Thermal boundary conductance across rough interfaces probed by molecular dynamics

In this article, we report the influence of the interfacial roughness on the thermal boundary conductance between two crystals, using molecular dynamics. We show evidence of a transition between two regimes, depending on the interfacial roughness: when the roughness is small, the boundary conductance is constant taking values close to the conductance of the corresponding planar interface. When the roughness is larger, the conductance becomes larger than the planar interface conductance, and the relative increase is found to be close to the increase of the interfacial area. The cross-plane conductivity of a superlattice with rough interfaces is found to increase in a comparable amount, suggesting that heat transport in superlattices is mainly controlled by the boundary conductance. These observations are interpreted using the wave characteristics of the energy carriers. We characterize also the effect of the angle of the asperities, and find that the boundary conductance displayed by interfaces having steep slopes may become important if the lateral period characterizing the interfacial profile is large enough. Finally, we consider the effect of the shape of the interfaces, and show that the sinusoidal interface displays the highest conductance, because of its large true interfacial area. All these considerations are relevant to the optimization of nanoscale interfacial energy transport

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

Critical angle for interfacial phonon scattering: Results from ab initio lattice dynamics calculations

Thermal boundary resistance is a critical quantity that controls heat transfer at the nanoscale, which is primarily related to interfacial phonon scattering. Here, we combine lattice dynamics calculations and inputs from first principles ab initio simulations to predict phonon transmission at the Si/Ge interface as a function of both the phonon frequency and the phonon wavevector. This technique allows us to determine the overall thermal transmission coefficient as a function of the phonon scattering direction and frequency. Our results show that the thermal energy transmission is highly anisotropic, while thermal energy reflection is almost isotropic. In addition, we found the existence of a global critical angle of transmission beyond which almost no thermal energy is transmitted. This critical angle around 50 degrees is found to be almost independent of the interaction range between Si and Ge, the interfacial bonding strength, and the temperature above 30 K. We interpret these results by carrying out a spectral and angular analysis of the phonon transmission coefficient and differential thermal boundary conductance

Thermal transport at a nanoparticle-water interface: A molecular dynamics and continuum modeling study

Heat transfer between a silver nanoparticle and surrounding water has been studied using molecular dynamics (MD) simulations. The thermal conductance (Kapitza conductance) at the interface between a nanoparticle and surrounding water has been calculated using four different approaches: transient with/without temperature gradient (internal thermal resistance) in the nanoparticle, steady-state non-equilibrium and finally equilibrium simulations. The results of steady-state non-equilibrium and equilibrium are in agreement but differ from the transient approach results. MD simulations results also reveal that in the quenching process of a hot silver nanoparticle, heat dissipates into the solvent over a length-scale of ~ 2nm and over a timescale of less than 5ps. By introducing a continuum solid-like model and considering a heat conduction mechanism in water, it is observed that the results of the temperature distribution for water shells around the nanoparticle agree well with MD results. It is also found that the local water thermal conductivity around the nanoparticle is greater by about 50 percent than that of bulk water. These results have important implications for understanding heat transfer mechanisms in nanofluids systems and also for cancer photothermal therapy, wherein an accurate local description of heat transfer in an aqueous environment is crucial.Comment: 22 pages, 7 figures

Numerical study of a slip-link model for polymer melts and nanocomposites

We present a numerical study of the slip link model introduced by Likhtman for describing the dy- namics of dense polymer melts. After reviewing the technical aspects associated with the implemen- tation of the model, we extend previous work in several directions. The dependence of the relaxation modulus with the slip link density and the slip link stiffness is reported. Then the nonlinear rheolog- ical properties of the model, for a particular set of parameters, are explored. Finally, we introduce excluded volume interactions in a mean field such as manner in order to describe inhomogeneous systems, and we apply this description to a simple nanocomposite model. With this extension, the slip link model appears as a simple and generic model of a polymer melt, that can be used as an alternative to molecular dynamics for coarse grained simulations of complex polymeric systems