There has been an ever-increasing need for soft, functional materials within areas of research such as soft robotics, flexible electronics, and wearable devices. These materials must be stretchable and/or flexible, thermally and electrically conductive, and robustly adhesive to a wide variety of substrates and surfaces. Over the past several decades, soft composites consisting of functional solid particles within an elastic matrix have been developed with the aim of achieving these properties. However, solid particulate fillers in elastomeric materials have various limitations which hinders the ability to achieve the aforementioned properties simultaneously. In this work, two novel approaches to developing soft conductive adhesives are introduced in an effort to solve mechanical, thermal, electrical, and adhesive trade-offs. The composites developed herein utilize liquid metal (LM) inclusions and a combination of LM with solid silver (Ag) flakes within deformable polymer matrices to maintain mechanical compliance while also achieving thermal and electrical functionality. Furthermore, adhesive properties of LM composites are enhanced through a chemical anchoring technique, while the composition and microstructure of LM-Ag composites are designed to control functional and adhesive properties. There are several demonstrations throughout which show the ability to robustly integrate the novel soft composites with rigid materials and electronic components for the creation of resilient and functional hybrid electronic systems. / Master of Science / There has been an ever-increasing need for soft, functional materials within areas of research such as soft robotics, flexible electronics, and wearable devices. These materials must be stretchable and/or flexible, thermally and electrically conductive, and robustly adhesive to a wide variety of substrates and surfaces. Over the past several decades, soft composites consisting of functional solid particles within an elastic matrix have been developed with the aim of achieving these properties. However, solid particulate fillers in elastomeric materials have various limitations which hinders the ability to achieve the aforementioned properties simultaneously. In this work, two novel approaches to developing soft conductive adhesives are introduced in an effort to solve mechanical, thermal, electrical, and adhesive trade-offs. The composites developed herein utilize liquid metal (LM) inclusions and a combination of LM with solid silver (Ag) flakes within deformable polymer matrices to maintain mechanical compliance while also achieving thermal and electrical functionality. Furthermore, adhesive properties of LM composites are enhanced through a chemical anchoring technique, while the composition and microstructure of LM-Ag composites are designed to control functional and adhesive properties. There are several demonstrations throughout which show the ability to robustly integrate the novel soft composites with rigid materials and electronic components for the creation of resilient and functional hybrid electronic systems. Fabrication and Characterization of Multifunctional Soft Composites for Hybrid Electronic Systems Tyler A. Pozarycki (GENERAL AUDIENCE ABSTRACT) Composites are materials which are made up of two or more components with characteristics that exceed their counterparts. Steel reinforced concrete is a common example, where the steel helps to reinforce the concrete while the concrete itself gives shape to the structure. One cannot exist without the other, as the steel alone would create a meaningless skeleton and the concrete alone would not be able to withstand weights of heavier objects such as vehicles. In recent years, soft composites have become an emerging paradigm. These materials are stretchable and flexible due to their main component typically being an elastomer, while their inner component can consist of various materials that give desired functionality. For example, iron particles can grant magnetic properties and carbon can allow the material to conduct heat and/or electricity. As a result, these materials have captured the interest of scientists and researchers in various fields such as robotics, electronics, and biomedicine. However, there exists a unique challenge in developing such a material for applications in these areas. That is, the material needs to possess three critical properties simultaneously: 1) it must be compliant to various surfaces, meaning it must assume complex shapes such as those found on the human body, 2) it must be able to efficiently conduct electricity and heat, and 3) it must be able to adhere, or stick strongly to a variety of surfaces and materials for assembly. Typically, solving this problem has been attempted by fabricating soft composites with inner components consisting of metallic and ceramic particles, powders, or flakes. However, the use of these materials within elastomers, gels, and the like often create a composite which falls short of the aforementioned requirements, as the rigid inner structure and soft outer material are uncomplimentary to each other. Additionally, silicone elastomers and other similar materials typically do not adhere to a wide variety of surfaces, which further complicates the problem. In this work, two novel materials are produced in an effort to solve these long-standing issues. The first utilizes room-temperature liquid metal (LM) as the inner component to preserve overall material integrity while also using a chemical anchoring process to adhere the composites to several plastics and metals. The second consists of a flexible epoxy (naturally adhesive material) which incorporates both LM and silver flakes to create an as-prepared thermally and electrically conductive adhesive. Both soft composites are shown integrated with rigid electronic components and other materials to demonstrate the feasibility of using the composites to fabricate hybrid electronic systems.

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<p dir="ltr">This work investigates four aspects of Ga-based low melting temperature alloys in their role as TIMs: the interaction between Ga and metal substrates, the change in the thermodynamic behavior of the liquid metal alloy, the evolution of the thermal performance, and mitigation strategies against Ga corrosion.</p>

High performance RF/mixed signal systems require new interconnect strategies to combine high frequency (microwave/mm-wave) circuitry with silicon mixed-signal and baseband digital processing. In such systems, heterogeneous vertical integration, in which circuits in different technologies can be stacked on top of each other within the system architecture, can reduce the overall system size and power consumption. Chip stacking also enables optimally-performing heterogeneous systems, because each level of the stack can consist of components fabricated in their most suited device or substrate technology. Two novel approaches for the vertical interconnection of heterogeneous integrated systems are proposed in this work. These approaches are related to flip-chip bonding techniques used in Radio-Frequency (RF)/microwave integrated circuits. The first proposed approach involves an interlocking mechanical structure that supports flip-chip assembled Monolithic Microwave Integrated Circuits (MMICs). Photolithographically patterned thick-film SU-8 structures are applied to both the chip and the carrier such that the chip self-aligns into place and mates with the carrier. Gold bumps embedded within the structures electrically connect the chip pads to the carrier pads. This method is demonstrated through the assembly of a SiGe power amplifier MMIC onto a high resistivity silicon carrier. The second proposed approach involves vertical interconnects consisting of room temperature liquid-state metals. The fluid nature of the liquid bumps allows them to be robust in the presence of thermo-mechanical stresses, such as Coefficient of Thermal Expansion (CTE) mismatch between the interconnected chips. SU-8 structures are used to form a shaping mold on the bottom carrier that contains the liquid metal. Gold posts are electroplated on the top chip, then mated with the SU-8 mold, thereby making contact with the liquid metal to form the electrical continuity. For each of these proposed methods, design and fabrication considerations are discussed in detail. RF measurements on prototype structures up to Ka band are performed to verify the functionality of the proposed methods. Given the results of these proof-of-concept efforts, electrical characteristics of the materials used in these methods are determined, and recommendations are provided for future improvements and refinements to these two techniques. / Master of Science

Flip chip interconnections have superior performance for microwave applications compared to wire bond interconnections because of their reduced parasitics, more compact architecture, and flexibility in laying out flip chip bond pads. Reduction in interconnect parasitics enables these interconnects to support broadband signals, therefore increasing the bandwidth capabilities of flip chip-assembled systems. Traditional flip chip designs provide mechanical and electrical connections from a top chip to a carrier substrate with rigid solder joints. For heterogeneous assemblies, flip chip connections suffer from thermo-mechanical failures caused by coefficient of thermal expansion mismatches. As an alternative, flexible flip chip interconnections incorporating a metal, which is liquid at room temperature, mitigates the possibility of such thermo-mechanical failures. Additionally, liquid metal, flip chip interconnections allow for room temperature assembly, simplifying assembly and rework processes. This dissertation focuses on the design and characterization of liquid metal interconnections, specifically using Galinstan, an alloy of gallium indium and tin, for the heterogeneous assembly of active monolithic microwave integrated circuits (MMICs) onto a CTE mismatched substrate. Carrier substrates designed for liquid metal transitions were fabricated on high resistivity Si and on three dimensional copper structures. The three dimensional copper structures were fabricated in the PolyStrata™ process. Individual MMIC chips were post-processed to mate with carrier substrates in a liquid metal, flip chip configuration. S-parameter measurements of prototype MMIC assemblies with liquid metal, flip chip interconnections showed an average transition loss of 0.7dB over the MMIC's frequency of operation (4.9 - 8.5 GHz). Passive assemblies were also fabricated to characterize the power and temperature performance of liquid metal transitions. Liquid metal interconnections show excellent power handling, maintaining consistent RF performance while transmitting 100W of continuous wave power for an hour. Liquid metal interconnections were also tested following 200 temperature cycles over the -140°C – 125°C range. A comparison of S parameter measurements taken before and after temperature cycling, over a frequency range of 10MHz - 40GHz showed no significant changes in performance. These passive assemblies were also used to develop a lumped element model of the interconnection which is useful for the verification the interconnection\'s performance and for comparison of liquid metal interconnection parasitic to wire bond and flip chip interconnect parasitics. The experimental results presented in this dissertation confirm that liquid metal interconnect are viable for wider use in military and commercial applications. In the future, additional environmental testing and further refinement of the processing flow, such as improved contact metallurgy, are needed to make this interconnect approach more viable for large volume manufacturing. / Ph. D.

The delayed neutron spectra for LMFBRs are not as well known as those for LWRs. These spectra are necessary for kinetics calculations which play an important role in safety and accident analyses. In this study, a sensitivity analysis was performed to study the sensitivity of the reactor power and power density to uncertainties in the delayed neutron spectra during a rod ejection accident. The generalised methodology, developed by Cacuci et. al., was used to derive a set of sensitivity derivatives. This method is based on the use of adjoints so that it is not necessary to repeatedly solve the governing (kinetics) equations to obtain the sensitivity derivatives. This is of particular importance when large systems of equations are used. A two-energy multigroup and two precursor group model was formulated for the INFCE reference design MOX-fuelled LMFBR. The accidents studied were central control rod ejections with ejection times of 2, 10, and 30 seconds. The power and power density responses were found to be most sensitive to uncertainties in the spectrum of the second delayed neutron precursor group, resulting from the fission of U-238, producing neutrons in the first energy group. It was found, for example, that for a rod ejection time of 30 seconds, an uncertainty of 7.2% in the fast components of the spectra resulted in a 24% uncertainty in the predicted power and power density. These responses were recalculated by repeatedly solving the kinetics equations. The maximum discrepancy was only 1.6%. The versatility and accuracy of Cacuci’s methodology has been demonstrated. The results of the sensitivity analysis indicates the need for improved delayed neutron spectral data in order to reduce the uncertainties in the accident analyses. The model can be extended by using more energy groups, more precursor groups, and more spatial dimensions. Other important responses that may be studied are the linear power density, linear heat rate, and reactivity worths. / Ph. D.

Efficient reconfigurable antennas are highly sought after in all communication applications for the ability to reduce space cost while maintaining the ability to control the frequency, gain, and polarization. The ability to control these parameters allows a single antenna to maximize its performance in a wide range of scenarios to satisfy changing operating requirements. This thesis will describe reconfigurable antennas using liquid-metal alloys that give the system this ability by injecting or retracting the liquid metal from various channels. After simulations were performed in an electromagnetic simulation software, proof-of-concept models were built, tested, and compared to the simulations to verify the results. / Master of Science

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 1980. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE. / Includes bibliographical references. / by King-Wo Thomas Chiu. / M.S.

Thesis. 1979. Ph.D. cn--Massachusetts Institute of Technology. Dept. of Nuclear Engineering. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND SCIENCE. / Includes bibliographical references. / Ph.D.cn