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Phase boundaries, most often between crystalline and amorphous phases of a material, are ubiquitous lattice imperfections in thin films, particularly deposited by low temperature processes such as atomic layer deposition (ALD). Thermal resistances from these boundaries may impede phonon transport and reduce thermal conductivity. However, to date, there have been no reports on the direct measurement of the phase boundary thermal resistance, due in part to challenges associated with fabricating an atomically flat yet geometrically planar interface between different phases within a film. Here, we present a systematic study to simultaneously determine the phase boundary thermal resistance and thermal conductivity in a series of phase-separated titanium oxide (TiO 2) thin films prepared via plasma-enhanced ALD (PEALD). A precise implementation of per-cycle plasma treatment enables extremely localized crystallization—i.e., plasma-assisted atomic layer annealing (ALA)—and the fabrication of crystalline/amorphous layered structures with their respective thicknesses systematically varied. Frequency-domain thermoreflectance measures the effective thermal conductivity of each film and its top and bottom interfaces, confirmed independently using time-domain thermoreflectance. We simultaneously solve a system of linear equations generated by applying a series resistor model to each film, yielding the phase boundary thermal resistance of 5.1 m2 K GW–1 between crystalline and amorphous TiO 2 , as well as the thermal conductivities of 5.1 and 2.1 W m–1 K–1 for crystalline and amorphous TiO 2 , respectively. The results suggest that the crystalline/amorphous phase boundary acts like a layer of crystalline (amorphous) TiO 2 with equivalent thickness ∼26 (∼11) nm. [Display omitted] [ABSTRACT FROM AUTHOR]

The solid solution phases have a large influence on the thermal conductivity of alloys, but the physics behind this effect is complicated. Existing works mainly utilize experimental methods to study the thermal conductivity of alloys. However, due to the coexistence of multi-phase and insufficient understanding of the microscopic property variations in different alloy systems, it remains unclear how different solute atoms can affect the thermal transport of solid solutions. In this work, we employ first-principles simulations based on the Korringa–Kohn–Rostoker coherent potential approximation method and the Boltzmann transport equation to calculate the thermal conductivity of 38 binary magnesium solid solutions with dilute solute atom concentration. Solute atoms can greatly reduce the thermal conductivity of magnesium. Phonon contribution is found to be less than 10 %. The machine learning feature screening methods are further applied to identify the key properties that influence their thermal conductivity the most. Different from the commonly used Linde's law, which states that the resistivity is proportional to the square of valence difference, the machine learning screening indicates that thermal conductivity is predominantly influenced by six features in total. Among them, the variance of the density of states of s + p, d + f orbitals at Fermi level is highly correlated to the electronic thermal conductivity of magnesium alloys. These correlations underscore the role of virtual bond states formed in magnesium alloys due to the presence of solute atoms. The developed method and uncovered physical mechanism can potentially be applied to other solid solution systems, which is an important step to achieve predictive design of high thermal conductivity metallic alloy. [Display omitted] [ABSTRACT FROM AUTHOR]

Chalcogenide perovskites are expected to be promising thermoelectric materials, since they not only possess efficient carrier transport and defect tolerance, but also demonstrate unique advantages of high thermodynamic stability, eco-friendly and earth-abundant constituents. Especially, theoretical reports have predicted their "glass-like" thermal conductivities. However, experimental investigation on thermoelectric performances of chalcogenide perovskite BaZrS 3 is extremely scarce due to the difficulty in preparing high-quality bulk samples, which originates from the brittle nature, high melting point, and the large difference in melting points between Ba/S and Zr. In this work, pure phase BaZrS 3 bulks with high relative density reaching 100 % are realized by optimized sulfurization from low-cost BaZrO 3 powders combined with fast spark plasma sintering. A maximum zT value of 0.37 at 623 K in BaZrS 3 bulks is achieved, which is the record-high value among the reported sulfide, halide, and hybrid perovskite materials. A room-temperature electron mobility up to 385 cm2V−1s−1 is among the highest values for perovskites due to the high phase purity, dense morphology and corner-sharing ZrS 6 octahedral three-dimensional network as effective carrier channels. Meanwhile, a measured low lattice thermal conductivity of 1.11 Wm−1K−1 at 623 K is attributed to the intense phonon scattering from the intrinsic distorted-perovskite structure and the lattice defects by sulfur deficiency. Moreover, the BaZrS 3 bulks in this work are stable against moisture/air and high temperature test. This work provides new insights into the fundamental electrical and thermal properties of chalcogenide perovskites, and highlights their great potential in the practical thermoelectric applications. [Display omitted] [ABSTRACT FROM AUTHOR]

GeTe is emerging as promising medium-temperature thermoelectric material due to its highly competitive performance and good mechanical properties. Strong Rashba spin splitting was harnessed to markedly improve the Seebeck coefficient and power factor of Bi and Sn codoped GeTe at low-medium temperature. Moreover, it is found that Bi-Sn-Cu doping reduces the phase-transition temperature to extend better electrical transport behavior of cubic phase to low temperature. As a result, the electrical transport properties in low-medium temperature were overall enhanced. In the meanwhile, endotaxial hetero-nanostructures efficiently scatter phonons and play a dominant role on affecting phonon propagation. The lattice thermal conductivity was reduced to 0.2 W m −1 K −1 at 673 K. Drive by strengthening Rashba effect and endotaxial hetero-nanostructures, a record-high average ZT (300–823 K) of 1.6 and a high ZT of 2.1 were obtained in lead-free GeTe-based compounds. The vast increase of ZT promotes GeTe as a promising candidate for a wide range of applications in waste heat recovery and power generation. [Display omitted] [ABSTRACT FROM AUTHOR]

In this article we present a thermal coupling to a recently published two-time scale phase field crystal model (MPFC) to describe latent heat contributions of systems undergoing rapid phase transformations. We call this formulation Temperature Field Crystal (TFC), which permits the study of coupled density, vacancy and temperature field dynamics at the length-scale of atomic ordering. Following a derivation of the new thermo-density coupled MPFC model, several physical properties of the model are demonstrated. We reproduce the thermo-density interface profiles encountered in the steady state solidification growth limit. We further illustrate the existence of a vacancy concentration inherent in the phase field crystal amplitude. It is shown that with a frozen thermal gradient, of the form often used in directional solidification studies, a vacancy gradient is established along with an associated thermal stress. In particular, we show that within the isochoric limit of the model, effects of thermal expansion are incorporated through the change of amplitude with respect to temperature, rather than by increasing the equilibrium lattice length. The TFC model is then applied to the study of select solidification processes. It is shown that the release of latent heat during recalescence is accompanied by a change in the average thermal pressure. Further, we illustrate, to our knowledge for the first time, that modulations of the recalescence curve can be indicative of plastic deformation, dislocation activity, and phonon scattering. Notably, the temperature evolution may be used as a marker for the grain distribution attained from nucleation following a system quench. [Display omitted] [ABSTRACT FROM AUTHOR]

With the perfect and orderly atomic arrangement, nanotwins could effectively scatter mid- and low-frequency phonons while minimizing the scattering for carriers, beneficial for enhancing the thermoelectric properties of materials. To unlock these features, we explored the twin structural characteristics in Cu 2 SnS 3 and found that nanotwins formed along [100] direction on the glide-set plane, and further delved into the mechanism of twin formation and found that twinning possessed a lower nucleation barrier than the slipping. In addition, we reduced the unstable stacking fault energy in the stacking stage and amplified the probability of twin formation by introducing Se atoms at the S sites. In spite of more nanotwins, a slight increase in carrier mobility was realized, leading to a maximum power factor of 5.5 μW cm−1 K−2 at 773 K for the Cu 2 SnS 2 Se material. Meanwhile, the lattice thermal conductivity across the entire temperature range was effectively reduced due to the introduction of nanotwins and Se S points defects, which is approximately 0.46 W m−1 K−1 at 773 K for Cu 2 SnS 2 Se. Further optimization of the power factor was realized by utilizing a high-efficient doping strategy via introducing Sn vacancies. The carrier concentration became significantly augmented while minimizing ionized impurities scattering, giving rise to an exceptionally high power factor of 8.2 μW cm−1 K−2 and a highly-ranked thermoelectric figure of merit of 0.98 at 773 K for Cu 2 Sn 0.99 S 2 Se. Our work demonstrates an intriguing route towards the high thermoelectric performance of Cu 2 SnS 3 through the introduction of nanotwins as well as high-efficient doping techniques. [Display omitted] [ABSTRACT FROM AUTHOR]

Abnormal thermal expansion denotes intriguing negative and zero thermal expansion (NTE and ZTE). The integration of abnormal thermal expansion with low thermal conductivity is highly desirable for high-end devices. However, the coexistence of seemingly contradictory features presents a grand challenge for abnormal-thermal-expansion materials. Herein, a unique NTE, for the first time, is reported in Cd 2 Re 2 O 7 with a volumetric coefficient of thermal expansion, α V of -5.0 × 10−6/K (5 - 210 K). Moreover, a remarkable ZTE is attained in Cd 1.95 Ni 0.05 Re 2 O 7 with α V of 1.1 × 10−6/K (5 - 180 K). Concomitantly, the attained ultralow thermal conductivities of 0.38 and 0.56 W/mK at 300 K of the two compositions are also highly competitive, exceeding prevailing abnormal-thermal-expansion materials. Concurrently, Cd 2 Re 2 O 7 -based oxides demonstrate orders of magnitude higher electrical conductivity than representative NTE materials. By a combined investigation of synchrotron X-ray techniques, aberration-corrected scanning transmission electron microscope, and phonon calculations, the synergy of local structure distortion highlighted by ordered Re-Re dimer and soft acoustic phonons is integral to the simultaneous accomplishment of distinctive thermal expansion and thermal conductivity, which is totally distinct from predominant optical-phonon driven NTE. This integration of intriguing thermal properties effectively resolves thermal issues and permits excellent size and temperature stability. This work opens up an unusual strategy to delicately control thermal expansion and thermal conductivity simultaneously. [Display omitted] [ABSTRACT FROM AUTHOR]

Microstructure control is crucial for thermoelectrics since it is intimately related to the scattering mechanism of both electrons and phonons. Herein, we propose a new strategy to modify microstructure by phase regulation that simultaneously induces high carrier mobility and low lattice thermal conductivity. As demonstrated in layered SnSe 2 , the addition of Cu can induce a phase transition from space group P 3 ¯ m 1 to P 6 3 mc. Due to the enlarged formation energy of stacking faults in the later phase, the stacking fault density is greatly reduced after heat treatment that leads to an increased grain size. Accordingly, the carrier mobility of SnSe 1.97 Br 0.03 –3 % Cu sample is enhanced by 100 % at room temperature. Furthermore, the reduction of stacking fault density is accompanied by the formation of pores in the matrix, which results in low lattice thermal conductivity. As a result, a record peak zT of 1.13 for SnSe 2 -based materials is achieved at 773 K, and the attained ZT ave of 0.62 is a record-high value among n -type polycrystalline layered materials working in intermediate-to-high temperature region. This work provides a feasible strategy to decouple the electron and phonon transport in layered thermoelectric compounds by phase-dependent microstructure modification. A new phase (SG P 6 3 mc) of SnSe 2 with a larger stacking fault formation energy can be formed by the addition of Cu, which leads to significantly reduced stacking fault density and thereby increased carrier mobility after heat treatment, finally resulting in a record ZT ave value among n -type polycrystalline layered TE materials working in mediate-to-high temperature region. [Display omitted] [ABSTRACT FROM AUTHOR]

In this work, the effect of chemical bonding on thermal transport in superhard BC 2 N polymorphs has been comparatively evaluated by the first-principles calculations. Compared with the high-symmetry phase, a cliff-like drop of lattice thermal conductivity occurs in the C 121 phase with the monoclinic lattice. The findings can be attributed to the heterogeneous C C bonding induced suppression of thermal transport via the transverse phonon modes. The crystal orbital Hamilton population analysis shows that there exists an extremely weak C C bond in the C 121 phase of BC 2 N. Despite the presence of stronger B N and C N bonds in the C 121 phase, the weak C C bond predominantly results in softened acoustic phonon branches, strong lattice anharmonicity, and low elastic moduli. These factors enhance the phonon–phonon scattering and are thereby responsible for the suppression of phonon thermal conductivity. This study gives fundamental insights into structure and chemical bonding effects on phonon transport and provides guidance toward designing superhard materials with tunable thermal conductivity under extreme conditions. [Display omitted] • Phonon thermal transport properties of superhard BC 2 N polymorphs are analyzed by solving BTEs. • Phonon thermal conductivity of the R 3 m phase is almost 50 times higher than that of the C 121 BC 2 N. • The heterogeneous C C bonding induces large suppression of thermal transport in the C 121 BC 2 N. • The weak C C bond in the C 121 BC 2 N results in softened acoustic phonon and strong anharmonicity. [ABSTRACT FROM AUTHOR]

Quantum well (QW) is one of the proposals to improve the thermoelectric properties. However, previous studies have shown that the large electron and phonon tunneling across the layers in semiconductor-based QWs and oxide-based QWs, respectively, are the two main challenges that need to be addressed. In this work, PbSe/amorphous-SrTiO 3 (α -STO) supperlattices with a multiple quantum-well (MQW) structure can overcome both limitations. We demonstrate that the PbSe/ α -STO MQW shows a high power factor of ∼ 24.5 μW cm−1K−2 at room temperature due to the strong quantum confine effect. In addition, the amorphous-SrTiO 3 layer can engender intrinsically low thermal conductivity (0.27± 0.05 W m−1 K−1). The maximum estimated zT value is 2.7 for PbSe/ α -STO MQW at room temperature, which is more than 1800 % higher than that of pure PbSe films (zT = 0.15). High quality n -type PbSe/amorphous-STO supperlattices with MQW structure have been fabricated. By formation of PbSe/ amorphous-STO structure, the infinite barrier height produce by the amorphous-STO can fabricate an ideal quantum well with quantized energy level. Quantum confinement of electrons in PbSe quantum well result in an enhancement of the density of electron states per unit volume. Thus, the Seebeck coiffcient rapidly increased to 271 μVK−1 as the kinetc energy increased per carrier. An enhancement of power factor (24.5 μW cm−1 K−2) was observed. [Display omitted] [ABSTRACT FROM AUTHOR]

Today, ab-initio calculations are becoming a powerful tool to perform virtual experiments that have the capacity to predict and to reproduce experimentally observed non-periodic features, such as interfaces, that are responsible for quantum properties of materials. In our paper we investigate 2D quantum-well structures, known as inversion boundaries (IB). Combining atomistic modeling, DFT calculations and HRTEM analysis we provide a new fundamental insight into the structure and stability of Sb - rich basal - plane IBs in ZnO. DFT screening for potential IB model was based on the known stacking deviations in originating wurtzite structure. The results show that the model with Aβ−Bα−AβC−γB−βC sequence (IB 3) is the most stable translation for Sb - doping, as opposed to previously accepted Aβ−Bα−AβC−γA−αC (IB 2) model. The key to the stability of IB structures has been found to lie in their cationic stacking. We show that the energies of constituting stacking segments can be used to predict the stability of new IB structures without the need of further ab-initio calculations. DFT optimized models of IBs accurately predict the experimentally observed IB structures with lateral relaxations down to a precision of ~1 pm. The newly determined cation sublattice expansions for experimentally confirmed IB 2 and IB 3 models, Δ IB(Zn-Zn) are +81 pm and +77 pm, whereas the corresponding O-sublattice contractions Δ IB(O-O) are –53 pm and –57 pm, respectively. The refined structures will help to solve open questions related to their role in electron transport, phonon scattering, p-type conductivity, affinity of dopants to generate IBs and the underlying formation mechanisms, whereas the excellent match between the calculations and experiment demonstrated in our study opens new perspectives for prediction of such properties from first principles. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Fundamental material parameters governing the carrier transport in thermoelectric materials are affected by microstructural characteristics. We have investigated the effects of compaction duration on microstructure and the thermoelectric properties of Sb doped Mg 2 Si 0.4 Sn 0.6. The transport properties show drastic changes with increasing compaction duration from 10 min to 40 min. A TEM-EDS analysis on samples sintered for 20 min and 40 min highlights Mg depleted grain boundaries and local compositional inhomogeneities but gives no indications for dopant loss. The transport properties were analyzed using a single parabolic band (SPB) model, and the observed changes can be attributed to carrier (n) loss, diminished carrier mobility (μ 0) and a reduction in lattice thermal conductivity (κ lat). Comparatively stronger carrier scattering in longer sintered sample is a combined effect of increasing electron-phonon interaction (higher E Def) and local compositional inhomogeneities in the material which are both linked to Mg depletion. The transport behavior of these samples can be fully captured by the SPB model only after the addition of grain boundary scattering in conjunction to acoustic phonon and alloy scattering. Furthermore, compensation between a lower κ lat and μ 0 of the longer sintered sample led to a similar z T m a x = 1.3 ± 0.18 and an only marginally reduced performance parameter β. While it is evident that Mg deficiency modifies the transport properties, the thermoelectric performance is only mildly affected and a Mg 2 (Si,Sn) based TE device can therefore withstand some Mg loss without a deterioration of its performance. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Pure phase NbFeSb 1-x Sn x (where x = 0, 0.04, 0.08, 0.12, 0.16) half-Heusler samples were prepared by direct mechanical alloying followed by spark plasma sintering. The results showed that the substitution of Sb by Sn can effectively enhance the peak figure of merit (ZT) to 0.55 at 923 K in NbFeSb 0.88 Sn 0.12 through optimized carrier concentration. To further reduce the lattice thermal conductivity (κ L), we substituted Nb by Hf to enhance the point defect scattering of phonon transport. A maximum reduction of ~80% in κ L was observed in Nb 0.8 Hf 0.2 FeSb 0.88 Sn 0.12 when compared to undoped NbFeSb at 573 K. We realized a minimum κ L of ~2.96 W m−1 K−1 at 673 K and a peak ZT of ~0.82 at 973 K for Nb 0.88 Hf 0.12 FeSb 0.88 Sn 0.12. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Nanoparticles in thermoelectric alloys has been considered as one of the most important ingredients to enhance their thermoelectric figure of merit zT mainly by reducing the lattice thermal conductivity due to intensified phonon scattering. However, the scattering mechanism of phonon with respect to wavelengths, which provides the comprehensive design rules for nanocomposites with enhanced zT , has not been fully understood. Here, we report a critical role of nanoparticles for the lattice thermal conductivity reduction from the theoretical and experimental analysis of the temperature-dependent thermal and electronic transport properties of p -type Ag/Cu nanoparticles-embedded Bi 0.5 Sb 1.5 Te 3 with respect to their electronic, bipolar, and lattice thermal conductivities. It was found that the introduction of the Ag/Cu nanoparticles reduced the lattice thermal conductivity through the additional phonon scattering based on the changeover between the Rayleigh and geometrical scatterings, indicating the indispensability of nanoparticles to scatter phonons that cannot be scattered effectively by either point defects or grain boundaries. Image, graphical abstract [ABSTRACT FROM AUTHOR]

ICME approaches provide decision support for materials design by establishing quantitative process-structure-property relations. Confidence in the decision support, however, must be achieved by establishing uncertainty bounds in ICME model chains. The quantification and propagation of uncertainty in computational materials science, however, remains a rather unexplored aspect of computational materials science approaches. Moreover, traditional uncertainty propagation frameworks tend to be limited in cases with computationally expensive simulations. A rather common and important model chain is that of CALPHAD-based thermodynamic models of phase stability coupled to phase-field models for microstructure evolution. Propagation of uncertainty in these cases is challenging not only due to the sheer computational cost of the simulations but also because of the high dimensionality of the input space. In this work, we present a framework for the quantification and propagation of uncertainty in a CALPHAD-based elastochemical phase-field model. We motivate our work by investigating the microstructure evolution in Mg 2 Si x Sn 1 − x thermoelectric materials. We first carry out a Markov Chain Monte Carlo-based inference of the CALPHAD model parameters for this pseudobinary system and then use advanced sampling schemes to propagate uncertainties across a high-dimensional simulation input space. Through high-throughput phase-field simulations we generate 200,000 time series of synthetic microstructures and use machine learning approaches to understand the effects of propagated uncertainties on the microstructure landscape of the system under study. The microstructure dataset has been curated in the Open Phase-field Microstructure Database (OPMD), available at http://microstructures.net. [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]

Suppressing the thermal conductivity of materials through the reduction of the particle sizes has long been applied to enhance the thermoelectric performance of nanostructured materials. Maintaining a high electrical conductivity in the as-formed nanostructure that is comparable to its bulk materials, is equally important in determining the final thermoelectric behavior of the nanostructure. However, this is not always attainable since the phononic and electronic transport in solids is often interdependent on each other. Naturally occurring with a complex layered structure, bulk polycrystalline Ca 3 Co 4 O 9+δ has been selected to investigate the subtle correlation between electronic and phononic subsystems, which is still challenging for complex thermoelectrics. By combining high-energy ball milling and spark plasma sintering (SPS), we generate a novel structure of CCO matrix with in-situ precipitates, in which the bar-shaped CCO grains form a three-dimensional network, and the Co 3 O 4 polyhedrons are found to be attached to the CCO grain surfaces with CaCO 3 nano-precipitates filling the network space. The CCO grains serve as electrical paths to maintain the high electrical conductivities, while the in-situ precipitates, especially CaCO 3 , have significantly blocked the phonon transport in the bulk CCO samples with nanocrystalline grains, thus greatly reducing the thermal conductivity. Electrochemical analysis and the modified Debye-Callaway calculation provide a substantial understanding for the transport of charge carriers and phonons. A record z T value of 0.5 with ultra-low thermal conductivity of 0.496 Wm−1K−1 at 973 K are achieved in the CCO-66 sample. To date, this is the highest reported z T value in undoped polycrystalline CCO material systems. [Display omitted] [ABSTRACT FROM AUTHOR]

Thermoelectric materials usually contain heavy metal elements to ensure low lattice thermal conductivity κ l. However, such elements like Pb, Bi and Sb have the expenses of high prices and potential risks of pollution. Herein, we propose a novel design strategy for thermoelectrics by constructing new light element dominated materials, such as quasi-antifluorite Li 4 OTe, with a high figure of merit surpassing 2.2. The introduction of O atom in Li 4 OTe crystal causes optical phonon softening and large phase space for three-phonon scattering processes, leading to much lower κ l (< 1 W m−1 K−1) than antifluorite Li 2 X (X=O, Te). This picture can be extended to other quasi-antifluorites, which will stimulate intense interests in the exploration of the light element dominated compounds, as new environmentally friendly and low-cost thermoelectric materials. [Display omitted] [ABSTRACT FROM AUTHOR]

Owing to the high near-room-temperature thermoelectric performance, Bi 2 Te 2.7 Se 0.3 (BTS)-based semiconductors have gained significant attention and acted as n-type materials in practical thermoelectric devices. To achieve higher thermoelectric performance in polycrystalline BTS, in this work, we employ SnSe as multifunctional micro/nanoinclusions via co-shear-exfoliating SnSe and BTS ingots and sintering the as-achieved hybrid powders into bulk materials. Experimental results indicate that the 2D-structured SnSe reduces the average size of shear-exfoliated BTS powders, leading to strengthened anisotropy of the as-sintered bulk materials, which can significantly improve the carrier mobility and in turn, the electrical conductivity. As well, introducing p-type SnSe into n-type BTS enhances the bipolar temperature of BTS, leading to higher Seebeck coefficients at higher temperatures, which contributes to a boosted power factor of >32 μW cm−1 K−2 at 448 K. Besides, the introduced SnSe micro/nanoinclusions with intrinsic low lattice thermal conductivities suppress the overall thermal conductivity of the hybrid bulk materials by providing denser phase boundaries in the matrix, and in turn strengthen the phonon scattering, which contributes to a competitive figure-of-merit ZT of ∼1.04 at 448 K and a high average ZT of 0.92, indicating the great potential for applying to practical devices. [Display omitted] [ABSTRACT FROM AUTHOR]

Phase transition induced by swift heavy ions in HfO 2 are detrimental to the reliability of HfO 2 -based aerospace devices. In this paper, the monoclinic to tetragonal phase transition in HfO 2 has been simulated by molecular dynamics combined with the two-temperature model. The specific roles of thermal spike, oxygen defects, and their coupling effects which have been considered as the key factors contributing to the phase transition in HfO 2 were reported. Thermal spike results in sharply enhanced atomic stress, triggering the phase transition. The introduction of oxygen vacancies leads to significant reduction in the stress and corresponding electronic stopping threshold for phase transition. In addition, the presence of the oxygen defects results in reduced thermal conductivity and stronger electron-phonon coupling, which enhances the energy transmission to the lattice and further lowers the electronic stopping threshold. Sufficient number of oxygen vacancies are necessary for the survival of tetragonal phase to reduce the average potential energy of atoms. [Display omitted] [ABSTRACT FROM AUTHOR]

The n -type I-V-VI 2 AgBiSe 2 features intrinsically low κ due to the anharmonicity of chemical bonds. Experimentally-determined isothermal section guides the starting compositions for the following AgBiSe 2 -based alloys. Among the undoped alloys, the Ag 25 Bi 25 Se 50 exhibits a highest peak of zT ∼0.75, and yet the neighboring Ag 20 Bi 27.5 Se 52.5 , which involves a Se-rich liquid phase, has a much lower zT ∼0.3 at 748 K, respectively. With the incorporation of Ge, the (GeSe) 0.03 (AgBiSe 2 ) 0.97 exhibits an ultralow κ∼0.3 (W/mK), owing to the formation of Bi 2 Se 3 nano-precipitate in the size of 20–40 nm. Additionally, the moiré fringes with a periodicity of 0.25 nm are observed in the Bi 2 Se 3 nano-precipitate, implying the presence of local mass fluctuation and superlattice, which could further lead to enhancing phonon scattering and reduced κ. As a result, the ultra-low κ∼0.3 (W/mK) boosts the peak of zT up to zT ∼1.05 in n -type (GeSe) 0.03 (AgBiSe 2 ) 0.97 , which shows a 140% enhancement compared with that of the undoped AgBiSe 2 . [ABSTRACT FROM AUTHOR]

Low thermal conductivity and a ferroelastic tetragonal-monoclinic phase transition are essential for the use of YTaO 4 in thermal barrier coatings. The phonon scattering mechanism of HfO 2 alloying YTaO 4 is elucidated via the analysis of microstructural characteristics, and a limit thermal conductivity (1.3 W·m−1·K−1) is achieved. Furthermore, a revised model is developed to successfully derive high-temperature phonon thermal conductivity, showing a decrease in the phonon scattering coefficient as strain field fluctuations decrease. This proves that the phonon scattering coefficient is temperature dependent. Atomic weight disorder plays a significant role in thermal conductivity reduction, whereas HfO 2 alloying enhances lattice symmetry and weakens the phonon scattering, and thus mitigates thermal conductivity reduction. Additionally, domain boundary width and spacing mismatch between neighboring domains cause the scattering of phonons and further reduce the thermal conductivity. Finally, evidence of a diffusive domain boundary validates that tetragonal-monoclinic transition is a second-order process. [Display omitted] [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]

Ag 2 Se is a promising near-room-temperature thermoelectric material with prospective applications in solid-state cooling and waste heat recovery. To advance the practical application of Ag 2 Se, it is imperative yet challenging to further boost its average dimensionless figure of merit (zT) and mechanical performance. Herein, we effectively enhance the two types of performance by constructing Ag 2 Se/AgSbSe 2 composite architectures with uniform distribution of AgSbSe 2 inclusions, which was realized by sintering solution-synthesized mixtures of Ag 2 Se nano/micro-particles and Sb 2 Se 3 nanorods. Particulate AgSbSe 2 and as-built Ag 2 Se/AgSbSe 2 interfaces enable intensified phonon scattering and impeded crack propagation, and refined grain size additionally contributes to optimized thermal and mechanical properties. As a result, (Ag 2 Se) 0.98 (2AgSbSe 2) 0.02 composite approaches a low lattice thermal conductivity of 0.15 W m–1 K–1 at 375 K and an excellent average zT of ∼0.89 between 300 and 375 K; mechanically, this composite obtains a compressive strength of 197.3 MPa, which is improved by ∼125% compared to that of Ag 2 Se and represents the highest value ever reported for this compound. This study sheds light on the efficacy of introducing secondary-phase particles on simultaneously boosting the thermoelectric and mechanical performance of Ag 2 Se. [Display omitted] [ABSTRACT FROM AUTHOR]

Al 2 O 3 nanoparticles-embedded single-crystalline Bi nanowires were successfully synthesized without using any templates via the spontaneous growth of Bi thin films covered with Al 2 O 3 nanoparticles. It is experimentally confirmed that Al 2 O 3 nanoparticles were embedded into the single-crystalline Bi nanowires by using the high-resolution transmission electron microscopy and energy dispersive X-ray spectrometry. The temperature-dependent thermal conductivities of individual Al 2 O 3 nanoparticles-embedded single-crystalline Bi nanowires were measured directly using suspended micro-devices. The thermal conductivities of the Al 2 O 3 nanoparticles-embedded Bi nanowires were found to be extremely low compared with those of pure Bi nanowires of similar diameters. Moreover, the thermal conductivity of the Al 2 O 3 nanoparticles-embedded Bi nanowires was not size-dependent, i.e., it did not vary for nanowires with significantly different diameters. This result suggests that the phonon-boundary scattering is not the dominant phonon scattering mechanism in these systems while the phonon-boundary scattering is dominant in the pure single-crystalline nanowires. From the experimental measurements and theoretical calculation, these drastic reduction and unique tendency in the thermal conductivities of Al 2 O 3 nanoparticles-embedded Bi nanowires were explained by the combined effect of the phonon-boundary scattering and impurity scattering occurring between the embedded Al 2 O 3 nanoparticles and Bi matrix. [ABSTRACT FROM AUTHOR]

Various approaches have been applied to improve the thermoelectric performance of bismuth antimony telluride based materials. Here, we prepare the bulk pellets of nominal Te-excess p-type Bi 0.4 Sb 1.6 Te 3+ x ( x = 0, 0.2, 0.4, 0.6, 0.8, 1) samples by a high-temperature solid state reaction followed by a hot-press sintering, and performe structural investigations including X-ray diffraction measurements and a high-resolution transmission electron microscopy analysis as well as a thermoelectric property characterization including electrical resistivity, Seebeck coefficient, thermal conductivity, and Hall effect. We demonstrate that a liquid-phase sintering of Te-excess samples can provide a preferential orientation of a bulk pellet along a measured direction, an improvement in hole carrier concentration, and sub-micron sized Sb precipitates acting as a phonon scattering center, resulting in an enhancement in power factor and a reduction in lattice thermal conductivity compared to the pristine x = 0 sample. Consequently, the hot-pressed bulk pellet of the nominal Bi 0.4 Sb 1.6 Te 3.4 composition exhibits an impressively high ZT of 1.41 at 417 K along the vertical direction, which is about 31 % enhancement in ZT compared to the pristine Bi 0.4 Sb 1.6 Te 3 sample. The significantly improved values of practical thermoelectric properties such as engineering (ZT) eng , maximum efficiency η max , and output power density P d indicates a practical applicability in thermoelectric energy harvesting technologies. [ABSTRACT FROM AUTHOR]

Phase diagrams provide important guidance for the optimal design of thermoelectric materials. Previous studies demonstrated a specific solubility of Ni atoms in TiNiSn and ZrNiSn, which can impede phonon transport and regulate the electronic band structure. But the thermodynamic investigation and the effect of Ni content on thermoelectric properties for the HfNiSn system are absent. In this work, we constructed an isothermal section phase diagram of the Hf-Ni-Sn ternary system at 1173 K and determined the solubility of Ni in the vacant position to be 0.04 < x < 0.16. At this temperature profile, the stoichiometric ratio HfNiSn is located in the two-phase region of HfNi 1+ x Sn + Sn when it is in equilibrium, and the homogeneity region may be broken by the process of ball milling and sintering. Calculation results indicate that the excess-Ni in HfNiSn will generate impurity levels in the forbidden band and localized phonon modes above the acoustic branch. The intermediate-Heusler phase formed by extra Ni atoms can work as a phonon scattering center and reduce the lattice thermal conductivity near room temperature, but the Sn impurities generated by the disrupted thermal equilibrium will neutralize this effect at high temperatures. Overall, the impact of the Ni-rich environment on HfNiSn-based half-Heusler is very different from that of alloys TiNiSn and ZrNiSn, which is of great significance for the comprehensive understanding of (Ti, Zr, Hf)NiSn system. [Display omitted] [ABSTRACT FROM AUTHOR]

Numerous intrinsic Ge vacancies in thermoelectric GeTe not only lead to overhigh carrier concentration but also seriously deteriorate carrier mobility, which shackles its thermoelectric performance. The efficient strategy and the related underlying mechanism in suppressing intrinsic Ge vacancy, however, are rarely researched yet. Herein, we demonstrated that lattice strain could be employed to regulate the defects concentration and then optimize electrical transport performance. Theoretically, the calculated results showed that lattice strain could efficiently raise the formation energy of Ge vacancies, weakening the carrier scattering and improving the carrier mobility. Calculated band structure revealed that Sb doping and Ge vacancy introduction could promote band convergence and thus efficiently decouple the electrical transport parameters. Experimentally, lattice strain was constructed through high-energy ball milling combined with spark plasma sintering to reduce the concentration of Ge vacancy and relaxation time of phonons, leading to high carrier mobility and low lattice thermal conductivity. Additionally, we carefully modulated the nominal content of Ge, and then a high ZT of 2.0 at 723 K in Ge 0.90 Sb 0.08 Te alloy was obtained. This work highlights that the lattice strain can be utilized to simultaneously optimize thermal and electrical transport properties. [Display omitted] [ABSTRACT FROM AUTHOR]

Liquid-assisted shear exfoliation (LASE) has been proved previously to be a facile approach for micro-texture engineering and TE performance optimization for TiS 2. Furtherly in this work, ionized impurity scattering was introduced by incorporating 4 mol% nano-AgSnSe 2 via LASE. In a specific temperature range of 373–573 K, Seebeck coefficient increased exceptionally in correspondence to an exceptional variation of electrical conductivity with growing temperature, featuring an ionized impurity scattering characteristic and contributing to a record high power factor of 18.2 μW cm−1 K−2 at 573 K in the LASE-derived nanocomposite. Moreover, lattice thermal conductivity was further reduced because of enhanced phonon scattering due to intercalation, impurity, and enhanced grain boundary density. Ultimately, a highest ZT of 0.78 at 673 K as a renewed record for TiS 2 -based materials was achieved, demonstrating an improved effectiveness of the combination of LASE and ionized impurity scattering in optimization of TE performance of TiS 2 -like layered TE materials. [Display omitted] [ABSTRACT FROM AUTHOR]

Mechanical strain is expected to impact the lattice thermal conductivity of crystalline solids by modifying lattice vibration properties. Tin oxide is one key base material in the family of transparent conducting oxides, and its thermal transport properties are of technological importance to device performance. In this work, we investigate the lattice thermal transport of tin oxide (SnO 2) under uniaxial strain by solving the phonon Boltzmann transport equation based on first-principles calculations. We find that the lattice thermal conductivity of tin oxide decreases monotonically as moving from uniaxial compression to tension. We disclose that strain affects remarkably the phonon group velocity and lifetime of optical modes, with two dominant optical modes identified. The behavior is in-depth understood by looking into the influence of strain on the number of the allowed three-phonon scattering processes, lattice anharmonicity, and bonding strength. This work provides quantitative understanding of strain dependent thermal conductivity of tin oxide, with insights into the strain effect on phonon properties of bulk crystalline materials. [Display omitted] [ABSTRACT FROM AUTHOR]

This work proves that scattering of elastic waves in the dislocation core induces an unusual behavior called stress-drop , which is defined for the first time in this study as a phenomenon where an externally applied stress drops inside a nano-sized cubic metal during dislocation motion. We develop a theoretical phonon scattering model based on discrete lattice dynamics and derive simple analytical equations to quantify the magnitude of the stress-drop. The proposed model is supported by atomistic simulations of perfect dislocations in bcc iron, where the stress-drop resulting from bond breaking is inversely proportional to the square of the dislocation speed. The derived equation is in excellent agreement with direct atomistic simulations for edge and screw dislocations. Next, we extend the equation to the edge dislocation in fcc aluminum where the dislocation exists as a stacking fault ribbon surrounded by two Shockley partial dislocations and thus the interaction between them is important. The extended equation accurately predicts the phonon scattering, resulting in the stress-drop by the oscillation of the two partials as well as bond breaking. In addition to the discussion on the validity of our model and equations at absolute zero, we investigate the temperature and size effects on our equations. As temperature increases, the magnitude of the stress-drop decreases and converges to zero even at very low temperature because of the extremely small Peierls barrier. Moreover, the magnitude of the stress-drop decreases, as the thickness of the nanoplate increases, which shows that the stress-drop is a mechanical behavior that is prominent on the nanoscale. [ABSTRACT FROM AUTHOR]

• An experimental and computational study on half-Heusler V(Fe 1 − x Ru x)Sb compounds are investigated. • An increase in room-temperature PF by ca. 25% was obtained for the x = 0.02 sample, which is consistent with the predicted doping level of Ru substitution in from the electronic structure analysis. • The minimum κ L value of 6.0 W/mK was achieved for the x = 0.08 sample at room temperature. • The room-temperature z T is improved from 0.1 to the value close to 0.2. [Display omitted] The effects of ruthenium (Ru) substitution on the thermoelectric properties of half-Heusler (HH) V(Fe 1 − x Ru x)Sb compounds have been theoretically and experimentally investigated. All V(Fe 1 − x Ru x)Sb samples with deficient HH structures reveal deficiencies in V 4 a and Fe 4 c sites, interstitial Fe occupied 4 d sites, and Ru substituted for Fe 4 c sites, by Rietveld analysis. Based on the refined crystal structures and electronic structure analysis, the Ru content x with a maximum power factor (PF) is predicted to be around x = 0.02. A small amount of Ru-substitution improves the PF by 26% at room temperature for the V(Fe 0.98 Ru 0.02)Sb sample, which is coincident with the predicted range of Ru content for maximum PF. The lattice thermal conductivity κ L of the V(Fe 1 − x Ru x)Sb samples decreases with an increase in x. The atomic mass and radius of Ru are substantially larger than those of Fe; therefore, κ L of the V(Fe 1 − x Ru x)Sb samples decreases mainly due to phonon scattering strengthened by the introduction of Ru at the Fe 4 c sites. A significantly reduced κ L of 5.8 W/Km at room temperature is obtained for the V(Fe 0.92 Ru 0.08)Sb sample, which is a ca. 40% decrease compared with the pristine HH-VFeSb sample. The dimensionless thermoelectric figure-of-merit reaches close to 0.2 at 300 K for the V(Fe 0.98 Ru 0.02)Sb sample. [ABSTRACT FROM AUTHOR]

This work aims at a quantitative and mechanistic understanding of the dynamic process of the phonon-dislocation interaction in PbTe/PbSe (001) heterostructures using the Concurrent Atomistic-Continuum (CAC) method as the simulation tool. The misfit dislocation network and the atomic-scale dislocation core structure obtained in the simulations are found to agree reasonably well with the experimental observations of the PbTe/PbSe (001) interface. Through visualizing the dynamic interaction between phonons and dislocations, as well as quantifying the dislocation vibration amplitude, the phonon energy transmission, and the thermal resistance of the misfit interfaces, this work has illustrated and quantified two mechanisms for phonon-dislocation interaction: (1) phonon scattering by the strain field of dislocations, and (2) phonon scattering by dislocations that vibrate via the local modes of a dislocation network; the latter, leads to resonant phonon-dislocation interaction, which is manifested as local maxima of out-of-phase vibration of the atoms on the two sides of the slip plane, leading to local minima of the energy transmission in the heterostructure that contains one interface. The local vibrational modes are found to be excited only by shear stress induced by transverse phonons. Among various resonant modes, the one with the lowest frequency has the strongest effect. This work has also demonstrated the collective motion of dislocations under ultrafast phonon pulses. In addition, the dynamic properties of the misfit dislocation network localized within one interface are found to be significantly altered by the presence of misfit dislocations at other interfaces, thus further confirming the cooperative dynamic nature of the motion of dislocations and phonons. [Display omitted] [ABSTRACT FROM AUTHOR]

Rare-earth niobates and tantalates possess low thermal conductivities, which can be further reduced in high-entropy compositions. Here, a large number of 40 compositions are synthesized to investigate the origin of low thermal conductivity. Of these, 29 possess single (nominally cubic) fluorite phases and most of them are new compositionally complex (medium- or high-entropy) compositions. Furthermore, doping 2% of light element cations can further reduce thermal conductivity. This large data set enables the discovery of a negative correlation between the thermal conductivity and averaged radius ratio of the 3+/5+ cations. While this ratio is still below the threshold for forming long-range ordered weberite-type phases, this correlation suggests the reduced thermal conductivity is related to short-range weberite-type order, which is indeed revealed by diffuse scattering in X-ray diffraction and neutron total scattering. Specifically, neutron total scattering is used to characterize five selected specimens. A better fit to a weberite-type structure is found at the nanoscale. The characteristic length (domain size) is appears to be larger in more insulative materials. As it approaches the Ioffe-Regel limit, the phonon limit breaks down and "diffusons" give rise to the observed amorphous-like thermal conductivity. [Display omitted] [ABSTRACT FROM AUTHOR]

Thermoelectric power generation (TEG) represents one of the cleanest methods of energy conversion available today. It can be used in applications ranging from the harvesting of waste heat to conversion of solar energy into useful electricity. Remarkable advances have been achieved in recent years for various thermoelectric (TE) material systems. The introduction of nanostructures is used to tune the transport of phonons, while band structure engineering allows for the tailoring of electron transport. In this overview, top-down approaches to phonon engineering, such as atomic construction of new materials, will be reviewed. Bottom-up approaches to electron engineering, such as the formation of ordered nanostructures, will also be discussed. The assembly of TEG devices is still particularly challenging, and consequently, thermal-to-electric conversion utilizing these devices has been realized only in niche applications. In this review paper, we will discuss some of the challenges that must be overcome to enable widespread use of TE devices. These include thermal stability at the material level, and reliable contact at the device level. [ABSTRACT FROM AUTHOR]

Oxygen-containing semiconductors are considered to be promising thermoelectric materials because of the high physicochemical stability. New layered oxyselenide Bi 6 Cu 2 Se 4 O 6 possesses intrinsic low lattice thermal conductivity and can be modulated to n -type semiconductor through halogen doping, which is considered as a potential n -type thermoelectric oxide. In this work, first-principles calculations are utilized to obtain the electronic and phonon band structures for deeply recognizing the transport features of intrinsic Bi 6 Cu 2 Se 4 O 6. Furthermore, Nb is selected as donor dopant in Bi 6 Cu 2 Se 3.6 Cl 0.4 O 6 , realizing a significantly improved power factor of ∼ 4.3 μV cm−1 K−2, which originates from the simultaneously optimized carrier concentration by effective doping and strengthened effective mass by inducing resonant levels around Fermi level. Meanwhile, the enhancement of phonon scattering are introduced, leading to a low lattice thermal conductivity of ∼ 0.68 W m−1 K−1. Finally, thanks to the effective doping of Nb and the introduction of resonant levels, a maximum ZT ∼ 0.4 at 873 K and an average ZT ∼ 0.21 from 303 to 873 K can be obtained in n -type Bi 5.91 Nb 0.09 Cu 2 Se 3.6 Cl 0.4 O 6. This study broadens the applicability of resonant levels to optimize performances of thermoelectric oxides, and promotes the potential application of n -type Bi 6 Cu 2 Se 4 O 6 -based thermoelectric materials in medium temperature range. [Display omitted] [ABSTRACT FROM AUTHOR]

As a kind of promising medium-temperature thermoelectric materials, SnTe has been widely studied in recent years. Herein, we investigate the effect of LiSbTe 2 and LiBiTe 2 alloying on the thermoelectric and mechanical properties of SnTe. Hall effect measurement and DFT calculation show that both LiSbTe 2 and LiBiTe 2 alloying induce band convergence, optimizing the electrical transport properties. Nevertheless, the excessive narrowing of band gap caused by LiBiTe 2 alloying deteriorates the high-temperature performance significantly. Numerous nano-precipitates and Li-rich nano-domains are formed and play important roles in enhancing phonon scattering. Furthermore, via introducing resonant state by Indium doping, relatively high PF ave of 19.54 µW cm−1 K −2 and ZT ave of 0.64 from 300 K to 873 K are achieved in Sn 0.99 In 0.01 Li 0.125 Sb 0.125 Te 1.25 sample. However, behind the high thermoelectric performance is the sacrifice of mechanical properties. The fracture toughness falls off a cliff in heavily alloyed sample due to the high density of point defects and nanostructures within the matrix, which greatly increases the risk of fracture under external force or thermal stress during processing or service. This work proposes that the compromise between thermoelectric properties and mechanical properties should be paid attention to during heavily alloying, especially for those materials with inherent poor mechanical performance. [Display omitted] [ABSTRACT FROM AUTHOR]

Metastable cubic SnSe has received extensive attention in terms of theoretical predictions since orthorhombic SnSe was experimentally developed as one of the most promising thermoelectric candidates in recent years. In this work, we successfully stabilized the cubic SnSe through AgSbTe 2 alloying, after which the thermoelectric transport properties were significantly optimized by simultaneously suppressing lattice and electronic thermal conductivities via introducing Pb. The maximum power factor of (SnSe) 0.6 (AgSbTe 2) 0.4 reached to 13 μWcm−1K−2 at 723 K, with a high average power factor of ∼ 12 μWcm−1K−2 at 300–773 K. Based on 40% AgSbTe 2 alloying, the strong phonon-defect scattering and low electronic thermal conductivity caused by Pb alloying contribute greatly to the reduction of total thermal conductivity, while the excellent electrical transport properties were substantially maintained. Finally, the peak ZT value reached to 1.20 and a high average ZT value of ∼ 0.90 over 300–773 K were achieved in the cubic phase (SnSe) 0.6 (AgSbTe 2) 0.4 with 10% Pb alloyed. [Display omitted] [ABSTRACT FROM AUTHOR]

We present a study on the dimensionless thermoelectric figure-of-merit (ZT) of nanostructured YbxCo4Sb12 (x = 0.2, 0.35, 0.45. 0.5 and 0.6) made by hot pressing the nanopowder obtained from ball milling an ingot in comparison with that made directly from ball milling the elements. It was found that the electrical conductivity of the sample made from the ingot is higher and the thermal conductivity is lower compared to the sample made from directly ball milling the elements. It is believed that the higher electrical conductivity is due to the presence of fewer defects, while the lower thermal conductivity may be due to stronger phonon scattering from the different grain sizes. We have achieved a peak ZT of ~1.4 at 550 °C in Yb0.35Co4Sb12. [ABSTRACT FROM AUTHOR]

Abstract: Ge-rich Ge1− x Pb x Te0.5Se0.5 (x =0, 0.1, 0.2, 0.25, 0.3, 0.4 and 0.5) alloys with high thermoelectric performance were prepared through a conventional melting, solutioning and quenching route, followed by a spark plasma sintering technique. Se doping extends the solid solubility of Pb in GeTe. The samples maintain the rhombohedral GeTe-based solid solution up to x =0.3, while the samples with x =0.4 and 0.5 are composites containing the major rhombohedral GeTe-based supersaturated solid solution and a mixture of the minor NaCl-type PbTe-based and rhombohedral GeTe-based phases separated from the supersaturated solid solution. All the samples show p-type conduction. The solute Pb and Se atoms in the GeTe lattice may modify the carrier concentration, leading to a significant increase in the Seebeck coefficient. The solute Pb and Se atoms, twinning microstructures and the grain boundaries or secondary phase may form broad-based phonon scattering centers, leading to alloys with extremely low thermal conductivity. In the results, the overall high thermoelectric properties were obtained in the alloys due to (Pb, Se) co-doping. A maximum ZT of 1.6 at 673K was eventually achieved in Ge0.75Pb0.25Te0.5Se0.5; this value is higher than that of pure PbTe or GeTe compounds and any other reported PbTe–GeTe composites. [Copyright &y& Elsevier]

Abstract: Nanostructured thermoelectric materials offer promising levels of conversion efficiency due to enhanced phonon scattering at interfaces, and a good understanding of the formation and evolution of their microstructure is important. In this paper, we analyze precipitate morphologies of pseudobinary – thermoelectric compounds using energetics analysis, combining the equivalent inclusion method and phase-field simulations. It is found that precipitates embedded in matrix tend to be lamellar, ribbon-like or needle-like in shape to minimize the potential energy, and tend to lie on a crystallographical cone around the body diagonal of the lattice. Furthermore, phase-field simulations reveal that interfacial energy plays an important role in precipitate morphology, with low interfacial energy favoring needle-like precipitates, while intermediate interfacial energy favors ribbon-like ones. Good agreements with experimental data and compatibility analysis are observed. [Copyright &y& Elsevier]