The panoscopic approach to high performance thermoelectrics

GeTe Thermoelectrics

Thermoelectric materials and generators, enabling to convert thermal energy into electrical energy or vice versa, offer great potentials in solving the energy problem from an environmental-sustainable perspective. As one of emerging thermoelectric materials, tin telluride (SnTe) shows uniuqe characterisitcs, such as low-toxicity and eco-friendly behaviour. The recent trend shows that band engineering and nanostructuring can enable to achieve enhanced thermoelectric performance of SnTe in the temperature range from 300 to 800 K, which provides an alternative for toxic PbTe with the same operational temperature. This thesis firstly highlights the key strategies to enhance the thermoelectric performance of SnTe materials through band engineering, carrier concentration optimization, synergistic engineering and structure design. On the basis of the fundamental analysis, the underpinnings for the property improvement are elucidated and can boost the relevant research with a view to work on further performance enhancement of SnTe materials.While most of the reported work on SnTe uses conventional melting method, this thesis governs with the solvothermal synthesis method which has unique advantages over melting method. In solvothermal method, the reactant ions and/or molecules react in solution which can lead to different final structures of products even if the same reactants are used with the melting method. In addition to this, solvothermal method can yield well-controlled nanomaterials with low energy consumption. In theoretical perspective, it is important to see the electronic band structure of doped and undoped SnTe, hence, density functional theory calculations are preformed to see the effect of suitable dopants including In, In/Sr, In/Ag on the band structure and density of states (DOS) of SnTe. The success of the solvothermal method in this thesis with synergistic band engineering and structure engineering is summarized as followsTo improve the thermoelectric performance of pristine SnTe, single In dopant was introduced to modify the crystal structure and band structure of SnTe. In dopant creates InTe nanoprecipiate in the Sn1-xInxTe matrix and the structure of this nanoprecipitates has been clearly identified by extensive transmission electron microscopy analysis. It is found that the structure of InTe (a = 6.14 A) is face-centred cubic which is the similar crystal structure with pristine SnTe (a = 6.32 A). These nanoprecipitates together with the point defects and grain boundaries significantly reduce lattice thermal conductivity to ~0.45 W m-1 K-1. Density functional theory calculation shows that the distortion of DOS (resonance energy level) near the Fermi level leads to enhanced seebeck coefficient (S) from ~23 mV K-1 to ~88 mV K-1. Finally, a high power factor of ~21.8 mW cm-1 K-2 and a corresponding figure of merit, a figure of merit (ZT) of ~ 0.78 have been obtained in Sn0.99In0.01Te at 773 K.The co-dopants of In and Cd with extra Te were used to further improve the thermoelectric performance of pristine SnTe. In and Cd rich nanoprecipitates can be found in the SnTe matrix and can significantly ameliorate the thermal transport properties. Pisarenko plot shows higher S in the In/Cd co-doped SnTe compared with the pristine SnTe, revealing that the significant valence band convergence and the resonant energy effect co-exist in the Sn(CdIn)xTe1+2x. Consequently, a high ZT of ~1.12 is obtained at 773 K in the p-type SnIn0.03Cd0.03Te1.06.A systematic theoretical and extensive experimental analyses have been successfully perceived in the In/Sr co-doped SnTe. The synergistic band and structure engineering significantly improve the electrical and thermal transport properties of Sn1-3xInxSr2xTe. The contribution of different phonon scattering centers such as grain boundaries, point defects and nanoprecipitates on the reduction of the lattice thermal conductivity has been well understood by using phonon modelling calculations. Besides, strain field associated with the In/Sr rich nanoprecipates is also analyzed by geometrical phase analysis (GPA). As a result, a record high power factor of ~33.88 mWcm-1K-2 and a peak ZT of ~1.31 has been achieved at 823 K for the Sn0.925In0.025Sr0.05Te pellet.A solvothermal synthesis method was developed to increase the solubility of In/Ag co-dopant in SnTe. It is has been found that In/Ag co-doping with appropriate dopant ratio (In:Ag = 1:2) can significantly improve the valence band convergence and resonant energy effect by using extensive density functional theory calculations. High-density strain field, dislocations, point defects and grain boundaries are observed in the matrix, which significantly scatter heat carrying phonons and yield low lattice thermal conductivity in a whole temperature range. Consequently, a high peak ZT of ~1.38 at 823 K has been achieved in Sn0.85In0.05Ag0.10Te, outperforming most of SnTe-based materials.In summary, this thesis successfully demonstrates the effectiveness of the facile and reliable solvothermal method for synthesising high-performance SnTe based thermoelectric materials by using synergistic band engineering and structure engineering. The simulation investigations on electronic transport and the phonon modeling fundamentally illustrate the effect of the proposed concepts, which will direct the future development of other thermoelectric system.

CuInTe2 and CuAlTe2, which are ternary chalcogenide compounds with the same tetragonal structure, are considered as thermoelectric materials owing to high Seebeck coefficients with large bandgaps of ∼1.08 and 1.96 eV, respectively. In this study, the electrical, thermal, and thermoelectric properties of a CuInTe2-CuAlTe2 solid solution alloy system were systematically investigated by synthesizing a series of CuIn1-xAlxTe2 (x = 0, 0.2, 0.4, 0.6, 0.8, and 1.0) compositions. CuInTe2 and CuAlTe2 form the complete solid solutions as reported, and the electrical conductivity and Seebeck coefficient decrease simultaneously to x = 0.8 due to a significant reduction in carrier mobility, thereby reducing the power factor. For CuAlTe2, the power factor suddenly increased owing to its very high electrical conductivity. On the other hand, the total and lattice thermal conductivity is greatly reduced by additional phonon scattering originating from solid-solution alloying. For instance, the largely reduced lattice thermal conductivity was measured to be 1.8 and 1.9 W m-1 K-1 for the sample with x = 0.4 and x = 0.6 at 300 K, whereas those for CuInTe2 and CuAlTe2 were 4.8 and 5.6 W m-1 K-1, respectively. Nevertheless, the thermoelectric figure of merit zT was significantly reduced by the solid solution alloying due to a significant reduction of power factors despite the reduction in thermal conductivity.

Transport and thermoelectric properties of Nb-doped FeV0.64Hf0.16Ti0.2Sb half-Heusler alloys synthesized by two ball milling regimes

Phonon scattering by point defects has been proven as an effective strategy for thermoelectric performance enhancements through reducing lattice thermal conductivity. This type of scattering largely relies on the mass and strain fluctuations between host and guest atoms, both of which can be maximized by vacancies as demonstrated in a few thermoelectric solid solutions showing a significant reduction in the lattice thermal conductivity. Here we show Cu2SnSe4, a new compound with intrinsic vacancies on the cation site, as a promising thermoelectric material due to the low lattice thermal conductivity of 0.6 W m–1 K–1 intrinsically resulting from the vacancies. A peak thermoelectric figure of merit (zT) of ∼0.6 is achievable in this compound without relying on additional approaches such as nanostructuring or band engineering. This result demonstrates the existence of intrinsic vacancies as an important guidance for exploring new thermoelectric materials.

Significantly improved thermoelectric properties of Nb-doped ZrNiSn half-Heusler compounds

Sb2Si2Te6 is a promising 2D material for medium-temperature thermoelectric applications, with the thermoelectric figure of merit zT approaching 1 at 823 K. However, its widespread use has been limited by relatively low power factor values. In this study, we successfully enhanced the performance of Sb2Si2Te6 by introducing Yttrium nanocomposites. This modification fine-tuned the carrier concentration and electrical conductivity, and increased the power factor up to 946 μW K-1 at 570 K. Jonker plot analysis revealed that increased carrier concentration did not affect the intrinsic electronic properties. SEM and TEM analyses revealed that Y nano-compositing introduced secondary phases, reducing the lattice thermal conductivity to values close to simulated ones using the Debye-Callaway model. Sb1.98Y0.02Si2Te6 exhibited the highest zT of 1.49 at 773 K due to the ultralow lattice thermal conductivity of 0.29 W m-1 K-1 and a moderate power factor of 858 μW K-1 at the same temperature. The single parabolic band (SPB) model suggests that with further optimization of the Fermi level and additional reduction in lattice thermal conductivity, the zT value could potentially increase to 1.55. These results demonstrate the potential of Y nanocompositing for enhancing Sb2Si2Te6 as an efficient medium-temperature thermoelectric material.

Compacts of 2N-lead telluride with grain sizes of <5 μm and <0.5 μm have been prepared by hot pressure sintering at a temperature of approximately 1100 K and pressure of 10 MPa. The densities of the compacts were in excess of 8.0 gm cm-3 compared with the “single crystal” value of 8.25 gm cm-3. Compacts having densities closer to “singe crystal” values can be obtained but they exhibit cracks along planes perpendicular to the pressing direction. The room temperature thermal conductivity of 2N-lead telluride with a grain size <0.5 μm it is about 9% lower than equivalent “single crystal” material. After allowing for porosity effects the reduction in thermal conductivity is in excess of 8%. This experimental data lends support to the predicted reduction in lattice thermal conductivity with grain size. Within the experimental error, the Seebeck coefficient of annealed samples does nos change with reduction in grain size. However, both the Seebeck coefficient and electrical resistivity of “as compacted” material are significantly higher than single crystal material. It is concluded that the thermoelectric figure of merit of materials based upon lead telluride could be improved through the use of very small grain size material, provided the electrical properties can be retained close to single crystal values.

Enhancing the thermoelectric (TE) performance of materials requires simultaneous optimization of the power factor (PF) and reduction of thermal conductivity (κ), a challenging task due to their interdependent nature. This work demonstrates a concurrent enhancement of PF and reduction of κ in p-type half-Heusler compound, ZrCoBi. Using ab initio electronic structure calculations, we investigate the impact of isotropic tensile and compressive strains on the TE properties of materials. Our analysis encompasses electronic structure, mechanical properties, lattice dynamics, Seebeck coefficient, electrical and thermal conductivity, PF, and the figure of merit (ZT). The results reveal a unique strain-dependent behavior in the electronic properties of ZrCoBi, including a spurious band convergence effect, as identified through first-principles calculations incorporating spin-orbit coupling (SOC). Furthermore, the analysis shows that tensile strain effectively tunes the band gap, significantly altering the electronic structure and improving PF. At the same time, tensile strain induces significant changes in lattice dynamics, reducing lattice thermal conductivity (κL). These combined effects result in a fourfold increase in ZT at high temperatures. This study also provides a comprehensive framework for leveraging isotropic strain to achieve high-performance thermoelectric materials and guides the experimental exploration of enhanced ZT compounds.

The limitation of the single crystal melt growth method to tune the microstructure of the materials in a controlled way and the need for enhancing the thermoelectric properties of single crystal grown Bismuth telluride (Bi 2 Te 3 ), through defect and microstructural engineering, has motivated this work. In this work, we address this limitation through a controlled doping strategy using antimony (Sb) and selenium (Se) to introduce targeted defects and microstructural modifications within single-crystalline Bi 2 Te 3 . Sb and Se substitutions create atomic scale strain, point defects, and micro-grain structures, enhancing phonon scattering without significantly disrupting the crystalline order. The resulting defect-engineered single crystals exhibit improved thermoelectric performance, with a notable reduction in lattice thermal conductivity and retention of excellent electrical properties. The co-doped compositions, Bi 2 Te 2.7 Se 0.3 and (Bi0.98Sb 0.02 ) 2 Te 2.7 Se 0.3 , exhibited significantly enhanced thermoelectric performance, with Seebeck coefficients reaching ~ 253 μV/K and − 211 μV/K, respectively, over the 10–400 K range. The power factor improved remarkably, showing a ~ 30-fold increase for Bi 2 Te 2.7 Se 0.3 and ~ 20-fold for the Sb-doped variant, while the figure of merit (ZT) improved by ~ 28.5 and ~ 14 times, respectively. Further, a flexible thermoelectric device fabricated from these optimized materials generated output power of 2.7 nW and 3.35 nW at ambient temperature. The non-monotonic variation of the Seebeck coefficient with Sb content, showing an optimal enhancement at x = 0.04, highlights the delicate balance between carrier concentration and band structure modification, emphasizing moderate Sb substitution achieves the most favorable conditions for thermoelectric performance. Our results present a scalable strategy for bridging the performance gap between pristine single crystals and heavily nanostructured thermoelectrics, opening new avenues for high-efficiency energy harvesting devices.

Striving for improvements of the thermoelectric (TE) properties of the technologically important lead telluride (PbTe) compound, we investigate the influence of different doping elements on the thermal conductivity, Seebeck coefficient, and electrical conductivity applying density functional theory calculations. Our approach combines total-energy calculations yielding lattice vibrational properties with the Boltzmann transport theory to obtain electronic transport properties. We find that doping with elements from the 1st and 3rd columns of the periodic table reduces the sound velocity and, consequently, the lattice thermal conductivity, while 2nd column dopants have no such influence. Furthermore, 1.6 at. % doping with 4th and 5th column elements provides the highest reduction of lattice thermal conductivity. Out of this group, Hf doping results in maximum reduction of the sound velocity from 2030 m s−1 for pure PbTe to 1370 m s−1, which is equivalent to ca. 32% reduction of lattice thermal conductivity. The highest power factor values calculated for 1.6 at. % doping range between 40 and 56 μW cm−1 K−2, and are obtained for substitution with dopants having the same valence as Pb or Te, such as those located at the 2nd, 14th, and 16th columns of the periodic table. We demonstrate how this method may be generalized for dopant-selection-oriented materials design aimed at improving TE performance of other compounds.

Nanocomposites have become a new paradigm for thermoelectric research in recent years and have resulted in the reduction of thermal conductivity via the nano-inclusion and grain boundary scattering. In this work, we report the preparation and thermoelectric study of SiGe-yttria stabilized zirconia (YSZ) nanocomposites prepared by Spark Plasma Sintering (SPS). We experimentally investigated the reduction of lattice thermal conductivity (κL) in the temperature range (30–800 K) of n-type Si80Ge20P2 alloys with the incorporation of YSZ nanoparticles (20–40 nm diameter) into the Si-Ge matrix. These samples synthesized by using the SPS technique were found to have densities &amp;gt; 95% of the theoretical density. The thermal conductivity, at both low and high temperatures, was measured by steady state and laser flash techniques, respectively. At room temperature, we observed approximately a 50% reduction in the lattice thermal conductivity as result of adding 10% YSZ by volume to the Si80Ge20P2 host matrix. A phenomenological model developed by Callaway was used to corroborate both the temperature dependence and reduction of κL over the measured temperature range (30–800 K) of both Si80Ge20P2 and Si80Ge20P2 + YSZ samples. The observed κL is discussed and interpreted in terms of various phonon scattering mechanisms such as alloy disorder, the Umklapp phonon scattering, and boundary scattering. In addition, a contribution from the phonon scattering by YSZ nanoparticles was further included to account for the κL of Si80Ge20P2 + YSZ sample. The theoretical calculations are in reasonably good agreement with the experimental results for both the Si80Ge20P2 and Si80Ge20P2 + YSZ alloys.

Phonon and electronic transport of buckled structured SiGe monolayer and halogenated SiGe monolayers (X2-SiGe, X = F, Cl, and Br) are investigated for the first-time using ab initio density functional theory (DFT). The phonon calculations reveal complete dynamical stability of SiGe and fluorinated (F2-SiGe) monolayers in contrast to earlier reported works, where a small magnitude of imaginary frequency in SiGe monolayer near the zone centre of the Brillouin zone (BZ) is observed. The phonon calculations of chlorinated and brominated SiGe reveal no dynamical stability even with very high convergence parameters and better computational accuracy. The lower value of lattice thermal conductivity in the case of F2-SiGe is attributed to the strong phonon anharmonic scattering and larger contribution of the three phonon process to anharmonic scattering. The semimetallic nature of the SiGe monolayer turns to semiconducting after halogenation. We have also calculated the electron relaxation time to study their precise thermoelectric parameters. The enhancement of the Seebeck coefficient and reduction in lattice thermal conductivity in the SiGe monolayer is observed after halogenation which results in the improvement of the thermoelectric figure of merit (ZT). The room temperature figure of merit, ZT, which is 0.112 for the SiGe monolayer, enhances significantly to 0.737 after addition of fluorine atoms. Our study suggests that the halogenation of two-dimensional materials can improve their thermoelectric properties.

Theoretical thermoelectric figure of merit of highly disordered lead telluride type materials

Surface and strain engineering are among the cheaper ways to modulate structure property relations in materials. Due to their compositional flexibilities, MXenes, the family of two-dimensional materials, provide enough opportunity for surface engineering. In this work, we have explored the possibility of improving thermoelectric efficiency of MXenes through these routes. The Janus MXenes obtained by modifications of the transition metal constituents and the functional groups passivating their surfaces are considered as surface engineered materials on which bi-axial strain is applied in a systematic way. We find that in the three Janus compounds Zr2COS, ZrHfCO2and ZrHfCOS, tensile strain modifies the electronic and lattice thermoelectric parameters such that the thermoelectric efficiency can be maximised. A remarkable reduction in the lattice thermal conductivity due to increased anharmonicity and elevation in Seebeck coefficient are obtained by application of moderate tensile strain. With the help of first-principles electronic structure method and semi-classical Boltzmann transport theory we analyse the interplay of structural parameters, electronic and dynamical properties to understand the effects of strain and surface modifications on thermoelectric properties of these systems. Our detailed calculations and in depth analysis lead not only to the microscopic understanding of the influences of surface and strain engineering in these three systems, but also provide enough insights for adopting this approach and improve thermoelectric efficiencies in similar systems.