As the extension of high-entropy alloy, entropy engineering has been already extensively used in thermoelectrics because it can guide the optimization of thermoelectric (TE) performance from the aspects of both electrical and thermal transports. Due to the inherent material gene-like feature, entropy can be used as a performance indicator to rapidly screen new multicomponent TE materials. In this review, we first reveal the reason why entropy can be used as the performance indicator of TE materials. The physical mechanisms of enhanced structure symmetry, improved Seebeck coefficient, and suppressed lattice thermal conductivity as a result of the increased configurational entropy are discussed. Then, the applications of entropy engineering in typical TE materials, such as liquid-like materials and IV-VI semiconductors, are outlined, and the approach to screen and identify candidate multicomponent TE materials with high configurational entropy is introduced. Finally, the future directions for entropy engineering are highlighted.

PbTe recently attracted extensive attentions as potential candidates for applications at intermediate temperatures. However, it is difficult to significantly improve the thermoelectric performance of n-type PbTe due to its relatively low carrier concentration and complicated band structures. In this study, the stepwise addition of dual components was utilized to verify the possibility for modulating the thermal and electrical transport characteristics of n-type PbTe-based materials. The results indicated that PbS and Sb₂Se₃ can improve the power factor and reduce thermal conductivity of PbTe, respectively. Optimization of band structures and enhancement of phonon scattering were realized by means of expanding band gap, producing point defects and secondary dispersoids. As a result, the merit of figure ZT was remarkably improved. In particular, (PbTe)_0.94(PbS)_0.05(Sb₂Se₃)_0.01 exhibited the highest value of ZT=1.7 at 700 K simultaneously with almost doubled average ZT compared to the pristine PbTe. Accordingly, it seems that the stepwise addition of adequate dual components provides possible technological approach for improving thermoelectric performance of other materials.

Mg₃Sb₂ compound has attracted much attention due to the promising thermoelectric properties and cost advantage. However, it is quite difficult to control Mg content during synthesizing processes because of high saturation vapor pressure and chemical reactivity of Mg element. Herein, Mg_3(1+z)Sb₂ (z=0, 0.02, 0.04, 0.06 and 0.08) samples were prepared by combination of solid state reaction, ball milling and spark plasma sintering (SPS). Their effects of Mg content on thermoelectric properties of Mg₃Sb₂ compounds were investigated in this study. Results indicate that actual Mg content rises with nominal Mg content increasing, and their point defect type changes from Mg vacancy(${{\text{{V}''}}_{\text{Mg}}}$) to interstitial Mg($\text{Mg}_{\text{i}}^{\centerdot \centerdot }$), leading to transition of transport behavior from p type (hole carriers predominated) for Mg_3(1+z)Sb₂ (z=0, 0.02, 0.04) samples to n type (electron carriers predominated) for Mg_3(1+z)Sb₂ (z=0.06, 0.08) samples. Besides, Mg_3(1+0.04)Sb₂ sample shows the highest ZT value from room temperature to 770 K, and achieves maximum ZT of 0.28 at 800 K. Additionally, Mg_3(1+0.04)Sb₂ sample exhibits intrinsic p-type transport behavior for Mg₃Sb₂ compound, which could serve as matrix to be extrinsically doped in the future study for further improvements of electrical properties and ZT value.

The high temperature interfacial stability of thermoelectric (TE) elements, which is mainly evaluated by the inter-diffusion and interfacial resistivity at the interface between the barrier layer and the TE material, is one of the key factors determining the service performance and application prospects of TE devices. In this study, a screening method based on high-throughput strategy was employed to further improve the interfacial stability of P-type bismuth telluride TE devices, and Fe was proved the preferred barrier layer material for P-type Bi_0.5Sb_1.5Te₃ (P-BT). Then Fe/P-BT TE elements were prepared by one-step sintering. Evolution of the Fe/P-BT interfacial microstructure during high temperature accelerated aging was systematically studied, and stability of the interfacial resistivity was explored. It is found that during aging, the Fe/P-BT interface is well bonded and the composition of the ternary Fe-Sb-Te diffusion layer remains basically unchanged. The diffusion layer thickness increases linearly with the square root of the aging time and the growth activation energy is 199.6 kJ/mol. The initially low interfacial resistivity of the Fe/P-BT interface increases slowly with the prolonged aging time but remains below 10 μΩ·cm² even after 16 d at 350 ℃. The life prediction based on the interfacial diffusion kinetics indicates that Fe is a suitable barrier layer material for Bi_0.5Sb_1.5Te₃ TE elements.

In general, (GeTe)_n(Bi₂Te₃)_m compounds in GeTe-Bi₂Te₃ pseudo-binary system possess a relatively low thermal conductivity, however, the thermoelectric properties of these compounds have not been evaluated systematically. In this study, a series of single-phase (GeTe)_nBi₂Te₃ (n=10, 11, 12, 13, 14) compounds were prepared by a melting-quenching-annealing process combined with spark plasma sintering. The phase compositions and thermoelectrical properties of these samples were characterized. It is found that doping with Bi₂Te₃ intensifies the phonon scattering and significantly reduces the lattice thermal conductivities of these samples, producing a low total thermal conductivity of 1.63 W?m ^-1?K ^-1 at 723 K for (GeTe)₁₃Bi₂Te₃ compound. Moreover, the effective mass of these compounds is enhanced through adjustment of the relative amount of Bi₂Te₃ and GeTe. Therefore, the Seebeck coefficient and power factor of these samples remain superior even at high carrier concentration. At 723 K, the maximum power factor of (GeTe)₁₃Bi₂Te₃ compound is 2.88×10 ^-3 W?m ^-1?K ^-2 and the maximum ZT of (GeTe)₁₃Bi₂Te₃ is 1.27, which is 16% higher than that of pristine GeTe.

As a new promising thermoelectrical material in the range of intermediate temperature, BiCuSeO attracts much attention due to the combination of low intrinsic thermal conductivity and relatively high Seebeck coefficient. In this study, the effects of substituting variable-valence rare-earth element Eu for Bi site on the microstructure and thermoelectric performance of BiCuSeO-based material were investigated. The results indicate that ions of two valence states, Eu²⁺ and Eu³⁺, coexist in the doped BiCuSeO samples. The doping of Eu not only improves the concentration of the carriers, but also modifies the band structure of BiCuSeO matrix, resulting in effective improvement of electrical transport properties. The electrical conductivity of Bi_0.85Eu_0.15CuSeO reaches 98 S·cm^-1 at 823 K, which is 6 times as high as that of the undoped sample. The power factor of 0.32 mW·m^-1·K^-2 and ZT of 0.49 can be achieved at 823 K for Bi_0.975Eu_0.025CuSeO sample. This study shows that the doping of variable-valence rare-earth elements can effectively improve the thermoelectric properties of BiCuSeO.

The resonant levels can be introduced into GeTe by In element, however, the effect of its microstructure on thermoelectric properties still remained unclear. In this study, a series of Ge_1-xIn_xTe samples were prepared by smelting-quenching-annealing combined with spark plasma sintering (SPS). The XRD, SEM, laser thermal conductivity instrument and thermoelectric performance analysis system (ZEM-3) were applied to study the microstructure and thermoelectric properties. Results show that, with the incorporation of In content, the unit cell volume decreases, and Herringbone structure has become smaller and grain boundaries increase, which result in a decrease in the lattice thermal conductivity. Thereby, a minimum thermal conductivity of 2.16 W·m ^-1·K ^-1 is obtained. Meanwhile, In doping introduces the resonant levels and decreases the carrier concentration, so the Seebeck coefficient and the power factor increase. Consequently, the maximum ZT value of 1.15 is obtained in the 0.03 sample at 600 K, which is 26.4% higher than that of GeTe. This indicates that the thermoelectric properties of Ge_1-xIn_xTe can be effectively improved by the microstructure regulation.

Ⅳ-Ⅵ SnSe single crystal is an attractive thermoelectric (TE) material due to its outstanding TE behavior and environment friendly charactistic. In this work, an effective way for SnSe single crystal growth was explored, and the mechanical property of the as-prepared product was investigated. Undoped SnSe single crystal with accurate stoichiometric ratio was successfully grown via a vertical Bridgman method. The as-grown SnSe single crystal has standard orthorhombic Pnma space group at room temperature. It is easy to cleave along (100) plane because of its weak link between adjacent Sn-Se layers. Microindentation test reveals that SnSe single crystal is a very soft material as its average Vickers microhardness HV is only 53 MPa under 0.01-0.05 kg applied loads. However, it displays excellent fracture toughness due to strong heteropolar bonds between Sn and Se atoms inside (100) plane. The friction coefficient COF on (100) is increased from 0.09 to 0.8 as the scratch load is added from 5 to 300 mN. This work is of great significance to provide the mechanical property of SnSe single crystal.

With the fascinating properties observed in high entropy alloys, the idea of high entropy design has been applied to many material fields. Thermoelectric materials have some particular requirements for high entropy structure according to their transport characteristics. Here, we revealed that the high entropy structure for thermoelectrics required less lattice distortion, and the doping sites should have less influence on the Fermi surface. In the designed compound of Cu_0.8Ag_0.2Zn_0.1Ga_0.4Ge_0.1In_0.4Te₂, the room-temperature thermal conductivity is reduced by 80% as compared to the matrix, and the maximum ZT is enhanced to 1.02. In SnTe, the solid solution of AgSbSe₂ reduces the room-temperature thermal conductivity by 80%, reaching 1.3 W·m^-1·K^-1. This study shows that the high entropy structure following the proposed designing rules could be an important strategy for thermoelectrics.

It is of great significance to find the ultra-rapid preparation technology of materials and realize the optimization of electroacoustic transport properties in the research of thermoelectric materials. In this study, BiAgSeS compounds were successfully prepared by self-propagating high temperature synthesis (SHS), of which the kinetic process was systematically studied. It is found that the melting of Bi is the key to activate and initiate SHS reaction. In addition, the high concentrations of nano- and atomic-scale strain field regions, and screw dislocations produced in the non-equilibrium SHS process provide an everlasting step source for material growth and make the grains possess the layered structure. In the process of material densification, the step source continues to play a role in dominating grain growth, and thus leaving nanopores at the grain boundary. Because of these defects, compared with samples via melting-quenching (MQ) combined with plasma activated sintering (PAS), the SHS+PAS samples can slightly increase the electrical conductivity and significantly reduce the lattice thermal conductivity by ~6%. Finally, the thermoelectric properties are optimized, and the ZT is improved in the whole temperature range with the maximum value of 0.5 obtained at 773 K.