Enhancement in Thermoelectric Figure‐Of‐Merit of an N‐Type Half‐Heusler Compound by the Nanocomposite Approach

An enhancement in the dimensionless thermoelectric figure‐of‐merit (ZT) of an n‐type half‐Heusler material is reported using a nanocomposite approach. A peak ZT value of 1.0 was achieved at 600 °C–700 °C, which is about 25% higher than the previously reported highest value. The samples were made by ball‐milling ingots of composition Hf_0.75Zr_0.25NiSn_0.99Sb_0.01 into nanopowders and hot‐pressing the powders into dense bulk samples. The ingots were formed by arc‐melting the elements. The ZT enhancement mainly comes from reduction of thermal conductivity due to increased phonon scattering at grain boundaries and crystal defects, and optimization of antimony doping.     

Subtle Roles of Sb and S in Regulating the Thermoelectric Properties of N‐Type PbTe to High Performance

A high ZT (thermoelectric figure of merit) of ≈1.4 at 900 K for n‐type PbTe is reported, through modifying its electrical and thermal properties by incorporating Sb and S, respectively. Sb is confirmed to be an amphoteric dopant in PbTe, filling Te vacancies at low doping levels (<1%), exceeding which it enters into Pb sites. It is found that Sb‐doped PbTe exhibits much higher carrier mobility than similar Bi‐doped materials, and accordingly, delivers higher power factors and superior ZT . The enhanced electronic transport is attributed to the elimination of Te vacancies, which appear to strongly scatter n‐type charge carriers. Building on this result, the ZT of Pb_0.9875Sb_0.0125Te is further enhanced by alloying S into the Te sublattice. The introduction of S opens the bandgap of PbTe, which suppresses bipolar conduction while simultaneously increasing the electron concentration and electrical conductivity. Furthermore, it introduces point defects and induces second phase nanostructuring, which lowers the lattice thermal conductivity to ≈0.5 W m^?1 K^?1 at 900 K, making this material a robust candidate for high‐temperature (500–900 K) thermoelectric applications. It is anticipated that the insights provided here will be an important addition to the growing arsenal of strategies for optimizing the performance of thermoelectric materials.     

Lead‐Free Thermoelectrics: High Figure of Merit in p‐type AgSnmSbTem+2

Thermoelectric materials based on Pb‐free compositions are of considerable current interest in environmentally friendly power‐generation applications derived from waste‐heat sources. Here, a new study of the thermoelectric properties of the tin‐based compositions with the general formula AgSn_mSbTe_m+2 (m = 2, 4, 5, 7, 10, 14, 18) is presented, where the m value is used as the tuning parameter of the thermoelectric properties. The electrical conductivity, Seebeck coefficient, and thermal conductivity are measured from 300 K to 723 K and the resulting thermoelectric figure of merit is determined as a function of the SnTe/AgSbTe₂ ratio. A thermoelectric figure of merit ZT ≈1 is obtained at 710 K for m = 4, indicating that the system AgSn_mSbTe_m+2 holds great promise as an alternative p‐type, lead‐free, thermoelectric material.     

Effect of Hf Concentration on Thermoelectric Properties of Nanostructured N‐Type Half‐Heusler Materials HfxZr1–xNiSn0.99Sb0.01

Half‐Heusler n‐type thermoelectric materials MNiSn (M = Hf, Zr) have been shown to exhibit peak thermoelectric dimensionless figure‐of‐merit (ZT) of ～1.0 at 600–700 °C with a composition of Hf_0.75Zr_0.25NiSn_0.99Sb_0.01. This work demonstrates that it is possible to achieve the same ZT by reducing the concentration of the most expensive component Hf to one third of the previously reported best composition, i.e., Hf_0.25Zr_0.75NiSn_0.99Sb_0.01, which corresponds to an overall 50% reduction on material cost. The samples are prepared by ball milling the arc melted ingot and hot pressing the finely ground powders. The reduction of Hf concentration is crucial for such materials to be used in large‐scale waste heat recovery.     

n‐Type SnSe2 Oriented‐Nanoplate‐Based Pellets for High Thermoelectric Performance

It is reported that electron doped n‐type SnSe₂ nanoplates show promising thermoelectric performance at medium temperatures. After simultaneous introduction of Se deficiency and Cl doping, the Fermi level of SnSe₂ shifts toward the conduction band, resulting in two orders of magnitude increase in carrier concentration and a transition to degenerate transport behavior. In addition, all‐scale hierarchical phonon scattering centers, such as point defects, nanograin boundaries, stacking faults, and the layered nanostructures, cooperate to produce very low lattice thermal conductivity. As a result, an enhanced in‐plane thermoelectric figure of merit ZT_max of 0.63 is achieved for a 1.5 at% Cl doped SnSe_1.95 pellet at 673 K, which is much higher than the corresponding in‐plane ZT of pure SnSe₂ (0.08).     

Increase in the Figure of Merit by Cd‐Substitution in Sn1–xPbxTe and Effect of Pb/Sn Ratio on Thermoelectric Properties

The effects of Cd‐doping on the thermoelectric properties of Sn_1–xPb_xTe are investigated and compared to the properties of the corresponding Sn_1–xPb_xTe solid solutions. The addition of Cd results in a reduction in the carrier concentration and changes in the physical properties, as well as in the conduction type of Sn_1–xPb_xTe. A significant increase in the power factor accompanied by a reduction in the thermal conductivity result in a higher figure of merit (ZT) for (Sn_1–xPb_x)_0.97Cd_0.03Te than that of undoped Sn_1–xPb_xTe. The maximum ZT (～0.7) values are observed for p‐type material with x = 0.36 at 560 K. Much higher values (ZT ～ 1.2 at 560 K for x = 0.73) are obtained on n‐type samples.     

Grain Boundary Scattering of Charge Transport in n‐Type (Hf,Zr)CoSb Half‐Heusler Thermoelectric Materials

High thermoelectric figure of merit zT of ≈1.0 has been reported in both n‐ and p‐type (Hf,Zr)CoSb‐based half‐Heusler compounds, and further improvement of thermoelectric performance relies on the insightful understanding of electron and phonon transport mechanisms. In this work, the thermoelectric transport features are analyzed for (Hf_0.3Zr_0.7)_1?xNb_xCoSb (x = 0.02–0.3) with a wide range of carrier concentration. It is found that, although both temperature and energy dependencies of charge transport resemble ionized impurity scattering, the grain boundary scattering is the dominant scattering mechanism near room temperature. With increasing carrier concentration and grain size, the influence of the grain boundary scattering on electron transport weakens. The dominant scattering mechanism changes from grain boundary scattering to acoustic phonon scattering as temperature rises. The lattice thermal conductivity decreases with increasing Nb doping content due to the increased strain field fluctuations. These results provide an in‐depth understanding of the transport mechanisms and guidance for further optimizing thermoelectric properties of half‐Heusler alloys and other thermoelectric systems.     

Fine‐Grained and Nanostructured AgPbmSbTem+2 Alloys with High Thermoelectric Figure of Merit at Medium Temperature

AgPb_mSbTe_m+2 (abbreviated as LAST) has received tremendous attention as a promising thermoelectric material at medium temperature. It can be synthesized by a simple process combining mechanical alloying (MA) and spark plasma sintering (SPS). This work reveals that the thermoelectric figure of merit (ZT value) of LAST can be increased by 50%, benefiting from enhanced electrical conductivity and thermopower due to refined grains and from nanostructuring realized by repeating the milling and SPS processes. This modified process and further compositional optimization enables ZT values of the LAST alloys up to 1.54 at 723 K. This supports the potential of the LAST alloy as a promising medium‐temperature thermoelectric material and reveals the validity of ZT enhancement by a simple microstructural refining and nanostructuring method.     

High‐Performance GeTe‐Based Thermoelectrics: from Materials to Devices

High‐performance GeTe‐based thermoelectrics have been recently attracting growing research interest. Here, an overview is presented of the structural and electronic band characteristics of GeTe. Intrinsically, compared to low‐temperature rhombohedral GeTe, the high‐symmetry and high‐temperature cubic GeTe has a low energy offset between L and Σ points of the valence band, the reduced direct bandgap and phonon group velocity, and as a result, high thermoelectric performance. Moreover, their thermoelectric performance can be effectively enhanced through either carrier concentration optimization, band structure engineering (bandgap reduction, band degeneracy, and resonant state engineering), or restrained lattice thermal conductivity (phonon velocity reduction or phonon scattering). Consequently, the dimensionless figure of merit, ZT values, of GeTe‐based thermoelectric materials can be higher than 2. The mechanical and thermal stabilities of GeTe‐based thermoelectrics are highlighted, and it is found that they are suitable for practical thermoelectric applications except for their high cost. Finally, it is recognized that the performance of GeTe‐based materials can be further enhanced through synergistic effects. Additionally, proper material selection and module design can further boost the energy conversion efficiency of GeTe‐based thermoelectrics.     

Skutterudite Unicouple Characterization for Energy Harvesting Applications

Skutterudites are promising thermoelectric materials because of their high figure of merit, ZT, and good thermomechanical properties. This work reports the effective figure of merit, ZT_eff, and the efficiency of skutterudite legs and a unicouple working under a large temperature difference. The p‐ and n‐type legs are fabricated with electrodes sintered directly to the skutterudite during a hot pressing process. CoSi₂ is used as the electrode for the n‐type skutterudite (Yb_0.35Co₄Sb₁₂) and Co₂Si for the p‐type skutterudite (NdFe_3.5Co_0.5Sb₁₂). A technique is developed to measure the ZT_eff of individual legs and the efficiency of a unicouple. An ZT_eff of 0.74 is determined for the n‐type legs operating between 52 and 595 °C, and an ZT_eff of 0.51 for the p‐type legs operating between 77 and 600 °C. The efficiency of the p–n unicouple is determined to be 9.1% operating between ～70 and 550 °C.     

High Performance Thermoelectricity in Earth‐Abundant Compounds Based on Natural Mineral Tetrahedrites

Thermoelectric materials can convert waste heat into electricity, potentially improving the efficiency of energy usage in both industry and everyday life. Unfortunately, known good thermoelectric materials often are comprised of elements that are in low abundance and require careful doping and complex synthesis procedures. Here, we report dimensionless thermoelectric figure of merit near unity in compounds of the form Cu_12‐xM_xSb₄S₁₃, where M is a transition metal such as Zn or Fe, for wide ranges of x. The compounds investigated here span the range of compositions of the natural mineral family of tetrahedrites, the most widespread sulfosalts on Earth, and we further show that the natural mineral itself can be used directly as an inexpensive source thermoelectric material. Thermoelectrics comprised of earth‐abundant elements will pave the way to many new, low cost thermoelectric energy generation opportunities.     

Unique Role of Refractory Ta Alloying in Enhancing the Figure of Merit of NbFeSb Thermoelectric Materials

NbFeSb‐based half‐Heusler alloys have been recently identified as promising high‐temperature thermoelectric materials with a figure of merit zT > 1, but their thermal conductivity is still relatively high. Alloying Ta at the Nb site would be highly desirable because the large mass fluctuation between them could effectively scatter phonons and reduce the lattice thermal conductivity. However, practically it is a great challenge due to the high melting point of refractory Ta. Here, the successful synthesis of Ta‐alloyed (Nb_1?xTa_x)_0.8Ti_0.2FeSb (x = 0 – 0.4) solid solutions with significantly reduced thermal conductivity by levitation melting is reported. Because of the similar atomic sizes and chemistry of Nb and Ta, the solid solutions exhibit almost unaltered electrical properties. As a result, an overall zT enhancement from 300 to 1200 K is realized in the single‐phase Ta‐alloyed solid solutions, and the compounds with x = 0.36 and 0.4 reach a maximum zT of 1.6 at 1200 K. This work also highlights that the isoelectronic substitution by atoms with similar size and chemical nature but large mass difference should reduce the lattice thermal conductivity but maintain good electrical properties in thermoelectric materials, which can be a guide for optimizing the figure of merit by alloying.     

Realizing High‐Ranged Out‐of‐Plane ZTs in N‐Type SnSe Crystals through Promoting Continuous Phase Transition

Thermoelectric technology enables direct conversion between heat and electricity. The conversion efficiency of a thermoelectric device is determined by the average dimensionless figure of merit ZT_ave. Here, a record high ZT_ave of ≈1.34 in the range of 300–723 K in n‐type SnSe based crystals is reported. The remarkable thermoelectric performance derives from the high power factor and the reduced thermal conductivity in the whole temperature range. The high power factor is realized by promoting the continuous phase transition in SnSe crystals through alloying PbSe, which results in a higher symmetry of the crystal structure and the correspondingly modified electronic band structure. Moreover, PbSe alloying induces mass and strain fluctuations, which enables the suppression of thermal transport. These findings provide a new strategy to enhance the thermoelectric performance for the continuous phase transition materials.     

Tuning Multiscale Microstructures to Enhance Thermoelectric Performance of n‐Type Bismuth‐Telluride‐Based Solid Solutions

Microstructure manipulation plays an important role in enhancing physical and mechanical properties of materials. Here a high figure of merit zT of 1.2 at 357 K for n‐type bismuth‐telluride‐based thermoelectric (TE) materials through directly hot deforming the commercial zone melted (ZM) ingots is reported. The high TE performance is attributed to a synergistic combination of reduced lattice thermal conductivity and maintained high power factor. The lattice thermal conductivity is substantially decreased by broad wavelength phonon scattering via tuning multiscale microstructures, which includes microscale grain size reduction and texture loss, nanoscale distorted regions, and atomic scale lattice distotions and point defects. The high power factor of ZM ingots is maintained by the offset between weak donor‐like effect and texture loss during the hot deformation. The resulted high zT highlights the role of multiscale microstructures in improving Bi₂Te₃‐based materials and demonstrates the effective strategy in enhancing TE properties.     

High Thermoelectric Efficiency of n‐type PbS

PbS shares several features with the other lead chalcogenides PbX (X: Te, Se), which are good thermoelectric materials. PbS has a potential advantage in that it is quite earth abundant and inexpensive. In this work we tune the transport properties in n‐type, single‐phase polycrystalline PbS_1‐xCl_x (x ≤ 0.008) with different carrier densities. Lead chloride provides a nearly 100% efficient doping control up to 1.2 × 10²⁰ cm^?3. The maximum zT achieved at 850 K is 0.7 with a predicted zT ～ 1 at 1000 K. This is about twice as high as what was previously reported (～0.4) for binary PbS. Compared with the other lead chalcogenides the higher effective mass and higher lattice thermal conductivity makes binary PbS an inferior thermoelectric material. However this study also predicts greater potential of zT improvement in PbS by material engineering such as alloying or nanostructuring compared to PbSe or PbTe. Considering their abundance and low cost, PbS based materials are quite competitive among the lead chalcogenides for thermoelectric applications.     

High Band Degeneracy Contributes to High Thermoelectric Performance in p‐Type Half‐Heusler Compounds

Half‐Heusler (HH) compounds are important high temperature thermoelectric (TE) materials and have attracted considerable attention in the recent years. High figure of merit zT values of 0.8 to 1.0 have been obtained in n‐type ZrNiSn‐based HH compounds. However, developing high performance p‐type HH compounds are still a big challenge. Here, it is shown that a new p‐type HH alloy with a high band degeneracy of 8, Ti‐doped FeV_0.6Nb_0.4Sb, can achieve a high zT of 0.8, which is one of the highest reported values in the p‐type HH compounds. Although the band effective mass of this system is found to be high, which may lead to a low mobility, its low deformation potential and low alloy scattering potential both contribute to a reasonably high mobility. The enhanced phonon scattering by alloying leads to a reduced lattice thermal conductivity. The achieved high zT demonstrates that the p‐type Ti doped FeV_0.6Nb_0.4Sb HH alloys are promising as TE materials and offer an excellent TE performance match with n‐type ones for high temperature power generation.     

High-Entropy Cubic Pseudo-Ternary Ag2(S,Se, Te) Materials With Excellent Ductility and Thermoelectric Performance

The discovery of ductile Ag₂(S, Se, Te) materials opens a new avenue toward high-performance flexible/hetero-shaped thermoelectrics. Specifically, the cubic-structured materials are quite attractive by combining remarkable plasticity, decent thermoelectric figure of merit (zT), and no phase transition above room temperature. However, such materials are quite few and the understanding is inadequate on their mechanical and thermoelectric properties. Enlightened by the high-entropy principles, a series of pseudo-ternary Ag₂S-Ag₂Se-Ag₂Te alloys is designed and comprehensive diagrams of composition-structure-plasticity-zT are compiled. Subsequently, the compositional region for the cubic phase is outlined. As a high-entropy example featuring with anion-site alloying and disordered Ag ions, Ag_2-xS_1/3Se_1/3Te_1/3 materials exhibit impressively large elongations of 60–97%, ultralow lattice thermal conductivities of ≈0.2 W m⁻¹ K⁻¹, and decent zT values of 0.45 at 300 K, 0.8 at 460 K. The materials can be readily rolled into thin foils, showing excellent flexibility. Finally, a six-leg in-plane device is fabricated, achieving an output voltage of 13.6 mV, a maximal power of 12.8 µW, and a power density of 14.3 W m⁻² under the temperature difference of 30 K, much higher than the organic counterparts. This study largely enriches the members of cubic ductile inorganic materials for the applications in flexible and hetero-shaped energy and electronic devices.     

Improved Bulk Materials with Thermoelectric Figure‐of‐Merit Greater than 1: Tl10–xSnxTe6 and Tl10–xPbxTe6

Noting the steadily worsening problem of depleted fossil fuel sources, alternate energy sources have become increasingly important; these include thermoelectrics, which may use waste heat to generate electricity. To be economically viable, the thermoelectric figure‐of‐merit, zT, which is related to the energy conversion efficiency, needs to be in excess of unity (zT > 1). Tl₄SnTe₃ and Tl₄PbTe₃ were reported to attain a thermoelectric figure‐of‐merit zT _max = 0.74 and 0.71, respectively, at 673 K. Here, the thermoelectric properties of both materials are presented as a function of x in Tl_10–x Sn _x Te₆ and Tl_10–x Pb _x Te₆, with x varying between 1.9 and 2.05, culminating in zT values in excess of 1.2. These materials are charge balanced when x = 2, according to (Tl⁺)₈(Sn²⁺)₂(Te^2?)₆ and (Tl⁺)₈(Pb²⁺)₂(Te^2?)₆ (or: (Tl⁺)₄Pb²⁺(Te^2?)₃). Increasing x causes an increase in valence electrons, and thus a decrease in the dominating p‐type charge carriers. Larger x values occur with a smaller electrical conductivity and a larger Seebeck coefficient. In each case, the lattice thermal conductivity remains under 0.5 W m^?1 K^?1, resulting in several samples attaining the desired zT _max > 1. The highest values thus far are exhibited by Tl_8.05Sn_1.95Te₆ with zT = 1.26 and Tl_8.10Pb_1.90Te₆ with zT = 1.46 around 685 K.     

High Thermoelectric Performance in Polycrystalline SnSe Via Dual‐Doping with Ag/Na and Nanostructuring With Ag8SnSe6

Single crystalline SnSe is one of the most intriguing new thermoelectric materials but the thermoelectric performance of polycrystalline SnSe seems to lag significantly compared to that of a single crystal. Here an effective strategy for enhancing the thermoelectric performance of p‐type polycrystalline SnSe by Ag/Na dual‐doping and Ag₈SnSe₆ (STSe) nanoprecipitates is reported. The Ag/Na dual‐doping leads to a two orders of magnitude increase in carrier concentration and a convergence of valence bands (VBM₁ and VBM₅), which in turn results in sharp enhancement of electrical conductivities and high Seebeck coefficients in the Ag/Na dual‐doped samples. Additionally, the SnSe matrix becomes nanostructured with dispersed nanoprecipitates of the compound Ag₈SnSe₆, which further strengthens the scattering of phonons. Specifically, ≈20% reduction in the already ultralow lattice thermal conductivity is realized for the Sn_0.99Na_0.01Se–STSe sample at 773 K compared to the thermal conductivity of pure SnSe. Consequently, a peak thermoelectric figure of merit ZT of 1.33 at 773 K with a high average ZT (ZT_ave) value of 0.91 (423–823 K) is achieved for the Sn_0.99Na_0.01Se–STSe sample.     

Thermoelectric Property Studies on Cu‐Doped n‐type CuxBi2Te2.7Se0.3 Nanocomposites

Combining high energy ball‐milling and hot‐pressing, significant enhancements of the thermoelectric figure‐of‐merit (ZT) have been reported for p‐type Bi_0.4Sb_1.6Te₃ nanocomposites. However, applying the same technique to n‐type Bi₂Te_2.7Se_0.3 showed no improvement on ZT values, due to the anisotropic nature of the thermoelectric properties of n‐type Bi₂Te_2.7Se_0.3. Even though texturing was effective in improving peak ZT of Bi₂Te_2.7Se_0.3 from 0.85 to 1.04, reproducibility from batch to batch remains unsatisfactory. Here, we show that good reproducibility can be achieved by introducing an optimal concentration of 0.01 copper (Cu) per Bi₂Te_2.7Se_0.3 to make Cu_0.01Bi₂Te_2.7Se_0.3 samples. A peak ZT value of 0.99 was achieved in Cu_0.01Bi₂Te_2.7Se_0.3 samples without texturing. With texturing by re‐pressing, the peak ZT was increased to 1.06. Aging in air for over 5 months did not deteriorate but further improved the peak ZT to 1.10. The mechanism by which copper improves the reproducibility, enhances the carrier mobility, and reduces the lattice thermal conductivity is also discussed.