performance shows substantial output power in comparison with the pristine sample. The outcomes stemming from EMMCA are documented as significantly impactful, contributing to superior overall thermoelectric performance. © 2024 American Chemical Society.

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