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Energy Harvesting

Heat Transfer in Nanostructured Materials(Super-Thermal Barriers)  

     We are investigating energy transport in nanostructured materials, especially nanolaminates consisting of alternating layers of disparate materials. Nanolaminates are promising as thermal barrier coatings and components of thermoelectric devices because heat conduction is strongly impeded by the thermal interface resistance. Nanolaminates are attractive as thermal protection coatings for gas turbine blades, machining tool bits, and gun barrels since they do not incorporate pores or microscale defects that compromise mechanical or chemical protection properties. They are also well-suited as thermal barrier layers in nanoscale electronic and data storage devices that have stringent roughness requirements. Heat conduction across metal-dielectric interfaces is also of great fundamental interest. The type of dominant heat carriers switches between phonons and electrons across the interfaces, a situation very different from semiconductor superlattices and quantum wells where phonons are dominant heat carriers throughout the entire thickness.
    We have experimentally determined the thermal resistance of nanolaminates synthesized through controlled oxidation of nanoscale metal films. The thermal conductivity of the nanolaminates is well below the minimum thermal conductivity limit of the constituent materials and among the lowest achieved for nonporous inorganic materials. Our work demonstrates that nanolaminates offer a promising approach to realizing superior thermal barrier coatings.

Our theoretical work also demonstrates the impact of spatial nonequilibrium between electrons and phonons near the interface on heat conduction across metal-dielectric nanolaminates. The spatial non-equilibrium results in extra thermal resistance for a metal layer sandwiched between two dielectric materials.The extra thermal resistance is a function of the electron phonon-coupling length

as well as the metal layer thickness L. The extra thermal resistance is approximately constant when L>>δ. But it decreases rapidly with decreasing metal layer thickness when L is comparable to or smaller than δ, which is typically a few nanometers. Our model provides important design guidelines in selecting material combinations and optimum thicknesses of individual layers to maximize the benefit of nanolaminates.

UCLA Department of Mechanical and Aerospace Engineering, Last update: January 31st, 2008