Abstract: Effective control over thermal transport is often a limiting factor in the development of energy efficient thermal/optical/electronic devices, especially thermoelectric materials that require ultra-low thermal conductivity. In the classic theory of thermal transport by atomic vibrations (phonons) in crystalline materials, heat moves in a manner analogous to a gas, described by a velocity and by scattering events. This is known as the phonon gas model. Thermal transport in amorphous materials is fundamentally different, where heat moves in a manner analogous to an atomic-scale random walk. These phonons can be called diffusons. Only recently have these two perspectives been unified within one theoretical framework, which subsequently shows that phonons in complex crystalline materials can exhibit both gas-like and diffuson-like character. Effective tuning of thermal conductivity thus requires an understanding of both transport “channels.” This talk will discuss materials design strategies in the context of two-channel thermal transport. It will be shown that many ultralow thermal conductivity materials likely have both types of phonon transport, whose relative contributions depend strongly on temperature. Furthermore, factors like chemical composition and material defects may be used to prefer or suppress one channel or the other, which is experimentally demonstrated in several thermoelectric silver argyrodite compounds. Given the structural complexity of many solid-state battery materials, the prevalence of two-channel thermal transport is also likely to play a crucial role in thermal battery management.

Biography: Dr. Matthias T. Agne received his Bachelor and Master degrees in Materials Science and Engineering from Drexel University in 2015. While at Drexel, Matthias spent 4 years researching MAX phase ceramic composites under Professor Michel Barsoum. He completed his Ph.D under the advisory of Professor Jeffrey Snyder and graduated from Northwestern University in 2020. There, his research was concentrated in thermodynamics and materials physics with applications to solid-state transport phenomena, thermoelectric materials and measurements. Since then, Matthias has been an Alexander von Humboldt Postdoctoral Fellow in the chemistry department at the WWU Münster in the group of Professor Wolfgang Zeier. Currently, he investigates the fundamental mechanisms of ionic transport in solid electrolytes, as well as the interrelations of thermal and ionic transport.