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Mg3Sb2-based thermoelectrics show great promise for next-generation thermoelectric power generators and coolers owing to their excellent figure of merit (zT) and earth-abundant composition elements. However, the complexity of the defect microstructure hinders the advancement of high performance. Here, the defect microstructure is modified via In doping and prolonged sintering time to realize the reduced structural disorder and microstructural evolution, synergistically optimizing electron and phonon transport via a delocalization effect. As a result, an excellent carrier mobility of ~174 cm2 V−1 s−1 and an ultralow κ l a t of ~0.42 W m−1 K−1 are realized in this system, leading to an ultrahigh zT of ~2.0 at 723 K. The corresponding single-leg module demonstrates a high conversion efficiency of ~12.6% with a 425 K temperature difference, and the two-pair module of Mg3Sb2/MgAgSb displays ~7.1% conversion efficiency with a 276 K temperature difference. This work paves a pathway to improve the thermoelectric performance of Mg3Sb2-based materials, and represents a significant step forward for the practical application of Mg3Sb2-based devices. The authors modify complex defect microstructure in Mg3Sb2-based thermoelectrics to reduce structural disorder and promote microstructural evolution, synergistically optimizing electron and phonon transport via a delocalization effect for high performance. [ABSTRACT FROM AUTHOR]

Oxides are of interest for thermoelectrics due to their high thermal stability, chemical inertness, low cost, and eco‐friendly constituting elements. Here, adopting a unique synthesis route via chemical co‐precipitation at strongly alkaline conditions, one of the highest thermoelectric performances for ZnO ceramics (PFmax=$PF_{\text{max}} =$ 21.5 µW cm−1 K−2 and zTmax=$zT_{\text{max}} =$ 0.5 at 1100 K in Zn0.96Al0.04O${\rm Zn}_{0.96} {\rm Al}_{0.04}{\rm O}$) is achieved. These results are linked to a distinct modification of the electronic structure: charge carriers become trapped at the edge of the conduction band due to Anderson localization, evidenced by an anomalously low carrier mobility, and characteristic temperature and doping dependencies of charge transport. The bi‐dimensional optimization of doping and carrier localization enable a simultaneous improvement of the Seebeck coefficient and electrical conductivity, opening a novel pathway to advance ZnO thermoelectrics. [ABSTRACT FROM AUTHOR]

Higher manganese silicides (denoted as MnSiγ ) are a prominent class of intermetallic compound for thermoelectric applications that crystallizes into a Nowotny Chimney Ladder (NCL) phase and constitutes a higher concentration of silicon with odd stoichiometry, typically represented by an irrational number. In this work, the genesis of Si odd stoichiometry and secondary phase relations in silicon-rich and silicon-deficient MnSix is presented to clarify the significance of labile (i.e., easily alterable) Si-subsystem in inheriting the modulated lattice structure with characteristic anharmonicity. The transport properties show an interrelation to the presence of a secondary phase, which is driven by the odd stoichiometry, where the thermoelectric figure of merit (zT) is found to be optimal for MnSi1.74. The structural characterization of rapidly solidified melt-spun ribbons and spark plasma sintered bulk specimens reveal a characteristic presence of MnSi and Si as secondary phases in Si-deficient (x ≈ 1.5) and Si-excess (x ≈ 2) compositions of nominal MnSix, respectively, that coexists with the major MnSiγ based NCL phase. This study provides the fundamental basis of phase evolution and the relevance of odd stoichiometry in silicon-rich manganese silicides which exhibit fascinating phonon dynamics in optimal MnSi1.74 stoichiometry. [ABSTRACT FROM AUTHOR]

Developing thermoelectric (TE) performance is impeded by the compromise of TE parameters, resulting in inadequate conversion efficiency of heat to electricity. Herein, this work reports that Mo is a particularly effective additive in Mg3Sb2‐based alloys with significantly improved electronic transport via grain‐boundary engineering and band‐structure regulation synergy. In addition, phonon transport is simultaneously suppressed by employing multiple effects, lattice imperfection scattering, reduced phonon group velocity, and enhanced atomic disorder, leading to a minimum κlat = 0.52 W m−1 K−1 at 723 K. As a result, an outstanding ZT peak of ≈1.84 at 723 K and ZTav = 1.34 within 323–723 K are achieved in Mg3Sb2‐based alloys, and the corresponding fabricated single‐leg TE module shows an exceptionally high conversion efficiency of ≈12% under a hot‐side temperature of 450 °C. These results demonstrate the great potential for advancing mid‐temperature heat harvesting in Mg3Sb2‐based materials. [ABSTRACT FROM AUTHOR]

The crystallographic and transport properties of thin films fabricated by pulsed laser deposition and belonging to the Sm y (Fe x Ni1- x )4Sb12 filled skutterudite system were studied with the aim to unveil the effect exerted by temperature and duration of thermal treatments on structural and thermoelectric features. The importance of annealing treatments in Ar atmosphere up to 523 K was recognized, and the thermal treatment performed at 473 K for 3 h was selected as the most effective in improving the material properties. With respect to the corresponding bulk compositions, a significant enhancement in phase purity, as well as an increase in electrical conductivity and a drop in room temperature thermal conductivity, were observed in annealed films. The low thermal conductivity, in particular, can be deemed as deriving from the reduced dimensionality and the consequent substrate/film interfacial stress, coupled with the nanometric grain size. [ABSTRACT FROM AUTHOR]

We employed the p-type semiconductor InTe as an extrinsic phase mixing in the GeTe matrix for thermoelectric properties investigations on the (GeTe) 1-x (InTe) x (x=0, 0.02, 0.04, 0.06, and 0.08) composites. We observed notable phenomena that a small amount of InTe has a significant influence on the thermoelectric properties as the primary host matrix in the (GeTe) 1-x (InTe) x (x=0.02, 0.04, 0.06, and 0.08) composites. Even for small concentrations of InTe which possess lower thermal and electrical conductivities compared to those of GeTe, the thermoelectric properties of the (GeTe) 1-x (InTe) x (x=0.02, 0.04, 0.06, and 0.08) composites are primarily influenced by the properties of InTe. We systematically elucidated the electrical conductivity of the (GeTe) 1-x (InTe) x (x=0.02, 0.04, 0.06, and 0.08) composites through the application of effective medium theory (EMT). It is found that the effective media of the (GeTe) 1-x (InTe) x (x=0.02, 0.04, 0.06, and 0.08) composites is an asymmetric medium insulator. The addition of InTe into GeTe reveals a significant decrease in total thermal conductivity due to reductions in both electronic and lattice thermal conductivity. Particularly, the decrease in lattice thermal conductivity was attributed to an increase in internal strain within the lattice induced by the addition of InTe. Consequently, the ZT values of the composite significantly increase across all temperature ranges. This study suggests that developing composites is an effective approach for enhancing thermoelectric performance, comparable to elemental doping, and demonstrates the ability to analyze the electrical properties of composite materials using EMT. • We synthesized the extrinsic phase mixing composite of (GeTe)1-x(InTe)x. • Small InTe concentration significantly changes thermal and electrical transport properties. • Effective medium theory states that the composite follows an asymmetric medium insulator. • Decrease of lattice and electronic thermal conductivities enhances ZT value over a wide temperature range. [ABSTRACT FROM AUTHOR]

Although the thermoelectric effect was discovered around 200 years ago, the main application in practice is thermoelectric cooling using the traditional Bi2Te3. The related studies of new and efficient room-temperature thermoelectric materials and modules have, however, not come to fruition yet. In this work, the electronic properties of n-type Mg3.2Bi1.5Sb0.5 material are maximized via delicate microstructural design with the aim of eliminating the thermal grain boundary resistance, eventually leading to a high zT above 1 over a broad temperature range from 323 K to 423 K. Importantly, we further demonstrated a great breakthrough in the non-Bi2Te3 thermoelectric module, coupled with the high-performance p-type α-MgAgSb, for room-temperature power generation and thermoelectric cooling. A high conversion efficiency of ~2.8% at the temperature difference of 95 K and a maximum temperature difference of 56.5 K are experimentally achieved. If the interfacial contact resistance is further reduced, our non-Bi2Te3 module may rival the long-standing champion commercial Bi2Te3 system. Overall, this work represents a substantial step towards the real thermoelectric application using non-Bi2Te3 materials and devices. The awaited studies of new and efficient thermoelectric modules have not come to fruition yet. Here, the authors demonstrate a high thermoelectric performance of non-Bi2Te3 module for room-temperature power generation and thermoelectric cooling. [ABSTRACT FROM AUTHOR]

GeTe is a promising mid-temperature thermoelectric compound but inevitably contains excessive Ge vacancies hindering its performance maximization. This work reveals that significant enhancement in the dimensionless figure of merit (ZT) could be realized by defect structure engineering from point defects to line and plane defects of Ge vacancies. The evolved defects including dislocations and nanodomains enhance phonon scattering to reduce lattice thermal conductivity in GeTe. The accumulation of cationic vacancies toward the formation of dislocations and planar defects weakens the scattering against electronic carriers, securing the carrier mobility and power factor. This synergistic effect on electronic and thermal transport properties remarkably increases the quality factor. As a result, a maximum ZT > 2.3 at 648 K and a record-high average ZT (300-798 K) were obtained for Bi0.07Ge0.90Te in lead-free GeTe-based compounds. This work demonstrates an important strategy for maximizing the thermoelectric performance of GeTe-based materials by engineering the defect structures, which could also be applied to other thermoelectric materials. The intrinsic high-concentration Ge vacancies in GeTe-based thermoelectric materials hinder their performance maximization. Here, the authors find that defect structure engineering strategy is effective for performance enhancement. [ABSTRACT FROM AUTHOR]

The last decade created tremendous advances in new and unique thermoelectric generation materials, devices, fabrication techniques, and technologies via various global research and development. This article seeks to elucidate and highlight some of these advances to lay foundations for future research work and advances. New advanced methods and demonstrations in TE device and material measurement, materials fabrication and composition advances, and device design and fabrication will be discussed. Other articles in this Special Issue present additional new research into materials fabrication and composition advances, including multi-dimensional additive manufacturing and advanced silicon germanium technologies. This article will discuss the most recent results and findings in thermoelectric system economics, including highlighting and quantifying the interrelationships between thermoelectric (TE) material costs, TE manufacturing costs and most importantly, often times dominating, the heat exchanger costs in overall TE system costs. We now have a methodology for quantifying the competing TE system cost-performance effects and impacts. Recent findings show that heat exchanger costs usually dominate overall TE system cost-performance tradeoffs, and it is extremely difficult to escape this condition in TE system design. In regard to material performance, novel or improved enhancement principles are being effectively implemented. Furthermore, in addition to further advancements in properties and module developments of relatively established champion materials such as skutterudites, several high performance ZT ≈≥ 2 new material systems such as GeTe, Mg3(Sb,Bi)2 have also been relatively recently unearthed and module applications also being considered. These recent advancements will also be covered in this review. [ABSTRACT FROM AUTHOR]

The pillars of Green Chemistry necessitate the development of new chemical methodologies and processes that can benefit chemical synthesis in terms of energy efficiency, conservation of resources, product selectivity, operational simplicity and, crucially, health, safety, and environmental impact. Implementation of green principles whenever possible can spur the growth of benign scientific technologies by considering environmental, economical, and societal sustainability in parallel. These principles seem especially important in the context of the manufacture of materials for sustainable energy and environmental applications. In this review, the production of energy conversion materials is taken as an exemplar, by examining the recent growth in the energy‐efficient synthesis of thermoelectric nanomaterials for use in devices for thermal energy harvesting. Specifically, "soft chemistry" techniques such as solution‐based, solvothermal, microwave‐assisted, and mechanochemical (ball‐milling) methods as viable and sustainable alternatives to processes performed at high temperature and/or pressure are focused. How some of these new approaches are also considered to thermoelectric materials fabrication can influence the properties and performance of the nanomaterials so‐produced and the prospects of developing such techniques further. [ABSTRACT FROM AUTHOR]

BiSbTe‐based nanocomposites hybridized with ceramic nanoparticles have recently revealed their higher efficiency as thermoelectric materials for near room‐temperature applications, as compared to the traditional BiTe‐SbTe alloys. Herein, the incorporation of ultra‐small Bi nanoprecipitates into the microstructure of a Bi0.5Sb1.5Te3/Sb2O3 nanocomposite and the consequential effects on the thermal and electrical transport properties of this three‐phase hybrid system is reported. Electron microscopy characterization and systematic analysis of the thermoelectric properties of Bi0.5Sb1.5Te3/Sb2O3/Bi nanoprecipitate samples suggest that the minute Bi nanoprecipitates increase the charge carrier concentration of the nanocomposites, act as effective scattering sites throughout the microstructure, and shift the maximum Seebeck coefficient values to higher temperatures. [ABSTRACT FROM AUTHOR]

Doping of external elements can substantially optimize the carriers, modulate the electronic structure, or affect the phonon dynamics[4] and, thus, have a significant impact on the TE performance. MODULATION OF ELECTRICAL TRANSPORT Carrier concentration optimization has always been an efficient route to improve TE performance in solids. This special issue of I Applied Physics Letters i presents pioneering articles in the field of thermoelectric (TE) materials and devices. In this regard, Wang I et al. i have shown that the strong anharmonic phonon scattering through the Umklapp process and acoustic-optical phonon coupling causes very low I i SB lat sb in single crystalline Bi SB 2 sb O SB 2 sb Se and the maximum zT reaches ~0.188 at 390 K.[19] The van der Waals heterostructure of layered Bi SB 4 sb GeTe SB 7 sb also exhibits intrinsically ultra-low I i ~ 0.42 W m SP -1 sp K SP -1 sp at 380 K due to the presence of large anharmonicity in the lattice.[20] NEW THERMOELECTRIC MATERIALS The involvement of theoretical studies and predictions in materials have remarkably advanced this field of thermoelectrics. [Extracted from the article]

We investigated the effects on thermoelectric properties of Cu doping in Fe2‐xCuxVAl at Fe site of full‐Heusler type compound. It is found that the Cu doping for Fe sites causes a significant increase in the absolute value of Seebeck coefficient |S| and a decrease in thermal conductivity. The Seebeck coefficient (S)=‐148μV/K and the Power factor (PF)=4.0 mWK−2m−1 have been observed for Fe1.9Cu0.1VAl (x=0.1) at 300 K. To further improve it, we fixed the Cu doping level at x=0.1 in Fe2‐xCuxVAl and co‐doped the material with Si at Al site, namely, Fe1.9Cu0.1VAl1‐ySiy. The thermoelectric properties have been improved by Si doping to a certain limit. We observed a decrease in electrical resistivity and lattice thermal conductivity by Si doping for Al. The maximum power factor of 4.5 mWK−2m−1 has been achieved for Fe1.9Cu0.1Al0.9Si0.1 at 350 K. More precisely, the thermoelectric performance has been improved with co‐doping of Cu for Fe sites and Si for Al sites. The largest ZT value is 0.13 for Fe1.9Cu0.1VAl1‐ySiy (y=0.15). Magnetic susceptibility suggests that all the measured compounds are showing paramagnetic behavior. The magnetic character is the most pronounced in Fe1.9Cu0.1VAl among the materials investigated, pointing to a possible correlation between the magnetic character due to electronic correlation and the larger Seebeck coefficient in this sample. [ABSTRACT FROM AUTHOR]

Discovered more than 200 years ago in 1821, thermoelectricity is nowadays of global interest as it enables direct interconversion of thermal and electrical energy via the Seebeck/Peltier effect. In their seminal work, Mahan and Sofo mathematically derived the conditions for 'the best thermoelectric'—a delta-distribution-shaped electronic transport function, where charge carriers contribute to transport only in an infinitely narrow energy interval. So far, however, only approximations to this concept were expected to exist in nature. Here, we propose the Anderson transition in a narrow impurity band as a physical realisation of this seemingly unrealisable scenario. An innovative approach of continuous disorder tuning allows us to drive the Anderson transition within a single sample: variable amounts of antisite defects are introduced in a controlled fashion by thermal quenching from high temperatures. Consequently, we obtain a significant enhancement and dramatic change of the thermoelectric properties from p-type to n-type in stoichiometric Fe2VAl, which we assign to a narrow region of delocalised electrons in the energy spectrum near the Fermi energy. Based on our electronic transport and magnetisation experiments, supported by Monte-Carlo and density functional theory calculations, we present a novel strategy to enhance the performance of thermoelectric materials. The mathematical conditions for the best thermoelectric is well known but never realised in real materials. Here, the authors propose the Anderson transition in a narrow impurity band as a physical realisation of this seemingly unrealisable scenario. [ABSTRACT FROM AUTHOR]

GeTe is a promising candidate for the fabrication of high‐temperature segments for p‐type thermoelectric (TE) legs. The main restriction for the widespread use of this material in TE devices is high carrier concentration (up to ∼ 1021 cm−3), which causes the low Seebeck coefficient and high electronic component of thermal conductivity. In this work, the band structure diagram and phase equilibria data have been effectively used to attune the carrier concentration and to obtain the high TE performance. The Ge1−xBixTe (x = 0.04) material prepared by the Spark plasma sintering (SPS) technique demonstrates a high power factor accompanied by moderate thermal conductivity. As a result, a significantly higher dimensionless TE figure of merit ZT = 2.0 has been obtained at ∼ 800 K. Moreover, we are the first to propose that application of the developed Ge1−xBixTe (x = 0.04) material in the TE unicouple should be accompanied by SnTe and CoGe2 transition layers. Only such a unique solution for the TE unicouple makes it possible to prevent the negative effects of high contact resistance and chemical diffusion between the segments at high temperatures. [ABSTRACT FROM AUTHOR]

Alongiside the growing demand for wearable and implantable electronics, the development of flexible thermoelectric (FTE) materials holds great promise and has recently become a highly necessitated and efficient method for converting heat to electricity. Conductive polymers were widely used in previous research; however, n-type polymers suffer from instability compared to the p-type polymers, which results in a deficiency in the n-type TE leg for FTE devices. The development of the n-type FTE is still at a relatively early stage with limited applicable materials, insufficient conversion efficiency, and issues such as an undesirably high cost or toxic element consumption. In this work, as a prototype, a flexible n-type rare-earth free skutterudite (CoSb3)/poly(3,4-ethylenedioxythiophene)-polystyrene sulfonate (PEDOT:PSS) binary thermoelectric film was fabricated based on ball-milled skutterudite via a facile top-down method, which is promising to be widely applicable to the hybridization of conventional bulk TE materials. The polymers bridge the separated thermoelectric particles and provide a conducting pathway for carriers, leading to an enhancement in electrical conductivity and a competitive Seebeck coefficient. The current work proposes a rational design towards FTE devices and provides a perspective for the exploration of conventional thermoelectric materials for wearable electronics. [ABSTRACT FROM AUTHOR]

SiGe‐based thermoelectric (TE) materials are well‐known for high‐temperature utilization, but rarely relevant in the low temperature region. Here a Ge quantum dots (QDs) and Sn precipitation SiGeSn hybrid film are constructed via ultrafast high temperature annealing (UHA) of a treated P‐ion implantation SiGeSn film on Si/SiO2 substrate. Combining the modulation doping effect dominated by Sn precipitates and the energy filtering effect caused by Ge QDs, the optimized SiGe films achieve a giant power factor as high as 91 µW cm−1 K−2 @300 K, room temperature, while maintaining low thermal conductivity. This strategy on film construction provides a novel insight for TE materials with striking performance. [ABSTRACT FROM AUTHOR]

In this work, we present the influence of Ga doping in Mg2Sn thin epitaxial films on sapphire (0001) substrates. Our results suggest that epitaxial nature is essential for achieving high mobility. Furthermore, we found that Ga incorporation influences the carrier concentration and acts as a phonon-scattering center. The optimal power factor and figure of merit values obtained were 1.49 × 10−3 W·m−1·K−1 and 0.08 at 300 K for Mg2Sn0.97Ga0.03. The values are in the same range as the bulk material of Mg-based II–IV semiconductors, suggesting that the combination of doping and epitaxial nature in thin films can be a promising route for miniaturization of thermoelectric devices based on Mg-based materials. [ABSTRACT FROM AUTHOR]

High-performance thermoelectric materials are currently being sought after to recycle waste heat. Copper chalcogenides in general are materials of great interest because of their naturally low thermal conductivity and readily modifiable electronic properties. The compounds Cu5Sn2Q7 were previously reported to have metal-like properties, which is not a desirable characteristic for thermoelectric materials. The aim of this study was to reduce the carrier concentration of these materials by Zn-doping, and then investigate the electronic and thermoelectric properties of the doped materials in comparison to the undoped ones. The compounds were synthesized using both the traditional solid-state tube method and ball-milling. The crystal structures were characterized using powder X-ray diffraction, which confirmed that all materials crystallize in the monoclinic system with the space group C2. With the partial substitution of zinc for copper atoms, the compounds exhibited an overall improvement in their thermoelectric properties. Figure of merit values were determined to be 0.20 for Cu4ZnSn2Se7 at 615 K and 0.05 for Cu4ZnSn2Te7 at 575 K. [ABSTRACT FROM AUTHOR]

A unified structure model in (3 + 1)‐dimensional superspace proved suitable for identification of a homologous phase (Ga1−αAlα)2O3(ZnO)m by the profile fitting of powder X‐ray diffraction intensities for thermoelectric composite materials in the pseudoternary system ZnO–Al2O3–Ga2O3. A homologous compound of the phase parameter m ≃ 37 was found to coexist with (Al,Ga)‐doped ZnO in samples sintered at 1723 K in air. The thermoelectric properties of the composite materials were closely related to the phase fractions. The higher the phase fraction of (Al,Ga)‐doped ZnO with the wurtzite structure, the higher the electrical conductivity. On the other hand, the homologous compound with the long‐period structure was effective in lowering the thermal conductivity of the materials. [ABSTRACT FROM AUTHOR]

Modification of crystal symmetry induced by chemical doping or alloying is well known to optimize the charge carrier transport properties in thermoelectric materials. Historically, the altered defect chemistry of the targeted materials, however, has long been neglected or underestimated, which may, in turn, favorably or adversely affect the thermoelectric properties. Herein, using a case study of thermoelectric GeTe, first, the hidden role of rhombohedral distortion degree on the Ge‐vacancy formation energy is theoretically unraveled, and it is experimentally found that Sc and Bi codoping realize a superior thermoelectric performance. Density functional theory calculations demonstrate that the distorted rhombohedral lattice closer to cubic would unexpectedly induce the significantly decreased formation energy of Ge vacancies, resulting in the spontaneous formation of higher‐concentration Ge vacancies. Sc doping is found to be the most effective dopant to reduce the carrier concentration without obviously affecting the rhombohedral distortion degree, leading to the realization of a record‐high power factor. Benefiting from the suppressed thermal conductivity by further Bi doping, an ultrahigh average ZT ≈1.2 from 300 to 723 K is achieved. These discoveries provide new insights into the relationship between crystal symmetry and defect chemistry, and also promote GeTe materials for future thermoelectric applications. [ABSTRACT FROM AUTHOR]

We report on the thermoelectric properties of n-type homologous boron cluster compounds REB17CN and REB22C2N (RE=rare earth) in the temperature range of 300–1000 K. Boron cluster compounds like boron carbide have predominantly exhibited p-type behavior. REB17CN and REB22C2N exhibit n-type behavior without doping and are possible counterparts to boron carbide, which is known to be an exemplar p-type high temperature thermoelectric compound. With the hot press method it was only possible to prepare samples with low density and extremely low power factors. Two methods for improving the properties were investigated: spark plasma sintering of HoB17CN and YB22C2N and seeding of ErB17CN and ErB22C2N samples with metal borides like ErB4 and ErB6. Although absolute values of the power factor are still low, both methods are shown to yield routes for large increases of the power factor (a factor of 31 to 106 times for the samples measured) compared to conventional hot pressed samples. [ABSTRACT FROM AUTHOR]

B12 icosahedral cluster-containing rare earth boride TbB44Si2, ErB44Si2, YbB44Si2 crystals were grown by the floating zone method and their high temperature thermoelectric properties (Seebeck coefficient, resistivity) measured. REB44Si2 is isostructural to the magnetic TbB50 compound. These compounds are attractive as high temperature materials due to their stability. An ErB66 crystal was also measured for comparison. Seebeck coefficients exceeding 200 μV/K are observed at temperatures above 1000 K for the REB44Si2 compounds and the conductivity increases rapidly as temperature is increased through the variable range hopping mechanism. REB44Si2 compounds were found to have a larger power factor compared to ErB66. The thermal conductivity of ErB44Si2 was also measured and determined to take the low value of 0.027 W/cm/K at room temperature. [ABSTRACT FROM AUTHOR]

Magnesium stannide (Mg 2 Sn) is a small bandgap semiconductor of interest as a promising thermoelectric or optoelectronic material. Thin films of Mg 2 Sn were epitaxially grown on sapphire (0001) surfaces using molecular beam epitaxy. The epitaxial relationship is (111) Mg 2 Sn ∥ (0001) Al 2 O 3 and [ 11 2 ¯ ] Mg 2 Sn ∥ [ 10 1 ¯ 0 ] Al 2 O 3 , with a small amount of stacking faults. A relatively high growth rate of 0.21–0.27 nm/s was attainable. [ABSTRACT FROM AUTHOR]

Several interesting physical properties which are closely related to the interesting crystal structures of rare earth borides will be reviewed. Experimentally it has been indicated that the B 12 icosahedral cluster enhances magnetic interaction and mediates it in dilute magnetic systems. There is interest in the potential of borides as high-temperature thermoelectric materials for use in topping cycles for power plants and other applications. Boron icosahedral compounds generally exhibit large Seebeck coefficients. They also tend to possess relatively small thermal conductivities despite the typically strong covalent bonding and high sound velocities of the materials. The paper reviews possible mechanisms proposed to lower the lattice thermal conductivity in borides, p–n controllable compounds, and several borides recently found with enhanced thermoelectric properties. Boron cluster and layered compounds have strong structure-property relationships. Striking thermoelectric and magnetic properties of rare earth borides are comprehensively reviewed. Image 1 [ABSTRACT FROM AUTHOR]

Thermoelectrics, in particular solid-state conversion of heat to electricity, is expected to be a key energy harvesting technology to power ubiquitous sensors and wearable devices in the future. A comprehensive review is given on the principles and advances in the development of thermoelectric materials suitable for energy harvesting power generation, ranging from organic and hybrid organic-inorganic to inorganic materials. Examples of design and applications are also presented. [ABSTRACT FROM AUTHOR]

To examine the potential of organic thermoelectrics (TEs) for energy harvesting, we fabricated an organic TE module to achieve 250 mV in the open-circuit voltage which is sufficient to drive a commercially available booster circuit designed for energy harvesting usage. We chose the π-type module structure to maintain the temperature differences in organic TE legs, and then optimized the p- and n-type TE materials' properties. After injecting the p- and n-type TE materials into photolithographic mold, we eventually achieved 250 mV in the open-circuit voltage by a method to form the upper electrodes. However, we faced a difficulty to reduce the contact resistance in this material system. We conclude that TE materials must be inversely designed from the viewpoints of the expected module structures and mass-production processes, especially for the purpose of energy harvesting. [ABSTRACT FROM AUTHOR]

Increasing energy demands require new materials, e.g., thermoelectrics, for efficient energy conversion of fossil fuels. However, their low figure of merit (ZT) limits widespread applications. Nanostructuring has been an effective way of lowering the thermal conductivity. However, grain growth at elevated temperature is still a big concern, for otherwise expected to be long-lasting thermoelectric generators. Here, we report a porous architecture containing nano- to micrometer size irregularly shaped and randomly oriented pores, scattering a wide spectrum of phonons without employing the conventional rattling phenomenon. Lattice thermal conductivity reaches the phonon glass limit. This design yields >100% enhancement in ZT, as compared to the pristine sample. An unprecedented and very promising ZT of 1.6 is obtained for Co23.4Sb69.1Si1.5Te6.0 alloy, by far the highest ZT ever reported for un-filled skutterudites, with further benefits, i.e. rare-earth-free and improved oxidation resistance enabling simple processing. [ABSTRACT FROM AUTHOR]

The reduction of the diameter of Bi nanowires below 10 nm has been an important target because of the theoretical prediction with regard to significant enhancement in thermoelectric performance by size reduction. In this study, we have demonstrated the usefulness of mesoporous silica with tunable pore size as a template for the preparation of thin Bi nanowires with diameters below 10 nm. Bi was deposited within the templates through a liquid phase deposition using hexane and 1,1,3,3-tetramethyldisiloxane as a solvent and reducing agent, respectively. Bundles of thin Bi nanowires with non-crystalline frameworks were successfully obtained after the template removal. The diameter was precisely controlled between about 6 nm and 9 nm. The judicious choices of mesoporous silica and deposition conditions are critical for the successful preparation. The reliable formation of such thin Bi nanowires reported here opens up exciting new possibilities. [ABSTRACT FROM AUTHOR]

A unified structure model for the homologous compounds Ga2O3(ZnO)m in (3 + 1)‐dimensional superspace was modified by refinements for samples of Ga2O3:ZnO = 1:n (n = 15, 23, 28, 33, 38). The first‐order sine terms in the modulation functions decreased with increasing phase index m. The diffraction intensities of the thermoelectric material Ga2O3:ZnO = 1:98 were best fitted by a two‐phase model containing the wurtzite structure (Ga‐doped ZnO) and the homologous compound Ga2O3(ZnO)s, where s = 38. [ABSTRACT FROM AUTHOR]

Current high-performance thermoelectric materials require elaborate doping and synthesis procedures, particularly in regard to the artificial structure, and the underlying thermoelectric mechanisms are still poorly understood. Here, we report that a natural chalcopyrite mineral, Cu1+xFe1−xS2, obtained from a deep-sea hydrothermal vent can directly generate thermoelectricity. The resistivity displayed an excellent semiconducting character, and a large thermoelectric power and high power factor were found in the low x region. Notably, electron-magnon scattering and a large effective mass was detected in this region, thus suggesting that the strong coupling of doped carriers and antiferromagnetic spins resulted in the natural enhancement of thermoelectric properties during mineralization reactions. The present findings demonstrate the feasibility of thermoelectric energy generation and electron/hole carrier modulation with natural materials that are abundant in the Earth's crust. [ABSTRACT FROM AUTHOR]

Layered materials have several properties that make them suitable as high-performance thermoelectric materials. In this study, we elucidated the potential of d0-electron layered complex metal nitrides, AMN2 (A = Sr or Ba; M = Ti, Zr, Hf), with KCoO2-type and α-NaFeO2-type crystal structures. The detailed electronic structures and electronic transport coefficients for AMN2 compounds were calculated by using density-functional theory and Boltzmann theory, respectively. The KCoO2-type AMN2 compounds show highly anisotropic thermoelectric properties. The effective masses at the lowest conduction bands in both KCoO2-type and α-NaFeO2-type AMN2 compounds were significantly smaller than that in three-dimensional perovskite SrTiO3, a well-known compound with good thermoelectric properties. The results suggest that the excellent thermoelectric transport properties arise from appropriate electronic structures and small effective masses in AMN2 layered complex metal nitrides. [ABSTRACT FROM AUTHOR]

Cover Feature: Thermoelectric properties of Cu-Doped Heusler compound Fe2-xCuxVAl (Z. Anorg. Keywords: Heusler compounds; Electrical resistivity; Seebeck coefficient; Thermal conductivity; ZT EN Heusler compounds Electrical resistivity Seebeck coefficient Thermal conductivity ZT 1 1 1 08/19/22 20220812 NES 220812 The cover picture shows co-doping of Cu for Fe and Si for Al site in Fe SB 2 sb VAl, and the effect of Cu doping to significantly enhance the thermoelectric power factor. Heusler compounds, Electrical resistivity, Seebeck coefficient, Thermal conductivity, ZT. [Extracted from the article]

A review is given on some investigations to enhance ZT through very effective selective scattering of phonons from bottom-up nanostructuring methods. Novel concepts are also being utilized in an attempt to overcome the conventional tradeoff in the power factor. [ABSTRACT FROM AUTHOR]

Synthesis conditions, morphology, and thermoelectric properties of Y1-xB28.5C4 were investigated. Y1-xB28.5C4 is the compound with the lowest metal content in a series of homologous rare earth borocarbonitrides, which have been attracting interest as high temperature thermoelectric materials because they can embody the long-awaited counterpart to boron carbide, one of the few thermoelectric materials with a history of commercialization. It was revealed that the presence of boron carbide inclusions was the origin of the p-type behavior previously observed for Y1-xB28.5C4 in contrast to Y1-xB15.5CN and Y1-xB22C2N. In comparison with that of previous small flux-grown single crystals, a metal-poor composition of YB40C6 (Y0.71B28.5C4) in the synthesis successfully yielded sintered bulk Y1-xB28.5C4 samples apparently free of boron carbide inclusions. "Pure" Y1-xB28.5C4 was found to exhibit the same attractive n-type behavior as the other rare earth borocarbonitrides even though it is the most metal-poor compound among the series. Calculations of the electronic structure were carried out for Y1-xB28.5C4 as a representative of the series of homologous compounds and reveal a pseudo gap-like electronic density of states near the Fermi level mainly originating from the covalent borocarbonitride network. [ABSTRACT FROM AUTHOR]

Synthesis conditions, morphology, and thermoelectric properties of Y1-xB28.5C4 were investigated. Y1-xB28.5C4 is the compound with the lowest metal content in a series of homologous rare earth borocarbonitrides, which have been attracting interest as high temperature thermoelectric materials because they can embody the long-awaited counterpart to boron carbide, one of the few thermoelectric materials with a history of commercialization. It was revealed that the presence of boron carbide inclusions was the origin of the p-type behavior previously observed for Y1-xB28.5C4 in contrast to Y1-xB15.5CN and Y1-xB22C2N. In comparison with that of previous small flux-grown single crystals, a metal-poor composition of YB40C6 (Y0.71B28.5C4) in the synthesis successfully yielded sintered bulk Y1-xB28.5C4 samples apparently free of boron carbide inclusions. "Pure" Y1-xB28.5C4 was found to exhibit the same attractive n-type behavior as the other rare earth borocarbonitrides even though it is the most metal-poor compound among the series. Calculations of the electronic structure were carried out for Y1-xB28.5C4 as a representative of the series of homologous compounds and reveal a pseudo gap-like electronic density of states near the Fermi level mainly originating from the covalent borocarbonitride network. [ABSTRACT FROM AUTHOR]

Layered materials have several properties that make them suitable as high-performance thermoelectric materials. In this study, we focus on the complex metal nitrides SrZrN2 and SrHfN2, which have an a-NaFeO2 layered crystal structure. To determine their electronic band structure features and potential thermoelectric transport properties, we calculated the electronic band structures and electronic transport coefficients for SrZrN2 and SrHfN2 using density-functional theory and Boltzmann transport theory, respectively. Despite the layered crystal structure, SrZrN2 and SrHfN2 both had three-dimensional electronic structures and isotropic electronic transport because of the contribution of the Sr 4dx²-y² + 4dxy orbitals to the bottom of the conduction bands in addition to that of the Zr 4dz² (Hf 5dz²) orbital. The three-dimensional electronic structures predict the appearance of large Seebeck coefficients (-145 µV K-1 at 300 K, -370 µV K-1 at 1200 K) and large electronic thermoelectric figures of merit. [ABSTRACT FROM AUTHOR]

Recently, based on measurement results below 400 K, we suggested that chalcopyrite CuFeS-based alloys hold promise as thermoelectric materials. In this study, we have investigated the phase stability of such compounds and measured their thermoelectric properties at temperatures above 400 K. Thermogravimetric data indicate that the samples synthesized by a spark plasma sintering method were stable up to 700 K, above which sulfur deficiency becomes prominent. The electrical resistivity of the electron-doped samples showed metallic behavior up to 700 K. The Seebeck coefficients show large negative values of about −300 μV/K above 400 K. As a result, the power factor of CuFeS is ~1 mW/Km in the temperature range of 400 K to 600 K. [ABSTRACT FROM AUTHOR]

The misfit layer compounds (BiS)(TiS), with a natural superlattice structures, are of substantial interest as thermoelectric materials. In this work we doped the Ti sites of (BiS)(TiS) with a series of transition metal (TM) elements (V, Cr, Mn, Fe, Co, Ni, Cu, and Zn), to optimize its transport properties and thermoelectric performance. X-ray diffraction confirmed all the resulting compositions were single-phase. X-ray photoelectron spectroscopy revealed the valence states of the doping elements, indicating they behave as acceptors and reduce the carrier concentration; this was also apparent from Hall measurements. However, because of the non-parabolic band structure of (BiS)(TiS), reduction of carrier concentration by doping with most of the TM elements did not improve the Seebeck coefficient. Exceptions were V and Cr. For these elements, the effective mass of electrons was maintained, or even enhanced, resulting in improvement of the Seebeck coefficient. Furthermore, the stacking disorder present in undoped (BiS)(TiS) was not observed for the TM element-doped samples, resulting in increased lattice thermal conductivity. Although the power factor of these materials was not optimized, because of the large reduction in electronic thermal conductivity upon doping, the Cr-doped sample had a higher figure of merit than the undoped material. [ABSTRACT FROM AUTHOR]

Dense samples of the higher boride YBCN have been fabricated through the spark plasma sintering (SPS) method with different sintering aids. YBCN is a representative of a series of newly discovered rare-earth borocarbonitrides, which may be the long-awaited n-type counterpart of boron carbide, 'BC.' The effect of Si, SiC, Al, and TiC additions on the sintering process of YBCN has been studied. The best sintered bodies with densities higher than 90% of theoretical density were obtained by means of SPS at 1700°C. We show that the additive choice and pressure have an effect on grain size and density. An investigation of the effect of atmosphere on the sintering behavior has also been carried out. It was found that sinterability is enhanced under nitrogen atmosphere. Thermoelectric properties of the materials sintered with additives have been evaluated, and we discuss their dependences on the fabrication process route. [ABSTRACT FROM AUTHOR]

In this work, a series of Bi2Te3/X mol% MoS2 (X = 0, 25, 50, 75) bulk nanocomposites were prepared by hydrothermal reaction followed by reactive spark plasma sintering (SPS). X-ray diffraction analysis (XRD) indicates that the native nanopowders, comprising of Bi2Te3/MoS2 heterostructure, are highly reactive during the electric field-assisted sintering by SPS. The nano-sized MoS2 particles react with the Bi2Te3 plates matrix forming a mixed-anion compound, Bi2Te2S, at the interface between the nanoplates. The transport properties characterizations revealed a significant influence of the nanocomposite structure formation on the native electrical conductivity, Seebeck coefficient, and thermal conductivity of the initial Bi2Te3 matrix. As a result, enhanced ZT values have been obtained in Bi2Te3/25 mol% MoS2 over the temperature range of 300–475 K induced mainly by a significant increase in the electrical conductivity. [ABSTRACT FROM AUTHOR]

Nanostructured β-FeSi2 and β-Fe0.95Co0.05Si2 specimens with a relative density of up to 95% were synthesized by combining a top-down approach and spark plasma sintering. The thermoelectric properties of a 50 nm crystallite size β-FeSi2 sample were compared to those of an annealed one, and for the former a strong decrease in lattice thermal conductivity and an upshift of the maximum Seebeck's coefficient were shown, resulting in an improvement of the figure of merit by a factor of 1.7 at 670 K. For β-Fe0.95Co0.05Si2, one observes that the figure of merit is increased by a factor of 1.2 at 723 K between long time annealed and nanostructured samples mainly due to an increase in the phonon scattering and an increase in the point defects. This results in both a decrease in the thermal conductivity to 3.95 W/mK at 330 K and an increase in the power factor to 0.63 mW/mK2 at 723 K. [ABSTRACT FROM AUTHOR]

Filled skutterudites are currently studied as promising thermoelectric materials due to their high power factor and low thermal conductivity. The latter property, in particular, can be enhanced by adding scattering centers, such as the ones deriving from low dimensionality and the presence of interfaces. This work reports on the synthesis and characterization of thin films belonging to the Smy(FexNi1−x)4Sb12-filled skutterudite system. Films were deposited under vacuum conditions by the pulsed laser deposition (PLD) method on fused silica substrates, and the deposition temperature was varied. The effect of the annealing process was studied by subjecting a set of films to a thermal treatment for 1 h at 423 K. Electrical conductivity σ and Seebeck coefficient S were acquired by the four-probe method using a ZEM-3 apparatus performing cycles in the 348–523 K temperature range, recording both heating and cooling processes. Films deposited at room temperature required three cycles up to 523 K before being stabilized, thus revealing the importance of a proper annealing process in order to obtain reliable physical data. XRD analyses confirm the previous result, as only annealed films present a highly crystalline skutterudite not accompanied by extra phases. The power factor of annealed films is shown to be lower than in the corresponding bulk samples due to the lower Seebeck coefficients occurring in films. Room temperature thermal conductivity, on the contrary, shows values comparable to the ones of doubly doped bulk samples, thus highlighting the positive effect of interfaces on the introduction of scattering centers, and therefore on the reduction of thermal conductivity. [ABSTRACT FROM AUTHOR]

CrSi 2 is a promising thermoelectric material constituted of non-toxic and earth abundant elements that offer good perspectives for the mass production of inexpensive and reliable thermoelectric modules for waste heat recovery. Realization of robust metallic contacts with low electrical and thermal resistances on thermoelectric materials is crucial to maximize the conversion efficiency of such a device. In this article, the metallization of an undoped CrSi 2 with Ti and Nb using a conventional Spark Plasma Sintering process is explored and discussed. These contact metals were selected because they have compatible thermal expansion coefficients with those of CrSi 2 , which were determined in this study by X-ray Diffraction in the temperature range 299–899 K. Ti was found to be a promising contact metal offering both strong adhesion on CrSi 2 and negligible electrical contact resistance (<1 μ Ω cm 2 ). However, metallization with Nb resulted in the formation of cracks caused by large internal stress inside the sample during the fabrication process and the diffusion of Si in the metallic layer. A maximum conversion efficiency of 0.3% was measured for a sandwiched Ti/CrSi 2 /Ti thermoelectric leg placed inside a thermal gradient of 427 K. The preliminary results obtained and discussed in this article on a relatively simple case study aim to initiate the development of more reliable and efficient CrSi 2 thermoelectric legs with an optimized design. [ABSTRACT FROM AUTHOR]

An unusual self‐assembly pattern is observed for highly ordered 1500‐nm‐thick films of monodisperse 13‐nm‐sized colloidal PbSe quantum dots, originating from their faceted truncated cube‐like shape. Specifically, self‐assembled PbSe dots exhibited attachment to the substrate by <001> planes followed by an interconnection through the {001} facets in plan‐view and {110}/{111} facets in cross‐sectional‐view, thus forming a cubic superlattice. The thermoelectric properties of the PbSe superlattice thin films are investigated by means of frequency domain thermoreflectance, scanning thermal probe microscopy, and four‐probe measurements, and augmented by computational efforts. Thermal conductivity of the superlattice films is measured as low as 0.7 W m−1 K−1 at room temperature due to the developed nanostructure. The low values of electrical conductivity are attributed to the presence of insulating oleate capping ligands at the dots’ surface and the small contact area between the PbSe dots within the superlattice. Experimental efforts aiming at the removal of the oleate ligands are conducted by annealing or molten‐salt treatment, and in the latter case, yielded a promising improvement by two orders of magnitude in thermoelectric performance. The result indicates that the straightforward molten‐salt treatment is an interesting approach to derive thermoelectric dot superlattice thin films over a centimeter‐sized area. [ABSTRACT FROM AUTHOR]

The structural and thermoelectric properties of the filled skutterudite (Sm,Gd)y(FexNi1-x)4Sb12 were investigated and critically compared to the ones in the Sm-containing system with the aim of unravelling the effect of double filling on filling fraction and thermal conductivity. Several samples (x = 0.50–0.90 and y = 0.15–0.48) were prepared by melting-sintering, and two of them were densified by spark plasma sintering in order to study their thermoelectric features. The crystallographic study enables the recognition of the role of the filler size in ruling the filling fraction and the compositional location of the p/n crossover: It has been found that the former lowers and the latter moves toward lower x values with the reduction of the filler ionic size, as a consequence of the progressively weaker interaction of the filler with the Sb12 cavity. The analysis of thermoelectric properties indicates that, despite the Sm3+/Gd3+ small mass difference, the contemporary presence of these ions in the 2a site significantly affects the thermal conductivity of both p- and n-compositions. This occurs by reducing its value with respect to the Sm-filled compound at each temperature considered, and making the overall thermoelectric performance of the system comparable to several multi-filled (Fe, Ni)-based skutterudites described in the literature. [ABSTRACT FROM AUTHOR]

High-performance thermoelectric materials are currently being sought after to recycle waste heat. Copper chalcogenides in general are materials of great interest because of their naturally low thermal conductivity and readily modifiable electronic properties. The compounds Cu5Sn2Q7 were previously reported to have metal-like properties, which is not a desirable characteristic for thermoelectric materials. The aim of this study was to reduce the carrier concentration of these materials by Zn-doping, and then investigate the electronic and thermoelectric properties of the doped materials in comparison to the undoped ones. The compounds were synthesized using both the traditional solid-state tube method and ball-milling. The crystal structures were characterized using powder X-ray diffraction, which confirmed that all materials crystallize in the monoclinic system with the space group C2. With the partial substitution of zinc for copper atoms, the compounds exhibited an overall improvement in their thermoelectric properties. Figure of merit values were determined to be 0.20 for Cu4ZnSn2Se7 at 615 K and 0.05 for Cu4ZnSn2Te7 at 575 K. [ABSTRACT FROM AUTHOR]

First-principles calculations were carried out to elucidate the excellent control of p–n characteristics recently reported for yttrium aluminoborides YxAlyB14 with different occupancies of Al sites . Such control of the occupancy of metal sites in borides is unusual. Calculations based on detailed x-ray diffraction data reveal a stable configuration of the atomic sites, indicating that such variation in occupancy is possible. A shift from positive through zero to negative values of the Seebeck coefficient is also clearly illustrated by determining the density of states for different configurations. [ABSTRACT FROM AUTHOR]