Practical High-Performance (Bi,Sb)2Te3-Based Thermoelectric Nanocomposites Fabricated by Nanoparticle Mixing and Scrap Recycling

Thermoelectric Performance Enhancement in SnS Polycrystals Owing to Hole Doping Combined with Textured Microstructures

Recently, the earth-abundant tin sulfide (SnS) has emerged as a promising thermoelectric material due to its phonon and electron structure similar to that of tin selenide (SnSe). However, compared with SnSe, limited progress has been achieved in the thermoelectric property enhancement of SnS. Textured SnS polycrystals with an enhanced thermoelectric performance have been developed in this work. The high carrier mobility benefited from the enhanced texture through the repressing strategy of spark plasma sintering, improving the electrical conductivity. In addition, Sn atom deficiencies in the texture sample led to an increased hole concentration, further boosting the electrical conductivity and power factor. The power factor exceeded 4.10 μW/cm·K2 at 423 K and 5.50 μW/cm·K2 at 850 K. The phonon scattering was strengthened by adjusting the multiscale microstructures including dislocations, defect clusters, etc., leading to an ultralow lattice thermal conductivity of 0.23 W/m·K at 850 K. A figure of merit zT > 1.3 at 850 K and an average zTave of 0.58 in the temperature range 373-850 K were achieved in the SnS polycrystal.

Recent trends and future perspectives of thermoelectric materials and their applications

This review explores the ever-evolving landscape of thermoelectric materials, focusing on the latest trends and innovations in ceramics, thermally conductive gel-like materials, metals, nanoparticles, polymers, and silicon. Thermoelectric materials have garnered significant attention for their capability to convert waste heat into electrical power, positioning them as promising candidates for energy harvesting and cooling applications. This review distinguishes itself by highlighting recent advancements in synthesis methods, advanced doping strategies, and nanostructuring techniques that have markedly enhanced material performance. It provides a comprehensive analysis of the controlled properties concerning their synthesis parameters, such as electrical conductivity, Seebeck coefficient, and thermal conductivity. Furthermore, this work delves into the emerging applications of thermoelectric devices across diverse fields, including automotive, aerospace, wearable electronics, and industrial waste heat recovery. By offering forward-looking insights, this review outlines thermoelectric devices' challenges and future prospects, underscoring their potential to contribute to sustainable energy solutions and efficient thermal management systems. By integrating current trends with future projections, this review offers a timely and comprehensive roadmap for researchers and engineers dedicated to advancing next-generation thermoelectric technologies.

High-Performance p-Type Bi2Te3-Based Thermoelectric Materials Enabled via Regulating Bi-Te Ratio

Bi2Te3-based alloys, as the sole commercial thermoelectric (TE) material, play an irreplaceable role in the thermoelectric field. However, the low TE efficiency, poor mechanical properties, and high cost have limited its large-scale applications. Here, high-performance p-type Bi2Te3-based materials were successfully prepared by ball milling and hot pressing. The optimized p-type Bi0.55Sb1.45Te3 + 2.5 wt % Bi shows a peak zT value of 1.45 at 360 K, and the average zT value of up to 1.24 at 300-480 K, which is completely comparable with previously reported Bi2Te3-based alloys with excellent performance. Such performance mainly results from the enhanced electrical conductivity and decreased lattice thermal conductivity via regulating carrier and phonon transport. Furthermore, this material shows good mechanical properties, in which the Vickers hardness and compressive strength are up to 0.95 GPa and 94.6 MPa, respectively. Overall, both the thermoelectric and mechanical performance of the materials fabricated by our processing technology are quite competitive. This may enlighten researchers concentrating on Bi2Te3-based alloys, thus further promoting their industrial applications.

N-Type Thermoelectric AgBiPbS3 with Nanoprecipitates and Low Thermal Conductivity

Thermoelectric sulfide materials are of particular interest due to the earth-abundant and cost-effective nature of sulfur. Here, we report a new n-type degenerate semiconductor sulfide, AgBiPbS3, which adopts a Fm3̅m structure with a narrow band gap of ∼0.32 eV. Despite the homogeneous distribution of elements at the scale of micrometer, Ag2S nanoprecipitates with dimensions of several nanometers were detected throughout the matrix. AgBiPbS3 exhibits a low room-temperature lattice thermal conductivity of 0.88 W m-1 K-1, owing to the intrinsic low lattice thermal conductivity of Ag2S and the effective scattering of phonons at nanoprecipitate boundaries. Moreover, compared to AgBiS2, AgBiPbS3 demonstrates a significantly improved weighted mobility of >16 cm2 V-1 s-1 at 300 K, leading to an enhanced PF of 1.6 μW cm-1 K-2 at 300 K. The superior electrical transport in AgBiPbS3 can be attributed to the high valley degeneracy of the L point (the conduction band minimum), which is contributed by the Pb s and Pb p orbitals. Further, Ga doping is found to be effective in modulating the Fermi levels of AgBiPbS3, leading to further enhancement of PF with a PFave of 2.7 μW cm-1 K-2 in the temperature range of 300-823 K. Consequently, a relatively high ZTave of 0.22 and a peak ZT of ∼0.4 at 823 K have been achieved in 3% Ga-doped AgBiPbS3, highlighting the potential of AgBiPbS3 as an n-type thermoelectric sulfide.

High Thermoelectric Performance of p-Type Bi0.4Sb1.6Te3+x Synthesized by Plasma-Assisted Ball Milling

The exploration of new synthesis methods is important for the improvement of the thermoelectric property of a material for the different mechanisms of microstructure fabrication, surface activity modulation, and particle refinement. Herein, we prepared p-Bi₂Te₃ bulk materials by a simple synthesis method of the plasma-assisted ball milling, which yielded finer nanopowders, higher texture of in-plane direction, and higher efficiency compared to the traditional ball milling, favoring the simultaneous improvement of electrical and thermal properties. When combined with the Te liquid sintering, nano-/microscale hierarchical pores were fabricated and the carrier mobility was also increased, which together resulted in the low lattice thermal conductivity of 0.52 W·m^-1·K^-1 and the high power factor of 43.4 μW·cm^-1·K^-2 at 300 K, as well as the ranking ahead zT of 1.4@375 K. Thus, this work demonstrated the advantages of plasma-assisted ball milling in highly efficient synthesis of p-type Bi₂Te₃ with promising thermoelectric performance, which can also be utilized to prepare other thermoelectric materials.

Microstructure Evolution in Plastic Deformed Bismuth Telluride for the Enhancement of Thermoelectric Properties

Thermoelectric generators are solid-state energy-converting devices that are promising alternative energy sources. However, during the fabrication of these devices, many waste scraps that are not eco-friendly and with high material cost are produced. In this work, a simple powder processing technology is applied to prepare n-type Bi₂Te₃ pellets by cold pressing (high pressure at room temperature) and annealing the treatment with a canning package to recycle waste scraps. High-pressure cold pressing causes the plastic deformation of densely packed pellets. Then, the thermoelectric properties of pellets are improved through high-temperature annealing (500 ∘C) without phase separation. This enhancement occurs because tellurium cannot escape from the canning package. In addition, high-temperature annealing induces rapid grain growth and rearrangement, resulting in a porous structure. Electrical conductivity is increased by abnormal grain growth, whereas thermal conductivity is decreased by the porous structure with phonon scattering. Owing to the low thermal conductivity and satisfactory electrical conductivity, the highest ZT value (i.e., 1.0) is obtained by the samples annealed at 500 ∘C. Hence, the proposed method is suitable for a cost-effective and environmentally friendly way.

Nanotwins Strengthening High Thermoelectric Performance Bismuth Antimony Telluride Alloys

Bi₂ Te₃ based thermoelectric alloys have been commercialized in solid-state refrigeration, but the poor mechanical properties restrict their further application. Nanotwins have been theoretically proven to effectively strengthen these alloys and could be sometimes constructed by strong deformation during synthesis. However, the obscure underlying formation mechanism restricts the feasibility of twin boundary engineering on Bi₂ Te₃ based materials. Herein, thorough microstructure characterizations are employed on a series of Bi_0.4 Sb_1.6 Te_{3+ δ} alloys to systematically investigate the twins' formation mechanism. The results show that the twins belong to the annealing type formed in the sintering process, which is sensitive to Te deficiency, rather than the deformation one. The Te deficiency combined with mechanical deformation is prerequisite for constructing dense nanotwins. By reducing the δ below -0.01 and undergoing strong deformation, samples with a high density of nanotwins are obtained and exhibit an ultrahigh compressive strength over 250 MPa, nearly twice as strong as the previous record reported in hierarchical nanostructured (Bi, Sb)₂ Te₃ alloy. Moreover, benefitting from the suppressed intrinsic excitation, the average zT value of this robust material could reach near 1.1 within 30-250 °C. This work opens a new pathway to design high-performance and mechanically stable Bi₂ Te₃ based alloys for miniature device development.

Synergistic Optimization of the Thermoelectric and Mechanical Properties of Large-Size Homogeneous Bi0.5Sb1.5Te3 Bulk Samples via Carrier Engineering for Efficient Energy Harvesting

Manufacturing an economically viable, efficient commercial thermoelectric (TE) module is essential for power generation and refrigeration. However, mediocre TE properties, lack of good mechanical stability of the material, and significant difficulties involved in the manufacturing of large-scale powder as well as bulk samples hinder the potential applications of the modules. Herein, an economically feasible single-step water atomization (WA) is employed to synthesize BST powder (2 kg) by Cu doping within a short time and consolidated into large-scale bulk samples (500 g) for the first time with a diameter of 50 mm and a thickness of about 40 mm using spark plasma sintering (SPS). The incorporation of Cu into BST greatly boosts the carrier concentration, leading to a significant increase in electrical conductivity, and inhibits the bipolar thermal conductivity by 73%. Synchronously, the lattice contribution (κ_L) is greatly reduced by the effective scattering of phonons by comprising fine-grain boundaries and point defects. Therefore, the peak ZT is shifted to the mid-temperature range and obtained a maximum of ∼1.31 at 425 K and a ZT_ave of 1.24 from 300 to 500 K for the BSTCu_0.05 sample, which are considerably greater than those of the bare BST sample. Moreover, the maximum compressive mechanical strength of large-size samples manufactured by the WA-SPS process is measured as 102 MPa, which is significantly higher than commercial zone melting samples. The thermoelectric module assembled with WA-SPS-synthesized BSTCu_0.05 and commercial n-type BTS material manifests an outstanding cooling performance (-19.4 °C), and a maximum output power of 6.91 W is generated at ΔT ∼ 200 K. These results prove that the BSTCu_x samples are eminently suitable for the fabrication of industrial thermoelectric modules.

Optimized Thermoelectric Properties of Bi0.48Sb1.52Te3 through AgCuTe Doping for Low-Grade Heat Harvesting

Zone-melted Bi₂Te₃-based alloys are the only commercially available thermoelectric (TE) materials, but they suffer from mediocre figure of merit (ZT) values and brittleness. In this work, we prepared Bi_0.48Sb_1.52Te₃ sintered samples using a hot-pressing method and added tiny AgCuTe to improve the comprehensive properties. Because the carrier concentration is boosted by the AgCuTe addition, the bipolar effect at higher temperature is explicitly suppressed and the power factor is also improved in a broad temperature scope. Simultaneously, κ_lat is mostly diminished by the introduced phonon scattering centers comprising point defects, dislocations, and grain boundaries. Consequently, we achieved a ZT_max of 1.25 at 350 K and its average ZT_ave of 1.1 from 300 to 500 K in the (Bi_0.48Sb_1.52Te₃ + 3 wt % Te) + 0.12 wt % AgCuTe sample. Composed of this sample and commercial Bi₂Te_2.5Se_0.5, the fabricated TE module manifests a maximum power output density of 0.31 W cm^-2 (T_cold = 300 K and T_hot = 500 K). This work suggests that AgCuTe-doped Bi_0.48Sb_1.52Te₃ is promising for recovering low-grade thermal energy near room temperature.

Enhanced Thermoelectric and Mechanical Performances in Sintered Bi0.48Sb1.52Te3-AgSbSe2 Composite

Bismuth telluride alloys have dominated the industrial application of thermoelectric cooling, but the relatively poor mechanical performance of commercial zone-melting material seriously limits the device integration and stability. Here, we exhibit synergistically enhanced thermoelectric and mechanical performances of sintered Bi_0.48Sb_1.52Te₃-AgSbSe₂ composites. It is found that the increased hole concentration improves the S²σ to 40 μW cm^-1 K^-2 at room temperature, and the emerged various defects effectively suppress the κ_l to 0.57 W m^-1 K^-1 at 350 K. All effects harvest a highest ZT = 1.2 at 350 K along with an average ZT = 1.0 between 300-500 K in the x = 0.2 sample. Notably, AgSbSe₂ addition not only optimizes the thermoelectric properties, but also enhances the mechanical performance with a Vickers hardness of 0.75 GPa. Furthermore, the isotropy of thermoelectric properties is also observably promoted by solid-phase reaction combined with high-energy ball milling and hot pressing. Our study reveals a viable strategy to improve the comprehensive performance of sintered bismuth telluride materials.

Phonon Engineering for Thermoelectric Enhancement of p-Type Bismuth Telluride by a Hot-Pressing Texture Method

Phonon engineering is a core stratagem to improve the thermoelectric performance, and multi-scale defects are expected to scatter a broad range of phonons and compress the lattice thermal conductivity. Here, we demonstrate obviously enhanced thermoelectric properties in Bi_0.48Sb_1.52Te₃ alloy by a hot-pressing texture method along the axial direction of a zone-melted ingot. It is found that a plastic deformation of grain refinement and rearrangement occurs during the textured pressing process. Although the obtained power factor is slightly decreased, a large amount of grain boundaries emerges in the textured samples and dense dislocations are observed around the boundaries and inside the grains. These additional phonon scattering centers can effectively scatter the low- and mid-frequency phonons, and the corresponding lattice thermal conductivity is significantly reduced to only 50% of that of zone-melted samples. Consequently, the maximum figure of merit (ZT) reaches 1.44 at 330 K and the average ZT (300-380 K) reaches 1.38. This study suggests that the simple hot-pressing texture technique is a promising method to significantly optimize the cooling capacity of Bi_0.48Sb_1.52Te₃-based thermoelectric refrigeration.