Bulk Thermoelectric Materials Research Articles

Comparative study on properties of n-PbS bulk thermoelectric materials prepared by chemical method

Comparative study on properties of n-PbS bulk thermoelectric materials prepared by chemical method

Modulation doping promoted ultrahigh electron mobility and enhanced thermoelectric performance in PbTe

Modulation doping was proven to be a quite effective strategy to realize high electrical conductivity at a relative low charge carrier concentration in bulk thermoelectric materials. In this work, we used lightly doped PbTe-0.1 %Cu with minimal ionized impurity scattering as matrix material and PbS-0.8 %Cu with adequate electron concentration as electron reservoir to construct a modulation doping scenario. Such modulation doped PbTe/PbS heterostructures were characterized with high-angle angular dark field scanning transmission electron microscopy (HAADF-STEM). We achieved significantly improved carrier mobility in modulation doped (PbTe)1-x(PbS)x than homogeneously doped PbTe materials reported in literatures; meanwhile, we also obtained obviously reduced lattice thermal conductivity owing to the strong phonon scattering by the dislocation arrays at PbTe/PbS phase boundaries. An ultrahigh room temperature (323 K) figure of merit ZTRT ∼0.63 as well as remarkable average ZTavg ∼1.17 at 323–773 K were simultaneously realized in the sample (0.1 %Cu-PbTe)0.9(0.8 %Cu-PbS)0.1, both of which are among the highest ones for all reported PbTe-based thermoelectric materials so far.

Multifactor roadmap for designing low‐power‐consumed micro thermoelectric thermostats in a closed‐loop integrated 5G optical module

AbstractAs the core components of fifth‐generation (5G) communication technology, optical modules should be consistently miniaturized in size while improving their level of integration. This inevitably leads to a dramatic spike in power consumption and a consequent increase in heat flow density when operating in a confined space. To ensure a successful start‐up and operation of 5G optical modules, active cooling and precise temperature control via the Peltier effect in confined space is essential yet challenging. In this work, p‐type Bi0.5Sb1.5Te3 and n‐type Bi2Te2.7Se0.3 bulk thermoelectric (TE) materials are used, and a micro thermoelectric thermostat (micro‐TET) (device size, 2 × 9.3 × 1.1 mm3; leg size, 0.4 × 0.4 × 0.5 mm3; number of legs, 44) is successfully integrated into a 5G optical module with Quad Small Form Pluggable 28 interface. As a result, the internal temperature of this kind of optical module is always maintained at 45.7°C and the optical power is up to 7.4 dBm. Furthermore, a multifactor design roadmap is created based on a 3D numerical model using the ANSYS finite element method, taking into account the number of legs (N), leg width (W), leg length (L), filling atmosphere, electric contact resistance (Rec), thermal contact resistance (Rtc), ambient temperature (Ta), and the heat generated by the laser source (QL). It facilitates the integrated fabrication of micro‐TET, and shows the way to enhance packaging and performance under different operating conditions. According to the roadmap, the micro‐TET (2 × 9.3 × 1 mm3, W = 0.3 mm, L = 0.4 mm, N = 68 legs) is fabricated and consumes only 0.89 W in cooling mode (QL = 0.7 W, Ta = 80°C) and 0.36 W in heating mode (Ta = 0°C) to maintain the laser temperature of 50°C. This research will hopefully be applied to other microprocessors for precise temperature control and integrated manufacturing.

Enhanced thermoelectric properties of In-filled Co4Sb12 by dispersion of reduced graphene oxide.

Uniform dispersion of nanosized secondary phases in bulk thermoelectric materials has proven to be an effective strategy to reduce the lattice part of thermal conductivity and improve the thermoelectric efficiency. In this work, reduced graphene oxide (rGO) was uniformly dispersed in the In0.5Co4Sb12 bulk material by ultrasonication. The formation of impurity phases of InSb and CoSb2 in the In-filled Co4Sb12 is inevitable, as observed from XRD and EPMA analyses. The Raman spectra of the nanocomposites showed broad peaks suggesting phonon softening and additional peaks corresponding to rGO. Electron transport was not affected by rGO addition, resulting in little change in the electrical resistivity and Seebeck coefficient. The lattice thermal conductivity of the bulk material was significantly reduced by the addition of a small amount of rGO, primarily attributed to the interface scattering of phonons. Hence, the highest zT of ∼1.53 at 773 K was achieved for the In0.5Co4Sb12/0.25 vol% rGO composite in the temperature range from 723 K to 773 K.

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

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

Unique Semi‐Coherent Nanostructure Advancing Thermoelectrics of N‐Type PbSe

AbstractIntroducing nanostructures into bulk thermoelectric materials is an effective strategy for reducing lattice thermal conductivity (κlat). However, large numbers of hanging bonds emerge on account of lattice mismatch usually at the interfaces between nanostructures and matrix, thereby significantly deteriorating carrier mobility (µH) and seriously limiting thermoelectric performance. Here, utilizing the cointroduction of Gd‐Cu2Te into n‐type PbSe as a paradigm, the zT could be greatly improved. The conduction band flattening caused by Gd doping can successfully compensate for the small Seebeck coefficient in n‐type PbSe, thus achieving extraordinary electrical performance. Further advanced electron microscopy and X‐ray absorption fine structure spectra directly demonstrate that most of the Cu forms nanoscale Cu2Se precipitates, especially a unique semi‐coherent interface between the PbSe matrix and the Cu2Se nanoprecipitate is observed for the first time. Such nanoprecipitate accompanied by the semi‐coherent interface not only ensures higher µH as an excellent charge transfer mediator but also collectively scatters the heat‐carrying phonons over a wide frequency range to obtain the ultralow κlat, resulting in the prominent improvement of the ratio µH/κlat. The present finding represents an important step forward in electron–phonon decoupling via constructing semi‐coherent nanostructure and interface engineering, which should be applicable for other thermoelectric materials.

3D printing of bulk thermoelectric materials: Laser powder bed fusion of N-type silicon germanium

Among Additive Manufacturing (AM) methods, Laser Powder Bed Fusion (L-PBF), also called Selective Laser Melting (SLM), is prevalent to printing complex metal parts in small and medium series. Recent studies in L-PBF processing develops the manufacturing of new materials, including thermoelectric (TE) materials. This study presents manufacturing of an N type Si80Ge20 powder by L-PBF. Silicon germanium alloy is a TE material intended for high temperature applications. It is the first time that this semiconductor material is studied by AM technology. Dense samples of various shapes and sizes were produced, and a first process window was identified. Structural analyses have been performed, highlighting good densification. Unfortunately, mechanical cracking occurs in all samples. TE properties were investigated on as built samples, displaying low values (ZT = 0.11 at 600 °C), due to poor electrical conductivity. Overall, these results show that L-PBF of silicon germanium is possible, which could open up its scope of applications.© 2023 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of Materials science in semiconductor processing.

Thermoelectric Materials and Applications: A Review

Solid-state energy conversion has been established as one of the most promising solutions to address the issues related to conventional energy generation. Thermoelectric materials allow direct energy conversion without moving parts and being deprived of greenhouse gases emission, employing lightweight and quiet devices. Current applications, main thermoelectric material classes, and manufacturing methods are the topics of this work; the discussion revolves around the crucial need for highly performing materials in the mid-temperature range, and around the development of more scalable fabrication technologies. The different manufacturing methods for thermoelectric bulk materials and films are also discussed. Small-scale technologies are generating increasing interest in research; the high potential of aerosol jet printing is highlighted, stressing the many advantages of this technology. A promising approach to scale the production of miniaturized thermoelectric devices that combines high energy ball milling and aerosol jet printing is proposed in the conclusion.

Reduced thermal conductivity and improved ZT of Cu-doped SnS-based bulk thermoelectric materials via compositing SnS nano-fiber strategy

Tin sulfide (SnS), as a low-cost and eco-friendly thermoelectric (TE) material, exhibits relatively poor phonon and charge transport capacities when the long-range ordered layered crystal structure is broken by the grain boundaries of polycrystalline. Nevertheless, doping and compositing are effective strategies for improving the electrical and thermal conductivity. Herein, utilizing the composite powders synthesized via Cu-doped SnS nanosheets grown in-situ on the surface of Cu-doped SnS nanofibers, we design a homogeneous nanofibers composited Cu-doped SnS polycrystalline bulk to boost its TE performance. Comparing with pure phase SnS, Cu doping can effectively increase the carrier concentration and mobility of SnS, realizing an increase of electrical conductivity from 4.11 S·cm−1 to 31.57 S·cm−1. Simultaneously, quite a number of fiber/matrix composite interfaces for phonon scattering are formed due to incorporating of composite nanofibers, which significantly reduce the lattice thermal conductivity of Cu-doped SnS polycrystalline bulk. The results show that thermal conductivity reduced as low as 0.64 W·m−1 K−1 at 324 K when the additive amount of nanofibers was fix at 2 wt%. Ultimately, the power factor and ZT value are markedly enhanced at low and medium temperature ranges (300–500 K) compared with SnS polycrystalline bulk without fiber incorporation. This provides an effective strategy for optimizing the thermoelectric performance of SnS-based polycrystalline.

Two-dimensional Nowotny-Juza NaZnX (X = P, As, Sb) as high-performance thermoelectric materials

Ideal thermoelectric material requires high power factor and low thermal conductivity. Two-dimensional structuring of bulk thermoelectric materials has been reported to be an effective way to achieving excellent thermoelectric performance. In this work, we proposed three two-dimensional Nowotny-Juza materials: NaZnP, NaZnAs, and NaZnSb. Based on the state-of-the-art first-principles calculations, it is revealed that the monolayers possess high stability, low cleavage energy, and excellent electronic transport performance with power factor of 0.65, 0.15 and 0.06 Wm−1K−2 respectively. Meanwhile, the monolayers exhibited ultra-low thermal conductivities of 1.16, 1.38, and 0.31 W⋅m−1⋅K−1 at 300 K, respectively. Such low thermal conductivities are mainly due to the larger scattering rates, resulting from the special cage structure of NaZnX (X = P, As, Sb) and weak ionic bond. As a result, their ZT values reach 2.59, 1.78, and 0.05 (at 300 K), respectively, and increase further to 6.03, 3.23, and 0.13 (at 700 K), respectively. The exceptional performance of NaZnX monolayers sheds light on their immense potential applications in 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).

Knowledge extraction and performance improvement of Bi2Te3-based thermoelectric materials by machine learning

Major advances in materials research often require serendipity and chemical intuition. Traditional trial-and-error-based experimental studies are becoming insufficient for designing novel high-performance materials because of staggering degree of freedom in materials' processing and composition. This work employed machine learning to aid the design of hot-extruded Bi2Te2.85Se0.15 bulk thermoelectric materials based on a small amount of in-house experimental data. Surprisingly, it was found that a combination of processing/composition design strategy with higher extrusion temperatures, more Cu dopants, and deficiency of Te, which is contradictory to experimental experiences, could enhance the materials’ figure of merit (ZT). Experimental validation demonstrated that such a processing/composition solution effectively suppressed formation of point defects, refined microstructure, and promoted evolution of a “fiber texture”, thus simultaneously improving thermoelectric and mechanical properties. The data-driven strategy breaks the rules of thumb in traditional experimental studies and extracts new knowledge to guide the design of high-performance Bi2Te3-based bulk thermoelectric materials.

Emerging homogeneous superlattices in CaTiO3 bulk thermoelectric materials.

The thermal conductivity of superlattices is strongly reduced as compared to that of the parent materials due to phonon-scattering and thermal boundary resistances at the superlattice period interfaces. Herein, homogenous superlattices consisting of homogenous structural CeδCa1-δTiO3 and CaTi1-δCeδO3 alternate layers were obtained through a variable-valence Ce doping, providing multi-quantum well interfaces between the alternate layers due to Ce-substitution at Ca and Ti sites, respectively. This material comprising these homogenous superlattices displayed a significantly reduced lattice thermal conductivity of 1.82 W m-1 K-1 and a record high zT value of 0.405 at 1031 K in CaTiO3-based thermoelectric materials. This strategy of synthesizing homogeneous superlattices provides a cost advantage over heterogeneous superlattices prepared by the molecular beam epitaxy method and paves a route for preparing bulk superlattices with unique thermoelectric properties rooting in the quantum domain limiting effect.

Microstructure change and thermal conductivity reduction in p-type Bi–Sb–Te thermoelectric materials using a metal fatty acid as process control agent

Nanostructuring of bulk thermoelectric materials has attracted interest as a promising approach to improve the thermoelectric performance by lowering the thermal conductivity through phonon scattering. In the present study, a metal fatty acid, which is widely used as a precursor that decomposes to form nanoparticles in colloidal chemistry, was used as a process control agent (PCA) in the ball milling for nanostructured bulk thermoelectric materials. A p-type Bi–Sb–Te alloy was ball-milled with different amounts of iron stearate PCA and sintered into bulk pellets. Microstructural analysis clearly showed that particle growth during sintering was suppressed and nanoinclusions were introduced in matrix, which resulted in an increased interface density and low thermal conductivity. As a result of further analysis of the decrease in thermal conductivity, it was found that the bipolar thermal conductivity was reduced, and thus the ZT was improved at high temperature.

Time-dependent finite-difference model for transient and steady-state analysis of thermoelectric bulk materials

A mathematical model of coupled thermoelectricity is presented to investigate the transient and steady-state behaviour of thermoelectric bulk material. Governing partial differential equations (PDEs) for the coupled thermal and electrical behaviour of the thermoelectric model are discretised using the explicit finite-difference method. Differencing schemes like Upwind and Lax–Wendroff methods are employed to obtain solutions for the first-order hyperbolic PDEs, whereas FTCS (Forward Time, Centred Space) scheme is employed to solve second-order parabolic PDEs. Courant-Friedrichs-Lewy and Von Neumann stability analyses are done to ensure the stability and convergence of the model. The model considers the temperature dependency of thermal conductivity, electrical conductivity, and Seebeck coefficient of the P/N materials separately. and accounts for the Seebeck, Peltier, and Joule-Thomson effects in thermoelectric materials. The new model is practically useful to predict the transient and steady-state behaviours of a thermoelectric device with multiple P-N elements. The results of the presented finite-difference model are proven to agree well with experimental values as well as 3D simulations with ANSYS®.

Inorganic‐Based Printed Thermoelectric Materials and Devices

One of the simplest ways to generate electric power from waste heat is thermoelectric (TE) energy conversion. So far, most of the research on thermoelectrics has focused on inorganic bulk TE materials and their device applications. However, high production costs per power output and limited shape conformity hinder applications of state‐of‐the‐art thermoelectric devices (TEDs). In recent years, printed thermoelectrics has emerged as an exciting pathway for their potential in the production of low‐cost shape‐conformable TEDs. Although several inorganic bulk TE materials with high performance are successfully developed, achieving high performance in inorganic‐based printed TE materials is still a challenge. Nevertheless, significant progress has been made in printed thermoelectrics in recent years. In this review article, it is started with an introduction signifying the importance of printed thermoelectrics followed by a discussion of theoretical concepts of thermoelectricity, from fundamental transport phenomena to device efficiency. Afterward, the general process of inorganic TE ink formulation is summarized, and the current development of the inorganic and hybrid inks with the mention of their TE properties and their influencing factors is elaborated. In the end, TEDs with different architecture and geometries are highlighted by documenting their performance and fabrication techniques.

Additive Manufacturing of Bulk Thermoelectric Architectures: A Review

Additive manufacturing offers several opportunities for thermoelectric energy harvesting systems. This new manufacturing approach enables customized leg geometries, minimized thermal boundary resistances, less retooling, reduced thermoelectric material waste, and strong potential to manipulate microstructure for higher values of figure of merit. Although additive manufacturing has been used to fabricate thin thermoelectric films, there has been comparatively limited demonstrations of additive manufacturing for bulk thermoelectric structures. This review provides insights about the current progress of bulk thermoelectric material and device additive manufacturing. Each additive manufacturing technique used to produce bulk thermoelectric structures is discussed in detail along with future directions and challenges.

Boosting thermoelectric performance of SnTe by selective alloying and band tuning

SnTe-based compounds have attracted increasing attention in recent years as lead-free thermoelectric materials. However, the thermoelectric performance of pristine SnTe is poor because of its overhigh lattice thermal conductivity and low Seebeck coefficient. In this study, we demonstrate that SnSe alloying considerably frustrates heat conduction in SnTe while preserving high charge carrier mobilities, by taking GeTe alloying as a comparative study. Based on these findings, we proceed to substitute Sn atoms by Cd in SnTe–SnSe alloys. This proves to be an efficient way of converging the two valence bands of SnTe, leading to remarkably improved Seebeck coefficients with the highest value approaching 200 μV/K at elevated temperatures. Consequently, the thermoelectric dimensionless figure of merit ZT reaches ∼0.9 at 873 K for the sample with nominal composition of Sn0.96Cd0.04Te0.88Se0.12, which is over 60% improvement over pristine SnTe. Our work highlights the importance of selective alloying and band tuning in engineering bulk thermoelectric materials.

Seeing Structural Mechanisms of Optimized Piezoelectric and Thermoelectric Bulk Materials through Structural Defect Engineering.

Aberration-corrected scanning transmission electron microscopy (AC-STEM) has evolved into the most powerful characterization and manufacturing platform for all materials, especially functional materials with complex structural characteristics that respond dynamically to external fields. It has become possible to directly observe and tune all kinds of defects, including those at the crucial atomic scale. In-depth understanding and technically tailoring structural defects will be of great significance for revealing the structure-performance relation of existing high-property materials, as well as for foreseeing paths to the design of high-performance materials. Insights would be gained from piezoelectrics and thermoelectrics, two representative functional materials. A general strategy is highlighted for optimizing these functional materials’ properties, namely defect engineering at the atomic scale.

Recent Advances in Ultrahigh Thermoelectric Performance Material SnSe

This perspective discusses the surprising discovery and development of SnSe thermoelectrics. Undoped, hole-doped, and electron-doped SnSe single crystals have successively represented an extraordinarily high thermoelectric figure of merit (ZT) ranging from 2.6 to 2.9, revitalizing efforts on finding new high-performance thermoelectric systems. Their unprecedented performance is mainly attributed to ultralow thermal conductivity arising from the uniquely anisotropic and anharmonic crystal chemistry of SnSe. Soon after the publications on SnSe single crystals, substantial debates were raised on their thermoelectric performance, especially on truth in ultralow thermal conductivity. Very recently, polycrystalline SnSe samples were synthesized, exhibiting lower lattice thermal conductivity and higher ZT than the single crystal samples. This work clearly addressed many questions that have arisen on the intrinsic thermal and charge transport properties of SnSe-based materials. It shows a peak ZT of ~3.1 at 783 K and an average ZT of ~2.0 from 400 to 783 K, which are the record-breaking performances of all bulk thermoelectric materials in any form ever reported.