We present a comprehensive experimental and theoretical study of phonon scattering by nanoprecipitates in potassium-doped PbTe, PbSe, and PbS. We highlight the role of the precipitate size distribution measured by microscopy, whose tuning allows for thermal conductivities lower than the limit achievable with a single size. The correlation between the size distribution and the contributions to thermal conductivity from phonons in different frequency ranges provides a physical basis to the experimentally measured thermal conductivities, and a criterion to estimate the lowest achievable thermal conductivity. The results have clear implications for efficiency enhancements in nanostructured bulk thermoelectrics.Keywords: lead chalcogenide; phonon; precipitate interface; thermal transport; thermoelectric;

AbstractLead telluride has long been realized as an ideal p-type thermoelectric material at an intermediate temperature range; however, its commercial applications are largely restricted by its n-type counterpart that exhibits relatively inferior thermoelectric performance. This major limitation is largely solved here, where it is reported that a record-high ZT value of ≈1.83 can be achieved at 773 K in n-type PbTe-4%InSb composites. This significant enhancement in thermoelectric performance is attributed to the incorporation of InSb into the PbTe matrix resulting in multiphase nanostructures that can simultaneously modulate the electrical and thermal transport. On one hand, the multiphase energy barriers between nanophases and matrix can boost the power factor in the entire temperature range via significant enhancement of the Seebeck coefficient and moderately reducing the carrier mobility. On the other hand, the strengthened interface scattering at the intensive phase boundaries yields an extremely low lattice thermal conductivity. This strategy of constructing multiphase nanostructures can also be highly applicable in enhancing the performance of other state-of-the-art thermoelectric systems.

•We directly observed a large number of off-stoichiometric point defects in single crystalline SnSe.•Our findings help to understand why theoretical simulations show higher thermal conductivities than experimental ones.•We present a quantitative explanation of the experimentally observed ultralow thermal conductivity in SnSe single crystals.Single crystalline tin selenide (SnSe) recently emerged as a very promising thermoelectric material for waste heat harvesting and thermoelectric cooling, due to its record high figure of merit ZT in mediate temperature range. The most striking feature of SnSe lies in its extremely low lattice thermal conductivity as ascribed to the anisotropic and highly distorted Sn-Se bonds as well as the giant bond anharmonicity by previous studies, yet no theoretical models so far can give a quantitative explanation to such low a lattice thermal conductivity. In this work, we presented direct observation of an astonishingly vast number of off-stoichiometric Sn vacancies and Se interstitials, using sophisticated aberration corrected scanning transmission electron microscope; and credited the previously reported ultralow thermal conductivity of the SnSe single crystalline samples partly to their off-stoichiometric feature. To further validate the conclusion, we also synthesized stoichiometric SnSe single crystalline samples, and illustrated that the lattice thermal conductivity is deed much higher as compared with the off-stoichiometric single crystals. The scattering efficiency of individual point defect on heat-carrying phonons was then discussed in the state-of-art Debye-Callaway model.Download high-res image (296KB)Download full-size image

The relatively inferior performance of n-type legs impedes the application of PbTe materials in intermediate temperature thermoelectric devices. In order to elevate the thermoelectric properties of n-type PbTe, we added some Sb phases into 0.1% PbI2-doped PbTe by a conventional melting method. Transmission electron microscopy (TEM) analysis together with density function theory (DFT) calculations showed that endotaxial Sb nanoprecipitates were produced in the PbTe samples at room temperature, and that part of these nanoprecipitates gradually dissolved into the PbTe matrix to form SbPb–SbTe dual-site substitutional point defects as temperature increased. A maximum ZT of about 1.8 was achieved at 773 K for the n-type PbTe0.998I0.002–3%Sb composite due to a simultaneous improvement in power factor and reduction in lattice thermal conductivity. In the PbTe0.998I0.002–x%Sb (x = 1–4) composite samples, the Seebeck coefficient was much higher than that of the reported single-phase PbTe samples with similar carrier concentration, which mainly originated from distortion of the density-of-states caused by Sb dual-site doping. Simulations based on the Callaway model suggested that the SbPb–SbTe dual-site substitutional point defects also played an important role in decreasing the lattice thermal conductivity at elevated temperature. We propose that the synergistic role of Sb in both electrical and thermal transport should be highly applicable in other bulk thermoelectric materials.

In the past several years, metal sulfides have been the subject of extensive research as promising thermoelectric materials with high potential in future commercial applications due to their low cost, low toxicity, and abundance. This review summarizes recent developments and progress in the research of metal sulfides, particularly for binary metal sulfides such as Bi2S3, Cu2−xS, and PbS. Methods for improving the thermoelectric properties of these binary sulfides are emphasized, and promising strategies are suggested to further enhance the thermoelectric figure of merit of these materials.

In this contribution, we report that an extremely low lattice thermal conductivity can be achieved in a Ge0.55Pb0.45Te-based thermoelectric system through gradually replacing Te by Se. It was revealed that substitution of Se promotes the solid solubility of Pb in GeTe, hence leading to a reduction of PbTe precipitation. The PbTe precipitation was found to completely disappear when 60% of the Te was replaced by Se. In the optimized composition alloy (x = 0.5), the substantial percentage of solute Pb and Se atoms in the GeTe-based phase, together with the twinning microstructures in the GeTe-based matrix phase, enhances phonon scattering sufficiently, leading to extremely low thermal conductivities (0.67 and 0.45 W m–1 K–1 at at 300 and 723 K, respectively), which are the lowest ever reported. The maximum ZT of 1.55 at 723 K was obtained in the alloy Ge0.55Pb0.45Te0.5Se0.5, which is 8 times higher than that of its Se-free counterpart Ge0.55Pb0.45Te.

A recipe for facile preparation of high-performance low-temperature magnetic refrigerants based on gadolinium-hydroxy-chloride (GHC), which exhibits a large magnetic entropy change of up to 61.8 J kg−1 K−1 or 318.9 mJ cm−3 K−1, high thermal stability, strong alkali-resistance and good thermal conductivity, is reported.

•Microstructure features of BiCuSeO system were investigated by Cs corrected STEM.•DFT calculations revealed that Ba–Bi point defects are easy to form.•Callaway's model was employed to calculate the phonon transport.•Microstructure with different length scale effectively scattered phonons.Heavily doped Bi0.875Ba0.125CuSeO alloys were found to exhibit the highest ever ZT value in oxygenous thermoelectric systems, attributed to the extremely low thermal conductivity. In this report, we investigated the microstructural reason of the thermal conductivity in Ba- heavily doped BiCuSeO through scanning transmission electron microscopy (STEM). We found a large amount of nano-scale BaSeO3 precipitates dispersed widely in BiCuSeO matrix grains; besides, for the first time, we provide visual evidence of Ba substituting Bi atoms in Bi–O layers. Combined with DFT calculations, we conclude that intrinsic lattice vibration anharmonicity, together with Ba–Bi alloying and excess BaSeO3 precipitation, is responsible for the observed low lattice thermal conductivity in experiments.Table of contents Introduced Ba partially dissolves in the BiCuSeO matrix through occupying Bi lattices in (Bi2O2)2+ layers, while the excessive part forms high density precipitates, resulting in the coexistence of BaSeO3 precipitates and BaBi substitutional point defects that are responsible for the extremely low lattice thermal conductivity in Ba doped BiCuSeO system.

Lead chalcogenides are dominant thermoelectric materials in the medium-temperature range due to their highly favorable electronic band structures and low thermal conductivities. An important system is the PbTe–PbS pseudo-binary, and its low thermal conductivity originates largely from the coexistence of both alloying and nanostructuring through phase-separation. To better understand the competition between the alloying and phase separation and its pronounced effects on the thermoelectric performance in PbTe–PbS, we systematically studied, via transmission electron microscopy (TEM) observations and theoretical calculations, the samples of Spark Plasma Sintered (SPSed) 3 at% Na-doped (PbTe)1−x(PbS)x with x = 10%, 15%, 20%, 25%, 30% and 35%. The highest figure of merit, viz., ZT ∼ 2.3 was obtained at 923 K, when the PbS phase-fraction, x, was 20%, which corresponds to the lowest lattice thermal conductivity of the series. The consistently lower lattice thermal conductivities in the SPSed samples as compared with the corresponding ingots originates from the mesostructured nature of the former, which contributes significantly to their superior ZT. We also studied the onset of carrier concentration modulation at ∼600 K, which leads to the observed saturation of electrical transport properties due to the diffusion and re-dissolution of excessive Na into the PbTe–PbS matrix. This carrier concentration modulation is equally crucial to achieve very high power factors (up to 26.5 μW cm−1 K−2 at 623 K) and outstanding thermoelectric performances in SPSed PbTe–PbS binaries.

•Structural imaging and chemical determination of “imperfectness” from grain to atom for phonon transport.•Electronic structure investigation to unravel the physics of electron transport.•3-D imaging of nanostructures, dynamic of microstructures and atomic-scale mapping of Seebeck coefficient and defects.•Calculations on structure–property relationship.•Perspectives of new advanced microscopy techniques in thermoelectric materials.Thermoelectric (TE) materials can interconvert waste heat into electricity, thus are promising for power generation and solid-state refrigeration. The thermoelectric properties of a certain material strongly correlate with its chemical, structural and electronic features; therefore, a thorough characterization of these features is not only crucial to profoundly understand the material itself, but also helps to design new materials with desired properties. Under this circumstance, various electron microscopy (EM) techniques are developed, from micro-scale to atomic-scale, two-dimensional (2-D) to 3-D, and static to dynamic. In this review, we review advanced EM techniques already applied in and also look into the perspective of introducing more EM techniques into the field of thermoelectrics. Specifically, we firstly summarize “what have been done” involving: structural and chemical characterizations of all-scale “imperfectness”, electronic structure investigation, 3-D morphology and dynamic evolution of nanostructures, and atomic-scale mapping of Seebeck coefficient and defects; based on these characterized features, we then briefly review the calculations on electrical and thermal transport properties to illustrate the structure–property correlations. In what follows, we propose “what can be done” in TEs via EM techniques including: valence-electron distribution, quantitative measurement of atomic displacement, point defect characterization, local band gap measurement, phonon excitation detection, electrostatic potential determination, thermal stability of nanostructures, and in-situ observation and measurement of local TE effects.Thermoelectric power generation, harvesting the widely distributed heat and converting it into electricity, is a promising energy-crisis solution. "Thermoelectric" is a research hotspot in recent fifty years. The properties of a thermoelectric material strongly correlate with its structural features, e.g., external “imperfectness” and intrinsic electronic configurations. Researchers have been realizing the importance of “structure” on “TE”. Advanced electron microscopy can provide chemical, structural and electronic information of materials, and are receiving growing attentions to characterize various structural features of TE materials, which are crucial to understand the material physically and design new materials with desired properties.

As a lead-free material, GeTe has drawn growing attention in thermoelectrics, and a figure of merit (ZT) close to unity was previously obtained via traditional doping/alloying, largely through hole carrier concentration tuning. In this report, we show that a remarkably high ZT of ∼1.9 can be achieved at 773 K in Ge0.87Pb0.13Te upon the introduction of 3 mol % Bi2Te3. Bismuth telluride promotes the solubility of PbTe in the GeTe matrix, thus leading to a significantly reduced thermal conductivity. At the same time, it enhances the thermopower by activating a much higher fraction of charge transport from the highly degenerate Σ valence band, as evidenced by density functional theory calculations. These mechanisms are incorporated and discussed in a three-band (L + Σ + C) model and are found to explain the experimental results well. Analysis of the detailed microstructure (including rhombohedral twin structures) in Ge0.87Pb0.13Te + 3 mol % Bi2Te3 was carried out using transmission electron microscopy and crystallographic group theory. The complex microstructure explains the reduced lattice thermal conductivity and electrical conductivity as well.

We report a greatly enhanced thermoelectric performance in a BiCuSeO system, realized by improving carrier mobility through modulation doping. The heterostructures of the modulation doped sample make charge carriers transport preferentially in the low carrier concentration area, which increases carrier mobility by a factor of 2 while maintaining the carrier concentration similar to that in the uniformly doped sample. The improved electrical conductivity and retained Seebeck coefficient synergistically lead to a broad, high power factor ranging from 5 to 10 μW cm–1 K–2. Coupling the extraordinarily high power factor with the extremely low thermal conductivity of ∼0.25 W m–1 K–1 at 923 K, a high ZT ≈ 1.4 is achieved in a BiCuSeO system.

We present nanocrystalline PbS, which was prepared using a solvothermal method followed by spark plasma sintering, as a promising thermoelectric material. The effects of grains with different length scales on phonon scattering of PbS samples, and therefore on the thermal conductivity of these samples, were studied using transmission electron microscopy and theoretical calculations. We found that a high density of nanoscale grain boundaries dramatically lowered the thermal conductivity by effectively scattering long-wavelength phonons. The thermal conductivity at room temperature was reduced from 2.5 W m−1 K−1 for ingot-PbS (grain size >200 μm) to 2.3 W m−1 K−1 for micro-PbS (grain size >0.4 μm); remarkably, thermal conductivity was reduced to 0.85 W m−1 K−1 for nano-PbS (grain size ~30 nm). Considering the full phonon spectrum of the material, a theoretical model based on a combination of first-principles calculations and semiempirical phonon scattering rates was proposed to explain this effective enhancement. The results show that the high density of nanoscale grains could cause effective phonon scattering of almost 61%. These findings shed light on developing high-performance thermoelectrics via nanograins at the intermediate temperature range.

One of the intellectual challenges for next generation thermoelectric materials revolves around the synthesis and fabrication of hierarchically organized microstructures that do not appreciably compromise the innate high power factor of the chosen thermoelectric system, but significantly reduce lattice thermal conductivity to enhance the overall figure of merit, ZT. An effective emerging strategy is to introduce nanostructures into bulk thermoelectric materials, which allow for diverse phonon scattering mechanisms to reduce thermal conductivity. In this review, we present key examples to show the intricate but tractable relationship across all relevant length-scales between various microstructural attributes (point, line, interfacial and mesoscale defects; as well as associated elastic and plastic strain) and lattice thermal conductivity in systems based on PbTe matrices. We emphasize the need for an overarching panoscopic approach that enables specific design strategies for the next generation of thermoelectric materials.

The solubility of sodium and its effects on phonon scattering in lead chalcogenide PbQ (Q = Te, Se, S) family of thermoelectric materials was investigated by means of transmission electron microscopy and density functional calculations. Among these three systems, Na has the highest solubility limit (∼2 mol %) in PbS and the lowest ∼0.5 mol %) in PbTe. First-principles electronic structure calculations support the observations, indicating that Na defects have the lowest formation energy in PbS and the highest in PbTe. It was also found that in addition to providing charge carriers (holes) for PbQ, Na introduces point defects (solid solution formation) and nanoscale precipitates; both reduce the lattice thermal conductivity by scattering heat-carrying phonons. These results explain the recent reports of high thermoelectric performance in p-type PbQ materials and may lead to further advances in this class of materials.

We present in this manuscript that enhanced thermoelectric performance can be achieved in polycrystalline SnSe prepared by hydrothermal reaction and spark plasma sintering (SPS). X-ray diffraction (XRD) patterns revealed strong orientation along the [l 0 0] direction in bulk samples, which was further confirmed by microstructural observation through transmission electron microscopy (TEM) and field emission scanning electron microscopy (FESEM). It was noticed that the texturing degree of bulk samples could be controlled by sintering temperature during the SPS process. The best electrical transport properties were found in the sample which sintered at 450 °C in the direction vertical to the pressing direction, where the highest texturing degree and mass density were achieved. Coupled with the relatively low thermal conductivity, an average ZT of ∼ 0.38, the highest ever reported in pristine polycrystalline SnSe was obtained. This work set up a forceful example that a texture-control approach can be utilized to enhance the thermoelectric performance effectively.

A recipe for facile preparation of high-performance low-temperature magnetic refrigerants based on gadolinium-hydroxy-chloride (GHC), which exhibits a large magnetic entropy change of up to 61.8 J kg−1 K−1 or 318.9 mJ cm−3 K−1, high thermal stability, strong alkali-resistance and good thermal conductivity, is reported.