Average Thermoelectric Figure Of Merit Research Articles

Ultrafast generation and detection of coherent acoustic phonons in SnS0.91Se0.09

The rapid growth of attention to tin sulfide (SnS) is due to its efficient application in the field of thermoelectricity, and its doping product SnS0.91Se0.09 can obtain a high average thermoelectric figure of merit (ZTave≈ 1.25). Although many efforts have been made to reveal the mechanisms associated with electron transport and heat conduction, the role of lattice vibration remains poorly understood. Here, we studied the coherent vibration dynamics in SnS0.91Se0.09 bulk single crystal by using the femtosecond two-color pump-probe optical reflectivity technique. The oscillation signal in the time domain of SnS0.91Se0.09 is considered to be an electron–phonon interaction. The coherent dynamics of longitudinal acoustic phonons in this coupling behavior is closely related to probe polarization and excitation power. The stability of oscillation frequency reveals the microscopic mechanism of phonon conduction, and further understands the lattice nonharmonic caused by electron–phonon interaction in light-induced.

Engineering an atomic-level crystal lattice and electronic band structure for an extraordinarily high average thermoelectric figure of merit in n-type PbSe

Multiscale defect structures driven by interstitial Cu, off-centered Pb and Se atoms and scarce anion vacancies in the new CuxPb(Se0.8Te0.2)0.95 give a record-high average ZT among all polycrystalline n-type thermoelectric materials due to high PF.

Vacancy-Ordered Double Perovskites Cs2BI6 (B = Pt, Pd, Te, Sn): An Emerging Class of Thermoelectric Materials.

Vacancy-ordered double perovskites (A2BX6), being one of the environmentally friendly and stable alternatives to lead halide perovskites, have garnered considerable research attention in the scientific community. However, their thermal transport has not been explored much, despite their potential applications. Here, we explore Cs2BI6 (B = Pt, Pd, Te, Sn) as potential thermoelectric materials using state-of-the-art first-principles-based methodologies, viz., density functional theory combined with many-body perturbation theory (G0W0) and spin-orbit coupling. The absence of polyhedral connectivity in vacancy-ordered perovskites gives rise to additional degrees of freedom, leading to lattice anharmonicity. The presence of anharmonic lattice dynamics leads to strong electron-phonon coupling, which is well-captured by the Fröhlich mesoscopic model. The lattice anharmonicity is further studied using ab initio molecular dynamics and the electron localization function. The maximum anharmonicity is observed in Cs2PtI6, followed by Cs2PdI6, Cs2TeI6, and Cs2SnI6. Also, the computed average thermoelectric figure of merit (zT) for Cs2PtI6, Cs2PdI6, Cs2TeI6, and Cs2SnI6 is 0.88, 0.85, 0.95, and 0.78, respectively, which reveals their promising renewable energy applications.

Reduced Thermal Conductivity in Nanostructured AgSbTe2 Thermoelectric Material, Obtained by Arc-Melting.

AgSbTe2 intermetallic compound is a promising thermoelectric material. It has also been described as necessary to obtain LAST and TAGS alloys, some of the best performing thermoelectrics of the last decades. Due to the random location of Ag and Sb atoms in the crystal structure, the electronic structure is highly influenced by the atomic ordering of these atoms and makes the accurate determination of the Ag/Sb occupancy of paramount importance. We report on the synthesis of polycrystalline AgSbTe2 by arc-melting, yielding nanostructured dense pellets. SEM images show a conspicuous layered nanostructuration, with a layer thickness of 25-30 nm. Neutron powder diffraction data show that AgSbTe2 crystalizes in the cubic Pm-3m space group, with a slight deficiency of Te, probably due to volatilization during the arc-melting process. The transport properties show some anomalies at ~600 K, which can be related to the onset temperature for atomic ordering. The average thermoelectric figure of merit remains around ~0.6 from ~550 up to ~680 K.

Atomic Level Defect Structure Engineering for Unusually High Average Thermoelectric Figure of Merit in n-Type PbSe Rivalling PbTe.

Realizing high average thermoelectric figure of merit (ZTave ) and power factor (PFave ) has been the utmost task in thermoelectrics. Here the new strategy to independently improve constituent factors in ZT is reported, giving exceptionally high ZTave and PFave in n-type PbSe. The nonstoichiometric, alloyed composition and resulting defect structures in new Pb1+ x Se0.8 Te0.2 (x = 0-0.125) system is key to this achievement. First, incorporating excess Pb unusually increases carrier mobility (µH ) and concentration (nH ) simultaneously in contrast to the general physics rule, thereby raising electrical conductivity (σ). Second, modifying charge scattering mechanism by the authors' synthesis process boosts a magnitude of Seebeck coefficient (S) above theoretical expectations. Detouring the innate inverse proportionality between nH and µH ; and σ and S enables independent control over them and change the typical trend of PF to temperature, giving remarkably high PFave ≈20 µW cm-1 K-2 from 300 to 823 K. The dual incorporation of Te and excess Pb generates unusual antisite Pb at the anionic site and displaced Pb from the ideal position, consequently suppressing lattice thermal conductivity. The best composition exhibits a ZTave of ≈1.2 from 400 to 823 K, one of the highest reported for all n-type PbQ (Q = chalcogens) materials.

Enhanced Thermoelectric Efficiency through Li-Induced Phonon Softening in CuGaTe2

Thermoelectric materials convert thermal energy into electrical energy and can be a solution for the global climate crisis. For advanced thermoelectric applications, the conversion efficiency has to be high, motivating the search for materials with a high average thermoelectric figure of merit. To achieve such large thermoelectric figures of merit, the electronic properties must be maximized, and the thermal transport must be minimized over a wide temperature range. The chalcopyrite CuGaTe2 exhibits promising electronic properties but suffers from poor thermoelectric performance due to its high lattice thermal conductivity. In the present study, we perform compressive sensing lattice dynamics (CSLD) and ShengBTE calculations, which suggest that the high room temperature lattice thermal conductivity is a result of high longitudinal group velocities. To effectively reduce the thermal conductivity, we introduce lithium into three variants of CuGaTe2: pristine, Sb-doped, and Ag-doped. All compositions exhibited a significant reduction in the lattice thermal conductivity with the inclusion of lithium without any compromise to the electronic properties. By comparing the elastic moduli, we demonstrate that the reduction in the lattice thermal conductivity is to some extent the result of phonon softening. The low thermal conductivity and high power factor in Cu0.90Li0.05Ag0.05GaTe2 lead to a 56% increase in the average zT compared to the pristine sample. Due to the low cost of lithium, this approach can be adapted to chalcopyrite compounds and other thermoelectric systems to develop sustainable and affordable applications for waste heat recovery.

Influence of geometric parameter and contact resistances on the thermal-electric behavior of a segmented TEG

Thermoelectric (TE) generation technology plays an increasingly significant role in a global environment. A desirable approach to improve the efficiency of thermoelectric generator (TEG) is to segment the n- and/or p-leg into several parts with different materials for increasing the average thermoelectric figure of merit of the legs, and to operate with a relatively larger temperature gradient. In this study, a three-dimensional model is developed for the performance analysis of a segmented TEG, and the geometric design optimization of the TEG is examined numerically with the use of the temperature-dependent thermoelectric materials. Specifically, the temperature, heat flow, electric potential, electric current and Joule heating in the thermoelectric modules are investigated in detail. Also, the efficiency and effectiveness of the TEG is analyzed with the design variables such as the ratio of cross-sectional area of p-leg and n-leg, the length ratio of different materials in the segmented leg, and the geometry configurations of p- and n-legs. Furthermore, the effect of the contact resistance on the TEG performance is considered. The results show that a proper design of the geometric parameters can lead to an optimal design of thermoelectric generation systems with higher efficiency.

Development of the high performance thermoelectric unicouple based on Bi2Te3 compounds

Bismuth telluride-based compounds are the most common thermoelectric (TE) materials for power generating systems, which can operate in the temperature range of 300–600 K. One of the main disadvantages of Bi2Te3-based TE modules is the low energy conversion efficiency (3–6%). For this reason, we offer a unique thermoelectric power generation unicouple based on the developed n-type Bi2Te2.7Se0.3 and cut perpendicularly to the main crystallographic axis p-type Bi0.5Sb1.5Te3 thermoelectric materials. The application of the optimized chemical compositions in combination with Spark Plasma Sintering (SPS) technique provided the high average thermoelectric figure of merit of up to 0.97 and 0.72 for n-Bi2Te2.7Se0.3 and pꓕ-Bi0.5Sb1.5Te3, respectively. A three-dimensional finite-element simulation was employed to account for the different transport properties of the n- and p-type materials. As a result of the optimized design, the outstanding energy conversion efficiency ηmax ∼8% was experimentally measured for the thermoelectric unicouple device at the relatively small temperature gradient of 250 K (Tc = 350 K). The output unicouple parameters were recorded using the developed herein high-precision original setup. Furthermore, a power generation TE unicouple with an electric power of 0.04 W at the 0.5 V was designed and fabricated offering significant industrial potential.

Evaluation of the double-tuned functionally graded thermoelectric material approach for the fabrication of n-type leg based on Pb0.75Sn0.25Te

Thermoelectric (TE) technologies realize the generation of electrical energy from the waste heat. The one bottleneck, which significantly restricts the wide use of these technologies, relates to the low energy conversion efficiency of the commercial devices. In this work, the double-tuned functionally graded thermoelectric material (DT-FGTM) approach was proposed to achieve the high-performance TE leg through the increase in the average TE figure of merit (ZT)ave. The essence of this idea is connected with the precise control of the bandgap Eg and chemical potential μc over the entire temperature range. Considering Pb0.75Sn0.25Te solid solution, as an example, and using the three band Kane model, we evaluated the best conditions for the highest thermoelectric performance in this material. Within the offered herein DT-FGTM approach, we fabricated the thermoelectric n-type Pb0.75Sn0.25Te1−xIx leg and measured its output energy characteristics. The efficiency of energy conversion for the prepared DT-FGTM leg reaches a very high value of ∼12.0% at temperature difference ΔT = 540 K. Furthermore, the thermal treatment of the fabricated leg should not injure the carrier concentration distribution through the leg, as the hot end of the leg is heavily doped, and the chemical diffusion between segments would be only beneficial. Our demonstration shows that the DT-FGTM approach has significant practical interest and can be utilized for the other TE materials.

Seeking high energy conversion efficiency in a fully temperature-dependent thermoelectric medium

The material parameters associated with a thermoelectric medium show strong temperature dependence when the medium is subjected to a wide range of temperatures. This motivates the need to investigate the thermal-electric conversion efficiency of a temperature-dependent medium. In this paper, we develop the temperature field and electric potential associated with a one-dimensional thermoelectric medium with particular attention paid to the influence of various material parameters on the thermal-electric conversion efficiency. We use both theoretical and numerical methods and illustrate our results using several detailed numerical examples. Our numerical results show that the Thomson heat induced by a variable Seebeck coefficient is responsible for more than 20% of total energy leading to a 13% effect on the thermal-electric conversion efficiency. This remarkable result indicates that the thermal-electric conversion efficiency can be effectively optimized by controlling the heat exchange between the thermoelectric medium and its surroundings. In addition, our results indicate that when dealing with a large temperature range, the maximum thermal-electric conversion efficiency should not be taken as a function solely of an average thermoelectric figure of merit since it can vary considerably with different material parameters.

First-principles predictions of low lattice thermal conductivity and high thermoelectric performance of AZnSb (A = Rb, Cs).

Here, two compounds, AZnSb (A = Rb, Cs), have been predicted to be potential materials for thermoelectric device applications at high temperatures by using first-principles calculations based on density functional theory (DFT), density functional perturbation theory (DFPT), and Boltzmann transport theory. The layered structure, and presence of heavier elements Rb/Cs and Sb induce high anharmonicity (larger values of mode Grüneisen parameter), low Debye temperature, and intense phonon scattering. Thus, these compounds possess intrinsically low lattice thermal conductivity (κl), ∼0.5 W m−1 K−1 on average at 900 K. Highly non-parabolic bands and relatively wide bandgap (∼1.37 and 1.1 eV for RbZnSb and CsZnSb, respectively, by mBJ potential including spin–orbit coupling effect) induce large Seebeck coefficient while highly dispersive and two-fold degenerate bands induce high electrical conductivity. Large power factor and low values of κl lead to a high average thermoelectric figure of merit (ZT) of RbZnSb and CsZnSb, reaching 1.22 and 1.1 and 0.87 and 1.14 at 900 K for p-and n-type carriers, respectively.

Thermoelectric Transport Properties of TmAgxCu1-xTe2 solid solutions

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

Improved carrier transport properties by I-doping in n-type Cu0.008Bi2Te2.7Se0.3 thermoelectric alloys

The addition of Cu becomes essential in polycrystalline n-type Bi2(Te,Se)3-based alloys since it is known to enhance stability of carrier transport properties as well as thermoelectric performance. However, a way to further optimize is necessary owing to the limited controllability of transport parameters by Cu addition. Herein, we present improved carrier transport properties via I-doping in Cu0.008Bi2Te2.7Se0.3. Weighted mobility and effective mass are increased simultaneously by small amount doping of I at Te/Se-site. As a result, ~15% increased power factor and high average thermoelectric figure of merit (zT) of 0.79 were obtained in a wide temperature range.

Manipulation of Band Degeneracy and Lattice Strain for Extraordinary PbTe Thermoelectrics.

Maximizing band degeneracy and minimizing phonon relaxation time are proven to be successful for advancing thermoelectrics. Alloying with monotellurides has been known to be an effective approach for converging the valence bands of PbTe for electronic improvements, while the lattice thermal conductivity of the materials remains available room for being further reduced. It is recently revealed that the broadening of phonon dispersion measures the strength of phonon scattering, and lattice dislocations are particularly effective sources for such broadening through lattice strain fluctuations. In this work, a fine control of MnTe and EuTe alloying enables a significant increase in density of electron states near the valence band edge of PbTe due to involvement of multiple transporting bands, while the creation of dense in-grain dislocations leads to an effective broadening in phonon dispersion for reduced phonon lifetime due to the large strain fluctuations of dislocations as confirmed by synchrotron X-ray diffraction. The synergy of both electronic and thermal improvements successfully leads the average thermoelectric figure of merit to be higher than that ever reported for p-type PbTe at working temperatures.

Enhancing the average thermoelectric figure of merit of elemental Te by suppressing grain boundary scattering

Suppressed grain boundary scattering contributes to enhanced electrical conductivity and device zT in elemental Te based thermoelectric materials.

Insight on the Interplay between Synthesis Conditions and Thermoelectric Properties of α-MgAgSb.

α-MgAgSb is a very promising thermoelectric material with excellent thermoelectric properties between room temperature and 300 °C, a range where few other thermoelectric materials show good performance. Previous reports rely on a two-step ball-milling process and/or time-consuming annealing. Aiming for a faster and scalable fabrication route, herein, we investigated other potential synthesis routes and their impact on the thermoelectric properties of α-MgAgSb. We started from a gas-atomized MgAg precursor and employed ball-milling only in the final mixing step. Direct comparison of high energy ball-milling and planetary ball-milling revealed that high energy ball milling already induced formation of MgAgSb, while planetary ball milling did not. This had a strong impact on the microstructure and secondary phase fraction, resulting in superior performance of the high energy ball milling route with an attractive average thermoelectric figure of merit of 0.9. We also show that the formation of undesired secondary phases cannot be avoided by a modification of the sintering temperature after planetary ball milling, and discuss the influence of commonly observed secondary phases on the carrier mobility and on the thermoelectric properties of α-MgAgSb.

A squeeze on the perovskite structure improves the thermoelectric performance of Europium Calcium Titanates

Polycrystalline Eu1–xCaxTiO3–δ (0 ≤ x ≤ 1) samples were synthesised to investigate the interrelations among the crystal structure, local structural disorder, and thermoelectric properties. The Ca2+ substitution is locally modifying (squeezing) the crystal structure, resulting in distinct differences between the long-range and local scales, e.g., the sample with x = 0.2 shows a cubic structure in long-range scale, while tetragonal distortions are observed locally. Additionally, the contraction of the unit cell volume with an accompanying reduction of the overall symmetry facilitates the accommodation of smaller Eu3+ (instead of Eu2+). The lattice imperfections induced by Ca2+ substitution significantly improve electron concentration and simultaneously dramatically reduce thermal conductivity (i.e., as large as 50% compared with pristine sample) at room temperature. The average thermoelectric figure of merit of Eu0.2Ca0.8TiO3–δ is enhanced by almost 100% compared with that of the pristine EuTiO3. This work demonstrates that controlling lattice deformation offers new ways to enhance the thermoelectric performance of titanates.

Thermoelectric Transport Properties of Cd xBi yGe1- x- yTe Alloys.

Band convergence has been proven as an effective approach for enhancing thermoelectric performance, particularly in p-type IV-VI semiconductors, where the superior electronic performance originates from the contributions of both L and Σ band valleys when they converge to have a small energy offset. When alloying with cubic IV-VI semiconductors, CdTe has been found as an effective agent for achieving such a band convergence. This work focuses on the effect of CdTe-alloying on the thermoelectric transport properties of GeTe, where the carrier concentration can be tuned in a broad range through Bi-doping on Ge site. It is found that CdTe-alloying indeed helps to converge the valence bands of GeTe in both low- T rhombohedral and high- T cubic phases for an increase in Seebeck coefficient with a decrease in mobility. In addition, the strong phonon scattering due to the existence of high-concentration Cd/Ge and Bi/Ge substitutions leads the lattice thermal conductivity to be reduced to as low as 0.6 W/(m-K). These lead to an effectively increased average thermoelectric figure of merit ( ZTave ∼ 1.2) at 300-800 K, which is higher than that of many IV-VI materials with CdTe-alloying or alternatively with MnTe-, MgTe-, SrTe-, EuTe-, or YbTe-alloying for a similar band convergence effect.

Enhancement of average thermoelectric figure of merit by increasing the grain-size of Mg3.2Sb1.5Bi0.49Te0.01

Zintl compound n-type Mg3(Sb,Bi)2 was recently found to exhibit excellent thermoelectric figure of merit zT (∼1.5 at around 700 K). To improve the thermoelectric performance in the whole temperature range of operation from room temperature to 720 K, we investigated how the grain size of sintered samples influences electronic and thermal transport. By increasing the average grain size from 1.0 μm to 7.8 μm, the Hall mobility below 500 K was significantly improved, possibly due to suppression of grain boundary scattering. We also confirmed that the thermal conductivity did not change by increasing the grain size. Consequently, the sample with larger grains exhibited enhanced average zT. The calculated efficiency of thermoelectric power generation reaches 14.5% (ΔT = 420 K), which is quite high for a polycrystalline pristine material.

Ultrahigh Average Thermoelectric Figure of Merit, Low Lattice Thermal Conductivity and Enhanced Microhardness in Nanostructured (GeTe)x (AgSbSe2 )100-x.

Waste heat sources are generally diffused and provide a range of temperatures rather than a particular temperature. Thus, thermoelectric waste heat to electricity conversion requires a high average thermoelectric figure of merit (ZTavg ) of materials over the entire working temperature along with a high peak thermoelectric figure of merit (ZTmax ). Herein an ultrahigh ZTavg of 1.4 for (GeTe)80 (AgSbSe2 )20 [TAGSSe-80, T=tellurium, A=antimony, G=germanium, S=silver, Se=selenium] is reported in the temperature range of 300-700 K, which is one of the highest values measured amongst the state-of-the-art Pb-free polycrystalline thermoelectric materials. Moreover, TAGSSe-80 exhibits a high ZTmax of 1.9 at 660 K, which is reversible and reproducible with respect to several heating-cooling cycles. The high thermoelectric performance of TAGSSe-x is attributed to extremely low lattice thermal conductivity (κlat ), which mainly arises due to extensive phonon scattering by hierarchical nano/meso-structures in the TAGSSe-x matrix. Addition of AgSbSe2 in GeTe results in κlat of ≈0.4 W mK-1 in the 300-700 K range, approaching to the theoretical minimum limit of lattice thermal conductivity (κmin ) of GeTe. Additionally, (GeTe)80 (AgSbSe2 )20 exhibits a higher Vickers microhardness (mechanical stability) value of ≈209 kgf mm-2 compared to the other state-of-the-art metal chalcogenides, making it an important material for thermoelectrics.