343 research outputs found
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 conductivity reduction in core-shell nanowires
Nanostructuring of thermoelectric materials bears promise for manipulating physical parameters to improve the energy conversion efficiency of thermoelectrics. Using nonequilibrium molecular dynamics, we investigate how the thermal conductivity can be altered in core-shell nanocomposites of Si and Ge. By calculating the phonon vibrational density of states and performing normal mode analysis, we show that the thermal conductivity decreases when phonon-transport becomes diffusion-dominated and unveil a competition between modes from the various regions of the nanocomposite (core, interface, and shell). The effects of nanowire length, cross section, and temperature on thermal conductivity are explicitly considered. Surprisingly, the thermal conductivity variation with nanowire length is much weaker than in pure nanowires. Also, the thermal conductivity is almost independent of temperature in the wide region between 50 and 600 K, a direct result of confinement of the core by the shell. These results suggest that core-shell nanowires are promising structures for thermoelectrics
Significant Reduction of Thermal Conductivity in Si/Ge Core-Shell Nanowires
We report on the effect of germanium (Ge) coatings on the thermal transport properties of silicon (Si) nanowires using nonequilibrium
molecular dynamics simulations. Our results show that a simple
deposition of a Ge shell of only 1 to 2 unit cells in thickness on a
single crystalline Si nanowire can lead to a dramatic 75% decrease in
thermal conductivity at room temperature compared to an uncoated Si
nanowire. By analyzing the vibrational density states of phonons and
the participation ratio of each specific mode, we demonstrate that the
reduction in the thermal conductivity of Si/Ge core hell nanowire stems
from the depression and localization of long-wavelength phonon modes at
the Si/Ge interface and of high frequency nonpropagating diffusive
modes
Methodology for determining the electronic thermal conductivity of metals via direct non-equilibrium ab initio molecular dynamics
Many physical properties of metals can be understood in terms of the free
electron model, as proven by the Wiedemann-Franz law. According to this model,
electronic thermal conductivity () can be inferred from the
Boltzmann transport equation (BTE). However, the BTE does not perform well for
some complex metals, such as Cu. Moreover, the BTE cannot clearly describe the
origin of the thermal energy carried by electrons or how this energy is
transported in metals. The charge distribution of conduction electrons in
metals is known to reflect the electrostatic potential (EP) of the ion cores.
Based on this premise, we develop a new methodology for evaluating
by combining the free electron model and non-equilibrium ab
initio molecular dynamics (NEAIMD) simulations. We demonstrate that the kinetic
energy of thermally excited electrons originates from the energy of the spatial
electrostatic potential oscillation (EPO), which is induced by the thermal
motion of ion cores. This method directly predicts the of pure
metals with a high degree of accuracy.Comment: 7 pages, 3 figures, with Supplementary Information of 19 pages, 7
figures and 7 table
Fundamental Structure of General Stochastic Dynamical Systems: High-Dimension Case
No one has proved that mathematically general stochastic dynamical systems have a special structure. Thus, we introduce a structure of a general stochastic dynamical system. According to scientific understanding, we assert that its deterministic part can be decomposed into three significant parts: the gradient of the potential function, friction matrix and Lorenz matrix. Our previous work proved this structure for the low-dimension case. In this paper, we prove this structure for the high-dimension case. Hence, this structure of general stochastic dynamical systems is fundamental
THE GENERALIZED LYAPUNOV FUNCTION as AOâS POTENTIAL FUNCTION: EXISTENCE in DIMENSIONS 1 and 2
By using Ao\u27s decomposition for stochastic dynamical systems, a new notion of potential function has been introduced by Ao and his collabora-tors recently. We show that this potential function agrees with the generalized Lyapunov function of the deterministic part of the stochastic dynamical sys-tem. We further prove the existence of Ao\u27s potential function in dimensions 1 and 2 via the solution theory of first-order partial differential equations. Our framework reveals the equivalence between Ao\u27s potential function and Lyapunov function, the latter being one of the most significant central notions in dynamical systems. Using this equivalence, our existence proof can also be interpreted as the proof of existence of Lyapunov function for a general dynamical system
Using canopy greenness index to identify leaf ecophysiological traits during the foliar senescence in an oak forest
© The Author(s), 2018. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Ecosphere 9 (2018): e02337, doi:10.1002/ecs2.2337.Cameraâbased observation of forest canopies allows for lowâcost, continuous, high temporalâspatial resolutions of plant phenology and seasonality of functional traits. In this study, we extracted canopy color index (green chromatic coordinate, Gcc) from the timeâseries canopy images provided by a digital camera in a deciduous forest in Massachusetts, USA. We also measured leafâlevel photosynthetic activities and leaf area index (LAI) development in the field during the growing season, and corresponding leaf chlorophyll concentrations in the laboratory. We used the Bayesian change point (BCP) approach to analyze Gcc. Our results showed that (1) the date of starting decline of LAI (DOY 263), defined as the start of senescence, could be mathematically identified from the autumn Gcc pattern by analyzing change points of the Gcc curve, and Gcc is highly correlated with LAI after the first change point when LAI was decreasing (R2 = 0.88, LAI < 2.5 m2/m2); (2) the second change point of Gcc (DOY 289) started a more rapid decline of Gcc when chlorophyll concentration and photosynthesis rates were relatively low (13.4 ± 10.0% and 23.7 ± 13.4% of their maximum values, respectively) and continuously reducing; and (3) the third change point of Gcc (DOY 295) marked the end of growing season, defined by the termination of photosynthetic activities, two weeks earlier than the end of Gcc curve decline. Our results suggested that with the change point analysis, cameraâbased phenology observation can effectively quantify the dynamic pattern of the start of senescence (with declining LAI) and the end of senescence (when photosynthetic activities terminated) in the deciduous forest.Priority Academic Program Development of Jiangsu Higher Education Institutions in Discipline of Environmental Science and Engineer in Nanjing Forest University;
China Scholarship Council Grant Number: 201506190095;
Brown University Seed Funds for International Research Projects on the Environmen
A 2D hybrid method for interfacial transport of passive scalars
A hybrid Eulerian-Lagrangian method is proposed to simulate passive scalar
transport on arbitrary shape interface. In this method, interface deformation
is tracked by an Eulerian method while the transport of the passive scalar on
the material interface is solved by a single-layer Lagrangian particle method.
To avoid particle clustering, a novel remeshing approach is proposed. This
remeshing method can resample particles, adjust the position of particles by a
relaxation process, and transfer mass from pre-existing particles to resampled
particles via a redistribution process, which preserves mass both globally and
locally. Computational costs are controlled by an adaptive remeshing strategy.
Accuracy is assessed by a series of test cases.Comment: 32 pages 1nd 14 figure
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