Matching Items (51)
Filtering by
- All Subjects: Mechanical Engineering
- All Subjects: Materials Science
- Creators: Wang, Robert
Description
In this research, our goal was to fabricate Josephson junctions that can be stably processed at 300°C or higher. With the purpose of integrating Josephson junction fabrication with the current semiconductor circuit fabrication process, back-end process temperatures (>350 °C) will be a key for producing large scale junction circuits reliably, which requires the junctions to be more thermally stable than current Nb/Al-AlOx/Nb junctions. Based on thermodynamics, Hf was chosen to produce thermally stable Nb/Hf-HfOx/Nb superconductor tunnel Josephson junctions that can be grown or processed at elevated temperatures. Also elevated synthesis temperatures improve the structural and electrical properties of Nb electrode layers that could potentially improve junction device performance. The refractory nature of Hf, HfO2 and Nb allow for the formation of flat, abrupt and thermally-stable interfaces. But the current Al-based barrier will have problems when using with high-temperature grown and high-quality Nb. So our work is aimed at using Nb grown at elevated temperatures to fabricate thermally stable Josephson tunnel junctions. As a junction barrier metal, Hf was studied and compared with the traditional Al-barrier material. We have proved that Hf-HfOx is a good barrier candidate for high-temperature synthesized Josephson junction. Hf deposited at 500 °C on Nb forms flat and chemically abrupt interfaces. Nb/Hf-HfOx/Nb Josephson junctions were synthesized, fabricated and characterized with different oxidizing conditions. The results of materials characterization and junction electrical measurements are reported and analyzed. We have improved the annealing stability of Nb junctions and also used high-quality Nb grown at 500 °C as the bottom electrode successfully. Adding a buffer layer or multiple oxidation steps improves the annealing stability of Josephson junctions. We also have attempted to use the Atomic Layer Deposition (ALD) method for the growth of Hf oxide as the junction barrier and got tunneling results.
ContributorsHuang, Mengchu, 1987- (Author) / Newman, Nathan (Thesis advisor) / Rowell, John M. (Committee member) / Singh, Rakesh K. (Committee member) / Chamberlin, Ralph (Committee member) / Wang, Robert (Committee member) / Arizona State University (Publisher)
Created2013
Description
The study of high energy particle irradiation effect on Josephson junction tri-layers is relevant to applications in space and radioactive environments. It also allows us to investigate the influence of defects and interfacial intermixing on the junction electrical characteristics. In this work, we studied the influence of 2MeV Helium ion irradiation with doses up to 5.2×1016 ions/cm2 on the tunneling behavior of Nb/Al/AlOx/Nb Josephson junctions. Structural and analytical TEM characterization, combined with SRIM modeling, indicates that over 4nm of intermixing occurred at the interfaces. EDX analysis after irradiation, suggests that the Al and O compositions from the barrier are collectively distributed together over a few nanometers. Surprisingly, the IV characteristics were largely unchanged. The normal resistance, Rn, increased slightly (<20%) after the initial dose of 3.5×1015 ions/cm2 and remained constant after that. This suggests that tunnel barrier electrical properties were not affected much, despite the significant changes in the chemical distribution of the barrier's Al and O shown in SRIM modeling and TEM pictures. The onset of quasi-particle current, sum of energy gaps (2Δ), dropped systematically from 2.8meV to 2.6meV with increasing dosage. Similarly, the temperature onset of the Josephson current dropped from 9.2K to 9.0K. This suggests that the order parameter at the barrier interface has decreased as a result of a reduced mean free path in the Al proximity layer and a reduction in the transition temperature of the Nb electrode near the barrier. The dependence of Josephson current on the magnetic field and temperature does not change significantly with irradiation, suggesting that intermixing into the Nb electrode is significantly less than the penetration depth.
ContributorsZhang, Tiantian (Author) / Newman, Nathan (Thesis advisor) / Rowell, John M (Committee member) / Singh, Rakesh K. (Committee member) / Chamberlin, Ralph (Committee member) / Wang, Robert (Committee member) / Arizona State University (Publisher)
Created2012
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
Solar energy has become one of the most popular renewable energy in human’s life because of its abundance and environment friendliness. To achieve high solar energy conversion efficiency, it usually requires surfaces to absorb selectivity within one spectral range of interest and reflect strongly over the rest of the spectrum. An economic method is always desired to fabricate spectrally selective surfaces with improved energy conversion efficiency. Colloidal lithography is a recently emerged way of nanofabrication, which has advantages of low-cost and easy operation.
In this thesis, aluminum metasurface structures are proposed based on colloidal lithography method. High Frequency Structure Simulator is used to numerically study optical properties and design the aluminum metasurfaces with selective absorption. Simulation results show that proposed aluminum metasurface structure on aluminum oxide thin film and aluminum substrate has a major reflectance dip, whose wavelength is tunable within the near-infrared and visible spectrum with metasurface size. As the metasurface is opaque due to aluminum film, it indicates strong wavelength-selective optical absorption, which is due to the magnetic resonance between the top metasurface and bottom Al film within the aluminum oxide layer.
The proposed sample is fabricated based on colloidal lithography method. Monolayer polystyrene particles of 500 nm are successfully prepared and transferred onto silicon substrate. Scanning electron microscope is used to check the surface topography. Aluminum thin film with 20-nm or 50-nm thickness is then deposited on the sample. After monolayer particles are removed, optical properties of samples are measured by micro-scale optical reflectance and transmittance microscope. Measured and simulated reflectance of these samples do not have frequency selective properties and is not sensitive to defects. The next step is to fabricate the Al metasurface on Al_2 O_3 and Al films to experimentally demonstrate the selective absorption predicted from the numerical simulation.
In this thesis, aluminum metasurface structures are proposed based on colloidal lithography method. High Frequency Structure Simulator is used to numerically study optical properties and design the aluminum metasurfaces with selective absorption. Simulation results show that proposed aluminum metasurface structure on aluminum oxide thin film and aluminum substrate has a major reflectance dip, whose wavelength is tunable within the near-infrared and visible spectrum with metasurface size. As the metasurface is opaque due to aluminum film, it indicates strong wavelength-selective optical absorption, which is due to the magnetic resonance between the top metasurface and bottom Al film within the aluminum oxide layer.
The proposed sample is fabricated based on colloidal lithography method. Monolayer polystyrene particles of 500 nm are successfully prepared and transferred onto silicon substrate. Scanning electron microscope is used to check the surface topography. Aluminum thin film with 20-nm or 50-nm thickness is then deposited on the sample. After monolayer particles are removed, optical properties of samples are measured by micro-scale optical reflectance and transmittance microscope. Measured and simulated reflectance of these samples do not have frequency selective properties and is not sensitive to defects. The next step is to fabricate the Al metasurface on Al_2 O_3 and Al films to experimentally demonstrate the selective absorption predicted from the numerical simulation.
ContributorsGuan, Chuyun (Author) / Wang, Liping (Thesis advisor) / Azeredo, Bruno (Committee member) / Wang, Robert (Committee member) / Arizona State University (Publisher)
Created2019
Description
Laser Powder Bed Fusion (LPBF) is an additive manufacturing (AM) technology that has emerged as the predominant technology for metal 3D printing. An alloy of particular interest to the aerospace industry is the nickel-based superalloy, Inconel 718 (IN718), which is widely used for its superior performance in elevated temperature conditions, particularly for gas-turbine engine blades and heat exchangers. With LPBF providing new ways of exploiting complex part geometry, the high-temperature properties of the AM version of the alloy must be understood. Of additional interest is how these properties change as a function of geometry and post-processing. This research focuses on the behavior of LPBF IN718 as a function of hot isostatic pressing (HIP) and specimen thickness at elevated temperatures. These results and behavior were compared to the behavior of IN718 sheet metal for properties such as True Ultimate Tensile Strength (UTS), Yield Strength, Young’s Modulus, percent elongation, and necking. The results showed dependence of strength on both thickness and HIP condition, and also exhibited a steep drop in UTS and yield strength at 1600 °F, linearly declining modulus, and excess dynamic strain ageing (DSA) behavior at certain temperatures.
ContributorsTemes, Samuel (Author) / Bhate, Dhruv (Thesis advisor) / Azeredo, Bruno (Committee member) / Das, Partha (Committee member) / Arizona State University (Publisher)
Created2021
Description
Additively Manufactured Thin-wall Inconel 718 specimens commonly find application in heat exchangers and Thermal Protection Systems (TPS) for space vehicles. The wall thicknesses in applications for these components typically range between 0.03-2.5mm. Laser Powder Bed Fusion (PBF) Fatigue standards assume thickness over 5mm and consider Hot Isostatic Pressing (HIP) as conventional heat treatment. This study aims at investigating the dependence of High Cycle Fatigue (HCF) behavior on wall thickness and Hot Isostatic Pressing (HIP) for as-built Additively Manufactured Thin Wall Inconel 718 alloys. To address this aim, high cycle fatigue tests were performed on specimens of seven different thicknesses (0.3mm,0.35mm, 0.5mm, 0.75mm, 1mm, 1.5mm, and 2mm) using a Servohydraulic FatigueTesting Machine. Only half of the specimen underwent HIP, creating data for bothHIP and No-HIP specimens. Upon analyzing the collected data, it was noticed that the specimens that underwent HIP had similar fatigue behavior to that of sheet metal specimens. In addition, it was also noticed that the presence of Porosity in No-HIP specimens makes them more sensitive to changes in stress. A clear decrease in fatigue strength with the decrease in thickness was observed for all specimens.
ContributorsSaxena, Anushree (Author) / Bhate, Dhruv (Thesis advisor) / Liu, Yongming (Committee member) / Kwon, Beomjin (Committee member) / Arizona State University (Publisher)
Created2021
Description
Inspired by the design of lightweight cellular structures in nature, humans have made cellular solids for a wide range of engineering applications. Cellular structures composed of solid and gaseous phases, and an interconnected network of solid struts or plates that form the cell's edges and faces. This makes them an ideal candidate for numerous energy absorption applications in the military, transportation, and automotive industries. The objective of the thesis is to study the energy-absorption of multi-material cellular structures. Cellular structures made from Acrylonitrile-Butadiene-Styrene (ABS) a thermoplastic polymer and Thermoplastic Polyurethane (TPU) a thermoplastic elastomer were manufactured using dual extrusion 3D printing. The surface-based structures were designed with partitions to allocate different materials using Matlab and nTopology. Aperiodicity was introduced to the design through perturbation. The specimens were designed for two wall thicknesses - 0.5mm and 1mm, respectively. In total, 18 specimens were designed and 3D printed. All the specimens were tested under quasi-static compression. A detailed analysis was performed to study the energy absorption metrics and draw conclusions, with emphasis on specific energy absorbed as a function of relative density, efficiency, and peak stress of the specimens to hypothesize and validate mechanisms for observed behavior. All the specimens were analyzed to draw comparisons across designs.
ContributorsVarma, Rajeshree Pawan (Author) / Bhate, Dhruv (Thesis advisor) / Shuaib, Abdelrahman (Committee member) / Nian, Qiong (Committee member) / Arizona State University (Publisher)
Created2021
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
Achieving a viable process for advanced manufacturing of ceramics and metal-ceramic composites is a sought-after goal in a wide range of fields including electronics and sensors for harsh environments, microelectromechanical devices, energy storage materials, and structural materials, among others. In this dissertation, the processing, and manufacturing of ceramics and ceramic composites are addressed, specifically, a process for three-dimensional (3D) printing of polymer-derived ceramics (PDC), and a process for low-cost manufacturing as well as healing of metal-ceramic composites is demonstrated.Three-dimensional printing of ceramics is enabled by dispensing the preceramic polymer at the tip of a moving nozzle into a gel that can reversibly switch between fluid and solid states, and subsequently thermally cross-linking the entire printed part “at once” while still inside the same gel was demonstrated. The solid gel converts to fluid at the tip of the moving nozzle, allowing the polymer solution to be dispensed and quickly returns to a solid state to maintain the geometry of the printed polymer both during printing and the subsequent high-temperature (160 °C) cross-linking. After retrieving the cross-linked part from the gel, the green body is converted to ceramic by high-temperature pyrolysis. This scalable process opens new opportunities for low-cost and high-speed production of complex three-dimensional ceramic parts and will be widely used for high-temperature and corrosive environment applications, including electronics and sensors, microelectromechanical systems, energy, and structural applications.
Metal-ceramic composites are technologically significant as structural and functional materials and are among the most expensive materials to manufacture and repair. Hence, technologies for self-healing metal-ceramic composites are important. Here, a concept to fabricate and heal co-continuous metal-ceramic composites at room temperature were demonstrated. The composites were fabricated by infiltration of metal (here Copper) into a porous alumina preform (fabricated by freeze-casting) through electroplating; a low-temperature and low-cost process for the fabrication of such composites. Additionally, the same electroplating process was demonstrated for healing damages such as grooves and cracks in the original composite, such that the healed composite recovered its strength by more than 80%. Such technology may be expanded toward fully autonomous self-healing structures.
ContributorsMahmoudi, Mohammadreza (Author) / Minary-Jolandan, Majid (Thesis advisor) / Rajagopalan, Jagannathan (Committee member) / Cramer, Corson (Committee member) / Kang, Wonmo (Committee member) / Bhate, Dhruv (Committee member) / Arizona State University (Publisher)
Created2022
Description
Dehumidifiers are ubiquitous and essential household appliances in many parts of the world. They are used extensively in tropical and sub-tropical environments to lower humidity in living spaces, where high ambient humidity can lead to numerous negative health effects from mild physical discomfort to more serious conditions such as mold build up in structures and dangerous illnesses in humans. Most common dehumidifiers are based on conventional mechanical refrigeration cycles, where the effects of condensation heat transfer play a critical role in their effectiveness. In these devices, humid ambient air flows over a cold evaporator, which lowers the temperature of the humid ambient air below its dew point temperature and therefore decreases its water content by causing liquid water condensation on the evaporator surface. The rate at which humidity can be extracted from the ambient air is governed in part by how quickly the evaporator can shed the condensed droplets. Recent advances in soft, stretchable, thermally enhanced (through the addition of liquid metals) silicone tubing offer the potential to use these stretchable tubes in place of conventional copper pipe for applications such as dehumidification. Copper is a common material choice for dehumidifier evaporator tubing owing to its ubiquity and its high thermal conductivity, but it has several thermal downsides. Specifically, copper tubes remain static and typically rely on gravity alone to remove water droplets when they reach a sufficient mass. Additionally, copper’s naturally hydrophilic surface promotes film-wise condensation, which is substantially less effective than dropwise condensation. In contrast to copper, thermally enhanced soft stretchable tubes have naturally hydrophobic surfaces that promote the more effective dropwise condensation mode and a soft surface that offers higher nucleation density. However, soft surfaces also increase droplet pinning, which inhibits their departure. This work experimentally explores the effects of periodic axial stretching and retraction of soft tubing internally cooled with water on droplet condensation dynamics on its exterior surface. Results are discussed in terms of overall system thermal performance and real-time condensation imaging. An overall null result is discovered, and recommendations for future experiments are made.
Contributorsnordstog, thomas (Author) / Rykaczewski, Konrad (Thesis advisor) / Wang, Robert (Committee member) / Devasenathipathy, Shankar (Committee member) / Arizona State University (Publisher)
Created2022