Merit ZT Research Articles

Thermoelectric algebra made simple for thermoelectric generator module performance prediction under constant Seebeck coefficient approximation

Although thermoelectric material performance can be estimated using the dimensionless figure of merit ZT, predicting the performance of thermoelectric generator modules (TGMs) is complex owing to the nonlinearity and nonlocality of the thermoelectric differential equations. Here, we present a simplified thermoelectric algebra framework for predicting TGM performance within the constant Seebeck coefficient approximation (CSA). First, we revisit the constant Seebeck coefficient model (CSM) to transform the differential equations into exact algebraic equations for thermoelectric heat flux and conversion efficiency in terms of the load resistance ratio and relative Fourier heat flux. Next, we introduce the CSA, where the Thomson term is neglected and the device parameters are assumed to be fixed. We define the average thermoelectric properties and device parameters under the zero-current condition using a simple temperature integral. Finally, we derive approximate thermoelectric algebraic equations for voltage, resistance, heat flux, and conversion efficiency as functions of current. We numerically validate that the CSA formalism is superior to other single-parameter theories, such as peak-ZT, integral-ZT, and generic engineering-ZT, in predicting efficiency. The relative standard error of the optimal efficiency is less than 11% for average ZT values not exceeding 2. By combining the CSM and CSA, TGM performance can be easily estimated without requiring calculus or solving differential equations. Therefore, this simplified thermoelectric algebra under the CSA framework has the potential to significantly enhance future TGM analysis and design, facilitating more efficient device-level research and development beyond the traditional focus on material properties.

Optimized Interface Engineering Enhances Carrier and Phonon Scattering for Superior Thermoelectric Performance in Yb-Filled Skutterudites.

Thermoelectric (TE) performance in materials is often constrained by the strong coupling between carrier and phonon transport, necessitating trade-offs between electrical and thermal properties that limit improvements in the figure of merit (zT). Herein, a novel strategy is proposed to achieve simultaneous energy filtering and enhanced phonon scattering, effectively optimizing the TE properties of CoSb3-based skutterudites. By introducing Cu2Te nanoprecipitates into the Yb0.3Co4Sb12 matrix, interfacial barriers are formed, which selectively filter low-energy charge carriers, significantly improving the Seebeck coefficient while maintaining high carrier mobility. As a consequence, a substantial enhancement of the power factor occurs. Furthermore, the multiscale precipitates inhibit grain boundary migration, leading to grain refinement, and effectively scatter phonons, consequently decreasing the lattice thermal conductivity. These synergistic improvements in electronic and phonon transport yield a peak zT of ∼1.47 at 823 K for the Yb0.3Co4Sb12 + 0.5%Cu2Te sample. Furthermore, a fabricated 7-pair TE module attains a maximum conversion efficiency of ∼6.1% under a temperature difference of 400 K. This work introduces a straightforward and effective approach for designing high-performance TE material systems through the collaborative tuning of electrical and thermal properties.

Achieving Ultralow Lattice Thermal Conductivity and High Thermoelectric Performance in Bi2Te2.7Se0.3 Alloys via Introducing Organic & Inorganic Nanoparticles.

N-type Bi2Te2.7Se0.3(BTS) is a state-of-the-art thermoelectric material owing to its excellent thermoelectric properties near room temperatures for commercial applications. However, its performance is restricted by its comparatively low figure of merit ZT. Here, it is shown that a 14% increase in power factor (PF) (at 300 K) can be reached through incorporation of inorganic GaAs nanoparticles due to enhanced thermopower originating from the energy-dependent carrier scattering. Besides, further incorporation of organic nanophase PEDOT: PSS can reduce its lattice thermal conductivity by 59% due to the strong scattering of middle- and low-frequency phonons. As a result, a peak ZT value of ZTmax ≈ 1.31 (at 373 K) and an average ZTave ≈ 1.10 (300-473 K) are achieved for the BTS/(0.4 wt.% GaAs + 0.5 wt.% PEDOT: PSS) sample. The present work demonstrates that incorporation of organic-inorganic nanophase is an effective way to improve the performance of BTS.

High power output density organic thermoelectric devices for practical applications in waste heat harvesting.

Organic thermoelectric (TE) materials are of great interest for researchers in waste heat recovery, especially for waste heat harvesting at near room temperature. Significant progress has been achieved in terms of their figure of merit (zT) values recently, which has presented new insights into the development of organic TE materials. For numerous practical applications of thermoelectric generators, where waste heat is unlimited and cost negligible, the primary goal has been switched to achieve high power output density rather than improving their heat-to-electricity conversion efficiency. In this review, we first discussed the theoretical relationship between the thermoelectric properties of organic materials and the power output density of devices. Then, we analyzed the state-of-the-art strategies for improving the power factor of organic materials. Methods for modulating the structure of the TE legs were discussed with an aim to maintain the temperature difference between the hot side and the cold side of the devices. Finally, some representative devices were summarized for the potential applications of the TEGs along with an outlook on the future of organic thermoelectric materials and devices.

Solvothermally optimizing Ag2Te/Ag2S composites with high thermoelectric performance and plasticity.

Silver-based fast ionic conductors show promising potential in thermoelectric applications. Among these, Ag2S offers unique high plasticity but low electrical conductivity, whereas Ag2Te exhibits high intrinsic electrical conductivity yet faces limitations due to high thermal conductivity and poor plasticity. Developing a composite thermoelectric material that combines the benefits of both is therefore essential. Here, this study reports the successful synthesis of Ag2Te/Ag2S composites via a facile and low-cost solvothermal method. By finely adjusting the composition of Ag2S and Ag2Te to obtain the optimized carrier concentration and the enhanced mobility, the figure of merit ZT of Ag2Te/Ag2S composites reached ∼0.42 at 373 K and ∼0.38 at 298 K, both surpassing those of pure Ag2S and Ag2Te. This increase in ZT also benefits from lattice defects created by the solvothermally synthesized biphasic composition, effectively scattering phonons of various wavelengths and reducing thermal conductivity compared to pure Ag2Te. Additionally, the plasticity of the Ag2Te/Ag2S composites improved considerably over pure Ag2Te, achieving a bending strain of ∼2.5% (versus ∼1.2% for intrinsic Ag2Te). This study can fill a critical gap in the research on composite silver-based fast ionic conductors synthesized via wet chemical methods and provide valuable guidance for future exploration.

Effects of the Fe/Ni ratio in double half-Heusler composition HfFe1−xNixSb

The adjustment of the Fe/Ni ratio in the double half-Heusler composition HfFexNi1−xSb leads to a p-type to n-type transition. The thermoelectric figures of merit zT = 0.36 and 0.22 at 950 K for n- and p-type, respectively, were demonstrated.

Impact of Boron Nitride on the Thermoelectric Properties and Service Stability of Cu2-xSe.

Improving the thermoelectric performance and service stability is essential for the effective use of cuprous selenide (Cu2-xSe). In this study, hexagonal boron nitride (h-BN) was incorporated into nano-Cu2-xSe, with the goal of enhancing thermoelectric performance and service stability. It was found that Cu2-xSe-0.005 wt % BN showed a higher thermoelectric figure of merit (zT) value (∼1.76 at 923 K), which was 11% greater than that of pure Cu2-xSe, mainly due to a significant reduction in lattice thermal conductivity (kL) to about 50% (0.13 W m-1 K-1). Additionally, the formation of an ion-blocking interface by hexagonal boron nitride effectively shortens the migration path of Cu+ ions, improving service stability while maintaining the contribution of Cu+ transitions to thermal conductivity. Finally, an increase in hardness of ∼11.1% was observed, reaching 0.7 GPa in Cu2-xSe-0.02 wt % BN. This research is a feasible approach to improving the service stability of Cu2-xSe.

Thermoelectric Material Performance (zT) Predictions with Machine Learning.

Research efforts using the tools in machine- and deep learning models have begun to show success in predicting target properties such as thermoelectric (TE) properties, including the figure of merit (zT). These models were trained on various data sources that used experimental, crystallographic, and density functional theory (DFT) data. We developed an interpretable model on a huge experimental data set of ∼160,000 data points to predict the performance of thermoelectric materials. The model predicts the results of three different test sets with high accuracy, such as the root-mean-square error (RMSE) ranging from 0.15 to 0.20 and the evaluation coefficients (R2) ranging from 0.80 to 0.67. Furthermore, we highlight probable reasons such as literature error, varied synthesis routes for the same material, different forms of crystallinity and morphology, and different particle sizes and densities for the deviation of predicted zT from the experimental zT results of the test sets. Lastly, using an experimental data set, our study is one of the few examples that predict a complex zT property directly across the entire gamut of TE materials.

Microstructure evolution and thermoelectric behaviour of directionally solidified Bi2Te3 based multiphase thermoelectric

Thermoelectric materials are vital for sustainable energy applications, yet improving their efficiency remains a significant challenge. This study investigates the influence of unidirectional solidification on the Bi2Te3-Cu3Te2 eutectic alloy to enhance thermoelectric performance through microstructural control. Alloys of the same composition of Bi, Cu, and Te were processed using a Bridgman-type furnace under varying solidification velocities, with the resulting phases characterized as rhombohedral (Bi2Te3) and tetragonal (Cu3Te2). A distinctive eutectic columnar structure was formed at higher solidification rates, optimizing the balance between electrical and thermal transport. Alloy Solidified at 20µm/s, exhibited a peak figure of merit (zT) of 0.93 at 442K due to improved power factor and reduced thermal conductivity. This work highlights unidirectional solidification as an effective approach to advancing thermoelectric material design.

Enhancing thermoelectric efficiency of highly conductive CuSbTe2 alloy by reducing electrical thermal conductivity via heavy Se doping.

Recently, CuSbTe2, one of the I-V-VI-based compounds, has received attention as a promising thermoelectric (TE) material that exhibits a narrow bandgap with high electrical conductivity. In this study, the evolution of electrical and thermal transport properties of CuSbTe2 by heavy Se doping was investigated by synthesizing a series of CuSb(Te1-xSex)2 (x = 0, 0.1, 0.2, 0.3, and 0.4) compositions. The high electrical conductivity of CuSbTe2 (5400 S/cm) is gradually decreased to 1800 S/cm by Se doping with x = 0.4 at 300K with decreased carrier concentration and mobility. Due to this large reduction in electrical conductivity, the power factor of pristine CuSbTe2 significantly decreased to 0.98 mW/mK2 for x = 0.4 by 25%, along with reduced density-of-states effective mass at 550K. Nevertheless, the lattice thermal conductivity was reduced by 5%, and the electrical thermal conductivity was significantly reduced by 67% for x = 0.4 at 550K. Consequently, the total thermal conductivity of pristine CuSbTe2 (2.76 W/mK) is significantly reduced to 1.65 W/mK for x = 0.4 by 40%, mainly owing to the significant reduction of electrical thermal conductivity, which originates from the reduced electrical conductivity. Therefore, an enhanced TE figure of merit (zT) of 0.33 at 550K is observed for CuSb(Te0.6Se0.4)2 (x = 0.4), which was 26% higher than that of CuSbTe2. In addition, the expected zT for various carrier concentrations is calculated by using a single parabolic band model. It was found that the zT could be further enhanced by reducing the carrier concentration, which can be achieved by further doping of electrons.

Improving the thermoelectric performance of the SnS monolayer by Na doping: A theoretical study

Aiming at exploring new thermoelectric (TE) materials of high performance, we theoretically calculate the TE transport properties including the Seebeck coefficient, electrical conductance, thermal conductance, power factor, and figure of merit ZT of the SnS monolayer. Different from the heavy-metal doping, the economical and environment-friendly Na doping is adopted to improve the ZT of the monolayer SnS. It is shown that the Na doping can increase the maximum ZT along the armchair and zigzag directions, respectively, and the highest ZT peak is moved to the proximity of chemical potential μ=0eV, which indicates that the corresponding TE device can work at a low bias voltage. As the temperature increases from 300 K to 800 K, the maximum ZT of the pristine SnS monolayer is increased from 0.89 to 2.26 (from 1.33 to 2.89) along the armchair (zigzag) direction, and the maximum ZT of the Na-doped one is increased from 1.24 to 2.45 (from 1.44 to 2.86) along the armchair (zigzag) direction. This implies that the Na-doped SnS monolayer can be utilized to design promising TE devices working in a broad temperature scope and at a lower bias voltage.

Enhancing zT in Organic Thermoelectric Materials through Nanoscale Local Control Crystallization.

Organic thermoelectric materials are promising for wearable heating and cooling devices, as well as near-room-temperature energy generation, due to their nontoxicity, abundance, low cost, and flexibility. However, their primary challenge preventing widespread use is their reduced figure of merit (zT) caused by low electrical conductivity. This study presents a method to enhance the thermoelectric performance of solution-processable organic materials through confined crystallization using the lithographically controlled wetting (LCW) technique. Using PEDOT as a benchmark, we demonstrate that controlled crystallization at the nanoscale improves electrical conductivity by optimizing chain packing and grain morphology. Structural characterizations reveal the formation of a highly compact PEDOT arrangement, achieved through a combination of confined crystallization and DMSO post-treatment, leading to a 4-fold increase in the power factor compared to spin-coated films. This approach also reduces the thermal conductivity dependence on electrical conductivity, improving the zT by up to 260%. The LCW technique, compatible with large-area and flexible substrates, offers a simple, green, and low-cost method to boost the performance of organic thermoelectrics, advancing the potential for sustainable energy solutions and advanced organic electronic devices.

High‐performance and flexible thermoelectric generator based on a robust carbon nanotube/BiSbTe foam

AbstractOrganic thermoelectric generators (TEGs) are flexible and lightweight, but they often have high electrical resistance, poor output power, and low mechanical durability, because of which their thermoelectric performance is poor. We used a facile and rapid solvent evaporation process to prepare a robust carbon nanotube/Bi0.45Sb1.55Te3 (CNT/BST) foam with a high thermoelectric figure of merit (zT). The BST sub‐micronparticles effectively create an electrically conductive network within the three‐dimensional porous CNT foam to greatly improve the electrical conductivity and the Seebeck coefficient and reinforce the mechanical strength of the composite against applied stresses. The CNT/BST foam had a zT value of 7.8 × 10−3 at 300 K, which was 5.7 times higher than that of pristine CNT foam. We used the CNT/BST foam to fabricate a flexible TEG with an internal resistance of 12.3 Ω and an output power of 15.7 µW at a temperature difference of 21.8 K. The flexible TEG showed excellent stability and durability even after 10,000 bending cycles. Finally, we demonstrate the shapeability of the CNT/BST foam by fabricating a concave TEG with conformal contact on the surface of a cylindrical glass tube, which suggests its practical applicability as a thermal sensor.

Energy-efficient synthesis and thermoelectric properties of Ni1-xCrx (x = 0.08, 0.10, and 0.17) alloys

Ni1-xCrx alloys are predominantly used as the p-leg in type E and K thermocouples and have promising applications in energy harvesting, including powering low-power devices and deep space power production. Their high electrical conductivity, long-term stability, and cost-effectiveness also make them attractive for spin caloritronic devices due to the modification of the density of states at the Fermi level by Cr impurities. This study introduces an energy-efficient and sustainable approach for manufacturing Ni1-xCrx alloys (with x = 0.08, 0.10, and 0.17) by incorporating KOH as a mineralizer. Nanosized Ni powders, synthesized via hydrothermal methods, are alloyed with commercial Cr powders, promoting rapid diffusion and alloy formation, followed by consolidation at 1110ºC for 6 hours in a 20 % H2 and 80 % N2 atmosphere. Compared to the traditional melting process, which typically takes over 124 hours for alloy formation, our method offers significant time savings. The Ni1-xCrx alloy with x = 0.10 exhibits the highest thermopower, achieves a maximum power factor of 4.56 μW/cm-K2 at 330 K, and exhibits reduced thermal conductivity compared to alloys produced by powder metallurgy. The dimensionless figure of merit (zT) reaches 0.029 at 330 K and 0.022 at 693 K for the Ni0.90Cr0.10 alloy, surpassing values obtained through powder metallurgy.

Predictions of new KTaSn half-Heusler compound for spintronic, thermoelectric and optoelectronic applications: A first-principles study

Structural, electronic, magnetic, elastic, optical, thermodynamic, and thermoelectric characteristics of KTaSn half-Heusler compound are investigated using density functional theory (DFT). GGA-mBJ approximation calculations showed a direct minority spin band gap energy of 1.28 eV and a spin polarization of 100 %. The calculated total magnetic moment and Curie temperature are 2 μB and 380 K respectively. Elastic constants study and formation energy calculation revealed that the material is mechanically and thermodynamically stable and has a ductile nature. 0ptical properties of KTaSn half-Heusler alloy were similarly investigated to highlight its potential for optoelectronic applications. Finally, utilizing Boltzmann's quasi-classical theory, transport properties were studied and yielded a high figure of merit ZT value of 0.62 indicating that the material is suitable for applications in thermoelectric devices.

Thermoelectric Properties of Benzothieno-Benzothiophene Self-Assembled Monolayers in Molecular Junctions.

We report a combined experimental (C-AFM and SThM) and theoretical (DFT) study of the thermoelectric properties of molecular junctions made of self-assembled monolayers on Au of thiolated benzothieno-benzothiophene (BTBT) and alkylated BTBT derivatives (C8-BTBT-C8). We measure the thermal conductance per molecule at 15 and 8.8 pW/K, respectively, among the lowest values for molecular junctions so far reported (10-50 pW/K). The lower thermal conductance for C8-BTBT-C8 is consistent with two interfacial thermal resistances introduced by the alkyl chains, which reduce the phononic thermal transport in the molecular junction. The Seebeck coefficients are 36 and 245 μV/K, respectively, the latter due to the weak coupling of the core BTBT with the electrodes. We deduce a thermoelectric figure of merit ZT up to ≈10-4 for the BTBT molecular junctions at 300 K, on a par with the values reported for archetype molecular junctions (oligo(phenylene ethynylene) derivatives).

Enhancement of thermoelectric transport in surface halogenated Ti2O MOenes via electron–phonon drag effect

Designing efficient and environmentally friendly thermoelectric materials near room temperature is critical, and the electron–phonon drag effect on thermoelectric transport in monolayers remains largely unexplored. This manuscript systematically investigates the electron–phonon drag effect in surface halogenated Ti2O MOenes (Ti2OX2, X = F, Cl) by solving fully coupled electron–phonon Boltzmann transport equations. We find that the phonon drag effect significantly enhances the total Seebeck coefficient and various electronic transport coefficients, including the thermal response of electrons to an electric field, electrical conductivity, and electronic thermal conductivity at zero field, while the electron drag effect notably increases the lattice thermal conductivity, especially at high carrier concentrations and low temperatures. Furthermore, the electron–phonon drag effect significantly increases the thermoelectric figure of merit (zT) across 100–900 K, with the greatest enhancement at low temperatures. At room temperature, zT increases by 13.73 times for Ti2OF2 and 2.82 times for Ti2OCl2, achieving maximum values of 0.92 and 0.84, respectively. Our work underscores the superior thermoelectric performance of surface halogenated Ti2O MOenes near room temperature and the potential of leveraging electron–phonon drag effects to enhance the electrical, thermal and thermoelectric transport in monolayers.

First-principle’s calculations to investigate structural, electronic, mechanical, optical, and thermoelectric properties of halide double perovskites K2InScX6 (X = Cl, Br, and I) for thermoelectric and optoelectronic applications

Physical properties of double perovskites K2InScX6 (X = Cl, Br, and I) are investigated using density functional theory (DFT) with generalized gradient approximation (GGA) and perdew-burke-ernerhof (PBE) exchange co-relational functional. Structural stability is confirmed through geometry optimization. Band gaps are calculated using Trans Blaha-modified Becke-Johnson (TB-mBJ) potential and GGA-PBE methods. The obtained band gaps have direct nature. The structural stability has also been confirmed by calculating enthalpy formation (ΔHf), cohesive energy (Ecoh), stiffness coefficients (Cij), tolerance (τG) and octahedral factor (µ). Two materials K2InScCl6, and K2InScBr6 are ductile and one is K2InScI6 brittle. The optical parameters inferred that the maximum absorption of photon energy lies within the visible and ultraviolet regions enabling their use in UV photodetectors and optoelectronics, where light absorption is important for performance. The thermoelectric (TE) parameters like Seebeck coefficient, thermal and electrical conductivity, carrier concentration, power factor and figure of merit ZT parameters are calculated and ZT values are 0.75, 0.94, and 0.91 for K2InScCl6, K2InScBr6, and K2InScI6 at room temperature. These results enable the development of more stable, and high-conversion efficiency devices for TE applications such as thermo-electric power generators.

First-principles insights into thermoelectric behavior of XNiAs (X = Sc, Y) half-Heusler compounds

Abstract Half-Heusler (HH) compounds are regarded as potential candidates for thermoelectric devices to convert waste heat energy into electrical power, which could help mitigate the ongoing energy crisis. In this study, we investigate the structural, electronic, magnetic, and thermoelectric properties of HH compounds XNiAs (X = Sc, Y) using Density Functional Theory (DFT) and semi-classical Boltzmann transport theory (BTE). The compounds are non-magnetic semiconductors with an indirect band gap of 0.48 ( 0.50) eV for ScNiAs (YNiAs) respectively. Both materials show dynamic and mechanical stability, with ScNiAs displaying brittleness and YNiAs exhibiting ductility. At room temperature, the lattice thermal conductivity (κ l ) for ScNiAs is 28.67 W/mK, while for YNiAs, it is 15.21 W/mK and both decrease as the temperature rises. The highest value of power factors is 29.03 μW/cmK2 and 29.74 μW/cmK2 for ScNiAs and YNiAs respectively at 1100 K for n-type doping. The optimal values of the figure of merit (zT) for n-type doping are 0.33 for ScNiAs and 0.58 for YNiAs at 1100 K. Both compounds exhibit better thermoelectric performance for n-type doping compared to p-type doping. The presence of flat bands and higher κ l limits these materials from achieving higher zT values.

Demonstration of efficient Thomson cooler by electronic phase transition.

In the 1850s, Lord Kelvin predicted the existence of a thermoelectric cooling effect inside a whole material (the Thomson effect) according to thermodynamics1, in addition to the Peltier effect that enables cooling at the junction between dissimilar materials. However, the Thomson effect is usually negligible (ΔT/T < 2%) in conventional thermoelectric materials because the entropy change in charge carriers is fairly small2, leading to the guiding principles for advancing thermoelectric cooling to be based on the framework of the Peltier effect and that the figure of merit ZT should be maximized to optimize performance. Here, we demonstrate a Thomson-effect-enhanced thermoelectric cooler using a large Thomson coefficient (τ) induced by the direct manipulation of charge entropy through an electronic phase transition in YbInCu4. The devices achieve a steady temperature span (ΔT) of >5 K from T = 38 K. Our findings suggest not only another approach to advance thermoelectric coolers in addition to improving ZT but also technologically opens opportunities for solid-state cryogenic cooling applications.