Elyes Nefzaoui

Simple far-field radiative thermal rectifier using Fabry–Perot cavities based infrared selective emitters

We present a thermal rectification device concept based on far-field radiative exchange between two selective emitters. Rectification is achieved due to the fact that one of the selective emitters radiative properties are independent on temperature whereas the other emitter properties are strongly temperature dependent. A simple device constituted by two multilayer samples made of metallic (Au) and semiconductor (Si and HDSi) thin films is proposed. This device shows a rectification up to 70% with a temperature difference \Delta T = 200 K, a rectification ratio that has never been achieved so far with radiation-based rectifiers. Further optimization would allow larger rectification values. Presented results might be useful for energy conversion devices, smart radiative coolers / insulators engineering and thermal modulators development.

Selective emitters design and optimization for thermophotovoltaic applications

Among several solutions to exploit solar energy, thermophotovoltaics have been popularized and have known great breakthroughs during the past two decades. Yet, existing systems still have low efficiencies since the wavelength range of optimal photovoltaic (PV) conversion is very small compared to the emitter spectral range. Selective emitters are a very promising solution to this problem. We developed numerical tools to design and optimize such emitters. Some of the resulting structures composed of two or four layers of metals and semiconductors are presented in this paper. We also show that the usual PV devices efficiency limits (30% for crystalline silicon under solar radiation, according to Shockley-Queisser model) can be easily overcome thanks to these structures.

Far field coherent thermal emission from a bilayer structure

Recent years, there has been an increased interest in the conception of micro/nanostructures with unusual radiative properties, far away from those of blackbody, especially thermal sources with temporal and/or spatial coherent emission. Such structures are indeed extremely interesting for energy conversion systems, radiative cooling devices, etc. The present study numerically investigates temporal coherent emission from a very simple structure composed of one layer of germanium and one of silicon carbide. Our investigation shows that, for well-defined thicknesses, this two-layer structure is able to emit in narrow spectral peak.

Radiative thermal rectification using superconducting materials

Thermal rectification can be defined as an asymmetry in the heat flux when the temperature difference between two interacting thermal reservoirs is reversed. In this Letter, we present a far-field radiative thermal rectifier based on high-temperature superconducting materials with a rectification ratio up to 80%. This value is among the highest reported in literature. Two configurations are examined: a superconductor (Tl2Ba2CaCu2O8) exchanging heat with (1) a black body and (2) another superconductor, YBa2Cu3O7 in this case. The first configuration shows a higher maximal rectification ratio. Besides, we show that the two-superconductor rectifier exhibits different rectification regimes depending on the choice of the reference temperature, i.e., the temperature of the thermostat. Presented results might be useful for energy conversion devices, efficient cryogenic radiative insulators engineering, and thermal logical circuits’ development.

Maximal near-field radiative heat transfer between two plates

Near-field radiative transfer is a promising way to significantly and simultaneously enhance both thermo-photovoltaic (TPV) devices power densities and efficiencies. A parametric study of Drude and Lorentz models performances in maximizing near-field radiative heat transfer between two semi-infinite planes separated by nanometric distances at room temperature is presented in this paper. Optimal parameters of these models that provide optical properties maximizing the radiative heat flux are reported and compared to real materials usually considered in similar studies, silicon carbide and heavily doped silicon in this case. Results are obtained by exact and approximate (in the extreme near-field regime and the electrostatic limit hypothesis) calculations. The two methods are compared in terms of accuracy and CPU resources consumption. Their differences are explained according to a mesoscopic description of nearfield radiative heat transfer. Finally, the frequently assumed hypothesis which states a maximal radiative heat transfer when the two semi-infinite planes are of identical materials is numerically confirmed. Its subsequent practical constraints are then discussed. Presented results enlighten relevant paths to follow in order to choose or design materials maximizing nano-TPV devices performances.

Design of micro-fabricated thermal flow-rate sensor for water network monitoring

We report on micro-machined flow-rate sensors as part of autonomous multi-parameter sensing devices for water network monitoring. Three different versions of the flow-rate sensors have been designed, fabricated and experimentally characterized. Those sensors are made of identical micrometric platinum resistors deposited on two different substrates-glass and silicon with and without insulation layer. The sensors were tested under the anemometric operating scheme. They were characterized under a water velocity range from 0 to 3.68 m/s. We highlight the fact that the glass substrate device is more sensitive and less power-consuming than the silicon one under the identical operating condition, which requires further design strategies when using silicon as the substrate material. Experimental results are analyzed with respect to CFD simulations with the Finite Element Method.

Sensitivity optimization of micro-machined thermo-resistive flow-rate sensors on silicon substrates

We report on an optimized micro-machined thermal flow-rate sensor as part of an autonomous multi-parameter sensing device for water network monitoring. The sensor has been optimized under the following constraints: low power consumption and high sensitivity, while employing a large thermal conductivity substrate, namely silicon. The resulting device consists of a platinum resistive heater deposited on a thin silicon pillar ~100 µm high and 5 µm wide in the middle of a nearly 100 µm wide cavity. Operated under the anemometric scheme, the reported sensor shows a larger sensitivity in the velocity range up to 1 m s−1 compared to different sensors based on similar high conductivity substrates such as bulk silicon or silicon membrane with a power consumption of 44 mW. Obtained performances are assessed with both CFD simulation and experimental characterization.

Evaluation of Tenax thin films as adsorbent material in a micro-preconcentrator and its operation as a valve-less multiple injection system in micro-gas chromatography

This paper aims to evaluate Tenax TA in the form of thin films as an adsorbent material as an alternative to its solid form counterpart (granular form) for micro-preconcentrators. The micro preconcentrator (μPC) consists of embedded high-aspect-ratio three-dimensional micro-pillars coated with Tenax adsorbent. This μPC has inner dimensions of 5 mm × 5 mm with a total inner volume of ∼6.5 μL. This layered-form adsorbent was tested to retain a specific class of hazardous pollutants named BTEX for Benzene, Toluene, Ethylbenzene, and Xylene respectively. To increase homogeneity of the films, the inner surface of silicon was chemically treated prior to Tenax coating. Experimental results demonstrate a practical preconcentration factor (PF) up to 96. Furthermore, a micro-GC platform is also presented, in which the developed preconcentrator is operated under fast thermal pulses demonstrating valve-less multiple injection capabilities.

On the co-integration of a thermo-resistive flow-rate sensor in a multi-parameter sensing chip for water network monitoring

Motivated by the need for a multi-parameter sensing chip for water networks monitoring, we address here the specific case of a flow-rate sensor where the main challenge is the substrate material. Instead of using conventional low thermal conductivity materials such as glass, silicon has to be used. Indeed, a silicon substrate enables the co-integration of various kinds of sensors on the same chip as reported in this contribution. However, it increases the flow-rate sensor power consumption due to larger thermal leaks. We therefore design and study an optimized low power micro-machined thermal flow-rate sensor based on a silicon substrate and operating according to hot-wire anemometry. It can be considered as an alternative to other well established sensors for liquid flow-rate measurement when both the use of a silicon substrate and a low power consumption are needed.

Thermal transport phenomena beyond the diffusive regime

Heat conduction in semiconductors is mediated by thermally-excited phonons. When dimensions smaller than the mean free path are involved, nondiffusive heat conduction arises. In addition, surface effects related to thermal boundary resistances become significant when the dimensions of the considered media decrease. These two phenomena significantly alter thermal transport in comparison to predictions made with standard heat diffusion and result in larger temperature levels at the heat source, which can be detrimental for electronics devices. We tackle few examples where these effects are observed. By using the Boltzmann Transport Equation (BTE) for phonons or approximated solutions, we show that effective cross-plane thermal conductivity reduction takes place. We then present results of heat conduction from a metallic line of nanometer-scale width towards a flat bulk. We show that 2D ballistic heat conduction takes place and that a ballistic reduction factor associated to the phonon rarefaction effect should be included. The dissipated heat fluxes are reduced in comparison to the Fourier prediction. The consequence is that strong hot spots may arise. We then analyze the effect of surface imperfect transmission in thermal boundary resistance and introduce a method based on acoustics to compute it. We show that confinement and imperfect transmission lead to similar reduction of the effective thermal conductivity.