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- Genre: Academic theses
Description
Recent advancements in the field of light wavefront engineering rely on complex 3D metasurfaces composed of sub-wavelength structures which, for the near infrared range, are challenging to manufacture using contemporary scalable micro- and nanomachining solutions. To address this demand, a novel parallel micromachining method, called metal-assisted electrochemical nanoimprinting (Mac-Imprint) was developed. Mac-Imprint relies on the catalysis of silicon wet etching by a gold-coated stamp enabled by mass-transport of the reactants to achieve high pattern transfer fidelity. This was realized by (i) using nanoporous catalysts to promote etching solution diffusion and (ii) semiconductor substrate pre-patterning with millimeter-scale pillars to provide etching solution storage. However, both of these approaches obstruct scaling of the process in terms of (i) surface roughness and resolution, and (ii) areal footprint of the fabricated structures. To address the first limitation, this dissertation explores fundamental mechanisms underlying the resolution limit of Mac-Imprint and correlates it to the Debye length (~0.9 nm). By synthesizing nanoporous catalytic stamps with pore size less than 10 nm, the sidewall roughness of Mac-Imprinted patterns is reduced to levels comparable to plasma-based micromachining. This improvement allows for the implementation of Mac-Imprint to fabricate Si rib waveguides with limited levels of light scattering on its sidewall. To address the second limitation, this dissertation focuses on the management of the etching solution storage by developing engineered stamps composed of highly porous polymers coated in gold. In a plate-to-plate configuration, such stamps allow for the uniform patterning of chip-scale Si substrates with hierarchical 3D antireflective and antifouling patterns. The development of a Mac-Imprint system capable of conformal patterning onto non-flat substrates becomes possible due to the flexible and stretchable nature of gold-coated porous polymer stamps. Understanding of their mechanical behavior during conformal contact allows for the first implementation of Mac-Imprint to directly micromachine 3D hierarchical patterns onto plano-convex Si lenses, paving the way towards scalable fabrication of multifunctional 3D metasurfaces for applications in advanced optics.
ContributorsSharstniou, Aliaksandr (Author) / Azeredo, Bruno (Thesis advisor) / Chan, Candace (Committee member) / Rykaczewski, Konrad (Committee member) / Petuskey, William (Committee member) / Chen, Xiangfan (Committee member) / Arizona State University (Publisher)
Created2022
Description
The thermal conductivity of cadmium sulfide (CdS) colloidal nanocrystals (NCs) and magic-sized clusters (MSCs) have been investigated in this work. It is well documented in the literature that the thermal conductivity of colloidal nanocrystal assemblies decreases as diameter decreases. However, the extrapolation of this size dependence does not apply to magic-sized clusters. Magic-sized clusters have an anomalously high thermal conductivity relative to the extrapolated size-dependence trend line for the colloidal nanocrystals. This anomalously high thermal conductivity could probably result from the monodispersity of magic-sized clusters. To support this conjecture, a method of deliberately eliminating the monodispersity of MSCs by mixing them with colloidal nanocrystals was performed. Experiment results showed that mixtures of nanocrystals and MSCs have a lower thermal conductivity that falls approximately on the extrapolated trendline for colloidal nanocrystal thermal conductivity as a function of size.
ContributorsSun, Ming-Hsien (Author) / Wang, Robert (Thesis advisor) / Rykaczewski, Konrad (Committee member) / Wang, Liping (Committee member) / Arizona State University (Publisher)
Created2022
Description
District heating plays an important role in improving energy efficiency and providing thermal heat to buildings. Instead of using water as an energy carrier to transport sensible heat, this dissertation explores the use of liquid-phase thermochemical reactions for district heating as well as thermal storage. Chapters 2 and 3 present thermodynamic and design analyses for the proposed district heating system. Chapter 4 models the use of liquid-phase thermochemical reactions for on-site solar thermal storage. In brief, the proposed district heating system uses liquid-phase thermochemical reactions to transport thermal energy from a heat source to a heat sink. The separation ensures that the stored thermochemical heat can be stored indefinitely and/or transported long distances. The reactant molecules are then pumped over long distances to the heat sink, where they are combined in an exothermic reaction to provide heat. The product of the exothermic reaction is then pumped back to the heat source for re-use. The key evaluation parameter is the system efficiency.
The results demonstrate that with heat recovery, the system efficiency can be up to 77% when the sink temperature equals 25 C. The results also indicate that the appropriate chemical reaction candidates should have large reaction enthalpy and small reaction entropy. Further, the design analyses of two district heating systems, Direct District Heating (DDH) system and Indirect District Heating (IDH) system using the solvated case shows that the critical distance is 106m. When the distance is shorter than 1000,000m, the factors related to the chemical reaction at the user side and factors related to the separation process are important for the DDH system. When the distance is longer than 106m, the factors related to the fluid mechanic become more important. Because the substation of the IDH system degrades the quality of the energy, when the distance is shorter than 106m, the efficiency of the substation is significant. Lastly, I create models for on-site solar thermal storage systems using liquid-phase thermochemical reactions and hot water. The analysis shows that the thermochemical reaction is more competitive for long-duration storage applications. However, the heat recovery added to the thermochemical thermal storage system cannot help improving solar radiation absorption with high inlet temperature of the solar panel.
ContributorsZhang, Yanan (Author) / Wang, Robert (Thesis advisor) / Milcarek, Ryan (Committee member) / Parrish, Kristen (Committee member) / Phelan, Patrick (Committee member) / Rykaczewski, Konrad (Committee member) / Arizona State University (Publisher)
Created2022
Description
Dissimilar metal joints such as aluminum-steel joints are extensively used in automobile, naval and aerospace applications and these are subjected to corrosive environmental and mechanical loading resulting in eventual failure of the structural joints. In the case of aluminum alloys under aggressive environment, the damage accumulation is predominantly due to corrosion and is accelerated in presence of other metals. During recent years several approaches have been employed to develop models to assess the metal removal rate in the case of galvanic corrosion. Some of these models are based on empirical methods such as regression analysis while others are based on quantification of the ongoing electrochemical processes. Here, a numerical model for solving the Nernst- Planck equation, which captures the electrochemical process, is implemented to predict the galvanic current distribution and, hence, the corrosion rate of a galvanic couple. An experimentally validated numerical model for an AE44 (Magnesium alloy) and mild steel galvanic couple, available in the literature, is extended to simulate the mechano- electrochemical process in order to study the effect of mechanical loading on the galvanic current density distribution and corrosion rate in AE44-mild steel galvanic couple through a multiphysics field coupling technique in COMSOL Multiphysics®. The model is capable of tracking moving boundariesy of the corroding constituent of the couple by employing Arbitrary Langrangian Eulerian (ALE) method.Results show that, when an anode is under a purely elastic deformation, there is no apparent effect of mechanical loading on the electrochemical galvanic process. However, when the applied tensile load is sufficient to cause a plastic deformation, the local galvanic corrosion activity at the vicinity of the interface is increased remarkably. The effect of other factors, such as electrode area ratios, electrical conductivity of the electrolyte and depth of the electrolyte, are studied. It is observed that the conductivity of the electrolyte significantly influences the surface profile of the anode, especially near the junction. Although variations in electrolyte depth for a given galvanic couple noticeably affect the overall corrosion, the change in the localized corrosion rate at the interface is minimal. Finally, we use the model to predict the current density distribution, rate of corrosion and depth profile of aluminum alloy 7075-stainless steel 316 galvanic joints, which are extensively used in maritime structures.
ContributorsMuthegowda, Nitin Chandra (Author) / Solanki, Kiran N (Thesis advisor) / Rykaczewski, Konrad (Committee member) / Jiao, Yang (Committee member) / Arizona State University (Publisher)
Created2015
Description
Contact angle goniometer is one of the most common tools in surfaces science. Since the introduction of this instrument by Fox and Zisman1 in 1950, dispensing the liquid using a syringe has generated pendant drops. However, using such approach at conditions significantly deviating from standard pressure and temperature would require an elaborate and costly fluidic system. To this end, this thesis work introduces alternative design of a goniometer capable of contact angle measurement at wide pressure and temperature range. In this design, pendant droplets are not dispensed through a pipette but are generated through localized condensation on a tip of a preferentially cooled small metal wire encapsulated within a thick thermal insulator layer. This thesis work covers experimental study of the relation between the geometry of the condensation-based pendant drop generator geometry and subcooling, and growth rate of drops of representative high (water) and low (pentane) surface tension liquids. Several routes that the generated pendant drops can be used to measure static and dynamic contact angles of the two liquids on common substrates well as nanoengineered superhydrophobic and omniphobic surfaces are demonstrated.
ContributorsMohan, Ajay Roopesh (Author) / Rykaczewski, Konrad (Thesis advisor) / Herrmann, Marcus (Committee member) / Wang, Robert (Committee member) / Arizona State University (Publisher)
Created2015
Description
In nature, it is commonly observed that animals and birds perform movement-based thermoregulation activities to regulate their body temperatures. For example, flapping of elephant ears or plumage fluffing in birds. Taking inspiration from nature and to explore the possibilities of such heat transfer enhancements, augmentation of heat transfer rates induced by the vibration of solid and well as novel flexible pinned heatsinks were studied in this research project. Enhancement of natural convection has always been very important in improving the performance of the cooling mechanisms. In this research, flexible heatsinks were developed and they were characterized based on natural convection cooling with moderately vibrating conditions. The vibration of heated surfaces such as motor surfaces, condenser surfaces, robotic arms and exoskeletons led to the motivation of the development of heat sinks having flexible fins with an improved heat transfer capacity. The performance of an inflexible, solid copper pin fin heat sink was considered as the baseline, current industry standard for the thermal performance. It is expected to obtain maximum convective heat transfer at the resonance frequency of the flexible pin fins. Current experimental results with fixed input frequency and varying amplitudes indicate that the vibration provides a moderate improvement in convective heat transfer, however, the flexibility of fins had negligible effects.
ContributorsPrabhu, Saurabh (Author) / Rykaczewski, Konrad (Thesis advisor) / Phelan, Patrick (Committee member) / Wang, Robert (Committee member) / Arizona State University (Publisher)
Created2019
Description
Droplet-structure interactions play a pivotal role in many engineering applications as droplet-based solutions are evolving. This work explores the physical understanding of these interactions through systematic research leading to improvements in thermal management via dropwise condensation (DWC), and breathable protective wearables against chemical aerosols for better thermoregulation.
In DWC, the heat transfer rate can be further increased by increasing the nucleation and by optimally ‘refreshing’ the surface via droplet shedding. Softening of surfaces favor the former while having an adverse effect on the latter. This optimization problem is addressed by investigating how mechanical properties of a substrate impact relevant droplet-surface interactions and DWC heat transfer rate. The results obtained by combining droplet induced surface deformation with finite element model show that softening of the substrates below a shear modulus of 500 kPa results in a significant reduction in the condensation heat transfer rate.
On the other hand, interactions between droplet and polymer leading to polymer swelling can be used to develop breathable wearables for use in chemically harsh environments. Chemical aerosols are hazardous and conventional protective measures include impermeable barriers which limit the thermoregulation. To solve this, a solution is proposed consisting of a superabsorbent polymer developed to selectively absorb these chemicals and closing the pores in the fabric. Starting from understanding and modeling the droplet induced swelling in elastomers, the extent and topological characteristic of swelling is shown to depend on the relative comparison of the polymer and aerosol geometries. Then, this modeling is extended to a customized polymer, through a simplified characterization paradigm. In that, a new method is proposed to measure the swelling parameters of the polymer-solvent pair and develop a validated model for swelling. Through this study, it is shown that for this polymer, the concentration-dependent diffusion coefficient can be measured through gravimetry and Poroelastic Relaxation Indentation, simplifying the characterization effort. Finally, this model is used to design composite fabric. Specifically, using model results, the SAP geometry, base fabric design, method of composition is optimized, and the effectiveness of the composite fabric highlighted in moderate-to-high concentrations over short durations.
In DWC, the heat transfer rate can be further increased by increasing the nucleation and by optimally ‘refreshing’ the surface via droplet shedding. Softening of surfaces favor the former while having an adverse effect on the latter. This optimization problem is addressed by investigating how mechanical properties of a substrate impact relevant droplet-surface interactions and DWC heat transfer rate. The results obtained by combining droplet induced surface deformation with finite element model show that softening of the substrates below a shear modulus of 500 kPa results in a significant reduction in the condensation heat transfer rate.
On the other hand, interactions between droplet and polymer leading to polymer swelling can be used to develop breathable wearables for use in chemically harsh environments. Chemical aerosols are hazardous and conventional protective measures include impermeable barriers which limit the thermoregulation. To solve this, a solution is proposed consisting of a superabsorbent polymer developed to selectively absorb these chemicals and closing the pores in the fabric. Starting from understanding and modeling the droplet induced swelling in elastomers, the extent and topological characteristic of swelling is shown to depend on the relative comparison of the polymer and aerosol geometries. Then, this modeling is extended to a customized polymer, through a simplified characterization paradigm. In that, a new method is proposed to measure the swelling parameters of the polymer-solvent pair and develop a validated model for swelling. Through this study, it is shown that for this polymer, the concentration-dependent diffusion coefficient can be measured through gravimetry and Poroelastic Relaxation Indentation, simplifying the characterization effort. Finally, this model is used to design composite fabric. Specifically, using model results, the SAP geometry, base fabric design, method of composition is optimized, and the effectiveness of the composite fabric highlighted in moderate-to-high concentrations over short durations.
ContributorsPhadnis, Akshay (Author) / Rykaczewski, Konrad (Thesis advisor) / Wang, Robert (Committee member) / Wang, Liping (Committee member) / Oswald, Jay (Committee member) / Burgin, Timothy (Committee member) / Arizona State University (Publisher)
Created2019
Description
Deformable heat exchangers could provide a multitude of previously untapped advantages ranging from adaptable performance via macroscale, dynamic shape change (akin to dilation/constriction seen in blood vessels) to enhanced heat transfer at thermal interfaces through microscale, surface deformations. So far, making deformable, ‘soft heat exchangers’ (SHXs) has been limited by the low thermal conductivity of materials with suitable mechanical properties. The recent introduction of liquid-metal embedded elastomers by Bartlett et al1 has addressed this need. Specifically, by remaining soft and stretchable despite the addition of filler, these thermally conductive composites provide an ideal material for the new class of “soft thermal systems”, which is introduced in this work. Understanding such thermal systems will be a key element in enabling technology that require high levels of stretchability, such as thermoregulatory garments, soft electronics, wearable electronics, and high-powered robotics. Shape change inherent to SHX operation has the potential to violate many conventional assumptions used in HX design and thus requires the development of new theoretical approaches to predict performance. To create a basis for understanding these devices, this work highlights two sequential studies. First, the effects of transitioning to a surface deformable, SHX under steady state static conditions in the setting of a liquid cooling device for thermoregulation, electronics and robotics applications was explored. In this study, a thermomechanical model was built and validated to predict the thermal performance and a system wide analysis to optimize such devices was carried out. Second, from a more fundamental perspective, the effects of SHXs undergoing transient shape deformation during operation was explored. A phase shift phenomenon in cooling performance dependent on stretch rate, stretch extent and thermal diffusivity was discovered and explained. With the use of a time scale analysis, the extent of quasi-static assumption viability in modeling such systems was quantified and multiple shape modulation regime limits were defined. Finally, nuance considerations and future work of using liquid metal-silicone composites in SHXs were discussed.
ContributorsKotagama, Praveen (Author) / Rykaczewski, Konrad (Thesis advisor) / Wang, Robert (Committee member) / Phelan, Patrick (Committee member) / Herrmann, Marcus (Committee member) / Green, Matthew (Committee member) / Arizona State University (Publisher)
Created2020
Description
As experiencing hot months and thermal stresses is becoming more common, chemically protective fabrics must adapt and provide protections while reducing the heat stress to the body. These concerns affect first responders, warfighters, and workers regularly surrounded by hazardous chemical agents. While adapting traditional garments with cooling devices provides one route to mitigate this issue, these cooling methods add bulk, are time limited, and may not be applicable in locations without logistical support. Here I take inspiration from nature to guide the development of smart fabrics that have high breathability, but self-seal on exposure to target chemical(s), providing a better balance between cooling and protection.
Natural barrier materials were explored as a guide, focusing specifically on prickly pear cacti. These cacti have a natural waxy barrier that provides protection from dehydration and physically changes shape to modify surface wettability and water vapor transport. The results of this study provided a basis for a shape changing polymer to be used to respond directly to hazardous chemicals, swelling to contain the agent.
To create a stimuli responsive material, a novel superabsorbent polymer was synthesized, based on acrylamide chemistry. The polymer was tested for swelling properties in a wide range of organic liquids and found to highly swell in moderately polar organic liquids. To help predict swelling in untested liquids, the swelling of multiple test liquids were compared with their thermodynamic properties to observe trends. As the smart fabric needs to remain breathable to allow evaporative cooling, while retaining functionality when soaked with sweat, absorption of water, as well as that of an absorbing liquid in the presence of water were tested.
Micron sized particles of the developed polymer were deposited on a plastic mesh with pore size and open area similar to common clothing fabric to establish the proof of concept of using a breathable barrier to provide chemical protection. The polymer coated mesh showed minimal additional resistance to water vapor transport, relative to the mesh alone, but blocked more than 99% of a xylene aerosol from penetrating the barrier.
Natural barrier materials were explored as a guide, focusing specifically on prickly pear cacti. These cacti have a natural waxy barrier that provides protection from dehydration and physically changes shape to modify surface wettability and water vapor transport. The results of this study provided a basis for a shape changing polymer to be used to respond directly to hazardous chemicals, swelling to contain the agent.
To create a stimuli responsive material, a novel superabsorbent polymer was synthesized, based on acrylamide chemistry. The polymer was tested for swelling properties in a wide range of organic liquids and found to highly swell in moderately polar organic liquids. To help predict swelling in untested liquids, the swelling of multiple test liquids were compared with their thermodynamic properties to observe trends. As the smart fabric needs to remain breathable to allow evaporative cooling, while retaining functionality when soaked with sweat, absorption of water, as well as that of an absorbing liquid in the presence of water were tested.
Micron sized particles of the developed polymer were deposited on a plastic mesh with pore size and open area similar to common clothing fabric to establish the proof of concept of using a breathable barrier to provide chemical protection. The polymer coated mesh showed minimal additional resistance to water vapor transport, relative to the mesh alone, but blocked more than 99% of a xylene aerosol from penetrating the barrier.
ContributorsManning, Kenneth (Author) / Rykaczewski, Konrad (Thesis advisor) / Burgin, Timothy (Committee member) / Emady, Heather (Committee member) / Green, Matthew (Committee member) / Thomas, Marylaura (Committee member) / Arizona State University (Publisher)
Created2020
Description
Buildings release an abundance of waste heat that is left unused. Thermogalvaniccells (TGCs) can take advantage of waste heat to generate electricity with a low
temperature gradient. In this dissertation, I simulated the thermal transport of TGCs
containing different triply periodic minimal surface (TPMS) structures, compared it
to measured values and conducted a mesh convergence study to examine the viability
of the computational fluid dynamics (CFD) solutions. Natural convection effects are
one of the driving forces in TGCs. Using the Bousinesq approximation, I was able to
capture those effects in the CFD simulations as it accounts for the density variations
of the fluid. Upon simulating the TGC using the Schwarz P TPMS geometry, the
cathode temperature converged as I refined the mesh and approached the measured
value. As for the IWP TPMS structure, the solution converged as I refined the mesh,
despite having a deviation to the measured values. This was due to the abundance of
sharp regions along the walls of the TPMS that ANSYS had difficulty to accurately
model.
Furthermore, I simulated the TGCs using different boundary condition (BC) approximations to observe the cathode and anode temperatures as well as their overall
∆T across the cell. For the TGC containing the Schwarz P geometry, Case C (constant anode temperature BC with TPMS conduction) was the most accurate while
Case D (convection BC at anode with TPMS conduction) deviated from the measured
values, had the most accurate ∆T and was well within the uncertainty bounds of the
measured values. Larger temperature fluctuations were seen closer to the cathode
while the effects steadily decrease as the fluid approaches the anode.
Moreover, the TGC containing the IWP structures presented interesting results.
The main deviation was from the cathode temperatures because a higher temperature
readings meant that more cells in the fluid domain were prone to diverging, thereby
resulting in a higher calculated cathode temperature. Simulating the TGC with the
Schwarz P geometry produced satisfactory results while the TGC using the IWP
geometry deviated due to the software limitations. Finally, the effects of natural
convection and TPMS on TGCs were studied and it was found that the absence of
natural convection lead to a higher ∆T while the absence of TPMS resulted in a more
uniform temperature distribution throughout the domain
Contributorsalweqayyan, yousef (Author) / Phelan, Patrick (Thesis advisor) / Rykaczewski, Konrad (Committee member) / Milcarek, Ryan (Committee member) / Arizona State University (Publisher)
Created2021