Anisotropic Thermoelectric Properties Research Articles

Grain boundary density on realizing anisotropic thermoelectric properties of Ca3Co4O9-based ceramics with excellent texturation

Texturation and anisotropic performance of Ca3Co4O9-based ceramics fabricated by spark plasma sintering were reported, showing promising thermoelectric properties. With template seeds simultaneous additives, the high degree of texturing was obtained. Electrical resistivity decreased from 16.9 to 6.1 mΩ cm and Seebeck coefficient increased from 75.8 to 187.6 μV/K for the sample with 10 wt% templates, which were dominated by a synergistic combination of the grain orientation and grain boundary density. The obvious anisotropic power factors presented, mainly coming from the texturing characteristic. A low total thermal conductivity of 1.16 W/(m·K) were greatly suppressed by tuning broad wavelength phonon scattering, which was ascribed to the layered nanostructures at grain boundaries. Furthermore, the ZT value of 0.31 was obtained at 1073 K for the sample perpendicular to pressure direction, yielding 138 % enhancement compared with that of the other direction. The work confirmed clear evidence that the textured ceramics with template seeds addition have a great influence on thermoelectricity in oxides materials.

Boosting the thermoelectric properties of layered SnSb2Te4 compound by microstructure regulation combined with heterovalent halogen substitution

The layered IVV2Te4-based compounds have recently received significant attention due to their intrinsically low lattice thermal conductivities. However, the influence of microstructure regulation and halogen doping on their thermoelectric properties has been scarcely studied. In this work, we systematically investigated the anisotropic thermoelectric transport properties of pristine SnSb2Te4 compounds with varied microstructures manipulated by ball milling. With the help of thermoelectric transport models, the optimal ball milling time is estimated based on the calculation of quality factor. Furthermore, the remarkably improved Seebeck coefficient is achieved by the introduction of halogen elements into Te sites, which is attributed to reduced carrier concentration and converged carrier pockets. As a result, a maximal zT ∼0.4 at 725 K is achieved in SnSb2(Te0·94I0.06)4 sample aligned with the SPS pressure direction with a ball-milling time of 1 h, which demonstrates a 35 % enhancement over the pristine sample. This work provides an insightful guide to elevate the thermoelectric properties of layered IVVI2Te4-based compounds.

Enhanced thermoelectric properties of layered anisotropic PbBi2Te4 through the suppression of bipolar diffusion effect via Se alloying

The layered ternary PbBi2Te4 is regarded as a promising thermoelectric material due to its intricate crystal structure. However, current research on PbBi2Te4 primarily focuses on theoretical calculations and predictions, with limited investigations conducted on its actual thermoelectric transport properties. In this study, we synthesized the single-phase PbBi2Te4 compound using the high-temperature melting method. The research findings indicate that PbBi2Te4 exhibits significant anisotropic thermoelectric properties and a pronounced bipolar diffusion effect, which can be attributed to its unique layered crystal structure and relatively narrow bandgap. This bipolar diffusion effect remarkably exacerbates the Seebeck coefficient of PbBi2Te4 and amplifies its thermal conductivity. To address these challenges, we effectively manipulated the electronic band structure of PbBi2Te4 and enlarged the bandgap through the incorporation of Se alloying. The thermoelectric performance of PbBi2Te4 has been significantly enhanced as a result of the increase in Seebeck coefficient and reduction in thermal conductivity. Ultimately, a superior ZT value of ∼0.52 at 673 K is achieved in PbBi2(Te0.85Se0.15)4, representing an impressive improvement of ∼ 49% compared to the pure PbBi2Te4. These results demonstrate the promising potential of ternary PbBi2Te4 as a thermoelectric material.

Achieving high quality factor and enhanced thermoelectric performance in polycrystalline SnS by Ag doping and Se alloying

The polycrystalline SnS with a similar layered crystal structure and band structure to SnSe exhibits enormous commercial thermoelectric potential due to its lower cost and environmentally friendly characteristics. However, the wider bandgap of SnS leads to low carrier concentration and inferior electrical transport performance. The two-dimensional interlayer hinders carrier transport, leading to interesting and mysterious anisotropic thermoelectric properties. Herein, we reported the optimized electron–phonon transport in anisotropic polycrystalline SnS by Ag doping and Se alloying, realizing a high quality factor B by multiple strategies of optimizing carrier concentration, modifying band structure, and introducing various defects; further potential performance is predicted by the single parabolic band model. Specifically, Ag-doped SnS not only significantly increases the carrier concentration and weighted mobility μw in both directions but also induces multi-scale precipitates proven by the Debye–Callaway model to suppress phonon transport. Moreover, additional Se alloying optimizes the electronic band structure and increases the Seebeck coefficient, further improving μW and boosting the maximum power factor to ∼3.72 μW cm−1 K−2 at 873 K in the out-of-plane direction. Consequently, the synergistic optimization of carrier and phonon transport achieved a high B of 0.7 and a maximum zTmax of ∼0.8 at 873 K in Ag0.02Sn0.98S0.99Se0.01. Additionally, the high B predicted a high zTmax∼1.5 based on optimized carrier transport characteristics, demonstrating the potential great-performance polycrystalline SnS. This work provides a promising avenue for optimizing the zT of polycrystalline SnS by transport engineering.

Effects of heavy bromine doping on the thermoelectric performance and dynamic stability of SnSe2 polycrystals

SnSe2 is a material with a layered crystal structure, which is abundant in the Earth's crust and is non-toxic. It exhibits anisotropic thermoelectric properties due to the anisotropy between its interlayer and intralayer electrical and thermal transport properties. However, its thermoelectric performance is intrinsically poor due to its lower electrical transport properties. In our study, we successfully synthesized n-type Bromine-doped SnSe2 polycrystals using a combined method of mechanical alloying (MA) and spark plasma sintering (SPS). Bromine (Br) acts as an electron donor, and its heavy doping has been achieved by substituting it with selenium (Se) atoms. This substitution results in a significant increase in carrier concentrations, thereby improving the thermoelectric performance of SnSe2. This Br doping has led to a notable improvement in the power factor, which increased to approximately 755 µWm−1K−2 at 573 K. The most significant anisotropy was observed in thermal conductivity, and Br doping led to an ultralow thermal conductivity of approximately 0.6 Wm−1K−1 at 748 K. Furthermore, density functional theory (DFT) was used to investigate the electronic structure and phonon dispersions. The results of our DFT calculations showed a shrinking bandgap and increased degeneracy, which supports the observed improvement in the thermoelectric performance of Bromine-doped SnSe2 polycrystals. Moreover, the absence of imaginary modes in the phonon dispersion confirmed the dynamical stability of the system after heavy Br doping. Finally, a maximum figure of merit (ZT) of 0.59 was obtained at 748 K for the SnSe1.95Br0.05 sample parallel to the SPS pressing direction, which is nearly sixfold higher than the value of pure SnSe2.

Anisotropic thermoelectric figure of merit in [formula omitted] monolayer

Herein we have studied the anisotropic thermoelectric properties of MoTe2 monolayer using density functional and semiclassical Boltzmann transport theory. The electrical and thermal properties are evaluated by considering electron–phonon and phonon–phonon scattering mechanisms. Enhancement of figure of merit (ZT) is observed along the armchair direction for both n-type and p-type MoTe2 monolayer. Strong anisotropic ZT is found for p-type carriers due to anisotropic nature of electrical conductivity. The calculated thermoelectric figure of merit (ZT) is 1.33 for n-type and 0.83 for p-type monolayer at 700 K along the armchair orientation. ZT of MoTe2 monolayer is ∼ 30 times larger than the ZT value for bulk MoTe2 (0.034 at 700 K). This high anisotropic ZT will be useful to design 2D material based transverse TE devices.

Bridgman Growth of Mg2Sn1–xBixSingle Crystals and Anisotropic Thermoelectric Properties

Mg2X-based (X = Si, Ge, Sn) phases are promising for thermoelectric applications due to their nontoxicity, low cost, and high performance. Substantial experimental work has already been carried out based on the polycrystalline samples; however, studies on related single-crystal growth and corresponding properties are still very limited owing to the high volatility and reactivity of Mg, which makes it very challenging to obtain high-quality single crystals with big sizes. In this report, for the first time, the Bridgman method was successfully applied in the single-crystal growth of the Bi-doped Mg2Sn materials. The crystals are of high quality and related anisotropic thermoelectric properties were systematically investigated. The Bi-doped Mg2Sn0.99Bi0.01 single crystal exhibited a maximum zT value of 0.58 along the [111] orientation at 623 K, which is about 30 times that of the pristine Mg2Sn single crystal.

Intrinsically Low Lattice Thermal Conductivity and Anisotropic Thermoelectric Performance in In‐doped GeSb2Te4 Single Crystals

AbstractLayer‐structured GeSb2Te4 is a promising thermoelectric candidate, while its anisotropy of thermal and electrical transport properties is still not clear. In this study, Ge1–xInxSb2Te4 single crystals are grown by Bridgman method, and their anisotropic thermoelectric properties are systematically investigated. Lower electrical conductivity and higher Seebeck coefficient are observed in the c‐axis due to the higher effective mass in this direction. Intrinsically low lattice thermal conductivity is also observed in the c‐axis due to the weak chemical bonding and the strong lattice anharmonicity proved by density functional theory calculation. Indium doping introduces an impurity band in the bandgap of GeSb2Te4 and leads to the locally distorted density of states near the Fermi level, which contributes to enhanced Seebeck coefficient and improved power factor. Ultimately, a peak zT value of 1 at 673 K and an average zT value of 0.68 within 323–773 K are obtained in Ge0.93In0.07Sb2Te4 along the c‐axis direction, which are 54% and 79% higher than that of the pristine GeSb2Te4 single crystal, respectively. This study clarified the origin of intrinsic low lattice thermal conductivity and anisotropy transport properties in GeSb2Te4, and shed light on the performance optimization of other layered thermoelectric materials.

Anisotropic thermoelectric properties in hydrogenated nitrogen-doped porous graphene nanosheets.

By using density functional theory calculations combined with the nonequilibrium Green's function method and machine learning, we systematically studied the thermoelectric properties of four kinds of porous graphene nanosheets (PGNS) before and after nitrogen doping. The results show that the thermoelectric performance of porous graphene nanosheets along the armchair or zigzag chiral direction is improved due to the dramatically enhanced power factor caused by nitrogen doping. The calculated ZT values of nitrogen-doped porous graphene nanosheets are boosted by about one order of magnitude compared with those of undoped porous graphene nanosheets at room temperature. More importantly, an anisotropic thermoelectric transport is found in the nitrogen-doped porous graphene nanosheets. The results show that the ZT values of nitrogen-doped porous graphene nanosheets along the zigzag transport direction are nearly 11 times larger than those of them along the armchair transport direction. These results reveal that the thermoelectric properties of porous graphene nanosheets can be well regulated by nitrogen doping, and provide a good theoretical guidance for their application in thermoelectric devices.

Quasi-One-Dimensional van der Waals Transition Metal Trichalcogenides.

The transition metal trichalcogenides (TMTCs) are quasi-one-dimensional (1D) MX3-type van der Waals layered semiconductors, where M is a transition metal element of groups IV and V, and X indicates chalcogen element. Due to the unique quasi-1D crystalline structures, they possess several novel electrical properties such as variable bandgaps, charge density waves, and superconductivity, and highly anisotropic optical, thermoelectric, and magnetic properties. The study of TMTCs plays an essential role in the 1D quantum materials field, enabling new opportunities in the material research dimension. Currently, tremendous progress in both materials and solid-state devices has been made, demonstrating promising applications in the realization of nanoelectronic devices. This review provides a comprehensive overview to survey the state of the art in materials, devices, and applications based on TMTCs. Firstly, the symbolic structure, current primary synthesis methods, and physical properties of TMTCs have been discussed. Secondly, examples of TMTC applications in various fields are presented, such as photodetectors, energy storage devices, catalysts, and sensors. Finally, we give an overview of the opportunities and future perspectives for the research of TMTCs, as well as the challenges in both basic research and practical applications.

A ferromagnetic composite of PEDOT:PSS and nitrogen-graphene decorated with copper oxide nanoparticles with high anisotropic thermoelectric properties

PP/CuONG system has been designed for high TE properties. CuONG plays a dual role in increasing S by the dipole and interfacial polarizations and the spin Seebeck coefficient and lowering k by phonons scattering and gaseous (N-/static air) environs.

Comparing investigation of the anisotropic thermoelectric properties of textured KSr2Nb5O15 ceramics

High ZT at a certain direction is highly desirable for tungsten bronze structure material which has anisotropic lattice structure. The anisotropic thermoelectric properties of KSr2Nb5O15 were studied using first-principles calculations. KSr2Nb5O15 has a high ZT of 1.379 in the ∥c direction, which is 16.65 times larger than that in the ⊥c direction. In addition, KSr2Nb5O15 textured ceramics were prepared by reactive templated grain growth, and their anisotropic thermoelectric properties were studied. The sample along the ∥c direction has a power factor of 0.236 mW/(mK2), which exhibits much larger thermoelectric properties than that along the ⊥c direction. Both the calculated results and experimental results have good consistency and verify the obvious anisotropic thermal properties of KSr2Nb5O15. Furthermore, the mechanism of the anisotropy of the thermoelectric properties of KSr2Nb5O15 is simply clarified. This research provides a basis for improving the thermoelectric performance of KSr2Nb5O15 ceramics.

Design of N-Type Textured Bi2 Te3 with Robust Mechanical Properties for Thermoelectric Micro-Refrigeration Application.

Thermoelectric refrigeration is one of the mature techniques used for cooling applications, with the great advantage of miniaturization over traditional compression refrigeration. Due to the anisotropic thermoelectric properties of n-type bismuth telluride (Bi2 Te3 ) alloys, these two common methods, including the liquid phase hot deformation (LPHD) and traditional hot forging (HF) methods, are of considerable importance for texture engineering to enhance performance. However, their effects on thermoelectric and mechanical properties are still controversial and not clear yet. Moreover, there has been little documentation of mechanical properties related to micro-refrigeration applications. In this work, the above-mentioned methods are separately employed to control the macroscopic grain orientation for bulk n-type Bi2 Te3 samples. The HF method enabled the stabilization of both composition and carrier concentration, therefore yielding a higher quality factor to compare with that of LPHD samples, supported by DFT calculations. In addition to superior thermoelectric performance, the HF sample also exhibited robust mechanical properties due to the presence of nano-scale distortion and dense dislocation, which is the prerequisite for realizing ultra-precision machining. This work helps to pave the way for the utilization of n-type Bi2 Te3 for commercial micro-refrigeration applications.

Anisotropic Optical, Mechanical, and Thermoelectric Properties of Two-Dimensional Fullerene Networks

Nanoclusters like fullerenes as the unit to build intriguing two-dimensional (2D) topological structures is of great challenge. Here we propose three bridged fullerene monolayers and comprehensively investigate the novel fullerene monolayer (α-C60-2D) as synthesized experimentally [Hou et al. Nature 2022, 606, 507-510] by state-of-the-art first-principles calculations. Our results show that α-C60-2D has a direct band gap of 1.55 eV close to the experimental value, an optical linear dichroism with strong absorption in the long-wave ultraviolet region, a small anisotropic Young's modulus, a large hole mobility, and an ultrahigh Seebeck coefficient at middle-low temperatures. It is unveiled that the anisotropic optical, mechanical, electrical, and thermoelectric properties of α-C60-2D originate from the asymmetric bridging arrangements between C60 clusters. Our study promises potential applications of monolayer fullerene networks in lots of fields.

Germanium Monosulfide as a Natural Platform for Highly Anisotropic THz Polaritons.

Terahertz (THz) electromagnetic radiation is key to access collective excitations such as magnons (spins), plasmons (electrons), or phonons (atomic vibrations), thus bridging topics between optics and solid-state physics. Confinement of THz light to the nanometer length scale is desirable for local probing of such excitations in low-dimensional systems, thereby circumventing the large footprint and inherently low spectral power density of far-field THz radiation. For that purpose, phonon polaritons (PhPs) in anisotropic van der Waals (vdW) materials have recently emerged as a promising platform for THz nanooptics. Hence, there is a demand for the exploration of materials that feature not only THz PhPs at different spectral regimes but also host anisotropic (directional) electrical, thermoelectric, and vibronic properties. To that end, we introduce here the semiconducting vdW-material alpha-germanium(II) sulfide (GeS) as an intriguing candidate. By employing THz nanospectroscopy supported by theoretical analysis, we provide a thorough characterization of the different in-plane hyperbolic and elliptical PhP modes in GeS. We find not only PhPs with long lifetimes (τ > 2 ps) and excellent THz light confinement (λ0/λ > 45) but also an intrinsic, phonon-induced anomalous dispersion as well as signatures of naturally occurring, substrate-mediated PhP canalization within a single GeS slab.

Study on the Thermoelectric Properties of n-Type Polycrystalline SnSe by CeCl3 Doping

SnSe is a promising medium-temperature thermoelectric material due to its extremely low thermal conductivity and tunable electronic transport properties. In this work, n-type SnSe0.95 polycrystals doped with CeCl3 are synthesized by a melting method combined with a spark plasma sintering (SPS) process. The samples have highly textured structures and anisotropic thermoelectric properties. CeCl3 doping has a favorable effect on the electronic transport above 473 K, which may benefit from the excitation of the introduced impurity energy level. Moreover, the thermal conductivity is significantly reduced due to the phonon scattering by point defects. Finally, a peak ZT value of 1.17 is obtained at 773 K in SnSe0.95–0.5 mol % CeCl3, which paves the way for improving the thermoelectric performance of n-type SnSe via rare-earth halide doping.

A study of anisotropic thermoelectric properties of bulk Germanium Sulfide in its Pnma phase: a combined first-principles and machine-learning approach

This work reports a detailed and systematic theoretical study of the anisotropic thermoelectric properties of bulk Germanium Sulfide (GeS) in its orthorhombic Pnma phase. Density functional theory (DFT), employing the generalized gradient approximation (GGA), has been used to examine the structural and electronic band structure properties of bulk GeS. Electronic transport properties have been studied by solving semiclassical Boltzmann transport equations. A machine-learning approach has been used to estimate the temperature-dependent lattice part of thermal conductivity. The study reveals that GeS has a direct band gap of 1.20 eV. Lattice thermal conductivity is lowest along crystallographic a-direction, with a minimum of ∼0.98 Wm −1 K −1 at 700 K. We have obtained the maximum figure of merit (ZT) ∼ 0.73 at 700 K and the efficiency ∼7.86% in a working temperature range of 300 K–700 K for pristine GeS along crystallographic a-direction.

Preparation and thermoelectric transport properties of BiSe and Sb-doped BiSe single crystals

<p indent="0mm">Exploring high thermoelectric performance in single crystal materials with intrinsic low lattice thermal conductivity has become an important field for thermoelectric research. BiSe, as a member of (Bi₂Se₃)_{<italic>m</italic>}(Bi₂)_{<italic>n</italic>} series superlattice compounds, has a very low intrinsic thermal conductivity due to the existence of Bi₂ bilayer in its layered crystal structure. Moreover, it is a new and promising thermoelectric material due to the non-toxic and environmentally friendly chemical compositions. The special layered structure of BiSe makes its thermoelectric transport properties significantly anisotropic. In this paper, BiSe single crystal was prepared by Bridgman method, and its anisotropic thermoelectric transport properties were investigated. The results show that the sample has higher conductivity and thermal conductivity in the in-plane direction, while the lower Seebeck coefficient makes the thermoelectric figure of merit <italic>ZT</italic> value in this direction lower than that in the interlamellar direction. The maximum values of <italic>ZT</italic> in the two directions are 0.11 <sc>(573 K)</sc> and 0.16 <sc>(623 K),</sc> respectively. Compared with polycrystalline BiSe, the single crystal sample exhibits lower electric transport performance, which can be attributed to the difference of scattering coefficient. The carrier concentration of BiSe single crystal is regulated by Sb doping, which effectively improves the Seebeck coefficient of the samples and optimizes the electrical transport performance. In the meanwhile, Sb doping can produce lattice distortion and reduce the lattice thermal conductivity. The final <italic>ZT</italic> value of Bi_0.8Sb_0.2Se single crystal increases by about 1 fold, reaching 0.34 <sc>(573 K)</sc> in the interlamellar direction and 0.20 <sc>(573 K)</sc> in the in-plane direction.

Grain orientation evolution and multi-scale interfaces enhanced thermoelectric properties of textured Sr0.9La0.1TiO3 based ceramics

Sr0.9La0.1TiO3 based textured ceramics (SLTT-S3T) with a texture fraction of 0.81 are successfully fabricated by the reactive template grain growth method, in which Sr0.9La0.1TiO3/20 wt%Ti was used as matrix and 10 wt% plate-like Sr3Ti2O7 template seeds were used as templates. The phase transition, microstructure evolution, and the anisotropic thermoelectric properties of SLTT-S3T ceramics were investigated. The results show that the ceramics are mainly composed of Sr0.9La0.1TiO3 and rutile TiO2 phases. Grains grow with a preferred orientation along (h00). A maximum ZT of 0.26 at 1073 K was achieved in the direction perpendicular to the tape casting direction. The low lattice thermal conductivity of 1.9 W/(m K) at 1073 K was obtained decreased by 34%, 40%, and 38% compared with non-textured, SrTiO3 and Sr0.9La0.1TiO3 ceramics prepared by the same process, can be attributed to the enhanced phonon scattering by the complex multi-scale boundaries and interfaces. This work provides a strategy of microstructural design for thermoelectric oxides to decrease intrinsic lattice thermal conductivity and further regulate thermoelectric properties via texture engineering.

Janus penta-PdSeTe: A two-dimensional candidate with high thermoelectric performance

Inspired by the successful preparation of Janus transition metal dichalcogenides MoSSe with hexagonal structure, we investigated the electronic and thermoelectric properties of a new Janus Pd-based material with pentagonal crystal structure, Janus PdSeTe, by first-principles calculations together with Boltzmann transport theory. This work reports that the Janus PdSeTe is a direct band gap semiconductor, and exhibits anisotropic electronic and thermoelectric properties. The weak electron-phonon coupling of hole leads to excellent electronic transport properties for the p-doping. A high power factor of 104 mWm−1K−2 at 300 K is obtained for the p-type PdSeTe along y-direction. The enhanced electronic transport properties and low thermal conductivity lead to a high ZT value and the ZT maxima of p-type PdSeTe reaches to 3.1 at 700 K. The results suggest that the Janus materials with pentagonal crystal structure are the potential promising thermoelectric candidates.