Single-and Two-band Transport Properties Crossover in Bi_(2)Te_(3)Based Thermoelectrics

Based on Peltier effect,Bi_(2)Te_(3)-based alloy is widely used in commercial solid-state refrigeration at room temperature.The mainstream strategies for enhancing room-temperature thermoelectric performance in Bi_(2)Te_(3)focus on band and microstructure engineering.However,a clear understanding of the modulation of band structure and scattering through such engineering remains still challenging,because the minority carriers compensate partially the overall transport properties for the narrow-gap Bi_(2)Te_(3)at room temperature(known as the bipolar effect).The purpose of this work is to model the transport properties near and far away from the bipolar effect region for Bi_(2)Te_(3)-based thermoelectric material by a two-band model taking contributions of both majority and minority carriers into account.This is endowed by shifting the Fermi level from the conduction band to the valence band during the modeling.A large amount of data of Bi_(2)Te_(3)-based materials is collected from various studies for the comparison between experimental and predicted properties.The fundamental parameters,such as the density of states effective masses and deformation potential coefficients,of Bi_(2)Te_(3)-based materials are quantified.The analysis can help find out the impact factors(e.g.the mobility ratio between conduction and valence bands)for the improvement of thermoelectric properties for Bi_(2)Te_(3)-based alloys.This work provides a convenient tool for analyzing and predicting the transport performance even in the presence of bipolar effect,which can facilitate the development of the narrow-gap thermoelectric semiconductors.

Ga intercalation in van der Waals layers for advancing p-type Bi_(2)Te_(3)-based thermoelectrics

Tetradymite-structured chalcogenides,such as Bi_(2)Te_(3) and Sb_(2)Te_(3),are quasi-two-dimensional(2D)layered compounds,which are significant thermoelectric materials applied near room temperature.The intercalation of guest species in van der Waals(vdW)gap implemented for tunning properties has attracted much attention in recent years.We attempt to insert Ga atoms in the vdW gap between the Te layers in p-type Bi_(0.3)Sb_(1.7)Te_(3)(BST)for further improving thermoelectrics.The vdW-related defects(including extrinsic interstitial and intrinsic defects)induced by Ga intercalation can not only modulate the carrier concentration but also enhance the texture,thereby yielding excellent electrical properties,which are reflected in the power factor PF~4.43 mW·m^(-1)·K^(-2).Furthermore,the intercalation of Ga produces multi-scale lattice imperfections such as point defects,Te precipitations,and nanopores,realizing the low lattice thermal conductivity in BST-Ga samples.Ultimately,a peak zT~1.1 at 373 K is achieved in the BST-1%Ga sample and greatly improved by~22%compared to the pristine BST.The weak bonding of vdW interlayer interaction can boost the synergistic effect for advancing BST-based or other layered thermoelectrics.

Tunable anharmonicity versus high-performance thermoelectrics and permeation in multilayer(GaN)_(1-x)(ZnO)_(x)

Nonisovalent(GaN)_(1-x)(ZnO)_(x)alloys are more technologically promising than their binary counterparts because of the abruptly reduced band gap.Unfortunately,the lack of two-dimensional(2D)configurations as well as complete stoichiometries hinders to further explore the thermal transport,thermoelectrics,and adsorption/permeation.We identify that multilayer(GaN)_(1-x)(ZnO)_(x)stabilize as wurtzite-like Pm-(GaN)_(3)(ZnO)_(1),Pmc2_(1)-(Ga N)_(1)(ZnO)_(1),P3m1-(GaN)_(1)(ZnO)_(2),and haeckelite C2/m-(GaN)_(1)(ZnO)_(3)via structural searches.P3m1-(GaN)_(1)(ZnO)_(2)shares the excellent thermoelectrics with the figure of merit ZT as high as 3.08 at 900 K for the p-type doping due to the ultralow lattice thermal conductivity,which mainly arises from the strong anharmonicity by the interlayer asymmetrical charge distributions.The p–d coupling is prohibited from the group theory in C2/m-(Ga N)_(1)(ZnO)_(3),which thereby results in the anomalous band structure versus Zn O composition.To unveil the adsorption/permeation of H^(+),Na^(+),and OH^(-)ions in AA-stacking configurations,the potential wells and barriers are explored from the Coulomb interaction and the ionic size.Our work is helpful in experimental fabrication of novel optoelectronic and thermoelectric devices by 2D(GaN)_(1-x)(ZnO)_(x)alloys.

The Quantum Chemistry Calculation and Thermoelectricsof Bi-Sb-Te Series

The density junction theory and discrete variation method ( DFT - DVM) was used to study correlation between composition, structure, chemical bond, and property of thermoelectrics of Bi-Sb-Te series. 8 models of Bi20-xSbxTe32(x = 0,2,6,8,12,14,18 and 20) were calculated. The results show that there is less difference in the ionic bonds between Te( I)-Bi(Sb) and Te(Ⅱ)-Bi(Sb) , but the covalent bond of Te(Ⅰ)-Bi( Sb ) is stronger than that of Te(Ⅱ)-Bi( Sb ) . The interaction between Te(Ⅰ) and Te(Ⅰ) in different layers is the weakest and the interaction should be Van Der Wools power. The charge of Sb is lower than that of Bi, and the ionic bond of Te-Sb is weaker than that of Te-Bi. The covalent bond of Te-Sb is also weaker than that of Te-Bi. Therefore, the thermoelectric property may be imfiroved by adjusting the electrical conductivity and thermal conductivity through changing the composition in the compounds of Bi-Sb-Te. The calculated results are consistent with the experiments.

Advancing thermoelectrics by suppressing deep-level defects in Pb-doped AgCrSe_(2) alloys

AgCrSe2-based compounds have attracted much attention as an environmentally friendly thermoelectric material in recent years due to the intriguing liquid-like properties.However,the ultra-low carrier concentration and the high Ag_(Cr)deep-level defects limit the overall thermoelectric performance.Here,we successfully introduced Pb into Ag-deficient Ag_(0.97)CrSe_(2) alloys to tune the carrier concentration across a broad temperature range.The Pb^(2+) as an acceptor dopant preferentially occupies Cr sites,boosting the hole carrier concentration to 1.77×10^(19) cm^(-3) at room temperature.Furthermore,the Pb strongly inhibits the creation of intrinsic Ag_(Cr) defects,weakens the increased thermal excited ionization with the increasing temperature and slowed the rising trend of the carrier concentration.The designed carrier concentration matches the theoretically predicted optimized one over the entire temperature range,leading to a remarkable enhancement in power factor,especially the maximum power factor of ~500 μW·m^(-1)·K^(-2) at 750 K is superior to most previous results.Additionally,the abundant point defects promote phonon scattering,thus reducing the lattice thermal conductivity.As a result,the maximum figure of merit zT(~0.51 at 750 K) is achieved in Ag_(0.97)Cr_(0.995)Pb_(0.005)Se_(2).This work confirms the feasibility of manipulating deep-level defects to achieve temperature-dependent optimal carrier concentration and provides a valuable guidance for other thermoelectric materials.

The MatHub-3d first-principles repository and the applications on thermoelectrics

Following the Materials Genome Initiative project,materials research has embarked a new research paradigm centered around material repositories,significantly accelerating the discovery of novel materials,such as thermoelectrics.Thermoelectric materials,capable of directly converting heat into electricity,are garnering increasing attention in applications like waste heat recovery and refrigeration.To facilitate research in this emerging paradigm,we have established the Materials Hub with Three-Dimensional Structures(MatHub-3d)repository,which serves as the foundation for high-throughput(HTP)calculations,property analysis,and the design of thermoelectric materials.In this review,we summarize recent advancements in thermoelectric materials powered by the MatHub-3d,specifically HTP calculations of transport properties and material design on key factors.For HTP calculations,we develop the electrical transport package for HTP purpose,and utilize it for materials screening.In some works,we investigate the relationship between transport properties and chemical bonds for particular types of thermoelectric compounds based on HTP results,enhancing the fundamental understanding about interested compounds.In our work associated with material design,we primarily utilize key factors beyond transport properties to further expedite materials screening and speedily identify specific materials for further theoretical/experimental analyses.Finally,we discuss the future developments of the MatHub-3d and the evolving directions of database-driven thermoelectric research.

Cold sintering process:A green route to fabricate thermoelectrics

Cold sintering is a newly developed low-temperature sintering technique that has attracted extensive attention in the fabrication of functional materials and devices.Low sintering temperatures allow for a substantial reduction in energy consumption,and simple experimental equipment offers the possibility of large-scale fabrication.The cold sintering process(CSP)has been demonstrated to be a green and cost-effective route to fabricate thermoelectric(TE)materials where significant grain growth,secondary phase formation,and element volatilization,which are prone to occur during high-temperature sintering,can be well controlled.In this review,the historical development,understanding,and application of thermoelectric materials produced via cold sintering are highlighted.The latest attempts related to the cold sintering process for thermoelectric materials and devices are discussed and evaluated.Despite some current technical challenges,cold sintering provides a promising and sustainable route for the design of advanced high-performance thermoelectrics.

Quasi-Solid Thermoelectrics for Wearable Low-Grade Energy Harvesting

Ionic thermoelectricity(i-TE),as a new energy conversion and storage technology,has been widely discussed by the academic community.As one of the representatives of low-grade thermal energy recovery,i-TE has made remarkable progress and become an influential research direction in the energy field.Among them,thermoelectric ionogels have a wide range of applications in the field of energy recovery and utilization due to their excellent flexibility,stability,and thermoelectric conversion ability,providing many application possibilities for such materials.The development of highly efficient and stable ionic thermoelectric devices is largely dependent on the development of new materials and structural designs.This paper focuses on the recent strategies for improving the efficiency of thermoelectric conversion in the field of ionic thermoelectric gels,including new methods for material design,structural optimization,and innovative developments in the application of thermoelectric materials.The evaluation indicators of thermoelectric conversion efficiency are discussed,including ionic thermal voltage,ionic conductivity and power output,ductility,and self-healing properties.Additionally,various application devices based on thermoelectric materials with excellent thermoelectric conversion properties are highlighted.Further,different challenges and strategies that need to be addressed are presented in the hope of providing inspiration and guidance for the commercialization of i-TE.

Weavable thermoelectrics: advances, controversies, and future developments

Owing to the capability of the conversion between thermal energy and electrical energy and their advantages of light weight,compactness,noise-free operation,and precision reliability,wearable thermoelectrics show great potential for diverse applications.Among them,weavable thermoelectrics,a subclass with inherent flexibility,wearability,and operability,find utility in harnessing waste heat from irregular heat sources.Given the rapid advancements in this field,a timely review is essential to consolidate the progress and challenge.Here,we provide an overview of the state of weavable thermoelectric materials and devices in wearable smart textiles,encompassing mechanisms,materials,fabrications,device structures,and applications from recent advancements,challenges,and prospects.This review can serve as a valuable reference for researchers in the field of flexible wearable thermoelectric materials and devices and their applications.

Revolution in thermoelectric cooling using PbSe thermoelectrics by grid plainification

Thermoelectric materials and devices enable direct conversion between heat and electricity,holding potential applications in thermoelectric power generation,localized cooling,and electronic thermal management[1].However,despite widespread applications,thermoelectric technology remains constrained by material performance[2].

Close-packed layer spacing as a practical guideline for structure symmetry manipulation of IV-VI/I-V-VI_(2) thermoelectrics

The crystal-structure symmetry in real space can be inherited in the reciprocal space,making high-symmetry materials the top candidates for thermoelectrics due to their potential for significant electronic band degeneracy.A practical indicator that can quantitatively describe structural changes would help facilitate the advanced thermoelectric material design.In face-centered cubic structures,the spatial environment of the same crystallographic plane family is isotropic,such that the distances between the close-packed layers can be derived from the atomic distances within the layers.Inspired by this,the relationship between inter-and intra-layer geometric information can be used to compare crystal structures with their desired cubic symmetry.The close-packed layer spacing was found to be a practical guideline of crystal structure symmetry in IV-VI chalcogenides and I-V-VI_(2) ternary semiconductors,both of which are historically important thermoelectrics.The continuous structural evolution toward high symmetry can be described by the layer spacing when temperature or/and composition change,which is demonstrated by a series of pristine and alloyed thermoelectric materials in this work.The layerspacing-based guideline provides a quantitative pathway for manipulating crystal structures to improve the electrical and thermal properties of thermoelectric materials.

Clathrate structure of YB_(3)C_(3) for high-performance thermoelectrics with superior mechanical properties

Exploring high-performance thermoelectric materials with improved mechanical properties is important for broadening the application scope and the assembly requirement of stable devices.This work presents an effective strategy to discover hard thermoelectric material by inserting foreign atoms in the rigid covalent framework.We demonstrate this in boron-carbon clathrate VII structure,showing a promising candidate for highly efficient thermoelectric energy conversion,especially with Y atom filled in the cage,with a peak zT of 0.73 at 1,000 K.The ab initio calculations indicate that YB_(3)C_(3) system has low lattice thermal conductivity of 4.5 W/(m·K)at 1,000 K due to the strong rattling of encaged Y atom.The strongly covalent framework provides highly degenerate band structures consisting of heavy and light electron pockets,which can maintain high carrier mobility arising from small effective mass and thus large group velocity.Consequently,high power factor can be achieved in YB_(3)C_(3) for both electron and hole doping.In addition,it exhibits well mechanical properties and a Vickers hardness of 23.7 GPa because of the strong covalent boron-carbon framework.This work provides a novel avenue for the search of high-performance thermoelectric materials with excellent mechanical properties,based on boron-carbon clathrate structure.

Origin of off-centering effect and the influence on heat transport in thermoelectrics

Recently,off-centering behavior has been discovered in a series of thermoelectric materials.This behavior indicates that the constituent atoms of the lattice displace from their coordination centers,leading to the locally distorted state and local symmetry breaking,while the material still retains its original crystallographic symmetry.This effect has been proved to be the root cause of ultralow thermal conductivity in off-centering materials,and is considered as an effective tool to regulate the thermal conductivity and improve the thermoelectric performance.Herein,we present a collection of recently discovered off-centering compounds,discuss their electronic origins and local coordination structures,and illuminate the underlying mechanism of the off-centering effect on phonon transport and thermal conductivity.This paper presents a comprehensive view of our current understanding to the off-centering effect,and provides a new idea for designing high performance thermoelectrics.

Molecular Insights into n-Type Organic Thermoelectrics

Organic thermoelectrics(OTEs)have been recently intensively investigated as they hold promise for flexible,large-area,and low-cost energy generation or heating–cooling devices for appealing applications,for example,wearable energy harvesting.In the past 7 years,n-type OTEs have witnessed a sharp increase in their performance thanks to significant progress in developing and understanding the fundamental physical properties of n-type OTE materials as well as the working principle and physical processes of the TE devices.

Boosting thermoelectrics by alloying Cu_(2)Se in SnTe-CdTe compounds

The discovery of band convergence has opened an effective avenue for significantly enhancing thermoelectric performance of SnTe,while alloying CdTe in SnTe is evidenced efficient for improving the valley degeneracy.However,the thermoelectric transport properties are limited due to the low solubility of CdTe in SnTe(~3%).Inspired by the improvement of dimensionless figure of merit zT in Cu or Se-doped SnTe,investigating the effect of Cu_(2)Se on the electronic and phonon transport properties of SnTe-CdTe alloys is highly desired.Traditionally,improving the quality factor can trigger an increase of the potential of a compound for higher zT,which is of importance for design of thermoelectric materials.Here,alloyed 3%Cu_(2)Se in SnTe-3%CdTe system enables an increased peak zT,which is attributed by the optimization of electronic performance(~21μW cm^(-1)K^(-2)at 800 K),as well as the decreased lattice thermal conductivity owing to the enhanced mass and strain fluctuations.More importantly,alloying Cu_(2)Se not only improves the quality factor from~0.25 to~0.45,resulting in a higher maximum potential zT,but also effectively preserves the Fermi energy in a relative optimized level.The current findings demonstrate the role of Cu_(2)Se for manipulating thermoelectrics in SnTe.

Investigation on halogen-doped n-type SnTe thermoelectrics

Recent theoretical predictions and experimental findings on the transport properties of n-type SnTe have triggered extensive researches on this simple binary compound,despite the realization of n-type SnTe being a great challenge.Herein,Cl as a donor dopant can effectively regulate the position of Fermi level in Sn_(0.6)Pb_(0.4)Te matrix and successfully achieve the n-type transport behavior in SnTe.An outstanding power factor of~14.7μW·cm^(-1)·K^(-2) at 300 K was obtained for Cl-doped Sn_(0.6)Pb_(0.4)Te sample.By combining the experimental analysis with theoretical calculations,the transport properties of n-type SnTe thermoelectrics doped with different halogen dopants(Cl,Br,and I)were then systematically investigated and estimated.The results demonstrated that Br and I had better doping efficiencies compared with Cl,which contributed to the well-optimized carrier concentrations of~1.03×10^(19)and~1.11×10^(19)cm^(-3)at 300 K,respectively.The improved n-type carrier concentrations effectively lead to the significant enhancement on the thermoelectric performance of n-type SnTe.Our study further promoted the experimental progress and deep interpretation of the transport features in n-type SnTe thermoelectrics.The present results could also be crucial for the development of n-type counterparts for SnTe-based thermoelectric devices.

Grain boundary engineering with nano-scale InSb producing high performance In_(x)Ce_(y)Co_(4)Sb_(12+z)skutterudite thermoelectrics

Thermoelectric semiconductors based on CoSb_(3)hold the best promise for recovering industrial or automotive waste heat because of their high efficiency and relatively abundant,lead-free constituent elements.However,higher efficiency is needed before thermoelectrics reach economic viability for widespread use.In this study,n-type In_(x)Ce_(y)Co_(4)Sb_(12+z)skutterudites with high thermoelectric performance are produced by combining several phonon scattering mechanisms in a panoscopic synthesis.Using melt spinning followed by spark plasma sintering(MS-SPS),bulk In_(x)Ce_(y)Co_(4)Sb_(12+z)alloys are formed with grain boundaries decorated with nano-phase of InSb.The skutterudite matrix has grains on a scale of 100-200 nm and the InSb nano-phase with a typical size of 5e15 nm is evenly dispersed at the grain boundaries of the skutterudite matrix.Coupled with the presence of defects on the Sb sublattice,this multi-scale nanometer structure is exceptionally effective in scattering phonons and,therefore,InxCey-Co_(4)Sb_(12)/InSb nano-composites have very low lattice thermal conductivity and high zT values reaching in excess of 1.5 at 800 K.

Review on Fiber-Based Thermoelectrics:Materials,Devices,and Textiles

With the development and prosperity of Internet of Things(IoT)technology,wearable electronics have brought fresh changes to our lives.The demands for low power consumption and mini-type wearable power systems for wearable electronics are more urgent than ever.Thermoelectric materials can efficiently convert the temperature difference between body and environment into electrical energy without the need for mechanical components,making them one of the ideal candidates for wearable power systems.In recent years,a variety of high-performance thermoelectric materials and processes for the preparation of large-scale single-fiber devices have emerged,driving the application of flexible fiber-based thermoelectric generators.By weaving thermoelectric fibers into a textile that conforms to human skin,it can achieve stable operation for long periods even when the human body is in motion.In this review,the complete process from thermoelectric materials to single-fiber/yarn devices to thermoelectric textiles is introduced comprehensively.Strategies for enhancing thermoelectric performance,processing techniques for fiber devices,and the wide applications of thermoelectric textiles are summarized.In addition,the challenges of ductile thermoelectric materials,system integration,and specifications are discussed,and the relevant developments in this field are prospected.

Transport properties of p-type CaMg_(2)Bi_(2) thermoelectrics

Due to the complex crystal structure for low lattice thermal conductivity and the tunable valence bands for superior electronic performance,CaAl_(2)Si_(2)-structured AB_(2)C_(2) Zintl compounds have been frequently proven as promising p-type thermoelectric materials.In this work,thermoelectric properties of CaMg_(2)Bi_(2) are systematically investigated in a broad carrier concentration(10^(18)-10^(20) cm^(-3))through Agdoping for comprehensively evaluating its potential for thermoelectric applications.The broad carrier concentration enables a well assessment of the carrier transport properties by single parabolic band with acoustic phonon scattering and a revelation of the carrier transport by multiple valence orbitals when the carrier concentration higher than ~2×10^(19) cm^(-3),leading to a significant enhancement in electronic performance.With the help of additional point defect phonon scattering introduced by BaMg_(2)Bi_(2)-alloying,a reduction in lattice thermal conductivity in the entire temperature range and the lowest one of ~0.7 W/m-K are achieved,leading to a 100% enhancement in average zTave.in addition to the contribution of a multiband transport.This work not only demonstrates CaMg_(2)Bi_(2) as a promising thermoelectric material,but also provides a well understanding of its underlying material physics.

Slowing down the heat in thermoelectrics

Heat transport has various applications in solid materials.In particular,the thermoelectric technology provides an alternative approach to traditional methods for waste heat recovery and solid-state refrigeration by enabling direct and reversible conversion between heat and electricity.For enhancing the thermoelectric performance of the materials,attempts must be made to slow down the heat transport by minimizing their thermal conductivity(κ).In this study,a continuously developing heat transport model is reviewed first.Theoretical models for predicting the lattice thermal conductivity(κlat)of materials are summarized,which are significant for the rapid screening of thermoelectric materials with lowκlat.Moreover,typical strategies,including the introduction of extrinsic phonon scattering centers with multidimensions and internal physical mechanisms of materials with intrinsically lowκlat,for slowing down the heat transport are outlined.Extrinsic defect centers with multidimensions substantially scatter various-frequency phonons;the intrinsically lowκlat in materials with various crystal structures can be attributed to the strong anharmonicity resulting from weak chemical bonding,resonant bonding,low-lying optical modes,liquid-like sublattices,off-center atoms,and complex crystal structures.This review provides an overall understanding of heat transport in thermoelectric materials and proposes effective approaches for slowing down the heat transport to depressκlat for the enhancement of thermoelectric performance.