Symposium Organizers
Subhash L. Shinde Sandia National Laboratories
David H. Hurley Idaho National Laboratory
Gyaneshwar P. Srivastava University of Exeter
Masashi Yamaguchi Rensselaer Polytechnic Institute
W1: Phonon Modes and Dispersion Relations
Session Chairs
Subash Shinde
Gyaneshwar Srivastava
Monday PM, November 28, 2011
Room 313 (Hynes)
9:00 AM - **W1.1
Inelastic Neutron Scattering Measurements and Lattice Dynamics Simulations of Phonon Dispersion and Lifetimes in UO2.
Judy Pang 1 , William Buyers 2 , Alexsandr Chernatynskiy 3 , Mark Lumsden 4 , Doug Abernathy 4 , Bennett Larson 1 , Simon Phillpot 3
1 Materials Science & Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States, 2 National Research Council, Chalk River Laboratories, Chalk River, Ontario, Canada, 3 Department of Materials Science & Engineering, University of Florida, Gainesville, Florida, United States, 4 Neutron Scattering Science Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Show AbstractUnderstanding lattice thermal conductivity in oxides requires a correct accounting for a wide range of phonon scattering processes, including anharmonic phonon-phonon, phonon-point defect, and phonon-impurity scattering. Uranium oxide (UO2) has a low thermal conductivity as a result of these scattering processes. Over five decades there has been remarkably little high-temperature research on phonons in UO2 to underpin its widespread use as a reactor fuel. We have used reactor and spallation based inelastic neutron scattering measurements of phonon dispersion, group velocity, lifetime, and phonon density of states in UO2 at ambient (295 K) and high temperature (1200 K). High-resolution phonon linewidth measurements made along the [001] direction in UO2 on the HB-3 beamline at the High Flux Isotope Reactor (HFIR) showed that the lifetimes at both ambient and 1200 K (1) depend strongly on the phonon wave vector, (2) do not scale with energy within phonon branches, and (3) do not scale uniformly with temperature. Lattice dynamics third-order anharmonic simulations based on a rigid-ion model (with the short-range part modeled by a Buckingham potential parameterized by Catlow) found qualitatively similar effects, but were found to predict a larger linewidth increase with temperature than observed experimentally. These results will be discussed in relation to the initial momentum and energy constraints on phonon decay into a two-phonon continuum. Estimates of thermal conductivity for UO2 at 1200 K for individual phonon branches based on experimentally measured phonon group velocities and linewidths showed that the transverse acoustical (TA) and longitudinal optical (LO) branches transport heat as efficiently as the longitudinal acoustical (LA) branch. This experimental result will be compared with our lattice dynamics simulations, which predicted only the LO and LA branches to conduct heat significantly. The experimental results will also be compared with a recent first principles calculation that predicted only the LA phonons to contribute significantly to thermal transport in UO2 at high temperature. This research was supported as part of the Center for Materials Science of Nuclear Fuel, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science.
9:30 AM - W1.2
Finite-Size Effects in the Phonon Density of States of Nanostructured Germanium: A Comparative Study of Nanoparticles, Nanocrystals, Nanoglasses and Bulk Phases.
Daniel Sopu 1 , Karsten Albe 1
1 , TU-Darmstadt, Darmstadt Germany
Show AbstractMolecular dynamics simulations are used for studying finite-size effects in the phonon density of states (PDOS) of nanostructured germanium materials such as nanoparticles, nanocrystals, embedded nanoparticles and nanoglasses.By comparing with the PDOS of single crystalline and amorphous structures the physical origins of additional or vanishing vibrational modes or frequency shifts are identified.The changes in the PDOS are mostly due to three size effects: structural discontinuities such as surfaces, grain boundaries and interfaces, tensile surface stresses and phonon confinement due to the finite particle size.Each of these size effects is systematically studied for the different nanostructured materials, separately.In addition, the extension of phonon modes across and along glass-crystal interfaces is studied.Our findings provide a general view on the interplay of nanostructural features and lattice vibrations.
9:45 AM - W1.3
The Vibrational Properties of Ultrananocrystalline Diamond Based on Molecular Dynamics Simulations.
Shashishekar Adiga 1 , Vivekananda Adiga 2 , Robert Carpick 3 , Donald Brenner 4
1 Kodak Research Laboratories, Eastman Kodak Company, Rochester, New York, United States, 2 School of of Applied and Engineering Physics, Cornell University, Ithaca, New York, United States, 3 Department of Mechanical Engineering and Applied Mechanics, University of Pennsylvania, Philadelphia, Pennsylvania, United States, 4 Department of Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina, United States
Show AbstractWe investigate the phonon properties of ultrananocrystalline diamond (UNCD) using molecular dynamics simulations. The UNCD model was prepared by generating grains of random orientations and locations of average size 4 nm using the Voronoi construction. We have characterized the structure in terms of grain boundary and core atoms based on the local structure and the dynamical properties of atoms. We have computed the velocity autocorrelation function and phonon spectra to obtain the specific heat as a function of temperature. The specific heat of UNCD showed enhancements over that of single crystal diamond, and we found that this enhancement is approximately 20% at 300 K. The excess specific heat in UNCD in comparison to single crystal diamond is found to be maximum at approximately 350 K. Further, our calculations of the specific heat of the grain boundary and core atoms show that the excess specific heat arises predominantly due the grain boundary atoms. We will discuss the implications of these findings to thermal and mechanical properties of UNCD.
10:00 AM - W1.4
Phonons in Clathrate Crystals: Harmonic and Anharmonic Modes.
Katsumi Tanigaki 1 2 , Jingtao Xu 1 , Gang Mu 2 , Jiazhen Wu 2 , Satoshi Heguri 2 , Khuong Huynh 2 , Quynh Phan 2 , Youichi Tanabe 1
1 WPI-AIMR, Tohoku University, Sendai Japan, 2 Department of Physics, Graduate School of Science, Tohoku University, Sendai Japan
Show AbstractPhonons as well as electrons and magnons play a very important role as quantized quantities for controlling physical properties of materials. Lattice phonons have been the important issue for understanding temperature evolution of both thermal and electric transports in terms of electron and phonon scatterings for long years. Recently, the concept of phonon is broadly generalized ranging from a collective mode to a localized mode. The former is the typical Debye-mode phonons with a linear κ-ω phonon dispersion, while the latter is the Einstein-mode phonons with the same frequency of all oscillators. Both are generally categorized in the harmonic phonons. Other types of phonons, possibly created from nano-structure materials, have begun to be considered to play an essential role [1-4] and even more importantly such phonons can provide a way of tailoring phonons as phonon engineering. Historically, such typical examples can be found in crystals constructed from the building blocks of clusters as well as molecules consisting of carbon. Thanks to the inner nano spaces in polyhedral network crystals that can accommodate atoms, a large degree of freedom in motions of atomic oscillations can be generated and this is recently drawing much attention as a new mode of phonons to be categorized as anharmonic oscillations. Clathrate crystals have nano cage structure consisting of IVth group elements with face shared. Therefore, these crystals have sufficiently large inner spaces for atomic elements to be confined. Such endohedral atoms move under the anharmonic potentials made by cages and give rise to unique phonons recently known as rattling phonon. These phonons are greatly different from the conventional lattice phonons and therefore produce unique electron-phonon interactions [2,3]. In this talk, we would like to describe how electron-phonon interactions are modified and how heat transport can be influenced. Both of these factors are very important for designing excellent thermoelectric material, a class of materials that convert heat to energy, and this has recently become a subject of intensive scientific investigation. We will also discuss how these phonons can be tailored by changing the elements of the cage.[1] K. Tanigaki, et al, Nature Materials, 2, 653 (2004). [2] Y. Kohama, T. Rachi, Ju Jing, Z. Li, J. Tang, R. Kumashiro, S. Izumisawa, H. Kawaji, T. Atake, H. Sawa, Y. Murata, K. Komatsu, and K. Tanigaki, Phys. Rev. Lett., 102, 013001-013004 (2009).[3] J.Tang, J-T Xu, S. Heguri, H. Fukuoka, S. Yamanaka, K. Akai, and K. Tanigaki, Phys. Rev. Lett., 105, 176402 (2010).[4] J-T Xu, J. Tang, K. Sato, Y. Tanabe, H. Miyasaka, M. Yamashita, S. Heguri, and K. Tanigaki, Phys. Rev.B, 82, 085206 (2010).
10:15 AM - W1.5
Phonon Engineering in Silicon Nanowires Using Stable Isotopes.
Uri Givan 1 , Oussama Moutanabbir 1
1 , Max Planck Institute of Microstructure Physics, Halle Germany
Show AbstractSilicon nanowires have attracted a great deal of attention as powerful nanotechnological building blocks with a potential impact on various fields such as thermoelectrics, photovoltaics, biosensing, and quantum information to name a few. Natural silicon (NATSi) is composed of three stable isotopes: 28Si, 29Si, and 30Si with abundances of 92.23%, 4.67%, and 3.10%, respectively. Several physical properties of semiconductor crystals can be significantly influenced by their isotopic composition. A number of isotope effects are related to the change in the properties of phonons with atomic mass. The most drastic phonon-related isotope effect is found for thermal conductivity resulting from the role of isotope “impurities” as phonon scatterers [1, 2]. In this presentation, we exploit these effects to enable phonon engineering in silicon nanowires using enriched Si isotopes. Here silicon nanowires were grown through metal-catalyzed vapor phase epitaxy using isotopically enriched (>99.9 %) monsilane precursors 28SiH4, 29SiH4, and 30SiH4 [3]. By controlling the isotopic content during the growth process, monoisotopic and isotopically disordered (28Six30Si1-x) nanowires were synthesized. Vibrational spectroscopy was employed to probe the phonon behavior and heat transport in this novel family of nanowires. The implication of our work for silicon nanowire-based thermoelectrics will be discussed.References:[1] I. Pomeranchuk, J. Phys. (Moscow) 5, 237 (1942).[2] P. G. Klemens, Proc. Phys. Soc. London, Sect. A 68, 1113 (1955).[3] O. Moutanabbir et al., Nano Today 4, 393 (2009).
10:30 AM - W1.6
Phonons in Ab Initio Generated Nanoporous Carbon.
Ariel Valladares 1 , Cristina Romero 1 , R. Valladares 2 , Alexander Valladares 2
1 Estado Solido y Criogenia, Instituto de Investigaciones en Materiales, UNAM, Mexico, D.F. Mexico, 2 Departamento de Fisica, Facultad de Ciencias, UNAM, Mexico, D.F. Mexico
Show AbstractWe investigate the vibrational density of states (vDOS) in nanoporous carbon (np-C) obtained from a crystalline diamond-like supercell with 216 atoms. These crystalline structures were subjected to an ab initio molecular dynamics process at constant temperature (1000 K) after the supercell edges were lengthened until we got a density of 1.38 g/cm3, following the procedure implemented by Valladares et al. [1] to generate amorphous porous materials. The density mentioned above represents a porosity of 61% for this diamond-like structure. The resulting sample is fundamentally amorphous and the pore sizes fall into the nanometer length scale [2]. The vDOS obtained is compared to the experimental results for amorphous samples, since no experimental determinations for porous materials exist. Our phonon results are also compared to simulational results for crystalline, amorphous and porous samples found in the literature. [1] Valladares, A.A.; Valladares, A.; Valladares, R.M. Computer modeling of nanoporous materials: An ab initio novel approach for silicon and carbon. Mater. Res. Soc. Symp. Proc. 988 (2007) 97-102.[2] Romero, C.; Valladares, A.A.; Valladares, R.M.; Valladares, A.; Calles, A. Ab initio computationally generated nanoporous carbon and its comparison to experiment. Mater. Res. Soc. Symp. Proc. 1145 (2008) 69-74.
11:15 AM - **W1.7
Phonons in Quantum Dots and Quantum Wires.
Michael Stroscio 1 2 3 , Mitra Dutta 1 3 , Banani Sen 1 , Sushmita Biswas 1
1 ECE, Univ. of IL at Chicago, Chicago, Illinois, United States, 2 BioE Department, Univ. of IL at Chicago, Chicago, Illinois, United States, 3 Physics Department, Univ. of IL at Chicago, Chicago, Illinois, United States
Show AbstractThis presentation will cover recent experimental findings and supporting theory for phonons in nanowires as well as in colloidal quantum dots. The experimetal studies have determined the phonon frequencies in polycrystalline ZnO nanowires as well as confined and interface phonons in selected CdS and CdSe colloidal quantum dots. The supporting theory is based on the continuum model [1-2] and provides comparisons with experimentally determined phonon frequencies for interface and confined phonon modes in quantum dots. Theoretical results also include comparisons of piezoelectric interactions in wurtzite and zincblends wires. References[1] Michael A. Stroscio and Mitra Dutta, Phonons in Nanostructures, (Cambridge University Press, Cambridge, 2001), ISBN: 0521792797; also published in a Russia Language Edition by Dauka Press (2006) and in a Chinese Language Edition, Beijing (2005).[2] Vladimir V. Mitin, Viatcheslav V. Kochelap, and Michael A. Stroscio, Quantum Heterostructures for Microelectronics and Optoelectronics, (Cambridge University Press, Cambridge, 1999). ISBN: 0521631777.
11:45 AM - W1.8
Time-Resolved Optical Spectroscopy of the Vibrational Dynamics of Individual Gold Nanorings.
Renaud Marty 1 2 , Arnaud Arbouet 1 2 , Christian Girard 1 , Adnen Mlayah 1 2 , Vincent Paillard 1 2 , Vivian Kaixin Lin 3 , Siew Lang Teo 3 , Sudhiranjan Tripathy 3
1 , CEMES-CNRS, Toulouse France, 2 , Université de Toulouse, Toulouse France, 3 , Institute for Materials Research and Engineering, A*STAR, Singapore Singapore
Show AbstractIn noble metal nanoparticles (NPs), the existence of localized surface plasmon resonances (LSPR) is responsible for the strong enhancement of their linear and non-linear optical properties, and their ability to concentrate the electromagnetic energy at the nanometer scale. Such NPs are promising for applications in integrated optics, biosensing and nanomedecine. Due to the strong dependence of the LSPR on the size and shape of the metal NPs, surface plasmons are sensitive probes of the mechanical properties of NPs. To overcome the averaging in NPs assembly, high sensitivity optical spectroscopy experiments have been developed to measure the extinction cross-section of individual NPs, as well as perform time-resolved optical spectroscopy.Using femtosecond pump-probe spectroscopy, we investigated the acoustic vibrations of isolated gold nanorings fabricated by electron beam lithography. The transient probe transmission displays clear oscillations, allowing the precise determination of the period and damping times of the vibration modes. The measured periods are compared to those calculated using elastic theory and assuming a continuous elastic medium. Good agreement is found for the lower order axisymetric vibration mode. Finite element numerical simulations confirm this identification and allowed us to study the influence of the shape of the nano-object and the substrate on the detected vibration mode.The damping time of the acoustic vibrations provides a unique opportunity to get information on the mechanical coupling and nanoparticle/environment interface quality. Contrary to ensemble experiments affected by both size and shape distributions, our data on isolated objects allow to get rid of the inhomogeneous contribution, for an unambiguous investigation of the coupling between the object and its environment. The measured damping times vary significantly on the probed nanoring, although the e-beam lithography allows fabrication with unprecedented control. These fluctuations have been ascribed to fluctuations of the mechanical coupling between the nano-object and the substrate. Finally, by changing the environment of the nanoring, we provide a clear evidence of the impact of the surrounding medium on the damping of the acoustic vibrations and compare the strength of the mechanical coupling between the metal nanoparticle and the susbtrate to a reference elastic medium.Reference: Marty et al, Damping of the acoustic vibrations of individual gold nanoparticles, Nanoletters 2011, accepted.
12:00 PM - W1.9
Phonon Confinement in Semiconducting Nanostructures.
Pedro Alfaro 1 , Rodolfo Cisneros 1 , Montserrat Bizarro 1 , Miguel Cruz-Irisson 2 , Chumin Wang 1
1 Instituto de Investigaciones en Materiales, UNAM, Mexico D.F. Mexico, 2 ESIME-Culhuacan, Instituto Politécnico Nacional, Mexico D.F. Mexico
Show AbstractDuring the last decade great research efforts have been focused on the nanostructured semiconductors, such as porous semiconductors and semiconducting nanowires. These nanomaterials possess two unusual features: (1) an extremely high ratio of surface area per unit volume, which could significantly modify the boundary condition of phonon states through its surface reconstruction and atoms adsorbed on the surface, and (2) an important reduction of system size to nanometer scale, becoming the energy levels of most elementary excitations discrete enough to be measured at macroscopic scale. In this work, we study the confinement of optical phonons in nanostructures by using the Raman spectroscopy. In particular, a microscopic theory based on the local bond-polarizability model is presented and applied to the analysis of porous silicon, porous germanium, and nanowires. Within the linear response approximation, the Raman shift intensity is calculated by means of the displacement-displacement Green’s function and the Born model, including central and non-central interatomic forces [1]. For porous systems, the supercell method is used and ordered pores are produced by removing columns of Si or Ge atoms from their crystalline structures. This microscopic theory predicts a remarkable shift of the highest-frequency of first-order Raman peaks towards lower energies, in comparison with the crystalline case. This shift is discussed within the quantum confinement framework and quantitatively compared with the experimental results obtained from porous silicon samples, which were produced by anodizing p-type (100)-oriented crystalline Si wafers in a hydrofluoric acid bath [2]. A good agreement is observed between the theoretical and experimental data, showing a possible phonon confinement in semiconducting nanostructures [3].[1] C. Wang and R. A. Barrio, Phys. Rev. Lett. 61, 191 (1988).[2] R. Cisneros, H. Pfeiffer, and C. Wang, Nanoscale Res. Lett. 5, 686 (2010).[3] P. Alfaro, R. Cisneros, M. Bizarro, M. Cruz-Irisson, and C. Wang, Nanoscale 3, 1246 (2011).
12:15 PM - W1.10
Phonons in Crystals of Nano Materials: How We Separate Phonon Terms in Heat Capacity.
Gang Mu 1 , Jingtao Xu 1 , Jiazhen Wu 1 , Satoshi Heguri 1 , Khuong Huynh 1 , Quynh Phan 1 , Youichi Tanabe 1 , Katsumi Tanigaki 1
1 WPI-AIMR/Department of Physics, Tohoku University, Sendai Japan
Show AbstractPhonons play an important role for controlling physical properties, such as a charge density of wave phase transition as well as a phonon mediated BCS superconductivity. The important information on phonons is generally extracted from specific heat capacity (Cp) measurements and neutron diffraction experimental data. In Cp experiments, both phonons and electrons are generally involved in the entire temperature range. Therefore, the term of electrons can be extracted only by separating the phonon contributions supposing the Debye model and/or the Einstein model in the category of harmonic phonons. In the case of a conventional crystal, this generally gives a good estimate. However, this sometimes can no longer be a reasonable estimate in such special materials as glass and nano structure materials of clathrates (polyhedral network materials) and Fe pnictide (quasi two-dimensional materials) [1-3]. Especially in the case of complex glass structure, many potentials are ordinarily made as minima of the complex energetic curvature and the tunneling mode of electrons or particles should seriously be taken into account because the temperature (T) evolution of Cp becomes T-linear as suggested by Anderson. When the phonon density of states (PDOS) is available experimentally or theoretically, a good estimate of the phonon term in Cp can be evaluated and accordingly one can separate the electron terms accurately for describing the intrinsic electronic contributions. In this meeting, we would like to present how we can describe the phonon contribution in the case of nano materials by employing some examples of clathrate thermoelectrics and Fe pnictide superconductors. [1] J.Tang, J-T Xu, S. Heguri, H. Fukuoka, S. Yamanaka, K. Akai, and K. Tanigaki, Phys. Rev. Lett., 105, 176402_1-4 (2010).[2] J-T Xu, J. Tang, K. Sato, Y. Tanabe, H. Miyasaka, M. Yamashita, S. Heguri, and K. Tanigaki, Phys. Rev.B, 82, 085206_1-6 (2010).[3] G. Mu et al., condmat (2011).
12:30 PM - W1.11
Lattice Dynamics of Polonium: Symmetry Breaking Phase Transitions and Surface Phonons.
Matthieu Verstraete 1 2 , Giorgio Benedek 3 4
1 Physics, University of Liege, Liege Belgium, 2 , ETSF, Louvain la Neuve Belgium, 3 Materials Science, Universita Milano Bicocca, Milan Italy, 4 DIPC, Universidad del Pais Vasco, San Sebastian, Pais Vasco, Spain
Show AbstractThe physics of phase transitions in pure elemental phases is regularly revived by the discovery of strange phenomena in simple systems. Polonium shows very delicate physics, being the only simple cubic element at low temperature and pressure, due to the interplay between relativistic and chemical bonding effects. We show that this balance is broken at higher temperatures, where entropy trumps relativistic effects, exceptionally reducing symmetry upon increasing temperature. The strange vibrational structure of Po carries over to its surface, which also shows an interplay of spin-orbit coupling and surface nesting to stabilize the cubic [100] surface, a signature which should also appear in neutron or alpha particle scattering.
12:45 PM - W1.12
Hypersonic Properties of Polymer Films and Multi-Layers.
Paul Walker 1 , Eric Young 1 , Andrey Akimov 1 , James Sharp 1 , Vitaly Gusev 2 , Anthony Kent 1
1 School of Physics and Astronomy, University of Nottingham, Nottingham United Kingdom, 2 Laboratoire de Physique de l’Etat Condensé, Université du Maine, Le Mans France
Show AbstractPicosecond acoustic measurements were performed on ultrathin polymer films and thin film multilayers of polystyrene and polyvinylpyrollidone supported on silicon (Si) substrates using a state of the art THz acoustic technique. In these experiments, a femtosecond pulsed laser is used to excite picosecond duration strain pulses in an aluminium film evaporated on the reverse side of the Si substrate. These strain pulses then propagate through the substrate and interact with the polymer film/multi-layer. Vibrations in the film are detected optically using an optical probe pulse which is split form the pump laser beam, passed through an optical delay line and reflected from the surface of the polymer film/multi-layer. The reflected beam is detected using a photodiode and lock-in amplifier referenced to an optical chopper in the pump path.Ultrathin films of polystyrene and a styrene-butadiene-styrene block copolymer were found to exhibit quantized closed-pipe organ like modes in the 0- 50 GHz regime that were attributed to vibrations of the entire polymer film [1]. Thin film multilayer structures were found to display folded phonon dispersion curves that are characteristic of super-lattice structures [2]. These structures have potential applications in GHz and THz optical switching and biosensing applications.[1] A. V. Akmov, E. S. K. Young, J. S. Sharp, V. Gusev and A. J. Kent: "Coherent hypersonic closed-pipe organ like modes in supported polymer films", Appl. Phys. Lett., in press (2011).[2] P. M. Walker, J. S. Sharp, A. V. Akimov and A. J. Kent: "Coherent elastic waves in a one dimensional polymer hypersonic crystal", Appl. Phys. Lett. 97, 073106 (2010).
W2: Phonon Scattering
Session Chairs
David Hurley
Masashi Yamaguchi
Monday PM, November 28, 2011
Room 313 (Hynes)
2:30 PM - **W2.1
Understanding the Lattice Thermal Conductivity in Real Materials: Phonon-Phonon and Phonon-Defect Interactions from Atomistic Simulations.
Aleksandr Chernatynskiy 1 , Judy Pang 2 , Bowen Deng 1 , Ben Larsen 2 , William Buyers 3 , Simon Phillpot 1
1 Materials Science and Engineering, University of Florida, Gainesville, Florida, United States, 2 , ORNL, Oak-Ridge, Tennessee, United States, 3 National Research Council, Chalk River Laboratories, Chalk River, Ontario, Canada
Show AbstractDespite tremendous progress in understanding lattice thermal conductivity, there are still many open questions. Advances in the computer power and simulation techniques allow to address those from the fundamental level of the interactions between individual atoms. From the microscopic perspective the interactions of phonons with each other and with lattice defects of control thermal transport. In this talk we will discuss techniques for the calculation of the thermal conductivity and demonstrate their applications. We describe spectral analysis of phonon/phonon interactions, using fluorite-structured UO2 as a prototype. This analysis is performed using Lattice dynamics/Boltzmann Transport Equation based techniques. These methods also allow us to determine phonon lifetimes, which are compared with experimental results on single crystals. The effects of microstructural defects, including dislocations on the thermal-transport properties are analyzed, from non-equilibrium molecular dynamics simulations. Finally, we characterize the mechanistic details of phonon-defect interactions for the specific case of the Si/SiO2/Si interfacial system from phonon wave packet simulations. This work was co-authored by a subcontractor (SRP) of the U.S. Government under DOE Contract No. DE-AC07-05ID14517, under the Energy Frontier Research Center (Office of Science, Office of Basic Energy Science, FWP 1356). Accordingly, the U.S. Government retains and the publisher (by accepting the article for publication) acknowledges that the U.S. Government retains a nonexclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for U.S. Government purposes.
3:00 PM - W2.2
Predicting the Thermal Conductivity of Defected Systems Using the Spectral Energy Density.
Jason Larkin 1 , Alan McGaughey 1
1 , Carnegie Mellon, Pittsburgh, Pennsylvania, United States
Show AbstractAccurately predicting the thermal conductivity of a dielectric or semiconducting material requires the properties of phonons from the entire Brillouon zone. Of particular importance are the phonon lifetimes, which are not accessible in experiment. Common theoretical techniques (e.g., lattice dynamics calculations) require the use of a perfectly periodic crystal to predict the required phonon properties. These techniques, however, break down when the system’s periodicity is broken (e.g., through point defects or the presence of an external fluid). The spectral energy density technique, where the atomic velocities are projected onto traveling waves, can predict the phonon properties of systems with a small perturbation from periodicity. In this study, we demonstrate that the spectral energy density technique can be used to model A_{1-x}B_x Lennard-Jones alloys with mass and bond point defects up to concentrations of x=0.1. The phonon lifetimes of these defected systems show a fourth-power scaling with the inverse of the phonon frequency, consistent with Rayleigh scattering theory. The phonon dispersion is found to agree with predictions from a virtual crystal approximation.
3:15 PM - W2.3
Thermal Transport in Zinc Antimonides.
Lasse Bjerg 1 3 , Georg K. H. Madsen 2 , Jeffrey Grossman 3 , Bo Iversen 1
1 iNANO & Department of Chemistry, Aarhus University, Aarhus C Denmark, 3 Department of Materials Science & Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 ICAMS, Ruhr-Universität Bochum, Bochum Germany
Show AbstractThermoelectric materials are capable of interconverting heat and electricity. To obtain high conversion efficiency, it is desirable to have as low a thermal conductivity as possible in thermoelectric compounds. Zinc antimonides are, generally, interesting as thermoelectric materials. The electronic structure, phonon dispersion, and elastic constants of ZnSb have been calculated using DFT methods. An atomistic potential has been produced and compared to the DFT results for the harmonic energy surface in zinc antimonides. The use of this potential for the calculation of anharmonic effects and thermal conductivity is discussed.
3:30 PM - W2.4
Coherent Acoustic Vibrations and Electron-Phonon Interactions in Nanostructured Copper Arrays.
Andrej Halabica 1 , Jianxun Liu 1 , Pei-I Wang 1 , Masashi Yamaguchi 1 , Gwo-Ching Wang 1 , Toh-Ming Lu 1
1 Physics, Applied Physics & Astronomy, RPI, Troy, New York, United States
Show Abstract Nanoscale confinement effects on phonon properties are critical elements for the thermal and electrical transport properties. Nanoscale copper materials are important ingredient of nanoscale devices, and the electron-phonon interactions in nanoscale copper and related electrical resistivity have been a topic of extensive studies. In this presentation, we report the results of ultrafast spectroscopic study of coherent acoustic vibrations and electron-phonon interactions in copper nanostrips. The samples are periodic copper strips with periods of 100 -150 nm and various strip width of 10-100 nm. Both of polycrystalline and epitaxial single crystal nanostrips are grown on a high resistivity silicon substrate by e-beam lithography. Electron-phonon interactions in copper nanostripes were studied using ultrafast transient reflectivity measuremnets. Electrons near the Fermi surface were excited with optical pump pulses with wavelength of 800 nm to above the Fermi surface. Temporal change of the electron distribution due to the electron-phonon coupling following the excitation pulse was detected by the probe pulses with the energy close to the resonance to d-band to near Fermi surface transition. Electron energy decay was modeled using the two-temperature model.1 At a relatively high pump power, the signal decay profile of the transient reflectivity signal showed dependencies on the pump power and relative orientation of the stripes to the pump pulse polarization. However, at the low pump power limit, these signals taken with different polarization converge to the same decay time and the anisotropy disappears for the sample with the 40 nm width. These power and orientation dependences are due to the nonlinear temperature dependence of the reflectivity and temperature dependent electron heat capacity. In addition, convergence of the signal of different orientation agrees with the fact that the measurement is in the thermal regime within the electron system. Coherent acoustic vibrations were excited with pump pulse with various wavelengths and probed at 800 nm, which is off resonant to the d-band to Fermi energy transition. Two kinds of oscillations with frequencies of 6.0 and 60.2 GHz were observed. Amplitudes of both vibrational components depend on the relative directions of the probe polarization and long axis of the strips, suggesting that the detection mechanism of the vibration is strain induced reflectivity change. While the ratio of the vibrational modes and thermal background does not change with the pump pulse power density, the ratio strongly depends on the excitation pump wavelength. The results suggest that the excitation of the vibrational modes depend on the excited electron energy, and the excitation is not a simple photothermal process.1.M. I. Kaganov, I. M. Lifshitz and L. V. Tanatarov, "Relaxation between Electrons and the Crystalline Lattice", Sov. Phys. JETP, 4, 173 (1957).
4:15 PM - **W2.5
Interaction of Elastic Waves with Dislocations.
Agnes Maurel 1 , Fernando Lund 1 , Pagneux Vincent 1 , Barra Felipe 1
1 Agnes Maurel, LOA/Institut Langevin, Paris France
Show AbstractAn overview of recent work on the interaction of elastic waves with dislocations is given. The perspective is provided by the wish to develop nonintrusive tools to probe plastic behavior in materials. For simplicity, ideas and methods are first worked out in two dimensions, and the results in three dimensions are then described. These results explain a number of recent, hitherto unexplained, experimental findings. The latter include the frequency dependence of ultrasound attenuation in copper, the visualization of the scattering of surface elastic waves by isolated dislocations in LiNbO3, and the ratio of longitudinal to transverse wave attenuation in a number of materials.Specific results reviewed include the scattering amplitude for the scattering of an elastic wave by a screw, as well as an edge, dislocation in two dimensions, the scattering amplitudes for an elastic wave by a pinned dislocation segment in an infinite elastic medium, and the wave scattering by a sub-surface dislocation in a semi-infinite medium. Also, using a multiple scattering formalism, expressions are given for the attenuation coefficient and the effective speed for coherent wave propagation in the cases of anti-plane waves propagating in a medium filled with many, randomly placed screw dislocations; in-plane waves in a medium similarly filled with randomly placed edge dislocations with randomly oriented Burgers vectors; elastic waves in a three-dimensional medium filled with randomly placed and oriented dislocation line segments, also with randomly oriented Burgers vectors; and elastic waves in a model three-dimensional polycrystal, with only low angle grain boundaries modeled as arrays of dislocation line segments.The theory suggests a non-intrusive way of measuring dislocation density in materials, which is confirmed with Resonant Ultrasound Spectroscopy (RUS) experiments using aluminum.
4:45 PM - W2.6
Soft Mode Behavior of Interface-Related Modes in AlGaN/GaN HEMT Structures Probed by UV Raman Scattering.
Esther Alarcon Llado 1 2 3 , Vivian Lin 3 , Surani Dolmanan 3 , Siew Lang Teo 3 , Alois Krost 4 , Armin Dadgar 4 , Sudhiranjan Tripathy 3
1 , Lawrence Berkeley National Laboratory, Berkeley, California, United States, 2 , Ecole Polytechniqu Federale Lausanne, Lausanne Switzerland, 3 , Institute of Materials Research and Engineering, A*STAR, Singapore Singapore, 4 , Otto von Guericke Universitat, Magdeburg Germany
Show AbstractIn the AlGaN/GaN system, a strong bending of the electronic bands leads to the formation of a highly dense and confined electron gas in the GaN, very close to the heterointerface. In such systems, thus, the electronic transport mainly takes places at the interface between AlGaN and GaN. The importance of phonons and their interactions in electronics and optoelectronics is well known. In the case of AlGaN/GaN based devices, interface-related phonons may also play an important role in electrical properties and can provide information about interface properties.In this work we provide a temperature-dependent Raman study of the interface properties of AlGaN/GaN HEMT based structures with high Al contents. AlGaN very thin films (<20nm) grown by different systems (metal-organic chemical vapor deposition and molecular beam epitaxy) and on different kind of substrates were probed. TEM measurements show smooth interfaces for both samples. Raman spectra were recorded in a backscattering configuration using the 325 nm line of a He-Cd laser as excitation source. Under such conditions, the Raman signal is basically confined at the interface due to the strong resonance with GaN.At room temperature, the typical Raman spectrum of such structures shows four main intense features. Apart from the well-known E2h and A1(LO) modes from the GaN buffer, three additional peaks at around 610 (IF1), 703 cm-1 (IF2) and 720 cm-1 (IF3) are observed for both samples. While IF2 has been observed in bulk material and is attributed to a disorder-activated mode characteristic of the one-phonon density of states of the III-N system, IF1 and IF3 have never been reported before and we assign them to interface modes (IF). A low-temperature stage was used to vary the sample temperature from 80 to 450 K. The GaN-related modes, as well as IF2 and IF3, show a typical temperature-induced red-shift. By contrast, the IF1 Raman feature blue-shifts with increasing temperature by more than 12 cm-1 from 80 to 450 K. This soft mode behavior suggests that the mode is strongly related to the interface.By a voltage-dependent study, we also show the large polar character of the IF modes. We observe that the intensity of IF1 and IF2 significantly drops with increasing voltage. We suggest that these additional modes are related to and/or confined in the AlxGa1-xN/GaN interface. Their spectral weight should decrease with increasing film thickness. The very small thickness of the present AlxGa1-xN layers, the strong electric fields at the interface due to the high Al contents, and the use of near-resonant excitation, might be the reason that these modes are observed in our experiments. Finally, we discuss the possible effects of the presence of such modes on AlGaN/GaN-based devices.
5:00 PM - W2.7
Diffuse Scattering of X-Rays in Nanoscale Silicon.
Gokul Gopalakrishnan 1 , Josef Spalenka 1 , David Czaplewski 2 , Martin Holt 2 , Tobias Schulli 3 , Paul Evans 1
1 Materials Science and Engineering, University of Wisconsin, Madison, Wisconsin, United States, 2 Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois, United States, 3 , European Synchrotron Radiation Facility, Grenoble France
Show AbstractThe emerging ability to engineer thermal properties using nanostructures arises in large part from confinement effects on the phonon dispersion. While a number of theoretical predictions have been made for the modification of the phonon band structure due to quantum confinement, so far, only the high energy regime of optical phonons near the Brillouin Zone center have been accurately measured in nanoscale samples. While conventional techniques such as Inelastic x-ray or neutron scattering have been successfully used to probe large wave vectors in macroscopic samples, the signals become impractically weak in small ensembles of nanoscale objects. Advances in high luminosity x-ray sources at synchrotron facilities have revived interest in the technique of Thermal Diffuse Scattering, which collects information from elastic scattering of x-rays by phonons.In this talk we describe results of x-ray diffuse scattering measurements performed on silicon membranes of varying thickness. The membranes were fabricated by selective wet etching of silicon-on-insulator samples releasing suspended silicon windows which were further thinned by reactive ion etching. Samples of thickness from 5 microns down to 50 nanometers were studied using a zone plate focused microbeam of 11 keV x-rays in a transmission measurement geometry. Details of the fabrication process and results of the scattering measurements will be presented.
5:15 PM - W2.8
Manipulation of the Cooling Behavior on the Nanoscale.
Anja Hanisch-Blicharski 1 , Simone Wall 1 , Tim Frigge 1 , Friedrich Klasing 1 , Annika Kalus 1 , Martin Kammler 1 , Horn-von Hoegen Michael 1
1 Department of Physics, University of Duisburg-Essen, Duisburg Germany
Show AbstractHow does a nanoscale thin film on a substrate cool? In order to investigate this seemingly simple question we have used ultrathin epitaxial Bi(111) films on a silicon substrate as a model system to explore the limits of the well accepted and commonly used acoustic mismatch model (AMM). AMM describes the thermal boundary conductance σtbc of films with an abrupt and smooth interface. In the framework of AMM the energy is carried by phonons which were treated as elastic waves. Most of the phonons are trapped in the Bi film due to total internal reflection. Only a small fraction of phonons with incident angles smaller than the critical angle can escape the film and contribute to the heat transport resulting in a low σtbc of only 1400 W/(cm2K). We extended the study to film thicknesses as low as 2nm. σtbc is determined from the cooling rate of the Bi film upon heating with a fs-laser pulse in a pump-probe experiment. The transient temperature was determined via the Debye-Waller effect employing surface sensitive ultrafast electron diffraction in reflection geometry. For films thinner than the mean free path of the phonons, we found that the thermalisation and repopulation of phonons smaller than the critical angle causes the bottleneck for the phonon transmission through the interface. This effect slows down the cooling of the film considerably, by 250%.
5:30 PM - W2.9
Elastic Anomaly of Pd Thin Films at Low Temperatures Studied by Picosecond Ultrasounds.
Kenichi Tanigaki 1 , Naoki Omori 1 , Tatsuya Kusumoto 1 , Hirotsugu Ogi 1 , Nobutomo Nakamura 1 , Masahiko Hirao 1
1 Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka, Japan
Show AbstractThe elastic constants of solids generally increase with cooling down from room temperature, and they remain unchanged below about 100 K since invariability of interatomic distances caused by the zero-point energy. However, at low temperatures, the elastic constants of Pd show unusual temperature dependences in spite of their normal thermal-expansion behavior. Contribution of the change in electron band structure is suggested as one possible cause. However, its intrinsic mechanism remains unclear because it is hard to separate contributions of electron and phonon (band structure and lattice vibration).In this study, we investigate the elastic properties of Pd thin films using the picosecond laser ultrasound (PSLU) method at cryogenic temperatures. The out-of-plane atomic distance of Pd thin films become very short at low temperatures because of restriction of the in-plane deformation by the substrate, and the resultant elastic strain can be significant, which is never achieved for bulk materials. On the other hand, the band structure of thin films is commonly similar to that of bulk materials. Thus, with larger elastic strain, we can estimate the contribution of phonons through the unharmonicity of interatomic potential. We develop an optical system for PSLU measurements at low temperatures, which is equipped with a cryostat vacuumized by a turbo molecular pump. The specimen is set in the cryostat and cooled by liquid He through a Cu heat exchanger. We can conduct the PSLU measurements through a quartz window of the cryostat. A titanium-sapphire pulse laser at 800 nm wavelength and 140 fs pulse width is separated into the pump light and the probe light. The former is used to generate an ultrahigh-frequency (~50 GHz) acoustic pulse through an instantaneous thermal expansion, and the latter used is for detection. The reflectivity change in probe light shows multiple pulse echoes or acoustic-phonon resonances depending on film thickness, which is determined accurately by X-ray reflectivity measurements. They give us the sound velocity along the film thickness direction and eventually the out-of-plane elastic stiffness of films. The elastic modulus of Pd thin films increases remarkably compared with the predicted elastic constant of the textured Pd using its bulk elastic constants at low temperatures. This result indicates the phonon’s contribution is normal. Thus, the electron band structure is considered to play the dominant role in Pd’s elastic anomaly. Furthermore, Ab-initio calculations of the effect of Fermi-Dirac broadening on elastic constants are performed to enhance understanding of the experimental results.
5:45 PM - W2.10
Low Phonon Energy BaCl2 Nanocrystals in Nd3+-Doped Fluorozirconate Glasses and Their Influence on the Fluorescence Properties.
Charlotte Pfau 1 , Ulrich Skrzypczak 1 , Manuela Miclea 1 , Paul-Tiberiu Miclea 2 3 , Christian Bohley 1 , Stefan Schweizer 1 2
1 , Centre for Innovation Competence SiLi-nano®, Martin Luther University of Halle-Wittenberg, Halle (Saale) Germany, 2 , Fraunhofer Center for Silicon Photovoltaics, Halle (Saale) Germany, 3 , Institute of Physics, Martin Luther University of Halle-Wittenberg, Halle (Saale) Germany
Show AbstractMulti-phonon relaxation (MPR) is one of the major fluorescence quenching mechanisms in rare-earth doped glasses. The MPR rate is significantly reduced in hosts providing low phonon frequencies like fluorozirconate (FZ) glasses (< 590 cm-1 [1]), which are based on the standard ZBLAN formulation made from Zr, Ba, La, Al, and Na fluorides [2]. The rare-earth ion Nd3+ shows emissions in FZ-based glasses from energy levels that would be quenched in high-phonon energy glasses. The FZ glasses were additionally doped with Cl ions by partial substitution of the fluorides BaF2 and NaF for BaCl2 and NaCl, respectively. The Cl substitution itself does not change the phonon frequency of the base FZ glass significantly, but subsequent annealing leads to the formation of BaCl2 nanocrystals in the glass. BaCl2 has a distinct lower phonon frequency (< 200 cm-1 [3]) than the FZ base glass. An interaction of the Nd3+ ions with the low frequency phonons of BaCl2 should result in significant influence on the Nd3+ fluorescence. For a deeper understanding, the phonon spectra and the fluorescence properties of the Nd3+- and chlorine-doped fluorozirconate glasses are investigated. Raman measurements of the heat-treated glasses show additional phonon bands at low phonon energies, this can be attributed to BaCl2 nanocrystals. The phonon spectra of the nanocrystals change with the annealing conditions due to a structural phase transition of the BaCl2 nanocrystals. For comparison, the phonon spectra of BaCl2 in different structural phases are calculated from first principles. Furthermore, the influence of the nanocrystals on the fluorescence properties is analyzed and interpreted with respect to a modified MPR. Time-resolved spectroscopy is applied to determine the fluorescence lifetime of the excited Nd3+ levels in different environments. In addition, temperature dependent lifetime measurements are performed to obtain information on multi-phonon relaxations and the phonons involved in the relaxation process. It will be shown that there is a strong interaction between the Nd3+ ions and the phonons of the BaCl2 nanocrystals. The formation of the nanocrystals increases the fluorescence efficiency of several Nd3+ transitions significantly.[1] S. Aasland, M.-A. Einarsrud, and T. Grande, J. Phys. Chem. 100, 5457 (1996).[2] I. D. Aggarwal and G. Lu, Fluoride Glass Fiber Optics (Academic, London, 1991).[3] A. Sadoc and R. Guillo, C. R. Acad. Sci., Ser. B 273, 203 (1971).
Symposium Organizers
Subhash L. Shinde Sandia National Laboratories
David H. Hurley Idaho National Laboratory
Gyaneshwar P. Srivastava University of Exeter
Masashi Yamaguchi Rensselaer Polytechnic Institute
W3: Phonon Transport-Bulk Materials
Session Chairs
Anthony Kent
Ali Shakouri
Tuesday AM, November 29, 2011
Room 313 (Hynes)
9:00 AM - **W3.1
Experimental and Theoretical Studies on Phonon Transport: From Bulk Materials to Nanostructures.
Gang Chen 1
1 , MIT, Cambridge, Massachusetts, United States
Show AbstractIn this talk, we will present recent progress in experimental and theoretical investigationsof phonon transport in bulk materials, and across single interfaces and superlattices.We investigate spectral distribution of phonon mean free path in bulk materials viacombined theoretical and experimental studies. Theoretically, we use first-principlecalculations to extract anharmonic force constants, and compute the phonon relaxationtime due to phonon-phonon scattering. Experimentally, we extend an optical pump-and-probe technique to measure contributions of phonons with different mean free paths tothermal conductivity via systematically changing the size of the heated regions. Thepump-probe experimental system is also used to probe phonon transport across a singleinterface and superlattices. Our experiment design identifies coherent phonon transportin superlattices.This work is supported by S3TEC, a DOE BES funded EFRC.
9:30 AM - W3.2
A Detailed Theoretical Study of the Thermal Conductivity ofBi2(Te0.85Se0.15)3 Single Crystals.
Ovgu Yelgel 1 , Gyaneshwar Srivastava 1
1 Physics, University of Exeter, Exeter United Kingdom
Show AbstractWe present a theoretical investigation of the phonon conductivity of Bi2(Te0.85Se0.15)3 single crystals by using the Debye model within the single-mode relaxation-time approximation and a detailed account of alloy, electron-phonon, and three-phonon interactions [1]. Different levels of n-doping from SbI3 and CuBr dopants were considered. The total thermal conductivity was obtained by combining the electronic polar (κel), lattice (κph), and electronic bipolar (κbp) contributions. The κel contribution was calculated within the nearly-free electron model [2], and the κbp contribution was obtained by employing Price’s theory [3]. The ‘dip’ observed in the thermal conductivity-temperature curve for the alloy at around 350 K [4] is successfully explained to arise from the joint contribution from phonons, donor electrons, and electron-hole pairs. Fermi energy, Seebeck coefficient and electrical conductivity were calculated within the nearly-free electron approximation applying the Fermi-Dirac statistics. The computed results are used to explain the temperature variation of thermoelectric figure of merit (ZT) reported in the experimental measurements by Hyun et al. [4]. The ZT of the alloy doped with 0.1 wt.% CuBr is improved by more than 50% over the results obtained for the Bi2Te3 single crystal.[1] G. P. Srivastava, ‘The Physics of Phonons’, (Taylor and Francis Group, New York, 1990).[2] J. M. Ziman, ‘Electrons and Phonons’, (Clarendon Press, Oxford, 1960).[3] P. J. Price, Phil. Mag., 46, 1252 (1955).[4] D. B. Hyun et al., J.Mat.Sci., 33, 5595 (1998).
9:45 AM - W3.3
Thermal Conductivity of Gallium Arsenide from First-Principles.
Jivtesh Garg 1 , Tengfei Luo 1 , Keivan Esfarjani 1 , Junichiro Shiomi 1 2 , Gang Chen 1
1 Mechanical Engineering, MIT, Cambridge, Massachusetts, United States, 2 Mechanical Engineering, Tokyo University, Tokyo Japan
Show AbstractWe compute the thermal conductivity of GaAs from first-principles by using harmonic and anharmonic force constants which are derived from density-functional perturbation theory. Boltzmann transport equation is solved in the single-mode relaxation time approximation to obtain phonon relaxation times which are then used along with phonon frequencies, group velocities and Bose-Einstein populations to compute the thermal conductivity. The approach is found to yield excellent agreement with experimentally measured values. The predictive power of these first-principles calculations allows to access information of accumulative thermal conductivity with respect to phonon mean free path which can be used to lay out design rules for low thermal conductivity materials. For example at 300 K we show that 50% of the heat is conducted by phonons of mean free path larger than 150 nm. This information can be used to design nanostructured materials where additional scattering mechanisms introduced by the presence of grain boundaries or nanoparticles distributed around these optimal length scales can reduce the phonon mean free path and thus the thermal conductivity.(This material is based upon work supported as part of the S3TEC, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-FG02-09ER46577.)
10:00 AM - W3.4
Phonon Conduction in Lead Selenide from First Principles.
Zhiting Tian 1 , Keivan Esfarjani 1 , Takuma Shiga 2 , Jivtesh Garg 1 , Junichiro Shiomi 2 , Gang Chen 1
1 Mechanical Engineering, MIT, Cambridge, Massachusetts, United States, 2 , University of Tokyo, Tokyo Japan
Show AbstractLead selenide (PbSe) is much less frequently considered for thermoelectric than its sister material PbTe. Counter-intuitively, PbSe has even lower thermal conductivity despite the lighter mass. The low thermal conductivity of PbSe might lead to potential good thermoelectric performance. In this study, we performed first-principles calculations to detail the spectral phonon transport properties of PbSe and to understand the low thermal conductivity. We first extract harmonic and anharmonic force constants from density functional theory calculations within a supercell. After validating anharmonic force constants, perturbation theory is used to extract the phonon lifetimes and compute the thermal conductivity. The frequency and polarization dependent phonon lifetimes and thermal conductivity accumulation with phonon mean free paths will be presented. Compared to other modes, TO modes turn out to be highly anharmonic with very short lifetimes, especially near the center of the first Brillouin zone. Consequences related to thermal transport in PbSe will be discussed.This material is based upon work supported as part of the S3TEC, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-FG02-09ER46577
10:15 AM - W3.5
Influence of Impurities on Phonon Transport in ZnO and ZnS: Ab Initio Studies.
Michael Bachmann 1 , Michael Czerner 1 , Saeideh Edalati-Boostan 1 , Christian Heiliger 1
1 1.Physikalisches Institut, Justus-Liebig Universität, Giessen Germany
Show AbstractWe present ab initio calculations of ballistic phonon scattering at sulphide impurities in ZnO and of oxygen impurities in ZnS. The interatomic potential of the scatterer is calculated using density functional theory. In particular, the interatomic force constants are calculated, which are used in a atomistic Green’s function (AGF) method [1] to calculate the transmission function and the phonon density of states. Knowing the transmission function we calculate the thermal conductance within the linear response regime. We estimate the influence of such impurities on the conductance by comparing our results with calculation of pure ZnO and pure ZnS. In particular, we investigate the frequency dependence of the scattering.[1] W. Fisher, T. & Mingo, N.Numerical Heat Transfer, Part B: 2007, 51, 333
10:30 AM - W3.6
Effect of Mechanical Alloying on Lattice Thermal Conductivity of Uni-Axial Hot Pressed Chromium Disilicide.
Suresh Perumal 1 , Ujwala Ail 2 , Stephane Gorsse 2 , Arun Umarji 1
1 Materials Research Centre, Indian Institute of Science, Bangalore, Karnataka, India, 2 Institut de Chimie de la Matière Condensée de Bordeaux , ICMCB-CNRS, Bordeaux France
Show AbstractNowadays, searching for a renewable energy sources have been a major challenge for researcher working in a various fileds such as solar cell, fuel cells, hydrogen storage and so on. Among them, thermoelectric effect also contributes by converting waste heat into electricity with non-mechanical movable parts and receives much attention due to its thermal, chemical and mechanical stability. Transition metal silicides are being studied extensively these days due to its structural and thermal stability at high temperatures. Chromium disilicide (CrSi2) is degenerate semiconducting material with narrow band gap of 0.35eV and belongs to hexagonal C40 crystal structure with space group of P6222. CrSi2 can be a predominant material for high temperature thermoelectric applications because of its high electrical conductivity (σ = 1x105 S/m at 300K) and Seebeck coefficient (S = 100 µV/K at 300K) over a temperature range. However, its practical applications are limited due to high thermal conductivity (κ = 10 W/m.K). The total thermal conductivity can be reduced by decreasing the grain size which is done by mechanical alloying. Hence an attempt has been made for reducing lattice thermal conductivity by mechanical alloying which is largely contributing to total thermal conductivity in CrSi2. The stoichiometric composition of Cr