Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session R12: Focus Session: Thermoelectrics Phonons and Heat Conduction |
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Sponsoring Units: DMP GERA FIAP Chair: David Cahill, University of Illinois at Urbana-Champaign Room: 314 |
Wednesday, March 20, 2013 2:30PM - 3:06PM |
R12.00001: Phonon Thermal Transport in Thermoelectric Materials from First Principles Invited Speaker: David Broido Breakthroughs in nanoscience and materials fabrication technology have led to the creation of materials with very low lattice thermal conductivity [1, 2], a requirement for high thermoelectric efficiency. There is now an unprecedented need for quantitative, predictive theoretical approaches to provide fundamental understanding of lattice thermal transport in thermoelectric materials and insight into the design and development of new materials for enhanced thermoelectric applications. In this talk, I will describe our atomistic first principles approach for calculating lattice thermal conductivity of materials [3-6], which combines a complete solution of the Boltzmann transport equation for phonons with harmonic and anharmonic interatomic forces determined from density functional theory. I will present an overview of this theoretical approach along with some of our recent calculated results for a range of test materials such as Si, Ge and III-V compounds. I will also discuss results for thermoelectric alloys such as Si$_{\mathrm{x}}$Ge$_{\mathrm{1-x}}$ and Mg$_{\mathrm{2}}$Si$_{\mathrm{x}}$Sn$_{\mathrm{1-x}}$ and nanoparticle embedded in alloy thermoelectric (NEAT) materials. Finally, I will discuss insights gained from this effort such as the importance of anharmonic coupling of acoustic and optic phonon modes.\\[4pt] [1] Bed Poudel et al., Science 320, 634 (2008);\\[0pt] [2] D. T. Morelli et al., Phys. Rev. Lett. 101, 035901 (2008);\\[0pt] [3] D. A. Broido et al, Appl. Phys. Lett., 91, 231922 (2007);\\[0pt] [4] A. Kundu et al, Phys. Rev. B, 84, 125426 (2011);\\[0pt] [5] L. Lindsay et al., Phys. Rev. Lett. 109, 095901 (2012).\\[0pt] [6] W. Li et al, submitted (2012). [Preview Abstract] |
Wednesday, March 20, 2013 3:06PM - 3:18PM |
R12.00002: Beyond the constant Lorenz number for separating thermal conductivities of electrons and phonons: A DFT study Mingxing Chen, Raimund Podloucky Lorenz number is an important quantity for separating thermal conductivities of electrons and phonons in the field of thermoelectrics, which is material- and temperature-dependent. Combing DFT calculations with Boltzmann transport equations, we have derived the Lorenz number for realistic compound BaAu$_6$Ge$_{40}$, a good thermoelectric material. It is demonstrated that using the constant Lorenz number of the Wiedemann-Franz law for simple metals leads to strong discrepancies, in particular at higher temperatures. The results suggest that one has to rethink the way of extracting both $\kappa_{el}$ and $\kappa_{ph}$ as usually done based on the measured electrical conductivity. We propose a strategy of correcting the Wiedemann-Franz Lorenz number that subtracts the metallic limiting value by S$^2$ as obtained from Seebeck measurements. [Preview Abstract] |
Wednesday, March 20, 2013 3:18PM - 3:30PM |
R12.00003: Calculating Lattice Thermal Conductivity via Compressive Sensing Lattice Dynamics Weston Nielson Calculating the lattice contribution to thermal conductivity (TC) is of great importance in a range of materials applications, including thermoelectrics. Common simulation-based methods for calculating the TC typically require either very long simulation times, large system size, or both. These constraints make it difficult or impractical to use DFT-based methods for calculating the TC. Classical molecular dynamics (MD), however, is typically unburdened by these constraints but is instead limited by the accuracy of the interatomic potentials. We have developed a method that uses DTF, combined with compressive sensing, to calculate the higher-order force constants from the theory of lattice dynamics. These force constants are then used to calculate interatomic potentials in a classical MD program. We present our findings from applying this method to a variety of materials. [Preview Abstract] |
Wednesday, March 20, 2013 3:30PM - 3:42PM |
R12.00004: Phonon Surface Scattering in Monte Carlo Simulations Leon Maurer, Zlatan Aksamija, Edwin Ramayya, Amirhossein Davoody, Irena Knezevic Surface roughness has a significant impact on the thermal conductivity and thermoelectric properties of nanowires. We investigate the effect of surface roughness on thermal transport using a phonon Monte Carlo simulation. In addition to allowing us to simulate a wide range of wire dimensions and surface topographies, Monte Carlo enables us to investigate different models for surface scattering: constant specularity parameters, momentum-dependent specularity parameters, and specular scattering from randomly generated rough surfaces. We investigate the relative merits of different surface scattering models and the limitations on their validity. [Preview Abstract] |
Wednesday, March 20, 2013 3:42PM - 3:54PM |
R12.00005: First principles and force field calculations of thermal transport in bulk semiconductors and oxides: a comparative study Eamonn Murray, Ivana Savic, Giulia Galli At present, large scale calculations of thermal transport properties of materials are carried using empirical potentials\footnote{See, e.g. Y.He, I.Savic, D.Donadio and G.Galli PCCP 2012 ASAP (DOI: 10.1039/C2CP42394D)}, due to difficulties in scaling ab initio methods to directly compute the thermal conductivity of complex, nanostructured systems. It is therefore important to asses the predictive ability of empirical potentials for representative bulk systems, for which ab initio simulations are possible, and to establish their accuracy in yielding absolute values of computed thermal conductivities ($\kappa$) and trends within given classes of systems. We report on comparisons between thermal conductivities of elemental semiconductors and insulators (Si, C, Ge) and simple oxides (MgO and SiO2) as obtained using the Boltzman Transport equation with first principles, DFT Hamiltonians and Tersoff type empirical potentials. The second and third derivatives of the energy with respect to atomic displacements are obtained by finite difference calculations in supercells in all cases. A detailed discussion of the reasons why these empirical potentials appear to systematically overestimate $\kappa$ will be presented. [Preview Abstract] |
Wednesday, March 20, 2013 3:54PM - 4:06PM |
R12.00006: A Comparative Study of Ab-Initio Thermal Conductivity Approaches: The Case of Cubic Boron Nitride Saikat Mukhopadhyay, Lucas Lindsay, David Broido, Derek Stewart Given its high strength and large thermal conductivity, cubic boron nitride (cBN) provides an important complement to diamond films for heat spreading applications. However, cBN, in contrast to diamond, is a polar material with significant LO-TO splitting in the phonon dispersion. In this talk, we examine the lattice thermal conductivity of cBN using several approaches based on first principles calculations. These approaches include: (1) an analytic modified Callaway-Debye model that relies on parameters from ab-initio harmonic force constants, (2) a fully self-consistent calculation of the thermal conductivity that links an iterative solution of the phonon Boltzmann transport equation (BTE) with harmonic and anharmonic interatomic force constants. The force constants for the BTE are calculated using two approaches: density functional perturbation theory and a real-space supercell approach. We will compare the results from these approaches, highlight the role of normal phonon-phonon scattering, and also examine the impact of optical modes and LO-TO splitting. In addition, we will discuss how isotope scattering affects thermal conductivity and compare this to other boron nitride structures (hexagonal BN, BN sheets and BN nanotubes). [Preview Abstract] |
Wednesday, March 20, 2013 4:06PM - 4:18PM |
R12.00007: Improved Calculation of Vibrational Mode Lifetimes in Anharmonic Solids Murray Daw, Yang Gao, Doyl Dickel, David Harrison We propose and evaluate a formal foundation for practical calculations of vibrational mode lifetimes in solids. The approach is based on a recursion method analysis of the Liouvillian. From this we derive the lifetime of a vibrational mode in terms of moments of the power spectrum of the Liouvillian as projected onto the relevant subspace of phase space. In practical terms, the moments are evaluated as ensemble averages of well-defined operators, meaning that the entire calculation is to be done with Monte Carlo. These insights should lead to significantly shorter calculations and improved understanding of mode lifetimes and lattice thermal conductivity. Evaluation performed on model systems have been encouraging. [See Dickel \& Daw, Comp Mat Sci, v47 p698 and v49 p445 (2010)]. [Preview Abstract] |
Wednesday, March 20, 2013 4:18PM - 4:30PM |
R12.00008: Local Distortions in PbTe:Tl Trevor Keiber, Frank Bridges, Brian Sales Lead Telluride (PbTe) is a well characterized thermoelectric material. Tl doping increases the figure of merit with a maximum at 2\% Tl. Recent X-ray diffraction and total neutron scattering experiments suggest Pb moves off-center along the 100 axis as T increases. To investigate the local structure we present an Extended X-ray Absorption Fine Structure (EXAFS) analysis for 0-3\% Tl concentrations at the Tl and Pb L3 edges and at the Te K edge. At 10K the local structure about Pb is well ordered, the Pb-Te (Te-Pb) pair distribution function (PDF) broadens rapidly with T. Attempts to model the increase in $\sigma^{2}$(T) for the Pb-Te pair ($\sigma$ is the width of the PDF) with a 100 Pb off-center displacement, were not successful. However $\sigma^2$(T) for the Pb-Te pair is well described by a correlated Debye model with a low correlated Debye temperature. The Te edge shows increased disorder for the the Te-Te pair and later peaks which may be caused by a structural change around the Te atom. For Tl, the environment is distorted even at 10K within the host material. This indicates a large variation of the Tl-Te bond lengths, presumably as a result of the presence of Tl(+1). We discuss possible models for the disorder about Tl, Pb, and Te in PbTe:Tl. [Preview Abstract] |
Wednesday, March 20, 2013 4:30PM - 4:42PM |
R12.00009: Anomalous Coherent Oscillations in PbTe from Ultrafast Optical Pump-Probe Measurements Mason Jiang, Paula Giraldo, Ian Fisher, David Reis We report on the observation of anomalous coherent oscillations in single crystals of PbTe from ultrafast optical pump-probe measurements. PbTe is a leading thermoelectric material with an unusually low thermal conductivity, which has recently been attributed to strongly anharmonic phonon interactions. In an attempt to understand in greater detail the nature of these interactions, we perform time-resolved, optical pump-probe measurements on PbTe with femtosecond resolution in a range of temperatures from 77K to room temperature. We see previously unreported, low-frequency reflectivity oscillations that decay on the timescale of a few picoseconds and remain robust through a wide range of temperature variation. This talk will discuss possible origins and explanations for the appearance of these oscillations. [Preview Abstract] |
Wednesday, March 20, 2013 4:42PM - 4:54PM |
R12.00010: Phonon Dispersions and Relaxation Times in AgSbTe$_2$ and PbTe Olivier Delaire, Jie Ma, Andrew May, Chris Carlton, Michael McGuire, Lindsay VanBebber, Douglas Abernathy, Georg Ehlers, Tao Hong, Ashfia Huq, Wei Tian, Veerle Keppens, Yang Shao-Horn, Brian Sales The thermoelectric material AgSbTe$_{2}$ had attracted much interest due to its high thermoelectric figure-of-merit, and its anomalously low thermal conductivity for a nominally simple rock-salt structure, which is glass-like even in bulk single-crystals. We present results of systematic neutron scattering investigations of the phonon density-of-states, dispersions, and relaxation times in AgSbTe$_{2}$, and contrast these with PbTe. A detailed account of the thermal conductivity is obtained in terms of microscopic phonon mean-free-paths, providing good agreement with bulk transport measurements. [Preview Abstract] |
Wednesday, March 20, 2013 4:54PM - 5:06PM |
R12.00011: Phonon dynamics in SnTe Chen Li, Olivier Delaire, Xin Chen, David Singh, Andrew May, Jie Ma, Michael McGuire, Georg Ehlers, Andrew Christianson, Ashfia Huq Thermoelectric materials can convert waste heat into electrical energy, and have attracted much attention in recent years for power generation. IV-VI compounds in rock salt structure include some of the most efficient thermoelectric materials and giant phonon anharmonicity is believed to contribute to the low thermal conductivity. In this work, phonon dispersions and linewidths in single-crystalline SnTe were measured at a series of temperatures using time-of-flight and triple-axis neutron spectrometers to study the temperature dependence of the phonon dynamics and phonon anharmonicity. Phonon calculations and molecular dynamics simulations with first-principles methods were used to identify the anomalies in phonon modes and the results were compared to the measurements. Because the phonons involved have an important contribution to the lattice thermal conductivity in this system, the anharmonic coupling is likely to provide a key insight in understanding the surprisingly low thermal conductivity of the rocksalt tellurides in general. [Preview Abstract] |
Wednesday, March 20, 2013 5:06PM - 5:18PM |
R12.00012: Simulation of Nanostructure and Thermal Conductivity in Binary Alloys Yusuke Konishi, Tetsuya Fukushima, Kazunori Sato, Hiroshi Katayama-Yoshida, Yoshihiro Asai Thermoelectric materials attract much attention because of concerns about energy conservation. Recently, Sugihara et al. made nanostructures using phase separation of Ni-Cu binary alloy [1]. This structure is about 10nm and has the large Seebeck coefficient. However, the way to make better thermoelectric material is under discussion. For this purpose, we need a large Seebeck coefficient, large electric conductivity, and small phonon thermal conductivity. The goal of this study is finding the condition of making good thermoelectric materials. In our simulation, we made structures in various conditions and evaluated phonon thermal conductivity. First, we simulated quenching binary alloy at high temperature by using Monte Carlo method. The potential between atoms are determined by KKR-CPA method [2]. In this simulation, nanostructures have the size distribution between 1 nm and 50 nm. Next, we simulated phonon conduction by molecular dynamics. Heat baths were placed at both ends and the thermal gradient was made. By calculating energy flux, we determined the value of phonon thermal conductivity. [1] A Sugihara et al., Appl. Phys. Exp. 3, 065204 (2010). [2] H. Akai, J. Phys.: Condens. Matter 1, 211 (1989). [Preview Abstract] |
Wednesday, March 20, 2013 5:18PM - 5:30PM |
R12.00013: Landauer approach to thermoelectric transport across grain boundaries in Si Michael Shaughnessy, Doug Medlin, Francois Leonard, Catalin Spataru Thermoelectric transport is strongly influenced by electron and phonon scattering from defects, grain boundaries, and nano structuring. While scattering from point defects is relatively well understood, the impact of the detailed structure of grain boundaries is still poorly understood. We use a Landauer approach based on ab initio Density Functional Theory and classical Molecular Dynamics simulations to compute electron and phonon transport coefficients in the presence of grain boundaries. The approach allows the calculation of all the thermoelectric quantities, including thermal conductivity, electrical conductivity, Seebeck coefficient, and the overall figure of merit, ZT. The method is applied to grain boundaries in Si, focusing on the \textbraceleft 111\textbraceright twin in the high and low density regimes. For ordered arrays of \textbraceleft 111\textbraceright twins in Si a small change in ZT is predicted because of compensating differences between thermal conductivity on the one hand and electrical conductivity and Seebeck coefficient on the other.~ [Preview Abstract] |
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