<p dir="ltr">Increasing power density and power consumption in electronic devices require heat dissipating components with high thermal conductivity to prevent overheating and improve performance and reliability. Polymers offer the advantages of low cost and weight over conventional metallic components, but their intrinsic thermal conductivity is low. Previous studies have shown that the thermal conductivity of polymers can be enhanced by aligning the polymer chains or by adding high thermal conductivity fillers to create percolation paths within the polymeric matrix. To further enhance the in-plane thermal conductivity, the conductive fillers can be aligned preferentially, but this leads to a lower increase in performance in the cross-plane direction. Yet, the cross-plane thermal conductivity plays a vital role in dissipating heat from active devices and transmitting it to the surrounding environment. Alternatively, when the fillers are aligned to enhance cross-plane thermal transport, the enhancement in the in-plane direction is limited. There is a need to develop polymer composites with an approximately isotropic increase in thermal performance compared to their neat counterparts.</p><p dir="ltr">To achieve this goal, in this study, I combine conductive fibers and fillers to enhance thermal conductivity of polymers without significantly inducing thermal anisotropy while preserving the mechanical performance of the matrix. I employ three approaches to enhance the thermal conductivity () of thermoset polymeric matrices. In the first approach, I fabricate thermally conductive polymer composites by creating an emulsion consisting of eutectic gallium indium alloy (EGaIn) liquid metal in the uncured polydimethylsiloxane (PDMS) matrix. In the second approach, I infiltrate mats formed from chopped fibers of Ultra High Molecular Weight Polyethylene (UHMWPE) with an uncured epoxy resin. Finally, the third approach combines the two previous methods by infiltrating the UHMWPE fiber mat with an emulsion of the liquid metal and uncured epoxy matrix.</p><p dir="ltr">To evaluate the thermal performance of the composites, I use infrared thermal microscopy with two different experimental setups, enabling independent measurement of in-plane and cross-plane thermal conductivity. The results demonstrate that incorporating thermally conductive fillers enhances the overall conductivity of the polymer composite. Moreover, I demonstrate that the network structure achieved by the fiber mat, in combination with the presence of liquid metal, promotes a more uniform increase in the thermal conductivity of the composite in all directions. Additionally, I assess the impact of filler incorporation and filler concentration on matrix performance through tension, indentation, and bending tests for mechanical characterization of my materials.</p><p dir="ltr">This work demonstrates the potential of strategic composite design to achieve polymeric materials with isotropically high thermal conductivity. These new materials offer a solution to the challenges posed by higher power density and consumption in electronics and providing improved heat dissipation capabilities for more reliable devices.</p>

<p>An emerging technology in the field of transient thermal management is thermal energy storage, or TES, which enables temporary, on-demand heat rejection via storage as latent heat in a phase-change material.  Latent TES devices have enabled advances in many thermal management applications, including peak load shifting for reducing energy demand and cost of HVAC systems and providing supplemental heat rejection in transient thermal management systems.  However, the design of a transient thermal management system with integrated storage comprises many challenges which are yet to be solved.  For example, design approaches and performance metrics for determining the optimal dimensions of the TES device have only recently been studied.  Another area of active research is estimation of the internal temperature state of the device, which can be difficult to directly measure given the transient nature of the thermal storage process.  Furthermore, in contrast to the three main functions of a thermal-fluid system--heat addition, thermal transport, and heat rejection--thermal storage introduces the need for active, real-time control and automated decision making for managing the operation of the thermal storage device. </p> <p>In this thesis, I present the design process for integrating thermal energy storage into a single-phase thermal management system for rejecting transient heat loads, including design of the TES device, state estimation and control algorithm design, and validation in both simulation and experimental environments. Leveraging a reduced-order finite volume simulation model of a plate-fin TES device, I develop a design approach which involves a transient simulation-based design optimization to determine the required geometric dimensions of the device to meet transient performance objectives while maximizing power density.  The optimized TES device is integrated into a single-phase thermal-fluid testbed for experimental testing.  Using the finite volume model and feedback from thermocouples embedded in the device, I design and experimentally validate a state estimator based on the state-dependent Riccati equation approach for determining the internal temperature distribution to a high degree of accuracy.  Real-time knowledge of the internal temperature state is critical for making control decisions; to manage the operation of the TES device in the context of a transient thermal management system, I design and test, both in simulation and experimentally, a logic-based control strategy that uses fluid temperature measurements and estimates of the TES state to make real-time control decisions to meet critical thermal management objectives. Together, these advances demonstrate the potential of thermal energy storage technology as a component of thermal management systems and the feasibility of logic-based control strategies for real-time control of thermal management objectives.</p>