Research is presented which encompasses a study of epoxies based on diglycidyl ether of bisphenol A (DGEBA) cured with 2-ethyl-4-methylimidazole (EMI-24) in the presence of the nonionic surfactant Triton X-100. Interest in this epoxy system is due partially to the potential application as a waterborne replacement for solvent cast epoxies in E-glass laminated printed circuit boards. This research has revealed that the viscoelastic behavior of the cured epoxy is altered when serving as the matrix in a glass composite. The additional constraining and coupling of the E-glass fibers to the segmental motion of the epoxy matrix results in an increased level of viscoelastic cooperativity. Current research has determined that the cooperativity of an epoxy/E-glass composite is also sensitive to the surface chemistry of the glass fibers. Model single-ply epoxy/E-glass laminates were constructed in which the glass was pretreated with either 3-aminopropyltriethoxysilane (APS) or 3-glycidoxypropyltrimethoxysilane (GPS) coupling agents. Dynamic mechanical analysis (DMA) was then used to create master curves of the storage modulus (E') in the frequency domain. The frequency range of the master curves and resulting cooperativity plots clearly varied depending on the surface treatment of the glass fibers. It was determined that the surfactant has surprisingly little effect in the observed trends in cooperativity of the composites. However, the changes in cooperativity due to the surface pretreatment of the glass were lessened by the aqueous phase of the waterborne resin. Moisture uptake experiments were also performed on epoxy samples that were filled with spherical glass beads as well as multi-ply laminated composites. No increases in the diffusion constant could be attributed to the surfactant. However, the surfactant did enhance the final equilibrium moisture uptake levels. These equilibrium moisture uptake levels were also sensitive to the surface pretreatment of the E-glass. / Ph. D.

京都大学 / 0048 / 新制・課程博士 / 博士(人間・環境学) / 甲第19072号 / 人博第725号 / 新制||人||174(附属図書館) / 26||人博||725(吉田南総合図書館) / 32023 / 京都大学大学院人間・環境学研究科相関環境学専攻 / (主査)教授 内本 喜晴, 教授 加藤 立久, 教授 吉田 寿雄 / 学位規則第4条第1項該当 / Doctor of Human and Environmental Studies / Kyoto University / DGAM

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Recently there has been an increased focus on the use of microfluidics for the synthesis of different products. One of the products proposed for synthesis is quantum dots. Microfluidics often uses Polydimethylsiloxane for structure in microfluidic chips, but quantum dots use octadecene in several synthesis steps. The purpose of this work was to create a lab setup capable of anodically bonding 4” diameter wafers, and to characterize the bond formed using the Plaza test chip so that microfluidic devices using glass and silicon as substrates could be created. Two stainless steel electrodes placed on top of a hot plate were attached to a high power voltage supply to perform anodic bonding. A Plaza test mask was created and used to pattern P type silicon wafers. The channels etched were between 300 and 500nm deep and ranged between 1000µm and 50µm. These wafers were then anodically bonded to Corning 7740 glass wafers. Bonding stopped once the entire surface of the wafer was bonded, determined by visual inspection. All bonds were formed at 400°C and the bond strength and toughness between wafers bonded at 400V and 700V was compared. A beam model was used to predict the interfacial fracture toughness, and the stress at the bond was calculated with a parallel spring model. By measuring the crack length of the test structures under a light microscope the load conditions of the beam could be found. It was concluded that the electrostatic forces between the wafers give the best indication of what the bond quality will be. This was seen by the large difference in crack length between samples that were bonded using a thick glass wafer (1 mm) and a thin glass wafer (500µm). The observed crack lengths for the thick glass wafers were between 40 and 60µm. Thin glass wafers had a crack length between 20 and 40µm. The fracture toughness was calculated using the beam model approximation. Fracture toughness of the thin glass wafers was 7MPa m1/2, and of the thick glass wafers was 30 MPa m1/2. The fracture toughness of the thick glass wafers agreed with results found through the use of the double cantilever beam samples in literature. The maximum observed interfacial stress was 70 MPa. Finally, to measure the change in the size of the sodium depletion zone formed during bonding, samples were placed under a scanning electron microscope (SEM). Depletion zones were found to be between 1.1 and 1.4µm for thin glass samples that were bonded at 400 and 700 volts. This difference was not found to have a significant effect on the strength or fracture toughness observed. Thicker glass samples could not have their depletion zone measured due to SEM chuck size.

Interfacial mechanical properties of adhesive joints are very crucial in board applications, including composites, multilayer structures, and biomedical devices. Establishing traction-separation (T-S) relations for interfacial adhesion can evaluate mechanical and structural reliability, robustness, and failure criteria. Due to the short range of interfacial adhesion such as micro to nanoscale, accurate measurements of T-S relations remain challenging. The advent of machine learning (ML) became a promising tool to predict materials behaviors and establish data-driven mechanical models. In this study, we integrated a state-of-the-art ML method, finite element analysis (FEA), and standard experiments to develop data-driven models for characterizing the interfacial mechanical properties precisely. Macroscale force-displacement curves are derived from FEA with incorporation of double cantilever beam tests to generate the dataset for ML model. The eXtreme Gradient Boosting (XGBoost) multi-output regressions and classifier models are used to determine T-S relations with R2 score of 98.8% and locate imperfections at the interface with accuracy of around 80.8%. The outcome of the XGBoost models demonstrated accurate predictions and fast calculation speed, outperforming several other ML methods. Using 3D printed double cantilever beam specimens, the performance of the ML models is validated experimentally for different materials. Furthermore, a XGBoost model-based package is designed to obtain different adhesive materials T-S relations without creating a database or training a model.

The invention of Li-ion batteries (LIBs) marks a new era of energy storage and allows for the large-scale industrialization of electric vehicles. However, the flammable organic electrolyte in LIBs raises significant safety concerns and has resulted in numerous fires and explosion accidents. In the pursuit of more reliable and stable battery solutions, interests in aqueous batteries composed of high-energy cathodes and water-based electrolytes are surging. Limited by the narrow electrochemical stability window (ESW) of water, conventional aqueous batteries only achieve inferior energy densities. Current development mainly focuses on manipulating the properties of aqueous electrolytes through introducing excessive salts or secondary solvents, which enables an unprecedentedly broad ESW and more selections of electrode materials while also resulting in some compromises. On the other hand, the interaction between electrodes and aqueous electrolytes and associated electrode failure mechanism, as the key factors that govern cell performance, are of vital importance yet not fully understood. Owing to the high-temperature calcination synthesis, most electrode materials are intrinsically moisture-free and sensitive to the water-rich environment. Therefore, compared to the degradation behaviors in conventional LIBs, such as cracking and structure collapse, the electrode may suffer more severe damage during cycling and lead to rapid capacity decay. Herein, we adopted multi-scale characterization techniques to identify the failure modes at cathode–electrolyte interface and provide strategies for improving the cell capacity and life during prolonged cycling. In Chapter 1, we first provide a background introduction of conventional non-aqueous and aqueous batteries. We then show the current development of modern aqueous batteries through electrolyte modification and their merits and drawbacks. Finally, we present typical electrode failure mechanism in non-aqueous electrolytes and discuss how water can further impact the degradation behaviors. In Chapter 2, we prepare three types of aqueous electrolytes and systematically evaluate the electrochemical performance of LiNixMnyCo1-x-yO2, LiMn2O4 and LiFePO4 in the aqueous electrolytes. Combing surface- and bulk-sensitive techniques, we identify the roles played by surface exfoliation, structure degradation, transition metal dissolution and interface formation in terms of the capacity decay in different cathode materials. We also provide fundamental insights into the materials selection and electrolyte design in the aqueous batteries. In Chapter 3, we select LiMn2O4 as the material platform to study the transition metal dissolution behavior. Relying on the spatially resolved X-ray fluorescence microscopy, we discover a voltage-dependent Mn dissolution/redeposition (D/R) process during electrochemical cycling, which is confirmed to be related to the Jahn–Teller distortion and surface reconstruction at different voltages. Inspired by the findings, we propose an approach to stabilize the material performance through coating sulfonated tetrafluoroethylene (i.e., Nafion) on the particle, which can regulate the proton diffusion and Mn dissolution behavior. Our study discovers the dynamic Mn D/R process and highlights the impact of coating strategy in the performance of aqueous batteries. In Chapter 4, we investigate the diffusion layer formed by transition metals at the electrode–electrolyte interface. With the help of customized cells and XFM technique, we successfully track the spatiotemporal evolution of the diffusion layer during soaking and electrochemical cycling. The thickness of diffusion layer is determined to be at micron level, which can be readily diminished when gas is generated on the electrode surface. Our approach can be further expanded to study the phase transformation and particle agglomeration at the interfacial region and provide insights into the reactive complexes. In Chapter 5, we reveal the correlation between the electrolytic water decomposition and ion intercalation behaviors in aqueous batteries. In the Na-deficient system, we discover that overcharging in the formation process can introduce more cyclable Na ions into the full cell and allows for a boosted performance from 58 mAh/g to 124 mAh/g. The mechanism can be attributed to the water oxidation on the cathode and Na-ion intercalation on the anode when the charging voltage exceeds the normal oxidation potential of cathode. We emphasize the importance of unique formation process in terms of the cell performance and cycle life of aqueous batteries. In Chapter 6, we summarize the results of our work and propose perspectives of future research directions. / Doctor of Philosophy / Li-ion batteries (LIBs) have dominated the market for portable devices and electric vehicles owing to their high energy density and good cycle life. However, frequent battery explosion accidents have raised significant safety concerns for all customers. The root cause can be attributed to the flammable organic electrolytes in conventional LIBs. To address this issue, aqueous batteries based on water-rich electrolytes attract intensive attention recently. Recent research progress has dramatically improved the energy density of aqueous batteries dramatically by modifying the properties of electrolytes. However, most electrode materials are incompatible with water, leading to severe side reactions and an unstable cycle life. Therefore, understanding the failure mechanism of electrode materials in the presence of water is crucial while not fully studied yet. Our projects systematically evaluate the degradation behavior of various electrodes in aqueous electrolytes and uncover the root cause of transition metal dissolution in the electrodes. Our studies shed light on improving battery capacity and cycle life through a specialized formation cycle and polymer coating process. Furthermore, we also provide new approaches to investigate the dynamic process occurring at electrode–electrolyte interface, which is applicable to other solid–liquid systems. In summary, our research reveals the correlation between the failure mechanism and the capacity decay in various electrode materials, proposing effective approaches to enhance the battery performance.

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<p>Metal-organic frameworks (MOFs) are considered as a type of very useful materials for the gas separation/purification industries. However, control over the growing position and growing shape of the crystals remains a challenge and must be overcome in order to realize the commercial potentials of MOFs.</p> <p>In this thesis, a method based on interfacial coordination is developed to address this issue. Zinc-benzenedicarboxyl (Zn-BDC) is chosen as a model system for the proof of concept. In a typical liquid-liquid interface protocol, the MOF precursors, zinc nitrate [Zn(NO3)2] and terephthalic acid (TPA or H2BDC), and the catalyst, triethylamine (TEA), were dissolved into two immiscible solvents, dimethylformamide (DMF) and hexane, respectively. The reaction site, i.e. the MOF growing position could thereby be confined at the interface of the two solvents. It was found that a free-standing membrane could be formed with the combinations of high Zn-H2BDC and low TEA concentrations. The combinations of low Zn-H2BDC and high TEA concentrations yielded MOF particles precipitated out from DMF. Similar results were obtained by changing the liquid-liquid interface to liquid-gas interface, with the TEA-hexane solution replaced by saturated TEA vapor. The dependence of product shape on precursor and catalyst concentrations can be explained by the competition between MOF formation and TEA diffusion into the precursor phase.</p> <p>The morphology, constitution and surface area of the MOF products were characterized by SEM, XRD and nitrogen adsorption testing, respectively. The particles were found to be exclusively MOF-5. The membranes were characterized as asymmetric. The top layer was particulate while the bottom layer had a sheet-like morphology. This was further revealed by XRD data as MOF-5 and MOF-2 (ZnBDC·DMF), respectively. This asymmetry was caused by a change of TEA diffusion rate during the synthesis process, which might result in a change in pH value for the membrane growth. Decent surface areas of the particles and membranes were measured.</p> <p>Apart from the free-standing membranes, MOF membranes on Anodisc support were also synthesized employing the same interfacial techniques. The MOF formation site, i.e. the interface, was confined to the upper end Anodisc pores and sealing the pores after the reaction. The difference in wetting force between DMF and hexane with Anodisc membrane material resulted in the difference of MOF layer morphology from liquid-liquid protocol and liquid-gas protocol. The later gave a continuous MOF membrane due to the absence of air bubble interference.</p> / Master of Science (MSc)

Surface tension is an essential force for the functioning of the world and life. Centuries of study, and still, new applications and limits of surface tension are being explored. Water has always drawn attention for its high surface tension value, 72mN/m compared to ethanol's 20mN/m. The high surface tension allows for numerous applications, superhydrophobic surfaces being one that takes heavy advantage of that value. Superhydrophobicsurfaceshave a high surface energy cost with water, resulting in small contact areas with high advancing and receding contact angles and low contact angle hysteresis. This results in very low adhesion on the surfaces. Here we study the ability of superhydrophobic surfaces with their low adhesion to shed meltwater from frost, showing a decrease in frost thickness to below 3mm for the meltwater to shed. We then take another approach to removing water from a surface, rather than increasing the surface energy cost, we introduce a difference in surface energy cost. Introducing a porous surface across from a solid one, droplets transfer from the solid to the porous, removing over 90% of the volume of the droplet from the solid surface. We thoroughly examine and model the hydrodynamics of the transfer process, varying the solid surface, the donor surface, and the liquid. This bridging between surfaces is then applied to fog harps, examining the efficiencies of large-form fog harps. Fog harps have shown a 3 to 5 times increase in water collection compared to the industry-standard mesh collector. However, droplets from fog collected on the wires eventually grow large enough to touch neighboring wires. Tominimizetheirsurfaceenergy, they begin pulling wires together, "tangling" them. This can potentially reduce efficiency, but has not been applied to large-scale harps until here. Another application of surface tension is then examined, using lower surface tension oils, but trapping them in microstructures to make slippery liquid-infused porous surfaces (SLIPS). The oil coats the microstructure, due to its lower surface tension. This creates a lubricating layer on the surface, along with potential air pockets reducing friction further. These surfaces have been studied extensively with liquids being placed on them, but here we begin to examine them when solids are used instead, showing some interesting cases where increasing the viscosity of the oil actually decreases the friction force. / Doctor of Philosophy / Sponges are something everyone has used, and most people can tell you that they work using surface tension. And for most people, that's enough. It's actually more useful to know to squeeze your sponge dry when you're done to prevent mold than it is to know that it holds onto liquids because of surface tension. But the point here was to take the study of sponges and surface tension to the extreme. To the point that some knowledge is going to be gained solely for the sake of gaining knowledge. Not all knowledge will have immediate uses, but this doesn't take value away from the knowledge, or any eventual uses it might have. So we start this by looking at the building of scientific knowledge and noticing that a brick is missing. Superhydrophobic surfaces, surfaces that water doesn't want to touch, have been studied very extensively and their properties have been thoroughly explored. However, a direct comparison of the defrosting behaviors, the process of frost melting on a surface, between superhydrophobic and hydrophobic surfaces had not been done. Water does prefer to be on a hydrophobic surface compared to a superhydrophobic one, but it's still uncomfortable. A plate was treated so that half was hydrophobic and the other half was superhydrophobic. Frost was grown across the surface and then melted simultaneously, allowing us to characterize the differences in the behaviors, highlighting the ability of the superhydrophobic surface to shed water droplets at smaller sizes than other surfaces. Next is a pure fluid mechanics work supporting a heat transfer application. Evaporation, for enhanced heat transfer, and a hydrophilic wick, essentially a sponge, are paired to create a plate with one-way heat transfer. Heating side A can heat side B, but heating side B can't heat side A. Water in the wick gets heated, evaporates from side A and then condenses on side B, carrying heat with it. The condensation grows until it touches the wick, which then pulls it in, allowing it to be evaporated again and cycling more heat. When side B, the smooth surface, is heated, the water can evaporate off it and condense in the wick, but then it has no way to return, preventing further heat transfer. The process of droplets being pulled from side B to the wick in side A is key to the process. It's a sponge pulling water in using surface tension. However, all the smaller pieces have been taken for granted. The second piece is a systematic study of this capture mechanism, exploring the effects of changing liquids, donor surfaces, and receiving porous wicks. The third is a continuation of the lab's previous work on Fog Harps, arrays of vertical fibers held in place to let fog run into them. The droplets grow until they slide down and can be collected. The wires of the harp are close enough that the water can actually start to tangle them together. This tangling can increase the water needed for sliding and collection to begin. Tensioning the wires can help mitigate the tangling. Here we show harps on around 1,$text{m}^2$, using optimal wire size and spacing that is possible for mass manufacturing. The harps were tested in the lab using humidifiers to generate fog for the harps to collect. Finally, an initial study of solid objects being pulled across oil-infused microstructured surfaces. The microstructure helps keep the oil on the surface thanks to the surface energy of the oil. These oil-infused surfaces have been studied extensively when liquids are placed on them, but not with solid objects. Solid objects can exert significantly more pressure than liquids, which naturally want to spread when they reach a certain thickness. Experiments were performed with a variety of oil viscosities, microstructures, and oil excess thicknesses. This work is not entirely complete but a significant portion of it is presented here.