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Advanced materials 35(19), 2300893 (2023). doi:10.1002/adma.202300893, Published by Wiley-VCH, Weinheim

Nature materials (2021). doi:10.1038/s41563-021-01064-6, Published by Nature Publishing Group, Basingstoke

The population of all non-equilibrium lattice defects in materials is referred to as microstructure. Examples are point defects such as substitutional and interstitial atoms, and vacancies; line defects such as dislocations; planar defects such as interfaces and stacking faults; or mesoscopic defects such as second-phase precipitates. These types of lattice imperfections are usually described in terms of their structural features, breaking the periodicity of the otherwise regular crystalline structure. Recent analytical probing at the nanoscale has revealed that their chemical features are likewise important and characteristic. The structure of the defects as well as their individual chemical composition, that is their chemical decoration state, which results from elemental partitioning with the adjacent matrix, can significantly influence the electrical and thermal transport properties of thermoelectric materials. The emergence of atom probe tomography (APT) has now made routinely accessible the mapping of three-dimensional chemical composition with sub-nanometer spatial accuracy and elemental sensitivity in the range of tens of ppm. Here, we review APT-based investigations and results related to the local chemical decoration states of various types of lattice defects in thermoelectric materials. APT allows to better understand the interplay between thermoelectric properties and microstructural features, extending the concept of defect engineering to the field of segregation engineering so as to guide the rational design of high-performance thermoelectric materials.

The thermoelectric (TE) community has mainly focused on improving the figure of merit (ZT) of materials. However, the output power of TE devices directly depends on the power factor (PF) rather than ZT. Effective strategies of enhancing PF have been elusive for Bi2Te3-based compounds, which are efficient thermoelectrics operating near ambient temperature. Here, we report ultrahigh carrier mobility of ∼467 cm2 V-1 s-1 and power factor of ∼45 μW cm-1 K-2 in a new n-type Bi2Te3 system with nominal composition CuxBi2Te3.17 (x = 0.02, 0.04, and 0.06). It is obtained by reacting Bi2Te3 with surplus Cu and Te and subsequently pressing powder products by spark plasma sintering (SPS). The SPS discharges excess Te but stabilizes the high extent of Cu in the structure, giving unique SPS CuxBi2Te3.17 samples. The analyzed composition is close to "CuxBi2Te3". Their charge transport properties are highly unusual. Hall carrier concentration and mobility simultaneously increase with the higher mole fraction of Cu contrary to the typical carrier scattering mechanism. As a consequence, the electrical conductivity is considerably enhanced with Cu incorporation. The Seebeck coefficient is nearly unchanged by the increasing Cu content in contrast to the general understanding of inverse relationship between electrical conductivity and Seebeck coefficient. These effects synergistically lead to a record high power factor among all polycrystalline n-type Bi2Te3-based materials.

Bi2Te3-based materials are a representative thermoelectric system operating near ambient temperature. Their n-type family is very limited and underperforms the p-type counterpart, which is a major concern in the thermoelectric community. Here we report that alkali earth metals (AE = Mg, Ca, Sr, Ba) in Group 2 unusually induce n-type thermoelectric properties in Bi2Te3. Mg is the most efficient electron donor among them. The x = 0.01 sample of MgxBi2-xTe3 (x = 0.005, 0.01, 0.015) system exhibits remarkably enhanced power factor and ZT of ~0.8 at 350 K in comparison with pristine Bi2Te3. The improved performance is attributed to simultaneously enhanced electrical conductivity and reduced lattice thermal conductivity. Remarkably, MgxBi2-xTe3 materials show larger Seebeck coefficients than those expected by the theoretical Pisarenko relation.

Lattice defects are typically used to tailor the thermoelectric properties of materials. It is desirable that such defects improve the electrical conductivity, while, at the same time, reduce the thermal conductivity, for an overall improvement on the thermoelectric properties of materials. Here, we report on an extended defect in Sb2Te3 consisting of swapped bilayers with chemical intermixing of Sb and Te atoms, which can be generated and effectively manipulated in polycrystalline samples through synthetic methods and thermal treatments. The swapped bilayers bridge the spatial gaps between the Sb2Te3 quintuple-layer blocks, enhancing the charge carrier mobility and thus the electrical conductivity. These defects also result in a reduced lattice thermal conductivity through suppressing phonon transport. These synergistic effects contribute together to an improved thermoelectric quality factor and an enhanced figure of merit (zT) value in Sb2Te3.

Thermoelectric materials with high average power factor and thermoelectric figure of merit (ZT) has been a sought-after goal. Here, we report new n-type thermoelectric system CuxPbSe0.99Te0.01 (x =...

From a structural and economic perspective, tellurium-free PbSe can be an attractive alternative to its more expensive isostructural analogue of PbTe for intermediate temperature power generation. Here we report that PbSe0.998Br0.002-2%Cu2Se exhibits record high peak ZT 1.8 at 723 K and average ZT 1.1 between 300 and 823 K to date for all previously reported n- and p-type PbSe-based materials as well as tellurium-free n-type polycrystalline materials. These even rival the highest reported values for n-type PbTe-based materials. Cu2Se doping not only enhance charge transport properties but also depress thermal conductivity of n-type PbSe. It flattens the edge of the conduction band of PbSe, increases the effective mass of charge carriers, and enlarges the energy band gap, which collectively improve the Seebeck coefficient markedly. This is the first example of manipulating the electronic conduction band to enhance the thermoelectric properties of n-type PbSe. Concurrently, Cu2Se increases the carrier conce...

Bi0.5Sb1.5Te3/Cu core/shell powders were prepared by electroless plating and hydrogen reduction, and then sintered into bulk by spark plasma sintering. After electroless plating, with increasing the Cu content, the electrical conductivity keeps enhancing significantly. The highest electrical conductivity reaches 3341S/cm at room temperature in Bi0.5Sb1.5Te3 with 0.67wt% Cu bulk sample. Moreover, the lowest lattice thermal conductivity reaches 0.32 W/m·K at 572.2 K in Bi0.5Sb1.5Te3 with 0.67wt% Cu bulk sample, which is caused by the scattering of the rich-copper particles with different dimensions and massive grain boundaries. According to the results, the ZT values of all Bi0.5Sb1.5Te3/Cu bulk samples have improved in a high temperature range. In Bi0.5Sb1.5Te3 with 0.15wt% Cu bulk sample, the highest ZT value at 573.4 K is 0.81. When the Cu content increases to 0.67wt%, the highest ZT value reaches 0.85 at 622.2 K. Meanwhile, the microhardness increases with increasing the Cu content.

Introducing structural defects such as vacancies, nanoprecipitates, and dislocations is a proven means of reducing lattice thermal conductivity. However, these defects tend to be detrimental to carrier mobility. Consequently, the overall effects for enhancing ZT are often compromised. Indeed, developing strategies allowing for strong phonon scattering and high carrier mobility at the same time is a prime task in thermoelectrics. Here we present a high-performance thermoelectric system of Pb0.95(Sb0.033□0.017)Se1–yTey (□ = vacancy; y = 0–0.4) embedded with unique defect architecture. Given the mean free paths of phonons and electrons, we rationally integrate multiple defects that involve point defects, vacancy-driven dense dislocations, and Te-induced nanoprecipitates with different sizes and mass fluctuations. They collectively scatter thermal phonons in a wide range of frequencies to give lattice thermal conductivity of ∼0.4 W m–1 K–1, which approaches to the amorphous limit. Remarkably, Te alloying incr...

Highly robust and flexible n-type thermoelectric (TE) films based on Ag2Te nanoshuttle/polyvinylidene fluoride were prepared by a solution-processable method without a surfactant. A good power performance of over 30 μW (m K2)-1 at room temperature was achieved. Moreover, the synthesized fabrics also exhibited potential for application in flexible electronic devices with negligible performance change after 1000 bending cycles.

A series of typical samples based on β-Zn 4 Sb 3 have been prepared by spark plasma sintering. Single-phase β-Zn 4 Sb 3 sample is found to be a brittle material. Many hole defects are found in single-phase sample after chemical etching. A small amount of excess metallic Zn reduces the hole defects and improves the thermoelectric performance of Zn-rich sample. The maximum ZT can reach 1.31 at 673 K. The improvement of performance can be attributed to hierarchical nanometer precipitates and interface stress. Introducing soft metal into particle boundaries is a potential solution to improve the performance of brittle polycrystalline thermoelectric materials.

SnSe emerges as a new class of thermoelectric materials since the recent discovery of an ultrahigh thermoelectric figure of merit in its single crystals. Achieving such performance in the polycrystalline counterpart is still challenging and requires fundamental understandings of its electrical and thermal transport properties as well as structural chemistry. Here we demonstrate a new strategy of improving conversion efficiency of bulk polycrystalline SnSe thermoelectrics. We show that PbSe alloying decreases the transition temperature between Pnma and Cmcm phases and thereby can serve as a means of controlling its onset temperature. Along with 1% Na doping, delicate control of the alloying fraction markedly enhances electrical conductivity by earlier initiation of bipolar conduction while reducing lattice thermal conductivity by alloy and point defect scattering simultaneously. As a result, a remarkably high peak ZT of ∼1.2 at 773 K as well as average ZT of ∼0.5 from RT to 773 K is achieved for Na0.01(Sn1...

As an important candidate for thermoelectric (TE) applications, the preparation of PbTe-based one-dimensional nanostructures with controllable components and properties remains a challenge. Herein, we report a new approach for the scalable solution-based synthesis of component-controllable ultrathin Se-doped PbTe (PbTe1−xSex, 0 ≤ x ≤ 0.5) single-phase ternary nanowires (NWs), which serve as building blocks for fabricating bulk nanostructured TE materials. Unexpectedly, Se dopant stabilized the NWs and favored the formation of a unique long-range-ordered nanonetwork structure inside the bulk, which led to a peak figure of merit (ZT) of 1.1 at 643 K and an average ZT of 0.9 in the range of 300–673 K, which was one of the highest values reported for the n-type nanostructured PbTe-based materials.

In this work we demonstrate a solution-based approach to the synthesis of ternary metal chalcogenide nanowires (NWs) with high thermoelectric figure of merit (ZT) and power factors. Utilizing templates of Te1−xSex alloys to balance the chemical reactivity between Te and Se atoms, we have been able to add a number of different metal species while maintaining a single-phase ternary metal chalcogenide NW with small diameter (

Despite extensive studies on emerging thermoelectric material SnSe, its n-type form is largely underdeveloped mainly due to the difficulty in stabilizing the carrier concentration at the optimal level. Here, we dually introduce Cl and PbSe to induce n-type conduction in intrinsic p-type SnSe. PbSe alloying enhances the power factor and suppresses lattice thermal conductivity at the same time, giving a highest thermoelectric figure of merit ZT of 1.2 at 823 K for n-type polycrystalline SnSe materials. The best composition is Sn0.90Pb0.15Se0.95Cl0.05. Samples prepared by the solid-state reaction show a high maximum ZT ( ZTmax) ∼1.1 and ∼0.8 parallel and perpendicular to the press direction of spark plasma sintering, respectively. Remarkably, post-ball milling and annealing processes considerably reduce structural anisotropy, thereby leading to a ZTmax ∼1.2 along both the directions. Hence, the direction giving a ZTmax is controllable for this system using the specialized preparation methods for specimens. Spherical aberration-corrected scanning transmission electron microscopic analyses reveal the presence of heavily dense edge dislocations and strain fields, not observed in the p-type counterparts, which contribute to decreasing lattice thermal conductivity. Our theoretical calculations employing a Callaway-Debye model support the experimental results for thermal transport and microscopic structures.

Copper coated Bi 0.5 Sb 1.5 Te 3 powders were prepared through electroless plating with simplified pretreatment method. After hydrogen reduction, the powders are consolidated into bulk by spark plasma sintering. The lowest lattice thermal conductivity in Bi 0.5 Sb 1.5 Te 3 /Cu reduces to 0.25 W/m·K at 371.5 K after annealing. The figure of merit ( ZT ) of Bi 0.5 Sb 1.5 Te 3 /Cu ascends in a large scale and reaches 1.08 at 373 K and 423 K before and after annealing respectively. The average ZT values of Bi 0.5 Sb 1.5 Te 3 /Cu samples both rise from 0.36 to 0.82, which makes this material suitable for application in a wide temperature range.

Cobalt telluride branched nanostructures on carbon fiber paper (CFP) with two different morphologies were synthesized via solution-based conversion reaction. Both the CoTe2 with nanodendrite and CoTe with nanosheet morphologies on the CoTe2 nanotube (CoTe2 NDs/CoTe2 NTs and CoTe NSs/CoTe2 NTs) supported by CFP exhibit high activities toward hydrogen evolution reaction (HER). Particularly, the CoTe NSs/CoTe2 NTs only require an overpotential of 230.0 mV to deliver the current density of 100 mA cm(-2) in acid solution. After cycling for 5000 cycles or 20 h continual electrolysis, only a small performance loss is observed.

This short review discusses recent advances in n-type PbSe-based bulk thermoelectric materials. Their performance has rapidly progressed recently and emerges as one of most efficient n-type thermoelectrics in the mid-temperature range (500–800 K), rivaling PbTe-based materials. This success is mainly attributed to the development of new performance-enhancing strategies employing nanoscale defect structures. We introduce intrinsic thermoelectric properties of PbSe based on crystal and electronic band structures. We then discuss how nanoscale defect structures can modulate charge and phonon transport properties and serve as a key to achieving higher ZT in n-type PbSe with the examples of current state-of-the-art systems.

From a structural and economic perspective, tellurium-free PbSe can be an attractive alternative to its more expensive isostructural analogue of PbTe for intermediate temperature power generation. Here we report that PbSe

Highly robust and flexible n-type thermoelectric (TE) films based on Ag2Te nanoshuttle/polyvinylidene fluoride were prepared by a solution-processable method without a surfactant. A good power performance of over 30 μW (m K2)-1 at room temperature was achieved. Moreover, the synthesized fabrics also exhibited potential for application in flexible electronic devices with negligible performance change after 1000 bending cycles.

The ternary alloyed Co(SxSe1−x)2 nanowires (NWs) supported by carbon fiber paper (CFP) have been successfully synthesized. By adjusting the components of the NWs, it is found that the Co(S0.73Se0.27)2 NWs on CFP exhibit the highest electrocatalytic activity towards hydrogen evolution reaction (HER). To drive the current density of 100 mV cm−2, the Co(S0.73Se0.27)2 NWs only require a small overpotential of 157 mV in 0.5 M H2SO4 solution and the corresponding Tafel slope is only 45.3 mV dec−1. After 20 h continuous electrolysis or cycling for 5000 times in acid electrolyte, the robust catalytic activity is still perfectly preserved. DFT calculation results demonstrate that the high intrinsic activity of Co(S0.73Se0.27)2 NWs towards HER is derived from the significant reduction of kinetic energy barrier induced by anion doping. This work presents a new efficient catalyst for HER and the anion doping strategy implemented here could be extended to other earth-abundant electrocatalysts to improve the catalytic performance.

We report on the fabrication of flexible and freestanding n-type thermoelectric Cu intercalated Bi 2 Se 3 nanoplatelet/Polyvinylidene Fluoride (PVDF) composite films. The optimized power factor and figure of merit (ZT) of the Cu 0.1 Bi 2 Se 3 nanoplatelet/PVDF composites are 103 μW m −1 K −2 and 0.10 at 290 K, respectively, which are one of the highest values for n-type thermoelectric films. The mechanism by which the Seebeck coefficient and electrical conductivity can be partially decoupled is explained in details: PVDF can not only grantee the robust and flexibility but also create a high trap-state by introducing the energy barrier at the organic/inorganic interface, thus a high level of Seebeck coefficient is maintained for the composite system while a remarkable improvement on electrical conductivity was achieved. The thermoelectric films show high mechanical durability with only a 13% decrease in performance after 5000 bending cycles (bending curvature 1/2 mm −1 ). The overall performance of the n-type thermoelectric films approaches the values required for efficient flexible thermoelectric power generators.

Ultrathin trigonal Te/Se alloyed nanowires with a tunable composition and band gap were fabricated using a nonhazardous reducing agent, ascorbic acid. The as-synthesized nanowires displayed a tunable direct band gap (3.39 to 3.78 eV) and indirect band gap (1.99 eV to 2.93 eV).

Mullite whiskers coating on the surface of SiC ceramics was fabricated by chemical reactions between the SiC ceramic substrate and molten salts (Al2(SO4)3 + Na2SO4), which greatly improved the oxidation resistance of the porous SiC ceramic. Scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction, Fourier transform-infrared spectroscopy and high resolution transmission electron microscopy were employed to characterize the structural morphology and phase compositions of the modified porous SiC. The results show that the surface of SiC ceramic was covered by dense mullite whiskers with nanometer-sized (100–300 nm) diameters and micrometer-sized (10–20 μm) lengths. The molten salts chemical reactions were firstly proposed and demonstrated by thermo gravimetric (TG) analyzer coupled with mass spectrometry (MS) and fourier transform infrared spectrometry (FTIR).

Uniform Bi

The Fe-C composites were fabricated by a combination of high-energy ball milling of Fe-C powder mixtures and plasma activated sintering process. An orthogonal experiment in four factors (including original powder composition, sintering temperature, applied pressure and holding time) and three levels was employed to investigate the effects of preparation technology on mechanical properties (bending strength and hardness) of the Fe-C composite. The experimental results show that the crystalline Fe3C phase can be produced by the rapid sintering process, though it is cannot form theoretically due to the high Gibbs free energy, and more or less holes and composition segregation phenomenon coexist in the composite. The original powder composition plays the leading role in both the mechanical properties of the Fe-C composites. However, the effects of the other parameters on the bending strength and hardness of the composite are somewhat different. The optimal technology combinations for the bending strength and hardness are obtained as follows: 50Fe+50Fe3C/1373 K/400 s/20 MPa and 50Fe+50Fe3C/50 MPa/1273 K/400 s, respectively.

Product family planning has received much attention from both academia and industries. It aims at incorporating customers’ needs into design elements of product family. The main challenger for product family planning originates from difficulties in mapping customer needs to product family specifications. This paper intends to develop a method to improve the mapping process by reusing knowledge from purchased products according to the satisfied customer needs. A knowledge discovery model for product family planning is proposed, where clustering is adopted to partition the purchased products so that commonality of product family could be effectively addressed and rough set is employed to extract the more concise decision rules. A case study of air condition is reported to illustrate the feasibility of proposed approach and associated algorithms.

A general approach to fabricate nanowires based inorganic/organic composite flexible thermoelectric fabric using a simple and efficacious five-step vacuum filtration process is proposed. As an excellent example, the performance of freestanding flexible thermoelectric thin film using copper telluride nanowires/polyvinylidene fluoride (Cu1.75Te NWs/PVDF = 2:1) as building block is demonstrated. By burying the Cu1.75Te NWs into the PVDF polymer agent, the flexible fabric exhibits room-temperature Seebeck coefficient and electric conductivity of 9.6 μV/K and 2490 S/cm, respectively, resulting in a power factor of 23 μW/(mK(2)) that is comparable to the bulk counterpart. Furthermore, this NW-based flexible fabric can endure hundreds of cycles of bending tests without significant performance degradation.

Uniform Bi2Te3−x Se x nanowires (NWs) with tunable components are synthesized by a modified solution method free of any template, and inter-diffusion mechanism is proposed for the growth and transformation of ternary nanowires. Spark plasma sintering is adopted to fabricate the pellets of Bi2Te3−x Se x NWs and thermoelectric transport properties are measured. As compared to Bi2Te3 pellets, Se doping results in lowered electrical conductivity because of the reduced carrier concentration, both the Seebeck coefficient and the power factor are enhanced substantially. The Bi2Te2.7Se0.3 pellet exhibits the highest power factor at room temperature as a result of optimized carrier concentration (4.37 × 1019 cm−3) and mobility (60.22 cm2 V−1 s−1). As compared to Bi2Te3, the thermal conductivity of Bi2Te3−x Se x is lowered owing to the enhanced phonon scattering by dopants and grain boundaries. As a result, the ZT value at 300 K is substantially improved from 0.045 of Bi2Te3 to 0.42 of Bi2Te2.7Se0.3. It is suggested that Se doping is an effective way to enhance the thermoelectric performance of Bi2Te3 based materials.