Controlling and Understanding Thermal Energy Exchange at Single Domains of Functional Materials

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NON-TECHNICAL DESCRIPTION: Heat has been familiar and important to humankind for a much longer history than electricity, yet people's ability of controlling and understanding heat lags much behind that of electricity. The key lies in materials science and engineering: how to design, synthesize and develop materials and tools that support, direct and gauge heat flow (in the same manner in which semiconductors work with electric current). This project seeks to tackle this fundamental problem using a specific class of materials: nanoscale functional materials. As heat is carried by both electrons and atomic vibration (the so-called "phonons"), achieving this goal requires exquisite control of behavior of both electrons and phonons, as well as their interactions. Functional materials made in the nanoscale and with high quality allow such control because they can be used to regulate the direction and magnitude of heat flow (as well as its conversion from/to electric current and fields). Knowledge gained in this project is used to educate underrepresented students through a partnership between Prof. Wu's university and local middle-high schools. TECHNICAL DETAILS: Functional materials support extraordinary thermal energy exchange processes such as thermoelectrics, thermal rectification, and electrocaloric effect. Domains (including functional domains and crystal grains) and domain walls of various types ubiquitously exist in these materials and strongly influence, sometimes dominate, the thermal energy exchange processes. Hampered by challenges in measuring heat flow at the nanoscale, previous studies of these effects are mostly limited to materials with a large number of disordered domains. As a result, intrinsic effects are hidden by ensemble averaging over the domains, leaving many key questions unsettled. This project goes beyond this ensemble averaging, to gauge, understand, control and optimize the thermal energy exchange at the level of single domain or single domain wall of functional materials, using microfabricated tools and nanomaterials processing techniques recently developed in Prof. Wu's lab. The project enables exquisite control of heat flow with functional materials by engineering density and mobility of electrons as well as crystallinity and interface of the lattice; identifies fundamental limits in such thermal energy exchange processes as interfacial thermal rectification, composite thermoelectrics, and electrocaloric cooling; and opens tremendous opportunities for novel design and improved performance of various thermal devices. Integrated with the research activities, Prof. Wu is also running an education project that stimulates and prepares pre-college students for careers in materials science and engineering pertaining to thermal applications.