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Room-temperature thermoelectric materials provide promising solutions for energy harvesting from the environment, and deliver a maintenance-free power supply for the internet-of-things (IoTs). The currently available Bi2Te3 family discovered in the 1950s, still dominates industrial applications, however, it has serious disadvantages of brittleness and the resource shortage of tellurium (1 × 10−3 ppm in the earth's crust). The novel Mg3Sb2 family has received increasing attention as a promising alternative for room-temperature thermoelectric materials. In this review, the development timeline and fabrication strategies of the Mg3Sb2 family are depicted. Moreover, an insightful comparison between the crystallinity and band structures of Mg3Sb2 and Bi2Te3 is drawn. An outlook is presented to discuss challenges and new paradigms in designing room-temperature thermoelectric materials.

Bi2S3-based alloys are considered promising thermoelectric materials due to their large Seebeck coefficient and low lattice thermal conductivity. However, low electrical conductivity usually leads to poor electrical transport properties, which seriously restricts their further application in thermoelectric refrigeration and/or power generation. In this work, Bi2S3 with high electrical transport properties is synthesized hydrothermally via Se and Cl co-doping. The maximum electrical conductivity value of 483 S cm−1 was obtained for the Bi2S2.4Se0.4Cl0.20 sample at room temperature. The significant improvement of electrical conductivity gives rise to a high average power factor of 411 μW m−1 K−2 during the measuring temperature range and a peak value of 456 μW m−1 K−2 at 673 K. Benefiting from the largely improved electrical transport properties, a superior ZT value of approximately 0.66 and ZTave. of 0.36 were obtained for Bi2S2.4Se0.4Cl0.20, and the theoretically calculated conversion efficiency reached 5.7%. The results indicate that Bi2S3 is a promising candidate for thermoelectric applications at medium temperatures.

InGaO3(ZnO)m (m = 1, 2, 3, 4, and 5) ceramics are a series of n-type oxide thermoelectric materials with layered structures and low thermal conductivities. Herein, InGaO3(ZnO)m (m = 1, 2, 3, 4, and 5) ceramics were fabricated by spark plasma sintering (SPS). Two different trends in the thermoelectric properties of the InGaO3(ZnO)m (m = 1, 2, 3, 4, and 5) ceramics were observed depending on the odevity of the m value. The InGaO3(ZnO) sample exhibited a relatively high electrical conductivity and was therefore selected for vacuum annealing to further improve the electrical transport performance. Oxygen vacancy defects were introduced to the matrix during the annealing procedure, which improved the thermoelectric performance. A maximum ZT of 0.45 was obtained at 973 K for the InGaO3(ZnO) sample with a 96 h vacuum annealing treatment, which is 30 times higher than that of the pristine sample.

CuPbBi5S9 compounds have been investigated as gladite for years. However, there have been no significant studies on their physical and chemical properties. This work demonstrates that upon alloying with moderate Cu, Pb, Bi, and S using an appropriate preparation method, quaternary CuPbBi5S9 compounds can exhibit excellent figure of merit ZT within the temperature range 298–723 K. A low average velocity, low Young’s modulus and Debye temperature, and large Gruneisen parameter, determined experimentally, indicate strong lattice anharmonicity in CuPbBi5S9 crystals. Furthermore, density functional theory calculations (local vibration of low-frequency acoustic phonons) justify the low lattice thermal conductivity of CuPbBi5S9 compounds. Because of the low thermal conductivity (0.514 W m−1K−1) and a relatively high power factor (293 μW m−1K−2), a maximum ZT of 0.42 was achieved at 723 K for CuPbBi5S9 prepared by mechanical alloying combined with solid-state melting. Thus, CuPbBi5S9 materials are promising candidates for use as high-performance thermoelectric materials in the intermediate-temperature range.

Bi2S3 is composed of inexpensive and environmental friendliness elements, which has received extensive interests and been investigated as a promising mid-temperature thermoelectric material for years. Even pure Bi2S3 possesses a high Seebeck coefficient and low thermal conductivity, its low electrical conductivity leads to a low figure of merit (ZT) value. In this work, Bi2S3 fabricated by solid-state melting combined with spark plasma sintering can significantly enhance the thermoelectric performance via introducing small amounts of Cu and BiCl3. Cu interstitial doping and Cl substitution on S site result in a large increase in electrical conductivity. Additionally, the enhanced phonon scattering is derived from the point defects caused by element doping, the grain boundaries, and the small amount of secondary phase, which leads to the low thermal conductivity. Finally, a high ZT value of 0.7 is obtained at 773 K and reaches a large average ZT of 0.36 in the temperature range from room temperature (RT) to 773 K for the Cu-interstitial-doped and BiCl3-alloyed (Cu0.01Bi2S3 + 0.175 mol% BiCl3) sample. Furthermore, the mechanical properties of the Cu0.01Bi2S3 + 0.175 mol% BiCl3 sample are lower than those of other Bi2S3 samples, which stem from the weak chemical bonding strength.

Bi0.42Sb1.58Te3 + x wt % Cu1.8S (x = 0, 0.03, 0.05, and 0.1) bulk materials with enhanced thermoelectric and mechanical properties were fabricated by a solid-state reaction and spark plasma sintering. The thermoelectric properties, such as electrical transport properties and thermal conductivity, are highly dependent on the Cu1.8S content. The highest value of ZT obtained for Bi0.42Sb1.58Te3 with 0.05 wt % Cu1.8S is 1.23 at 373 K, and an optimistic average ZT of 1.2 is achieved at temperatures in the range of 323-448 K, which is 34% higher than that of the pristine sample. The highly enhanced ZT of the doped sample is attributed to the increased electrical conductivity and reduced lattice thermal conductivity caused by the effective element doping and the multiscale phonon scattering by quantities of point defects, twin boundaries, and nanopores. Further, the hardness obtained for this sample is 1.02 GPa, which is increased by 16% in comparison with that of the pristine sample. The conversion efficiency of the doped sample is also significantly higher than that of the pristine sample. Therefore, Cu1.8S is considered to be a promising dopant for enhancing the thermoelectric and mechanical properties of Bi-Sb-Te-based thermoelectric materials.

The management of thermal conductivity is of significant scientific interest, particularly for thermal barrier coatings (TBCs). Multifarious strategies have been used to regulate heat transportation, but it is hard to achieve limit thermal conductivity at elevated temperatures. A systematical investigation of weberite (La1-xGdx)3NbO7 was thus performed, and multiphonon scattering mechanisms were introduced to achieve limit thermal conductivity (0.92 W m−1 K−1). Phonon point defect scattering process accounted for thermal conductivity reduction at low temperatures. Additionally, lattice softening strongly contributed to the reduction of high-temperature thermal conductivity, and solid and stiff chemical bonds were beneficial for inhibiting thermal radiative conductivity. A novel strategy was presented to modify thermal transportation property of weberite RE3NbO7 ceramics. Also, the hardness, toughness, and modulus were improved to promote engineering applications of weberite RE3NbO7. This study also illuminates novel paths for thermal management and mechanical properties manipulation of TBCs, thermoelectric materials, and microelectronics.

Thermoelectric materials transfer heat and electrical energy, being useful for power generation or cooling applications. Many of these materials have narrow bandgaps, especially for cooling applications where this property has been seen as particularly important for enhancing the thermoelectric properties. We developed SnSe crystals with a wide bandgap Eg ~ 33 kBT with attractive thermoelectric properties through Pb alloying. The momentum and energy multiband alignment promoted by Pb alloying resulted in an ultra-high power factor ~75 μWcm–1K–2 at 300 K, and a ZTave ~ 1.90. We show that a 31-pair thermoelectric device can produce a power generation efficiency ~4.4% and a cooling ΔTmax ~ 45.7 K. These results demonstrate that wide bandgap compounds can be used for thermoelectric cooling applications.

Fabrication of nanoparticle-dispersed composites is an effective strategy for enhancing the performance of thermoelectric materials, and in particular SiC nanoparticles have been often used to create composites with Bi2Te3-based applied thermoelectric materials. However, the effect of particle size on the thermoelectric performance is unclear. This work systematically investigated the electrical and thermal properties of a series of (Bi,Sb)2Te3-based nanocomposites containing dispersed SiC nanoparticles of different sizes. It was found that particle size has a significant impact on the electrical properties with smaller SiC nanoparticles giving rise to higher electrical conductivity. Even though the dispersed SiC nanoparticles enhanced the Seebeck coefficient, no apparent dependence of the enhancement on the particle size was observed. It was also found that smaller SiC nanoparticles scatter phonons to some extent while the larger nanoparticles contribute to increased thermal conductivity. Eventually, the highest ZT value of 1.12 was obtained in 30 nm-SiC dispersed sample, corresponding to an increase by 18% from 0.95 for the matrix made from commercial scraps, and then the ZT was further boosted to 1.33 by optimizing the matrix composition and expelling excess Te during the optimized spark plasma sintering process. This work proves that the dispersion of smaller SiC nanoparticles in p-type (Bi,Sb)2Te3 materials is more effective than the dispersion of larger nanoparticles. In addition, it is revealed that additional compositional and/or processing optimization is vital and effective for obtaining further performance enhancement for nanocomposites of SiC nanoparticles dispersed in (Bi,Sb)2Te3.

Extremely low lattice thermal conductivity is always the pursuit of thermoelectric materials research. In this work, we reported an exceptional effect of Ag2S addition in MnTe, an emerging promising mid-temperature thermoelectric material, to enable the realization of minimum lattice thermal conductivity, namely ∼0.4 Wm−1K−1. Such a low lattice thermal conductivity is guaranteed by the incorporation of in-situ formed Ag rich phase (Ag2Te) with ultralow lattice thermal conductivity and further scattering of phonons from the partial doping effects induced point defects and boundaries between various phases. Apart from the dramatically decreased lattice thermal conductivity, the partial doping of Ag and S simultaneously enhance the electrical conductivity, further contributing to enhanced thermoelectric performance. Meanwhile, an inverse sign of Seebeck and Hall coefficient was observed and rationalized by the influence of highly electron-conductive Ag2Te phase. Thanks to the synergetic modulation of electrical and thermal transport properties by in-situ formed composite, a high ZT value of 1.1 was achieved in MnTe based thermoelectric materials, which also demonstrates the importance of compositing approaches to design state-of-the-art thermoelectric materials.

Thermoelectric technology has been applied in the fields of industrial waste heat power generation, special spatial power supply, semiconductor chip cooling, advanced refrigeration, etc. Comparing to other thermoelectric material systems, bismuth telluride (Bi2Te3) based thermoelectric materials are widely used in industrial and commercial fields due to their stable zT values and relatively high conversion efficiencies at room temperature range. However, the low hardness and easy dissociation of Bi2Te3 materials lead to the waste of raw materials in industrial processing. Therefore, substantial efforts have been made to find a balance between the zT value and the hardness of bismuth telluride. In this study, the mechanical properties of commercial Bi2Te3 were significantly enhanced by dispersing the second phase of the SiC particles in the matrix via a combined process of mechanical alloying (MA) and spark plasma sintering (SPS). Although the zT value of the composite was slightly decreased, the average hardness was increased from ~1.2 to 1.7 GPa, and the modulus of the composite was also improved. The thermal expansion rate and thermal expansion coefficients were also measured. The mechanism of the enhanced mechanical properties of the Bi2Te3 was discussed in detail. This work offered a simple strategy for improving the machinability of commercial Bi2Te3, which is of high importance for the large scale industrial application of thermoelectric materials.

Bi2Te3-based compounds are the most mature and widely used thermoelectric materials. However, industrial device fabrication will inevitably produce a lot of Bi2Te3 scraps, which results in wastes of expensive material resources. In this work, we recycled p-type (Bi,Sb)2Te3 scraps and reprocessed them by making nanocomposites with nano-SiC. The thermoelectric performance was enhanced, and a high ZT value of 1.07 was achieved, which is a significant improvement compared with commercial p-type (Bi,Sb)2Te3 ingots. Also, the hardness showed a notable increase, which is beneficial for device fabrication. In addition, we adjusted the proportion of Bi/Te of the commercial p-type (Bi,Sb)2Te3 scraps, thereby improving the thermoelectric performance and obtaining a higher ZT value of 1.2.

Eco-friendly tin sulfide (SnS) has attracted increasing attention in the thermoelectric community because of its elemental abundance and analogous crystal structure to SnSe as a new thermoelectric material. However, so far no high dimensionless thermoelectric figure of merit ZT > 1 was reported in SnS polycrystals. This work found an effective strategy for enhancing the thermoelectric performance of p-type polycrystalline SnS by Ag doping and vacancy engineering, leading to three orders of magnitude increase in carrier concentration and optimized effective mass and carrier mobility. As a result of the enhanced electrical conductivity, three times higher power factor ∼3.85 μW/cm K2 at 877 K is realized in Sn0.995Ag0.005S sample. Interestingly, nanostructuring with Ag nano-precipitates were formed in the Ag-doped SnS sample. Moreover, with introducing Sn vacancies in the crystal structure of Sn0.995-vacAg0.005S, the power factor further enhanced to ∼4.25 μW/cm K2. In addition to the low-frequency phonons scattering by Ag nano-precipitates, dislocations strengthens the scattering of mid-frequency phonon, leading to an ultralow lattice thermal conductivity < 0.5 W/m K above 800 K and a record high ZT up to 1.1 at 877 K in Sn0.99Ag0.005S polycrystals. Keywords: Thermoelectric tin sulfide, Spark plasma sintering, Nanoprecipitation, Dislocation

Thermoelectric materials can directly convert heat to electricity and vice versa. ZT ( = S 2 σ T / κ ) is usually used for evaluating the performance of thermoelectric materials. Improving ZT is a challenging task because of the strong interdependence among the Seebeck coefficient (S), electrical conductivity (σ) and thermal conductivity (κ). This review provides a summary of some promising thermoelectric materials in different developing stages with emphasis on the corresponding strategies for thermoelectric performance enhancement. In addition, the number of recent publications on some representative thermoelectric materials is summarized, addressing the current research interests in this field. Thermoelectricity still need intensified scientific efforts to meet the practical demand.

This paper focuses on the thermoelectric properties of natural chalcopyrite and provides methods for optimizing its thermoelectric properties. Natural chalcopyrite shows obvious anisotropic thermoelectric properties. Although the thermoelectric properties of natural chalcopyrite are low, they are found to be significantly enhanced by vacuum annealing. Natural chalcopyrite was proven to be feasible and potential thermoelectric materials.

Thermoelectric technology allows conversion between heat and electricity. Many good thermoelectric materials contain rare or toxic elements, so developing low-cost and high-performance thermoelectric materials is warranted. Here, we report the temperature-dependent interplay of three separate electronic bands in hole-doped tin sulfide (SnS) crystals. This behavior leads to synergistic optimization between effective mass (m*) and carrier mobility (μ) and can be boosted through introducing selenium (Se). This enhanced the power factor from ~30 to ~53 microwatts per centimeter per square kelvin (μW cm−1 K−2 at 300 K), while lowering the thermal conductivity after Se alloying. As a result, we obtained a maximum figure of merit ZT (ZTmax) of ~1.6 at 873 K and an average ZT (ZTave) of ~1.25 at 300 to 873 K in SnS0.91Se0.09 crystals. Our strategy for band manipulation offers a different route for optimizing thermoelectric performance. The high-performance SnS crystals represent an important step toward low-cost, Earth-abundant, and environmentally friendly thermoelectrics.

Bi2S3, with earth-abundant compositions and a low thermal conductivity, is regarded as a candidate thermoelectric material. In this work, Bi2S3 samples that were doped with x % mol InCl3 were successfully fabricated via a mechanical alloying and spark plasma sintering process. InCl3, as an n-type donor dopant, was added to the Bi2S3 system to improve its electrical transport properties and optimize its thermal conductivity. Upon doping, the electrical conductivity of Bi2S3 doped with 1 mol% InCl3 reaches up to 62 Scm−1, and the Seebeck coefficient maintains a relatively large value of −244 μV K−1 at 673 K, which results in a maximum power factor of 363 μW m−1 K−2. Furthermore, due to a simultaneously reduced thermal conductivity at high temperature, a ZT peak of 0.57 is obtained at 673 K along the parallel to the press direction for the sample doped with 1.0 mol% InCl3, which is almost four times higher than that of pristine Bi2S3 (0.14 at 673 K). The elastic properties and Debye temperature of Bi2S3 are also calculated to analyze the origin of the intrinsically low thermal conductivity and are compared to those of other thermoelectric materials with a low thermal conductivity.

In2O3(ZnO)k superlattice ceramics are promising oxide thermoelectric materials, as superlattice interfaces are effective in scattering phonons and filtering low-energy electrons to decrease thermal conductivity and improve the figure of merit (ZT) value. In this work, homologous compounds were prepared by solid-state reaction method using ZnO and In2O3 as raw materials. Phase purity and crystal structure were characterized by x-ray diffraction, revealing that In2O3(ZnO)k crystallizes in an R3 m space group for odd k values and P63/mmc for even k values. The study of thermoelectric (TE) properties showed that as k increased, the thickness of InO(ZnO) 3 + also increased, while the electrical conductivity, thermal conductivity, Seebeck coefficient, power factor and ZT value were all decreased. In2O3(ZnO)3 samples obtained a maximum power factor of 651 μW m−1 K−2 at 973 K, and also achieved a maximum ZT value of 0.24 at 973 K.

P-type Bi2-xSbxTe3 is one of the most mature and widely used thermoelectric (TE) materials. Enhancing its performance is important and has generated significant research attention, with studies ranging from chemical doping to various fabrication processes. In this work, we applied a method combining mechanical alloying and spark plasma sintering to synthesize Cu-doped Bi0.3Sb1.7Te3. By adjusting the doping amount of Cu, the electrical conductivity and power factor were improved with a reduced lattice thermal conductivity, leading to a high dimensionless figure of merit (ZT) up to 1.32 at 400 K. This high performance could be maintained even after the sintered sample was subjected to a 100 A electric current pulse (ECP) at 523 K for 30 min, despite some slight changes in the electrical transport properties. Interestingly, it was found that the ECP treatment induced grain refinement and nanoscale inhomogeneity that reduced the lattice thermal conductivity, which canceled the negative effect on the ZT that had been caused by the slight decline in power factor. This work validated the effectiveness of Cu-doping for Bi2-xSbxTe3, and it also demonstrated a facile method for the stability study of thermoelectric materials.

Although the Cu1.8S compound consisting of cheap and earth-abundant elements has been reported to be a potential candidate of thermoelectric material, its thermoelectric performance is not very high. In this work, a fast doping strategy is offered to introduce both point defects and nanostructures into thermoelectric materials to improve the performance. Ball-milled Cu1.8S powders were mixed with different amounts of In2S3 powders and were then sintered into bulk samples by the spark plasma sintering (SPS) technique. During the SPS process, In2S3 was doped into the Cu1.8S matrix at 723 K for 10 min, and the fast doping process results in point defects and nanostructures of the Cu1.8S bulk sample including nanopores and those with an included second phase. On the one hand, In doping increased the effective mass of charge carriers in Cu1.8S and improved the Seebeck coefficient, whereas on the other hand, the nanostructures reduced thermal conductivity significantly. The phase structure and microstructure of Cu1.8S bulks are highly dependent on the In2S3 content. Density functional theory calculations revealed that Cu1.8S has an intrinsically low lattice thermal conductivity because of low-frequency localized vibrations from the Cu ionic migration and Cu vacancies. As a result, Cu1.8S+3 wt% In2S3 bulk sample achieved a ZT value of ∼1.4 at 773 K compared with that of 0.45 at 773 K for the pristine Cu1.8S sample; this value is the highest ZT value in sulfide thermoelectric materials at this temperature. The fast doping strategy demonstrated in this work can also be applied for reducing thermal conductivity and improving ZT values of other thermoelectric systems.

Bismuth sulfide (Bi2S3) with an earth-abundant elemental composition has attracted wide attention as a promising n-type thermoelectric material in recent years. However, the thermoelectric performance of Bi2S3 is limited by its low electrical conductivity. It is therefore vital to choose an effective dopant for enhancing the electrical transport properties of Bi2S3. In this work, by using the solid-state reaction and the spark plasma sintering technique, Bi2S3 bulk materials doped with x mol% LaCl3 (x = 0, 1.0, 2.0, 3.0) were successfully fabricated. The conductivity of Bi2S3 samples doped with 2 mol% LaCl3 (about 153 S cm−1) was one order of magnitude higher than that of the pristine Bi2S3 sample, resulting in a high power factor of 483 μW m−1 K−2 at 625 K. The lattice thermal conductivity of the doped samples decreased due to more grain boundaries, point defects and nano-precipitates. Consequently, a peak thermoelectric figure-of-merit (ZT) of 0.50 at 625 K was obtained for the Bi2S3 sample with 2 mol% LaCl3 doping, which was over four times higher compared to that of pristine Bi2S3 (about 0.11 at 625 K). The results suggest that the dopant LaCl3 is effective for modifying carrier concentration and reducing lattice thermal conductivity.

GeTe has attracted extensive interest due to its promising mid-temperature thermoelectric performance, yet the high cost and ferroelectric phase transformation may limit its application. Here, we explore the thermoelectric properties of electron doped GeMnTe2, which is a much cheaper derivative of GeTe with a rock-salt structure and a high thermoelectric figure of merit (ZT value) of 1.4. It is substantiated that an all-scale hierarchical structure including point defects at the atomic scale, endotaxially embedded nano-inclusions and precipitates are responsible for the strong scattering of phonons and low lattice thermal conductivity. Further synergistic modulation of electrical and thermal transport properties is successfully realized by Pb doping, which is effective in optimizing the carrier concentration and suppressing the phonon propagation by lattice softening and enhanced point defect scattering. The present work revealed the potential of GeMnTe2 as a promising candidate for mid-temperature thermoelectric materials and demonstrated the effectiveness of nanostructuring to enhance the thermoelectric performance.

GeTe is a promising thermoelectric material at medium temperature, but its carrier concentration tends to go beyond the optimal range for thermoelectrics. This work realized a significant ZT enhancement from 1.0 to 2.0 by suppressing the formation of Ge vacancies and band convergence. By simply optimizing the amount of excessive Ge, the hole carrier concentration is greatly reduced. It is demonstrated that the suppression of Ge vacancies can not only optimize the carrier concentration but also recover the mobility to a high value of 90 cm2 V−1 s−1, which well exceeds the previously reported data and guarantees superior electrical transport properties, leading to a ZT of 1.6. Further Bi doping facilitates band convergence as featured by the increased band effective mass and high mobility, which in turn yields large power factors and low electronic thermal conductivity. Bi doping induced mass and strain fluctuation also favors the reduction of the lattice thermal conductivity. Consequently, a maximum ZT of ∼ 2.0 at 650 K with an average ZT of over 1.2 is achieved in the nominal composition Bi0.05Ge0.99Te, which is one of the best thermoelectric materials for medium temperature applications.

Modulating microstructures in a wide range from atomic defects to microscale structures independently can partially decouple the transport of charge carriers and phonons and thus enhance the figure of merit (zT) of thermoelectric materials. High mobility requires atomic scale purity, while introducing nanoscopic inhomogeneities leads to low thermal conductivity. Through a two-step sintering process with excess Te, lower reduction of mobility and decreased thermal conductivity were simultaneously achieved in (Bi,Sb)2Te3. Grain boundaries and defects that strongly impede charge carrier transport are reduced by the two-step sintering process leading to a higher mobility compared to that of the one-step sintered bulk. At the same time, removal of Te as well as Sb-rich inhomogeneities with lattice misfit to the matrix decreased the thermal conductivity. In this way, simultaneous maintenance of high mobility and low lattice thermal conductivity was illustrated. By further optimization of carrier concentration, the produced material showed an encouraging zT value of 1.38 at 323 K. The present work demonstrates a method for synthesizing high-efficiency thermoelectric materials through simultaneous optimization of the electrical and thermal transport properties.

Cu-based chalcogenides have attracted increasing attention as a potential thermoelectric material due to their unique structural features, high element abundance, and low toxicity. In this paper, a series of Cu1.8SbxSnyS compositions (0.01 < x < 0.04, 0.015 < y < 0.045) were prepared by a simple method combining mechanical alloying (MA) and spark plasma sintering (SPS). The results demonstrate that Sb/Sn co-doping can optimize the hole concentration and substantially improve the dimensionless figure of merit (ZT) of copper–sulfide Cu1.8S. The highest ZT, up to 1.2 at 773 K, was realized for the Cu1.8Sb0.02Sn0.03S sample and maintained a high power factor of 975 μW m−1 K−2. The reduction in thermal conductivity was linked to the heterogeneous phase of Cu12Sb4S13 and Cu4SnS4. Remarkably, after four testing cycles, the variation in ZT was less than 2%, indicating a good stability of the Sb/Sn co-doped Cu1.8S materials.

Shashlik-like Te–Bi2Te3 nanocomposites (consisting of Te nanorods and Bi2Te3 nanoplates) were successfully synthesized via a solvothermal method. Te nanorods first appear in the solvent followed by the formation of Bi2Te3 nanoplates on the surface of the Te nanorods. The lattice parameters of the Te phase and Bi2Te3 phase are almost identical (aBi2Te3 ≈ aTe3, cBi2Te3 ≈ 5cTe), resulting in the epitaxial growth of the nanoplates and ultimately forming the shashlik-like structure. With increasing reaction time, the Te nanorods were gradually dissolved, and only the pure Bi2Te3 nanoplates remained. The effect of different reaction times on crystal growth and the morphology evolution of products are extensively discussed. The Te–Bi2Te3 hetero-nanostructure powders were then densified into a bulk material by a spark plasma sintering technique, showing a promising figure of merit (ZT) value (0.54, 398 K). This study explores and provides a reliable one-pot method to synthesize novel shashlik-like Te–Bi2Te3 for application of high-performance thermoelectric materials.

Developing high-efficiency materials with earth-abundant and low-toxicity elements has become a popular trend in the field of thermoelectrics. Among these compounds, oxides and sulfides, the lighter, cheaper and green analogies of tellurides, have been extensively investigated and summarized as well defined classes. Nonetheless, the vast family of selenides with better electrical performance, lower thermal conductivity and higher thermoelectric efficiency have not been specially discussed. Here in this review, we present recent advances in binary and multinary selenide thermoelectric materials, covering traditional PbSe, liquid-like Cu2Se, layered SnSe, diamond-like and disordered multinary compounds. The features of selenides are discussed based on both environmental concerns and from the perspective of chemical bonding, transport properties and performance. Emphasis is put on the “composition-structure-processing-performance” relationship, and some interesting issues are addressed. Finally, challenges for thermoelectric selenides are discussed, and possible optimization strategies are also suggested. Keywords: Thermoelectric materials, Selenides, Transport properties

The inorganic halide perovskite CsSnI3 has attracted significant attention for use as a thermoelectric material owing to its ultralow thermal conductivity and unique electrical properties. In this study, we report lead iodide (PbI2) doping to optimize the thermoelectric performance of bulk CsSnI3. The addition of a small amount of PbI2 narrowed the bandgap of CsSnI3 and modulated its carrier concentration, improving the electrical conductivity of CsSnI3. Moreover, the lattice distortion caused by PbI2 doping led to a significant decrease in the lattice thermal conductivity of CsSnI3. Finally, the thermoelectric performance was enhanced, and the highest ZT value attained was 0.14 at 523 K.

Copper sulfide has received wide attention as a potential thermoelectric material with high performance and abundant resource. While the liquid-like Cu ions play an important role for the low κL and high ZT values, grain-boundary engineering is needed to further enhance the phonon scattering and the thermal stability of Cu2-xS. Here, we introduced the 3D graphene heterointerface into the Cu2-xS matrix by a facile technique combining mechanical alloying (MA) and spark plasma sintering (SPS). A significant ZT enhancement was realized with the highest ZT up to 1.56 and a high power factor of 1197 μW m−1 K−2 at 873 K in the sample with 0.75 wt% graphene. Additionally, the composite showed excellent reproducibility of PF after five cycles testing from room temperature to 873 K, which confirmed the practical application potentiality of this composite.

Cu12Sb4S13 tetrahedrite has received great attention as an earth-abundant and environmental-friendly thermoelectric material. This work aims to uncover the thermoelectric performance-enhancing effect and the mechanism of nickel doping on tetrahedrite. A series of Cu12− x Ni x Sb4S13− δ ( x = 0.5, 0.7, 1.0, 1.5 and 2.0) compounds were synthesized by mechanical alloying combined with spark plasma sintering. It is found that the thermal conductivity sharply reduces with increasing Ni content over the entire temperature range, accompanied with an enhanced thermoelectric power factor. The model predicted that the reduced lattice thermal conductivity is attributed to mid-frequency phonon scattering, caused by precipitates and dislocations resulting from Ni doping. Consequently, a high ZT value up to 0.95 at 723 K was achieved for Cu11NiSb4S13− δ , corresponding to a ~46% increase over non-doped Cu12Sb4S13− δ . Furthermore, the cyclic measurement showed that the Ni-doped tetrahedrites displayed high chemical stability.

Tin selenide (SnSe), a simple binary compound with low-cost, earth-abundant and eco-friendly elements, has aroused extensive interest in the thermoelectric community on account of its promising power generation. Herein, we report a much more advantageous SnS crystal with promising thermoelectric performance, as an alternative to SnSe. We found that the maximum ZT > 1.0 at 873 K and high device ZT (ZTdev) > 0.57 from 300 to 873 K can be achieved in hole-doped SnS crystals, projecting a conversion efficiency of ∼10.4%. We attribute the excellent performance of SnS to its remarkable electron and phonon band structures. SnS possesses multiple valence bands, which can be activated by hole doping through pushing the Fermi level deep into the valence band structure, and activating several Fermi pockets to produce enhanced Seebeck coefficients and high power factors ∼30 μW cm−1 K−2 at 300 K. Meanwhile, the anharmonic and anisotropic bonding of SnS leads to a low thermal conductivity, which ranges from 0.65 to 0.85 W m−1 K−1 at 873 K. Our results indicate that SnS is a promising thermoelectric material for energy conversion applications in low and moderate temperature ranges.

Strontium titanate (SrTiO3) is a promising n-type thermoelectric material at high temperature. However, to date, its reported high dimensional figure of merit (zT) > 0.4 has only been achieved in a narrow temperature range near 1000 K. In this study, zT values of >0.4 were achieved in the broad temperature range of 769–1009 K in bulk SrTiO3 co-doped with La and Nb with in situ precipitation of second phases of NbC and TiO2−δ. The electronic transport properties of the samples were optimized by adjusting the doping ratio, resulting in a large power factor of 1.82 mW m−1 K−2 at 622 K for 7 mol% La–7 mol% Nb-doped SrTiO3. Notably, the power factor (PF) decreased more gradually with increasing temperature, resulting in a high PF of 1.28 mW m−1 K−2 even at 1009 K. In addition, precipitation of the second phases occurred during sintering of the mixture of La–Nb doped SrTiO3 nano powder and carbon powder, which provided additional phonon scattering centers except for the phonon scattering centers of La and Nb point defects. This high thermoelectric performance achieved over a broad temperature range could be beneficial for broadening the range of application temperatures for bulk polycrystalline SrTiO3. Furthermore, the tailoring strategy of co-doping, an in situ second phase, and oxygen vacancies applied in this study may be applicable to other oxide thermoelectric materials.

MnTe has been found to exhibit good thermoelectric properties at medium temperature recently, but the electrical transport properties, especially the interaction between magnetism and carriers, are still not fully understood and the synthesis process reported is mainly composed of melting. Herein, we present a facile method combining mechanical alloying (MA) and spark plasma sintering (SPS) to fabricate high purity MnTe. Carrier concentration is well tuned by sodium doping, resulting in a high power factor over 900 μW m−1 K−2 and a maximum ZT value exceeding 1.0 at 873 K. The electrical transport properties are analyzed by the single parabolic band model. Additionally, different electrical transport properties induced by the magnetic transformation were discussed and clarified by first-principles calculation, including the increase of carrier concentration, decrease of mobility and enhancement of the density-of-state (DOS) effective mass. The high thermoelectric performance reveals the potential of MnTe as a promising candidate for medium-temperature thermoelectric materials.

The decrease in lattice thermal conductivity is one of the most effective methods for enhancing thermoelectric properties. Achieving lattice thermal conductivity close to the theoretical limit is extremely difficult while maintaining an impressive power factor. By introducing ultrafine bismuth sulfide nanograins, strong phonon scattering was generated by associated multiscale lattice defects to further gain a low lattice thermal conductivity of 0.244 W/m/K approaching Clarke's limitation of 0.242 W/m/K. Simultaneously, the electrical conductivity was increased by S diffusion and suppression of the antisite defects, and a high power factor was maintained to produce an improved ZT peak of 1.2 at 473 K for the Bi2Te2.7Se0.3 + 1 wt% Bi2S3 #2 sample. This sample was used to fabricate a small device with a p-leg, and the direct conversion efficiency reached 4.39%. Inverse changes in hardness and Young's modulus in conjunction with Bi2S3 content are first reported in thermoelectric materials, which has potential assistance in synchronously improving their mechanical and thermoelectric properties.

Pores in a solid can effectively reduce thermal conduction, but they are not favored in thermoelectric materials due to simultaneous deterioration of electrical conductivity. Conceivably, creating a porous structure may endow thermoelectric performance enhancement provided that overwhelming reduction of electrical conductivity can be suppressed. This work demonstrates such an example, in which a porous structure is formed leading to a significant enhancement in the thermoelectric figure of merit (zT). By a unique BiI3 sublimation technique, pore networks can be introduced into tetrahedrite Cu12 Sb4 S13 -based materials, accompanied by changes in their hierarchical structures. The addition of a small quantity of BiI3 (0.7 vol%) results in a ≈72% reduction in the lattice thermal conductivity, whereas the electrical conductivity is improved due to unexpected enhanced carrier mobility. As a result, an enhanced zT of 1.15 at 723 K in porous tetrahedrite and a high conversion efficiency of 6% at ΔT = 419 K in a fabricated segmented single-leg based on this porous material are achieved. This work offers an effective way to concurrently modulate the electrical and thermal properties during the synthesis of high-performance porous thermoelectric materials.

AgPb m SbTe m +2 (abbreviation as LAST) is a promising thermoelectric material based on PbTe for mid-temperature applications. The electrical and thermal transport properties of LAST alloys are greatly affected by the formation of nanostructured precipitation depending on the processing conditions, especially for spark plasma sintering (SPS) combined with mechanical alloying (MA). This study was focused on the optimization of SPS temperature ( T SPS ) and found that its variation in a narrow range (100 K) leads to apparent differences of thermoelectric performances. The sample sintered at an optimized temperature ( T SPS = 923 K) shows a low κ total (0.829 Wm −1 K −1 ) and a high PF (14.9 Wm −1 K −2 ), which is mainly attributed to the nanoscale composition segregation of Ag and Sb. The highest ZT value of 1.28 at 773 K was obtained when T SPS = 923 K, along with the highest ZT eng value of 0.26 and the corresponding η max value of 5.8% at the range of 323 K ( T cold )–773 K ( T hot ), which were increased almost twice by the optimization of sintering temperature.

Cu-Se compounds have attracted more attentions as a low-cost, low toxicity thermoelectric materials. In this work, p-type Cu 2 Se x compound powders were prepared by solution syntheses, including the wet chemistry method and hydrothermal synthesis method. The reaction mechanisms were investigated, showing that the NaOH contents affected the phase structure of the final product. Polycrystalline Cu 2 Se x bulk materials were obtained by densifying the powders using spark plasma sintering (SPS) and obtained the maximum ZT value of 0.75 at 623 K, which is currently the highest reported value for the Cu-Se system at this temperature. We also found that the introduced oxides could suppress the long-range migration of Cu ions in Cu-Se system to avoid the decomposition of the materials.