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Using laser flash analysis, the low thermal conductivity of the pressed Zirconium metal-organic framework (UiO-66) powder pellet was obtained. As a result, the density ρ, thermal diffusivity α, specific heat capacity cP, and low thermal conductivity κexp of the pressed UiO-66 powder pellet at 300 K are observed to be 1.258 g/cm3, 0.001 59 cm2/s, 0.7765 J/g K, and 0.156 W/m K, respectively. Due to the presence of the 12-coordinated nodes with six transfer pathways, the thermal transport of the UiO-66 particles is preferred through linkers to metal sites. The low thermal conductivity follows the trend of vacuum < argon (Ar) < air < helium (He) since the entrapped gas molecules provide additional heat transfer channels inside the particles and between the particles. The low thermal conductivity along with a weak temperature-dependent thermal conductivity are elucidated in terms of boundary scattering. [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]

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]

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]

Defects such as planar faults in thermoelectric materials improve their performance by scattering phonons with short and medium mean free paths (3–100 nm), thereby reducing the lattice thermal conductivity, κ l . Understanding statistically the microscopic distribution of these extended defects within the grains and in low angle grain boundaries is necessary to tailor and develop materials with optimal thermoelectric performance for waste heat harvesting. Herein, we analyze these defects from the millimeter down to the nanometer scale in a AgSbTe 2 thermoelectric material with low angle grain boundaries. The investigations were performed using electron channeling contrast imaging combined with transmission electron microscopy. The microstructure study was complemented by estimating the effect of planar faults on the phonon scattering using the Debye-Callaway model. AgSbTe 2 is a promising thermoelectric material, which exhibits extremely low thermal conductivity, κ , of 0.5 Wm−1K−1 at room temperature. In contrast to conventional alloys or intermetallic materials, in the present material small angle grain boundaries are not composed of individual dislocations but of a dense arrangement of stacked planar faults with fault densities up to N P F = 1.6 ⋅ 10 8 m − 1 . We explain their abundance based on their low interfacial energy of about 186 mJm−2 calculated ab-initio. The current findings show, that it is possible to reach very high densities of phonon-scattering planar faults by the correct microstructure engineering in AgSbTe 2 thermoelectric materials. Image 1 [ABSTRACT FROM AUTHOR]

Complex microstructures are found in many thermoelectric materials and can be used to optimize their transport properties. Grain boundaries in particular scatter phonons, but they often impede charge carrier transfer at the same time. Designing grain boundaries in order to offer a conductive path for electrons is a substantial opportunity to optimize thermoelectrics. Here, we demonstrate in TiCoSb half Heusler compounds that Fe-dopants segregate to grain boundaries and simultaneously increase the electrical conductivity and reduce the thermal conductivity. To explain these phenomena, three samples with different grain sizes are synthesized and a model is developed to relate the electrical conductivity with the area fraction of grain boundaries. The electrical conductivity of grain interior and grain boundaries is calculated and the atomic structure of grain boundaries is studied in detail. Segregation engineering in fine-grained thermoelectrics is proposed as a new design tool to optimize transport properties while achieving a lower thermal conductivity. [Display omitted] [ABSTRACT FROM AUTHOR]