Faculty Publications
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Item Effect of substrate temperature and film thickness on the thermoelectric properties of In2Te3 thin films(Elsevier Ltd, 2017) Vallem, V.; Bangera, K.V.; Shivakumar, G.K.Herein, the thermoelectric properties of vacuum deposited In2Te3 thin films were investigated by varying the substrate temperature and the thickness of the films. The thermo-electro motive force of the prepared films was found to increase with an increase in the substrate temperature up to 423 K and then decrease at 473 K due to the presence of mixed-phase structure. The maximum thermoelectric power of 220 ?V/K was observed for the films deposited at 423 K substrate temperature, which was found to decrease with increase in thickness. The films deposited at 423 K with 150 nm thickness showed maximum power factor of 27 ?Wm?1K?2 at 450 K. These observations are explained on the basis of structural, morphological and compositional changes. © 2017 Elsevier B.V.Item Drastic increase in thermoelectric power factor of mixed Sb2Te3-In2Te3 thin films(Academic Press, 2019) Vallem, S.; Bangera, K.V.; G.k, S.Thermoelectric power factor is an indicator of the performance of a thermoelectric material. Attempts have been made by various techniques, like doping, to improve the thermoelectric conversion efficiency of materials. In the present study, a layer structured thermoelectric material Sb2Te3 is alloyed to In2Te3 using vacuum deposition method at 423 K to significantly enhance the power factor of ?118 ?Wm-1K?2 (at 450 K). Structurally, all films were polycrystalline in nature as clearly reflected in XRD patterns. All films were showing p-type conductivity, and electrical conductivity of In2Te3 films increased with increasing Sb2Te3 content. The seebeck coefficient is found to be higher for un-doped In2Te3 than that of Sb2Te3-In2Te3 and pure Sb2Te3 films. However, the thermoelectric power factor of 25% Sb2Te3 alloyed In2Te3 films is enhanced by 11.9 times that of In2Te3 films and 4 times that of Sb2Te3 films at 320 K. It is interesting to note that efficiency of the mixed films is higher than that of the individual films. © 2019 Elsevier LtdItem Thermodynamic irreversibility and conjugate effects of integrated microchannel cooling device using TiO2 nanofluid(Springer, 2020) Narendran, G.; Gnanasekaran, N.; Arumuga Perumal, A.P.Thermal management is highly essential for the latest electronic devices to effectively dissipate heat in a densely packed environment. Usually, these high power devices are cooled by integrating micro scale cooling systems. Most of the works reported in the literature majorly concentrate on microchannel heat sink in which the characteristics of friction factor and enhancement of heat transfer are analyzed in detail. However, due to the advent of compact electronic devices a crucial investigation is required to facilitate an amicable environment for the neighboring components so as to improve the reliability of the electronic devices. Henceforth, in the present study a combined experimental and numerical analysis is performed to provide an insight to determine the performance of a copper microchannel integrated with aluminium block using TiO2 nanofluid for different particle configurations. Needless to say, the present study, which also focuses on entropy generation usually attributed to the thermodynamic irreversibility, is very much significant to design an optimum operating condition for better reliability and performance of the cooling devices. © 2019, Springer-Verlag GmbH Germany, part of Springer Nature.Item Mg/Ca doping ameliorates the thermoelectric properties of GeTe: Influence of electronic structure engineering(Elsevier Ltd, 2020) Bhat, D.K.; Shenoy, U.S.GeTe, though originally believed to be a poor thermoelectric material due to its inherent Ge vacancies has recently attracted the attention of the scientific community due to its tunable electronic structure. Herein, we study the electronic structure modifications of GeTe by means of doping it with Mg and Ca. Both Mg and Ca increases the band gap of GeTe and brings about valence band convergence decreasing the energy offset. The enhanced Seebeck co-efficient due to tuning of the electronic structure results in improved thermoelectric properties as predicted by the Boltzmann transport calculations. This strategy of doping could be very well extended to other dopants for improving the thermoelectric properties of GeTe. © 2020 Elsevier B.V.Item 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.Item Optimized Mn and Bi co-doping in SnTe based thermoelectric material: A case of band engineering and density of states tuning(Chinese Society of Metals, 2021) Kihoi, S.K.; Kahiu, J.N.; Kim, H.; Shenoy, U.S.; Bhat, D.K.; Yi, S.; Lee, H.S.Tin telluride (SnTe) overwhelmingly continues to be studied owing to its promising thermoelectric properties, tunable electronic structure, and its potential as an alternate to toxic lead telluride (PbTe) based materials. In this research, we engineer the electronic properties of SnTe by co-doping Mn and Bi below their individual solubility limit. The First principles density functional theory studies reveal that both Bi and Mn introduce resonance states, thereby increasing the density of states near the Fermi level leading to enhanced Seebeck coefficient. This unique combination of using two resonant dopants to introduce flatter bands is effective in achieving higher performance at lower temperatures manifesting into a large Seebeck value of ?91 ?V/K at room temperature in the present case. Both elements optimally co-doped results in a very high power factor value of ?24.3 ?W/cmK2 at 773 K when compared to other high performance SnTe based materials. A zT of ?0.93 at 773 K is achieved by tuning the proportion of the co-dopants Mn and Bi in SnTe. The hardness value of pristine SnTe was also seen to increase after doping. As a result, synergistic optimized doping proves to be a suitable means for obtaining thermoelectric materials of superior characteristics without the need for heavy doping. © 2021Item 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 Optimized electronic performance in half-Heusler Ti-doped NbFeSb materials by stoichiometric tuning at the Fe and Sb sites(Elsevier Ltd, 2022) Kahiu, J.N.; Shenoy, U.S.; Kihoi, S.K.; Kim, H.; Yi, S.; Bhat, D.K.; Lee, H.S.Electronic structure is known to be highly influenced by the site occupancy and the stoichiometry of the material which in turn largely effects the thermoelectric properties. Herein, we present electronic calculations using density functional theory (DFT) of non-stoichiometric Ti doped NbFeSb configuration, showing the effect of the anti-site Fe atoms on the electronic properties, and supporting them with experimental results of the prepared Nb0.8Ti0.2Fe1+xSb1−x samples. The electronic structure of the non-stoichiometric sample shows the introduction of two distinct peaks near the Fermi level by the Fe atoms at the Sb sites. These resonance states are known to cause an increase in the density of states effective mass near the Fermi level, which explains the increase in the Seebeck coefficient in the sample x = 0.03 compared to the sample x = 0.00. In addition, a comparatively higher electrical conductivity is reported from sample x = 0.03, which is attributed to the aliovalent substitution of Sb atoms by Fe atoms. The simultaneous increase in the Seebeck coefficient and electrical conductivity culminates in an increased power factor of ∼50.3 µW/cmK2 at 373 K, which is ∼46% higher than that of samples x = 0.00 and x = 0.05, highlighting the possibility of increasing the power density by stoichiometric variation to achieve the high joule-per-dollar performance of NbFeSb-based TE devices, the relevance of which is also currently emphasized in the quest for commercial viability. © 2021 Elsevier B.V.Item Selective co-doping improves the thermoelectric performance of SnTe: An outcome of electronic structure engineering(Elsevier Ltd, 2022) Shenoy, U.S.; Bhat, D.K.Thermoelectric materials which exhibit high performance throughout a range of temperature is required for successful scavenging of waste heat to generate electricity. Herein, we tailor the electronic structure of SnTe by co-doping Zn with three elements namely Ag, Ca and Mg. We observe that the dopants play complementary roles and improve the thermoelectric performance throughout the studied temperature range. Zn introduces resonance level and causes hyper-convergence to increase the Seebeck at low temperatures while M (M = Ag, Ca, Mg) enlarges the band gap preventing bipolar transport and also helps in the band convergence improving the performance at higher temperatures. The enhanced thermoelectric properties predicted by theoretical calculations is supported by experimental results. For the same concentration of doping, AgZn co-doped SnTe exhibits higher performance compared to the other two with an impressive ZT of ~1.54 at 840 K and average ZT of ~0.97 between 500 K and 840 K. © 2021 Elsevier B.V.Item Resonance states and hyperconvergence induced by tungsten doping in SnTe: Multiband transport leading to a propitious thermoelectric material(Elsevier Ltd, 2022) Shenoy, U.S.; D, G.K.; Bhat, D.K.Discovery of dopants which can engineer the electronic structure of the thermoelectric materials beneficially to improve the figure of merit has been receiving a lot of attention. In this work, we study one such unique dopant, tungsten in SnTe by implementing first principles density functional theory approach. We predict that tungsten is a n-type resonant dopant which not only increases the band gap but causes convergence of valence sub-bands leading to increased Seebeck co-efficient due to increase in the effective mass and decrease in the bipolar conduction. We show for the first time, the introduction of hyperconvergence in the conduction sub-bands, a feature which was observed only in valence bands of SnTe and GeTe. In addition to the above features, it also introduces multiple electronic valleys near the Fermi level excluding the use of a co-dopant to exploit the benefits of the electronic structure engineering. A maximum ZT of ~1.61 theoretically achieved by tuning the chemical potential at 800 K makes this material worth being explored experimentally. © 2022 Elsevier B.V.
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