Matching Items (35)
Filtering by
- All Subjects: Mechanical Engineering
- Genre: Doctoral Dissertation
- Creators: Solanki, Kiran
- Creators: Wang, Robert
Description
Recently, the use of zinc oxide (ZnO) nanowires as an interphase in composite materials has been demonstrated to increase the interfacial shear strength between carbon fiber and an epoxy matrix. In this research work, the strong adhesion between ZnO and carbon fiber is investigated to elucidate the interactions at the interface that result in high interfacial strength. First, molecular dynamics (MD) simulations are performed to calculate the adhesive energy between bare carbon and ZnO. Since the carbon fiber surface has oxygen functional groups, these were modeled and MD simulations showed the preference of ketones to strongly interact with ZnO, however, this was not observed in the case of hydroxyls and carboxylic acid. It was also found that the ketone molecules ability to change orientation facilitated the interactions with the ZnO surface. Experimentally, the atomic force microscope (AFM) was used to measure the adhesive energy between ZnO and carbon through a liftoff test by employing highly oriented pyrolytic graphite (HOPG) substrate and a ZnO covered AFM tip. Oxygen functionalization of the HOPG surface shows the increase of adhesive energy. Additionally, the surface of ZnO was modified to hold a negative charge, which demonstrated an increase in the adhesive energy. This increase in adhesion resulted from increased induction forces given the relatively high polarizability of HOPG and the preservation of the charge on ZnO surface. It was found that the additional negative charge can be preserved on the ZnO surface because there is an energy barrier since carbon and ZnO form a Schottky contact. Other materials with the same ionic properties of ZnO but with higher polarizability also demonstrated good adhesion to carbon. This result substantiates that their induced interaction can be facilitated not only by the polarizability of carbon but by any of the materials at the interface. The versatility to modify the magnitude of the induced interaction between carbon and an ionic material provides a new route to create interfaces with controlled interfacial strength.
ContributorsGalan Vera, Magdian Ulises (Author) / Sodano, Henry A (Thesis advisor) / Jiang, Hanqing (Committee member) / Solanki, Kiran (Committee member) / Oswald, Jay (Committee member) / Speyer, Gil (Committee member) / Arizona State University (Publisher)
Created2013
Description
Locomotion of microorganisms is commonly observed in nature and some aspects of their motion can be replicated by synthetic motors. Synthetic motors rely on a variety of propulsion mechanisms including auto-diffusiophoresis, auto-electrophoresis, and bubble generation. Regardless of the source of the locomotion, the motion of any motor can be characterized by the translational and rotational velocity and effective diffusivity. In a uniform environment the long-time motion of a motor can be fully characterized by the effective diffusivity. In this work it is shown that when motors possess both translational and rotational velocity the motor transitions from a short-time diffusivity to a long-time diffusivity at a time of pi/w. The short-time diffusivities are two to three orders of magnitude larger than the diffusivity of a Brownian sphere of the same size, increase linearly with concentration, and scale as v^2/2w. The measured long-time diffusivities are five times lower than the short-time diffusivities, scale as v^2/{2Dr [1 + (w/Dr )^2]}, and exhibit a maximum as a function of concentration. The variation of a colloid's velocity and effective diffusivity to its local environment (e.g. fuel concentration) suggests that the motors can accumulate in a bounded system, analogous to biological chemokinesis. Chemokinesis of organisms is the non-uniform equilibrium concentration that arises from a bounded random walk of swimming organisms in a chemical concentration gradient. In non-swimming organisms we term this response diffusiokinesis. We show that particles that migrate only by Brownian thermal motion are capable of achieving non-uniform pseudo equilibrium distribution in a diffusivity gradient. The concentration is a result of a bounded random-walk process where at any given time a larger percentage of particles can be found in the regions of low diffusivity than in regions of high diffusivity. Individual particles are not trapped in any given region but at equilibrium the net flux between regions is zero. For Brownian particles the gradient in diffusivity is achieved by creating a viscosity gradient in a microfluidic device. The distribution of the particles is described by the Fokker-Planck equation for variable diffusivity. The strength of the probe concentration gradient is proportional to the strength of the diffusivity gradient and inversely proportional to the mean probe diffusivity in the channel in accordance with the no flux condition at steady state. This suggests that Brownian colloids, natural or synthetic, will concentrate in a bounded system in response to a gradient in diffusivity and that the magnitude of the response is proportional to the magnitude of the gradient in diffusivity divided by the mean diffusivity in the channel.
ContributorsMarine, Nathan Arasmus (Author) / Posner, Jonathan D (Thesis advisor) / Adrian, Ronald J (Committee member) / Frakes, David (Committee member) / Phelan, Patrick E (Committee member) / Santos, Veronica J (Committee member) / Arizona State University (Publisher)
Created2013
Description
With increasing concerns of the intrinsic toxicity of lead (Pb) in electronics, a series of tin (Sn) based alloys involving silver (Ag) and copper (Cu) have been proposed as replacements for Pb-Sn solder and widely accepted by industry. However, they have a higher melting point and often exhibit poorer damage tolerance than Pb-Sn alloys. Recently, a new class of alloys with trace amount of rare-earth (RE) elements has been discovered and investigated. In previous work from Prof. Chawla's group, it has been shown that cerium (Ce)-based Pb-free solder are less prone to oxidation and Sn whiskering, and exhibit desirable attributes of microstructural refinement and enhanced ductility relative to lanthanum (La)-based Sn-3.9Ag-0.7Cu (SAC) alloy. Although the formation of RESn3 was believed to be directly responsible for the enhanced ductility in RE-containing SAC solder by allowing microscopic voids to nucleate throughout the solder volume, this cavitation-based mechanism needs to be validated experimentally and numerically. Additionally, since the previous study has exhibited the realistic feasibility of Ce-based SAC lead-free solder alloy as a replacement to conventional SAC alloys, in this study, the proposed objective focuses on the in in-depth understanding of mechanism of enhanced ductility in Ce-based SAC alloy and possible issues associated with integration of this new class of solder into electronic industry, including: (a) study of long-term thermal and mechanical stability on industrial metallization, (b) examine the role of solder volume and wetting behavior of the new solder, relative to Sn-3.9Ag-0.7Cu alloys, (c) conduct experiments of new solder alloys in the form of mechanical shock and electromigration. The research of this new class alloys will be conducted in industrially relevant conditions, and the results would serve as the first step toward integration of these new, next generation solders into the industry.
ContributorsXie, Huxiao (Author) / Chawla, Nikhilesh (Thesis advisor) / Krause, Stephen (Committee member) / Solanki, Kiran (Committee member) / Mirpuri, Kabir (Committee member) / Arizona State University (Publisher)
Created2012
Description
Structural features of canonical wall-bounded turbulent flows are described using several techniques, including proper orthogonal decomposition (POD). The canonical wall-bounded turbulent flows of channels, pipes, and flat-plate boundary layers include physics important to a wide variety of practical fluid flows with a minimum of geometric complications. Yet, significant questions remain for their turbulent motions' form, organization to compose very long motions, and relationship to vortical structures. POD extracts highly energetic structures from flow fields and is one tool to further understand the turbulence physics. A variety of direct numerical simulations provide velocity fields suitable for detailed analysis. Since POD modes require significant interpretation, this study begins with wall-normal, one-dimensional POD for a set of turbulent channel flows. Important features of the modes and their scaling are interpreted in light of flow physics, also leading to a method of synthesizing one-dimensional POD modes. Properties of a pipe flow simulation are then studied via several methods. The presence of very long streamwise motions is assessed using a number of statistical quantities, including energy spectra, which are compared to experiments. Further properties of energy spectra, including their relation to fictitious forces associated with mean Reynolds stress, are considered in depth. After reviewing salient features of turbulent structures previously observed in relevant experiments, structures in the pipe flow are examined in greater detail. A variety of methods reveal organization patterns of structures in instantaneous fields and their associated vortical structures. Properties of POD modes for a boundary layer flow are considered. Finally, very wide modes that occur when computing POD modes in all three canonical flows are compared. The results demonstrate that POD extracts structures relevant to characterizing wall-bounded turbulent flows. However, significant care is necessary in interpreting POD results, for which modes can be categorized according to their self-similarity. Additional analysis techniques reveal the organization of smaller motions in characteristic patterns to compose very long motions in pipe flows. The very large scale motions are observed to contribute large fractions of turbulent kinetic energy and Reynolds stress. The associated vortical structures possess characteristics of hairpins, but are commonly distorted from pristine hairpin geometries.
ContributorsBaltzer, Jon Ronald (Author) / Adrian, Ronald J (Thesis advisor) / Calhoun, Ronald (Committee member) / Gelb, Anne (Committee member) / Herrmann, Marcus (Committee member) / Squires, Kyle D (Committee member) / Arizona State University (Publisher)
Created2012
Description
An approach for modeling resistance spot welding of thin-gauge, dissimilar metal sheets with high electrical conductivity is presented in this work. In this scenario, the electrical and thermal contact resistances play a dominant role in heat generation and temperature evolution within the workpieces; these interactions ultimately control the weld geometry. Existing models are limited in modeling these interactions, especially for dissimilar and thin-gauge metal sheets, and at higher temperatures when the multiphysics becomes increasingly interdependent. The approach presented here uses resistivity measurements, combined with thermal modeling and known bulk resistance relationships to infer the relationship between electrical contact resistance and temperature for each of the different material interfaces in the welding process. Corresponding thermal contact resistance models are developed using the Wiedemann-Franz law combined with a scaling factor to account for nonmetallic behavior. Experimental and simulation voltage histories and final weld diameter were used to validate this model for a Cu/Al/Cu and a Cu/Al/Cu/Al/Cu stack-ups. This model was then used to study the effect of Ni-P coating on resistance spot welding of Cu and Al sheets in terms of weld formation, mechanical deformation, and contact resistance. Contact resistance and current density distribution are highly dependent on contact pressure and temperature distribution at the Cu/Al interface in the presence of alumina. The Ni-P coating helps evolve a partially-bonded donut shaped weld into a fully-bonded hourglass-shaped weld by decreasing the dependence of contact resistance and current density distribution on contact pressure and temperature distribution at the Cu/Al interface. This work also provides an approach to minimize distortion due to offset-rolling in thin aluminum sheets by optimizing the stiffening feature geometry. The distortion is minimized using particle swarm optimization. The objective function is a function of distortion and smallest radius of curvature in the geometry. Doubling the minimum allowable radius of curvature nearly doubles the reduction in distortion from the stadium shape for a quarter model. Reduction in distortion in the quarter model extends to the full-scale model with the best design performing 5.3% and 27% better than the corresponding nominal design for a quarter and full-scale model, respectively.
ContributorsVeeresh, Pawan (Author) / Oswald, Jay (Thesis advisor) / Carlson, Blair (Committee member) / Hoover, Christian (Committee member) / Rajagopalan, Jagannathan (Committee member) / Solanki, Kiran (Committee member) / Arizona State University (Publisher)
Created2022
Description
Energy storage technologies are essential to overcome the temporal variability in renewable energy. The primary aim of this thesis is to develop reactor solutions to better analyze the potential of thermochemical energy storage (TCES) using non-stoichiometric metal oxides, for the multi-day energy storage application. A TCES system consists of a reduction reactor and an insulated MOx storage bin. The reduction reactor heats (to ~ 1100 °C) and partially reduces the MOx, thereby adding sensible and chemical energy (i.e., charging it) under reduced pO2 environments (~10 Pa). Inert gas removes the oxygen generated during reduction. The storage bin holds the hot and partially reduced MOx (typically particles) until it is used in an energy recovery device (i.e., discharge). Irrespective of the reactor heat source (here electrical), or the particle-inert gas flows (here countercurrent), the thermal reduction temperature and inert gas (here N2) flow minimize when the process approaches reversibility, i.e., operates near equilibrium. This study specifically focuses on developing a reduction reactor based on the theoretical considerations for approaching reversibility along the reaction path. The proposed Zigzag flow reactor (ZFR) is capable of thermally reducing CAM28 particles at temperatures ~ 1000 °C under an O2 partial pressure ~ 10 Pa. The associated analytical and numerical models analyze the reaction equilibrium under a real (discrete) reaction path and the mass transfer kinetic conditions necessary to approach equilibrium. The discrete equilibrium model minimizes the exergy destroyed in a practical reactor and identifies methods of maximizing the energy storage density () and the exergetic efficiency. The mass transfer model analyzes the O2 N2 concentration boundary layers to recommend sizing considerations to maximize the reactor power density. Two functional ZFR prototypes, the -ZFR and the -ZFR, establish the proof of concept and achieved a reduction extent, Δδ = 0.071 with CAM28 at T~950 °C and pO2 = 10 Pa, 7x higher than a previous attempt in the literature. The -ZFR consistently achieved > 100 Wh/kg during >10 h. runtime and the -ZFR displayed an improved = 130 Wh/kg during >5 h. operation with CAM28. A techno-economic model of a grid-scale ZFR with an associated storage bin analyzes the cost of scaling the ZFR for grid energy storage requirements. The scaled ZFR capital costs contribute < 1% to the levelized cost of thermochemical energy storage, which ranges from 5-20 ¢/kWh depending on the storage temperature and storage duration.
ContributorsGhotkar, Rhushikesh (Author) / Milcarek, Ryan (Thesis advisor) / Ermanoski, Ivan (Committee member) / Phelan, Patrick (Committee member) / Wang, Liping (Committee member) / Wang, Robert (Committee member) / Arizona State University (Publisher)
Created2023
Description
The conversion of H2S enables the recycling of a waste gas into a potential source of hydrogen at a lower thermodynamic energy cost as compared to water splitting. However, studies on the photocatalytic decomposition of H2S focus on traditional deployment of catalyst materials to facilitate this conversion, and operation only when a light source is available. In this study, the efficacy of Direct Ink Written (DIW) luminous structures for H2S conversion has been investigated, with the primary objective of sustaining H2S conversion when a light source has been terminated. Additionally, as a secondary objective, improving light distribution within monoliths for photocatalytic applications is desired. The intrinsic illumination of the 3D printed monoliths developed in this work could serve as an alternative to monolith systems that employ light transmitting fiber optic cables that have been previously proposed to improve light distribution in photocatalytic systems. The results that were obtained demonstrate that H2S favorable adsorbents, a wavelength compatible long afterglow phosphor, and a photocatalyst can form viscoelastic inks that are printable into DIW luminous monolithic contactors. Additionally, rheological, optical and porosity analyses conducted, provide design guidelines for future studies seeking to develop DIW luminous monoliths from compatible catalyst-phosphor pairs. The monoliths that were developed demonstrate not only improved conversion when exposed to light, but more significantly, extended H2S conversion from the afterglow of the monoliths when an external light source was removed. Lastly, considering growing interests in attaining a global circular economy, the techno-economic feasibility of a H2S-CO2 co-utilization plant leveraging hydrogen from H2S photocatalysis as a feed source for a downstream CO2 methanation plant has been assessed. The work provides preliminary information to guide future chemical kinetic design characteristics that are important to strive for if using H2S as a source of hydrogen in a CO2 methanation facility.
ContributorsAbdullahi, Adnan (Author) / Andino, Jean (Thesis advisor) / Phelan, Patrick (Thesis advisor) / Bhate, Dhruv (Committee member) / Wang, Robert (Committee member) / Huang, Huei-Ping (Committee member) / Arizona State University (Publisher)
Created2023
Description
Thermal management of electronics is critical to meet the increasing demand for high power and performance. Thermal interface materials (TIMs) play a key role in dissipating heat away from the microelectronic chip and hence are a crucial component in electronics cooling. Challenges persist with overcoming the interfacial boundary resistance and filler particle connectivity in TIMs to achieve thermal percolation while maintaining mechanical compliance. Gallium-based liquid metal (LM) capsules offer a unique set of thermal-mechanical characteristics that make them suitable candidates for high-performance TIM fillers. This dissertation research focuses on resolving the fundamental challenges posed by integration of LM fillers in polymer matrix. First, the rupture mechanics of LM capsules under pressure is identified as a key factor that dictates the thermal connectivity between LM-based fillers. This mechanism of oxide “popping” in LM particle beds independent of the matrix material provides insights in overcoming the particle-particle connectivity challenges. Second, the physical barrier introduced due to the polymer matrix needs to be overcome to achieve thermal percolation. Matrix fluid viscosity impacts thermal transport, with high viscosity uncured matrix inhibiting the thermal bridging of fillers. In addition, incorporation of solid metal co-fillers that react with LM fillers is adopted to facilitate popping of LM oxide in uncured polymer to overcome this matrix barrier. Solid silver metal additives are used to rupture the LM oxide, form inter-metallic alloy (IMC), and act as thermal anchors within the matrix. This results in the formation of numerous thermal percolation paths and hence enhances heat transport within the composite. Further, preserving this microstructure of interconnected multiphase filler system with thermally conductive percolation pathways in a cured polymer matrix is critical to designing high-performing TIM pads. Viscosity of the precursor polymer solution prior to curing plays a major role in the resulting thermal conductivity. A multipronged strategy is developed that synergistically combines reactive solid and liquid fillers, a polymer matrix with low pre-cure viscosity, and mechanical compression during thermal curing. The results of this dissertation aim to provide fundamental insights into the integration of LMs in polymer composites and give design knobs to develop high thermally conducting soft composites.
ContributorsUppal, Aastha (Author) / Rykaczewski, Konrad (Thesis advisor) / Wang, Robert (Thesis advisor) / Kwon, Beomjin (Committee member) / Choksi, Gaurang (Committee member) / Phelan, Patrick (Committee member) / Arizona State University (Publisher)
Created2022
Description
The increasing demand for structural materials with superior mechanical properties has provided a strong impetus to the discovery of novel materials, and innovations in processing techniques to improve the properties of existing materials. Methods like severe plastic deformation (SPD) and surface mechanical attrition treatment (SMAT) have led to significant enhancement in the strength of traditional structural materials like Al and Fe based alloys via microstructural refinement. However, the nanocrystalline materials produced using these techniques exhibit poor ductility due to the lack of effective strain hardening mechanisms, and as a result the well-known strength-ductility trade-off persists. To overcome this trade-off, researchers have proposed the concept of heterostructured materials, which are composed of domains ranging in size from a few nanometers to several micrometers. Over the last two decades, there has been intense research on the development of new methods to synthesize heterostructured materials. However, none of these methods is capable of providing precise control over key microstructural parameters such as average grain size, grain morphology, and volume fraction and connectivity of coarse and fine grains. Due to the lack of microstructural control, the relationship between these parameters and the deformation behavior of heterostructured materials cannot be investigated systematically, and hence designing heterostructured materials with optimized properties is currently infeasible. This work aims to address this scientific and technological challenge and is composed of two distinct but interrelated parts. The first part concerns the development of a broadly applicable synthesis method to produce heterostructured metallic films with precisely defined architectures. This method exploits two forms of film growth (epitaxial and Volmer-Weber) to generate heterostructured metallic films. The second part investigates the effect of different microstructural parameters on the deformation behavior of heterostructured metallic films with the aim of elucidating their structure-property relationships. Towards this end, freestanding heterostructured Fe films with different architectures were fabricated and uniaxially deformed using MEMS stages. The results from these experiments are presented and their implications for the mechanical properties of heterostructured materials is discussed.
ContributorsBerlia, Rohit (Author) / Rajagopalan, Jagannathan (Thesis advisor) / Sieradzki, Karl (Committee member) / Peralta, Pedro (Committee member) / Crozier, Peter (Committee member) / Solanki, Kiran (Committee member) / Arizona State University (Publisher)
Created2021
Description
Progressive miniaturization in electronics demands advanced materials with excellent energy conversion and transport properties. Opportunities exist in novel material morphologies such as hierarchical structures, multi-functional composites and nanoscale architectures which may offer mechanical, thermal and electronic properties tailored to a wide range of applications (e.g., aerospace, robotics, biomedical etc.). However, the manufacturing capabilities have always posed a grand challenge in realizing the advanced material morphologies. Furthermore, the multi-scale modeling of complex material architectures has been extremely challenging owing to the limitations in computation methodologies and lack of understanding in nano-/micro-meter scale physics. To address these challenges, this work considers the morphology effect on carbon nanotube (CNT)-based composites, CNT fibers and thermoelectric (TE) materials. First, this work reports additively manufacturable TE morphologies and analyzes the thermo-electric transport behavior. This research introduces innovative honeycomb TE architectures that showed ~26% efficiency increase and ~25% density reduction compared to conventional rectangular TE architectures. Moreover, this work presents 3D printable compositionally segmented TE architecture which provides record-high efficiencies (up to 8.7%) over wide temperature ranges if the composition and aspect ratio of multiple TE materials are optimized within a single TE device. Next, this research proposes computationally efficient two-dimensional (2D) finite element model (FEM) to study the electrical and thermal properties in CNT based composites by simultaneously considering the stochastic CNT distributions, CNT fractions (upto 80%) and interfacial resistances. The FEM allows to estimate the theoretical maximum possible conductivities with corresponding interfacial resistances if the CNT morphologies are carefully controlled, along with appreciable insight into the energy transport physics. Then, this work proposes a data-driven surrogate model based on convolutional neural networks to rapidly approximate the composite conductivities in a second with accuracy > 98%, compared to FEM taking >100 minutes per simulation. Finally, this research presents a pseudo 2D FEM to approximate the electrical and thermal properties in CNT fibers at various CNT aspect ratios (up to 10,000) by simultaneously considering CNT-CNT interfacial effects along with the stochastic distribution of inter-bundle voids.
ContributorsEjaz, Faizan (Author) / Kwon, Beomjin (Thesis advisor) / Zhuang, Houlong (Committee member) / Song, Kenan (Committee member) / Wang, Robert (Committee member) / Kang, Wonmo (Committee member) / Arizona State University (Publisher)
Created2023