Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session C21: Thermoelectrics IIFocus

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Sponsoring Units: GERA DMP Chair: David Parker, Oak Ridge National Lab Room: LACC 309 
Monday, March 5, 2018 2:30PM  3:06PM 
C21.00001: Thermal phonon coherence in superlattices and the role of phonon scattering phase space Invited Speaker: Austin Minnich 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 firstorder impact of phonon coherence on thermal transport is instead the phonon scattering phase space, or the space that defines which modes can interact in threephonon scattering processes. Abinitio 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. 
Monday, March 5, 2018 3:06PM  3:18PM 
C21.00002: Effect of grain boundary scattering on the thermal conductivity of uranium dioxide Keshav Shrestha, Daniel Antonio, Jie Lian, Krzysztof Gofryk Uranium dioxide (UO_{2}) 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. UO_{2} is a MottHubbard insulator with an energy gap of ≈2 eV and its thermal and magnetovibrational properties are still under debate. The thermal conductivity of an insulating material depends on various scattering mechanisms such as defects, phononphonon, or grain boundaries. In this talk, we will present the effect of grain size and grain boundary scattering on the thermal conductivity of UO_{2}. It is observed that the grain boundary scattering is prominent at low temperatures (below 30 K) where thermal conductivity decreases systematically with lowering the grainsize. We have estimated various physical parameters contributing to the thermal conductivity using the Callawaymodel and their relation with the grainsize will be discussed. 
Monday, March 5, 2018 3:18PM  3:30PM 
C21.00003: Thermodynamics of alloys incorporating both the configurational and the vibrational degrees of freedom: the vibrational cluster expansion method Yusheng Kuo, Fei Zhou, Vidvuds Ozolins 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 vibrationconfiguration 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 systemindependent force field can then be used in MD calculation to accurately predict lattice thermal conductivity of PbTeSe, which VCA overestimates and few existing empirical potentials can model properly. 
Monday, March 5, 2018 3:30PM  3:42PM 
C21.00004: Highthroughput study of CuFeS minerals as new thermoelectric materials Ilaria Siloi, Priya Gopal, Paz Vaqueiro, Stefano Curtarolo, Marco Buongiorno Nardelli, Marco Fornari Largescale production of thermoelectrics devices requires the design of novel materials that not only exhibit high performance but also are low cost and earthabundant. 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 highthroughput framework (http://aflowlib.org/src/aflowpi/), including improved electronic structure using the new ACBN0 functional. Our results show that CuFeS minerals have specific vibrational features that indicate low thermal conductivity and good electronic properties making them promising candidates in thermoelectric applications for lowgrade waste heat recovery. 
Monday, March 5, 2018 3:42PM  3:54PM 
C21.00005: Portable Combustion Generator with Integrated Thermoelectric/Heat Exchanger Michael Adams, Yuanhua Zheng, Joseph P Heremans 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 counterflow 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. 
Monday, March 5, 2018 3:54PM  4:06PM 
C21.00006: Transverse Peltier effect in single crystals of the quasionedimensional conductor, Li_{0.9}Mo_{6}O_{17} Saeed Moshfeghyeganeh, Joshua Cohn, John Neumeier Lithium purple bronze (Li_{0.9}Mo_{6}O_{17}) is a quasionedimensional (q1D) conductor for which a large, ptype Seebeck coefficient transverse to its q1D chains coexists with ntype 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. 
Monday, March 5, 2018 4:06PM  4:18PM 
C21.00007: Thermal Conductivity of Group VA Puckered Monolayer Structures Deniz Cakir, Tugbey Kocabas, Oguz Gulseren, Feridun Ay, Nihan Perkgoz, Cem Sevik Lattice thermal transport properties of monolayer structures of the group VA elements 
Monday, March 5, 2018 4:18PM  4:30PM 
C21.00008: FirstPrinciples Study of the Thermal Properties and Phonon Anharmonicity in the Type II Clathrate Alloys Si_{34x}Ge_{x}_{ }(0≤x<34) Charles Myles, Dong Xue Alloy clathrates are a class of materials possessing a crystalline framework containing more than one Group IV element (eg. Si, Ge and Sn). The guestfree lattice frameworks of these materials are sp^{3} hybridized covalently bonded structures. Others [1] have studied the electronic properties of Type II silicongermanium clathrate alloys Si_{34x}Ge_{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 firstprinciples method which is based on the quasiharmonic approximation (QHA) and which uses the Vienna Ab initio simulation package (VASP). 
Monday, March 5, 2018 4:30PM  4:42PM 
C21.00009: Impurity doping and FermiLevel Tuning in Bi_{2}Te_{3}related Alloys Byungki Ryu, Jaywan Chung, JaeKi Lee, Sudong Park Fermi level (E_{F}) tuning of Bi_{2}Te_{3}related alloys is very critical for their applications to the thermoelectrics (TEs) or topological insulators (TIs). Bi_{2}Te_{3} 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 ~ 68 x 10^{19} cm^{3} can prevent the bipolar effect. Meanwhile, Bi_{2}Te_{3} 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.. 
Monday, March 5, 2018 4:42PM  4:54PM 
C21.00010: Unraveling the role of defect induced exciting charge and heat transport phenomena in thermoelectric clathrates Amrita Bhattacharya 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. 
(Author Not Attending)

C21.00011: Maximum cooling of a quantum thermocouple Marco Antonio Jimenez Valencia, Abhay Shastry, Charles Stafford 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 pielectrons 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 superexchange) in a quantum thermocouple are tested to determine the possibility of achieving the thermodynamic limit of Carnot efficiency. 
Monday, March 5, 2018 5:06PM  5:18PM 
C21.00012: Changes in Local Structure during the Metal to Insulator Phase Transition of Cu_{12}Sb_{4}S_{13} Cameron MacKeen, Frank Bridges Tetrahedrites such as Cu_{12}Sb_{4}S_{13} 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_{12x}M_{x}Sb_{4}S_{13}, with an increased electrical conductivity while remaining thermally glasslike. 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 Cu_{12}Sb_{4}S_{13} as a function of temperature by analyzing the Extended Xray Absorption FineStructure (EXAFS). Using EXAFS, we probe the Sb and Cu Kedge and find large discontinuities in bondlength and σ^{2} of neighboring atomic shells at the transition temperature. 
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