Anisotropic Thermoelectric Properties Research Articles

Synthesis of N-type Bi2Te2.7Se0.3 Compounds through Oxide-Reduction Process and Related Thermoelectric Transport Properties

We present a study on the synthesis of Bi2Te2.7Se0.3 powders through an oxide reduction process and also on the effect of the oxide reduction process on the thermoelectric transport properties of the resulting Bi2Te2.7Se0.3 compounds. Starting materials of Bi2O3, TeO2 and SeO2 were mechanical milled for homogeneous mixing, and then oxidized to prepare complex oxide powders. The complex oxide powders were reduced at different reduction temperatures under hydrogen atmosphere, and a single phase of Bi2Te2.7Se0.3 could be achieved in the reduced powder, with shorter reduction times as the reduction temperature was increased. The shape of the synthesized powder particles changed from granules to flakes with increasing reduction temperature and longer reduction time. Bi2Te2.7Se0.3 powders with different shapes were consolidated by spark plasma sintering (SPS). The SPSed Bi2Te2.7Se0.3 compounds exhibited anisotropic n-type thermoelectric transport properties along the vertical and parallel planes with respect to the pressing direction of the SPS. The degree of anisotropy in the thermoelectric properties was quite consistent with the degree of anisotropy in the microstructure, which originated from the anisotropic shapes in the Bi2Te2.7Se0.3 powders depending on the reduction conditions. Detailed thermoelectric transport properties of the n-type Bi2Te2.7Se0.3 compounds were interpreted in terms of anti-site defect formation and their structural anisotropy.

Effect of texturing on thermal, electric and elastic properties of MoAlB, Fe2AlB2, and Mn2AlB2

Herein, MoAlB, Fe2AlB2, and Mn2AlB2 with textured microstructures were sinter-forged from pre-reacted preforms in a hot press at 1400, 1200, and 1050 °C, respectively. X-ray diffraction and energy dispersive X-ray spectroscopy confirmed the textured nature and high phase purities. The effect of texturing on the thermoelectric properties was evaluated by measuring thermal diffusivities, electrical resistivities, and Seebeck coefficients on the sinter-forged samples and those of less textured, reactively hot-pressed samples. While the thermoelectric properties were essentially isotropic for sinter-forged Fe2AlB2 and Mn2AlB2, differences included electron-dominated thermal transport in Fe2AlB2 and phonon-dominated transport in Mn2AlB2 over most of the 323–873 K range. Magnetic phase transitions at 281 K in Fe2AlB2 and 308 K in Mn2AlB2 were identified in electrical resistivity measurements. MoAlB was found to have anisotropic thermoelectric properties, with notably small relative Seebeck coefficients ranging from − 2 μV/K at 323 K and increasing to 5 μV/K at 873 K.

Low thermal conductivity and high performance anisotropic thermoelectric properties of XSe (X = Cu, Ag, Au) monolayers.

Combining density functional theory (DFT) and semi-classic Boltzmann transport theory, we report the thermoelectric (TE) performance of a family of two-dimensional (2D) group IB-selenides XSe (X = Cu, Ag, Au). The results show that these monolayers exhibit small and anisotropic phonon velocities (0.98-3.84 km s-1), large Grüneisen parameters (up to 100), and drastic phonon scattering between the optical and acoustic phonons. These intrinsic properties originate from strong phonon anharmonicity and suppress the heat transport capacity, resulting in low lattice thermal conductivities (12.54 and 1.22 W m-1 K-1) along the x- and y-directions for a CuSe monolayer. Among our studied monolayers, the 2D CuSe monolayer possesses the most remarkable TE performance with ultrahigh ZT (3.26) for n-type doping along the y-direction at 300 K. CuSe monolayer can achieve higher thermoelectric conversion efficiency at a lower synthetic preparation cost than the expensive AgSe and AuSe monolayers, and our work provides a theoretical basis for paving the way for further experimental studies.

Controllable anisotropic thermoelectric properties in 2D covalent organic radical frameworks

By combining the density functional theory with the Boltzmann transport equation, thermoelectric properties of graphphenyl-based materials are investigated. The results show that anisotropic thermoelectric properties can be realized by breaking symmetry and twisting the phenyl ring. The ZT values reach 1.4 in both p- and n-type thermoelectric materials at room temperature. In addition, the thermoelectric properties of these materials can be further promoted by rotating the phenyl ring. These results demonstrate that these materials have excellent thermoelectric performance, two orders of magnitude greater than that of graphene, and have a wide range of suitable working temperatures. This work provides a way to optimize the thermoelectric performance of two-dimensional conjugated organic radical frameworks and provides theoretical support for the design of electrodes and thermoelectric components made of this organic material.

Anisotropic thermoelectric and superconducting properties of the bulk misfit-layered (SnSe)1.17(TaSe2) compound

We present the thermoelectric and superconducting properties of the bulk misfit-layered (SnSe)1.17(TaSe2) compound with the critical temperature (Tc ~ 3.8 K). From XRD and electrical resistivity, the anisotropic properties of the misfit-layered compound by the preferred orientation were observed. The zero-temperature-limit upper critical field Hc2(0) and coherence length ξ are obtained by Hc2(0) = 8.94 T and ξ = 6.1 nm. The electron-phonon coupling constant (λe-p = 0.735) and the specific heat jump (ΔCe/γTc ~ 1.4) imply that the (SnSe)1.17(TaSe2) compound is a weak coupled conventional s-wave superconductor. Density functional theory (DFT) calculation results show that some electronic charge transfers from the SnSe layers into the TaSe2 layers. In addition, the thermoelectric properties of the bulk misfit-layered (SnSe)1.17(TaSe2) compound show significant anisotropic properties.

Scattering Mechanisms and Suppression of Bipolar Diffusion Effect in Bi2Te2.85Se0.15Ix Compounds.

We investigated the anisotropic thermoelectric properties of the Bi2Te2.85Se0.15Ix (x = 0.0, 0.1, 0.3, 0.5 mol.%) compounds, synthesized by ball-milling and hot-press sintering. The electrical conductivities of the Bi2Te2.85Se0.15Ix were significantly improved by the increase of carrier concentration. The dominant electronic scattering mechanism was changed from the mixed (T ≤ 400 K) and ionization scattering (T ≥ 420 K) for pristine compound (x = 0.0) to the acoustic phonon scattering by the iodine doping. The Hall mobility was also enhanced with the increasing carrier concentration. The enhancement of Hall mobility was caused by the increase of the mean free path of the carrier from 10.8 to 17.7 nm by iodine doping, which was attributed to the reduction of point defects without the meaningful change of bandgap energy. From the electron diffraction patterns, a lattice distortion was observed in the iodine doped compounds. The modulation vector due to lattice distortion increased with increasing iodine concentration, indicating the shorter range lattice distortion in real space for the higher iodine concentration. The bipolar thermal conductivity was suppressed, and the effective masses were increased by iodine doping. It suggests that the iodine doping minimizes the ionization scattering giving rise to the suppression of the bipolar diffusion effect, due to the prohibition of the BiTe1 antisite defect, and induces the lattice distortion which decreases lattice thermal conductivity, resulting in the enhancement of thermoelectric performance.

Anisotropic thermoelectric transport properties in polycrystalline SnSe2 * *Project supported by the Beijing Natural Science Foundation, China (Grant No. JQ18004), the National Key Research and Development Program of China (Grant Nos. 2018YFA0702100 and 2018YFB0703600), the National Natural Science Foundation of China (Grant No. 51772012), Shenzhen Peacock Plan Team (Grant No. KQTD2016022619565991), and 111 Project (Grant No. B17002). This work was

It is reported that SnSe2 consisting of the same elements as SnSe, is a new promising thermoelectric material with advantageous layered structure. In this work, the thermoelectric performance of polycrystalline SnSe2 is improved through introducing SnSe phase and electron doping (Cl doped in Se sites). The anisotropic transport properties of SnSe2 are investigated. A great reduction of the thermal conductivity is achieved in SnSe2 through introducing SnSe phase, which mainly results from the strong SnSe2–SnSe inter-phase scattering. Then the carrier concentration is optimized via Cl doping, leading to a great enhancement of the electrical transport properties, thus an extraordinary power factor of ∼5.12 μW⋅cm−1⋅K−2 is achieved along the direction parallel to the spark plasma sintering (SPS) pressure direction (∥ P). Through the comprehensive consideration on the anisotropic thermoelectric transport properties, an enhanced figure of merit ZT is attained and reaches to ∼ 0.6 at 773 K in SnSe2-2% SnSe after 5% Cl doping along the ∥ P direction, which is much higher than ∼ 0.13 and ∼ 0.09 obtained in SnSe2-2% SnSe and pristine SnSe2 samples, respectively.

First-principles study of anisotropic thermoelectric properties of hexagonal KBaBi

Fast-track computational prediction of efficient thermoelectric materials is crucial for designing future thermoelectric generators. In the present study, an attempt has been made to estimate the thermoelectric figure of merit (ZT) of hexagonal KBaBi, a topological insulator by employing first-principles calculations based on Density Functional Theory (DFT) and Boltzmann transport theory. KBaBi, a member of ternary honeycomb compounds has been chosen due to its high band valley degeneracy and hence high density of states effective mass which may lead to possible high-power factor (S2σ). Therefore, to estimate the ZT, properties such as mechanical, phononic, electronic, and transport phenomena of KBaBi have been evaluated. It is observed that KBaBi exhibits low average sound velocity (va) and Debye temperature (θD) resulting from soft acoustic phonon modes. The study reveals larger Seebeck coefficients than the minimum requirement of 200 ​μV/K along a-axis (Sa) and c-axis (Sc) for n and p-type KBaBi respectively, over a broad range of temperature and carrier concentration. Moreover, the calculated relaxation time (τ) based on deformation potential theory suggests the higher p-type conductivity (σ) along c-axis. It also unveils reasonably low lattice thermal conductivities (klat) along a (2.09 Wm−1K−1) and c-axes (2.99 Wm−1K−1) at room temperature. Finally, we report a promising ZT of 1.04 for p-type KBaBi along c-axis despite having strong bipolar effect.

Investigating charge carrier scattering processes in anisotropic semiconductors through first-principles calculations: the case of p-type SnSe.

Efficient ab initio computational methods for the calculation of the thermoelectric transport properties of materials are of great interest for energy harvesting technologies. The constant relaxation time approximation (CRTA) has been largely used to efficiently calculate thermoelectric coefficients. However, CRTA usually does not hold for real materials. Here we go beyond the CRTA by incorporating realistic k-dependent relaxation time models of the temperature dependence of the main scattering processes, namely, screened polar and nonpolar scattering by optical phonons, scattering by acoustic phonons, and scattering by ionized impurities with screening. Our relaxation time models are based on a smooth Fourier interpolation of Kohn-Sham eigenvalues and its derivatives, taking into account non-parabolicity (beyond the parabolic or Kane models), degeneracy and multiplicity of the energy bands on the same footing, within very low computational cost. In order to test our methodology, we calculated the anisotropic thermoelectric transport properties of the low temperature phase (Pnma) of intrinsic p-type and hole-doped tin selenide (SnSe). Our results are in quantitative agreement with experimental data, regarding the evolution of the anisotropic thermoelectric coefficients with both temperature and chemical potential. Hence, from this picture, we also obtained the evolution and understanding of the main scattering processes of the overall thermoelectric transport in p-type SnSe.

In-Plane Optical and Electrical Anisotropy of 2D Black Arsenic.

Low-symmetry two-dimensional (2D) semiconductors have attracted great attention because of their rich in-plane anisotropic optical, electrical, and thermoelectric properties and potential applications in multifunctional nanoelectronic and optoelectronic devices. However, anisotropic 2D semiconductors with high performance are still very limited. Here, we report the systematic study of in-plane anisotropic properties in few-layered b-As that is a narrow-gap semiconductor, based on the experimental and theoretical investigations. According to experimental results, we have come up with a simple method for identifying the orientation of b-As crystals. Meanwhile, we show that the maximum mobility of electrons and holes was measured in the in-plane armchair (AC) direction. The measured maximum electron mobility ratio is about 2.68, and the hole mobility ratio is about 1.79.

Particle size dependent anisotropic thermoelectric properties of N-type Bi2(Te,Se)3 alloys on hot deformation

The anisotropy of thermoelectric properties and microstructures in Bi2Te2.7Se0.3 fabricated by repressing (hot deformation) is investigated. The initial particle and grain size decreased with high-energy ball milling time, which affected the degree of grain orientation during the hot deformation process. To achieve high-performance thermoelectric properties, the anisotropic TE properties are optimized with the control of the degree of grain orientation, carrier concentration and temperature-dependent transport properties. By optimizing synthesis processes, we achieved a highly anisotropic figure of merit (ZT), with peak ZT values of 0.7 and 1.05 at 423 K, in the out of plane and in-plane directions, respectively. The in-plane ZT of the hot deformation sample was enhanced by 24% compared with that of the as-pressed sample (ZT = 0.85) due to reduced electrical resistivity and thermal conductivity, which was attributed to the significant anisotropy in grain orientation.

Anisotropic thermoelectric transport properties of Bi0.5Sb1.5Te2.96+x zone melted ingots

In this study, p-type Bi0.5Sb1.5Te3 ingots were fabricated with a wide range of carrier concentrations through zone melting method. The anisotropy of thermoelectric properties was systematically investigated. The layered structure with weak van der Waals bonding, which selectively scatters charge carrier and phonon, produces the anisotropic thermoelectric properties. The intrinsic excitation generates electron-hole pairs with highly anisotropic transport behavior, resulting in a remarkable anisotropic Seebeck coefficient and bipolar thermal conductivity. The out-of-plane Seebeck coefficient is remarkably higher than that in-plane Seebeck coefficient in the intrinsic excitation regime. Doping with Te decreases the extrinsic carrier concentration of BiSbTe alloy, strengthening the intrinsic excitation and the contribution of the bipolar thermal conductivity. Optimization of carrier concentration via adjusting Te content significantly improves the thermoelectric performance. The maximum power factor of 5.2 ​mW ​m−1 ​K−2 is achieved for the Bi0.5Sb1.5Te3.08 sample at room temperature along the in-plane direction. A high ZT value of 1.06 ​at 375 ​K is attained for the Bi0.5Sb1.5Te3.04 sample along the in-plane direction, while the maximum out-of-plane ZT value is achieved as much as 0.94 ​at 450 ​K for Bi0.5Sb1.5Te3.02 sample.

In-plane anisotropic electronics based on low-symmetry 2D materials: progress and prospects.

Low-symmetry layered materials such as black phosphorus (BP) have been revived recently due to their high intrinsic mobility and in-plane anisotropic properties, which can be used in anisotropic electronic and optoelectronic devices. Since the anisotropic properties have a close relationship with their anisotropic structural characters, especially for materials with low-symmetry, exploring new low-symmetry layered materials and investigating their anisotropic properties have inspired numerous research efforts. In this paper, we review the recent experimental progresses on low-symmetry layered materials and their corresponding anisotropic electrical transport, magneto-transport, optoelectronic, thermoelectric, ferroelectric, and piezoelectric properties. The boom of new low-symmetry layered materials with high anisotropy could open up considerable possibilities for next-generation anisotropic multifunctional electronic devices.

Lattice distortion and anisotropic thermoelectric properties in hot-deformed CuI-doped Bi2Te2·7Se0.3

We investigated anisotropic thermoelectric properties of (CuI)xBi2Te2·7Se0.3 (x = 0.0, 0.3, 0.6, and 0.9 mol.%) compounds, synthesized by the hot-press and hot-deformation process. In spite of polycrystalline compound, the hot-deformed compounds exhibit preferred orientation along the c-axis, parallel with the applied press direction. The sample of x = 0.3 mol.% shows the maximum power factor (3.8 mW m−1 K−2 at 300 K) and ZT value (0.97 at 423 K), which is relatively high thermoelectric performance in n-type thermoelectric materials as a mild-temperature operation. Notably, the in-plane lattice thermal conductivity of the x = 0.3% compound with covalent bonding layer has lower value than the one of out-of-plane lattice thermal conductivity with van der Waals bonding layer. From the high resolution transmission electron microscopy and electron diffraction measurements, we observe the lattice distortion of the x = 0.3% compound. Therefore, the unconventional anisotropic lattice thermal conductivity can be associated with the lattice distortion along the in-plane on the compound driven by the CuI doping.

Achieving Out-of-Plane Thermoelectric Figure of Merit ZT = 1.44 in a p-Type Bi2Te3/Bi0.5Sb1.5Te3 Superlattice Film with Low Interfacial Resistance.

Recently, low-dimensional superlattice films have attracted significant attention because of their low dimensionality and anisotropic thermoelectric (TE) properties such as the Seebeck coefficient, electrical conductivity, and thermal conductivity. For these superlattice structures, both electrons and phonons show highly anisotropic behavior and exhibit much stronger interface scattering in the out-of-plane direction of the films compared to the in-plane direction. However, no detailed information is available in the literature for the out-of-plane TE properties of the superlattice-based films. In this report, we present the out-of-plane Seebeck coefficient, thermal conductivity, and electrical properties of p-type Bi2Te3/Bi0.5Sb1.5Te3 (bismuth telluride/bismuth antimony telluride, BT/BST) superlattice films in the temperature range of 77-500 K. Because of the synergistic combination of the energy filtering effect and low interfacial resistance of the superlattice structure, an impressively high ZT of 1.44 was achieved at 400 K for the 200 nm-thick p-type BT/BST superlattice film, corresponding to a 43% ZT enhancement compared to the pristine p-BST films with the same thickness.

Enhanced thermoelectric properties of natural chalcopyrite by vacuum annealing

This paper focuses on the thermoelectric properties of natural chalcopyrite and provides methods for optimizing its thermoelectric properties. Natural chalcopyrite shows obvious anisotropic thermoelectric properties. Although the thermoelectric properties of natural chalcopyrite are low, they are found to be significantly enhanced by vacuum annealing. Natural chalcopyrite was proven to be feasible and potential thermoelectric materials.

Anisotropic Thermoelectric Performance and Sustainable Thermal Stability in Textured Ca3Co4O9/Ag Nanocomposites

The Ca3Co4O9 (CCO) with layered structure has been considered as a potential candidate for high-temperature thermal energy harvesting application. However, CCO’s layered structure imparts anisotropy in transport properties, which results in anisotropic thermoelectric performance. So far, limited attempt has been made to understand the anisotropic thermoelectric performance of CCO, which often results in erroneous estimation of the thermoelectric response. Here, we fabricated highly textured CCO/x wt % Ag (x = 0, 1, 3, 5) nanoinclusion composites using the spark plasma sintering (SPS) technique and systematically investigated correlation between microstructure and anisotropic thermoelectric properties. The thermoelectric response was measured along both in-plane and out-of-plane directions (perpendicular and parallel to the pressure axis). We developed a two-step SPS method to achieve enhanced degree of texturing and increased electrical conductivity along ab-planes. The addition of Ag nanoinclusions was f...

Thermodynamic Stability, Thermoelectric, Elastic and Electronic Structure Properties of ScMN2-Type (M = V, Nb, Ta) Phases Studied by ab initio Calculations

ScMN2-type (M = V, Nb, Ta) phases are layered materials that have been experimentally reported for M = Ta and Nb, but they have up to now not been much studied. However, based on the properties of binary ScN and its alloys, it is reasonable to expect these phases to be of relevance in a range of applications, including thermoelectrics. Here, we have used first-principles calculations to study their thermodynamic stability, elastic, thermoelectric and electronic properties. We have used density functional theory to calculate lattice parameters, the mixing enthalpy of formation and electronic density of states as well as the thermoelectric properties and elastic constants (cij), bulk (B), shear (G) and Young’s (E) modulus, which were compared with available experimental data. Our results indicate that the considered systems are thermodynamically and elastically stable and that all are semiconductors with small band gaps. All three materials display anisotropic thermoelectric properties and indicate the possibility to tune these properties by doping. In particular, ScVN2, featuring the largest band gap exhibits a particularly large and strongly doping-sensitive Seebeck coefficient.

Highly anisotropic thermoelectric properties of carbon sulfide monolayers

Strain engineering applied to carbon monosulphide monolayers allows to control the bandgap, controlling electronic and thermoelectric responses. Herein, we study the semiconductor–metal phase transition of this layered material driven by strain control on the basis of first-principles calculations. We consider uniaxial and biaxial tensile strain and we find a highly anisotropic electronic and thermoelectonic responses depending on the direction of the applied strain. Our results indicate that strain-induced response could be an effective method to control the electronic response and the thermoelectric performance.

Anisotropic thermoelectric properties of Weyl semimetal NbX (X = P and As): a potential thermoelectric material.

Weyl semimetal, a newly developed thermoelectric material, has aroused much interest due to its extraordinary transport properties. In this work, the thermoelectric transport properties of NbX (X = P and As), a prototypical Weyl semimetal, are investigated using the first-principles calculations together with Boltzmann transport theory. The calculated room-temperature lattice thermal conductivities along the a and c directions are 2.0 W mK-1 and 0.6 W mK-1 for NbP and 1.4 W mK-1 and 0.4 W mK-1 for NbAs, respectively. The low thermal conductivities may be useful in the thermoelectric applications. It is found that the acoustic branches have obvious contribution to the total lattice thermal conductivity, and the size dependence of the thermal conductivities can provide guidance for designing thermoelectric nanostructures. Our results show that the anisotropic structures of these compounds bring about the anisotropy of transport coefficients along the a and c directions, and the preferred direction is the c direction in thermoelectric applications. Moreover, NbP and NbAs show high ZT values of 0.82 and 0.50 along the c direction for p-type at an optimal carrier concentration, indicating that they are potential thermoelectric materials.