Search

The (GeTe)x(AgSbTe2)100-x alloys (TAGS-x) have been demonstrated as a promising p-type candidate for thermoelectric applications, attracting numerous attentions on the advancements of thermoelectric performance. Manipulation of carrier concentration for optimizing thermoelectric performance in TAGS can be achieved by varying the ratio of Ag to Sb, and the Ag/Sb ratio of ~ 2/3 has been proven as the optimal composition. Therefore, this work focuses on the systematic investigation on thermoelectric properties of (GeTe)1-x[(Ag2Te)0.4(Sb2Te3)0.6]x alloys. The crystal structure for the alloys transfers from rhombohedral to cubic at room temperature as x ≥ 0.2. The evolution of band parameter is estimated using a single parabolic band (SPB) model with acoustic phonon scattering. The density of states effective mass increases with x increasing, which leads to an enhancement of Seebeck coefficient along with a reduction in Hall mobility due to the additional carrier scattering by point defects. Meanwhile, the lattice thermal conductivity of lower than ~ 0.7 W·m−1·K−1 in the entire temperature range and the lowest one of only 0.45 W·m−1·K−1 is achieved due to additional phonon scattering by point defects. As a result, a peak thermoelectric figure of merit (zT) of ~ 1.80 and an average one of ~ 1.37 in 300–800 K are realized in nonstoichiometric TAGS alloys here.

Ag/Cu-based argyrodites have recently received extensive attention as promising thermoelectric materials. This can be largely understood by the hierarchical chemical bonding structure that allows the existence of high-concentration and highly mobile Ag+/Cu+ ions distributed in the rigid polyanionic framework, since the diffusive cations enable an effective phonon scattering for phononically insulating as that in woods while the rigid framework ensures an electronic charge transport as that in trivial thermoelectric semiconductors. This review focused firstly on the synthesis of both single-crystalline and poly-crystalline argyrodites as well as the chemical compositions, phases and crystallographic features; secondly on the origins of the low lattice thermal conductivity including crystal structure complexity, weak bonded ions, strong lattice anharmonicity and low sound velocity; and thirdly on the electronic band structure characteristics including effective mass and band degeneracy for an evaluation on the electronic transport properties, as well as performance stability. These together lead a perspective on future development of thermoelectric argyrodites to emphasize the enhancement of electronic performance such as by carrier concentration and by manipulating the electronic band structure.

Intrinsic partial occupation of Cu induces strong phonon scattering for an extremely low lattice thermal conductivity in TmCuTe2, which leads this material to be a promising candidate for thermoelectric applications. This work focus on the electronic and phononic transport properties of TmCuTe2 with Ag-alloying on the Cu site. Such a Ag/Cu substitution is found to enable a decrease in Hall carrier concentration from 14 × 1019 to 8 × 1019 cm−3 for an evaluation of the charge transport based on a single parabolic band (SPB) model with acoustic scattering. In addition, Ag-substitution simultaneously introduces Cu/Ag point defects for extra phonon scattering, leading the lattice thermal conductivity to be as low as ∼0.25 W/m-K in a broad temperature. The optimization in carrier concentration and the reduction in lattice thermal conductivity contribute to a ∼40% improvement in average thermoelectric figure of merit (zT). The determination of band parameters enables a guidance for further advancements in this promising thermoelectric material.

GeTe that exhibits a strong anharmonicity and a ferroelectric phase transition between the rhombohedral and cubic structures has emerged as one of the leading thermoelectric materials. Herein, combining molecular dynamics simulations and inelastic neutron scattering measurements, the lattice dynamics in GeTe have been investigated to reveal the soft-mode mechanisms across the phase transition. We have constructed a first-principles-based machine-learning interatomic potential, which successfully captures the dynamical ferroelectric phase transition of GeTe by adopting the neural network technique. Although the low-energy acoustic phonons remain relatively unaffected at elevated temperatures, the high-energy optical, and longitudinal acoustic phonons demonstrate strong renormalizations as evidenced from the vibrational phonon spectra, which are attributed to the large anharmonicity accompanying the phase transition. Furthermore, our results reveal a nonmonotonic temperature dependence of the soft-modes beyond the perturbative regime. The insight provided by this work into the soft-modes may pave the way for further phonon engineering of GeTe and the related thermoelectrics.

Toward high-performance thermoelectric energy conversion, the electrons and holes must work jointly like two wheels of a cart: if not longitudinally, then transversely. The bipolar effect — the main performance restriction in the traditional longitudinal thermoelectricity, can be manipulated to be a performance enhancer in the transverse thermoelectricity. Here, we demonstrate this idea in semimetal Mg2Pb. At 30 K, a giant transverse thermoelectric power factor as high as 400 μWcm−1K−2 is achieved, a 3 orders-of-magnitude enhancement than the longitudinal configuration. The resultant specific heat pumping power is ~ 1 Wg−1, higher than those of existing techniques at 10~100 K. A large number of semimetals and narrow-gap semiconductors making poor longitudinal thermoelectrics due to severe bipolar effect are thus revived to fill the conspicuous gap of thermoelectric materials for solid-state applications. Heat pumping is in high demand at cryogenic temperature, but whether thermoelectricity can take on cryogenic heat pumping is an open question. Here, the authors answer this question by leveraging bipolar effect to enhance transverse thermoelectricity in semimetal Mg2Pb for cryogenic heat pumping.

Thermoelectric technology offers the unique advantages of being completely solid-state, silent and emission-free for waste-heat recovery applications. Despite this, existing thermoelectric modules, particularly for recovering low-grade but abundant heat of 10% with a heat source temperature of

Cu3SbSe3 that exhibits distinct liquid-like sublattice due to the heterogeneous bonding environment has emerged as a promising low cost superionic semiconductor with intrinsic ultralow thermal conductivity. However, the relationship between atomic dynamics resulting in liquid-like diffusion and anomalous phonon transport properties remains poorly understood. Herein, combing ab initio molecular dynamics with temperature-dependent Raman measurements, we have performed a thorough investigation on the lattice dynamics of Cu3SbSe3. Superionic transition is unveiled for both structurally inequivalent Cu atoms at elevated temperatures, while the Se-formed tetrahedral framework can simultaneously maintain. An intermediate state of Cu3SbSe3 through the mixture of quasi-1D/2D Cu nearest-neighbor vacancy hopping is discovered below the superionic transition temperature. Our results also manifest that phonons predominately involved with Cu contributions along diffusion channels have been strongly scattered during the superionic transition, whereas the liquid-like diffusion of Cu is too slow to completely breakdown the propagation of all transverse phonon modes. The insight provided by this work into the atomic dynamics and phonon scattering relationship may pave the way for further phonon engineering of Cu3SbSe3 and related superionic materials.

Summary Aliovalent defects are extremely effective in manipulating charge transport and atomic vibrational properties for thermoelectric enhancements. Electronic performance of thermoelectrics is optimized at a reduced Fermi level of ∼0.3, which causes the optimal carrier concentration (nopt) to be strongly temperature dependent. This motivates a dynamic doping approach for electronic enhancements through an increase with temperature of solubility of aliovalent dopants. In addition, the defects could simultaneously act as scattering sources of phonons for reducing the lattice thermal conductivity. These effects are illustrated in this work by the temperature-dependent excess Cu solubility in n-PbTe0.75Se0.25 thermoelectrics, in which both carriers and dislocations are induced for regulating the electronic and phononic transport properties for a realization of an extraordinary thermoelectric figure of merit. The resultant defect structures and temperature gradient doping effects (for aliovalent solutes) could in principle open extra possibilities for optimizing charge and phonon transport properties in thermoelectrics.

Ag-based argyrodites with a general formula of Ag(12–n)Bn+X6 (B = Ga/Al/Si/Ge/Sn and X = S/Se/Te) have recently attracted much attention as promising thermoelectrics because of their ultralow latti...

Defect engineering is an effective route to improve the performance of thermoelectric materials, including Sb doped PbSe, but the formation mechanism of defects remains unclear. In the thermoelectric material Pb1−xSb2x/3Se (0 < x ≤ 0.07), a large number of dislocations have been reported, and they enhance intermediate-frequency phonon scattering, thereby improving the zT value. However, the microstructural origin of dislocations remains unclear. In this paper, via a combination of atomic resolution scanning transmission electron microscopy and density functional theory, we successfully revealed the microstructure of Pb1−xSb2x/3Se (x = 0–0.07) for in-depth understanding of the formation mechanism of dislocations. Plenty of zinc blende (ZB) nanostructures are found in the PbSe matrix with a rock salt (RS) structure, and the theoretical calculations confirm its viability from the point of view of formation energy. A similar ZB structure is identified in the dislocation cores of Sb-doped materials as well, and thus the formation mechanism of dislocations is discussed for this PbSe system. This result provides important guidance to understand the structural evolution in compounds with a RS structure, especially in high-performance lead chalcogenide thermoelectric materials.

A high conduction band degeneracy (Nv = 6) leads to an extraordinary figure of merit (zT) in n-type Mg3Sb2 thermoelectrics, while the p-type ones show a much inferior zT due to its lower Nv of only 1 owing to the large energy offset (ΔE ∼ 0.4 eV) between the two valence bands. The derivatives of Mg3Sb2, ternary AB2C2 (A = Eu, Yb, Ca, Sr, Ba; B = Mg, Zn, Cd; C = Sb, Bi) Zintls, show a similar valence band structure but with a much smaller ΔE of

Next-generation thermal management requires the development of low lattice thermal conductivity materials, as observed in ionic conductors. For example, thermoelectric efficiency is increased when thermal conductivity is decreased. Detrimentally, high ionic conductivity leads to thermoelectric device degradation. Battery safety and design also require an understanding of thermal transport in ionic conductors. Ion mobility, structural complexity, and anharmonicity have been used to explain the thermal transport properties of ionic conductors. However, thermal and ionic transport are rarely discussed in direct comparison. Herein, the ionic conductivity of Ag+ argyrodites is found to change by orders of magnitude without altering the thermal conductivity. Thermal conductivity measurements and two-channel lattice dynamics modeling reveal that the majority of Ag+ vibrations have a non-propagating diffuson-like character, similar to amorphous materials. It is found that high ionic mobility is not a requirement for diffuson-mediated transport. Instead, the same bonding and structural traits that can lead to fast ionic conduction also lead to diffuson-mediated transport. Bridging the fields of solid-state ionics and thermal transport, it is proposed that a vibrational perspective can lead to new design strategies for functional ionic conducting materials. As a first step, the authors relate the so-called Meyer–Neldel behavior in ionic conductors to phonon occupations.

Texturization tuning is of crucial significance for designing and developing high-performance thermoelectric materials and devices. Here, we report for the first time that a strong texturization effect induces an in-plane high-performance thermoelectric and an out-of-plane low lattice thermal conductivity in Sb-substituted misfit-layered (SnS)1.2(TiS2)2 alloys. In the in-plane direction, the oriented texture promotes a high carrier mobility, contributing to the maximization of the power factor (∼0.90 mW K-2 m-1). Moreover, the in-plane lattice thermal conductivity dramatically reduces deriving from the point defects due to the Sb substitution and weakened transverse sound velocity owing to the softening of bonding, ultimately leading to one of the highest thermoelectric performances ever reported among misfit-layered chalcogenides. In particular, in the out-of-plane direction, the texturization triggers the lowest lattice thermal conductivity (∼0.39 W K-1 m-1), exceeding the theoretical limit of the Debye-Cahill model, which provides a precious opportunity to investigate this real Sb-substituted (SnS)1.2(TiS2)2 material. The present finding in misfit-layered chalcogenides provides a novel strategy for manipulating thermoelectrics via texturization engineering.

Ultra-low lattice thermal conductivity as often found in superionic compounds is greatly beneficial for thermoelectric performance, however, a high ionic conductivity can lead to device degradation. Conversely, high ionic conductivities are searched for materials in solid-state battery applications. It is commonly thought that ionic transport induces low thermal conductivity and that ion and thermal transport are not completely independent properties of a material. However, no direct comparison or underlying physical relationship has been shown between the two. Here we establish that ionic transport can be varied independent of thermal transport in Ag+ superionic conductors, even though both phenomena arise from atomic vibrations. Thermal conductivity measurements, in conjunction with two-channel lattice dynamics modeling, reveals that the vast majority of Ag+ vibrations have non-propagating diffuson-like character, which provides a rational for how these two transport properties can be independent. Our results provide conceptually novel lattice dynamical insights to ionic transport and confirm that ion transport is not a requirement for ultra-low thermal conductivity. Consequently, this work bridges the fields of solid state ionics and thermal transport, thus providing design strategies for functional ionic conducting materials from a vibrational perspective.

Ternary chalcogenides have attracted great attention for their potential applications in thermoelectric devices. Here, we investigate the pressure and doping effects on the structural stability of BaAg

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.

In recent years, the layered semiconductor tin selenide (SnSe) has been of great interest in the thermoelectric field because of its remarkable thermoelectric potential. Here, the as-grown Sn0.98Se...

For decades, significant thermoelectric enhancements at high temperatures have been witnessed in many materials. However, efficient materials working well near room temperature are still restricted...

Bi2Te3-based compounds are important near room temperature thermoelectric materials with commercial applications in thermoelectric modules. However, new routes leading to improved thermoelectric pe...

Summary Lattice vibrations in crystalline materials generate phonons as heat carriers for heat conduction, and the phonon dispersion (energy versus momentum) is fundamentally determined by the mass of lattice vibrators (atoms) and the interaction force between atoms. A significant manipulation of lattice thermal conductivity through a change in atomic mass usually requires a large variation in chemical composition, which is not always valid thermodynamically or may risk the resultant detriment of other functionalities (e.g., carrier mobility). Here we show a strategy of alternatively manipulating the interaction force between atoms through lattice strains without changing the composition, for remarkably reducing the lattice thermal conductivity without reducing carrier mobility, in Na0.03Eu0.03Sn0.02Pb0.92Te with stable lattice dislocations. This successfully leads to an extraordinarily high thermoelectric figure of merit, with the help of valence band convergence. This work offers both insights and solutions on lattice strain engineering for reducing lattice thermal conductivity, thus advancing thermoelectrics.

Recently, polycrystalline argyrodite Ag8SnSe6 has shown promising thermoelectric performance, which largely relies on the ultralow lattice thermal conductivity. However, the thermoelectric properti...

The anharmonic lattice dynamics of Te is studied based on perturbation theory using the third-order anharmonic force constants. The low lattice thermal conductivity of Te at high temperature is related to its low acoustic phonon lifetimes. In contrast to the expectation that the lattice thermal conductivity increases under compression, we find that it shows a counter-intuitive strain dependence in Te. The lattice thermal conductivity along the Te atomic chain is shown to decrease unexpectedly when a uniaxial compressive strain is applied in this direction. The lattice thermal conductivity along directions perpendicular to the Te atomic chain also shows an abnormal strain dependence. The lattice thermal conductivity under strain is analyzed based on the phonon lifetime and group velocity. Moreover, the variation of bond lengths under uniaxial strains is also discussed to provide further insight into the thermal transport behavior.

Alignment of doubly degenerate pxy and nondegenerate pz band orbitals at the Γ point in the Brillouin zone for high orbital degeneracy has been demonstrated as an effective strategy for advancing thermoelectric AB2C2 Zintl compounds in p-type conduction. Formation of solid solutions of different compounds with opposite energy offsets between pxy and pz orbitals would enable a minimization of energy offset to realize an orbital alignment, and thereby lead to an enhanced electronic performance. Here we show that alloying EuCd2Sb2 with EuZn2Sb2 not only effectively aligns the pxy and pz valence bands for superior electronic performance, but also significantly reduces the lattice thermal conductivity resulting from the phonon scattering by alloy defects. Eventually, the average thermoelectric figure of merit (zT) of the EuCd1.4Zn0.6Sb2 alloy is enhanced by at least 35% compared to that of pristine EuCd2Sb2 and the alloying agent EuZn2Sb2, demonstrating that this alloy is a promising thermoelectric material.

Manipulations of carrier and phonon scatterings through hierarchical structures have been proved to be effective in improving thermoelectric performance. Previous efforts in GeTe-based materials mainly focus on simultaneously optimizing the carrier concentration and band structure. In this work, a synergistic strategy to tailor thermal and electrical transport properties of GeTe by combination with the scattering effects from both Ge vacancies and other defects is reported. The addition of Fe in GeTe-based compounds introduces the secondary phase of FeGe

Advanced functional materials 31(20), 2101214 (2021). doi:10.1002/adfm.202101214, Published by Wiley-VCH, Weinheim