Energy Transport in Extreme Thermal Materials

Controlling thermal transport is critical for many applications including electronics and energy technologies. Discovering new materials and nanostructures with extreme conductivity that can efficiently insulate or dissipate heat are in urgent need. In this talk, I will first describe our recent effort in developing emerging high thermal conductivity semiconductors, including boron arsenide (BAs) and boron phosphide (BP) with a room temperature thermal conductivity of 1300 W/mK (Science 361, 575, 2018) and 500 W/mK (Nano Letters 17, 7507, 2017) respectively, beyond most common semiconductors and metals. Ultrafast spectroscopy study in conjunction with atomistic theory reveals that the unique band structure of BAs allows for very long phonon mean free paths and strong high-order anharmonicity through the four-phonon process. Our study establishes BAs and BP as benchmark materials for thermal management, and exemplifies the power of combining experiments and ab initio theory in new materials discovery. On the other hand, we investigated ultralow thermal conductivity materials with record-high thermoelectric performance. Examining the intrinsic crystals of tin selenide (Nano Letters 19, 4941, 2019), we observed abnormally strong phonon renormalization at room temperature due to high-order anharmonicity, as well as the failure of widely used quasi-harmonic model in predicting thermophysical properties. In addition, I will briefly describe our effort in developing in-situ techniques to characterize electrochemical materials (Nano Letters 17, 1431, 2017) and quantifying interface energy transport (Advanced Materials 31, 1901021, 2019) that enable better fundamental understanding of phonon spectra and defect scattering for electronics, sensors, batteries, and quantum information.