MSCA-COFUND-CLEAR-Doc - PhD Position #CD21-15 "Radiative properties of infrared meta-surfaces. Applications to radiative cooling, Dew Water Harvesting and chemical analysis"

Radiative cooling Radiative cooling, is a process of losing heat by surface radiation through infrared atmospheric transparency window (8-14 µm spectral range mainly) It is used in cooling various objects such as buildings, solar cells, power plants and even some liquids. It enables substrates to be cooled within a few degrees and even more below ambient temperature, a value which is large enough to address numerous applications. For example, it can be used to reach the dew point at night and condense water vapor for the purpose of water harvesting from air.

Every object continuously emits electromagnetic radiation outward whose wavelengths are determined by its temperature according to the Planck’s Law. Considering a substrate at temperature T, exposed to the clear sky, which is transparent in the atmospheric window (8-14 µm), to achieve the radiative cooling effect, its lost radiative power Prad should be larger than the power received from outside. According to Kirchhoff’s law, the emissivity is equal to absorptivity. This means that for a radiative cooler, the demand of enhancing Prad therefore translates into the selective absorption of electromagnetic waves in the atmospheric window wavebands, of which will be taken advantages to radiate heat towards the outer space.

High emissivity (or its equivalent high absorptivity) of electromagnetic waves appears as a key material property of the surfaces involved in radiative cooling. The same properties are also of paramount importance for producing broad spectrum infrared light sources, that are required for chemical analysis using optical spectroscopy in the mid-Infrared (MIR) range, where fundamental molecular vibrational tones have large absorption cross section, which brings high sensitivity in the measurement together with versatility for a wide variety of biochemicals.

Optimization of surface properties regarding their radiative properties (emissivity/absorptivity) can be done by designing engineered meta-surfaces, whose properties outperform those of natural materials.

This thesis work is aiming to provide deeper understanding of radiative properties, in particular, through measurement of their properties at different temperatures from ambient temperatures up to 800°C. Besides, we intend design, fabrication and experimental evaluation of various meta-surfaces that we identified regarding their potential applications to either radiative cooling or broad-band chemical analysis.

Note that for radiative cooling applications, the weak light absorptivity outside the atmospheric window is preferred to reduce the light absorption from atmospheric radiation. Especially, the daytime radiative cooler also needs to take the effect of sunlight into account, thereby requiring an extreme low absorptivity in visible and near-infrared range to minimize its absorption for solar irradiance. Various spectrally-selective materials for light absorption have been designed for nighttime or daytime radiative cooling, including multilayer film substrates, nanostructured metasurfaces, polymer-based metamaterials, bio-inspired structures, phase change materials, and new materials, which realize effective cooling with temperature drops of typically -5 °C down to -40 °C, depending on their different selective emission abilities and thermal insulation treatments.

The project deals with radiative properties of nanomaterials and meta-surfaces, in particular the spectral emissivity, which is a key property for radiative cooling and infrared sources and detectors, among the numerous applications in the mid-infrared spectral range.

Both broadband and spectrally-selective materials and infrared meta-surfaces will be considered. A first set of material candidates will be selected among the materials that are already available at ESYCOM Laboratory. A second batch of materials will be then considered (through collaboration with international partners including NTU-Singapore and Ain-Shams University in Egypt), which might require additional work on the design and the material synthesis.

Characterization of the spectral emissivity of the selected materials will be done, first at and near room temperature by an indirect method through measurement using FTIR spectroscopy of both spectral reflectivity and transmissivity, which allows obtaining absorptivity, and hence emissivity thanks to Kirchhoff Law.

Then, we will extend our study targeting the measurement of emissivity at different temperatures up to 800°C, which are very useful for many infrared applications such as blackbody infrared light sources, thermal imaging, thermophotovoltaic energy harvesting, etc... The literature is indeed very poor with this kind of data. We expect reaching this goal through a direct method, which consists of measuring the emissivity, by heating the material, collecting its emitted radiation and comparing it to a reference blackbody radiation. A dedicated experimental setup is under construction at ESYCOM Lab and will be used for this thesis work.

International Mobility:

A secondment of the ESR is planned for a total period of up to 6 months in our international partners, at Nanyang Technological University (NTU) in Singapore and at Ain-Shams University (ASU) in Egypt."

For more information, contact the PhD thesis supervisor.