Ultralow Lattice Thermal Conductivity Research Articles

Loose bonding induced ultralow lattice thermal conductivity of a metallic crystal KNaRb

In the domain of metallic systems, electronic thermal conductivity typically governs heat transfer, while lattice (phononic) thermal conductivity (LTC) remains non-negligible magnitude. This study introduces the exceptional metallic material, cubic half-Heusler-type KNaRb, via phonon Boltzmann transport equation (BTE) resolution and first-principles calculations. KNaRb exhibits an ultralow LTC of 0.114 W/mK at room temperature, comparable to the lowest reported for metallic materials, contributing merely 0.66 % to overall thermal transport. Analysis attributes KNaRb's low LTC to low group velocity and strong anharmonicity, originating from its loosely bonded electronic structure. Acoustic modes, primarily from Rb atoms, dominate thermal transport. Examination of mean square displacement and potential energy profiles reveals significant movement of loosely bonded Rb atoms within KNaRb, acting as intrinsic "rattlers," inducing pronounced phonon anharmonicity and ultrashort lifetimes. This study advances understanding of heat conduction in metals, offering insights into materials with extremely low lattice thermal conductivity for potential future applications.

Ultralow two-channel thermal conductivity in aikinite

Realizing glass-like ultralow lattice thermal conductivity κL in an ordered crystalline material is overwhelmingly appealing for various applications in thermoelectrics and thermal barrier coatings. Here, based on first-principles calculations, we take CuPbBiS3 as an example and systematically study the underlying physical mechanism of the ultralow and glass-like κL in aikinite. It is found that the strong anharmonic renormalization induces temperature-driven stiffening of the acoustic modes and low-lying optical modes from Pb, Cu, and Bi atoms, while leading to the softening of high-frequency optical phonon modes from S atoms. Further analyses highlight that CuPbBiS3 hosts metavalent bonding, stereochemical lone pair activity, and loosely bonded rattling atoms. These characteristics arouse partially liquid-like state, synergistically contribute to strong anharmonicity, and result in unexpectedly low thermal conductivity. Finally, our calculated κL at 300 K is 0.68 Wm−1K−1 with a non-standard dependency of κL ∝ T−0.7, which reasonably agrees well with experimental values in both magnitude (0.52 ± 0.05 Wm−1K−1 at 300 K) and temperature-dependence trend (∝ T−0.2). We demonstrate that the anomalous thermal transport originates from the dual particle-wave behavior of the heat-carrying phonons, in which wavelike tunneling contribute over 18% to the total κL when T exceeds 300 K. Our work underpins the microscopic origins of ultralow κL in CuPbBiS3, providing crucial insights into designing highly efficient aikinite materials for thermoelectric conversion.

Structural evolution and thermoelectric performance in (GeTe)m(Sb2Te3)n compounds

Exploring the relationship between crystal structure and thermoelectric performance is a pivotal topic in the thermoelectric field. In this study, we have comprehensively investigated the correlation between the structural evolution of (GeTe)m(Sb2Te3)n pseudo-binary system and the thermoelectric properties. The proportion of van der Waals bonds increases with the rising Sb2Te3 content, resulting in an increase in the anisotropy of the electrical conductivity and a decrease in the average sound velocity. Additionally, the cation sites in the crystal lattice of these compounds exhibit a mixed occupancy of Ge/Sb atoms, although the cation sites adjacent to the van der Waals gaps are predominantly occupied by Sb atoms. The ultra-low lattice thermal conductivity of the GST124 and GST147 compounds is mainly attributed to the high concentration of van der Waals bonds and enhanced phonon scattering arising from Ge/Sb mixed cation occupancy and high density of defect structures. The high electrical conductivity combined with the low lattice thermal conductivity enables GST124 and GST147 compounds to achieve a maximum ZT value of 0.56 and 0.57, respectively. Higher thermoelectric performance can be achieved through optimization of the microstructure as well as the carrier concentration.

N‐Type CuIn5Se8‐Based Thermoelectric Materials with All‐Scale Hierarchical Architectures

AbstractCopper (Cu)‐based thermoelectric (TE) materials have attracted great attention from both scientific and industrial societies, but for a long time, their real applications are greatly limited by the lack of high‐performance n‐type Cu‐based TE materials. Most recently, the novel n‐type Cu‐based TE material, CuIn5Se8, has been discovered to show a record‐high TE figure‐of‐merit (zT) to match the state‐of‐the‐art p‐type Cu‐based TE materials. However, the physical origin of such high zT is still unclear due to its complex phase compositions and crystal structures. In this work, it is revealed that the excellent TE performance is mainly contributed by the intrinsically ultralow lattice thermal conductivity originating from the unique all‐scale hierarchical architecture. It covers the ranges from atomic‐scale cation disorder in the tetragonal CuIn5Se8 phase and nanoscale diversified stacking units and stacking sequences in the hexagonal CuIn5Se8 phase, to mesoscale grain boundaries between the tetragonal phase and hexagonal phase. Doping Br at the Se‐sites can largely tune the electrical transports of CuIn5Se8 while maintaining the ultralow lattice thermal conductivity, leading to high zT reaching the optimal value predicted by the single parabolic model. This work will guide the investigation of n‐type Cu‐based TE materials in the future.

Silver Copper Chalcogenide Thermoelectrics: Advance, Controversy, and Perspective.

Thermoelectric technology, which enables a direct and pollution-free conversion of heat into electricity, provides a promising path to address the current global energy crisis. Among the broad range of thermoelectric materials, silver copper chalcogenides (AgCuQ, Q = S, Se, Te) have garnered significant attention in thermoelectric community in light of inherently ultra-low lattice thermal conductivity, controllable electronic transport properties, excellent thermoelectric performance across various temperature ranges, and a degree of ductility. This review epitomizes the recent progress in AgCuQ-based thermoelectric materials, from the optimization of thermoelectric performance to the rational design of devices, encompassing the fundamental understanding of crystal structures, electronic band structures, mechanical properties, and quasi-liquid behaviors. The correlation between chemical composition, mechanical properties, and thermoelectric performance in this material system is also highlighted. Finally, several key issues and prospects are proposed targeting further optimizing AgCuQ-based thermoelectric materials. This article is protected by copyright. All rights reserved.

Synergistically optimized electron and phonon transport in high-performance copper sulfides thermoelectric materials via one-pot modulation

Optimizing thermoelectric conversion efficiency requires the compromise of electrical and thermal properties of materials, which are hard to simultaneously improve due to the strong coupling of carrier and phonon transport. Herein, a one-pot approach realizing simultaneous second phase and Cu vacancies modulation is proposed, which is effective in synergistically optimizing thermoelectric performance in copper sulfides. Multiple lattice defects, including nanoprecipitates, dislocations, and nanopores are produced by adding a refined ratio of Sn and Se. Phonon transport is significantly suppressed by multiple mechanisms. An ultralow lattice thermal conductivity is therefore obtained. Furthermore, extra Se is added in the copper sulfide for optimizing electrical transport properties by inducing generating Cu vacancies. Ultimately, an excellent figure of merit of ~1.6 at 873 K is realized in the Cu1.992SSe0.016(Cu2SnSe4)0.004 bulk sample. The simple strategy of inducing compositional and structural modulation for improving thermoelectric parameters promotes low-cost high-performance copper sulfides as alternatives in thermoelectric applications.

Giant Deformation Induced Staggered-Layer Structure Promoting the Thermoelectric and Mechanical Performance in n-Type Bi2(Te, Se)3.

Bismuth telluride has long been recognized as the most promising near-room temperature thermoelectric material for commercial application; however, the thermoelectric performance for n-type Bi2(Te, Se)3-based alloys is far lagging behind that of its p-type counterpart. In this work, a giant hot deformation (GD) process is implemented in an optimized Bi2Te2.694Se0.3I0.006+3 wt%K2Bi8Se13 precursor and generates a unique staggered-layer structure. The staggered-layered structure, which is only observed in severely deformed crystals, exhibits a preferential scattering on heat-carrying phonons rather than charge-carrying electrons, thus resulting in an ultralow lattice thermal conductivity while retaining high-weight carrier mobility. Moreover, the staggered-layer structure is located adjacent to the van der Waals gap in Bi2(Te, Se)3 lattice and is able to strengthen the interaction between anion layers across the gap, leading to obviously improved compressive strength and Vickers hardness. Consequently, a high peak figure of merit ZT of ≈ 1.3 at 423 K, and an average ZT of ≈ 1.2 at 300-473 K can be achieved in GD sample. This study demonstrates that the GD process can successfully decouple the electrical and thermal transports with simultaneously enhanced mechanic performance.

Amorphous-Like Ultralow Thermal Transport in Crystalline Argyrodite Cu7PS6.

Due to their amorphous-like ultralow lattice thermal conductivity both below and above the superionic phase transition, crystalline Cu- and Ag-based superionic argyrodites have garnered widespread attention as promising thermoelectric materials. However, despite their intriguing properties, quantifying their lattice thermal conductivities and a comprehensive understanding of the microscopic dynamics that drive these extraordinary properties are still lacking. Here, an integrated experimental and theoretical approach is adopted to reveal the presence of Cu-dominated low-energy optical phonons in the Cu-based argyrodite Cu7PS6. These phonons yield strong acoustic-optical phonon scattering through avoided crossing, enabling ultralow lattice thermal conductivity. The Unified Theory of thermal transport is employed to analyze heat conduction and successfully reproduce the experimental amorphous-like ultralow lattice thermal conductivities, ranging from 0.43 to 0.58Wm-1 K-1, in the temperature range of 100-400 K. The study reveals that the amorphous-like ultralow thermal conductivity of Cu7PS6 stems from a significantly dominant wave-like conduction mechanism. Moreover, the simulations elucidate the wave-like thermal transport mainly results from the contribution of Cu-associated low-energy overlapping optical phonons. This study highlights the crucial role of low-energy and overlapping optical modes in facilitating amorphous-like ultralow thermal transport, providing a thorough understanding of the underlying complex dynamics of argyrodites.

Study of electronic, mechanical, thermoelectric, and optical aspects of K2AlAg(Br/I)6 for solar cells, and energy storage applications

Double perovskite materials exhibit considerable potential in addressing global energy challenges and advancing renewable energy technologies. This study focuses on K2AlAg(Br/I)6 and employs a comprehensive DFT approach to explore its structural, mechanical, thermodynamic, optical, and thermoelectric properties. The structural and thermodynamic stabilities of K2AlAg(Br/I)6 are accurately examined through tolerance factor (τF) and formation energy analysis. The mechanical stability and ductile characteristics are affirmed by studying elastic constants, Passion's, and Pugh's ratios. Additionally, insights into hardness, Debye temperature, as well as transverse and longitudinal sound velocities contribute to a deeper understanding of the material's thermodynamic response. The band gap of K2AlAgBr6 is identified as 2.71 eV, which undergoes a reduction to 1.17 eV when Br is substituted with I, attributed to the pd-hybridization of cations and anions. Exploring optical properties reveals valuable information through dielectric constants ε(ω), refractive index (n(ω)), absorption (α(ω)), and reflectivity (R(ω)). Notably, absorption in both visible and ultraviolet ranges underscores the material's significance for solar cell applications. Furthermore, the computed ultralow lattice thermal conductivity and higher figure of merit are noteworthy outcomes, driven by the substantial Seebeck coefficient and electrical conductivity of the material. These results collectively highlight the exclusive and promising characteristics of K2AlAg(Br/I)6 in the area of renewable energy materials.

Realizing Ultrahigh Thermoelectric Performance in n‐Type PbSe Through Lattice Planification and Introducing Liquid‐Like Cu Ions

AbstractThe coupling relationship between electrical and thermal transports makes it rather challenging to enhance thermoelectric performance. Here, electrical and thermal transports are successfully decoupled to realize high performance in n‐type PbSe by utilizing a stepwise strategy. First, the PbSe lattice is plained with extra Pb to compensate for the intrinsic Pb vacancies, which can weaken defect scattering and improve carrier mobility to ≈1230 cm2 V−1 s−1. The room‐temperature power factor triples and reaches ≈32 µW cm−1 K−2, and ZT is significantly enhanced to ≈0.6 in Pb1.006Se. Subsequently, liquid‐like interstitial Cu ions are introduced to inhibit heat conduction without damaging electrical transport. While maintaining a high power factor of ≈25 µW cm−1 K−2, Cu ions strongly suppress phonon transport at high temperature, leading to an ultralow lattice thermal conductivity of ≈0.28 W m−1 K−1 in Pb1.006Cu0.006Se, only 30% of the Cu‐free PbSe. Eventually, a remarkable peak ZT of ≈1.8 at 773 K is achieved along with a high average ZT of ≈1.1 from 300 to 823 K in Pb1.006Cu0.006Se. An outstanding experimental conversion efficiency of ≈7.1% is obtained in the single‐leg device, demonstrating great potential for PbSe as low‐ to mid‐temperature thermoelectrics.

Zinc Doping Induces Enhanced Thermoelectric Performance of Solvothermal SnTe.

The creation of hierarchical nanostructures can effectively strengthen phonon scattering to reduce lattice thermal conductivity for improving thermoelectric properties in inorganic solids. Here, we use Zn doping to induce a remarkable reduction in the lattice thermal conductivity in SnTe, approaching the theoretical minimum limit. Microstructure analysis reveals that ZnTe nanoprecipitates can embed within SnTe grains beyond the solubility limit of Zn in the Zn alloyed SnTe. These nanoprecipitates result in a substantial decrease of the lattice thermal conductivity in SnTe, leading to an ultralow lattice thermal conductivity of 0.50 W m-1  K-1 at 773 K and a peak ZT of ~0.48 at 773 K, marking an approximately 45 % enhancement compared to pristine SnTe. This study underscores the effectiveness of incorporating ZnTe nanoprecipitates in boosting the thermoelectric performance of SnTe-based materials.

Enhancement in thermoelectric performance of hydrothermally synthesized Ca and Sb co-doped Bi2Te3 nanostructures

−Ca & Sb co-doped Bi2Te3 compounds have been prepared by hydrothermal method at 210 °C for 24 h and investigated their thermoelectric properties. Phase purity and crystallinity were analyzed by XRD. All the prepared samples have rhombohedral crystal structure with space group R-3m. The hexagonal nanoplate-like morphology was examined by FESEM. Elemental analysis was done with EDX. Band gap energy of prepared samples has values in the range of ∼0.40–0.65 eV, obtained by Tauc plot. The Raman shift was obtained at a lower frequency with doping. Carrier concentration increased with doping from 3.18 × 1020 cm−1 to 9.34 × 1020 cm−1. The high value of power factor (PF) of ∼10.8 × 10–4 Wm−1K−2 was obtained due to high carrier concentration. An ultralow lattice thermal conductivity of ∼0.28 and ∼0.63 W mK−1 at 420 K, was obtained for Ca0.06Bi1.88Sb0.06Te3 and pure Bi2Te3, respectively. A maximum ZT of ∼0.78 at 386 K was obtained for Ca0.03Bi1.94Sb0.03Te3. The value of ZT thus obtained is about ∼ 51% higher than the ZT of pure Bi2Te3 (∼0.39 at 386 K).

Micromechanism for Copper-Deficiency Enhanced Stability: A Quasi In Situ TEM Study of Electric-Induced Structural Evolution in Cu2-x(S, Se) Liquid-Like Thermoelectric Materials.

Cu-based liquid-like thermoelectric materials have garnered tremendous attention due to their inherent ultralow lattice thermal conductivity. However, their practical application is hampered by stability issues under a large current or temperature gradient. It has been reported that introduction of copper vacancies can enhance the chemical stability, whereas the micromechanism behind this macroscopic improvement still remains unknown. Here, we have established a quasi in situ TEM method to examine and compare the structural evolution of Cu2-xS0.2Se0.8 (x = 0, 0.05) under external electric fields. It is then found that the preset Cu vacancies could favor the electric-induced formation of a more stable intermediate phase, i.e., the hexagonal CuSe-type structure in the form of either lamellar defects (majorly) or long-range order (minorly), in which ordering of S and Se also occurred. Thereby, copper and chalcogen atoms could largely be solidified into the matrix, and the elemental deposition and evaporation process is mitigated under an electric field.

Hydrogenation driven ultra-low lattice thermal conductivity in β 12 borophene

Borophene gathered large interest owing to its polymorphism and intriguing properties such as Dirac point, inherent metallicity, etc but oxidation limits its capabilities. Hydrogenated borophene was recently synthesised experimentally to harness its applications. Motivated by experimental work, in this paper, using first-principles calculations and Boltzmann transport theory, we study the freestanding β 12 borophene nanosheet doped and functionalised with hydrogen (H), lithium (Li), beryllium (Be), and carbon (C) atoms at different β 12 lattice sites. Among all possible configurations, we screen two stable candidates, pristine and hydrogenated β 12 borophene nanosheets. Both nanosheets possess dynamic and mechanical stability while the hydrogenated sheet has different anisotropic metallicity compared to pristine sheet leading to enhancement in brittle behaviour. Electronic structure calculations reveal that both nanosheets host Dirac cones (DCs), while hydrogenation leads to shift and enhancement in tilt of the DCs. Further hydrogenation leads to the appearance of additional Fermi pockets in the Fermi surface. Transport calculations reveals that the lattice thermal conductivity changes from 12.51 to 0.22 W m−1 K−1 (along armchair direction) and from 4.42 to 0.07 W m−1 K−1 (along zigzag direction) upon hydrogenation at room temperature (300 K), demonstrating a large reduction by two orders of magnitude. Such reduction is mainly attributed to decreased phonon mean free path and relaxation time along with the enhanced phonon scattering rates stemming from high frequency phonon flat modes in hydrogenated nanosheet. Comparatively larger weighted phase space leads to increased anharmonic scattering in hydrogenated nanosheet contributing to ultra-low lattice thermal conductivity. Consequently, hydrogenated β 12 nanosheet exhibits a comparatively higher thermoelectric figure of merit (∼0.75) at room temperature along armchair direction. Our study demonstrates the effects of functionalisation on transport properties of freestanding β 12 borophene nanosheets which can be utilised to enhance the thermoelectric performance in two-dimensional (2D) systems and expand the applications of boron-based 2D materials.

A New N‐Type High Entropy Semiconductor AgBiPbSe2S with High Thermoelectric and Mechanical Properties

AbstractExploring N‐type High‐entropy materials with both high thermoelectric and mechanical properties is highly desirable for all‐high‐entropy thermoelectric generators (TEGs) since the thermoelectric and mechanical properties of N‐type one are largely behind its P‐type counterparts. Herein, a new rock‐salt structure N‐type high entropy thermoelectric AgBiPbSe2S is introduced with a bandgap of ≈0.43 eV. The atomic radii difference of each component results in a large lattice distortion of 0.246, leading to a low thermal conductivity of 0.36 W m−1 K−1 at 823 K. The figure of merit (ZT) reaches 0.6 for AgBiPbSe2S at 823 K. Moreover, Ag2Se precipitates are included in AgBiPbSe2S to filter low energy carriers for high Seebeck coefficients and to scatter phonons with nanoscale wavelength for ultralow lattice thermal conductivities. Consequently, a peak ZT of ≈1.18 at 823 K and an average ZT of 0.60 at 400–823 K are obtained for Ag1.02BiPbSe2S. More importantly, high mechanical properties are also obtained in Ag1.02BiPbSe2S, of which the Vickers hardness and flexural strength are ≈209 Hv and 32 MPa, respectively, originating from the enhanced lattice friction by chemical short‐range disorder (i.e., high entropy effect) and dispersion strengthening caused by Ag2Se nanoprecipitates.

Temperature and composition insensitivity of thermoelectric properties of high-entropy half-heusler compounds

Composition modification by doping and solid solution is a well-studied strategy in thermoelectric (TE) materials to optimize their properties. Recently, the concept of entropy stabilization has offered the possibility of forming random solid solutions that have properties that go beyond the rule of mixture. In this study, we prepared a series of high-entropy half-Heusler solid solutions (HEHHs) with varying valence electron counts (VEC), (Ti0.33Zr0.33Hf0.33)1-x(V0.33Nb0.33Ta0.33)xCoSb (x = 0.5 to 0.75). Compared to their medium- and low-entropy counterparts, the TE properties of HEHHs are less sensitive to temperature and composition variation (charge carrier concentration efficiency of ∼10 %). An ultra-low lattice thermal conductivity for half-Heusler of 1.19 W·m−1·K−1 was achieved.

Multilayer Approach for Ultralow Lattice Thermal Conductivity in Low-Dimensional Solids

Multilayer Approach for Ultralow Lattice Thermal Conductivity in Low-Dimensional Solids

CdSe Quantum Dots Enable High Thermoelectric Performance in Solution-Processed Polycrystalline SnSe.

Here, a high peakZTof ≈2.0 is reported in solution-processed polycrystalline Ge and Cd codoped SnSe. Microstructural characterization reveals that CdSe quantum dots are successfully introduced by solution process method. Ultraviolet photoelectron spectroscopy evinces that CdSe quantum dots enhance the density of states in the electronic structure of SnSe, which leads to a large Seebeck coefficient. It is found that Ge and Cd codoping simultaneously optimizes carrier concentration and improves electrical conductivity. The enhanced Seebeck coefficient and optimization of carrier concentration lead to marked increase in power factor. CdSe quantum dots combined with strong lattice strain give rise to strong phonon scattering, leading to an ultralow lattice thermal conductivity. Consequently, high thermoelectric performance is realized in solution-processed polycrystalline SnSe by designing quantum dot structures and introducing lattice strain. This work provides a new route for designing prospective thermoelectric materials by microstructural manipulation in solution chemistry.

Nuanced dilute doping strategy enables high-performance GeTe thermoelectrics

In thermoelectrics, doping is essential to augment the figure of merit. Traditional strategy, predominantly heavy doping, aims to optimize carrier concentration and restrain lattice thermal conductivity. However, this tactic can severely hamper carrier transport due to pronounced point defect scattering, particularly in materials with inherently low carrier mean-free-path. Conversely, dilute doping, although minimally affecting carrier mobility, frequently fails to optimize other vital thermoelectric parameters. Herein, we present a more nuanced dilute doping strategy in GeTe, leveraging the multifaceted roles of small-size metal atoms. A mere 4% CuPbSbTe3 introduction into GeTe swiftly suppresses rhombohedral distortion and optimizes carrier concentration through the aid of Cu interstitials. Additionally, the formation of multiscale microstructures, including zero-dimensional Cu interstitials, one-dimensional dislocations, two-dimensional planar defects, and three-dimensional nanoscale amorphous GeO2 and Cu2GeTe3 precipitates, along with the ensuing lattice softening, contributes to an ultralow lattice thermal conductivity. Intriguingly, dilute CuPbSbTe3 doping incurs only a marginal decrease in carrier mobility. Subsequent trace Cd doping, employed to alleviate the bipolar effect and align the valence bands, yields an impressive figure-of-merit of 2.03 at 623 K in (Ge0.97Cd0.03Te)0.96(CuPbSbTe3)0.04. This leads to a high energy-conversion efficiency of 7.9% and a significant power density of 3.44 W cm−2 at a temperature difference of 500 K. These results underscore the invaluable insights gained into the constructive role of nuanced dilute doping in the concurrent tuning of carrier and phonon transport in GeTe and other thermoelectric materials.

Four-phonon scattering of so-As and improvement of the thermoelectric properties by increasing the buckling height.

In recent years, more and more thermoelectric (TE) materials have been discovered as the research boom of TE materials advances. However, due to the low conversion efficiency, most of the current TE materials cannot meet the commercial demand. The low-dimensional nanomaterials are promising to break the current status quo of low conversion efficiency of TE materials. Here, we predicted a stable two-dimensional TE material, namely so-As, based on density functional theory. The so-As has an ultra-low lattice thermal conductivity,κl= 1.829 W m-1K-1at 300 K, and when the temperature rises to 700 K theκlis only 0.788 W m-1K-1. This might be caused by the strong anharmonic interaction among the so-As phonon and the out-of-plane vibration of the low-frequency acoustic modes. Moreover, the maximumZTvalue of thep-type so-As is 0.18 at room temperature (0.45 at 700 K), while that of then-type can even reach 0.75 at 700 K. In addition, we have also studied the difference between the four- and three-phonon scattering rates. The increase of scattering channels leads to the ultra-lowκl, which is only 3.33 × 10-4W m-1K-1at room temperature, showing an almost adiabatic property. Finally, we adjust the TE properties of so-As by changing the buckling height. With the buckling height is increased by 2%, the scattering rate of so-As is extremely high. WhenTis 700 K, the maximumZTof then-type is 0.94 (p-type can also reach 0.7), which is 25% higher than the pristine one. Our work reveals the impact of buckling height on the TE figure of merit, which provides a direction for future search and regulation of the highZTTE materials.