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Proceedings Papers

*Proc. ASME*. HT2016, Volume 1: Heat Transfer in Energy Systems; Thermophysical Properties; Theory and Fundamentals in Heat Transfer; Nanoscale Thermal Transport; Heat Transfer in Equipment; Heat Transfer in Fire and Combustion; Transport Processes in Fuel Cells and Heat Pipes; Boiling and Condensation in Macro, Micro and Nanosystems, V001T04A006, July 10–14, 2016

Paper No: HT2016-7414

Abstract

In this work, we have observed 60% reduction in total interfacial resistance by adding an intermediate metal layer nickel between gold and aluminum oxide. Two temperature model is applied to explain the change of interfacial resistance, including both lattice mismatch with diffuse mismatch model and electron-phonon coupling effect. Simulation result agrees reasonably well with experimental data. Even though interfacial resistance due to electron-phonon coupling effect for Au-aluminum oxide is much larger than that of Ni-aluminum oxide interface, lattice mismatch is still the dominant factor for interfacial resistance.

Proceedings Papers

Ramez Cheaito, John C. Duda, Thomas E. Beechem, Jon F. Ihlefeld, Khalid Hattar, Edward S. Piekos, Amit Misra, Jon K. Baldwin, Patrick E. Hopkins

*Proc. ASME*. HT2013, Volume 1: Heat Transfer in Energy Systems; Thermophysical Properties; Theory and Fundamental Research in Heat Transfer, V001T03A009, July 14–19, 2013

Paper No: HT2013-17541

Abstract

The thermal conductivities of 1μm copper-niobium multilayer films of different period thicknesses are measured by time–domain thermoreflectance at room temperature. The values for thermal conductivity are then used to calculate the thermal conductance between Cu/Nb interfaces using a series resistors model. Results show that Cu/Nb interface conductance increases with the decrease in period thickness reaching a value as high as 20 GWm −2 K −1 for a 1×1 Cu/Nb multilayer. At shorter period thicknesses, ballistic electron transport dominates the thermal transport across this interface resulting in high interface conductance. The results are well described by a model that accounts for both ballistic and diffusive transport of electrons. This model assumes that an electron on one side of a metal-metal multilayer may not scatter at the interface but rather move ballistically on to the adjacent material and scatter in the adjacent material.

Proceedings Papers

*Proc. ASME*. HT2013, Volume 1: Heat Transfer in Energy Systems; Thermophysical Properties; Theory and Fundamental Research in Heat Transfer, V001T03A041, July 14–19, 2013

Paper No: HT2013-17297

Abstract

Thermal transport across solid/solid interfaces has been extensively studied, but heat transfer pathways other than phonon transmission and electron-phonon nonequilibrium in the metal were usually neglected. In this work, we aim to build a general and unified model including both the above transport channels and others such as electron transmission and electron-interface coupling. For a general solid/solid system with electrons and phonons existing on both sides, an analytical solution to the interfacial thermal resistance is obtained. We show that the relative contribution from different transport channels depends on both the local condition at the interface and the bulk properties of each side of the interface. We find that for a metallic thin film deposited on a semiconductor substrate, the contribution of electron transmission to thermal transport is negligible when the semiconductor is not heavily doped even though the electronic interfacial thermal boundary resistance can be lower than the phononic counterpart at high temperatures. In contrast, the electron-interface channel plays an important role in the intrinsic to low-doped regime, where substrate phonons can remove heat efficiently from the interface.

Proceedings Papers

*Proc. ASME*. HT2013, Volume 1: Heat Transfer in Energy Systems; Thermophysical Properties; Theory and Fundamental Research in Heat Transfer, V001T01A036, July 14–19, 2013

Paper No: HT2013-17003

Abstract

Transverse thermoelectric effect can be produced artificially by stacking at an angle layers of a thermoelectric material with another material that may or may not be a thermoelectric material. In this exploratory computational study, a new metamaterial, comprised of tilted alternating layers of an n -type thermoelectric alloy and a metal, is investigated to gain an understanding of how much cooling can be produced by transverse thermoelectric effect and the conditions under which maximum cooling is attainable. The governing conservation equations of energy and electric current, with the inclusion of thermoelectric effects, are solved on an unstructured mesh using the finite-volume method to simulate a transverse Peltier cooler under various operating conditions. First, the code is validated against experimental data for a n -Bi 2 Te 3 -Pb metamaterial, and subsequently explored. It is found that intermediate applied currents produce maximum temperature depression ( ΔT ). Optimum values of the geometric design parameters such as tilt angle and device aspect ratio are also established through parametric studies. Finally, it is shown that the ΔT can be amplified by constricting the phonon (heat) transport cross-section while keeping the electron (current) transport cross-section unchanged — a strategy that cannot be employed in conventional thermoelectric devices where electrons and phonons follow the same path. This makes transverse Peltier coolers particularly attractive for generating large ΔT without multi-stage cascading.

Proceedings Papers

*Proc. ASME*. HT2013, Volume 1: Heat Transfer in Energy Systems; Thermophysical Properties; Theory and Fundamental Research in Heat Transfer, V001T03A006, July 14–19, 2013

Paper No: HT2013-17235

Abstract

In this work we show the structure and application of a two carrier thermal model applied to a near field transducer, representative of that used in Heat Assisted Magnetic Recording (HAMR). As part of the HAMR device operation, high energy non-thermalized electrons are initially excited by laser incidence on a gold nanostructure. The high energy electrons can travel in a ballistic fashion over longer distances than the optical thickness of gold, resulting in a spreading of the local heat. During their travel the hot electrons collide with lower-energy electrons, thermalizing the hot electrons via inelastic scattering. The thermalized electrons then transfer energy to the lattice due to electron-phonon coupling, as captured in the two carrier model. Starting with an electromagnetic solution for local heating in a sub-micron-scale microfabricated gold structure, the chosen modeling technique applies physical effects of unique interest at the nanometer scale, including brief ballistic transport of hot electrons, experimentally-verified interface thermal resistance, and electron-phonon temperature mismatch. By design, the model is built to use far-field boundary conditions from conventional one-carrier FEMs as well as lubrication-flow computational fluid dynamics. The fundamental governing equations of the two carrier model are two versions of Poisson’s Equation for heat diffusion, coupled by empirically determined terms. These equations are combined with equations for interfacial discontinuities in the temperature fields, yielding a third degree of freedom. The continuous fields are discretized using the finite difference method, and solved using algorithms developed for linear algebra, such as Gaussian Elimination, or non-direct iterative methods. Through use of the model we explore effects of ballistic electron transport length, electron-phonon coupling, as well as interfacial thermal resistance between gold and neighboring ceramics. The model results show the relative impact of the nanoscale heat transfer phenomena in a nanometer scale metal-ceramic structure, allowing us to identify the relative importance of design features and compare candidate designs.

Proceedings Papers

*Proc. ASME*. HT2012, Volume 1: Heat Transfer in Energy Systems; Theory and Fundamental Research; Aerospace Heat Transfer; Gas Turbine Heat Transfer; Transport Phenomena in Materials Processing and Manufacturing; Heat and Mass Transfer in Biotechnology; Environmental Heat Transfer; Visualization of Heat Transfer; Education and Future Directions in Heat Transfer, 579-587, July 8–12, 2012

Paper No: HT2012-58350

Abstract

Heat transfer across nanoscale metal/dielectric multilayers involves multiple thermal conduction mechanisms. Electron or phonon interface scattering can augment the thermal conductivity anisotropy in multilayer composites. Weak electron-phonon coupling and quasi-ballistic phonon transport normal to the metal film further increase the anisotropy for metal-dielectric multilayers with period shorter than the relevant free paths. This paper models these physical mechanisms using an approximate thermal resistor network with support from the Boltzmann transport equation. We measure the in- and cross-plane thermal conductivity of a Mo/Si (2.8 nm/4.1 nm) multilayer as 15.4 and 1.2 W/mK, respectively, which agree with the proposed theoretical model. This work introduces a criterion for the transition from electron to phonon dominated heat conduction in metal films bounded by dielectrics.

Proceedings Papers

*Proc. ASME*. HT2012, Volume 1: Heat Transfer in Energy Systems; Theory and Fundamental Research; Aerospace Heat Transfer; Gas Turbine Heat Transfer; Transport Phenomena in Materials Processing and Manufacturing; Heat and Mass Transfer in Biotechnology; Environmental Heat Transfer; Visualization of Heat Transfer; Education and Future Directions in Heat Transfer, 897-898, July 8–12, 2012

Paper No: HT2012-58046

Abstract

In this paper, a critical point model with three Lorentzian terms for interband transition was proposed for dielectric permittivity of metal films. After validated, it was incorporated into a two-temperature model (TTM) to study transient optical and thermal response for a copper film irradiated by an ultrashort laser pulse. The dynamic changes of reflectivity ( R ) and absorptivity coefficient (α) during laser irradiation, electron and lattice temperature, and phase change were investigated. It was shown that for an ultrashort laser pulse with relatively high laser fluence, both R and α could drastically decrease, leading to significantly different thermal response than that described by using constant R and α at room temperature (RT).

Proceedings Papers

#### Study of Interesting Length and Temperature Effect on Ultrafast Electron Relaxation in CdSe Nanorods

*Proc. ASME*. HT2012, Volume 1: Heat Transfer in Energy Systems; Theory and Fundamental Research; Aerospace Heat Transfer; Gas Turbine Heat Transfer; Transport Phenomena in Materials Processing and Manufacturing; Heat and Mass Transfer in Biotechnology; Environmental Heat Transfer; Visualization of Heat Transfer; Education and Future Directions in Heat Transfer, 671-679, July 8–12, 2012

Paper No: HT2012-58565

Abstract

Time-domain non-adiabatic ab initio simulations are performed to study the phonon-assisted hot electron relaxation dynamics in CdSe QD, EQD and LQD, which are of the same diameter but an increasing length along c axis. Our work shows that both the length and system temperature have a strong impact on the electronic properties and electron relaxation dynamics of the CdSe QRs. Higher frequency phonons are excited and scattered with electrons at higher temperatures. The band gap shows a negative dependence on the temperature. The band gap decreases and the electron and hole density of states increase with increasing the length. However, not all the properties studied here vary with the length in a straight way. The band gap shows a stronger negative temperature dependence for the EQD than the QD and LQD. The electron-phonon couples stronger in the EQD than the QD and LQD. The hot electron relaxation proceeds faster in the EQD than the QD and LQD. Furthermore, the hot electron decay rate varies linearly with the average electron density of states and this linear relationship can be well described by the Fermi’s golden rule and of practical use in predicting the hot electron decay rate with the knowledge of the average NA coupling and electron density of states.

Proceedings Papers

*Proc. ASME*. HT2012, Volume 2: Heat Transfer Enhancement for Practical Applications; Fire and Combustion; Multi-Phase Systems; Heat Transfer in Electronic Equipment; Low Temperature Heat Transfer; Computational Heat Transfer, 685-693, July 8–12, 2012

Paper No: HT2012-58322

Abstract

In this study, a thermal and electrical coupled device solver is developed to simulate the energy transfer mechanism within a GaN FET with a gate length of 0.2 μm. The simulation simultaneously solves a set of hydrodynamic equations (derived from the Boltzmann Transport Equation) and the Poisson equation for electron, optical phonon and acoustic phonon energies, electron number density, electric field and electric potential. This approach has been previously established for gallium arsenide (GaAs) devices [36,37], but has not been extended to GaN due to the lack of readily available property values for GaN devices that are required. Via extensive literature study, high-fidelity properties for GaN were collected in analytical forms with respect to many dependencies, e.g. lattice temperature, electrical field, electron number density, doping rate, defects rate. These properties are then implemented into the developed code to provide a high accuracy sub-micron GaN device simulation. Simulations show that non-equilibrium heat generation is exhibited in a typical device while the drain current is reduced due to the decrease in electron mobility. Future analysis is needed to quantify the hot-electron effect on reducing the drain current and to discover more effective ways of heat removal.

Proceedings Papers

*Proc. ASME*. HT2009, Volume 1: Heat Transfer in Energy Systems; Thermophysical Properties; Heat Transfer Equipment; Heat Transfer in Electronic Equipment, 505-506, July 19–23, 2009

Paper No: HT2009-88347

Abstract

Elemental boron has many interesting properties, such as high melting point, low density, high hardness, high Young’s modulus, good oxidation resistance, resulting from its complex crystalline structure from its electron-deficient nature. Boron forms complex crystalline structures according to the various arrangements of B 12 icosahedra in the lattice, such as α (B 12 )- and β (B 105 )-rhombohedral and α (B 50 )- and β (B 196 )-tetragonal boron polymorphs, among others. Even though considerable materials research has been conducted over the past half century on boron and boron-based compounds, investigating their unique structures and corresponding properties, our understanding of this complex class of materials is still poor, compared to some other well-studied materials with much simpler structures such as silicon. Thermal transport studies through bulk boron have been performed mainly on β-rhombohedral and amorphous boron, because of the difficulty to grow high quality bulk α-rhombohedral boron samples [1–3]. Some efforts have been made to measure B 12 As 2 , B 12 P 2 , AlB 12 samples that have an α-rhombohedral form [2,3]. There is almost no information available on α-tetragonal boron. However, Slack predicted the thermal conductivity of α-boron should be ∼200 W/m-K at room temperature, which is 1/2 that of copper. Large phonon mean free path has been predicted for α-boron (from ∼200 nm at room temperature to 6 nm at the Debye temperature), which could lead to interesting thermal transport properties for low dimensional boron structures.

Proceedings Papers

*Proc. ASME*. HT2009, Volume 1: Heat Transfer in Energy Systems; Thermophysical Properties; Heat Transfer Equipment; Heat Transfer in Electronic Equipment, 421-430, July 19–23, 2009

Paper No: HT2009-88599

Abstract

In this paper, efficient spectral modules and random number databases are developed for atomic and diatomic species for use in photon Monte Carlo (PMC) modeling of hypersonic nonequilibrium flow radiation. To model nonequilibrium flow conditions, the quasi steady state (QSS) assumption was used to generate electronic state populations of atomic and diatomic gas species in the databases. For atomic species (N and O), both bound-bound transitions and continuum radiation were included, and were separately databased as a function of electron temperature and number density as well as the ratio of atomic ion to neutral number density. For the radiating diatomic species of N 2 + , N 2 , O 2 , and NO, databases were generated for each electronic molecular electronic system. In each molecular electronic system, the ro-vibrational transition lines were separately databased for each electronic upper state population forming the electronic system. The spectral module for the PMC method was optimized toward computational efficiency for emission calculations, wavelength selections of photon bundles and absorption coefficient calculations in the ray tracing scheme.

Proceedings Papers

*Proc. ASME*. HT2009, Volume 2: Theory and Fundamental Research; Aerospace Heat Transfer; Gas Turbine Heat Transfer; Computational Heat Transfer, 193-201, July 19–23, 2009

Paper No: HT2009-88270

Abstract

Electron-interface scattering during electron-phonon nonequilibrium in thin films creates another pathway for electron system energy loss as characteristic lengths of thin films continue to decrease. As power densities in nanodevices increase, excitations of electrons from sub-conduction-band energy levels will become more probable. These sub-conduction-band electronic excitations significantly affect the material’s thermophysical properties. In this work, the effects of d-band electronic excitations are considered in electron energy transfer processes in thin metal films. In thin films with thicknesses less than the electron mean free path, ballistic electron transport leads to electron-interface scattering. The ballistic component of electron transport, leading to electron-interface scattering, is studied by a ballistic-diffusive approximation of the Boltzmann Transport Equation. The effects of d-band excitations on electron-interface energy transfer is analyzed during electron-phonon nonequilibrium after short pulsed laser heating in thin films.

Proceedings Papers

*Proc. ASME*. HT2009, Volume 2: Theory and Fundamental Research; Aerospace Heat Transfer; Gas Turbine Heat Transfer; Computational Heat Transfer, 217-226, July 19–23, 2009

Paper No: HT2009-88280

Abstract

A recently developed Shastry formalism for energy transport is used to analyze the temporal behavior of the energy and heat transport in metals. Comparison with Cattaneo’s equation is performed. Both models show the transition between ballistic and diffusive regimes. Furthermore, because the new model considers the discrete character of the lattice, it highlights some new phenomena such as oscillations in the energy transport at very short time scales. The energy relaxation of the conduction band electrons in metals is considered to be governed by the electron-phonon scattering, and the scattering time is taken to be averaged over the Fermi surface. Using the new formalism, one can quantify the transfer from ballistic modes to diffusive ones as energy propagates in the material and it is transformed into heat. While the diffusive contribution shows an almost exponentially decaying behavior with time, the non-diffusive part shows a damped oscillating behavior. The origin of this oscillation will be discussed as well as the effect of temperature on the dynamics of the energy modes transport.

Proceedings Papers

*Proc. ASME*. HT2009, Volume 2: Theory and Fundamental Research; Aerospace Heat Transfer; Gas Turbine Heat Transfer; Computational Heat Transfer, 159-161, July 19–23, 2009

Paper No: HT2009-88210

Abstract

Upon filling, caged compounds like skutterudites can have their lattice thermal conductivity reduced by 4∼5 times compared with unfilled structures [1]. Recently, it was found that the thermal conductivity in Bi 2 Te 3 /Sb 2 Te 3 superlattice structure is also greatly reduced, even comparing with its corresponding alloy, in the cross-plane direction [2]. A fundamental understanding of thermal conductivity reduction in these structures is important due to their enhanced thermoelectric figure of merit. For filled skutterudites, “phonon-glass-electron-crystal (PGEC)” scheme was adopted to describe the role of guest atoms in the cages constructed by host atoms [3]. The localized and incoherent “rattling” behavior of guest atoms cuts down the mean free path of phonons, which results in reduced lattice thermal conductivity. In this study we apply ultrafast time resolved measurement technique to study coherent phonons in Bi 2 Te 3 /Sb 2 Te 3 superlattice and coherent vibrations in misch metal filled skutterudites, aim to reveal the mechanisms behind thermal conductivity reduction.

Proceedings Papers

*Proc. ASME*. HT2009, Volume 1: Heat Transfer in Energy Systems; Thermophysical Properties; Heat Transfer Equipment; Heat Transfer in Electronic Equipment, 129-136, July 19–23, 2009

Paper No: HT2009-88134

Abstract

Temperature dependent dynamics of phonon-assisted relaxation of hot carriers, both electrons and holes, is studied in a PbSe quantum dot using ab initio time-domain density functional theory. The electronic structure is first calculated, showing that the hole states are denser than the electron states. Fourier transforms of the time resolved energy levels show that the hot carriers couple to both acoustic and optical phonons. At higher temperature, more phonon modes in the high frequency range participate in the relaxation process due to their increased occupation number. The phonon-assisted hot carrier decay dynamics is predicted using non-adiabatic molecular dynamics, and the calculated relaxation rates clearly show a temperature-activation behavior. The complex temperature dependence is attributed to the combined effects of the phonon occupation number and thermal expansion. Comparing the simulation results with experiments, we suggest that the multiphonon relaxation channel is efficient at high temperature, while the Auger-like process may dominate the relaxation at low temperature. This combined mechanism can explain the weak temperature dependence at low temperature and stronger temperature dependence at higher temperature.

Proceedings Papers

*Proc. ASME*. HT2007, ASME/JSME 2007 Thermal Engineering Heat Transfer Summer Conference, Volume 3, 245-256, July 8–12, 2007

Paper No: HT2007-32475

Abstract

An interfacial tracking method is developed to model rapid melting and resolidification of a free-standing metal film subject to an ultrashort laser pulse. The laser energy is deposited to the electrons near thin film surface, and subsequently diffused into deeper part of the electron gas and transferred to the lattice. The energy equations for the electron and lattice are coupled through an electron-lattice coupling factor. Melting and resolidification are modeled by considering the interfacial energy balance and nucleation dynamics. An iterative solution procedure is employed to determine the elevated melting temperature and depressed solidification temperature in the ultrafast phase-change process. The predicted surface lattice temperature, interfacial location, interfacial temperature, and interfacial velocity are compared with those obtained by an explicit enthalpy model. The effects of the electron thermal conductivity models, ballistic range, and laser fluence on the melting and resolidification are also investigated.

Proceedings Papers

*Proc. ASME*. HT2007, ASME/JSME 2007 Thermal Engineering Heat Transfer Summer Conference, Volume 1, 7-11, July 8–12, 2007

Paper No: HT2007-32144

Abstract

It is well known that an emission of secondary electrons is observed in an ion collision process to a surface, such as the focused ion beam (FIB) process. However, the physical effect of secondary electron emission to energy and mass transfer is seldom considered and there are few examples of analysis of the secondary electron emission. It is one of interesting problems as an extreme small scale energy transfer problem how energy is transferred to the electron emitted from the surface by ionic collisions. In the present study the quantum molecular dynamics method was applied to an energy transfer problem to an electron during ionic surface collision process in order to elucidate how energy of ionic collision transfers to the emitted electrons. The energy transfer paths to the electron was discussed during the collision process of an ion with changing the interaction between the electron and ions and that between the electron and surface molecules by the quantum molecular dynamics method. Effects of various physical parameters, such as the collision velocity and interaction strength between the observed electron and the classical particles to the energy transfer to the electron were investigated by the quantum molecular dynamics method when the potassium ion was collided with the surface so as to elucidate the energy path to the electron and the predominant factor of energy transfer to the electron. Effects of potential energy between the ion and the electron and that between the surface molecule and the electron to the electronic energy transfer were shown in the present paper. The energy transfer to the observed secondary electron through the potential energy term between the ion and the electron was much dependent on the ion collision energy although the energy increase to the observed secondary electron was not monotonous through the potential energy between the ion and surface molecules with the change of the ion collision energy.

Proceedings Papers

*Proc. ASME*. HT2007, ASME/JSME 2007 Thermal Engineering Heat Transfer Summer Conference, Volume 1, 933-935, July 8–12, 2007

Paper No: HT2007-32843

Abstract

Optical properties of silver nanoparticles with different diameters are investigated based on the electronic structures of component silver atoms. Within the frame of tight binding method, the local density of states of each silver atom is obtained through a recursive approach that extracts the required information directly from the Hamilton matrix. Then the interaction between the electric field of incident light and electrons in the nanoparticles is simulated to characterize their optical features and the size effects were interpreted according the results.

Proceedings Papers

*Proc. ASME*. HT2007, ASME/JSME 2007 Thermal Engineering Heat Transfer Summer Conference, Volume 2, 485-492, July 8–12, 2007

Paper No: HT2007-32199

Abstract

An electron-phonon interaction model is proposed and applied to the transient thermal transport simulation during electrostatic discharge (ESD) event in the NMOS transistor. The high electron energy induced by the ESD in the transistor is transferred to the lattice phonons through electron-phonon interaction in the local region of the transistor. Due to this fact, a hot spot turns up, the size of which is much smaller than the phonon mean free path in the silicon layer. The full phonon dispersion model based on the Boltzmann transport equation (BTE) with the relaxation time approximation is applied to describe the interactions among different phonon branches and different phonon frequencies. The Joule heating by the electronphonon scattering is modeled through the intervalley and intravalley processes by introducing the average electron energy. In the simulation, the electron-phonon interaction model is used in the hot spot region, and then after a quasi-equilibrium state is achieved there, the temperature of lattice phonons in the silicon is calculated by using the phonon-phonon interaction model. The revolution of peak temperature in the hot spot during the ESD event is simulated and compared to that obtained by the previous full phonon dispersion model which treats the electron-phonon scattering as a volumetric heat source. The results show that the lower group velocity phonon modes (i.e. higher frequency) and optical mode of negligible group velocity obtain the highest energy density from electrons during the ESD event, which induces the devices melting phenomenon. The thermal response of phonon is also investigated, and it is found that the ratio of the phonon group velocity to the phonon specific heat can account for the phonon thermal response. If the ratio is higher than 2, the phonon have a good response to the heat input changes.

Proceedings Papers

*Proc. ASME*. HT2008, Heat Transfer: Volume 1, 35-36, August 10–14, 2008

Paper No: HT2008-56412

Abstract

At a finite temperature, electrons and ions in any matter are under constant thermal agitation, acting as the random current source for thermal emission. The thermally-excited electromagnetic waves have two forms: the propagating modes that can leave the surface of the emitter and radiate freely into the space, and the non-propagating modes (evanescent modes) that do not radiate. The contribution from the propagating modes, or the far-field radiation modes, to the radiative heat flux is well-known and its maximum is governed by Planck’s law of blackbody radiation. The non-propagating modes do not propagate and thus do not carry energy in the direction normal to the surface, unless a second surface is brought close to the first to enable photon tunneling. The contribution from the non-propagating modes to radiative heat flux is the near-field radiative flux.