Faculty Publications
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Item Bi and Zn co-doped SnTe thermoelectrics: Interplay of resonance levels and heavy hole band dominance leading to enhanced performance and a record high room temperature: ZT(Royal Society of Chemistry, 2020) Shenoy, U.S.; Bhat, D.K.Lead free SnTe with a tunable electronic structure has become the front runner in eco-friendly thermoelectrics. Herein, we show through first-principles density functional theory calculations that Bi and Zn doping introduces a resonance level in SnTe. The dominance of the heavy hole valence band at room temperature in Bi-Zn co-doped SnTe leads to a record high room temperature ZT of ?0.3 (at 300 K) for SnTe based materials. The increase in the Seebeck coefficient value due to the interaction between the resonance states and formation of the nanoprecipitates leading to an appreciably low lattice thermal conductivity of 0.68 W m-1 K-1 results in a peak ZT of ?1.6 at 840 K. A record high ZTaverage of ?0.86 with 300 K and 840 K as cold and hot ends, respectively, makes Bi-Zn co-doped SnTe a potential material for thermoelectric applications. This strategy of using two resonant dopants, to not only improve the room temperature ZT but also high temperature values, can very well be extended to other systems. This journal is © The Royal Society of Chemistry.Item SnTe thermoelectrics: Dual step approach for enhanced performance(Elsevier Ltd, 2020) Bhat, D.K.; Shenoy, U.S.Doping of SnTe to achieve desirable properties has been a wide spread approach in the recent past to enhance its thermoelectric performance. Herein, we apply a dual approach: Pb doping for reduction of thermal conductivity and Zn doping for improving the power factor. The theoretical prediction of enhanced Seebeck due to increase in the band gap, introduction of the resonance levels by Zn and dominance of the heavy hole valence band, is realized experimentally as improved power factor throughout the temperature range. The accompanying reduction in the thermal conductivity by co-doping Pb and Zn leads to a record high room temperature figure of merit, ZT of 0.35 (@ 300K) and ZT of 1.66 at 840 K. The ZTaverage of ?0.9 with 300 K as cold end and 840 K as hot end sets a new record for SnTe based materials. © 2020 Elsevier B.V.Item Electronic structure modulation of Pb0.6Sn0.4Te via zinc doping and its effect on the thermoelectric properties(Elsevier Ltd, 2021) Shenoy, U.S.; Bhat, D.K.Striking a balance between the high performance and detrimental environmental toxicity of PbTe materials in thermoelectrics (TE) has become a necessity in the current situation. In this context, improving the performance of materials with lower lead content to the level of PbTe is crucial. Herein, we engineer the electronic structure of Pb0.6Sn0.4Te, a well-known TCI but a poor TE material by doping Zn. The first principles calculation reveal that Zn doping introduces multiple electronic valleys while simultaneously opening the band gap of Pb0.6Sn0.4Te. Higher power factor with lower thermal conductivity is predicted by the transport property calculations in the doped material. The resonance level introduced along with features of hyper-convergence of the valence bands leads to improved Seebeck co-efficient throughout the studied temperature range. An experimental figure of merit, ZT of ~1.57 at 840 K promises us a TE material applicable for a broad temperature range for future energy applications. © 2021 Elsevier B.V.Item Ultralow Lattice Thermal Conductivity and Enhanced Mechanical Properties of Cu and Sb Co-Doped SnTe Thermoelectric Material with a Complex Microstructure Evolution(American Chemical Society, 2022) Kihoi, S.K.; Shenoy, U.S.; Kahiu, J.N.; Kim, H.; Bhat, D.K.; Lee, H.S.SnTe is an exceptionally promising eco-friendly thermoelectric material that continues to draw immense interest as a source of alternative energy recovered from waste heat energy. Here, we investigate the effect of introducing Cu as a single doping element rather than phase separated in SnTe followed by Sb co-doping to tune the lattice thermal conductivity. A microstructure evolution was observed which influences the thermoelectric performance of these SnTe-based materials. An overall power factor of ∼22 μW/cmK2 and an ultralow lattice thermal conductivity of 0.39 W/mK are reported. A maximum ZT of 0.86 is also reported with an all-time record high hardness value of 165 Hv among SnTe-based thermoelectric materials. Through DFT calculations, we show that Cu opens the band gap of SnTe, whereas Sb in the presence of Cu introduces resonance levels and causes band convergence. This kind of enhanced thermoelectric performance is paramount for the application of SnTe in recovery of heat into useful electrical energy. © 2022 American Chemical SocietyItem Pushing the limit of synergy in SnTe-based thermoelectric materials leading to an ultra-low lattice thermal conductivity and enhanced ZT(Royal Society of Chemistry, 2023) Kihoi, S.K.; Shenoy, U.S.; Kahiu, J.N.; Kim, H.; Bhat, D.K.; Lee, H.S.In the era of sustainable and environmentally friendly energy requirements, alternative sources of energy continue to be fervently sought after. Heat recovery into useful electrical energy from waste heat offers a readily available source of energy with humongous potential. Herein, a non-toxic thermoelectric material, SnTe, is explored. Promising thermoelectric performance is also communicated. Introducing Ge as a single dopant is shown for the first time in SnTe-based materials to introduce amorphous Ge (a-Ge) precipitates into the matrix. These act as an auxiliary contributor to the observed ultra-low lattice thermal conductivity of ∼0.33 W m−1 K−1 at 823 K, which is below the reported amorphous limit of SnTe. Bi, which is a known resonant dopant, was further co-doped to fine-tune the electrical properties where a high power factor of ∼25.7 μW cm−1 K−2 is reported. To push the limit of synergy, Sb was added raising the maximum figure of merit ZT to a value of ∼1.1 at 873 K. With co-doping, dual resonance levels are shown which distorts the density of states (DOS) contributing to an increased band effective mass. In conjunction with the introduction of an amorphous phase, co-doping is ascertained as a practical means for the synthesis of high-performance thermoelectric materials for effective waste-heat recovery applications. © 2023 The Royal Society of ChemistryItem High thermoelectric and mechanical performance achieved by a hyperconverged electronic structure and low lattice thermal conductivity in GeTe through CuInTe2 alloying(Royal Society of Chemistry, 2023) Kim, H.; Kihoi, S.K.; Shenoy, U.S.; Kahiu, J.N.; Shin, D.H.; Bhat, D.K.; Lee, H.S.GeTe-based thermoelectric materials have a very high hole carrier concentration (∼1021 cm−3), and thus, improving the figure of merit, ZT, is substantially challenging. In this work, we foremost dope Bi to lower the majority carrier concentration, followed by alloying CuInTe2 to further adjust the hole concentration to an optimal level (0.5-2.0 × 1020 cm−3). This strategy also improves the structural symmetry and leads to hyperconverged valence sub-bands and resonance levels, increasing the effective mass from 1.42 m0 to 1.95 m0. Consequently, a high power factor of ∼23 μW cm−1 K−2 at room temperature and ∼41 μW cm−1 K−2 at 623 K in the (Ge0.93Bi0.05Te0.98)(CuInTe2)0.01 sample is reported. Moreover, the introduced point defects and nano-deposits reduce the lattice thermal conductivity to amorphous levels. As a result, the (Ge0.93Bi0.05Te0.98)(CuInTe2)0.01 sample has a peak ZT value of ∼2.16 at 623 K and an average ZT value of ∼1.42 at 300-773 K. A record high hardness value (∼277 Hv) is achieved. Simultaneous Bi doping and CuInTe2 alloying appear to be an effective strategy for increasing the ZT values of GeTe-based compounds. © 2023 The Royal Society of Chemistry.Item Tailoring the Thermoelectric Performance of the Layered Topological Insulator SnSb2Te4 through Bi Positional Doping at the Sn and Sb Cation Sites(American Chemical Society, 2023) Kihoi, S.K.; Shenoy, U.S.; Kahiu, J.N.; Kim, H.; Bhat, D.K.; Lee, H.S.Ongoing research and development focus on emerging thermoelectric materials with enhanced performance, continually making the possibility of waste heat recovery a reality. In this work, we engineer the thermoelectric properties of the layered SnSb2Te4 topological insulators. To date, there is little research reporting on these materials as potential state-of-the-art thermoelectric materials. Thus, there is a need to formulate effective strategies to realize this potential. Since these materials are known to have intrinsically low lattice thermal conductivity, we shift our attention to improving the electrical transport properties. For the first time, positional Bi doping at both the Sn and Sb cation sites is adopted. The aliovalent and isovalent nature of Bi at these sites, respectively, is shown to cause significant improvements in the performance of these layered materials. The electronic band structure of the pure and doped samples, where we considered various occupancies, is studied whereby we reveal the occurrence of band convergence and resonant levels resulting in a high power factor of ∼10.8 μW cm-1 K-2 at 623 K. Overall, a high ZT of ∼0.46 at a relatively lower temperature of 673 K is recorded. The potential of these materials for thermoelectric applications is shown, especially in the case of Bi doping at the Sn cation site. Continued efforts to enhance the thermoelectric performance of these topological insulators are needed for them to gain a substantial competitive edge in comparison to other state-of-the-art thermoelectric materials. © 2023 American Chemical Society.Item Towards achieving an ideal convergence of light and heavy electron conduction bands in SnTe: Insights into copper doping(Elsevier B.V., 2024) Shenoy, U.S.; Bhat, D.K.In recent years, tin telluride has garnered significant attention in the field of thermoelectrics, offering a promising avenue for sustainable ecofriendly conversion of waste heat into electricity. The unique electronic structure of this material makes it a compelling candidate for exploring innovative strategies to enhance its transport properties by employing substitutional doping. Among myriad elements doped, copper has been considered an intriguing candidate due to its ability to lower the thermal conductivity. However, its impact on the electronic structure has not been thoroughly explored till date. Herein, we investigate a nuanced aspect of copper doping, specifically focusing on its impact on tuning the electronic structure of SnTe. Significantly, our findings reveal a novel dimension to copper doping, showcasing its potential to enhance n-type performance in SnTe through the near-perfect convergence of its conduction bands - a feature not observed when doped in GeTe. We also shed the light on improvement of the p-type performance by means of valence band convergence and increased band gap. Furthermore, we reveal that copper doping allows the contribution of low-lying bands in SnTe to participate in transport, ensuring a higher Seebeck coefficient across the entire temperature range. Overall, this work provides a panoramic view of role of copper in improving the Seebeck co-efficient of SnTe making it a potential lead-free material for several thermoelectric applications. © 2024 Elsevier B.V.Item In-situ synthesis of cuprous oxide nanofluid using ribose for enhanced thermal conductivity and stability(Elsevier B.V., 2024) Bhat, D.K.; Kumar, S.P.; Shenoy, U.S.Enhancing the thermal properties of conventional heat transfer fluids represents a significant technological challenge. In this context, nanofluids have emerged as a promising solution, emphasizing the need for simpler and more convenient synthesis methods. This study introduces a novel, eco-friendly, one-step synthesis method, overcoming the complexities of traditional two-step processes. The resulting nanofluid generated by using ribose as a reducing agent, consists of cuprous oxide particles at the nano scale, and the fluid itself exhibits Newtonian behavior. With an impressive thermal conductivity of 3.052 W m−1 K−1, the nanofluid exhibits stability for a noteworthy 4-month duration, achieved through the strategic addition of sodium lauryl sulfate. This breakthrough positions the nanofluid as a compelling option for diverse applications in thermal energy storage and management. © 2024 Elsevier Inc.Item Enhancing the thermal conductivity and stability of cuprous oxide nanofluids: Ribose-mediated single step chemical synthesis for solar energy applications(Elsevier B.V., 2025) Bhat, D.K.; Kumar, S.P.; Shenoy, U.S.The efficiency of photovoltaic (PV) panels can be compromised by rising temperatures, prompting extensive research into thermal management strategies aimed at maximizing power output. Recently, there has been growing interest in using nanofluids to enhance the cooling efficiency of photovoltaic thermoelectric generator (PV-TEG) systems compared to conventional water cooling. This underscores the potential of investigating innovative synthetic methods to improve the thermal conductivity and stability of nanofluids. We employed a simple straightforward method to synthesize cuprous oxide nanofluid. This solution-based technique constrains formation of cuprous oxide particles to the nanoscale dimensions using cetylammonium bromide as capping agent. Our investigation delved into the impact of various parameters on the formation and dispersion of nanoparticles within a base fluid comprised of a 1:1 mixture of water and ethylene glycol. The resulting nanofluid exhibited Newtonian behaviour and demonstrated remarkable stability of 9 months, accompanied by a notable increase in thermal conductivity upto 3.59 W m-1 K-1. This meticulous approach has proven to be not only straightforward and dependable but also efficient for the rapid synthesis of highly stable Newtonian nanofluids overcoming the complexities associated with traditional two-step processes and could be extended to other metal oxide nanofluids. Beyond its economic appeal, the nanofluid's improved thermal properties and stability position it for diverse applications requiring efficient heat transfer. © 2024