Active Regulation of Thermal Boundary Conductance

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CBET-1402845Kieffer (Univ. of Michigan, Ann Arbor)Molecular-scale simulations have shown that when applying an electric field to an electro-active polymer it causes it to constrict and it becomes mechanically stiffer. As a result, the thermal conductivity of the polymer increases by up to 40%. Similarly, when the polymer adheres to a metallic substrate, which serves to apply the field, the adhesive forces are intensified and simulations predict that the heat conductance across this interface increases by a factor of six. Accordingly, one can devise a heat valve by sandwiching a thin electro-active polymer film between two metallic films, and by applying a few tens of volts to these metal films, depending on film thickness, one can turn alter the heat flow rate across this multi-layered structure by a factor of three to four. The objective of this project is to demonstrate this heat flow switching mechanism experimentally, to clearly elucidate the underlying physical principles, and based on these insights, to improve the materials design, for example by creating nano-porous structures of molecularly bonded polymer and ceramic components that exhibit larger deformation amplitudes, so as to achieve bigger heat flow amplification ratios. The expected outcome of this research is a technology with application in numerous situations that require thermal management, including regulating heat flow in confined environments, e.g., living organisms, engines, fuel cells, sensors, and chemical reactors, and even thermal diodes, i.e., devices that allow one to control the direction of heat flow. The research findings may inspire technologies such as actuated membranes for controlled selective filtering, sensors with time-differential sampling capability, deployable medical devices for targeted drug and heat delivery, e.g., for localized cancer treatment. Finally, the research strategy employed here may impact science and technology beyond a specific discipline by validating an emerging research simulation-guided approach and by demonstrating an innovative materials development approach.The goal of this research is to explore novel nano-structural materials designs that allow for the in situ regulation of thermal transport properties at interfaces and surfaces, i.e., switching between extreme levels of heat transfer or continuously adjusting the thermal conductance within that range. This functional response is achieved by optimizing the structure and topology of electro-active polymers incorporated into dense and nano-porous hybrid materials, i.e., in which the organic and inorganic components are dispersed at the molecular level and chemically bonded to one another, so as to make most efficient use of their inherent properties. To accomplish this goal computation is used to explore the fundamental principles that govern materials behavior and determine the most effective molecular configurations for the targeted functional response. The design principles so obtained guide materials development and appropriate chemical synthesis routes, and are validated by characterizing the dielectric, mechanical, and thermal transport behavior of the resulting materials. First, the underling materials design concepts, which ensues from a molecular simulation-based proof-of-concept study, predicting a 40% thermal conductivity and a six-fold thermal boundary conductance increase when applying an electric field to an ultra-thin layer of piezoelectric polymer deposited on a metal substrate, is verified experimentally and the underlying mechanisms are identified. From these insights, blueprints for the design of materials design that yields maximal thermal transport regulating behavior are derived. Accordingly, nano-porous polymer-inorganic hybrid materials are explored as materials that potentially yield magnified changes in thermal transport properties because of their large strains in response to piezoelectric actuation. Conversely, large-amplitude actuation in purposely designed nano-porous structures are investigated for application as adjustable membranes for selective filtration, detection of pathogens, time-selective sampling, targeted drug delivery, and fluid flow regulation.