Engineering Multiple Microstructural Defects for Record-Breaking Thermoelectric Properties of Chalcopyrite Cu1-xAgxGaTe2

Defect engineering for vacancies, holes, nano precipitates, dislocations, and strain are efficient means of suppressing lattice thermal conductivity. Multiple microstructural defects are successfully designed in Cu_1-xAg_xGaTe₂ (0 ≤ x ≤ 0.5) solid solutions through high-ratio alloying and vibratory ball milling, to achieve ultra-low thermal conductivity and record-breaking thermoelectric performance. Extremely low total thermal conductivities of 1.28 W m⁻¹ K⁻¹ at 300 K and 0.40 W m⁻¹ K⁻¹ at 873 K for the Cu_0.5Ag_0.5GaTe₂ are observed, which are ≈79% and ≈58% lower than that of the CuGaTe₂ matrix. Multiple phonon scattering mechanisms are collectively responsible for the reduction of thermal conductivity in this work. On one hand, large amounts of nano precipitates and dislocations are formed via vibrating ball milling followed by the low-temperature hot press, which can enhance phonon scattering. On the other hand, the difference in atomic sizes, distorted chemical bonds, elements fluctuation, and strained domains are caused by the high substitution ratio of Ag and also function as a center for the strong phonon scattering. As a result, the Cu_0.7Ag_0.3GaTe₂ exhibits a record high ZT_max of ≈1.73 at 873 K and ZT_ave of ≈0.69 between 300–873 K, which are the highest values of CuGaTe₂-based thermoelectric materials.     

Selective Dissolution-Derived Nanoporous Design of Impurity-Free Bi2Te3 Alloys with High Thermoelectric Performance

Thermoelectric technology, which has been receiving attention as a sustainable energy source, has limited applications because of its relatively low conversion efficiency. To broaden their application scope, thermoelectric materials require a high dimensionless figure of merit (ZT). Porous structuring of a thermoelectric material is a promising approach to enhance ZT by reducing its thermal conductivity. However, nanopores do not form in thermoelectric materials in a straightforward manner; impurities are also likely to be present in thermoelectric materials. Here, a simple but effective way to synthesize impurity-free nanoporous Bi_0.4Sb_1.6Te₃ via the use of nanoporous raw powder, which is scalably formed by the selective dissolution of KCl after collision between Bi_0.4Sb_1.6Te₃ and KCl powders, is proposed. This approach creates abundant nanopores, which effectively scatter phonons, thereby reducing the lattice thermal conductivity by 33% from 0.55 to 0.37 W m⁻¹ K⁻¹. Benefitting from the optimized porous structure, porous Bi_0.4Sb_1.6Te₃ achieves a high ZT of 1.41 in the temperature range of 333–373 K, and an excellent average ZT of 1.34 over a wide temperature range of 298–473 K. This study provides a facile and scalable method for developing high thermoelectric performance Bi₂Te₃-based alloys that can be further applied to other thermoelectric materials.     

Simultaneous Boost of Power Factor and Figure‐of‐Merit in In–Cu Codoped SnTe

Significantly enhanced thermoelectric performance is achieved for eco‐friendly SnTe by a coorperative effect between a dopant resonant energy level and interstitial defects. By manipulating the band structure through indium doping, the Seebeck coefficient is remarkably improved, leading to an enhanced power factor, with a high level of ≈29 µW cm^?1 K^?2 at 873 K. Lattice thermal conductivity is sharply reduced, approaching the amorphous limit, through the strong phonon scattering induced by multiple scales of Cu₂Te nanoprecipitates, as well as Cu interstitials, leading to a high ZT value of ≈1.55 at 873 K.     

Phase diagram and thermoelectric properties of Ag3−x Sb1+x Te4 system

In order to obtain better thermoelectric performance in the composition domain should be stabilized, the phase diagram of the Ag_3–x Sb_1+x Te₄ system by varying the Ag:Sb ratio. The phase diagram is investigated using the differential thermal analysis and the powder X-ray diffraction techniques. The Seebeck coefficient and the electrical resistivity of the grown bulk crystals of the system are also measured. The phase diagram of the Ag_3–x Sb_1+x Te₄ system indicates that a mixed phase of AgSbTe₂ and Ag₂Te, which is expected to show higher thermoelectric performance, exists in a wide temperature range between 600 and 830 K at a composition of Ag_2.2Sb_1.8Te₄. The maximum of Seebeck coefficient for AgSbTe₂ (x = 1) is 0.73 mV/K at about 680 K. The thermoelectric performance is lowered by the compositional deviation from Ag:Sb:Te = 1:1:2.     

X-ray diffraction characterization and electrical properties of TlIn<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Ga<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Te<Subscript>2</Subscript> crystals

Single crystals of TlIn_{1 − x} Ga _x Te₂ solid solutions have been grown and characterized by X-ray diffraction, and their electrical conductivity, Hall coefficient, and thermoelectric power have been measured as functions of temperature. Partial substitution of gallium for indium in TlInTe₂ increases its unit-cell parameters.     

Microstructure evolution and thermoelectric properties of Te-poor and Te-rich (Bi,Sb)<Subscript>2</Subscript>Te<Subscript>3</Subscript> prepared via solidification

This paper explores in detail, the microstructures and thermoelectric properties of Te-rich and Te-poor (Bi,Sb)₂Te₃ alloys. We show that tuning the composition of ternary Bi–Sb–Te type alloys allows us to synthesize a range of microstructures containing a primary solid solution of (Bi,Sb)₂Te₃ with varying amounts of Te solid solution or a (Bi,Sb)Te compound. Te exists as a constituent of the multilayer domain while (Bi,Sb)Te appears in the thin intercellular regions of the (Bi,Sb)₂Te₃ dendritic cells. The presence of Te imparts an n-type behavior to the composite while the (Bi,Sb)₂Te₃ with a small amount of (Bi,Sb)Te exhibits p-type properties. A maximum ZT value of ≈0.4 at 425 K was achieved, opening up the possibility of using these alloys for thermoelectric device applications.     

All Cubic-Phase δ-TAGS Thermoelectrics Over the Entire Mid-Temperature Range

GeTe-based pseudo-binary (GeTe)_x(AgSbTe₂)_100−x (TAGS–x) is recognized as a promising p-type mid-temperature thermoelectric material with outstanding thermoelectric performance; nevertheless, its intrinsic structural transition and metastable microstructure (due to Ag/Sb/Ge localization) restrict the long-time application of TAGS-x in practical thermoelectric devices. In this work, a series of non-stoichiometric (GeTe)_x(Ag_1-δSb_1+δTe_2+δ)_100−x (x = 85∼50; δ = ≈0.20–0.23), referred to as δ-TAGS-x, with all cubic phase over the entire testing temperature range (300-773 K), is synthesized. Through optimization of crystal symmetry and microstructure, a state-of-the-art ZT_max of 1.86 at 673 K and average ZT_avg of 1.43 at ≈323–773 K are realized in δ-TAGS-75 (δ = 0.21), which is the highest value among all reported cubic-phase GeTe-based thermoelectric systems so far. As compared with stoichiometric TAGS-x, the remarkable thermoelectric achieved in cubic δ-TAGS-x can be attributed to the alleviation of highly (electrical and thermal) resistive grain boundary Ag₈GeTe₆ phase. Moreover, δ-TAGS-x exhibits much better mechanical properties than stoichiometric TAGS-x, together with the outstanding thermoelectric performance, leading to a robust single-leg thermoelectric module with η_max of ≈10.2% and P_max of ≈0.191 W. The finding in this work indicates the great application potential of non-stoichiometric δ-TAGS-x in the field of mid-temperature waste heat harvesting.     

Ag_2Te掺杂对p型Bi_(0.5)Sb_(1.5)Te_3块状合金热电性能的影响

采用真空熔炼、机械球磨及放电等离子烧结技术(SPS)制备得到了(Ag2Te)x(Bi0.5Sb1.5Te3)1-x(x=0,0.025,0.05,0.1)系列样品,性能测试表明,Ag2Te的掺入可以显著改变材料的热电性能变化趋势,掺杂样品在温度为450~550K范围内具有较未掺杂样品更优的热电性能.适当量的Ag2Te掺入能够有效地提高材料的声子散射,降低材料的热导率.在测试温度范围内,(Ag2Te)0.05(Bi0.5Sb1.5Te3)0.95具有最低的晶格热导,室温至575K范围内保持在0.2~0.3W/(m·K)之间,在575K时,(Ag2Te)0.05(Bi0.5Sb1.5Te3)0.95试样具有最大热电优值ZT=0.84,相较于未掺杂样品提高了约20%.     

Liquid‐Alloy‐Assisted Growth of 2D Ternary Ga2In4S9 toward High‐Performance UV Photodetection

2D ternary systems provide another degree of freedom of tuning physical properties through stoichiometry variation. However, the controllable growth of 2D ternary materials remains a huge challenge that hinders their practical applications. Here, for the first time, by using a gallium/indium liquid alloy as the precursor, the synthesis of high‐quality 2D ternary Ga₂In₄S₉ flakes of only a few atomic layers thick (≈2.4 nm for the thinnest samples) through chemical vapor deposition is realized. Their UV‐light‐sensing applications are explored systematically. Photodetectors based on the Ga₂In₄S₉ flakes display outstanding UV detection ability (R _λ = 111.9 A W^?1, external quantum efficiency = 3.85 × 10⁴%, and D* = 2.25 × 10¹¹ Jones@360 nm) with a fast response speed (τ_ring ≈ 40 ms and τ_decay ≈ 50 ms). In addition, Ga₂In₄S₉‐based phototransistors exhibit a responsivity of ≈10⁴ A W^?1@360 nm above the critical back‐gate bias of ≈0 V. The use of the liquid alloy for synthesizing ultrathin 2D Ga₂In₄S₉ nanostructures may offer great opportunities for designing novel 2D optoelectronic materials to achieve optimal device performance.     

Study of flash evaporated CuIn1 − xGaxTe2 (x = 0, 0.5 and 1) thin films

CuIn_1 − xGa_xTe₂ thin films with x = 0, 0.5 and 1, have been prepared by flash evaporation technique. These semiconducting layers present a chalcopyrite structure. The optical measurements have been carried out in the wavelength range 200-3000 nm. The linear dependence of the lattice parameters as a function of Ga content obeying Vegard's law was observed. The films have high absorption coefficients (4 · 10⁴ cm^− 1) and optical band gaps ranging from 1.06 eV for CuInTe₂ to 1.21 eV for CuGaTe₂. The fundamental transition energies of the CuIn_1 − xGa_xTe₂ thin films can be fitted by a parabolic equation namely E_g1(x) = 1.06 + 0.237x − 0.082x². The second transition energies of the CuInTe₂ and CuGaTe₂ films were estimated to be: E_g2 = 1.21 eV and E_g2 = 1.39 eV respectively. This variation of the energy gap with x has allowed the achievement of absorber layers with large gaps.     

Synthesis and high temperature transport properties of Te-doped skutterudite compounds

Te-doped skutterudite compounds Co₄Sb_12?x Te _x (x?=?0.4–0.7) have been fabricated by solid state reaction method and spark plasma sintering. The scanning electron microscope images indicate all samples are compact and the average particle size increases with the Te doping fraction. The carrier concentration and electrical conductivity exhibit positive doping fraction dependence, and a maximum electrical conductivity of 16.29?×?10⁴?Sm^?1 is obtained at 300?K for Co₄Sb_11.3Te_0.7. The values of the power factor (x?=?0.4–0.6) are greater than 4.0?×?10^?3?Wm^?1?K^?2 at the temperature range of 650–800?K, larger than previous literature reports. The lattice thermal conductivity decreases monotonously over the whole investigated temperature range and exhibits a negative doping fraction dependence except for Co₄Sb_11.3Te_0.7. The resultant dimensionless figure of merit of all the samples increases monotonously over the whole investigated temperature range, and a maximum value of 0.95 is achieved at 800?K for Co₄Sb_11.4Te_0.6.     

Enhanced thermoelectric properties in Co4Sb12 − xTex alloys prepared by HPHT

Skutterudite compounds Co₄Sb_12 ? xTe_x with bcc crystal structure were prepared by high pressure and high temperature (HTHP) method. The study explored chemical doping with Te at the Sb site in an attempt to optimize the thermoelectric figure of merit ZT in the system Co₄Sb_12 ? xTe_x. The electrical resistivities, Seebeck coefficients and thermal conductivities of the samples were measured in the temperature range of 300–710 K. We found that the presence of Te substantially decreased the electrical resistivity without any detrimental effect on the Seebeck coefficients, which improved the power factor. Among all the samples, Co₄Sb_11.5Te_0.5 shows the highest power factor of 35.3 µw/(cmK²) at 710 K, and the maximum ZT value reaches 0.67 at 710 K.     

Manipulating the Interfacial Band Bending For Enhancing the Thermoelectric Properties of 1T′-MoTe2/Bi2Te3 Superlattice Films

Interfacial charge effects, such as band bending, modulation doping, and energy filtering, are critical for improving electronic transport properties of superlattice films. However, effectively manipulating interfacial band bending has proven challenging in previous studies. In this study, (1T′-MoTe₂)_x(Bi₂Te₃)_y superlattice films with symmetry-mismatch were successfully fabricated via the molecular beam epitaxy. This enables to manipulate the interfacial band bending, thereby optimizing the corresponding thermoelectric performance. These results demonstrate that the increase of Te/Bi flux ratio (R) effectively tailored interfacial band bending, resulting in a reduction of the interfacial electric potential from ≈127 meV at R = 16 to ≈73 meV at R = 8. It is further verified that a smaller interfacial electric potential is more beneficial for optimizing the electronic transport properties of (1T′-MoTe₂)_x(Bi₂Te₃)_y. Especially, the (1T′-MoTe₂)₁(Bi₂Te₃)₁₂ superlattice film displays the highest thermoelectric power factor of 2.72 mW m⁻¹ K⁻² among all films, due to the synergy of modulation doping, energy filtering, and the manipulation of band bending. Moreover, the lattice thermal conductivity of the superlattice films is significantly reduced. This work provides valuable guidance to manipulate the interfacial band bending and further enhance the thermoelectric performances of superlattice films.     

Thermoelectric Properties of Chalcogenides with the Spinel Structure

Many compounds with the spinel structure type have been analyzed for their thermoelectric properties. Published data was used to augment experimental results presented here and to select promising thermoelectric spinels. Compounds studied here include Cu_0.5Al_0.5Cr₂Se₄, Cu_0.5Co_0.5Cr₂Se₄, Cu_0.5In_0.5Cr₂Se₄, and CuIr₂Se₄. Many exhibit low lattice thermal conductivity of about 20 mW/cmK, independent of temperature. Two representative series of compounds Ga_xCu_1-xCr₂Se₄ 0≤x ≤ 0.6 and Cu_xZn_1-xCr₂Se₄ 0≤x≤ 0.3 have been studied to examine the doping region of interest for both high and low temperature thermoelectric applications. Ferromagnetic, small polaron semiconductors are found in both series of compounds. Ga_xCu_1-xCr₂Se₄ shows a metal insulator transition, from a p-type metal at x=0 to a p-type and then n-type semiconductor as x crosses 1/2. The maximum figure of merit ZT is of order 0.1. Cu_xZn_1-xCr₂Se₄ transitions from a p-type semiconductor to a p-type metal as x increases. Received: 9 February 2001 / Reviewed and accepted: 9 April 2001     

Effects of Ga content on thermoelectric properties of P-type Ba8Ga16+ x Zn3Ge27? x type-I clathrates

P-type Ba₈Ga_16+x Zn₃Ge_27−x (x = 0.1, 0.2, 0.3, and 0.4) type-I clathrates were synthesized by combining solid-state reaction with spark plasma sintering (SPS) technology. The effects of slight increase of Ga content on thermoelectric properties have been investigated. The results show that at room temperature the carrier concentration N _p of p-type Ba₈Ga_16+x Zn₃Ge_27−x clathrates increases remarkably compared with that of Ba₈Ga₁₆Zn₃Ge₂₇ compound, which results in the increases of electrical conductivity although carrier mobility μ _H slightly decreases. The thermal conductivity κ of all samples increases with the increase of Ga content. Ba₈Ga_16.2Zn₃Ge_26.8 compound exhibits the highest ZT value of 0.43 at 700 K, which is increased by 13% compared with that of Ba₈Ga₁₆Zn₃Ge₂₇ compound.     

Microstructures and thermoelectric properties of p-type pseudo-binary AgxBi0.5Sb1.5−xTe3 (x = 0.05–0.4) alloys prepared by cold pressing

The p-type pseudo-binary Ag_xBi_0.5Sb_1.5−xTe₃ (x = 0.05–0.4) alloys were prepared by cold pressing. The thermal conductivities (κ) were calculated from the values of heat capacities, densities and thermal diffusivities measured, and range approximately from 0.66 to 0.56 (W K^− 1 m^− 1) for the Ag_xBi_0.5Sb_1.5−xTe₃ alloy with molar fraction x being 0.4. Combining with the electrical properties obtained in the previous study, the maximum dimensionless figure of merit ZT of 1.1 was obtained at the temperature of 558 K.     

Facile synthesis and thermoelectric studies of n-type bismuth telluride nanorods with cathodic stripping Te electrode

Bismuth telluride (Bi₂Te₃) nanorods (NRs) of n-type thermoelectric materials were prepared using an electrogenerated precursor of tellurium electrode in the presence of Bi³⁺ and mercapto protecting agent in aqueous solution under atmosphere condition. The optimal preparation conditions were obtained with ratio of Bi³⁺ to mercapto group and Te coulomb by photoluminescence spectra. The mechanism for generation of Bi₂Te₃ precursor was investigated via the cyclic voltammetry. The highly crystalline rhombohedral structure of as-prepared Bi₂Te₃ NRs with the shell of Bi₂S₃ was evaluated with high resolution transmission electron microscopy (HRTEM) and powder X-ray diffraction (XRD) spectroscopy. The near-infrared absorption of synthetic Bi₂Te₃ NRs was characterized with spectrophotometer to obtain information of electron at interband transition. The thermoelectric performance of Bi₂Te₃ NRs was assessed with the result of electrical resistivity, Seebeck coefficient, thermal conductivity, and the figure of merit ZT parameters, indicating that thermoelectric performance of as-prepared Bi₂Te₃ nanocrystals was improved by reducing thermal conductivity while maintaining the power factor.     

Toward Ultrahigh Thermoelectric Performance of Cu2SnS3-Based Materials by Analog Alloying

Cu₂SnS₃ is a promising thermoelectric candidate for power generation at medium temperature due to its low-cost and environmental-benign features. However, the high electrical resistivity due to low hole concentration severely restricts its final thermoelectric performance. Here, analog alloying with CuInSe₂ is first adopted to optimize the electrical resistivity by promoting the formation of Sn vacancies and the precipitation of In, and optimize lattice thermal conductivity through the formation of stacking faults and nanotwins. Such analog alloying enables a greatly enhanced power factor of 8.03 µW cm⁻¹ K⁻² and a largely reduced lattice thermal conductivity of 0.38 W m⁻¹ K⁻¹ for Cu₂SnS₃ – 9 mol.% CuInSe₂. Eventually, a peak ZT as high as 1.14 at 773 K is achieved for Cu₂SnS₃ – 9 mol.% CuInSe₂, which is one of the highest ZT among the researches on Cu₂SnS₃-based thermoelectric materials. The work implies analog alloying with CuInSe₂ is a very effective route to unleash superior thermoelectric performance of Cu₂SnS₃.     

Introduction of porous structure: A feasible and promising method for improving thermoelectric performance of Bi2Te3 based bulks

The porous p-type Bi_0.4Sb_1.6Te₃ bulks containing irregularly and randomly oriented pores were obtained by artificially controlling the relative density of sintered samples during resistance pressing sintering process. It is demonstrated that the thermoelectric performances are significantly affected by the porous structure, especially for the electrical and thermal conductivity due to the enhanced carrier scattering and phonon scattering. The increasing porosity resulted in the obvious decrease in electrical and thermal conductivity, and little change in Seebeck coefficients. It is encouraging that the reduction of thermal conductivity can compensate for the deterioration of electrical performance, leading to the enhancement in thermoelectric figure of merit (ZT). The maximum ZT value of 1.0 was obtained for the sample with a relative density of 90% at 333?K. Unfortunately, the increase in porosity also brought in obvious degradations in Vickers hardness from 51.71 to 27.74?HV. It is worth mentioning that although the Vickers hardness of the sample with a relative density of 90% decreased to 40.12?HV, it was still about twice as high as that of the zone melting sample (21.25?HV). To summarize, introducing pores structure into bulks properly not only enhances the ZT value of Bi₂Te₃ based alloys, but also reduces the use of raw materials and saves production cost.     

CuGaTe2–CuAlTe2 System

It was shown by x-ray diffraction and differential thermal analysis that CuGaTe₂ and CuAlTe₂ form a continuous series of solid solutions. CuGaTe₂, CuAlTe₂, and CuAl _x Ga_{1 – x} Te₂ crystals consisting of large blocks were grown by the horizontal Bridgman process, and their thermal expansion and near-edge transmission and reflection spectra were measured. The thermal expansion coefficient was shown to vary linearly across the solid-solution series, while the band gap is a nonlinear function of composition.