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Grain boundaries (GBs) form ubiquitous microstructures in polycrystalline materials which play a significant role in tuning the thermoelectric figure of merit (ZT). However, it is still unknown which types of GB features are beneficial for thermoelectrics due to the challenge of correlating complex GB microstructures with transport properties. Here, it is demonstrated that GB complexions formed by Ga segregation in GeTe-based alloys can optimize electron and phonon transport simultaneously. The Ga-rich complexions increase the power factor by reducing the GB resistivity with slightly improved Seebeck coefficients. Simultaneously, they lower the lattice thermal conductivity by strengthening the phonon scattering. In contrast, Ga2Te3 precipitates at GBs act as barriers to scatter both phonons and electrons and are thus unable to improve ZT. Tailoring GBs combined with the beneficial alloying effects of Sb and Pb enables a peak ZT of ≈2.1 at 773 K and an average ZT of 1.3 within 300–723 K for Ge0.78Ga0.01Pb0.1Sb0.07Te. The corresponding thermoelectric device fabricated using 18-pair p-n legs shows a power density of 1.29 W cm−2 at a temperature difference of 476 K. This work indicates that GB complexions can be a facile way to optimize electron and phonon transport, further advancing thermoelectric materials.

The thermoelectric material ZnSb utilizes elements that are inexpensive, abundant, and viable for mass production. While a high thermoelectric figure of merit zT, is difficult to achieve in Sn‐doped ZnSb, it is shown that this obstacle is primarily due to shortcomings in reaching high enough carrier concentrations. Sn‐doped samples prepared in different equilibrium phase spaces in the ternary Zn‐Sb‐Sn system are investigated using phase boundary mapping, and a range of achievable carrier concentrations is found in the doped samples. The sample with the highest zT in this study, which is obtained with a carrier concentration of 2 × 10¹⁹ cm⁻³ when the material is in equilibrium with Zn₄Sb₃ and Sn, confirms that the doping efficiency can be controlled by preparing the doped sample in a particular region of the thermodynamic phase diagram. Moreover, density functional theory calculations suggest that the doping efficiency is limited by the solubility of Sn in ZnSb, as opposed to compensation from native defects. Cognizance of thermodynamic conditions is therefore crucial for rationally tuning the carrier concentration, a quantity that is significant for many areas of semiconductor technologies.

Interest in high entropy alloy thermoelectric materials is predicated on achieving ultralow lattice thermal conductivity $\kappa\sub{L}$ through large compositional disorder. However, here we show that for a given mechanism, such as mass contrast phonon scattering, $\kappa\sub{L}$ will be minimized along the binary alloy with the highest mass contrast, such that adding an intermediate-mass atom to increase atomic disorder can increase thermal conductivity. Only when each component adds an independent scattering mechanism (such as adding strain fluctuation to an existing mass fluctuation) is there a benefit. In addition, both charge carriers and heat-carrying phonons are known to experience scattering due to alloying effects, leading to a trade-off in thermoelectric performance. We apply analytic transport models, based on perturbation and effective medium theories, to predict how alloy scattering will affect the thermal and electronic transport across the full compositional range of several pseudo-ternary and pseudo-quaternary alloy systems. To do so, we demonstrate a multicomponent extension to both thermal and electronic binary alloy scattering models based on the virtual crystal approximation. Finally, we show that common functional forms used in computational thermodynamics can be applied to this problem to further generalize the scattering behavior that is modeled.

GeSe is a IV-VI semiconductor, like the excellent thermoelectric materials PbTe and SnSe. Orthorhombic GeSe has been predicted theoretically to have good thermoelectric performance but is difficult to dope experimentally. Like PbTe, rhombohedral GeTe has a multivalley band structure, which is ideal for thermoelectrics and also promotes the formation of Ge vacancies to provide enough carriers for electrical transport. Herein, we investigate the thermoelectric properties of GeSe alloyed with AgSbSe2, which stabilizes a new rhombohedral structure with higher symmetry that leads to a multivalley Fermi surface and a dramatic increase in carrier concentration. The zT of GeAg0.2Sb0.2Se1.4 reaches 0.86 at 710 K, which is 18 times higher than that of pristine GeSe and over four times higher than doped orthorhombic GeSe. Our results open a new avenue towards developing novel thermoelectric materials via crystal phase engineering using a strategy of entropy stabilization of high symmetry alloys.

Effiziente thermoelektrische Materialien werden durch ein Zusammenspiel von stark voneinander abhangenden Transporteigenschaften erreicht. Um diese wissentlich zu beeinflussen, wird ein tieferes Verstandnis der Chemie und Physik in Festkorpern benotigt. Auf der Grundlage von Molekulorbitaldiagrammen, der “Tight‐Binding”‐Methode und dem klassischem Verstandnis von Bindungsstarken betrachten wir das gezielte Design thermoelektrischer Materialien. Hierbei werden Parameter wie Elektronegativitat, Bandbreite, Orbitaluberlappung sowie Bindungsenergie und ‐lange herangezogen, um Trends der elektronischen Eigenschaften wie Grose und Temperaturabhangigkeit von Bandlucken, effektive Masse der Ladungstrager sowie Bandentartung und Bandkonvergenz zu erklaren. Gitterwarmeleitfahigkeiten werden in Bezug auf die Kristallstruktur und Bindungsstarke behandelt, um den Einfluss von Bindungslangen zu verdeutlichen. Wir zeigen, wie Symmetrie und Starke von Bindungen den Transport von Elektronen und Phononen beeinflussen und wie gezielte Strategien zu Veranderungen und zur Verbesserung thermoelektrischer Effizienz fuhren konnen.

Phase transition in thermoelectric (TE) material is a double-edged sword-it is undesired for device operation in applications, but the fluctuations near an electronic instability are favorable. Here, Sb doping is used to elicit a spontaneous composition fluctuation showing uphill diffusion in GeTe that is otherwise suspended by diffusionless athermal cubic-to-rhombohedral phase transition at around 700 K. The interplay between these two phase transitions yields exquisite composition fluctuations and a coexistence of cubic and rhombohedral phases in favor of exceptional figures-of-merit zT. Specifically, alloying GeTe by Sb

Bismuth telluride is the working material for most Peltier cooling devices and thermoelectric generators. This is because Bi2Te3 (or more precisely its alloys with Sb2Te3 for p-type and Bi2Se3 for n-type material) has the highest thermoelectric figure of merit, zT, of any material around room temperature. Since thermoelectric technology will be greatly enhanced by improving Bi2Te3 or finding a superior material, this review aims to identify and quantify the key material properties that make Bi2Te3 such a good thermoelectric. The large zT can be traced to the high band degeneracy, low effective mass, high carrier mobility, and relatively low lattice thermal conductivity, which all contribute to its remarkably high thermoelectric quality factor. Using literature data augmented with newer results, these material parameters are quantified, giving clear insight into the tailoring of the electronic band structure of Bi2Te3 by alloying, or reducing thermal conductivity by nanostructuring. For example, this analysis clearly shows that the minority carrier excitation across the small bandgap significantly limits the thermoelectric performance of Bi2Te3, even at room temperature, showing that larger bandgap alloys are needed for higher temperature operation. Such effective material parameters can also be used for benchmarking future improvements in Bi2Te3 or new replacement materials.

We report a high-performance thermoelectric (TE) oxide-based module using the segmentation of half-Heusler Ti_(0.3)Zr_(0.35)Hf_(0.35)CoSb_(0.8)Sn_(0.2) and misfit-layered cobaltite Ca_3Co_4O_(9+δ) as the p-leg and 2 % Al-doped ZnO as the n-leg. The maximum output power of a 4-couple segmented module at ΔT=700 K attains a value of approximately 6.5 kW m^(−2), which is three times higher than that of the best reported non-segmented oxide module. The TE properties of individual legs, as well as the interfacial contact resistances, were characterized as a function of temperature. Numerical modeling was used to predict the efficiency and to evaluate the influence of the electrical and thermal losses on the performance of TE modules. Initial long-term stability tests of the module at the hot and the cold side temperatures of 1073 K and 444 K, respectively, showed a promising result with 4 % degradation for 48 h operating in air.

A new type of high performance thermoelectric material Cu_(2-x)S composed of non-toxic and earth-abundant elements Cu and S is reported. Cu_(2-x)S surprisingly has lower thermal conductivity and more strikingly reduced specific heat compared to the heavier Cu_(2)Se, leading to an increased zT to 1.7.

Experimentally, AZn_2Sb_2 samples (A=Ca, Sr, Eu, Yb) are found to have large charge carrier concentrations that increase with increasing electronegativity of A. Using density functional theory (DFT) calculations, we show that this trend can be explained by stable cation vacancies and the corresponding finite phase width in A1−xZn_2Sb_2 compounds.

Highly resistive grain boundaries significantly reduce the electrical conductivity that compromises the thermoelectric figure‐of‐merit zT in n‐type polycrystalline Mg_3Sb_2. In this work, discovered is a Mg deficiency near grain boundaries using atom‐probe tomography. Approximately 5 at% of Mg deficiency is observed uniformly in a 10 nm region along the grain boundary without any evidence of a stable secondary or impurity phase. The off‐stoichiometry can prevent n‐type dopants from providing electrons, lowering the local carrier concentration near the grain boundary and thus the local conductivity. This observation explains how nanometer scale compositional variations can dramatically determine thermoelectric zT, and provides concrete strategies to reduce grain‐boundary resistance and increase zT in Mg_3Sb_2‐based materials.

The influence of micro/nanostructure on thermal conductivity is a topic of great scientific interest, particularly to thermoelectrics. The current understanding is that structural defects decrease thermal conductivity through phonon scattering where the phonon dispersion and speed of sound are assumed to remain constant. Experimental work on a PbTe model system is presented, which shows that the speed of sound linearly decreases with increased internal strain. This softening of the materials lattice completely accounts for the reduction in lattice thermal conductivity, without the introduction of additional phonon scattering mechanisms. Additionally, it is shown that a major contribution to the improvement in the thermoelectric figure of merit (zT > 2) of high-efficiency Na-doped PbTe can be attributed to lattice softening. While inhomogeneous internal strain fields are known to introduce phonon scattering centers, this study demonstrates that internal strain can modify phonon propagation speed as well. This presents new avenues to control lattice thermal conductivity, beyond phonon scattering. In practice, many engineering materials will exhibit both softening and scattering effects, as is shown in silicon. This work shines new light on studies of thermal conductivity in fields of energy materials, microelectronics, and nanoscale heat transfer.

Microstructure control in functional materials draws from a historical reserve rich in established theory and experimental observation of metallurgy. Methods such as rapid solidification, eutectoid reaction, and nucleation and growth precipitation have all proven to be effective means to produce microstructure relevant for a wide array of applications. Here, the available parameters to control structure morphology, size, and spacing are discussed using thermoelectric composites as an example. Moreover, exploiting different aspects of a material system's phase diagram enables a controlled introduction of nanostructures. While much of this discussion is pertinent to the rapidly developing field of thermal conductivity control in thermoelectric composites, these techniques can be applied to a variety of other material systems where their use may lead to novel electrical, optical, as well as thermal properties of semiconductors and insulators as it has in the past for the mechanical properties of metals.

Electron and phonon transport characteristics determines the potential of thermoelectric materials for power generation or refrigeration. This work shows that, different from most of high performance thermoelectric materials with dominant acoustic phonon scattering, the promising ZrNiSn based half-Heusler thermoelectric solid solutions exhibit an alloy scattering dominated charge transport. A low deformation potential and a low alloy scattering potential are found for the solid solutions, which is beneficial to maintain a relatively high electron mobility despite of the large effective mass, and can be intrinsic favorable features contributing to the noticeably high power factors of ZrNiSn based alloys. A quantitive description of the different phonon scattering mechanisms suggests that the point defect scattering is the most important mechanism that determines the phonon transport process of the solid solutions. The present results indicate that alloying can be an effective approach for such materials systems to enhance thermoelectric figure of merit ZT by reducing phonon thermal conductivity, while minimizing the deterioration of charge mobility due to the low alloy scattering potential.

Understanding the electron and phonon transport characteristics is crucial for designing and developing high performance thermoelectric materials. Weak scattering effects on charge carriers, characterized by deformation potential and alloy scattering potential, are favorable for thermoelectric solid solutions to enable high carrier mobility and thereby promising thermoelectric performance. Mg_2(Si,Sn) solid solutions have attracted much attention due to their low cost and environmental compatibility. Usually, their high thermoelectric performance with ZT ∼ 1 is ascribed to the band convergence and reduced lattice thermal conductivity caused by alloying. In this work, both a low deformation potential Ξ = 13 eV and a low alloy scattering potential U = 0.7 eV are found for the thermoelectric alloys by characterizing and modeling of thermoelectric transport properties. The band convergence is also verified by the increased density-of-states effective mass. It is proposed that, in addition to band convergence and reduced lattice thermal conductivity, the low deformation potential and alloy scattering potential are additional intrinsic features that contribute to the high thermoelectric performance of the solid solutions.

The compatibility approach enables the description of thermoelectric (TE) effects in terms of intensive state variables. Based on the new concept of a “Thomson cooler” (using the particular case of a u = s cooler as a demonstration) the authors present practically relevant results about fully self-compatible cooler elements where the compatibility factor s is adjusted locally at any position along the element. By means of a comparison u = s cooler vs. conventional Peltier cooler with (Bi_(0.5)Sb_(0.5))_2Te_3 material it is shown that there is a similar self-compatibility effect for different ΔT. Therewith, the paper completes results published in Phys. Rev. B 86, 045202 (2012).

Heavy doping changes an intrinsic semiconductor into a metallic conductor by the introduction of impurity states. However, Ga impurities in thermoelectric skutterudite CoSb_3 with lattice voids provides an example to the contrary. Because of dual-site occupancy of the single Ga impurity charge-compensated compound defects are formed. By combining first-principle calculations and experiments, we show that Ga atoms occupy both the void and Sb sites in CoSb_3 and couple with each other. The donated electrons from the void-filling Ga (GaVF) saturate the dangling bonds from the Sb-substitutional Ga (Ga_(Sb)). The stabilization of Ga impurity as a compound defect extends the region of skutterudite phase stability toward Ga_(0.15)Co_4Sb_(11.95) whereas the solid–solution region in other directions of the ternary phase diagram is much smaller. A proposed ternary phase diagram for Ga-Co-Sb is given. This compensated defect complex leads to a nearly intrinsic semiconductor with heavy Ga doping in CoSb_3 and a much reduced lattice thermal conductivity (κ_L) which can also be attributed to the effective scattering of both the low- and high-frequency lattice phonons by the dual-site occupant Ga impurities. Such a system maintains a low carrier concentration and therefore high thermopower, and the thermoelectric figure of merit quickly increases to 0.7 at a Ga doping content as low as 0.1 per Co_4Sb_(12) and low carrier concentrations on the order of 10^(19) cm^(−3).

Thermoelectric materials can be used to directly and reversibly convert waste heat into electrical power and have attracted significant attention. Lead-free tin chalcogenide (SnTe, SnSe, SnS) thermoelectric materials are regarded as promising thermoelectric materials due to the great progress in the thermoelectric research field during the past several years. Analogous to PbTe, SnTe has a simple high symmetry NaCl-type cubic structure and a complex band structure, leading to its capability of achieving high thermoelectric performance by band structure and microstructure engineering. SnSe/SnS are simple compounds but exhibit an intrinsically ultralow thermal conductivity, suggesting that a high thermoelectric performance can be realized also in simple layered, anisotropic and anharmonic systems, without nanostructuring. This short perspective summarizes some current strategies that could enhance the thermoelectric performance of bulk lead-free tin chalcogenides, such as band structure engineering, nanostructuring, and synergistically improved approaches. Future possible strategies for further enhancing the thermoelectric performance of tin chalcogenides are also considered at the end of this perspective.

ZnxSb1–x systems are one of the most promising materials for thermoelectric applications due to their low cost, low toxicity, and high thermoelectric figure of merit. We report the discovery and characterization of novel ZnSb polymorphs with improved thermoelectric properties from a systematic ab initio structural search. The putative ground state at low temperature is a structure with Pbca symmetry. Corresponding electron and phonon transport properties are computed, where the lattice thermal conductivity was accurately obtained through third-order interatomic force constants. At 500 K, both the reduced figure of merit S2/L as well as the lattice thermal conductivity are superior to any previously reported numbers on ZnSb polymorphs, rendering the Pbca phase a promising candidate for an exceptionally good thermoelectric material.

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-x)Cl_x (x ≤ 0.008) with different carrier densities. Lead chloride provides a nearly 100% efficient doping control up to 1.2 × 10^(20) 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.

The microstructure of Bi_(90)Sb_(10) composites with separate additions of Ag and As was studied, and measurements of the room temperature transport properties were made to see the effects of hot pressing with and without precipitates. The most successful nanostructured Bi_(90)Sb_(10) samples were those containing As when annealed for durations of 3 days or less near the solidus temperature. The transport measurements showing similar electrical resistivity and decreased thermal conductivity with the addition of precipitates at room temperature demonstrates the potential of this bulk nanostructuring and synthesis approach in this material system. With further study, this work could be helpful in creating similar complex nanostructured systems which have the potential to yield high-efficiency thermoelectric materials for low temperature applications.

A mesoporous Bi_2Te_3 monolith with ≈20% porosity is fabricated by hot-pressing mesoporous powders. The mesostructures are able to transport carriers and scatter phonons efficiently. The reduction (≈60%) of the thermal conductivity is sufficient to compensate for the loss of electrical conductivity perpendicular to the direction of applied pressure, leading to an enhanced zT of 0.7, highest among all reported self-doped, n-type Bi_2Te_3 at similar temperatures.

A new route to self-assembled nanocomposite thermoelectric materials is proposed. High-energy mechanical alloying brings materials into a nonequilibrium intermediate state, such as a solid solution with an extended solubility. The large driving force for the transformation to the equilibrium state leads to nanometer-scale microstructure formation, which is ideal for reducing lattice thermal conductivity.

PbTe alloys have been the most efficient materials for thermoelectric power generation since the 1950s. In addition to the observed lattice thermal conductivity reduction by alloy scattering, recent efforts targeting electronic-transport enhancement through band structure engineering have demonstrated a design route to achieve extraordinary thermoelectric performance. Guided by these recent results, the current work discusses the influence of alloying CdTe in p-type PbTe where the solubility is strongly temperature-dependent. Beneficial changes to the band structure and effective scattering of phonons are concurrently inferred when the PbTe–CdTe precipitate composite is expected to become a solid solution at high temperature, enhancing the peak thermoelectric figure of merit to ~1.7. This work emphasizes the importance of solid solution alloys of PbTe for thermoelectric power-generation applications and also provides an upper limit of about 1 micrometer to the average distance between interfaces above which phonon scattering by composite interfaces is ineffectual.

Even though the ZnSb compound has been known for decades and used in the earliest thermoelectric devices, the potential of the material as a modern thermoelectric may be underestimated. We synthesized p-type doped samples using ball-milling and hot-pressing and measured their thermoelectric properties including mobility and carrier concentration. Establishing a single parabolic band (SPB) model using these measurements on the Cu, Sn, and self-doped samples allows for predictions on the optimum thermoelectric efficiency. It is projected to reach zT = 0.75 at 700 K. Deviations from the SPB model at low carrier concentrations are discussed and impurity band conduction is brought in as a possible explanation.

Thermoelectric materials can be optimized by tuning the carrier concentration with chemical doping. However, because the optimum dopant concentration typically increases with temperature, the optimum efficiency can not normally be achieved for a uniform material in a temperature gradient. Here, we show Ag-doped PbTe/Ag_2Te composites exhibit high thermoelectric performance (∼50% greater than La-doped composites) because of a temperature induced gradient in the doping concentration caused by the temperature-dependent solubility of Ag in the PbTe matrix. This demonstrates a new mechanism to achieve a higher thermoelectric efficiency afforded by a given material system, and should be applicable to other thermoelectric materials.

PbSe was expected to have a smaller bandgap and higher thermalconductivity than PbTe. Instead, these values are about the same at high temperature leading to comparable thermoelectric figure of merit, with zT> 1 achieved in heavily doped p-type PbSe.

Heavily p-doped PbTe is known to be a highly efficient thermoelectric material and theories explaining this have connected it to positive curvature in the Seebeck coefficient between 300 and 500 K and a peak in the Hall coefficient at 500 K due to band convergence. Recent work by Jaworski et al. has suggested that a pocket of high mobility electrons between the L and Sigma point in the Brillouin zone are responsible for these effects, inferred from an observed inversion of the Hall resistivity under small magnetic fields (

Thermoelectrics are being rapidly developed for waste heat recovery applications, particularly in automobiles, to reduce carbon emissions. PbTe-based materials with small ( 1.5 at 775 K.

Understanding transport in Zintl compounds is important due to their unusual chemistry, structural complexity, and potential for good thermoelectric performance. Resistivity measurements indicate that undoped Ca_5Al_2Sb_6 is a charge-balanced semiconductor with a bandgap of 0.5 eV, consistent with Zintl–Klemm charge counting rules. Substituting divalent calcium with monovalent sodium leads to the formation of free holes, and a transition from insulating to metallic electronic behavior is observed. Seebeck measurements yield a hole mass of ∼2m_e, consistent with a structure containing both ionic and covalent bonding. The structural complexity of Zintl compounds is implicated in their unusually low thermal conductivity values. Indeed, Ca_5Al_2Sb_6 possesses an extremely low lattice thermal conductivity (0.6 W mK^(−1) at 850 K), which approaches the minimum thermal conductivity limit at high temperature. A single parabolic band model is developed and predicts that Ca_(4.75)Na_(0.25)Al_2Sb_6 possesses a near-optimal carrier concentration for thermoelectric power generation. A maximum zT > 0.6 is obtained at 1000 K.Beyond thermoelectric applications, the semiconductor Ca_5Al_2Sb_6 possesses a 1D covalent structure which should be amenable to interesting magnetic interactions when appropriately doped.

Microstructure engineering is an effective strategy to reduce lattice thermal conductivity (κl ) and enhance the thermoelectric figure of merit (zT). Through a new process based on melt-centrifugation to squeeze out excess eutectic liquid, microstructure modulation is realized to manipulate the formation of dislocations and clean grain boundaries, resulting in a porous network with a platelet structure. In this way, phonon transport is strongly disrupted by a combination of porosity, pore surfaces/junctions, grain boundaries, and lattice dislocations. These collectively result in a ≈60% reduction of κl compared to zone melted ingot, while the charge carriers remain relatively mobile across the liquid-fused grains. This porous material displays a zT value of 1.2, which is higher than fully dense conventional zone melted ingots and hot pressed (Bi,Sb)2 Te3 alloys. A segmented leg of melt-centrifuged Bi0.5 Sb1.5 Te3 and Bi0.3 Sb1.7 Te3 could produce a high device ZT exceeding 1.0 over the whole temperature range of 323-523 K and an efficiency up to 9%. The present work demonstrates a method for synthesizing high-efficiency porous thermoelectric materials through an unconventional melt-centrifugation technique.

The compatibility approach introduced by Snyder and Ursell opens a new pathway for the improvement of thermoelectric (TE) device performance. It has been shown that sufficient compatibility is - besides an increase of the averaged figure of merit Z - essential for efficient operation of a TE device, and that compatibility will facilitate rational materials selection, device design, and the engineering of functionally graded materials (FGMs). In this paper, the authors give an overview of the fundamental results and present a new approach from the perspective of variational calculus. A particular focus is on the role of ideal self-compatibility, i.e., adjusting compatibility locally at any position along a TE leg.