Sebastian Volz

Molecular dynamics simulation of thermal conductivity of silicon nanowires

We investigate the thermal conductivity of silicon nanowires based on molecular dynamics (MD) simulations. The simulated thermal conductivities of nanowires with square cross sections are found to be about two orders of magnitude smaller than those of bulk Si crystals in a wide range of temperatures (200–500 K) for both rigid and free boundary conditions. A solution of the Boltzmann transport equation is used to explore the possibility of explaining the MD results based on boundary scattering.

Computation of thermal conductivity of Si/Ge superlattices by molecular dynamics techniques

The effective thermal conductivity of devices including superlattice structures like laser diodes is mostly governed by interface resistance. We carried out an atomic study to overcome the complexity of phonon-based models by using the molecular dynamics technique. We simulate multilayer configurations using the conjugate gradient method to minimise the structure energy. Results not using the conjugate gradient method are also presented. The cross plane heat flux and thermal conductivity are then deduced for both cases and the observed discrepancies are discussed.

Thermal conductivity calibration for hot wire based dc scanning thermal microscopy

Thermal conductivity characterization with nanoscale spatial resolution can be performed by contact probe techniques only. The technique based on a hot anemometer wire probe mounted in an atomic force microscope is now a standard setup. However, no rigorous calibration procedure is provided so far in basic dc mode. While in contact with the sample surface, the electrical current I injected into the probe is controlled so that electrical resistance or the wire temperature is maintained by the Joule effect. The variation in current is assumed to be linearly related to the heat flux lost towards the sample and traditional calibration is carried out by relating the thermal conductivity of a set of samples to the measured current I. We provide analytical and numerical thermal modeling of the tip and sample to estimate the key heat transfer in a conductivity calibration procedure. A simple calibration expression is established that provides thermal conductivity as a function of the probe current or voltage measured. Finally, experimental data allow us to determine the unknown quantities of the parametric form obtained, i.e., the mean tip-sample contact radius and conductance.

Functionalization mediates heat transport in graphene nanoflakes.

The high thermal conductivity of graphene and few-layer graphene undergoes severe degradations through contact with the substrate. Here we show experimentally that the thermal management of a micro heater is substantially improved by introducing alternative heat-escaping channels into a graphene-based film bonded to functionalized graphene oxide through amino-silane molecules. Using a resistance temperature probe for in situ monitoring we demonstrate that the hotspot temperature was lowered by ∼28u2009°C for a chip operating at 1,300u2009Wu2009cm−2. Thermal resistance probed by pulsed photothermal reflectance measurements demonstrated an improved thermal coupling due to functionalization on the graphene–graphene oxide interface. Three functionalization molecules manifest distinct interfacial thermal transport behaviour, corroborating our atomistic calculations in unveiling the role of molecular chain length and functional groups. Molecular dynamics simulations reveal that the functionalization constrains the cross-plane phonon scattering, which in turn enhances in-plane heat conduction of the bonded graphene film by recovering the long flexural phonon lifetime.

Highly efficient thermal glue for carbon nanotubes based on azide polymers

Equilibrium molecular dynamics (EMD) simulations and experimental data show that the thermal contact resistance (TCR) between carbon nanotube (CNT) and azide-functionalized polymer with C-N bond is significantly decreased compared to that with Van der Waals force interaction. EMD simulations indicate that C-N covalent bond between CNT and polymer is the most efficient way to reduce TCR, and we measured the lowest thermal interface resistance of Si/CNT/Polymer/Cu thermal interface material as 1.40 mm2 KW−1 with CNTs of 10u2009μm length. These results provide useful information for future designs of thermal glue for carbon-based materials with better thermal conduction.

Tuning Temperature and Size of Hot Spots and Hot-Spot Arrays

By using scanning thermal microscopy, it is shown that nanoscale constrictions in metallic microwires deposited on an oxidized silicon substrate can be tuned in terms of temperature and confinement size. High-resolution temperature maps indeed show that submicrometer hot spots and hot-spot arrays are obtained when the SiO(2) layer thickness decreases below 100 nm. When the SiO(2) thickness becomes larger, heat is less confined in the vicinity of the constrictions and laterally spreads all along the microwire. These results are in good agreement with numerical simulations, which provide dependences between silica-layer thickness and nanodot shape and temperature.

Calculation of inter-plane thermal resistance of few-layer graphene from equilibrium molecular dynamics simulations

Inter-plane thermal resistance in 5-layer graphene is calculated from equilibrium molecular dynamics (EMD) by calculating the autocorrelation function of temperature difference. Our simulated inter-plane resistance for 5-layer graphene is 4.83 × 10−9 m2K/W. This data is in the same order of magnitude with the reported values from NEMD simulations and Debye model calculations, and the possible reasons for the slight differences are discussed in details. The inter-plane resistance is not dependent on temperature, according to the results of the EMD simulation. Phonon density of states (DOSs) were plotted to better understand the mechanism behind the obtained values. These results provide a better insight in the heat transfer across a few layer graphene and yield useful information on the design of graphene based thermal materials.

Transient heat transfer in a Lennard-Jones crystal by molecular dynamics

The phenomenological description of heat conduction problems in non-continuous media by the Fourier law, is called into question when thermal transfers are studied at both temporal and space microscales. Several Equilibrium and Non Equilibrium Molecular Dynamics (EMD and NEMD) experiments have been carried out in order to specify the nature of the heat transfer at short times in a Lennard-Jones solid. Presented are (i) the hydrodynamic, the heat flux and the phonon relaxation times (respectively ru, rv and rp) computations done thanks to EMD experiments. ~v is deduced from a heat flux fluctuations description, and re is worked out from the Peierls thermal conductivity definition in the frame of the Deybe model. (ii) The response of the lattice to a kinetic energy step which displays wave-like phenomena of coherent and incoherent nature for times of the order of some phonon relaxation times. An analysis in terms of mode velocities confirms the presence of acoustic and thermal (second sound) wave propagation.

Transformational fluctuation electrodynamics: application to thermal radiation illusion

Thermal radiation is a universal property for all objects with temperatures above 0K. Every object with a specific shape and emissivity has its own thermal radiation signature; such signature allows the object to be detected and recognized which can be an undesirable situation. In this paper, we apply transformation optics theory to a thermal radiation problem to develop an electromagnetic illusion by controlling the thermal radiation signature of a given object. Starting from the fluctuation dissipation theorem where thermally fluctuating sources are related to the radiative losses, we demonstrate that it is possible for objects residing in two spaces, virtual and physical, to have the same thermal radiation signature if the complex permittivities and permeabilities satisfy the standard space transformations. We emphasize the invariance of the fluctuation electrodynamics physics under transformation, and show how this result allows the mimicking in thermal radiation. We illustrate the concept using the illusion paradigm in the two-dimensional space and a numerical calculation validates all predictions. Finally, we discuss limitations and extensions of the proposed technique.

Modelling and measurement of the thermal conductivity of composites with silver particles

The effective thermal conductivity of composites made up of silver micro-particles embedded in a resin matrix is modelled and measured. This is done for spherical and flake-like particles to analyse the effects of the particles geometry and concentration on the composite thermal performance. It is experimentally found that spherical particles yield a higher thermal conductivity than the one given by flakes, such that it takes the value of 16 Wm-1 K-1 for a 50% volume fraction of particles. Furthermore, this behaviour is well described by a simple and analytical model, which takes into account the particle-particle interactions through a crowding factor. The obtained results could be useful to optimize the design and manufacture of composites with metallic particles.