Frequency Resolved Phonon Transport in Si/Ge Nanocomposites

2011 ◽  
Author(s):  
Dhruv Singh ◽  
Jayathi Y. Murthy ◽  
Timothy S. Fisher
Keyword(s):  
Thermal Conductivity ◽  
Energy Exchange ◽  
Phonon Dispersion ◽  
Low Frequency ◽  
Phonon Transport ◽  
Ballistic Regime ◽  
Acoustic Phonons ◽  
Length Scales ◽  
Diffusive Regime ◽  
Heterogeneous Interfaces

In this paper, we analyze cross plane phonon transport and thermal conductivity in two-dimensional Si/Ge nanocomposites. A non-gray BTE model that includes full details of phonon dispersion, the spread in phonon mean free paths and the frequency dependent transmissivity is used to simulate thermal transport. The general conclusions inferred from gray BTE simulations that the thermal conductivity of the nanocomposite is much lower than its constituent materials and interfacial density as the parameter determining thermal conductivity remain the same. However, it is found that the gray BTE significantly overpredicts thermal conductivity in the length scales of interest and quantitatively reliable results are obtained only upon inclusion of the details of phonon dispersion. The transition of phonon transport from ballistic regime to near diffusive regime is observed by looking at a large range of length scales. Non-equilibrium energy exchange between optical and acoustic phonons and the granularity in phonon mean free paths are found to significantly affect thermal conductivity leading to departures from the frequently employed gray approximation. It is also found that the frequency content of thermal conductivity in the nanocomposite extends out to a much larger frequency range unlike bulk Si and Ge. Scattering against heterogeneous interfaces is very effective in suppressing thermal conductivity contribution from the low frequency acoustic phonons but less so for high frequency phonons, which have much smaller mean free paths.

A Graphene Chain Acts as a Long-Distance Ballistic Heat Conductor

2010 ◽  
Author(s):  
Koji Takahashi ◽  
Yohei Ito ◽  
Tatsuya Ikuta
Keyword(s):  
Thermal Conductivity ◽  
Carbon Nanofiber ◽  
Soft Drink ◽  
Three Dimensional ◽  
Low Frequency ◽  
Phonon Transport ◽  
Dynamics Simulation ◽  
Infinite Length ◽  
Unexpected Result ◽  
Long Distance

A carbon nanofiber material, consisting of bottomless graphene cups inside on each other in a line, like a set of soft-drink cups, has been discovered to have the potential to conduct heat ballistically over a long distance. Its longitudinal heat transport ability had been forecast to be extremely poor due to the weak van der Waals force operating between the graphene cups, but our measurements using nano thermal sensor showed that its thermal conductivity is much higher than that along the c-axis of bulk graphite. This unexpected result can be understood by its similarity to a one-dimensional (1D) harmonic-chain where no phonon is scattered even for an infinite length. The current graphene-based nanofiber resembles this type of “superconductive” chain due to the huge difference between the stiff covalent bonding in each cup and the weak inter-cup interaction. A non-equilibrium molecular dynamics simulation is conducted to explore the phonon transport in this fiber. The simulation results show that the thermal conductivity varies with the fiber length in a power law fashion with an exponent as large as 0.7. The calculated phonon density of states and atomic motions indicate that a low-frequency quasi-1D oscillation occurs there. Our investigations show that treating the current nanofiber as a 1D chain with three-dimensional oscillations explains well why this material has the most effective ballistic phonon transport ever observed.

scholarly journals Thermal transport in nanocrystalline Si and SiGe by ab initio based Monte Carlo simulation

2017 ◽  
Vol 7 (1) ◽  
Author(s):  
Lina Yang ◽  
Austin J. Minnich
Keyword(s):  
Thermal Conductivity ◽  
Monte Carlo ◽  
Ab Initio ◽  
Grain Boundaries ◽  
Thermal Transport ◽  
Phonon Scattering ◽  
Phonon Dispersion ◽  
Computational Study ◽  
Phonon Transport ◽  
Nanocrystalline Si

Abstract Nanocrystalline thermoelectric materials based on Si have long been of interest because Si is earth-abundant, inexpensive, and non-toxic. However, a poor understanding of phonon grain boundary scattering and its effect on thermal conductivity has impeded efforts to improve the thermoelectric figure of merit. Here, we report an ab-initio based computational study of thermal transport in nanocrystalline Si-based materials using a variance-reduced Monte Carlo method with the full phonon dispersion and intrinsic lifetimes from first-principles as input. By fitting the transmission profile of grain boundaries, we obtain excellent agreement with experimental thermal conductivity of nanocrystalline Si [Wang et al. Nano Letters 11, 2206 (2011)]. Based on these calculations, we examine phonon transport in nanocrystalline SiGe alloys with ab-initio electron-phonon scattering rates. Our calculations show that low energy phonons still transport substantial amounts of heat in these materials, despite scattering by electron-phonon interactions, due to the high transmission of phonons at grain boundaries, and thus improvements in ZT are still possible by disrupting these modes. This work demonstrates the important insights into phonon transport that can be obtained using ab-initio based Monte Carlo simulations in complex nanostructured materials.

Monte Carlo Modeling of Phonon Transport Using Scattering Phase Functions

2008 ◽  
Author(s):  
Neil Zuckerman ◽  
Jennifer R. Lukes
Keyword(s):  
Thermal Conductivity ◽  
Monte Carlo ◽  
Numerical Methods ◽  
Ballistic Transport ◽  
Simulation Method ◽  
Phonon Transport ◽  
Nanometer Scale ◽  
Boltzmann Transport ◽  
Length Scales ◽  
Scale Structures

The calculation of heat transport in nonmetallic materials at small length scales is important in the design of thermoelectric and electronic materials. New designs with quantum dot superlattices (QDS) and other nanometer-scale structures can change the thermal conductivity in ways that are difficult to model and predict. The Boltzmann Transport Equation can describe the propagation of energy via mechanical vibrations in an analytical fashion but remains difficult to solve for the problems of interest. Numerical methods for simulation of propagation and scattering of high frequency vibrational quanta (phonons) in nanometer-scale structures have been developed but are either impractical at micron length scales, or cannot truly capture the details of interactions with nanometer-scale inclusions. Monte Carlo (MC) models of phonon transport have been developed and demonstrated based on similar numerical methods used for description of electron transport [1-4]. This simulation method allows computation of thermal conductivity in materials with length scales LX in the range of 10 nm to 10 μm. At low temperatures the model approaches a ballistic transport simulation and may function for even larger length scales.

Micro- and Nanoscale Phonon Heat Transport in Silicon

Volume 4 ◽  
2004 ◽  
Author(s):  
Y. Ju
Keyword(s):  
Thermal Conductivity ◽  
Silicon Nanowires ◽  
Phonon Dispersion ◽  
Transport Model ◽  
Phonon Transport ◽  
Conductivity Data ◽  
Phonon Modes ◽  
Conductivity Model ◽  
Thermal Conductivity Data ◽  
Thermal Conductivity Model

Micro- and nanoscale energy transport in semiconductors is one of the critical research areas for emerging nano-electronics. Key features of phonon dispersion curves are re-examined, which motivates the use of phonon density of states obtained from ab initio calculations as a basis for constructing a semi-phenomenological thermal conductivity model. Thermal conductivity data on silicon nanowires are analyzed to identify dominant phonon modes. The consistency of the present thermal conductivity model is examined by comparing its prediction with the thermal conductivity data from bulk germanium samples with controlled amount of point defects. The thermal conductivity modeling study provides input parameters for a two-fluid phonon transport model for silicon and related semiconductors, which can play an important role in computer aided design of nanoelectronic devices and simulation of ultra-fast phenomena.

Spectral Detail of Phonon Conduction and Scattering in Graphene

2011 ◽  
Author(s):  
Dhruv Singh ◽  
Jayathi Y. Murthy ◽  
Timothy S. Fisher
Keyword(s):  
Thermal Conductivity ◽  
Phonon Scattering ◽  
Interatomic Potential ◽  
Phonon Dispersion ◽  
Transition Probabilities ◽  
Transition Probability ◽  
Phonon Transport ◽  
Temperature Perturbation ◽  
Boltzmann Transport ◽  
Wide Range

This paper examines the thermodynamic and thermal transport properties of the 2D graphene lattice. The interatomic interactions are modeled using the Tersoff interatomic potential and are used to evaluate phonon dispersion curves, density of states and thermodynamic properties of graphene as functions of temperature. Perturbation theory is applied to calculate the transition probabilities for three-phonon scattering. The matrix elements of the perturbing Hamiltonian are calculated using the anharmonic interatomic force constants obtained from the interatomic potential as well. An algorithm to accurately quantify the contours of energy balance for three-phonon scattering events is presented and applied to calculate the net transition probability from a given phonon mode. Under the linear approximation, the Boltzmann transport equation (BTE) is applied to compute the thermal conductivity of graphene, giving spectral and polarization-resolved information. Predictions of thermal conductivity for a wide range of parameters elucidate the behavior of diffusive phonon transport. The complete spectral detail of selection rules, important phonon scattering pathways, and phonon relaxation times in graphene are provided, contrasting graphene with other materials, along with implications for graphene electronics. We also highlight the specific scattering processes that are important in Raman spectroscopy based measurements of graphene thermal conductivity, and provide a plausible explanation for the observed dependence on laser spot size.

Lattice Boltzmann Modeling of Subcontinuum Energy Transport in Crystalline and Amorphous Microelectronic Devices

2006 ◽  
Vol 128 (2) ◽  
pp. 115-124 ◽  
Author(s):  
Rodrigo Escobar ◽  
Brian Smith ◽  
Cristina Amon
Keyword(s):  
Thin Films ◽  
Thermal Conductivity ◽  
Lattice Boltzmann Method ◽  
Lattice Boltzmann ◽  
Energy Transport ◽  
Phonon Dispersion ◽  
Numerical Models ◽  
Phonon Transport ◽  
Nonlinear Frequency ◽  
Boltzmann Method

Numerical simulations of time-dependent energy transport in semiconductor thin films are performed using the lattice Boltzmann method applied to phonon transport. The discrete lattice Boltzmann method is derived from the continuous Boltzmann transport equation assuming first gray dispersion and then nonlinear, frequency-dependent phonon dispersion for acoustic and optical phonons. Results indicate that a transition from diffusive to ballistic energy transport is found as the characteristic length of the system becomes comparable to the phonon mean free path. The methodology is used in representative microelectronics applications covering both crystalline and amorphous materials including silicon thin films and nanoporous silica dielectrics. Size-dependent thermal conductivity values are also computed based on steady-state temperature distributions obtained from the numerical models. For each case, reducing feature size into the subcontinuum regime decreases the thermal conductivity when compared to bulk values. Overall, simulations that consider phonon dispersion yield results more consistent with experimental correlations.

Estimating Entropy Generation Rate for Ballistic-Diffusive Phonon Transport Using Effective Thermal Conductivity

2021 ◽  
Vol 0 (0) ◽  
Author(s):  
Saad Bin Mansoor ◽  
Bekir S. Yilbas
Keyword(s):  
Thermal Conductivity ◽  
Effective Thermal Conductivity ◽  
Entropy Generation ◽  
Mathematical Formulation ◽  
Phonon Transport ◽  
Generation Rate ◽  
Entropy Generation Rate ◽  
Film Material ◽  
Diffusive Regime ◽  
Low Dimensional

Abstract The entropy generation rate in a low dimensional film is formulated incorporating the heat flux and effective thermal conductivity of the film material. In the analysis, the mathematical formulation employed is kept the same as that used in the diffusive regime. However, the entropy generation rate is corrected by replacing the bulk thermal conductivity with an effective thermal conductivity evaluated from the Boltzmann equation. The entropy generation rate using the phonon distribution from the equation of phonon radiative transport in the film material is employed. The results show that both formulations result in a very close match for the entropy generation rates.

scholarly journals Giant isotope effect on phonon dispersion and thermal conductivity in methylammonium lead iodide

2020 ◽  
Vol 6 (31) ◽  
pp. eaaz1842
Author(s):  
M. E. Manley ◽  
K. Hong ◽  
P. Yin ◽  
S. Chi ◽  
Y. Cai ◽  
...  
Keyword(s):  
Thermal Conductivity ◽  
High Performance ◽  
Phonon Scattering ◽  
Light Emission ◽  
Phonon Dispersion ◽  
Acoustic Phonons ◽  
Lead Iodide ◽  
Phonon Bottleneck ◽  
Optic Phonon ◽  
Methylammonium Lead Iodide

Lead halide perovskites are strong candidates for high-performance low-cost photovoltaics, light emission, and detection applications. A hot-phonon bottleneck effect significantly extends the cooling time of hot charge carriers, which thermalize through carrier–optic phonon scattering, followed by optic phonon decay to acoustic phonons and finally thermal conduction. To understand these processes, we adjust the lattice dynamics independently of electronics by changing isotopes. We show that doubling the mass of hydrogen in methylammonium lead iodide by replacing protons with deuterons causes a large 20 to 50% softening of the longitudinal acoustic phonons near zone boundaries, reduces thermal conductivity by ~50%, and slows carrier relaxation kinetics. Phonon softening is attributed to anticrossing with the slowed libration modes of the deuterated molecules and the reduced thermal conductivity to lowered phonon velocities. Our results reveal how tuning the organic molecule dynamics enables control of phonons important to thermal conductivity and the hot-phonon bottleneck.

scholarly journals Effect of Phonon Dispersion on Thermal Conduction Across Si/Ge Interfaces

2009 ◽  
Author(s):  
Dhruv Singh ◽  
Jayathi Y. Murthy ◽  
Timothy S. Fisher
Keyword(s):  
Thermal Conductivity ◽  
Volume Fraction ◽  
Phonon Dispersion ◽  
Boltzmann Transport ◽  
Host Materials ◽  
Wide Range ◽  
Heterogeneous Interfaces ◽  
Silicon And Germanium ◽  
First Time ◽  
Superlattice Period

We report finite volume simulations of the phonon Boltzmann transport equation (BTE) for heat conduction across the heterogeneous interfaces in SiGe superlattices. We employ the diffuse mismatch model with full details of phonon dispersion and polarization. Simulations are performed over a wide range of Knudsen numbers. Similar to previous studies we establish that thermal conductivity of a superlattice is much lower than the host materials for superlattice period in the submicron regime. Details of the non-equilibrium between optical and acoustic phonons that emerge due to the mismatch of phonon spectrum in silicon and germanium are delineated for the first time. Conditions are identified for which this can lead to a significant additional thermal resistance than that attributed primarily to boundary scattering of phonons. We report results for thermal conductivity for various volume fraction and superlattice periods.

Thermal Transport in Finite-Sized Nanocomposites

2008 ◽  
Author(s):  
Dhruv Singh ◽  
Jayathi Y. Murthy ◽  
Timothy S. Fisher
Keyword(s):  
Thermal Conductivity ◽  
Phonon Transport ◽  
Boltzmann Transport ◽  
Host Materials ◽  
Individual Host ◽  
Wide Range ◽  
Heterogeneous Interfaces ◽  
The Individual ◽  
Nanoscale Composites ◽  
Bulk Behavior

We report finite volume simulations of the phonon Boltzmann Transport Equation (BTE) for heat conduction in periodic nanowire composites. Models for phonon transport across heterogeneous interfaces are developed, and simulations are performed over a wide range of Knudsen numbers. Conditions are identified under which the thermal conductivity of the composite material is less than the bulk thermal conductivity of the individual host materials and under which the alloy limit of thermal conductivity is recovered. We also compute the length scale needed to achieve bulk behavior in nanoscale composites. The results of this study are expected to inform and improve applications such as thermoelectric devices and flexible macroelectronics.

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