ZT Peak Research Articles

Thermoelectric properties of monolayer GeTe with Au, Ni, and Co substrates

Since the basic thermoelectric (TE) units must be integrated on substrates to form large-scale devices, it would be more practical to study the TE properties of the TE devices with substrates. Based on the first-principles calculation and nonequilibrium Green’s function method, we comparatively investigate the TE properties of the GeTe monolayer with its leads deposited on Au, Ni, and Co substrates, which are called GeTe-X (X = Au, Ni, and Co). It is shown that in comparison with the pristine GeTe monolayer, the figures of merit ZTs of the GeTe-X are seriously changed with one ZT peak appearing near zero chemical potential. The GeTe-Au and GeTe-Ni are of higher TE performance near zero chemical potential than the GeTe-Co, indicating one can choose Au or Ni as the substrate. Moreover, we show that the TE properties of the GeTe-X are sensitively dependent on the substrate layer number, the central scattering length, and the temperature, which necessitates the synergistic optimization of the related parameters to obtain the best TE performance. This work should be an important reference for designing practical GeTe-based TE devices.

PREPARATION AND THERMOELECTRIC PROPERTIES OF p-TYPE Ag(1-x)Cu(1+x)Te SAMPLES BY SPARK PLASMA SINTERING METHOD

The combination of silver (Ag), copper (Cu), and telluride (Te) yields silver copper telluride (AgCuTe), a new type of ternary liquid-like thermoelectric material. Due to its ultra-low lattice thermal conductivity and highly symmetrical cubic crystal structure, it exhibits excellent thermoelectric properties in the medium-to-high temperature range (400-600 K). However, hexagonal crystal structures with lower symmetry near room temperature can lead to higher thermal conductivity. In addition, the high carrier concentration of the hexagonal crystal structure results in a lower Seebeck coefficient for the material. This seriously affects the application of AgCuTe-based thermoelectric materials in the room temperature range. Therefore, how to improve the thermoelectric performance of AgCuTe-based thermoelectric materials in the room temperature range has received widespread attention from researchers. AgCuTe-based thermoelectric materials were successfully prepared by mechanically alloying Ag, Cu, and Te powders using a planetary ball mill combined with spark plasma sintering. On this basis, the thermoelectric properties of AgCuTe-based materials were optimized by adjusting the Ag/Cu ratio and Ag<sub>2</sub>Te/CuTe content. The results indicated that at 600 K, the Ag<sub>0.93</sub>Cu<sub>1.07</sub>Te sample obtained a high Seebeck coefficient value of approximately 171 μV/K<sup>-1</sup>. Finally, the ZT peak obtained from the Ag<sub>093</sub>Cu<sub>1.07</sub>Te sample was 1.06; compared with AgCuTe (ZT = 0.78), it had increased by about 36%.

Thermoelectric and mechanical properties of Bi0.42Sb1.58Te3/SnO2 bulk composites with controllable ZT peak for power generation

Bi2Te3-based materials' potential for power generation at lower to moderate temperatures is curtailed by intrinsic excitation. This study seeks to address this issue. This is accomplished by fabricating bulk thermoelectric materials consisting of varying weights of SnO2 via solid-state reaction and spark plasma sintering. Results show that the composite material has an average ZT value of ∼0.85 in the 425–575 K temperature range. The lattice thermal conductivity of the composite material is reduced by approximately 54% compared to pure Bi0.42Sb1.58Te3 (BST) and almost 47% when compared to Cu1.8S modified- Bi0.42Sb1.58Te3 (m-BST) at 300 K. A high power factor is achieved, which almost doubled in the 425–575 K. The composite material also exhibits significantly enhanced hardness and modulus due to compositing. These findings provide a straightforward strategy for improving the low- to medium-temperature thermoelectric performance of commercial p-type (BiSb)2Te3, which is crucial for expanding the temperature range of Bi2Te3-based materials.

Realizing High Thermoelectric Performance in N‐Type Mg3(Sb, Bi)2‐Based Materials via Synergetic Mo Addition and Sb–Bi Ratio Refining

AbstractDeveloping thermoelectric (TE) performance is impeded by the compromise of TE parameters, resulting in inadequate conversion efficiency of heat to electricity. Herein, this work reports that Mo is a particularly effective additive in Mg3Sb2‐based alloys with significantly improved electronic transport via grain‐boundary engineering and band‐structure regulation synergy. In addition, phonon transport is simultaneously suppressed by employing multiple effects, lattice imperfection scattering, reduced phonon group velocity, and enhanced atomic disorder, leading to a minimum κlat = 0.52 W m−1 K−1 at 723 K. As a result, an outstanding ZT peak of ≈1.84 at 723 K and ZTav = 1.34 within 323–723 K are achieved in Mg3Sb2‐based alloys, and the corresponding fabricated single‐leg TE module shows an exceptionally high conversion efficiency of ≈12% under a hot‐side temperature of 450 °C. These results demonstrate the great potential for advancing mid‐temperature heat harvesting in Mg3Sb2‐based materials.

Strain-Induced Medium-Temperature Thermoelectric Performance of Cu4TiSe4 : The Role of Four-Phonon Scattering

Motivated by the synthesized ${\mathrm{Cu}}_{4}{\mathrm{Ti}\mathrm{Se}}_{4}$ with ultralow room-temperature thermal conductivity [Angew. Chem., Int. Ed. 60, 9106 (2021)], we systematically investigate its thermoelectric (TE) properties via combining Boltzmann transport equations and first-principles calculations. The results show that the thermal conductivity of ${\mathrm{Cu}}_{4}{\mathrm{Ti}\mathrm{Se}}_{4}$ determined considering only three-phonon scattering is reduced by about 40% after including four-phonon scattering at room temperature, indicating the importance of four-phonon scattering in phonon transport. On the other hand, the coexistence of high dispersion and valley degeneracy at the top valence band in the electronic structure causes a high power factor. Consequently, the isotropic figure of merit (ZT) value of 1.5 at 500 K is captured in the p-type doped ${\mathrm{Cu}}_{4}{\mathrm{Ti}\mathrm{Se}}_{4}$. In addition, the value of ZT can be further enhanced to 2.2 by applying 2.25% triaxial tensile strain, which is ascribed to the remarkably enhanced four-phonon scattering processes induced by the tensile strain. Meanwhile, the significant suppression of thermal conductivity allows the optimal carrier concentration for the ZT peak to be reduced, which is of important practical significance for the experimental preparation of ${\mathrm{Cu}}_{4}{\mathrm{Ti}\mathrm{Se}}_{4}$-based TE devices. Our results pave a way for the design of adjustable medium-temperature TE devices.

Development of Nanostructured Bi2Te3 with High Thermoelectric Performance by Scalable Synthesis and Microstructure Manipulations

Nanostructuring of thermoelectric (TE) materials leads to improved energy conversion performance; however, it requires a perfect fit between the nanoprecipitates' chemistry and crystal structure and those of the matrix. We synthesize bulk Bi2Te3 from molecular precursors and characterize their structure and chemistry using electron microscopy and analyze their TE transport properties in the range of 300-500 K. We find that synthesis from Bi2O3 + Na2TeO3 precursors results in n-type Bi2Te3 containing a high number density (Nv ∼ 2.45 × 1023 m-3) of Te-nanoprecipitates decorating the Bi2Te3 grain boundaries (GBs), which yield enhanced TE performance with a power factor (PF) of ∼19 μW cm-1 K-2 at 300 K. First-principles calculations validate the role of Te/Bi2Te3 interfaces in increasing the charge carrier concentration, density of states, and electrical conductivity. These optimized TE coefficients yield a promising TE figure of merit (zT) peak value of 1.30 at 450 K and an average zT of 1.14 from 300 to 500 K. This is one of the cutting-edge zT values recorded for n-type Bi2Te3 produced by chemical routes. We believe that this chemical synthesis strategy will be beneficial for future development of scalable n-type Bi2Te3 based devices.

Thermoelectric properties of C2P4 monolayer: A first principle study

We have theoretically explored the electronic and thermoelectric properties of the C2P4 monolayer with the interface of density functional theory and semi-classical transport theory. Our calculation shows a high Seebeck coefficient and low electronic thermal conductivity in the vicinity of zero chemical potential (μ = 0), resulting in a good power factor (PF) and a high figure of merit (ZT). More particularly, the electronic figure of merit (ZTe) exhibits two high peak values around μ = 0 due to the significant contribution of thermoelectric parameters. Furthermore, ZTe decreases by increasing the temperature, giving a peak value of 0.98 in the negative chemical potential (μ), whereas, for μ > 0, the peak value increases slightly with temperature. Additionally, the ZTe peak value is robust against ±10% of uni- and biaxial strains at room temperature. To make our calculation more realistic, we add phonon contributions to the thermal conductivity in pristine C2P4 and calculate the total ZT. We have found that phonon contribution dominates at low temperatures, and the ZT peak is reduced to 0.78. These optimal thermoelectric parameters of the C2P4 monolayer may be suitable for demonstrating the feasibility of a good thermoelectric material.

Thermoelectric Zintl Compound In1−xGaxTe: Pure Acoustic Phonon Scattering and Dopant‐Induced Deformation Potential Reduction and Lattice Shrink

AbstractWe report a Zintl phase thermoelectric material, coarse grain‐In0.99Ga0.01Te, achieving a ZT peak of 1.2 at 648 K and an average ZT=0.8 in 300–650 K, which outperforms all the known InTe‐based materials to date. The synergistic optimization of electronic property and phonon transport are achieved by the purification of grain boundary scattering, together with the Ga‐doping‐induced weak phonon‐electron coupling, which enhances the carrier mobility and carrier concentration simultaneously and consequently gives a remarkably increased power factor of 8.9 μW cm−1 K−2. The DFT phonon calculations indicate the dopant reduces the deformation potential coefficient and induces the lattice shrink, which reduces significantly the acoustic cutoff frequency, and enhances the scattering phase space. Moreover, the bonding hierarchy leads to the dense intragranular dislocation arrays, which suppresses the lattice thermal conductivity further and induces an ultralow lattice thermal conductivity (0.21 Wm−1 K−1).

Thermoelectric Zintl Compound In1-x Gax Te: Pure Acoustic Phonon Scattering and Dopant-Induced Deformation Potential Reduction and Lattice Shrink.

We report a Zintl phase thermoelectric material, coarse grain-In0.99 Ga0.01 Te, achieving a ZT peak of 1.2 at 648 K and an average ZT=0.8 in 300-650 K, which outperforms all the known InTe-based materials to date. The synergistic optimization of electronic property and phonon transport are achieved by the purification of grain boundary scattering, together with the Ga-doping-induced weak phonon-electron coupling, which enhances the carrier mobility and carrier concentration simultaneously and consequently gives a remarkably increased power factor of 8.9 μW cm-1 K-2 . The DFT phonon calculations indicate the dopant reduces the deformation potential coefficient and induces the lattice shrink, which reduces significantly the acoustic cutoff frequency, and enhances the scattering phase space. Moreover, the bonding hierarchy leads to the dense intragranular dislocation arrays, which suppresses the lattice thermal conductivity further and induces an ultralow lattice thermal conductivity (0.21 Wm-1 K-1 ).

Achieving high thermoelectric performance through carrier concentration optimization and energy filtering in Cu3SbSe4-based materials

The previous works commonly adjust the carrier concentration through acceptor doping, but at the same time, the decrease of the Seebeck coefficient limits the further improvement of electrical properties in Cu3SbSe4-based materials. In this work, a microwave-assisted hydrothermal synthesis method was used to synthesize Cu3SbSe4/TiO2 hollow microspheres. Part of TiO2 participates in the reaction, replaces the Sb site of Cu3SbSe4 to form holes, and the rest is dispersed in the matrix in the form of the second phase. The first-principles calculations reveal that the doping of Ti significantly changes the band structure and phonon spectrum, thereby regulating carrier concentration while increasing phonon scattering. In addition, experimental results show that the energy filtering effect generated by the extra-mixed TiO2 nano particles, which suppresses the decrease of Seebeck coefficient by acceptor doping. Consequently, the highest average power factor 897.5 μW m−1 K−2 and the zT peak value of 0.70 can be obtained in Cu3SbSe4/6%TiO2 sample at 298–623 K. This work provides a new sight to improve the thermoelectric properties in Cu3SbSe4 through carrier concentration regulation and nano-phase composition.

First-principles investigation of the significant anisotropy and ultrahigh thermoelectric efficiency of a novel two-dimensional Ga2I2S2 at room temperature

Two-dimensional (2D) thermoelectric (TE) materials have been widely developed; however, some 2D materials exhibit isotropic phonon, electron transport properties, and poor TE performance, which limit their application scope. Thus, exploring excellent anisotropic and ultrahigh-performance TE materials are very warranted. Herein, we first investigate the phonon thermal and TE properties of a novel 2D-connectivity ternary compound named Ga2I2S2. This paper comprehensively studies the phonon dispersion, phonon anharmonicity, lattice thermal conductivity, electronic structure, carrier mobility, Seebeck coefficient, electrical conductivity, and the dimensionless figure of merit (ZT) versus carrier concentration for 2D Ga2I2S2. We conclude that the in-plane lattice thermal conductivities of Ga2I2S2 at room temperature (300 K) are found to be 1.55 W mK−1 in the X-axis direction (xx-direction) and 3.82 W mK−1 in the Y-axis direction (yy-direction), which means its anisotropy ratio reaches 1.46. Simultaneously, the TE performance of p-type and n-type doping 2D Ga2I2S2 also shows significant anisotropy, giving rise to the ZT peak values of p-type doping in xx- and yy-directions being 0.81 and 1.99, respectively, and those of n-type doping reach ultrahigh values of 7.12 and 2.89 at 300 K, which are obviously higher than the reported values for p-type and n-type doping ternary compound Sn2BiX (ZT∼ 1.70 and ∼2.45 at 300 K) (2020 Nano Energy 67 104283). This work demonstrates that 2D Ga2I2S2 has high anisotropic TE conversion efficiency and can also be used as a new potential room-temperature TE material.

Thermoelectric properties of Bi2-xTixO2Se with the shear exfoliation-restacking process

Bi2O2Se is a potential medium-temperature n-type thermoelectric material owing to its lower lattice thermal conductivity and larger Seebeck coefficient. However, very low electrical conductivity of pristine polycrystalline Bi2O2Se hinders further enhancement of its thermoelectric performance. In this work, the thermoelectric properties of Bi2O2Se were investigated by Ti-doping with the shear exfoliation- restacking process. The results show that both the electron content and mobility can be further improved by Ti-doping with the shear exfoliation-restacking process, thus a higher electrical conductivity can be obtained. (e.g., 368 S cm−1 of Bi1.99Ti0.01O2Se at room temperature) On the other hand, the thermal conductivity of Bi1.99Ti0.01O2Se is suppressed to 0.84 Wm−1 K−1 at 772 K. Finally, the ZT peak of Bi1.99Ti0.01O2Se is raised to 0.56, which is 1.2 times of that of un-doped sample. The results show that the thermoelectric properties of Bi2O2Se can be further optimized by Ti doping with the shear exfoliation-restacking process.

Ultralow lattice thermal conductivity and enhanced power generation efficiency realized in Bi2Te2.7Se0.3/Bi2S3 nanocomposites

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.

Ab-initio investigation on the electronic and thermoelectric properties of new half-Heusler compounds KBiX (X = Ba and Sr)

Electronic structures and thermoelectric (TE) properties of KBiBa and KBiSr half-Heusler compounds are investigated by using the combined framework of first-principles and semi-classical Boltzmann transport theory. Elastic and phonon properties calculations reveal that these compounds are mechanically and dynamically stable. Band structures calculations, using the Tran and Blaha modified Becke–Johnson potential including spin–orbit coupling, show that KBiBa and KBiSr compounds are semiconductors with an indirect bandgap of 0.88 and 0.95 eV respectively. Performing calculations on lattice thermal conductivity, deformation potential, effective mass of charge carriers and their relaxation times allow us to deduce the dependence of the figure of merit ZT on doping concentration and temperature. At 1200 K, KBiBa and KBiSr compounds exhibit a ZT peak value of 2.68 and 1.56 at optimized n-doping values of 2.2 × 1019 and 1.7 × 1019 cm−3, respectively. Considering high ZT values, KBiBa and KBiSr are considered to be ideal TE candidates at high temperature.

Simultaneous enhancement of thermoelectric performance and mechanical properties in Bi2Te3 via Ru compositing

Recently, researchers have been exploring the optimization of the thermoelectric properties of composite materials. Composite technology with many successful cases of enhancement in thermoelectric properties have been confirming effectively to optimize thermoelectric materials. In this study, Ru nano-micro composite particles were successfully introduced in commercial bismuth telluride (Bi2Te3) as second phase to simultaneously optimize the thermoelectric and mechanical properties. In a Bi2Te3 sample containing 1 wt% Ru (+1 wt% Ru), the ZT peak was measured to be 0.93 (at 150 °C) and the maximum Vickers hardness was measured to be 2.38 GPa. These values exceeded the values of a pure sample by 45.3% and 36%, respectively. The average ZT of the +1 wt% Ru sample at 50–200 °C was 0.86, which is comparable with the Bi2Te3-xSex system. By varying the Ru content, the components, microstructure, thermoelectric and mechanical properties of Bi2Te3 were manipulated and their interdependence demonstrated. The extent to which the thermoelectric and mechanical properties of Bi2Te3 can be optimized, via Ru compositing and nano-Bi2Te3 producing, is related by the complex nano-micro composite structure. The superior thermoelectric properties of composite materials hold great the commercial value and may reduce the input of raw materials.

Enhanced thermoelectric properties of n-type Bi2Te2.7Se0.3 for power generation

The abundant waste heat (below 500 K) from industry and automobile exhaust stimulate the development of thermoelectric materials for power generation. The commercial Bi2Te3-based alloys are generally used near room temperature. Herein, n-type Bi2Te2.7+xSe0.3 was prepared by a method of mechanical alloying combined with hot pressing, which is more efficient, time-saving, and energy-saving than the method of melting and spark plasma sintering. The n-type Bi2Te2.7Se0.3 being suitable for power generation (below 500 K) is obtained by manipulating the intrinsic point defects to enhance carrier concentration and suppress the intrinsic excitation at elevated temperature using a tiny amount of excess Te. Enhanced carrier concentration improves electrical conductivity, while enhanced phonon scattering reduces lattice thermal conductivity, originating from increased anti-site defects Te.Bi in Te-rich condition. Consequently, the enhanced ZT peak gradually shifts to higher temperature while increasing the excess Te content. The maximum ZT peak of 0.9 at 373 K and an average ZT of 0.82 between 300 and 498 K are obtained in Bi2Te2.71Se0.3, indicating potential for thermoelectric power generation (below 500 K).

Development of p-type Bi2−xSbxTe3 thermoelectric materials for power generation application exploiting synergetic effect of Sb alloying and repress process

Bi–Te based alloy compounds are currently the best-performing thermoelectric materials near room temperature. However, with the temperature increase, their conversion efficiency drops dramatically due to intrinsic excitation, thereby limiting their power generation application. In this study, for low-temperature thermoelectric power generation, we develop p-type Bi–Sb–Te alloys with the highest performance at a temperature of ~473 K. Here, we note that increasing the amount of Sb can effectively suppress intrinsic excitation, shifting the ZT peak towards higher temperatures. Moreover, we apply the repress process, through which microstructures such as grain boundaries and dislocations are developed in the material, which in turn reduces the thermal conductivity. As a result, a peak ZT of ~1.32 at 473 K is achieved for Bi0.3Sb1.7Te3 in the direction parallel to the pressure, which indicates its large application prospect for low-temperature thermoelectric power generation.

Improved thermoelectric properties of Ag-doped polycrystalline SnSe with facile electroless plating

The thermoelectric (TE) performance of polycrystalline SnSe is limited by its low electrical properties, and normally doped with metals like Ag, Na etc by simply mixing with SnSe in melting. In this paper, a facile electroless plating is introduced to improve its TE performance for the first time. The results showed Ag was coated on the surface of SnSe powders homogeneously even after sintered with SPS. The influence of the Ag dopant on the electrical and thermal transport is analyzed. The silver coating with electroless plating could obviously improve average power factor (nearly fourfold) with minor effects (only 13% enhancement) on thermal conductivity. Therefore, a peak ZT value of 0.68 is achieved at 873 K in 1.0% Ag doped SnSe parallel to SPS pressing direction. In addition, the anisotropic properties are also found in this Ag doping SnSe by electroless plating. Compared with sample along parallel direction in 0.5% Ag coating, the power factor along the perpendicular direction improves over 50%, and thermal conductivity only increases 12.35%, resulting in a ZT peak value of 0.85 at 873 K. Compared with traditional mixing method, electroless plating is a facile and more homogenous doping method, and could be an improved way for doping to finer tune other thermoelectric materials performance.

Achieving high thermoelectric properties of Bi2S3 via InCl3 doping

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.

Effect of solder and barrier layer elements on the thermoelectric properties of Bi0.5Sb1.5Te3

The interfacial diffusion between the electrode and thermoelectric legs is a key factor to determine thermoelectric (TE) module performance. Metal elements M (Sn, Ni, Ag, Cu) are frequently used in lead-free solder SAC serials and barrier layers. M’s effects on thermoelectric properties are investigated in this work, through doping 0.1 wt% M in Bi0.5Sb1.5Te3 (BST) prepared by melting and spark plasma sintering method. The results show that the additive of Ag and Cu could enhance the average ZT value 69.4% and 75.5% in the temperature from 323 K to 573 K, and transfer ZT summit to 423 K, while Sn and Ni doping have little influence on the BST’s thermoelectric performance. The ZT peak value of Cu doped BST reaches 1.01 at 423 K, and Ag 0.96 at 423 K. This work exhibits a prospect that the efficiency of thermoelectric device can be improved by finely choosing different solders at cold and hot side of leg-electrode joint at different applied temperature.