Enhanced Thermoelectric Performance of p-Type Mg3Sb2 for Reliable and Low-Cost all-Mg3Sb2-Based Thermoelectric Low-Grade Heat Recovery

Bi₂Te₃-based devices have long dominated the commercial market for thermoelectric cooling applications, but their narrow operating temperature range and high cost have limited their possible applications for conversion of low-grade heat into electric power. The recently developed n-type Mg₃Sb₂-based compounds exhibit excellent transport properties across a wide temperature range, have low material costs, and are nontoxic, so it would be possible to substitute the conventional Bi₂Te₃ module with a reliable and low-cost all-Mg₃Sb₂-based thermoelectric device if a good p-type Mg₃Sb₂ material can be obtained to match its n-type counterpart. In this study, by comprehensively regulating the carrier concentration, carrier mobility, and lattice thermal conductivity, the thermoelectric performance of p-type Mg₃Sb₂ is significantly improved through Na and Yb doping in Mg_1.8Zn_1.2Sb₂. Moreover, p- and n-type Mg₃Sb₂ are similar in terms of their coefficients of thermal expansion and their good performance stability, thus allowing the construction of a reliable all-Mg₃Sb₂-based unicouple. The decent conversion efficiency (≈5.5% at the hot-side temperature of 573 K), good performance stability, and low cost of this unicouple effectively promote the practical application of Mg₃Sb₂-based thermoelectric generators for low-grade heat recovery.     

Exceptional Performance Driven by Planar Honeycomb Structure in a New High Temperature Thermoelectric Material BaAgAs

The discovery of new, high-performing thermoelectrics is of vital importance to promoting thermal energy conversion efficiency. Herein, a new p-type thermoelectric material BaAgAs with an exceptional figure of merit (zT) surpassing 1.1 at 970 K is present as a promising candidate for high-temperature applications. Verified by comprehensive experimental and theoretical investigations, BaAgAs possesses two intrinsic features in favoring zT: i) low lattice thermal conductivity, ascribed to the heavy element Ba in a loose mono-hexagonal layer, the large mass fluctuation in the Ag-As honeycomb layer, and the alternately interlayer stacking between mono-hexagonal and honeycomb layers; ii) good electrical properties contributed by multiple band transport, due to the small band offset between two valence band extremums and the strong anisotropic band effective mass. With enhanced phonon–phonon scattering via Sb/Bi substitution on the As sites, the lattice thermal conductivity is minimized, which results in significantly enhanced zT values. Additionally, an inspiring prediction via the first-principles calculation suggests that n-type BaAgAs can potentially outperform its p-type counterpart due to its higher conducting band degeneracy. This study will stimulate intense interests in the exploration of compounds with planar honeycomb structures as new high-performance thermoelectric materials.     

BiSbTe‐Based Nanocomposites with High ZT: The Effect of SiC Nanodispersion on Thermoelectric Properties

Thermoelectric materials have potential applications in energy harvesting and electronic cooling devices, and bismuth antimony telluride (BiSbTe) alloys are the state‐of‐the‐art thermoelectric materials that have been widely used for several decades. It is demonstrated that mixing SiC nanoparticles into the BiSbTe matrix effectively enhances its thermoelectric properties; a high dimensionless figure of merit (ZT) value of up to 1.33 at 373 K is obtained in Bi_0.3Sb_1.7Te₃ incorporated with only 0.4 vol% SiC nanoparticles. SiC nanoinclusions possessing coherent interfaces with the Bi_0.3Sb_1.7Te₃ matrix can increase the Seebeck coefficient while increasing the electrical conductivity, in addition to its effect of reducing lattice thermal conductivity by enhancing phonon scattering. Nano‐SiC dispersion further endows the BiSbTe alloys with better mechanical properties, which are favorable for practical applications and device fabrication.     

Dislocations Stabilized by Point Defects Increase Brittleness in PbTe

Dislocations and the residual strain they produce are instrumental for the high thermoelectric figure of merit, zT  ≈ 2, in lead chalcogenides. However, these materials tend to be brittle, barring them from practical green energy and deep space applications. Nonetheless, the bulk of thermoelectrics research focuses on increasing zT without considering mechanical performance. Optimized thermoelectric materials always involve high point defect concentrations for doping and solid solution alloying. Brittle materials show limited plasticity (dislocation motion), yet clear links between crystallographic defects and embrittlement are hitherto unestablished in PbTe. This study identifies connections between dislocations, point defects, and the brittleness (correlated with Vickers hardness) in single crystal and polycrystalline PbTe with various n- and p-type dopants. Speed of sound measurements show a lack of electronic bond stiffening in p-type PbTe, contrary to the previous speculation. Instead, varied routes of point defect–dislocation interaction restrict dislocation motion and drive embrittlement: dopants with low doping efficiency cause high defect concentrations, interstitial n-type dopants (Ag and Cu) create highly strained obstacles to dislocation motion, and highly mobile dopants can distribute inhomogeneously or segregate to dislocations. These results illustrate the consequences of excessive defect engineering and the necessity to consider both mechanical and thermoelectric performance when researching thermoelectric materials for practical applications.     

Effect of geometric dimensions on thermoelectric and mechanical performance for Mg2Si-based thermoelectric unicouple

The generating efficiency of thermoelectric generation (TEG) depends not only on the thermoelectric (TE) performance of TE device, but also on its mechanical performance. And choosing suitable TE materials and geometric dimension can improve the working performance of TE device. Mg₂Si is one of the most promising TE materials in the medium temperature range, and Mg₂Si-based TE devices have broad application prospects. In this paper, a three-dimensional finite model of the Mg₂Si-based TE unicouple used for recovering vehicle exhaust waste heat is constructed for the performance analysis. The TE performance and mechanical performance of the Mg₂Si-based TE unicouple under the influence of different geometric dimensions are investigated, respectively. The curves of the output power, the power conversion efficiency and the thermal stress distribution varying with different geometric dimensions are discussed in detail. The calculated result would be helpful for further understanding of the TE and mechanical properties of the Mg₂Si-based TE unicouple, and it can also provide guidance for further strength check and optimum geometric design of TE unicouples in general.     

Enhanced Thermoelectric Performance and Service Stability of Cu2Se Via Tailoring Chemical Compositions at Multiple Atomic Positions

Liquid‐like thermoelectric (TE) materials have the advantages of ultrahigh performance, low cost, and environment friendly, but their stability is greatly limited by the possible Cu/Ag deposition under a large current and/or temperature gradient. The pratical application based on liquid‐like TE materials requires both a high TE figure of merit (zT) for high energy conversion efficiency and large critical voltage for good stability, but they are very difficult to be simultaneously achieved in one material. In this work, both the zT and critical voltage are simultaneously optimized in Cu₂Se via tailoring chemical compositions at multiple atomic positions, i.e., introducing Cu deficiency at the Cu‐sites to lower Cu ion chemical potential and alloying sulfur at the Se‐sites to reduce carrier concentrations. A maximum zT of 2.0 at 1000 K has been successfully achieved for Cu_1.96Se_0.8S_0.2, about a 30% improvement over that for Cu₂Se. More importantly, Cu_1.96Se_0.8S_0.2 demonstrates a much higher critical voltage than Cu₂Se, yielding a greatly enhanced service stability under the conditions with/without a temperature gradient. An Ni/Mo/Cu_1.96Se_0.8S_0.2 TE unileg is successfully fabricated with a stable power output even after 400 thermal cycles between 473 and 873 K. This study greatly accelerates the real application of Cu₂Se‐based liquid‐like materials.     

Device-Level Optimization of n-Type Mg3(Sb,Bi)2-Based Thermoelectric Modules toward Applications: A Perspective

In recent decades, improvements in thermoelectric material performance have made it more practical to generate electricity from waste heat and to use solid-state devices for refrigeration. However, despite the development of successful strategies to enhance the figure-of-merit zT, optimizing devices for large-scale applications remains challenging. High zT values do not guarantee excellent device performance, and maintaining high zT over a wide temperature range is difficult. Thus, device-level structural optimization is crucial for maximizing overall energy conversion efficiency. Proper interfacial and structure design strategies, including contact layer selection, multi-stage optimization, and size matching for the n- and p-type thermoelectric legs, are necessary for advancing device performance. Additionally, thermal stability issues, device assembly techniques, mechanical properties, and manufacturing costs are crucial considerations for large-scale applications. To achieve actual applications, the thermoelectric community must look beyond simply aiming for high zT values. This article focuses on modules based on n-type Mg₃(Sb, Bi)₂, one of the most promising commercially available thermoelectric materials, and discusses the influence of various parameters on the modules and on the corresponding device-level optimization strategies.     

Flexible Thermoelectric Devices of Ultrahigh Power Factor by Scalable Printing and Interface Engineering

Printing is a versatile method to transform semiconducting nanoparticle inks into functional and flexible devices. In particular, thermoelectric nanoparticles are attractive building blocks to fabricate flexible devices for energy harvesting and cooling applications. However, the performance of printed devices are plagued by poor interfacial connections between nanoparticles and resulting low carrier mobility. While many rigid bulk materials have shown a thermoelectric figure of merit ZT greater than unity, it is an exacting challenge to develop flexible materials with ZT near unity. Here, a scalable screen‐printing method to fabricate high‐performance and flexible thermoelectric devices is reported. A tellurium‐based nanosolder approach is employed to bridge the interfaces between the BiSbTe particles during the postprinting sintering process. The printed BiSbTe flexible films demonstrate an ultrahigh room‐temperature power factor of 3 mW m^?1 K^?2 and ZT about 1, significantly higher than the best reported values for flexible films. A fully printed thermoelectric generator produces a high power density of 18.8 mW cm^?2 achievable with a small temperature gradient of 80 °C. This screen‐printing method, which directly transforms thermoelectric nanoparticles into high‐performance and flexible devices, presents a significant leap to make thermoelectrics a commercially viable technology for a broad range of energy harvesting and cooling applications.     

To enhance the performance of n-type organic thermoelectric materials

Organic thermoelectric(OTE)materials that can convert waste heat to electricity have aroused interests due to their unique advantages over traditional inorganic TE materials,such as light weight,mechanical flexibility,low thermal conductivity,and solution processability[1-4].In general,TE devices require both p-type and n-type semiconductors.The p-type polymers have been extensively studied,showing rapid advances,but there are few efficient n-type TE polymers[5,6].Therefore,the development of high-performance ndoped conjugated polymers is demanded.     

High Performance BiSbTe Alloy for Superior Thermoelectric Cooling

At present, the weak thermoelectric and mechanical performance of zone-melting bismuth telluride alloys cannot support the further improvement of cooling and processing performance of semiconductor refrigeration devices. Here, MnO₂ is added into high-strength Bi_0.4Sb_1.6Te₃ prepared by ball milling method to optimize its thermoelectric transport properties. Via in situ reaction, Sb₂O₃ nano-precipitates are formed in the matrix, which also leads to the surplus of Te element. As results, the donor-like effect is suppressed, thereby increasing carrier concentration and power factor. Besides, volatilization of Te-rich phases during sintering leaves plentiful nanopores, which together with Sb₂O₃ nano-precipitates significantly decrease the lattice thermal conductivity. Eventually, the maximum ZT reaches 1.43 at 75 °C for the Bi_0.4Sb_1.6Te₃+0.01MnO₂ sample. On this basis, a 31-pairs module made of the material and commercial n-type BiTeSe produces large temperature differences (ΔT) of 70.1, 80.8, and 89.4 K at the hot-side temperature (T_h) of 300, 325, and 350 K respectively, which are highly competitive. The maximum coefficient of performance of 8.6 and cooling capacity of 7 W are achieved when T_h is set as 325 K. This excellent progress will promote the further development of bismuth telluride refrigeration modules.     

Hierarchical Architectural Structures Induce High Performance in n-Type GeTe-Based Thermoelectrics

Compatible p- and n-type materials are necessary for high-performance GeTe thermoelectric modules, where the n-type counterparts are in urgent need. Here, it is reported that the p-type GeTe can be tuned into n-type by decreasing the formation energy of Te vacancies via AgBiTe₂ alloying. AgBiTe₂ alloying induces Ag₂Te precipitates and tunes the carrier concentration close to the optimal level, leading to a high-power factor of 6.2 µW cm⁻¹ K⁻² at 423 K. Particularly, the observed hierarchical architectural structures, including phase boundaries, nano-precipitates, and point defects, contribute an ultralow lattice thermal conductivity of 0.39 W m⁻¹ K⁻¹ at 423 K. Correspondingly, an increased ZT of 0.5 at 423 K is observed in n-type (GeTe)_0.45(AgBiTe₂)_0.55. Furthermore, a single-leg module demonstrates a maximum η of 6.6% at the temperature range from 300 to 500 K. This study indicates that AgBiTe₂ alloying can successfully turn GeTe into n-type with simultaneously optimized thermoelectric performance.     

掺锰对不同导电类型硅材料热敏特性的影响

采用电阻率为5 ·cm的p型单晶硅和n型单晶硅,通过高温扩散金属锰的方法,可得到两种类型的热敏材料。测试发现,选择适当的扩散温度和时间,这两种类型的热敏材料一致性都较好。对n型硅掺锰,得到的是小正温度系数热敏材料,其B值在620 K左右;对p型硅掺锰,得到的是负温度系数热敏材料,其B值在4 200 ~4300 K之间。     

Interface Microstructure and Performance of Sb Contacts in Bismuth Telluride-Based Thermoelectric Elements

A thermoelectric joint composed of p-type Bi_0.5Sb_1.5Te₃ (BiSbTe) material and an antimony (Sb) interlayer was fabricated by spark plasma sintering. The reliability of the thermoelectric joints was investigated using electron probe microanalysis for samples with different accelerated isothermal aging time. After aging for 30 days at 300°C in vacuum, the thickness of the diffusion layer at the BiSbTe/Sb interface was about 30 μm, and Sb₂Te₃ was identified to be the major interfacial compound by element analysis. The contact resistivity was 3 × 10^?6 ohm cm² before aging and increased to 8.5 × 10^?6 ohm cm² after aging for 30 days at 300°C, an increase associated with the thickness of the interfacial compound. This contact resistivity is very small compared with that of samples with solder alloys as the interlayer. In addition, we have also investigated the interface behavior of Sb layers integrated with n-type Bi₂Se_0.3Te_2.7 (BiSeTe) material, and obtained similar results as for the p-type semiconductor. The present study suggests that Sb may be useful as a new interlayer material for bismuth telluride-based power generation devices.     

Skutterudites: Thermoelectric Materials for Automotive Applications?

The power output of a thermoelectric generator (TEG) was investigated under engine partial-load operation based on measured exhaust gas temperatures and mass flow rates. Materials with properties required for highend temperature TE couples (>500°C) were evaluated. Various possible material combinations for p- and n-legs of these couples as well as the conflicting targets of high efficiency and low cost as required for automotive mass production are discussed. New skutterudite materials for both p- and n-legs as identified during a joint research project are presented, which can help to overcome this conflict. Efficiencies >10% were achieved with these new materials, which have potentially twofold lower production costs than telluride-based materials due to the price of their elements. Some potential for improvement in efficiency and costs has been identified by developing highly integrated TEG units, specifically designed for automotive applications. These initial results of the material development and the evaluation of different integration concepts will be applied in a subsequent step for the fabrication of a pilot number of TEG modules/units.     

Microwave Sintering of Bi2Te3- and PbTe-Based Alloys: Structure and Thermoelectric Properties

Microwave sintering is well known as an expeditious process in applications involving ceramics and biomaterials. For powders in the nanometer range, rapid microwave heating could reduce material exposure to elevated temperatures, thus preserving nanostructures in the resulting materials. To investigate the potential of this technique for thermoelectric (TE) materials, we have prepared samples of bismuth-telluride- and lead-telluride-based alloys from powders, for both materials, having sizes of partially agglomerated particles distributed from 0.15 μm to 7 μm. Sintering of the cold-pressed powders was carried out in a microwave furnace for 900 s at temperatures in the range of 583 K to 623 K for bismuth telluride and 793 K to 813 K for lead telluride specimens. For optimized sintering times and temperatures, the samples obtained showed relative densities of almost 95%. Scanning electron microscopy shows some residual porosity and a reduction of grain size, up to a factor of 5 for PbTe, compared with optimized hot-extruded specimens. For bismuth telluride samples, the TE performance in the range of 300 K to 460 K is poor, which is attributed to the arbitrary texture obtained from cold pressing of a highly anisotropic alloy prior to its sintering. In contrast, PbTe exhibits isotropic properties, hence deficiency of texturing is not expected to have a negative impact on its TE properties. Harman measurements show a value of ZT = 0.42 at 617 K for PbTe p-type sintered samples, which is comparable to hot-extruded alloys from similar powders. The present work demonstrates that microwave sintering is a promising alternative to other powder consolidation techniques for polycrystalline materials exhibiting isotropic TE properties.     

Coherent Sb/CuTe Core/Shell Nanostructure with Large Strain Contrast Boosting the Thermoelectric Performance of n-Type PbTe

The exploration of n-type PbTe as thermoelectric materials always falls behind its p-type counterpart, mainly due to their quite different electronic band structure. In this work, elemental Sb and Cu₂Te are introduced into an n-type base material (PbTe)₈₁-Sb₂Te₃. The introduction of extra Sb can effectively tune the concentration of electrons; meanwhile, Sb precipitates can also scatter low-energy electrons (negatively contribute to the Seebeck coefficient) thus enhance the overall Seebeck coefficient. The added Cu₂Te is found to always co-precipitate with Sb, forming an interesting Sb/CuTe core/shell structure; moreover, the interface between core/shell precipitates and PbTe matrix simultaneously shows coherent lattice and strong strain contrast, beneficial for electron transport but adverse to phonon transport. Eventually, a peak figure of merit ZT_max  ≈  1.6 @ 823K and simultaneously an average ZT  ≈  1.0 (323–823 K) are realized in the (PbTe)₈₁Sb₂Te₃-0.6Sb-2Cu₂Te sample, representing the state of the art for n-type PbTe-based thermoelectric materials. Moreover, for the first time the three existing forms of Cu atoms in Cu₂Te alloyed PbTe are unambiguously clarified with aberration-corrected scanning transmission electron microscopy (C_s-STEM).     

Thermoelectric Performance Enhancement in BiSbTe Alloy by Microstructure Modulation via Cyclic Spark Plasma Sintering with Liquid Phase

The widespread application of thermoelectric (TE) technology demands high-performance materials, which has stimulated unceasing efforts devoted to the performance enhancement of Bi₂Te₃-based commercialized thermoelectric materials. This study highlights the importance of the synthesis process for high-performance achievement and demonstrates that the enhancement of the thermoelectric performance of (Bi,Sb)₂Te₃ can be achieved by applying cyclic spark plasma sintering to Bi_xSb_2–xTe₃-Te above its eutectic temperature. This facile process results in a unique microstructure characterized by the growth of grains and plentiful nanostructures. The enlarged grains lead to high charge carrier mobility that boosts the power factor. The abundant dislocations originating from the plastic deformation during cyclic liquid phase sintering and the pinning effect by the Sb-rich nano-precipitates result in low lattice thermal conductivity. Therefore, a high ZT value of over 1.46 is achieved, which is 50% higher than conventionally spark-plasma-sintered (Bi,Sb)₂Te₃. The proposed cyclic spark plasma liquid phase sintering process for TE performance enhancement is validated by the representative (Bi,Sb)₂Te₃ thermoelectric alloy and is applicable for other telluride-based materials.     

A Power-Generation Test for Oxide-Based Thermoelectric Modules Using p-Type Ca3Co4O9 and n-Type Ca0.9Nd0.1MnO3 Legs

Metal oxides are considered to be promising thermoelectric (TE) materials, especially for high-temperature power-generation applications, because they have many advantages such as low price, light weight, thermal stability, nontoxicity, and high oxidation resistance. For these reasons, oxide-based TE modules were fabricated using p-type pure Ca₃Co₄O₉ and n-type Ca_0.9Nd_0.1MnO₃ legs for power generation at temperatures in excess of 1000?K. This study involved the use of Ag sheets with a Ag paste as electrode materials and alumina plates as a substrate for the modules. The p-type pure Ca₃Co₄O₉ legs were manufactured by spark plasma sintering, and the n-type Ca_0.9Nd_0.1MnO₃ legs were sintered by a conventional process at atmospheric pressure. From a unicouple, a power density as high as 93.2?mW/cm² under a temperature condition of ??T?=?727?K (T _hot?=?1175?K) was obtained. This high power density is believed to be a result of the modified contact of the electrode (notch process) and the optimized material properties (the SPS process and a dopant effect) along with the high ??T obtained in this study (reduced thermal losses because of good packing of thermal insulation). Areas of concern for future research include the following: (1) the measured open-circuit voltage from the present unicouples was only 94.3% of the theoretical voltage, and (2) the internal resistance value was as high as 490% of the theoretical resistance.     

Three-Stage Thin-Film Superlattice Thermoelectric Multistage Microcoolers with a Δ<Emphasis Type="Italic">T</Emphasis><Subscript>max</Subscript> of 102 K

Thin-film Bi₂Te₃- and Sb₂Te₃-based superlattice (SL) thermoelectric (TE) devices are an enabling technology for high-power and low-temperature applications, which include low-noise amplifier cooling, electronics hot-spot cooling, radio frequency (RF) amplifier thermal management, and direct sensor cooling. Bulk TE devices, which can pump heat loads on the order of 10 W/cm², are not suitable in these applications due to their large size and low heat pumping capacity. Recently, we have demonstrated an external maximum temperature difference, ΔT _max, as high as 58 K in an SL thin-film p–n couple. This state-of-the-art couple exhibited a cold-side minimum temperature, T _cmin, of −30.9°C. We regularly attain ΔT _max values in excess of 53 K, in spite of the many significant electrical and thermal parasitics that are unique to thin-film devices. These measurements do not use any complex thermal management at the heat sink to remove the heat flux from the TE device’s hot side. We describe here multistage SL cooling technologies currently being developed at RTI that can provide useful microcooling cold-side temperatures of 200 K. This effort includes a three-stage module employing independently powered stages which produced a ΔT _max of 101.6 K with a T _cmin of −75°C, as well as a novel two-wire three-stage SL cascade which demonstrated a T _cmin of −46°C and a ΔT _max of nearly 74 K. These RTI modules are only 2.5 mm thick, significantly thinner than a similar commercial three-stage module (5.3 mm thick) that produces a ΔT _max of 96 K. In addition, TE coolers fabricated from these thin-film SL materials perform significantly better than the extrapolated performance of similar thickness bulk alloy materials.     

Synergistic Effect of Multi-Walled Carbon Nanotubes and Ladder-Type Conjugated Polymers on the Performance of N-Type Organic Electrochemical Transistors

Organic electrochemical transistors (OECTs) have the potential to revolutionize the field of organic bioelectronics. To date, most of the reported OECTs include p-type (semi-)conducting polymers as the channel material, while n-type OECTs are yet at an early stage of development, with the best performing electron-transporting materials still suffering from low transconductance, low electron mobility, and slow response time. Here, the high electrical conductivity of multi-walled carbon nanotubes (MWCNTs) and the large volumetric capacitance of the ladder-type π-conjugated redox polymer poly(benzimidazobenzophenanthroline) (BBL) are leveraged to develop n-type OECTs with record-high performance. It is demonstrated that the use of MWCNTs enhances the electron mobility by more than one order of magnitude, yielding fast transistor transient response (down to 15 ms) and high μC* (electron mobility × volumetric capacitance) of about 1 F cm^?1 V^?1 s^?1. This enables the development of complementary inverters with a voltage gain of >16 and a large worst-case noise margin at a supply voltage of <0.6 V, while consuming less than 1 µW of power.