Low Lattice Thermal Conductivity Research Articles

Transport characteristics and lattice dynamics with phonon topology accentuation in layered CuTlX (X: S, Se)

In recent years, numerous Cu-based compounds have attracted a great deal of interest for enhanced thermoelectric energy conversion. Here, we demonstrate that CuTlX (X: S, Se), a layered semiconductor, exhibits low lattice thermal conductivity (κ l ) and a high thermoelectric figure of merit (ZT), using density functional theory calculations and Boltzmann transport theory beyond relaxation time approximation. To evaluate the absolute values of thermoelectric coefficients, different scattering mechanisms such as acoustic deformation potential scattering, impurity phonon scattering, and polar optical phonon scattering are analysed. This low lattice thermal conductivity, which is complemented by a low group velocity and a low phonon lifetime, accounts for the remarkable thermoelectric efficiency in these compounds. In CuTlS, the contribution of the in-plane optical phonon mode to κ l results in a decrease in its value, which might be attributed to the occurrence of Dirac-like crossings with non-trivial topological characteristics, as corroborated by the non-zero Berry curvature value. Overall, the thermoelectric behavior of both compounds is favorable at ambient temperature. Specifically, the out-of-plane direction in CuTlSe presents elevated thermoelectric performance with a high value for the thermoelectric figure of merit, with 1.08 and 1.16 for holes and electrons, respectively, at 300 K at the optimal carrier density of 1019 cm−3 , which well aids in both the electron and phonon transport. We also undertook monolayer examinations of these compounds due to the existence of van der Waals interactions, which predicted strong thermoelectric performance for both carrier concentrations at 300 K. As a result, our study presents a theoretical prediction on transport phenomena that requires experimental verification and should motivate additional research into prospective thermoelectric materials in the same crystal family for device applications.

The role of phonon anharmonicity on the structural stability and phonon heat transport of CrFeCoNiCux high-entropy alloys at finite temperatures

Chemical disorder and lattice distortion make high-entropy alloys (HEAs) promising to become a new generation of thermoelectric materials. However, the large parameter space of HEAs makes it still a great challenge to study their anharmonic unfolded phonon properties, and the role of anharmonicity in their structural stability and phonon heat transport is still unclear. Here we provide a new scheme to achieve a breakthrough in obtaining the unfolded anharmonic phonon properties from the dynamic trajectories. Through careful analysis of the unfolded phonon dispersions, we found that CrFeCoNiCu HEA has a special anharmonic phonon scattering distribution, which makes the contribution of short-wavelength and long-wavelength phonons to lattice thermal conductivity significantly different. In addition, we have developed a model for estimating the formation energy of HEAs based on the formation energy of binary alloy from first-principles calculation. Our results also show that anharmonic phonon scattering plays an important role in the stability of CrFeCoNiCux HEAs solid-solution phase, which also complements the dynamic mechanism of the formation and stability of dual-FCC phase. CrFeCoNiCu HEA has very low lattice thermal conductivity even at extremely low temperatures. The contribution of long-wavelength phonons to lattice thermal conductivity is more than ten times that of short-wavelength phonons, and they are the main contributors of phonon heat transport of CrFeCoNiCu HEA. This study makes it possible to study the unfolded phonon properties of HEAs at finite temperature and provides important theoretical support and information for the stability of solid-solution structure and phonon engineering for HEAs.

Rapid fabrication of Se-modified skutterudites obtained via self-propagating high-temperature synthesis and pulse plasma sintering route

Selenium is an effective dopant in skutterudite-based thermoelectric materials. It strongly influences thermal transport properties due to effective phonon scattering. This study proposes a short-term fabrication route to Se-modified CoSb3-based materials. Alloy synthesis was conducted via self-propagating high-temperature synthesis. Subsequently, pulse plasma sintering consolidated all materials. As a result, thermoelectric materials with high electrical properties homogeneity were obtained. Seebeck potential mapping showed the measured deviation of the Seebeck coefficient for all fabricated samples was between 5 and 7%. A very low thermal conductivity (1.59 W m−1 K−1, at 573 K) was achieved for the highest doped sample, and one of the lowest reported results obtained for bulk skutterudite-based thermoelectric materials ever. This resulted in a low lattice thermal conductivity (1.51 W m−1 K−1, at 573 K). This led to the highest ZT (0.27 at 623 K) for the highest doped sample.

Theoretical analysis of the thermoelectric properties of penta-PdX2 (X = Se, Te) monolayer.

Based on the successful fabrication of PdSe2 monolayers, the electronic and thermoelectric properties of pentagonal PdX2 (X = Se, Te) monolayers were investigated via first-principles calculations and the Boltzmann transport theory. The results showed that the PdX2 monolayer exhibits an indirect bandgap at the Perdew-Burke-Ernzerhof level, as well as electronic and thermoelectric anisotropy in the transmission directions. In the PdTe2 monolayer, P-doping owing to weak electron-phonon coupling is the main reason for the excellent electronic properties of the material. The low phonon velocity and short phonon lifetime decreased the thermal conductivity (κ l) of penta-PdTe2. In particular, the thermal conductivity of PdTe2 along the x and y transmission directions was 0.41 and 0.83Wm-1K-1, respectively. Owing to the anisotropy of κ l and electronic structures along the transmission direction of PdX2, an anisotropic thermoelectric quality factor ZT appeared in PdX2. The excellent electronic properties and low lattice thermal conductivity (κ l) achieved a high ZT of the penta-PdTe2 monolayer, whereas the maximum ZT of the p- and n-type PdTe2 reached 6.6 and 4.4, respectively. Thus, the results indicate PdTe2 as a promising thermoelectric candidate.

Enhanced thermoelectric properties of p-type Bi1−xCaxCuSeO achieved via microwave synthesis combined with spark plasma sintering

In this work, the nearly single-phase Bi1−xCaxCuSeO bulks are fabricated via 4 min microwave synthesis combined with 5 min spark plasma sintering (SPS). The phase composition, microstructure, electrical and thermal transport performance of the samples are studied, and the effect of Ca doping contents on thermoelectric performance is discussed in the temperature range of 298–873 K. The conductivity and power factor increase with increasing Ca concentration on account of synchronously increase of carrier concentration and mobility. The two-dimensional layered superlattice structure allows the sample to maintain relatively low lattice thermal conductivity, and the total thermal conductivity of all samples is less than 1.0 W/mK. As a result, a maximum ZT value of 0.85 in Bi0.93Ca0.07CuSeO is achieved at 873 K, and which is ∼70 % higher than that of the undoped BiCuSeO. This indicates that the synthesis method is not only simple and efficient, but also has improved performance compared with traditional synthesis methods.

Interlayer Coupling Induced Phonon-Glass–Electron-Crystal Behavior in van der Waals Heterostructure PtSe2/γ-GeSe

We report the electronic band structure and interlayer coupling induced phonon-glass–electron-crystal behavior in the van der Waals heterostructure PtSe2/γ-GeSe. The heterostructure is dynamically stable and possesses an indirect band gap of 0.63 eV (at the Heyd–Scuseria–Ernzerhof functional level) with type II band alignment. A low conduction band offset of 0.20 eV compared to the valence band offset of 0.92 eV suggests fluent electrons drive from γ-GeSe to PtSe2. Interlayer coupling induced strong phonon coupling and a unique “avoided crossing” feature between longitudinal acoustic and low-lying optical phonon modes between K and Γ points. Significant suppression of acoustic phonon modes and giant phonon scattering rates (a maximum value of 73.76 ps–1) results in a low lattice thermal conductivity of 1.20 W/(m·K) at 300 K after enforcing the mandatory rotational invariance condition. The calculated lattice thermal conductivity is 14-fold smaller than that of monolayer PtSe2 (16.97 W/(m·K)). The lattice thermal conductivity upsurges by 24% as the out-of-plane acoustic phonon dispersion undergoes linear dispersion, indicating the importance of pure quadratic flexural phonon dispersion. Our results disclose a strategy for having the phonon-glass–electron-crystal character in van der Waals heterostructures.

Defect engineering synergistically modulates power factor and thermal conductivity of CuGaTe2 for ultra-high thermoelectric performance

• The vacancy of Cu atom can increase the band gap and reduce the sound velocity. • Cu vacancies increase the density of the samples, and significantly increase the carrier concentration. • Cu vacancies generate many defects, which scatter phonons at multiple scales and realize thermoelectric decoupling. • The highest ZT value of Cu 0.96 GaTe 2 reaches 1.23 at 823 K, and the average ZT value reaches 0.575. The ternary chalcopyrite CuGaTe 2 has emerged as a promising p -type thermoelectric material with its advantages of low cost, good stability, and non-toxic elements. However, its thermoelectric performance is limited by the intrinsic low electrical conductivity and high lattice thermal conductivity. In this work, A deficiency of Cu in Cu 1– x GaTe 2 semiconductors can be used to optimize the electrical properties by improving the carrier concentration and to reduce thermal conductivity through multi-scale phonon scattering, which is predicted and guided by the First-principles density functional theory calculations. The carrier concentration is increased to 10 20 , which compensates for the low electrical performance caused by the intrinsic low n H of CuGaTe 2 . The average power factor of Cu 0.96 GaTe 2 reaches 106.3% higher than that of the original CuGaTe 2 . In addition, the lattice thermal conductivity of the defective samples is greatly reduced at high temperatures, which is mainly due to the reduction of sound speed and phonon scattering. All the above factors contribute to the highest dimensionless figure of merit (ZT) value of 1.23 at 823 K in Cu 0.96 GaTe 2 , which is 114% higher than the pristine CuGaTe 2 , and the average ZT is 171.4% higher.

Enhancing the thermoelectric performance of β-Zn4Sb3 via progressive incorporation of Zn interstitials

Owing to the intrinsically low lattice thermal conductivity, β-Zn4Sb3 is regarded as a promising candidate for thermoelectric applications. To further improve its thermoelectric performance, it is essential to increase the carrier mobility of β-Zn4Sb3 for enhanced electrical transport. While incorporating light ions is a useful approach for this purpose, the excess Zn ions tend to segregate in as-grown samples. Here, we present a design strategy to stabilize the excess Zn ions in the interstitial sites by applying electric current and uniaxial stress, which triggers a progressive directional migration of Zn ions towards one terminal of the sample. In this Zn-enriched end, a homogeneous distribution of excess Zn interstitials was achieved, as confirmed by atom probe tomography and transmission electron microscopy at the atomic scale. As revealed by ab initio simulations, the additional electrons brought by Zn interstitials connect the separated (Sb2)4- dimers, enhancing the carrier mobility and thus the electrical conductivity. In consequence, the detached Zn-enriched section shows a much improved thermoelectric figure of merit, a very high maximum ZT of ∼1.5 at 618 K, and an average ZT of ∼1.1 between 300 and 618 K, outperforming the majority of thermoelectric materials in this temperature range.

Achieving Low Lattice Thermal Conductivity in Half‐Heusler Compound LiCdSb via Zintl Chemistry

Half‐Heusler compounds usually possess ultrahigh power factors, while the large thermal conductivity hinders the further optimization of their thermoelectric properties. Herein, from the perspective of material design, a new half‐Heusler lattice with low lattice thermal conductivity by using Zintl chemistry based on the composition of LiCdSb is rationally constructed. The weak bonding within the polyanions combined with the resonance vibration modes of Li+ contributes to the small lattice thermal conductivity of pristine LiCdSb as low as 3.2 W m−1 K−1 at 303 K and 0.85 W m−1 K−1 at 573 K. Ag doping is further conducted for boosting the electronic quality factor BE from 2.5 to 5.2 μW cm−1 K−2 due to the energy band modulation. As a result, a high power factor up to 21.35 μW cm−1 K−2 at 393 K is achieved in LiCd0.94Ag0.06Sb. In view of the low thermal conductivity, the figure of merit zT reaches 0.79 at 633 K. Herein, it is demonstrated that the half‐Heusler compound LiCdSb is a competitive thermoelectric parent, and low thermal conductivity can indeed be realized in half‐Heusler compounds through Zintl chemistry.

Enhanced Thermoelectric Performance in Black Phosphorene via Tunable Interlayer Twist.

Twist-angle two-dimensional (2D) systems are attractive in their exotic and tunable properties by the formation of the moiré superlattices, allowing easy access to manipulating intrinsic electrical and thermal properties. Here, the angle-dependent thermoelectric properties of twisted bilayer black phosphorene (tbBP) by first-principles calculations are reported. The simulations show that significantly enhanced Seebeck coefficient and power factor can be achieved in p-type tbBP due to merging of the multi-valley electronic states and flat moiré bands. Moreover, the twisted layers bring in a strong anharmonic phonon scattering and thus very low lattice thermal conductivity of 4.51Wm-1 K-1 at 300K. Consequently, a maximal ZT value can be achieved in p-type 10.11° tbBP along the armchair direction up to 0.57 and 1.06 at 300 and 500K, respectively. The room-temperature ZT value along the zigzag direction is also significantly increased by almost 40 times compared to pristine BP when the twist angle is close to 70.68°. This work demonstrates a platform to manipulate thermoelectric performance in 2D materials by creating moiré patterns, leading tbBP as a promising eco-friendly candidate for thermoelectric applications.

Carrier concentration mediated enhancement in thermoelectric performance of various polymorphs of hafnium oxide: a plausible material for high temperature thermoelectric energy harvesting application

The optimization of figure of merit by tuning carrier concentrations is an effective way to realize efficient thermoelectrics (TEs). Recently, the feasibility of high p-type carrier concentration (order of ∼1022cm−3) is experimentally demonstrated in various polymorphs of hafnium oxide (HfO2). In light of these studies, using the first-principles calculation combined with the semi-classical Boltzmann transport theory and phonon dynamics, we realized high TE performance in various polymorphs of HfO2 in a range of carrier concentrations at high temperatures. The phonon dispersion calculations confirm the dynamical stability of all polymorphs. The observed values of the Seebeck coefficient are 945.27 mV K−1, 922.62 mV K−1, 867.44 mV K−1, and 830.81 mV K−1 for tetragonal (t), orthorhombic (o), monoclinic (m), and cubic (c) phases of HfO2, respectively, at 300 K. These values remain positive at all studied temperatures which ensures the p-type behaviour of HfO2 polymorphs. The highest value of electrical conductivity (2.34 × 1020 Ω−1m−1s−1) observed in c-HfO2 at 1200 K, and the lowest value of electronic thermal conductivity (0.37 × 1015 W mK s−1) observed in o-HfO2 at 300 K. The lattice thermal conductivities at room temperature are 5.56 W mK−1, 2.87 W mK−1, 4.32 W mK−1, and 1.75 W mK−1 for c-, m-, o- and t- HfO2, respectively which decrease to 1.58 W mK−1, 0.92 W mK−1, 1.12 W mK−1, 0.53 W mK−1 at 1200 K for respective phases. The low lattice thermal conductivities lead to the high values of the figure of merit, i.e. 0.97, 0.87, 0.83, and 0.77 at 1200 K for the m-, o-, t-, and c- HfO2, respectively, at the optimized carrier concentrations (∼1021 cm−3). The predicted optimized carrier concentrations for various phases are in close agreement with the experimental reports. The estimated high figure of merit can make HfO2 a potential material for TE energy harvesting applications at elevated temperatures.

First principle studies on electronic and thermoelectric properties of Fe2TiSn based multinary Heusler alloys

The alloys with 8/18/24 valence electron count (VEC) are promising candidates for efficient energy conversion and refrigeration applications at low as well as high temperatures. Recently Fe-based Heusler alloys attracted researchers due to their compelling electronic band structure i.e flat band along one direction of the Brillouin zone and highly dispersive bands along the other directions. Here we focus on the thermoelectric (TE) transport properties of isovalent/aliovalent substituted Fe2TiSn systems those may be potential TE materials. The multinary substitution has been done in such a way that it preserves the 24 VEC and hence the semiconducting nature. The calculated total energies with VASP-PAW potential within density functional theory with PBE-GGA functional were used to determine the ground state properties such as equilibrium lattice parameters, bulk modulus etc. We have also investigated the structural, electronic, lattice dynamic and TE transport properties by using PBE-GGA and TB-mBJ exchange–correlation functional. The full potential linearized augmented plane wave method as implemented in WIEN2k code was used to investigate electronic structure and TE transport properties with the PBE-GGA and TB-mBJ exchange potentials and Boltzmann transport theory. The calculated single crystal elastic constants, phonon dispersion and phonon density of states confirm that these systems are mechanically and dynamically stable. The TE transport properties is calculated by including the lattice part of thermal conductivity (κL) obtained from two methods one from the calculated elastic properties calculation (κLelastic) and the other from phonon dispersion curve (κLphonon). The strong phonon–phonon scattering by large mass difference/strain fluctuation of isovalent/aliovalent substitution at Ti/Sn sites of Fe2TiSn reduces the lattice thermal conductivity which results in high thermoelectric figure-of-merit (ZT) value of 0.81 at 900K for Fe2Sc0.25Ti0.5Ta0.25Al0.5Bi0.5. The comparative analysis of TE transport properties using the band structures calculated with the PBE-GGA and TB-mBJ functional shows that the ZT value obtained from TB-mBJ scheme is found to be significantly higher than that based on PBE-GGA. The calculated relatively low lattice thermal conductivity and high ZT values suggest that isovalent/aliovalent substituted Fe2TiSn are promising candidates for medium to high temperature waste heat recovery.

Synergistically optimized electron and phonon transport of p-type BiCuSeO oxyselenides via Pb dopant and Te composite

Layer-structured BiCuSeO oxyselenides with intrinsic low lattice thermal conductivity, non-toxicity, non-pollution and outstanding thermal stability is regarded as a promising thermoelectric material, but its application is limited by the poor electrical conductivity. In this paper, Pb-doped BiCuSeO (Bi1-xPbxCuSeO) was fabricated via a fast preparation method combined self-propagating high-temperature synthesis (SHS) with spark plasma sintering (SPS). Although Pb doping could greatly improve the electrical conductivity of BiCuSeO, the thermal conductivity increased as well, and it limits the thermoelectric performance. To further optimize the electron and phonon transport properties, Te powders and Te nanowires were introduced in Bi0·9Pb0·1CuSeO by ball milling, respectively. As a result, the thermal conductivity largely decreased with a maintained electrical performance. Ultimately, a maximum ZT value of 1.0 at 873 K was achieved for the 5 vol% Te nanowire + Bi0·9Pb0·1CuSeO sample, which is about 2.5 times as large as that for pristine BiCuSeO. Our work paves an alternative strategy for tuning the thermoelectric performance of BiCuSeO through Bi site doping combined low-dimension nanocomposites.

Fast preparation of AgxPb20SbTe20 (x = 0.5–3) component with high thermoelectric performance

PbTe-based quaternary AgPbmSbTem+2 (LAST) has been a kind of promising thermoelectric materials due to its relatively low lattice thermal conductivity and good electrical performance. However, traditional melting or mechanical alloying methods are time-consuming, which is not suitable for applications. To solve this problem, a fast method combined self-propagating high-temperature synthesis (SHS) with spark plasma sintering (SPS) has been adopted in this work. Nanostructured n-type AgxPb20SbTe20 (x = 0.5–3) materials with high thermoelectric performance have been successfully prepared. On one hand, power factors of all samples reached over 1.6 mW·m−1·K−2 at high temperature, because Ag assembles grain boundaries and forms Ag2Te precipitates in the SHS process, enhancing the thermoelectric properties by modulation doping, energy filtering and dynamic doping. On the other hand, the Ag2Te-PbTe coherent interfaces bring multiple phonon scattering resources to reduce the thermal conductivity. As a result, a high ZT value of ∼1.32 at 773 K is achieved in Ag1.25Pb20SbTe20. This work provides a facile, rapid and energy-saving way to prepare PbTe-based materials with good thermoelectric performance.

Two-Dimensional β-PdX2 (X=S, Se, and Te) Monolayers with Promising Potential for Thermoelectric Applications

Transition metal dichalcogenides have attracted extensive research attention due to their unique electronic, topological, and optical properties. Based on first-principles methods and Boltzmann transport theory, the thermoelectric performance of β-PdX2 (X = S, Se, and Te) monolayers was comprehensively studied in this work. It is worth noting that the structural anisotropy of β-PdX2 monolayers leads to obvious anisotropy of thermoelectric coefficients along the x and y directions. By investigating the thermal transport up to four-phonon scattering, it was found that the strong higher-order phonon scattering plays a decisive role in ultra-low lattice thermal conductivity of β-PdX2 monolayers. A low lattice thermal conductivity of 3.4 (1.0) W m–1 K–1 is predicted for β-PdTe2 at 300 K along the x (y) direction. In addition, due to the suitable band gap and high carrier mobility, the highest power factor of 25.7 mW m–1 K–2 is obtained in n-type β-PdX2 monolayers, which is superior to many other two-dimensional thermoelectric materials. Finally, a maximum ZT of 5.2 is achieved for the n-type β-PdS2 monolayer along the y direction at 600 K. Meanwhile, the maximum p-type ZT also reaches 3.3 along the y direction at 600 K in the β-PdSe2 monolayer. Our results demonstrate that β-PdX2 monolayers are promising candidates for two-dimensional thermoelectric materials with excellent conversion performance.

Phase Stability, Strong Four-Phonon Scattering, and Low Lattice Thermal Conductivity in Superatom-Based Superionic Conductor Na3OBH4.

Superatom-based superionic conductors are of current interest due to their promising applications in solid-state electrolytes for rechargeable batteries. However, much less attention has been paid to their thermal properties, which are vital for safety and performance. Motivated by the recent synthesis of superatom-based superionic conductor Na3OBH4 consisting of superhalogen cluster BH4, we systematically investigate its lattice dynamics and thermal conductivity using the density functional theory combined with a self-consistent phonon approach. We reveal the bonding hierarchy features by studying the electron localization function and potential energy surface and further unveil the rattling effect of the BH4 superatom, which introduces strong quartic anharmonicity and induces soft phonon modes in low temperatures by assisting Na displacements, thus calling for the necessity of quartic renormalization and four-phonon scattering in calculating the lattice thermal conductivity. We find that the contribution of four-phonon processes to the lattice thermal conductivity increases from 13 to 32% when the temperature rises from 200 to 400 K. At room temperature (300 K), the phonon scattering phase space is enlarged by 133% due to the four-phonon interactions, and the lattice thermal conductivity is evaluated to be 5.34 W/mK, reduced by 24% as compared with a value of 6.99 W/mK involving three-phonon scattering only. These findings provide a better understanding of the lattice stability and thermal transport properties of superionic conductor Na3OBH4, shedding light on the role of strong quartic anharmonicity played in superatom-based materials.

Ab initio study of two-dimensional MgAl2Se4 and MgIn2Se4 with high stability, high electron mobility, and high thermoelectric figure of merit

Various two-dimensional (2D) materials possess low thermal conductivity owing to the predominant quantum effects, making them promising candidates for high-performance thermoelectric devices. Herein, we proposed two 2D materials, MgAl2Se4 and MgIn2Se4, exhibiting low cleavage energies of 0.20 J·m−2 and 0.22 J·m−2, respectively. They are both direct-gap semiconductors with bandgaps of 1.93 eV and 1.37 eV, respectively. Further, they have a high Seebeck coefficient (0.57–0.87 mV·K−1) and electrical conductivity (106–107 Ω−1·m−1). Moreover, owing to strong phonon–phonon anharmonic scattering, the monolayers exhibit low intrinsic lattice thermal conductivities of 1.39 W/m·K and 0.54 W/m·K. Consequently, the ZT values of MgAl2Se4 and MgIn2Se4 reach 0.36 and 0.74 at 300 K and rise even further to 2.58 and 3.06 at 700 K, respectively, indicating potential applications in nanoelectronics and thermoelectric devices.

First-Principles Study of the Phonon Lifetime and Low Lattice Thermal Conductivity of Monolayer γ-GeSe: A Comparative Study

Germanium selenide (GeSe) is a unique two-dimensional (2D) material showing various polymorphs stable at ambient condition. Recently, a new phase with a layered hexagonal lattice ({\gamma}-GeSe) was synthesized with ambient stability and extraordinary electronic conductivity even higher than graphite while its monolayer is semiconducting. In this work, via using first-principles derived force constants and Boltzmann transport theory we explore the lattice thermal conductivity ({\kappa}_l) of the monolayer {\gamma}-GeSe, together with a comparison with monolayer {\alpha}-GeSe and {\beta}-GeSe. The {\kappa}_l of {\gamma}-phase is relatively low (5.50 W/mK), comparable with those of {\alpha}- and {\beta}- phases. The acoustic branches in {\alpha}-GeSe are well separated from the optical branches, limiting scattering channels in the phase space, while for \b{eta}-GeSe and {\gamma}-GeSe the acoustic branches are resonant with the low-frequency optical branches facilitating more phonon-phonon scattering. For {\gamma}-GeSe, the cumulative {\kappa}_l is isotropic and phononic representative mean free path (rMFP) is the shortest (17.07 nm) amongst the three polymorphs, indicating that the {\kappa}_l of the {\gamma} phase is less likely to be affected by the size of the sample, while for {\alpha}-GeSe the cumulative {\kappa}_l grows slowly with mean free path and the rMFP is longer (up to 20.56 and 35.94 nm along zigzag and armchair direction, respectively), showing a stronger size-dependence of {\kappa}_l. Our work suggests that GeSe polymorphs with overall low thermal conductivity are promising contenders for thermoelectric and thermal management applications.

Discovery of YbNiSb-Based Half-Heusler Alloys as Promising Thermoelectric Materials

Half-Heusler materials are promising candidates for high-temperature power generation and have relatively high lattice thermal conductivity compared to other thermoelectric material systems. In this work, we report novel p-type YbNiSb-based half-Heusler alloys with a low lattice thermal conductivity (∼3.6 W m–1 K–1 at 340 K) that resulted from their large Grüneisen parameter, low sound speed, and low Debye temperature. All YbNiSb-based alloys exhibit a high carrier mobility of 30–50 cm2 V–1 s–1 at room temperature because of their relatively small effective mass. Importantly, the structural analysis reveals that Yb-rich Yb1.3Ni0.9Sb0.8 exhibits Yb/Ni and Yb/Sb substitution, indicating a wide homogeneity region of the YbNiSb phase experimentally. The adjustable Yb and Ni contents in YbNiSb-based alloys can modify the band structure around the Fermi level and significantly affect electrical transport properties. Additionally, by doping Ta at Yb sites, the carrier concentration and lattice thermal conductivity of these alloys can be manipulated. Consequently, a peak zT value of 0.45 at 823 K was achieved for Yb0.95Ta0.05NiSb. Our work demonstrates that YbNiSb-based alloys are promising p-type thermoelectric materials and suggests the possibility of exploring novel thermoelectric alloys in rare-earth nickel pnictides via tuning their composition and crystal structure.

Magnetically Enhanced Thermoelectric Performance of Ti0.75NiSb+x mol % Fe (x = 0-5) Nanocomposites.

Ti0.75NiSb is a half-Heusler compound with low lattice thermal conductivity due to a large number of cation vacancies. However, the higher carrier concentration limits the improvement of its thermoelectric performance. In this paper, magnetic Fe nanoparticles with a size of 30 nm are composited into Ti0.75NiSb in the form of the second phase. The charge transfer between Fe nanoparticles and Ti0.75NiSb leads to a decrease in carrier concentration. The strong interaction between the magnetic moment and carriers enhances the electron scattering, so that the scattering factor increases and the mobility decreases. The combined effect results in an increase of about 10% in the Seebeck coefficient and a decrease by about 14% in the electronic thermal conductivity at 873 K for the composite Ti0.75NiSb+2 mol % Fe. Meanwhile, the magnetic Fe nanoparticles provide additional scattering centers, leading to a decrease in lattice thermal conductivity. As a result, a zT value of 0.4 at 873 K is achieved for the composite Ti0.75NiSb+2 mol % Fe, which is 21% higher than that of Ti0.75NiSb. This work demonstrates that the compositing magnetic nanoparticles Fe can enhance the thermoelectric performance of Ti0.75NiSb.