Achieving Ultralow Lattice Thermal Conductivity and High Thermoelectric Performance in GeTe Alloys via Introducing Cu2Te Nanocrystals and Resonant Level Doping

The binary compound of GeTe emerging as a potential medium-temperature thermoelectric material has drawn a great deal of attention. Here, we achieve ultralow lattice thermal conductivity and high thermoelectric performance in In and a heavy content of Cu codoped GeTe thermoelectrics. In dopants improve the density of state near the surface of Femi of GeTe by introducing resonant levels, producing a sharp increase of the Seebeck coefficient. In and Cu codoping not only optimizes carrier concentration but also substantially increases carrier mobility to a high value of 87 cm² V^-1 s^-1 due to the diminution of Ge vacancies. The enhanced Seebeck coefficient coupled with dramatically enhanced carrier mobility results in significant enhancement of PF in Ge_1.04-x-yIn_xCu_yTe series. Moreover, we introduce Cu₂Te nanocrystals' secondary phase into GeTe by alloying a heavy content of Cu. Cu₂Te nanocrystals and a high density of dislocations cause strong phonon scattering, significantly diminishing lattice thermal conductivity. The lattice thermal conductivity reduced as low as 0.31 W m^-1 K^-1 at 823 K, which is not only lower than the amorphous limit of GeTe but also competitive with those of thermoelectric materials with strong lattice anharmonicity or complex crystal structures. Consequently, a high ZT of 2.0 was achieved for Ge_0.9In_0.015Cu_0.125Te by decoupling electron and phonon transport of GeTe. This work highlights the importance of phonon engineering in advancing high-performance GeTe thermoelectrics.

Thermo-active Smart Electrospun Nanofibers

The recent burst of research on smart materials is a clear evidence of the growing interest of the scientific community, industry, and society in the field. The exploitation of the great potential of stimuli-responsive materials for sensing, actuation, logic, and control applications is favored and supported by new manufacturing technologies, such as electrospinning, that allows to endow smart materials with micro- and nano-structuration, thus opening up additional and unprecedented prospects. In this wide and lively scenario, this article systematically reviews the current advances in the development of thermo-active electrospun fibers and textiles, sorting them, according to their response to the thermal stimulus. Hence, several platforms including thermo-responsive systems, shape memory polymers, thermo-optically responsive systems, phase change materials, thermoelectric materials, and pyroelectric materials, have been described and critically discussed. The difference in active species and outputs of the aforementioned categories has been highlighted, evidencing the transversal nature of temperature stimulus. Moreover, the potential of novel thermo-active materials has been pointed out, revealing how their development could take to utmost interesting achievements. This article is protected by copyright. All rights reserved.

Lattice Thermal Conductivity: An Accelerated Discovery Guided by Machine Learning

In the present work, we used machine learning (ML) techniques to build a crystal-based model that can predict the lattice thermal conductivity (LTC) of crystalline materials. To achieve this, first, LTCs of 119 compounds at various temperatures (100-1000 K) were obtained based on density functional theory (DFT) and phonon calculations, and then, these data were employed in the next learning process to build a predictive model using various ML algorithms. The ML results showed that the model built based on the random forest (RF) algorithm with an R² score of 0.957 was the most accurate compared with the models built using other algorithms. Additionally, the accuracy of this model was validated using new cases of four compounds, which was not seen for the model before, where a good matching between calculated and predicted LTCs of the new compounds was found. To find candidates with ultralow LTCs (<1 W m^-1 K^-1) at room temperature, the model was used to screen compounds (32116) in the Inorganic Crystal Structure Database. From the screened compounds, Cs₂SnI₆ and SrS were selected to validate the ML prediction using the counterpart theoretical calculations (DFT and phonon), and it was found that the outcome behaviors by both methods (ML prediction and DFT/phonon calculations) are fairly consistent. Considering the type of employed feature, the prime novelty in this work is that the built model can credibly predict the LTC-temperature behaviors of new compounds that are constructed based on prototype structures and chemical compositions, without the use of any DFT-relaxed structure parameters. Accordingly, using the periodic table, prototype structures, and the RF-based model, the LTC-temperature behavior of a huge number of compounds can be predicated.

Extremely Fast and Cheap Densification of Cu2S by Induction Melting Method

The aim of this work was to obtain dense Cu₂S superionic thermoelectric materials, homogeneous in terms of phase and chemical composition, using a very fast and cheap induction-melting technique. The chemical composition was investigated via scanning electron microscopy (SEM) combined with an energy-dispersive spectroscopy (EDS) method, and the phase composition was established by X-ray diffraction (XRD). The thermoelectric figure of merit ZT was determined on the basis of thermoelectric transport properties, i.e., Seebeck coefficient, electrical and thermal conductivity in the temperature range of 300–923 K. The obtained values of the ZT parameter are comparable with those obtained using the induction hot pressing (IHP) technique and about 30–45% higher in the temperature range of 773–923 K in comparison with Cu₂S samples densified with the spark plasma sintering (SPS) technique.

Fe-Al-Si Thermoelectric (FAST) Materials and Modules: Diffusion Couple and Machine-Learning-Assisted Materials Development

To lower the introduction and maintenance costs of autonomous power supplies for driving Internet-of-things (IoT) devices, we have developed low-cost Fe-Al-Si-based thermoelectric (FAST) materials and power generation modules. Our development approach combines computational science, experiments, mapping measurements, and machine learning (ML). FAST materials have a good balance of mechanical properties and excellent chemical stability, superior to that of conventional Bi-Te-based materials. However, it remains challenging to enhance the power factor (PF) and lower the thermal conductivity of FAST materials to develop reliable power generation devices. This forum paper describes the current status of materials development based on experiments and ML with limited data, together with power generation module fabrication related to FAST materials with a view to commercialization. Combining bulk combinatorial methods with diffusion couple and mapping measurements could accelerate the search to enhance PF for FAST materials. We report that ML prediction is a powerful tool for finding unexpected off-stoichiometric compositions of the Fe-Al-Si system and dopant concentrations of a fourth element to enhance the PF, i.e., Co substitution for Fe atoms in FAST materials.

Enhancing Thermoelectrics for Small-Bandwidth n-Type PbTe-MnTe Alloys via Balancing Compromise

Small-bandwidth n-type PbTe-MnTe alloys effectively modify the valley shape, while it also inevitably aggravates the deterioration of carrier mobility as nonpolar phonons dominate the scattering. It is found that a trace amount of Cu doping can alleviate the compromises among thermoelectric parameters, thereby significantly optimizing the electrical-transport performance near room temperature of n-type PbTe-MnTe alloys. The single-Kane model reveals that the physical origin of performance improvement lies in the carrier mobility enhancement and self-optimization of carrier concentration. The Debye-Callaway model further quantifies the contribution of copper defect scattering to the lattice thermal conductivity. Notably, the high thermoelectric quality factor obtained in this work rationalizes their superior properties and reveals immense potential for achieving higher zT. Herein, an extremely high zT of ∼0.52 at room temperature and a maximum zTmax of ∼1.2 at 823 K are achieved in 0.3% Cu-intercalated n-type PbTe-MnTe. The mechanism in balancing compromise elaborated in principle contributes to an improvement of thermoelectric properties of the n-type PbTe alloys in a broad temperature range.

Ultralow Thermal Conductivity, Multiband Electronic Structure and High Thermoelectric Figure of Merit in TlCuSe

The entanglement of lattice thermal conductivity, electrical conductivity, and Seebeck coefficient complicates the process of optimizing thermoelectric performance in most thermoelectric materials. Semiconductors with ultralow lattice thermal conductivities and high power factors at the same time are scarce but fundamentally interesting and practically important for energy conversion. Herein, an intrinsic p-type semiconductor TlCuSe that has an intrinsically ultralow thermal conductivity (0.25 W m^-1 K^-1 ), a high power factor (11.6 µW cm^-1 K^-2 ), and a high figure of merit, ZT (1.9) at 643 K is described. The weak chemical bonds, originating from the filled antibonding orbitals p-d* within the edge-sharing CuSe₄ tetrahedra and long TlSe bonds in the PbClF-type structure, in conjunction with the large atomic mass of Tl lead to an ultralow sound velocity. Strong anharmonicity, coming from Tl⁺ lone-pair electrons, boosts phonon-phonon scattering rates and further suppresses lattice thermal conductivity. The multiband character of the valence band structure contributing to power factor enhancement benefits from the lone-pair electrons of Tl⁺ as well, which modify the orbital character of the valence bands, and pushes the valence band maximum off the Γ-point, increasing the band degeneracy. The results provide new insight on the rational design of thermoelectric materials.

Weighted Mobility Ratio Engineering for High-Performance Bi-Te-Based Thermoelectric Materials via Suppression of Minority Carrier Transport

Thermoelectrics, which can generate electricity from a temperature difference, or vice versa, is a key technology for solid-state cooling and energy harvesting; however, its applications are constrained owing to low efficiency. Since the conversion efficiency of thermoelectric devices is directly obtained via a figure of merit of materials, zT, which is related to the electronic and thermal transport characteristics, the aim here is to elucidate physical parameters that should be considered to understand transport phenomena in semiconducting materials. It is found that the weighted mobility ratio of the majority and minority carrier bands is an important parameter that determines zT. For nanograined Bi-Sb-Te alloy, the unremarked role of this parameter on temperature-dependent electronic transport properties is demonstrated. This analysis shows that the control of the weighted mobility ratio is a promising way to enhance zT of narrow bandgap thermoelectric materials.

Recent Progress in Multiphase Thermoelectric Materials

Thermoelectric materials, which directly convert thermal energy to electricity and vice versa, are considered a viable source of renewable energy. However, the enhancement of conversion efficiency in these materials is very challenging. Recently, multiphase thermoelectric materials have presented themselves as the most promising materials to achieve higher thermoelectric efficiencies than single-phase compounds. These materials provide higher degrees of freedom to design new compounds and adopt new approaches to enhance the electronic transport properties of thermoelectric materials. Here, we have summarised the current developments in multiphase thermoelectric materials, exploiting the beneficial effects of secondary phases, and reviewed the principal mechanisms explaining the enhanced conversion efficiency in these materials. This includes energy filtering, modulation doping, phonon scattering, and magnetic effects. This work assists researchers to design new high-performance thermoelectric materials by providing common concepts.

Boron-Mediated Grain Boundary Engineering Enables Simultaneous Improvement of Thermoelectric and Mechanical Properties in N-Type Bi2 Te3

Powder metallurgy introduces small structures of high-density grain boundaries into Bi₂ Te₃ -based alloys, which promises to enhance their mechanical and thermoelectric performance. However, due to the strong donor-like effect induced by the increased surface, Te vacancies form in the powder-metallurgy process. Hence, the as-sintered n-type Bi₂ Te₃ -based alloys show a lower figure of merit (ZT) value than their p-type counterparts and the commercial zone-melted (ZM) ingots. Here, boron is added to one-step-sintered n-type Bi₂ Te₃ -based alloys to inhibit grain growth and to suppress the donor-like effect, simultaneously improving the mechanical and thermoelectric (TE) performance. Due to the alleviated donor-like effect and the carrier mobility maintained in our n-type Bi₂ Te_2.7 Se_0.3 alloys upon the addition of boron, the maximum and average ZT values within 298-473 K can be enhanced to 1.03 and 0.91, respectively, which are even slightly higher than that of n-type ZM ingots. Moreover, the addition of boron greatly improves the mechanical strength such as Vickers hardness and compressive strength due to the synergetic effects of Hall-Petch grain-boundary strengthening and boron dispersion strengthening. This facile and cost-effective grain boundary engineering by adding boron facilitates the practical application of Bi₂ Te₃ -based alloys and can also be popularized in other thermoelectric materials.

Thermoelectric Cu12 Sb4 S13 -Based Synthetic Minerals with a Sublimation-Derived Porous Network

Pores in a solid can effectively reduce thermal conduction, but they are not favored in thermoelectric materials due to simultaneous deterioration of electrical conductivity. Conceivably, creating a porous structure may endow thermoelectric performance enhancement provided that overwhelming reduction of electrical conductivity can be suppressed. This work demonstrates such an example, in which a porous structure is formed leading to a significant enhancement in the thermoelectric figure of merit (zT). By a unique BiI₃ sublimation technique, pore networks can be introduced into tetrahedrite Cu₁₂ Sb₄ S₁₃ -based materials, accompanied by changes in their hierarchical structures. The addition of a small quantity of BiI₃ (0.7 vol%) results in a ≈72% reduction in the lattice thermal conductivity, whereas the electrical conductivity is improved due to unexpected enhanced carrier mobility. As a result, an enhanced zT of 1.15 at 723 K in porous tetrahedrite and a high conversion efficiency of 6% at ΔT = 419 K in a fabricated segmented single-leg based on this porous material are achieved. This work offers an effective way to concurrently modulate the electrical and thermal properties during the synthesis of high-performance porous thermoelectric materials.

Wearable Thermoelectric Materials and Devices for Self-Powered Electronic Systems

The emergence of artificial intelligence and the Internet of Things has led to a growing demand for wearable and maintenance-free power sources. The continual push toward lower operating voltages and power consumption in modern integrated circuits has made the development of devices powered by body heat finally feasible. In this context, thermoelectric (TE) materials have emerged as promising candidates for the effective conversion of body heat into electricity to power wearable devices without being limited by environmental conditions. Driven by rapid advances in processing technology and the performance of TE materials over the past two decades, wearable thermoelectric generators (WTEGs) have gradually become more flexible and stretchable so that they can be used on complex and dynamic surfaces. In this review, the functional materials, processing techniques, and strategies for the device design of different types of WTEGs are comprehensively covered. Wearable self-powered systems based on WTEGs are summarized, including multi-function TE modules, hybrid energy harvesting, and all-in-one energy devices. Challenges in organic TE materials, interfacial engineering, and assessments of device performance are discussed, and suggestions for future developments in the area are provided. This review will promote the rapid implementation of wearable TE materials and devices in self-powered electronic systems.

Recent Progress in Designing Thermoelectric Metal-Organic Frameworks

Thermoelectrics that enable direct heat-electricity conversion possess unique advantages for green and renewable energy revolution and have received rapidly growing attention in the past decade. Among various thermoelectric materials, metal-organic frameworks (MOFs) with intrinsic high porosity and tunable physical/chemical properties are emerging as a promising class of materials that have been demonstrated to exhibit many unique merits for thermoelectric applications. Their structural topologies and thermoelectric properties can be facilely regulated by precisely selecting and arranging metal centers and organic ligands. Besides, a large variety of guest molecules can be incorporated within their pores, giving rise to novel avenues of raising energy-conversion efficiency. This review focuses on the recent advances in designing conductive MOFs and MOF-based composites for thermoelectric applications. It first introduces the fundamental thermoelectric parameters and the underlying regulation mechanisms specifically effective for MOFs, then summarizes the related studies conducted in recent years, where the structural design strategies of tuning thermoelectric properties are demonstrated and discussed. In the final part, conclusions and perspectives with the envision of an outlook for this promising area are presented.

Some Thermoelectric Phenomena in Copper Chalcogenides Replaced by Lithium and Sodium Alkaline Metals

This review presents thermoelectric phenomena in copper chalcogenides substituted with sodium and lithium alkali metals. The results for other modern thermoelectric materials are presented for comparison. The results of the study of the crystal structure and phase transitions in the ternary systems Na-Cu-S and Li-Cu-S are presented. The main synthesis methods of nanocrystalline copper chalcogenides and its alloys are presented, as well as electrical, thermodynamic, thermal, and thermoelectric properties and practical application. The features of mixed electron–ionic conductors are discussed. In particular, in semiconductor superionic copper chalcogenides, the presence of a “liquid-like phase” inside a “solid” lattice interferes with the normal propagation of phonons; therefore, superionic copper chalcogenides have low lattice thermal conductivity, and this is a favorable factor for the formation of high thermoelectric efficiency in them.

Realize High Thermoelectric Properties in n-Type Bi2Te2.7Se0.3/Y2O3 Nanocomposites by Constructing Heterointerfaces

Due to the excellent thermoelectric performance, bismuth telluride (Bi₂Te₃) compounds are highly promising for the thermoelectric conversion in the room temperature range. However, the inferior thermoelectric performance of the n-type leg severely restricts the applications of Bi₂Te₃-based thermoelectric couples. Herein, n-type Bi₂Te_2.7Se_0.3 (BTS)-based thermoelectric materials incorporated with nanosized Y₂O₃ (0.5-3 wt %) are prepared and their thermoelectric properties are systematically studied. The dramatically improved thermoelectric performance is ascribed to the realization of a multiscale feature of Y₂O₃ nanoparticle (NP)-induced interfacial decorations distributed along grain boundaries, which creates massive BTS/Y₂O₃ interfaces for the manipulation of carrier and phonon transport properties. The geometric phase analysis is employed to further confirm the condition of local strain in the BTS composite incorporated with Y₂O₃ NPs. Due to the presence of heterointerfaces and high density of dislocations in BTS matrices, the minimum lattice thermal conductivity (κ_l) of the nanocomposites (NCs) is dramatically suppressed from 0.76 to 0.37 W m^-1 K^-1. With the incorporation of 3 wt % Y₂O₃ NPs, the Vickers hardness of the BTS/Y₂O₃ NC is increased by about 32%. Overall, the BTS + 1.5 wt % Y₂O₃ NC maintains excellent thermoelectric properties (ZT_ave = 1.1) in the whole operative temperature range (300-500 K). The present strategy of implementing high-density heterogeneous interfaces by Y₂O₃ NP addition offers an applicable pathway for fabricating high-performance thermoelectric materials with both optimized thermoelectric properties and mechanical properties.

Enhancing Near-Room-Temperature GeTe Thermoelectrics through In/Pb Co-doping

The traditional thermoelectric material GeTe has drawn much attention recently because of the reported high thermoelectric performance of the rhombohedral phase in low-temperature ranges, where the split L and Σ band can be reconverged to have a small energy offset and thus high density of state effective mass according to the rhombohedral angle. In addition, In doping in GeTe is also reported to enhance the density of effective mass and therefore increase the Seebeck coefficient because of the induced resonant levels. In this work, In and Pb are doped in GeTe, and In doping leads to an increase in the rhombohedral angle and thus enhanced density of state effective mass in addition to the resonant effect. However, the improved Seebeck coefficient result from In doping is compensated for by a sharp reduction of Hall mobility, leading to no significant enhancement of electronic performance in the rhombohedral phase. By further Pb/Ge doping in the matrix Ge_0.95In_0.05Te for the optimization of carrier concentration and reduction of lattice thermal conductivity (as low as 0.7 W/mK), a zT as high as ∼1.2 at 550 K and average zT of ∼0.75 between 300 and 550 K are realized in this work, demonstrating GeTe as a promising candidate for near-room-temperature application.

Effects of Doping Ni on the Microstructures and Thermoelectric Properties of Co-Excessive NbCoSn Half-Heusler Compounds

The half-Heusler (HH) compound NbCoSn with 18 valence electrons is a promising thermoelectric (TE) material due to its appropriate electrical properties as well as its suitable thermal and chemical stability. Nowadays, doping/substitution and tailoring of microstructures are common experimental approaches to enhance the TE performance of HH compounds. However, detailed theoretical insights into the effects of doping on the microstructures and TE properties are still missing. In this work, the microstructure of NbCoSn was tailored through precipitating the full-Heusler phases in the matrix by changing the nominal ratio of Co and Ni on the Co sites, focusing on the resulting TE properties. Further, first-principles calculations were employed to understand the relationship between the microstructure and the TE properties from the thermodynamic point of view. Detailed analysis of the electronic structure reveals that the presence of excess Co/Ni contributes to the increasing carrier concentration. Through an increase in the electrical conductivity and a reduction in the thermal conductivity, the TE performance is improved. Therefore, the present work offers a new pathway and insights to enhance the TE properties by modifying the microstructure of HH compounds via tailoring the chemical compositions.

Point Defect Engineering: Co-Doping Synergy Realizing Superior Performance in n-Type Bi2 Te3 Thermoelectric Materials

Bi₂ Te₃ has attracted great attention because of its excellent thermoelectric (TE) performance around room temperature. However, the TE property of the n-type Bi₂ Te₃ is still relatively low compared to the p-type counterpart, which seriously hinders its commercial application with a combination of the n-type and p-type materials. Herein, an effective process of Cl and W co-doping is employed into the n-type Bi₂ Te₃ materials to enhance its TE properties. The Bi_1.996 W_0.004 Te_2.476 Cl_0.024 Se_0.5 sample achieves a peak and average ZT over 1.3 and 1.2, respectively, at temperature range of 300-575 K. A 24-leg TE module of this n-type material and a home-made p-type Bi₂ Te₃ sample can produce a high efficiency over 6% at a temperature gradient of 235 K, which possesses a 71% improvement compared with a commercial Bi₂ Te₃ module. This high performance is ascribed to the effect of the Cl and W doping. This co-doping not only significantly increases the Grüneisen parameter but also successfully induces interstitial atoms in the van der Waals gap, which lead to a low lattice thermal conductivity (κ_l ) of 0.31W m^-1 K^-1 and a boosted charge transport. This finding represents an important step to promote the development of the n-type Bi₂ Te₃ materials.

Thermoelectric Properties of Cu2Se Synthesized by Hydrothermal Method and Densified by SPS Technique

The aim of the work was to obtain copper (I) selenide Cu₂Se material with excellent thermoelectric properties, synthesized using the hydrothermal method and densified by the spark plasma sintering (SPS) method. Chemical and phase composition studies were carried out by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) methods. Measurements of thermoelectric transport properties, i.e., electrical conductivity, the Seebeck coefficient, and thermal conductivity in the temperature range from 300 to 965 K were carried out. Based on these results, the temperature dependence of the thermoelectric figure of merit ZT as a function of temperature was determined. The obtained, very high ZT parameter (ZT~1.75, T = 965 K) is one of the highest obtained so far for undoped Cu₂Se.

Strong interlayer coupling in two-dimensional PbSe with high thermoelectric performance

It was generally believed that weak van der Waals interactions exist between neighboring layers in the two-dimensional group-IV chalcogenides. Using PbSe as a prototypal example, we find additional strong coupling between the Pb-Pb layers, as evidenced by detailed analysis of the differential charge density plot. The coupling is covalent-like and can be fine-tuned to obviously reduce the phonon thermal conductivity but slightly change the electronic transport of PbSe layer. As a consequence, a maximumZTvalue of 2.5 can be realized at 900 K for thep-type system. Our work also offers an effective and feasible design strategy to enhance the thermoelectric performance of similar layered structures.

Effect of Powder Heat Treatment on Chemical Composition and Thermoelectric Properties of Bismuth Antimony Telluride Alloys Fabricated by Combining Water Atomization and Spark Plasma Sintering

In this work, Bi₀.₅Sb₁.₅Te₃ materials were produced by an economically viable and time efficient water atomization process. The powder samples were heat treated at different temperatures (673 K, 723 K, 743 K, 773 K, 803 K, and 823 K) followed by spark plasma sintering (SPS). It was found that the Te evaporated slightly at 723 K and 743 K and became dominated at 773 K, 803 K, and 823 K, which severely influences the thermoelectric properties. The electrical conductivity was significantly improved for over 803 K heat treated samples due to the remarkable improvement in hole concentration. The power factor values for the 803 K and 823 K samples were significantly larger at T > 350 K compared to other samples. Consequently, the peak ZT of 0.92 at 350 K was obtained for the 803 K sample, which could be useful in commercial thermoelectric power generation.

Design of thermoelectric materials with high electrical conductivity, high Seebeck coefficient, and low thermal conductivity

Thermoelectric materials with a high Seebeck coefficient, high electrical conductivity, and low thermal conductivity are required to directly and efficiently convert unused heat into electricity. In this study, we construct models predicting the Seebeck coefficient, electrical conductivity, and thermal conductivity using existing material databases. In addition to the ratios of atoms in the crystals and temperature at which the materials are used, the values from the X-ray diffraction (XRD) spectra were used as inputs to represent the crystal structure of the materials. It was confirmed that the constructed models could predict the properties with high accuracy using the X-ray diffraction values. Additionally, using the constructed models, we succeeded in proposing promising new candidate materials with high Seebeck coefficients, high electric conductivities, and low thermal conductivities.

Structural Evolution of High-Performance Mn-Alloyed Thermoelectric Materials: A Case Study of SnTe

Mn alloying in thermoelectrics is a long-standing strategy for enhancing their figure-of-merit through optimizing electronic transport properties by band convergence, valley perturbation, or spin-orbital coupling. By contrast, mechanisms by which Mn contributes to suppressing thermal transports, namely thermal conductivity, is still ambiguous. A few precedent studies indicate that Mn introduces a series of hierarchical defects from the nano- to meso-scale, leading to effective phonon scattering scoping a wide frequency spectrum. Due to insufficient insights at the atomic level, the theory remains as phenomenological and cannot be used to quantitatively predict the thermal conductivity of Mn-alloyed thermoelectrics. Herein, by choosing the SnTe as a case study, aberration-corrected transmission electron microscopy (TEM)/scanning transmission electron microscopy (STEM) to characterize the lattice complexity of Sn_{1.02- x} Mn_x Te is employed. Mn as a "dynamic" dopant that plays an important role in SnTe with respect to different alloying levels or post treatments is revealed. The results indicate that Mn precipitates at x = 0.08 prior to reaching solubility (≈10 mol%), and then splits into Mn_Sn substitution and γ-MnTe hetero-phases via mechanical alloying. Understanding such unique crystallography evolution, combined with a modified Debye-Callaway model, is critical in explaining the decreased thermal conductivity of Sn_{1.02- x} Mn_x Te with rational phonon scattering pathways, which should be applicable for other thermoelectric systems.

Lattice Strain Leads to High Thermoelectric Performance in Polycrystalline SnSe

Polycrystalline SnSe materials with ZT values comparable to those of SnSe crystals are greatly desired due to facile processing, machinability, and scale-up application. Here manipulating interatomic force by harnessing lattice strains was proposed for achieving significantly reduced lattice thermal conductivity in polycrystalline SnSe. Large static lattice strain created by lattice dislocations and stacking faults causes an effective shortening in phonon relaxation time, resulting in ultralow lattice thermal conductivity. A combination of band convergence and resonance levels induced by Ga incorporation contribute to a sharp increase of Seebeck coefficient and power factor. These lead to a high thermoelectric performance ZT ∼ 2.2, which is a record high ZT reported so far for solution-processed SnSe polycrystals. Besides the high peak ZT, a high average ZT of 0.72 and outstanding thermoelectric conversion efficiency of 12.4% were achieved by adopting nontoxic element doping, highlighting great potential for power generation application at intermediate temperatures. Engineering lattice strain to achieve ultralow lattice thermal conductivity with the aid of band convergence and resonance levels provides a great opportunity for designing prospective thermoelectrics.

Generic Approach for Contacting Thermoelectric Solid Solutions: Case Study in n- and p-Type Mg2Si0.3Sn0.7

Metallization (known as contacting) of thermoelectric (TE) legs is vital to the long-term performance of a TE device. It is often observed that the compositional changes in a TE solid solution may render a given contact material unsuitable due to a mismatch in the thermal expansion coefficient values. Finding suitable contact materials for TE solid solutions (which often are the best TE materials) remains a challenge. In this work, we propose a multilayer single-step approach in which the same combination of contact materials can be used for a wide compositional range in a solid solution. The outer layer is a metal foil, which helps in creating an Ohmic contact with the interconnects. The intermediate layer is a mixture of the TE material and a metal powder, which results in the formation of the diffusion barrier. The innermost layer is the TE material, which is the active component of the device. The strategy was applied on n- and p-doped Mg₂Si_0.3Sn_0.7 with elemental Cu and Ni providing the desired interface functionalities. Single-step compaction was carried out using the monoblock sintering technique. Microscopic investigation reveals the formation of a well-bonded crack-free interface. Various intermetallic phases were identified at the interface, and the formation of the MgNi₂Sn phase was found to be critical to prevent any interdiffusion of elements. Electrical contact resistance (r_c) measurements were conducted, and low values of 3 and 19 μΩ cm² were measured in n- and p-type legs, respectively. The contacted TE legs were further annealed at 400 °C for 7 days to check their stability. Microstructural and electrical resistance measurements reveal minimal changes in the interface layer and r_c values, indicating the workability of the multilayer technique.