Peak ZT Research Articles

High wide-temperature-range thermoelectric performance in n-PbSe integrated with quantum dots

An extraordinary average ZT of ∼1 as well as a high peak ZT of 1.44 is realized in n-PbSe integrated with quantum dots.

From stoichiometric to off-stoichiometric GeTe: Phase diagram reconstruction and thermoelectric performance reassessment

This study investigates the structure, microstructure, and transport properties of off-stoichiometric GeTe (off-GeTe). In a narrow range of 50–53 at% Te, both the rhombohedral a-GeTe and orthorhombic g-GeTe phases coexist. Despite their similar chemical composition, GeTe and off-GeTe alloys exhibit distinct microstructural and thermal/electronic properties. Theoretical density functional theory (DFT) calculations were employed to verify that changes in the Ge/Te ratios influence the concentration of Ge vacancies, leading to a significant alteration in transport properties despite minor variations in chemical compositions. The off-GeTe alloy, which is free of Ge precipitates, displays defective domain boundaries, showcasing a non-typical herringbone nanostructure that is unprecedented for GeTe-based materials. Notably, the phase transition temperature of off-GeTe, at 620K, differs from its peak zT temperature of 698K. Moreover, a TE device incorporating off-GeTe demonstrates superior interfacial stability and higher energy conversion efficiency compared to its stoichiometric GeTe counterpart. Consequently, off-GeTe demonstrates superior TE performance and enhanced interfacial stability compared to stoichiometric GeTe. The addition of Sb to off-GeTe further improves its potential for TE applications by lifting the single-leg conversion efficiency greater than 3%.

Improved Thermoelectric Performance of p-Type PbTe by Entropy Engineering and Temperature-Dependent Precipitates.

Entropy engineering is aneffective scheme to reduce the thermal conductivity of thermoelectric materials, but it inevitably deteriorates the carrier mobility. Here, we report the optimization of thermoelectric performance of PbTe by combining entropy engineering and nanoprecipitates. In the continuously tuned compounds of Pb0.98Na0.02Te(1-2x)SxSex, we show that the x = 0.05 sample exhibits an exceptionally low thermal conductivity relative to its configuration entropy. By introducing Mn doping, the produced temperature-dependent nanoprecipitates of MnSe cause the high-temperature thermal conductivity to be further reduced. A very low lattice thermal conductivity of 0.38 W m-1 K-1 is achieved at 825 K. Meanwhile, the carrier mobility of the samples is only slightly influenced, owing to the well-controlled configuration entropy and the size of nanoprecipitates. Finally, a high peak zT of ∼2.1 at 825 K is obtained in the Pb0.9Na0.04Mn0.06Te0.9S0.05Se0.05 alloy.

Optimization of the thermoelectric properties of Nb-doped Bi2S3 via high pressure and high temperature

Bi2S3 is considered a promising n-type thermoelectric (TE) material due to its inherent extremely low thermal conductivity and earth-abundant elements. Herein, we have successfully synthesized a series of Nb-doped Bi2S3 (Bi2-xNbxS3, x = 0, 0.01, 0.02, 0.03) bulk materials by combining mechanical alloying (MA) method and high pressure and high temperature (HPHT) technology. The enhanced electrical transport properties of the Nb-doped Bi2S3 samples is derived from the collaborative optimization of carrier concentration and effective mass, which indicated that the Nb dopants acted as donors. In addition, differences in the direction of the lattice fringes were observed in the samples, and a large number of lattice distortions and dislocation defects occurred, which significantly reduced the lattice thermal conductivity of the Bi2S3 material. Consequently, a peak zT of 0.22 was achieved for the Bi1.98Nb0.02S3 sample at 543 K, which is 120% higher than that of pure Bi2S3.

Functionally separated electronic band engineering via multi-element doping plus high-density defects advances board-temperature-range thermoelectric performance in GeTe

GeTe has attracted widespread attention as an ideal mid-temperature thermoelectric (TE) material, but its peak performance within a narrow mid-temperature range limits the conversion efficiency improvement of future TE devices. Here, to achieve the overall enhancement of GeTe over a broad temperature range, especially near room temperature, we demonstrate the functionally separated electronic band engineering strategy (resonance energy levels and band convergence) coupling the construction of high-density defects by designing multi-element SnSe-In-Sb co-doping. Fundamentally, to distinguish the functions of selected elements, it was found that SnSe alloying mainly promotes the band convergence of GeTe in elevated temperatures, In doping can create the resonance energy level for enhancing the TE performance near room temperature, and Sb can optimize symmetry and the overall carrier concentration, thereby jointly improving the effective mass and Seebeck coefficient. Moreover, the construction of various phonon scattering centers including high-density Ge micro/nano-precipitates, van der Waals gaps, dislocations, and strong stress fields, strongly inhibits phonon transport. Benefiting from these synergistic effects, a peak ZT of ∼ 2.0 at 653 K, ZTave of ∼ 1.2 in the range from 303 to 803 K, and Vickers microhardness of ∼ 234 Hv are obtained in (Ge0.91In0.01Sb0.08Te)0.98(SnSe)0.02 sample. This study demonstrates a promising strategy to optimize thermoelectric performance by selectively co-doping functionally separated elements for developing high-performance TE materials.

P-type SiGe-based composite produced by mechanical alloying and spark plasma sintering

We report on a study of bulk SiGe-MnTe composites obtained via spark plasma sintering of mixed powders composed of SiGe that was fabricated by mechanical alloying and MnTe prepared via solid-state reaction. The presence of MnTe in the SiGe matrix led to an increase in the electrical conductivity by ∼ 50 % and a decrease in the lattice thermal conductivity of the samples by ∼ 28 %, while the Seebeck coefficient was reduced from 252 μV K−1 to 197 μV K−1 at 1000 K. As a result, the peak zT of 0.75 was obtained for pristine Si80Ge20B2, and 0.8 for Si80Ge20B2 + 4 wt% MnTe at 1000 K.

Decoupling thermoelectric parameters by the energy-dependent carrier and phonon scattering based on the nano-structuring interface design

Thermoelectric materials can transfer heat energy to electricity energy or vice versa. However, the transfer efficiency is restricted by the interdependent thermoelectric parameters. In this work, all the thermoelectric parameters are simultaneously optimized via the energy-dependent carrier and phonon scattering based on the nano-structuring interface design. As a result, not only the electrical conductivity is significantly enhanced by increasing the electron concentration, the Seebeck coefficient is maintained in a high value due to the energy-dependent carrier scattering mechanism. Otherwise, based on the stronger phonon scattering by the nanoinclusions and dislocations, the lattice thermal conductivity is markedly suppressed. And thus, a peak ZT=1.5 and an outstanding conversion efficiency η=9 % are achieved for Pb0.995Bi0.005Te/0.1 wt.%Fe3O4 composite, indicating that the thermoelectric property of n-type PbTe can be elevated greatly by embedding Fe3O4 nanoinclusions. This strategy of nano-structuring interface design can also be highly applicable in improving the performance of other state-of-the-art thermoelectric materials.

Pseudopolymorphic Phase Engineering for Improved Thermoelectric Performance in Copper Sulfides.

Polymorphism (and its extended form - pseudopolymorphism) in solids is ubiquitous in mineralogy, crystallography, chemistry/biochemistry, materials science, and the pharmaceutical industries. Despite the difficulty of controlling (pseudo-)polymorphism, the realization of specific (pseudo-)polymorphic phases and associated boundary structures is an efficient route to enhance material performance for energy conversion and electromechanical applications. Here, this work applies the pseudopolymorphic phase (PP) concept to a thermoelectric copper sulfide, Cu2- x S (x ≤ 0.25), via CuBr2 doping. A peak ZT value of 1.25 is obtained at 773K in Cu1.8 S + 3wt% CuBr2 , which is 2.3 times higher than that of a pristine Cu1.8 S sample. Atomic-resolution scanning transmission electron microscopy confirms the transformation of pristine Cu1.8 S low digenite into PP-engineered high digenite, as well as the formation of (semi-)coherent interfaces between different PPs, which is expected to enhance phonon scattering. The results demonstrate that PP engineering is an effective approach for achieving improved thermoelectric performance in Cu-S compounds. It is also expected to be useful in other materials.

Multifunctional GeMnTe2 Synergistically Optimizes Thermoelectric Properties of SnTe-In2Te3 Alloys.

SnTe-In2Te3 alloys ensure excellent electrical properties in the whole temperature region due to the resonant level. Nevertheless, temperature-sensitive resonance states and single phonon scattering restrict further improvement of thermoelectric performance. Consequently, it is anticipated that additional electrically independent scattering sources should be introduced to impede phonon transport. Here, the SnTe-In2Te3-GeMnTe2 alloy is prepared by further solidifying cubic GeMnTe2, which demonstrates multiple modulation effects. The highly redissolved Mn2+ promotes the valence band convergence, enhances the Seebeck coefficient at higher temperature, and balances the possible weakened resonance level effect at higher carrier concentrations, and a high average power factor (1.94 mW m-1 K-2) is realized over the entire temperature range. Additionally, compensatory vacancies, substitutions, and Ge/Mn precipitates are easily constructed with GeMnTe2 alloying, leading to a further reduction in lattice thermal conductivity, which reaches κl ∼ 0.6 W m-1 K-1 at 850 K. Ultimately, a high peak zT of ∼1.25 (850 K) and a zTave of 0.72 (300-850 K) are realized in (SnTe)2.91(In2Te3)0.03(Ge0.5Mn0.5Te)1.2, and the maximum thermoelectric conversion efficiency of ∼2.8% (ΔT ∼ 450 K) is achieved. The present results indicate multiple effects of GeMnTe2 in enhancing the thermoelectric performance of SnTe-In2Te3 alloys.

Enhanced Thermoelectric Performance of P-Type (Bi,Sb)2 Te3 by Incorporating Non-Stoichiometric Ag5 Te3 and Refining Te-Se Ratio.

Power generation modules utilizing thermoelectric (TE) materials are suitable for recycling widespread low-grade waste heat (<600K), highlighting the immediate necessity for advanced Bi2 Te3 -based alloys. Herein, the substantial enhancement in TE performance of the p-type Bi0.4 Sb1.6 Te3 (BST) sintered sample is realized by subtly incorporating the non-stoichiometric Ag5 Te3 and counteractive Se. Specifically, Ag atoms diffused into the BST lattice improve the density-of-states effective mass (md * ) and boost the hole concentration for the suppressed bipolar effect. The addition of Se further improves md * prompting the room-temperature power factor upgrade to 46Wcm-1 K-2 . Concurrently, the lattice thermal conductivity is considerably lowered by multiple scattering sources exemplified by Sb-rich nanoprecipitates and dense dislocations. These synergistic results yield a high peak ZT of 1.44 at 375K and an average ZT of 1.28 between 300 and 500K in the Bi0.4 Sb1.6 Te2.95 Se0.05 + 0.05wt.% Ag5 Te3 sample. More significantly, when coupled with n-type zone-melted Bi2 Te2.7 Se0.3 , the integrated 17-pair TE module achieves a competitive conversion efficiency of 6.1% and an output power density of 0.40Wcm-2 at a temperature difference of 200K, demonstrating great potential for practical applications.

Wide-temperature-range thermoelectric n-type Mg3(Sb,Bi)2 with high average and peak zT values

Mg3(Sb,Bi)2 is a promising thermoelectric material suited for electronic cooling, but there is still room to optimize its low-temperature performance. This work realizes >200% enhancement in room-temperature zT by incorporating metallic inclusions (Nb or Ta) into the Mg3(Sb,Bi)2-based matrix. The electrical conductivity is boosted in the range of 300–450 K, whereas the corresponding Seebeck coefficients remain unchanged, leading to an exceptionally high room-temperature power factor >30 μW cm−1 K−2; such an unusual effect originates mainly from the modified interfacial barriers. The reduced interfacial barriers are conducive to carrier transport at low and high temperatures. Furthermore, benefiting from the reduced lattice thermal conductivity, a record-high average zT > 1.5 and a maximum zT of 2.04 at 798 K are achieved, resulting in a high thermoelectric conversion efficiency of 15%. This work demonstrates an efficient nanocomposite strategy to enhance the wide-temperature-range thermoelectric performance of n-type Mg3(Sb,Bi)2, broadening their potential for practical applications.

High Performance Thermoelectric Power of Bi0.5Sb1.5Te3 Through Synergistic Cu2GeSe3 and Se Incorporations.

Bi2Te3-based alloys are the benchmark for commercial thermoelectric (TE) materials, the widespread demand for low-grade waste heat recovery and solid-state refrigeration makes it imperative to enhance the figure-of-merits. In this study, high-performance Bi0.5Sb1.5Te3 (BST) is realized by incorporating Cu2GeSe3 and Se. Concretely, the diffusion of Cu and Ge atoms optimizes the hole concentration and raises the density-of-states effective mass (md *), compensating for the loss of "donor-like effect" exacerbated by ball milling. The subsequent Se addition further increases md *, enabling a total 28% improvement of room-temperature power factor (S2σ), reaching 43.6µWcm-1K-2 compared to the matrix. Simultaneously, the lattice thermal conductivity is also significantly suppressed by multiscale scattering sources represented by Cu-rich nanoparticles and dislocation arrays. The synergistic effects yield a peak ZT of 1.41 at 350K and an average ZT of 1.23 (300-500K) in the Bi0.5Sb1.5Te2.94Se0.06 + 0.11 wt.% Cu2GeSe3 sample. More importantly, the integrated 17-pair TE module achieves a conversion efficiency of 6.4%, 80% higher than the commercial one at ΔT=200K. These results validate that the facile composition optimization of the BST/Cu2GeSe3/Se is a promising strategy to improve the application of BST-based TE modules.

Phase-dependent microstructure modification leads to high thermoelectric performance in n-type layered SnSe2

Microstructure control is crucial for thermoelectrics since it is intimately related to the scattering mechanism of both electrons and phonons. Herein, we propose a new strategy to modify microstructure by phase regulation that simultaneously induces high carrier mobility and low lattice thermal conductivity. As demonstrated in layered SnSe2, the addition of Cu can induce a phase transition from space group P3¯m1 to P63mc. Due to the enlarged formation energy of stacking faults in the later phase, the stacking fault density is greatly reduced after heat treatment that leads to an increased grain size. Accordingly, the carrier mobility of SnSe1.97Br0.03–3 % Cu sample is enhanced by 100 % at room temperature. Furthermore, the reduction of stacking fault density is accompanied by the formation of pores in the matrix, which results in low lattice thermal conductivity. As a result, a record peak zT of 1.13 for SnSe2-based materials is achieved at 773 K, and the attained ZTave of 0.62 is a record-high value among n-type polycrystalline layered materials working in intermediate-to-high temperature region. This work provides a feasible strategy to decouple the electron and phonon transport in layered thermoelectric compounds by phase-dependent microstructure modification.

Filtering low‐energy carriers to enhance thermoelectric performance of Cd‐doped SnSe via incorporation of MXene sheets

AbstractSnSe‐based materials have attracted widespread attention in thermoelectrics due to their outstanding thermoelectric performance. However, the pristine and unmodified polycrystalline SnSe reveals poor electrical properties. Doping and constructing nanostructured composite architectures to produce energy filtering effect proved to be an effective method to strengthen thermoelectric performance. In this study, Ti3C2/Sn0.98Cd0.02Se composites are successfully fabricated by the solvothermal method combined with the electrostatic self‐assembly method and spark plasma sintering. The phase interface introduced by incorporating Ti3C2 into Sn0.98Cd0.02Se can effectively filter low‐energy carriers due to its generation of energy barriers, thereby the Seebeck coefficient of x wt% Ti3C2/Sn0.98Cd0.02Se x = (0.05, 0.5, 1) samples is better than that of the pristine Sn0.98Cd0.02Se over the whole temperature range. Meanwhile, high conductivity was also obtained in 1 wt% Ti3C2/Sn0.98Cd0.02Se sample so that the high power factor of 3.31 μWcm−1K−2 was acquired at 773 K. Ultimately, a peak ZT value of 0.41 was obtained at 773 K, compared with pristine Sn0.98Cd0.02Se, and the thermoelectric performance improved by 24%. This study offers an available approach to efficiently enhance the thermoelectric properties of polycrystalline SnSe‐based materials.

N-Type Thermoelectric AgBiPbS3 with Nanoprecipitates and Low Thermal Conductivity.

Thermoelectric sulfide materials are of particular interest due to the earth-abundant and cost-effective nature of sulfur. Here, we report a new n-type degenerate semiconductor sulfide, AgBiPbS3, which adopts a Fm3̅m structure with a narrow band gap of ∼0.32 eV. Despite the homogeneous distribution of elements at the scale of micrometer, Ag2S nanoprecipitates with dimensions of several nanometers were detected throughout the matrix. AgBiPbS3 exhibits a low room-temperature lattice thermal conductivity of 0.88 W m-1 K-1, owing to the intrinsic low lattice thermal conductivity of Ag2S and the effective scattering of phonons at nanoprecipitate boundaries. Moreover, compared to AgBiS2, AgBiPbS3 demonstrates a significantly improved weighted mobility of >16 cm2 V-1 s-1 at 300 K, leading to an enhanced PF of 1.6 μW cm-1 K-2 at 300 K. The superior electrical transport in AgBiPbS3 can be attributed to the high valley degeneracy of the L point (the conduction band minimum), which is contributed by the Pb s and Pb p orbitals. Further, Ga doping is found to be effective in modulating the Fermi levels of AgBiPbS3, leading to further enhancement of PF with a PFave of 2.7 μW cm-1 K-2 in the temperature range of 300-823 K. Consequently, a relatively high ZTave of 0.22 and a peak ZT of ∼0.4 at 823 K have been achieved in 3% Ga-doped AgBiPbS3, highlighting the potential of AgBiPbS3 as an n-type thermoelectric sulfide.

Clathrate structure of YB3C3 for high-performance thermoelectrics with superior mechanical properties

Exploring high-performance thermoelectric materials with improved mechanical properties is important for broadening the application scope and the assembly requirement of stable devices. This work presents an effective strategy to discover hard thermoelectric material by inserting foreign atoms in the rigid covalent framework. We demonstrate this in boron-carbon clathrate VII structure, showing a promising candidate for highly efficient thermoelectric energy conversion, especially with Y atom filled in the cage, with a peak zT of 0.73 at 1,000 K. The ab initio calculations indicate that YB3C3 system has low lattice thermal conductivity of 4.5 W/(m·K) at 1,000 K due to the strong rattling of encaged Y atom. The strongly covalent framework provides highly degenerate band structures consisting of heavy and light electron pockets, which can maintain high carrier mobility arising from small effective mass and thus large group velocity. Consequently, high power factor can be achieved in YB3C3 for both electron and hole doping. In addition, it exhibits well mechanical properties and a Vickers hardness of 23.7 GPa because of the strong covalent boron-carbon framework. This work provides a novel avenue for the search of high-performance thermoelectric materials with excellent mechanical properties, based on boron-carbon clathrate structure.

Realizing high thermoelectric performance in CeCl3-doped n-type SnSe polycrystals

SnSe is a promising thermoelectric material because of its intrinsic low lattice thermal conductivity. However, the inferior electrical transport properties restrict the thermoelectric performance of n-type SnSe polycrystals especially at lower temperature range. In this work, an effective strategy to enhance the performance of n-type SnSe polycrystals by CeCl3 doping is reported. CeCl3 doping increased both the carrier concentration and electrical conductivity. The high weighted mobility contributed to the high power factor over the whole temperature range. Combined with the reduced thermal conductivity, a peak zT of ∼1.3 at 823 K was obtained in SnSe0.92+1 mol% CeCl3 with an average zT of ∼0.36 in the temperature range of 300–773 K. This work suggests that CeCl3 doping is an effective way to optimize the thermoelectric properties of n-type SnSe polycrystals.

Enhancement on thermoelectric performance by Ti doping and vacancies

Due to the exceptionally low lattice thermal conductivity, AgSbTe2 has emerged as a promising thermoelectric material. However, unlike other IV-VI compounds, most reported AgSbTe2 samples exhibit relatively poor electrical performance due to low carrier concentration. Herein, an unusually strategy of introducing dopants and vacancies on the same site of Sb is adopted, which significantly improves the electrical performance of AgSbTe2. It is found that Ti doping on Sb site can increase the carrier concentration and promote band convergence, based on which the introduction of Sb vacancy further enlarges the carrier concentration to approach the optimal value without affecting the carrier mobility. Consequently, an excellent average power factor of 1.4 mW m−1K−2 is achieved in AgSb0.94 Ti0.05Te2 from 323 K to 623 K. Combining with reduced thermal conductivity due to strengthened phonon scattering induced by the abundant lattice defects, a peak zT of 1.8 and a high average zT of 1.3 are attained. These results provide some new insights for improving the thermoelectric performance of AgSbTe2 based compounds.

Enhancement of Thermoelectric Performance in Robust ZnO‐Based Composite Ceramics Driven by A Stepwise Optimization Strategy

AbstractZnO is a promising high‐temperature thermoelectric (TE) material due to its superior stability and earth abundance. However, the coupling relation between Seebeck coefficient and electrical conductivity on carrier concentration limits the optimum TE performance for most TE materials, especially ZnO. Herein, enhancement of TE performance is achieved via a stepwise optimization strategy composed of carrier concentration optimization and carrier filtering effect for fabricating robust ZnO‐based composite ceramics through a self‐developed specific high‐pressure synthesis followed by spark plasma sintering. Specifically, doping SnO2 provide substantial electrons to surge the carrier concentration. The subsequent compositing Si3N4 nanoparticles results in the unique reaction‐generated Zn2SiO4 nanoprecipitates with a larger bandgap and intrinsically low thermal conductivity, which introduce an excellent carrier filtering effect to increase the Seebeck coefficient by 57.7% at 300 K without compromising electrical conductivity much and enhance phonon scattering to cause an ultralow lattice thermal conductivity of 1.39 W m−1 K−1 achieving amorphous limits of ZnO (1.4±0.1 W m−1 K−1). Consequently, a high peak ZT (figure of merit) of 0.691 at 873 K is obtained, which is higher than that of previously reported ZnO‐based TE materials. This work demonstrates a feasible and effective strategy to fabricate high‐performance TE materials, especially those with inferior electrical conductivity.

Strained Lamellar Structures Leading to Improved Thermoelectric Performance in Mg3Sb1.5Bi0.5.

Mg3Sb2-xBix solid-solutions represent an important class of thermoelectric (TE) materials due to their high efficiency and variable operating temperature range. Of particular significance for midtemperature applications is the Mg3Sb1.5Bi0.5 composition whose superior thermoelectric (TE) performance is attributed to the complex conduction band edge in conjunction with alloy dominated phonon scattering. In this work, we show that microstructure also plays a significant role in lowering the lattice thermal conductivity which in turn affects the overall TE performance (change in peak zT values between 1.1 and 1.4 have been observed). Temperature dependent TE properties of Mg3+xSb1.5Bi0.5 compositions with varying nominal Mg content (x = 0.2, 0.3, 0.4) have been studied. A marked reduction of the lattice thermal conductivity (κL) is observed in compositions with low nominal Mg content (x = 0.2), which is due to the presence of lamellar structures within the grains. These lamellar regions are isostructural to the matrix with a low misfit angle and represent compositional fluctuations in the Bi to Sb ratio. Both the size (200 nm-500 nm) and the interfacial strain contribute to the enhanced phonon scattering. A quantitative estimate of κL reduction due to these structures have been carried out using a mean free path (MFP) spectrum analysis which reveal a good match with experiments at room temperature. Further, the electrical properties are not influenced by these lamellar structures as observed from the similar power-factor (S2σ) and weighted mobilities in all of the compositions. This is due to their similar orientation to the adjacent matrix region. Thus, the zT parameter in the various compositions with similar carrier concentration can be significantly altered (∼25%) by adjusting the nominal Mg content. The results demonstrate that preferential phonon scattering by microstructure modification can be a new route for property improvement in Mg3+xSb2-yBiy solid-solutions.