High-performance SnTe-based thermoelectric materials via synergistically band and nanostructure engineering

GeTe Thermoelectrics

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.

Band engineering and rational design of high-performance thermoelectric materials by first-principles

This review discusses recent developments and current research in high performance bulk thermoelectric materials, comprising nanostructuring, mesostructuring, band alignment, band engineering and synergistically defining key strategies for boosting the thermoelectric performance. To date, the dramatic enhancements in the figure of merit achieved in bulk thermoelectric materials have come either from the reduction in lattice thermal conductivity or improvement in power factors, or both of them. Here, we summarize these relationships between very large reduction of the lattice thermal conductivity with all-scale hierarchical architecturing, large enhanced Seebeck coefficients with intra-matrix electronic structure engineering, and control of the carrier mobility with matrix/inclusion band alignment, which enhance the power factor and reduce the lattice thermal conductivity. The new concept of hierarchical compositionally alloyed nanostructures to achieve these effects is presented. Systems based on PbTe, PbSe and PbS in which spectacular advances have been demonstrated are given particular emphasis. A discussion of future possible strategies is aimed at enhancing the thermoelectric figure of merit of these materials.

Solvothermal synthesis of cerium oxides

Solvothermal preparation and gas permeability of an IRMOF-3 membrane

Charge carrier and phonon dynamics tuning are promising approaches for transport property modulation. Improvement of thermoelectric (TE) parameters via atomic substitution and band engineering is prevalent. Besides, carrier effective mass (m*) is a crucial factor that has a noteworthy influence on TE properties. Transition metals (TMs), possessing outstanding valence electronic properties and distinctive electronic state distributions, have recently been used as potential candidates for enhancing TE performance. Here, we report the structural, electronic, and TE characteristics of p-type Sb2Te3 by TM (=Fe, Co, Ni) doping. Consequently, the synergistic amelioration of electrical and thermal transport properties is elucidated. Structural and phonon vibrational properties are characterized by synchrotron powder x-ray diffraction and Raman spectroscopic (RS) measurements. Raman peak position and full-width at half-maximum provide insight into electron–phonon interactions and phonon anharmonicity. Anharmonic phonon–phonon interaction is illustrated via a four-phonon decay model. Furthermore, optothermal RS measurement is used to estimate the thermal conductivity κ of the samples. A reduction in the lattice thermal conductivity, κL, is observed after TM doping. Experimentally measured transport parameters, viz., S(T), ρ(T), and nH(T) are simulated via the Boltzmann transport equation (BTE), and reasonable quantitative agreement between the experimental and simulated data is obtained. The role of m* and valence band convergence, as estimated from BTE analysis, is highlighted. Weighted mobility and m* are found to be increased, significantly enhancing the power factor in the Co-doped sample. As compared to its pristine counterpart, around three times the improvement of the ZT value in Sb1.97Co0.03Te3 is reported.

Among thermoelectric materials, tin telluride has been attracting significant interest for its nontoxic and eco-friendly nature. Meanwhile, band engineering and ultrathin film technologies have been reported to be promising in improving the performance of thermoelectric materials. In this work, we explore the potential of two-dimensional (2D) SnTe’s thermoelectric performance, following the recent successful synthesis of 2D SnTe. It is found that pristine 2D SnTe is more likely to be a p-type semiconductor due to the existence of Sn vacancies. The electrical transport properties of 2D SnTe when it is (i) under compressive stress and (ii) doped with either nitrogen group or halogen group elements (including As, Sb, Bi, Br, and I) have been studied from first-principles electronic structure calculations. Boltzmann transport study illustrates that equibiaxial compressive stress may enhance the electrical transport properties of 2D SnTe. Moreover, our calculations suggest that iodine and arsenic can be effective n-type and p-type dopants, respectively.

Structural and morphological studies of Se doped SnTe thermoelectric material

A facile and effective method is developed for preparing ZnO twins with perfect mirror symmetry through the assembly of the supermolecular compound julolidine in N,N-diethylformamide by a solvothermal synthesis method.

Thermoelectric materials can generate electricity by harnessing the temperature gradient and lowering the temperature through applying electromotive force. Lead chalcogenides based materials, especially PbTe-based ones, have shown extremely high thermoelectric performance. PbSe has a similar crystal structure and band structure to PbTe. Compared with the commonly-used PbTe, PbSe possesses a high melting point and has an abundant reserve of Se, making it attractive to high temperature thermoelectric applications. It has been theoretically proposed that Mn-doping in lead chalcogenide should be able to lower the temperature of band degeneracy, and experimental evidences have been represented in Mn-PbTe. However, such an experimental study as well as the investigations of influences of Mn on microstructure, mechanical, electrical and thermal properties has not been conducted in Mn-PbSe. In this work, Pb0.98-xMnxNa0.02Se (0 x 0.12) materials are prepared by the melting-quenching techniques combined with rapid hot-press sintering. Effects of Mn doping on the microstructures, mechanical and thermoelectric properties of PbSe samples are systematically studied. The refined lattice parameters from X-ray powder diffraction patterns show that the solubility of Mn in the matrix is in a range from 0 to 0.04. The back-scattered electron images and elemental maps reveal that the MnSe-rich impurity phases exist in the PbSe matrix, which makes the PbSe-MnSe system a nano-composite system. Pb0.96Mn0.02Na0.02Se has also such microstructures, implying that the solubility of Mn should be below 0.02. Cubic-phase MnSe-rich precipitates have the sizes ranging from 50 nanometers to 1-5 micrometers. They are well dispersed in the PbSe-rich matrix, as round or layered microstructures. The mechanical properties of the nanocomposites can be determined by micro-hardness measurements. Interestingly, the average Vickers hardness values of the PbSe-MnSe nanocomposites are significantly improved, which are 16.6% and 51.6% harder respectively in x= 0.02 and 0.06 samples than those of pristine PbSe. Smaller Mn content can optimize the figure of merit ZT due to the band convergence and additional phonon scattering by precipitates, while higher Mn content has little influence on ZT because of the saturated Seebeck coefficient and anomalous increase in lattice thermal conductivity. As a result, the highest figure of merit is 0.52 at 712 K, which is achieved in the Pb0.96Mn0.02Na0.02Se sample. By further adjusting the Na content from 2% to 0.7%, the carrier concentration is optimized. Thus, the Seebeck coefficient and power factor become higher. A figure of merit of 0.65 is achieved at 710 K in the PbSe-MnSe nano-composite with a nominal composition of Pb0.973Mn0.02Na0.007Se. We suggest that further optimizing the electrical properties may achieve a higher thermoelectric performance in the PbSe-MnSe system.

Thermoelectric materials based on porous hybrid and MOFs constitute a very recent development, with many new and undiscovered phenomena for the research field. These porous Hybrid materials and MOF (Metal-Organic-Framework) films represent modern designer materials that exhibit many requirements of a near ideal and tunable future thermoelectric (TE) material. It is anticipated that especially the inherent porosity can be used advantageously in the design and simulation predictions of future thermoelectric materials. In principle, porous Hybrid and MOFs materials can comply with most of the requirements of an ideal and tunable future thermoelectric (TE) material. In contrast to traditional semiconducting bulk thermoelectric materials, porous hybrid MOF templates can be used to overcome some of the constraints of physics in bulk thermoelectric materials. These porous hybrid systems are amenable for simulation and modeling to design novel optimized electron-crystal phonon-glass materials with potentially very high Figure of Merit (ZT) numbers. Porous MOF and Hybrid materials possess an ultra-low thermal conductivity, which can be further modulated by phonon engineering within their complex porous and hierarchical architecture to advance the thermoelectric figure of merit ZT. This class of novel materials offers new approaches to the design of complex and hierarchical porous structures, possessing ultra-low thermal conductivity, which are able to trap, and localize phonons, thus resulting in a new generation of thermoelectric materials with a high ZT number. This work is demonstrating recent results of MOF thermoelectric Materials and provides a future outlook to the search for the next generation thermoelectric porous Hybrid and MOF materials, which could be part of a green renewable energy revolution with novel materials of sustainable high ZT values.

Thermoelectric materials enables the harvest of waste heat and directly conversion into electricity. In search of high efficient thermoelectric materials, low thermal conductivity of a material is essential and critical. Here, we have theoretically investigated the lattice thermal conductivity and thermoelectric properties of layered intermetallic Na$_2$MgSn and Na$_2$MgPb based on the density functional theory and linearized Boltzmann equation with the single-mode relaxation-time approximation. It is found that both materials exhibit very low and anisotropic intrinsic lattice thermal conductivity. Despite of the very low mass density and simple crystal structure of Na$_2$MgSn, its lattice thermal conductivities along $a$ and $c$ axes are only 1.75 and 0.80 W/m$\cdot$K respectively at room temperatures. When Sn is replaced by the heavier element Pb, its lattice thermal conductivities decrease remarkably to 0.51 and 0.31 W/m$\cdot$K respectively along $a$ and $c$ axes at room temperatures. We show that the low lattice thermal conductivities of both materials are mainly due to their very short phonon lifetimes, which are roughly between 0.4 to 4.5 ps. Combined with previous experimental measurements, the metallic Na$_2$MgPb can not be a good thermoelectric material. However, we predict that the semiconducting Na$_2$MgSn is a potential room-temperature thermoelectric material with a considerable $ZT$ of 0.34 at 300 K. Our calculations not only imply that the intermetallic Na$_2$MgSn is a potential thermoelectric material, but also can motivate more theoretical and experimental works on the thermoelectric researches in simple layered intermetallic compounds.

Half-Heusler (HH) compounds, with a valence electron count of 8 or 18, have gained popularity as promising high-temperature thermoelectric (TE) materials due to their excellent electrical properties, robust mechanical capabilities, and good high-temperature thermal stability. With the help of first-principles calculations, great progress has been made in half-Heusler thermoelectric materials. In this review, we summarize some representative theoretical work on band structures and transport properties of HH compounds. We introduce how basic band-structure calculations are used to investigate the atomic disorder in n-type MNiSb (M = Ti, Zr, Hf) compounds and guide the band engineering to enhance TE performance in p-type FeRSb (R = V, Nb) based systems. The calculations on electrical transport properties, especially the scattering time, and lattice thermal conductivities are also demonstrated. The outlook for future research directions of first-principles calculations on HH TE materials is also discussed.

Understanding and manipulating magnetic damping, particularly in magnetic heterostructures, is crucial for fundamental research, versatile engineering, and optimization. Although magnetic damping can be enhanced by the band hybridization between ferromagnetic and nonmagnetic materials at the interface, the contribution of individual subbands on the hybridized bands to magnetic damping is fully unexplored. Here, it is found that magnetic damping αeff is modified by the Fermi level in Fe/GeTe heterostructures via Bi doping. By combining angle‐resolved photoemission spectroscopy and density functional theory calculations, the enhancement of damping originated from the strongly hybridized band structures between Fe and the surface Rashba bands of GeTe are unveiled. More interestingly, the Fermi level modulates the density of states (DOS) ratio between the subbands of GeTe and the total DOS of hybridized states, which is directly proportional to the magnetic damping. This work gives an insightful physical understanding of the magnetic damping influenced by the hybridized band structures and opens a novel avenue to manipulate magnetic damping by band engineering.