Controlling Thermal Transport with Thermal Metamaterials

To ensure electronic devices and systems operate at appropriate temperatures, efficient heat conductors are required to move the heat away from hot spots. On the other hand, to protect components with different thermal management needs in compact electronic and thermal systems and devices, thermally insulating materials are required. Moreover, many devices, such as batteries, operate effectively in a relatively narrow temperature range, hence they require thermal regulation where cooling is needed when the ambient is hot and insulation is needed when the ambient is cold. Overall, such systems and devices are subject to complex thermal challenges such as self-heating, over-heating, or over-cooling, which requires materials used for thermal management to be more versatile and even dynamic. This dissertation addresses three such thermal management challenges with the development of novel thermal metamaterials.

This dissertation first develops multi-layer thermal barrier coatings for thermal insulation in high temperature applications. In aerospace, higher inlet temperature of turbines enable higher power and efficiency. Thermal barrier coatings protect turbine blade alloys and push the limit of the operation temperature. Previously, doping has been used to improve certain properties and multi-layer films composed of different materials have been studied to combine their outstanding functions into a single system. However, the thermal transport properties of the optimum multi-layered systems have not been well understood. The author worked with a multidisciplinary research group to design a multi-layer TBC with optimized thermal and optical properties to protect turbine blades by reducing heat transfer through conduction and radiation. The thermal properties of the multi-layer stacks and the constituent materials are characterized using time-domain thermoreflectance from room temperature to 500 degree Celsius. The results for the individual materials agree well with literature reported data. Data for the multi-layer TBC is compared with predictions based on a thermal resistance network model with bulk thermal conductivity of each layer obtained by lattice dynamics and anharmonic phonon scattering calculations.

Going beyond thermal insulation, this dissertation then demonstrates a bi-layer anisotropic material for heat spreading in one direction while insulation in the vertical direction for thermal management of heterogeneous integration. Cooling of heterogeneous integrated electronic devices present unique challenges due to high power density, closely placed components, and limited space for installation in a compact package. Thermal issues such as thermal crosstalk and differences in operation temperature of adjacent components do not present in flip-chip and cannot be easily solved by increasing cooling power of the heat sink. In this dissertation, a bi-layer metamaterial with heat spreader and thermal diode functions is designed, fabricated, and tested. The designed bi-layer metamaterial demonstrates superior anisotropic ratio and outperforms conventional epoxy underfill material in regulating memory and logic die temperatures in a compact 3D IC package in numerical simulations and a benchtop experimental demonstration.

Finally, this dissertation goes beyond static thermal management approaches and pursues active thermal management, using continuously tunable thermal switches between conduction and insulation. For instance, batteries, such as those in electric vehicles, have a strict operational temperature range for the best performance and safety considerations. Thermal switches and regulators enable tuning of thermal resistance and change between insulating and conducting behavior. One promising approach is a continuously-tunable thermal switch based on compressible graphene/polymer composite foam to regulate transport through a material. In this dissertation, a model is developed to understand the switching mechanism of thermal conductivity of porous foam and provide insight for the search and design of a porous material with higher intrinsic thermal conductivity and switching ratio. The model is validated through molecular dynamic simulations, finite element analysis, and experimental demonstration using polyurethane foam.