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- Member of: ASU Electronic Theses and Dissertations
Description
The behavior of a solid oxide fuel cell (SOFC) cermet (ceramic-metal composite) anode under reaction conditions depends significantly on the structure, morphology and atomic scale interactions between the metal and the ceramic components. In situ environmental transmission electron microscope (ETEM) is an important tool which not only allows us to perform the basic nanoscale characterization of the anode materials, but also to observe in real-time, the dynamic changes in the anode material under near-reaction conditions. The earlier part of this dissertation is focused on the synthesis and characterization of Pr- and Gd-doped cerium oxide anode materials. A novel spray drying set-up was designed and constructed for preparing nanoparticles of these mixed-oxides and nickel oxide for anode fabrication. X-ray powder diffraction was used to investigate the crystal structure and lattice parameters of the synthesized materials. Particle size distribution, morphology and chemical composition were investigated using transmission electron microscope (TEM). The nanoparticles were found to possess pit-like defects of average size 2 nm after subjecting the spray-dried material to heat treatment at 700 °C for 2 h in air. A novel electron energy-loss spectroscopy (EELS) quantification technique for determining the Pr and Gd concentrations in the mixed oxides was developed. Nano-scale compositional heterogeneity was observed in these materials. The later part of the dissertation focuses mainly on in situ investigations of the anode materials under a H2 environment in the ETEM. Nano-scale changes in the stand-alone ceramic components of the cermet anode were first investigated. Particle size and composition of the individual nanoparticles of Pr-doped ceria (PDC) were found to affect their reducibility in H2 gas. Upon reduction, amorphization of the nanoparticles was observed and was linked to the presence of pit-like defects in the spray-dried material. Investigation of metal-ceramic interactions in the Ni-loaded PDC nanoparticles indicated a localized reduction of Ce in the vicinity of the Ni/PDC interface at 420 °C. Formation of a reduction zone around the interface was attributed to H spillover which was observed directly in the ETEM. Preliminary results on the fabrication of model SOFCs and in situ behavior of Ni/Gd-doped ceria anodes have been presented.
ContributorsSharma, Vaneet (Author) / Crozier, Peter A. (Thesis advisor) / Sharma, Renu (Thesis advisor) / Adams, James B (Committee member) / Dey, Sandwip (Committee member) / Arizona State University (Publisher)
Created2011
Description
In-situ environmental transmission electron microscopy (ETEM) is a powerful tool for following the evolution of supported metal nanoparticles under different reacting gas conditions at elevated temperatures. The ability to observe the events in real time under reacting gas conditions can provide significant information on the fundamental processes taking place in catalytic materials, from which the performance of the catalyst can be understood. The first part of this dissertation presents the application of in-situ ETEM studies in developing structure-activity relationship in supported metal nanoparticles. In-situ ETEM studies on nanostructures in parallel with ex-situ reactor studies of conversions and selectivities were performed for partial oxidation of methane (POM) to syngas (CO+H2) on Ni/SiO2, Ru/SiO2 and NiRu/SiO2 catalysts. During POM, the gas composition varies along the catalyst bed with increasing temperature. It is important to consider these variations in gas composition in order to design experiments for in-situ ETEM. In-situ ETEM experiments were performed under three different reacting gas conditions. First in the presence of H2, this represents the state of the fresh catalyst for the catalytic reaction. Later in the presence of CH4 and O2 in 2:1 ratio, this is the composition of the reacting gases for the POM reaction and this composition acts as an oxidizing environment. Finally in the presence of CH4, this is the reducing gas. Oxidation and reduction behavior of Ni, Ru and NiRu nanoparticles were followed in an in-situ ETEM under reacting gas conditions and the observations were correlated with the performance of the catalyst for POM. The later part of the dissertation presents a technique for determining the gas compositional analysis inside the in-situ ETEM using electron energy-loss spectroscopy. Techniques were developed to identify the gas composition using both inner-shell and low-loss spectroscopy of EELS. Using EELS, an "operando TEM" technique was successfully developed for detecting the gas phase catalysis inside the ETEM. Overall this research demonstrates the importance of in-situ ETEM studies in understanding the structure-activity relationship in supported-metal catalysts for heterogeneous catalysis application.
ContributorsChenna, Santhosh (Author) / Crozier, Peter A. (Thesis advisor) / Carpenter, Ray (Committee member) / Sieradzki, Karl (Committee member) / Petuskey, William (Committee member) / Arizona State University (Publisher)
Created2011
Description
HgCdTe is currently the dominant material for infrared sensing and imaging, and is usually grown on lattice-matched bulk CdZnTe (CZT) substrates. There have been significant recent efforts to identify alternative substrates to CZT as well as alternative detector materials to HgCdTe. In this dissertation research, a wide range of transmission electron microscopy (TEM) imaging and analytical techniques was used in the characterization of epitaxial HgCdTe and related materials and substrates for third generation IR detectors. ZnTe layers grown on Si substrates are considered to be promising candidates for lattice-matched, large-area, and low-cost composite substrates for deposition of II-VI and III-V compound semiconductors with lattice constants near 6.1 Å. After optimizing MBE growth conditions including substrate pretreatment prior to film growth, as well as nucleation and growth temperatures, thick ZnTe/Si films with high crystallinity, low defect density, and excellent surface morphology were achieved. Changes in the Zn/Te flux ratio used during growth were also investigated. Small-probe microanalysis confirmed that a small amount of As was present at the ZnTe/Si interface. A microstructural study of HgCdTe/CdTe/GaAs (211)B and CdTe/GaAs (211)B heterostructures grown using MBE was carried out. High quality MBE-grown CdTe on GaAs(211)B substrates was demonstrated to be a viable composite substrate platform for HgCdTe growth. In addition, analysis of interfacial misfit dislocations and residual strain showed that the CdTe/GaAs interface was fully relaxed. In the case of HgCdTe/CdTe/ GaAs(211)B, thin HgTe buffer layers between HgCdTe and CdTe were also investigated for improving the HgCdTe crystal quality. A set of ZnTe layers epitaxially grown on GaSb(211)B substrates using MBE was studied using high resolution X-ray diffraction (HRXRD) measurements and TEM characterization in order to investigate conditions for defect-free growth. HRXRD results gave critical thickness estimates between 350 nm and 375 nm, in good agreement with theoretical predictions. Moreover, TEM results confirmed that ZnTe layers with thicknesses of 350 nm had highly coherent interfaces and very low dislocation densities, unlike samples with the thicker ZnTe layers.
ContributorsKim, Jae Jin (Author) / Smith, David J. (Thesis advisor) / McCartney, Martha R. (Committee member) / Alford, Terry L. (Committee member) / Crozier, Peter A. (Committee member) / Arizona State University (Publisher)
Created2012
Description
Dealloying, the selective dissolution of an elemental component from an alloy, is an important corrosion mechanism and a technological significant means to fabricate nanoporous structures for a variety of applications. In noble metal alloys, dealloying proceeds above a composition dependent critical potential, and bi-continuous structure evolves "simultaneously" as a result of the interplay between percolation dissolution and surface diffusion. In contrast, dealloying in alloys that show considerable solid-state mass transport at ambient temperature is largely unexplored despite its relevance to nanoparticle catalysts and Li-ion anodes. In my dissertation, I discuss the behaviors of two alloy systems in order to elucidate the role of bulk lattice diffusion in dealloying. First, Mg-Cd alloys are chosen to show that when the dealloying is controlled by bulk diffusion, a new type of porosity - negative void dendrites will form, and the process mirrors electrodeposition. Then, Li-Sn alloys are studied with respect to the composition, particle size and dealloying rate effects on the morphology evolution. Under the right condition, dealloying of Li-Sn supported by percolation dissolution results in the same bi-continuous structure as nanoporous noble metals; whereas lattice diffusion through the otherwise "passivated" surface allows for dealloying with no porosity evolution. The interactions between bulk diffusion, surface diffusion and dissolution are revealed by chronopotentiometry and linear sweep voltammetry technics. The better understanding of dealloying from these experiments enables me to construct a brief review summarizing the electrochemistry and morphology aspects of dealloying as well as offering interpretations to new observations such as critical size effect and encased voids in nanoporous gold. At the end of the dissertation, I will describe a preliminary attempt to generalize the morphology evolution "rules of dealloying" to all solid-to-solid interfacial controlled phase transition process, demonstrating that bi-continuous morphologies can evolve regardless of the nature of parent phase.
ContributorsChen, Qing (Author) / Sieradzki, Karl (Thesis advisor) / Friesen, Cody (Committee member) / Buttry, Daniel (Committee member) / Chan, Candace (Committee member) / Arizona State University (Publisher)
Created2013
Description
Hexavalant chromium (Cr(VI)) poses an emerging concern in drinking water treatment with stricter regulations on the horizon. Photocatalytic reduction of Cr(VI) was investigated as an engineering scale option to remove hexavalent chromium from drinking or industrial waters via a UV/titanium dioxide (TiO2) process. Using an integrated UV lamp/ceramic membrane system to recirculate TiO2, both hexavalent and total chromium levels were reduced through photocatalytic processes without additional chemicals. Cr(VI) removal increased as a function of higher energy input and TiO2 dosage, achieving above 90% removal for a 1g/L dose of TiO2. Surface analysis of effluent TiO2 confirmed the presence of chromium species.
ContributorsStancl, Heather O'Neal (Author) / Westerhoff, Paul K (Thesis advisor) / Chan, Candace (Committee member) / Hristovski, Kiril (Committee member) / Arizona State University (Publisher)
Created2013
Description
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.
ContributorsYang, Ting (Author) / Chan, Candace K. (Thesis advisor) / Crozier, Peter A. (Committee member) / Sieradzki, Karl (Committee member) / Arizona State University (Publisher)
Created2012
Description
A system for illuminating a sample in situ with visible and UV light inside a transmission electron microscope was devised to study photocatalysts. There are many factors which must be considered when designing and building such a system. These include both mechanical, optical, and electron optical considerations. Some of the restrictions posed by the electron microscope column are significant, and care must be taken not to degrade the microscope's electron optical performance, or to unduly restrict the other current capabilities of the microscope. The nature of these various design considerations is discussed in detail. A description of the system that has been added to the microscope at ASU, an FEI Tecnai F20 environmental transmission electron microscope is also given. The system includes a high brightness broadband light source with optical filters, a fiber to guide the light to the sample, and a system for precisely aligning the fiber tip. The spatial distribution and spectrum of the light reaching the sample has been characterized, and is described in detail.
ContributorsMiller, Benjamin (Author) / Crozier, Peter A. (Thesis advisor) / McCartney, Martha (Committee member) / Rez, Peter (Committee member) / Arizona State University (Publisher)
Created2012
Description
The research of this dissertation has involved the nanoscale quantitative characterization of patterned magnetic nanostructures and devices using off-axis electron holography and Lorentz microscopy. The investigation focused on different materials of interest, including monolayer Co nanorings, multilayer Co/Cu/Py (Permalloy, Ni81Fe19) spin-valve nanorings, and notched Py nanowires, which were fabricated via a standard electron-beam lithography (EBL) and lift-off process. Magnetization configurations and reversal processes of Co nanorings, with and without slots, were observed. Vortex-controlled switching behavior with stepped hysteresis loops was identified, with clearly defined onion states, vortex states, flux-closure (FC) states, and Omega states. Two distinct switching mechanisms for the slotted nanorings, depending on applied field directions relative to the slot orientations, were attributed to the vortex chirality and shape anisotropy. Micromagnetic simulations were in good agreement with electron holography observations of the Co nanorings, also confirming the switching field of 700-800 Oe. Co/Cu/Py spin-valve slotted nanorings exhibited different remanent states and switching behavior as a function of the different directions of the applied field relative to the slots. At remanent state, the magnetizations of Co and Py layers were preferentially aligned in antiparallel coupled configuration, with predominant configurations in FC or onion states. Two-step and three-step hysteresis loops were quantitatively determined for nanorings with slots perpendicular, or parallel to the applied field direction, respectively, due to the intrinsic coercivity difference and interlayer magnetic coupling between Co and Py layers. The field to reverse both layers was on the order of ~800 Oe. Domain-wall (DW) motion within Py nanowires (NWs) driven by an in situ magnetic field was visualized and quantified. Different aspects of DW behavior, including nucleation, injection, pinning, depinning, relaxation, and annihilation, occurred depending on applied field strength. A unique asymmetrical DW pinning behavior was recognized, depending on DW chirality relative to the sense of rotation around the notch. The transverse DWs relaxed into vortex DWs, followed by annihilation in a reversed field, which was in agreement with micromagnetic simulations. Overall, the success of these studies demonstrated the capability of off-axis electron holography to provide valuable insights for understanding magnetic behavior on the nanoscale.
ContributorsHe, Kai (Author) / McCartney, Martha R. (Thesis advisor) / Smith, David J. (Thesis advisor) / Chamberlin, Ralph V. (Committee member) / Crozier, Peter A. (Committee member) / Drucker, Jeff (Committee member) / Arizona State University (Publisher)
Created2010
Description
Topological insulators with conducting surface states yet insulating bulk states have generated a lot of interest amongst the physics community due to their varied characteristics and possible applications. Doped topological insulators have presented newer physical states of matter where topological order co&ndashexists; with other physical properties (like magnetic order). The electronic states of these materials are very intriguing and pose problems and the possible solutions to understanding their unique behaviors. In this work, we use Electron Energy Loss Spectroscopy (EELS) – an analytical TEM tool to study both core&ndashlevel; and valence&ndashlevel; excitations in Bi2Se3 and Cu(doped)Bi2Se3 topological insulators. We use this technique to retrieve information on the valence, bonding nature, co-ordination and lattice site occupancy of the undoped and the doped systems. Using the reference materials Cu(I)Se and Cu(II)Se we try to compare and understand the nature of doping that copper assumes in the lattice. And lastly we utilize the state of the art monochromated Nion UltraSTEM 100 to study electronic/vibrational excitations at a record energy resolution from sub-nm regions in the sample.
ContributorsSubramanian, Ganesh (Author) / Spence, John (Thesis advisor) / Jiang, Nan (Committee member) / Chen, Tingyong (Committee member) / Chan, Candace (Committee member) / Arizona State University (Publisher)
Created2013
Description
Recent advancements in the field of light wavefront engineering rely on complex 3D metasurfaces composed of sub-wavelength structures which, for the near infrared range, are challenging to manufacture using contemporary scalable micro- and nanomachining solutions. To address this demand, a novel parallel micromachining method, called metal-assisted electrochemical nanoimprinting (Mac-Imprint) was developed. Mac-Imprint relies on the catalysis of silicon wet etching by a gold-coated stamp enabled by mass-transport of the reactants to achieve high pattern transfer fidelity. This was realized by (i) using nanoporous catalysts to promote etching solution diffusion and (ii) semiconductor substrate pre-patterning with millimeter-scale pillars to provide etching solution storage. However, both of these approaches obstruct scaling of the process in terms of (i) surface roughness and resolution, and (ii) areal footprint of the fabricated structures. To address the first limitation, this dissertation explores fundamental mechanisms underlying the resolution limit of Mac-Imprint and correlates it to the Debye length (~0.9 nm). By synthesizing nanoporous catalytic stamps with pore size less than 10 nm, the sidewall roughness of Mac-Imprinted patterns is reduced to levels comparable to plasma-based micromachining. This improvement allows for the implementation of Mac-Imprint to fabricate Si rib waveguides with limited levels of light scattering on its sidewall. To address the second limitation, this dissertation focuses on the management of the etching solution storage by developing engineered stamps composed of highly porous polymers coated in gold. In a plate-to-plate configuration, such stamps allow for the uniform patterning of chip-scale Si substrates with hierarchical 3D antireflective and antifouling patterns. The development of a Mac-Imprint system capable of conformal patterning onto non-flat substrates becomes possible due to the flexible and stretchable nature of gold-coated porous polymer stamps. Understanding of their mechanical behavior during conformal contact allows for the first implementation of Mac-Imprint to directly micromachine 3D hierarchical patterns onto plano-convex Si lenses, paving the way towards scalable fabrication of multifunctional 3D metasurfaces for applications in advanced optics.
ContributorsSharstniou, Aliaksandr (Author) / Azeredo, Bruno (Thesis advisor) / Chan, Candace (Committee member) / Rykaczewski, Konrad (Committee member) / Petuskey, William (Committee member) / Chen, Xiangfan (Committee member) / Arizona State University (Publisher)
Created2022