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Many thermoelectric materials benefit from complex microstructures. Grain boundaries (GBs) in nanocrystalline thermoelectrics cause desirable reduction in the thermal conductivity by scattering phonons, but often lead to unwanted loss in the electrical conductivity by scattering charge carriers. Therefore, modifying GBs to suppress their electrical resistivity plays a pivotal role in the enhancement of thermoelectric performance, zT. In this work, different characteristics of GB phases in Ti‐doped NbFeSb half‐Heusler compounds are revealed using a combination of scanning transmission electron microscopy and atom probe tomography. The GB phases adopt a hexagonal close‐packed lattice, which is structurally distinct from the half‐Heusler grains. Enrichment of Fe is found at GBs in Nb0.95Ti0.05FeSb, but accumulation of Ti dopants at GBs in Nb0.80Ti0.20FeSb, correlating to the bad and good electrical conductivity of the respective GBs. Such resistive to conductive GB phase transition opens up new design space to decouple the intertwined electronic and phononic transport in thermoelectric materials. [ABSTRACT FROM AUTHOR]

Engineering semiconductor devices requires an understanding of the effective mass of electrons and holes. Effective masses have historically been determined in metals at cryogenic temperatures estimated using measurements of the electronic specific heat. Instead, by combining measurements of the Seebeck and Hall effects, a density of states effective mass can be determined in doped semiconductors at room temperature and above. Here, a simple method to calculate the electron effective mass using the Seebeck coefficient and an estimate of the free electron or hole concentration, such as that determined from the Hall effect, is introduced mS∗me=0.924(300KT)(nH1020cm−3)2/3[3(exp[|S|kB/e−2]−0.17)2/31+exp[−5(|S|kB/e−kB/e|S|)]+|S|kB/e1+exp[5(|S|kB/e−kB/e|S|)]] here mS∗ is the Seebeck effective mass, nH is the charge carrier concentration measured by the Hall effect (nH = 1/eRH, RH is Hall resistance) in 1020 cm−3, T is the absolute temperature in K, S is the Seebeck coefficient, and kB/e = 86.3 μV K−1. This estimate of the effective mass can aid the understanding and engineering of the electronic structure as it is largely independent of scattering and the effects of microstructure (grain boundary resistance). It is particularly helpful in characterizing thermoelectric materials. [ABSTRACT FROM AUTHOR]

Dislocations and the residual strain they produce are instrumental for the high thermoelectric figure of merit, zT ≈ 2, in lead chalcogenides. However, these materials tend to be brittle, barring them from practical green energy and deep space applications. Nonetheless, the bulk of thermoelectrics research focuses on increasing zT without considering mechanical performance. Optimized thermoelectric materials always involve high point defect concentrations for doping and solid solution alloying. Brittle materials show limited plasticity (dislocation motion), yet clear links between crystallographic defects and embrittlement are hitherto unestablished in PbTe. This study identifies connections between dislocations, point defects, and the brittleness (correlated with Vickers hardness) in single crystal and polycrystalline PbTe with various n‐ and p‐type dopants. Speed of sound measurements show a lack of electronic bond stiffening in p‐type PbTe, contrary to the previous speculation. Instead, varied routes of point defect–dislocation interaction restrict dislocation motion and drive embrittlement: dopants with low doping efficiency cause high defect concentrations, interstitial n‐type dopants (Ag and Cu) create highly strained obstacles to dislocation motion, and highly mobile dopants can distribute inhomogeneously or segregate to dislocations. These results illustrate the consequences of excessive defect engineering and the necessity to consider both mechanical and thermoelectric performance when researching thermoelectric materials for practical applications. [ABSTRACT FROM AUTHOR]

Thermoelectric properties are frequently manipulated by introducing point defects into a matrix. However, these properties often change in unfavorable directions owing to the spontaneous formation of vacancies at high temperatures. Although it is crucial to maintain high thermoelectric performance over a broad temperature range, the suppression of vacancies is challenging since their formation is thermodynamically preferred. In this study, using PbTe as a model system, it is demonstrated that a high thermoelectric dimensionless figure of merit, zT ≈ 2.1 at 723 K, can be achieved by suppressing the vacancy formation via dopant balancing. Hole‐killer Te vacancies are suppressed by Ag doping because of the increased electron chemical potential. As a result, the re‐dissolution of Na2Te above 623 K can significantly increase the hole concentration and suppress the drop in the power factor. Furthermore, point defect scattering in material systems significantly reduces lattice thermal conductivity. The synergy between defect and carrier engineering offers a pathway for achieving a high thermoelectric performance by alleviating the power factor drop and can be utilized to enhance thermoelectric properties of thermoelectric materials. [ABSTRACT FROM AUTHOR]

Dislocations play an important role in thermal transport by scattering phonons. Nevertheless, for materials with intrinsically low thermal conductivity, such as thermoelectrics, classical models require exceedingly high numbers of dislocations (>1012 cm–2) to further impede thermal transport. In this work, a significant reduction in thermal conductivity of Na0.025Eu0.03Pb0.945Te is demonstrated at a moderate dislocation density of 1 × 1010 cm–2. Further characteristics of dislocations, including their arrangement, orientation, and local chemistry are shown to be crucial to their phonon‐scattering effect and are characterized by correlative microscopy techniques. Electron channeling contrast imaging reveals a uniform distribution of dislocations within individual grains, with parallel lines along four <111> directions. Transmission electron microscopy (TEM) shows the parallel networks are edge‐type and share the same Burgers vectors within each group. Atom probe tomography reveals the enrichment of dopant Na at dislocation cores, forming Cottrell atmospheres. The dislocation network is demonstrated to be stable during in situ heating in the TEM. Using the Callaway transport model, it is demonstrated that both parallel arrangement of dislocations and Cottrell atmospheres make dislocations more efficient in phonon scattering. These two mechanisms provide new avenues to lower the thermal conductivity in materials for thermal‐insulating applications. [ABSTRACT FROM AUTHOR]

The widespread application of thermoelectric (TE) technology demands high‐performance materials, which has stimulated unceasing efforts devoted to the performance enhancement of Bi2Te3‐based commercialized thermoelectric materials. This study highlights the importance of the synthesis process for high‐performance achievement and demonstrates that the enhancement of the thermoelectric performance of (Bi,Sb)2Te3 can be achieved by applying cyclic spark plasma sintering to BixSb2–xTe3‐Te above its eutectic temperature. This facile process results in a unique microstructure characterized by the growth of grains and plentiful nanostructures. The enlarged grains lead to high charge carrier mobility that boosts the power factor. The abundant dislocations originating from the plastic deformation during cyclic liquid phase sintering and the pinning effect by the Sb‐rich nano‐precipitates result in low lattice thermal conductivity. Therefore, a high ZT value of over 1.46 is achieved, which is 50% higher than conventionally spark‐plasma‐sintered (Bi,Sb)2Te3. The proposed cyclic spark plasma liquid phase sintering process for TE performance enhancement is validated by the representative (Bi,Sb)2Te3 thermoelectric alloy and is applicable for other telluride‐based materials. [ABSTRACT FROM AUTHOR]

Half-Heusler (HH) alloys are among the best promising thermoelectric (TE) materials applicable for the middle-to-high temperature power generation. Despite of the large thermoelectric power factor and decent figure-of-merit ZT (≈1), their broad applications and enhancement on TE performance are limited by the high intrinsic lattice thermal conductivity (κL) due to insufficiencies of phonon scattering mechanisms, and the fewer powerful strategies associated with the microstructural engineering for HH materials. This study reports a bottom-up nanostructure synthesis approach for these HH materials based on the displacement reaction between metal chlorides/bromides and magnesium (or lithium), followed by vacuum-assisted spark plasma sintering process. The samples are featured with dense dislocation arrays at the grain boundaries, leading to a minimum κL of ≈1 W m−1 K−1 at 900 K and one of the highest ZT (≈1) and predicted η (≈11%) for n-type Hf0.25Zr0.75NiSn0.97Sb0.03. Further manipulation on the dislocation defects at the grain boundaries of p-type Nb0.8Ti0.2FeSb leads to enhanced maximum power factor of 47 × 10−4 W m−1 K−2 and the predicted η of ≈7.5%. Moreover, vanadium substitution in FeNb0.56V0.24Ti0.2Sb significantly promotes the η to ≈11%. This strategy can be extended to a broad range of advanced alloys and compounds for improved properties. [ABSTRACT FROM AUTHOR]

Segmentation of thermoelectric (TE) materials is a widely used solution to improve the efficiency of thermoelectric generators over a wide working temperature range. However, the improvement can only be obtained with appropriate material selections. In this work, we provide an overview of the theoretical efficiency of the best performing unicouples designed from segmenting the state-of-the-art TE materials. The efficiencies are evaluated using a 1D numerical model which includes all thermoelectric effects, heat conduction, Joule effects and temperature dependent material properties, but neglects contact resistance and heat losses. The calculations are performed for a fixed cold side temperature of 300 K and different hot side temperatures of 700, 900, and 1100 K. We confirm that without taking into account the compatibility of TE materials, segmentation can even decrease the total efficiency. Choosing the TE materials carefully, one is, however, rewarded by a significant improvement in the total efficiency. [ABSTRACT FROM AUTHOR]

The performance of thermoelectric materials is typically assessed using the dimensionless figure of merit, zT. Increasing zT is challenging due to the intricate relationships between electrical and thermal transport properties. This study focuses on Y2Te3‐based thermoelectric materials, which are predicted to be promising for high‐temperature applications due to their inherently low lattice thermal conductivity. A series of Y2+xTe3 compositions with excess Y is synthesized to explore the effects on electronic and structural characteristics. Density functional theory calculations suggest that additional Y atoms increase charge carriers, thereby enhancing electrical conductivity and boosting thermoelectric performance. X‐ray diffraction analysis reveals that the presence of excess Y reduces lattice volume and alters bonding structures. Furthermore, the addition of Bi significantly enhances the power factor by promoting the segregation of elemental Bi particles and the formation of Y‐Bi‐rich grain boundaries, which improve weighted mobility. This microstructural optimization leads to a fourfold increase in the Seebeck coefficient, resulting in a peak zT of 1.23 at 973 K and a predicted maximum conversion efficiency of 10.3% under a temperature difference of 673 K. These findings highlight the potential of Y2Te3 for high‐temperature thermoelectric applications and demonstrate the effectiveness of grain boundary engineering in enhancing thermoelectric performance. [ABSTRACT FROM AUTHOR]

Lattice softening is an intricate mechanism utilized to modulate lattice thermal conductivity (κlat). However, experimental observations are often complicated by numerous factors including thermal regimes, elemental matrices, and crystalline topographies, making the fundamental mechanisms complex. In this study, the temperature gradients are meticulously harnessed during phase transitions, both in heating and cooling trajectories, to ascertain that atomic configuration acts as the paramount factor modulating phonon propagation. Within CuTeI, the predilection between the tetragonal (β) and orthorhombic (α) phases is deftly manipulated via specific thermal pathways to a juncture of 273 K. A salient 44% variance in κlat is observed consequent to a singular alteration in the structural disposition of the bridging Cu atoms. Such atomic configurations delineate pronounced differential effects on the transmission dynamics of transverse and longitudinal phonons. The theoretical analysis indicates that the transverse acoustic velocity plays a more pivotal role in dictating κlat than its longitudinal counterpart due to its greater contribution to the Grüneisen parameter. The synergistic interplay of lattice softening and anharmonicity enhancement culminates in an exceptionally diminished κlat in α‐CuTeI, registering a record low κlat of 0.21 W/(m × K) among inorganic materials dominated by phonon–phonon scattering at 273 K. The revelations proffer avant‐garde perspectives for the nuanced modulation of phonon velocities and κlat. [ABSTRACT FROM AUTHOR]

Nanostructuring to reduce thermal conductivity is among the most promising strategies for designing next‐generation, high‐performance thermoelectric materials. In practice, electrical grain boundary resistance can overwhelm the thermal conductivity reduction induced by nanostructuring, which results in worse overall performance. Since a large body of work has characterized the transport of both polycrystalline ceramics and single crystals of SrTiO3, it is an ideal material system for conducting a case study of electrical grain boundary resistance. An effective mass model is used to characterize the transport signatures of electrical grain boundary resistance and evaluate thermodynamic design principles for controlling that resistance. Treating the grain boundary as a secondary phase to the bulk crystallites explains the transport phenomena. Considering that the interface can be engineered by controlling oxygen partial pressure, temperature, and the addition of extrinsic elements into the grain boundary phase, the outlook for SrTiO3 as a nanostructured thermoelectric is promising, and the zT could be greater than 0.5 at room temperature. [ABSTRACT FROM AUTHOR]

The Ag and In co‐doped PbTe, AgnPb100InnTe100+2n (LIST), exhibits n‐type behavior and features unique inherent electronic levels that induce self‐tuning carrier density. Results show that In is amphoteric in the LIST, forming both In3+ and In1+ centers. Through unique interplay of valence fluctuations in the In centers and conduction band filling, the electron carrier density can be increased from ≈3.1 × 1018 cm−3 at 323 K to ≈2.4 × 1019 cm−3 at 820 K, leading to large power factors peaking at ≈16.0 µWcm−1 K−2 at 873 K. The lone pair of electrons from In+ can be thermally continuously promoted into the conduction band forming In3+, consistent with the amphoteric character of In. Moreover, with rising temperature, the Fermi level shifts into the conduction band, which enlarges the optical band gap based on the Moss–Burstein effect, and reduces bipolar diffusion and thermal conductivity. Adding extra Ag in LIST improves the electrical transport properties and meanwhile lowers the lattice thermal conductivity to ≈0.40 Wm−1 K−1. The addition of Ag creates spindle‐shaped Ag2Te nanoprecipitates and atomic‐scale interstitials that scatter a broader set of phonons. As a result, a maximum ZT value ≈1.5 at 873 K is achieved in Ag6Pb100InTe102 (LIST). [ABSTRACT FROM AUTHOR]

The discovery of a semiconducting ground state XyYZ (y = 0.8 or 0.75) in nominally 19‐electron half‐Heusler materials warrants a closer look at their apparently metallic properties that often make them good thermoelectric (TE) materials. By systematically investigating the temperature dependence of off‐stoichiometry (x) in V0.8+xCoSb, Nb0.8+xCoSb, and Ti0.75+xNiSb it is found that x invariably increases with increasing temperature, leading to an n‐type self‐doping behavior. In addition, there is also a large phase width (range of x) associated with each phase that is temperature dependent. Thus, unlike in typical 18‐electron half‐Heuslers (e,g, TiNiSn), the temperature dependence of vacancy and carrier concentration (n) in nominally 19‐electron half‐Heuslers links its transport properties to synthesis conditions. The temperature dependence of x and n are understood using density functional theory based defect energies (Ed) and phase diagrams. Ed are calculated for 21 systems which can be used in predicting solubility in this family of compounds. Using this simple strategy, suitable composition and temperature synthesis conditions are devised for obtaining an optimized n to engineer TE properties in phase‐pure V0.8+xCoSb, and the previously unexplored Ta0.8+xCoSb. Defect and doping concentrations in nominally 19‐electron half‐Heuslers are strongly dependent on the annealing temperature. A simple strategy based on calculated defect energies to exploit this temperature degree‐of‐freedom for engineering thermoelectric transport is realized. [ABSTRACT FROM AUTHOR]