Advanced GeSe-based thermoelectric materials: Progress and future challenge

A powerful economy and a sustainable society are based upon the available energy resources. A nonrenewable energy source is limited in its availability as a result of high consumption rates, output efficiency, and when used at large scale generates a large amount of waste heat and toxic gases. The eco-friendly and long lasting human society has always worked hard to find an alternate source of renewable and clean energy. Material scientists have found possible solutions with the discovery of thermoelectric materials that utilize the waste heat and convert it into useful electricity. This chapter focuses on thermoelectric (TE) materials. The content of this chapter is presented with the assumption that readers have a basic understanding of material’s structural, chemical, and transport properties, as well as the mechanisms responsible for displaying these properties based on computational results. We begin the chapter with an overview of thermoelectric materials and their applications in waste heat energy harvesting. The physical parameters and selection criteria for screening high-performance TE materials are also discussed. To gain a better understanding of TE properties, a brief description of the computational and experimental methods for investigating a material’s crystal structure, electronic and heat transport properties is provided before classifying the category of different materials. With a better understanding of materials properties, a wide range of emergent materials, discovered in recent past years, with high TE performance over a wide temperature range, are covered. We concentrated on the TE properties of chalcogenides, Si–Ge-based alloys, and oxides, in this chapter. For the realization of practical applications, everything from materials to device fabrication and output performance is elaborated. The limitations of conventional approaches and commonly available tools used in TE research, as well as potential solutions, are discussed, paving the way for further investigation of the exceptional properties governed by unusual crystal and electronic structure of materials. Finally, we present a summary of thermoelectric materials as a potential candidate for renewable energy sources in next-generation technological applications.KeywordsThermoelectricityThermoelectric generatorsTransport propertiesThermal diodesEnergy materials

Achieving an excellent thermoelectric performance in nanostructured copper sulfide bulk via a fast doping strategy

The performance of a thermoelectric material is given by its so-called figure-of-merit z ˆ AE2o=o, where AE is the Seebeck coefficient, and o and o are the electrical and thermal conductivities, respectively. Since the pioneering work of Ioffe et al. [1] efforts of researchers have concentrated on identifying and developing materials with higher figures-ofmerit in order to increase the competitiveness of thermoelectric devices in generation and refrigeration applications. Crucial to the search for improved thermoelectric materials is the provision of realistic guidelines to assist in identifying materials combinations, structures and mechanisms, which can increase the thermoelectric figure-of-merit. Over the past 40 years an improved understanding of electrical and thermal transport processes has resulted in the establishment of some guidelines based upon a deterministic approach. These include: the use of point defects as a phonon scattering mechanism to increase the ratio of electrical to thermal conductivities [1]; control of the energy band gap to determine suitable operating temperature ranges of thermoelectric materials [2]; use of the material parameter in assessment of thermoelectric properties [3]; and, more recently, the study of quantum well structures [4] and multiple potential barriers [5] in identifying desirable materials structures. An alternative approach outlined in this letter, which may prove fruitful, is based upon a statistical analysis of the physical properties of known materials to identify materials’ trends which can be used to identify new thermoelectric materials. It has been reported that the thermal conductivity appears to depend upon the materials’ atomic weight and melting point [6, 7]; while the electrical conductivity exhibits some correlation with the electronegativity difference of constituent elements [8]. Recently, progress has also been reported in identifying crystal structures using a similar phenomenological approach based on a so-called ‘‘quantum structural diagram’’ (QSD) [9–12]. The applications of QSDs have also provided encouraging results in predicting ferroelectric, high-Tc superconductors and stable quasicrystals [13–15]. In this work, a similar QSD approach has been employed to identify trends in materials and provide guidelines in the search for new thermoelectric materials. The difference in electronegativities, X, of two atoms in a chemical bond determines how the electrons in the bond are shared and thus one can use the electronegativity difference to predict the type of chemical bonds. It is expected that the crystal structure and physical properties are affected by the electronegativity difference. Previous investigations by Slack [8] showed that some properties such as energy band gap and ‘‘weighted mobility’’, correlate with the electronegativity difference. As a result, X < 0.5 has been proposed as an initial screening value in the search for good thermoelectric materials. Since the electrical power factor, AE2o, is affected significantly by the ‘‘weighted mobility’’ [16], it is also expected to be related to the electronegativity difference. Fig. 1 shows the electrical power factor of bismuth based binary compounds and alloys as a function of the electronegativity difference. The materials were prepared using mechanical alloying followed by cold pressing and sintering. (Although materials prepared by the mechanical alloying exhibit thermoelectric properties which are, in general, inferior to corresponding ‘‘single crystals’’, they are satisfactory for use in comparative studies.) The Seebeck coefficient and electrical conductivity were measured at room temperature using hot probe and four probe apparatus, respectively. Binary compounds and alloys based upon tellurium, antimony or germanium have also been investigated [17]. The electrical power factors of those binary compounds and alloys exhibit a similar trend to that shown in Fig. 1. The results

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.

Driven by the ability to harvest waste heat into reusable electricity and the exclusive role of serving as the power generator for deep spacecraft, intensive endeavors are dedicated to enhancing the thermoelectric performance of ecofriendly materials. Herein, the most recent progress in superhigh-performance GeTe-based thermoelectric materials is reviewed with a focus on the crystal structures, phase transitions, resonant bondings, multiple valance bands, and phonon dispersions. These features diversify the degrees of freedom to tune the transport properties of electrons and phonons for GeTe. On the basis of the optimized carrier concentration, strategies of alignment of multiple valence bands and density-of-state resonant distortion are employed to further enhance the thermoelectric performance of GeTe-based materials. To decrease the thermal conductivity, methods of strengthening intrinsic phonon-phonon interactions and introducing various lattice imperfections as scattering centers are highlighted. An overview of thermoelectric devices assembled from GeTe-based thermoelectric materials is then presented. In conclusion, possible future directions for developing GeTe in thermoelectric applications are proposed. The achieved high thermoelectric performance in GeTe-based thermoelectric materials with rationally established strategies can act as a reference for broader materials to tailor their thermoelectric performance.

Semiconductor InSe with wide bandgap and layered crystal structure is expected to be a promising thermoelectric material, and its excellent plasticity brings great potential applications in flexible and wearable thermoelectric devices. To advance its thermoelectric performance, this work systematically investigates the carrier and phonon transport properties in n‐type InSe. It is found that InSe compound presents an exceptional dynamic carrier transport property due to the amphoteric indium (In+ and In3+), which contributes to favorable temperature‐dependent increasing carrier concentration. More importantly, with S alloying in InSe, the carrier concentration can be further enhanced from ∼3.2 × 1013 cm–3 in InSe to ∼4.8 × 1015 cm–3 in InSe0.97S0.03 at 300 K, because S (χP ∼ 2.58) with larger Pauling electronegativity than Se (χP ∼ 2.55) can induce more In3+ state to increase carrier concentration in matrix. This boosted dynamic carrier transport property benefits an obviously enhanced power factor. Additionally, InSe compound presents intrinsically low thermal conductivity ∼1.6 W m–1 K–1 at 300 K due to low‐symmetry crystal structure and strong anharmonicity. This work indicates that the special dynamic carrier transport property and intrinsically low thermal conductivity in InSe make it as a worth‐expecting thermoelectric material.

Dopant‐induced microstructure in thermoelectric materials significantly affects thermoelectric properties and offers a potential to break the interdependence between electron and phonon transport properties. However, identifying all‐scale dopant‐induced microstructures and correlating them with thermoelectric properties remain a huge challenge owing to a lack of detailed microstructural characterization encompassing all length scales. Here, the hierarchical boron (B)‐induced microstructures in B‐doped Si80Ge20 alloys with different B concentrations are investigated to determine their precise effects on thermoelectric properties. By adopting a multi‐scale characterization approach, including X‐ray diffraction, scanning and transmission electron microscopy, and atom probe tomography, five distinctive B‐induced phases within Si80Ge20 alloys are identified. These phases exhibit different sizes, compositions, and crystal structures. Furthermore, their configuration is comprehensively determined according to B doping concentrations to elucidate their consequential impact on the unusual changes in carrier concentration, density‐of‐states effective mass, and lattice thermal conductivity. The study provides insights into the intricate relationship between hierarchical dopant‐induced microstructures and thermoelectric properties and highlights the importance of investigating all‐scale microstructures in excessively‐doped systems for determining the precise structure‐property relationships.

An insight into β-Zn4Sb3 from its crystal structure, thermoelectric performance, thermal stability and graded material

Facing the growing scarcity of traditional fossil fuels and the inefficiency of energy utilization, thermoelectric materials have garnered increasing attention due to their ability to convert between electrical and thermal energy. However, the strong coupling between thermoelectric parameters presents a significant challenge for simultaneously reducing thermal conductivity and maintaining electrical performance in bulk materials. The solution-based synthesis of nanostructured materials offers a promising approach for the decoupling regulation of electronic and phonon transport properties by regulating grain size and morphology, second phases, and surface ligands. The strategies for optimizing thermoelectric performance outlined above are founded upon several pivotal elements: the enhancement of grain boundary effects, precise regulation of grain stacking, utilization of heterogeneous interface effects, and generation of metastable phases and novel structural configurations facilitated by ligand management approaches. We have also comprehensively addressed the challenges associated with solution-based synthesis, particularly material oxidation and grain coarsening, along with their corresponding mitigation strategies. In addition, machine learning can effectively accelerate solution synthesis and the exploration of composite materials. This review summarizes and generalizes the research related to these strategies, providing recommendations for future research directions based on observed trends.

Impact of defect migration on electrical and dielectric properties in molten salt synthesized CaCu3Ti4O12 and customizing the properties by compositional engineering with Mg doping

Enhanced thermoelectric performance of two dimensional MS2 (M = Mo, W) through phase engineering

Inspired by recent experimental synthesis of prototypical Sn(1−x)Pb(x)O under ambient and high-temperature conditions, we systematically construct layered Sn(1−x)Pb(x)O alloys across a range of compositions. Herein, we study the thermoelectric properties of layered Sn(1−x)Pb(x)O compositions using the first-principles-based density functional theory and density functional perturbation theory calculations. Our results show that Sn(1−x)Pb(x)O compositions exhibit very low lattice thermal conductivity (κl) of 2.94 and 1.83 W/m K for x = 0.5 and 1, respectively, at 300 K. Such remarkably low κl of Sn0.5Pb0.5O and PbO primarily originates from pronounced phonon softening and coupling of phonon branches driven by the presence of mass anisotropy and symmetry breaking induced by Pb substitution. Furthermore, in our present study, the inclusion of Fröhlich interaction addresses limitations in the conventional deformation potential theory formalism, allowing for a more rigorous and accurate assessment of carrier mobility. Interestingly, Sn0.5Pb0.5O reveals superior Seebeck values for both charge types, with 1127.4 μV/K (p-type) and −1078.4 μV/K (n-type) at 300 K, compared to SnO and PbO, attributed to its higher band effective mass and moderate carrier concentration within the system. Moreover, our results reveal that Pb substitution in Sn0.5Pb0.5O benefits n-type transport by increasing carrier concentrations; however, it adversely affects p-type conductivity due to heavier hole effective masses and reduced mobility. Henceforth, leveraged by the superior phononic and electronic transport properties, Sn0.5Pb0.5O composition emerges as a superior n-type thermoelectric material that outperforms the other two systems, as its ZT reaches a significant maximum value of 3.19 at 900 K.

Chemical bonds determine electron and phonon transport in solids. Tailoring chemical bonding in thermoelectric materials causes desirable or compromise thermoelectric transport properties. In this work, taking an example of CaMg2 Bi2 with covalent and ionic bonds, density functional theory calculations uncover that element Zn, respectively, replacing Ca and Mg sites cause the weakness of ionic and covalent bonding. Electrically, Zn doping at both Ca and Mg sites increases carrier concentration, while the former leads to higher carrier concentration than that of the latter because of its lower vacancy formation energy. Both doping types increase density-of-state effective mass but their mechanisms are different. The Zn doping Ca site induces resonance level in valence band and Zn doping Mg site promotes orbital alignment. Thermally, point defect and the change of phonon dispersion introduced by doping result in pronounced reduction of lattice thermal conductivity. Finally, combining with the further increase of carrier concentration caused by Na doping and the modulation of band structure and the decrease of lattice thermal conductivity caused by Ba doping, a high figure-of-merit ZT of 1.1 at 823 K in Zn doping Ca sample is realized, which is competitive in 1-2-2 Zintl phase thermoelectric systems.

Understanding and controlling vibrations in condensed matter is emerging as an essential necessity both at fundamental level and for the development of a broad variety of technological applications. Intelligent design of the band structure and transport properties of phonons at the nanoscale and of their interactions with electrons and photons impact the efficiency of nanoelectronic systems and thermoelectric materials, permit the exploration of quantum phenomena with micro- and nanoscale resonators, and provide new tools for spectroscopy and imaging. In this colloquium we assess the state of the art of nanophononics, describing the recent achievements and the open challenges in nanoscale heat transport, coherent phonon generation and exploitation, and in nano- and optomechanics. We also underline the links among the diverse communities involved in the study of nanoscale phonons, pointing out the common goals and opportunities.

Enhanced thermoelectric properties of earth-abundant Cu2SnS3 via In doping effect