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High efficiency GeTe-based materials and modules for thermoelectric power generation
Abstract
GeTe-based materials have a great potential to be used in thermoelectric generators for waste heat recovery due to their excellent thermoelectric performance, but their module research is greatly lagging behind the material research. In this work, we successfully fabricate a GeTe-based thermoelectric module and report a high energy conversion efficiency of 7.8% under a temperature gradient of 500 K. The Sb-, Bi-, and Se-included GeTe-based material, with the chemical composition of Ge0.92Sb0.04Bi0.04Te0.95Se0.05 and a peak ZT of 2.0 at 700 K, is used to make the p-type legs in the module. By using a high-throughput strategy, Mo is screened from 12 pure metals as an effective diffusion barrier material between GeTe-based material and electrode. Based on the optimal geometry predicted by the three-dimensional numerical analysis, one eight-couple Ge0.92Sb0.04Bi0.04Te0.95Se0.05/Yb0.3Co4Sb12 TE module is fabricated and evaluated, which shows a comparable energy conversion efficiency with those of skutterudites- and half-Heusler-based modules in the similar temperature range. This study opens the door for the development of GeTe-based modules.
... An ideal barrier layer material should not only provide an interface with low contact resistance to minimize losses in thermoelectric conversion but also with high bonding strength and service inertness to withstand thermal attack during operation. Recently, several strategies, such as powder co-sintering experiments [4][5][6][7] , interfacial reaction and diffusion energy criteria 8,9 , and phase diagram calculations [10][11][12] , have been proposed for the efficient screening of suitable barrier layers. Although these efforts have led to significant progress, materials for accessing thermoelectric interfacial barrier layers that are characterized by both high bonding activity and high operation reliability have rarely been developed. ...
... Based on this envision, we compared the T s and T o of a variety of thermoelectric materials 1,5,6,8,9, . It is evident that T s is much higher than T o in some thermoelectric systems, as exemplified by Mg 3 Sb 2 , whose T s can be 500 K greater than T o (Fig. 1c). ...
... TE is an abbreviation for thermoelectric. c One-step sintering temperature (T s ) and thermoelectric device operating temperature (T o ) of various thermoelectric materials 1,5,6,8,9, . HH is an abbreviation for half-Heusler. ...
Development of efficient and reliable thermoelectric generators is vital for the sustainable utilization of energy, yet interfacial losses and failures between the thermoelectric materials and the electrodes pose a significant obstacle. Existing approaches typically rely on thermodynamic equilibrium to obtain effective interfacial barrier layers, which underestimates the critical factors of interfacial reaction and diffusion kinetics. Here, we develop a desirable barrier layer by leveraging the distinct chemical reaction activities and diffusion behaviors during sintering and operation. Titanium foil is identified as a suitable barrier layer for Mg3Sb2-based thermoelectric materials due to the creation of a highly reactive ternary MgTiSb metastable phase during sintering, which then transforms to stable binary Ti-Sb alloys during operation. Additionally, titanium foil is advantageous due to its dense structure, affordability, and ease of manufacturing. The interfacial contact resistivity reaches below 5 μΩ·cm², resulting in a Mg3Sb2-based module efficiency of up to 11% at a temperature difference of 440 K, which exceeds that of most state-of-the-art medium-temperature thermoelectric modules. Furthermore, the robust Ti foil/Mg3(Sb,Bi)2 joints endow Mg3Sb2-based single-legs as well as modules with negligible degradation over long-term thermal cycles, thereby paving the way for efficient and sustainable waste heat recovery applications.
... The heightened energy needs, climate change, and increasing emission of greenhouse gases instigate the need to find non-toxic, environment-friendly energy sources [1]. Thermoelectric energy generation technology is one such way to transform heat liberated as waste from multiple natural and industry-based sources into electrical energy [2]. A triboelectric nanogenerator is another way of generating energy. ...
Thermoelectric materials hold significant promise for converting waste heat energy into electrical energy. The performance of these materials and devices is assessed using a quantitative measure known as the figure of merit, which relies on the Seebeck coefficient, thermal conductivity, and electrical conductivity of the material. Different classes of thermoelectric materials have their own merits and demerits. High temperature thermoelectric materials are useful for space exploration, automobile applications, etc Many materials have been explored within temperature range of 300–900 K, showing suitable properties for thermoelectric applications. Germanium, an inorganic material is investigated in details, due to its high Seebeck coefficient and better thermal stability. Silicon-Germanium alloys are thermoelectric materials suitable for operating at high temperatures. These materials help in reduction of emission of green house gases. Extensive efforts have been devoted to enhance the efficiency of Germanium-based thermoelectric materials and devices through various techniques such as doping, nanostructuring, electron energy filtering, and band engineering. Recently, a new material Ge0.94Sm0.06Te has been introduced, reporting a high figure of merit value of 2.5 at 730 K. Many theoretical studies are also reported showing the potential of new Germanium-based thermoelectric materials. Further, 2D Germanium-based materials show enhanced thermoelectric properties as well. These findings underscore the significance of Germanium as a thermoelectric material. This review provides an overview of the latest developments in Germanium-based thermoelectric materials and focuses on different strategies to enhance their thermoelectric performance. Additionally, the suitability of various Germanium-based thermoelectric materials in comparison to other materials for energy harvesting applications is extensively discussed in this review.
... Notwithstanding this, in the past, thermoelectric devices practically achieved a scant success; this was largely the consequence of their very low efficiencies. The rise of nanotechnologies has been a real breakthrough for thermoelectricity, because they allow us to currently develop new thermoelectric materials [32][33][34]; by obtaining the device's sizes comparable to the mean-free path of the different heat carriers (phonons, electrons, holes, etc.), in fact, it is now possible to significantly enhance the thermoelectric features of a material. ...
The aim of this paper is twofold. From the practical point of view, an enhanced model for the description of thermoelectric effects at nanoscale is proposed. From the theoretical point of view, instead, in the particular case of the proposed model, the equivalence between two classical techniques for the exploitation of the second law of thermodynamics is shown, i.e., Onsager’s method and Liu’s technique. An analysis of the heat-wave propagation is performed as well.
... Achieving a large temperature gradient across the TE device and ultimately maximizing energy conversion efficiency requires the device-level design of TE generator 13,14 . The key design parameters in the conventional cuboid TE leg-based devices include the length and aspect ratio of the cuboid legs and their fill factor [15][16][17][18][19][20] . Another important design strategy can involve the optimization of non-cuboid geometries for individual TE legs. ...
Waste heat, an abundant energy source generated by both industries and nature, has the potential to be harnessed into electricity via thermoelectric power generation. The performance of thermoelectric modules, typically composed of cuboid-shaped materials, depends on both the materials’ intrinsic properties and the temperature difference created. Despite significant advancements in the development of efficient materials, macroscopic thermal designs capable of accommodating larger temperature differences have been largely underexplored because of the challenges associated with processing bulk thermoelectric materials. Here we present the design strategy for Cu2Se thermoelectric materials for high-temperature power generation using a combination of finite element modelling and 3D printing. The macroscopic geometries and microscopic defects in Cu2Se materials are precisely engineered by optimizing the 3D printing and post-treatment processes, leading to notable enhancements in the material efficiency and temperature difference across legs, where the hourglass geometry exhibits maximized output powers and efficiencies. The proposed approach paves the way for designing efficient thermoelectric power generators.
... A real breakthrough on the road towards 2050 will be attained if it will be possible to considerably enhance the efficiency in the conversion of heat into electricity by means of thermoelectric effects. This could be done, in principle, by discovering and/or developing ever more efficient thermoelectric materials [4][5][6]. Several research groups currently confirm that both one-dimensional (1D), and two-dimensional (2D) materials, significantly exhibit higher thermoelectric effects than the usual threedimensional (3D) materials: the rise of nano-technology, therefore, has given a very important propulsion to the development of the aimed new thermoelectric materials [7][8][9][10]. Their usage, in principle, allows to better control the transport coefficients of the materials by getting the sizes comparable to the mean-free path (MFP) of the different heat carriers (phonons, electrons, holes, etc.). ...
The analysis of coupled transport phenomena is one of the most outstanding aspects of non-equilibrium thermodynamics. In this paper the attention is put on thermoelectricity, i.e., the coupling of heat and electricity. We propose a theoretical model which goes beyond the usual relations employed at macro-scale to describe thermoelectric effects. It introduces the non-local effects which should be taken into account in view of the possible applications of thermoelectric effects at nano-scale. The proposed model is here employed to investigate how non-local effects may influence the propagation of waves.
Achieving a low-resistance and highly stable connection between high-temperature electrodes and thermoelectric legs is a major challenge in module fabrication. Here we propose a method of co-sintering p-type and n-type...
GeTe has good thermoelectric performance at relatively high temperatures, but the low‐symmetry structure near room temperature and phase transition limit its service stability for power generation applications. Here, the thermoelectric properties and mechanical hardness of GeTe are improved by microstructure manipulation and phase transition engineering. The incorporation of Cr, Pb, and Sb into the GeTe lattice modulates the phase transition, increasing the cubic phase fraction from 49% to 73% at 300 K. These modifications introduce shear strains and refined herringbone structures, enhancing phonon scattering while simultaneously improving carrier mobility through dislocation accumulation and band convergence. Consequently, the optimal materials achieve a maximum zT of 2.1 at 700 K and an average zT of 1.5 across the 300–773 K temperature range, along with a Vickers hardness of 1.88 GPa. Paired with n‐type PbTe, the fabricated seven‐pair module achieves a superior efficiency of 9.7% under a 500 K temperature gradient. This study demonstrates microstructure manipulation is a promising strategy for GeTe‐based thermoelectric applications.
Weyl and Dirac semimetals, characterized by their unique band structures with linear energy dispersion (E vs k) near the Fermi level (EF), have emerged as promising candidates for next-generation technology based on thermoelectric materials. Their exceptional electronic properties, notably high carrier mobility and substantial Berry curvature, offer the potential to surmount the limitations inherent in conventional thermoelectric materials. A comprehensive understanding of the fundamental physics underlying these materials is essential. This chapter mainly focused into the topological properties and distinctive electronic band structures of Weyl and Dirac semimetals, providing a theoretical framework for comprehending their thermoelectric transport properties such as Seebeck coefficients, electrical and thermal conductivity. The pivotal role of Berry curvature in enhancing Seebeck coefficients while reducing thermal conductivity is a key focus. Experimental advancements in synthesizing single crystals and characterizing these materials have been significant. Recent development in material growth and characterization techniques have propelled research forward. The intricate relationship between material properties, such as carrier concentration, electronic bandgap, and crystal structure, and thermoelectric performance is explored. Realizing the potential of Weyl and Dirac semimetals for practical thermoelectric applications necessitates overcoming specific challenges. This chapter outlines strategies to optimize thermoelectric figures of merit (ZT) through band engineering, carrier doping, and nanostructuring. Moreover, the exploration of hybrid materials and heterostructures offers promising avenues for enhancing thermoelectric performance for renewable energy applications.
Lead‐containing GeTe‐based thermoelectric (TE) materials exhibit outstanding performance, but the toxicity of lead (Pb) limits their practical applications. Lead‐free GeTe‐based TE materials present a promising alternative for environmentally sustainable and scalable applications. Here, Bi doping is used to reduce the majority carrier concentration in GeTe, combined with uniform incorporation of nanoscale layered MXene via high‐energy ball milling. This approach significantly enhances the structural symmetry of GeTe, while MXene further reduces carrier concentration and improves carrier mobility. Consequently, the Ge0.93Bi0.07Te‐0.6 mass% MXene sample achieves an impressive average power factor of ≈28.40 µW m⁻¹ K⁻² across 303–603 K. Moreover, point defects, multilayer nanostructures, and grain boundaries reduce thermal conductivity to ≈1.04 W m⁻¹ K⁻¹ at 603 K. A maximum ZTmax of ≈2.1 at 603 K, an average ZTavg of ≈1.1, and a Vickers microhardness of ≈ 236.75 Hv are obtained. In particular, a high power density of ≈1.54 W cm⁻² and a maximum conversion efficiency of ≈7.5% at a temperature difference of 300 K are achieved in a 7‐pair TE module. These outcomes represent the highest performance levels for lead‐free GeTe‐based materials. This work uncovers a straightforward method to enhance the structural symmetry of GeTe‐based compounds, providing insights for advancing lead‐free TE technologies.
Thermoelectric technology plays an important role in developing sustainable clean energy and reducing carbon emissions, offering new opportunities to alleviate current energy and environmental crises. Nowadays, GeTe has emerged as a highly promising thermoelectric candidate for mid-temperature applications, due to its remarkable thermoelectric figure of merit (ZT) of 2.7. This review presents a thorough overview of the advancements in GeTe thermoelectric materials, meticulously detailing the crystal structure, chemical bonding characteristics, band structure, and phonon dynamics to elucidate the underlying mechanisms that contribute to their exceptional performance. Moreover, the phase transition in GeTe introduces unique degrees of freedom that enable multiple pathways for property optimization. In terms of electrical properties, noticeable enhancement can be realized through strategies such as band structure modulation, carrier concentration engineering, and vacancy engineering. For phonon transport properties, by incorporating defect structures with varying dimensions and constructing multi-scale hierarchical architectures, phonons can be effectively scattered across different wavelengths. Additionally, we provide a summary of current research on devices and modules of GeTe. This review encapsulates historical progress while projecting future development trends that will facilitate the practical application of GeTe in alignment with environmentally sustainable objectives.
Te-free thermoelectrics have garnered significant interest due to their immense thermoelectric potential and low cost. However, most Te-free thermoelectrics have relatively low performance because of the strong electrical and thermal transport conflicts and unsatisfactory compatibility of interfaces between device materials. Here, we develop lattice defect engineering through Cu doping to realize a record-high figure of merit of ~1.9 in n-type polycrystalline PbSe. Detailed micro/nanostructural characterizations and first-principles calculations demonstrate that Cu-induced interstitial defects and nanoprecipitates simultaneously optimize electron and phonon transport properties. Moreover, a robust Co/PbSe interface is designed to effectively prevent chemical reactions/diffusion; this interface exhibited a low electrical contact resistivity of ~10.9 μΩ cm², excellent durability, and good stability in the thermoelectric module, which achieves a record-high conversion efficiency of 13.1% at a temperature difference of 460 K in segmented thermoelectric modules. This study lays the groundwork for advancing the development of Te-free selenide-based thermoelectric materials.
Due to the high intrinsic zT of≈1.0 by multi‐band valleys, GeTe‐based thermoelectric materials are promising for medium‐temperature applications. In this work, the extraordinary role of the new dopant Hg on the electro‐phonon transport of GeTe is comprehensively elucidated. According to the first‐principles calculations, Hg doping can effectively reduce the energy offset between light and heavy valence bands, and increase the density‐of‐states effective mass md∗ from 1.45 m0 to 2.58 m0. Experimentally, the (GeTe)1‐x(HgTe)x samples are demonstrated to possess an enhanced room‐temperature Seebeck coefficient from 30.3 to 59.9 µV K⁻¹, plus an outstanding average power factor (300–773 K) of 28.8 µW cm⁻¹ K⁻². The Pb‐Bi codoping further optimizes the hole concentration and significantly suppresses the lattice thermal conductivity to 0.47 W m⁻¹ K⁻¹ by the induced various phonon scattering centers including dense point defects, grain boundaries, dislocations, and nano‐precipitates. Consequently, a high zTmax of 2.31 at 650 K and an average zTavg of 1.52 between 300 and 773 K are obtained in (Ge0.89Pb0.08Bi0.03Te)0.97(HgTe)0.03. Utilizing the optimized materials, achieving a high energy conversion efficiency of 10.8% at a temperature difference of 500 K in the fabricated 7‐pair thermoelectric module. This study clarifies the multiple benefits of Hg doping in GeTe‐based derivatives and provides a framework for designing high‐performance medium‐temperature thermoelectric materials.
n‐Type Mg3(Sb, Bi)2 shows great prospects in mid‐temperature power generation due to its extraordinary thermoelectric (TE) performance, low‐cost, and environment‐friendly attributes. However, its practical application is hindered by the slow progress of its p‐type counterpart. Herein, a high ZT of ≈0.9 at 773 K is achieved for p‐type Na0.01Mg1.7Zn1Yb0.25Sb2 through manipulation of the electronic transport properties. Then thermal expansion matched barrier material Fe75Sb25 is subtly obtained for Na0.01Mg1.7Zn1Yb0.25Sb2. The contact resistivity no longer increases significantly after aging for 72 hours at 673 K and tends to stabilize, remaining below ≈17 µΩ cm² after 168 hours. Paired with the high‐performance n‐type Mg3(Sb, Bi)2, the transient liquid phase bonding technique is adopted to connect the hot side of the all‐Mg3Sb2‐based legs to electrode at low temperature, which enables service at high temperature. This all‐Mg3Sb2‐based module displays a high efficiency of ≈8.3% at a temperature difference of ≈430 K and shows good thermal stability when the hot side temperature is maintained at 673 K. This work merits the great potential of the all‐Mg3Sb2‐based device for heat recovery.
Superior electronic performance due to the highly degenerated Σ valence band (Nv∼12) makes rhombohedral GeTe a promising low‐temperature (<600 K) thermoelectric candidate. Minimizing lattice thermal conductivity (κL) is an essential route for enhancing thermoelectric performance, but the temperature‐dependent κL, corelated to T⁻¹, makes its reduction difficult at low temperature. In this work, a room‐temperature κL of ≈0.55 W m⁻¹‐K⁻¹, the lowest ever reported in GeTe‐based thermoelectric, is realized in (Ge1‐ySbyTe)1‐x(Cu8GeSe6)x, primarily due to strong phonon scattering induced by point defects and precipitates. Simultaneously, Cu8GeSe6‐alloying effectively suppresses the precipitation of Ge, enabling the optimization of carrier concentration with the additional help of aliovalent Sb doping. As a result, an extraordinary peak zT of up to 2.3 and an average zTavg. of ≈1.2 within 300–625 K are achieved, leading to a conversion efficiency of ≈9% at a temperature difference of 282 K. This work robustly demonstrates its potential as a promising component in thermoelectric generator utilizing low‐grade waste heat.
Lead-free SnTe with naturally non-stoichiometric vacancies has a limited thermoelectric performance due to a deviated carrier concentration from the optimum. In this paper, we experimentally demonstrated that Gd with + 3 valence state as a novel n-type dopant is an effective solution for reducing carrier concentration in SnTe. A lowest value of 7.6 × 1018 cm−3 has been achieved. Yet with the involvement of Gd doping, the slightly modified band structure requires a further Sn-deficiency compensation to enhance the overall figure of merit zT. As a consequence, in the specific sample Sn0.91Gd0.07Te, we successfully achieved a low lattice thermal conductivity of 0.8 W/(m K) due to the high doping level and an improved zT approaching 0.8 at 850 K.
Defect engineering is an effective method for tuning the performance of thermoelectric materials and shows significant promise in advancing thermoelectric performance. Given the rapid progress in this research field, this Review summarizes recent advances in the application of defect engineering in thermoelectric materials, offering insights into how defect engineering can enhance thermoelectric performance. By manipulating the micro/nanostructure and chemical composition to introduce defects at various scales, the physical impacts of diverse types of defects on band structure, carrier and phonon transport behaviors, and the improvement of mechanical stability are comprehensively discussed. These findings provide more reliable and efficient solutions for practical applications of thermoelectric materials. Additionally, the development of relevant defect characterization techniques and theoretical models are explored to help identify the optimal types and densities of defects for a given thermoelectric material. Finally, the challenges faced in the conversion efficiency and stability of thermoelectric materials are highlighted and a look ahead to the prospects of defect engineering strategies in this field is presented.
The rapid advancement of Internet of Things (IoT) technology and AI microchips has increased the demand for efficient and cost‐effective cooling solutions. However, traditional Bi2Te3‐based thermoelectric modules face the challenge of low abundance tellurium (Te). Although Te‐free modules using MgAgSb have been explored, they still suffer from a low price‐to‐performance ratio. To address these issues, this study investigates the development of alternative thermoelectric materials that are both cost‐effective and Te free. In this work, the potential of cost‐effective Te‐free alternatives is explored for thermoelectric applications by developing a high‐performance module composed of amorphous carbon‐modulated Mg3(Bi,Sb)2 and electron‐poor CdSb. The modules of CdSb/Bi2Te3 and CdSb/Mg3(Bi,Sb)2 demonstrate superior refrigeration performance, achieving a maximum temperature difference (ΔTmax) of 49.2 and 46 K, respectively. Notably, the material cost of CdSb/Mg3(Bi,Sb)2 module is only 5.5% of Te‐free modules built on MgAgSb, highlighting a significant economic advantage. This work provides a viable, ultralow‐cost approach to meet general refrigeration needs, thereby enhancing the practical value and application potential of thermoelectric materials.
Eliminating the ferroelectric phase transition to obtain a cubic phase under ambient conditions is deemed an ultimate goal for p-type GeTe-based materials to be feasible for thermoelectric generator application at...
Thermoelectric refrigeration, utilizing Peltier effect, has great potential in all‐solid‐state active cooling field near room temperature. The performance of a thermoelectric cooling device is highly determined by the power factor of consisting materials besides the figure of merit. In this work, it is demonstrated that successive addition of Cu and Nd can realize non‐trivial modulation of deformation potential in n‐type room temperature thermoelectric material Bi2Te2.7Se0.3 and result in a significant increment of electron mobility and remarkably enhanced power factor. Following giant hot deformation process improves grain texturing and strengthens inter‐layer interaction in Bi2Te2.7Se0.3 lattice, further pushing the power factor to ≈47 µW cm⁻¹ K⁻² at 300 K and maximal figure of merit ZTmax to ≈1.34 at 423 K with average ZTave of ≈1.27 at 300–473 K. Moreover, robust compressive strength is enhanced to ≈146.6 MPa. The corresponding finite element simulations demonstrate large temperature differences ΔT of ≈70 K and a maximal coefficient of performance COP ≈ 10.6 (hot end temperature at 300 K), which can be achieved in a ten‐pair thermoelectric cooling virtual module. The strategies and results as shown in this work can further advance the application of n‐type Bi2Te3 for thermoelectric cooling.
Entropy engineering in thermoelectric materials involves a deliberate manipulation of entropy-related effects to boost performance. It revolves around designing materials to capitalize on entropy-driven changes, breaking conventional trade-offs between properties like electrical and thermal conductivity for improved efficiency. Entropy engineering fosters higher crystal symmetry, altering the Seebeck coefficient by augmenting degenerate valleys in the band structure. The introduction of significant mixing entropy mitigates strain energy, enhancing structural stability. Conversely, severe lattice distortion, atomic mass fluctuations, lattice anharmonicity, multiscale microstructures, and point defects lead to potent scattering of phonons, which suppresses thermal transport properties. This study comprehensively explores the effectiveness of entropy engineering in diverse compounds, aligning with the status and challenges in this field. These insights will guide researchers in refining material design and properties, advancing high-performance thermoelectric materials and devices to revolutionize energy conversion and stimulate sustainable technological advancements.
Segmented thermoelectric (TE) legs are promising for improving heat‐electricity conversion efficiency, but their practical applications are still limited by the lack of cost‐effective interface‐connection technology. Here, an interface‐connection method for one‐step sintering of GeTe‐Bi2Te3 segmented TE legs is developed using mixtures of Al0.88Si0.12 and Ni (ASN) as diffusion barrier materials. Although the interface‐connection performance using Ni or Al0.88Si0.12 alone is poor, their mixtures can realize a compromise optimization of interface‐connection properties, simultaneously achieving a matched coefficient of thermal expansion, low contact resistivity, high shear strength, and high reliability. These robust interface‐connection properties can be attributed to the activation of the eutectic‐alloy Al0.88Si0.12 and the formation of appropriate reaction‐diffusion layers between ASN and GeTe/Bi2Te3 and between Al0.88Si0.12 and Ni in ASN. Finally, a remarkable energy conversion efficiency of ≈15.5% at a temperature difference of 449 K can be obtained in this segmented TE leg. Moreover, ASN can also be applied in fabricating n‐type PbTe‐Bi2Te3 segmented legs and multi‐pair TE devices, demonstrating the universality of this methodology. This work accelerates the development of robust, low‐cost, one‐step‐sintered segmented TE legs and promotes the use of eutectic alloys as “alloy glues” for advancing TE‐device connection technology.
Electronic band convergence can have a beneficial impact on thermoelectric performance, but finding the right band-converged compositions is still time-consuming. We propose a method for designing a series of compositions with simultaneous band convergence in the high-entropy Yb x Ca 1− x Mg y Zn 2− y Sb 2 material by zeroing the weighted sum of crystal-field splitting energies of the parent compounds. We found that so-designed compositions have both larger power factors and lower thermal conductivities and that one of these compositions exhibits a large thermoelectric figure of merit value in comparison with to other p-type Zintls. Our material shows high stability both thermally and temporally. We then assembled an all-Zintl single-stage module, nontoxic and free of tellurium, that demonstrates an exceptional heat-to-electricity conversion efficiency exceeding 10% at a temperature difference of 475 kelvin.
Resonant levels (RLs) are expected to increase the density of states (DOS) abruptly, thereby increasing the Seebeck coefficient for a high thermoelectric figure of merit (ZT). However, the negative effects of RLs on reducing the carrier mobility and band gap have rarely been explored. In this work, the pros and cons of resonant‐state doping of group IIIA elements (In, Ga) in GeTe‐based alloys are studied by experimental transport properties and density‐functional‐theory calculations. RLs effects of Fermi level pinning and increased effective mass are strong in Ge0.9‐xInxSb0.1Te but weak in Ge0.9‐y(In0.5Ga0.5)ySb0.1Te, which shows dependence on the hump DOS relative to the background, location of RLs, and energy width of RLs. Using the In‐Ga co‐doping strategy to make a compromise between the DOS, carrier mobility, and bipolar transport, combined with beneficial alloying effects of Pb and Sb, a peak ZT≈ 2.1 at 773 K and a high average ZT≈ 1.43 within 300–773 K can be obtained in Ge0.78In0.005Ga0.005Pb0.1Sb0.07Te. The corresponding single‐leg device shows a remarkable energy conversion efficiency of 13.7% at a temperature difference of 451 K. This work suggests that RLs are not always beneficial for improving ZT, and the compromise design of RLs should be carefully considered to advance ZT.
The utilization of thermoelectric (TE) technology for eco-friendly energy harvesting presents a promising solution for off-grid power generation from waste heat.
The IV–VI compound GeTe is considered as a promising alternative to the toxic PbTe for high-efficiency mid-temperature thermoelectric applications. However, pristine GeTe suffers from a high concentration of Ge vacancies, resulting in an excessively high hole concentration (> 1 × 1021 cm−3), which greatly limits its thermoelectric enhancement. To address this issue, CuBiTe2 alloying is introduced to increase the formation energy of Ge vacancies in GeTe, thereby inhibiting the high carrier concentration. The carrier scattering caused by the electronegativity difference between different elements is suppressed due to the similar electronegativity of Cu and Ge atoms. A relatively high hole mobility is obtained, which ultimately leads to a high power factor. Additionally, by introducing Se as an alloying element at the anionic site in GeTe, dense point defects with mass/strain-field fluctuations are induced. This contributes to the strengthening of phonon scattering, thereby reducing the lattice thermal conductivity from 1.44 W·m−1·K−1 for pristine GeTe to 0.28 W·m−1·K−1 for Ge0.95Cu0.05Bi0.05Te0.9Se0.15 compound at 623 K.
Germanium telluride (GeTe) with ultrafast ferroelectric transition, Rashba‐like electronic transport, and anomalous phonon anharmonicity are historically studied for potential memorizing and thermoelectric applications. Due to recent breakthroughs in spintronics, valleytronics, orbitronics, pre‐eminent GeTe thermoelectrics have re‐attracted enormous interest from both academia and industries, with increasing reports of significant figure‐of‐merit over 2.7 and the maximum efficiency of up to 17.0%. Here, the emerging trends in advancing GeTe thermoelectrics, starting from fundamentals of phase transformation, crystal structure, bonding mechanisms, and transport characteristics, with a highlight on the roles of Ge_4s² lone pairs, are timely overviewed. Technical insights in synthesis, characterization, property measurement, and computation are then summarized. After that, several innovative strategies for increasing the figure‐of‐merit, including entropy engineering, nanostructuring, and hybridization, which will further benefit near‐room‐temperature and n‐type performance, are examined. Moreover, high‐density and high‐efficiency devices with broad working temperatures are discussed as a result of rational configurational and interfacial design. In the end, perspective remarks on the challenges and outlook envisaging for next‐generation GeTe thermoelectrics, which will play a prominent role in future energy and environmental landscapes, are provided.
The coupling relationship between electrical and thermal transports makes it rather challenging to enhance thermoelectric performance. Here, electrical and thermal transports are successfully decoupled to realize high performance in n‐type PbSe by utilizing a stepwise strategy. First, the PbSe lattice is plained with extra Pb to compensate for the intrinsic Pb vacancies, which can weaken defect scattering and improve carrier mobility to ≈1230 cm² V⁻¹ s⁻¹. The room‐temperature power factor triples and reaches ≈32 µW cm−1 K⁻², and ZT is significantly enhanced to ≈0.6 in Pb1.006Se. Subsequently, liquid‐like interstitial Cu ions are introduced to inhibit heat conduction without damaging electrical transport. While maintaining a high power factor of ≈25 µW cm−1 K⁻², Cu ions strongly suppress phonon transport at high temperature, leading to an ultralow lattice thermal conductivity of ≈0.28 W m⁻¹ K⁻¹ in Pb1.006Cu0.006Se, only 30% of the Cu‐free PbSe. Eventually, a remarkable peak ZT of ≈1.8 at 773 K is achieved along with a high average ZT of ≈1.1 from 300 to 823 K in Pb1.006Cu0.006Se. An outstanding experimental conversion efficiency of ≈7.1% is obtained in the single‐leg device, demonstrating great potential for PbSe as low‐ to mid‐temperature thermoelectrics.
Sustained thermoelectric efforts have concentrated on the enhancement of conversion efficiency of power generators, while simultaneously achieving high output power continues to lag. Specifically, the highest output power density of emerging Mg‐based modules reported so far is only half of that of commercial Bi2Te3‐based, mainly due to the low power factor of MgAgSb. Herein, homogenously distributed MgCuSb in situ nanoprecipitates in the MgAgSb matrix effectively optimized carrier concentration due to the effect of carrier injection from the metal–semiconductor Ohmic contact. As a result, a record‐high average power factor of 27.2 µW cm⁻¹ K⁻² is obtained in MgCu0.1Ag0.87Sb0.99 composite within the temperature range of 300–550 K, which is much higher than ever reported values of MgAgSb system. Benefiting from the combination of the optimized average power factor of p‐leg and low interface resistivity, a fabricated eight‐pair MgCu0.1Ag0.87Sb0.99/Mg3.2Bi1.5Sb0.5 module demonstrates an unprecedently high output power density of 2.9 W cm⁻² under a temperature difference of 300 K, outperforming all low‐temperature advanced thermoelectric modules. Meanwhile, a competitive conversion efficiency of 7.65% is obtained simultaneously. The work significantly advances high‐power thermoelectric applications of Mg‐based modules in the field of low‐temperature sustainable energy harvesting.
As a highly promising thermoelectric material in the mid‐temperature region, pristine GeTe shows deteriorated thermoelectric properties due to the low Seebeck coefficient and the intrinsic high thermal conductivity. Herein, it is discovered that the introduction of strongly correlated d, f electrons by alloying rare‐earth atoms at Ge sites will increase the band effective mass and promote band convergency, which can largely improve the electric transport property. Moreover, the heavy atoms (Pb, Bi) doping induces optical phonon softening and the avoided crossing between acoustic and optical phonon branches, decelerating both optical and acoustic phonon velocities. The modulated composition wave from the fluctuated multi‐component distribution introduces a complicated hierarchical structure including point defects and nanoprecipitates, which provides all‐scale scattering sources for heat‐carrying phonons and results in low lattice thermal conductivity. Consequently, a high zT of 2.4 at 800 K can be obtained in the Ge0.86Pb0.09Bi0.03Ce0.005Te sample due to the combination of strong correlation for d, f electrons and the full‐spectrum scattering for phonons. This work provides an effective strategy to increase the thermoelectric performance of IV–VI compounds by combining the modulated band structure and the softening phonon dispersion.
The liquid-like feature of thermoelectric superionic conductors is a double-edged sword: the long-range migration of ions hinders the phonon transport, but their directional segregation greatly impairs the service stability. We report the synergetic enhancement in figure of merit (ZT) and stability in Cu1.99Se-based superionic conductors enabled by ion confinement effects. Guided by density functional theory and nudged elastic band simulations, we elevated the activation energy to restrict ion migrations through a cation–anion co-doping strategy. We reduced the carrier concentration without sacrificing the low thermal conductivity, obtaining a ZT of ∼3.0 at 1,050 K. Notably, the fabricated device module maintained a high conversion efficiency of up to ∼13.4% for a temperature difference of 518 K without obvious degradation after 120 cycles. Our work could be generalized to develop electrically and thermally robust functional materials with ionic migration characteristics.
GeTe‐based materials exhibit superior thermoelectric performance, while the development of power generation devices has mainly been limited by the challenge of designing the interface due to the phase transition in GeTe. In this work, via utilizing the low‐temperature nano‐Ag sintering technique and screening suitable Ti‐Al alloys, a reliable interface with excellent connection performance has been realized. The Ti‐Al intermetallic compounds effectively inhibit the diffusion process at Ti‐34Al/Ge0.9Sb0.1Te interface. Thus, the thickness of the interfacial reaction layer only increases by ≈2.08 µm, and the interfacial electrical contact resistivity remains as low as ≈15.2 µΩ cm² even after 30 days of isothermal aging at 773 K. A high conversion efficiency of ≈11.3% has been achieved in the GeTe/PbTe module at a hot‐side temperature of 773 K and a cold‐side temperature of 300 K. More importantly, the module's performance and the reliability of the interface remain consistently stable throughout 50 thermal cycles and long‐term aging. This work promotes the application of high‐performance GeTe materials for thermoelectric power generation.
To achieve the commercial application of high‐performance skutterudite (SKD) thermoelectric (TE) devices, breakthroughs in the batch fabrication of TE material and the realization of its stable bonding to the barrier layer are necessary. In this work, the large‐scale SKD bulks with excellent TE performance and decent homogeneity are prepared by melt‐spinning (MS) combined with a hot pressing process. Then, a modified two‐step sintering method is employed to effectively suppress the degree of initial interfacial reaction, resulting in a barrier layer/SKD joint that exhibits low contact resistivity (≈2µΩ·cm²) and high bonding strength (≈30 MPa) with no significant change after aging at 773 K for 30 days. Encouragingly, the joints welded with Mo50Cu50 electrodes also show high stability under the same aging conditions. Finally, the as‐fabricated 8‐pair module shows a highly competitive conversion efficiency of 9% under a temperature difference of 580 K. In addition, during a 360 h aging test with hot‐side temperature of 773 K, the module shows excellent stability. Overall, this work paves the way for batch fabrication of high‐performance SKD using MS and lays a solid foundation for the stable operation of SKD‐based modules.
Although the CoSb3-based skutterudite thermoelectric devices have been highly expected for wide uses such as waste heat recovery and space power supply, the limited long-term service stability majorly determined by the degradation of electrode interface obstructs its applications. Here, we built up an effective criterion for screening barrier layer based on the combination of negative interfacial reaction energy and high activation energy barrier of Sb migration through the formed interfacial reaction layer. Accordingly, we predicted niobium as a promising barrier layer. The experimental results show the skutterudite/Nb joint has the slowest interfacial reaction layer growth rate and smallest interfacial electrical resistivity. The fabricated 8-pair skutterudite module using Nb as barrier layer achieves a recorded conversion efficiency of 10.2% at hot-side temperature of 872 K and shows excellent stability during long-time aging. This simple criterion provides an effective guidance on screening barrier layer with bonding-blocking-conducting synergetic functions for thermoelectric device integration.
Half‐Heusler (HH) compounds have shown great potential in waste heat recovery. Among them, p‐type NbFeSb and n‐type ZrNiSn based alloys have exhibited the best thermoelectric (TE) performance. However, TE devices based on NbFeSb‐based HH compounds are rarely studied. In this work, bulk volumes of p‐type (Nb0.8Ta0.2)0.8Ti0.2FeSb and n‐type Hf0.5Zr0.5NiSn0.98Sb0.02 compounds are successfully prepared with good phase purity, compositional homogeneity, and matchable TE performance. The peak zTs are higher than 1.0 at 973 K for Hf0.5Zr0.5NiSn0.98Sb0.02 and at 1200 K for (Nb0.8Ta0.2)0.8Ti0.2FeSb. Based on an optimal design by a full‐parameters 3D finite element model, a single stage TE module with 8 n‐p HH couples is assembled. A high conversion efficiency of 8.3% and high power density of 2.11 W cm⁻² are obtained when hot and cold side temperatures are 997 and 342 K, respectively. Compared to the previous TE module assembled by the same materials, the conversion efficiency is enhanced by 33%, while the power density is almost the same. Given the excellent mechanical robustness and thermal stability, matchable thermal expansion coefficient and TE properties of NbFeSb and ZrNiSn based HH alloys, this work demonstrates their great promise for power generation with both high conversion efficiency and high power density.
GeTe-based compounds have been intensively studied recently due to their superior thermoelectric performance, but their real applications are still limited so far due to the drastic volume variation that occurs during the rhombohedral-cubic phase transition, which may break the material or the material/electrode interface during service. Here, superior performance and high service stability for GeTe-based thermoelectric compounds are achieved by co-doping Mg and Sb into GeTe. The linear coefficient of thermal expansion before phase transition is greatly improved to match that after phase transition, yielding smooth volume variation around the phase transition temperature. Likewise, co-doping (Mg, Sb) in GeTe successfully tunes the carrier concentration to the optimal range and effectively suppresses the lattice thermal conductivity. A peak zT of 1.84 at 800 K and an average zT of 1.2 in 300-800 K have been achieved in Ge 0.85 Mg 0.05 Sb 0.1 Te. Finally, a Ni/Ti/Ge 0.85 Mg 0.05 Sb 0.1 Te thermoelectric uni-leg is fabricated and tested, showing quite good service stability even after 450 thermal cycles between 473 K and 800 K. This study will accelerate the application of GeTe-based compounds for power generation in the mid-temperature range.
The (Bi,Sb)2Te3 (BST) compounds have long been considered as the benchmark of thermoelectric (TE) materials near room temperature especially for refrigeration. However, their unsatisfactory TE performances in wide‐temperature range severely restrict the large‐scale applications for power generation. Here, using a self‐assembly protocol to deliver a homogeneous dispersion of 2D inclusion in matrix, the first evidence is shown that incorporation of MXene (Ti3C2Tx) into BST can simultaneously achieve the improved power factor and greatly reduced thermal conductivity. The oxygen‐terminated Ti3C2Tx with proper work function leads to highly increased electrical conductivity via hole injection and retained Seebeck coefficient due to the energy barrier scattering. Meanwhile, the alignment of Ti3C2Tx with the layered structure significantly suppresses the phonon transport, resulting in higher interfacial thermal resistance. Accordingly, a peak ZT of up to 1.3 and an average ZT value of 1.23 from 300 to 475 K are realized for the 1 vol% Ti3C2Tx/BST composite. Combined with the high‐performance composite and rational device design, a record‐high thermoelectric conversion efficiency of up to 7.8% is obtained under a temperature gradient of 237 K. These findings provide a robust and scalable protocol to incorporate MXene as a versatile 2D inclusion for improving the overall performance of TE materials toward high energy‐conversion efficiency.
In order to locate the optimal carrier concentrations for peaking the thermoelectric performance in p-type group IV monotellurides, existing efforts focus on aliovalent doping, either to increase (in PbTe) or to decrease (in SnTe and GeTe) the hole concentration. The limited solubility of aliovalent dopants usually introduces insufficient phonon scattering for thermoelectric performance maximization. With a decrease in the size of cation, the concentration of holes, induced by cation vacancies in intrinsic compounds, increases rapidly from ≈1018 cm−3 in PbTe to ≈1020 cm−3 in SnTe and then to ≈1021 cm−3 in GeTe. This motivates a strategy here for reducing the carrier concentration in GeTe, by increasing the mean size of cations and vice-versa decreasing the average size of anions through isovalent substitutions for increased formation energy of cation vacancy. A combination of the simultaneously resulting strong phonon scattering due to the high solubility of isovalent impurities, an ultrahigh thermoelectric figure of merit, zT of 2.2 is achieved in GeTe–PbSe alloys. This corresponds to a 300% enhancement in average zT as compared to pristine GeTe. This work not only demonstrates GeTe as a promising thermoelectric material but also paves the way for enhancing the thermoelectric performance in similar materials.
Widespread application of thermoelectric devices for waste heat recovery requires low-cost high-performance materials. The currently available n-type thermoelectric materials are limited either by their low efficiencies or by being based on expensive, scarce or toxic elements. Here we report a low-cost n-type material, Te-doped Mg3Sb1.5Bi0.5, that exhibits a very high figure of merit zT ranging from 0.56 to 1.65 at 300−725 K. Using combined theoretical prediction and experimental validation, we show that the high thermoelectric performance originates from the significantly enhanced power factor because of the multi-valley band behaviour dominated by a unique near-edge conduction band with a sixfold valley degeneracy. This makes Te-doped Mg3Sb1.5Bi0.5 a promising candidate for the low- and intermediate-temperature thermoelectric applications.
A promising thermoelectric figure of merit, zT, of ∼1.3 at 725 K was obtained in high quality crystalline ingots of Ge1−xBixTe. The substitution of Bi³⁺ in a Ge²⁺ sublattice of GeTe significantly reduces the excess hole concentration due to the aliovalent donor dopant nature of Bi³⁺. Reduction in carrier density optimizes electrical conductivity, and subsequently enhances the Seebeck coefficient in Ge1−xBixTe. More importantly, a low lattice thermal conductivity of ∼1.1 W m⁻¹ K⁻¹ for Ge0.90Bi0.10Te was achieved, which is due to the collective phonon scattering from meso-structured grain boundaries, nano-structured precipitates, nano-scale defect layers, and solid solution point defects. We have obtained a reasonably high mechanical stability for the Ge1−xBixTe samples. The measured Vickers microhardness value of the high performance sample is ∼165 HV, which is comparatively higher than that of state-of-the-art thermoelectric materials, such as PbTe, Bi2Te3, and Cu2Se.
Solid-state thermoelectric technology offers a promising solution for converting waste heat to useful electrical power. Both high operating temperature and high figure of merit zT are desirable for high-efficiency thermoelectric power generation. Here we report a high zT of ∼1.5 at 1,200 K for the p-type FeNbSb heavy-band half-Heusler alloys. High content of heavier Hf dopant simultaneously optimizes the electrical power factor and suppresses thermal conductivity. Both the enhanced point-defect and electron-phonon scatterings contribute to a significant reduction in the lattice thermal conductivity. An eight couple prototype thermoelectric module exhibits a high conversion efficiency of 6.2% and a high power density of 2.2 W cm(-2) at a temperature difference of 655 K. These findings highlight the optimization strategy for heavy-band thermoelectric materials and demonstrate a realistic prospect of high-temperature thermoelectric modules based on half-Heusler alloys with low cost, excellent mechanical robustness and stability.
Simultaneously optimizing electrical and thermal transport properties of bulk thermoelectric materials remains a key challenge due to the conflicting combination of material traits. Here, we have explored the electrical and thermal transport features of In-filled CoSb3 through X-ray absorption fine structure, X-ray photoemission spectra, transport measurement and theoretical calculation. The results provide evidence of three types of coexisting multi-localization transport behaviours in the material; these are heat-carrying phonon-localized resonant scattering, accelerated electron movement and increase in density of states near the Fermi level. The 5p-orbital hybridization between In and Sb is discovered in the In-filled CoSb3 compound, which results in a charge transfer from Sb to In and the enhancement of p-d orbital hybridization between Co and Sb. Our work demonstrates that the electrical and thermal properties of filled skutterudite bulk thermoelectric materials can be simultaneously optimized through the three types of coexisting multi-localization transport behaviours in an independent way.
Driven by materials science development, thermoelectric performance has been enhanced. However, only increasing the figure of merit to enhance thermoelectric efficiency becomes more challenging. Here, we combine the enhanced figure of merit and the geometry optimization of the device by computer-aided design to achieve a record-high thermoelectric efficiency of 16 %. A figure of merit over 2.6 in p-type Ge1-x-yCrxSbyTe alloys is achieved resulting from the convergence of three valence edges induced by Cr doping to enhance power factor and superlattice precipitates to lower thermal conductivity. Using finite element analysis simulations, we optimize the geometry of a segmented thermoelectric device made of as-developed Ge1-x-yCrxSbyTe and other reported materials, leading to recorded high efficiency. Furthermore, our simulations on over 70 existing n-type thermoelectric materials can serve as the library to bridge the gap between materials science and device engineering to achieve high-efficiency thermoelectric devices.
To build high-performance thermoelectric (TE) devices for power generation, a suitable diffusion-barrier layer between the electrodes and the TE materials in a TE device is generally required for achieving good interfacial connection with high reliability, high mechanical strength but low electrical and thermal contact resistivities. The GeTe-based materials have attracted great attention recently due to their high TE performance in the mid-temperature range, but studies on their TE devices are still limited. Here, we have selected the Al66Si34 alloy as diffusion barrier for GeTe-based TE legs based on the matching test of coefficient of thermal expansion. The well connection between the Al66Si34 and Ge0.9Sb0.1TeB0.01 is realized by the interfacial reaction, where the randomly distributed Al2Te3 and Ge precipitates are formed within the interface of the joint. The as-prepared interfacial electrical contact resistivity can be as low as 20.7 μΩcm2 and only slightly increase to 26.1 μΩcm2 after 16-day aging at 500 oC. Moreover, the shear strength of the joints can be as high as 26.6 MPa and unexpectedly increase to 41.7 MPa after 16-day aging. The thickness of the reaction layer tends to be stabilized after 8-day aging and nearly do not change after further aging to 16 days, which may be ascribed to the drag effect from Si and the secondary Ge phases. These results demonstrate the great potential of Al-Si alloy as diffusion barrier for GeTe-based TE devices with high performance.
Combining high thermoelectric (TE) performance, excellent mechanical properties, and good thermal stability, half-Heusler materials show great potential in real applications, such as industrial waste heat recovery. However, the materials synthesis technology developed in the laboratory scale environment cannot fulfil the requirements of massive device fabrication. In this work, the batch synthesis utilizing the self-propagating high-temperature synthesis (SHS) method was used to prepare the state-of-the-art n-type Zr0.5Hf0.5NiSn0.985Sb0.015 and p-type Zr0.5Hf0.5CoSb0.8Sn0.2 half-Heusler alloys. Due to the nonequilibrium reaction process, dense dislocation arrays were introduced in both n-type and p-type materials, which greatly depressed the lattice thermal conductivity. As a consequence, zT values of samples cut from ingots weighing a few hundreds of grams compared favorably with those prepared from a few gram laboratory size pellets. Based on the high TE performance, three-dimensional finite element model encompassing all relevant parameters was applied to optimize the topological structures of both a half-Heusler single-stage module and a half-Heusler/Bi2Te3 segmented module. The optimized modules attained the record-high conversion efficiencies of 9.6% and 12.4% for the single-stage and the segmented module, respectively. The work documents a comprehensive processing of novel TE materials culminating in the assembly of efficient TE modules. As such, it paves the way for widespread commercial applications of TE power generation.
GeTe and its derivatives have recently attracted wide attention as promising thermoelectric materials. The principle challenge in optimizing the thermoelectric figure of merit, zT, is the low Seebeck coefficient (S) and high thermal conductivity of GeTe. Here, we report a high zT of ∼2.1 at 723 K in In and Bi co-doped GeTe along with an extremely high TE conversion efficiency of ∼12.3% in a single-leg thermoelectric generator for the temperature difference of 445 K. In and Bi play a distinct but complementary role. In doping significantly enhances the S through the formation of resonance level, which is confirmed with first-principles density functional theory calculations and Pisarenko plot considering two valance band model. However, Bi doping markedly reduces the lattice thermal conductivity due to the formation of extensive solid solution point defects and domain variants. Moreover, a high value of Vickers microhardness (∼200 Hv, Hv = kgf/mm²) reveals excellent mechanical stability.
Thermoelectric technology provides an alternative way to utilize fossil energy more efficiently through converting the waste heat from industrial or automobile exhaust gas into electricity. The usual thermoelectric module design only needs to achieve high-energy conversion efficiency. This is not enough for many novel thermoelectric materials with low thermodynamic stability such as liquid-like materials and some Mg/Zn-containing compounds, which may lead to possible module instability during service. Here, we successfully achieve a thermoelectric module based on high-performance liquid-like materials with both good stability and high efficiency up to 9.1%, more than 50% higher than those made by half-Heusler and SiGe. A module’s stability is included in the design through tuning the geometry of the legs to ensure that the voltage applied on the liquid-like material is below the threshold for stable usage. This study provides an effective strategy to achieve efficient and stable modules based on various new thermoelectric materials.
GeTe is a promising thermoelectric material at medium temperature, but its carrier concentration tends to go beyond the optimal range for thermoelectrics. This work realized a significant ZT enhancement from 1.0 to 2.0 by suppressing the formation of Ge vacancies and band convergence. Through simply optimizing the amount of excessive Ge, the hole carrier concentration is greatly reduced. It is demonstrated that the suppression of Ge vacancy can not only optimize the carrier concentration but also recover the mobility to a high value of 90 cm²V⁻¹s⁻¹, which well exceeds the previously reported data and guarantees the superior electrical transport properties, leading to a ZT of 1.6. Further Bi doping facilitates band convergence as featured by the increased band effective mass and high mobility, which in turn yields large power factors and low electronic thermal conductivity. Bi doping induces mass and strain fluctuation also favors the reduction of lattice thermal conductivity. Consequently, a maximum ZT ~ 2.0 at 650 K with an average ZT over 1.2 is achieved in the nominal composition Bi0.05Ge0.99Te, which is one of the best thermoelectric material for medium temperature applications
Thermoelectric (TE) power generation technology is highly expected for various applications such as special power supply, green energy, energy harvesting from the environment and harvesting of industrial waste heat. Over the past years, the record of zT values of TE materials has been continuously updated, which would bode well for widespread practical applications of TE technology. However, the TE device as the core technology for the TE application lags behind the development of TE materials. Especially, the large-scale application of TE power generation technology is facing bottlenecks and new challenges. This review presents an overview of the recent progress on TE device design and integration with particular attentions on device optimization design, electrode fabrication, interface engineering, and service behavior. The future challenges and development strategies for large-scale application of thermoelectric power generation are also discussed.
Diffusion barrier materials play an important role in both structure reliability and performance stability of thermoelectric (TE) modules. Preferred barrier materials are screened out from various candidates by comparing the interdiffusion at the barrier material/TE substrate interfaces. Traditionally, for each barrier material candidate, complicated fabrication processing of TE elements (electrode/barrier material/TE material) must be finished to obtain relative interfaces, which makes the screening costly and time consuming. In this article, using a high-throughput strategy, we developed a high-efficiency screening method of barrier materials. By cosintering the mixture of TE substrate material and various barrier material candidates simply following the TE material's sintering parameters, various microinterfaces were integrated to one single sample. This enables parallel aging and microstructure characterization of different interfaces, and preferred barrier materials can be swiftly screened out. As a result, it makes the design and optimization of TE modules much more efficient and economical.
The ability of substitution atoms to decrease thermal conductivity is usually ascribed to the enhanced phonon-impurity scattering by assuming the original phonon dispersion relations. In this study, we find that 10% SbGe alloying in GeTe modifies the phonon dispersions significantly, closes the acoustic-optical phonon band gap, increases the phonon-phonon scattering rates, and reduces the phonon group velocities. These changes, together with grain boundaries, nanoprecipitates, and planar vacancies, lead to a significant decrease in the lattice thermal conductivity. In addition, extra 2% – 6% Zn alloying decreases the energy offset between valence band edges at L and Σ points in Ge1-xSbxTe that is found to be induced by the Ge 4_s² lone pairs. Since Zn is free of s² lone pair electrons, substituting Ge with Zn atoms can consequently diminish the Ge 4_s² lone pair characters and reduce the energy offset, resulting in two energetically merged valence band maxima. The refined band structures render a power factor up to 40 μWcm⁻¹K⁻² in Ge0.86Sb0.1Zn0.04Te. Ultimately, a super-high zT of 2.2 is achieved. This study clarifies the impacts of high-concentration substitutional atoms on phonon band structure, phonon-phonon scattering rates, and the convergence of electron valence band edges, which could provide guidelines for developing high-performance thermoelectric materials.
Optimization of carrier concentration plays an important role on maximizing thermoelectric performance. Existing efforts mainly focus on aliovalent doping, while intrinsic defects (e.g. vacancies) provide extra possibilities. Thermoelectric GeTe intrinsically forms in off-stoichiometric with Ge-vacancies and Ge-precipitates, leading to a hole concentration significantly higher than required. In this work, Sb2Te3 having a smaller cation-to-anion ratio, is used as a solvend to form solid solutions with GeTe for manipulating the vacancies. This is enabled by the fact that each substitution of 3 Ge²⁺ by only 2 Sb³⁺ creates 1 Ge vacancy, because of the overall 1:1 cation-to-anion ratio of crystallographic sites in the structure and by the charge neutrality. The increase in the overall Ge-vacancy concentration facilitates Ge-precipitates to be dissolved into the matrix for reducing the hole concentration. In a combination with known reduction in hole concentration by Pb/Ge-substitution, a full optimization on hole concentration is realized. In addition, the resultant high-concentration point defects including both vacancies and substitutions strongly scatter phonons and reduce the lattice thermal conductivity to the amorphous limit. These enable a significantly improved thermoelectric figure of merit at working temperatures of thermoelectric GeTe.
The multivalence bands in GeTe provide an additional handle to manipulate the thermoelectric performance. Herein, the density‐functional‐theory calculation indicates that Cd doping enables the convergence of these multivalence bands. Plus, the additional Bi dopant serving as the electron donors optimizes the carrier concentration, leading to an enhanced power‐factor in Ge1−x−yCdxBiyTe. Moreover, comprehensive electron microscopy characterizations demonstrate the array of high‐density planar vacancies in Ge1−x−yCdxBiyTe stemming from the absence of {111} Ge atomic planes, which is driven by the reduced formation energy in the scenario of Cd/Bi codoping. Simulations of phonon transport confirm the significant role of planar vacancies in scattering mid‐frequency phonons. Such high‐density planar vacancies, in tandem with grain boundaries and point defects, lead to a lattice thermal conductivity of 0.4 W m⁻¹ K⁻¹ in Ge1−x−yCdxBiyTe, reaching the amorphous limit. Ultimately, a peak zT of 2.2 is realized, which promotes GeTe into the first echelon of cutting‐edge thermoelectric materials. The strategy of combining band convergence and planar vacancies opens an avenue to develop Pb‐free derivatives with superhigh thermoelectric efficiency.
Significance
Phase-transition behavior in thermoelectric materials is detrimental for their application in thermoelectric devices. Here we designed, and experimentally realized the high thermoelectric performance of cubic GeTe-based material by suppressing the phase transition from a cubic to a rhombohedral structure to below room temperature through a simple Bi and Mn codoping on the Ge site. Bi doping reduced the hole concentration while Mn alloying largely suppressed the phase-transition temperature and also induced modification of the valence bands. Our work provides the basis for studying phase transitions in other thermoelectric materials to optimize these materials for applications.
Defect engineering and nano-structuring are the core stratagems for improving thermoelectric properties. In bismuth telluride alloys nanosizing individual crystallites has been extensively studied in efforts to reduce the thermal conductivity but nanostructuring with second phases has been more challenging. In this study, we demonstrate a thermoelectric figure of merit ZT of 1.4 at 400 K, realized in Zn-containing BiSbTe alloys (specifically Bi0.46Sb1.54Te3) by integrating defect complexity with nanostructuring. We have succeeded in creating nanostructured BiSbTe alloys containing ZnTe nanoprecipitates. We present a melt-spinning-based synthesis that forms in-situ ZnTe nanoprecipitates to produce extremely low lattice thermal conductivity of ~0.35 Wm-1K-1 at 400 K, approaching the amorphous limit in the Bi1-xSbxTe3 system, while preserving the high power factor of Bi0.46Sb1.54Te3. These samples show excellent repeatability and thermal stability at temperatures up to 523 K. DFT calculations and experimental results show that Zn is inclined to form dual site defects, including two substitutional defects Zn’Bi/Sb and a Te vacancy, to achieve full charge compensation, which was further explicitly corroborated by Positron annihilation measurement. The strong enhancement of thermoelectric properties was validated in a thermoelectric module fabricated with the melt-spun p-legs (ZnTe-nanostructured BiSbTe) and zone-melt n-legs (conventional BiTeSe) which achieved a thermoelectric conversion efficiency of 5.0% when subjected to a temperature gradient of 250 K, representing about 40% improvement compared with a commercial zone-melt-based module. The results presented here represent a significant step forward for applications in thermoelectric power generation.
In this study, a series of Ge1-xMnxTe (x=0-0.21) compounds were prepared by melting-quenching-annealing process combined with Spark Plasma Sintering (SPS). The effect of alloying MnTe into GeTe on the structure and thermoelectric properties of Ge1-xMnxTe is profound. With increasing content of MnTe, the structure of the Ge1-xMnxTe compounds gradually changes from rhombohedral to cubic, and the known R3m to Fm-3m phase transition temperature of GeTe moves from 700 K closer to room temperature. First-principles density functional theory calculations show that alloying MnTe into GeTe decreases the energy difference between the light and heavy valence bands in both the R3m and the Fm-3m structures, enhancing a multi-band character of the valence band edge that increases the hole carrier effective mass. The effect of this band convergence is a significant enhancement in the carrier effective mass from 1.44 m0 (GeTe) to 6.15 m0 (Ge0.85Mn0.15Te). In addition, alloying with MnTe decreases the phonon relaxation time by enhancing alloy scattering, and reduces the phonon velocity, and increases Ge vacancies all of which result in an ultralow lattice thermal conductivity of 0.13 Wm-1K-1 at 823 K. Subsequent doping of the Ge0.9Mn0.1Te compositions with Sb lowers the typical very high hole carrier concentration and brings it closer to its optimal value enhancing the power factor, which combined with the ultralow thermal conductivity yield a maximum ZT value of 1.61 at 823 K (for Ge0.86Mn0.10Sb0.04Te). The average ZT value of the compound over the temperature range 400 K-800 K is 1.09, making it the best GeTe-based thermoelectric material.
Bi2Te3-based alloys are the most well-known thermoelectric materials for room-temperature application. However, the maximum ZT values of Bi2Te3-based materials are usually achieved near room temperature. The ZT values dramatically drop above 400 K because of material’s intrinsic thermal excitation in Bi2Te3-based materials, which seriously restricts their applications as thermoelectric power generators. In this study, by doping tiny amount of Ag into Bi0.5Sb1.5Te3, we successfully suppressed the intrinsic excitation in p-type Bi2Te3-based materials and shifted the ZT peak temperature to high temperatures. Eventually, due to the enhanced power factor and greatly depressed bipolar thermal conductivity at high temperature, a maximum ZT of 1.25 at 400 K was obtained in Ag0.002Bi0.5Sb1.498Te3, and an average ZT value of approximately 1.03 was achieved between 300 and 600 K. The theoretical energy conversion efficiency of TE module fabricated with Ag0.002Bi0.5Sb1.498Te3 achieves a maximum value of 11.0% when ∆T = 300 K, which makes p-type Bi2Te3-based devices attractive for the applications in TE power generation.
In recent decades, by continuously enhancing the figure of merit ZT of various thermoelectric (TE) materials, solid state TE technology has matured and is on the verge of making an impact in real industrial settings as a promising approach to harvest waste industrial heat and convert it to useful electricity. Nevertheless, actual TE module development has remained stagnant with rather poor efficiencies. This has raised an urgent need to design rational module structures that rely on complex parameter optimization and utilization of efficient integration technologies that minimize energy losses during bonding of various interfaces. Here, we demonstrate a three-dimensional numerical analysis model of a segmented TE power-generating device, which takes into account the temperature-dependent materials' properties and various parasitic losses. The model generates an optimized design with predictive performance to realize maximum conversion efficiency. Combined with the developed bonding schemes and assembly techniques, the segmented modules consisting of Bi2Te3-based alloys and CoSb3-based filled skutterudites were successfully fabricated with a record-high efficiency of up to 12% when operating under a temperature difference of 541°C. The rational structure design based on the numerical analysis model and the extremely low thermal and electrical losses enable the heat-to-electricity conversion efficiency to reach up to 96.9% of the theoretical efficiency based on the TE materials themselves. These findings highlight the importance of the optimization strategy for TE power generation devices based on the TE materials' intrinsic properties and demonstrate that realistic high temperature TE modules with predictive high efficiency and high power density can be fabricated, which provides a useful guide to achieve a high conversion efficiency in large-scale TE applications.
High thermoelectric power factor not only enables potentially high figure of merit ZT but also leads to large output power density, hence it is pivotal to find an effective route to improve power factor. Previous reports on manipulation of carrier scattering mechanisms (e.g., ionization scattering) were mainly focused on enhancing the Seebeck coefficient. On the contrary, here we demonstrate that by tuning the carrier scattering mechanism in n-type Mg3Sb2-based materials, it is possible to noticeably improve the Hall mobility, from ~19 cm2 V-1 s-1 to ~77 cm2 V-1 s-1, and hence substantially increasing the power factor by a factor of 3, from ~5 to ~15 µW cm-1 K-2. The enhancement in mobility is mainly due to the reason that ionization scattering has been shifted into mixed scattering between ionization scattering and acoustic phonon scattering, which less effectively scatters the carriers. The strategy of tuning carrier scattering mechanism to improve the mobility should be widely applicable to various materials systems for achieving better thermoelectric performance.
GeTe-based alloys have been intensively considered as p-type thermoelectrics for about 50 years, yet existing literature barely discussed the thermoelectric properties of pristine GeTe at high temperatures (300~800 K). This work firstly backs to a fundamental understanding on the thermoelectric transport properties inherent to p-type GeTe, based on more than 50 samples synthesized with expected carrier concentrations ranging from 1 to 20×1020 cm-3. A thermoelectric figure of merit zT as high as ~1.7 is found inherent to this compound when it is optimally doped with a Hall carrier concentration of 2.2±10% ×1020 cm-3, offering a reference substance to expose the origins for the high zT in historical GeTe-based alloys. Guided by the above knowledge, further alloying Te with Se in samples with an optimal carrier concentration, enables a reduction on the lattice thermal conductivity by ~40%, and eventually leads to a further enhancement on zT (up to 2.0) by ~20%. This work demonstrates not only GeTe as an inherently high performance thermoelectric matrix compound, but also its availability for further improvements by additional strategies.
Skutterudite materials are widely considered for thermoelectric waste heat recovery. While the skutterudite structure effectively scatters the high frequency phonons, grain-boundary engineering is needed to further reduce the thermal conductivity beyond simply decreasing grain size. Here, we show that reduced graphene oxide (rGO) increases the grain boundary thermal resistivity by a factor of 3 to 5 compared to grain boundaries without graphene. Wrapping even micron sized grains with graphene leads to such a significant reduction in the thermal conductivity that a high thermoelectric figure of merit zT = 1.5 was realized in n-type YbyCo4Sb12, while a zT of 1.06 was achieved in p-type CeyFe3CoSb12. A 16 leg thermoelectric module was made by using n- and p-type skutterudite-graphene nanocomposites that exhibited conversion efficiency 24% higher than a module made without graphene. Engineering grain boundary complexions with 2-D materials introduces a new strategy for advanced thermoelectric materials.
To resolve the controversy in the literature, we studied the extensively twinned domain structure of ferroelectric germanium telluride (GeTe) to formulate a comprehensive three-dimensional domain description. The observed herringbone-domain structure arises due to the displacive phase transformation from a cubic to rhombohedral symmetry upon cooling. Using a simple model based on minimizing global shape change and local (interface) strain, a mixed system of {010} and {011} twin boundaries is argued to be the most energetically favorable structure, and the only solution to obtain a fully compatible domain structure. Using scanning- and transmission- electron microscopy, we identified the twin boundaries orientations together with the arrangement of local distortion directions in the sub-micron size domains, and confirm that the proposed domain structure is indeed the prevalent one. Because our model and analysis do not assume material-specific parameters other than the phase transition, this analysis is argued to be valid for a wide range of materials showing domain formation after a cubic to rhombohedral phase transformation. Indeed, we demonstrate that our model also holds in the case of LaAlO3.
By suppressing intrinsic excitation in p-type Bi2Te3-based materials, we report maximum and average zT values of up to 1.4 and 1.2 between 100 and 300 °C, respectively. Thermoelectric modules based on these high performance materials show energy conversion efficiencies of up to 6.0% under a temperature gradient of 217 K, and are greatly superior to current Bi2Te3-based modules.
Excellent thermoelectric performance is obtained over a broad temperature range from 300 K to 800 K by doping single crystals of SnSe. The average value of the figure of merit ZT, of more than 1.17, is measured from 300 K to 800 K along the crystallographic b-axis of 3 at% Na-doped SnSe, with the maximum ZT reaching a value of 2 at 800 K. The room temperature value of the power factor for the same sample and in the same direction is 2.8 mW mK−2, which is an order of magnitude higher than that of the undoped crystal. Calculations show that Na doping lowers the Fermi level and increases the number of carrier pockets in SnSe, leading to a collaborative optimization of the Seebeck coefficient and the electrical conductivity. The resultant optimized carrier concentration and the increased number of carrier pockets near the Fermi level in Na-doped samples are believed to be the key factors behind the spectacular enhancement of the average ZT.
In this work, we demonstrate the use of high performance nanostructured PbTe-based materials in high conversion efficiency thermoelectric modules. We fabricated the samples of PbTe–2% MgTe doped with 4% Na and PbTe doped with 0.2% PbI2 with high thermoelectric figure of merit (ZT) and sintered them with Co–Fe diffusion barriers for use as p- and n-type thermoelectric legs, respectively. Transmission electron microscopy of the PbTe legs reveals two shapes of nanostructures, disk-like and spherical. The reduction in lattice thermal conductivity through nanostructuring gives a ZT of ∼1.8 at 810 K for p-type PbTe and ∼1.4 at 750 K for n-type PbTe. Nanostructured PbTe-based module and segmented-leg module using Bi2Te3 and nanostructured PbTe were fabricated and tested with hot-side temperatures up to 873 K in a vacuum. The maximum conversion efficiency of ∼8.8% for a temperature difference (ΔT) of 570 K and ∼11% for a ΔT of 590 K have been demonstrated in the nanostructured PbTe-based module and segmented Bi2Te3/nanostructured PbTe module, respectively. Three-dimensional finite-element simulations predict that the maximum conversion efficiency of the nanostructured PbTe-based module and segmented Bi2Te3/nanostructured PbTe module reaches 12.2% for a ΔT of 570 K and 15.6% for a ΔT of 590 K respectively, which could be achieved if the electrical and thermal contact between the nanostructured PbTe legs and Cu interconnecting electrodes is further improved.
Thermoelectric (TE) devices for power generation have been attracting increasing attention on account of their advantages such as solid-state operation, good stability, and high reliability. This paper presents an overview of the design principle, fabrication methods and testing technology of TE power generation devices. Particular attention is paid to skutterudite-based devices regarding electrode fabrication, barrier layer design, interface optimization, protective coating, and evaluation of elements and modules. The development of Bi2Te3-based devices for power generation focusing specifically on the optimization of Bi2Te3/electrode joints and fabrication and evaluation of Bi2Te3-based modules is summarized. The future challenges concerning TE devices for power generation are discussed.
High thermoelectric figure of merit, zT, of ∼1.85 at 725 K along with significant cyclable temperature stability was achieved in Pb-free p-type Ge1-xSbxTe samples through simultaneous enhancement in Seebeck coefficient and reduction of thermal conductivity. Sb doping in GeTe decreases the carrier concentration due to the donor dopant nature of Sb and enhances the valence band degeneracy by increasing the cubic nature of the sample, which collectively boost Seebeck coefficient in the temperature range of 300-773 K. Significant thermal conductivity reduction was achieved due to collective phonon scattering from various meso-structured domain variants, twin and inversion boundaries, nanostructured defect layers, and solid solution point defects. The high performance Ge0.9Sb0.1Te sample shows mechanical stability (Vickers microhardness) of ∼206 Hv, which is significantly higher compared to other popular thermoelectric materials such as Bi2Te3, PbTe, PbSe, Cu2Se, and TAGS.
Filled skutterudites RxCo4Sb12 are excellent n-type thermoelectric materials owing to their high electronic mobility and high effective mass, combined with low thermal conductivity associated with the addition of filler atoms into the void site. The favourable electronic band structure in n-type CoSb3 is typically attributed to threefold degeneracy at the conduction band minimum accompanied by linear band behaviour at higher carrier concentrations, which is thought to be related to the increase in effective mass as the doping level increases. Using combined experimental and computational studies, we show instead that a secondary conduction band with 12 conducting carrier pockets (which converges with the primary band at high temperatures) is responsible for the extraordinary thermoelectric performance of n-type CoSb3 skutterudites. A theoretical explanation is also provided as to why the linear (or Kane-type) band feature is not beneficial for thermoelectrics.
Microstructure manipulation plays an important role in enhancing physical and mechanical properties of materials. Here a high figure of merit zT of 1.2 at 357 K for n-type bismuth-telluride-based thermoelectric (TE) materials through directly hot deforming the commercial zone melted (ZM) ingots is reported. The high TE performance is attributed to a synergistic combination of reduced lattice thermal conductivity and maintained high power factor. The lattice thermal conductivity is substantially decreased by broad wavelength phonon scattering via tuning multiscale microstructures, which includes microscale grain size reduction and texture loss, nanoscale distorted regions, and atomic scale lattice distotions and point defects. The high power factor of ZM ingots is maintained by the offset between weak donor-like effect and texture loss during the hot deformation. The resulted high zT highlights the role of multiscale microstructures in improving Bi2Te3-based materials and demonstrates the effective strategy in enhancing TE properties.
The successful research strategies to enhance the dimensionless figure of merit zT above 2 rely on either bulk nanomaterials or on single crystals. Here, a new physical mechanism of nanoscale mosaicity is shown that goes beyond the approaches in single crystals or conventional nanomaterials. A zT value of 2.1 at 1000 K in bulk nanomaterials is achieved.
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Iodine-doped n-type SnSe polycrystalline by melting and hot pressing is prepared. The prepared material is anisotropic with a peak ZT of ≈0.8 at about 773 K measured along the hot pressing direction. This is the first report on thermoelectric properties of n-type Sn chalcogenide alloys. With increasing content of iodine, the carrier concentration changed from 2.3 × 1017 cm−3 (p-type) to 5.0 × 1015 cm−3 (n-type) then to 2.0 × 1017 cm−3 (n-type). The decent ZT is mainly attributed to the intrinsically low thermal conductivity due to the high anharmonicity of the chemical bonds like those in p-type SnSe. By alloying with 10 at% SnS, even lower thermal conductivity and an enhanced Seebeck coefficient were achieved, leading to an increased ZT of ≈1.0 at about 773 K measured also along the hot pressing direction.
A thermoelectric (TE) device is basically fabricated by joining p- and n-type thermoelectric materials with electrodes. For skutterudite (SKD) based TE devices, Ti and alloys like Co-Fe-Ni are widely used as the barrier layer joining the SKD and the electrode. In this paper, TE joints composed of Yb0.6Co4Sb12 (Yb-SKD) and a Ti-Al barrier layer with various Al contents (0-15 at.%) were fabricated by a one-step SPS process. The influence of the Al content on the evolution of the interfacial microstructure during accelerated aging at 600 degrees C was studied. During sintering and aging, Ti and Al diffused into each other and formed inert Ti-Al alloys, which hindered the interfacial diffusion and resulted in a lower growth rate of the diffusion layers. On the other hand, excess Al at the Ti-Al/Yb-SKD interface encouraged the interfacial diffusion. The interplay of the above two mechanisms decides the inter-diffusion behaviors at the interface. In this work, the two mechanisms reached balanced when the Al content was 6 at.%, which resulted in the best interfacial stability and the highest shear strength after aging. At the same time, with the increase of the Al content in the Ti-Al barrier layer, the CTE mismatch between the Yb-SKD and the Ti-Al barrier layer was reduced, consequently the interfacial integrity after aging was improved. As a result, the contact resistivity of the Ti-Al/Yb-SKD joints (Al: 6-12 at.%) remained below 12 mu Omega cm(2) after 16 days of aging.
Traditional processes of making contacts (metallization layer) onto bulk crystalline Bi2Te3-based materials do not work for nanostructured thermoelectric materials either because of weak bonding strength or an unstable contact interface at temperatures higher than 200 °C. Hot pressing of nickel contact onto nanostructured thermoelectric legs in a one-step process leads to strong bonding. However, such a process results in large contact resistance in n-type Ni/Bi2Te2.7Se0.3/Ni legs, although not in p-type Ni/Bi0.4Sb1.6Te3/Ni legs. A systematic study was carried out to investigate the detailed reaction and diffusion at the interface of the nickel layer and n-type Bi2Te3-based thermoelectric material layer. We found that a p-type region formed within the n-type Bi2Te2.7Se0.3 during hot pressing due to Te deficiency and Ni doping, leading to a large contact resistance.
We demonstrate the potential of metallurgical controlling of the phase separation reaction, by means of spark plasma sintering consolidation and subsequently controlled heat treatments sequence, for enhancement the thermoelectric properties of the p-type Ge0.87Pb0.13Te composition. Very high ZTs of up to ∼2, attributed to the nucleation of sub-micron phase separation domains and to comparable sized twinning and dislocation networks features, were observed. Based on the experimentally measured transport properties, combined with the previously reported phase separated n-type (Pb0.95Sn0.05Te)0.92(PbS)0.08 composition, a maximal efficiency value of ∼11.5% was theoretically calculated. These ZT and efficiency values are among the highest reported for single composition non-segmented bulk material legs.
As a lead-free material, GeTe has drawn a growing attention in thermoelectrics, and the figure of merit ZT close to unity was previously obtained via traditional doping/alloying, largely owing to the hole carrier concentration tuning. In this report, we show that a remarkably high ZT of ~1.9 can be achieved at 773K in Ge0.87Pb0.13Te upon the introduction of 3mol% Bi2Te3. Bismuth Telluride promotes the solubility of PbTe in the GeTe matrix thus leading to a significantly reduced thermal conductivity. At the same time, it enhances the thermopower by activating a much higher fraction of charge transport from the highly degenerate Σ valence band, as evidenced by Density Functional Theory calculations. These mechanisms are incorporated and discussed in a 3-band (L+Σ+C) model, and found to well explain the experimental results. The detailed microstructure (including rhombohedral twin structures) analysis in Ge0.87Pb0.13Te+3mol%Bi2Te3 is carried out using transmission electron microscopy (TEM) and crystallographic group theory. The complex microstructure explains the reduced lattice thermal conductivity, and electrical conductivity as well.
The thermoelectric effect enables direct and reversible conversion between thermal and electrical energy, and provides a viable route for power generation from waste heat. The efficiency of thermoelectric materials is dictated by the dimensionless figure of merit, ZT (where Z is the figure of merit and T is absolute temperature), which governs the Carnot efficiency for heat conversion. Enhancements above the generally high threshold value of 2.5 have important implications for commercial deployment, especially for compounds free of Pb and Te. Here we report an unprecedented ZT of 2.6 ± 0.3 at 923 K, realized in SnSe single crystals measured along the b axis of the room-temperature orthorhombic unit cell. This material also shows a high ZT of 2.3 ± 0.3 along the c axis but a significantly reduced ZT of 0.8 ± 0.2 along the a axis. We attribute the remarkably high ZT along the b axis to the intrinsically ultralow lattice thermal conductivity in SnSe. The layered structure of SnSe derives from a distorted rock-salt structure, and features anomalously high Grüneisen parameters, which reflect the anharmonic and anisotropic bonding. We attribute the exceptionally low lattice thermal conductivity (0.23 ± 0.03 W m(-1) K(-1) at 973 K) in SnSe to the anharmonicity. These findings highlight alternative strategies to nanostructuring for achieving high thermoelectric performance.
Multifilling with La, Ba, Ga, and Ti in p-type skutterudite and Yb, Ca, Al, Ga, and In in n-type skutterudite remarkably reduces their thermal conductivity, resulting in enhancement of their dimensionless figure of merit ZT to ZT = 0.75 for p-type (La,Ba,Ga,Ti)1(Fe,Co)4Sb12 and ZT = 1.0 for n-type (Yb,Ca,Al,Ga,In)0.7(Co,Fe)4Sb12. A thermoelectric module technology suitable for these skutterudites including diffusion barrier and electrode materials has been established. The diffusion barrier materials allow the electrode to coexist stably with the p/n skutterudites in the module’s working temperature range of room temperature to 600°C. Under conditions of hot/cold-side temperatures of 600°C/50°C, a skutterudite module with size of 50 mm × 50 mm × 7.6 mm exhibited generation performance of 32 W power output and 8% thermoelectric conversion efficiency.
A CoSb3-based thermoelectric module was fabricated using Ce0.45Co2.5Fe1.5Sb12 p-type leg and Yb0.25Co4Sb12/Yb2O3 n-type leg. Ag–Cu foil was used to construct the junction of hot side legs. With two p–n couples, the module generated a maximum output power (Pmax) of 140mW and a maximum open-circuit voltage (Vo) of 210mV under the thermal condition of hot side temperature Th=810K and a temperature difference ΔT=490K. No deterioration in output power in vacuum was seen when thermal cycle of five times for the module was carried out under Th=810K and ΔT=490K with natural cooling to room temperature, which shows the module has high durability.
We report the thermoelectric characteristics of individual p-type SiGe alloy nanowires for diameters of 100 to 300 nm and temperatures between 40 to 300 K. A technique that allows for electrical and thermal characterization on the same nanowire was developed in this work. Experimental data provide evidence of the scattering of low-frequency phonons by the boundary of the nanowires. The thermal conductivities for SiGe alloy nanowires with different free carrier concentrations reveal that the long free path phonons are also scattered by hole-phonon interactions. Combined boundary and hole-phonon scattering mechanisms with alloy scattering resulted in thermal conductivities as low as 1.1 W/m-K at 300 K, which is one of the lowest measured for SiGe alloys and is comparable to that of bulk silica. The enhanced thermal properties observed in this work yielded ZT close to 0.18 at 300 K—more than a factor of 2 higher than the bulk SiGe alloy.
A method of preparation has been developed which allows the growth of large homogeneous single phase ingots of such alloy systems as GeTe-MnTe, GeTe-SnTe, etc. … We have measured the thermoelectric power and the thermal conductivity of both GeTe and two alloys of the system Ge1−xMnxTe, in the temperature interval 2.5 to 110 °K, the thermoelectric measurements being extended to 350 °K. Using conventional transport theory both the lattice and the electronic contributions to these effects have been obtained. The temperature dependence of these properties was found to be of similar shape for the GeTe-MnTe samples as for the pure GeTe, even below the known magnetic transition temperature of these samples. To explain the behaviour of the diffusion term of the thermoelectric power, however, the influence of the incomplete Mn d-band on the electronic properties of the system must be invoked. The lattice thermoelectric power has the same temperature dependence as the phonon-drag term in metals. From the analysis of both this component and the lattice thermal conductivity, some informations on the predominant phonon scattering mechanisms have been obtained.
We present a model calculation of the lattice thermal conductivity of ZrNiSn -based half-Heusler thermoelectric compounds for temperatures where phonon scattering is dominated by Umklapp and point defect scattering. The difference in mass between impurity and host atoms dominates point defect scattering for alloying Hf on the Zr sublattice, whereas differences in size and interatomic coupling forces between impurity and host atoms dominate point defect scattering for alloying Pd on the Ni sublattice. Because Pt is heavier and larger than Pd , we predict that Pt will further reduce lattice thermal conductivity when alloyed on the Ni sublattice of these half-Heusler compounds.
Advanced thermoelectric technology offers a potential for converting waste industrial heat into useful electricity, and an emission-free method for solid state cooling. Worldwide efforts to find materials with thermoelectric figure of merit, zT values significantly above unity, are frequently focused on crystalline semiconductors with low thermal conductivity. Here we report on Cu(2-x)Se, which reaches a zT of 1.5 at 1,000 K, among the highest values for any bulk materials. Whereas the Se atoms in Cu(2-x)Se form a rigid face-centred cubic lattice, providing a crystalline pathway for semiconducting electrons (or more precisely holes), the copper ions are highly disordered around the Se sublattice and are superionic with liquid-like mobility. This extraordinary 'liquid-like' behaviour of copper ions around a crystalline sublattice of Se in Cu(2-x)Se results in an intrinsically very low lattice thermal conductivity which enables high zT in this otherwise simple semiconductor. This unusual combination of properties leads to an ideal thermoelectric material. The results indicate a new strategy and direction for high-efficiency thermoelectric materials by exploring systems where there exists a crystalline sublattice for electronic conduction surrounded by liquid-like ions.