The microelectronics industry is actively focusing on advanced packaging technologies, notably on three-dimensional stacking of heterogeneous integrated (3D-HI) circuits for enhanced performance. Despite its computational performance benefits, this approach faces challenges in thermal management due to increased power density and heat generation. Conventional cooling methods struggle to address this issue effectively. This study investigates microfluidic intralayer cooling techniques using analytical correlation and computational fluid dynamics (CFD) principles to propose a method capable of managing thermal performance across varying load conditions. The proposed configuration achieved a dissipation of 40 W/cm2 with a volumetric flow rate of 200 mL/min, maintaining chip temperature at 315K. Additionally, extreme hotspot conditions generating 1kW/cm2, along with the presence of thermal resistance from redistribution layers (RDLs), are analyzed. This research aims to establish a model for understanding geometric property variations under different heat flux conditions in 3D heterogeneous integration of semiconductor packaging.

Soft thermal interface materials (TIMs) are critical for improving the thermal management of advanced microelectronic devices. Despite containing high thermal conductivity filler materials, TIM performance is limited by thermal resistances between fillers, filler-matrix, and external contact resistance. Recently, room-temperature liquid metals (LMs) started to be adapted as an alternative TIM for their low thermal resistance and fluidic nature. However, LM-based TIMs face challenges due to their low viscosity, non-wetting qualities, chemical reactivity, and corrosiveness towards aluminum.To address these concerns, this dissertation research investigates fundamental LM properties and assesses their utility for developing multiphase LM composites with strong thermal properties. Augmentation of LM with gallium oxide and air capsules lead to LM-base foams with improved spreading and patterning. Gallium oxides are responsible for stabilizing LM foam structures which is observed through electron microscopy, revealing a temporal evolution of air voids after shear mixing in air. The presence of air bubbles and oxide fragments in LM decreases thermal conductivity while increasing its viscosity as the shear mixing time is prolonged. An overall mechanism for foam generation in LM is presented in two stages: 1) oxide fragment accumulation and 2) air bubble entrapment and propagation. To avoid the low thermal conductivity air content, mixing of non-reactive particles of tungsten or silicon carbide (SiC) into LM forms paste-like LM-based mixtures that exhibit tunable high thermal conductivity 2-3 times beyond the matrix material. These filler materials remain chemically stable and do not react with LM over time while suspended. Gallium oxide-mediated wetting mechanisms for these non-wetting fillers are elucidated in oxygen rich and deficient environments. Three-phase composites consisting of LM and Ag-coated SiC fillers dispersed in a noncuring silicone oil matrix address LM-corrosion related issues. Ag-coated SiC particles enable improved wetting of the LM, and the results show that applied pressure is necessary for bridging of these LM-coated particles to improve filler thermal resistance. Compositional tuning between the fillers leads to thermal improvements in this multiphase composite. The results of this dissertation work aim to advance our current understanding of LMs and how to design LM-based composite materials for improved TIMs and other soft thermal applications.

Phononic crystals are artificially engineered materials that can forbid phonon propagation in a specific frequency range that is referred to as a “phononic band gap.” Phononic crystals that have band gaps in the GHz to THz range can potentially enable sophisticated control over thermal transport with “phononic devices”. Calculations of the phononic band diagram are the standard method of determining if a given phononic crystal structure has a band gap. However, calculating the phononic band diagram is a computationally expensive and time-consuming process that can require sophisticated modeling and coding. In addition to this computational burden, the inverse process of designing a phononic crystal with a specific band gap center frequency and width is a challenging problem that requires extensive trial-and-error work.

In this dissertation, I first present colloidal nanocrystal superlattices as a new class of three-dimensional phononic crystals with periodicity in the sub-20 nm size regime using the plane wave expansion method. These calculations show that colloidal nanocrystal superlattices possess phononic band gaps with center frequencies in the 102 GHz range and widths in the 101 GHz range. Varying the colloidal nanocrystal size and composition provides additional opportunities to fine-tune the phononic band gap. This suggests that colloidal nanocrystal superlattices are a promising platform for the creation of high frequency phononic crystals.

For the next topic, I explore opportunities to use supervised machine learning for expedited discovery of phononic band gap presence, center frequency and width for over 14,000 two-dimensional phononic crystal structures. The best trained model predicts band gap formation, center frequencies and band gap widths, with 94% accuracy and coefficients of determination (R2) values of 0.66 and 0.83, respectively.

Lastly, I expand the above machine learning approach to use machine learning to design a phononic crystal for a given set of phononic band gap properties. The best model could predict elastic modulus of host and inclusion, density of host and inclusion, and diameter-to-lattice constant ratio for target center and width frequencies with coefficients of determinations of 0.94, 0.98, 0.94, 0.71, and 0.94 respectively. The high values coefficients of determination represents great opportunity for phononic crystal design.

This thesis explores the possibility of fabricating superconducting tunnel junctions (STJ) using double angle evaporation using an E-beam system. The traditional method of making STJs use a shadow mask to deposit two films requires the breaking of the vacuum of the main chamber. This technique has given bad results and proven to be a tedious process. To improve on this technique, the E-beam system was modified by adding a load lock and transfer line to perform the multi-angle deposition and in situ oxidation in the load lock without breaking the vacuum of the main chamber. Bilayer photolithography process was used to prepare a pattern for double angle deposition for the STJ. The overlap length could be easily controlled by varying the deposition angles. The low-temperature resistivity measurement and scanning electron microscope (SEM) characterization showed that the deposited films were good. However, I-V measurement for tunnel junction did not give expected results for the quality of the fabricated STJs. The main objective of modifying the E-beam system for multiple angle deposition was achieved. It can be used for any application that requires angular deposition. The motivation for the project was to set up a system that can fabricate a device that can be used as a phonon spectrometer for phononic crystals. Future work will include improving the quality of the STJ and fabricating an STJs on both sides of a silicon substrate using a 4-angle deposition.

Soft polymer composites with improved thermal conductivity are needed for the thermal management of electronics. Interfacial thermal boundary resistance, however, prevents the efficient use of many high thermal conductivity fill materials. Magnetic alignment of ferrous fill material enforces percolation of the high thermal conductivity fill, thereby shifting the governing boundary resistance to the particle- particle interfaces and increasing the directional thermal conductivity of the polymer composite. Magnetic alignment maximizes the thermal conductivity while minimizing composite stiffening at a fill fraction of half the maximum packing factor. The directional thermal conductivity of the composite is improved by more than 2-fold. Particle-particle contact engineering is then introduced to decrease the particle- particle boundary resistance and further improve the thermal conductivity of the composite.

The interface between rigid fill particles is a point contact with very little interfacial area connecting them. Silver and gallium-based liquid metal (LM) coatings provide soft interfaces that, under pressure, increase the interfacial area between particles and decrease the particle-particle boundary resistance. These engineered contacts are investigated both in and out of the polymer matrix and with and without magnetic alignment of the fill. Magnetically aligned in the polymer matrix, 350nm- thick silver coatings on nickel particles produce a 1.8-fold increase in composite thermal conductivity over the aligned bare-nickel composites. The LM coatings provide similar enhancements, but require higher volumes of LM to do so. This is due to the rapid formation of gallium oxide, which introduces additional thermal boundaries and decreases the benefit of the LM coatings.

The oxide shell of LM droplets (LMDs) can be ruptured using pressure. The pressure needed to rupture LMDs matches closely to thin-walled pressure vessel theory. Furthermore, the addition of tungsten particles stabilizes the mixture for use at higher pressures. Finally, thiols and hydrochloric acid weaken the oxide shell and boost the thermal performance of the beds of LMDs by 50% at pressures much lower than 1 megapascal (MPa) to make them more suitable for use in TIMs.

The rapid progress of solution-phase synthesis has led colloidal nanocrystals one of the most versatile nanoscale materials, provided opportunities to tailor material's properties, and boosted related technological innovations. Colloidal nanocrystal-based materials have been demonstrated success in a variety of applications, such as LEDs, electronics, solar cells and thermoelectrics. In each of these applications, the thermal transport property plays a big role. An undesirable temperature rise due to inefficient heat dissipation could lead to deleterious effects on devices' performance and lifetime. Hence, the first project is focused on investigating the thermal transport in colloidal nanocrystal solids. This study answers the question that how the molecular structure of nanocrystals affect the thermal transport, and provides insights for future device designs. In particular, PbS nanocrystals is used as a monitoring system, and the core diameter, ligand length and ligand binding group are systematically varied to study the corresponding effect on thermal transport.

Next, a fundamental study is presented on the phase stability and solid-liquid transformation of metallic (In, Sn and Bi) colloidal nanocrystals. Although the phase change of nanoparticles has been a long-standing research topic, the melting behavior of colloidal nanocrytstals is largely unexplored. In addition, this study is of practical importance to nanocrystal-based applications that operate at elevated temperatures. Embedding colloidal nanocrystals into thermally-stable polymer matrices allows preserving nanocrystal size throughout melt-freeze cycles, and therefore enabling observation of stable melting features. Size-dependent melting temperature, melting enthalpy and melting entropy have all been measured and discussed.

In the next two chapters, focus has been switched to developing colloidal nanocrystal-based phase change composites for thermal energy storage applications. In Chapter 4, a polymer matrix phase change nanocomposite has been created. In this composite, the melting temperature and energy density could be independently controlled by tuning nanocrystal diameter and volume fractions. In Chapter 5, a solution-phase synthesis on metal matrix-metal nanocrytal composite is presented. This approach enables excellent morphological control over nanocrystals and demonstrated a phase change composite with a thermal conductivity 2 - 3 orders of magnitude greater than typical phase change materials, such as organics and molten salts.

Waste heat energy conversion remains an inviting subject for research, given the renewed emphasis on energy efficiency and carbon emissions reduction. Solid-state thermoelectric devices have been widely investigated, but their practical application remains challenging because of cost and the inability to fabricate them in geometries that are easily compatible with heat sources. An intriguing alternative to solid-state thermoelectric devices is thermogalvanic cells, which include a generally liquid electrolyte that permits the transport of ions. Thermogalvanic cells have long been known in the electrochemistry community, but have not received much attention from the thermal transport community. This is surprising given that their performance is highly dependent on controlling both thermal and mass (ionic) transport. This research will focus on a research project, which is an interdisciplinary collaboration between mechanical engineering (i.e. thermal transport) and chemistry, and is a largely experimental effort aimed at improving fundamental understanding of thermogalvanic systems. The first part will discuss how a simple utilization of natural convection within the cell doubles the maximum power output of the cell. In the second part of the research, some of the results from the previous part will be applied in a feasibility study of incorporating thermogalvanic waste heat recovery systems into automobiles. Finally, a new approach to enhance Seebeck coefficient by tuning the configurational entropy of a mixed-ligand complex formation of copper sulfate aqueous electrolytes will be presented. Ultimately, a summary of these results as well as possible future work that can be formed from these efforts is discussed.