Advanced Thermoelectric Materials Research Articles

Advanced Thermoelectric Materials and Devices

Advanced Thermoelectric Materials and Devices

Unlocking the Potential of Porous Bi2Te3-Based Thermoelectrics Using Precise Interface Engineering through Atomic Layer Deposition.

Porous thermoelectric materials offer exciting prospects for improving the thermoelectric performance by significantly reducing the thermal conductivity. Nevertheless, porous structures are affected by issues, including restricted enhancements in performance attributed to decreased electronic conductivity and degraded mechanical strength. This study introduces an innovative strategy for overcoming these challenges using porous Bi0.4Sb1.6Te3 (BST) by combining porous structuring and interface engineering via atomic layer deposition (ALD). Porous BST powder was produced by selectively dissolving KCl in a milled mixture of BST and KCl; the interfaces were engineered by coating ZnO films through ALD. This novel architecture remarkably reduced the thermal conductivity owing to the presence of several nanopores and ZnO/BST heterointerfaces, promoting efficient phonon scattering. Additionally, the ZnO coating mitigated the high resistivity associated with the porous structure, resulting in an improved power factor. Consequently, the ZnO-coated porous BST demonstrated a remarkable enhancement in thermoelectric efficiency, with a maximum zT of approximately 1.53 in the temperature range of 333-353 K, and a zT of 1.44 at 298 K. Furthermore, this approach plays a significant role in enhancing the mechanical strength, effectively mitigating a critical limitation of porous structures. These findings open new avenues for the development of advanced porous thermoelectric materials and highlight their potential for precise interface engineering through the ALD.

Close‐packed layer spacing as a practical guideline for structure symmetry manipulation of IV‐VI/I‐V‐VI2 thermoelectrics

AbstractThe crystal‐structure symmetry in real space can be inherited in the reciprocal space, making high‐symmetry materials the top candidates for thermoelectrics due to their potential for significant electronic band degeneracy. A practical indicator that can quantitatively describe structural changes would help facilitate the advanced thermoelectric material design. In face‐centered cubic structures, the spatial environment of the same crystallographic plane family is isotropic, such that the distances between the close‐packed layers can be derived from the atomic distances within the layers. Inspired by this, the relationship between inter‐ and intra‐layer geometric information can be used to compare crystal structures with their desired cubic symmetry. The close‐packed layer spacing was found to be a practical guideline of crystal structure symmetry in IV‐VI chalcogenides and I‐V‐VI2 ternary semiconductors, both of which are historically important thermoelectrics. The continuous structural evolution toward high symmetry can be described by the layer spacing when temperature or/and composition change, which is demonstrated by a series of pristine and alloyed thermoelectric materials in this work. The layer‐spacing‐based guideline provides a quantitative pathway for manipulating crystal structures to improve the electrical and thermal properties of thermoelectric materials.

Analysis of different Hybrid TEG/TEC Configurations

The growing demand for electricity and the looming environmental crisis are the main challenges to be faced and solved today. This calls for innovative energy conversion systems, for which efficiency and reliability are among the most sought-after features. Thermoelectric generators are devices that can offer partial or complete solutions to these challenges of the new millennium. The advantages of these technologically advanced devices are many: they are environmentally friendly, reliable and have a long service life. Furthermore, by applying thermoelectric generators, it is possible to improve the efficiency of existing systems or meet the electricity demand of different systems with high flexibility. At the same time, their low conversion efficiency has so far prevented their wide application, limiting them mainly to research. However, recent advances in thermoelectric materials and devices are pushing this technology to find its place among state-of-the-art energy conversion systems. This review explores the state of the art in terms of research and solutions already on the market, in order to illustrate a comprehensive and realistic perspective of the PV-TEG/TEC structures.

Rethinking SnSe Thermoelectrics from Computational Materials Science.

ConspectusThe growing energy crisis and the adverse environmental impacts caused by carbon-based energy consumption have spurred the exploration of green and sustainable energy. Researchers have been devoted to developing thermoelectric technology that could directly and reversibly convert heat into electricity. By virtue of zero emissions, nonmoving parts, precise temperature control, and long service life, thermoelectrics exhibit broad application in power generation and refrigeration. Nevertheless, traditional narrow-bandgap thermoelectrics exhibit high performance within a narrow temperature range, limiting the overall energy conversion. Consequently, a selection rule for exploring advanced thermoelectrics was proposed: materials with wide-bandgap, crystals form, asymmetry, and anisotropic structure. According to the rules, we conducted much research and found some promising materials.As the lead-free, cost-effective, and stable thermoelectric candidates, layered SnSe crystals with wide-bandgap and covalent bonding have gained significant attention due to their ultralow thermal conductivity resulting from strong bonding anharmonicity, via strong polar covalent bonding, because of the electronegativity difference between the Sn and Se atoms. This was proved to be the result from the unique structure of layered SnSe crystals, a distorted rock-salt structure with high and anisotropic Grüneisen parameters. In this Account, we introduce and rethink our recent advancements in developing high-performance thermoelectric SnSe crystals from computational materials science, involving p- and n-type SnSe crystals, respectively. For p-type SnSe crystals, according to the complex valence band structures, we utilized the multiband synglisis via electronic structure calculations and multiband simulations to activate valence bands to participate in electrical transport of in-plane direction, achieving an ultrahigh power factor (PF) of ∼75 μW cm-1 K-2 at room temperature and an average figure-of-merit ZTave of ∼1.9 for Sn0.91Pb0.09Se. Besides, on the basis of defect chemistry, the characteristics of p-type SnSe crystals are determined by intrinsic Sn vacancies. We thus used point-defect calculations to achieve the lattice plainification, and we fixed the lattice intrinsic defects to weaken defect scattering of carriers along the in-plane direction, facilitating further a PF > 100 μW cm-1 K-2 and a ZT of ∼1.5 at room temperature for SnCu0.001Se. For n-type SnSe crystals, inspired by the anisotropic characteristics of the layered materials, we analyzed charge density and proposed the insight of 3D charge and 2D phonon transports and calculated the deformation potential to manipulate layered phonon-electron decoupling to achieve high performance, ultimately Pb-alloyed and Cl-doped SnSe (SnSe-Cl-PbSe) reaching a ZTave of ∼1.7 from 300 to 773 K. In the end, we offer potential perspectives on high-throughput calculations (HTC) and machine learning (ML), combined with our proposed insights, which could be a promising avenue for future thermoelectrics. By virtue of our theoretical and experimental understanding of thermoelectrics, integrating these insights as rules and descriptors for HTC and ML will accelerate the research and development of thermoelectrics. We want to share our recent works and latest perspectives in SnSe thermoelectrics, and we expect to inspire enthusiasm for innovative research on advanced thermoelectric materials and devices.

Machine learning accelerated design of 2D covalent organic frame materials for thermoelectrics

Both thermodynamic stability and high thermoelectric figure of merit are required for organic thermoelectric materials for practical application. However, it is extremely time-consuming to design advanced organic thermoelectric materials via conventional theoretical calculation. In this work, combining first-principles calculations, machine learning fitted potential and solving phonon Boltzmann transport equation, we propose a new group of two-dimensional covalent organic frame semiconductors 2AL-PR-X (X = 2H, Ni, Pt, Zn) as promising organic thermoelectric materials. We found that the embedding metal atoms in the center can enhance remarkably the stability of porphyrin ring, demonstrated by evaluating the system total energy and phonon dispersion relationship. Moreover, the interaction between the metal atoms and the porphyrin ring will affect the phonon modes in the porphyrin ring, result in reduction in thermal conductivity. We found that 2AL-PR-Pt is the best thermoelectric candidate material among the studied 2AL-PR-X, which has a peak ZT of 0.32 at room temperature and can reach around 0.9 at 800 K. Our work provides strategy for the thermal transport regulation of two-dimensional covalent organic frame materials and demonstrates the large potential of 2AL-PR-X materials for thermoelectric applications.

Accelerated Discovery of Advanced Thermoelectric Materials via Transfer Learning (Adv. Energy Mater. 23/2023)

Thermoelectrics In article 2300049, Huijun Liu, Ying Fang, and co-workers leverage the knowledge from a pre-training on electronic bandgaps to propose a transfer learning strategy that allows ready and accurate predictions of the ZT values of any candidate materials at arbitrary temperature, which is very beneficial for the accelerated discovery of advanced thermoelectric materials.

Accelerated Discovery of Advanced Thermoelectric Materials via Transfer Learning

AbstractThermoelectric (TE) technology can realize direct conversion of widely distributed heat into useful electricity, which provides a promising route to solve the global energy crisis that is increasingly severe. However, it is extremely complex and time‐consuming to discover advanced TE materials via conventional trial‐and‐error approaches. In this work, using a pre‐trained neural network architecture for the electronic bandgap, a transfer learning (TL) strategy that allows ready and accurate prediction on the ZT values of any TE materials at arbitrary temperature is proposed. Compared with direct machine learning algorithms, the TL‐driven model exhibits significantly enhanced predictive power beyond the initial dataset, as characterized by improved Pearson correlation coefficient (reduced mean absolute error) from 23% to 95% (0.35 to 0.07) for the p‐type systems, and 46% to 94% (0.23 to 0.06) for the n‐type systems. By screening 6353 possible candidates in the AFLOW repository that having relatively smaller gaps, 925 p‐type and 788 n‐type systems are quickly identified to exhibit ZT exceeding 2.0. Equally importantly, the established TL model is highly adaptable to ZT prediction in an even larger search space, where the constituent atoms and/or stoichiometric compositions of the screened systems may be variously tuned to further optimize their TE performance.

Toughening Thermoelectric Materials: From Mechanisms to Applications.

With the tendency of thermoelectric semiconductor devices towards miniaturization, integration, and flexibility, there is an urgent need to develop high-performance thermoelectric materials. Compared with the continuously enhanced thermoelectric properties of thermoelectric materials, the understanding of toughening mechanisms lags behind. Recent advances in thermoelectric materials with novel crystal structures show intrinsic ductility. In addition, some promising toughening strategies provide new opportunities for further improving the mechanical strength and ductility of thermoelectric materials. The synergistic mechanisms between microstructure-mechanical performances are expected to show a large set of potential applications in flexible thermoelectric devices. This review explores enlightening research into recent intrinsically ductile thermoelectric materials and promising toughening strategies of thermoelectric materials to elucidate their applications in the field of flexible thermoelectric devices.

Tuning thermoelectric figure of merit in Ag doped nanostructured PbSnTeSe alloy by entropy and band engineering phenomena

Advanced thermoelectric materials with novel chemistry and structure that exhibit superior functional properties are in high demand in today's scientific scenario. Engineered configurational entropy in bulk thermoelectric materials through complex chemical disorder is an emerging and potential approach to achieve exceptional thermoelectric properties over traditional material systems. In the current study, mechanical alloying (MA) and spark plasma sintering (SPS) were used to synthesize the high-density (>96%) nanocrystalline lead tin tellurium selenium (PbSnTeSe) high entropy alloy (HEA) with varied atomic concentrations of silver (Ag) doping (0–0.9 at%). The high entropy, band engineering and nanostructuring approach synergistically show a remarkable decrease to ultralow thermal conductivity (0.814 W/mK), resulting in a substantial increase in the power factor (14.16×10−4W/mK2). Thus, an excellent figure of merit (ZT=0.891) is achieved in PbSnTeSe doped with 0.9Ag HEA at 573 K, which is estimated as a 225% increase over the pristine PbSnTeSe HEA (ZT=0.396).

Flexible hydrogel with a coupling enhanced thermoelectric effect for low-grade heat harvest

Harvesting abundant and ubiquitous low-grade thermal energy and simultaneously converting it into electrical energy hold the potential of solving the existing energy crisis. Advances in thermoelectric materials now allow thermoelectric ionogel electrolyte to enhance the energy conversion and storage capacities under a thermal gradient. Here, we report an ionic thermoelectric (i-TE) material based on polyacrylamide (PAM) carboxymethyl cellulose (CMC) double network gel substrate with flexible and high thermal-electrical conversion properties. By combining the thermodiffusion effect of lithium sulfate (Li2SO4) ions and the thermogalvanic effect of [Fe(CN)64−/Fe(CN)63−], which achieved a coupled ionic Seebeck effect of up to 11.58 mV/K, exhibiting a high ionic conductivity (18.4 mS cm−1) and low thermal conductivity (0.47 W m−1/K), resulting in a high ionic power factor (198.2 μW m−1K−2) at room temperature. It exhibited an excellent tensile property (634%) and adhesion, and avoids electrolyte leakage to a large extent. Moreover, the application in ionic capacitors also demonstrates the superior thermoelectric conversion capability of i-TE gels. We believe that i-TE materials will pave a new pathway for low-grade thermal harvesting and self-driven flexible wearable electronics.

A coupled experimental and thermodynamic assessment of the Mg-Bi-Sb and Zn-Bi-Sb ternary system and extrapolate to the Mg-Bi-Zn-Sb system

The magnesium-bismuth-antimony (Mg-Bi-Sb) system as a promising system for developing the alloy-type anode of magnesium recharge batteries and creep-resistant magnesium alloys has drawn widespread attention. Additionally, zinc-bismuth-antimony (Zn-Bi-Sb) alloys are candidates of the advanced thermoelectric materials. Hereby, the phase equilibria and transformation temperatures of the Mg-Bi-Sb ternary system were studied via thermodynamic modeling coupled with key experiments. The isothermal section of the Mg-rich corner at 500°C in the Mg-Bi-Sb ternary system was experimentally constructed and no new ternary compound was observed in this work. The calculated phase equilibria of the Mg-Bi-Sb ternary system can reproduce most of the measured data in this work using the current optimized thermodynamic parameters. Meanwhile, the binary Zn-Sb and ternary Zn-Bi-Sb systems were re-assessed based on the available literature data, respectively. A set of self-consistent parameters describing the ternary Zn-Bi-Sb system were achieved, and the present calculated results are in good agreement between phase equilibria as well as thermodynamic properties. The tentative Mg-Zn-Sb and Mg-Bi-Zn-Sb systems were then developed using the extrapolation method.

Microstructure evolution of Si nanoparticles during the melting process: Insights from molecular dynamics simulation

Si nanomanufacturing has attracted considerable attention in the manufacturing of advanced thermoelectric materials and microelectronics owing to its compatibility with the semiconductor industry. The microstructure determines physical and chemical characteristics of a material; however, it is difficult to observe the microstructural evolution using traditional experimental methods. In this study, the melting process of Si nanoparticles is studied in depth by classical molecular dynamics (MD) simulation. Using the entropy of the largest standard cluster and the pair distribution function, the microstructural evolution characteristics can be determined. The transformation of Si-like structures was examined by largest standard cluster analysis (LaSCA). The results show that the melting process begins at the surface and rapidly penetrates into the nanoparticles at a high heating rate, indicating a liquid nucleation and growth (LNG) mode. At a higher heating rate, the temperature range of the solid and liquid coexistence will be larger. This study provides an understanding of the melting behavior of Si nanoparticles at the atomic scale and a useful reference for the preparation and processing of nanodevices.

Processing High-Performance Thermoelectric Materials in a Green Way: A Proof of Concept in Cold Sintered PbTe0.94Se0.06.

For years, most of the advanced polycrystalline thermoelectric (TE) materials are fabricated by spark plasma sintering (SPS) in the research field, mainly because of its high processing efficiency. However, issues like high energy consumption and an expensive apparatus have prevented the application of this strategy in industry. Herein, taking PbTe0.94Se0.06 (PTS) as a typical n-type mid-temperature material, we demonstrate that the cold sintering process (CSP) can serve as a green and cost-effective technology for preparing advanced TE materials. By selecting the solvothermal precursors as liquid sintering aids, the CSP-densified PTS shows a maximum figure of merit of 0.96 at 700 K, which is on par with, if not better than, the reported similar materials prepared by SPS. This remarkable performance is ascribed to the distinct densification procedure in the CSP: (1) the ultralow temperature alleviates the precipitation of Pb, which preserves the high carrier concentration of PTS; (2) the transient liquid phase forms intimate grain boundaries comparable to the high-temperature sintered one, leading to a high carrier mobility; (3) the dissolution-precipitation process greatly restrains the coarsening of precipitates, which effectively suppresses the bipolar effect and lattice thermal conductivity due to enhanced scattering. We believe that these results can greatly encourage the application of CSP in the future development of TE materials.

Probing the Carrier Dynamics of Polymer Composites with Single and Hybrid Carbon Nanotube Fillers for Improved Thermoelectric Performance

The incorporation of carbon nanotubes (CNTs) within polymer hosts offers a great platform for the development of advanced thermoelectric (TE) composite materials. Over the years, several CNT/polymer composite formulations have been investigated on an effort to maximize the TE performance. Meanwhile, several studies focused on the decay dynamics of the charged excitons within CNTs itself and therefrom derived structures, aiming to investigate the lifetimes and the corresponding recombination processes of free charge carriers. The latter physical phenomena play a crucial role in the performance of various types of energy converting and scavenging materials. Nevertheless, up to this date, there is no systematic study on the combination of TE parameters and the critical charge carrier dynamics within CNT containing TE polymer composites. Herein, a variety of composites with single and hybrid CNT fillers based on polycarbonate (PC) and polyether ether ketone (PEEK) polymer matrices were prepared by melt-mixing in small scale. At the same loading, the addition of single fillers in PC results in higher Seebeck coefficients and similar conductivities when compared to the use of hybrid filler systems. In contrast, with hybrid filler systems in PEEK composites, higher power factors could be reached than in single filler composites. Moreover, the PC-based composites are studied using ultrafast laser time-resolved transient absorption spectroscopy (TAS), for the investigation of the exciton lifetimes and the physical origins of free charge carrier transport within the TE films. The findings of this study reveal interesting links between the TE parameters and the obtained charge carrier dynamics.

A Novel Electronegative Guest‐Filled Skutterudite‐Based Thermoelectric Generator with an Efficiency of 6.0%

Electronegative guest‐filled skutterudites (EN‐SKDs) are considered one of the most promising thermoelectric materials for next‐generation thermoelectric generators (TEGs). However, the practical application of EN‐SKDs in TEGs is still a huge challenge because of their unsatisfactory zT value (less than 1.0). Herein, Ge, Te‐codoped electronegative sulfur‐filled skutterudites of S x Co4Sb11Ge0.2Te0.8 (x = 0, 0.05, 0.1, 0.2, 0.25) are synthesized. The optimal zT value of as high as 1.45 at 823 K with a low room‐temperature lattice thermal conductivity of 1.98 W m−1 K−1 is achieved by S0.25Co4Sb11Ge0.2Te0.8. Such performance is competitive with those of traditional TE materials and among one of the best EN‐SKDs. The excellence is attributed to the synergistic effect of the disorder caused by Ge, Te co‐doping at Sb‐site and the “rattling” effect of electronegative element S. Furthermore, a novel EN‐SKD‐based TEG is prepared for the first time through an integrated welding process and exhibits a high efficiency of 6.0% at the temperature difference of 475 K. This work provides valuable guidance for designing advanced thermoelectric materials and paves a way toward high‐performance EN‐SKD‐based TEGs.

A general White–Box strategy for designing thermoelectric cooling system

AbstractThermoelectric cooling (TEC) is critically important in thermal management of laser modules or chips and potentially for personalized thermoregulation. The formulae for efficiency in standard textbooks can only describe the performance of a TEC module with ideal thermal conditions, that is, fixed terminal temperatures, but are unable to deal with a real TEC system where heat transfer at its interfaces with the heat source and sink are finite and with thermal resistances. Here, we define the TEC system‐level performance indices, that is, the maximum cooling power, temperature difference, and coefficient of performance, by introducing a set of explicit formulae. The external heat transfer conditions are taken into account as dimensionless thermal resistance parameters. With these formulae, the TEC system performances are evaluated elegantly with errors well within ±5% over broad operating conditions. We further optimize the cooling power and the coefficient of performance in practical scenarios and establish a general White–Box design procedure for TEC systems, which enables a transparent design process and straightforward analysis of performance bottlenecks. A set of cooling experiments are performed to validate the analytical model and to illustrate the dependence of system design on realistic thermal conditions. By choosing the suitable TEC module parameter under given external heat transfer conditions, the cooling power can be improved by more than 100%. This work sheds some light on the integral design of TEC systems for broad applications to take full advantage of the advanced thermoelectric materials in the cooling field.image

Cubic-spinel AgIn5S8-based thermoelectric materials: synthesis, phonon transport and defect chemistry

Cubic-spinel Ag1-xCuxIn5S8 is promising as it is a kind of In–S-based thermoelectric material using eco-friendly elements like In, Cu and S. Here, we develop a melting method to synthesize single-phase Ag1-xCuxIn5S8 alloys using metal sulfides as precursors, and we also comprehensively study their thermoelectric properties, chemical bonding, electronic and phonon structures, and defect chemistry. Owing to the complex crystal structures, weak chemical bonding, and “rattling-like” phonon modes, the Ag1-xCuxIn5S8 alloys show low lattice thermal conductivity of ∼0.63–0.72 Wm−1K−1 at 873 K. The optimal figure of merit (ZT) of the n-type Ag1-xCuxIn5S8 can reach ∼0.31 at 873 K, which is higher than the theoretical prediction. The diamond-like crystal structures also enable them to have good mechanical properties, with a hardness of ∼3.95 GPa, comparable to those thermoelectric materials with the highest mechanical strength. Our understanding of the “rattling-like” phonon modes and defect chemistry in AgIn5S8 can also guide the further development of the more promising p-type AgIn5S8 and other advanced cubic-spinel thermoelectric materials.

Strategies for boosting thermoelectric performance of PbSe: A review

• General physical properties of typical IV-VI compounds are summarized. • Phase diagram, crystal and electrical band structures of PbSe are presented. • Recent ways of adjusting charge and phonon transport processes in PbSe are updated. • Typical preparation methods for PbSe and their scalability are discussed. • Key challenges and future research directions of PbSe are discussed. Thermoelectric materials enable the direct conversion between waste heat and electric energy, playing an important role in alleviating energy crisis. Many excellent thermoelectrics developed so far contain expensive and scarce Te element, largely limiting their applications. Therefore, exploring Te-free compounds with extraordinary thermoelectric performance becomes a vital topic in thermoelectric community in recent years. PbSe is an ideal candidate that meets above criteria and has advanced rapidly in the last decade with reported peak ZT s close to 2.0. Herein we review the recent research progress of PbSe-based thermoelectric materials. This review article starts with a general introduction of the properties of IV-VI semiconductors as advanced thermoelectric materials by comparing their cost, crustal abundance, mechanical strength and chemical bonding. Following that, phase diagram, crystal and electronic band structures of PbSe are comprehensively summarized. Then we discuss how the frequently used dopants regulate its carrier concentrations. Subsequently, electronic band structure engineering (including resonant levels, band flattening, band convergence, band inversion, etc. ) and microstructural architecturing (including atomic arrangements, dislocation arrays, nanoscale precipitates, etc. ) approaches and their impacts on charge and phonon transport properties of PbSe are elaborated. Finally, we summarize the typical production processes of PbSe and comment on their scalability. The future directions for how to further improve the thermoelectric properties of PbSe and promote its applications are discussed at the end of this article.

Recent advances in designing thermoelectric materials

Strategies for designing advanced thermoelectric materials.