Mode-Decay Molecular Dynamics for Frequency-Dependent Phonon Scattering Rates

2014 ◽  
Author(s):  
M. D. Gerboth ◽  
D. G. Walker
Keyword(s):  
Thermal Conductivity ◽  
Molecular Dynamics ◽  
Standing Wave ◽  
Phonon Scattering ◽  
Relaxation Rates ◽  
Phonon Modes ◽  
Crystalline Materials ◽  
Transmission Coefficients ◽  
Energy Dependent ◽  
Scattering Rates

The thermal conductivity of crystalline materials can be determined in a statistical mechanical framework as long as phonon relaxation rates are known. Unfortunately, these quantities are difficult if not impossible to measure directly, and attempts to deduce these quantities yield gross averages not energy dependent relationships. Consequently, researchers often rely on heuristic models such as Holland’s suite of scattering rates for various phonon modes. A new molecular dynamics method was developed to estimate mode-dependent scattering rates by tracking the decay of an initially imposed standing wave. The wave vector is systematically changed and the corresponding decay is collected. Ultimately, the the thermal conductivity can be reconstructed using a Landauer formalism. The phonon scattering rates of a LJ crystal are calculated using this method. The standing wave decay approach allows scattering rates to be probed more directly than wave packet simulations, which are often used to obtain transmission coefficients.

A Molecular Dynamics Study of Thermal Conductivity in Nanocomposites via the Phonon Wave Packet Method

2009 ◽  
Author(s):  
Zhiting Tian ◽  
Sang Kim ◽  
Ying Sun ◽  
Bruce White
Keyword(s):  
Thermal Conductivity ◽  
Molecular Dynamics ◽  
Wave Packet ◽  
Molecular Dynamics Simulations ◽  
Phonon Scattering ◽  
Flow Direction ◽  
Normal Incidence ◽  
Material Interfaces ◽  
Non Equilibrium Molecular Dynamics ◽  
Dynamics Simulations

The phonon wave packet technique is used in conjunction with the molecular dynamics simulations to directly observe phonon scattering at material interfaces. The phonon transmission coefficient of nanocomposites is examined as a function of the defect size, thin film thickness, orientation of interface to the heat flow direction. To generalize the results based on phonons in a narrow frequency range and at normal incidence, the effective thermal conductivity of the same nanocomposite structure is calculated using non-equilibrium molecular dynamics simulations for model nanocomposites formed by two mass-mismatched Ar-like solids and heterogeneous Si-SiCO2 systems. The results are compared with the modified effective medium formulation for nanocomposites.

scholarly journals Reduction of Lattice Thermal Conductivity in PbTe Induced by Artificially Generated Pores

2015 ◽  
Vol 2015 ◽  
pp. 1-6 ◽  
Author(s):  
Jae-Yeol Hwang ◽  
Eun Sung Kim ◽  
Syed Waqar Hasan ◽  
Soon-Mok Choi ◽  
Kyu Hyoung Lee ◽  
...  
Keyword(s):  
Thermal Conductivity ◽  
Pore Structure ◽  
Transport Properties ◽  
Thermoelectric Materials ◽  
Thermal Transport ◽  
Lattice Thermal Conductivity ◽  
Phonon Scattering ◽  
Phonon Modes ◽  
Thermal Transport Properties

Highly dense pore structure was generated by simple sequential routes using NaCl and PVA as porogens in conventional PbTe thermoelectric materials, and the effect of pores on thermal transport properties was investigated. Compared with the pristine PbTe, the lattice thermal conductivity values of pore-generated PbTe polycrystalline bulks were significantly reduced due to the enhanced phonon scattering by mismatched phonon modes in the presence of pores (200 nm–2 μm) in the PbTe matrix. We obtained extremely low lattice thermal conductivity (~0.56 W m−1 K−1at 773 K) in pore-embedded PbTe bulk after sonication for the elimination of NaCl residue.

Monte Carlo Study of Phonon Heat Conduction in Silicon Thin Films Including Contributions of Optical Phonons

2010 ◽  
Vol 132 (5) ◽  
Author(s):  
Arpit Mittal ◽  
Sandip Mazumder
Keyword(s):  
Thin Films ◽  
Thermal Conductivity ◽  
Monte Carlo ◽  
Heat Conduction ◽  
Phonon Scattering ◽  
Computational Domain ◽  
Optical Phonons ◽  
Phonon Modes ◽  
Silicon Thin Film ◽  
Silicon Thin Films

Abstract The Monte Carlo method has found prolific use in the solution of the Boltzmann transport equation for phonons for the prediction of nonequilibrium heat conduction in crystalline thin films. This paper contributes to the state-of-the-art by performing a systematic study of the role of the various phonon modes on thermal conductivity predictions, in particular, optical phonons. A procedure to calculate three-phonon scattering time-scales with the inclusion of optical phonons is described and implemented. The roles of various phonon modes are assessed. It is found that transverse acoustic (TA) phonons are the primary carriers of energy at low temperatures. At high temperatures (T>200 K), longitudinal acoustic (LA) phonons carry more energy than TA phonons. When optical phonons are included, there is a significant change in the amount of energy carried by various phonons modes, especially at room temperature, where optical modes are found to carry about 25% of the energy at steady state in silicon thin films. Most importantly, it is found that inclusion of optical phonons results in better match with experimental observations for silicon thin-film thermal conductivity. The inclusion of optical phonons is found to decrease the thermal conductivity at intermediate temperatures (50–200 K) and to increase it at high temperature (>200 K), especially when the film is thin. The effect of number of stochastic samples, the dimensionality of the computational domain (two-dimensional versus three-dimensional), and the lateral (in-plane) dimension of the film on the statistical accuracy and computational efficiency is systematically studied and elucidated for all temperatures.

scholarly journals Ultra-low Thermal Conductivity in Si/Ge Hierarchical Superlattice Nanowire

2015 ◽  
Vol 5 (1) ◽  
Author(s):  
Xin Mu ◽  
Lili Wang ◽  
Xueming Yang ◽  
Pu Zhang ◽  
Albert C. To ◽  
...  
Keyword(s):  
Thermal Conductivity ◽  
Molecular Dynamics ◽  
Silicon Germanium ◽  
Phonon Scattering ◽  
Low Thermal Conductivity ◽  
Structural Hierarchy ◽  
Si Nanowire ◽  
Hierarchical Structuring ◽  
Simulation Results ◽  
Dynamics Simulations

Abstract Due to interfacial phonon scattering and nanoscale size effect, silicon/germanium (Si/Ge) superlattice nanowire (SNW) can have very low thermal conductivity, which is very attractive for thermoelectrics. In this paper, we demonstrate using molecular dynamics simulations that the already low thermal conductivity of Si/Ge SNW can be further reduced by introducing hierarchical structure to form Si/Ge hierarchical superlattice nanowire (H-SNW). The structural hierarchy introduces defects to disrupt the periodicity of regular SNW and scatters coherent phonons, which are the key contributors to thermal transport in regular SNW. Our simulation results show that periodically arranged defects in Si/Ge H-SNW lead to a ~38% reduction of the already low thermal conductivity of regular Si/Ge SNW. By randomizing the arrangement of defects and imposing additional surface complexities to enhance phonon scattering, further reduction in thermal conductivity can be achieved. Compared to pure Si nanowire, the thermal conductivity reduction of Si/Ge H-SNW can be as large as ~95%. It is concluded that the hierarchical structuring is an effective way of reducing thermal conductivity significantly in SNW, which can be a promising path for improving the efficiency of Si/Ge-based SNW thermoelectrics.

Mechanism of Thermal Conductivity Reduction From Suspended to Supported Graphene: A Quantitative Spectral Analysis of Phonon Scattering

2011 ◽  
Author(s):  
Bo Qiu ◽  
Xiulin Ruan
Keyword(s):  
Thermal Conductivity ◽  
Spectral Analysis ◽  
Phonon Scattering ◽  
Md Simulations ◽  
Mean Free Path ◽  
Spectral Energy ◽  
Phonon Modes ◽  
Suspended Graphene ◽  
Out Of Plane ◽  
Predicted Values

In this work, we perform molecular dynamics (MD) simulations together with phonon spectral analysis to predict the thermal conductivity of both suspended and supported graphene. We quantitatively address the relative importance of different types of phonon in thermal transport and explain why thermal conductivity is significantly reduced in supported graphene compared to that in suspended graphene. Within the framework of equilibrium MD, we perform spectral energy density analysis to obtain the phonon mean free path of each individual phonon mode. The contribution of each mode to thermal conductivity is then calculated and summed to obtain the lattice thermal conductivity in the temperature range 300–650 K. Our predicted values and temperature dependence for both suspended and supported graphene agree with experimental data well. In contrast to prior studies, our results suggest that the contribution from out-of-plane acoustic (ZA) branch to thermal conductivity is around 25–30% in suspended graphene at room temperature. The thermal conductivity of supported graphene is predicted to be largely reduced, which is consistent with experimental observations. Such reduction is shown to be due to stronger scattering of all phonon modes rather than only the ZA mode in the presence of the substrate.

Coupling Between Phonons and Fluid Particles in Water/Carbon Nanotube Systems

2009 ◽  
Author(s):  
A. J. H. McGaughey ◽  
J. A. Thomas ◽  
J. Turney ◽  
R. M. Iutzi
Keyword(s):  
Thermal Conductivity ◽  
Molecular Dynamics ◽  
Carbon Nanotube ◽  
Relaxation Times ◽  
Mean Free Path ◽  
Phonon Transport ◽  
Dynamics Simulation ◽  
Spectral Energy ◽  
Total Thermal Conductivity ◽  
Phonon Modes

We investigate thermal transport in water/carbon nanotube (CNT) composite systems using molecular dynamics simulations. Carbon-carbon interactions are modeled using the second-generation REBO potential, water-water interactions are modeled using the TIP4P potential, and carbon-water interactions are modeled using a Lennard-Jones potential. The thermal conductivities of empty and water-filled CNTs with diameters between 0.83 nm and 1.66 nm are predicted using molecular dynamics simulation and a direct application of the Fourier law. For empty CNTs, the thermal conductivity decreases with increasing CNT diameter. As the CNT length approaches 1 micron, a length-independent thermal conductivity is obtained, indicative of diffusive phonon transport. When the CNTs are filled with water, the thermal conductivity decreases compared to the empty CNTs and transitions to diffusive phonon transport at shorter lengths. To understand this behavior, we calculate the spectral energy density of the empty and water-filled CNTs and calculate the mode-specific group velocities, relaxation times, and thermal conductivity. For the empty 1.10 nm diameter CNT, we show that the acoustic phonon modes account for 65 percent of the total thermal conductivity. This behavior is attributed to their long mean-free paths. When the CNT is filled with water, interactions with the water molecules shorten the acoustic mode mean-free path and lower the overall CNT thermal conductivity.

scholarly journals Crystallographic Orientation Effect On Hole Polar Optical Phonon Scattering Rates in Thin Gaas/Alxga1-Xas Quantum Wells

2019 ◽  
Author(s):  
Mohamed Boumaza
Keyword(s):  
Quantum Wells ◽  
Optical Phonon ◽  
Phonon Scattering ◽  
Strain Effect ◽  
Polar Optical Phonon ◽  
Phonon Modes ◽  
Bulk Gaas ◽  
Hole Energy ◽  
Optical Phonon Scattering ◽  
Scattering Rates

We report on hole polar optical phonon scattering processes in thin GaAs/AlxGa1-xAs quantum wells grown in various crystallographic directions, such as [001], [110]. Using the dielectric continuum model we focus on how the different scattering processes of holes with interface phonon modes depend on the initial hole energy. In our work, we use the Luttinger-Kohn (LK) 6×6 k.p Hamiltonian with the envelope function approximation, from which we compute numerically the electronic structure of holes for a thin quantum well sustaining only one bound state for each type of hole. Due to mixing between the heavy, light, and split off bands, hole subbands exhibit strong nonparabolicity and important warping that have their word to say on physical properties. Detailed and extensive calculations that the rates for intra-subband scattering processes differ significantly from those of bulk GaAs because of quantization and reduced dimensionality. Moreover, the study of scattering as a function of hole energy shows that the trend of the scattering rates is governed mostly by i) overlap integrals and ii) the density of the final states to which the hole scatters. The influence of warping, in the hole energy dispersion, on the phonon scattering rates is also explored and found to be important when the initial hole energy is high. Our calculations show evidence of strong anisotropy in the scattering rates especially for processes involving the heavy hole subband, which anisotropy is in fact quite important and far from being negligible. However, strain effect can reduce scattering rates.

Study of the thermal conductivity of a metal-coated multi-walled carbon nanotube using molecular dynamics atomistic simulations

MRS Advances ◽  
2019 ◽  
Vol 4 (08) ◽  
pp. 507-513 ◽  
Author(s):  
Dinesh Bommidi ◽  
Ravindra Sunil Dhumal ◽  
Iman Salehinia
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Molecular Dynamics ◽  
Carbon Nanotube ◽  
Phonon Scattering ◽  
Metal Coating ◽  
Atomistic Simulations ◽  
Multi Walled Carbon Nanotube

ABSTRACTThermal conductivity of a nickel-coated tri-wall carbon nanotube was studied using molecular dynamics where both the phonon and electron contributions were considered. Simulations predicted a significant effect of the metal coating on the thermal conductivity, i.e. 50% decrease for 1.2 nm of Ni coating. However, the decreasing rate of the thermal conductivity is minuscule for the metal thicker than 1.6 nm. The smaller thermal conductivity of the metal coating, phonon scattering at the interface, and less impacted heat transfer on the inner tubes of the carbon nanotube rationalized the observed trends.

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