Peak ZT Value Research Articles

Highly enhanced thermoelectric properties of Bi2S3 via (Se, Cl)-co doping in hydrothermal synthesis process

Bi2S3-based materials with low thermal conductivity and high Seebeck coefficient has been regarded as promising thermoelectric materials for years. To optimize the thermoelectric properties of Bi2S3 materials, (Se, Cl)-co-doped nanostructured Bi2S3 bulks were prepared by hydrothermal method combined with spark plasma sintering. The doping contents of Se and Cl in Bi2S3 depend on the reaction time of the hydrothermal process. A high power factor of 450 μWm−1 K−2 was achieved for Bi2S3 sample with the reaction time of 24 h owing to the substitution of S by Se and Cl. Meanwhile, a low thermal conductivity of 0.35 Wm−1 K−1 is obtained for the Bi2S3 sample with the reaction time of 18 h, owing to the strong phonon scattering caused by the high density of grain boundaries, pores and point defect. Therefore, benefiting from the improved electrical transport properties and reduced thermal conductivity, a peak ZT value of ~ 0.57 was obtained at 673 K for the Bi2S3 sample with the reaction time of 24 h.

Efficient Si Doping Promoting Thermoelectric Performance of Yb-Filled CoSb3-Based Skutterudites.

Nanocomposites have become a widely popular way to assist in the enhancement of thermoelectric performance for filled skutterudites. Herein, we unveil the distinctive effect of Si doping on the classic Yb0.3Co4Sb12. On the one hand, the reduced Yb filling fraction is accompanied by the in-situ precipitated CoSi nanoparticles, which not only enhances the power factor in the intermediate-low temperature range but also reduces electronic thermal conductivity for decreasing the carrier concentration. On the other hand, CoSi nanoparticles intensively disrupt the phonon transport, hiding the increased lattice thermal conductivity due to reduced Yb filling fraction. Although the residual YbSb2 second phases have an adverse effect on the thermoelectric properties, the integration effects achieve a peak ZT value of 1.37 at 823 K and increase ZTave by 21% for the Yb0.3Co4Sb12/0.1Si sample.

Synergistic Manipulation of Interdependent Thermoelectric Parameters in SnTe-AgBiTe2 Alloys by Mn Doping.

In the mid-temperature region, SnTe is a promising substitute for PbTe, whereas the thermoelectric (TE) property of pristine SnTe is severely limited by the good thermal conductivity and inferior Seebeck coefficient. In this research, we synergistically manipulate the interdependent TE parameters of SnTe-AgBiTe2 alloys by Mn doping to increase the ZT value. The AgBiTe2 alloying is found to greatly reduce the electrical conductivity and electronic contribution for thermal transport by reducing the carrier mobility, while Mn doping obviously improves the Seebeck coefficient by effectively decreasing the valence band offset. The lowest κl of Mn-doped SnTe-AgBiTe2 alloys is 0.49 W m-1 K-1 at 823 K since the various defects strengthen the phonon scattering. Collectively, these manipulations yield a peak ZT value of 1.40 at 823 K and an average ZT value of 0.73 (300-823 K) in the Mn-doped SnTe-AgBiTe2 alloys. This research suggests that Mn doping is a valid scheme to constantly improve the thermoelectric property of SnTe-AgBiTe2 alloys in a wide temperature range.

The role of Ge vacancies and Sb doping in GeTe: A Comparative Study of Thermoelectric Transport Properties in SbxGe1-1.5xTe and SbxGe1-xTe Compounds

Optimizing the thermoelectric properties of GeTe-based compounds usually involves the modulation of Ge vacancies and dopants. However, the mixture of vacancies and dopants makes the understanding of the doping mechanism complicated. Herein, two batches of SbxGe1-1.5xTe and SbxGe1-xTe compounds were prepared and the role of Ge vacancies and Sb were systematically studied. Alloying Sb2Te3 in GeTe increases the concentration of cationic vacancies, which boosts the solubility limit of Sb in the GeTe compound. Moreover, such vacancies aggregate and form planar vacancies in SbxGe1-1.5xTe compounds. Both the cationic vacancies and Sb doping in the structure weaken the degree of the rhombohedral distortion. Additionally, doping with Sb in both cases lower the hole concentration effectively but with different regulation mechanism. Apart from the donor characteristic of Sb dopant in the SbxGe1-xTe compounds, the dissolution of Ge-precipitates facilitated by the additional Ge vacancies introduced via alloying with Sb2Te3 reduces the carrier concentration. Furthermore, all-scale hierarchical architectures, including point defects, planar vacancies, and inherent grain boundaries, effectively scatter heat-carrying phonons, resulting in a low lattice thermal conductivity of 1.07 W m−1 K−1 for the Sb0.10Ge0.85Te compound. All these produce a peak ZT value of 1.85 at 724 K for the Sb0.10Ge0.85Te compound.

Improved Thermoelectric Performance of p-Type Argyrodite Cu8GeSe6 via the Simultaneous Engineering of the Electronic and Phonon Transports.

Guided by the concept of "phonon-liquid electron-crystal", many n-type argyrodite compounds have been developed as candidates for thermoelectric (TE) materials. In recent years, the p-type Cu8GeSe6 (CGS) compound has attracted some attention in TEs due to the presence of very strong atomic vibrational arharmonicity inside the sublattice, which is caused by the weak bonding between Cu ions and [GeSe6]8-. However, its TE performance is still poor, with a ZT value of only 0.2 at 623 K. Therefore, in this work, we propose to engineer both the electronic and phonon transports in CGS by incorporating the species In2Te3. This strategy tunes the carrier concentration and at the same time increases the phonon scattering on the point defects (InGe, Ininterstitial, and TeSe) and randomly distributed tetrahedra ([InSe4]5- and [GeTeSe3]4-). As a result, the phase transformation at 329 K in CGS is eliminated, and the peak ZT value is enhanced from 0.27 for CGS to ∼0.92 for (Cu8SnSe6)0.9(In2Te3)0.1 at 774 K; this thus proves that the incorporation of In2Te3 in CGS is an effective way of regulating its TE performance.

Enhanced Thermoelectric Performance in SmMg2Bi2 via Ca-Alloying and Ge-Doping

Zintl phase compounds with a CaAl2Si2 structure are promising thermoelectric materials. In this paper, enhanced thermoelectric performance was achieved in SmMg2Bi2, a new member of AB2X2 compounds, via Ca-alloying and Ge-doping. The introduction of point defects by alloying Ca on the Sm site significantly reduces the lattice thermal conductivity, and the lowest value was achieved when the Sm/Ca molar ratio is 1:1. Doping Ge on the Bi site increases the hole concentration of the material, which successfully suppresses the bipolar effect and significantly improves the power factor (PF) in the whole temperature range. Due to the decrease of thermal conductivity and the increase of PF over 323–873 K, the peak zT value of Sm0.5Ca0.5Mg2.15Bi1.99Ge0.01 reached 0.71 at 873 K and the zTave approached 0.44, about 51 and 47% enhancement compared with the pristine sample, respectively. This work provides a thermoelectric performance optimization strategy that can be used for other AB2X2 compounds.

Integrating band engineering with point defect scattering for high thermoelectric performance in Bi2Si2Te6

A layer-structure compound Bi2Si2Te6 is introduced as a promising thermoelectric material. The intrinsic low lattice thermal conductivity of Bi2Si2Te6 is because of the low phonon group velocities near the Γ point and the overlap of optical and acoustic phonon branches. A further decreased thermal conductivity is obtained by alloying Sb at the Bi site for the additional point-defect scattering of phonons. This Sb alloying also enhances the density-of-states near the valence band maximum for a larger carrier effective mass, and suppresses the bipolar effect by increasing the carrier concentration and widening the band gap. Benefitting from both the enhancement of power factor and the decrease of lattice thermal conductivity, the peak zT value is improved from ∼ 0.45 at 573 K for Bi2Si2Te6 to ∼ 1.2 at 773 K for BiSbSi2Te6.

Synergistically optimizing carrier and phonon transport properties in n-type PbTe through I doping and SnSe alloying

PbTe is a promising thermoelectric material, but the properties in n-type PbTe need further improvement to match its advanced p-type material. In this work, the thermoelectric performance in n-type PbTe is synergistically enhanced with iodine (I) doping and SnSe alloying. The experiments show that I element is a good donor dopant to tune the carrier density and SnSe alloying can effectively balance the triple-relations between carrier mobility, carrier effective mass, and lattice thermal conductivity. Thus, high carrier mobility of ∼755 cm2V−1s−1 results in an optimum power factor (PF) of ∼27.0 μWcm−1K−2 in PbTe–2%SnSe at 573 K. Microstructure observation reveals the dispersively embedded Sn-rich nanoprecipitates in PbTe–2%SnSe, which largely suppresses lattice thermal conductivity at room temperature, from ∼3.41 Wm−1K−1 in PbTe to ∼1.01 Wm−1K−1 in PbTe–2%SnSe. Combined the optimized PF and decreased thermal conductivity, the peak ZT value of ∼1.5 can be obtained at 773 K in PbTe–2%SnSe. The obtained high thermoelectric performance in our work is comparable with other advanced n-type PbTe thermoelectrics and has great application potential.

Unconventional Doping Effect Leads to Ultrahigh Average Thermoelectric Power Factor in Cu3 SbSe4 -Based Composites.

Thermoelectric materials are typically highly degenerate semiconductors, which require high carrier concentration. However, the efficiency of conventional doping by replacing host atoms with alien ones is restricted by solubility limit, and, more unfavorably, such a doping method is likely to cause strong charge-carrier scattering at ambient temperature, leading to deteriorated electrical performance. Here, an unconventional doping strategy is proposed, where a small trace of alien atoms is used to stabilize cation vacancies in Cu3 SbSe4 by compositing with CuAlSe2 , in which the cation vacancies rather than the alien atoms provide a high density of holes. Consequently, the hole concentration enlarges by sixtimes but the carrier mobility is well maintained. As a result, a record-high average power factor of 19µWcm-1 K-2 in the temperature range of 300-723K is attained. Finally, with further reduced lattice thermal conductivity, a peak zT value of 1.4and a record-high average zT value of 0.72are achieved within the diamond-like compounds. This new doping strategy not only can be applied for boosting the average power factor for thermoelectrics, but more generally can be used to maintain carrier mobility for a variety of semiconductors that need high carrier concentration.

Super-structured defects modulation for synergistically optimizing thermoelectric property in SnTe-based materials

Among the intriguing p-type thermoelectric IV-VI compounds, SnTe is an important alternative to both PbTe and GeTe for free of toxicity of Pb and scarcity of Ge. However, pristine SnTe has poor thermoelectric performance because of its intrinsic high hole concentration, big energy difference between L and Σ bands and high lattice thermal conductivity. In this work, In and Cd co-doping SnTe material, which combines the effect of resonant level and band convergence together, was obtained by self-propagating high-temperature synthesis (SHS) and plasma activated sintering (PAS). The generated Te vacancies are regularly arranged to form super-structured defects in the subsequent constrained hot compressing process, which include regular array of atomic defects and nanoscale precipitates. As a result, the hole concentration of the compressed SnTe-based material is reduced due to the donor doping effect of Te vacancies, and the hole mobility is remarkably improved to 372.94 cm2V−1s−1 and 66.19 cm2V−1s−1 at room temperature for the SnTe and In0.01Cd0.02Sn0.97Te material with compressibility of 35%, respectively. These atomic and nanoscale super-structured defects not only ensure superior electrical transport properties, but also bring much stronger scattering effect on phonons, thus leading to extremely low lattice thermal conductivity of 1.2 Wm−1K−1 at 888 K for the 35% compressed In0.01Cd0.02Sn0.97Te material. With synergistically optimized carrier and phonon transport behavior through resonant level engineering, band convergence engineering and super-structured defects engineering, a peak ZT value of 0.8 is achieved in In0.01Cd0.02Sn0.97Te material with compressibility of 35%. This work provides an effective strategy to realize the decoupling of electron and phonon scattering through Te vacancy-based super-structured defects modulation, and makes up an important part of multiscale microstructure tailoring for thermoelectric materials.

Improved Thermoelectric Performance of P-type SnTe through Synergistic Engineering of Electronic and Phonon Transports

SnTe has been regarded as a potential alternative to PbTe in thermoelectrics because of its environmentally friendly features. However, it is a challenge to optimize its thermoelectric (TE) performance as it has an inherent high hole concentration (nH∼2 × 1020 cm-3) and low mobility (μH∼18 cm2 V-1 s-1) at room temperature (RT), arising from a high intrinsic Sn vacancy concentration and large energy separation between its light and heavy valence bands. Therefore, its TE figure of merit is only 0.38 at ∼900 K. Herein, both the electronic and phonon transports of SnTe were engineered by alloying species Ag0.5Bi0.5Se and ZnO in succession, thus increasing the Seebeck coefficient and, at the same time, reducing the thermal conductivity. As a result, the TE performance improves significantly with the peak ZT value of ∼1.2 at ∼870 K for the sample (SnGe0.03Te)0.9(Ag0.5Bi0.5Se)0.1 + 1.0 wt % ZnO. This result proves that synergistic engineering of the electronic and phonon transports in SnTe is a good approach to improve its TE performance.

Band convergence and phonon engineering to optimize the thermoelectric performance of CaCd2Sb2

Alignment of valence bands has been demonstrated to be effective in promoting the thermoelectric performance of p-type AB2X2 zintl phases. In this work, the degeneracy of the valence bands is manipulated by alloying CaCd2Sb2 with CaMg2Sb2. It is found that the Γ(pxy) band and the Γ(pz) band were effectively converged in CaCd1.5Mg0.5Sb2. By further doping Ag at the Cd site, the carrier concentration can be maintained when the alloying concentration varies. The room-temperature Seebeck coefficient increased from ∼150 μV K−1 in CaCd2Sb2 to ∼190 μV K−1 in CaCd1.5Mg0.5Sb2 when the carrier concentration was maintained at ∼2.5 × 1019 cm−3. In addition, Cd/Mg substitutional point defects with substantial atomic mass difference induced significant phonon scattering; thus, a lattice thermal conductivity as low as ∼0.5 W m−1 K−1 was achieved at 750 K. Eventually, a peak zT value of ∼1.3 was realized in CaCd1.494Ag0.006Mg0.5Sb2.

Enhanced thermoelectric perfromance in cubic form of SnSe stabilized through enformatingly alloying AgSbTe2

Metastable cubic SnSe has received extensive attention in terms of theoretical predictions since orthorhombic SnSe was experimentally developed as one of the most promising thermoelectric candidates in recent years. In this work, we successfully stabilized the cubic SnSe through AgSbTe2 alloying, after which the thermoelectric transport properties were significantly optimized by simultaneously suppressing lattice and electronic thermal conductivities via introducing Pb. The maximum power factor of (SnSe)0.6(AgSbTe2)0.4 reached to 13 μWcm−1K−2 at 723 K, with a high average power factor of ∼ 12 μWcm−1K−2 at 300–773 K. Based on 40% AgSbTe2 alloying, the strong phonon-defect scattering and low electronic thermal conductivity caused by Pb alloying contribute greatly to the reduction of total thermal conductivity, while the excellent electrical transport properties were substantially maintained. Finally, the peak ZT value reached to 1.20 and a high average ZT value of ∼ 0.90 over 300–773 K were achieved in the cubic phase (SnSe)0.6(AgSbTe2)0.4 with 10% Pb alloyed.

Routes to High-Ranged Thermoelectric Performance

Thermoelectric technology has immense potential in enabling energy conversion between heat and electricity, and its conversion efficiency is mainly determined by the wide-temperature thermoelectric performance in a given material. Therefore, it is more meaningful to pursue high ZT values in a wide temperature range (namely high average ZT) rather than the peak ZT value at a temperature point. Herein, taking lead chalcogenides as paradigm, some rational routes to high average ZT value in thermoelectric materials are introduced, such as bandgap tuning and dynamic doping. This perspective will emphasize the importance of dynamically optimizing carrier and phonon transport properties to high-ranged thermoelectric performance, which could judiciously be extended to other thermoelectric systems.

Strain-induced enhancement in the electronic and thermal transport properties of the tin sulphide bilayer.

The enhancement in the thermoelectric figure of merit (ZT) of a material is limited by the interplay between the electronic transport coefficients. Here we report the greatly enhanced thermoelectric performance of the SnS bilayer with the application of isotropic strain, due to the simultaneous increase in the Seebeck coefficient and low lattice thermal conductivities. Based on first-principles calculations combined with Boltzmann transport theory, we predict that the band structure of the SnS bilayer can be effectively tuned using the strain, and the Seebeck coefficient is significantly improved for the tensile strain. The lattice thermal conductivities for the bilayer under the tensile strain are quite low (0.21-1.89 W m-1 K-1 at 300 K) due to the smaller frequencies of the acoustic phonon modes. Along the zigzag (armchair) direction, the room temperature peak ZT value of 4.96 (2.40) is obtained at a strain of 2% (4%), which is 5.3 (2.03) times higher than the peak ZT of the unstrained bilayer along the zigzag (armchair) direction. Thus the strain-tuned SnS bilayer is a good thermoelectric material with low lattice thermal conductivities and promising ZT values at room temperature.

Improving thermoelectric performance by constructing a SnTe/ZnO core-shell structure.

SnTe is becoming a new research focus as an intermediate temperature thermoelectric material for its environment-friendly property. Herein, the SnTe/ZnO core–shell structure prepared by a facile hydrothermal method is firstly constructed to enhance the thermoelectric performance. The characterization results demonstrate that ZnO nanosheets are coated on the surface of SnTe particles by in situ synthesis and converted into ZnO nano-dots by spark plasma sintering. The energy barriers built by the SnTe/ZnO core–shell structure improve the Seebeck coefficient effectively. Additionally, the increased density of interfaces induced by ZnO can effectively scatter low/medium frequency phonons, reducing the lattice thermal conductivity in the low/medium temperature region. Further, the point defects caused by Cu2Te-alloying strengthen the scattering of high frequency phonons. The lattice thermal conductivity reaches 0.48 W m−1 K−1, which is close to the amorphous limit of pristine SnTe. As a result, a peak ZT value of 0.94 is achieved at 823 K for SnTe(Cu2Te)0.06–1.5% ZnO, benefiting from the synergistic optimization of thermal and electrical properties. This provides a new idea for exploring an optimization strategy of thermoelectric performance.

Synergistically Enhanced Thermoelectric Performance of Cu2SnSe3-Based Composites via Ag Doping Balance.

As an ecofriendly and low-cost thermoelectric material, Cu2SnSe3 has recently drawn much attention. In this work, the thermoelectric properties of Cu2SnSe3-based materials have been synergistically optimized. Ag doping at the Cu site enables strong phonon scattering via large strain field fluctuations and increases the effective mass of carriers through band engineering, which results in a reduced lattice thermal conductivity and enhanced Seebeck coefficient. Thus, a peak ZT value of 1.04 at 800 K is obtained for Cu1.85Ag0.15SnSe3. Then, In is further doped at the Sn site to further increase the carrier concentration and power factor; the ZT values of Cu1.85Ag0.15Sn1-yInySe3 samples are highly improved in the temperature range of 300-800 K, and a peak ZT value of 1.12 is obtained at 800 K for the Cu1.85Ag0.15Sn0.91In0.09Se3 sample. Considering that Ag2S decomposes to Ag and S completely under vacuum and high current field, the sulfur vapor would be pumped away by a vacuum pump, and the generated Ag not only enriches at the grain boundary of the Cu1.85Ag0.15Sn0.91In0.09Se3 bulk material but also enters the matrix and occupies the Cu site, leading to the extruding Cu and Se forming the second phase of CuSe nanoparticles. Thus, the power factor greatly improves to 13.8 μW cm-1 K-2 at 700 K, and the lattice thermal conductivity is as low as 0.12 W m-1 K-1 at 800 K for the Cu1.85Ag0.15Sn0.91In0.09Se3/4% Ag2S composite. Finally, a high ZT value of 1.58 is obtained at 800 K for the Cu1.85Ag0.15Sn0.91In0.09Se3/4% Ag2S composite, which is nearly an increase of 204% compared to that of Cu2SnSe3. This work provides an effective solution to optimize the conflicting material properties for Cu2SnSe3-based thermoelectric materials.

Ce Filling Limit and Its Influence on Thermoelectric Performance of Fe3CoSb12-Based Skutterudite Grown by a Temperature Gradient Zone Melting Method.

CoSb3-based skutterudite is a promising mid-temperature thermoelectric material. However, the high lattice thermal conductivity limits its further application. Filling is one of the most effective methods to reduce the lattice thermal conductivity. In this study, we investigate the Ce filling limit and its influence on thermoelectric properties of p-type Fe3CoSb12-based skutterudites grown by a temperature gradient zone melting (TGZM) method. Crystal structure and composition characterization suggests that a maximum filling fraction of Ce reaches 0.73 in a composition of Ce0.73Fe2.73Co1.18Sb12 prepared by the TGZM method. The Ce filling reduces the carrier concentration to 1.03 × 1020 cm−3 in the Ce1.25Fe3CoSb12, leading to an increased Seebeck coefficient. Density functional theory (DFT) calculation indicates that the Ce-filling introduces an impurity level near the Fermi level. Moreover, the rattling effect of the Ce fillers strengthens the short-wavelength phonon scattering and reduces the lattice thermal conductivity to 0.91 W m−1 K−1. These effects induce a maximum Seebeck coefficient of 168 μV K−1 and a lowest κ of 1.52 W m−1 K−1 at 693 K in the Ce1.25Fe3CoSb12, leading to a peak zT value of 0.65, which is 9 times higher than that of the unfilled Fe3CoSb12.

Hierarchical twinning and light impurity doping enable high-performance GeTe thermoelectrics

Microstructure engineering fulfilled by phase transformation elicits the reduced thermal conductivity κ in thermoelectric materials. We demonstrate that the multiscale hierarchical twinning structure could exist in the GeTe alloys as the phase transformation from cubic β-GeTe to rhombohedral α-GeTe is manipulated by two-step cooling. The coexistence of nanoscale and microscale herringbone structures yields the low-lying κ, attributing to the reorientation of martensitic variants and pile-up of martensitic twinning. Dilute dopants of Cu and Bi/Sb further suppresses hole carrier concentration nH to the order of 1020 cm−3, leading to the enhanced power factor PF = S2ρ−1 of α-GeTe. Meanwhile, the phase diagram engineering probes the solubility limit of Cu in the single-phase GeTe, diminishing the formation of undesired impurity phases. These strategies boost the peak zT value to 1.5 at 698 K in the light-doped Bi0.01Cu0.01Ge0.98Te alloy. The multiscale twin hierarchy and carrier optimization synergistically enhance the thermoelectric performance of GeTe-based alloys that provide a new chapter in search of green energy resources out from phase change materials.

High thermoelectric performance bismuth telluride prepared by cold pressing and annealing facilitating large scale application

Bi2Te3-based alloys, generally prepared by traditional zone melting or hot-pressing, have been used in commercial applications for decades. However, the zone melting method is time-consuming and usually accompanied by poor thermoelectric performance, while the hot-pressing sintering needs complex and high-cost equipment. In this work, high-performance p-type Bi0.4Sb1.6Te3 alloy is prepared by cold pressing the ball milled Bi0.4Sb1.6Te3 powder followed by vacuum annealing, which is more suitable for large-scale promoting with cost saving. Owing to the low lattice thermal conductivity brought by fine grains, a peak ZT value of ∼1.15 at 50 °C and an average ZT value of ∼0.84 within 30–250 °C are realized, which are comparable with the excellent performance achieved by zone melting and hot-pressing, and are even better than that of the optimized cold-pressed sample. More importantly, the thermoelectric performance here shows good repeatability from batch to batch. Our work makes the widespread application of Bi0.4Sb1.6Te3 alloy in thermoelectric power generation and refrigeration an important step forward.