Thermoelectric Figure Of Merit ZT Research Articles

Low thermal conductivity and good thermoelectric performance in mercury chalcogenides

Thermoelectric (TE) property refers to converting thermal energy into electrical energy via Seebeck effect. Theoretically, the presence of a low thermal conductivity and a high thermoelectric power factor in a same material promises a good thermoelectric performance. In this paper, we investigate the thermal transport and thermoelectric properties of crystalline mercury chalcogenides (HgX, X = O, S, Se, Te) based on first-principles calculations combined with Boltzmann transport equation and electron–phonon interaction (EPI). Remarkably, the calculated lattice thermal conductivity κL of the semiconducting mercury chalcogenides (α-HgO and α-HgS) are fairly low (κL~0.60 W/mK at 300 K, which is about 38% of the value for the typical thermoelectric material PbTe), while the corresponding power factors S2σ for α-HgS are relatively high, which, as a result, leads to a good thermoelectric performance in α-HgS, with the thermoelectric figure of merit ZT even exceeding 1.28. However, the highest ZT of α-HgO is only 0.58 due to the relatively low S2σ. For comparison, the lattice thermal conductivity κL of the semimetallic mercury chalcogenides (β-HgS, β-HgSe, and β-HgTe) are much higher than that of PbTe, limiting the applications in thermoelectricity. These results indicate that semiconducting mercury chalcogenides may be potential candidates for the design of thermoelectric generators.

Porous Metal-Organic-Framework (MOF) Based Hybrid Materials for Thermoelectric Applications

Thermoelectric materials based on porous hybrid and MOFs constitute a very recent development, with many new and undiscovered phenomena for the research field. These porous Hybrid materials and MOF (Metal-Organic-Framework) films represent modern designer materials that exhibit many requirements of a near ideal and tunable future thermoelectric (TE) material. It is anticipated that especially the inherent porosity can be used advantageously in the design and simulation predictions of future thermoelectric materials. In principle, porous Hybrid and MOFs materials can comply with most of the requirements of an ideal and tunable future thermoelectric (TE) material. In contrast to traditional semiconducting bulk thermoelectric materials, porous hybrid MOF templates can be used to overcome some of the constraints of physics in bulk thermoelectric materials. These porous hybrid systems are amenable for simulation and modeling to design novel optimized electron-crystal phonon-glass materials with potentially very high Figure of Merit (ZT) numbers. Porous MOF and Hybrid materials possess an ultra-low thermal conductivity, which can be further modulated by phonon engineering within their complex porous and hierarchical architecture to advance the thermoelectric figure of merit ZT. This class of novel materials offers new approaches to the design of complex and hierarchical porous structures, possessing ultra-low thermal conductivity, which are able to trap, and localize phonons, thus resulting in a new generation of thermoelectric materials with a high ZT number. This work is demonstrating recent results of MOF thermoelectric Materials and provides a future outlook to the search for the next generation thermoelectric porous Hybrid and MOF materials, which could be part of a green renewable energy revolution with novel materials of sustainable high ZT values.

Probing Efficient N-Type Lanthanide Dopants for Mg3Sb2 Thermoelectrics.

The recent discovery of n‐type Mg3Sb2 thermoelectrics has ignited intensive research activities on searching for potential n‐type dopants for this material. Using first‐principles defect calculations, here, a systematic computational screening of potential efficient n‐type lanthanide dopants is conducted for Mg3Sb2. In addition to La, Ce, Pr, and Tm, it is found that high electron concentration (≳1020 cm−3 at the growth temperature of 900 K) can be achieved by doping on the Mg sites with Nd, Gd, Ho, and Lu, which are generally more efficient than other lanthanide dopants and the anion‐site dopant Te. Experimentally, Nd and Tm are confirmed as effective n‐type dopants for Mg3Sb2 since doping with Nd and Tm shows higher electron concentration and thermoelectric figure of merit zT than doping with Te. Through codoping with Nd (Tm) and Te, simultaneous power factor improvement and thermal conductivity reduction are achieved. As a result, high zT values of ≈1.65 and ≈1.75 at 775 K are obtained in n‐type Mg3.5Nd0.04Sb1.97Te0.03 and Mg3.5Tm0.03Sb1.97Te0.03, respectively, which are among the highest values for n‐type Mg3Sb2 without alloying with Mg3Bi2. This work sheds light on exploring promising n‐type dopants for the design of Mg3Sb2 thermoelectrics.

Electrical conductivity increase by order of magnitude through controlling sintering to tune hierarchical structure of oxide ceramics

Perovskite calcium manganate CaMnO3-δ is representative of an essential group of semiconductors with unique physical phenomena including colossal magnetoresistance and thermoelectric properties. For the large-scale applications that require the utilization of oxide ceramics, the polycrystalline materials’ physical properties such as conductivity can be controlled and ultimately optimized through tuning the ceramics sintering conditions. In the present study, the impact of the sintering temperature on the structure and thermoelectric performance of CaMnO3-δ is systematically studied. For the ceramics pellets made of precursors powders synthesized using the chemical sol-gel reaction, the increase in the sintering temperature dramatically increases the electrical conductivity by order of magnitude and simultaneously increases the Seebeck coefficient. Meanwhile, the thermal conductivity increases with the rise of the sintering temperature. Among the samples sintered at different temperatures, the peaking thermoelectric Figure of Merit ZT of pristine CaMnO3-δ reached 0.28, which is a factor of 2.5 higher than the highest reported ZT for pristine CaMnO3-δ and approached that of the doped single-phase CaMnO3-δ. The electrical and thermal properties changes were interpreted based on the oxide ceramics hierarchical structure evolutions, from unit cell level to micron scales, induced by changes of sintering temperatures.

Thermoelectric and Plasmonic Properties of Metal Nanoparticles Linked by Conductive Molecular Bridges

Thermoelectric and plasmonic properties of systems comprising small golden nanoparticles (NPs) linked by narrow conductive polymer bridges are studied using the original hybrid quantum‐classical model. The bridges are considered here to be either conjugated polyacetylene, polypyrrole, or polythiophene chain molecules terminated by thiol groups. The parameters required for the model are obtained using density functional theory and density functional tight‐binding simulations. Charge‐transfer plasmons in the considered dumbbell structures are found to possess frequency in the infrared region for all considered molecular linkers. The appearance of plasmon vibrations and the existence of charge flow through the conductive molecule, with manifestation of quantum properties, are confirmed using frequency‐dependent polarizability calculations implemented in the coupled perturbed Kohn–Sham method. To study the thermoelectric properties of the 1D periodical systems, a universal equation for the Seebeck coefficient is derived. The phonon part of the thermal conductivity for the periodical – system is calculated by the classical molecular dynamics. The thermoelectric figure of merit ZT is calculated by considering the electrical quantum conductivity of the systems in the ballistic regime. It is shown that for nanoparticles connected by polyacetylene, polypyrrole, or polythiophene chains at T = 300 K, the ZT value is {0.08;0.45;0.40}, respectively.

Enhancement of Thermoelectric Figure of Merit of p‐Type Nb0.9Ti0.1FeSb Half‐Heusler Compound by Nanostructuring

Half‐Heusler compounds with 18 valence electron count show a large thermoelectric power factor. However, the thermal conductivity is high, so if the thermal conductivity can be reduced, the thermoelectric figure of merit zT can be enhanced. The key point to reduce the thermal conductivity is effective phonon scattering. Nanostructuring is a well‐known method of introducing phonon scattering centers by creating high‐density grain boundaries in nanoscale. Herein, NbFeSb, which is one of the best p‐type half‐Heusler compounds in thermoelectrics, is focused on. Nanostructured bulk Nb0.9Ti0.1FeSb is synthesized by melt spinning followed by spark plasma sintering. The cooling speed during melt spinning is changed to obtain nanostructures with different sizes. This study reveals that the zT at high temperatures is enhanced by nanostructuring. The nanostructured Nb0.9Ti0.1FeSb shows an ≈20% higher zT value at 1073 K than the non‐nanostructured one, mainly due to the effective reduction of the thermal conductivity.

Thermoelectric properties of cubic Ba-substituted strontium disilicide, Sr1-xBaxSi2, with Ba content above solid solubility limit

In the interest of the sustainable development of society, it is preferable to construct next-generation thermoelectric devices using materials from earth-abundant elements. In conventional thermoelectric devices for low temperature applications, Bi2Te3-based materials that consists of scarce elements have been used. Cubic strontium disilicide (SrSi2) is a potential thermoelectric material for low temperature applications that consists of earth-abundant elements. To enhance the value of dimensionless thermoelectric figure of merit ZT of cubic SrSi2, partial substitution of Sr atoms with Ba atoms was examined. Cubic Ba-substituted SrSi2, Sr1-xBaxSi2 (x = 0.00, 0.09, and 0.19), was synthesized by spark plasma sintering at 250 MPa and 1023 K, and its thermoelectric properties, such as electrical resistivity ρ, Seebeck coefficient S, and thermal conductivity κ, were measured at temperatures ranging from 10 to 380 K. The sample with Ba content x above the Ba solid solubility limit in cubic SrSi2 (0.13) was successfully synthesized. With increasing x, ρ, and S increase while κ decreases, leading to an increase of ZT with x. ZT exhibits a maximum at a temperature Tmax, which shifts from 313 to 357 K when x increases from 0 to 0.19. The maximum ZT value for cubic Sr0.81Ba0.19Si2 in the present study was calculated to be 0.21 at 357 K. The ZT value obtained is higher than previously reported values for cubic Sr1-xBaxSi2 and cubic Sr1-xCaxSi2. These results demonstrate that cubic SrSi2 has properties suitable for low temperature applications.

Influence of Te Vacancies on the Thermoelectric Properties of n-type Cu0.008Bi2Te2.7-xSe0.3

In this study, we report the influence of Te vacancy formation on the thermoelectric properties of n-type Cu0.008Bi2Te2.7Se0.3 alloys, including their electronic and thermal transport properties. Te-deficient Cu0.008Bi2Te2.7-xSe0.3 (x = 0, 0.005, 0.01 and 0.02) samples were systematically synthesized and characterized. Regarding electronic transport properties, carrier concentration was increased with Te vacancies, while carrier mobility was maintained. As a result, the electrical conductivity significantly increased while the Seebeck coefficient reduced moderately, thus, the power factor was enhanced from 3.04 mW/mK<sup>2</sup> (pristine) to 3.22 mW/mK<sup>2</sup> (x = 0.02) at 300 K. Further analysis based on a single parabolic band model revealed that the weighted mobility of the conduction band increased, which is favorable for electron transport, as Te vacancies were generated. Regarding thermal transport properties, lattice thermal conductivity decreased with Te vacancies due to additional point defect phonon scattering, however, total thermal conductivity increased due to larger electronic contribution as Te vacancies increased. Analysis using the Debye-Callaway model suggests that the phonon scattering by the Te vacancies is as efficient as the substitution point defect scattering. Consequently, the thermoelectric figure of merit zT increased at all temperatures for x = 0.005 and 0.01. The maximum zT of 0.95 was achieved for Te-deficient Cu0.008Bi2Te2.69Se0.3 (x = 0.01) at 400 K.

Influence of shear strain on HPT-processed n-type skutterudites yielding ZT=2.1

After our successful new and energy-saving production route to prepare bulk skutterudites with high ZTs from commercial powders via cold pressing (CP) and high-pressure torsion (HPT), we present in this paper a detailed study of the influence of shear strain and the preparation parameters, especially the temperature, on the micro-structural, thermoelectric and mechanical properties by analyzing specimens cut from various positions of the HPT disks of (Sm,Mm)0.15Co4Sb12. Across these large discs (30 mm in diameter and about 1 mm thickness) the shear strain γ rises linearly from the center of the sample to the rim reaching γ ∼ 450 (for 5 revolutions), a value first time realized for skutterudites. HPT-straining is essential for the introduction of high densities of crystal lattice defects, particularly dislocations, which result in a significant refining of the grains. Whereas the processing temperature mainly influences the density of the HPT consolidated samples and as a consequence, physical and mechanical properties, the applied shear strain rules the thermoelectric parameters. As an important feature, the Seebeck coefficient remained roughly unchanged under strain. Although the increasing shear strain caused the thermal conductivity to significantly decrease, this beneficial effect is accompanied by a concomitant significant increase of the electrical resistivity. Consequently, the interplay of the two controversial influences results in a maximum and record high thermoelectric figure of merit ZT ∼2.1.

Теплопроводность и термоэдс соединений системы Cu-Ge-As-Se

The effect of temperatures (300 - 400 K) and concentrations on electrical conductivity, thermoelectric power, and thermal conductivity of crystalline materials based on copper chalcogenides with the general formula (GeSe)1-x (CuAsSe2)x is analyzed. The mechanisms of heat transfer are determined. The non-monotonicity of the temperature dependence of the thermal conductivity with an anomaly at 358 K. The thermoelectric figure of merit ZT is calculated.

A Study on N-Type Bismuth Sulphochloride (BiSCl): Efficient Synthesis and Characterization

Irregular rod-like bismuth sulphochloride (BiSCl) was synthesized by a high efficiency process in which BiCl3 and Bi2S3 were fully ground and mixed in N2 atmosphere followed by a sintering at 227∘C for 9 h to achieve crimson BiSCl powder. The products were characterized by X-ray powder diffraction (XRD) and scanning electronic microscopy coupled with energy dispersive X-ray spectroscopy (SEM-EDX). The BiSCl is an n-type semiconductor with a bandgap of 1.9 eV. In the temperature range of 300–500 K, thermal conductivity, electrical conductivity, Seebeck coefficient and thermoelectric figure of merit ZT of the BiSCl were tested and calculated. It is resulted that the BiSCl has a thermal conductivity of 0.45 Wm−1K−1 at 500 K and a Seebeck coefficient of −579 μVK−1 at 400 K. This work will help inform future in-depth studies on possible applications of BiSCl in the fields of thermoelectricity, optoelectronics and photocatalyticity.

Low thermal conductivity and good thermoelectric performance in mercury chalcogenides

Thermoelectric (TE) property refers to converting thermal energy into electrical energy via Seebeck effect. Theoretically, the presence of a low thermal conductivity and a high thermoelectric power factor in a same material promises a good thermoelectric performance. In this paper, we investigate the thermal transport and thermoelectric properties of crystalline mercury chalcogenides (HgX, X = O, S, Se, Te) based on first-principles calculations combined with Boltzmann transport equation and electron–phonon interaction (EPI). Remarkably, the calculated lattice thermal conductivity κL of the semiconducting mercury chalcogenides (α-HgO and α-HgS) are fairly low (κL~0.60 W/mK at 300 K, which is about 38% of the value for the typical thermoelectric material PbTe), while the corresponding power factors S2σ for α-HgS are relatively high, which, as a result, leads to a good thermoelectric performance in α-HgS, with the thermoelectric figure of merit ZT even exceeding 1.28. However, the highest ZT of α-HgO is only 0.58 due to the relatively low S2σ. For comparison, the lattice thermal conductivity κL of the semimetallic mercury chalcogenides (β-HgS, β-HgSe, and β-HgTe) are much higher than that of PbTe, limiting the applications in thermoelectricity. These results indicate that semiconducting mercury chalcogenides may be potential candidates for the design of thermoelectric generators.

High Thermoelectric Performance in the New Cubic Semiconductor AgSnSbSe3 by High-Entropy Engineering

We investigate the structural and physical properties of the AgSnmSbSem+2 system with m = 1-20 (i.e., SnSe matrix and ∼5-50% AgSbSe2) from atomic, nano, and macro length scales. We find the 50:50 composition, with m = 1 (i.e., AgSnSbSe3), forms a stable cation-disordered cubic rock-salt p-type semiconductor with a special multi-peak electronic valence band structure. AgSnSbSe3 has an intrinsically low lattice thermal conductivity of ∼0.47 W m-1 K-1 at 673 K owing to the synergy of cation disorder, phonon anharmonicity, low phonon velocity, and low-frequency optical modes. Furthermore, Te alloying on Se sites creates a quinary high-entropy NaCl-type solid solution AgSnSbSe3-xTex with randomly disordered cations and anions. The extra point defects and lattice dislocations lead to glass-like lattice thermal conductivities of ∼0.32 W m-1 K-1 at 723 K and higher hole carrier concentration than AgSnSbSe3. Concurrently, the Te alloying promotes greater convergence of the multiple valence band maxima in AgSnSbSe1.5Te1.5, the composition with the highest configurational entropy. Facilitated by these favorable modifications, we achieve a high average power factor of ∼9.54 μW cm-1 K-2 (400-773 K), a peak thermoelectric figure of merit ZT of 1.14 at 723 K, and a high average ZT of ∼1.0 over a wide temperature range of 400-773 K in AgSnSbSe1.5Te1.5.

Intrinsically high thermoelectric figure of merit of half-Heusler ZrRuTe

The electronic structure and thermoelectric properties of ZrRuTe-based half-Heusler compounds are studied using density functional theory and Boltzmann transport formalism. Based on rigorous computations of electron relaxation time τ considering electron–phonon interactions and lattice thermal conductivity κl considering phonon–phonon interactions, we find ZrRuTe to be an intrinsically good thermoelectric material. It has a high power factor of ∼2 × 10−3 W m−1 K−2 and low κl ∼ 10 W m−1 K−1 at 800 K. The thermoelectric figure of merit ZT ∼ 0.13 at 800 K is higher than similar other compounds. We have also studied the properties of the material as a function of doping and find the thermoelectric properties to be substantially enhanced for p-doped ZrRuTe with the ZT value raised to ∼0.2 at this temperature. The electronic, thermodynamic, and transport properties of the material are thoroughly studied and discussed.

Hf-Doping Effect on the Thermoelectric Transport Properties of n-Type Cu0.01Bi2Te2.7Se0.3

Polycrystalline bulks of Hf-doped Cu0.01Bi2Te2.7Se0.3 are prepared via a conventional melt-solidification process and subsequent spark plasma sintering technology, and their thermoelectric performances are evaluated. To elucidate the effect of Hf-doping on the thermoelectric properties of n-type Cu0.01Bi2Te2.7Se0.3, electronic and thermal transport parameters are estimated from the measured data. An enlarged density-of-states effective mass (from ~0.92 m0 to ~1.24 m0) is obtained due to the band modification, and the power factor is improved by Hf-doping benefitting from the increase in carrier concentration while retaining carrier mobility. Additionally, lattice thermal conductivity is reduced due to the intensified point defect phonon scattering that originated from the mass difference between Bi and Hf. Resultantly, a peak thermoelectric figure of merit zT of 0.83 is obtained at 320 K for Cu0.01Bi1.925Hf0.075Te2.7Se0.3, which is a ~12% enhancement compared to that of the pristine Cu0.01Bi2Te2.7Se0.3.

Electronic origin of the enhanced thermoelectric efficiency of Cu2Se

Thermoelectric materials (TMs) can uniquely convert waste heat into electricity, which provides a potential solution for the global energy crisis that is increasingly severe. Bulk Cu2Se, with ionic conductivity of Cu ions, exhibits a significant enhancement of its thermoelectric figure of merit zT by a factor of ~3 near its structural transition around 400 K. Here, we show a systematic study of the electronic structure of Cu2Se and its temperature evolution using high-resolution angle-resolved photoemission spectroscopy. Upon heating across the structural transition, the electronic states near the corner of the Brillouin zone gradually disappear, while the bands near the centre of Brillouin zone shift abruptly towards high binding energies and develop an energy gap. Interestingly, the observed band reconstruction well reproduces the temperature evolution of the Seebeck coefficient of Cu2Se, providing an electronic origin for the drastic enhancement of the thermoelectric performance near 400 K. The current results not only bridge among structural phase transition, electronic structures and thermoelectric properties in a condensed matter system, but also provide valuable insights into the search and design of new generation of thermoelectric materials.

First-principles calculation of lattice thermal conductivity and thermoelectric figure of merit in ferromagnetic half-Heusler alloy CoMnSb

Half-Heusler (HH) alloys are an important and well-studied class of thermoelectric, magnetic, and spintronic materials. However, few studies have reported on thermal conductivity of magnetic HH alloys. In this study, we have performed first-principles calculation of the thermoelectric properties of a magnetic HH alloy CoMnSb. The lattice thermal conductivity of CoMnSb was found to be smaller than typical nonmagnetic HH alloys (CoTiSb, CoZrSb). The reason for this was found to be the small group velocity and relaxation time of acoustic phonons. Moreover, we estimated the electronic thermal conductivity and power factor, and finally, we evaluated the thermoelectric figure of merit ZT of CoMnSb.

Enhanced Thermoelectric Performance in Black Phosphorus Nanotubes by Band Modulation through Tailoring Nanotube Chirality.

Black phosphorus (BP) has attracted great attention for applications in thermoelectric devices, owing to its unique in-plane anisotropic electrical and thermal properties. However, its limited conversion efficiency hinders practical application. Here, the thermoelectric properties of 1D BP nanotubes (BPNTs) with different tube chirality are investigated using first-principles calculations and Boltzmann transport theory. The results reveal that variation of crystallographic orientation has a distinct impact on band dispersions, which provides a wide tunability of electronic transport. It is shown that (1,1)-oriented BPNT structure can yield an order-of-magnitude enhanced thermoelectric figure of merit ZT at room temperature (as high as 1.0), compared with the bulk counterpart. The distinct enhancement is attributed to the favorable multiple band structures that lead to high carrier mobility of 2430 cm2 V-1 s-1 . Further performance improvement can be realized by suitable doping, such as N-alloying, reaching an increase of room-temperature ZT by a factor of 3 over that of pristine BPNT. The work provides an applicable method to achieve band engineering design, and presents a new strategy of designing 1D BPNT that are promising candidates for flexible, eco-friendly, and high-performance thermoelectrics.

Microstructure and properties of n-type Bi2Te3-based thermoelectric material fabricated by selective laser sintering

An n-type Bi2Te3-based thermoelectric material Bi2Te2.85Se0.15 was prepared through an SLS (Selective Laser Sintering) process with E12/Bi2Te2.85Se0.15 as composite powder (weight ratio 1:7), from which E12 was degreased after the SLS process. It was studied the effect of laser power, scanning speed and scanning interval on the density and thermoelectric properties of the sintered samples. The laser power was found the most important influential factor, while the scanning interval was the second and the scanning speed the least. The maximum dimensionless thermoelectric figure of merit ZT is about 0.88 in the condition when the laser power, scanning interval and scanning speed were 20 W, 0.26 mm and 3000 mm s−1, respectively.

Enhanced thermoelectric properties in Ag-rich AgSbSe2

In this paper, we report remarkably improved thermoelectric properties of p-type Ag-rich AgSbSe2. Polycrystalline bulk samples of Ag1+xSb1-xSe2 (x ​= ​0, 0.01, 0.02, 0.03) were prepared from high purity elements and densified by spark plasma sintering. The X-ray diffraction analysis indicates the formation of solid solution and Ag2Se second phase in Ag-rich samples. The existence of Ag2Se precipitates is further confirmed by differential scanning calorimetric measurements and scanning electron microscopy images. The increased carrier concentrations combined with the modified band structures lead to the enhanced power factor in Ag1+xSb1-xSe2. Ag2Se particles act as phonon scattering centres and reduce the lattice thermal conductivity. This, together with the increased power factor, further enhances the thermoelectric figure of merit ZT. The maximum ZT value at 673 ​K increases from 0.44 for pristine AgSbSe2 to 1.02 for Ag1.02Sb0.98Se2, which is more than two times enhancement.