238 research outputs found
Neutral-cluster implantation in polymers by computer experiments
In this work we perform atomistic model potential molecular dynamics
simulations by means of state-of-the art force-fields to study the implantation
of a single Au nanocluster on a Polydimethylsiloxane substrate. All the
simulations have ben performed on realistic substrate models containing up to
4.6 millions of atoms having depths up to 90 nm and lateral dimensions up to 25
nm. We consider both entangled-melt and cross-linked Polydimethylsiloxane
amorphous structures. We show that even a single cluster impact on the
Polydimethylsiloxane substrate remarkably changes the polymer local temperature
and pressure. Moreover we observe the presence of craters created on the
polymer surface having lateral dimensions comparable to the cluster radius and
depths strongly dependent on the implantation energy. Present simulations
suggest that the substrate morphology is largely affected by the cluster impact
and that most-likely such modifications favor the the penetration of the next
impinging clusters
Thermal transport in nanocrystalline graphene investigated by approach-to-equilibrium molecular dynamics simulations
Approach-to-equilibrium molecular dynamics simulations have been used to
study thermal transport in nanocrystalline graphene sheets. Nanostructured
graphene has been created using an iterative process for grain growth from
initial seeds with random crystallographic orientations. The resulting cells
have been characterized by the grain size distribution based on the radius of
gyration, by the number of atoms in each grain and by the number of atoms in
the grain boundary. Introduction of nanograins with a radius of gyration of 1
nm has led to a significant reduction in the thermal conductivity to 3% of the
value in single crystalline graphene. Analysis of the vibrational density of
states has revealed a general reduction of the vibrational intensities and
broadening of the peaks when nanograins are introduced which can be attributed
to phonon scattering in the boundary layer. The thermal conductivity has been
evaluated as a function of the grain size with increasing size up to 14 nm and
it has been shown to follow an inverse rational function. The grain size
dependent thermal conductivity could be approximated well by a function where
transport is described by a connection in series of conducting elements and
resistances (at boundaries).Comment: 9 pages, 9 figure
Thermal boundary resistance from transient nanocalorimetry: a multiscale modeling approach
The Thermal Boundary Resistance at the interface between a nanosized Al film
and an Al_{2}O_{3} substrate is investigated at an atomistic level. A room
temperature value of 1.4 m^{2}K/GW is found. The thermal dynamics occurring in
time-resolved thermo-reflectance experiments is then modelled via macro-physics
equations upon insertion of the materials parameters obtained from atomistic
simulations. Electrons and phonons non-equilibrium and spatio-temporal
temperatures inhomo- geneities are found to persist up to the nanosecond time
scale. These results question the validity of the commonly adopted lumped
thermal capacitance model in interpreting transient nanocalorimetry
experiments. The strategy adopted in the literature to extract the Thermal
Boundary Resistance from transient reflectivity traces is revised at the light
of the present findings. The results are of relevance beyond the specific
system, the physical picture being general and readily extendable to other
heterojunctions.Comment: 12 pages, 8 figure
Modeling the Coupled MassâHeat Transport in LennardâJonesâLike Binary Mixtures by ApproachâtoâEquilibrium Molecular Dynamics
A general theoretical framework to address the coupled heat-mass transport and predict the corresponding Soret and Dufour coefficients is presented. It is shown that by starting from microscopical definitions of heat and mass currents, conservation laws dictate the form of the differential
equations governing the time evolution of the temperature and mass density profiles along the sample. The present theoretical device is finally validated using as benchmark system a two-component LennardâJones (LJ) liquid system, for which generalized diffusivities are estimated in different reduced temperature and density regions of phase diagram
Assessing the anomalous superdiffusive heat transport in a single one-dimensional PEDOT chain
We present a computational investigation on heat transport in a single polymer chain of poly-3,4- ethylenedioxythiophene (PEDOT). By applying equilibrium and nonequilibrium molecular dynamics simulations to evaluate the thermal conductivity, as well as by investigating how the polymer chain approaches equilibrium upon a local thermal excitation, we provide a robust picture assessing the anomalous superdiffusive (i.e., intermediate between ballistic and diffusive) character of its thermal transport. This assessment is provided by the present simulations showing that three scaling laws with unlike physical meaning and characterizing the thermal energy transport in one-dimensional systems indeed occur in the very same polymer chain with consistent critical exponents. In order to disentangle the effect of dimensionality, we perform a systematic comparison of transport features for a single one-dimensional (1D) PEDOT chain and a three-dimensional (3D) PEDOT crystal. Present simulations suggest that by increasing the dimensionality, the anomalous regime is completely removed as due to the occurrence of strong interchains anharmonic interactions. Finally, we prove that thermal transport in isolated single PEDOT chains belongs to a novel universality class of superdiffusion characterized by a critical exponent ÎČ = 1/2
Lattice strain at c-Si surfaces: a density functional theory calculation
The measurement of the Avogadro constant by counting Si atoms is based on the
assumption that Si balls of about 94 mm diameter have a perfect crystal
structure up to the outermost atom layers. This not the case because of the
surface relaxation and reconstruction, the possible presence of an amorphous
layer, and the oxidation process due to the interaction with the ambient. This
paper gives the results of density functional calculations of the strain
components orthogonal to crystal surface in a number of configurations likely
found in real samples
Molecular Dynamics Simulations of Thermal Transport in Solid State Systems
In this chapter, we provide a synoptic review of the theoretical/computational approaches currently used to characterize thermal transport at the nanoscale, a topic of paramount importance for several applications and technological thermal management requirements. We focus in particular on the description of the atomistic techniques based on equilibrium (EMD), non-equilibrium (NEMD), and approach to equilibrium (AEMD) molecular dynamics (MD), which allow to efficiently describe relatively large and structurally complex systems with a reduced computational cost as compared to fully "ab-initio" techniques. We describe the theoretical background for each simulation strategy, as well as their implementation in state-of-the-art MD codes by underlying their intrinsic limitations and providing strategies to control some of them. We finally perform a series of benchmark calculations on bulk crystalline silicon by showing that the estimated thermal conductivity is weakly dependent on the specific strategy actually employed, while the overall computational cost is largely dependent on it
Multiscale Modelling of Resistive Switching in Gold Nanogranular Films*
Metallic nanogranular films display a complex dynamical response to a constant bias, typically showing up as a resistive switching mechanism which, in turn, could be used to create electrical components for neuromorphic applications. To model such a phenomenon we use a multiscale computational approach blending together (i) an ab initio treatment of the electric current at the nanoscale, (ii) a molecular dynamics approach dictating structural rearrangements, and (iii) a finite-element solution of the heat equation for heat propagation in the sample. We also consider structural changes due to electromigration which are modelled on the basis of experimental observations on similar systems. Within such an approach, we manage to describe some distinctive features of the resistive switching occurring in a nanogranular film and provide a physical interpretation at the microscopic level
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