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Beyond the Fourier Law of Heat Diffusion in Phonon-Engineered Materials:Ballistic, Coherent, Localized, Hydrodynamic, and Divergent Modes
Fourier law of heat conduction is one of the bases for analyzing thermal problems. Although the failure of the Fourier law as reflected in the reduced thermal conductivity of materials have been known and extensively studied, going beyond this classical size effect is difficult. In this talk, I will discuss prediction and experimental observation of several different modes of heat conduction beyond classical boundary scattering effect, enabled by advances in simulation and experimental tools. Density-functional-based tools can now reliably predict thermal conductivity of crystalline materials, as demonstrated by recent prediction and discovery of new high thermal conductivity materials such as boron arsenide. These simulations show that the mean free path distributions of phonons in many materials are broad, leading to experimental observation of quasi-ballistic phonon transport even when no boundaries exist. In superlattice structures, ballistic phonon transport through many interfaces across the entire thickness of the superlattices implies phase coherence. We observed this coherent transport in GaAs/AlAs superlattices. Accessing the coherent heat conduction regime opens a new venue for phonon engineering. I will further show that Anderson localization in phonon heat conduction happens in GaAs/AlAs superlattice by placing ErAs nanodots at interfaces. In the opposite direction, we will discuss phonon hydrodynamic transport mode in graphene and graphite. In this mode, phonons drift with an average velocity under a temperature gradient, similar to fluid flow in a pipe under a pressure gradient. We have conducted simulation to guide experimental observation of second sound in graphite above 100 K, which is a manifestation of collective phonon hydrodynamic transport. Furthermore, it is also possible to have infinite heat thermal conductivity, as suggested by the Fermi-Pasta-Ulam discovery of nonergodic behavior in one-dimensional nonlinear atomic chain in 1950s. This divergence could exist in real materials such as polymer chains. New understandings in these nondiffusive heat conduction phenomena are stimulating developments of new materials. One example is thermoelectric energy conversion for which phonon thermal conductivity is to be minimized while maintaining electron transport properties. Another example is high thermal conductivity in ultra-drawn polyethylene nanofibers and sheets. Progress in these areas will be highlighted.
Dr. Gang Chen is the Carl Richard Soderberg Professor of Power Engineering at Massachusetts Institute of Technology (MIT). He served as the Department Head of the Department of Mechanical Engineering at MIT from 2013 to 2018, and as the director of the "Solid-State Solar-Thermal Energy Conversion Center (S3TEC Center)" - an Energy Frontier Research Center funded by the US Department of Energy from 2009 to 2019. He obtained his PhD degree from the Mechanical Engineering Department at UC Berkeley. He was a faculty member at Duke University and UCLA, before joining MIT in 2001. He received an NSF Young Investigator Award, an R&D 100 award, an ASME Heat Transfer Memorial Award, a Nukiyama Memorial Award by the Japan Heat Transfer Society, a World Technology Network Award in Energy, an Eringen medal from the Society of Engineering Science, and the Capers and Marion McDonald Award for Excellences in Mentoring and Advising from MIT. He is a fellow of American Association for the Advancement of Science, APS, ASME, the Guggenheim Foundation, and the American Academy of Arts and Sciences. He is an academician of Academia Sinica and a member of the US National Academy of Engineering.