Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session Z25: Focus Session: Thermoelectrics - Phonons and Heat Conduction III |
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Sponsoring Units: DMP GERA FIAP Chair: Austin Minnich Room: 503 |
Friday, March 7, 2014 11:15AM - 11:51AM |
Z25.00001: Phonon Mean Free Path Spectra Measured by Broadband Frequency Domain Thermoreflectance Invited Speaker: Jonathan Malen Nonmetallic crystalline materials conduct heat by the transport of quantized atomic lattice vibrations called phonons. Thermal conductivity depends on how far phonons travel between scattering events$\relbar $their mean free paths (MFPs). Due to the breadth of the phonon MFP spectrum, nanostructuring of materials and devices can reduce thermal conductivity from bulk by scattering long MFP phonons, while short MFP phonons are unaffected. We have developed a novel approach called Broadband Frequency Domain Thermoreflectance (BB-FDTR) that uses high-frequency laser heating to generate non-Fourier heat conduction that can sort phonons based on their MFPs. BB-FDTR outputs thermal conductivity as a function of heating frequency. Through non-equilibrium Boltzmann Transport Equation models this data can be converted to thermal conductivity accumulation, which describes how thermal conductivity is summed from phonons with different MFPs. Relative to alternative approaches, BB-FDTR yields order-of-magnitude improvements in the resolution and breadth of the thermal conductivity accumulation function. We will present data for GaAs, GaN, AlN, Si, and SiC that show interesting commonalities near their respective Debye temperatures and suggest that there may be a universal phonon MFP spectrum for small unit cell non-metals in the high temperature limit. At the time of this abstract submission we are also working on measurements of semiconductor alloys and select metals that will be presented if completed by the conference. [Preview Abstract] |
Friday, March 7, 2014 11:51AM - 12:03PM |
Z25.00002: Measurement of surface acoustic wave velocity using phase shift mask and application on thin film of thermoelectric material Dongyao Li, Peng Zhao, Noel Gunning, David Johnson, Ji-Cheng Zhao, David Cahill We describe a convenient approach for measuring the velocity $v_{SAW}$ of surface acoustic waves (SAWs) of the near-surface layer of a material through optical pump-probe measurements and apply this method, in combination with conventional picosecond acoustics, to determine a subset of the elastic constants of thin films of semiconducting misfit layered compounds. SAWs with a wavelength of 700 nm are generated and detected using an elastomeric polydimethylsiloxane (PDMS) phase-shift mask which is fabricated using a commercially-available Si grating as a mold. The velocity of SAWs of [(SnSe)$_{1.04}$]$_{m}$[MoSe$_{2}$]$_{n}$ synthesized by elemental reactants show subtle variations in their elastic constants as a function of m and n. Precise measurements of elastic constants will enable a better understanding of interfacial stiffness in nanoscale multilayers and the effects of phonon focusing on thermal conductivity. [Preview Abstract] |
Friday, March 7, 2014 12:03PM - 12:15PM |
Z25.00003: Coherent phonon transport in Epitaxial Oxide Heterostructures Ajay Yadav, Aaron Swarz, Ramez Cheaito, Jayakanth Ravichandran, Patrick Hopkins, Arun Majumdar, Joel Moore, Ramamoorthy Ramesh Coherent transport of phonons was unambiguously observed [1] in superlattices of complex oxides, (SrTiO$_{3})_{\mathrm{m}}$/(CaTiO$_{3})_{\mathrm{n}}$ and (SrTiO$_{3})_{\mathrm{m}}$/(BaTiO$_{3})_{\mathrm{n}}$ manifested by a minimum in thermal conductivity as a function of superlattice interface density. To gain further insights into coherent regime of phonon transport, we systematically changed acoustic impedance mismatch between superlattice constituents and studied its effect on the transition from incoherent to coherent regime of heat transport. Further, in an attempt to manipulate transport of a broad range of phonons in the coherent regime, controllable disorder is introduced to attain both short-range and long-range phonon scattering. Quasi-periodic, controllable disorder is introduced by randomly stacking alternating layers of SrTiO$_{3}$ and CaTiO$_{3}$, for a given average interface density and volume fraction. In conclusion, our studies elucidate the effect of periodicity and impedance mismatch on both coherent and incoherent phonon scattering in epitaxial oxide heterostructures.\\[4pt] [1] J. Ravichandran, A. Yadav, R. Cheaito et al., accepted in \textit{Nature Materials }(2013). [Preview Abstract] |
Friday, March 7, 2014 12:15PM - 12:27PM |
Z25.00004: Phonon transmission and reflection antiresonances at the interface between solids with impurities as interference phenomena in atomic-scale phononic metamaterials Yury Kosevich, Haoxue Han, Sebastian Volz We study theoretically phonon transmission through the interface between two solid crystals, which contains heavy isotopic impurities and/or soft-force-constant defects. We perform analytical calculations of plane wave transmission and numerical molecular dynamics simulation of wave packet transmission, which give consistent with each other results. If the impurities do not fill completely the interface plane, longitudinal and transverse phonons have two passes to cross such interface, through the host and through the impurity atoms bonds. Destructive interference between these passes can result in total resonance reflection of the phonon. The phonon transmission antiresonance is followed by phonon reflection antiresonance at higher frequency. The random distribution of the defects at the interface and nonlinearity of atomic bonds do not deteriorate the reflection and transmission antiresonances. Such Fano-like phonon interference antiresonances can affect heat transport through interfaces and contacts between nanostructures with impurities. The antiresonances are realized in phonon transmission through a planar defect in Si crystal with segregated Ge atoms. The phonon antiresonances can be considered as interference phenomena in atomic-scale phononic metamaterials. [Preview Abstract] |
Friday, March 7, 2014 12:27PM - 12:39PM |
Z25.00005: Directed phonon engineering in nanostructured Mn-Ge superlattices: Towards a description of heat transport in device-like structures Claudia Mangold, Joerg Behler, Davide Donadio Poor performance of thermoelectric materials severely limits the application of Peltier devices. Our work aims at the improvement of the efficiency of such devices by replacing standard p-n junctions with a membrane structure with nanofeatures. The low dimensionality of the membranes and the nanofeatures will ensure a reduction of the phononic thermal conductivity $\kappa$, thus enhancing the thermoelectric figure of merit, ZT=S$^2\sigma$T/$\kappa$. Mn-Ge compounds turned out to be excellent candidates for nanostructuring due to the broad structural variety[1]. We performed first-principles electronic structure calculations, in particular density functional theory, to characterize various Mn-Ge bulk species as well as Mn-Ge superlattices. To reach larger length scales we have constructed a transferable neural network potential[2] for Mn-Ge compounds to characterize nanostructured membranes up to device-like size and determine their thermal transport properties. This multiscale modeling approach is a powerful tool to design materials and devices with specifically engineered phonon properties and enhanced thermoelectric performances. [1]Jamet et al Nature Mat.5,653(2006);Jain et al J.Appl.Phys. 109,013911(2011) [2]Behler et al PRL 98,146401(2007);Phys.Stat.Sol.B 245,261(2008) [Preview Abstract] |
Friday, March 7, 2014 12:39PM - 12:51PM |
Z25.00006: First principles study of thermal conductance across the MgO/TiN interface Derek Stewart, Saikat Mukhopadhyay MgO and TiN are well lattice-matched crystals and their interface has one of the lowest thermal resistances currently measured. As such, it represents a key test for atomistic models for thermal interfacial resistance. In this work, we examine the phonon contribution to thermal transport across the epitaxial MgO/TiN (001) and (111) interfaces using an atomistic Green's function approach that incorporates interatomic force constants calculated using density functional theory. Since TiN is a metal, this approach will allow us to isolate the direct phonon contribution to thermal conductance across the interface. Calculated phonon dispersions for bulk MgO and TiN show good agreement with experiment. We will discuss how the predicted thermal interface resistance compares with values calculated using standard acoustic mismatch and diffusive mismatch models. We will also examine the impact of TiN nitrogen vacancies on both the bulk phonon dispersion and MgO/TiN thermal conductance. [Preview Abstract] |
Friday, March 7, 2014 12:51PM - 1:03PM |
Z25.00007: Phonon impedance matching: minimizing interfacial thermal resistance of thin films Carlos Polanco, Jingjie Zhang, Avik Ghosh The challenge to minimize interfacial thermal resistance is to allow a broad band spectrum of phonons, with non-linear dispersion and well defined translational and rotational symmetries, to cross the interface. We explain how to minimize this resistance using a frequency dependent broadening matrix that generalizes the notion of acoustic impedance to the whole phonon spectrum including symmetries. We show how to ``match'' two given materials by joining them with a single atomic layer, with a multilayer material and with a graded superlattice. Atomic layer ``matching'' requires a layer with a mass close to the arithmetic mean (or spring constant close to the harmonic mean) to favor high frequency phonon transmission. For multilayer ``matching,'' we want a material with a broadening close to the geometric mean to maximize transmission peaks. For graded superlattices, a continuous sequence of geometric means translates to an exponentially varying broadening that generates a wide-band antireflection coating for both the coherent and incoherent limits. Our results are supported by ``first principles'' calculations of thermal conductance for $GaAs/Ga_xAl_{1-x}As/AlAs$ thin films using the Non-Equilibrium Greens Function formalism coupled with Density Functional Perturbation Theory. [Preview Abstract] |
Friday, March 7, 2014 1:03PM - 1:15PM |
Z25.00008: Nanoscale thermal transport measurements: Bridging ultrafast and steady-state Brian Green, Mark Siemens Macroscale thermal transport is explained by classical thermal diffusion, but as nanostructure length scales are reduced towards the order of the phonon mean free path, transport of thermal energy takes on a fundamentally different character. Nanoscale effects emerge such as sensitivity to the presence of surfaces and the onset of ballistic transport. We investigate nanoscale thermal physics by comparing results from two different transport measurement techniques applied to systems of highly confined thermal transport: metallic thin films deposited on a suspended bridge structure. One technique uses the transient thermoreflectance (TTR) method to measure picosecond cooling dynamics following ultrafast laser heating in a micron-sized region of the metallic film deposited on the bridge; the second is a DC technique that measures transport driven by an Ohmically-generated thermal gradient across the bridge through the full volume of the film. We find that these very different methods give similar results of significantly reduced thermal conductivity relative to macroscale values. We compare TTR and DC results between differing film thicknesses, and evaluate conductance uniformity across film surfaces. In combination, the TTR and DC methods are powerful tools for investigating and understanding thermal transport at the nanoscale. [Preview Abstract] |
Friday, March 7, 2014 1:15PM - 1:27PM |
Z25.00009: Nanocrystalline silicon thin films for thermoelectric applications Daniel Queen, Battogtokh Jugdersuren, Jim Culberston, Qi Wang, William Nemeth, Tom Metcalf, Xiao Liu Recent advances in thermoelectric materials have come from reductions in thermal conductivity by manipulating both chemical composition and nanostructure to limit the phonon mean free path. However, wide spread applications for some of these materials may be limited due to high raw material and integration costs. In this talk we will discuss our recent results on nanocrystalline silicon thin films deposited by both hot-wire and plasma enhanced chemical vapor deposition where the nanocrystal size and crystalline volume fraction are varied by dilution of the silane precursor gas with hydrogen. Nanocyrstalline silicon is an established material technology used in multijunction amorphous silicon solar cells and has the potential to be a low cost and scalable material for use in thermoelectric devices. [Preview Abstract] |
Friday, March 7, 2014 1:27PM - 1:39PM |
Z25.00010: Phonon Thermal Transport in SiGe-based Nanocomposites for Thermoelectric Applications Zlatan Aksamija Silicon-germanium (SiGe) and Si/Si$_{1-x}$Ge$_{x}$ superlattices (SLs) have been proposed for application as efficient thermoelectrics because of their low thermal conductivity, below that of bulk Si$_{1-x}$Ge$_{x}$ alloys. However, the cost of growing SLs is prohibitive, so nanocomposites, made by a ball-milling and sintering, have been proposed as a cost-effective replacement with similar properties. Lattice thermal conductivity in SiGe SLs is reduced by scattering from the rough interfaces between layers. Therefore, it is expected that interface properties, such as roughness, orientation, and composition, will play a significant role in thermal transport in nanocomposites and offer many additional degrees of freedom to control the thermal conductivity in nanocompoosites by tailoring grain size, shape, and crystal angle distributions. We previously demonstrated the sensitivity of the lattice thermal conductivity in SLs to the interface properties, based on solving the phonon Boltzmann transport equation under the relaxation time approximation. Here we adapt the model to a broad range of SiGe nanocomposites. We model nanocomposite structures using a Voronoi tessellation to mimic the grains and their distribution in the nanocomposite and show excellent agreement with experimentally observed structures. In order to accurately treat phonon scattering from a series of atomically rough interfaces between the grains in the nanocomposite, we employ a \textit{momentum-dependent} specularity parameter p(\textbf{q})$=$exp(-4$\pi^{2}\Delta ^{2}$q$^{2}$cos$^{2}\theta )$. Our results show highly anisotropic thermal transport in SiGe nanocomposites below their bulk alloy counterparts. [Preview Abstract] |
Friday, March 7, 2014 1:39PM - 1:51PM |
Z25.00011: Phonon Drag in Thin Films, Cases of Bi$_{2}$Te$_{3}$ and ZnTe Hang Chi, Ctirad Uher At low temperatures, in (semi-)conductors subjected to a thermal gradient, charge carriers (electrons and holes) are swept (dragged) by out-of-equilibrium phonons due to strong electron-phonon interaction, giving rise to a large contribution to the Seebeck coefficient called the phonon-drag effect. Such phenomenon was surprisingly observed in our recent transport study of highly mismatched alloys as potential thermoelectric materials: a significant phonon-drag thermopower reaching 1.5--2.5 mV/K was recorded for the first time in nitrogen-doped ZnTe epitaxial layers on GaAs (100). In thin films of Bi$_{2}$Te$_{3}$, we demonstrate a spectacular influence of substrate phonons on charge carriers. We show that one can control and tune the position and magnitude of the phonon-drag peak over a wide range of temperatures by depositing thin films on substrates with vastly different Debye temperatures. Our experiments also provide a way to study the nature of the phonon spectrum in thin films, which is rarely probed but clearly important for a complete understanding of thin film properties and the interplay of the substrate and films. [Preview Abstract] |
Friday, March 7, 2014 1:51PM - 2:03PM |
Z25.00012: High Temperature Thermoelectric Properties of Gd doped InGaAs Thin Films Rachel Koltun, Ryan Need, Ashton Meginnis, Brian Schultz, Chris Palmstrom, John Bowers Doping III-As thin films with rare earths has been shown to increase the thermoelectric figure of merit (ZT) at high temperatures. Above the solubility limit, rare earth - arsenide nanoparticles precipitate out of molecular beam epitaxy grown films. These particles scatter phonons to reduce the thermal conductivity and act as a source of thermally activated carriers at high temperature. In this study, we compare the thermoelectric properties of Gd doped InGaAs to traditional doping methods (Si). Gd doped samples were grown to explore the doping effects below and above the solubility limit in InGaAs. This range also captures the peak ZT for these structures. Electrical conductivity and Seebeck coefficient were measured as a function of temperature. Gd doped InGaAs shows a higher doping efficiency than Er doped InGaAs, leading to better thermoelectric performance. However, Si has a much higher doping efficiency than any of the rare earths, leading to overall peak room temperature thermoelectric performance of Si doped InGaAs. Temperature dependent hall suggests that there may be a crossover point where enough carriers are thermally generated from nanoparticles to surpass the thermoelectric performance of Si doped InGaAs. [Preview Abstract] |
Friday, March 7, 2014 2:03PM - 2:15PM |
Z25.00013: Nanophononic metamaterial: Thermal conductivity reduction by local resonance Bruce Davis, Mahmoud Hussein Engineered manipulation of phonons can yield beneficial thermal properties in semiconducting materials. One pivotal application relates to thermoelectric materials, or the concept of converting energy in the form of heat into electricity and vice-versa. The ability to use nanostructuring to reduce the thermal conductivity without negatively impacting the power factor provides a promising avenue for achieving high values of the thermoelectric energy conversion figure-of-merit, ZT. In this work, we propose a novel nanostructured material configuration that seeks to achieve this goal. Termed nanophononic metamaterial, the configuration is based on a silicon thin-film with a periodic array of pillars erected on one or two of the free surfaces. The pillars qualitatively alter the base thin-film phonon spectrum due to a hybridization mechanism between their local resonances and the underlying atomic lattice dispersion. Using an experimentally-fitted lattice-dynamics-based model, we conservatively predict a drop in the thermal conductivity to as low as 50{\%} of the corresponding uniform thin-film value despite the fact that the pillars add more phonon modes to the spectrum. [Preview Abstract] |
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