The Zintl compound Mg₃Sb₂ has been recently identified as promising thermoelectric material owing to its high thermoelectric performance and cost-effective, nontoxicity and environment friendly characteristics. However, the intrinsically p-type Mg₃Sb₂ shows low figure of merit (zT=0.23 at 723 K) for its poor electrical conductivity. In this study, a series of Mg_3-xLi_xSb₂ bulk materials have been prepared by high-energy ball milling and spark plasma sintering (SPS) process. Electrical transport measurements on these materials revealed significant improvement on the power factor with respect to the undoped sample, which can be essentially attributed to the increased carrier concentration, leading to a maximum zT of 0.59 at 723 K with the optimum doping level x=0.01. Additionally, the engineering zT and energy conversion efficiency are calculated to be 0.235 and 4.89%, respectively. To our best knowledge, those are the highest values of all reported p-type Mg₃Sb₂-based compounds with single element doping.

We have systematically studied the thermoelectric properties in Zn-doped SnTe. Strikingly, band convergence and embedded precipitates arising from Zn doping, can trigger a prominent improvement of thermoelectric performance. In particular, the value of dimensionless figure of merit zT has increased by 100% and up to~0.5 at 775 K for the optimal sample with 2% Zn content. Present findings demonstrate that carrier concentration and effective mass play crucial roles on the Seebeck coefficient and power factor. The obvious deviation from the Pisarenko line (Seebeck coefficient versus carrier concentration) due to Zn-doping reveals the convergence of valence bands. When the doping concentration exceeds the solubility, precipitates occur and lead to a reduction of lattice thermal conductivity. In addition, bipolar conduction is suppressed, indicating an enlargement of band gap. The Zn-doped SnTe is shown to be a promising candidate for thermoelectric applications.

Nb-doped SrTiO₃ thermoelectric ceramics with different niobium concentrations, sintering temperatures and Sr-site vacancies are successfully prepared by high energy ball milling combined with carbon burial sintering. For fully understanding the effect of niobium doping on SrTiO₃, thermoelectric transport properties are systematically investigated in a temperature range from 300 K to 1100 K. The carrier mobility can be significantly enhanced, and the electrical conductivity is quadrupled, when the sintering temperature rises from 1673 K to 1773 K (beyond the eutectic temperature (1713 K) of SrTiO₃-TiO₂). The lattice vibration can be suppressed by the lattice distortion introduced by the doped niobium atoms. However, Sr-site vacancies compensate for the lattice distortion and increase the lattice thermal conductivity more or less. Finally, we achieve a maximum value of figure-of-merit zT of 0.21 at 1100 K for SrTi_0.9Nb_0.1O₃ ceramic sintered at 1773 K.

In this work, we report that the thermoelectric properties of Bi_0.52Sb_1.48Te₃ alloy can be enhanced by being composited with MnTe nano particles (NPs) through a combined ball milling and spark plasma sintering (SPS) process. The addition of MnTe into the host can synergistically reduce the lattice thermal conductivity by increasing the interface phonon scattering between Bi_0.52Sb_1.48Te₃ and MnTe NPs, and enhance the electrical transport properties by optimizing the hole concentration through partial Mn²⁺ acceptor doping on the Bi³⁺ sites of the host lattice. It is observed that the lattice thermal conductivity decreases with increasing the percentage of MnTe and milling time in a temperature range from 300 K to 500 K, which is consistent with the increasing of interfaces. Meanwhile, the bipolar effect is constrained to high temperatures, which results in the figure of merit zT peak shifting toward higher temperature and broadening the zT curves. The engineering zT is obtained to be 20% higher than that of the pristine sample for the 2-mol% MnTe-added composite at a temperature gradient of 200 K when the cold end temperature is set to be 300 K. This result indicates that the thermoelectric performance of Bi_0.52Sb_1.48Te₃ can be considerably enhanced by being composited with MnTe NPs.

We report the synthesis of Nd-filled and Fe substituted p-type Nd_xFe_3.2Co_0.8Sb₁₂ (x=0.5, 0.6, 0.7, 0.8, and 0.9) skutterudites by the solid-state reaction method. The influences of Nd filler on the electrical and thermal transport properties are investigated in a temperature range from room temperature to 850 K. A lowest lattice thermal conductivity of 0.88 W·m^-1·K^-1 is obtained in Nd_0.8Fe_3.2Co_0.8Sb₁₂ at 673 K, which results from the localized vibration modes of fillers and the increase of grains boundaries. Meanwhile, the maximum power factor is 2.77 mW·m^-1·K^-2 for the Nd_0.9Fe_3.2Co_0.8Sb₁₂ sample at 668 K. Overall, the highest dimensionless figure of merit zT=0.87 is achieved at 714 K for Nd_0.9Fe_3.2Co_0.8Sb₁₂.

Nanocomposite is proved to be an effective method to improve thermoelectric performance. In the present study, graphene is introduced into p-type skutterudite La_0.8Ti_0.1Ga_0.1Fe₃CoSb₁₂ by plasma-enhanced chemical vapor deposition (PECVD) method to form skutterudite/graphene nanocomposites. It is demonstrated that the graphene has no obvious effect on the electrical conductivity of La_0.8Ti_0.1Ga_0.1Fe₃CoSb₁₂, but the Seebeck coefficient is slightly improved at high temperature, thereby leading to high power factor. Furthermore, due to the enhancement of phonon scattering by the graphene, the lattice thermal conductivity is reduced significantly. Ultimately, the maximum zT value of La_0.8Ti_0.1Ga_0.1Fe₃CoSb₁₂/graphene is higher than that of graphene-free alloy and reaches to 1.0 at 723 K. Such an approach raised by us enriches prospects for future practical application.

Strontium titanate (STO) is an n-type oxide thermoelectric material, which has shown great prospects in recent years. The doping of La and Nb into STO can improve its power factor, whereas its thermal conductivity is still very high. Thus, in order to obtain a high thermoelectric figure-of-merit zT, it is very important to reduce its thermal conductivity. In this paper, using a combination of a hydrothermal method and a high-efficiency sintering method, we succeed in preparing a composite of pure STO and LaNb-doped STO, which simultaneously realizes lower thermal conductivity and higher Seebeck coefficient, therefore, the thermoelectric properties of STO are significantly improved. In the SrTiO₃/LaNb-SrTiO₃ bulk samples, the lowest thermal conductivity is 2.57 W·m^-1·K^-1 and the highest zT is 0.35 at 1000 K for the STO/La10Nb20-STO sample.

Binary Cu-based chalcogenide thermoelectric materials have attracted a great deal of attention due to their outstanding physical properties and fascinating phase sequence. However, the relatively low figure of merit zT restricts their practical applications in power generation. A general approach to enhancing zT value is to produce nanostructured grains, while one disadvantage of such a method is the expansion of grain size in heating-up process. Here, we report a prominent improvement of zT in Cu₂Te_0.2Se_0.8, which is several times larger than that of the matrix. This significant enhancement in thermoelectric performance is attributed to the formation of abundant porosity via cold press. These pores with nano-to micrometer size can manipulate phonon transport simultaneously, resulting in an apparent suppression of thermal conductivity. Moreover, the Se substitution triggers a rapid promotion of power factor, which compensates for the reduction of electrical properties due to carriers scattering by pores. Our strategy of porosity engineering by phonon scattering can also be highly applicable in enhancing the performances of other thermoelectric systems.

Thermoelectric selenides have attracted more and more attentions recently. Herein, p-type SnSe polycrystalline bulk materials with good thermoelectric properties are presented. By using the SnSe₂ nanostructures synthesized via a wet-chemistry route as the precursor, polycrystalline SnSe bulk materials were successfully obtained by a combined heat-treating process under reducing atmosphere and following spark plasma sintering procedure. As a reference, the SnSe nanostructures synthesized via a wet-chemistry route were also fabricated into polycrystalline bulk materials through the same process. The thermoelectric properties of the SnSe polycrystalline transformed from SnSe₂ nanostructures indicate that the increasing of heattreating temperature could effectively decrease the electrical resistivity, whereas the decrease in Seebeck coefficient is nearly invisible. As a result, the maximum power factor is enhanced from 5.06×10^-4 W/m·K² to 8.08×10^-4 W/m·K² at 612℃. On the other hand, the reference sample, which was obtained by using SnSe nanostructures as the precursor, displays very poor power factor of only 1.30×10^-4 W/m·K² at 537℃. The x-ray diffraction (XRD), scanning electron microscope (SEM), x-ray fluorescence (XRF), and Hall effect characterizations suggest that the anisotropic crystal growth and existing Sn vacancy might be responsible for the enhanced electrical transport in the polycrystalline SnSe prepared by using SnSe₂ precursor. On the other hand, the impact of heat-treating temperature on thermal conductivity is not obvious. Owing to the boosting of power factor, a high zT value of 1.07 at 612℃ is achieved. This study provides a new method to synthesize polycrystalline SnSe and pave a way to improve the thermoelectric properties of polycrystalline bulk materials with similar layered structure.