Session L29: Thermoelectrics III

The High-throughput First-principles Materials Informatics Platform and the Applications on Thermoelectric Materials

In recent years, high throughput material screenings, represented by the materials genome initiative proposed in 2011, have been ever important in the search of novel functional materials. These methods aim at new candidates with top performance, which tremendously speed up the material research. As for the high throughput work for the thermoelectric (TE) application, researchers usually focused on the proposals of new TE candidates based on some simplified high throughput algorithms for electrical and thermal transports. Another topic is the development of the method to compute the scattering terms based on the generated big data. In the Materials Genome Institute of Shanghai University, we build up our own high-throughput platform, Material Informatics Platform (MIP). It will have automated calculations from the structural relaxation all the way to the evaluation of transport properties for TE applications. The first mission of MIP is to screen new TE candidates in chalcogenides with diamond-like structures. A universal conductive network dominated by the anion sublattice has been revealed. Because of this, these diamond-like chalcogenides have close power factors. New candidates are then proposed based on the analysis of other factors, and one particular compound has been experimentally verified to possess dimensionless ZT over unity.   

Screening for potential thermoelectric materials using an electronic fitness function

High performance thermoelectric (TE) materials require materials with complex electronic structures which may come from complex iso-energy surfaces, multiple valleys, valley anisotropy, heavy-light band combination and band convergence. Here we present a simple transport function that can be directly evaluated based on the first-principles electronic structures and the Boltzmann transport theory and it is a universal function that incorporates all these features. We applied the transport function for the screening of 75 potential TE materials and we find an efficient screening with the transport function. The well-known binary TE materials (GeTe, PbTe) can be efficiently screened amomg all the candidates. Moreover, we also identified two alkali metal Zintl phases (Na₂AuBi, KSnSb), two half-Heuslers (RhNbSn,IrNbSn) and one full-Heusler, Li₂NaSb with favorable electronic properties.
  

Atomistic Study of the Electronic Contact Resistivity Between the Half-Heusler Alloys ( HfCoSb, HfZrCoSb and HfZrNiSn ) and the Metal Ag

Half-Heusler(HH) alloys have shown promising thermoelectric properties in the medium and high temperature range. To harness these material properties for thermoelectric applications, it is important to realize electrical contacts with low electrical contact resistivity. However, little is known about the detailed structural and electronic properties of such contacts, and the expected values of contact resistivity. Here, we employ atomistic ab initio calculations to study electrical contacts in a subclass of HH alloys consisting of the compounds HfCoSb, HfZrCoSb, and HfZrNiSn. By using Ag as an example metal, we show that the termination of the HH material critically determines the presence or absence of strong deformations at the interface. We also study the cases of contacts to the p-type and n-type materials, and our results indicate that the p-type materials generally form ohmic contacts while the n-type materials have a small Schottky barrier. We predict the temperature dependence of the contact resistivity as well as quantitative values that set lower limits for these systems.   

Quartic Anharmonicity of Rattlers and its Effects on Exceptional Thermal Transport in Intermetallic Clathrates: A First-Principles Investigation

The rattling atoms in host-guest structures, such as clathrates and skutterudites, cause many intriguing thermal properties including low-frequency vibrational modes showing strong temperature (T) dependence and very low lattice thermal conductivity κ_L. While small κ_L values of host-guest systems can mainly be attributed to the strong Umklapp scattering induced by rattlers [1], the atomistic origin of anomalous T-dependence of κ_L still remains unclear. In this study, we have investigated the role of the strong quartic anharmonicity of rattlers in lattice dynamics and thermal transport of type-I clathrate Ba₈Ga₁₆Ge₃₀ based on ab initio self-consistent phonon calculations [2]. We show that the strong quartic anharmonicity of rattling guest atoms causes the hardening of vibrational frequencies of low-lying optical modes and thereby affects calculated lattice thermal conductivities κ_L significantly, resulting in an improved agreement with experimental results including the deviation from κ_L∝ T at high temperature [3]. The origin of the glasslike κ_L is also discussed.
[1] T. Tadano, Y. Gohda, and S. Tsuneyuki, Phys. Rev. Lett. 114, 095501 (2015).;[2] T. Tadano and S. Tsuneyuki, Phys. Rev. B 92, 054301 (2015).; [3] T. Tadano and S. Tsuneyuki, arXiv:1710.0031.   

Prediction of Fe2TiSi Alloys as Efficient p-and-n-type Thermoelectrics at Low Temperatures via Explicit Treatment of Electron-phonon Scattering

Using first-principles methods based on DFT, we predict that alloys of Fe₂TiSi full-Heusler compound are potentially very efficient thermoelectric materials at low temperatures from 100 to 400 K. With about 4% of Ti replaced with Hf, p-type zT around 1 could be achieved, as well as slightly lower but still respectible n-type zT well beyond 0.5. p-and-n-type efficiencies are both high but for two different sets of physical reasons. High n-type power factor results from the well-known flat-and-dispersive conduction band. Even higher p-type power factor results from extremely high electron-phonon scattering lifetime at the valence band maximum and intrinsic Fermi level largely skewed towards the valence band. A large difference in effective masses of valence and conduction bands leads to the latter, allowing conductivity and Seebeck coefficient to be simultaneously high at low hole-doping concentrations. We explicitly calculate electron-phonon scattering matrix elements and lifetime by density functional perturbation theory and Wannier interpolation. Alloying reduces inherently high lattice thermal conductivity by mass-disorder scattering. If realized, these alloys of Fe₂TiSi would be very efficient low-temperature thermoelectrics as both p-and-n-types.   

Experimentally probing thermoelectric energy conversion at the molecular scale

The study of thermal energy transport and conversion at the nanoscale is of fundamental interest, and holds great promise for the development of a variety of technologies, including heat management in nanoelectronics, refrigeration and thermoelectrics . Although much attention has been directed towards studying nanoscale optical and electronic properties, thermoelectric properties at the molecular scale has been poorly characterized due to experimental challenges. In my presentation I will describe a series of novel experimental techniques and how they we leveraged them to systematically answer how the thermopower can be enhanced by quantum effects, and the electricity-to-heat conversion occurring at the molecular scale. These findings set the stage for rational design of thermally-efficient nanoscale devices and are expected to enable future development of environmentally-friendly energy saving solutions.   

Electronic transport simulations in materials with embedded nano-inclusions for enhanced thermoelectric power factors

Nanostructured thermoelectric materials have shown the potential to provide much larger thermoelectric performance compared to pristine materials. In this work we perform Non-Equilibrium Green’s Function electronic transport simulations to study thermoelectric transport through 2D materials with embedded nanoinclusions. The goal is to optimize the conditions that improve the power factor, to complement the reduction in the thermal conductivity caused by the nanoinclusions. We find that optimal designs can be achieved, in which the power factor is resilient, or even improved compared to pristine materials. We discuss in addition the influence of variability in the geometry and location of the nanoinclusions on the power factor, the influence of the non-uniform dopant distribution in the power factor in these materials, and finally we compare the possibility of achieving energy filtering in such materials compared to energy filtering in superlattices.   

Transformation Optics for Thermoelectric Transport

Transformation optics has proven to be an effective technique in manipulating electromagnetic fields, heat and electrical currents, and other physical phenomena by utilizing coordinate transformation methods. This has been a powerful approach in the design of cloaks, field rotators, and concentrators, which have also been implemented in novel devices. We show that transformation optics can also be applied to coupled thermal and electric transport as captured in thermoelectricity. The description relies on the invariance of the thermodynamic governing and constitutive equations for thermoelectricity. We demonstrate that the thermoelectric flow can be cloaked, rotated, or concentrated by specifying appropriate coordinate transformations. The design of metamaterial composites constructed using bilayer components with specified transport properties realizing such thermoelectric flow is also considered. The proposed thermoelectric cloak, rotator, and concentrator are independent of the particular boundary conditions and can also operate in (decoupled) electric or heat modes.   

Prediction of High Thermoelectric Performance of Pnictogen-dichalcogenide Layered Compounds with Quasi-one-dimensional Gapped-Dirac-like Band Dispersion

A theoretical guiding principle for designing high-performance thermoelectric materials has long been pursued. In this study [1], we theoretically demonstrate that pnictogen-dichalcogenide layered compounds, which originally attracted attention as a family of superconductors and have recently been investigated as thermoelectric materials [2], can exhibit very high thermoelectric performance. In particular, we clarify a promising guiding principle for materials design and find that LaOAsSe₂, a material that has yet to be synthesized, has a powerfactor that is six times as large as that of the known compound LaOBiS₂, and can exhibit a very large ZT under some plausible assumptions. This large enhancement of the thermoelectric performance originates from the quasi-one-dimensional gapped-Dirac-like band dispersion, which is realized by the square-lattice network of the p-orbitals. Our study offers one ideal limit of the band structure for thermoelectric materials. Because our target materials have high controllability of constituent elements and feasibility of carrier doping, experimental studies along this line are strongly awaited. [1] M. Ochi, H. Usui, and K. Kuroki, arXiv: 1706.09271 (2017). [2] Y. Mizuguchi et al., Cog. Phys. 3, 1156281 (2016).   

Experimental Determination of Phonon Thermal Conductivity and Lorenz Ratio of Single Crystal Bismuth Telluride at Intermediate Temperatures

A magneto-thermal resistance method is used to extract the phonon thermal conductivity (TC) of single crystal bismuth telluride from 5 - 60 K. Phonon TC is calculated by extrapolating the total TC vs. electrical conductivity curve to a zero electrical conductivity value. Knowledge of the phonon and electron components of TC in thermoelectric materials allows us to understand if either component dominates and what particular strategy may lead to an increased thermal conversion efficiency (ZT). Our results show that the phonon thermal conductivity follows the e^{Δ_min/T} temperature dependence and the Lorenz ratio corresponds to the modified Sommerfeld value in the intermediate temperature range. Our low-temperature data and analysis of bismuth telluride are a complement to previous measurements of Goldsmid (Proc. of the Phys. Soc. Sec. B 69, 1956) and theoretical calculations by Hellman, et al. (Phys. Rev. B 90, 2014) at higher temperatures (100 - 300 K). In addition to our results for bismuth telluride, data for a parallel study on single crystal Cu, Al and Zn will be presented.   

Thermoelectric Properties of All-Inorganic Halide Perovskite Nanowires

Controlling the flow of thermal energy is crucial to numerous applications ranging from microelectronic devices to energy storage and energy conversion devices. I will present experimental ultralow lattice thermal conductivities of solution-synthesized, single-crystalline all-inorganic halide perovskite nanowires composed of CsPbI₃, CsPbBr₃, and CsSnI₃. I will also discuss mechanisms behind ultralow thermal conductivitiy values of all-inorganic halide perovskites. Further, we found that CsSnI₃ possesses a rare combination of ultralow thermal conductivity and high electrical conductivity. CsSnI3 is a promsing candidate for thermoelectric applications.   

Thermoelectric properties of monoclinic NaSbSe2

We explored the thermoelectric properties of NaSbSe₂ using first principles and transport calculations. Previously known to exist in its disordered rock-salt type structure which has too low band gap for thermoelectric applications, its stable nanocrystalline (layered) monoclinic form with suitable band gap was synthesized recently. Our calculation suggests a favorable combination of conductivity and thermopower. The iso-energy surface plots have certain degree of anisotropy reflected in the electrical conductivity arising from the layered structure. The electron fitness function t supports the other results indicating good thermoelectric properties, especially for the n-doped system.   

Giant improvement of thermoelectric properties of PdS under pressure

Application of pressure can significantly affect the electronic and lattice properties of a material without introducing impurity, especially the transport properties. Measurements of electrical conductivity, Seebeck coefficient, and thermal conductivity under pressure are conducted on PdS. An enhanced electrical conductivity and Seebeck coefficient are observed upon compression. On the contrary, thermal conductivity has a downward trend with increasing pressure. These results demonstrate that the value of zT can be significantly enhanced with the application of pressure in the studied system.