High-efficiency Thermoelectric Materials Research Articles

High thermoelectric performance in cubic inorganic halide perovskite material AgCdX3 (X = F and Cl) from first principles

The need for high-performance thermoelectric materials is a hot topic under research to harvest the waste heat generated in the environment. In the present work, we have studied the inorganic halide perovskite AgCdX3 (X = F and Cl) in the cubic phase with the Pm − 3 m space group. We have explored the structural, electronic, elastic, and thermoelectric properties for both materials using first -principles calculations based on density functional theory. The thermoelectric transport coefficients have been calculated from band structure results with the aid of the BoltzTraP2 code based on the semiclassical Boltzmann theory under the constant relaxation time approximation. The deformation potential theory has been implemented to estimate the relaxation time for charge carriers. The Slack equation is solved to evaluate the contribution of lattice thermal conductivity. The results show that the AgCdCl3 has a high value of the figure of merit (ZT) in comparison to that of AgCdF3 at any temperature irrespective of the type of charge carriers. Our findings will provide a guide for the experimentalists to develop high-efficient thermoelectric materials.

Geometry optimization of solar thermoelectric generator under different operating conditions via Taguchi method

The commercial solar energy market is dominated by non-concentrating photovoltaics and concentrating solar thermal systems. With the development of high efficiency thermoelectric materials, the solar thermoelectric generator is becoming to be a competitive alternative solar energy technology in certain applications. This paper proposes a simulation model and a design methodology for solar thermoelectric generator. Geometry parameters, namely the height, the fill-ratio, the ratio of the cross-section area of n-type material over p-type material of thermoelectric module, solar concentration ratio, were analyzed under different operating conditions. In order to perform the analysis, an L27 (35) orthogonal array was employed to assess all of the design parameters returning the maximum output power. By the analysis of variance, the effect of each design parameter on the output power performance and the interaction between design parameters are identified. The optimal design parameter set is also obtained and it is found that the optimal design performs can potentially achieve 5.47 W output power compared with the output power of 1.95 W from the original design in most operating conditions.

Experimental study on thermoelectric power generation based on cryogenic liquid cold energy

A large amount of cold energy can be recovered while in use because of the large temperature difference between the cryogenic fluid and the ambience. This study designed and built a thermoelectric power generation system for cold energy. Taking liquid nitrogen as the cryogenic liquid, the thermoelectric characteristics of the system were studied experimentally. In addition, the influence of the heat transfer process of the cryogenic fluid on the thermoelectric characteristics was determined. The results revealed that in the liquid-phase and two-phase regions, the temperature difference between the hot and cold end increased in the gasification process. However, in the gas-phase region, it first decreased and then increased. The temperature difference between the hot and cold module ends was small, being only 7.2–28.1 °C. The poor heat transfer capacity at the hot end of the generator restricted the improvement of the temperature difference at the hot and cold ends. Nevertheless, the flow rate change had a relatively small impact on the open-circuit voltage. Although the thermoelectric conversion efficiency remains low, it is expected that the efficiency can be significantly improved by developing high-efficiency low-temperature thermoelectric materials and by improving the heat exchange performance of the hot end of the generator.

Effect of Single Crystallization on Thermoelectric Performance Improvement of Ba8CuxSi46−x Clathrate

The thermoelectric properties of single-crystalline and polycrystalline n-type Ba8CuxSi46−x type I clathrate were investigated and compared. Single-crystalline Ba8CuxSi46−x clathrate was synthesized by the Czochralski (CZ) method with a Cu content gradient, while spark plasma sintering (SPS) was used for the reorganization of polycrystalline materials. The same Cu content resulted in almost the same Seebeck coefficient in both single-crystalline and polycrystalline crystals, while the power factor $$ \left( {{\text{PF}} = S^{2} /\rho } \right) $$ of single-crystalline clathrate is twice that of polycrystalline clathrate. Thus, single crystallization is effective for the synthesis of high-efficiency thermoelectric materials in a type I Ba8CuxSi46−x clathrate ternary system.

A general strategy for designing two-dimensional high-efficiency layered thermoelectric materials

Introducing lone pairs occupied in the pz-orbital not only effectively improves the electronic transport properties, but also increases low-frequency and high-frequency phonon scattering simultaneously.

Synthesis and characterization of bulk Si–Ti nanocomposite and comparisons of approaches for enhanced thermoelectric properties in nanocomposites composed of Si and various metal silicides

Si-based materials are considered as promising candidates for thermoelectric materials, owing to several advantages such as non-toxicity, abundance, and high stability. One of the strategies to develop high-efficiency Si-based thermoelectric materials with enhanced figure of merit zT is increasing the μH/κlat ratio, where μH and κlat are the Hall mobility and lattice thermal conductivity, respectively. In the present study, we synthesize a bulk Si–Ti nanocomposite composed of Si and Ti silicide by a combined method of melt-spinning and spark plasma sintering and examine the thermoelectric properties. The obtained results are compared with the data for Si–V and Si–Ni nanocomposites reported previously by the authors’ group. In the comparison, we focus on (1) size and distribution of the precipitates, (2) the interface between Si and the precipitates, and (3) volume fraction of the precipitates; and investigate the effects of these three factors on keeping high μH and reduction of κlat. Based on the comparison, it is revealed that the uniform distribution of the precipitates and the coherent interface between Si and metal silicide lead to large μH/κlat ratio and thus enhanced zT in the nanocomposites composed of Si and metal silicides.

Half-Heusler thermoelectric materials: NMR studies

We report 59Co, 93Nb, and 121Sb nuclear magnetic resonance measurements combined with density functional theory (DFT) calculations on a series of half-Heusler semiconductors, including NbCoSn, ZrCoSb, TaFeSb, and NbFeSb, to better understand their electronic properties and general composition-dependent trends. These materials are of interest as potentially high efficiency thermoelectric materials. Compared to the other materials, we find that ZrCoSb tends to have a relatively large amount of local disorder, apparently antisite defects. This contributes to a small excitation gap corresponding to an impurity band near the band edge. In NbCoSn and TaFeSb, Curie–Weiss-type behavior is revealed, which indicates a small density of interacting paramagnetic defects. Very large paramagnetic chemical shifts are observed associated with a Van Vleck mechanism due to closely spaced d bands splitting between the conduction and valence bands. Meanwhile, DFT methods were generally successful in reproducing the chemical shift trend for these half-Heusler materials, and we identify enhancement of the larger-magnitude shifts, which we connect to electron interaction effects. The general trend is connected to changes in d-electron hybridization across the series.

Thermoelectric properties of La- and Sc-doped Mg3Sb2 synthesized via pulsed electric current sintering

High-efficiency thermoelectric materials that convert heat into electricity are considered as a countermeasure against greenhouse gas emissions and global warming. In this regard, Mg3Sb2-based Zintl compounds, such as Mg3(Sb, Bi)2 and Mg3Sb2–Mg3Bi2, are inexpensive yet suitable candidates in the medium-temperature range due to low-toxicity and relative abundance of their constituent elements. However, n-type Mg3Sb2 without Bi possesses superior oxidation resistance. This study aimed to investigate the suitability of Sc and La as candidates for n-type Mg3Sb2 dopants. La-doped and Sc-doped Mg3Sb2 (Mg3Sb2Mx [M = La, Sc]; x = 0.000 to 0.05) were synthesized by a one-step, rapid pulsed electric current sintering using Mg, Sb, and a small amount of hydrate (La(OH)3) or oxide (Sc2O3). Electron backscatter diffraction revealed that the average grain sizes for the La-doped (x = 0.03) and Sc-doped (x = 0.05) Mg3Sb2 were 17 and 38 μm, respectively. The room temperature maximum electron concentration for the La-doped and Sc-doped Mg3Sb2 increased up to 2.3 × 1019 and 3.7 × 1019 cm−3, respectively, which is comparable to or higher than the maximum values obtained for the reported n-type Te-doped Mg3Sb2 (approximately 2 × 1019 cm−3). The maximum dimensionless thermoelectric figure of merit values for the La-doped and Sc-doped Mg3Sb2 at 770 K were 0.93 and 0.80, respectively, indicating that Sc and La are promising doping candidates for achieving higher ZT values in n-type Mg3Sb2 without Bi.

Beneficial influence of iodine substitution on the thermoelectric properties of Mo3Sb7

Mo3Sb7 has been known as a p-type metal with commonly poor thermoelectric properties. However, Mo3Sb7 can be a high-efficiency thermoelectric material, owing to its capability of a metal–semiconductor transition, which can be realized by adding two valence electrons through elemental substitutions. Among the Mo3Sb7-based compounds, Mo3Sb5.4Te1.6 shows the highest figure of merit, zT, but additional valence electrons are needed for further improvement of the figure of merit. Here, we try to enhance the figure of merit of Mo3Sb7 by iodine-doping and by synthesizing and characterizing Mo3Sb7Ix with x = 0, 0.50, 0.75, 1.00, 1.25, and 1.50, where antimony (valence electrons = 5) is replaced by iodine (valence electrons = 7). We confirmed that the solubility limit for iodine in Mo3Sb7Ix was 1.25 < x < 1.50, and the figure of merit was enhanced by approximately 65% in maximum in x = 1.25.

A combined experiment and first-principles study on lattice dynamics of thermoelectric CuInTe2

Chalcopyrite compounds are promising high efficient thermoelectric materials. However, the relatively high lattice thermal conductivity at modest temperatures limits their performance. Here, we investigate the lattice dynamics in a polycrystalline CuInTe2 with a combined experimental and computational approach. The phonon dispersion and density of states are computed using the density functional theory. Raman scattering is performed to investigate the phonon dynamical properties. Together with the bulk modulus from X-ray diffraction, the mode-Grüneisen parameters are determined. The low energy B11 and E2 modes characterize the weak anharmonicity, thus responsible for the high lattice thermal conductivity. Meanwhile, B11 and E2 modes display energy redshift under pressure. The softening and enhanced anharmonicity of these modes naturally result in the reduction of the thermal conductivity. Our study suggests that pressure is a routine to reduce the phonon heat conduction at modest temperatures in chalcopyrites.

Enhanced thermoelectric properties of cobalt silicide-silicon heterostructured nanowires

Silicon nanowires (SiNWs) have attracted attention as promising high efficiency thermoelectric materials by significantly improving the thermoelectric efficiency of silicon. Here, we have fabricated cobalt silicide/silicon heterostructure on both ends of nanowires using self-aligned silicide (salicide) process. By forming cobalt silicide (CoSi2) layer on SiNWs, the thermal conductivity of SiNWs with diameters of 200, 350, and 500 nm decrease to 25.1, 31.3, and 38.1 W·m−1·K−1, respectively, which is about 8% reduction on average. Since the phonon nanoinclusion scattering is influenced by the density of the nanoinclusions, the thermal conductivity tends to decrease as the volume fraction of CoSi2 in SiNWs increases. The Seebeck coefficient of the heterostructured nanowires increases to 255 μV/K, which is mainly attributed to the low-energy charge carrier filtering effect due to the Schottky barrier at the CoSi2/Si interfaces. The measurements show that the figure-of-merit ZT of the heterostructured nanowires is improved by 10% on average compared with the conventional SiNWs. Consequently, the CoSi2/Si heterostructured nanowires enhance the thermoelectric properties of SiNWs effectively by suppressing phonon transport and improving electron flow.

Influence Na in GeTe for thermoelectric applications

Germanium telluride (GeTe), a semiconductor material, is a complex solid state system with interesting fundamental properties and it serves as a base for high-efficiency thermoelectric materials. Generally, thermoelectric conversion efficiency can be enhanced with appropriate strategies such as doping, alloying, and nanostructuring in the thermoelectric materials. Since there is a significant discrepancy between the electronic and thermal transport data for GeTe-based materials reported in the literature obscuring the baseline knowledge and preventing a clear understanding of the effect of alloying GeTe with various elements, we introduce pentavelant ions into GeTe to enhance the Seebeck coefficient and reduction of thermal conductivity to overcome the discrepancy between the electronic and thermal transport data. A complete study including XRD, Seebeck coefficient, electrical resistivity, and thermal conductivity on Ge rich Ge1-xNaxTe (x = 0.01, 0.02 &0.03) samples was conducted. All compounds were prepared by solid state reaction and hot press method. From the results, Na doping enhances the Seebeck coefficient in the low temperature range and drastically decreased at high temperature. Our data show that at 723 K, Ge0.99Na0.01 Te & Ge0.98Na0.02Te has power factor about 25 µWcm-1K−2. In addition the influence of Na in GeTe to enhance the electrical conductivity.

Influence of the reducing agent on the formation and morphology of the bismuth telluride nanostructures by using template assisted chemical process: From nanowires to ultrathin nanotubes

Abstract One-dimensional telluride based materials have recently attracted significant attention for development of the high-efficiency thermoelectric materials. Among one dimensional nanostructures, nanotubular structure has shown significant enhancement in the energy conversion efficiency due to higher reduction in the therml conductivity as compared to other one dimensional nanostructrues. Though some progress has already been establsihed to prepare nanotubes still synthesis of ultra-thin, smoother surface and stoichiometry Bi2Te3 nanotubes are still big challenges. Herein, we have reported that properly adjusting concentration of the reducing agent, different types of one-dimensional Bi2Te3 nanostructures such as nanowires to hollow nanostructures can be synthesized by using a template assisted chemical process. Furthermore, ultra-thin, smoother surface and close to stoichiometry Bi2Te3 nanotubes with a diameter of about 50 nm have been obtained by properly adjusting concentration of the reducing agent. Our experimental results evidenced that concentration of the reducing agent (hydrazine hydrate) plays a crucial role (morphology controlling agent) for synthesis of different types one-dimensional nanostructures from nanowires to ultrathin smooth surface nanotubes. A possible formation mechanism has been proposed to explain formation of the different one-dimensional Bi2Te3 nanostructures and the specific role of the reducing agent based on the experimental results.

Molecular Control of Charge Carrier and Seebeck Coefficient in Hybrid Two-Dimensional Nanoparticle Superlattices

The development of high efficiency thermoelectric materials would revolutionize energy harvesting capabilities and be useful for a large number of applications. Hybrid organic–inorganic nanostructu...

Maximizing Thermoelectric Figures of Merit by Uniaxially Straining Indium Selenide

A majority of the electricity currently generated is regrettably lost as heat. Engineering high-efficiency thermoelectric materials which can convert waste heat back into electricity is therefore vital for reducing our energy fingerprint. ZT, a dimensionless figure of merit, acts as a beacon of promising thermoelectric materials. However, engineering materials with large ZT values is practically challenging, since maximizing ZT requires optimizing many interdependent material properties. Motivated by recent studies on bulk indium selenide that suggest it may have favorable thermoelectric properties, here we present the thermoelectric properties of monolayer indium selenide in the presence of uniaxial strain using first-principles calculations conjoined with semiclassical Boltzmann transport theory. Our calculations indicate that conduction band convergence occurs at a compressive strain of −6% along the zigzag direction and results in an enhancement of ZT for p-type indium selenide at room temperature. Fu...

Enhanced thermoelectric properties of YbZn2Sb2−xBix through a synergistic effect via Bi-doping

Recently, extensive research has been done to explore the potential thermoelectric performance of p-Type Zintl compounds AB2Sb2 (A = Ca, Yb, Eu, Sr, Mg; B = Zn, Mn, Cd, Mg), mainly focus on A-site substitution and/or B-site substitution effect. Herein, we report that both power factors and thermal conductivities are simultaneously optimized to increase ZT values via substitution of Sb atoms with the isoelectronic Bi atoms in YbZn2Sb2−xBix compounds. We found that the power factor could be enhanced by 27%, as a result of improved electrical conductivities and the maintained Seebeck coefficients. The former is obtained from an increase of carrier density through a shift of Fermi level via Bi doping, which is supported by XPS analysis. The Seebeck coefficients are maintained due to the increased effective mass. Moreover, the lowest lattice thermal conductivity (0.47 Wm−1 K−1 at 723K) is below the lattice thermal conductivity limit due to enhanced phonon scattering via Bi-doping, which is analyzed by the Callaway model. Benefiting from the synergistic effect, the ZT of Bi-doping samples are significantly enhanced compared with that of the pristine one. These results probably offer a synergistic strategy to enhance ZT for p-Type Zintl compounds AB2Sb2 and other high-efficiency thermoelectric materials.

Marcus Theory of Thermoelectricity in Molecular Junctions

Thermoelectric energy conversion is perhaps the most promising of the potential applications of molecular electronics. Ultimately, it is desirable for this technology to operate at around room temperature, and it is therefore important to consider the role of dissipative effects in these conditions. Here, we develop a theory of thermoelectricity which accounts for the vibrational coupling within the framework of Marcus theory. We demonstrate that the inclusion of lifetime broadening is necessary in the theoretical description of this phenomenon. We further show that the Seebeck coefficient and the power factor decrease with increasing reorganisation energy, and identify the optimal operating conditions in the case of non-zero reorganisation energy. Finally, with the aid of DFT calculations, we consider a prototypical fullerene-based molecular junction. We estimate the maximum power factor that can be obtained in this system, and confirm that C$_{60}$ is an excellent candidate for thermoelectric heat-to-energy conversion. This work provides general guidance that should be followed in order to achieve high-efficiency molecular thermoelectric materials.

Designing chemical analogs to PbTe with intrinsic high band degeneracy and low lattice thermal conductivity

High-efficiency thermoelectric materials require simultaneously high power factors and low thermal conductivities. Aligning band extrema to achieve high band degeneracy, as realized in PbTe, is one of the most efficient approaches to enhance power factor. However, this approach usually relies on band structure engineering, e.g., via chemical doping or strain. By employing first-principles methods with explicit computation of phonon and carrier lifetimes, here we show two full-Heusler compounds Li2TlBi and Li2InBi have exceptionally high power factors and low lattice thermal conductivities at room temperature. The expanded rock-salt sublattice of these compounds shifts the valence band maximum to the middle of the Σ line, increasing the band degeneracy by a factor of three. Meanwhile, resonant bonding in the PbTe-like sublattice and soft Tl–Bi (In–Bi) bonding interaction is responsible for intrinsic low lattice thermal conductivities. Our results present an alternative strategy of designing high performance thermoelectric materials.

Low lattice thermal conductivity and high thermoelectric figure of merit in Na2MgSn

Thermoelectric materials enables the harvest of waste heat and directly conversion into electricity. In search of high efficient thermoelectric materials, low thermal conductivity of a material is essential and critical. Here, we have theoretically investigated the lattice thermal conductivity and thermoelectric properties of layered intermetallic Na$_2$MgSn and Na$_2$MgPb based on the density functional theory and linearized Boltzmann equation with the single-mode relaxation-time approximation. It is found that both materials exhibit very low and anisotropic intrinsic lattice thermal conductivity. Despite of the very low mass density and simple crystal structure of Na$_2$MgSn, its lattice thermal conductivities along $a$ and $c$ axes are only 1.75 and 0.80 W/m$\cdot$K respectively at room temperatures. When Sn is replaced by the heavier element Pb, its lattice thermal conductivities decrease remarkably to 0.51 and 0.31 W/m$\cdot$K respectively along $a$ and $c$ axes at room temperatures. We show that the low lattice thermal conductivities of both materials are mainly due to their very short phonon lifetimes, which are roughly between 0.4 to 4.5 ps. Combined with previous experimental measurements, the metallic Na$_2$MgPb can not be a good thermoelectric material. However, we predict that the semiconducting Na$_2$MgSn is a potential room-temperature thermoelectric material with a considerable $ZT$ of 0.34 at 300 K. Our calculations not only imply that the intermetallic Na$_2$MgSn is a potential thermoelectric material, but also can motivate more theoretical and experimental works on the thermoelectric researches in simple layered intermetallic compounds.

CsCu5S3: Promising Thermoelectric Material with Enhanced Phase Transition Temperature.

Cu2S, featuring low cost, nontoxicity, and earth abundance, has been recently recognized as a high efficiency thermoelectric (TE) material. However, before reaching the maximum of the figure of merit ( ZT), Cu2S undergoes three phase transformations starting at 370 K, which give rise to severe problems, such as possible decomposition and low reliability. Herein, we discover CsCu5S3 with phase transformation at 823 K, which is significantly higher than the 370 K value of Cu2S. Single crystal diffraction data reveal that its two phases are constructed by the same Cu4S4 columnar building unit via propagating either at the opposite sides into a layered o-CsCu5S3, or at the four apexes into a 3D t-CsCu5S3, respectively. Interestingly, the o-to- t transformation is quick, but the reverse one is relatively slow. Theoretical studies reveal that the Cu4S4 column exhibits not only the most condensed atomic aggregation ( Dcolumn) but also the lightest effective mass ( m*), along which higher σ is realized. More interestingly, both phases exhibit remarkable ZT enhancements, 0.46 at 800 K for o-CsCu5S3, and 0.56 at 875 K for t-CsCu5S3, which are 170% and 175% that of Cu2S at the same temperature.