Session C21: Thermoelectrics II

Thermal phonon coherence in superlattices and the role of phonon scattering phase space

Phonon coherence in heterostructures like superlattices is a topic of intense interest due to the potential to realize new materials that do not occur naturally. Most prior works have focused on how coherence can open up phonon bandgaps and modify group velocities. Here, we show that the first-order impact of phonon coherence on thermal transport is instead the phonon scattering phase space, or the space that defines which modes can interact in three-phonon scattering processes. Ab-initio calculations show that the thermal transport properties of heterogeneous solids can be made drastically different, both higher and lower, than those of the constituents by manipulating this phase space. Our work provides a new viewpoint into how phonon coherence can be exploited to realize materials with exceptional thermal properties for applications such as thermoelectricity.
  

Effect of grain boundary scattering on the thermal conductivity of uranium dioxide

Uranium dioxide (UO₂) is a primary nuclear fuel used in the light water reactors. The thermal conductivity of the fuel material is an important physical parameter that governs the conversion of heat generated via nuclear fission events into electricity. UO₂ is a Mott-Hubbard insulator with an energy gap of ≈2 eV and its thermal and magneto-vibrational properties are still under debate. The thermal conductivity of an insulating material depends on various scattering mechanisms such as defects, phonon-phonon, or grain boundaries. In this talk, we will present the effect of grain size and grain boundary scattering on the thermal conductivity of UO₂. It is observed that the grain boundary scattering is prominent at low temperatures (below 30 K) where thermal conductivity decreases systematically with lowering the grain-size. We have estimated various physical parameters contributing to the thermal conductivity using the Callaway-model and their relation with the grain-size will be discussed.
  

Thermodynamics of alloys incorporating both the configurational and the vibrational degrees of freedom: the vibrational cluster expansion method

Characterizing thermal conductivity for alloy thermoelectric systems has been considered a challenging task due to the many degrees of freedom in the atomic configurational space needed to approximate truly random alloys. Virtual crystal approximation (VCA) and empirical potentials have been widely applied in lattice dynamics (LD) and molecular dynamics (MD), respectively, to address this problem. However, inaccurate description of strain disorder and strong system dependence limit the applicability of aforementioned methods. We address this problem by constructing a unified model, vibrational cluster expansion (VCE), which explicitly takes both vibrational and configurational degrees of freedom into account by forming coupled vibration-configuration clusters. To deal with the vast number of variables in the model, we apply compressed sensing technique to robustly recover important force constants using training data acquired from density functional theory (DFT) calculations. We demonstrate that our method can efficiently reproduce DFT forces in alloys. An accurate and system-independent force field can then be used in MD calculation to accurately predict lattice thermal conductivity of PbTe-Se, which VCA overestimates and few existing empirical potentials can model properly.   

High-throughput study of Cu-Fe-S minerals as new thermoelectric materials

Large-scale production of thermoelectrics devices requires the design of novel materials that not only exhibit high performance but also are low cost and earth-abundant. In this context sulfide minerals are a largely unexplored field. Here we investigate the electronic and vibrational properties for 6 minerals in the copper iron sulfide family including Mawsonite (Cu_{6}Fe_{2}SnS_{8}), Mooihoekite (Cu_{9}Fe_{9}S_{16}) and Nukundamite (Cu_{3.38}Fe_{0.62}S_{4}). Typically, such materials have complex crystal structures with large unit cells, where interstitial atoms in the tetrahedral sites can be partially occupied. Electronic and thermal transport results are obtained using the recently developed PAOFLOW package (www.aflow.org/paoflow) integrated in AFLOWπ high-throughput framework (http://aflowlib.org/src/aflowpi/), including improved electronic structure using the new ACBN0 functional. Our results show that Cu-Fe-S minerals have specific vibrational features that indicate low thermal conductivity and good electronic properties making them promising candidates in thermoelectric applications for low-grade waste heat recovery.   

Portable Combustion Generator with Integrated Thermoelectric/Heat Exchanger

Unlike semiconductors in traditional thermoelectric generators (TEGs), metals with large thermoelectric power factor can fully integrate with the heat exchangers and structural parts of a TEG, because they can be formed into complex shapes and welded. This integrated TEG consists of a combustion chamber with counter-flow heat exchanger, all constructed from commercially available metals. Heat of combustion from the premixed fuel and air is converted directly into electricity by the walls of the heat exchanger. The cold side of the generator is maintained by the entry of the premixed gas flow, which in turn is preheated by the exhaust from the combustion.
Demonstrated here are several variations of the generator, including Monel and Inconel tubes for the heat exchanger, or Constantan and Chromel ribbons. Schematics of each generator are given, along with video demonstrations. We report the thermopower and power factor of available metals, and the performance of the generators including temperature, power output, and efficiency.   

Transverse Peltier effect in single crystals of the quasi-one-dimensional conductor, Li0.9Mo6O17

Lithium purple bronze (Li_0.9Mo₆O₁₇) is a quasi-one-dimensional (q1D) conductor for which a large, p-type Seebeck coefficient transverse to its q1D chains coexists with n-type metallic behavior along the chains; a substantial transverse Peltier effect has been demonstrated.^a We will report on measurements of the transverse Peltier effect in a series of single crystals cut with their body axes at varying angles to the q1D chain direction. Comparisons of the data to theoretical models that employ known transport coefficients along the mutually perpendicular directions will be discussed.

^a J. L. Cohn, S. Moshfeghyeganeh, C. A. M. dos Santos, and J. J. Neumeier, Phys. Rev. Lett. 112, 186602 (2014).   

Thermal Conductivity of Group VA Puckered Monolayer Structures

Lattice thermal transport properties of monolayer structures of the group VA elements
with black phosphorus like puckered structure were systematically investigated by first principles
calculations and an iterative solution of the Boltzmann transport equation for phonons.
Surprisingly, we determined that the thermal conductivity of all the considered materials can be modeled by
a simple formula, κ=c1 + c2/m2. As expected, phosphorene has the highest thermal
conductivity due to its low atomic massand strong bonding between the P atoms. We found that alloying of
the pristine monolayers with P, As, Sb and Bi atoms improves (suppresses) the electronic (thermal)
conductivity. We can achieve a 1.5 Wm⁻¹K⁻¹ at 300 K when we dope phosphorene with Bi.
The contribution of ZA mode (< 50%) to the thermal conductivity is markedly small as compared
to graphene (80%), and decreases as the average atomic mass in the unit cell increases. Moreover,
our calculations on alloyed structures clearly resemble that thermoelectric potential of these mate-
rials can be improved by suppressing its thermal properties. The presence of the ultra-low and
high electrical conductivity makes these class of monolayers
promising candidates for thermoelectric applications.   

First-Principles Study of the Thermal Properties and Phonon Anharmonicity in the Type II Clathrate Alloys Si34-xGex (0≤x<34)

Alloy clathrates are a class of materials possessing a crystalline framework containing more than one Group IV element (eg. Si, Ge and Sn). The guest-free lattice frameworks of these materials are sp³ hybridized covalently bonded structures. Others [1] have studied the electronic properties of Type II silicon-germanium clathrate alloys Si_34-xGe_x (0 ≤ x <34) for some compositions. Motivated by this previous work, we have performed our calculations using the same x values as reference [1]. Our calculations have explored the effect of phonon anharmonicity on the thermal expansion properties of these materials. Our calculations have focused on the acoustic phonon frequency shift due to clathrate framework deformation. This has led us to explore the anharmonic features of NTE via an analysis of the mode Gruneisen parameters (MGP). To explore the underlying mechanisms governing NTE behavior, we have used a first-principles method which is based on the quasi-harmonic approximation (QHA) and which uses the Vienna Ab initio simulation package (VASP).
[1] Koji Moriguchi, Shinji Munetoh, and Akira Shiniani, Phys. Rev. B 62, 7138 (2000).   

Impurity doping and Fermi-Level Tuning in Bi2Te3-related Alloys

Fermi level (E_F) tuning of Bi₂Te₃-related alloys is very critical for their applications to the thermoelectrics (TEs) or topological insulators (TIs). Bi₂Te₃ and its alloys are known as good TE materials at near room temperature. The room temperature thermoelectric performance is very large in these materials. However due to the narrow band gap nature and the large bipolar effect, the thermoelectric performance is rapidly vanishing with temperature. Recent studies revealed that the high doping concentration of ~ 6-8 x 10¹⁹ cm^-3 can prevent the bipolar effect. Meanwhile, Bi₂Te₃ is known as the TI material. For the use of the topological surface states for future device, the E_F should be positioned at the mid gap to reduce the bulk transport while maintaining surface transport..
In this work, by performing the density functional calculations, we investigated the impurity doping for Fermi level tuning in Bi₂Te₃-based alloys. We considered the alkali and halogen point impurities and calculated their formation energies and band structures to understand their electrical properties. Based on the results, we will discuss the dopability of impurity dopants and the Fermi-level tuning in Bi₂Te₃-based alloys.   

Unraveling the role of defect induced exciting charge and heat transport phenomena in thermoelectric clathrates

Thermoelectric clathrates are generally referred to have a semiconducting cage like framework of group -IV (host) elements enclosing metal (guest) atoms, whereby the electronic and vibrational transport can be decoupled by means of the host and guest respectively. Density functional theory calculations are perfectly suitable to analyse the electronic and vibrational transport phenomena in this class. Using the simple binary [1] and complex ternary clathrates [2] as examples, we unravel the role of rattling of guest, mutual coupling of guest and host, and implications of defects on the charge and heat transport of this material class. Our study reveals the complex interplay of materials phenomena that goes beyond the usual concepts in this field.
[1] Amrita Bhattacharya, Christian Carbogno, Bodo Bohme, Michael Baitinger, Yuri Grin, and Matthias Scheffler. “Formation of vacancies in Si/Ge Clathrates: The importance of broken symmetries”, Phys. Rev. Lett. 118, 236401 (2017).
[2] Amrita Bhattacharya and Saswata Bhattacharya. Unraveling the role of vacancies in the potentially promising thermoelectric clathrates Ba₈Zn_xGe_46-x-y■_y clathrates. Phys. Rev. B, 94, 094305 (2016).   

Maximum cooling of a quantum thermocouple

A quantum thermocouple is investigated beyond linear response using the method of nonequilibrium Green’s functions (NEGF). The maximum cooling power achievable is calculated through an effective field theory of the interacting pi-electrons which characterize molecular junctions’ transport properties. The cooling limit is determined by the interplay of the Peltier effect and Joule heating. Different junction couplings (such as sequential exchange and super-exchange) in a quantum thermocouple are tested to determine the possibility of achieving the thermodynamic limit of Carnot efficiency.   

Changes in Local Structure during the Metal to Insulator Phase Transition of Cu12Sb4S13

Tetrahedrites such as Cu₁₂Sb₄S₁₃ are prime candidates for thermoelectric devices as they can exhibit a high figure of merit (ZT), and further are naturally abundant. Currently, researchers look to dope these compounds with a transition metal (M) in search of a compound, Cu_12-xM_xSb₄S₁₃, with an increased electrical conductivity while remaining thermally glass-like. The pure tetrahedrite exhibits a metal to insulator transition at T~85K, and doping generally supresses this transition. In an effort to understand the physical mechanism that leads to the peculiar properties of this class of materials, we have studied the changes in local structure of Cu₁₂Sb₄S₁₃ as a function of temperature by analyzing the Extended X-ray Absorption Fine-Structure (EXAFS). Using EXAFS, we probe the Sb and Cu K-edge and find large discontinuities in bond-length and σ² of neighboring atomic shells at the transition temperature.