arXiv subjects

Younes Ezzahri

Publications and source records attributed to Younes Ezzahri.

7 recordsLinked to original sources

arXiv2018

Radiative cooling by tailoring surfaces with microstructures

We propose in this article a method to generate radiative coolers which are reflective in the solar spectrum and emissive in the transparency window of the atmosphere (8-13 $μ$m). We choose an approach combining thermal control capacity of gratings and multi-layers. We use optimized BN, SiC and SiO 2 gratings, which have emissivity peak in the transparency window. We place under these gratings a metal/dielectric multi-layer structure to obtain a near perfect reflectivity in the solar spectrum and to enhance the emissivity in the transparency window. The optimized structures produce a good radiative cooling power density up to 80 W.m --2 .

physics.app-ph↗

arXiv2017

VO 2 -based radiative thermal transistor in the static regime

We study a near-field radiative thermal transistor analogous to an electronic one made of a VO 2 base placed between two silica semi-infinite plates playing the roles of the transistor collector and emitter. The fact that VO 2 exhibits an insulator to metal transition is exploited to modulate and/or amplify heat fluxes between the emitter and the collector, by applying a thermal current on the VO 2 base. We study the transistor behavior in 4 typical regimes where the emitter-base and base-collector separation distances can be larger or smaller than the thermal wavelength, and in which the VO 2 layer can be opaque or transparent. Thermal currents variations with the base temperatures are calculated and analyzed. An optimum configuration for base thickness and separation distance maximizing the thermal transistor modulation factor is found.

physics.optics↗

arXiv2016

Quantum Thermal Rectification to design thermal diodes and transistors

We study in this article how heat can be exchanged between two level systems (TLS) each of them being coupled to a thermal reservoir. Calculation are performed solving a master equation for the density matrix using the Born markov-approximation. We analyse the conditions for which a thermal diode and a thermal transistor can be obtained as well as their optimization.

quant-ph↗

arXiv2015

Radiative Thermal Rectification between SiC and SiO2

By means of fluctuationnal electrodynamics, we calculate radiative heat flux between two pla-nar materials respectively made of SiC and SiO2. More specifically, we focus on a first (direct) situation where one of the two materials (for example SiC) is at ambient temperature whereas the second material is at a higher one, then we study a second (reverse) situation where the material temperatures are inverted. When the two fluxes corresponding to the two situations are different, the materials are said to exhibit a thermal rectification, a property with potential applications in thermal regulation. Rectification variations with temperature and separation distance are here reported. Calculations are performed using material optical data experimentally determined by Fourier transform emission spectrometry of heated materials between ambient temperature (around 300 K) and 1480 K. It is shown that rectification is much more important in the near-field domain, i.e. at separation distances smaller than the thermal wavelength. In addition, we see that the larger is the temperature difference, the larger is rectification. Large rectification is finally interpreted due to a weakening of the SiC surface polariton when temperature increases, a weakening which affects much less SiO2 resonances.

cond-mat.mes-hall↗

arXiv2014

Vacuum induced phonon transfer between two solid dielectric materials: Illustrating the case of Casimir force coupling

The natural transition from the radiative regime to the conductive regime of heat transfer between two identical isotropic dielectric solid materials, is questioned by investigating the possibility of induced phonon transfer in vacuum. We describe the process in a general way assuming a certain phonon coupling mechanism between the two identical solids, then we particularly illustrate the case of coupling through Casimir force. We analyze how this mechanism of heat transfer compares and competes with the near field thermal radiation using a local model of the dielectric function. We show that the former mechanism can be very effective and even overpass the latter mechanism depending on the nature of the solid dielectric materials, the distance gap between them as well as the operating temperature regime.

cond-mat.mes-hall↗

arXiv2013

Dynamical lattice thermal conductivity, Shastry sum rule and second sound in bulk semiconductor crystals

The paper discusses the fundamental behavior of the dynamical lattice thermal conductivity k(W) of bulk cubic semiconductor crystals. The calculation approach is based on solving Boltzmann-Peierls Phonon Transport Equation in the frequency domain after excitation by a dynamical temperature gradient, within the framework of the single relaxation time approximation and using modified Debye-Callaway model. Our model allows us to obtain a compact expression for k(W) that captures the leading behavior of the dynamical thermal conduction by phonons. This expression fulfills the causality requirement and leads to a convolution type relationship between the heat flux density current and the temperature gradient in the real space-time domain in agreement with Gurtin-Pipkin theory. The dynamical behavior of k(W) is studied by changing ambient temperature as well as different intrinsic and extrinsic parameters including the effect of embedding semiconductor nanoparticles as extrinsic phonon scattering centers. The paper investigates also the applicability of Shastry Sum Rule and the possibility of existence and propagation of second sound in the frame work of Boltzmann theory.

cond-mat.mtrl-sci↗

arXiv2010

Transient Energy and Heat Transport in Metals: Effect of the Discrete Character of the Lattice

A recently developed Shastry's formalism for energy transport is used to analyze the temporal and spatial behaviors of the energy and heat transport in metals under delta function excitation at the surface. Comparison with Cattaneo's model is performed. Both models show the transition between nonthermal (ballistic) and thermal (ballistic-diffusive) regimes. Furthermore, because the new model considers the discrete character of the lattice, it highlights some new phenomena such as damped oscillations in the energy transport both in time and space. The energy relaxation of the conduction band electrons in metals is considered to be governed by the electron-phonon scattering, and the scattering time is taken to be averaged over the Fermi surface. Using the new formalism, one can quantify the transfer from nonthermal modes to thermal ones as energy propagates in the material and it is transformed into heat. While the thermal contribution shows a wave-front and an almost exponentially decaying behavior with time, the nonthermal part shows a wave-front and a damped oscillating behavior. Two superimposed oscillations are identified; a fast oscillation that is attributed to the nonthermal nature of energy transport at very short time scales and a slow oscillation that describes the nature of the transition from the nonthermal regime to the thermal regime of energy transport.

cond-mat.mes-hall↗