129 research outputs found
Quantitatively Analyzing Phonon Spectral Contribution of Thermal Conductivity Based on Non-Equilibrium Molecular Dynamics Simulation II: From Time Fourier Transform
From nano-scale heat transfer point of view, currently one of the most
interesting and challenging tasks is to quantitatively analyzing phonon mode
specific transport properties in solid materials, which plays vital role in
many emerging and diverse technological applications. It has not been so long
since such information can be provided by the phonon spectral energy density
(SED) or equivalently time domain normal mode analysis (TDNMA) methods in the
framework of equilibrium molecular dynamics simulation (EMD). However, until
now it has not been realized in non-equilibrium molecular dynamics simulations
(NEMD), the other widely used computational method for calculating thermal
transport of materials in addition to EMD. In this work, a computational scheme
based on time Fourier transform of atomistic heat current, called frequency
domain direct decomposed method (FDDDM), is proposed to analyze the
contributions of frequency dependent thermal conductivity in NEMD simulations.
The FDDDM results of Lennard-Jones (LJ) Argon and Stillinger-Weber (SW) Si are
compared with TDNMA method from EMD simulation. Similar trends are found for
both cases, which confirm the validity of our FDDDM approach. Benefiting from
the inherent nature of NEMD and the theoretical formula that does not require
any temperature assumption, the FDDDM can be directly used to investigate the
size and temperature effect. Moreover, the unique advantage of FDDDM prior to
previous methods (such as SED and TDNMA) is that it can be straightforwardly
used to characterize the phonon frequency dependent thermal conductivity of
disordered systems, such as amorphous materials. The FDDDM approach can also be
a good candidate to be used to understand the phonon behaviors and thus
provides useful guidance for designing efficient structures for advanced
thermal management
Quantitatively Analyzing Phonon Spectral Contribution of Thermal Conductivity Based on Non-Equilibrium Molecular Dynamics Simulation I: From Space Fourier Transform
Probing detailed spectral dependence of phonon transport properties in bulk
materials is critical to improve the function and performance of structures and
devices in a diverse spectrum of technologies. Currently, such information can
only be provided by the phonon spectral energy density (SED) or equivalently
time domain normal mode analysis (TDNMA) methods in the framework of
equilibrium molecular dynamics simulation (EMD), but has not been realized in
non-equilibrium molecular dynamics simulations (NEMD) so far. In this paper we
generate a new scheme directly based on NEMD and lattice dynamics theory,
called time domain direct decomposition method (TDDDM), to predict the phonon
mode specific thermal conductivity. Two benchmark cases of Lennard-Jones (LJ)
Argon and Stillinger-Weber (SW) Si are studied by TDDDM to characterize
contributions of individual phonon modes to overall thermal conductivity and
the results are compared with that predicted using SED and TDNMA. Excellent
agreements are found for both cases, which confirm the validity of our TDDDM
approach. The biggest advantage of TDDDM is that it can be used to investigate
the size effect of individual phonon modes in NEMD simulations, which cannot be
tackled by SED and TDNMA in EMD simulations currently. We found that the phonon
modes with mean free path larger than the system size are truncated in NEMD and
contribute little to the overall thermal conductivity. The TDDDM provides
direct physical origin for the well-known strong size effects in thermal
conductivity prediction by NEMD
Thermal Transfer in Amorphous Superionic Systems
Using direct atomic simulations, the vibration scattering time scales are
characterized, and then the nature and the quantitative weight of thermal
excitations are investigated in an example system Li2S from its amorphous solid
state to its partial-solid partial-liquid and, liquid states. For the amorphous
solid state at 300 K, the vibration scattering time ranges a few femtoseconds
to several picoseconds. As a result, both the progagons and diffusons are the
main heat carriers and contribute largely to the total thermal conductivity.
The enhancement of scattering among vibrations and between vibrations and free
ions flow due to the increase of temperature, will lead to a large reduction of
the scattering time scale and the acoustic vibrational thermal conductivity,
i.e., 0.8 W/mK at 300 K to 0.56 W/mK in the partial solid partial liquid Li2S
at 700 K. In this latter state, the thermal conductivity contributed by
convection increases to the half of the total, as a result of the usually
neglected cross-correlation between the virial term and the free ions' flow.
The vibrational scattering time can be as large as ~ 1.5 picoseconds yet, and
the vibrational conductivity is reduced to a still significant 0.42 W/mK
highlighting the unexpected role of acoustic transverse and longitudinal
vibrations in liquid Li2S at 1100 K. At this same temperature, the convection
heat transfer takes overreaching 0.63 W/mK. Our study provides a fundamental
understanding of the thermal excitations at play in amorphous materials from
solid to liquid
Quantitatively Predicting Modal Thermal Conductivity of Nanocrystalline Si by full band Monte Carlo simulations
Thermal transport of nanocrystalline Si is of great importance for the
application of thermoelectrics. A better understanding of the modal thermal
conductivity of nanocrystalline Si will be expected to benefit the efficiency
of thermoelectrics. In this work, the variance reduced Monte Carlo simulation
with full band of phonon dispersion is applied to study the modal thermal
conductivity of nanocrystalline Si. Importantly, the phonon modal transmissions
across the grain boundaries which are modeled by the amorphous Si interface are
calculated by the mode-resolved atomistic Greens function method. The predicted
ratios of thermal conductivity of nanocrystalline Si to that of bulk Si agree
well with that of the experimental measurements in a wide range of grain size.
The thermal conductivity of nanocrystalline Si is decreased from 54 percent to
3 percent and the contribution of phonons with mean free path larger than the
grain size increases from 30 percent to 96 percnet as the grain size decreases
from 550 nm to 10 nm. This work demonstrates that the full band Monte Carlo
simulation using phonon modal transmission by the mode-resolved atomistic
Greens function method can capture the phonon transport picture in complex
nanostructures, and therefore can provide guidance for designing high
performance Si based thermoelectrics
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