Thermoelectric Compounds Research Articles

Role of volume change on the physics of thermoelectric half-Heusler compounds

Role of volume change on the physics of thermoelectric half-Heusler compounds

Enhanced thermoelectric performance of Ru2TiSi Heusler compounds with Ta substitution

The Ru2TiSi Heusler compound can be classified as a semimetal, with a deep pseudogap near the Fermi level, because of the possession of a valence electron concentration VEC of 6.0. In this study, we investigate the electrical and thermal transport properties of Ru2Ti1-xTaxSi (x = 0 – 0.20) compounds and evaluate their thermoelectric performance up to a temperature of 1000 K. The pristine Ru2TiSi compound displays a p-type Seebeck coefficient (S) that peaks at S = 185 μV/K around 700 K. Notably, the power factor for Ru2TiSi gradually increases with temperature and reaches a value of 4.4 × 10–3 W/mK2 at 830 K. Upon partial substitution of Ta, a significant change occurs, resulting in n-type thermoelectric conduction with a Seebeck coefficient of S = –155 μV/K at 680 K for x = 0.03. The thermal conductivity (κ) of Ru2TiSi is approximately 20 W/mK at 300 K, but the substitution of Ta dramatically reduces κ to 4.5 W/mK for x = 0.20, which remains almost unchanged with temperature. Through thermoelectric measurements on the sintered alloys, we determined the dimensionless figure of merit, ZT, to be 0.42 for p-type Ru2TiSi at 1000 K and 0.42 for n-type Ru2Ti0.80Ta0.20Si at 900 K, both of which are some of the highest values ever achieved for bulk thermoelectric Heusler compounds.

Oxide semiconductors for thermoelectric: The challenges and future

AbstractThe oxide thermoelectric (TE) ceramics have continued to attract the focus due to their high stability and low cost. However, their TE performance falls obviously backward with the typical TE compounds such as PbTe. Here, we revealed the reason for the difficulty of optimized performance is the large electronegativity difference between the metal and O ions in oxide, which results in strong electron localization and bond energy. The former leads to unfavorable electrical properties and the latter results in a high lattice thermal conductivity. Focusing on these issues, we briefly reviewed the strategies for electrical optimization including carrier concentration optimization, band gap tuning, and the density of state resonance, as well as the lattice thermal suppression strategies involving weakened bond, high entropy, grain size engineering, hierarchical architecture, textured polycrystal, and composite strategies for oxide TE ceramics. Finally, we proposed several possible perspectives for n‐type oxide TE ceramics, strategies for bond anisotropy, and predicted new TE oxides for further breakthroughs in oxide TE properties.

Obtaining and characterisation of thermoelectric Mg2Si compound via wet and dry mechanical alloying and spark plasma sintering

Mg2Si thermoelectric compound was obtained by dry and wet (isohexane and benzene) mechanical alloying route. The Mg2Si compound was characterised by X-ray diffraction, scanning electron microscopy (SEM), differential scanning calorimetry (DSC), laser particle size analysis and thermoelectric measurements. After 14 h of milling, the complete reaction of the elements is achieved by both routes. The powders particle size distribution obtained after 14 h of dry or wet milling (using benzene) reveals a bimodal curve. The wet milling moves the particle size distribution towards smaller particle sizes. The SEM analysis confirms the results of particles size analysis. DSC analyses performed for samples milled up to 14 h present stress relief and recrystallisation thermal events. For the benzene wet-milled sample, the DSC curve shows an additional thermal event at 350 °C, associated with benzene removal. Thermoelectric properties were determined on spark plasma sintered compacts. The milling process with benzene leads to a higher value of Seebeck coefficient (≈580 μV/K). The electrical conductivity is low at room temperature and increases exponentially with temperature.

High-Throughput Screening of High-Performance Thermoelectric Materials with Gibbs Free Energy and Electronegativity.

Thermoelectric (TE) materials are an important class of energy materials that can directly convert thermal energy into electrical energy. Screening high-performance thermoelectric materials and improving their TE properties are important goals of TE materials research. Based on the objective relationship among the molar Gibbs free energy (Gm), the chemical potential, the Fermi level, the electronegativity (X) and the TE property of a material, a new method for screening TE materials with high throughput is proposed. This method requires no experiments and no first principle or Ab initio calculation. It only needs to find or calculate the molar Gibbs free energy and electronegativity of the material. Here, by calculating a variety of typical and atypical TE materials, it is found that the molar Gibbs free energy of Bi2Te3 and Sb2Te3 from 298 to 600 K (Gm = -130.20~-248.82 kJ/mol) and the electronegativity of Bi2Te3 and Sb2Te3 and PbTe (X = 1.80~2.21) can be used as criteria to judge the potential of materials to become high-performance TE materials. For good TE compounds, Gm and X are required to meet the corresponding standards at the same time. By taking Gm = -130.20~-248.82 kJ/mol and X = 1.80~2.21 as screening criteria for high performance TE materials, it is found that the Gm and X of all 15 typical TE materials and 9 widely studied TE materials meet the requirement very well, except for the X of Mg2Si, and 64 pure substances are screened as potential TE materials from 102 atypical TE materials. In addition, with reference to their electronegativity, 44 pure substances are selected directly from a thermochemical data book as potential high-performance TE materials. A particular finding is that several carbides, such as Be2C, CaC2, BaC2, SmC2, TaC and NbC, may have certain TE properties. Because the Gm and X of pure substances can be easily found in thermochemical data books and calculated using the X of pure elements, respectively, the Gm and X of materials can be used as good high-throughput screening criteria for predicting TE properties.

Re‐Doped p‐Type Thermoelectric SnSe Polycrystals with Enhanced Power Factor and High ZT > 2

AbstractThermoelectric technology enables the direct interconversion between heat and electricity. SnSe has received increasing interest as a new promising thermoelectric compound due to its exceptionally high performance reported in crystals. SnSe possesses intrinsic low thermal conductivity as a congenital advantage for thermoelectric, but high thermoelectric performance can be hardly achieved due to the difficulty to realize efficient doping to raise its low carrier concentration to an optimal level. In this work, it is found that a series of rare earth elements are effective dopants for SnSe, which can greatly improve the electrical transport properties of p‐type polycrystalline SnSe. In particular, the remarkable enhancement in electrical conductivity and power factor is achieved by Na/Er co‐doping at 873 K. The lattice thermal conductivity is reduced due to the presence of abundant defects (dislocations, stacking faults, and twin boundaries). Consequently, a peak thermoelectric figure of merit ZT (2.1) as well as a high average ZT (0.77) are achieved in polycrystalline SnSe.

Insight to structural, electronic, optical and thermoelectric propertiesof NaCaSb and KCaSb half Heusler compounds: a DFT approach

Insight to structural, electronic, optical and thermoelectric propertiesof NaCaSb and KCaSb half Heusler compounds: a DFT approach

Copper-Based Diamond-like Thermoelectric Compounds: Looking Back and Stepping Forward.

The research on thermoelectric (TE) materials has a long history. Holding the advantages of high elemental abundance, lead-free and easily tunable transport properties, copper-based diamond-like (CBDL) thermoelectric compounds have attracted extensive attention from the thermoelectric community. The CBDL compounds contain a large number of representative candidates for thermoelectric applications, such as CuInGa2, Cu2GeSe3, Cu3SbSe4, Cu12SbSe13, etc. In this study, the structure characteristics and TE performances of typical CBDLs were briefly summarized. Several common synthesis technologies and effective strategies to improve the thermoelectric performances of CBDL compounds were introduced. In addition, the latest developments in thermoelectric devices based on CBDL compounds were discussed. Further developments and prospects for exploring high-performance copper-based diamond-like thermoelectric materials and devices were also presented at the end.

Highly Stable Metal─Na0.02Pb0.98Te Contacts for Medium Temperature Thermoelectric Devices.

In the medium temperature (600-850 K) range, Na0.02Pb0.98Te is a highly efficient p-type thermoelectric compound. Device fabrication utilizing this compound for power generation demands highly stable low-contact resistance contacts with metal electrodes. This work investigates the microstructural, electrical, mechanical, and thermochemical stability of Na0.02Pb0.98Te-metal (Ni, Fe, and Co) contacts made by a one-step vacuum hot pressing process. Direct contact mostly resulted in either an interface with poor mechanical integrity, as in Co and Fe, or poisoning of the TE compound, as in the case of Ni, which results in high specific contact resistance (rc). In Ni and Co, adding a SnTe interlayer lowers the rc and strengthens the contact. It does not, however, effectively stop Ni from diffusing into Na0.02Pb0.98Te. The bonding is poor in the Fe/SnTe/Na0.02Pb0.98Te contacts due to the absence of any reaction at the Fe/SnTe interface. A composite buffer layer Co + 75 vol % SnTe with SnTe improves the mechanical stability of the Co contact with moderately lesser rc than pure SnTe alone. However, a similar approach with Fe does not yield stable contact. The Co/Co + 75 vol % SnTe/SnTe/Na0.02Pb0.98Te contact exhibits rc less than 50 μΩ cm2 and has good microstructural and mechanical stability after annealing at 723 K for 170 h.

Bi-Deficiency Leading to High-Performance in Mg3 (Sb,Bi)2 -Based Thermoelectric Materials.

Mg3 (Sb,Bi)2 is a potential nearly-room temperature thermoelectric compound composed of earth-abundant elements. However, complex defect tuning and exceptional microstructural control are required. Prior studies have confirmed the detrimental effect of Mg vacancies in Mg3 (Sb,Bi)2 . This study proposes an approach to mitigating the negative scattering effect of Mg vacancies (VMg ) by Bi deficiency, synergistically modulating the electrical and thermal transport properties to enhance the thermoelectric performance. Positron annihilation spectrometry and Cs -corrected scanning transmission electron microscopy analyses indicated that the VMg tends to coalesce due to the introduced VBi . The defects created by Bi deficiency effectively weaken the scattering of electrons from the intrinsic VMg and enhance phonon scattering. A peak zT of 1.82 at 773 K and high conversion efficiency of 11.3% at ∆T = 473 K are achieved in the optimized composition of Mg3 (Sb,Bi)2 by tuning the defect combination. This work demonstrates a feasible and effective approach to improving the performance of Mg3 (Sb,Bi)2 as an emerging thermoelectric material. This article is protected by copyright. All rights reserved.

Multiple Valence Bands Convergence and Localized Lattice Engineering Lead to Superhigh Thermoelectric Figure of Merit in MnTe.

MnTe has been considered a promising candidate for lead-free mid-temperature range thermoelectric clean energy conversions. However, the widespread use of this technology is constrained by the relatively low-cost performance of materials. Developing environmentally friendly thermoelectrics with high performance and earth-abundant elements is thus an urgent task. MnTe is a candidate, yet a peak ZT of 1.4 achieved so far is less satisfactory. Here, a remarkably high ZT of 1.6 at 873 K in MnTe system is realized by facilitating multiple valence band convergence and localized lattice engineering. It is demonstrated that SbGe incorporation promotes the convergence of multiple electronic valence bands in MnTe. Simultaneously, the carrier concentration can be optimized by SbGeS alloying, which significantly enhances the power factor. Simultaneously, MnS nanorods combined with dislocations and lattice distortions lead to strong phonon scattering, resulting in a markedly low lattice thermal conductivity(κlat ) of 0.54Wm K-1 , quite close to the amorphous limit. As a consequence, extraordinary thermoelectric performance is achieved by decoupling electron and phonon transport. The vast increase in ZT promotes MnTe as an emerging Pb-free thermoelectric compound for a wide range of applications in waste heat recovery and power generation.

Enhanced thermoelectricity in Bi-sprayed bismuth sulphide particles

Bismuth sulphide (Bi2S3), an n-type semiconductor that critically demonstrates the Seebeck effect with Seebeck coefficients of about 300 μVK−1. However, its poor electrical conductivity makes it unsuitable for thermoelectric applications. In this study, we present a facile preparation method for fabricating Bi-sprayed Bi2S3 particles that alters their thermoelectric properties. Samples were created with differing Bi concentrations into the Bi2S3 compound to test for enhanced thermoelectric properties of the resulting Bi/Bi2S3 composites. The incorporation of excess Bi into Bi2S3 significantly improves the compound's electrical conductivity and optimises overall thermoelectric performance. The electrical conductivity of the Bi/Bi2S3 composites improved from 6.5 Scm−1 (for pristine Bi2S3) to 154 Scm−1 (for highest Bi added Bi2S3). Although the Seebeck coefficient of samples decreased with Bi incorporation, a high power factor (∼390 μWm−1K−2) has been achieved for an optimised composition of the composite. Incorporation of metallic Bi has led to an increase in the thermal conductivity of the samples, but the increase is not significant for the optimised composition of the composites where a high thermoelectric performance has been observed. Therefore, enhanced power factor and moderate thermal conductivity have resulted in a peak ZT value of 0.11 at room temperature. The strategy proposed here improves the thermoelectricity in Bi2S3 and shows excellent potential for developing better-performing thermoelectric compounds with excess elemental contents.

Enhancement of Thermoelectric Performance for InTe by Selective Substitution and Grain Size Modulation

The different masses, ionic radii, and chemical valences of the nonequivalent crystallographic sites of thermoelectric (TE) compounds provide an effective way to modulate the thermoelectric performance by selective substitution. In this work, the selective substitution of In+ by Pb for the binary InTe material monotonically reduces the carrier concentration, which is greatly beneficial to the mechanism investigation of serious grain boundary scattering (GBS). This is the first time this point has been mentioned with regard to InTe material. As a result, we found that GBS was dominated by the grain size when the carrier concentration was higher than 0.7 × 1019 cm−3 but was inversely governed by the carrier concentration when the carrier was situated at a lower density. In particular, the occupation of Pb on the targeted In+ site could further reduce the lattice thermal conductivity. Finally, In0.9999Pb0.0001Te achieved the improved power factor and average zT value, which could contribute to high-power generation below a medium temperature. This effect of increasing the carrier concentration on the suppression of GBS sheds light on the possibility of improving electron mobility by increasing the carrier concentration.

Morphology and Thermoelectric Properties of Mg<sub>3+δ</sub>Sb<sub>2</sub> Foams Manufactured Using Combustion Synthesis

Magnesium antimonide (Mg3Sb2) is a promising thermoelectric compound utilized for thermoelectric power generation. We have found out that the Mg3Sb2 compound can be made from Mg and Sb powder mixture via the combustion synthesizing reaction. In this study, we manufactured the compounds from the compacted bodies with different Mg fractions between 60.0 and 75.0 at% in an argon gas flow at 650°C for 1 hour via the combustion synthesis process. Morphology, porosity, phases and thermoelectric properties (electrical resistivity and Seebeck coefficient at room temperature to 500°C) were investigated for the manufactured samples. All samples manufactured with different Mg fractions mainly consisted of Mg3Sb2 and became foamed bodies with a porosity between 50 and 70%. The thermoelectric properties changed with the Mg fraction. The maximum power factor of 42.1 µW/mK2 at 488°C was obtained in the foamed body with the Mg fraction of 64 at%. The dimensionless figure of merit (ZT) of the sample was also estimated using the thermal conductivity, which was calculated considering its large porosity. The maximum dimensionless figure of merit (ZTmax) would be 0.13 at 488°C.

Enhancement in Power Factor of Al-Doped Cu2Se Thermoelectric Compound Prepared by Combustion Synthesis via Spark Plasma Sintering Technique

Enhancement in Power Factor of Al-Doped Cu2Se Thermoelectric Compound Prepared by Combustion Synthesis via Spark Plasma Sintering Technique

First-principles calculations to investigate optical and electrical properties of the half-Heusler materials TiXSn (X = Ni, Pt)

ABSTRACT Thermoelectric compounds have the potential to produce electricity from waste heat, these materials are environmentally-friendly, which can play a key role on sustainable energy solution. This paper reports an investigation of electronic, optical and thermoelectric properties of two Half Heusler compounds TiNiSn and TiPtSn by employing density functional theory (DFT). Both compounds i.e. TiNiSn and TiPtSn exhibit an overestimated band gap with respective values 0.40 and 0.95 eV obtained by Tran Blaha modified Becke–Johnson functional (TB-mBJ) approximation, while, we found 0.113 and 0.84 eV within GGA + U (U = 6.13 eV) approximation, which are around other theoretical and experimental results. A detailed investigation of the obtained optical properties particularly, dielectric function, absorption, refractive index, and reflectivity of these compounds are given and discussed. Additionally, the transport properties of these compounds have been examined within Boltzmann statistics through BoltzTrap code, as well as certain thermoelectric parameters such as electrical conductivity, Seebeck coefficient and figure of merits are also determined. The obtained results reveal that the studied compounds are quite promising materials for thermoelectric applications.

Highly deformable Ag2Te1-xSex-based thermoelectric compounds

Thermoelectric semiconductors are crucial for many critical applications, but are also universally fragile, which greatly limits the development of room temperature flexible thermoelectric devices. Herein, an extraordinary deformability at room temperature and high thermoelectric performance are simultaneously realized in wide composition of Ag2Te1-xSex (x = 0–0.2) compounds with monoclinic structure. An average compressive strain rate of 24% at room temperature is attained for the pristine Ag2Te compound. Moreover, a tensile strain rate of 43% is measured at room temperature using the in-situ transmission electron microscopy. First Principles calculations and in-situ transmission electron microscope tensile test results indicate that the Ag2Te compound is inclined to slip along the (−102) [010] plane under external forces due to a larger ratio between the cleavage energy (Ec) and the slip energy barrier. The slipping process generates high concentrations of dislocations, deformations, and amorphous structures, which together dissipate the externally applied load energy, yielding superior deformability. Meanwhile, the maximum dimensionless figure of merit ZT of 0.61 is obtained at 380 K. This study demonstrates that Ag2Te-based compounds are promising for the fabrication of flexible thermoelectric devices in applications, such as wearable, room temperature electronic gadgets.

Extended phase homogeneity and improved out-of-plane charge transfer in Sb and Te co-alloyed n-type BiSe layered compound with extraordinary thermoelectric performance

BiSe is a promising n-type layered thermoelectric compound near room temperature. However, its full potential is not realized because of its overall high electron density and inferior interlayer charge transport. In this study, we demonstrate that these drawbacks are well addressed by a feasible Sb and Te co-alloying approach. Sb, preferably substituting Bi atoms in the Bi2Se3 quintuplet layer, effectively neutralizing excess electrons of BiSe by suppressing positively charged Se vacancies. On the other hand, Te exclusively replaces Se, and promotes interlayer charge transfer by increasing chemical bond covalency. In addition, Te alloying significantly impedes phonon propagation of BiSe by introducing considerable size and mass disorders. More importantly, it is found that co-alloying expands the phase homogeneity of BiSe, which further intensifies the respective functions of Sb and Te alloying. As a consequence, a peak ZT value of 0.6 at 425 K is realized in the sample Bi0.6Sb0.4Se0.6Te0.4, which is 2.5 times that of pristine BiSe. Our work suggests that Sb and Te co-alloyed BiSe is a robust candidate for thermoelectric application near room temperature.

Cu-doping boosts the thermoelectric properties of layered compound LaOBiS2 with weak anisotropy

Layered compounds like Bi2Te3 and SnSe are attracting great interest for applications in thermoelectric devices, mainly due to their low lattice thermal conductivity from large anharmonicity. Herein, we report the thermoelectric properties of the quaternary layered compound LaOBiS2, which is a n-type semiconductor constructed by stacking of fluorite-type [LaO]+ layers and rock-salt-type [BiS2]- layers. Our results reveal that the as-sintered LaOBiS2 sample features weak anisotropy due to the almost random arrangement of crystalline grains with nonpreferred-orientation growth. The pristine LaOBiS2 has high intrinsic electrical conductivity and low Seebeck coefficient as a result of a high electron concentration (n = 3.25 ×1020 cm−3, 300 K) from anionic vacancy. Meanwhile, the total thermal conductivity of LaOBiS2 is as low as 1.10 W m−1 K−1 at 873 K. Cu-doping improve the overall thermoelectric properties, which induces a decrease in both electrical conductivity and thermal conductivity. The maximum zT is 0.24 at 873 K for the sample with dopant Cu of 1 %, indicating a 33 % increase compared with pristine LaOBiS2. Our work demonstrates that constructing semiconductors by functional building blocks is an effective approach to design thermoelectric compounds.

Electronic Transport Properties of Bi<sub>2</sub>Te<sub>2.7</sub>Se<sub>0.3</sub> Fabricated by Hot Extrusion

Herein we report the optimized processing conditions of hot extrusion for fabricating an <i>n</i>-type Bi<sub>2</sub>Te<sub>2.7</sub>Se<sub>0.3</sub> thermoelectric compound, with high electronic transport properties as well as improved mechanical reliability. We fabricated a Bi<sub>2</sub>Te<sub>2.7</sub>Se<sub>0.3</sub> extrudate that was 3.8 mm in diameter and 700 mm in length by controlling the processing parameters of temperature and pressure. A 3-point bending strength of over 70 MPa, which is 7 times higher that of the commercial zone melting ingot, was obtained in the samples prepared at 460 <sup>o</sup>C temperature under 6–6.5 MPa pressure. The samples benefitted from the formation of a highly-dense microstructure (relative density > 98%). It is noted that the electronic transport properties (electrical conductivity and Seebeck coefficient) could be manipulated by controlling the applied pressure of hot extrusion at 460 <sup>o</sup>C, mainly due to the change in the characteristics of the 00<i>l</i> crystal orientation, which originated from grain rotation and rearrangement. Power factor values of ~2.9 mW/mK<sup>2</sup> at 300 K and ~2.95 mW/mK<sup>2</sup> at 320 K, similar to those of sintered bulks, were obtained in the hot extrudate fabricated under processing parameters of 460 <sup>o</sup>C and 6 MPa. Moreover, a high power factor value of 2.25 mW/mK<sup>2</sup> was observed even at the high temperature of 480 K, which is 70% higher than that of Bi<sub>2</sub>Te<sub>2.7</sub>Se<sub>0.3</sub> bulk fabricated by hot pressing.