Maximum ZT Value Research Articles

Optimized Thermoelectric Properties of Sulfide Compound Bi2SeS2 by Iodine Doping.

The Te-free compound Bi2SeS2 is considered as a potential thermoelectric material with less environmentally hazardous composition. Herein, the effect of iodine (I) substitution on its thermoelectric transport properties was studied. The electrical conductivity was enhanced due to the increased carrier concentration caused by the carrier provided defect Ise. Thus, an enhanced power factor over 690 μWm−1K−2 was obtained at 300 K by combining a moderate Seebeck coefficient above 150 µV/K due to its large effective mass, which indicated iodine was an effective n-type dopant for Bi2SeS2. Furthermore, a large drop in the lattice thermal conductivity was observed due to the enhanced phonon scattering caused by nanoprecipitates, which resulted in a low total thermal conductivity (<0.95 Wm−1K−1) for all doped samples. Consequently, a maximum ZT value of 0.56 was achieved at 773 K for a Bi2Se1−xIxS2 (x = 1.1%) sample, a nearly threefold improvement compared to the undoped sample.

High Thermoelectric Performance of Janus Monolayer and Bilayer HfSSe

For 2D transition metal dichalcogenides (TMDs), the layer thickness has a greater impact on its thermoelectric properties. Herein, the thermoelectric properties of Janus monolayer and bilayer HfSSe are studied based on first‐principles calculations combined with Boltzmann equations, and the thermoelectric properties of bilayer HfSSe are also investigated with different stacking types. Owing to the influence of the interlayer coupling, the degradation of the phonon dispersion curve and the change of the phonon scattering intensity of the bilayer HfSSe lead to the decrease of its lattice thermal conductivity. In addition, the increase in the degeneracy of the electronic band structure of the bilayer HfSSe and the change in the density of states also change the thermoelectric transport properties of electrons. Integrating the coupling of high power factor and low thermal conductivity, the maximum ZT values at room temperature for n‐type doping are predicted as 3.24 and 5.54 for monolayer and SeHfS/SHfSe, respectively. And the ZT values increase to 3.24 and 5.54 as the temperature increases to 600 K. The result manifests that both monolayer and bilayer HfSSe are promising thermoelectric materials, and the layer thickness and stacking types also significantly affect the thermoelectric properties of the 2D TMDs materials.

Regulating Multiscale Defects to Enhance the Thermoelectric Performance of Ca0.87Ag0.1Dy0.03MnO3 Ceramics.

Achieving high thermoelectric properties of CaMnO3 ceramics is significant for its applications at high temperature. Herein, Ca0.87Ag0.1Dy0.03MnO3 ceramics with plate-like template seeds additives were prepared by using a solid-state reaction method. The multiscale defects, including grain boundaries, oxygen defects, and Ag nanoprecipitations, which were regulated by the different sintering atmospheres, were beneficial for electron transport and phonon scattering. The grain boundaries as coherent interfaces could act as an alternative phonon scattering source. Oxygen vacancies coupled with Ag nanoprecipitations were verified by geometric phase analysis and annular bright-field analysis. The decrement in oxygen vacancies concentration strongly depended on the enriched oxygen environment, which could reduce electrical resistivities. Compared to the sample sintered at Ar atmosphere, a 17.5 times increment in power factor and a 20.1% reduction of the total thermal conductivity were obtained for the sample sintered at O2 atmosphere. As a result, the maximum ZT value of 0.22 was obtained at 500 °C. It is an effective way for improving the thermoelectric performance of oxide-based thermoelectric materials.

Strong anharmonicity induced low lattice thermal conductivity and high thermoelectric performance in (CuInTe2)1−x(AgSbTe2)x system

The suppression of lattice thermal conductivity of thermoelectric CuInTe2 was often realized by complex micrometer-structure control. Herein, we demonstrate that simple alloying AgSbTe2 in CuInTe2 is capable of strongly strengthening lattice anharmonicity, which gives rise to an extremely low lattice thermal conductivity of ∼ 0.62 W m−1 K−1 at 823 K for (CuInTe2)0.95(AgSbTe2)0.05. In addition, the intentionally introduced copper vacancies can effectively increase the carrier concentration and dramatically improve the electrical performance of (CuInTe2)0.95(AgSbTe2)0.05. As a result, the sample with nominal composition of (Cu0.95InTe2)0.95(AgSbTe2)0.05 reaches a maximum zT value of ∼ 1.3 at 823 K. This study provides some insight into designing thermoelectric compounds in the diamond-like semiconductor family by inducing strong anharmonicity through a small amount of alloying.

Significantly improved thermoelectric properties of Nb-doped ZrNiSn half-Heusler compounds

ZrNiSn-based n-type half-Heusler (HH) compounds are listed among the mid-high temperature thermoelectric materials with excellent thermoelectric properties because they have a high power factor (PF). However, the thermal conductivity of HH compounds remains high, especially for the lattice thermal conductivity and bipolar thermal conductivity, which limits their application. In this work, we have enhanced the thermoelectric and mechanical properties of ZrNiSn-based compounds by Nb doping, which is realized by replacing Zr sites with Nb. As a consequence, the PF is enhanced from 3.25 mW mK−2 for pristine ZrNiSn to 4.78 mW mK−2 for Zr0.96Nb0.04NiSn, while the lattice thermal conductivity respectively declines from 4.5 W m-1K−1 to 2.58 W m-1K-1at 1123 K. Combined experimental results and ab-initio calculations revealed the increase in PF is because of that Nb-doping can significantly improve the electrical conductivity through a lot of carrier injection. The reduction of lattice thermal conductivity is primarily due to that Nb doping can generate quantum dots in the matrix causing the phonon scattering for the Nb-rich quantum dots and cause the mass and stress field fluctuations, and Nb-doping can efficiently weaken the bipolar thermal effect further reducing the thermal conductivity. A maximum ZT value of 0.89 within single-doped ZrNiSn compounds is achieved at 1123 K for Zr0.96Nb0.04NiSn, which is 53% higher than the pristine ZrNiSn. Furthermore, Nb doping effectively improves the microhardness, shear modulus, and Young's modulus of the ZrNiSn HH samples. The synthesized Hf-free HH compounds establish Nb as an effective dopant for attaining a high ZT and implying the excellent potential of these materials for industrial applications.

The electronic structure, elastic properties, dynamical stability and thermoelectric properties of rock-salt and orthorhombic phases of CdS: First-principles calculations

Combined with first-principles calculation and Boltzmann transport theory, we investigated the electronic structure, dynamic stability, mechanical and thermoelectric properties of rock-salt (rs-CdS) and orthorhombic CdS (orth-CdS). Both phases are mechanical and dynamical stable, showing semiconductor characteristics with an indirect band gap of ∼0.2 eV and excellent resistance to deformation and external pressure. Due to the strong coupling of optical and acoustic phonon branches, the predicted lattice thermal conductivities of these phases are quite low. The electron transport properties including Seebeck coefficient and power factor of p-type doping are excellent. From 300 K to 800 K, the maximum ZT values are in the range of 0.56 to 0.71 for p-type doping, hinting that the rs-CdS and orth-CdS are superior thermoelectric materials.

Enhanced thermoelectric performance of n-type polycrystalline SnSe via NdCl3 doping

A series of n-type polycrystalline SnSe0.95 - xmol% NdCl3 samples were synthesized using the melting method combined with spark plasma sintering (SPS). Doping NdCl3 significantly improves the electrical conductivity and suppresses the deterioration of the Seebeck coefficient after thermal excitation, which leads to significantly higher power factors than SnSe0.95. In addition, excessive doping of NdCl3 not only introduces a large number of point defects but also produces NdCl3 precipitates, which largely enhances the phonon scattering and significantly reduces the thermal conductivity. As a result, a maximum ZT value of 1.3 at 773 K has been obtained in SnSe0.95 - 1.5 mol% NdCl3 sample, indicating that the doping of rare earth halides can effectively enhance the thermoelectric performance of n-type SnSe0.95 by simultaneously improving the power factor and reducing the lattice thermal conductivity.

Improvement of anisotropic thermoelectric performance in polycrystalline SnSe by metallic AgSnSe2 compositing

SnSe is an emerging star material in thermoelectric community owing to its attractive electronic band structure and ultralow intrinsic lattice thermal conductivity. By now, aliovalent doping is often employed to be a route strategy to optimize the thermoelectric properties of SnSe via improving its poor electrical conductivity. In this work, AgSnSe2, as a metallic composite, is introduced with the aim to increase the hole concentration of SnSe via metal-semiconductor contact. It is demonstrated that compositing of AgSnSe2 can effectively increase the hole concentration of polycrystalline SnSe hosts by two orders of magnitude (from 1017 to 1019 cm−3) without deteriorating the carrier mobilities. Therefore, a lager power factor of 5.72 μW cm−1 K−2 is achieved at 773 K for the SnSe + 0.020AgSnSe2 sample perpendicular to the hot-pressing direction. Combined with the intrinsic low thermal conductivity, the maximum zT value of the sample can be boosted to 0.68 at 773 K, which is increased by 124% compared to that of the uncomposited counterpart. This work proves that composite engineering should be an efficient alternative strategy in optimizing the thermoelectric performance of polycrystalline SnSe and can be synergistically used with other sophisticated approaches.

Modulating the valence of Ga and the deep level impurity for high thermoelectric performance of n-type Pb0.98Ga0.02Te1-xSex compounds

It has been found that the doped Ga has a distinct role in PbTe and PbSe compounds though they possess the same crystal structure. Therefore, to understand how the thermoelectric performance evolves in the Ga-doped PbTe1-xSex solid solution is crucial. Herein, we report that a maximum ZT value of 1.57 at 721 K and a high average ZT value of 1.13 in the temperature range of 300–843 K are attained for Pb0.98Ga0.02Te0.96Se0.04 compound. The experimental studies show that doping Ga into PbTe introduces a shallow level impurity state Ga3+ and a deep level associated with the Ga + state simultaneously. Interestingly, upon Se-alloying all of the Ga + are oxidized into Ga3+ at low temperatures, which increases the room-temperature carrier concentration from 1.16 × 1019 cm−3 to 2.17 × 1019 cm−3. This leads to an improved power factor of 3.04 mW m−1 K−2 for Pb0.98Ga0.02Te0.96Se0.04 sample at 482 K. Additionally, Se-alloying further enhances the point defect phonon scattering and lowers the lattice thermal conductivity to 0.46 W m−1 K−1 for Pb0.98Ga0.02Te0.96Se0.04 sample at 690 K. As a result, the ZT value is increased by about 112% compared with the maximum ZT of 0.74 for pristine PbTe. This work provides a new avenue for tuning the doping behavior of cation site dopant via alloying on anion site to further improve the ZT value in thermoelectric materials.

Influence of temperatures on structure, thermoelectric, and mechanical properties of nanocrystalline SnSe thin films deposited by thermal evaporation

The Tin Selenide (SnSe) thermoelectric materials’ high heat-electricity inter-conversion capability makes it a potential energy resource material to tap the waste heat from different industrial processing. The present work has optimized parameters for the deposition of single-phase nanocrystalline SnSe thin films using the thermal evaporation technique. The XRD, Raman Spectroscopy, SEM, EDS, Seebeck Coefficient, Electrical Conductivity, and Thermal Conductivity data analyses were used to optimize and establish the structure-thermoelectric property relationship for nanocrystalline SnSe films. The phase analysis of the SnSe thin films deposited at various substrate heating temperatures (Ts) reveals a significant influence of Ts in the evolution of a single phase of SnSe film. The films deposited at Ts ≤ 200 °C evolved with the phases of Sn, Se, and SnSe, whereas films deposited at Ts ≥ 300 °C grew with single-phase polycrystalline SnSe. An increase in crystallite size with a shape transformation from circular to elongated grains was observed with Ts. The maximum ZT value of 0.64 with a power factor value of ~2.2 μWcm-1K-2 at 750 K measuring temperature (Ta) was obtained for the SnSe film deposited at Ts = 300 °C. The change in thermoelectric properties with Ta, including a p-type to n-type transition observed at ~600 K, was correlated with the alteration of structure and the elemental composition of the deposited films after heating at 350 °C. The heating temperature significantly influenced the hardness and elastic modulus values of the films deposited at Ts ≤ 300 °C.

Enhanced thermoelectric properties of I-doped polycrystalline Bi2O2Se oxyselenide

Bi2O2Se has an intrinsically low thermal conductivity and a high Seebeck coefficient, which suggests as potential thermoelectric materials. However, pristine Bi2O2Se polycrystalline exhibits very low electrical conductivity. In this study, we investigated the influence of I doping in enhancing electrical conductivity of Bi2O2Se by synthesizing a series of Bi2O2Se1-xIx polycrystalline samples (x = 0, 0.0025, 0.005, and 0.0075). By I doping, the electrical conductivity was enhanced by significant increase in carrier concentration. The carrier concentration was increased more than 10 times 2.1–4.6 × 1019 cm-3 from 2.1 × 1018 of the undoped sample. As results, the maximum power factor was enhanced to 0.24 mW/mK2 for x = 0.0025 from 0.086 mW/mK2 of undoped Bi2O2Se at 300 K. The maximum power factor of 0.42 mW/mK2 was achieved for x = 0.05 sample at 790 K. These increases are discussed in terms of the increase in effective mass and weighted mobility. Additionally, the moderate reduction of thermal conductivity was observed in the I doped samples due to the point defect scattering. As a result, a maximum zT value was enhanced to 0.32, 0.35, and 0.33 for x = 0.0025, 0.005, and 0.0075) at 790 K, representing an improvement of more than 50% compared to 0.21 of the undoped Bi2O2Se (x = 0).

Optimizing thermoelectric performance of CoSbS0.85Se0.15 by doping 3d transition metal ions M (M = Cr, Mn, Fe and Ni)

Transitional metal compound CoSbS attracted much attention as a thermoelectric material in the past decades. The configuration of the transition metal octahedra has a close relationship with the thermoelectric performance. This study reports the optimized thermoelectric performance of CoSbS through tuning Co octahedra distortion. The substitution of M (M ​= ​Cr, Mn, Fe and Ni) on Co sites could remarkably reduce the lattice thermal conductivity, and typically, Cr substitution endows the lowest lattice thermal conductivity owing to its strongest effect on the octahedra distortion, whereas Ni substitution induces to a relatively weak octahedra distortion at the same substitution level. Additionally, band structure calculations demonstrate that Ni-3d orbitals contribute shallow impurity levels near the conduction band minimum, strikingly increasing the carrier concentration and power factor. As a result, CoSbS0.85Se0.15 with Ni substitution on Co sites exhibits a much higher thermoelectric performance compared with the other counterparts. Furthermore, by optimizing Ni content, a maximum zT value of 0.52 ​at 876 ​K was achieved in Ni0.10Co0.90SbS0.85Se0.15. This study provides an exemplary approach to optimize the thermoelectric performance via polyhedral distortion.

First-principles insights into thermoelectric properties of topological nontrivial semimetal LiAuTe material

Structural, electronic and thermoelectric properties of LiAuTe ternary compound are studied using density functional theory (DFT) and semi-classical Boltzmann transport theory. The cubic -phase (space group F3m) is predicted to be ground state structure with a significant energy difference compared to honeycomb structure (space group P63mmc). The mechanical and dynamical stability of the -phase is confirmed by calculating the elastic constants and phonon dispersion frequencies. At equilibrium lattice, with and without spin–orbit coupling, the LiAuTe compound band structure calculations show an s-p band inversion at Γ point, leading to a topological nontrivial semimetal phase. Thermoelectric parameters, such as Seebeck coefficient (S), electrical conductivity (σ), electronic (κ e ) and lattice (κ L ) thermal conductivities are computed. Electrons and holes relaxation times (τ) are also predicted. Hence, LiAuTe compound exhibits a low κ L value of 1.76 W mK−1 at room temperature which decreases with temperature increasing. At 900 K, κ L falls to 0.58 W mK−1 leading to a maximum ZT value of 0.52 at optimized n-doping concentration of 2.5 × 1020 cm−3. The present study reveals that LiAuTe compound is a suitable candidate for thermoelectric applications and will open new horizons for further researches on similar types of topological thermoelectric materials with better ZT.

Excellent Medium-Temperature Thermoelectric Performance of Monolayer BiOCl.

Recently, a ternary-layered material BiOCl has elicited intense interest in photocatalysis, environmental remediation, and ultraviolet light detection because of its unique band gap of around 3.6 eV, low toxicity, and earth abundance. In particular, Gibson et al. reported a measurement of the in-plane thermal conductivity of BiOCl experimentally using a four-point-probe method [Science, 373, 1017-1022 (2021)], which is only 1.25 W/m K at 300 K. Motivated by the work, we studied the thermoelectric property of monolayer BiOCl using first-principles calculations combined with the Boltzmann transport equation. The calculated phonon thermal conductivity of monolayer BiOCl is 3 W/m K at 300 K, which is far below that of other promising 2D thermoelectric materials like graphyne and MoS2. A comprehensive analysis of phonon modes is conducted to reveal the low thermal conductivity. Moreover, the maximal ZT value is as high as 1.8 at 300 K and 5.7 at 800 K for the p-type doping with the 2 × 1015 cm-2 concentration. More importantly, we found that the thermoelectric efficiency of such 2D materials is significantly enhanced to 8 at 800 K by applying 1.5% tensile strain, which clearly outperforms that of the reported 2D thermoelectric material SnSe. The results shed light on the promising application in medium-temperature (600-900 K) thermoelectric devices.

Improving electrical and thermal properties synchronously via introducing CsPbBr3 QDs into higher manganese silicides

Higher manganese silicide (HMS) is a P-type medium temperature thermoelectric (TE) material, which has attracted widespread attention over the past few decades due to its remarkable mechanical properties, excellent chemical and thermal stability, as well as the non-toxicity, abundance and competitive price. The peak power factor (PF) of HMS is as high as ∼1.50 × 10−3 W m−1 K−2 because of its intrinsic high electrical conductivity and Seebeck coefficient. However, the thermal conductivity of HMS is also high, resulting in relatively low zT values. Introducing nano-dispersion in the matrix is one of the most effective methods to enhance the TE properties via reducing the lattice thermal conductivity significantly without drastic changes on the other parameters. In this study, CsPbBr3 QDs with uniform size were synthesized and introduced into HMS bulks. The PF (at 823 K) was enhanced to 1.71 × 10−3 W m−1 K−2, which is improved 14.0% approximately compared with that of pure HMS owing to the combined effect of element doping and energy filtering. The lattice thermal conductivity (at 823 K) decreased from 2.56 W m−1 K−1 to 1.99 W m−1 K−1 synchronously (∼22.0%) due to the intensive phonon scattering caused by Cs doping, and the embedding of Pb riched CsPbBr3 QDs and Pb QDs. A maximum zT value of 0.57 (823 K) is achieved in CsPbBr3 QDs/HMS composites, which is 36.0% higher than that of pure HMS. Predictably, for other TE materials, it is also feasible to improve the TE properties via introducing metastable quantum dots.

Thermoelectric performance of porphyrin molecular junction with graphene nanoribbon electrodes

Thermoelectric performance of porphyrin molecular junction is investigated by using first-principles calculation combined with nonequilibrium Green function (NEGF) method. It is found that porphyrin molecule rotation (90[Formula: see text] rotated) has no obvious effect on its thermoelectric conversion efficiency due to the conjugate structure of central molecule. Through the functions of energy and thermal conductivity, it is found that phonon thermal conductivity accounts for the main part, which provides a potential for further optimizing thermoelectric performance. Moreover, it can be seen from phonon transmission spectrum that porphyrin molecule has a certain filtering effect on high-frequency phonon mode. For power factor and figure of merit (ZT), wave functions for two cases highly coincide. The maximum ZT values of por1 and por2 are 0.38 and 0.34 near Fermi level (FL), respectively. Porphyrin molecular junction could have a good application prospect in nanoscale thermoelectric devices.

Synthesis Optimization and Thermoelectric Properties of S-Filled and Te-Se Double-Substituted Skutterudite under High Pressure.

In recent years, skutterudite filled with electronegative elements (S, Se, Cl, Br) has attracted the extensive attention of researchers. By doping electron donors (Pb, Ni, or Te, S, Se) at the Co or Sb sites, the electronegative elements can form thermodynamically stable compounds in the intrinsic pores of the skutterudite, substantially expanding the research scope of skutterudite. In this study, S0.05Co4Sb11.3Te0.6Se0.1 skutterudite was synthesized at high pressure and high temperature, with a pressure range of 2.0-3.5 GPa. The phase composition, micro-morphology, and electrical and thermal transport properties were systematically characterized. The micromorphology analysis shows that the introduction of S element and substituting Te and Se at the Sb sites inhibit the grain growth in a suitable high-pressure environment. Substantial differences are observed in the directions of the lattice stripes in the samples, and rich grain boundaries and many lattice distortions and dislocation defects occur. The carrier concentration can be optimized by filling the voids of the skutterudite with a few S atoms, and the thermoelectric properties can be optimized by scattering phonons through resonance scattering and defect scattering. The samples synthesized at a pressure of 3.0 GPa and a temperature of 900 K have a maximum power factor of 23.85 × 10-4 W m-1 K-2 and a maximum zT value of 1.30 at a test temperature of 773 K.

Electronic structure, magnetic, and thermoelectric properties of BaMn2As2 compound: a first-principles study

We report the magnetic stability, antiferromagnetic ordering, electronic, magnetic, and thermoelectric properties of BaMn2As2 employing the full-potential linearized augmented-plane wave method under the framework of density functional theory. The exchange-correlation energy was treated using the Perdew–Burke–Ernzerhof generalized gradient approximation (PBE-GGA) and GGA plus Hubbard U parameter method. From structural relaxation, we reveal that antiferromagnetic (A-AFM) state is more appropriate for BaMn2As2 than other known configurations. Under electronic properties, BaMn2As2 shows metallic nature in paramagnetic (PM) and antiferromagnetic phase (AFM). Further, the decrease in electrical conductivity over the entire temperature range characterize the metallic nature of BaMn2As2. The electronic band structure calculation demonstrates that Mn-3d and As-4p orbital hybridization are essential for the band gap formation, suggesting BaMn2As2, a hybridization-gap semiconductor. The total magnetic moment of BaMn2As2 in ferromagnetic phase is ~9.54 μB, with a major contribution from Mn atom. In thermoelectric, we obtain a negative Seebeck coefficient (S), n-type electrical conductivity, and a maximum ZT value of 0.40. Our study suggests BaMn2As2 as a novel candidate for spintronics and waste heat management.

Enhanced Thermoelectric Properties of Te Doped Polycrystalline Sn0.94Pb0.01Se.

Thermoelectric materials can directly convert heat and electricity, which is a kind of promising energy material. In view of cost and mechanical properties, polycrystalline SnSe material with high zT value is greatly desired. In this study, polycrystalline Sn0.94Pb0.01Se1-xTex samples were prepared by the vacuum melting–hot pressing sintering method. Sn vacancies, Pb and Te atoms were simultaneously introduced into the polycrystalline SnSe. The power factor of Sn0.94Pb0.01Se1-xTex samples was decreased, which could be attributed to the generation of n-type semiconductor SnSe2. In addition, the phonons were strongly scattered by point defects and dislocations, which led to the decrease of thermal conductivity—from 0.43 Wm−1K−1 to 0.29 Wm−1K−1 at 750 K. Finally, the polycrystalline Sn0.94Pb0.01Se0.96Te0.04 sample achieved the maximum zT value of 0.60 at 750 K.

Effect of perovskite template seeds on microstructures and thermoelectric properties of Ca0.87Ag0.1Dy0.03MnO3 ceramics

Calcium manganite (CaMnO3) ceramics, members of the perovskite family, have attracted considerable attention in the field of thermoelectric materials owing to their environmental friendliness, chemical stability, long lifetime, and low cost. Ruddlesden–Popper Ca3Mn2O7 and plate-like perovskite CaMnO3 templates were synthesized using the molten salt method. Perovskite CaMnO3 template seeds with a radius-thickness ratio of 16.8 were obtained via topochemical conversion process, demonstrating an obviously anisotropy. The template seeds were introduced into Ca0·87Ag0.1Dy0.03MnO3 ceramics to promote the grain growth. The bulk density of the sample containing 20 wt% template seeds was 96.0%. An electrical resistivity of 5.35 mΩ cm, a Seebeck coefficient of − 196 μV/K, and a power factor of 0.73 mW/m·K2 were obtained. Such enhancements in thermoelectric properties could be attributed to the formation of grain boundaries between stripe-like and equiaxial grains. Additionally, the maximum ZT value of 0.35 reached at 500 °C, which is 57% higher than that of the sample without template seeds. Therefore, it is believed that this study can lay the foundation for textured CaMnO3 ceramics with superior thermoelectric properties.