Promising Thermoelectric Performance Research Articles

Large Transverse Thermoelectric Power Factor in Topological Semimetal NbAs2

AbstractThe distinctive properties of topological semimetals, including linear band dispersion and compensated electron‐hole carriers, have positioned these materials at the forefront of research in power generation and solid‐state cooling due to their remarkable magneto‐thermoelectric performance. In this work, the transverse thermoelectric characteristics of the topological semimetal NbAs2 are studied. Specifically, under a magnetic field of 9 T, the Nernst coefficient displays a linear and unsaturated trend, reaching a peak of 600 µV K−1 at 35 K. Consequently, this engenders a substantial transverse power factor (tPF) of 850 µW cm−1 K−2 under a 5 T magnetic field. The exceptional attributes of NbAs2 driven by its remarkably high carrier mobility and compensated electron‐hole concentration near the Fermi level are revealed by band structure analysis based on theoretical calculations and quantum oscillations. This work not only underscores the immense potential of topological semimetals as transverse thermoelectric materials for niche applications where magnetic fields exist but also provides valuable guidance for the discovery and optimization of topological materials for promising thermoelectric performance.

Machine Learning-Based Predictions of Power Factor for Half-Heusler Phases

A support vector regression model for predictions of the thermoelectric power factor of half-Heusler phases was implemented based on elemental features of ions. The training subset was composed of 53 hH phases with 18 valence electrons. The target values were calculated within the density functional theory and Boltzmann equation. The best predictors out of over 2000 combinations regarded for the p-type power factor at room temperature are: electronegativity, the first ionization energy, and the valence electron count of constituent ions. The final results of support vector regression for 70 hH phases are compared with data available in the literature, revealing good ability to determine favorable thermoelectric materials, i.e., VRhGe, TaRhGe, VRuSb, NbRuAs, NbRuBi, LuNiAs, LuNiBi, TaFeBi, YNiAs, YNiBi, TaRuSb and NbFeSb. The results and discussion presented in this work should encourage further fusion of ab initio investigations and machine learning support, in which the elemental features of ions may be a sufficient input for reasonable predictions of intermetallics with promising thermoelectric performance.

Origin of low lattice thermal conductivity in promising ternary PbmBi2S3+m (m = 1–10) thermoelectric materials

Ternary Pb-Bi-S compounds emerge as potential thermoelectric materials owing to low thermal conductivity, but the origin of their intrinsic low lattice thermal conductivities lacks further investigation. Herein, a series of ternary PbmBi2S3+m (m = 1–10) compounds are synthesized and their crystal structure evolutions with increasing m values are clearly unclosed. The room-temperature lattice thermal conductivities in PbBi2S4, Pb3Bi2S6 and Pb6Bi2S9 can reach at 0.57, 0.56 and 0.80 W m−1 K−1, respectively, outperforming other ternary sulfur-based compounds. Theoretical calculations show that the low lattice thermal conductivities in PbmBi2S3+m (m = 1–10) mainly originate from soft phonon dispersion caused by strong lattice anharmonicity, and both asymmetric chemical bond and lone pair electrons (Pb 6s2 and Bi 6s2) can favorably block phonon propagation. Furthermore, the elastic measurements also confirm relatively low sound velocities and shear modulus, and the Grüneisen parameter (γ) calculated by sound velocities can reach at 1.67, 1.85 and 1.94 in PbBi2S4, Pb3Bi2S6 and Pb6Bi2S9, respectively. Finally, the intrinsic low lattice thermal conductivities in PbmBi2S3+m (m = 1–10) contribute to promising thermoelectric performance, and the maximum ZT values of 0.47, 0.38 and 0.45 can be achieved in undoped PbBi2S4, Pb3Bi2S6 and Pb6Bi2S9, respectively.

Achieving high carrier mobility and thermoelectric performance in nearly twin-free rhombohedral GeTe (00l) films

GeTe-based thermoelectric (TE) films have garnered significant attentions due to their promising TE performance near room temperature. However, it is challenging to further optimizing the TE performance due to the inferior carrier mobility (μ) and the excessively high hole density (p). Herein, we developed a novel method based on molecular beam epitaxy technique to successfully fabricate nearly twin-free GeTe (00l) films via incorporating Bi2Te3 buffer layers to alleviate epitaxial strain. Consequently, μ was significantly enhanced. Additionally, through comprehensively investigating the processing conditions, we found that substrate temperature and Te/GeTe flux ratio can shape intrinsic atomic defects and further decrease p. With the optimal synthesis and processing conditions, the GeTe film achieves optimized p of 3.44 × 1020 cm−3 and a high μ of 73.31 cm2/V/s, which lead to the highest room-temperature power factor of 2.67 mWm/K2, outperforming the values of other GeTe films. This work provides important guidance on fabricating twin-free GeTe films and on further improving their TE performance.

Stoichiometric manipulation to enhance the thermoelectric and mechanical performance of Ag2Se1+x

Thermoelectric silver selenide alloys have attracted significant attention owing to their high carrier mobility and promising thermoelectric performance. In this study, the phase diagram window is utilized to synthesize Ag2Se1+x samples within the range of −0.02 ≤ x ≤ 0.03. It is found that an excess of Se is helpful to fabricate pure-phase Ag2Se samples at room temperature. Combined with the optimized carrier concentration and reduced thermal conductivity, Ag2Se1.03 sample exhibits an average zT ∼ 0.72 in the temperature range of 310–380 K, as well as a 25 % improvement of the Vickers hardness compared to that of the stoichiometric Ag2Se sample.

Prediction of novel ground-state structures and analysis of phonon transport in two-dimensional GexSy compounds.

We conducted this study to explore the ground-state structures of two-dimensional (2D) variable-composition GexSy compounds, driven by the polymorphic characteristics of bulk germanium sulfides and the promising thermoelectric performance of 2D GeS (Pmn21). To accomplish this, we utilized the highly successful evolutionary-algorithm-based code USPEX in conjunction with VASP for total energy calculations, leading to the discovery of three previously unexplored structures of Ge2S (P2/c), GeS (P3̄m1), and GeS2 (P21/c). These 2D materials exhibit significantly lower formation energies compared to their reported counterparts. We thoroughly scrutinized the structural stability and subsequently analyzed their electronic structures. Our analysis reveals a nearly direct band gap of 0.12/0.84 eV with the PBE/HSE06 functional for 2D Ge2S and an indirect band gap for 2D GeS and GeS2. Their semiconducting nature highlights the crucial importance of lattice thermal conductivity (κl), which we determined by solving the Boltzmann transport equation for phonons. Importantly, we predict a room temperature κl value of 6.82 W m-1 K-1 for GeS, lower than its 2D orthorhombic counterpart. In the case of GeS2, we observed an anisotropic κl value of 16.95/10.68 W m-1 K-1 along the zigzag/armchair directions at 300 K, with an in-plane anisotropy ratio of 1.59, surpassing that of 2D IV-VI compounds. We delve into detailed discussions regarding the role of lattice anharmonicity, group velocities, phonon lifetimes, and three-phonon weighted phase space in the overall thermal conductivity analysis.

Promising thermoelectric performance in p-type AgBiSe2 doping with alkaline-earth metals

P-type AgBiSe2 is a promising thermoelectric material because of its intrinsic low lattice thermal conductivity and multiple valence band behavior. However, realizing high performance in p-type AgBiSe2 is challenging due to the doping bottleneck. In this work, the enhancement of thermoelectric performance for p-type AgBiSe2 has been successfully realized by adding an excess amount of Se and doping with alkaline-earth metals. We show that Ca or Mg doping at the Bi site effectively increases the room-temperature hole concentration and improves the electrical transport properties of p-type AgBiSe2. Combined with the intrinsic lower thermal conductivity, the maximum zT values of ∼0.74 and ∼0.63 at 448 K were achieved in AgBi0.98Ca0.02Se2 and AgBi0.98Mg0.02Se2, respectively. This work demonstrates a crucial strategy for improving the thermoelectric properties of p-type AgBiSe2-based materials.

Progressive carrier density optimization contributes to enhanced thermoelectric performance in Bi(Te, Se) compound

Layered BiTe and BiSe compounds present promising n-type thermoelectric performance, but the intrinsic high carrier density limits ZT value near room temperature. In this work, thermoelectric performance in Bi(Te, Se) compound is significantly improved by progressively optimizing its carrier density. The room-temperature carrier density is reduced from ∼5.5 × 1020 cm−3 in Bi0.99Te to ∼2.21 × 1020 cm−3 in Bi0.92Sn0.07Te0.4Se0.6-2%Cu, which can lead to obvious enhancement of Seebeck coefficient and power factor, and thus the room-temperature power factor increases from ∼4.17 μW cm−1 K−2 in Bi0.99Te to ∼9.78 μW cm−1 K−2 in Bi0.92Sn0.07Te0.4Se0.6-2%Cu. Moreover, the largely suppressed total thermal conductivity in Bi0.92Sn0.07Te0.4Se0.6-2%Cu also benefit from lowered carrier density because of reduced electronic thermal conductivity, as well as intensified phonon scattering caused by imported point defects. Finally, a room-temperature ZT of ∼0.29 and a peak value of ∼0.41 at 373 K in Bi0.92Sn0.07Te0.4Se0.6-2%Cu are achieved successfully, which are ∼6 and ∼3 times higher than that in Bi0.99Te, respectively.

Enhanced thermoelectric performance of n-type PbTe through introducing PbSe by a fast preparation method

N-type PbTe-based materials exhibit promising thermoelectric performance around 600–900 K, while the near room-temperature thermoelectric performance still needs to be improved due to their high thermal conductivity. Forming solid solutions is an effective strategy to strengthen phonon scatterings and results in lower lattice thermal conductivity. However, the existing preparation methods used in PbTe-based solid solutions are time-consuming and energy-intensive, which limits their further applications. Herein, we report a fast method combined self-propagating high-temperature synthesis (SHS) with spark plasma sintering (SPS) to successfully fabricate PbTe–PbSe solid solutions. Due to the mass and strain fluctuations, a largely reduced room temperature lattice thermal conductivity of 1.27 W m−1 K−1 was achieved in (Pb0.995Bi0.005Te)0.8-(PbSe)0.2, which was 40% lower than that of pristine Pb0.995Bi0.005Te. Besides, carrier mobility increased after PbSe alloying, leading to comparable electrical performance. Consequently, a maximum average thermoelectric figure of merit ZTave reached 0.57 at 300–623 K of (Pb0.995Bi0.005Te)0.8-(PbSe)0.2 and was 21% and 24% higher than those of pristine Pb0.995Bi0.005Te and PbSe, respectively. Such a simple and effective method is expected to be further adopted in other thermoelectric solid solutions.

Enhanced the thermoelectric power factor of n-type Bi2Te3 thin film via energy filtering effect

In recent years, Bi2Te3 thin films have gained interest in the field of thermoelectric power generation due to their promising thermoelectric performance. However, the poor performance of Bi2Te3 has limited its widespread applicability. To address this, we deposited Bi2Te3 films on soda lime glass substrates via thermal evaporation and annealed them at different temperature ranges between 100 and 350 °C. Our X-ray diffraction analysis showed that the best quality film was produced at a post-annealing temperature of 350 °C, while the as-deposited sample yielded a low-quality film. We then analyzed the surface morphology of the prepared sample using scanning electron microscopy and found that the grains were highly intact with a uniformly aligned granular surface morphology and no porosity. By controlling the density and grain size, we were able to improve the Seebeck coefficient and electrical conductivity. Specifically, the Seebeck coefficient value increased up to 135 μV/°C due to the energy filtering effect at small grain boundaries, while the electrical conductivity improved by raising the charge carrier mobility and reducing grain size, thus increasing the conductivity of the tiny domain. Consequently, the Bi2Te3 thin film with a small grain size exhibited the highest thermoelectric power factor of 15.30 μW cm−1 C−2 at room temperature. Our findings suggest that the thermal evaporation technique is an effective method for growing Bi2Te3 thin films with improved thermoelectric properties.

Electronic and Thermoelectric Transport Properties of CaAgP from First Principles

Recent experimental and theoretical progress on semiconducting CaAgP prompted us to investigate in detail the electrical and thermal transport properties of this hexagonal pnictide, using first-principles calculations based on the density functional theory. In contrast to using a standard generalized gradient approximation, employing a hybrid Heyd–Scuseria–Ernzerhof functional yields its semiconducting nature, in agreement with the experimental observation with a bandgap of ∼0.15 eV. The narrow band gap semiconductor CaAgP, which under negative (chemical) pressure has been shown to turn into a nodal-line semimetal, is found to be dynamically stable, with a gapped electronic band structure when a hybrid functional is used. This is in an attractive range for applications in thermoelectric devices, and we have determined the lattice and electronic conductivity as a function of doping, which indeed predicts a promising thermoelectric performance, particularly for p-doped CaAgP.

Realizing n-type CdSb with promising thermoelectric performance

Realizing n-type CdSb with promising thermoelectric performance

Crystal structure modulation of SnSe thermoelectric material by AgBiSe2 solid solution

Although low thermal conductivity and high band degeneracy bring promising thermoelectric performance to cubic SnSe, its preparation strategies remain elusive. Here, a series of Sn1–2x(AgBi)xSe samples are synthesized using the vacuum melting (1173 K) and spark plasma sintering (723 K) methods. Owing to the increased configurational entropy caused by AgBiSe2 solid solution, the cubic structure is obtained when x exceeds 0.2. The optimized carrier concentration significantly enhances electrical conductivity (σ), and maximal σ of 350 Scm−1 is obtained in Sn0.5(AgBi)0.25Se. Combined with the low lattice thermal conductivity caused by the small sound velocity and strong anharmonicity, the figure of merit of 0.08 is reached in Sn0.6(AgBi)0.2Se at 500 K. The mechanical performances are also improved, and a high Vicker hardness of 1.5 GPa is obtained in Sn0.4(AgBi)0.3Se. This work demonstrates the importance of the configurational entropy in phase regulation and provides insights into the design of new thermoelectrics.

Thermoelectric properties of layered oxyselenides with 3d transition metal ions

AbstractLayered oxychalcogenides have received extensive attention in the fields of magnetism, superconductivity, lithium battery, and luminescence due to their unique electronic, magnetic properties, and layered structure, among which layered oxyselenides have excellent and promising thermoelectric performance, such as BiCuSeO and Bi2LnO4Cu2Se2. Here, we successfully synthesized Sr2MO2Cu2Se2 (M = Co, Ni, Zn) and Sr2FeO3CuSe and investigated the thermoelectric properties at a wide temperature range (298–923 K). They have a relatively high Seebeck coefficient (>300 μV K−1) in medium to high temperature range and possess a low thermal conductivity. The power factor and ZT reach 65 μW m−1 K−1 and 0.07 at 923 K for intrinsic Sr2NiO2Cu2Se2, and a higher performance is expected to be achieved by strategies like carrier concentration optimization and band structure engineering.

Synergistic modulation of electrical and thermal transport toward promising n-type MgOCuSbSe2 thermoelectric performance by MO-intercalated CuSbSe2.

The layered ternary CuSbSe2 semiconductor with ultralow thermal conductivity is particularly suitable for thermoelectric applications. Nevertheless, its poor electrical conductivity greatly lowers the dimensionless figure of merit ZT and accordingly limits its thermoelectric applications. Here, we use first-principles calculations combined with semi-classical Boltzmann transport theory to evaluate the thermoelectric properties of MO-intercalated (M = Mg, Ca, Sr, and Ba) CuSbSe2. Compared with CuSbSe2, MO-intercalated CuSbSe2 semiconductors, as a new class of semiconductors, host distorted lattices with low symmetry monoclinic structures. Such a structure feature provides desired channels for electron transport between adjacent layers and accordingly enhances electrical transport properties. Meanwhile, the MO intercalation effectively softens phonons and gives rise to an ultralow lattice thermal conductivity in MOCuSbSe2. These synergistically yield a high figure of merit ZT of ∼4.17 for MgO-intercalated CuSbSe2 at 200 K with electron doping being n = 1018 cm-3. Our study provides an effective route to improve the thermoelectric performance of layered CuSbSe2 by designing new multicomponent thermoelectric compounds with alternatively stacked [CuSbSe2] (electronic conduction units) and [MO] (electronic insulation units) layers. The approach can be extended to similar chalcostibite compounds for screening and designing thermoelectric materials.

Evidence of pressure-induced multiple electronic topological transitions in BiSe

BiSe, an exciting member of homologous family (Bi2)m (Bi2Se3)n [where, m = 1; n = 2], has recently gained attention in thermoelectric research due to its intrinsically ultralow lattice thermal conductivity and promising n-type thermoelectric performance near room temperature. It is experimentally observed to be a weak topological insulator and possesses a fascinating crystal structure where Bi2 bilayer is sandwiched between two Bi2Se3 quintuple layers, thus the structure resembles natural van der Waals heterostructure (Bi2Se3–Bi2–Bi2Se3). Herein, we have studied the pressure induced electronic topological transition (ETT) in BiSe through high pressure (0–10 GPa) synchrotron X-ray diffraction, Raman spectroscopy and first-principles density functional theory (DFT). We have observed clear anomalies at ∼1 and ∼2.2 GPa in the pressure dependent lattice parameters (a, c, and c/a ratio), cell volume, Raman mode shift and line width data suggesting the presence of two ETTs which is further verified by electronic structures and Fermi surface calculations under pressure. The origin of these ETTs is associated with the two different vibrational modes arising from Bi2 bilayer and Bi2Se3 quintuple layers of BiSe, respectively.

High Thermoelectric Performance of p-Type Bi0.4Sb1.6Te3+x Synthesized by Plasma-Assisted Ball Milling.

The exploration of new synthesis methods is important for the improvement of the thermoelectric property of a material for the different mechanisms of microstructure fabrication, surface activity modulation, and particle refinement. Herein, we prepared p-Bi2Te3 bulk materials by a simple synthesis method of the plasma-assisted ball milling, which yielded finer nanopowders, higher texture of in-plane direction, and higher efficiency compared to the traditional ball milling, favoring the simultaneous improvement of electrical and thermal properties. When combined with the Te liquid sintering, nano-/microscale hierarchical pores were fabricated and the carrier mobility was also increased, which together resulted in the low lattice thermal conductivity of 0.52 W·m-1·K-1 and the high power factor of 43.4 μW·cm-1·K-2 at 300 K, as well as the ranking ahead zT of 1.4@375 K. Thus, this work demonstrated the advantages of plasma-assisted ball milling in highly efficient synthesis of p-type Bi2Te3 with promising thermoelectric performance, which can also be utilized to prepare other thermoelectric materials.

Manipulation of phase structure and Se vacancy to enhance the average thermoelectric performance of AgBiSe2

The high-temperature cubic phase of AgBiSe2 has been revealed to enable promising thermoelectric performance because of the high symmetry. In this work, it is shown that the cubic phase could be stabilized at room temperature by alloying SnSe. Although the effective mass increased, the decreased carrier concentration and mobility as well as the aggravated bipolar effect deteriorated the thermoelectric properties of the pure cubic phase. In contrast, the room-temperature rhombohedral AgBiSe2 obtained a peak zT value of ∼0.66 at 723 K, which is attributed to the decreased thermal conductivity and increased power factor. Furthermore, reducing Se content not only increased the carrier concentration, but also increased the mobility due to the reduced energy barrier, and hence substantially improved the power factor over the whole temperature range. As a result, the peak zT and average zT were further enhanced in Ag0.94Bi0.94Sn0.06Se1.92. This work provides a novel strategy to optimize the thermoelectric performance of AgBiSe2 material.

Theoretical investigations of Janus WSeTe monolayer and related van der Waals heterostructures with promising thermoelectric performance

Recently, two-dimensional (2D) Janus MXY monolayers have received much attention due to their novel properties induced by the antisymmetric structures. In this work, the thermoelectric transport properties of the WSe2 and the Janus WSeTe monolayers are comparatively investigated based on first-principles calculations. The carriers of holes with higher mobility result in the better p-type power factor of WSeTe than WSe2. In addition, the broken of the mirror-asymmetry enhances the coupling strength between acoustic and optical branches and thus reduces the lattice thermal conductivity in WSeTe. The obtained ZT value of 1.53 is 5.9 times larger than that of WSe2 (0.26). Meanwhile, the thermoelectric performance of heterostructure MoSSe/WSeTe can be greatly enhanced by the biaxial compressive strain resulting from the band convergence. At the strain of −3%, the optimal ZT value of 1.62 is achieved for the p-type doped MoSSe/WSeTe at 300 K, which is 2.05 times higher than that of the unstrained structure. These results reveal that Janus monolayer WSeTe and its van der Waals heterostructure MoSSe/WSeTe are promising thermoelectric materials.

Promising thermoelectric performance in CaAgP with intrinsic Ag vacancies

Semimetals have recently emerged at the forefront of contemporary thermoelectrics research. In this paper, we show that the candidate Dirac semimetal CaAgP forms with a stable CaAg0.9P composition. Intentionally, Ag-deficient CaAg0.9P shows a promising thermoelectric figure of merit, achieving zT = 0.43 at 660 K. This derives from moderate power factors, S2/ρ = 1.25 mW m−1 K−2, and a low lattice thermal conductivity, κlat = 1.2 W m−1 K−1. The thermoelectric properties of CaAg0.9P are consistent with that of a small bandgap semiconductor. High levels of doping are key to suppressing bipolar transport, enabling promising zT values, despite the low ∼0.2 eV bandgap. Unusually, the onset of bipolar transport coincides with a transition to a more electrically resistive state, indicating a fundamental change in electronic properties at high temperature.