Numerical Calculation for Phonon Properties of a Nano-Porous Si

2009 ◽  
Author(s):  
Koji Miyazaki ◽  
Daisuke Nagai ◽  
Yohei Kido ◽  
Hiroshi Tsukamoto
Keyword(s):  
Thermal Conductivity ◽  
Fourier Transform ◽  
Dispersion Curve ◽  
Phonon Dispersion ◽  
Mean Free Path ◽  
Atomic Scale ◽  
Phonon Modes ◽  
Velocity Correlation ◽  
Nano Structure ◽  
Time Space

We carried out molecular dynamics simulations (MD) of heat conduction in Si with a nano-hole to represent the nano-structure, in order to investigate the mechanism of the thermal conductivity reduction of nano-structured materials. The Stillinger-Weber potential is used in this study. The temperature is kept constant at 300K by velocity scaling. Periodic boundary conditions are applied in the x, y and z directions. Phonon dispersion curves are calculated by using the time-space 2D Fourier transform. The phonon group velocity is calculated from the slope of the dispersion curve. The velocity is reduced by nano-holes, even if those are random. Phonon mean free path can be evaluated from the width of dispersion curve, and the long waves are clearly scattered by nano-holes. Phonon density of states (DOS) is also calculated by the Fourier transform of a velocity correlation. The DOS of Si with periodic nano-holes are slightly smaller than that of a single crystal Si. In other words, the specific heat is reduced by periodic nano-holes due to the reduced phonon modes. We discuss the mechanism of the reduction of the thermal conductivity of nano-porous material on the atomic scale.

Thermal Design of Highly-Efficient Thermoelectric Materials With Atomic-Scale Three-Dimensional Phononic Crystals

2007 ◽  
Author(s):  
Jean-Numa Gillet ◽  
Yann Chalopin ◽  
Sebastian Volz
Keyword(s):  
Thermal Conductivity ◽  
Bulk Material ◽  
Phononic Crystal ◽  
Three Dimensional ◽  
Mean Free Path ◽  
Dispersion Curves ◽  
Phononic Crystals ◽  
Thermal Design ◽  
Atomic Scale ◽  
Thermal Insulating

Owing to their thermal insulating properties, superlattices have been extensively studied. A breakthrough in the performance of thermoelectric devices was achieved by using superlattice materials. The problem of those nanostructured materials is that they mainly affect heat transfer in only one direction. In this paper, the concept of canceling heat conduction in the three spatial directions by using atomic-scale three-dimensional (3D) phononic crystals is explored. A period of our atomic-scale 3D phononic crystal is made up of a large number of diamond-like cells of silicon atoms, which form a square supercell. At the center of each supercell, we substitute a smaller number of Si diamond-like cells by other diamond-like cells, which are composed of germanium atoms. This elementary heterostructure is periodically repeated to form a Si/Ge 3D nanostructure. To obtain different atomic configurations of the phononic crystal, the number of Ge diamond-like cells at the center of each supercell can be varied by substitution of Si diamond-like cells. The dispersion curves of those atomic configurations can be computed by lattice dynamics. With a general equation, the thermal conductivity of our atomic-scale 3D phononic crystal can be derived from the dispersion curves. The thermal conductivity can be reduced by at least one order of magnitude in an atomic-scale 3D phononic crystal compared to a bulk material. This reduction is due to the decrease of the phonon group velocities without taking into account that of the phonon average mean free path.

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.

Mode-Wise Thermal Conductivity of Bismuth Telluride

2013 ◽  
Vol 135 (9) ◽  
Author(s):  
Yaguo Wang ◽  
Bo Qiu ◽  
Alan J. H. McGaughey ◽  
Xiulin Ruan ◽  
Xianfan Xu
Keyword(s):  
Thermal Conductivity ◽  
Bismuth Telluride ◽  
Thermal Transport ◽  
Phonon Dispersion ◽  
Relaxation Times ◽  
Phonon Modes ◽  
Time Resolved ◽  
Transport Control ◽  
Reported Data ◽  
Phonon Properties

Thermal properties and transport control are important for many applications, for example, low thermal conductivity is desirable for thermoelectrics. Knowledge of mode-wise phonon properties is crucial to identify dominant phonon modes for thermal transport and to design effective phonon barriers for thermal transport control. In this paper, we adopt time-domain (TD) and frequency-domain (FD) normal-mode analyses to investigate mode-wise phonon properties and to calculate phonon dispersion relations and phonon relaxation times in bismuth telluride. Our simulation results agree with the previously reported data obtained from ultrafast time-resolved measurements. By combining frequency-dependent anharmonic phonon group velocities and lifetimes, mode-wise thermal conductivities are predicted to reveal the contributions of heat carriers with different wavelengths and polarizations.

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.

Phononic Engineering for Hot Carrier Solar Cells

2012 ◽  
pp. 214-242
Author(s):  
Sana Laribi ◽  
Arthur Le Bris ◽  
Lun Mei Huang ◽  
Par Olsson ◽  
Jean Francois Guillemoles
Keyword(s):  
Solar Cells ◽  
Solar Cell ◽  
Phonon Dispersion ◽  
Dispersion Relations ◽  
Phonon Modes ◽  
Nano Structure ◽  
Hot Carrier ◽  
Phonon Bottleneck ◽  
Nano Structures ◽  
Phonon Dispersion Relations

In this chapter, the authors first analyse the operation of a hot carrier solar cell and lay down the general principles. They then discuss the opportunity of phonon engineering to improve the phonon bottleneck. Finally, they present how these can be modeled in nanostuctures comprising several thousand atoms, where true 3D phonon dispersion relations for Si-Ge nano-structures are obtained using first principles methods. The effects of the nano-structure size and geometry on the phonon dispersion relations are investigated. The possible phonon decay processes in the nano-structures are discussed and compared with the bulk crystal materials. The performance of calculated nano-structures on the hot carrier solar cell is evaluated with the acquired knowledge of phonon modes.

Heat Conduction of a Porous Material

2012 ◽  
Vol 134 (5) ◽  
Author(s):  
Koji Miyazaki ◽  
Saburo Tanaka ◽  
Daisuke Nagai
Keyword(s):  
Thermal Conductivity ◽  
Heat Conduction ◽  
Porous Material ◽  
Boundary Conditions ◽  
Periodic Boundary ◽  
Phonon Dispersion ◽  
Bulk Material ◽  
Periodic Boundary Conditions ◽  
Boltzmann Transport ◽  
Time Space

In this study, we introduce our numerical and experimental works for the thermal conductivity reduction by using a porous material. Recently thermal conductivity reduction has been one of the key technologies to enhance the figure of merit (ZT) of a thermoelectric material. We carry out numerical calculations of heat conduction in porous materials, such as phonon Boltzmann transport (BTE) and molecular dynamics (MD) simulations, in order to investigate the mechanism of the thermal conductivity reduction of a porous material. In the BTE, we applied the periodic boundary conditions with constant heat flux to calculate the effective thermal conductivity of porous materials.In the MD simulation, we calculated the phonon properties of Si by using the Stillinger–Weber potential at constant temperature with periodic boundary conditions in the x, y, and z directions. Phonon dispersion curves of single crystal of Si calculated from MD results by time-space 2D FFT are agreed well with reference data. Moreover, the effects of nanoporous structures on both the phonon group velocity and the phonon density of states (DOS) are discussed. At last, we made a porous p-type Bi2Te3 by nanoparticles prepared by a beads milling method. The thermal conductivity is one-fifth of that of a bulk material as well as keeping the same Seebeck coefficient as the bulk value. However, electrical conductivity was much reduced, and the ZT was only 0.048.

Dielectric Nanocomposite Layering Configurations for Thermal Conductivity Reduction

2008 ◽  
Author(s):  
E. S. Landry ◽  
A. J. H. McGaughey ◽  
M. I. Hussein
Keyword(s):  
Thermal Conductivity ◽  
Phonon Dispersion ◽  
Phononic Crystal ◽  
Thermal Characteristics ◽  
Atomic Scale ◽  
Bulk Property ◽  
Space Applications ◽  
Structure And Property ◽  
Dynamics Simulations ◽  
Crystal Design

Thermal transport in crystals is governed by dynamic phenomena that take place at the atomic scale, namely phonon dispersion and scattering. A growing understanding of these mechanisms, coupled with increasingly capable nanofabrication and characterization technologies, provide a not-too-distant opportunity for designing a new class of materials with tailored thermal characteristics such as thermal conductivity, among other physical characteristics. Focusing on layered nanocomposites, also known as superlattices, modeled using the Lennard-Jones potential as a starting platform, we examine the effects of layering topology on the bulk property of thermal conductivity. We use molecular dynamics simulations to examine the link between structure and property; and employ ideas from phononic crystal design to investigate the potential of realizing dielectric crystals with exceedingly low thermal conductivities. This work potentially targets a range of applications such as thermal insulators for space applications and thermoelectrics for energy harvesting.

Lattice Boltzmann Modeling of Phonon Heat Conduction in Superlattice Structures

2010 ◽  
Author(s):  
Cristian J. San Marti´n ◽  
Amador M. Guzma´n ◽  
Rodrigo A. Escobar
Keyword(s):  
Thermal Conductivity ◽  
Free Path ◽  
Lattice Boltzmann ◽  
Phonon Dispersion ◽  
Effective Conductivity ◽  
Mean Free Path ◽  
One Dimension ◽  
The Mean ◽  
Superlattice Structures ◽  
Boltzmann Method

The results of temperature prediction and determination of effective thermal conductivity in periodic Si-Ge superlattice in one dimension, at length scale comparable to the mean free path are presented. Classical heat transfer models such as Fourier’s law do not represent what actually happens within electronic devices at these length scales. Phonon-border and phonon-interface scattering effects provide discontinuous jumps in temperature distribution when the mean free path is comparable with the device’s characteristic length, a relation given by the Knudsen number (Kn). For predicting the temperature within the periodic Si-Ge superlattice use is made of the lattice Boltzmann method in one dimension, using Debye’s model in the phonon dispersion relation. The predictions show that as Kn increases, so do the jumps at the borders, the same as at the interfaces. The prediction also shows that the effective conductivity of the Si-Ge superlattice decreases as Kn and the number of layers of material increase, and that keff decreases as the magnitude of p increases, a factor that allows heat flow between one layer and another. Use of gray LBM leads to good approximations of the actual temperature field and thermal conductivity values for the superlattice materials model when the physics of phonons established by Debye’s model is used.

Molecular Dynamics Simulations of Heat Conduction in Nano-Structured Silicon

2007 ◽  
Author(s):  
Koji Miyazaki ◽  
Yoshizumi Iida ◽  
Daisuke Nagai ◽  
Hiroshi Tsukamoto
Keyword(s):  
Thin Film ◽  
Thermal Conductivity ◽  
Molecular Dynamics ◽  
Heat Conduction ◽  
Molecular Dynamics Simulations ◽  
Atomic Scale ◽  
Dynamics Simulation ◽  
Phonon Modes ◽  
Dynamics Simulations ◽  
Si Thin Film

We carried out molecular dynamics simulations (MD) of heat conduction in Si thin film and Si films with a nano-hole to represent the nano-structure, in order to investigate the mechanism of the thermal conductivity reduction of nano-structured materials. The Stillinger-Weber potential is used in this study. Different temperatures are applied at the both sides of boundaries of the calculation domain in the z-direction, and periodic boundary conditions are applied in the x and y directions. The calculated temperature profile of a Si thin film of 10.86nm thickness is compared to that calculated by using the phonon Boltzmann transport equation (BTE). These agreed reasonably well with each other, and the phonon mean free path of Si is estimated to be several tens of nanometers. Molecular dynamics simulation of Si at the uniform temperature of 800K is also carried out. Phonon dispersion curves are calculated by using the time-space 2D Fourier transform. The phonon modes at high frequency are not present in nano-structures of Si. We discuss the mechanism of the reduction of the thermal conductivity of nano-structured material on the atomic scale.

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