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.

Metal sulfide Ag₂S is an attractive semiconductor due to its excellent physical and chemical property that enable it with wide applications in fields of catalysis, sensing, optoelectronics in past years. In present work, ϕ18 mm× 50 mm Ag₂S ingot was successfully prepared using zone melting method and its thermoelectric (TE) behavior was investigated. Ag₂S has standard monoclinic P2₁/c space group (α-Ag₂S phase) below 450 K and transfer to cubic structure (β-Ag₂S phase) over this temperature. Ag₂S is a n-type semiconductor as the Seebeck coefficient S is always negative due to the Ag interstitial ions in the material that can provide additional electrons. S is about -1200 µV·K^-1near room temperature, declines to -680 µV·K ^-1 at 440 K and finally decreases to ~-100 µV·K ^-1at β-Ag₂S state. The electrical conductivity (σ) of α-Ag₂S is almost zero. However, the value sharply jumps to ~40000.5 S·m ^-1 as the material just changes to β-Ag₂S at 450 K and then gradually deceases to 33256.2 S·m ^-1 at 650 K. Hall measurement demonstrates that carrier concentration n_H of Ag₂S is suddenly increased from the level of ~10 ¹⁷ cm^-3 to ~10¹⁸ cm^-3during phase transition. Total thermal conductivity κ of α-Ag₂S is ~0.20 W·m ^-1·K^-1 but is ~0.45 W·m^-1·K^-1of β-Ag₂S. Ultimately, a maximum ZT=0.57 is achieved around 580 K that means Ag₂S might be a promising middle-temperature TE material.

MnTe is a promising candidate for the p-type lead-free thermoelectric material in middle temperature application. However, its thermoelectric performance isn’t qualified for some conventional n-type materials to form efficient thermoelectric devices. In this study, Mn_1.06-xGe_xTe (x=0, 0.01, 0.02, 0.03, 0.04) polycrystalline block samples with different Ge doping contents were efficiently synthesized by vacuum melting quenching and spark plasma sintering. The as-obtained Mn_1.06-xGe_xTe bulk was dense and consisted of homogeneous composition. Tiny extensive Mn can effectively restrict the formation of the second phase of MnTe₂ and improve the thermoelectric properties of the matrix phase. Electrical conductivity of the materials increasing to 7×10³ S∙cm^-1 results from the enhanced carrier concentration 7.328×10¹⁸ cm^-3 at 873 K, which contributed to a power factor of 620 μW∙m^-1∙K^-2 by 4% Ge doping. Meanwhile, Mn_1.06-xGe_xTe showed the reduced thermal conductivity of 0.62 W∙m^-1∙K^-1 by enhanced phonons scattering intensified with point defects, realizing the effective regulation of both electrical- and thermal-transport properties. Mn_1.02Ge_0.04Te achieved a thermoelectric performance of 0.86 at 873 K, which evolved by 43% compared with the pristine sample.

The thermoelectric properties of ZrNiSn-based half-Heusler materials were hindered due to their high thermal conductivity. In order to reduce the lattice thermal conductivity, the high-entropy alloys ZrNiSn and Zr_0.5Hf_0.5Ni_1-xPt_xSn (x=0, 0.1, 0.15, 0.2, 0.25, 0.3) were prepared by levitation melting and spark plasma sintering. Configurational entropy of the alloys was manipulated by Hf substitution for Zr and Pt substitution for Ni. Effects of configuration entropy on the thermoelectric properties were investigated. The reslults showed that the minimum sum of lattice thermal conductivity and bipolar thermal conductivity (κ_l+κ_b) at 673 K for Zr_0.5Hf_0.5Ni_0.85Pt_0.15Sn was optimized at 2.1 W·m^-1·K^-1, which was significantly reduced by about 58% when compared with ZrNiSn. This finding provides an effective strategy for reducing lattice thermal conductivity of ZrNiSn-based alloy to offer great potential for further improvement of thermoelectrics.

As being a good photoelectric and gas sensitive material, In₂O₃ is of great interest to the thermoelectric community due to its excellent thermoelectric properties at high temperature. In this study, the second phase InNbO₄ was successfully induced into the In₂O₃ matrix in situ by solid-state reaction combined with spark plasma sintering (SPS) to optimize the preparation process of bulk samples. It is found that introducing InNbO₄ distinctly affects the electrical transport properties of the In₂O₃/InNbO₄ composite samples, and its carrier concentration is dramatically increased. The highest electrical conductivity is 1548 S·cm^-1 at 1023 K, which is higher than those of most element-doped samples. The power factor of 0.67 mW·m^-1·K^-2 and the highest ZT value of 0.187 are achieved for the 0.998In₂O₃/0.002InNbO₄ sample at 1023 K. In conclusion, the electrical properties of In₂O₃ ceramics can be effectively improved by introducing in-situ InNbO₄ second phase, and thus the thermoelectric property at high temperatures is further regulated.