Point Defect Engineering of High‐Performance Bismuth‐Telluride‐Based Thermoelectric Materials

Developing high‐performance thermoelectric materials is one of the crucial aspects for direct thermal‐to‐electric energy conversion. Herein, atomic scale point defect engineering is introduced as a new strategy to simultaneously optimize the electrical properties and lattice thermal conductivity of thermoelectric materials, and (Bi,Sb)₂(Te,Se)₃ thermoelectric solid solutions are selected as a paradigm to demonstrate the applicability of this new approach. Intrinsic point defects play an important role in enhancing the thermoelectric properties. Antisite defects and donor‐like effects are engineered in this system by tuning the formation energy of point defects and hot deformation. As a result, a record value of the figure of merit ZT of ≈1.2 at 445 K is obtained for n‐type polycrystalline Bi₂Te_2.3Se_0.7 alloys, and a high ZT value of ≈1.3 at 380 K is achieved for p‐type polycrystalline Bi_0.3Sb_1.7Te₃ alloys, both values being higher than those of commercial zone‐melted ingots. These results demonstrate the promise of point defect engineering as a new strategy to optimize thermoelectric properties.     

Mg3+δSbxBi2−x Family: A Promising Substitute for the State‐of‐the‐Art n‐Type Thermoelectric Materials near Room Temperature

The Bi₂Te_3?xSe_x family has constituted n‐type state‐of‐the‐art thermoelectric materials near room temperature (RT) for more than half a century, which dominates the active cooling and novel heat harvesting application near RT. However, the drawbacks of a brittle nature and Te‐content restricts the possibility for exploring potential applications. Here, it is shown that the Mg_3+δSb_xBi_2?x family ((ZT)_avg = 1.05) could be a promising substitute for the Bi₂Te_3?xSe_x family ((ZT)_avg = 0.9–1.0) in the temperature range of 50–250 °C based on the comparable thermoelectric performance through a synergistic effect from the tunable bandgap using the alloy effect and the suppressible Mg‐vacancy formation using an interstitial Mn dopant. The former is to shift the optimal thermoelectric performance to near RT, and the latter is helpful to partially decouple the electrical transport and thermal transport in order to get an optimal RT power factor. The positive temperature dependence of the bandgap suggests this family is also a superior medium‐temperature thermoelectric material for the significantly suppressed bipolar effect. Furthermore, a two times higher mechanical toughness, compared with the Bi₂Te_3?xSe_x family, allows for a promising substitute for state‐of‐the‐art n‐type thermoelectric materials near RT.     

On Intensifying Carrier Impurity Scattering to Enhance Thermoelectric Performance in Cr‐Doped CeyCo4Sb12

The beneficial effect of impurity scattering on thermoelectric properties has long been disregarded even though possible improvements in power factor have been suggested by Ioffe more than a half century ago. Here it is theoretically and experimentally demonstrated that proper intensification of ionized impurity scattering to charge carriers can benefit the thermoelectric figure of merit (ZT) by increasing the Seebeck coefficient and decreasing the electronic thermal conductivity. The optimal strength of ionized impurity scattering for maximum ZT depends on the Fermi level and the density of states effective mass. Cr‐doping in Ce_yCo₄Sb₁₂ progressively increases the strength of ionized impurity scattering, and significantly improves the Seebeck coefficient, resulting in high power factors of 45 μW cm^?1 K^?2 with relatively low electrical conductivity. This effect, combined with the increased Ce‐filling fraction and thus decreased lattice thermal conductivity by charge compensation of Cr‐dopant, gives rise to a maximum ZT of 1.3 at 800 K and a large average ZT of 1.1 between 500 and 850 K, ≈30% and ≈20% enhancements as compared with those of Cr‐free sample, respectively. Furthermore, this study also reveals that carrier scattering parameter can be another fundamental degree of freedom to optimize electrical properties and improve thermal‐to‐electricity conversion efficiencies of thermoelectric materials.     

High Thermoelectric Performance in Supersaturated Solid Solutions and Nanostructured n‐Type PbTe–GeTe

Sb‐doped and GeTe‐alloyed n‐type thermoelectric materials that show an excellent figure of merit ZT in the intermediate temperature range (400–800 K) are reported. The synergistic effect of favorable changes to the band structure resulting in high Seebeck coefficient and enhanced phonon scattering by point defects and nanoscale precipitates resulting in reduction of thermal conductivity are demonstrated. The samples can be tuned as single‐phase solid solution (SS) or two‐phase system with nanoscale precipitates (Nano) based on the annealing processes. The GeTe alloying results in band structure modification by widening the bandgap and increasing the density‐of‐states effective mass of PbTe, resulting in significantly enhanced Seebeck coefficients. The nanoscale precipitates can improve the power factor in the low temperature range and further reduce the lattice thermal conductivity (κ_lat). Specifically, the Seebeck coefficient of Pb_0.988Sb_0.012Te–13%GeTe–Nano approaches ?280 µV K^?1 at 673 K with a low κ_lat of 0.56 W m^?1 K^?1 at 573 K. Consequently, a peak ZT value of 1.38 is achieved at 623 K. Moreover, a high average ZT_avg value of ≈1.04 is obtained in the temperature range from 300 to 773 K for n‐type Pb_0.988Sb_0.012Te–13%GeTe–Nano.     

Preparation and Thermoelectric Properties of Doped Bi2Te3-Bi2Se3 Solid Solutions

Since Bi₂Te₃ and Bi₂Se₃ have the same crystal structure, they form a homogeneous solid solution. Therefore, the thermal conductivity of the solid solution can be reduced by phonon scattering. The thermoelectric figure of merit can be improved by controlling the carrier concentration through doping. In this study, Bi₂Te_2.85Se_0.15:D _m (D: dopants such as I, Cu, Ag, Ni, Zn) solid solutions were prepared by encapsulated melting and hot pressing. All specimens exhibited n-type conduction in the measured temperature range (323 K to 523 K), and their electrical conductivities decreased slightly with increasing temperature. The undoped solid solution showed a carrier concentration of 7.37 × 10¹⁹ cm^?3, power factor of 2.1 mW m^?1 K^?1, and figure of merit of 0.56 at 323 K. The figure of merit (ZT) was improved due to the increased power factor by I, Cu, and Ag dopings, and maximum ZT values were obtained as 0.76 at 323 K for Bi₂Te_2.85Se_0.15:Cu_0.01 and 0.90 at 423 K for Bi₂Te_2.85Se_0.15:I_0.005. However, the thermoelectric properties of Ni- and Zn-doped solid solutions were not enhanced.     

High-Efficiency Thermoelectric Module Based on High-Performance Bi0.42Sb1.58Te3 Materials

Bismuth-telluride-based alloy is the sole thermoelectric candidate for commercial thermoelectric application in low-grade waste heat harvest near room temperature, but the sharp drop of thermoelectric properties at higher temperature and weak mechanical strength in zone-melted material are the main obstacles to its wide development for power generation. Herein, an effective approach is reported to improve the thermoelectric performance of p-type Bi_0.42Sb_1.58Te₃ hot-pressed sample by incorporating Ag₅SbSe₄. A peak ZT of 1.40 at 375 K and a high average ZT of 1.25 between 300 and 500 K are achieved. Such outstanding thermoelectric performance originates from the synergistic effects of improved density-of-states effective mass, reduced bipolar thermal conductivity by the boosted carrier concentration, and suppressed lattice thermal conductivity by the induced phonon scattering centers including substitute point defects, dislocations, stress–strain clusters, and grain boundaries. Comprised of the p-type Bi_0.42Sb_1.58Te₃ + 0.10 wt% Ag₅SbSe₄ and zone-melted n-type Bi₂Te_2.7Se_0.3, the thermoelectric module exhibits a high conversion efficiency of 6.5% at a temperature gradient of 200 K, indicating promising applications for low-grade heat harvest near room temperature.     

BiSbTe‐Based Nanocomposites with High ZT: The Effect of SiC Nanodispersion on Thermoelectric Properties

Thermoelectric materials have potential applications in energy harvesting and electronic cooling devices, and bismuth antimony telluride (BiSbTe) alloys are the state‐of‐the‐art thermoelectric materials that have been widely used for several decades. It is demonstrated that mixing SiC nanoparticles into the BiSbTe matrix effectively enhances its thermoelectric properties; a high dimensionless figure of merit (ZT) value of up to 1.33 at 373 K is obtained in Bi_0.3Sb_1.7Te₃ incorporated with only 0.4 vol% SiC nanoparticles. SiC nanoinclusions possessing coherent interfaces with the Bi_0.3Sb_1.7Te₃ matrix can increase the Seebeck coefficient while increasing the electrical conductivity, in addition to its effect of reducing lattice thermal conductivity by enhancing phonon scattering. Nano‐SiC dispersion further endows the BiSbTe alloys with better mechanical properties, which are favorable for practical applications and device fabrication.     

Effect of Electric Current on the Performance of Bi<Subscript>1.2</Subscript>Sb<Subscript>4.8</Subscript>Te<Subscript>9</Subscript> Thermoelectric Material Prepared by a Field-Activated Pressure-Assisted Sintering Process

Field-activated pressure-assisted sintering (FAPAS) was applied to sinter Bi_1.2Sb_4.8Te₉ thermoelectric materials under different conditions, including no-current sintering (NCS), low-density current sintering (LCS), and high-density current sintering (HCS). The effect of the current density on the final thermoelectric performance of the products was investigated. Applying a higher-density electric current and shorter dwell time can improve the thermoelectric performance of the sample by increasing its electric conductivity and decreasing its thermal conductivity. The maximum figure of merit ZT values of the NCS, LCS, and HCS samples were 0.46, 0.48, and 0.57, respectively. Therefore, applying a high-density electric current in the sintering process may be an effective way to obtain Bi_1.2Sb_4.8Te₉ thermoelectric material with high ZT value.     

Effects of Cooling Rate on Thermoelectric Properties of <Emphasis Type="Italic">n</Emphasis>-Type Bi<Subscript>2</Subscript>(Se<Subscript>0.4</Subscript>Te<Subscript>0.6</Subscript>)<Subscript>3</Subscript> Compounds

A series of Bi₂(Se_0.4Te_0.6)₃ compounds were synthesized by a rapid route of melt spinning (MS) combined with a subsequent spark plasma sintering (SPS) process. Measurements of the Seebeck coefficient, electrical conductivity, and thermal conductivity were performed over the temperature range from 300 K to 520 K. The measurement results showed that the cooling rate of melt spinning had a significant impact on the transport properties of electrons and phonons, effectively enhancing the thermoelectric properties of the compounds. The maximum ZT value reached 0.93 at 460 K for the sample prepared with the highest cooling rate, and infrared spectrum measurement results showed that the compound with lower tellurium content, Bi₂(Se_0.4Te_0.6)₃, possesses a larger optical forbidden gap (E _g) compared with the traditional n-type zone-melted material with formula Bi₂(Se_0.07Te_0.93)₃. Our work provides a new approach to develop low-tellurium-bearing Bi₂Te₃-based compounds with good thermoelectric performance.     

Strain-Mediated Lattice Rotation Design for Enhancing Thermoelectric Performance in Bi2S2Se

A unique strain-mediated lattice rotation strategy is introduced via nanocompositing to upsurge the optimized limits in the composition-to-structural pathway on rationally engineering the efficient thermoelectric material. In this study, a special lattice rotation via strain engineering is realized to optimize the desired electronic and chemical environment for enhancing thermoelectric properties in n-type Bi₂S₂Se. This approach results in a unique transport phenomenon to assist high-energy electrons in transferring through the optimized transport channels, and appropriate structure disparity to significantly localize phonons. As a result, Sb over Cl doping in Bi₂S₂Se gently reduces E_g and introduces defect states in bandgap with shifting down the Fermi level, thus causing increase in carrier concentration, which contributes to a higher power factor of ≈7.18 µW cm⁻¹ K⁻² (at T = 773 K). Besides, a lower thermal conductivity of ≈0.49 W m⁻¹ K⁻¹ is driven through lattice strain and defect engineering. Consequently, an ultra-high ZT_max = 1.13 (at T = 773 K) and a high ZT_ave = 0.54 (323 K-773 K) are realized. This study not only leads to an extraordinary thermoelectric performance but also reveals a unique paradigm for electron transportation and phonon localization via lattice strain engineering.     

Synergistic Compositional–Mechanical–Thermal Effects Leading to a Record High zT in n‐Type V2VI3 Alloys Through Progressive Hot Deformation

Here a progressive hot deformation procedure that endows the benchmark n‐type V₂VI₃ thermoelectric materials with short range disorder (multiple defects), long range order (crystallinity), and strong texture (nearly orientation order) is reported. Not only it is rare for these structural features to coexist but also these structural features elicit the synergistic compositional–mechanical–thermal effects, i.e., a profound interplay among the counts, magnitude, and temperature of hot deformation in relation to the as formed point defects, dislocations, textures, strain clusters, and distortions. Using progressively larger die sets and relatively low hot deformation temperature, rich multiscale microstructures concurrently with a high level of texture comparable to that of zone melted ingot are obtained. The strong donor‐like effect significantly increases the majority carrier concentration, suppressing the detrimental bipolar effect. In addition, the multiscale microstructures yield an ultralow lattice thermal conductivity ≈0.31 W m⁻¹ K⁻¹ at 405 K. A record zT ≈ 1.3 at 450 K are attained in progressively hot deformed n‐type Bi_1.95Sb_0.05Te_2.3Se_0.7 through the synergistic effects. These results not only promise a better pairing between n‐type and p‐type legs in device fabrication but also bring our understanding of n‐type V₂VI₃ alloys and hot deformation technique to a new level.     

Thermoelectric and mechanical properties of Bi0.5Sb1.5Te3 solid solution prepared by melt solidification in liquid

The influence of the conditions for preparing samples from granules of Bi_0.5Sb_1.5Te₃ solid solution obtained by melt solidification in liquid on the mechanical and thermoelectric properties of these samples is investigated. The microstructure and surface morphology of sample cleavages are analyzed by optical and scanning electron microscopy. The mechanical properties of the samples are investigated by compression tests at temperatures from 300 to 620 K. The thermoelectric characteristics (Seebeck coefficient and electrical and thermal conductivities) are measured both at room temperature and in the temperature range of 100–700 K. The samples with the highest thermoelectric figure of merit, (ZT)_max ≈ 1.3 at 370 K, are obtained.

     

Enhancement of Thermoelectric Figure of Merit for Bi0.5Sb1.5Te3 by Metal Nanoparticle Decoration

Introducing nanoinclusions in thermoelectric (TE) materials is expected to lower the lattice thermal conductivity by intensifying the phonon scattering effect, thus enhancing their TE figure of merit ZT. We report a novel method of fabricating Bi_0.5Sb_1.5Te₃ nanocomposite with nanoscale metal particles by using metal acetate precursor, which is low cost and facile to scale up for mass production. Ag and Cu particles of ??40?nm were successfully near-monodispersed at grain boundaries of Bi_0.5Sb_1.5Te₃ matrix. The well-dispersed metal nanoparticles reduce the lattice thermal conductivity extensively, while enhancing the power factor. Consequently, ZT was enhanced by more than 25% near room temperature and by more than 300% at 520?K compared with a Bi_0.5Sb_1.5Te₃ reference sample. The peak ZT of 1.35 was achieved at 400?K for 0.1?wt.% Cu-decorated Bi_0.5Sb_1.5Te₃.     

Thermoelectric Properties of Nano/microstructured p-Type Bi0.4Sb1.6Te3 Powders Fabricated by Mechanical Alloying and Vacuum Hot Pressing

Two kinds of Bi_0.4Sb_1.6Te₃ powder with different particle and grain sizes were fabricated by high-energy ball milling. Powder mixtures with varied weight ratios were consolidated by vacuum hot pressing (HP) to produce nano/ microstructured composites of identical chemical composition. From measurements of the Seebeck coefficient, electrical resistivity, and thermal conductivity of these composites, a figure of merit (ZT) value of up to 1.19 was achieved at 373 K for the sample containing 40% nanograin powder. This ZT value is higher than that of monolithic nanostructured Bi_0.4Sb_1.6Te₃. It is further noted that the ZT value of this sample in the temperature range of 450 K to 575 K is in the range of 0.7 to 1.1. Such ZT characteristics are suitable for power generation applications as no other material with a similar high ZT value in this temperature range has been observed until now. The achieved high ZT value can probably be attributed to the unique nano/microstructure, in which the dispersed nanograin powder increases the number of phonon scattering sites, which in turn results in a decrease of the thermal conductivity while simultaneously increasing the electrical conductivity, owing to the existence of the microsized powder that can provide a fast carrier transportation network. These results indicate that the nano/microstructured Bi_0.4Sb_1.6Te₃ alloy can serve as a high-performance material for application in thermoelectric devices.     

Enhanced Thermoelectric Properties Obtained by Compositional Optimization in p-Type BixSb2?xTe3 Fabricated by Mechanical Alloying and Spark Plasma Sintering

Bi _x Sb_2−x Te₃ bulk alloys are known as the best p-type thermoelectric materials near room temperature. In this work, single-phase Bi _x Sb_2−x Te₃ (x = 0.2, 0.25, 0.3, 0.34, 0.38, 0.42, 0.46, and 0.5) alloys were prepared by spark plasma sintering (SPS) using mechanical alloying (MA)-derived powders. A small amount (0.1 vol.%) of SiC nanoparticles was added to improve the mechanical properties and to reduce the thermal conductivity of the alloys. The electrical resistivity decreases significantly with increasing ratio of Sb to Bi in spite of the weaker decreasing trend in Seebeck coefficient, whereby the power factor at 323 K reaches 3.14 × 10⁻³ W/mK² for a sample with x = 0.3, obviously higher than that at x = 0.5 (2.27 × 10⁻³ W/mK²), a composition commonly used for ingots. Higher thermal conductivities at low temperatures are obtained at the compositions with lower x values, but they tend to decrease with temperature. As a result, higher ZT values are obtained for Bi_0.3Sb_1.7Te₃, with a maximum ZT value of 1.23 at 423 K, about twice the ZT value (about 0.6) of Bi_0.5Sb_1.5Te₃ at the same temperature.     

Thermoelectric Performance of Sb- and La-Doped Mg2Si0.5Ge0.5

La _x Mg_2?x Si_0.49Ge_0.5Sb_0.01 compounds (x?=?0, 0.005, 0.01, 0.02) were synthesized by solid-state reaction followed by spark plasma sintering. The thermoelectric properties, such as the Seebeck coefficient, the electrical and thermal conductivities, and ZT, of these compounds have been studied in the temperature range of 300?K to 823?K. The figure of merit of this n-type compound has been raised above unity at 823?K for the sample with x?=?0.01, a value 60% higher than that of Mg₂Si_0.49Ge_0.5Sb_0.01. The reduction of the thermal conductivity via increasing phonon scattering is considered as the main reason for the enhanced ZT. These observations demonstrate an opportunity to improve the thermoelectric performance of Mg₂Si_1?x Ge _x solid solutions.     

Atomic Disorders Induced by Silver and Magnesium Ion Migrations Favor High Thermoelectric Performance in α‐MgAgSb‐Based Materials

Thermoelectric devices can directly convert thermal energy to electricity or vice versa with the efficiency being determined by the materials’ dimensionless figure of merit (ZT). Since the revival of interests in the last decades, substantial achievements have been reached in search of high‐performance thermoelectric materials, especially in the high temperature regime. In the near‐room‐temperature regime, MgAgSb‐based materials are recently obtained with ZT ≈ 0.9 at 300 K and ≈1.4 at 525 K, as well as a record high energy conversion efficiency of 8.5%. However, the underlying mechanism responsible for the performance in this family of materials has been poorly understood. Here, based on structure refinements, scanning transmission electron microscopy (STEM), NMR experiments, and density function theory (DFT) calculations, unique silver and magnesium ion migrations in α‐MgAg_0.97Sb_0.99 are disclosed. It is revealed that the local atomic disorders induced by concurrent ion migrations are the major origin of the low thermal conductivity and play an important role in the good ZT in MgAgSb‐based materials.     

Enhancement of Thermopower of TAGS‐85 High‐Performance Thermoelectric Material by Doping with the Rare Earth Dy

Enhancement of thermopower is achieved by doping the narrow‐band semiconductor Ag_6.52Sb_6.52Ge_36.96Te₅₀ (acronym TAGS‐85), one of the best p‐type thermoelectric materials, with 1 or 2% of the rare earth dysprosium (Dy). Evidence for the incorporation of Dy into the lattice is provided by X‐ray diffraction and increased orientation‐dependent local fields detected by ¹²⁵Te NMR spectroscopy. Since Dy has a stable electronic configuration, the enhancement cannot be attributed to 4f‐electron states formed near the Fermi level. It is likely that the enhancement is due to a small reduction in the carrier concentration, detected by ¹²⁵Te NMR spectroscopy, but mostly due to energy filtering of the carriers by potential barriers formed in the lattice by Dy, which has large both atomic size and localized magnetic moment. The interplay between the thermopower, the electrical resistivity, and the thermal conductivity of TAGS‐85 doped with Dy results in an enhancement of the power factor (PF) and the thermoelectric figure of merit (ZT) at 730 K, from PF = 28 μW cm^?1 K^?2 and ZT ≤ 1.3 in TAGS‐85 to PF = 35 μW cm^?1 K^?2 and ZT ≥ 1.5 in TAGS‐85 doped with 1 or 2% Dy for Ge. This makes TAGS‐85 doped with Dy a promising material for thermoelectric power generation.     

Enhancing the Thermoelectric Properties of p-Type Bulk Bi-Sb-Te Nanocomposites via Solution-Based Metal Nanoparticle Decoration

Embedding nanosized particles in bulk thermoelectric materials is expected to lower the lattice thermal conductivity by enhancing the degree of interface phonon scattering, thus improving their thermoelectric figure of merit ZT. We have developed a wet chemical process to fabricate Bi_0.5Sb_1.5Te₃-based thermoelectric nanocomposites which include nanometer-sized metal particles. By simple solution mixing of metal acetate precursors and Bi_0.5Sb_1.5Te₃ powders in ethyl acetate as a medium for homogeneous incorporation, it is possible to apply various types of metal nanoparticles onto the surfaces of the thermoelectric powders. Next, bulk Bi_0.5Sb_1.5Te₃ nanocomposites with homogeneously dispersed metal nanoparticles were fabricated using a spark plasma sintering technique. The lattice thermal conductivities were reduced by increasing the long-wavelength phonon scattering in the presence of metal nanoparticles, while the Seebeck coefficients increased for a few selected metal-decorated nanocomposites, possibly due to the carrier-energy-filtering effect. Finally, the figure of merit ZT was enhanced to 1.4 near room temperature. This approach highlights the feasibility of incorporating various types of nanoparticles into an alloy matrix starting by wet chemical routes, which is an effective means of improving the thermoelectric performance of Bi-Te-based alloys.     

Ultrahigh Responsivity of Ternary Sb–Bi–Se Nanowire Photodetectors

High‐quality single‐crystalline ternary (Sb_1‐xBi_x)₂Se₃ nanowires (NWs) (x = 0–0.88) are synthesized by chemical vapor deposition. Nanowires with x from 0 to 0.75 are indexed as an orthorhombic structure. With increasing Bi incorporation ratio, (Sb_1‐xBi_x)₂Se₃ NWs exhibit remarkable photoresponsivities, which originate from growing surface Se vacancies and augmented oxygen chemisorptions. Notably, spectra responsivity and external quantum efficiency of an (Sb_0.44Bi_0.56)₂Se₃ NW photodetector reach as high as ≈8261.4 A/W and ≈1.6 × 10⁶ %, respectively. Those excellent performances unambiguously demonstrate that Sb–Bi–Se NWs are promising for the utilizations of high‐sensitivity and high‐speed photodetectors and photoelectronic switches.