Dimitris N. Chorafas Foundation Award 2020 – Michele Simoncelli
Thermal transport beyond Fourier, and beyond Boltzmann
Thesis director: Prof. N. Marzari
For the quantum-mechanical generalization of the Peierls-Boltzmann transport equation, providing a unified theory of thermal transport applicable to any crystalline, disordered, or glassy material.
Crystals and glasses exhibit fundamentally different heat-conduction mechanisms: the periodicity of crystals allows for the excitation of propagating vibrational waves that carry heat, as first discussed by Peierls in 1929, while in glasses the lack of periodicity breaks Peierls’s picture and heat is mainly carried by the coupling of vibrational modes, often described by a harmonic theory introduced by Allen and Feldman in 1989.
In this thesis, we developed a unified microscopic equation that describes on an equal footing heat conduction in crystals, glasses, and anything in-between. In particular, it predicts correctly and in agreement with experiments the thermal conductivity in crystals, glasses, and most importantly in the mixed regime of complex thermoelectrics. Consequently, this equation has found straightforward applications in the field of renewable energy, setting the stage to systematically enhance the efficiency of devices used for thermoelectric conversion of waste heat into electricity.
We also showed how in the crystalline regime such a microscopic transport equation can be coarse-grained into a set of “viscous heat equations” that generalize Fourier's macroscopic equation (formulated in 1822), accounting for both diffusion and heat hydrodynamics, and rationalize the recent discovery of heat transfer via temperature waves in graphitic devices.