VO2-based radiative thermal transistor with a semi-transparent base

International audienc

Radiative cooling by tailoring surfaces with microstructures: Association of a grating and a multi-layer structure

International audienceWe propose in this article a method to conceive radiative coolers that are reflective in the solar spectrum and emissive in the transparency window of the atmosphere (8–13 µm). We choose an approach combining thermal control capacities of gratings and multi-layers. It is the first time that simple gratings are used for radiative cooling applications. We use optimized BN, SiC and SiO2 gratings, which have emissivity peaks 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 at night and a mean daytime radiative cooling power density of 55 W.m-2 with local atmospherical and solar conditions in Poitiers

Modeling of the electrical conductivity, thermal conductivity, and specific heat capacity of VO 2

International audienceBased on Bruggeman's symmetric effective-medium formula and an explicit expression derived for the temperature evolution of the volume fractions of the metallic and isolating domains appearing during the heating and cooling of VO2, respectively, we develop a model to describe the hysteresis of its electrical and thermal conductivities as well as of its specific heat capacity. The model takes into account the average value and standard deviation of the transition temperatures of the individual domains, as well as their activation energies, which represent the driving force for the existence of the VO2 hysteresis. It is shown that the model's predictions driven by these three parameters related to the microstructure of VO2 are in good agreement with robust experimental data. Furthermore, as these parameters are intrinsically correlated to the doping, defect, strain, and interface effects of VO2, the proposed model enables the seamless integration of these effects, and therefore, its predictions are also expected to be useful for describing the thermal and electrical properties of composites based on VO2

Periodic amplification of radiative heat transfer

International audienceWe demonstrate that the direction and values of the radiative heat flux exchanged between a non-phase-change material and a phase-change one excited with a temperature difference modulated in time can efficiently be tuned by means of their common steady-state temperature. This heat-flux modulation occurs in both the far- and near-field regimes as a result of the strong temperature dependence of the emissivity and permittivity of the phase-change material, respectively. It is shown that the heat pumping into or out of the phase-change material can not only be amplified but also canceled out for temperatures around its critical temperature. This nullification of the radiative heat flux can be used as a mechanism to rectify heat currents and to insulate the two bodies from each other, even when their temporal temperature difference is different than zero. The obtained results thus open a new pathway for the heat-flux control of nonequilibrium radiating systems

NIR and MIR Absorption of Ultra-Black Silicon(UBS) Application To High Emissivity, All-Silicon, Light Source

International audienceWe present the Near-Infra-Red (NIR) and Mid-Infrared (MIR) absorption properties of Ultra-Black Silicon obtained by wafer-level cryogenic plasma processing. We found that when using highly-doped silicon, the spectral range of near-unity full absorption of light is extended from the visible range till a wavelength of 10 µm. This MIR wavelength range coincides with that of the maximum of black-body radiation from room temperature up to a few thousand Kelvin. Therefore, according to Kirchhoff's Law, we take advantage of the enhanced properties of black silicon to realize ultra-compact light-sources of high efficiency, which are operated in combination with a MEMS-FTIR spectrometer