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S. Volz

Publications and source records attributed to S. Volz.

17 recordsLinked to original sources
arXiv2020

Revisiting thermal conductivity and interface conductance at the nanoscale

A semi-analytical model for studying thermal transport at the nanoscale, able to accurately describe both the effect of out of equilibrium transport and the thermal transfer at interfaces, is presented. Our approach is based on the definition of pseudo local temperatures distinguishing the phonon populations according to the direction of their velocity. This formalism leads to a complete set of equations capturing the heat transfer in nanostructures even in the case of hetero-structures. This model only requires introducing a new intrinsic thermal parameter called ballistic thermal conductance and a geometric one called the effective thermal conductivity. Finally, this model is able to reproduce accurately advanced numerical results of Monte Carlo simulation for phonons in all phonon transport regime: diffusive (as the Fourier heat transport regime is included), ballistic, and intermediate ones even if thermal interface are involved. This formalism should provide new insights in the interpretation of experimental measurements.

cond-mat.mes-hall↗
arXiv2018

Heat transfer in rough nanofilms and nanowires using Full Band Ab Initio Monte Carlo simulation

The Boltzmann transport equation is one of the most relevant framework to study the heat transport at the nanoscale, beyond the diffusive regime and up to the micrometer-scale. In the general case of three-dimensional devices, the particle Monte Carlo approach of phonon transport is particularly powerful and convenient, and requires reasonable computational resources. In this work, we propose an original and versatile particle Monte Carlo approach parametrized by using ab-initio data. Both the phonon dispersion and the phonon-phonon scattering rates have been computed by DFT calculation in the entire 3D Brillouin zone. To treat the phonon transport at rough interfaces, a combination of specular and diffuse reflections has been implemented in phase space. Thermal transport has been investigated in nanowires and thin films made of cubic and hexagonal Silicon, including edge roughness, in terms of effective thermal conductivity, phonon band contributions and heat flux orientation. It is shown that the effective thermal conductivity in quasi-ballistic regime obtained from our Monte Carlo simulation cannot be accurately fitted by simple semi-analytical Matthiessen-like models and that spectral approaches are mandatory to get good results. Our Full Band approach shows that some phonon branches exhibiting a negative group velocity in some parts of the Brillouin zone may contribute negatively to the total thermal flux. Besides, the thermal flux clearly appears to be oriented along directions of high density of states. The resulting anisotropy of the heat flux is discussed together with the influence of rough interfaces.

cond-mat.mes-hall↗
arXiv2016

Vibrational mean free paths and thermal conductivity of amorphous silicon from non-equilibrium molecular dynamics simulations

The frequency-dependent mean free paths (MFPs) of vibrational heat carriers in amorphous silicon are predicted from the length dependence of the spectrally decomposed heat current (SDHC) obtained from non-equilibrium molecular dynamics simulations. The results suggest a (frequency)$^{-2}$ scaling of the room-temperature MFPs below 5 THz. The MFPs exhibit a local maximum at a frequency of 8 THz and fall below 1 nm at frequencies greater than 10 THz, indicating localized vibrations. The MFPs extracted from sub-10 nm system-size simulations are used to predict the length-dependence of thermal conductivity up to system sizes of 100 nm and good agreement is found with separate molecular dynamics simulations. Weighting the SDHC by the frequency-dependent quantum occupation function provides a simple and convenient method to account for quantum statistics and provides reasonable agreement with the experimentally-measured trend and magnitude.

physics.comp-ph↗
arXiv2016

Spectral mapping of heat transfer mechanisms at liquid-solid interfaces

Thermal transport through liquid-solid interfaces plays an important role in many chemical and biological processes, and better understanding of liquid-solid energy transfer is expected to enable improving the efficiency of thermally driven applications. We determine the spectral distribution of thermal current at liquid-solid interfaces from nonequilibrium molecular dynamics, delivering a detailed picture of the contributions of different vibrational modes to liquid-solid energy transfer. Our results show that surface modes located at the Brillouin zone edge and polarized along the liquid-solid surface normal play a crucial role in liquid-solid energy transfer. Strong liquid-solid adhesion allows also for the coupling of in-plane polarized modes in the solid with the liquid, enhancing the heat transfer rate and enabling efficient energy transfer up to the cut-off frequency of the solid. Our results provide fundamental understanding of the energy transfer mechanisms in liquid-solid systems and enable detailed investigations of energy transfer between, e.g., water and organic molecules.

physics.comp-ph↗
arXiv2015

Nonequilibrium phonon mean free paths in anharmonic chains

Harnessing the power of low-dimensional materials in thermal applications calls for a solid understanding of the anomalous thermal properties of such systems. We analyze thermal conduction in one-dimensional systems by determining the frequency-dependent phonon mean free paths (MFPs) for an anharmonic chain, delivering insight into the diverging thermal conductivity observed in computer simulations. In our approach, the MFPs are extracted from the length-dependence of the spectral heat current obtained from nonequilibrium molecular dynamics simulations. At low frequencies, the results reveal a power-law dependence of the MFPs on frequency, in agreement with the diverging conductivity and the recently determined equilibrium MFPs. At higher frequencies, however, the nonequilibrium MFPs consistently exceed the equilibrium MFPs, highlighting the differences between the two quantities. Exerting pressure on the chain is shown to suppress the mean free paths and to generate a weaker divergence of MFPs at low frequencies. The results deliver important insight into anomalous thermal conduction in low-dimensional systems and also reveal differences between the MFPs obtained from equilibrium and nonequilibrium simulations.

physics.comp-ph↗
arXiv2015

Frequency-dependent phonon mean free path in carbon nanotubes from non-equilibrium molecular dynamics

Owing to their long phonon mean free paths (MFPs) and high thermal conductivity, carbon nanotubes (CNTs) are ideal candidates for, e.g., removing heat from electronic devices. It is unknown, however, how the intrinsic phonon MFPs depend on vibrational frequency in non-equilibrium. We determine the spectrally resolved phonon MFPs in isotopically pure CNTs from the spectral phonon transmission function calculated using non-equilibrium molecular dynamics, fully accounting for the resistive phonon-phonon scattering processes through the anharmonic terms of the interatomic potential energy function. Our results show that the effective room temperature MFPs of low-frequency phonons ($f<0.5$ THz) exceed $10$ $μ$m, while the MFP of high-frequency phonons ($f\gtrsim 20$ THz) is in the range 10--100 nm. Because the determined MFPs directly reflect the resistance to energy flow, they can be used to accurately predict the thermal conductivity for arbitrary tube lengths by calculating a single frequency integral. The presented results and methods are expected to significantly improve the understanding of non-equilibrium thermal transport in low-dimensional nanostructures.

cond-mat.mes-hall↗
arXiv2014

Role of anharmonic phonon scattering in the spectrally decomposed thermal conductance at planar interfaces

Detailed understanding of vibrational heat transfer mechanisms between solids is essential for the efficient thermal engineering and control of nanomaterials. We investigate the frequency dependence of anharmonic scattering and interfacial thermal conduction between two acoustically mismatched solids in planar contact by calculating the spectral decomposition of the heat current flowing through an interface between two materials. The calculations are based on analyzing the correlations of atomic vibrations using the data extracted from non-equilibrium molecular dynamics simulations. Inelastic effects arising from anharmonic interactions are shown to significantly facilitate heat transfer between two mass-mismatched face-centered cubic lattices even at frequencies exceeding the cut-off frequency of the heavier material due to (i) enhanced dissipation of evanescent vibrational modes and (ii) frequency-doubling and frequency-halving three-phonon energy transfer processes at the interface. The results provide substantial insight into interfacial energy transfer mechanisms especially at high temperatures, where inelastic effects become important and other computational methods are ineffective.

cond-mat.mtrl-sci↗
arXiv2008

Interface heat transfer between crossing carbon nanotubes, and the thermal conductivity of nanotube pellets

We theoretically compute the interface thermal resistance between crossing single walled carbon nanotubes of various chiralities, using an atomistic Green's function approach with semi-empirical potentials. The results are then used to model the thermal conductivity of three dimensional nanotube pellets in vacuum. For an average nanotube length of 1 $μ$m, the model yields an upper bound for the thermal conductivity of densely compacted pellets, of the order of a few W/m-K. This is in striking contrast with the ultra-high thermal conductivity reported on individually suspended nanotubes. The results suggest that nanotube pellets might have an application as thermal insulators.

cond-mat.mtrl-sci↗
arXiv2007

Polymer nanoparticles to decrease thermal conductivity of phase change materials

Microparticles including paraffin are currently used for textiles coating in order to deaden thermal shocks. We will show that polymer nanoparticles embedded in those microsized capsules allow for decreasing the thermal conductivity of the coating and enhance the protection in the stationary regime. A reasonable volume fraction of polymer nanoparticles reduces the conductivity more than predicted by Maxwell mixing rules. Besides, measurements prove that the polymer nanoparticles do not affect the latent heat and even improve the phase change behaviour as well as the mechanical properties.

cond-mat.mtrl-sci↗
arXiv2007

Increase of Thermal Resistance Between a Nanostructure and a Surface due to Phonon Multireflections

The thermal resistance between a nanostructure and a half-body is calculated in the framework of particle-phonons physics. The current models approximate the nanostructure as a thermal bath. We prove that the multireflections of heat carriers in the nanostructure significantly increase resistance in contradiction with former predictions. This increase depends on the shape of the nanostructure and the heat carriers mean free path only. We provide a general and simple expression for the contact resistance and examine the specific cases of nanowires and nanoparticles.

physics.class-ph↗
arXiv2007

Thermodiffusion in model nanofluids by molecular dynamics simulations

In this work, a new algorithm is proposed to compute single particle (infinite dilution) thermodiffusion using Non-Equilibrium Molecular Dynamics simulations through the estimation of the thermophoretic force that applies on a solute particle. This scheme is shown to provide consistent results for simple Lennard-Jones fluids and for model nanofluids (spherical non-metallic nanoparticles + Lennard-Jones fluid) where it appears that thermodiffusion amplitude, as well as thermal conductivity, decrease with nanoparticles concentration. Then, in nanofluids in the liquid state, by changing the nature of the nanoparticle (size, mass and internal stiffness) and of the solvent (quality and viscosity) various trends are exhibited. In all cases the single particle thermodiffusion is positive, i.e. the nanoparticle tends to migrate toward the cold area. The single particle thermal diffusion 2 coefficient is shown to be independent of the size of the nanoparticle (diameter of 0.8 to 4 nm), whereas it increases with the quality of the solvent and is inversely proportional to the viscosity of the fluid. In addition, this coefficient is shown to be independent of the mass of the nanoparticle and to increase with the stiffness of the nanoparticle internal bonds. Besides, for these configurations, the mass diffusion coefficient behavior appears to be consistent with a Stokes-Einstein like law.

cond-mat.stat-mech↗
arXiv2007

Temperature measurement of sub-micrometric ICs by scanning thermal microscopy

Surface temperature measurements were performed with a Scanning Thermal Microscope mounted with a thermoresistive wire probe of micrometrSurface temperature measurements were performed with a Scanning Thermal Microscope mounted with a thermoresistive wire probe of micrometric size. A CMOS device was designed with arrays of resistive lines 0.35$μ$m in width. The array periods are 0.8micron and 10micron to study the spatial resolution of the SThM. Integrated Circuits with passivation layers of micrometric and nanometric thicknesses were tested. To enhance signal-to-noise ratio, the resistive lines were heated with an AC current. The passivation layer of nanometric thickness allows us to distinguish the lines when the array period is 10micron. The results raise the difficulties of the SThM measurement due to the design and the topography of ICs on one hand and the size of the thermal probe on the other hand.ic size. A CMOS device was designed with arrays of resistive lines 0.35$μ$m in width. The array periods are 0.8micron and 10micron to study the spatial resolution of the SThM. Integrated Circuits with passivation layers of micrometric and nanometric thicknesses were tested. To enhance signal-to-noise ratio, the resistive lines were heated with an AC current. The passivation layer of nanometric thickness allows us to distinguish the lines when the array period is 10micron. The results raise the difficulties of the SThM measurement due to the design and the topography of ICs on one hand and the size of the thermal probe on the other hand.

physics.class-ph↗
arXiv2007

Quantitative 3w-Scanning Thermal Microscopy: Modelling the AC/DC coupling and the sample heat conduction

A way to increase the Scanning Thermal Microscope (SThM) sensitivity in the harmonic 3w mode is to heat the probe with an AC current sufficiently high to generate a coupling between the AC and the DC signals. We detail in this paper how to properly take into account this coupling with a Wollaston-probe SThM. We also show how to link correctly the thermal conductivity to the thermal conductance measured by the SThM.

cond-mat.mtrl-sci↗
arXiv2005

Two-phonon 1- state in 112Sn observed in resonant photon scattering

Results of a photon scattering experiment on 112Sn using bremsstrahlung with an endpoint energy of E_0 = 3.8 MeV are reported. A J = 1 state at E_x = 3434(1) keV has been excited. Its decay width into the ground state amounts to Gamma_0 = 151(17) meV, making it a candidate for a [2+ x 3-]1- two-phonon state. The results for 112Sn are compared with quasiparticle-phonon model calculations as well as the systematics of the lowest-lying 1- states established in other even-mass tin isotopes. Contrary to findings in the heavier stable even-mass Sn isotopes, no 2+ states between 2 and 3.5 MeV excitation energy have been detected in the present experiment.

nucl-ex↗
arXiv2001

Measurement of the (gamma,n) reaction rates of the nuclides 190Pt, 192Pt, and 198Pt in the astrophysical gamma-process

The nucleosynthesis of heavy neutron-deficient nuclei in a stellar photon bath at the temperatures relevant for the astrophysical gamma process was investigated. In order to derive (gamma,n) cross sections and reaction rates, the stellar photon bath was simulated by the superposition of several bremsstrahlung spectra with different endpoint energies. As a first test for this method, the (gamma,n) reaction rates of the platinum isotopes 190Pt, 192Pt, and 198Pt were derived. The results are compared to other experimental data and theoretical calculations.

astro-ph↗