Lithium-sulfur (Li-S) batteries are faced with practical drawbacks of poor cycle life and low charge efficiency which hinder their advancements. Those drawbacks are primarily caused by the intrinsic issues of the cathodes (sulfur) and the anodes (Li metal). In attempt to resolve the issues found on the cathodes, this work discusses the method to prepare a binder-free three-dimensional carbon nanotubes-sulfur (3D CNTs-S) composite cathode by a facile and a scalable approach. Here, the 3D structure of CNTs serves as a conducting network to accommodate high loading amounts of active sulfur material. The efficient electron pathway and the short Li ions (Li+) diffusion length provided by the 3D CNTs offset the insulating properties of sulfur. As a result, high areal and specific capacities of 8.8 mAh cm−2 and 1068 mAh g−1, respectively, with the sulfur loading of 8.33 mg cm−2 are demonstrated; furthermore, the cells operated at a current density of 1.4 mA cm−2 (0.1 C) for up to 150 cycles. To address the issues existing on the anode part of Li-S batteries, this work also covers the novel approach to protect a Li metal anode with a thin layer of two-dimensional molybdenum disulfide (MoS2). With the protective layer of MoS2 …

High entropy alloys (HEAs) are a new class of solid solution alloys that contain multiple principal elements and possess excellent mechanical properties, from corrosion resistance to fatigue and wear resistance. Even more recently, twinning induced plasticity (TWIP) and transformation induced plasticity (TRIP) non-equiatomic high entropy alloys have been engineered, promising increased strength and ductility as compared to their equiatomic counterparts. However, impact and fracture resistance of these HEAs has not been studied as much as their other mechanical properties. In this thesis, the hardness, tensile properties, and Charpy impact energy of Al0.3CoCrFeNi, a TWIP HEA, and 50Fe-30Mn-10Co-10Cr (at.%), a TRIP HEA, was explored. First, three processing conditions, (1) as-received, (2) recrystallized, and (3) peak hardness, were chosen for each alloy and verified with Vickers microhardness measurements. Next, the tensile properties of each alloy and condition were investigated. Charpy impact specimen size was then selected based on the final plate thickness, and the machined samples were tested. Plastic zone size and change in sample thickness in the deformed region of each condition after testing was measured. Post-impact test inspection of the samples in all conditions showed that the samples were in tension near the V-notch root and in compression at the …

High entropy alloys (HEAs) are a relatively new class of solid solution alloys that contain multiple principal elements to take advantage of their high configurational entropy, sluggish diffusion, lattice distortion, and the cocktail effect. In recent development, work hardening mechanisms known as twinning induced plasticity (TWIP) and transformation induced plasticity (TRIP) have been found active in Al0.3CoCrFeNi (molar fraction) and Fe50Mn30Co10Cr10 (at %) HEA compositions. Friction-stir processing was done to increase the mechanical properties and improve the microstructure of the alloys for the purpose of high strain rate performance. Quasi-static tensile tests as well as top-hat geometry Split-Hopkinson pressure bar tests were conducted to view the mechanical properties as well as view the microstructural evolution at dynamic strain rates. Overall, the Al0.3CoCrFeNi condition after friction-stir processing and heat treatment has proved to have the best mechanical properties, and selecting from the conditions in this study, Al0.3CoCrFeNi has better shear localization resistance.

This dissertation tested the hypothesis that pulsed laser deposition (PLD) could be used to create targeted dopant profiles in few layered WS2 films based on congruent evaporation of the target. At the growth temperatures used, 3D Volmer-Weber growth was observed. Increased energy transfer from the PLD plume to the growing films degraded stoichiometry (desorption of sulfur) and mobility. Sulfur vacancies act as donors and produce intrinsic n-type conductivity. Post deposition annealing significantly improved the crystallinity, which was accompanied by a mobility increase from 6.5 to 19.5 cm2/Vs. Preparation conditions that resulted in excess sulfur, possibly in the form of interstitials, resulted in p-type conductivity. Current-voltage studies indicated that Ohmic contacts were governed by surface properties and tunneling. Extrinsic p-type doping of few layered WS2 films with Nb via pulsed laser deposition using ablation targets fabricated from WS2, S and Nb powders is demonstrated. The undoped controls were n-type, and exhibited a Hall mobility of 0.4 cm2/Vs. Films doped at 0.5 and 1.1 atomic percentages niobium were p-type, and characterized by Fermi levels at 0.31 eV and 0.18 eV from the valence band edge. That is, the Fermi level moved closer to the valence band edge with increased doping. With increased …

The current understanding of the mechanical properties and deformation behavior of some individual phases in titanium alloys is limited due to the fine scale at which these phases precipitate within the β-phase matrix. The α and ω phases represent the most widely observed phases in titanium alloys depending on the alloy composition and also the heat treatment procedure adopted during processing. The possibility of precipitating ω-phase depends on the content of the β-stabilizers within the system. Although a significant compositional partitioning occurs within ω-phase upon aging treatment, the knowledge of ω-phase mechanical properties as a function of composition is very limited. The initial part of the current work focuses on the effect of common β-stabilizers elements on the phase stability and mechanical properties of the ω-phase using first-principles calculations. A relation between the bonding nature, the phase stability, and elastic properties was proposed. Thereafter αʺ martensitic phase was investigated in Ti-Nb and Ti-Nb-O alloys. The phase stability and martensitic start temperature of αʺ-phase was studied as a function of Nb and oxygen content. Also, the effect of the lattice shear distortion induced by oxygen atom on stabilizing β-phase was investigated. Subsequently the effect of the β-stabilizers' elements on stacking faults …

High entropy alloys (HEAs) based on refractory elements have shown a great potential for high temperature structural applications. In particular, the ones containing Al, exhibits a microstructure similar to the γ-γ' in Ni-based superalloys. While these alloys exhibit impressive strengths at room temperature (RT) and at elevated temperatures, the continuous B2 matrix in these alloys is likely to be responsible for their brittle behavior at RT. Phase stability of five such alloys are studied by thermo-mechanical treatments and characterization techniques using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Two of these alloys showed an inverted microstructure, where the disordered BCC phase becomes continuous, and therefore, they were characterized in detail using SEM, TEM, atom probe tomography (APT) and synchrotron x-ray diffraction experiments. The phenomenon of phase inversion lead to a better combination of strength and ductility as compared to the non-inverted microstructure.To enhance the stability of B2 intermetallic phase which provides the strength when present in a BCC matrix, multicomponent B2 phase compositions stable at 1000°C in some of the above studied alloys, were melted separately. The aim was to establish a single phase B2 at 1000°C and understand the mechanical behavior of these single-phase multicomponent B2 intermetallic …

Sintering parameter effects on the microstructure of boron carbide and boron carbide/titanium diboride composites are investigated. The resulting microstructure and composition are characterized by scanning electron microscopy (SEM), x-ray microscopy (XRM) and x-ray diffraction (XRD). Starting powder size distribution effects on microstructure are present and effect the mechanical properties. Reactive spark plasma sintering introduces boron nitride (BN) intergranular films (IGF's) and their effects on fracture toughness, hardness and flexural strength are shown. Mechanical testing of Vickers hardness, 3-point bend and Chevron notch was done and the microstructural effects on the resulting mechanical properties are investigated.

In the current study, we have investigated the cyclic loading behavior of conventional as well as novel alloy system exhibiting fine and ultrafine-grained structure. While in case of conventional alloy systems (here aluminum alloy AA5024), the effect of three different grain sizes was investigated. Improvement in fatigue properties was observed with decreasing grain size. The unique microstructure produced via Friction stir processing was responsible for the improved fatigue response. Additionally, microstructures consisting of a high fraction of special boundaries within the fine and ultrafine-grained regime were also subjected to cyclic loading. The hierarchical features introduced in the eutectic high entropy alloy deflected the persistent slip bands, responsible for fatigue cracking, thus resulted in delayed crack initiation and improved fatigue life. The selective nature of fatigue was learnt in the fine grain Al0.5CoCrFeNi, where the introduction of hierarchical features did not result in improved fatigue properties. The weak links in the microstructure, while not affecting the tensile properties, got exposed during cyclic loading. Further study on the medium entropy alloy revealed the inherent reason for the improved fatigue properties. The medium entropy alloys utilized the benefit of UFG single-phase FCC matrix. The UFG matrix showed signs of transformation of FCC phase …

Hafnium based high-κ dielectrics are considered potential candidates to replace SiO2 or SiON as the gate dielectric in complementary metal oxide semiconductor (CMOS) devices. Hydrogen is one of the most significant elements in semiconductor technology because of its pervasiveness in various deposition and optimization processes of electronic structures. Therefore, it is important to understand the properties and behavior of hydrogen in semiconductors with the final aim of controlling and using hydrogen to improve electronic performance of electronic structures. Trap transformations under annealing treatments in hydrogen ambient normally involve passivation of traps at thermal SiO2/Si interfaces by hydrogen. High-κ dielectric films are believed to exhibit significantly higher charge trapping affinity than SiO2. In this thesis, study of hydrogen trapping in alternate gate dielectric candidates such as HfO2 during annealing in hydrogen ambient is presented. Rutherford backscattering spectroscopy (RBS), elastic recoil detection analysis (ERDA) and nuclear reaction analysis (NRA) were used to characterize these thin dielectric materials. It was demonstrated that hydrogen trapping in bulk HfO2 is significantly reduced for pre-oxidized HfO2 prior to forming gas anneals. This strong dependence on oxygen pre-processing is believed to be due to oxygen vacancies/deficiencies and hydrogen-carbon impurity complexes that originate from organic precursors used in …

Recent commercial developments have created a need for alternative materials and methods for imparting oil/grease resistance to paper and/or paperboard used in packaging. The performance of a novel grease resistant functional coating comprised of polyvinyl alcohol (PVA), sodium tetraborate pentahydrate (borate) and acetonedicarboxylic acid (ACDA) and the application of said coating by means of flexographic press is presented herein. Application criteria is developed, testing procedures described, and performance assessment of the developed coating materials are made. SEM images along with contact angle data suggest that coating performance is probably attributable to decreased mean pore size in conjunction with a slightly increased surface contact angle facilitated by crosslinking of PVA molecules by both borate ions and ACDA.

Photoresist stripping with oxygen plasma ashing destroys the functional groups in organosilicate glass films and induce moisture uptake, causing low-k dielectric degradation. In this study, hexamethyldisilazane (HMDS), triethylchlorosilane and tripropylchlorosilane are used to repair the damage to organosilicate glass by the O2 plasma ashing process. The optimization of the surface functionalization of the organosilicate glass by the silanes and the thermal stability of the functionalized surfaces are investigated. These experimental results show that HMDS is a promising technique to repair the damage to OSG during the photoresist removal processing and that the heat treatment of the functionalized surfaces causes degradation of the silanes deteriorating the hydrophobicity of the films.

This paper deals with various topics in micro electromechanical systems (MEMS) technology beginning with microactuation, MEMS processing, and MEMS design engineering. The fabrication and testing of three separate MEMS devices are described. The first two devices are a linear stepping motor and a continuous rotary motor, respectively; and were designed for the purpose of investigating the frictional and wear properties of silicon components. The third device is a bi-stable microrelay, in which electrical current conducts through a secondary circuit, via a novel probe-interconnect mechanism. The second half focuses on engineering a carbon nanotube / SU-8 photoepoxy nanocomposite for fabricating MEMS devices. A processing method for this material as well as the initial results of characterization, are discussed.

I have used computer simulations to investigate the behavior of polymeric materials at the molecular level. The simulations were performed using the molecular dynamics method with Lennard-Jones potentials defining the interactions between particles in the system. Significant effort was put into the creation of realistic materials on the computer. For this purpose, an algorithm was developed based on the step-wise polymerization process. The resulting computer-generated materials (CGMs) exhibit several features of real materials, such as molecular weight distribution and presence of chain entanglements. The effect of the addition of a liquid crystalline (LC) phase to the flexible matrix was also studied. The concentration and distribution of the second phase (2P) were found to influence the mechanical and tribological properties of the CGMs. The size of the 2P agglomerates was found to have negligible influence on the properties within the studied range. Moreover, although the 2P reinforcement increases the modulus, it favors crack formation and propagation. Regions of high LC concentration exhibit high probability of becoming part of the crack propagation path. Simulations of the tensile deformation under a uniaxial force have shown that the molecular deformation mechanisms developing in the material depend on several variables, such as the magnitude of …