Thesis (M.S.)--Cleveland State University, 2009. / Abstract. Title from PDF t.p. (viewed on June 25, 2010). Includes bibliographical references (p. 37-38). Available online via the OhioLINK ETD Center and also available in print.

abstract: This thesis discusses the use of low temperature microwave anneal as an alternative technique to recrystallize materials damaged or amorphized due to implantation techniques. The work focuses on the annealing of high-Z doped Si wafers that are incapable of attaining high temperatures required for recrystallizing the damaged implanted layers by microwave absorption The increasing necessity for quicker and more efficient processing techniques motivates study of the use of a single frequency applicator microwave cavity along with a Fe2O3 infused SiC-alumina susceptor/applicator as an alternative post implantation process. Arsenic implanted Si samples of different dopant concentrations and implantation energies were studied pre and post microwave annealing. A set of as-implanted Si samples were also used to assess the effect of inactive dopants against presence of electrically active dopants on the recrystallization mechanisms. The extent of damage repair and Si recrystallization of the damage caused by arsenic and Si implantation of Si is determined by cross-section transmission electron microscopy and Raman spectroscopy. Dopant activation is evaluated for the As implanted Si by sheet resistance measurements. For the same, secondary ion mass spectroscopy analysis is used to compare the extent of diffusion that results from such microwave annealing with that experienced when using conventional rapid thermal annealing (RTA). Results show that compared to susceptor assisted microwave annealing, RTA caused undesired dopant diffusion. The SiC-alumina susceptor plays a predominant role in supplying heat to the Si substrate, and acts as an assistor that helps a high-Z dopant like arsenic to absorb the microwave energy using a microwave loss mechanism which is a combination of ionic and dipole losses. Comparisons of annealing of the samples were done with and without the use of the susceptor, and confirm the role played by the susceptor, since the samples donot recrystallize when the surface heating mechanism provided by the susceptor is not incorporated. Variable frequency microwave annealing was also performed over the as-implanted Si samples for durations and temperatures higher than the single frequency microwave anneal, but only partial recrystallization of the damaged layer was achieved. / Dissertation/Thesis / M.S. Materials Science and Engineering 2011

Solid oxide fuel cells (SOFCs) convert chemical energy directly into electrical energy, leading to significantly higher conversion efficiencies. The oxygen reduction reaction (ORR) at the cathode is often the rate-controlling step in the electrochemical reactions occurring in the SOFCs. Strontium doped lanthanum cobalt ferrite (LSCF) is a widely used cathode material due to its high electronic and ionic conductivity, and reasonable oxygen surface exchange coefficient. However, LSCF can have long-term stability issues such as surface segregation of Sr during SOFC operation, which can adversely affect the electrochemical performance. Thus, understanding the nature of the Sr surface segregation phenomenon, and how it is affected by the composition of LSCF, strain, and the CO2 in the gas phase at the cathode, are critical. In this research, heteroepitaxial thin films of La1-x SrxCo0.2Fe0.8O3- with various Sr contents (x = 0.4, 0.3, 0.2) were deposited by Pulsed Laser Deposition (PLD) on single crystal NdGaO3, SrTiO3 and GdScO3 substrates, leading to different strains in the films. The extent of Sr segregation at the film surface was quantified using the synchrotron-based total reflection X-ray fluorescence (TXRF) technique, and by Atomic Force Microscopy (AFM). The microstructure and the electronic structure of the Sr-rich phases formed on the surface were investigated by scanning/transmission electron microscopy (S/TEM) and hard X-ray photoelectron spectroscopy (HAXPES), respectively. These studies revealed that the surface phases consisted of SrO covered with a capping layer of SrCO3. The presence of CO2 in the atmosphere was found to enhance the kinetics of Sr surface segregation in LSCF. The extent of Sr segregation was found to be a function of the Sr content in bulk. Lowering the Sr content from 40% to 30% reduced the surface segregation, but further lowering the Sr content to 20% increased the segregation. The strains of LSCF thin films on various substrates were measured using high-resolution X-ray diffraction (HRXRD) and the Sr surface segregation was found to be reduced with compressive strain and enhanced with tensile strain present within the thin films. A model was developed correlating the Sr surface segregation with Sr content and strain effects to explain the experimental results.

Lanthanum chromite (LaCrO3), Lanthanum manganite (LaMnO3) and 40% strontium doped lanthanum cobalt iron oxide, La0.6Sr0.4Co0.2Fe0.8O3 (LSCF-6428) are perovskite oxides which are widely used as interconnect or cathode materials in solid oxide fuel cells(SOFCs) due to their high electrical conductivity, good oxygen reduction kinetics, and good chemical stability. The solid state reaction route is the most commonly used method for synthesis of these materials. However, the solid state reaction method usually involves long-time mixing and high synthesis temperature (typically, >1200 ºC), which makes it time-consuming and costly. Molten salt synthesis, which occurs at much lower temperatures (350 ºC – 550 ºC) can offer better particle size and compositional control and reduced energy usage during materials synthesis. In this study, LaCrO3, LaMnO3, and LSCF were synthesized in a molten salt eutectic of LiCl-KCl. A range of reaction temperature from 370 ºC to 600 ºC was investigated. It was found that a pure LaMnO3 perovskite phase can be formed at as 400 ºC using the molten salt method and that LSCF powders were successfully synthesized at 500 ºC. When forming LaCrO3 using the molten salt method, LaOCl was also formed at or above 400 ºC. The X-ray diffraction (XRD) results show this is an attractive alternative route of synthesis to decrease the reaction temperature. Both Scanning Electron Microscopy (SEM) images and XRD patterns for LaCrO3 showed that only cubic structures were formed at low temperature (400 ºC and 450 ºC) and then hexagonal structures started to appear at temperatures above 500 ºC. The molten salt synthesis method was then used to prepare core-shell structures with LaCrO3 or LSCF particles as the core and LaMnO3 as the shell. Core-shell structures were characterized by Scanning Electron Microscopy (SEM), Scanning Transmission Electron Microscope (STEM) and Energy Dispersive X-ray Spectroscopy (EDS). It was found that the expected core-shell structures were successfully formed with the overall cubic structures. Therefore, the molten salt synthesis method is a feasible method to decrease the operation temperature and form the core-shell structure.

abstract: The goal of this work is to develop low cost and highly efficient hybrid solar cells based on semiconductor nanoparticles (NPs). Hybrid solar cells have been demonstrated to take advantages of both inorganic and organic semiconductors by employing simple soluble process. In order to improve the power conversion efficiency (PCE), the bulk heterojunction (BHJ) of cadmium selenide (CdSe) tetrapods (TPs) and poly (3-hexylthiophene) (P3HT) are introduced as an electron acceptor and donor, respectively. The dimension of CdSe TPs and the 3D spatial distribution of CdSe TPs:P3HT photoactive blends are investigated to improve optical and electrical properties of photovoltaic devices. Hybrid solar cells having long-armed CdSe TPs and P3HT establish higher PCE of 1.12% when compared to device employing short-armed TPs of 0.80%. The device performance are improved by using longer armed CdSe TPs, which aids in better percolation connectivity and reduced charge hopping events, thus leading to better charge transport. The device architecture of hybrid solar cells is examined to assist vertical phase separation (VPS). Improvement of VPS in hybrid solar cells using CdSe TPs:P3HT photoactive blends is systematically manipulated by solution processed interfacial layers, resulting in enhanced device performance. Multi-layered hybrid solar cells assist better light absorption, efficient charge carrier transport, and increase of the surface contact area. In this work, hole transport assisting layer (HTAL)/BHJ photoactive layer (BPL)/electron transport assisting layer (ETAL) or HTAL/BPL/ETAL (HBE) multi-layered structure is introduced, similarly to p-type layer/intermixed photoactive layer/n-type layer (p-i-n) structure of organic photovoltaic devices. To further control the improvement of the device performance, the effects of nano-scale morphology from solvents having different boiling points, the various shapes of semiconductor NPs, and the emergence of blending NPs are demonstrated. The formation of favorable 3D networks in photoactive layer is attributed to enhance the efficient charge transport by the optimized combination of semiconductor NPs in polymer matrix. / Dissertation/Thesis / Ph.D. Materials Science and Engineering 2012

abstract: 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. / Dissertation/Thesis / M.S. Materials Science and Engineering 2012

<p> Lithium-sulfur (Li-S) batteries have been considered as an attractive alternative to current Li-ion batteries due to their large theoretical capacity (1672 mA-h/g) and theoretical energy density (2600 Wh/kg) while having a low cost, an abundance of the material, and relatively non-toxic properties. However, the low cyclability and significant capacity fading during the first several cycles prevent Li-S rechargeable batteries from being commercialized. During discharge, elemental sulfur is reduced to the final product Li2S through a series of soluble intermediate species, lithium polysulfides (Li₂S_x, 2 &le; x &le; 8). Lithium polysulfides dissolved into the electrolyte in the separator can no longer participate in redox reductions, resulting in a loss of active materials, as well as a &ldquo;shuttling effect&rdquo; that causes capacity fading and low coulombic efficiency. Despite the fact that decades of research have attempted to solve this, the problem is still not resolved due to a lack of fundamental understanding of the system. This includes how lithium polysulfides are produced during discharge interactions with other components in the cell and the reaction mechanisms (the electrochemical and chemical processes) during cycling. The objective of this dissertation is to provide a fundamental understanding of lithium polysulfides produced during discharge of a Li-S cell. This is an essential piece of knowledge when designing and identifying the issues associated with Li-S batteries.</p><p> To begin, the morphology, thermal properties, and ionic conductivity of an ether-based nanostructured block copolymer containing lithium polysulfides were investigated. Previous work has shown that nanostructured block copolymer electrolytes containing an ion-conducting block and modulus-strengthening block has the potential of enabling solid-state lithium metal rechargeable batteries. This is of particular interest for a lithium-sulfur battery to fully explore its high energy density and capacity. Understanding the thermal and electrochemical properties of these block copolymer electrolytes containing lithium polysulfides is essential for evaluating their potential use in Li-S batteries. A systematic study of polystyrene-<i>b</i>-poly(ethylene oxide) (SEO) block copolymer mixed with Li₂S_x with an average x value of 4 and 8 was conducted. Small angle X-ray scattering, differential scanning calorimetry, and ac impedance spectroscopy were used to measure the morphology, thermal properties, and ionic conductivities of all samples. The ionic conductivity of SEO/Li₂S_x mixtures were compared with those of poly(ethylene oxide) (PEO) mixed with Li₂S_x to quantify the effect of nanostructuring on ion transport. The conductivities of both SEO and PEO samples containing polysulfides with a longer average chain length higher than the same polymer containing polysulfides with a shorter average chain length at all salt concentrations, indicating that dissociation of long-chain polysulfides occurs more readily than short-chain polysulfides. Normalized conductivity was used to quantify the effect of morphology on ion transport. The results showed that SEO suppressed the migration of polysulfides relative to PEO. However, this suppression is inadequate for practical applications. In other words, cathode architectures that prevent polysulfides from entering the electrolyte are necessary for enabling Li-S batteries with block copolymer electrolytes. Nevertheless, the results obtained in this study are important as they enable quantification of polysulfide migration in Li-S batteries with imperfect polysulfide encapsulation, a limitation that applies to all known Li-S batteries.</p><p> Next, UV-vis spectroscopy with radiation wavelength in the range 200 - 800 nm was used to study different polysulfides in ether. Ex-situ UV-vis spectra were measured for chemically synthesized lithium polysulfides in TEGDME, Li₂ S_{x_mix} | TEGDME solutions for x_mix values of 4, 6, 8, and 10 and sulfur concentrations of 10, 50, and 100 mM. The peaks are generally more resolved at lower concentrations than at higher concentrations for all x_mix values, suggesting a concentration dependence of spectra shape. The peak at 617 nm was used to confirm the existence of S₃^&bull;- radical anion, which supports the argument that polysulfide radical anions are stable in ether-based electrolytes, and may play an important role in Li-S reaction mechanism. Using in-situ UV-vis method was discussed and challenges for Li-S reaction mechanism study were evaluated. A new fluorinated-ether based electrolyte was explored. Its low polysulfide solubility makes it a good candidate to be used in in-situ Li-S reaction studies because UV-vis radiations do not have a large penetration path through high concentration of polysulfide-containing materials. However, the main challenge in using UV-vis spectroscopy to study Li-S reaction mechanism is the ambiguity in peak assignments arised both from a lack of spectra standards for different polysulfides. It is difficult to experimentally obtain polysulfide spectra standards because polysulfides cannot be separated. (Abstract shortened by ProQuest.) </p><p>

abstract: This research reports on the investigation into the synthesis and stabilization of iron oxide nanoparticles for theranostic applications using amine-epoxide polymers. Although theranostic agents such as magnetic nanoparticles have been designed and developed for a few decades, there is still more work that needs to be done with the type of materials that can be used to stabilize or functionalize these particles if they are to be used for applications such as drug delivery, imaging and hyperthermia. For in-vivo applications, it is crucial that organic coatings enclose the nanoparticles in order to prevent aggregation and facilitate efficient removal from the body as well as protect the body from toxic material. The objective of this thesis is to design polymer coated magnetite nanoparticles with polymers that are biocompatible and can stabilize the iron oxide nanoparticle to help create mono-dispersed particles in solution. It is desirable to also have these nanoparticles possess high magnetic susceptibility in response to an applied magnetic field. The co- precipitation method was selected because it is probably the simplest and most efficient chemical pathway to obtain magnetic nanoparticles. In literature, cationic polymers such as Polyethylenimine (PEI), which is the industry standard, have been used to stabilize IONPs because they can be used in magnetofections to deliver DNA or RNA. PEI however is known to interact very strongly with proteins and is cytotoxic, so as mentioned previously the Iron Oxide nanoparticles i (IONPs) synthesized in this study were stabilized with amine-epoxide polymers because of the limitations of PEI. Four different amine-epoxide polymers which have good water solubility, biodegradability and less toxic than PEI were synthesized and used in the synthesis and stabilization of the magnetic nanoparticles and compared to PEI templated IONPs. These polymer-templated magnetic nanoparticles were also characterized by size, surface charge, Iron oxide content (ICP analysis) and superconducting quantum interference devices (SQUID) analysis to determine the magnetization values. TEM images were also used to determine the shape and size of the nanoparticles. All this was done in an effort to choose two or three leads that could be used in future work for magnetofections or drug delivery research. / Dissertation/Thesis / Masters Thesis Chemical Engineering 2014

abstract: LiNbO3 and ZnO have shown great potential for photochemical surface reactions and specific photocatalytic processes. However, the efficiency of LiNbO3 is limited due to recombination or back reactions and ZnO exhibits a chemical instability in a liquid cell. In this dissertation, both materials were coated with precise thickness of metal oxide layers to passivate the surfaces and to enhance their photocatalytic efficiency. LiNbO3 was coated with plasma enhanced atomic layer deposited (PEALD) ZnO and Al2O3, and molecular beam deposited TiO2 and VO2. On the other hand, PEALD ZnO and single crystal ZnO were passivated with PEALD SiO2 and Al2O3. Metal oxide/LiNbO3 heterostructures were immersed in aqueous AgNO3 solutions and illuminated with ultraviolet (UV) light to form Ag nanoparticle patterns. Alternatively, Al2O3 and SiO2/ZnO heterostructures were immersed in K3PO4 buffer solutions and studied for photoelectrochemical reactions. A fundamental aspect of the heterostructures is the band alignment and band bending, which was deduced from in situ photoemission measurements. This research has provided insight to three aspects of the heterostructures. First, the band alignment at the interface of metal oxides/LiNbO3, and Al2O3 or SiO2/ZnO were used to explain the possible charge transfer processes and the direction of carrier flow in the heterostructures. Second, the effect of metal oxide coatings on the LiNbO3 with different internal carrier concentrations was related to the surface photochemical reactions. Third is the surface passivation and degradation mechanism of Al2O3 and SiO2 on ZnO was established. The heterostructures were characterized after stability tests using atomic force microscopy (AFM), scanning electron microscopy (SEM), and cross-section transmission electron microscopy (TEM). The results indicate that limited thicknesses of ZnO or TiO2 on polarity patterned LiNbO3 (PPLN) enhances the Ag+ photoinduced reduction process. ZnO seems more efficient than TiO2 possibly due to a higher carrier mobility. However, an increase of the ZnO thickness (≥ 4 nm) reduced the effect of the PPLN substrate on the Ag nanoparticle pattern. For the case of Al2O3 and SiO2/ZnO heterostructures, SiO2 remains intact through 1 h stability tests. Unlike SiO2, Al2O3 shows surface degradation after a short stability test of a few minutes. Thus, SiO2 provides improved passivation over Al2O3. A detailed microscopy analysis indicates the underneath ZnO photocorrodes in the SiO2/ZnO samples, which is possibly due to transport of ions through the SiO2 protective layer. / Dissertation/Thesis / Doctoral Dissertation Materials Science and Engineering 2016

abstract: Stress corrosion cracking (SCC) is a materials degradation phenomena resulting from a combination of stress and a corrosive environment. Among the alphabet soup of proposed mechanism of SCC the most important are film-rupture, film-induced cleavage and hydrogen embrittlement. This work examines various aspects of film-induced cleavage in gold alloys for which the operation of hydrogen embrittlement processes can be strictly ruled out on thermodynamic grounds. This is so because in such alloys SCC occurs under electrochemical conditions within which water is stable to hydrogen gas evolution. The alloy system examined in this work is AgAu since the corrosion processes in this system occur by a dealloying mechanism that results in the formation of nanoporous gold. The physics behind the dealloying process as well as the resulting formation of nanoporous gold is today well understood. Two important aspects of the film-induced cleavage mechanism are examined in this work: dynamic fracture in monolithic nanoporous gold and crack injection. In crack injection there is a finite thickness dealloyed layer formed on a AgAu alloy sample and the question of whether or not a crack that nucleates within this layer can travel for some finite distance into the un-corroded parent phase alloy is addressed. Dynamic fracture tests were performed on single edge-notched monolithic nanoporous gold samples as well as “infinite strip” sample configurations for which the stress intensity remains constant over a significant portion of the crack length. High-speed photography was used to measure the crack velocity. In the dynamic fracture experiments cracks were observed to travel at speeds as large as 270 m/s corresponding to about 68% of the Raleigh wave velocity. Crack injection experiments were performed on single crystal Ag77Au23, polycrystalline Ag72Au28 and pure gold, all of which had thin nanoporous gold layers on the surface of samples. Through-thickness fracture was seen in both the single crystal and polycrystalline samples and there was an indication of ~ 1 μm injected cracks into pure gold. These results have important implications for the operation of the film-induced cleavage mechanism and represent a first step in the development of a fundamental model of SCC. / Dissertation/Thesis / Doctoral Dissertation Materials Science and Engineering 2016