CAREER: Pushing the Extremes of Heat Conduction via Multiscale Phonon Modeling from First-Principles

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Recent advances in nanomaterials and processing technologies have enabled the creation of a large number of heterogeneous nanostructures for a variety of applications including thermoelectric energy generation, microelectronics cooling, thermal barrier materials, solar cells, and energy storage. The number of complex systems consisting of hierarchical structures spanning the nano-, meso- and macro- scales is increasing rapidly. Thermal modeling of these devices requires attention to a broad range of length scales and physical phenomena. Small scale transport in these hierarchical materials is poorly understood mainly due to the lack of a proper description of energy transport by vibrations in the structures, called phonons, across these multiple scales. A rigorous understanding of multiscale phonon transport is crucial for pushing the extremes of heat conduction for the advancement of diverse, transformative applications such as economical thermoelectric energy conversion, which requires ultralow thermal conductivity, and more efficient electronics cooling, which demands ultrahigh thermal conductivity. By improving the efficiency of energy conversion and heat rejection, the project can essentially contribute to global sustainable energy solutions. The educational objective of this CAREER project is to promote academic diversity and equal educational opportunities and to prepare a highly educated workforce in the STEM fields by encouraging interest in thermal science and engineering via a creative museum exhibit for the general public, engaging in research activities via a novel international collaborative course and outreach activities for kindergarten-to-college students, and sharing of the state-of-the-art research findings with industrial partners. The research objective of this CAREER project is to obtain a comprehensive understanding of multiscale phonon transport from first-principles in order to push the upper and lower boundaries of thermal conductivity. Despite well-established theories at the macroscale and the significant progress made at the nanoscale over the past few decades, mesoscale thermal transport remains poorly understood. This project focuses on mesoscale phonon transport to bridge the knowledge gap between nanoscale and macroscale phonon transport. The research tasks are below: (1) Generate the key input parameters for mesoscale simulations, phonon mean free path and interface transmittance, from atomic- and nano-scale first-principles calculations using density functional theory, atomistic Green?s function method, and ab initio molecular dynamics simulations; (2) Solve the Boltzmann transport equation using Monte Carlo simulations for mesoscale transport; (3) Validate multiscale simulation results using time-domain thermoreflectance measurements; (4) Develop a compact robust analytical model for thermal engineers and heat transfer researchers. The outcome of this project is expected to be a major leap in the fundamental understanding of multiscale phonon transport, enabling the creation of novel materials with unprecedented thermal transport properties for numerous applications including thermal energy conversion and management.