Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session S23: Thermoelectrics Theory IFocus

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Sponsoring Units: DMP GERA FIAP Chair: Joseph Feser, University of Delaware Room: 322 
Thursday, March 17, 2016 11:15AM  11:51AM 
S23.00001: Phonon Heat Conduction In Nanostructures: Ballistic, Coherent, Localized, Hydrodynamic, and Divergent Modes Invited Speaker: Gang Chen In this talk, we will discuss different modes of heat conduction in nanostructures. Ballistic transport happens when phonon mean free path is longer than the characteristic size of the structure. We will discuss how we compute phonon mean free path distributions based on firstprinciples and measure the distributions with optical pumpprobe techniques by exploring ballistic phonon transport processes. In superlattice structures, ballistic phonon transport across the whole thickness of the superlattices implies phase coherence. We observed this coherent transport in GaAs/AlAs superlattices with fixed periodic thickness and varying number of periods. Simulations show that although high frequency phonons are scattering by roughness, remaining long wavelength phonons maintain their phase and traverse the superlattices ballistically. Accessing the coherent heat conduction regime opens a new venue for phonon engineering. We show further that phonon heat conduction localization happens in GaAs/AlAs superlattice by placing ErAs nanodots at interfaces. This heatconduction localization phenomenon is confirmed by nonequilibrium atomic Green's function simulation. These ballistic and localization effects can be exploited to improve thermoelectric energy conversion materials via reducing their thermal conductivity. In another opposite, we will discuss phonon hydrodynamic transport mode in graphene via firstprinciple simulations. In this mode, phonons drift with an average velocity under a temperature gradient, similar to fluid flow in a pipe. Conditions for observing such phonon hydrodynamic modes will be discussed. Finally, we will talk about the onedimensional nature of heat conduction in polymer chains. Such 1D nature can lead to divergent thermal conductivity. Inspired by simulation, we have experimentally demonstrated high thermal conductivity in ultradrawn polyethylene nanofibers and sheets. [Preview Abstract] 
Thursday, March 17, 2016 11:51AM  12:03PM 
S23.00002: GreenKubo Modal Analysis Asegun Henry A new method for direct calculation of the modal contributions to thermal conductivity, which is termed GreenKubo modal analysis (GKMA) will be presented. The GKMA method combines the lattice dynamics formalism with the GreenKubo formula for thermal conductivity, such that the thermal conductivity becomes a direct summation of modal contributions, where one need not define the phonon velocity. As a result, the GKMA method can be applied to any material/group of atoms, where the atoms vibrate around stable equilibrium positions, which includes crystalline line compounds, nonstoichiometric compounds, random alloys, amorphous materials and even rigid molecules. By using molecular dynamics simulations to obtain the time history of each mode's contribution to the heat current, one naturally includes anharmonicity to full order and can obtain insight into the interactions between different modes through the crosscorrelations. Several example materials will be discussed and the specific attention will be devoted to new fundamental questions that arise from the changes in mode character that occur in disordered systems. The GKMA method provides new insight into the nature of phonon transport, as it casts the problem in terms of modemode correlation instead of scattering, and provides a general unified formalism that can be used to understand phononphonon interactions in essentially any class of materials or structures where the atoms vibrate around stable equilibrium sites. [Preview Abstract] 
(Author Not Attending)

S23.00003: On The Solenoidal Heat Flux in QuasiBallistic Thermal Conduction Ashok Ramu, John Bowers The Boltzmann transport equation for phonons is recast directly in terms of the heatflux by means of iteration followed by truncation at the second order in the spherical harmonic expansion of the distribution function. This procedure displays the heatflux in an explicitly coordinateinvariant form, and leads to a natural decomposition into two components, namely the solenoidal component in addition to the usual irrotational component. The solenoidal heatflux is explicitly shown to arise in a rightcircular cylinder when the transport is in the quasiballistic regime. These findings are important in the context of phonon resonators that utilize the strong quasiballistic thermal transport reported recently in silicon membranes at room temperature. Effects due to circulating heat fluxes are noted in the effective thermal conductivity of silicon discs. [Preview Abstract] 
Thursday, March 17, 2016 12:15PM  12:27PM 
S23.00004: Active Extraction of Nearfield Thermal Radiation Ding Ding, Taeyong Kim, Austin Minnich Radiative heat transport between materials supporting surfacephonon polaritons is greatly enhanced when the materials are placed at subwavelength separation as a result of the contribution of nearfield surface modes. However, the enhancement is limited to small separations due to the evanescent decay of the surface waves. In this work, we propose and numerically demonstrate an active radiative cooling (ARC) scheme to extract these modes to the farfield. Our approach exploits the monochromatic nature of nearfield thermal radiation to drive a transition in a laser gain medium, which, when coupled with external optical pumping, allows the resonant surface mode to be emitted into the farfield. We also provide further insights into our ARC scheme by applying the theoretical framework used for laser cooling of solids (LCS) to ARC. We show that LCS and ARC can be described with the same mathematical formalism by replacing the electronphonon coupling parameter in LCS with the electronphoton coupling parameter in ARC. Using this framework, we examine the predictions of the formalism for LCS and ARC using realistic parameters and find that ARC can achieve higher efficiency and extracted power over a wide range of conditions. Our study demonstrates a new approach to manipulate nearfield thermal radiation for thermal management. [Preview Abstract] 
Thursday, March 17, 2016 12:27PM  12:39PM 
S23.00005: Spectral Analysis of Surface Controlled Phonon Transport in Nanophononic Metamaterials Sanghamitra Neogi, Davide Donadio Phonon engineering in nanostructured semiconductors has shown promises to further advance the performance of energy applications beyond the stateoftheart limit. In nanostructured materials, phonon transport is greatly affected by the surface nanoscale character[1]. The concept of nanophononic metamaterial (NPM) was introduced recently [2] to affect nanoscale thermal transport with the inclusion of local surface resonators. We carried out a systematic investigation of phonon transport in locally resonant siliconbased NPMs. We used classical equilibrium molecular dynamics and a Boltzmann transport equation approach with the relaxation time approximation to investigate the nature of phononic thermal transport in nanopatterned silicon membranes with thicknesses of the order of 10 nm and below. We find the presence of local surface resonators has a significant effect on the phonon dispersion and has a direct consequence of suppression of group velocities of phonons in the NPMs. We completed the investigation by relating nanoscale resonant character (geometry and material composition) with phonon scattering, and consequently, phonon transport in the locally resonant silicon membrane NPMs. [1] Neogi et al, ACS nano, 9(4), 38203828 (2015) [2] Davis & Hussein, PRL, 12, 055505 (2014) [Preview Abstract] 
Thursday, March 17, 2016 12:39PM  12:51PM 
S23.00006: Nanophononic metamaterial: Thermal conductivity reduction by fullspectrum resonance hybridizations. Mahmoud Hussein, Hossein Honarvar, Lina Yang 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 viceversa. 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 figureofmerit, ZT. Here, we propose a novel nanostructured material configuration that seeks to achieve this goal. Termed ``nanophononic metamaterial,'' the configuration is based on a freestanding silicon membrane with a periodic array, or random forest, of nanopillars erected on the surface. The nanopillars qualitatively alter the base membrane phonon spectrum due to a hybridization mechanism between their local resonances and the underlying atomic lattice dispersion. Using equilibrium molecular dynamics simulations, we predict a factor of 10 drop in the thermal conductivity compared to the corresponding uniform membrane value despite the fact that the nanopillars add more phonon modes to the spectrum. [Preview Abstract] 
Thursday, March 17, 2016 12:51PM  1:03PM 
S23.00007: Design principles of interfacial thermal conductance Carlos Polanco, Rouzbeh Rastgarkafshgarkolaei, Jingjie Zhang, Nam Le, Pamela Norris, Avik Ghosh We explore fundamental principles to design the thermal conductance across solid interfaces by changing the composition and disorder of an intermediate matching layer. In absence of phononphonon interactions, the layer addition involves two competing effects that influence the conductance. The layer can act as an impedance matching ‘bridge’ to increase the modeaveraged phonon transmission. However, it also reduces the relevant modes that conserve their momenta transverse to the interface, so that the net result depends on features such as the overlap of conserving modes and the dispersivity of the transverse subbands. Moving into the interacting anharmonic regime, we find that the added layer aids conductance when the decreased resistances at the contactlayer boundaries compensate for the layer resistance. In fact, we show that the maximum conductance corresponds to an exact matching of the two separate contactlayer resistances. For instance, if we vary just the atomic mass across layers, then maximum conductance happens when the intervening layer mass is the geometric mean of the contact masses. We conjecture that the best interfacial layer is one that is compositionally graded into many geometric means – in other words, an exponential variation in thermal impedance. [Preview Abstract] 
Thursday, March 17, 2016 1:03PM  1:15PM 
S23.00008: Modal Contributions to Heat Conduction across Crystalline and Amorphous Si/Ge Interfaces Kiarash Gordiz, Asegun Henry Until now, our entire understanding of interfacial heat transfer has been based on the phonon gas model and Landauer formalism. Based on this framework, it is difficult to offer any intuition on heat transfer between two solid materials if one side of the interface is an amorphous structure. Here, using the interface conductance modal analysis (ICMA) method, we investigate the modal contributions to thermal interface conductance (TIC) through crystalline (c) and amorphous (a) Si/Ge interfaces. It is revealed that around 15{\%} of the conductance through the cSi/cGe interface arises from less than 0.1{\%} of the modes of vibration in the structure that exist between 1213THz and because of their large eigenvectors around the interface are classified as interfacial modes. Correlation maps show that these interfacial modes exhibit strong correlations with all the other modes. The physics behind this strong coupling ability is studied by calculating the modelevel harmonic and anharmonic energy distribution among all the atoms in the system. It is found that these interfacial modes are enabled by the large degree of anharmonicity near the interface, which is higher than the bulk and ultimately allows this small group of modes to couple to other modes of vibration. In addition, unlike the cSi/cGe, correlation maps for aSi/cGe, cSi/aGe, and aSi/aGe interfaces show that the majority of contributions to TIC arise from autocorrelations instead of crosscorrelations. The provided analysis sheds light on the nature of localized vibrations at interfaces and can be enlightening for other investigations of localization. [Preview Abstract] 
Thursday, March 17, 2016 1:15PM  1:27PM 
S23.00009: SiCSi interfacial thermal and mechanical properties of reaction bonded SiC/Si ceramic composites Chunyen Hsu, Fei Deng, Prashant Karandikar, Chaoying Ni Reaction bonded SiC/Si (RBSC) ceramic composites are broadly utilized in military, semiconductor and aerospace industries. RBSC affords advanced specific stiffness, hardness and thermal. Interface is a key region that has to be considered when working with any composites. Both thermal and mechanical behaviors of the RBSC are highly dependent on the SiCSi interface. The SiCSi interface had been found to act as a thermal barrier in restricting heat transferring at room temperature and to govern the energy absorption ability of the RBSC. However, up to present, the role of the SiCSi interface to transport heat at higher temperatures and the interfacial properties in the nanoscale have not been established. This study focuses on these critically important subjects to explore scientific phenomena and underlying mechanisms. The RBSC thermal conductivity with volume percentages of SiC at 80 and 90 vol{\%} was measured up to 1,200 \textdegree C, and was found to decrease for both samples with increasing environmental temperature. The RBSC with 90 vol{\%} SiC has a higher thermal conductivity than that of the 80 vol{\%}; however, is still significantly lower than that of the SiC. The interfacial thermal barrier effect was found to decrease at higher temperatures close 1200 \textdegree C. A custommade \textit{insitu} tensile testing device which can be accommodated inside a ZEISS Auriga 60 FIB/SEM has been setup successfully. The SiCSi interfacial bonding strength was measured at 98 MPa. The observation and analysis of crack propagation along the SiCSi interface was achieved with \textit{insitu} TEM. [Preview Abstract] 
Thursday, March 17, 2016 1:27PM  1:39PM 
S23.00010: Kapitza resistance at segregated boundaries in $\beta $SiC Nipun Goel, Edmund Webb III, Alparslan Oztekin, Jeffrey Rickman, Sudhakar Neti Silicon Carbide is a candidate material for hightemperature thermoelectric applications for harvesting waste heat associated with exhaust from automotive and furnaces as well hot surfaces in solar towers and power electronics. However, for SiC to be a viable thermoelectric material, its thermoelectric figure of merit must be improved significantly. In this talk we examine the role of grainboundary segregation on phononic thermal transport, an important factor in determining the figure of merit, via nonequilibrium molecular dynamics simulations. In particular, we consider the role of dopant concentration and dopant/matrix interactions on the enhancement of the Kapitza resistance of symmetric tilt grain boundaries. We find that the calculated resistance depends on the segregation profile, with increases of more than a factor of 50 (relative to an unsegregated boundary) at the highest dopant concentrations. Finally, we relate the calculated phonon density of states to changes in the Kapitza resistance. [Preview Abstract] 
Thursday, March 17, 2016 1:39PM  1:51PM 
S23.00011: Heat current characteristics in nanojunctions: The effect of external magnetic fields D. Melisa Dominguez, Juliana Restrepo, Boris A. Rodriguez, R Chitra We study the heat current in the simplest hybrid device of a two level system weakly coupled to two heat baths. We consider both metallic and semiconducting baths with external magnetic fields applied on the central spin and the baths. By using a reduced density matrix approach together with a simple BornMarkov approximation we calculate the heat current. Our goal is to investigate the effect of the applied fields in the transient and steady state heat current, the ensuing rectification and the possibility of using our setup as a building block for a quantum thermal diode. [Preview Abstract] 
Thursday, March 17, 2016 1:51PM  2:03PM 
S23.00012: Estimating thermal conductivity and thermoelectricity in PbTiO$_3$ from first principles Anindya Roy A combination of density functional theory and Boltzmann transport equation is used in this study to calculate the lattice thermal conductivity ($\kappa_{\rm L}$) of PbTiO$_3$ (PTO). We cannot apply this procedure to determine $\kappa_{\rm L}$ in presence of imaginary phonon modes ("soft modes"). Hence the tetragonal structure of PTO is used in these calculations, and the predicted $\kappa_{\rm L}$ is extrapolated to higher temperature using insights from experiments. The computed $\kappa_{\rm L}$ of PTO is low, possibly due to the anharmonicity associated with the ferroelectric/paraelectric transition. Electronic transport parameters such as the Seebeck coefficient and the electrical conductivity are also determined (under constant scattering time approximation in semiclassical Boltzmann theory) for PTO. The low $\kappa_{\rm L}$ and the electronic transport parameters together indicate excellent thermoelectric properties of PTO ($zT >$ 1.5 at 1000 K). As a technologically important ferroelectric/piezoelectric material, PTO is used in alloys and in layered structures. These morphologies could bring down the $\kappa_{\rm L}$ further, improving its thermoelectric performance. Synthesis of electrically conducting samples of PTO would allows us to verify the above predictions. [Preview Abstract] 
Thursday, March 17, 2016 2:03PM  2:15PM 
S23.00013: Rayleigh surface waves, phonon mode conversion, and thermal transport in nanostructures Leon Maurer, Irena Knezevic We study the effects of phonon mode conversion and Rayleigh (surface) waves on thermal transport in nanostructures. We present a technique to calculate thermal conductivity in the elasticsolid approximation: a finitedifference timedomain (FDTD) solution of the elastic or scalar wave equations combined with the GreenKubo formula. The technique is similar to an equilibrium molecular dynamics simulation, captures phonon wave behavior, and scales well to nanostructures that are too large to simulate with many other techniques. By imposing fixed or free boundary conditions, we can selectively turn off mode conversion and Rayleigh waves to study their effects. In the example case of graphenelike nanoribbons with rough edges, we find that mode conversion among bulk modes has little effect on thermal transport, but that conversion between bulk and Rayleigh waves can significantly reduce thermal conductivity. With increasing surface disorder, Rayleigh waves readily become trapped by the disorder and draw energy away from the propagating bulk modes, which lowers thermal conductivity. We discuss the implications on the accuracy of popular phononsurface scattering models that stem from scalar wave equations and cannot capture mode conversion to Rayleigh waves. [Preview Abstract] 
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