High Temperature Thermoelectric Materials Research Articles

Improved thermoelectric properties of n-type ZrNiCu0.05Sn by doping Y2O3

Half-Heusler (HH)-based alloys are promising candidates for high-temperature thermoelectric (TE) energy conversion materials due to their excellent adaptability in high-temperature regions (900–1100 K). However, their TE performance is greatly limited owing to the inherently high lattice thermal conductivity (κl). In this study, we introduce a synergistic strategy to reduce thermal conductivity while modifying electrical transport properties through yttrium oxide (Y2O3) dispersion. The binary NiSn compound precipitations with sub-micron dimension together with Y2O3 nanoparticles synergistically intensify the phonon scattering and enhance the seebeck coefficient (S) effectively via energy filtering effect (EFE) at hetero-interfaces, leading to a remarkable reduction in κl and a slight improvement in the power factor (PF). Consequently, upon 1.5 wt% Y2O3 introduction, the sample incorporating Y2O3 exhibits a ∼13 % reduction in total thermal conductivity (κt) as compared to the HH matrix, reaching a peak ZT of 0.86 at 938 K, thereby successfully optimizing its TE performance. The presented strategy provides a feasible approach to simultaneously modulate the thermal and electrical transport behaviors to improve the TE performance of HH-based materials.

Few-layer Bi2O2Se: a promising candidate for high-performance near-room-temperature thermoelectric applications

Advancements in high-temperature thermoelectric (TE) materials have been substantial, yet identifying promising near-room-temperature candidates for efficient power generation from low-grade waste heat or TE cooling applications has become critical but proven exceedingly challenging. Bismuth oxyselenide (Bi2O2Se) emerges as an ideal candidate for near-room-temperature energy harvesting due to its low thermal conductivity, high carrier mobility and remarkable air-stability. In this study, the TE properties of few-layer Bi2O2Se over a wide temperature range (20-380 K) are investigated, where a charge transport mechanism transitioning from polar optical phonon to piezoelectric scattering at 140 K is observed. Moreover, the Seebeck coefficient (S) increases with temperature up to 280 K then stabilizes at∼-200μV K-1through 380 K. Bi2O2Se demonstrates high mobility (450 cm2V-1s-1) within the optimum power factor (PF) window, despite itsT-1.25dependence. The high mobility compensates the minor reduction in carrier densityn2Dhence contributes to maintain a robust electrical conductivity∼3 × 104S m-1. This results in a remarkable PF of 860μW m-1K-2at 280 K without the necessity for gating (Vg= 0 V), reflecting the innate performance of the as-grown material. These results underscore the considerable promise of Bi2O2Se for room temperature TE applications.

Enhanced thermoelectric properties for eco-friendly CaTiO3 by band sharpening and atomic-scale defect phonon scattering

CaTiO3-based compounds have emerged as a promising thermoelectric material, renowned for their environmentally benign, thermally stable, and cost-efficient merits. Non-etheless, the pristine CaTiO3 manifests inherently low electronic transport properties. Herein, the thermoelectric properties of Ca1-xDyxTiO3 (x = 0, 0.05, 0.10, 0.15, 0.20) compounds are systematically investigated. The electrical transport properties are markedly enhanced by synergistic optimization of the carrier concentration, mobility, and density-of-states effective mass. Density functional theory results demonstrate that the conduction band tends to be sharper and that the lighter band participates in carrier transport after Dy doping. The large discrepancy in atomic mass results in considerable mass fluctuations, which give rise to intense phonon scattering. Benefitting from the modulated band structure and reduced thermal conductivity, the highest thermoelectric figure of merit (ZT) of 0.31 is achieved at 1073 K, enhanced by 287.5% in contrast with pristine CaTiO3 (ZTmax = 0.08). The defect and energy band modulation strategies proposed to optimize thermoelectric performance are applicable to other thermoelectric materials. This investigation inspires the exploration of high-performance and eco-friendly high-temperature thermoelectric material.

A first principles study of stability, electronic and thermoelectric characteristics of novel high temperature half- heusler alloys ScAgX (X=Si,Ge,Sn)

To meet the ever-increasing renewable energy demand Half- Heusler (HH) materials can be a potential candidate. Heusler materials are seeking popularity as efficient high-temperature thermoelectric materials. Here we systematically investigated the structural, electronic, mechanical and thermoelectric properties of novel HH compounds ScAgX (X = Si,Ge,Sn) in the framework of the first principles approach. These materials are found to be chemically, mechanically and dynamically stable. Born-Oppenheimer molecular dynamics simulations (BOMD) verify the thermal stability of these materials from 300 K to 1100 K. The semiconducting nature of these materials is confirmed by the electronic band structure. ScAgX (X = Si,Ge,Sn) have an indirect band gap of 0.37 eV, 0.42 eV and 0.38 eV respectively. A high value of Seebeck coefficient of 6351 μV/K, 6955 μV/K and 6558 μV/K has been obtained at room temperature. ScAgSi shows a high value of electrical conductivity (∼106 S/m) among these compounds at room temperature. The calculated value of the figure of merit (ZT) of ScAgX (X = Si,Ge,Sn) is 0.12, 0.08 and 0.16 at 1100 K respectively. Our present investigation shows that these materials can be a potential candidate for high-temperature thermoelectric power generation.

Principles and Methods for Improving the Thermoelectric Performance of SiC: A Potential High-Temperature Thermoelectric Material.

Thermoelectric materials that can convert thermal energy to electrical energy are stable and long-lasting and do not emit greenhouse gases; these properties render them useful in novel power generation devices that can conserve and utilize lost heat. SiC exhibits good mechanical properties, excellent corrosion resistance, high-temperature stability, non-toxicity, and environmental friendliness. It can withstand elevated temperatures and thermal shock and is well suited for thermoelectric conversions in high-temperature and harsh environments, such as supersonic vehicles and rockets. This paper reviews the potential of SiC as a high-temperature thermoelectric and third-generation wide-bandgap semiconductor material. Recent research on SiC thermoelectric materials is reviewed, and the principles and methods for optimizing the thermoelectric properties of SiC are discussed. Thus, this paper may contribute to increasing the application potential of SiC for thermoelectric energy conversion at high temperatures.

Utilization of doping and compositing strategy for enhancing the thermoelectric performance of CaMnO3 perovskite

The performance of high-temperature thermoelectric material CaMnO3 is primarily limited by its high electrical resistivity and thermal conductivity. In this study, we synergistically optimized its electrical and thermal transport properties by employing a combination of doping and compositing strategies. The results suggest that Yb doping promotes the transfer of electrons to the Mn d orbital and facilitates the double exchange interactions between neighbor Mn3+ and Mn4+, leading to decreased electrical resistivity and a transition of electron transport mechanism from semiconductor to metal. The further addition of CoAl2O4 nanoparticles improves the Seebeck coefficient by reducing carrier concentration and increasing electron scattering, ultimately enhancing the power factor. Meanwhile, the fluctuations in atomic mass and increased interface density strengthen the interface phonon scattering, which results in a significant reduction in lattice thermal conductivity. As a result, the Ca0.95Yb0.05MnO3 with 2 wt% CoAl2O4 nanoparticles exhibits a maximum ZT value of 0.12 at 850 K, representing a 180 % improvement compared to the pristine material. This study highlights the effectiveness of the combined doping and compositing strategy in enhancing the thermoelectric performance of CaMnO3 materials and offers a promising approach for optimizing other oxide thermoelectric materials.

Two-dimensional Li-based ternary chalcogenides LiMTe2 (M = Al, Ga, and In): Promising high-temperature thermoelectric materials

High-efficiency thermoelectric materials require two essential characteristics; low lattice thermal conductivity (κl) and high power factor (PF). Although group-III chalcogenides exhibit extremely high PF, their high κl hinders their practical applications. Through first-principles calculations, we have discovered that the LiMTe2 (M = Al, Ga, and In) monolayers, which have a similar structure stacking as the group-III chalcogenides, simultaneously possess the two key characteristics mentioned above. The κl values of the LiMTe2 monolayers are as low as 1.4−2.0 W m−1 K−1 at room temperature. Detailed analysis of bonding characteristics and phonon scattering rates suggests that the ultralow κl can be mainly attributed to the strong bond anharmonicity, which is a result of the cooperative endeavor of bond heterogeneity (coexistence of strong covalent bonds and weak ionic bonds) and the existence of lone-pair electrons. Additionally, the “pudding-mold” type of the conduction band near the Fermi level contributes to the high power factor of LiMTe2 monolayers. Therefore, the ZT values of n-type doped LiMTe2 monolayers exceed 1 at T = 800 K and carrier conentration of 1.71 × 1012–3.32 × 1012 cm−2, indicating the great potential of the 2D LiMTe2-family under n-type doping as high-temperature thermoelectric materials. The current work suggests that increasing bond complexity and the introduction of lone-pair electrons are strategic pathways for designing high-performance thermoelectric materials.

Effects of Magnetic Fe Doping on the Thermoelectric Properties of TiNiSn Nanomaterials prepared via melt spinning method

As a kind of high temperature thermoelectric material, TiNiSn compound has the characteristics of high power factor and high thermal conductivity. Its thermoelectric performance is largely limited by the high thermal conductivity. In this paper, TiNiFexSn(x=0-0.05) samples are prepared by melt spinning combined with spark plasma sintering method. The results show that there are nanocrystals existed in the samples with a size of around 200nm. The grain size of the samples is refined through the melt-spinning process, and the lattice thermal conductivity is lower compared with the arc-melted samples. With the increase of Fe content, the lattice thermal conductivity of TiNiFexSn samples decreases, reaching a minimum value of 3.31W/mK when x=0.02, which is 44% lower than that of TiNiSn matrix. The carrier concentration of TiNiFexSn samples increases due to the doping of Fe, resulting in an increase of electrical conductivity and decrease of Seebeck coefficient and finally improvement of the power factor. The maximum zT value of TiNiFe0.03Sn sample is 0.54 at 900K, which is 35% higher than that of TiNiSn samples. This work shows that the melt spinning and magnetic Fe doping can effectively improve the thermoelectric properties of TiNiSn.

Thermoelectric properties of half-Heusler alloys

Half-Heusler alloys (HHs) as medium- and high-temperature thermoelectric materials have attracted increasing considerable attention, owing to their excellent mechanical properties and thermal stability. Nonetheless, the intercoupling between the Seebeck coefficient and electrical conductivity, and their high thermal conductivity limit the furthermore enhancement of their thermoelectric properties. This review systemically summarizes and analyses the various current strategies for improving thermoelectric properties, such as manipulating band structure, optimizing electron and phonon transports, and constructing high configurational entropy, etc. Moreover, the possible deficiencies of the existing electric-phonon transport mechanisms are highlighted and discussed, which aiming to provide a meaningful guide for future efforts to accelerate the development of HHs. Finally, based on current research, a new potential future for the integration of single crystal technology and high-entropy effect is introduced for HHs.

Thermoelectric properties of heavily Co-doped β-FeSi2

Element doping is a widely employed strategy to enhance the thermoelectric (TE) properties of various materials. β-FeSi2 is a promising low-cost high-temperature TE material with exceptional thermal stability; however, the doping limit of β-FeSi2 is usually very low, which limits the tunability of electrical and thermal properties. Recently, a high doping content of 0.16 in β-FeSi2 has been achieved by the introduction of iridium (Ir), leading to the highest reported figure of merit (zT) of 0.6 in β-FeSi2. Motivated by the successful heavy doping with Ir, this work aims to explore element heavy doping in β-FeSi2 with cobalt (Co), a cheaper, more readily available dopant with a smaller atomic radius and closer electronegativity to iron (Fe). In this study, we successfully obtained a record-high doping content of 0.24 in Co-doped β-FeSi2 through a prolonged annealing process. Despite the absence of a substantial enhancement in the zT of Co-doped β-FeSi2 at high doping levels, with a maximum zT of 0.3 at 900 K in Fe0.92Co0.08Si2, we observed a transition in the carrier transport mechanism as a function of Co doping content, attributed to changes in the band structure. At a low Co doping content (x ≤ 0.12), Fe1–xCoxSi2 demonstrates dominant carrier transport via impurity levels within the band gap, exhibiting hopping conduction. As the Co doping content increases (x > 0.16), the impurity levels overlap and form an impurity band, and the carrier transport turns into the impurity band conduction. The observed band conduction behavior of Fe1–xCoxSi2 (x > 0.16) mirrors that of Ir-doped β-FeSi2, but Fe1–xCoxSi2 shows much lower mobility, which can be attributed to the localized feature of the impurity band introduced by the Co doping. Overall, this study provides insights into the heavy Co doping and its influence on the TE properties and carrier conduction mechanisms in β-FeSi2, helpful for the further development of this TE system.

Theoretical exploration of novel boron–carbon clathrate in Sr(B,C)10 at high pressure

SrB8C2 is a hard, high-temperature thermoelectric multifunctional material.

Decomposed defect formation energy for analysis of doping process: The case of n-type and p-type doping of β-FeSi2

Defect formation energy governs the thermodynamics of a specific dopant within the host material. Here, we introduce an approach to decomposing the defect formation energy into intuitive components, each representing a distinct physical step in the process of defect formation. Through this approach, we illustrate that adhering solely to conventional criteria, such as ionic radius, may overlook potential dopants. Taking β-FeSi2, a promising high-temperature thermoelectric material, as an example, we demonstrate that non-intuitive chemical interactions can play a more significant role in lowering the defect formation energy. As a result, Ir on Fe site is found to exhibit unexpected low defect formation energy among the 26 candidate dopants and has been employed in experiment to enhance the thermoelectric figure of merit of n-type β-FeSi2. The understanding gained from this work could be of general interest for addressing the doping limit issue for other potential thermoelectric materials.

Dopant dependent microstructure of hot-pressed SiGe alloys and its implications on thermoelectric properties

SiGe is a proven high temperature thermoelectric material for space application. We have investigated the role of dopants (P and B) on the densification of SiGe alloys. It was observed that on P doping there is a formation of liquidous SiP phase in the SiGe matrix during hot pressing that improves the diffusion across grain boundaries and as a result ∼98 % of theoretical density was achieved. In case of identically prepared B doped samples, the segregation of B resulted in porous microstructure and low density ∼89 %. Such a difference in microstructure resulted a figure-of-merit of ∼1.29 (at 1100K) for n-type SiGe (P doped) and ∼0.86 (at 1100K) for p-type SiGe (B doped) at a hot press temperature of 1273 K. The study suggests that in case of SiGe apart from doping the densification of materials plays a very important role in improving its thermoelectric performance.

Tweaking the interplay of the layers by substitution in Ca3Co4O9+δ for high efficiency thermoelectric materials

Ca3Co4O9 is a well-known high temperature thermoelectric material owing to its sandwich type layered structure consisting of both conducting and insulating slabs. In the present study we attempt to enhance the electrical conductivity without affecting the thermal conductivity by suitable doping in these layers. Compositions with a general formula Ca3−xSrxCo4−xMgx/2Tix/2O9+δ (0 ≤ x ≤ 0.15) were prepared by conventional solid state method and subjected to routine characterization techniques like XRD, SEM and XPS. Sr substitution in the insulating layer and Mg, Ti substitution in the conductive layer led to a decrease in the electrical resistivity from 17.86 mΩ cm to 9.74 mΩ cm, which resulted in an increase in the power factors. The dimensionless thermoelectric figure of merit ZT is found to be 0.13 at 873 K for x = 0.15, which is ̴2.1 times higher than the parent compound attributed to the excess oxygen in the doped composition.

Impact of Lu-Substitution in Yb14-xLuxZnSb11: Thermoelectric Properties and Oxidation Studies.

Yb14ZnSb11 is one of the newest additions to the high-performance Yb14MSb11 (M = Mn, Mg, and Zn) family of p-type high-temperature thermoelectric materials and shows promise for forming passivating oxide coatings. Work on the oxidation of rare earth (RE)-substituted Yb14-xRExMnSb11 single crystals suggested that substituting late RE elements may form more stable passivation oxide coatings. Yb14-xLuxZnSb11 (x = 0.1, 0.2, 0.3, 0.4, 0.5, and 0.7) samples were synthesized, and Lu-substitution's effects on thermoelectric and oxidation properties are investigated. The solubility of Lu within the system was found to be quite low with xmax ∼ 0.3; samples with x > 0.3 contained impurities of LuSb. Goldsmid-Sharp band gap estimations show that introducing Lu reduces the apparent band gap. Because of this, the Lu-substituted samples show a reduction in the maximum Seebeck coefficient, decreasing the high-temperature zT. This contrasts with the impact of Lu3+ substitution in Yb14MnSb11, where the addition of Lu3+ for Yb2+ results in increases in both resistivity and the Seebeck coefficient. Oxidation of the x = 0.3 solid solution was studied by thermogravimetric- differential scanning calorimetry , powder X-ray diffraction, scanning electron microscopy-energy-dispersive spectroscopy, and optical images. The samples show no mass gain before 785 K, and ensuing oxidation reactions are proposed. At the highest temperatures, significant amounts of Yb14-xLuxZnSb11 remained beneath an oxide coating, suggesting that passivation may be achievable in oxygen environments.

The filled tetrahedral compound NaZnP with strong quartic anharmonicity and good thermoelectric properties

Based on first-principles calculations, self-consistent phonon (SCP) theory, and the Boltzmann transport equation, we investigate the thermal transport and thermoelectric properties of the Nowotny-Juza filled tetrahedral compound NaZnP. The effects of anharmonic phonon renormalization and four-phonon scattering on the heat transport of NaZnP are also considered. The weak binding and rattling-like vibrational behavior of Na atoms lead to strong lattice anharmonicity, and the results show that NaZnP has low lattice thermal conductivity. Furthermore, the coexistence of high dispersion and high degeneracy of valence band maxima leads to high power factor under p-type doping. The combination of low lattice thermal conductivity and good electrical transport properties results in high thermoelectric performance. This indicates that NaZnP is a promising p-type high-temperature thermoelectric material.

Enhancement of thermoelectric performance in n-type Si90Ge10-based alloy by metallic Zn doping

Silicon–germanium (SiGe) alloy has become one of the representative high-temperature thermoelectric (TE) materials due to its advantages of stability, non-toxicity, oxidation resistance, and high mechanical strength. However, the high thermal conductivity and expensive Ge greatly limit the enhancement of zT value and its application. In this paper, n-type Si90Ge10P2Znx nanocomposites were prepared by ball milling and spark plasma sintering. By adjusting the Zn content and sintering time, multiple phonon-scattering centers, such as Zn precipitates, nano-pores, and layered structures, have been introduced into the SiGe matrix. The thermal conductivity was significantly reduced to 2.59 W m−1 K−1 without deteriorate power factor (PF), thus leading to a high zT value of 1.23 at 873 K. At 323–873 K, the average zT value (zTavg) also reached 0.6, increased by approximately 25% in comparison to the reported value using the same ratio of Si90Ge10. Compared with the conventional radioisotope TE generator with Si80Ge20 composition, the zTavg value increased by nearly 30% with only half of Ge, giving strong impetus to the application of SiGe-based TE materials.

First-principles studies on the structural, electronic and thermal transport characteristics of half-Heusler compounds LiXN (X=Mg, Zn)

The electronic structures and thermal transport properties of the half-Heusler (HH) compounds LiMgN and LiZnN are calculated in detail by combining first-principles calculations with Boltzmann transport theory. Our phonon calculations showed that LiMgN and LiZnN are dynamic stability. The calculated bulk moduli and equilibrium lattice constants are in good agreement with the experimental and available theoretical data. The electronic band structures obtained by PBE (HSE06) method indicate that both LiMgN and LiZnN are direct bandgap semiconductors with band gaps of 2.33 eV (3.23 eV) and 0.53 eV (1.42 eV), respectively. It is shown that the Li–N，Li–Mg and Li–Zn are ionic bond via ELF analysis. At room temperature, the calculated lattice thermal conductivities of LiMgN and LiZnN compounds are 19.31 Wm−1K−1 and 18.74 Wm−1K−1, respectively; But with the external temperature increases to 1100 K, their lattice thermal conductivities are decrease rapidly to 4.96 Wm−1K−1 and 4.32 Wm−1K−1, respectively. Moreover, we analyzed the mean free paths, phase volume spaces (P3), Gruneisen parameters (γ), scattering rates and group velocities in detail in order to further understand the origins of their relatively low lattice thermal conductivities and corresponding microscopic mechanism. We also investigated thermoelectric performances of LiMgN and LiZnN under different temperatures, and our results showed that with the external temperature increasing from 300 K to 1100 K, the maximum ZT values can increase from 0.05 (0.03) and 1.83 (1.32) for LiMgN (LiZnN), indicating that they are potential high-temperature thermoelectric materials. Our work can offer some theoretical references for improving thermoelectric performance and for future experimental studies.

Evolution of Thermoelectric and Oxidation Properties in Lu-Substituted Yb14MnSb11

Yb14MnSb11 is one of the state-of-the-art high-temperature p-type thermoelectric materials with reported zTs of 1.2–1.3 at 1273 K. Site substitution of Yb14MnSb11 provides a means to control carrier concentration and impacts the oxidation kinetics. Substitution of Lu3+ for Yb2+ in Yb14MnSb11 single crystals has been shown to provide faster oxidation kinetics compared with the pristine phase, and it has been speculated that this may lead to surface passivation. Polycrystalline samples of Yb14–xLuxMnSb11 were synthesized from the elements and through a route utilizing YbH2, MnSb, and Yb4Sb3 as reactive precursors. The solubility of Lu was found to be less than x = 0.5, and at that composition and above LuSb impurities are observed in the powder X-ray diffraction. Because Lu3+ is substituting for Yb2+ in a p-type system, there is an increase in both the electrical resistivity and Seebeck coefficient with Lu substitution. As Yb14MnSb11 has been predicted to be optimally doped ∼1.5 × 1021 h+ cm–3, this reduction in carrier concentration in the solid solution of Yb14–xLuxMnSb11 decreases the peak zT. Oxidation of Yb13.7Lu0.3MnSb11 as a function of temperature was studied. At low temperatures, the Lu-substituted sample may have improved oxidation resistance through forming a passivating oxide film on the surface.

Enhancing thermoelectric properties of MCoSb-based alloys by entropy-driven energy-filtering effects and band engineering

MCoSb-based medium-entropy half-Heusler (HH) alloy with low lattice thermal conductivity (κL) is a promising medium- and high-temperature thermoelectric material. However, the unexpected band structure of the alloy leads to a sudden decrease in the Seebeck coefficient (S), which severely restricts the optimization of the figure-of-merit (ZT). In this study, we demonstrate a new concept for decoupling S and electrical conductivity (σ) based on entropy engineering. This method involves band engineering and the energy-filtering effect, where the entropy-driven quantum confinement effect is manipulated to filter low-energy electrons, and the increasing entropy promotes a rapidly changing density of state near Fermi level. Consequently, an excellent S of ∼ −211.6 μV K−1 at 923 K was achieved for the M0.82Nb0.18CoSb HH alloy, which is 62.3% higher than that of the pristine M0.85N0.15CoSb (M = Ti, Zr, Hf; N= V, Ta; equimolar) HH alloy (∼−130.4 μV K−1). Moreover, benefiting from the optimization of S and κL, a peak ZT was enhanced from ∼0.17 for the pristine M0.85N0.15CoSb medium-entropy HH alloy to 0.58 for the M0.85Nb0.15CoSb high-entropy HH alloy. These results broaden the applications of entropy-engineering to decrease κL and decoupling between S and σ.