Lanthanide Contraction as a Design Factor for High‐Performance Half‐Heusler Thermoelectric Materials

Forming solid solutions, as an effective strategy to improve thermoelectric performance, has a dilemma that alloy scattering will reduce both the thermal conductivity and carrier mobility. Here, an intuitive way is proposed to decouple the opposite effects, that is, using lanthanide contraction as a design factor to select alloying atoms with large mass fluctuation but small radius difference from the host atoms. Typical half‐Heusler alloys, n‐type (Zr,Hf)NiSn and p‐type (Nb,Ta)FeSb solid solutions, are taken as paradigms to attest the validity of this design strategy, which exhibit greatly suppressed lattice thermal conductivity and maintained carrier mobility. Furthermore, by considering lanthanide contraction, n‐type (Zr,Hf)CoSb‐based alloys with high zT of ≈1.0 are developed. These results highlight the significance of lanthanide contraction as a design factor in enhancing the thermoelectric performance and reveal the practical potential of (Zr,Hf)CoSb‐based half‐Heusler compounds due to the matched n‐type and p‐type thermoelectric performance.     

Promoting SnTe as an Eco‐Friendly Solution for p‐PbTe Thermoelectric via Band Convergence and Interstitial Defects

Compared to commercially available p‐type PbTe thermoelectrics, SnTe has a much bigger band offset between its two valence bands and a much higher lattice thermal conductivity, both of which limit its peak thermoelectric figure of merit, zT of only 0.4. Converging its valence bands or introducing resonant states is found to enhance the electronic properties, while nanostructuring or more recently introducing interstitial defects is found to reduce the lattice thermal conductivity. Even with an integration of some of the strategies above, existing efforts do not enable a peak zT exceeding 1.4 and usually involve Cd or Hg. In this work, a combination of band convergence and interstitial defects, each of which enables a ≈150% increase in the peak zT, successfully accumulates the zT enhancements to be ≈300% (zT up to 1.6) without involving any toxic elements. This opens new possibilities for further improvements and promotes SnTe as an environment‐friendly solution for conventional p‐PbTe thermoelectrics.     

Thermoelectric Performance of IV–VI Compounds with Octahedral‐Like Coordination: A Chemical‐Bonding Perspective

Thermoelectric materials provide a challenge for materials design, since they require optimization of apparently conflicting properties. The resulting complexity has favored trial‐and‐error approaches over the development of simple and predictive design rules. In this work, the thermoelectric performance of IV–VI chalcogenides on the tie line between GeSe and GeTe is investigated. From a combination of optical reflectivity and electrical transport measurements, it is experimentally proved that the outstanding performance of IV–VI compounds with octahedral‐like coordination is due to the anisotropy of the effective mass tensor of the relevant charge carriers. Such an anisotropy enables the simultaneous realization of high Seebeck coefficients, due to a large density‐of‐states effective mass, and high electrical conductivity, caused by a small conductivity effective mass. This behavior is associated to a unique bonding mechanism by means of a tight‐binding model, which relates band structure and bond energies; tuning the latter enables tailoring of the effective mass tensor. The model thus provides atomistic design rules for thermoelectric chalcogenides.     

Carbon‐Nanotube‐Based Thermoelectric Materials and Devices

Conversion of waste heat to voltage has the potential to significantly reduce the carbon footprint of a number of critical energy sectors, such as the transportation and electricity‐generation sectors, and manufacturing processes. Thermal energy is also an abundant low‐flux source that can be harnessed to power portable/wearable electronic devices and critical components in remote off‐grid locations. As such, a number of different inorganic and organic materials are being explored for their potential in thermoelectric‐energy‐harvesting devices. Carbon‐based thermoelectric materials are particularly attractive due to their use of nontoxic, abundant source‐materials, their amenability to high‐throughput solution‐phase fabrication routes, and the high specific energy (i.e., W g^?1) enabled by their low mass. Single‐walled carbon nanotubes (SWCNTs) represent a unique 1D carbon allotrope with structural, electrical, and thermal properties that enable efficient thermoelectric‐energy conversion. Here, the progress made toward understanding the fundamental thermoelectric properties of SWCNTs, nanotube‐based composites, and thermoelectric devices prepared from these materials is reviewed in detail. This progress illuminates the tremendous potential that carbon‐nanotube‐based materials and composites have for producing high‐performance next‐generation devices for thermoelectric‐energy harvesting.     

Ink Processing for Thermoelectric Materials and Power‐Generating Devices

The growing concern over the depletion of hydrocarbon resources, and the adverse environmental effects associated with their use, has increased the demand for renewable energy sources. Thermoelectric (TE) power generation from waste heat has emerged as a renewable energy source that does not generate any pollutants. Recently, ink‐based processing for the preparation of TE materials has attracted tremendous attention because of the simplicity in design of power generators and the possibility of cost‐effective manufacturing. In this progress report, recent advances in the development of TE inks, processing techniques, and ink‐fabricated devices are reviewed. A summary of typical formulations of TE materials as inks is included, as well as a discussion on various ink‐based fabrication methods, with several examples of newly designed devices fabricated using these techniques. Finally, the prospects of this field with respect to the industrialization of TE power generation technology are presented.     

热电材料中的高熵结构设计

高熵合金的设计思想在诸多材料领域都有广泛的应用,本研究从高熵结构对热电输运性质的影响出发,着重讨论热电材料对高熵结构设计的一些要求。以CuInTe₂为实例,提出了热电材料的高熵结构应当尽量减小晶格畸变,尽量选择在不影响费米面结构的格点位进行高熵掺杂。依据这些准则,设计的高熵化合物Cu_0.8Ag_0.2Zn_0.1Ga_0.4Ge_0.1In_0.4Te₂的室温热导率降到了2.1 W·m^-1·K^-1,比基体材料降低70%,最高ZT值达到1.02,较基体提升90%。在二元化合物SnTe中进行了AgSbSe₂固溶,其室温热导率降到1.3 W·m^-1·K^-1,比基体降低80%以上。本研究表明,遵循一定准则设计的高熵结构对于提升热电材料性能具有重要的意义。     

Extraordinary Thermoelectric Performance Realized in n‐Type PbTe through Multiphase Nanostructure Engineering

Lead telluride has long been realized as an ideal p‐type thermoelectric material at an intermediate temperature range; however, its commercial applications are largely restricted by its n‐type counterpart that exhibits relatively inferior thermoelectric performance. This major limitation is largely solved here, where it is reported that a record‐high ZT value of ≈1.83 can be achieved at 773 K in n‐type PbTe‐4%InSb composites. This significant enhancement in thermoelectric performance is attributed to the incorporation of InSb into the PbTe matrix resulting in multiphase nanostructures that can simultaneously modulate the electrical and thermal transport. On one hand, the multiphase energy barriers between nanophases and matrix can boost the power factor in the entire temperature range via significant enhancement of the Seebeck coefficient and moderately reducing the carrier mobility. On the other hand, the strengthened interface scattering at the intensive phase boundaries yields an extremely low lattice thermal conductivity. This strategy of constructing multiphase nanostructures can also be highly applicable in enhancing the performance of other state‐of‐the‐art thermoelectric systems.     

熵调控抑制ZrNiSn基half-Heusler热电材料的晶格热导率

ZrNiSn基half-Heusler热电材料具有较高的热导率, 限制了其热电性能进一步提高。为了降低晶格热导率, 本研究采用磁悬浮熔炼和放电等离子烧结的方法制备ZrNiSn和Zr_0.5Hf_0.5Ni_1-xPt_xSn (x=0, 0.1, 0.15, 0.2, 0.25, 0.3)高熵half-Heusler热电合金。在Zr位进行Hf原子替代, Ni位进行Pt原子替代以调控该合金的构型熵, 并研究构型熵对热电性能的影响。本工作优化了Zr_0.5Hf_0.5Ni_0.85Pt_0.15Sn在673 K的最小晶格热导率和双极扩散热导率之和为2.1 W·m^-1·K^-1, 与ZrNiSn相比降低了约58%。这一发现为降低ZrNiSn基合金的晶格热导率提供了一种有效的策略, 有助于改善材料的热电性能。     

Mechanochemical‐Assisted Synthesis of High‐Entropy Metal Nitride via a Soft Urea Strategy

Crystalline high‐entropy ceramics (CHC), a new class of solids that contain five or more elemental species, have attracted increasing interest because of their unique structure and potential applications. Up to now, only a couple of CHCs (e.g., high‐entropy metal oxides and diborides) have been successfully synthesized. Here, a new strategy for preparing high‐entropy metal nitride (HEMN‐1) is proposed via a soft urea method assisted by mechanochemical synthesis. The as‐prepared HEMN‐1 possesses five highly dispersed metal components, including V, Cr, Nb, Mo, Zr, and simultaneously exhibits an interesting cubic crystal structure of metal nitrides. By taking advantage of these unique features, HEMN‐1 can function as a promising candidate for supercapacitor applications. A specific capacitance of 78 F g^?1 is achieved at a scan rate of 100 mV s^?1 in 1 m KOH. In addition, such a facile synthetic strategy can be further extended to the fabrication of other types of HEMNs, paving the way for the synthesis of HEMNs with attractive properties for task‐specific applications.     

Phase‐Transformation Ductilization of Brittle High‐Entropy Alloys via Metastability Engineering

High‐entropy alloys (HEAs) in which interesting physical, chemical, and structural properties are being continuously revealed have recently attracted extensive attention. Body‐centered cubic (bcc) HEAs, particularly those based on refractory elements are promising for high‐temperature application but generally fail by early cracking with limited plasticity at room temperature, which limits their malleability and widespread uses. Here, the “metastability‐engineering” strategy is exploited in brittle bcc HEAs via tailoring the stability of the constituent phases, and transformation‐induced ductility and work‐hardening capability are successfully achieved. This not only sheds new insights on the development of HEAs with excellent combination of strength and ductility, but also has great implications on overcoming the long‐standing strength–ductility tradeoff of metallic materials in general.     

Direct Observation of Inherent Atomic‐Scale Defect Disorders responsible for High‐Performance Ti1−xHfxNiSn1−ySby Half‐Heusler Thermoelectric Alloys

Structural defects often dominate the electronic‐ and thermal‐transport properties of thermoelectric (TE) materials and are thus a central ingredient for improving their performance. However, understanding the relationship between TE performance and the disordered atomic defects that are generally inherent in nanostructured alloys remains a challenge. Herein, the use of scanning transmission electron microscopy to visualize atomic defects directly is described and disordered atomic‐scale defects are demonstrated to be responsible for the enhancement of TE performance in nanostructured Ti_1?x Hf_x NiSn_1?y Sb_y half‐Heusler alloys. The disordered defects at all atomic sites induce a local composition fluctuation, effectively scattering phonons and improving the power factor. It is observed that the Ni interstitial and Ti,Hf/Sn antisite defects are collectively formed, leading to significant atomic disorder that causes the additional reduction of lattice thermal conductivity. The Ti_1?x Hf_x NiSn_1?y Sb_y alloys containing inherent atomic‐scale defect disorders are produced in one hour by a newly developed process of temperature‐regulated rapid solidification followed by sintering. The collective atomic‐scale defect disorder improves the zT to 1.09 ± 0.12 at 800 K for the Ti_0.5Hf_0.5NiSn_0.98Sb_0.02 alloy. These results provide a promising avenue for improving the TE performance of state‐of‐the‐art materials.