Pushing the Limits of Thermal Transport to Address Electronics, Energy and Climiate Challenges

This talk will overview our recent effort in pushing the limits of thermal transport to address challenges in electronics thermal management, high temperature applications, sustainable energy, and climate crisis. The first part of the talk will cover our development of the general formulation and computational method of four-phonon scattering, and prediction of its unexpected significance in thermal conductivity, thermal radiative properties, and Raman spectra. Our predictions have been confirmed by a wide range of experiments, including high and low thermal conductivity materials. For complex crystals at high temperature, the conventional phonon mean free path concept is insufficient, and we propose a dual-phonon transport theory to better describe their thermal transport. The second part will cover the invention of ultrawhite paints that can cool below the ambient temperature under direct sunlight without consuming power. We have fabricated CaCO3-acrylic, BaSO4-acrylic, and hBN-acrylic paints that reflect 95.5%, 98.1%, and 97.9% of sunlight respectively. The sky window emissivity is high. Therefore, these paints can emit more infrared heat than the absorbed solar irradiation, cooling the surfaces up to 4.5 °C below the ambient temperature and achieving a cooling power up to 117 W/m2. Our predictions show that such high performance is due to pushing a few factors to the limit simultaneously: moderately high electronic band gaps that eliminate solar absorption while yielding reasonably high refractive index, abundant vibrational modes in acrylic and BaSO4, strong four-phonon scattering in BaSO4, particle size in the neighborhood of solar photon wavelength, a broad particle size distribution, and high particle concentration. Radiative cooling paints can have broad implications from saving energy to mitigating climate crisis. The talk will close by exploring future opportunities.