Thèses

Lundi 14 Mars 2022 à 14h00.

Understanding Thermal Transport in Advanced Nanophononic Structures for Energy Applications

''The hazardous effects of pollution have many aspects such as health effects and environmental changes. The main source of pollution is the emission of toxic gases from energy sources like fuel and coal. Numerous fields of research attempt to tackle the addressed problems. We are here interested in photocatalysis as a promising way for producing a green source of energy. In this thesis, we investigate the concept of nanostructured materials for the optimization of thermal energy distribution in order to thermally boost the photo-induced processes. This process, taking place at the interface between optics (photons) and thermics (phonons), belongs to the field of phoxonics. To this aim, in the framework of an "Idex Lyon Breakthrough" project called IPPON, " Incoherent light and Phonon management in micro-nanopatterned materials for efficient depollution and artificial PhOtosyNthesis.", we have investigated thermal transport in nanostructured amorphous SiN thin films, by means of pump-probe techniques to measure thermal conductivity and understand the microscopic mechanisms below thermal transport in such materials.

As a first research axis, we report the study of the effect of the temperature deposition on the thermal conductivity in amorphous SiNx films of thicknesses between 200 and 500 nm, as measured by the thermoreflectance technique with temperatures up to 773 K. Surprisingly, for all deposition temperatures between 300 and 573 K, thermal conductivity exhibits a steep decrease above 473 K, decreasing by more than 30 % down to a minimum around 673 K, before increasing back to values comparable with the room temperature one. This behavior, observed only for a first heating of the sample, is associated to an irreversible modification of the thin film, and may be related to a partial desorption of the hydrogen trapped in a-SiNx during the deposition. A second research axis has been to investigate directly the quasiparticle responsible for heat transport, the phonon, in order to understand the effect of nanostructuration on its individual properties (energy and lifetime).
The study shows that the reduction of the phonon lifetime in the nanostructured material as compared to the uniform material can be ascribed to the increase of the phonon scattering from the interfaces. Our work shows that the phonon lifetime reduction strongly depends on phonon wavelength, as related to the typical nanostructure lengthscale. As such, depending on phonon wavelength, the ratio between the lifetime in the nanostructured material and the uniform one ranges between 10 to 60 %.

Membres DU JURY :
PAILHÈS Stéphane, Directeur de thèse - Chargé de Recherche CNRS, Institut Lumière Matière, CNRS
GIORDANO Valentina, Co-Directrice de thèse - Chargé de Recherche CNRS, Institut Lumière Matière, CNRS
BOURGEOIS Olivier, Rapporteur - Directeur de Recherche CNRS, INP, Institut Néel
ZARDO Ilaria, Rapporteure - Professeur, Département de Physique, Université de Basel
BENCIVENGA Filippo, Examinateur - Chercheur, FERMI FEL, Elettra-Sincrotrone Trieste S.C.p.A.
DEL FATTI Natalia, Examinatrice - Professeur, Institut Lumière Matière, Université Lyon 1

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