Soutenance de thèse : Paul DESMARCHELIER

Atomistic Simulations of Nano-Architectured Semiconductors: Thermal and Vibrational Properties / Simulations Atomistiques de Semi-conducteurs Nanoarchitecturés : Propriétés Thermique et Vibratoire

Doctorant : Paul DESMARCHELIER

Laboratoire INSA : CETHIL / LaMCoS

Ecole doctorale : ED162 : Mécanique, Energétique, Génie civil et Acoustique

At the nanoscale, thermal and vibrational properties are intimately linked and depend on the shape and composition of the material. Thanks to the nanostructuration, nanocomposites allow a better control of the heat transfer. This can be used to improve the performances of thermoelectric generators through a better insulation. In this work, the thermal properties of some nanocomposites are studied using atomistic level simulations. In a first part, the focus is laid on nanocomposites composed of an amorphous matrix and crystalline nano-inclusions. The approach separating the propagative and diffusive contribution, developed for amorphous materials is used. The ballistic contribution where the heat is propagated by plane waves is systematically impacted by the nanostructuration. Whereas affecting the diffusive contribution, that spreading the heat slowly at the nanoscale, is more challenging. This can be done, through pores or inclusions softer than the matrix but in variable proportions. A second part is dedicated to the study of silicon nanowires, and the impact of amorphization. An amorphous shell causes the apparition of diffusive transport and the decrease in transmission at low frequencies. The structuration of the external amorphous layer in a conical shape can be used to obtain thermal rectification, that is, a spatial asymmetry in thermal transport. This rectification appears to be caused by the perturbation of transmission at low frequencies. Finally, molecular dynamics are coupled to hydrodynamic heat equations. This is used to study the radial distribution of flux in cylindrical nanowires with a constant thickness amorphous shell. This analysis suggests that the reduction of the thermal conductivity upon the addition of shell cannot be linked solely to changing of interface properties, but are rather due to a global effect of the shell on the mean free path of heat carriers.