73 research outputs found
Influence of the electron-phonon interfacial conductance on the thermal transport at metal/dielectric interfaces
Thermal boundary conductance at a metal-dieletric interface is a quantity of
prime importance for heat management at the nanoscale. While the boundary
conductance is usually ascribed to the coupling between metal phonons and
dielectric phonons, in this work we examine the influence of a direct coupling
between the metal electrons and the dielectric phonons. The effect of electron-
phonon processes is generally believed to be resistive, and tends to decrease
the overall thermal boundary conductance as compared to the phonon-phonon
conductance {\sigma}p . Here, we find that the effect of a direct coupling
{\sigma}e is to enhance the effective thermal conductance, between the metal
and the dielectric. Resistive effects turn out to be important only for thin
films of metals having a low electron-phonon coupling strength. Two approaches
are explored to reach these conclusions. First, we present an analytical
solution of the two-temperature model to compute the effective conductance
which account for all the relevant energy channels, as a function of {\sigma}e
, {\sigma}p and the electron-phonon coupling factor G. Second, we use numerical
resolution to examine the influence of {\sigma}e on two realistic cases: gold
film on silicon or silica substrates. We point out the implications for the
interpretation of time-resolved thermoreflectance experiments
Effective temperatures of a heated Brownian particle
We investigate various possible definitions of an effective temperature for a
particularly simple nonequilibrium stationary system, namely a heated Brownian
particle suspended in a fluid. The effective temperature based on the
fluctuation dissipation ratio depends on the time scale under consideration, so
that a simple Langevin description of the heated particle is impossible. The
short and long time limits of this effective temperature are shown to be
consistent with the temperatures estimated from the kinetic energy and Einstein
relation, respectively. The fluctuation theorem provides still another
definition of the temperature, which is shown to coincide with the short time
value of the fluctuation dissipation ratio
Heat transfer from nanoparticles: a corresponding state analysis
In this contribution, we study situations in which nanoparticles in a fluid
are strongly heated, generating high heat fluxes. This situation is relevant to
experiments in which a fluid is locally heated using selective absorption of
radiation by solid particles. We first study this situation for different types
of molecular interactions, using models for gold particles suspended in octane
and in water. As already reported in experiments, very high heat fluxes and
temperature elevations (leading eventually to particle destruction) can be
observed in such situations. We show that a very simple modeling based on
Lennard-Jones interactions captures the essential features of such experiments,
and that the results for various liquids can be mapped onto the Lennard-Jones
case, provided a physically justified (corresponding state) choice of
parameters is made. Physically, the possibility of sustaining very high heat
fluxes is related to the strong curvature of the interface that inhibits the
formation of an insulating vapor film
Heterogeneous nanofluids: natural convection heat transfer enhancement
Convective heat transfer using different nanofluid types is investigated. The domain is differentially heated and nanofluids are treated as heterogeneous mixtures with weak solutal diffusivity and possible Soret separation. Owing to the pronounced Soret effect of these materials in combination with a considerable solutal expansion, the resulting solutal buoyancy forces could be significant and interact with the initial thermal convection. A modified formulation taking into account the thermal conductivity, viscosity versus nanofluids type and concentration and the spatial heterogeneous concentration induced by the Soret effect is presented. The obtained results, by solving numerically the full governing equations, are found to be in good agreement with the developed solution based on the scale analysis approach. The resulting convective flows are found to be dependent on the local particle concentration φ and the corresponding solutal to thermal buoyancy ratio N. The induced nanofluid heterogeneity showed a significant heat transfer modification. The heat transfer in natural convection increases with nanoparticle concentration but remains less than the enhancement previously underlined in forced convection case
Thermal conductivity and thermal boundary resistance of nanostructures
International audienceWe present a fabrication process of low-cost superlattices and simulations related with the heat dissipation on them. The influence of the interfacial roughness on the thermal conductivity of semiconductor/semiconductor superlattices was studied by equilibrium and non-equilibrium molecular dynamics and on the Kapitza resistance of superlattice's interfaces by equilibrium molecular dynamics. The non-equilibrium method was the tool used for the prediction of the Kapitza resistance for a binary semiconductor/metal system. Physical explanations are provided for rationalizing the simulation results
Confinement-Induced Stiffening of Thin Elastomer Films: Linear and Nonlinear Mechanics vs Local Dynamics
A mesoscopic model for (de)wetting
We present a mesoscopic model for simulating the dynamics of a non-volatile liquid on a solid substrate. The wetting properties of the solid can be tuned from complete wetting to total non-wetting. This model opens the way to study the dynamics of drops and liquid thin films at mesoscopic length scales of the order of the nanometer. As particular applications, we analyze the kinetics of spreading of a liquid drop wetting a solid substrate and the dewetting of a liquid film on a hydrophobic substrate. In all these cases, very good agreement is found between simulations and theoretical predictions
Heterogeneous dynamics at the glass transition in van der Waals liquids: Determination of the characteristic scale
Heterogeneous dynamics at the glass transition in van der Waals liquids: Determination of the characteristic scale
Recent experiments have demonstrated that the
dynamics in liquids close to the glass transition temperature
is strongly heterogeneous. The characteristic size of these
heterogeneities has been measured to be a few nanometers at .
We extend here a recent model for describing the heterogeneous
nature of the dynamics which allows both
to derive this length scale and the right orders of magnitude
of the heterogeneities of the dynamics close to the glass transition.
Our model allows then to interpret quantitatively small probes
diffusion experiments
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