UC Irvine

Fundamental thermal management problems, from macroscopic issues such as global warming to microscale problems such as electronic cooling, demonstrate the need for unconventional tools to manipulate heat flow across a wide range of length scales. The ability to fabricate materials that exhibit highly anisotropic thermal properties, ideally tunable under service conditions, would be game-changing in several applications. Metamaterials, i.e., porous solids consisting of periodic repetitions in three dimensions of carefully designed unit cells, could provide an avenue towards these goals. This doctoral thesis investigates thermal transport mechanisms in complex additively manufactured metamaterials, enabling a new strategy for manipulating heat flow and leading to programmable thermal behavior.A common challenge of conventional cooling solutions in electronic packaging is thermal cross-talk and the formation of local hotspots due to uniform heat dissipation from the heat source to the heat sink. Thermal metamaterials can address these challenges by providing an optimized solution for non-uniform heat dissipation. A suitable approach for the design of metamaterials enabling ideal heat routing while maintaining mechanical stability is topology optimization. A density-based topology optimization method is presented to identify thermally conductive and mechanically stable structures for optimal heat guiding under various heat source-sink arrangements. As a proof-of-concept experiment, we fabricate thermo-mechanically optimized 3D heat guiding structures using Laser Powder Bed Fusion (LPBF) technique and develop an infrared (IR) thermography methodology to characterize their thermal properties. Our results reveal that the thermal resistance of the topology optimized structures outperforms that of reference structures at the same volume. This approach enables new avenues for optimal heat guiding, thus advancing state-of-the-art thermal management in electronics. Periodic metamaterials (e.g., microlattices) provide another design framework to generate mechanically stable thermal metamaterials for preferential heat routing, whereby the effective thermal properties are determined by the interplay of metamaterials topology and constituent material properties. This thesis investigates heat transfer mechanisms in architected metamaterials by exploring the role of architecture on the conduction and radiation contributions. We model and characterize extremely low-density metallic hollow microlattices and polymeric shape memory lattices, both of which enable recoverable deformations in excess of 50% when compressed. An analytical model previously proposed for highly porous foams is adapted to regular lattice topologies, allowing expressions for conduction as a function of volume fraction, geometrical factor, constituent material properties, and radiation as a function of volume fraction, surface emissivity, view factor of the struts, and absolute temperature. Through IR thermography, we experimentally identify the individual contribution of each heat transfer mode. In particular, the impact of high surface emissivity and high surface-to-volume ratio on the resultant radiation contribution are explored. In contrast with monolithic solid materials, we find that radiation has a more significant contribution to heat transfer, even at low absolute temperatures. We experimentally demonstrate the possibility of modulating the thermal properties of architected metamaterials by geometrical reconfiguration using an external mechanical stimulus. We show that large-scale compression of stretching-dominated lattice topologies, characterized by deformation behavior involving buckling and densification, increases contact among micro-struts, thus increasing conductive thermal transport. In contrast, deformation in bending-dominated lattice topologies leads to a change in view factor among micro-struts without altering the physical contacts, thus decreasing radiative thermal transport. These findings suggest that thermal transport in mechanically recoverable architected materials can be programmed upon external stimuli, a viable strategy for designing future dynamic or adaptive thermal control devices and thermal information processing systems. In closing, we explore the possibility of generating superior thermal metamaterials by architecting solids at the nanoscale, thus allowing the exploitation of size effects on the thermal conduction mechanism.