Impact of Four-Phonon Scattering on Thermal Conductivity and Radiative Cooling Properties

This talk will overview our recent developments on first principles predictions of four-phonon scattering and its significant impact on thermal conductivity and radiative cooling properties, which are relevant for many thermal management and sustainable energy applications. By developing the quantum mechanical formalism for four-phonon scattering and mitigating the severe computational challenges, we have established four-phonon scattering as a significant intrinsic mechanism beyond three-phonon scattering, in governing thermal conductivity of nearly all materials at high temperature, and 2D materials, low thermal conductivity materials, and certain high-thermal conductivity materials even at room temperature. We also show how the spectral optical and thermal radiative properties of nanocomposites can be predicted by combining first principles and Monte Carlo simulations. The optical response for dielectrics is due to photon-electron interaction in the UV-VIS-NIR band and photon-phonon interaction in the MIR band. Here four-phonon scattering can have a leading role in determining infrared and Raman linewidth in dielectrics, which are beneficial for achieving high and broadband emissivity in the "sky window". These predictions can explain the strong sub-ambient radiative cooling under direct sunlight we achieved in particle-matrix ultrawhite paints.