Thermal Properties for Bulk Silicon Based on the Determination of Relaxation Times Using Molecular Dynamics

2009 ◽  
Vol 132 (1) ◽  
Author(s):  
Javier V. Goicochea ◽  
Marcela Madrid ◽  
Cristina Amon

Molecular dynamics simulations are performed to estimate acoustical and optical phonon relaxation times, dispersion relations, group velocities, and specific heat of silicon needed to solve the Boltzmann transport equation (BTE) at 300 K and 1000 K. The relaxation times are calculated from the temporal decay of the autocorrelation function of the fluctuation of total energy of each normal mode in the ⟨100⟩ family of directions, where the total energy of each mode is obtained from the normal mode decomposition of the motion of the silicon atoms over a period of time. Additionally, silicon dispersion relations are directly determined from the equipartition theorem obtained from the normal mode decomposition. The impact of the anharmonic nature of the potential energy function on the thermal expansion of the crystal is determined by computing the lattice parameter at the cited temperatures using a NPT (i.e., constant number of atoms, pressure, and temperature) ensemble, and are compared with experimental values reported in the literature and with those computed analytically using the quasiharmonic approximation. The dependence of the relaxation times with respect to the frequency is identified with two functions that follow the functional form of the relaxation time expressions reported in the literature. From these functions a simplified version of relaxation times for each normal mode is extracted. Properties, such as group and phase velocities, thermal conductivity, and mean free path, needed to further develop a methodology for the thermal analysis of electronic devices (i.e., from nano- to macroscales) are determined once the relaxation times and dispersion relations are obtained. The thermal properties are validated by comparing the BTE-based thermal conductivity against the predictions obtained from the Green–Kubo method. It is found that the relaxation times closely resemble the ones obtained from perturbation theory at high temperatures; the contribution to the thermal conductivity of the transverse acoustic, longitudinal acoustic, and longitudinal optical modes being approximately 30%, 60%, and 10%, respectively, and the contribution of the transverse optical mode negligible.

Author(s):  
Bo Qiu ◽  
Hua Bao ◽  
Xiulin Ruan

In this paper, thermoelectric properties of bulk PbTe are calculated using first principles calculations and molecular dynamics simulations. The Full Potential Linearized Augmented Plane Wave (FP-LAPW) method is first employed to calculate the PbTe band structure. The transport coefficients (Seebeck coefficient, electrical conductivity, and electron thermal conductivity) are then computed using Boltzmann transport equation (BTE) under the constant relaxation time approximation. Interatomic pair potentials in the Buckingham form are also derived using ab initio effective charges and total energy data. The effective interatomic pair potentials give excellent results on equilibrium lattice parameters and elastic constants for PbTe. The lattice thermal conductivity of PbTe is then calculated using molecular dynamics simulations with the Green-Kubo method. In the end, the figure of merit of PbTe is computed revealing the thermoelectric capability of this material, and the multiscale simulation approach is shown to have the potential to identify novel thermoelectric materials.


Author(s):  
Majid S. al-Dosari ◽  
D. G. Walker

Yttrium Aluminum Garnet (YAG, Y3Al5O12) and its varieties have applications in thermographic phosphors, lasing mediums, and thermal barriers. In this work, thermal properties of crystalline YAG where aluminum atoms are substituted with gallium atoms (Y3(Al1−xGax)5O12) are explored with molecular dynamics simulations. For YAG at 300K, the simulations gave values close to experimental values for constant-pressure specific heat, thermal expansion, and bulk thermal conductivity. For various values of x, the simulations predicted no change in thermal expansion, an increase in specific heat, and a decrease in thermal conductivity for x = 50%. Furthermore, the simulations predicted a decrease in thermal conductivity with decreasing system size.


2019 ◽  
Author(s):  
Hui Yang ◽  
Jia-Yue Yang ◽  
Christopher Savory ◽  
Jonathan Skelton ◽  
Benjamin Morgan ◽  
...  

<div>LiCoO<sub>2</sub> is the prototype cathode in lithium ion batteries. It adopts a crystal structure with alternating Li<sup>+</sup> and CoO<sub>2</sub><sup>-</sup> layers along the hexagonal <0001> axis. It is well established that ionic and electronic conduction is highly anisotropic; however, little is known regarding heat transport. We analyse the phonon dispersion and lifetimes of LiCoO<sub>2</sub> using anharmonic lattice dynamics based on quantum chemical force constants. Around room temperature, the thermal conductivity in the hexagonal ab plane of the layered cathode is ≈ 6 times higher than that along the c axis based on the phonon Boltzmann transport. The low thermal conductivity (< 10Wm<sup>-1</sup>K<sup>-1</sup>) originates from a combination of short phonon lifetimes associated with anharmonic interactions between the octahedral face-sharing CoO<sub>2</sub><sup>-</sup> networks, as well as grain boundary scattering. The impact on heat management and thermal processes in lithium ion batteries based on layered positive electrodes is discussed.</div>


Author(s):  
Keivan Esfarjani ◽  
Gang Chen ◽  
Asegun Henry

Based on first-principles density-functional calculations, we have developed and tested a force-field for silicon, which can be used for molecular dynamics simulations and the calculation of its thermal properties. This force field uses the exact Taylor expansion of the total energy about the equilibrium positions up to 4th order. In this sense, it becomes systematically exact for small enough displacements, and can reproduce the thermodynamic properties of Si with high fidelity. Having the harmonic force constants, one can easily calculate the phonon spectrum of this system. The cubic force constants, on the other hand, will allow us to compute phonon lifetimes and scattering rates. Results on equilibrium Green-Kubo molecular dynamics simulations of thermal conductivity as well as an alternative calculation of the latter based on the relaxation-time approximation will be reported. The accuracy and ease of computation of the lattice thermal conductivity using these methods will be compared. This approach paves the way for the construction of accurate bulk interatomic potentials database, from which lattice dynamics and thermal properties can be calculated and used in larger scale simulation methods such as Monte Carlo.


Author(s):  
Carlos J. Gomes ◽  
Marcela Madrid ◽  
Cristina H. Amon

We have implemented a parallel molecular dynamics algorithm, which incorporates the Stillinger-Weber interatomic potential. The code was parallelized using a ghost cell atomic division approach, ensuring scaling with the number of processors and a significant increase in speed with respect to the serial version. The methodology is validated by computing the thermal conductivity and phonon frequency spectra of bulk silicon single crystals for different domain sizes at 1000K. The predicted thermal conductivities are consistent with the experimental value at that temperature. In addition, the phonon frequency spectra capture the properties expected from the dispersion relations for silicon.


2012 ◽  
Vol 2012 (DPC) ◽  
pp. 000706-000737
Author(s):  
Ravi M. Bhatkal ◽  
Ranjit Pandher ◽  
Anna Lifton ◽  
Paul Koep ◽  
Hafez Raeisi Fard

LED chip-on-board applications typically involve assembling an LED die stack directly on to a high thermal conductivity substrate such as a Metal Core PCB. If solder is used for die-substrate attach for such chip-on-board applications, one needs to consider the CTE mismatch between the die stack and the MCPCB and its impact on thermal cycle-induced creep fatigue of the solder material. This paper presents a methodology to compare relative performance of different solder materials with varying thermo-mechanical properties, and compare the impact of CTE mismatch and temperature swings on transient thermal properties and relative reliability of the solder attach materials. Implications for LED chip-on-board applications are discussed.


2013 ◽  
Vol 8 (3) ◽  
pp. 155892501300800 ◽  
Author(s):  
Abolfazl Mirdehghan ◽  
Siamak Saharkhiz ◽  
Hooshang Nosraty

This paper describes an experimental study of the impact of yarn structure on the thermal properties of worsted fabric. In this study, four different spun yarn structures (Solo, Siro, and single ply and two ply Ring) were woven into four fabric structures (Plain, Twill2/1, Twill2/2 and Basket2/2) and their thermal properties were studied. In addition, the thermal behavior of finished and unfinished samples was also evaluated. Results showed that the finishing process causes an increase in thermal conductivity and warmth to weight factor and a decrease in thermal insulation. Different spinning systems, also affect the thermal properties of the worsted fabrics. Samples with Siro yarns in the weft were found to have the highest thermal conductivity and those made from single ply weft yarn the lowest thermal conductivity. A relation between fabric thermal insulation and air permeability and thickness was also found.


2015 ◽  
Vol 1727 ◽  
Author(s):  
M. Rifu ◽  
K. Shintani

ABSTRACTThe thermal conductivities of pillared-graphene nanostructures (PGNSs) are obtained using nonequilibrium molecular-dynamics simulation. It is revealed their thermal conductivities are much smaller than the thermal conductivities of carbon nanotubes (CNTs). This fact is explained by examining the density of states (DOS) of the local phonons of PGNSs. It is also found the thermal conductivity of a PGNS linearly decreases with the increase of the inter-pillar distance.


Author(s):  
A. J. H. McGaughey ◽  
J. A. Thomas ◽  
J. Turney ◽  
R. M. Iutzi

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.


2017 ◽  
Vol 72 (2) ◽  
pp. 189-192 ◽  
Author(s):  
Konstantinos Termentzidis ◽  
Maxime Verdier ◽  
David Lacroix

AbstractThe majority of the silicon devices contain amorphous phase and amorphous/crystalline interfaces which both considerably affect the transport of energy carriers as phonons and electrons. In this article, we investigate the impact of amorphous phases (both amorphous silicon and amorphous SiO2) of silicon nanoporous membranes on their thermal properties via molecular dynamics simulations. We show that a small fraction of amorphous phase reduces dramatically the thermal transport. One can even create nanostructured materials with subamorphous thermal conductivity, while keeping an important crystalline fraction. In general, the a-SiO2 shell around the pores reduces the thermal conductivity by a factor of five to ten compared to a-Si shell. The phonon density of states for several systems is also given to give the impact of the amorphisation on the phonon modes.


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