Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session B35: Focus Session: Nanoscale Thermal, Thermoelectric and Mass Transport: Theory and Simulation |
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Sponsoring Units: DMP Chair: Joel E. Moore, University of California, Berkeley Room: Baltimore Convention Center 338 |
Monday, March 13, 2006 11:15AM - 11:51AM |
B35.00001: Electromigration Forces on Ions in Carbon Nanotubes Invited Speaker: Due to their unique structural, electronic, and optical properties carbon nanotubes (CNs) are promising candidates for future nanoelectronic devices. Recently, field-effect transistors (FETs) from single-wall CNs have been a research focus. In particular, ballistic transport has been demonstrated [1] and key transport parameters compare well with state-of-the-art silicon FETs.\\ In many cases improved CN-FET performance has been achieved by the use of dopants such as alkali metal atoms (see e.g.~[2]). However, during transistor operation a current-induced, {\sl electromigration}, force will be exerted on the alkali metal ions. Due to the low diffusion barriers the alkali ions may move along the CN which can influence the FET characteristics. On the other hand, electromigration forces can be used to intentionally transport atoms along a CN~[3].\\ Here, we present self-consistent non-equilibrium Greens function calculations to treat a ballistic CN-FET within a tight-binding approximation. We use a cylindrically symmetric device and calculate the current-induced forces on ions located either inside or outside of the CN. We observe that the forces are especially large in the turn-on regime of the transistor, and much smaller in the off- and on-states. The electromigration forces are mainly due to momentum transfer from the charge carriers, i.e. due to the ``wind'' forces. The sign of the ``effective valence'' $Z^*$ is independent of the actual charge sign, but can be reversed with gate voltage, providing a dramatic illustration of the quantum character of the wind force.\\ \protect{[1]} A.~Javey, J.~Guo, Q.~Wang {\it et al.}, Nature {\bf 424}, 654 (2003).\\ \protect{[2]} M.~Radosavljevi\'c, J.~Appenzeller, and Ph.~Avouris, Appl.~Phys.~Lett.~{\bf 84}, 3693 (2004).\\ \protect{[3]} B.~C.~Regan, S.~Aloni, R.~O.~Ritchie, U.~Dahmen, and A.~Zettl, Nature {\bf 428}, 924 (2004). [Preview Abstract] |
Monday, March 13, 2006 11:51AM - 12:03PM |
B35.00002: Molecular Dynamics Simulations of the Thermal Conductivities of Group IV Bulk Materials and Nanowires John Reed, Andrew Williamson, Giulia Galli We present the results of equilibrium molecular dynamics simulations of the thermal conductivities of bulk C, Si, Ge, and SiC using the Green-Kubo formalism. We use an empirical interatomic potential developed by Tersoff [1] and investigate the effects of modifications to this potential suggested by Porter et al [2]. We also investigate the effects of choosing a symmetric versus nonsymmetric definition of the local heat. A generalization of this approach to study the dependence of the thermal conductivity of SiGe nanowires on their size and composition will also be presented. [1] J. Tersoff, PRB 39 (8), 5566-5568 [2] L. Porter, J. Li, S. Yip, J. Nuc. Matl. 246 (1997) 53-59 This work was performed under the auspices of the U.S. Dept. of Energy at the University of California/Lawrence Livermore National Laboratory under contract no. W-7405-Eng-48. [Preview Abstract] |
Monday, March 13, 2006 12:03PM - 12:15PM |
B35.00003: The Nanocomposite Approach to Enhanced Thermoelectric Performance G. Chen, R. Yang, H. Lee, Q. Hao, M. Tang, M. S. Dresselhaus, D. Wang, Z. Ren, J. P. Fleurial, P. Gogna Model calculations and experimental results confirm that a nanocomposite approach leads to an enhancement in the thermoelectric performance of a bulk nanocomposite sample based on Si-Ge relative to its 3D alloy counterpart, though the predictions are quite general and should be applicable to a variety of nanocomposite systems. Modeling and experimental results here are reported for nanocomposites of Si-Ge made of Si and Ge nanoparticles (typically the particles are 20 nm or less in size prepared from the liquid phase or by ball milling or other techniques) and consolidated by hot press in an inert argon atmosphere to theoretical density. Most important is the large decrease in the thermal conductivity, well below that of the alloy of the same composition, both at room temperature and up to 1000K. Although the electrical conductivity decreases somewhat, the selective filtering of the high energy electron carriers enhances the Seebeck coefficient much more than the decrease in electrical conductivity, resulting in a gain in the power factor as well over a large temperature range. Emphasis is given to physical phenomena associated with nanostructures that serve to enhance the thermoelectric performances generally, and can be used for other Nanocomposite systems. The authors acknowledge support from NASA under the Radio Isotope Power Conversion program. [Preview Abstract] |
Monday, March 13, 2006 12:15PM - 12:27PM |
B35.00004: Theory of the thermoelectric properties of semiconductor-matrix nanocomposites Natalio Mingo, David Broido We theoretically investigate the thermoelectric properties of a nanocomposite nanowire array where the matrix material is a semiconductor. We take InSb to be the nanocomposite matrix that surrounds an array of cylindrical holes, and we calculate the density-optimized power factor, P, and the lattice thermal conductivity, k, employing a relaxation time approach and including band nonparabolicity. For fixed aspect ratio of wire diameter to cylindrical-hole period, we obtain universal curves for P. For small period, we find that P is enhanced above the bulk value with the magnitude of this enhancement increasing with the aspect ratio. For k, we model the phonon scattering by a frequency-dependent relaxation time [1] and use a boundary-scattering geometry introduced by Prasher [2]. For fixed aspect ratio and small periods we find reductions in k of around 50\%. Our results for P and k suggest that choosing a thermoelectric material as the matrix of a nanowire composite can contribute to enhance the composite’s ZT. [1] N. Mingo and D. A. Broido, Phys. Rev. Lett. 93, 246106 (2004). [2] R. Prasher, submitted. [Preview Abstract] |
Monday, March 13, 2006 12:27PM - 12:39PM |
B35.00005: Molecular dynamics simulation of shock-induced chemical, mechanical and thermal processes in Ni/Al nanolaminates Shijin Zhao, Timothy Germann, Alejandro Strachan Nanostructured metastable intermolecular composites (MICs) are a new class of energetic materials with a wide range of applications. MICs can be made to react to form a more stable compound while releasing a large amount of energy and exhibit several unique properties, for example, extremely fast propagation of the chemical reactions when the initial components are intermixed at the nanometer scales. The fundamental molecular-level mechanisms that govern the unique properties of these materials are to a large extent unknown. We employ molecular dynamics to characterize the chemical and mechanical response of MICs induced by shock loading. We use detailed analysis methods to characterize the atomic level processes responsible for the initiation and propagation of the chemical reactions. Our simulations are designed to characterize the role of composition and nanostructure on the initiation and subsequent ultra-fast propagation of chemical reactions in nanostructured MICs as well as their mechanical and thermal properties. [Preview Abstract] |
Monday, March 13, 2006 12:39PM - 12:51PM |
B35.00006: Vibrations and thermal conductivity in inorganic and polymeric glasses. Sergei Shenogin, Arun Bodapati, Pawel Keblinski The mechanism of thermal transport in amorphous materials was studied by means of vibrational mode analysis and classical nonequilibrium molecular dynamics (MD) simulations. We~ studied four different model systems of (a) Lennard-Jones glass, (b) bead-spring model of an amorphous polymer, (c) amorphous silicon with Stillinger-Weber potential; and (d) all-atom model of glassy polystyrene with PCFF-type force field. For all structures we evaluated thermal conductivity from the harmonic theory of disordered solids [P.B.Allen, and J.L.Feldman, Phys.Rev.B 48, 12581 (1993)] and from direct MD simulations. We found that for all models but polystyrene, the harmonic theory accurately predicts thermal conductivity. By contrast, in the case of polystyrene, only $\sim $1/2 of thermal conductivity can be explained within the harmonic approximation. Consequently, a major part of the transport has to be attributed to anharmonic coupling between vibrational modes. The reasons for the failure of harmonic theory of disordered solids to model amorphous glassy polymers will be discussed. [Preview Abstract] |
Monday, March 13, 2006 12:51PM - 1:03PM |
B35.00007: Anomalous thermal transport in the low-conductivity phase of granular metals Vikram Tripathi, Yen Lee Loh We study the thermal conductivity of a nonmagnetic, nonsuperconducting granular metal in the low-conductivity phase using the Kubo formula approach, and compare it with the electrical conductivity. We find that the physical mechanisms and the temperature dependences of the two are very different. In a regular granular array, electrical transport, which takes place through the intergrain hopping of quasiparticles, obeys an Arrhenius law due to Coulomb blockade of quasiparticle hopping. Certain many-particle processes such as particle-hole cotunneling do not suffer Coulomb blockade due to their charge-neutrality and show a much slower power-law decrease with temperature; however, because of their charge-neutrality, these processes make no qualitative difference to the electrical conductivity. Cotunneling of particle-hole pairs does transport heat, and therefore, the thermal conductivity decreases only algebraically with temperature. This picture is reminiscent of excess thermal transport in disordered semiconductors due to low-energy excitons. [Preview Abstract] |
Monday, March 13, 2006 1:03PM - 1:15PM |
B35.00008: Molecular heat pump Dvira Segal A heat pump is a device that transfers heat from a low to a high temperature reservoir by applying an external work that modulates the system's parameters. In this work we discuss a novel molecular machine of this kind. The system consists of a molecular element connecting two thermal reservoirs that are characterized by different spectral properties. The pumping action is achieved by applying an external force that periodically modulates molecular levels. This modulation affects periodic oscillations of the internal temperature of the molecule and the strength of its coupling to each reservoir resulting in a net heat flow in the desired direction. The heat flow is examined in the slow and fast modulation limits and for different modulation waveforms, thus making it possible to optimize the device performance. [Preview Abstract] |
Monday, March 13, 2006 1:15PM - 1:27PM |
B35.00009: Transport and Noise in Mesoscopic Conductors Coupled to Quantized Electro-Magnetic Fields. A.D. Stone, M.G. Vavilov Previous analyses of the effects of electro-magnetic (EM) fields on transport through mesoscopic systems have employed a classical treatment of the fields. To describe experiments on circuit electrodynamics [A. Wallraff et al., Nature 431, 162 (2004)] this treatment is no longer applicable. In this talk we discuss how the current and current noise through mesoscopic conductors are modified by the presence of such non-classical EM fields. For example when the EM field corresponds to a thermal state with temperature different from the electron temperature typically a steady-state current will flow through the system at zero bias due to the lack of detailed balance. The magnitude and direction of this current can be used to measure the temperature of the EM field. More generally, this current and its noise will allow measurement of non-classical properties of the EM field coupled to the system. [Preview Abstract] |
Monday, March 13, 2006 1:27PM - 1:39PM |
B35.00010: Elastomeric Network/Air Structures for Mechanically Tunable Hypersonic Phononic Crystals Taras Gorishnyy, Ji-Hyun Jang, Chaitanya K. Ullal, Edwin L. Thomas Hypersonic phononic crystals allow control over high frequency phonons, which is crucial for a whole range of applications from acousto-optics to thermal management and high resolution nondestructive evaluation techniques. The ability to fabricate phononic crystals with a band diagram that can be modified reversibly and repeatedly opens an interesting possibility to create tunable acoustic devices. In this talk we will describe the use of submicron elastomeric PDMS (poly(dimethylsiloxane))/air network structures as tunable phononic crystals operating in hypersonic frequency regime. The structures were fabricated from interference lithography templates, which were infiltrated with PDMS precursor and then after crosslinking the photoresist template was removed in water-based basic solution. Brillouin light scattering was used to monitor the modification of the phononic band diagram of these elastomeric structures as a function of the direction and degree of reversible mechanical deformation. The influence of symmetry and anisotropic sound velocities on the features of the phononic band diagram will be discussed. [Preview Abstract] |
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