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.

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.

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.

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.

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.

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.

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.

Grain refinement is an effective way to improve the mechanical properties of Bi_0.5Sb_1.5Te₃ based alloys. However, the donor-like effect caused by the grain refinement during powder metallurgy severely degrades the thermoelectric properties of the material, which limits the application of Bi_0.5Sb_1.5Te₃-based alloys in micro thermoelectric devices. Therefore, in this study, focusing on p-type Bi_0.5Sb_1.5Te₃-base alloy, the impact of grinding and desorption atmosphere on donor-like effect and thermoelectric transport properties of sintered samples during powder metallurgy process were systematically studied through experiments combined with theoretical calculations. Surface defects $~\text{V}_{\text{Te}}^{\cdot \cdot }$ and ${{\text{{V}'''}}_{\text{Sb}}}$^， generated by the grinding process of Bi_0.5Sb_1.5Te₃-based alloy, reacts with the adsorbed O₂ during sintering process. This generates a large number of $\text{V}_{\text{Te}}^{\cdot \cdot }$ vacancies and free electrons, leading to a donor-like effect and a significant decrease in carrier concentration. The use of protective atmosphere (Ar atmosphere) during grinding to avoid exposure to air or O₂ desorption under protective atmosphere after grinding effectively suppresses the donor-like effect, maintaining high carrier concentration and electrical conductivity of the sample. Moreover, the performance is stable up to 473 K. The samples treated in air or sintered using powder placed in air exhibit a significant donor-like effect, and the carrier concentration decreases from 4.49×10¹⁹ cm^–3 to 3.21×10¹⁹ cm^–3. The maximum thermoelectric ZT of 1.03 is obtained at 402 K for the samples sintered with the powders treated under protective atmosphere, and the overall average ZT_ave reaches 0.92. This study provides a new avenue to regulate the donor-like effect of p-type polycrystalline Bi₂Te₃-based compounds and to optimize their thermoelectric properties.

Thermoelectric materials are functional materials that can realize the direct conversion between heat and electricity, which have great prospects in the field of green refrigeration and waste heat recovery. To date, researches on thermoelectric materials mainly focus on semiconducting inorganic materials and conductive polymers. Although great progress has been made regarding material design and performance improvement, it is still of great significance to explore and expand thermoelectric candidates for potential application. Metal-organic frameworks (MOFs) are porous extended solids formed by coordination bonds between organic ligands and metal ions or metal clusters. They are promising candidates in the field of thermoelectrics due to their unique porous structure as well as tunable composition and structure, which could meet the requirement of "electron crystal-phonon glass". In this work, conductive polymer, poly(3, 4-vinyl dioxythiophene) (PEDOT) was in-situ polymerized in Zr-based MOFs UiO-67 through “conductive guest-promoted transport” approach. The confined effects originated from porous structures of MOFs on molecular chains of PEDOT effectively improve electrical conductivity of the composites. As a result, the prepared composites exhibit an electrical conductivity up to 5.96×10⁻³ S·cm⁻¹ at room temperature, which is one order of magnitude higher than the corresponding PEDOT. Correspondingly, their power factor (PF) is up to 3.67×10⁻² nW·m⁻¹·K⁻² at room temperature. In conclusion, this work uses ordered porous structures of MOFs as reaction platform and constructs conductive polymer/MOFs conductive materials by facile in-situ polymerization methods, providing a reference for further development of MOFs-based thermoelectric materials.

Despite the growing research in CaTiO₃ as a novel high-temperature oxide thermoelectric material, effects of various elements doping on the microstructure and thermoelectric performance of CaTiO₃ have not been fully understood. Here, a combination of hydrothermal synthesis and vacuum hot-press sintering techniques was employed to fabricate polycrystalline bulks of CaTiO₃ doped with six elements: Cr, Nb, Eu, Dy, Ce, and La. Cr doping resulted in substantial precipitation of nanoscale Cr phases, leading to a severely compromised power factor and a ZT of only 0.012 at 983 K due to insufficient donor element concentration in the matrix. Incorporating Eu as a donor carrier in the matrix is proved ineffective, resulting in a marginal ZT enhancement of 0.141 at 1031 K. Nb doping resulted in the formation of micrometer-scale Nb phases with high thermal conductivity, leading to an elevation in thermal conductivity. However, the relatively higher Nb concentration in the matrix provided carriers, resulting in a noticeable ZT improvement to 0.263 at 1013 K. On the contrary, Dy, Ce, and La doping exhibited remarkable dual functionality as donor dopants and point defects, thereby significantly enhancing the power factor and concurrently reducing the lattice thermal conductivity. These improvements were achieved through efficient manipulation of carrier concentration and implementation of phonon scattering. As a result, the thermoelectric figure of merit (ZT) reached 0.357, 0.398, and 0.329 at 1031 K for Dy, Ce, and La-doped CaTiO₃ bulks, respectively. These values represent an extraordinary improvement of 296%, 342%, and 265%, respectively, as compared to that of the pristine CaTiO₃ (0.096 @1031 K). Notably, Dy-doped samples exhibited significantly reduced lattice thermal conductivity and comparatively higher power factors over the entire temperature range. Regulating Dy content and enhancing the second phase at grain boundaries enabled the decoupling of electrical and thermal transport properties, potentially surpassing the current ZT record of CaTiO₃. This study provides valuable insights into the relationships among composition, structure, and performance in CaTiO₃ doped with various elements, offering theoretical support for high-temperature thermoelectric applications.

Bi₂Te₃-based thermoelectric (TE) materials have already been commercialized, of which the hygrothermal stability has a direct impact on the service reliability of TE devices, but is still confronted many challenges. This work investigated the degradation behavior of commercial n-type Bi₂Se_0.21Te_2.79 and p-type Bi_0.4Sb_1.6Te₃ TE materials during storage in 85 ℃, 85% RH hygrothermal environment for 600 h. The surfaces of n-type Bi₂Se_0.21Te_2.79 and p-type Bi_0.4Sb_1.6Te₃ TE materials were oxidized with reaction process of Bi₂Te₃+O₂→Bi₂O₃+TeO₂ and Bi₂Te₃+Sb₂Te₃+O₂→Bi₂O₃+Sb₂O₃+TeO₂, respectively. The oxidation process creates nanoscale holes and even microcracks inside the material, which leads to an overall deterioration of the electrical and thermal properties. At room temperature, the electrical conductivity of the n-type Bi₂Se_0.21Te_2.79 material drops from 9.45×10⁴ S·m^-1 to 7.79×10⁴ S·m^-1 after exposure, and ZT decreases from 0.97 to 0.79, while Seebeck coefficient of the p-type Bi_0.4Sb_1.6Te₃ material declines from 243 μV·K^-1 to 220 μV·K^-1, correspondingly, ZT decreases from 1.24 to 0.97. In conclusion, Bi₂Te₃-based TE materials have extremely poor hygrothermal stability, and their corresponding micro-TE devices need to be strictly encapsulated in service to prevent complex redox reactions between the TE materials themselves and the environmental water vapor and air.

n-Type AgBiSe₂-based compounds are considered as promising high-performance thermoelectric (TE) materials due to the low lattice thermal conductivity. However, their two phase transitions between 300 and 700 K limits their applications. Therefore, it is crucial to obtain AgBiSe₂-based compounds with stable structures and optimized TE properties. In this work, the Pb-free group IV-VI compound SnTe is selected for alloying with AgBiSe₂. Introduction of SnTe not only reduces the cubic phase transition temperature, but also effectively suppresses the reversible phase transition of AgBiSe₂. At room temperature, reduction of the lattice thermal conductivity from 0.76 to 0.51 W·m^-1·K^-1 results from highly disordered distribution of atoms. Furthermore, Nb dopant to replace Ag, significantly improves carrier concentration of AgBiSe₂-based compounds, which promotes the effective mass and increases the electrical conductivity from 77.7 S·cm^-1 to 158.1 S·cm^-1 at room temperature. Meanwhile, the defect scattering at high temperature is enhanced with the increase of impurity point defects, leading to the lattice thermal conductivity reduced. At 700 K, the lattice thermal conductivity is reduced from 0.56 to 0.43 W·m^-1·K^-1, obtaining stable cubic phase compound (Ag_0.98Nb_0.02BiSe₂)_0.75(SnTe)_0.25 with a ZT of 0.32 at 650 K. These results indicate that the (AgBiSe₂)_0.75(SnTe)_0.25 compound is a promising n-type TE compound with low lattice thermal conductivity and a stable cubic structure. Such efforts provide a scheme for the crystal structure regulation of high-performance TE materials with phase transition and promotion of its application.

Combining thermoelectric materials with magnetocaloric materials enables a potential new all-solid-state cooling technology based on coupling enhancement of thermoelectric cooling and magnetic cooling, which is highly expected to achieve a technological change from thermoelectric cooling to thermoelectromagnetic cooling. However, the stability of thermal-electro-magnetic composites in service environment is still unknown. Herein, a series of Gd/BST thermo-electro-magnetic gradient composites were prepared by combining Bi_0.5Sb_1.5Te₃ (BST) thermoelectric material and Gd magnetocaloric material via spark plasma sintering technology. Evolution of phase composition, microstructure, thermoelectric, and cooling performance of the gradient composites during the 12 d aging process at 338 K and 80% relative humidity(RH) were systematically studied. The results show that the phase composition and microstructure of Gd/BST thermal-electro-magnetic gradient composites have excellent service stability. The chemical composition and average thickness (~4.5 μm) of Gd-Te diffusion layer at Gd/BST heterogeneous interface doesn’t exhibit obvious change during the aging process. The test of thermoelectric and cooling performance along different Gd concentration gradient indicates that ZT of the materials negligibly changed before and after aging treatment, and the cooling temperature difference of the single-leg device is stable at about 6.5 K under the threshold current of 2.5 A. These results show that thermoelectric and cooling performance of the Gd/BST gradient composites have excellent service stability.

Zintl phase Mg₃X₂ (X=Sb, Bi) based thermoelectric materials have attracted much attention because of their non-toxic, low cost and high performance. Compared with polycrystalline materials, the Mg₃X₂ crystals are of great value in revealing material’s intrinsic and anisotropic thermoelectric properties, as well as providing effective strategies for enhancing electrical and thermal transport properties. Therefore, the recent progress of single crystal growth and thermoelectric properties for Mg₃X₂ crystals are systematically summarizes in this paper. Due to the volatility and causticity of Mg element, several different methods such as slow cooling method, directional solidification method, flux method, and flux Bridgman method are widely used for synthesizing Mg₃X₂ crystals, in which the flux Bridgman method is more competitive to prepare large size bulk crystals. Researchers found that both n-type and p-type Mg₃Sb₂ crystals show an anisotropy thermoelectric transport property. The crystal growth rate, the concentration of self-doped Mg element, the concentration of impurity doping or alloying elements have a great impact on both electrical and thermal transport properties for Mg₃Sb₂ crystals. So far, the p-type and n-type Mg₃Sb₂ crystals with ZT value of 0.68 and 0.82 are achieved, respectively. This paper reviews the recent progress of growth and thermoelectrics properties of Zintl phase Mg₃X₂-based crystals, revealing that the flux Bridgman method is the most effective method to produce large-sized Mg₃X₂-based crystals. Tuning chemical composition of Mg₃X₂-based crystal by doping and forming solid solution for optimal carrier concentration and band structure engineering is expected to further improve the thermoelectric performance of Mg₃X₂-based crystal. The above-mentioned growth method and research strategies provide a significant guidance for the in-depth understanding of the Mg₃X₂-based crystal in the future.

Microstructure plays a key role in tuning physical properties of materials. Here YbAl₃ materials with high figure of merit ZT of 0.35 at 300 K was directly synthesized with Yb and Al pure powders through one-step spark plasma sintering process in 10 min. The excellent thermoelectric performance is attributed to the simultaneous reduction in the lattice thermal conductivity by 47% and electronic thermal conductivity by 27% at 300 K. The remarkable decrease in the electronic thermal conductivity is ascribed to the enhanced scattering of electrons by nanocrystals with 5-20 nm in diameter, strip-like non-crystal with several nanometers in width and various atomic-scale distortions. The substantial decline in the lattice thermal conductivity originates from the enhanced scattering of phonons due to multi-scale microstructures spanning from nanoscale to mesoscale. This work demonstrates that one-step spark plasma sintering process is an efficient strategy to rapidly synthesize YbAl₃ materials with multi-scale microstructures and enhanced thermoelectric performance.

The smaller the size of the Bi₂Te₃-based micro thermoelectric device, the more significant the effect of interface bonding strength and contact resistance on the mechanical properties, open circuit voltage and output power of the device. It is of great significance to develop a thermoelectric unit preparation technology with low cost and simple process, and to enable the interface between n-type Bi₂Te₃ bulk materials and barrier layer with low contact resistance and high bonding strength. Here, surface of n-type Bi₂Te₃-based thermoelectric material was treated in mixed acid solution (pH~3), followed by electroless plating Ni (5 μm), and then welded with Cu electrode to prepare thermoelectric unit. After corrosion, the anchoring effect between large gully on the surface of n-type Bi₂Te₃-based thermoelectric materials and Ni barrier layer contributes to the interface bonding strength of 15.88 MPa for the material corroded for 6 min. Furthermore, nano-holes between the Ni barrier layer and the fine branches corroded by further corrosion significantly increase the interface contact resistance, resulting in 2.23 μΩ·cm² for the material corroded for 2 min. Finally, the output power of the micro thermoelectric device prepared by n-type Bi₂Te₃-based bulk material for 4 min corrosion treatment is as high as 3.43 mW at 20 K temperature difference (306 K at high temperature end and 286 K at low temperature end). Compared to device with the same size prepared by commercial electroplating coating, the output power is increased by 31.92%. This work provides support to optimize the performance of micro thermoelectric devices.