β-FeSi₂, an environmentally friendly and high temperature oxidation-resistant thermoelectric material, has potential applications in the field of industrial waste heat recovery. Previous studies have shown that phosphorus (P), an ideal n-type dopant in the silicon (Si) site of β-FeSi₂, can easily lead to the formation of a secondary phase, thereby limiting the enhancement of thermoelectric performance. In this study, a series of FeSi_2-xP_x (x=0, 0.02, 0.04, 0.06) samples were synthesized using an induction melting method, which greatly inhibited the formation of the secondary phase. Then, the influence of P doping on the electrical and thermal transport properties of β-FeSi₂ was studied. The results indicate that the solubility limit of P in β-FeSi₂ is about 0.04, consistent with earlier theoretical predictions based on the defect formation energy. It is also discovered that P doping enhanced the thermoelectric performance of β-FeSi₂, culminating in an optimal figure of merit (ZT) of FeSi_1.96P_0.04 approximately 0.12 at 850 K, which is much higher than the previous results (ZT about 0.03 at 673 K). However, compared to β-FeSi₂ doped with other n-type elements like cobalt (Co) and iridium (Ir), which can achieve carrier concentrations up to 10²² cm^-3, P-doped β-FeSi₂ exhibits lower carrier concentrations, with the highest of only 10²⁰ cm^-3. This results in a weaker electron-phonon scattering effect, which in turn constrains the overall enhancement of the thermoelectric performance. If the carrier concentration could be further increased, the thermoelectric performance of the material is expected to evolve significantly.

Full-Heusler Fe₂VAl alloy has received significant attention for thermoelectric (TE) applications due to its high mechanical strength, favorable electrical transport behavior, and earth-abundant constituent elements. However, its intrinsically high lattice thermal conductivity hinders the enhancement of the figure of merit (zT). In this study, a series of bulk materials with the nominal composition of Fe₂V_1+xAl_1-x (x=0-0.21) were prepared by the arc-melting method. Effects of substituting Al site with V on the phase composition, microstructure, band structure, and TE transport properties were systematically investigated. All materials exhibit a single phase with a partially disordered B2 structure. V-doping shifts the Fermi level into the conduction band, significantly enhancing the carrier concentration, and resulting in a high power factor of 4.5 mW·K^-2·m^-1. Additionally, the lattice thermal conductivity is substantially reduced due to enhanced phonon scattering induced by the mass and stress fluctuations. Ultimately, a maximum zT of 0.14 is achieved for the material with x=0.15, which is nearly 280 times larger than that of undoped Fe₂VAl. This work demonstrates that substituting Al site with V can effectively improve the TE performance of Fe₂VAl alloy.

MgAgSb exhibits excellent thermoelectric performance in the near-room temperature range of 300-573 K, making it a key research focus for p-type materials in recent years. Due to the low level of atomic doping, the improvement of thermoelectric performance through carrier concentration optimization via doping alone is limited. In this study, Nb and Ta nano-secondary phases were introduced into the MgAg_0.97Sb_0.99 matrix, and two series of alloys, xNb/MgAg_0.97Sb_0.99 and yTa/MgAg_0.97Sb_0.99, were prepared using high-energy ball milling mechanical alloying. The introduction of Nb nano-secondary phase significantly altered carrier and phonon transport properties of the material. It was the presence of nano-secondary phase Nb in the 0.005Nb/MgAg_0.97Sb_0.99 alloy that optimized its unique microstructure and electrical performance, achieving a maximum power factor of 24.1 μW·cm^-1·K^-2 at 533 K. Additionally, the introduction of secondary phase enhanced phonon scattering, significantly reducing thermal conductivity. As a result, the 0.005Nb/MgAg_0.97Sb_0.99 sample achieved an optimal zT of 1.09 at 483 K, improved by 9.0% compared to MgAg_0.97Sb_0.99 at the same temperature. Furthermore, the introduction of Ta, a homologue of Nb, into the MgAg_0.97Sb_0.99 sample showed a similar trend, with 0.005Ta/MgAg_0.97Sb_0.99 achieving zT of 1.02 at 483 K. This study demonstrates an effective nanocomposite strategy for optimizing thermoelectric transport properties and enhancing thermoelectric performance of p-type MgAg_0.97Sb_0.99.

Thermoelectric materials can realize the direct conversion of heat and electric energy, and have broad application prospects in the fields of thermoelectric power generation and semiconductor refrigeration. Both SnTe and PbTe thermoelectric materials belong to the Ⅳ-Ⅵ group, and have the same NaCl-type crystal structure, but SnTe possesses poor thermoelectric properties. In this work, SnTe-based thermoelectric materials were prepared by a fast method, known as self-propagating high-temperature synthesis under high-gravity field (HG-CS) combined with spark plasma sintering (SPS). The effect and mechanism of multi-element doping on the thermoelectric properties of SnTe compounds were also studied. Multi-element doping, equivalent ions Ge²⁺ and Pb²⁺ in cation of SnTe and anionic S^2- and Se^2-, causes a large number of lattice distortion point defects. At the same time, rapid solidification under the supergravity field brings about plastic deformation and introduces a stress field and a large number of dislocations, which results in the formation of multilevel microstructural defects and strong scattering of medium- and high-frequency phonons. As a result, the room-temperature thermal conductivity decreases dramatically from 7.28 W·m^-1·K^-1 (undoped SnTe) to 2.74 W·m^-1·K^-1 (Sn_0.70Ge_0.15Pb_0.15Te_0.80Se_0.10S_0.10), with a minimum thermal conductivity of only 1.38 W·m^-1·K^-1 at 873 K. These microstructural defects scatter phonons and carriers, leading to a decrease in carrier mobility and conductivity. It is worth mentioning that doping decreases the bandgap of SnTe and increases the Seebeck coefficient, so that the power factor PF of the doped material remains at a high value. Finally, the peak thermoelectric figure of merit ZT of Sn_0.70Ge_0.15Pb_0.15Te_0.80Se_0.10S_0.10 sample is greatly improved to 1.02 (873 K).

As group ⅣA tellurides, SnTe has the same crystal structure and similar bivalent band structure as PbTe, making it a promising thermoelectric material. However, the main concern of softening at elevated temperature and lower ZT at low temperatures has been hindering its application. Therefore, it is significant to expand the service temperature range of SnTe by improving its average ZT. It has been reported that the thermoelectric performance of SnTe is improved by regulating the power factor and lattice thermal conductivity based on band and lattice engineering. In this study, MgSe alloying strategy was used to prepare a series of Sn_1-yPb_yTe-x%MgSe(0.01≤y≤0.05, 0≤x≤6) samples by combining melting and Spark Plasma Sintering (SPS) techniques. The results show that alloying MgSe leads to an increase in the band gap, effectively suppressing the bipolar effect of intrinsic SnTe, improving the Seebeck coefficient in the high-temperature range, and reducing lattice thermal conductivity through phonon scattering as well. As a result, ZT at 873 K is improved by 100%. The incorporation of Pb effectively modulates the carrier concentration, successfully suppressing electronic thermal conductivity, and thereby improving average thermoelectric performance of SnTe. Among them, Sn_0.96Pb_0.04Te-4%MgSe possesses a ZT value of 1.5 at 873 K and an average ZT value of 0.8 at 423-873 K, displaying superior performance compared to literature.

Based on Peltier effect, Bi₂Te₃-based alloy is widely used in commercial solid-state refrigeration at room temperature. The mainstream strategies for enhancing room-temperature thermoelectric performance in Bi₂Te₃ focus on band and microstructure engineering. However, a clear understanding of the modulation of band structure and scattering through such engineering remains still challenging, because the minority carriers compensate partially the overall transport properties for the narrow-gap Bi₂Te₃ at room temperature (known as the bipolar effect). The purpose of this work is to model the transport properties near and far away from the bipolar effect region for Bi₂Te₃-based thermoelectric material by a two-band model taking contributions of both majority and minority carriers into account. This is endowed by shifting the Fermi level from the conduction band to the valence band during the modeling. A large amount of data of Bi₂Te₃-based materials is collected from various studies for the comparison between experimental and predicted properties. The fundamental parameters, such as the density of states effective masses and deformation potential coefficients, of Bi₂Te₃-based materials are quantified. The analysis can help find out the impact factors (e.g. the mobility ratio between conduction and valence bands) for the improvement of thermoelectric properties for Bi₂Te₃-based alloys. This work provides a convenient tool for analyzing and predicting the transport performance even in the presence of bipolar effect, which can facilitate the development of the narrow-gap thermoelectric semiconductors.

As a typical multi-layered compound thermoelectric (TE) material, BiSbSe_1.50Te_1.50, can be utilized to fabricate p-n junctions with the same chemical composition. It has great potential in the development and design of high-performance TE devices due to its ability to avoid lattice mismatch incompatibility and harmful band misalignment. However, the TE performance of n-type BiSbSe_1.50Te_1.50 is limited due to poor electrical transport properties, which hinders its further application in TE devices. Therefore, it is of great significance to improve the TE performance by enhancing the electrical transport properties while maintaining low thermal conductivity. In this work, a series of n-type BiSbSe_1.50Te_1.50 hot deformation samples were prepared by solid-state reaction combined with hot pressed sintering. It is found that the preferred orientation and nanoscale lamellar structures with large surface areas form in hot-deformed samples. The donor-like effect elevates the carrier concentration, while these lamellar structures facilitate higher carrier mobility by providing expressways for carriers, giving rise to the enhanced electrical conductivity. Additionally, various and abundant multiscale defects are introduced into samples, evoking strong phonon scattering with different frequencies and thus lowering the thermal conductivity. The electrical and thermal transport properties have been synergistically optimized by hot deformation, realizing the improvement of TE properties for n-type BiSbSe_1.50Te_1.50. As a result, a peak thermoelectric figure of merit (ZT) of 0.50 at 500 K is achieved for the hot-deformed sample, which increased ~138% compared to the undeformed sample (0.21). This work establishes a foundation for further advancement of the preparation for BiSbSe_1.50Te_1.50 TE devices with high conversion efficiency and homogeneous structure.

p-type GeTe based thermoelectric (TE) materials have attracted significant attention owing to their remarkable TE performance in medium and low temperature range (300-800 K). However, the material undergoes a phase transition from rhombohedral to cubic in the temperature range of 600-700 K, inducing changes in the coefficient of thermal expansion that limit its application in TE devices. Consequently, it is imperative to develop stable GeTe based thermoelectric material free from phase transitions. In this study, high temperature melting combined with spark plasma sintering was employed to synthesize GeMnTe₂. The as-synthesized samples contain the secondary phase of MnTe₂, which increases total thermal conductivity from 1.34 W·m^-1·K^-1 to 1.81 W·m^-1·K^-1 at 800 K. By optimizing the chemical stoichiometry of different elements, the formation of MnTe₂ secondary phase is suppressed, resulting in stable pure cubic phase GeMnTe_1.96. GeMnTe_1.96 achieves a maximum zT of ~0.85 at 800 K. This TE material exhibits significant potential for efficient and stable waste heat utilization in the medium temperature range.

Thermoelectric materials can directly convert thermal energy into electrical energy, offering significant potential for waste heat recovery applications. Ag₂Se as a novel low-temperature thermoelectric material has attracted considerable attention due to its unique crystal structure and excellent electronic transport properties. In this study, micro-sized Ag₂Se powders were synthesized via solvothermal method, which contributes to precise control over the powders’ structure, composition, and grain size. The results indicate that the synthesized Ag₂Se powders exhibit irregular micro-columnar structure with a complete crystal lattice, and the Ag-to-Se ratio is close to the theoretical value (approximately 2 : 1 (in atom)). Furthermore, spark plasma sintering (SPS) was employed to densify the Ag₂Se powders at various temperatures, which reveals that the sintering temperature has a significant impact on the microstructure and thermoelectric performance. The fracture surface of the sample sintered at 473 K is relatively dense, whereas samples sintered at 573 and 673 K display the emergence of nanopores. Notably, the nanopores are particularly pronounced in the sample sintered at 673 K, which effectively reduces the material’s lattice thermal conductivity (κ₁) to 0.33 W·m^-1·K^-1 at room temperature. Ultimately, the Ag₂Se bulk prepared at a sintering temperature of 573 K exhibits the best thermoelectric performance, achieving a room-temperature thermoelectric figure of merit (ZT) of 0.5. This study not only demonstrates the feasibility of the solvothermal method for fabricating high-performance micro-sized thermoelectric powders but also provides valuable experimental evidence for further optimizing the microstructure and thermoelectric properties of Ag₂Se materials.

Bismuth telluride-based materials have been extensively studied due to their outstanding thermoelectric performance at room temperature. However, bismuth telluride is inherently brittle, which poses a significant challenge in developing flexible bismuth telluride-based materials with high thermoelectric performance remains in thermoelectric research. In this study, a non(00l) layered flexible p-type thermoelectric thin film was fabricated by depositing Cu_0.005Bi_0.5Sb_1.495Te₃ onto polyimide (PI) substrates using magnetron sputtering technology, with a systematic investigation of the effect of sputtering pressure on thermoelectric properties. The results show that at 0.7 Pa sputtering pressure, its mobility is enhanced due to the large grain size and high crystallinity, its carrier concentration is optimized to 5.78×10¹⁹ cm⁻³, and its room-temperature power factor (PF) reaches 1660 μW·m⁻¹·K⁻². In addition, the film exhibits excellent mechanical flexibility, showing less than 10% variation in resistivity at a bending radius of 5 mm and less than 5% variation in Seebeck coefficient after 600 cycles of bending. Furthermore, a flexible thermoelectric device comprising four p-type thermoelectric legs (5 mm×25 mm×767 nm) was designed and fabricated based on this film. The device demonstrates promising performance, generating an output voltage of 18.5 mV, with a power density reaching 44.80 μW·cm⁻² under a temperature difference of 30 K. Touch-sensitive linguistic output design of the device demonstrates promising potential for language assistance applications. This work provides valuable insights into the magnetron sputtering preparation of high-performance flexible bismuth telluride-based thermoelectric materials and the optimization of their properties.

Zone melting technique is an important method for commercial preparation of Bi₂Te₃-based thermoelectric materials. The zone melting purification process is affected by segregation of materials. However, so far, the effect of zone melting process on the segregation mechanism of Bi₂Te₃-based materials has not yet formed unified understanding. In particular, increase of material components significantly affects the segregation process and uniformity. In this study, n-type Bi_1.96Sb_0.04Te_2.694Se_0.3Br_0.006 material was used as the research object, and the melting-zone melting-annealing process was used to systematically explore the influence of zone melting temperature on the composition and uniformity of thermoelectric performance. It was found that the zone melting temperature had a great influence on uniformity of the ingot, and the axial composition segregation was an important factor affecting its uniformity. At high zone melting temperature (≥988 K), the segregation of Bi₂Te₃-rich phase appeared at the top of the ingot, which made the poor uniformity of thermoelectric properties of the material. The maximum difference of ZT at room temperature in different regions (center top, edge top (ET), center bottom, edge bottom) of samples with zone melting temperatures of 988 and 1003 K reached 31.5% and 28.6%, respectively. Reducing the zone melting temperature to 958 K significantly inhibited the segregation of Bi₂Te₃-enriched phase, and a cylindrical ingot (inner diameter of 16 mm, height of 55 mm) with excellent thermoelectric properties and uniformity was prepared. The maximum difference of ZT at room temperature in different regions was only 14%, and the maximum ZT of 958 K-ET sample was 1.05 at 350 K. This study reveals the regulation mechanism of zone melting temperature on the composition and thermoelectric performance uniformity of multi-component n-type (Bi, Sb)₂(Te, Se)₃-based materials, which provides important guidance for the preparation of high-performance thermoelectric materials with excellent uniformity.

Though Te has excellent figure of merit (ZT), the severe element diffusion and reaction at the Te/metallic-electrodes interface render high contact resistivity (ρ_c) and low device conversion efficiency (η). Therefore, it is critical to develop suitable barrier layers for optimizing the bonding between Te and metallic electrodes. In this work, an appropriate barrier layer, NiTe_2-m (Ni_xTe (x=0.500~0.908)), was screened based on gradient structure. No reaction layers and defects at the interface of Ni_0.5Te/Te_0.985Sb_0.015/Ni_0.5Te were detected before and after aging at 473 K. Low ρ_c (less than 10 μΩ·cm²) and high η (about 75% of the theoretical value under a temperature difference of 180 K (hot end: 473 K)) were achieved and maintained stable during aging, showing excellent thermal stability of the interface. When x>0.500, the thickness of the interface reaction layer decreased with x increasing, showing the retarding effect dominating the growth behavior of interface reaction layer not from the usual thermodynamic factors, such as interface reaction energy and composition gradient, but from the “atom vacancy” on formation of the reaction layer.

Ag₂Se-based thermoelectric films and devices have become a popular research topic in the field of wearable thermoelectric energy conversion due to their narrow bandgap semiconductor properties, which exhibit good thermoelectric properties at room temperature. These films are typically constructed by stacking nanoparticles, and the dimensions of nanomaterials significantly impact the thermoelectric transport properties of the network. In this study, Ag₂Se nanomaterials with different dimensions were prepared by solvothermal and template methods, and flexible Ag₂Se thermoelectric films were constructed on polyimide substrates using Ag₂Se nanomaterials with different dimensions by spraying process combined with high-temperature post processing. The effects of the dimensionality of Ag₂Se nanomaterials on the microstructures and thermoelectric properties of the films were then systematically investigated. Zero dimensional Ag₂Se nanoparticles exhibited superior conductive networks and thermoelectric properties in comparison to one dimensional nanowire structures. Furthermore, the room temperature power factor of the films reached 199.6 μW·m^-1·K^-2, and the power factor was 257.9 μW·m^-1·K^-2 at the temperature of 375 K, which indicated the good thermoelectric properties of the films. Additionally a device was designed and integrated based on the Ag₂Se films, with four thermoelectric arms and excellent performance. The device exhibited good mechanical flexibility and output performance with internal resistance increased by only 8.2% after 1000 bending cycles (bending radius: 20 mm), and the device displayed an open-circuit voltage of 9.1 mV and a max output power of 43.7 nW at a temperature difference of 30 K. This study presents a novel approach for the preparation of flexible Ag₂Se-based thermoelectric thin-film materials and devices.

Flexible thermoelectric devices, capable of generating electricity from the slight temperature difference between the human body and the environment, demonstrate significant potential for continuous power supply in wearable devices. However, the poor thermoelectric performance still limits their widespread application. This study reports a method for fabricating high-performance flexible thermoelectric thin films using inkjet printing. AgCuTe nanowires prepared by a chemical transfer method were dispersed in ethanol to form the ink with no significant sedimentation, which could be stably and continuously sprayed to print p-type thermoelectric films on polyimide substrates. Dense thermoelectric films were then obtained through thermal treatment by a spark plasma sintering furnace, and the effect of sintering temperature on thermoelectric properties was studied. The results showed that the film sintered at a pressure of 10 MPa and a temperature of 400 ℃ for 15 min possessed a room temperature power factor of 432 µW·m^-1·K^-2, which is 182% higher than that of inkjet-printed p-type Bi₂Te₃ films (a room temperature power factor of 153 µW·m^-1·K^-2) reported in literature. This advancement further expands the application of inkjet printing in the field of flexible thermoelectrics and provides more possibilities for the fabrication of a new generation of high-performance flexible thermoelectric devices.