Synthesis and thermal characterization of multifunctional porous metal nanomaterials [electronic resource]
Keywords: Mechanical Engineering.

Heat dissipation is one of the primary challenges facing the increasing performance of modern electronic devices. The thermal management of such devices is a multiscale materials challenge that ranges from the ever-shrinking transistors to the increasingly prevalent portable electronics. Many thermal challenges can be addressed using thermal metamaterials, which are multifunctional thermal conductors that exhibit unique combinations of properties not available in nature. Thermal metamaterials can be used to achieve unprecedented thermal performance in applications ranging from microelectronics and solar cells to solid-state lighting and thermal batteries. This work introduces a materials-oriented design approach using complex material architectures to achieve the extreme limits of thermal metamaterials. Through a combination of templated electrodeposition and electrothermal characterization methods, we synthesize and characterize the thermal properties of porous metal nanomaterials to address three specific challenges in thermal management: thermal interfaces, microscale heat exchangers, and thermal capacitors. Each challenge is further deconstructed into three distinct areas of research: materials synthesis, thermal characterization, and physics-based modeling. First we demonstrate the use of vertically-aligned copper nanowire (NW) arrays as high-performance, long-lifetime thermal interface materials. Dense arrays of vertically-aligned metal NWs offer the unique combination of thermal conductance from the constituent metal and mechanical compliance from the high aspect ratio geometry, which facilitates interfacial heat transfer and improves device reliability. We measure the thermal conductivity of freestanding copper NW arrays to be as high as 70 W m-1 K-1, which is more than an order of magnitude larger than most commercial interface materials and enhanced-conductivity nanocomposites reported in the literature. Second, we synthesize and measure the thermal conductivity of metal inverse opals (IOs) for applications in high heat flux microscale heat exchangers and heat pipes. Metal IOs are thermally-conductive thin films that have a large fluid-accessible surface area derived from a periodic arrangement of interconnected spherical pores. The combination of geometric tortuosity and nanoscale conduction pathways leads to quasi-ballistic electron transport in IOs having submicron pore sizes. Third, we examine the use of porous metals infiltrated with a phase change material as high-rate thermal capacitors to buffer thermal transients. Any open-cell porous metal can be infiltrated with an active interstitial material to provide additional functionality, and we demonstrate the use of metal NWs and IOs for both solid-liquid and liquid-vapor heat transfer. While each of these challenges demonstrates the ongoing importance of thermal engineering at decreasing length scales, the goal of this work is to provide a comprehensive framework for the design of thermal metamaterials, which will continue to play a necessary role in the ever-miniaturization and increasing performance demands of modern electronic devices.