Matching Items (158)
- All Subjects: Materials Science
- Genre: Masters Thesis
- Member of: ASU Electronic Theses and Dissertations
- Status: Published
The focus of this research is to investigate methods for material substitution for the purpose of re-engineering legacy systems that involves incomplete information about form, fit and function of replacement parts. The primary motive is to extract as much useful information about a failed legacy part as possible and use fuzzy logic rules for identifying the unknown parameter values. Machine elements can fail by any number of failure modes but the most probable failure modes based on the service condition are considered critical failure modes. Three main parameters are of key interest in identifying the critical failure mode of the part. Critical failure modes are then directly mapped to material properties. Target material property values are calculated from material property values obtained from the originally used material and from the design goals. The material database is searched for new candidate materials that satisfy the goals and constraints in manufacturing and raw stock availability. Uncertainty in the extracted data is modeled using fuzzy logic. Fuzzy member functions model the imprecise nature of data in each available parameter and rule sets characterize the imprecise dependencies between the parameters and makes decisions in identifying the unknown parameter value based on the incompleteness. A final confidence level for each material in a pool of candidate material is a direct indication of uncertainty. All the candidates satisfy the goals and constraints to varying degrees and the final selection is left to the designer's discretion. The process is automated by software that inputs incomplete data; uses fuzzy logic to extract more information and queries the material database with a constrained search for finding candidate alternatives.
This thesis focuses on the theoretical work done to determine thermodynamic properties of a chalcopyrite thin-film material for use as a photovoltaic material in a tandem device. The material of main focus here is ZnGeAs2, which was chosen for the relative abundance of constituents, favorable photovoltaic properties, and good lattice matching with ZnSnP2, the other component in this tandem device. This work is divided into two main chapters, which will cover: calculations and method to determine the formation energy and abundance of native point defects, and a model to calculate the vapor pressure over a ternary material from first-principles. The purpose of this work is to guide experimental work being done in tandem to synthesize ZnGeAs2 in thin-film form with high enough quality such that it can be used as a photovoltaic. Since properties of photovoltaic depend greatly on defect concentrations and film quality, a theoretical understanding of how laboratory conditions affect these properties is very valuable. The work done here is from first-principles and utilizes density functional theory using the local density approximation. Results from the native point defect study show that the zinc vacancy (VZn) and the germanium antisite (GeZn) are the more prominent defects; which most likely produce non-stoichiometric films. The vapor pressure model for a ternary system is validated using known vapor pressure for monatomic and binary test systems. With a valid ternary system vapor pressure model, results show there is a kinetic barrier to decomposition for ZnGeAs2.
This work focuses on simulation of electrical resistivity and optical behaviors of thin films, where an Ag or Au thin layer is embedded in zinc oxide. Enhanced conductivity and transparency were earlier achieved with multilayer structured transparent conducting oxide (TCO) sandwich layer with metal (TCO/metal/TCO). Sputtering pattern of metal layer is simulated to obtain the morphology, covered area fraction, and the percolation strength. The resistivity as a function of the metal layer thickness fits the modeled trend of covered area fraction beyond the percolation threshold. This result not only presents the robustness of the simulation, but also demonstrates the influence of metal morphology in multilayer structure. Effective medium coefficients are defined from the coverage and percolation strength to obtain simulated optical transmittance which matches experimental observation. The coherence of resistivity and optical transmittance validates the simulation of the sputtered pattern and the incorporation of percolation theory in the model.
Pb-free solder joints are commonly used as interconnects in semiconductor packaging. One of the major defects affecting the mechanical performance of solder joints are reflow pores that form during processing. These pores exhibit significant variability in size and distribution, and understanding the effects of pore geometry on failure is an important reliability concern. In this thesis, the pore microstructures of solder joint samples and the localized plastic deformation around individual pores was characterized in 3D using lab scale X-ray Microtomography. To observe the deformation of a solder joint in 3D, a solder joint was imaged with Microtomography after reflow and then deformed in shear in several loading steps with additional tomography data taken between each. The 3D tomography datasets were then segmented using the 3D Livewire technique into regions corresponding to solder and pores, and used to generate 3D models of the joint at each strain value using Mimics software. The extent of deformation of individual pores in the joint as a function of strain was quantified using sphericity measurements, and correlated with the observed cracking in the joint. In addition, the error inherent in the data acquisition and 3D modeling process was also quantified. The progression of damage observed with X-ray Microtomography was then used to validate the deformation and failure predicted by a Finite Element (FE) simulation. The FE model was based on the as-reflowed tomography data, and incorporated a ductile damage failure model to simulate fracture. Using the measured sphericity change and cracking information obtained from the tomography data, the FE model is shown to correctly capture the broad plastic deformation and strain localization seen in the actual joint, as well as the crack propagation. Lastly, Digital Image Correlation was investigated as a method of obtaining improved local strain measurements in 3D. This technique measures the displacement of the inherent microstructural features of the joint, and can give localized strain measurements that can be directly comparable to that predicted by modeling. The technique is demonstrated in 2D on Pb-Sn solder, and example 3D data is presented for future analysis.
This thesis discusses the evolution of conduction mechanism in the silver (Ag) on zinc oxide (ZnO) thin film system with respect to the Ag morphology. As a plausible substitute for indium tin oxide (ITO), TCO/Metal/TCO (TMT) structure has received a lot of attentions as a prospective ITO substitute due to its low resistivity and desirable transmittance. However, the detailed conduction mechanism is not fully understood. In an attempt to investigate the conduction mechanism of the ZnO/Ag/ZnO thin film system with respect to the Ag microstructure, the top ZnO layer is removed, which offers a better view of Ag morphology by using scanning electron microscopy (SEM). With 2 nm thick Ag layer, it is seen that the Ag forms discrete islands with small islands size (r), but large separation (s); also the effective resistivity of the system is extremely high. This regime is designated as dielectric zone. In this regime, thermionic emission and activated tunneling conduction mechanisms are considered. Based on simulations, when "s" was beyond 6 nm, thermionic emission dominates; with "s" less than 6 nm, activated tunneling is the dominating mechanism. As the Ag thickness increases, the individual islands coalesce and Ag clusters are formed. At certain Ag thickness, there are one or several Ag clusters that percolate the ZnO film, and the effective resistivity of the system exhibits a tremendous drop simultaneously, because the conducting electrons do not need to overcome huge ZnO barrier to transport. This is recognized as percolation zone. As the Ag thickness grows, Ag film becomes more continuous and there are no individual islands left on the surface. The effective resistivity decreases and is comparable to the characteristics of metallic materials, so this regime is categorized as metallic zone. The simulation of the Ag thin film resistivity is performed in terms of Ag thickness, and the experimental data fits the simulation well, which supports the proposed models. Hall measurement and four point probe measurement are carried out to characterize the electrical properties of the thin film system.
Gas diffusion layers (GDLs) are a critical and essential part of proton exchange membrane fuel cells (PEMFCs). They carry out various important functions such as transportation of reactants to and from the reaction sites. The material properties and structural characteristics of the substrate and the microporous layer strongly influence fuel cell performance. The microporous layer of the GDLs was fabricated with the carbon slurry dispersed in water containing ammonium lauryl sulfate (ALS) using the wire rod coating method. GDLs were fabricated with different materials to compose the microporous layer and evaluated the effects on PEMFC power output performance. The consistency of the carbon slurry was achieved by adding 25 wt. % of PTFE, a binding agent with a 75:25 ratio of carbon (Pureblack and vapor grown carbon fiber). The GDLs were investigated in PEMFC under various relative humidity (RH) conditions using H2/O2 and H2/Air. GDLs were also fabricated with the carbon slurry dispersed in water containing sodium dodecyl sulfate (SDS) and multiwalled carbon nanotubes (MWCNTs) with isopropyl alcohol (IPA) based for fuel cell performance comparison. MWCNTs and SDS exhibits the highest performance at 60% and 70% RH with a peak power density of 1100 mW.cm-2 and 850 mW.cm-2 using air and oxygen as an oxidant. This means that the gas diffusion characteristics of these two samples were optimum at 60 and 70 % RH with high limiting current density range. It was also found that the composition of the carbon slurry, specifically ALS concentration has the highest peak power density of 1300 and 500mW.cm-2 for both H2/O2 and H2/Air at 100% RH. However, SDS and MWCNTs demonstrates the lowest power density using air and oxygen as an oxidants at 100% RH.
Alkali treated aluminosilicate (geopolymer) was functionalized by surfactant to increase the hydrophobicity for making Pickering emulsion for the first part of this work. In the first part of this study, alkali treated metakaolin was functionalized with cetyltrimethylammonium bromide ((C16H33)N(CH3)3Br, CTAB). The electrostatic interaction between this quaternary ammonium and the surface of the aluminosilicate which has negative charge has taken place. The particles then were used to prepare Pickering emulsion. The resulting stable dispersions, obtained very fast at very simple conditions with low ratio of aluminosilicate to liquid phase. In the second part, the interaction between geopolymer and glycerol was studied to see the covalent grafting of the geopolymer for making geopolymer composite. The composite material would be the basis material to be used as support catalyst, thin coating reagent and flame retardant material and so on, Variety of techniques, Thermogravimetric (TGA), Particle-induced X-ray emission (PIXE), FTIR, Solid state NMR, Powder X-ray diffraction (PXRD), BET surface area, Elemental analysis (CHN), TEM, SEM and Optical microscopy were used to characterize the functionalized geopolymer.
Lithium-ion batteries can fail and catch fire when overcharged, exposed to high temperatures or short-circuited due to the highly flammable organic liquid used in the electrolyte. Using inorganic solid electrolyte materials can potentially improve the safety factor. Additionally, nanostructured electrolyte materials may further enhanced performance by taking advantage of their large aspect ratio. In this work, the synthesis of two promising nanostructured solid electrolyte materials was explored. Amorphous lithium niobate nanowires were synthesized through the decomposition of a niobium-containing complex in a structure-directing solvent using a reflux method. Lithium lanthanum titanate was obtained via solid state reaction with titanium oxide nanowires as the titanium precursor, but the nanowire morphology could not be preserved due to high temperature sintering. Hyperbranched potassium lanthanum titanate was synthesized through hydrothermal route. This was the first time that hyperbranched nanowires with perovskite structure were made without any catalyst or substrate. This result has the potential to be applied to other perovskite materials.
This study investigates the fabrication and mechanical properties of semicontinuous, hemp fiber reinforced thermoset composites. This research determines if off-the-shelf refined woven hemp fabric is suitable as composite reinforcement using resin pre-impregnated method. Industrial hemp was chosen for its low cost, low resource input as a crop, supply chain from raw product to refined textile and biodegradability potential. Detail is placed on specimen fabrication considerations. Lab testing of tension and compression is conducted and optimization considerations are examined. The resulting composite is limited in mechanical properties as tested. This research shows it is possible to use woven hemp reinforcement in pre-impregnated processed composites, but optimization in mechanical properties is required to make the process commercially practical outside niche markets.
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.