View All Items

Title: Phase transformation and growth kinetics in reaction zone between uranium alloy and zirconium diffusion barrier.
Creator: Park, Young Joo, Sohn, Yongho, Coffey, Kevin, Fang, Jiyu, University of Central Florida
Abstract / Description: U-10wt.%Mo (U10Mo) alloy as a part of monolithic fuel system is being developed under Material Management and Minimization Reactor Conversion (MMMRC) program, tasked with replacing high-enriched uranium (HEU) fuel with low-enriched uranium (LEU) fuel in civilian research and test reactors. Use of U10Mo fuel alloy entails a Zr diffusion barrier to avoid the undesirable interdiffusion and reactions between the U10Mo and Al-alloy cladding. To better understand the interaction between these fuel...
Show moreU-10wt.%Mo (U10Mo) alloy as a part of monolithic fuel system is being developed under Material Management and Minimization Reactor Conversion (MMMRC) program, tasked with replacing high-enriched uranium (HEU) fuel with low-enriched uranium (LEU) fuel in civilian research and test reactors. Use of U10Mo fuel alloy entails a Zr diffusion barrier to avoid the undesirable interdiffusion and reactions between the U10Mo and Al-alloy cladding. To better understand the interaction between these fuel system constituents, microstructural development and diffusion kinetics in U-Mo-Zr, U-Zr and fuel plate assembly processed by co-rolling and hot isostatic pressing (HIP) were investigated using a variety of analytical techniques accompanying scanning electron microscopy and transmission electron microscopy.Phase constituents, microstructure and diffusion kinetics between U10Mo and Zr were examined using solid-to-solid diffusion couples annealed at 650 (&)deg;C for 240, 480 and 720 hours. Concentration profiles were mapped as diffusion paths on the isothermal ternary phase diagram. Within the diffusion zone, single-phase layers of (?U,?Zr) were observed along with a discontinuous layer of Mo2Zr between the ?Zr and ?U layers. In the vicinity of Mo2Zr phase, islands of ?Zr phase were also found. In addition, acicular ?Zr and U6Zr3Mo phases were observed within the ?U(Mo). Growth rate of the interdiffusion-reaction zone was determined to be 1.81 (&)#215; 10-15 m2/sec at 650 (&)deg;C, however with an assumption of a certain incubation period.Investigation for interdiffusion and reaction between U and Zr were carried out using solid-to-solid diffusion couples annealed at 580, 650, 680 and 710 (&)deg;C. The interdiffusion and reaction layer consisted of ?U containing Zr acicular precipitate, ?' (oC4-variant) and (?U,?Zr) solid solution at 650, 680 and 710 (&)deg;C. The ?-UZr2 phase, instead of (?U,?Zr) solid solution phase, was observed in the couple annealed at 580 (&)deg;C. The interdiffusion fluxes and coefficients were determined for the ?U, (?U,?Zr) and ?-UZr2 (580 ?C only) phases using both Sauer-Freise and Boltzmann-Matano analyses. For the ?'-phase with negligible concentration gradient, integrated interdiffusion coefficients were determined via Wagner method. Marker plane was found in (?U,?Zr) (cI2) solid solution from the couples annealed at 650, 680 and 710 (&)deg;C and ?-UZr2 from the couple at 580 (&)deg;C. Intrinsic diffusion coefficients at the compositions corresponding to the marker plane were determined based on Heumann analysis: U intrinsically diffused an order magnitude faster than Zr. Arrhenius temperature-dependence, Darken relation, and comparison to existing literature data demonstrated consistency in results.Monolithic fuel plate assembly was fabricated by sequential process of (1) co-rolling to laminate the Zr barrier onto the U10Mo fuel alloy and (2) HIP to encase the fuel laminated with Zr, within the Al-alloy 6061 (AA6061). In this study, HIP process was carried out as functions of temperature (520, 540, 560 and 580 (&)deg;C for 90 minutes), time (45, 60, 90, 180 and 345 minutes at 560 (&)deg;C) with ramp-cool rate (35, 70 and 280 (&)deg;C/hour). At the interface between the U10Mo and Zr, following the co-rolling, the UZr2 phase was observed to develop adjacent to Zr, and the ?U phase was found between the UZr2 and U10Mo. Mo2Zr was found as precipitates mostly within the ?U phase. Observable growth due to HIP was only observed for the (Al,Si)3Zr phase found at the Zr/AA6061 interface, however, with a large activation energy of 457 kJ/mol. Decomposition of ?U into ?U and ?' was observed in the U10Mo alloy. The volume fraction of ? and ?' increased as the HIP temperature and ramp-cool rate decreased. The UC-UO2 inclusions within the U10Mo fuel alloy were observed, but the volume percent of the UC-UO2 inclusions within the U10Mo alloy, ranging from approximately 0.5 to 1.8, did not change as functions of HIP temperature and holding time. However, the inclusions located near the surface of the U10Mo alloy, were frequently observed to interfere the uniformity of interdiffusion and reaction between the U10Mo alloy and Zr diffusion barrier. The regions of limited interaction between the U10Mo and Zr barrier associated with UC-UO2 inclusions decreased with an increase in HIP temperature, however no significant trend was observed with an increase in HIP duration at 560 (&)deg;C.
Show less
Date Issued: 2016
Identifier: CFE0006371, ucf:51499
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0006371

Title: Growth and doping of MoS2 thin films for electronic and optoelectronic applications.
Creator: Abouelkhair, Hussain, Peale, Robert, Kaden, William, Stolbov, Sergey, Coffey, Kevin, University of Central Florida
Abstract / Description: MoS2 high absorption coefficient, high mobility, mechanical flexibility, and chemical inertness is very promising for many electronic and optoelectronic applications. The growth of high-quality MoS2 by a scalable and doping compatible method is still lacking. Therefore, the suitable dopants for MoS2 are not fully explored yet. This dissertation consists mainly of four main studies. The first study is on the growth of MoS2 thin films by atmospheric pressure chemical vapor deposition. Scanning...
Show moreMoS2 high absorption coefficient, high mobility, mechanical flexibility, and chemical inertness is very promising for many electronic and optoelectronic applications. The growth of high-quality MoS2 by a scalable and doping compatible method is still lacking. Therefore, the suitable dopants for MoS2 are not fully explored yet. This dissertation consists mainly of four main studies. The first study is on the growth of MoS2 thin films by atmospheric pressure chemical vapor deposition. Scanning electron microscope images revealed the growth of microdomes of MoS2 on top of a smooth MoS2 film. These microdomes are very promising as a broadband omnidirectional light trap for light harvesting applications. The second study is on the growth of MoS2 thin films by low pressure chemical vapor deposition (LPCVD). Control of sulfur vapor flow is essential for the growth of a pure phase of MoS2. Turning off sulfur vapor flow during the cooling cycle at 700 (&)#186;C leads to the growth of highly textured MoS2 with a Hall mobility of 20 cm2/Vs. The third study was on the growth of Ti-doped MoS2 thin films by LPCVD. The successful doping was confirmed by Hall effect measurement and secondary ion mass spectrometry (SIMS). Different growth temperatures from 1000 to 700 ? were studied. Ti act as a donor in MoS2. The fourth study is on fluorine-doped SnO2 (FTO) which has many technological applications including solar cells and transistors. FTO was grown by an aqueous-spray-based method. The main objective was to compare the actual against the nominal concentration of fluorine using SIMS. The concentration of fluorine in the grown films is lower than the concentration of fluorine in the aqueous solution.?
Show less
Date Issued: 2017
Identifier: CFE0006847, ucf:51767
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0006847

Title: Stress-induced phase change sintering: A novel approach to the fabrication of barium chloride transparent ceramic scintillators.
Creator: Shoulders, William, Gaume, Romain, Coffey, Kevin, Sohn, Yongho, University of Central Florida
Abstract / Description: For the precise in-field identification of dangerous radioisotopes, the desire for higher resolution, cheaper, and more rugged gamma-ray scintillator detectors has pushed the materials science community to investigate new compounds and processing techniques.(&)nbsp; One particular solution is Eu:BaCl2 transparent ceramics processed by the novel phase-change sintering technique. Typically, optical transparency in BaCl2 would be limited by birefringence scattering at grain boundaries due to...
Show moreFor the precise in-field identification of dangerous radioisotopes, the desire for higher resolution, cheaper, and more rugged gamma-ray scintillator detectors has pushed the materials science community to investigate new compounds and processing techniques.(&)nbsp; One particular solution is Eu:BaCl2 transparent ceramics processed by the novel phase-change sintering technique. Typically, optical transparency in BaCl2 would be limited by birefringence scattering at grain boundaries due to mismatch in refractive index.(&)nbsp; Traditional ceramic processing routes would also predispose this volatile and hygroscopic material to the formation of defects, which interrupt the energy transfer in the scintillation process. Literature shows that these challenges have prevented halide gamma-ray scintillator ceramics, including Ce:LaBr3, Eu:SrI2, and Ce:Cs2LiYCl6, from reaching the performance of their single-crystalline counterparts. The sintering method explored in this thesis utilizes a polymorphic orthorhombic to cubic phase transition followed by plastic deformation.(&)nbsp; The experimental implementation of this method necessitated the design of a unique airtight hot-pressing device, capable of developing conditions for this phase conversion, and the synthesis of high purity powders. Systematic experiments on powder synthesis and on densification were carried out to demonstrate the feasibility of this approach and understand the conditions for phase-change sintering. These experiments, supported by characterizations including x-ray diffraction, electron microscopy, and thermal analysis, lead to the production of optically isotropic cubic barium chloride ceramic samples. Finally, the optical and scintillation properties of Eu:BaCl2 ceramic samples were investigated, revealing an energy resolution of 6% at 662 keV, an unprecedented value for a halide ceramic scintillator.
Show less
Date Issued: 2016
Identifier: CFE0006844, ucf:51779
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0006844

Title: Quantification of the Effect of Degassing on the Microstructure, Chemistry and Estimated Strength of Nanocrystalline AA5083 Powder.
Creator: Hofmeister, Clara, Sohn, Yongho, Challapalli, Suryanarayana, Coffey, Kevin, University of Central Florida
Abstract / Description: Degassing is a critical heat treatment process in aluminum powder metallurgy, where powders are subjected to high temperature in vacuum to remove volatile gaseous species absorbed in and adsorbed on powders. For cryomilled aluminum alloy powders with nanoscale features, degassing can cause accelerated microstructural and chemical changes including removal of volatiles, grain growth, dislocation annihilation, and formation of dispersoids. These changes can significantly alter the mechanical...
Show moreDegassing is a critical heat treatment process in aluminum powder metallurgy, where powders are subjected to high temperature in vacuum to remove volatile gaseous species absorbed in and adsorbed on powders. For cryomilled aluminum alloy powders with nanoscale features, degassing can cause accelerated microstructural and chemical changes including removal of volatiles, grain growth, dislocation annihilation, and formation of dispersoids. These changes can significantly alter the mechanical behavior of consolidated components based on their contributions to strength. In this study, cryomilled AA5083 (0.4 wt.% Mn; 4.5 wt.% Mg; minor Si, Fe, Cu, Cr, Zn, Ti; balance Al) powders were degassed at 200, 300, 350, 410 and 500(&)deg;C at a ramp rate of 68.3 (&)deg;C?hr-1 for a soak time of 8 hours with a vacuum at or below 6.5 x 10-3 Pa. Grain size, dislocation density and dispersoid phase constituents were examined as a function of degassing temperature by X-ray diffraction, scanning electron microscopy and transmission electron microscopy, equipped with high angle annular dark field detector and X-ray energy dispersive spectroscopy. Inert gas fusion and thermal conductivity analysis were employed to determine the oxygen, nitrogen and hydrogen concentrations as a function of degassing temperature. Grain size in as-cryomilled powders (21 ~ 34 nm) increased as a function of degassing temperature, and reached a maximum value of 70 ~ 80 nm for powders degassed at 500(&)deg;C for 8 hours. The dislocation density of 1.11 x 1015 m-2 in as-cryomilled powders decreased to 1.56 x 1014 m-2 for powders degassed at 500(&)deg;C for 8 hours. The Al6(MnFeCr) phase was the most commonly observed dispersoid, mostly on samples degassed at or above 300(&)deg;C. Volume fraction increased with degassing temperature up to 5 vol.% and the size of the dispersoids grew up to ~ 280 nm. Oxygen and nitrogen content after cryomilling were unaffected by the change in degassing temperature, but the hydrogen content decreased and reached a minimum of 45 (&)#177; 3.16 ppm for cryomilled powders degassed at 500(&)deg;C for 8 hours. Grain growth was quantitatively analyzed based on the general grain growth formula and Burke's model in the presence of pinning forces. Degassing occurred in two different kinetic regimes: Harrison A kinetics at higher temperatures and Harrison B in the lower with a transition temperature of about 287(&)deg;C. Burke's model exhibited a poor fit to the experimental results in higher temperature regime. Desorption of impurities during degassing was analyzed using Fickian diffusion in a spherical coordinate system and an empirical expression based on the exponential decay of average concentration. The activation energy for degassing was estimated to be 16.2 (&)#177; 1.5 kJ?mol-1. Evolutions in composition and microstructure in cryomilled powders as a function of degassing temperature were further analyzed and quantitatively correlated to the strengthening mechanisms of solid solution, grain size reduction (i.e., Hall-Petch), dislocation forest and Orowan. For consolidated AA5083 derived from cryomilled powders, strengthening by grain size reduction was the dominant mechanism of strengthening.
Show less
Date Issued: 2016
Identifier: CFE0006461, ucf:51426
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0006461

Title: Interdiffusion Study of Mg-AA6061 System.
Creator: Fu, Mian, Sohn, Yongho, Coffey, Kevin, An, Linan, University of Central Florida
Abstract / Description: Magnesium (Mg) is a light-weight metal that has extraordinary physical and chemical properties for many potential applications in automobile, military, and electronics. Aluminum alloys, because of its light-weight, high strength and corrosion resistance have a wide range of commercial applications. Given these two, sometime competing, alloy systems, there are now many applications where the metallurgical compatibility of Mg- and Al-alloys are required for engineering applications. One such...
Show moreMagnesium (Mg) is a light-weight metal that has extraordinary physical and chemical properties for many potential applications in automobile, military, and electronics. Aluminum alloys, because of its light-weight, high strength and corrosion resistance have a wide range of commercial applications. Given these two, sometime competing, alloy systems, there are now many applications where the metallurgical compatibility of Mg- and Al-alloys are required for engineering applications. One such case is the development of diffusion barrier for U-Mo metallic fuel in Al-alloy cladding, where Mg, with its complete immiscibility with U and Mo is being considered as the diffusion barrier. While negligible diffusional interaction between Mg and U-Mo alloys have been reported, diffusional interaction between the Mg and Al-alloy cladding has not been investigated. In this study, solid-to-solid diffusion couples were assembled using discs of pure Mg (99.999 %) and AA6061 Al-alloy. After preparation, Mg was diffusion bonded to AA6061 in sealed quartz capsule at 300(&)deg;, 350(&)deg;, and 400(&)deg;C for 720, 360, and 240 hours, respectively. Scanning electron microscopy was used to inspect the interdiffusion zone, while phase identification was performed using X-ray energy dispersive spectroscopy. One specific phase that exists in the binary Mg-Al system, labeled (")epsilon(") was observed and characterized by transmission electron microscopy. From the preceding data, the growth rates as well as interdiffusion coefficients of the intermetallic phases were extracted and compared to previous investigations using pure Mg and Al.
Show less
Date Issued: 2013
Identifier: CFE0005333, ucf:50521
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0005333

Title: The Influence of Alloying Additions on Diffusion and Strengthening of Magnesium.
Creator: Kammerer, Catherine, Sohn, Yongho, Coffey, Kevin, Challapalli, Suryanarayana, Gordon, Ali, University of Central Florida
Abstract / Description: Magnesium alloys are being developed as advanced materials for structural applications where reduced weight is a primary motivator. Alloying can enhance the properties of magnesium without significantly affecting its density. Essential to alloy development, inclusive of processing parameters, is knowledge of thermodynamic, kinetic, and mechanical behavior of the alloy and its constituents. Appreciable progress has been made through conventional development processes, but to accelerate...
Show moreMagnesium alloys are being developed as advanced materials for structural applications where reduced weight is a primary motivator. Alloying can enhance the properties of magnesium without significantly affecting its density. Essential to alloy development, inclusive of processing parameters, is knowledge of thermodynamic, kinetic, and mechanical behavior of the alloy and its constituents. Appreciable progress has been made through conventional development processes, but to accelerate development of suitable wrought Mg alloys, an integrated Materials Genomic approach must be taken where thermodynamics and diffusion kinetic parameters form the basis of alloy design, process development, and properties-driven applications.The objective of this research effort is twofold: first, to codify the relationship between diffusion behavior, crystal structure, and mechanical properties; second, to provide fundamental data for the purpose of wrought Mg alloy development. Together, the principal deliverable of this work is an advanced understanding of Mg systems. To that end, the objective is accomplished through an aggregate of studies. The solid-to-solid diffusion bonding technique is used to fabricate combinatorial samples of Mg-Al-Zn ternary and Mg-Al, Mg-Zn, Mg-Y, Mg-Gd, and Mg-Nd binary systems. The combinatorial samples are subjected to structural and compositional characterization via Scanning Electron Microscopy with X-ray Energy Dispersive Spectroscopy, Electron Probe Microanalysis, and analytical Transmission Electron Microscopy. Interdiffusion in binary Mg systems is determined by Sauer-Freise and Boltzmann-Matano methods. Kirkaldy's extension of the Boltzmann-Matano method, on the basis of Onsager's formalism, is employed to quantify the main- and cross-interdiffusion coefficients in ternary Mg solid solutions. Impurity diffusion coefficients are determined by way of the Hall method. The intermetallic compounds and solid solutions formed during diffusion bonding of the combinatorial samples are subjected to nanoindentation tests, and the nominal and compositionally dependent mechanical properties are extracted by the Oliver-Pharr method.In addition to bolstering the scantly available experimental data and first-principles computations, this work delivers several original contributions to the state of Mg alloy knowledge. The influence of Zn concentration on Al impurity diffusion in binary Mg(Zn) solid solution is quantified to impact both the pre-exponential factor and activation energy. The main- and cross-interdiffusion coefficients in the ternary Mg solid solution of Mg-Al-Zn are reported wherein the interdiffusion of Zn is shown to strongly influence the interdiffusion of Mg and Al. A critical examination of rare earth element additions to Mg is reported, and a new phase in thermodynamic equilibrium with Mg-solid solution is identified in the Mg-Gd binary system. It is also demonstrated that Mg atoms move faster than Y atoms. For the first time the mechanical properties of intermetallic compounds in several binary Mg systems are quantified in terms of hardness and elastic modulus, and the influence of solute concentration on solid solution strengthening in binary Mg alloys is reported. The most significant and efficient solid solution strengthening is achieved by alloying Mg with Gd. The Mg-Nd and Mg-Gd intermetallic compounds exhibited better room temperature creep resistance than intermetallic compounds of Mg-Al. The correlation between the concentration dependence of mechanical properties and atomic diffusion is deliberated in terms of electronic nature of the atomic structure.
Show less
Date Issued: 2015
Identifier: CFE0005815, ucf:50043
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0005815

Title: Development of Nitrogen Concentration During Cryomilling of Aluminum Composites.
Creator: Hofmeister, Clara, Sohn, Yongho, Suryanarayana, Challapalli, Coffey, Kevin, University of Central Florida
Abstract / Description: The ideal properties of a structural material are light weight with extensive strength and ductility. A composite with high strength and tailorable ductility was developed consisting of nanocrystalline AA5083, boron carbide and coarser grained AA5083. The microstructure was determined through optical microscopy and transmission electron microscopy. A technique was developed to determine the nitrogen concentration of an AA5083 composite from secondary ion mass spectrometry utilizing a nitrogen...
Show moreThe ideal properties of a structural material are light weight with extensive strength and ductility. A composite with high strength and tailorable ductility was developed consisting of nanocrystalline AA5083, boron carbide and coarser grained AA5083. The microstructure was determined through optical microscopy and transmission electron microscopy. A technique was developed to determine the nitrogen concentration of an AA5083 composite from secondary ion mass spectrometry utilizing a nitrogen ion-implanted standard. Aluminum nitride and amorphous nitrogen-rich dispersoids were found in the nanocrystalline aluminum grain boundaries. Nitrogen concentration increased as a function of cryomilling time up to 72hours. A greater nitrogen concentration resulted in an enhanced thermal stability of the nanocrystalline aluminum phase and a resultant increase in hardness. The distribution of the nitrogen-rich dispersoids may be estimated considering their size and the concentration of nitrogen in the composite. Contributions to strength and ductility from the Orowan relation can be more accurately modeled with the quantified nitrogen concentration.
Show less
Date Issued: 2013
Identifier: CFE0004864, ucf:49702
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0004864

Title: Theoretical And Computational Studies Of Diffusion Of Adatom Islands And Reactions Of Molecules On Surfaces.
Creator: Shah, Syed Islamuddin, Rahman, Talat, Kara, Abdelkader, Schelling, Patrick, Coffey, Kevin, University of Central Florida
Abstract / Description: The work presented in this dissertation focuses on the study of post deposition spatial and temporal evolution of adatom islands and molecules on surfaces using ab initio and semiemperical methods. It is a microscopic study of the phenomena of diffusion and reaction on nanostructured surfaces for which we have developed appropriate computational tools,as well as implemented others that are available. To map out the potential energy surface on which the adatom islands and molecules move, we...
Show moreThe work presented in this dissertation focuses on the study of post deposition spatial and temporal evolution of adatom islands and molecules on surfaces using ab initio and semiemperical methods. It is a microscopic study of the phenomena of diffusion and reaction on nanostructured surfaces for which we have developed appropriate computational tools,as well as implemented others that are available. To map out the potential energy surface on which the adatom islands and molecules move, we have carried out ab initio electronic structure calculations based on density functional theory (DFT) for selected systems. For others, we have relied on semiempirical interatomic potentials derived from the embedded atom method. To calculate the activation energy barriers, we have employed the (")drag(") method in most cases and verified its reliability by employing the more accurate nudged elastic band method for selected systems. Temporal and spatial evolution of the systems of interest have been calculated using the kinetic Monte Carlo (KMC), or the more accurate (complete) Self Learning kinetic Monte Carlo (SLKMC) method in the majority of cases, and ab initio molecular dynamics simulations in others. We have significantly enhanced the range of applicability of the SLKMC method by introducing a new pattern recognitionscheme which by allowing occupancy of the (")fcc(") and (")hcp(") sites (and inclusion of (")top(") site in the pattern recognition as well) is capable of simulating the morphological evolution of three dimensional adatom islands, a feature not feasible via the earlier - proposed SLKMC method. Using SLKMC (which allows only fcc site occupancy on fcc(111) surface), our results of the coarsening of Ag islands on the Ag(111) surface show that during early stages, coarsening proceeds as a sequence of selected island sizes, creating peaks and valleys in the island-size distribution. This island size selectivity is independent of initial conditions andresults from the formation of kinetically stable islands for certain sizes as dictated by the relative energetics of edge atom detachment/attachment processes together with the large activation barrier for kink detachment.On applying the new method, SLKMC-II, to examine the self diffusion of smalladatom islands (1-10 atoms) of Cu on Cu(111), Ag on Ag(111) and Ni on Ni(111), wefind that for the case of Cu and Ni islands, diffusion is dominated by concerted processes(motion of island as a whole), whereas in the case of Ag, islands of size 2-9 atoms diffusethrough concerted motion whereas the 10-atom island diffuses through single atom processes.Effective energy barriers for the self diffusion of these small Cu islands is 0.045 eV/atom,for Ni it is 0.060 eV/atom and for Ag it is 0.049 eV/atom, increasing almost linearly withisland size.Application of DFT based techniques have allowed us to address a few issues stemmingfrom experimental observations on the effect of adsorbates such as CO on the structure and stability of bimetallic systems (nanoparticles and surfaces). Total energy calculationsof Ni-Au nanoparticles show Ni atoms to prefer to be in the interior of the nanoparticle.CO molecules, however, prefer to bind to a Ni atom if present on the surface. Using abinitio molecular dynamics simulations, we confirm that the presence of CO molecule induces diffusion of Ni atom from the core of the Ni-Au nanoparticle to its surface, making the nanoparticle more reactive. These results which help explain a set of experimental data are rationalized through charge transfer analysis.Similar to the case of Ni-Au system, it is found that methoxy (CH$_{3}$O) may also induce diffusion of inner atoms to the surface on bimetallic Au-Pt systems. Our total energy DFT calculations show that it is more favorable for methoxy to bind to a Pt atom in the top Au layer than to a Au atom in Au-Pt system thereby explaining experimental observations.To understand questions related to the dependence of product selectivity on ambientpressure for ammonia decomposition on RuO2(110), we have carried out an extensivecalculation of the reaction pathways and energy barriers for a large number of intermediate products. On combining the reaction energetics from DFT, with KMC simulations, we showthat under UHV conditions, selectivity switches from N2 ( ? 100 % selectivity) at T = 373Kto NO at T = 630K, whereas under ambient conditions, N2 is still the dominant productbut maximum selectivity is only 60%. An analysis based on thermodynamics alone shows a contradiction between experimental data at UHV with those under ambient pressure. Ourcalculations of the reaction rates which are essential for KMC simulations removes this apparentinconsistency and stresses the need to incorporate kinetics of processes in order toextract information on reaction selectivity.
Show less
Date Issued: 2013
Identifier: CFE0005254, ucf:50584
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0005254

Title: Barium Based Halide Scintillator Ceramics for Gamma Ray Detection.
Creator: Shoulders, William, Gaume, Romain, Coffey, Kevin, Sohn, Yongho, University of Central Florida
Abstract / Description: As our understanding of ceramic processing methods for the purpose of fabricating polycrystalline optical materials has increased over the past few decades, the race is on to bring ceramic technology to markets where single crystalline materials have traditionally been used. One such market is scintillators. This Master's thesis focuses specifically on a class of materials attractive for use as gamma-ray scintillators. These barium based halides can potentially be utilized in fields...
Show moreAs our understanding of ceramic processing methods for the purpose of fabricating polycrystalline optical materials has increased over the past few decades, the race is on to bring ceramic technology to markets where single crystalline materials have traditionally been used. One such market is scintillators. This Master's thesis focuses specifically on a class of materials attractive for use as gamma-ray scintillators. These barium based halides can potentially be utilized in fields applications ranging from ionizing radiation detection in the field to high-energy physics experimentation. Barium bromide iodide and barium chloride single crystals have already showed high light yield, fast scintillation decay, and high energy resolution, all desirable properties for a scintillator. This work attempts to show the likelihood of moving towards polycrystalline scintillators to take advantage of the lower processing temperature, higher manufacturing output, and overall reduced cost. The experiments begin with identifying appropriate sintering conditions for hot pressed ceramics of BaBrI and BaCl2. Possible sources of optical loss in the first phase of hot pressed samples are investigated using a wide range of characterization tools. Preliminary luminescence and scintillation measurements are reported for a translucent sample of BaBrI. Recommendations are made to move toward highly transparent ceramics with scintillation properties approaching those measured in single crystal samples.
Show less
Date Issued: 2013
Identifier: CFE0005258, ucf:50576
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0005258

Title: Time-Resolved Phosphorescence Spectroscopy at Cryogenic Temperatures for the Environmental Analysis of Polycyclic Aromatic Sulfur Heterocycles in Oil Contaminated Sites.
Creator: Al Tameemi, Maha, Campiglia, Andres, Zou, Shengli, Harper, James, Heider, Emily, Coffey, Kevin, University of Central Florida
Abstract / Description: The large volume of crude oil released into the Gulf of Mexico by the Deepwater Horizon (DWH) accident has raised considerable concerns over potential ecosystem impacts. The dispersion of harmful oil components into the ocean waters could pose long term risks to flora and fauna. Due to the complexity of oil contaminated sites, the unambiguous identification and quantitation of environmental pollutants often requires the sequence of high-performance liquid chromatography (HPLC) and gas...
Show moreThe large volume of crude oil released into the Gulf of Mexico by the Deepwater Horizon (DWH) accident has raised considerable concerns over potential ecosystem impacts. The dispersion of harmful oil components into the ocean waters could pose long term risks to flora and fauna. Due to the complexity of oil contaminated sites, the unambiguous identification and quantitation of environmental pollutants often requires the sequence of high-performance liquid chromatography (HPLC) and gas chromatography-mass spectrometry (GC-MS). A classic example is the analysis of the sixteen polycyclic aromatic hydrocarbons included in the priority pollutants list of the U.S. Environmental Protection Agency (EPA-PAHs).This dissertation tackles a different aspect of environmental analysis as is focuses on the photoluminescence spectroscopy of polycyclic aromatic sulfur heterocycles (PASHs). Since considering the EPA-PAHs alone can lead to drastic underestimations of potential toxic effects of oil spills, a strong case can be made for including hetero-aromatic compounds in risk assessments of contaminated sites. PASHs exist in an even greater variety of chemical structures than PAHs and, because of the asymmetry imposed by the heteroatom, the number of PASHs isomers is usually large. The existence of numerous isomers of the same molecular weight increases the difficulty of separation and identification by chromatographic methods.This dissertation demonstrates the capability to differentiate individual PASHs isomers of MW 234 g mol-1 via vibrational spectroscopy at liquid nitrogen (77 K) and liquid helium (4.2K) temperatures. Fluorescence and phosphorescence spectra are presented for isomer determination at the parts-per-billion (ng. mL-1) concentration levels. It is demonstrated that the relatively long phosphorescence decays of PASHs facilitate the time discrimination of strong fluorescence.interference from PAHs and methylated-PAHs often present in Normal-Phase HPLC fractions. The spectral and lifetime databases compiled in this dissertation have paved the road to explore the full dimensionality of photoluminescence spectroscopy.
Show less
Date Issued: 2017
Identifier: CFE0007275, ucf:52183
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0007275

Title: Fundamental core effects in Co-Cr-Fe-Ni based high entropy alloys.
Creator: Mehta, Abhishek, Sohn, Yongho, Coffey, Kevin, Kushima, Akihiro, Jiang, Tengfei, Stolbov, Sergey, University of Central Florida
Abstract / Description: High entropy alloys (HEAs) are near equiatomic multi-principal-element-alloys (MPEAs) which are different from traditional solvent-based multicomponent alloys. Based on initial work by Yeh and Co-workers, they were proposed to exhibit four (")core(") effects: high entropy, sluggish diffusion, lattice distortion, and cocktail effect. Present work investigates two of the four (")core(") effects, i.e. high entropy and sluggish diffusion effects, in Co-Cr-Fe-Ni based transition metal high entropy...
Show moreHigh entropy alloys (HEAs) are near equiatomic multi-principal-element-alloys (MPEAs) which are different from traditional solvent-based multicomponent alloys. Based on initial work by Yeh and Co-workers, they were proposed to exhibit four (")core(") effects: high entropy, sluggish diffusion, lattice distortion, and cocktail effect. Present work investigates two of the four (")core(") effects, i.e. high entropy and sluggish diffusion effects, in Co-Cr-Fe-Ni based transition metal high entropy alloys. Solid-to-solid diffusion couple approach was adopted to investigate, these core effects. Experimental results contradicts the (")high entropy(") effect based on thermodynamics analysis: that the HEAs with low entropy of mixing may be thermodynamically more stable than the HEA of similar constituent elements with high entropy of mixing. In such cases, enthalpy of mixing can also play a vital role in stabilizing the HEA with lower entropy of mixing. Measurement of diffusion coefficients (i.e. both interdiffusion and tracer diffusion coefficients) in HEAs and its comparison with conventional solvent-based multicomponent alloys suggests that diffusion is not always sluggish in high entropy alloys. Contrary to previous findings, larger fluctuations in lattice potential energy (LPE) of an alloy may not always result in anomalously slow diffusion, in comparison to alloy systems which exhibits smaller fluctuation in LPE. Findings from his dissertation provide a (")controversial(") understanding of high entropy alloys, and alloy development strategies in the future for the most aggressive applications such as those found in gas turbines and nuclear reactors. As these applications will certainly require the knowledge of high temperature stability and nature of diffusion under extreme application environment.
Show less
Date Issued: 2019
Identifier: CFE0007501, ucf:52645
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0007501

Title: Simultaneous Measurement of Isotope-free Tracer and Interdiffusion Coefficients in Sandwich Type Diffusion Couples.
Creator: Schulz, Esin, Sohn, Yongho, Bai, Yuanli, Florczyk, Stephen, Coffey, Kevin, Orlovskaya, Nina, University of Central Florida
Abstract / Description: The experimental determination of the tracer or self-diffusion coef?cient as a function of composition can be quite burdensome in alloys since separate measurements must be carried out for each alloy composition. A new formalism recently developed by I.V. Belova, N.S. Kulkarni, Y.H. Sohn and G.E. Murch, based on linear response theory combined with the Boltzmann(-)Matano method allows determination of tracer and interdiffusion coef?cients simultaneously from a single, isotope-free solid to...
Show moreThe experimental determination of the tracer or self-diffusion coef?cient as a function of composition can be quite burdensome in alloys since separate measurements must be carried out for each alloy composition. A new formalism recently developed by I.V. Belova, N.S. Kulkarni, Y.H. Sohn and G.E. Murch, based on linear response theory combined with the Boltzmann(-)Matano method allows determination of tracer and interdiffusion coef?cients simultaneously from a single, isotope-free solid to solid diffusion couple experiment. In this study, for the first time, an experimental investigation with an analytical approach based on the new formalism has been carried out in the binary Cu-Ni system. Pure Cu and Ni thin films were deposited in between several binary diffusion couples with varying terminal alloy compositions (such as Cu, Cu-25Ni, Cu-50Ni, Cu-75Ni, Ni). Diffusion couples were then annealed at 800(&)deg;C, 900(&)deg;C and 1000(&)deg;C. After annealing, the couples were water quenched, cross-sectioned, and prepared for compositional characterization. Scanning Electron Microscopy was employed to examine the interdiffusion zone. Energy Dispersive X-ray Spectroscopy was conducted to obtain concentration profiles for quantitative analysis. The superposition of the concentration profiles of thin film and interdiffusion were analyzed for the simultaneous determination of tracer and interdiffusion coefficients. The tracer diffusion coefficient of Cu, tracer diffusion coefficient of Ni and inter-diffusion coefficients simultaneously determined using the experimental methodology based on the novel formalism derived, and produced results consistent with previously reported values determined independently by radiotracer and interdiffusion experiments.
Show less
Date Issued: 2018
Identifier: CFE0007368, ucf:52101
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0007368

Title: Advancements in Liquid Chromatography for the Determination of Polycyclic Aromatic Compounds in Environmental Samples.
Creator: Hayes, Hugh, Campiglia, Andres, Harper, James, Yestrebsky, Cherie, Frazer, Andrew, Coffey, Kevin, University of Central Florida
Abstract / Description: The purpose of this dissertation is to provide a new and improved analytical methodology for the analysis of polycyclic aromatic hydrocarbons (PAHs) and polycyclic aromatic sulfur heterocycles (PASHs) in environmental samples. Hundreds of PAHs and PASHs are present in the environment and have great toxicological importance and the chemical determination of these components is a priority. The complete normal-phase liquid chromatography (NPLC) retention behavior has been explored for these...
Show moreThe purpose of this dissertation is to provide a new and improved analytical methodology for the analysis of polycyclic aromatic hydrocarbons (PAHs) and polycyclic aromatic sulfur heterocycles (PASHs) in environmental samples. Hundreds of PAHs and PASHs are present in the environment and have great toxicological importance and the chemical determination of these components is a priority. The complete normal-phase liquid chromatography (NPLC) retention behavior has been explored for these compounds. Retention indices were determined for 124 PAHs, 62 methyl-PAHs (MePAHs), 67 PASHs, and 80 alkyl-PASHs on an aminopropyl (NH2) stationary phase which represents the most comprehensive study of polycyclic aromatic compounds in normal phase conditions to date. NPLC retention behavior for PAHs and PASHs directly correlated to the total number of aromatic carbons in the parent structures. The normal-phase retention behavior information was used to develop an NPLC fractionation procedure to aid in the sample cleanup for complex environmental matrices which can later be analyzed by gas chromatography/mass spectrometry (GC/MS). Standard Reference Material (SRM) 1597a (complex mixture of PAHs in coal tar), SRM 1991 (coal tar/petroleum extract), and SRM 1975 (diesel particulate extract) were analyzed before and after NPLC fractionation by using GC/MS. In SRM 1597a, the NPLC-GC/MS method allowed for the identification of 72 PAHs, 56 Me-PAHs, 35 PASHs, and 59 alkyl-PASHs. The NPLC-GC/MS procedure also provided the tentative identification of 74 PAHs and 117 MePAHs based on the molecular ion peak only. Furthermore, this method allowed for the following identification breakdown: SRM 1991(-)31 PASHs and 58 alkyl-PASHs; and SRM 1975(-)13 PASHs and 25 alkyl-PASHs.Additional work related to the NPLC fractionation of SRM 1597 included the collection of room-temperature fluorescence spectra for the 21 PAHs with molecular mass (MM) 302 Da known to be in SRM 1597a during reversed-phase liquid chromatography (RPLC) separation. Adding spectral features to a chromatographic run provided the tentative identification of 20 PAHs based on retention times and the presence of 18 were confirmed on the basis of spectral profiles. The advancements in liquid chromatography presented in this dissertation via NPLC fractionation along with RPLC stop-flow fluorescence spectra collection shows potential for becoming routine methodologies for PAC determination in complex environmental samples.
Show less
Date Issued: 2018
Identifier: CFE0007190, ucf:52244
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0007190

Title: Diffusion and reaction in selected uranium alloy system.
Creator: Huang, Ke, Sohn, Yongho, An, Linan, Xu, Chengying, Coffey, Kevin, Heinrich, Helge, University of Central Florida
Abstract / Description: U-Mo metallic fuels with Al alloys as the matrix/cladding are being developed as low enriched uranium fuels under the Reduced Enrichment for Research and Test Reactor (RERTR) program. Significant interactions have been observed to occur between the U-Mo fuel and the Al alloy during fuel processing and irradiation. U-Zr metallic fuels with stainless steel claddings have been developed for the generation IV sodium fast reactor (SFR). The fuel cladding chemical interaction (FCCI) induced by the...
Show moreU-Mo metallic fuels with Al alloys as the matrix/cladding are being developed as low enriched uranium fuels under the Reduced Enrichment for Research and Test Reactor (RERTR) program. Significant interactions have been observed to occur between the U-Mo fuel and the Al alloy during fuel processing and irradiation. U-Zr metallic fuels with stainless steel claddings have been developed for the generation IV sodium fast reactor (SFR). The fuel cladding chemical interaction (FCCI) induced by the interdiffusion of components was also observed. These interactions induce deleterious effects on the fuel system, such as thinning of the cladding layer, formation of phases with undesirable properties, and thermal cracking due to thermal expansion mismatches and changes in molar volume. The interaction between the fuel and the cladding involves multi-component interdiffusion. To determine the ternary interdiffusion coefficients using a single diffusion couple, a new method based on regression via the matrix transformation approach is proposed in this study. This new method is clear in physical meaning and simple in mathematical calculation. The reliability and accuracy of this method have been evaluated through application to three case studies: a basic asymptotic concentration profile, a concentration profile with extrema and a smoothed concentration profile with noise. Generally, this new method works well in all three cases.In order to investigate the interdiffusion behavior in U-Mo alloys, U vs. Mo diffusion couples were assembled and annealed in the temperature range of 650 to 1000(&)deg;C. The interdiffusion microstructures and concentration profiles were examined via scanning electron microscopy (SEM) and electron probe microanalysis (EPMA), respectively. Interdiffusion coefficients and activation energies were calculated as functions of temperature and Mo composition. The intrinsic diffusion coefficients of U and Mo at the marker composition were also determined. The activity of U and the thermodynamic factor of the U-Mo alloy have been calculated using the ideal solution, the regular solution, and the subregular solution models based on the molar excess Gibbs free energy of the U-Mo alloy. The calculated intrinsic diffusivities of U and Mo along with the thermodynamic factor of the U-Mo alloy were employed to estimate the atomic mobilities and the vacancy wind effects of U and Mo according to Manning's description.To explore potential diffusion barrier materials for reducing the fuel cladding chemical interaction between the U-Mo fuel and the Al alloy matrix/cladding, the interdiffusion behavior between U-Mo alloys and Mo, Zr, Nb and Mg were systematically studied. U-10wt.%Mo vs. Mo, Zr and Nb diffusion couples were annealed in the temperature range from 600 to 1000(&)deg;C. A diffusion couple between U-7wt.%Mo and Mg was annealed at 550(&)deg;C for 96 hours. SEM and transmission electron microscopy (TEM) were applied to characterize the microstructure of the interdiffusion zone. X-ray energy dispersive spectroscopy (XEDS) and EPMA were utilized to examine the concentration redistribution and the phase constituents. For the U-Mo vs. Mo diffusion couples, the interdiffusion coefficients at high Mo concentrations ranging from 22 to 32 at.%Mo were determined for the first time. In the U-Mo vs. Zr diffusion couples, the Mo2Zr phase was found at the interface. The diffusion paths were estimated and investigated according to the Mo-U-Zr ternary phase diagram. Thermal cracks and pure U precipitates were found within the diffusion zone in the U-Mo vs. Nb system. The growth rate of the interdiffusion zone was found to be lower by about 103 times for Zr, 105 times for Mo and 106 times for Nb compared to those observed in the U-10wt.%Mo vs. Al or Al-Si systems. For the diffusion couple of U-Mo vs. Mg, the U-Mo was bonded very well to the Mg and there was negligible diffusion observed even after 96 hours annealing at 550(&)deg;C.For a more fundamental understanding of the complex diffusion behavior between U-Zr fuels and their stainless steel claddings, U vs. Fe, Fe-15wt.%Cr and Fe-15wt.%Cr-15wt.%Ni diffusion couples were examined to investigate the interdiffusion behaviors between U and Fe and the effects of the alloying elements Cr and Ni. The diffusion couples were annealed in the temperature range from 580 to 700(&)deg;C for various times. Two intermetallic phases, U6Fe and UFe2, developed in all of the diffusion couples with the U6Fe layer growing faster than the UFe2 layer. For the diffusion couples of U vs. Fe, extrinsic growth constants, intrinsic growth constants, integrated interdiffusion coefficients and activation energies in each phase were calculated. The results suggest that U6Fe impeded the growth of UFe2, and the boundary condition change caused by the allotropic transformation of U played a role in the growth of the U6Fe and UFe2 layers. The reasons why U6Fe grew much faster than UFe2 are also discussed. The additions of Cr and Ni into Fe affected the growth rates of U6Fe and UFe2. The solubility of Cr and Ni in U6Fe and UFe2 were determined, and it was found that Cr diffused into U more slowly than Fe or Ni.
Show less
Date Issued: 2012
Identifier: CFE0004548, ucf:49238
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0004548

Title: Rare Earth Oxide Coating with Controlled Chemistry Using Thermal Spray.
Creator: Singh, Virendra, Seal, Sudipta, Coffey, Kevin, Raghavan, Seetha, Heinrich, Helge, Zhai, Lei, University of Central Florida
Abstract / Description: Cerium oxide (Ceria) at nano scale has gained significant attention due to its numerous technological applications. Ceria in both doped and undoped forms are being explored as oxygen sensor, catalysis, protective coating against UV and corrosion, solid oxide fuel cell (SOFC) electrolyte and newly discovered antioxidant for biomedical applications. Therefore, there is an imminent need of a technology which can provide a cost effective, large scale manufacturing of nanoceria and its subsequent...
Show moreCerium oxide (Ceria) at nano scale has gained significant attention due to its numerous technological applications. Ceria in both doped and undoped forms are being explored as oxygen sensor, catalysis, protective coating against UV and corrosion, solid oxide fuel cell (SOFC) electrolyte and newly discovered antioxidant for biomedical applications. Therefore, there is an imminent need of a technology which can provide a cost effective, large scale manufacturing of nanoceria and its subsequent consolidation, specially using thermal spray.This dissertation aims to develop a scientific understanding towards the development of pure and doped ceria- based coating for a variety of technological applications, from SOFC applications to corrosion resistant coating. Atmospheric plasma spray (APS) and solution precursor plasma spray (SPPS) techniques for the fabrication of nano ceria coating were investigated. For feedstock powder preparation, a spray drying technique was used for the agglomeration of cerium oxide nano particles to achieve high density coating. Deposition efficiencies and coating porosity as a function of processing parameters were analyzed and optimized using a statistical design of experiment model. The coating deposition efficiency was dependent on the plasma temperature and vaporization pressure of the ceria nanoparticles. However, low standoff distance and high carrier gas flow rate were responsible for the improved density upto 86 (&)#177;3%.An alternative novel SPPS technique was studied for a thin film of cerium oxide deposition from various cerium salt precursors in doped and undoped conditions. The SPPS process allows controlling the chemistry of coating at a molecular level. The deposition mechanism by single scan experiments and the effect of various factors on coating microstructure evolution were studied in terms of splats formation. It was found that the precursor salt (nitrate of cerium) with lower thermal decomposition temperatures was suitable for a high density coating. The high concentration and low spray distance significantly improve the splat morphology and reduced porosity (upto 20%). The feasibility of the trivalent cations (Sm 3+ and Gd 3+) doping into cerium oxide lattice in high temperature plasma was discussed and experimentally studied. XRD analysis revealed the nano crystalline characteristic of the coating and lattice expansion due to doping. The extensive transmission electron microscopy, Scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and thermo gravimetric were conducted to evaluate the precursors, and coating microstructure.Due to facial switching between Ce4+ and Ce3+ oxidation state, the cerium oxide surface becomes catalytically active. Thus, the APS ceria coatings were investigated for their applicability under extreme environmental conditions (high pressure and temperature). The air plasma sprayed coated 17-4PH steel was subjected to high pressure (10 Kpsi) and temperature (300 oF) corrosive environment. The coated steel showed continuous improvement in the corrosion resistance at 3.5 wt% NaCl at ambient temperature for three months study whereas, high pressure did not reveal a significant role in the corrosion process, and however, one needs to do further research. The ceria coated steel also revealed the improvement in corrosion protection (by 4 times) compared to the bare steel at low pH, 300 oF and 4000 Psi environment. This study projects the importance of cerium oxide coatings, their fabrication, optimization and applications.
Show less
Date Issued: 2012
Identifier: CFE0004230, ucf:49023
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0004230

Title: Direct measurement of thicknesses, volumes or compositions of nanomaterials by quantitative atomic number contrast in high-angle annular dark-field scanning transmission electron microscopy.
Creator: Yuan, Biao, Heinrich, Helge, Sohn, Yongho, Coffey, Kevin, Fang, Jiyu, Roldan Cuenya, Beatriz, University of Central Florida
Abstract / Description: The sizes, shapes, volumes and compositions of nanoparticles are very important parameters determining many of their properties. Efforts to measure these parameters for individual nanoparticles and to obtain reliable statistics for a large number of nanoparticles require a fast and reliable method for 3-D characterization. In this dissertation, a direct measurement method for thicknesses, volumes or compositions of nanomaterials by quantitative atomic number contrast in High-Angle Annular...
Show moreThe sizes, shapes, volumes and compositions of nanoparticles are very important parameters determining many of their properties. Efforts to measure these parameters for individual nanoparticles and to obtain reliable statistics for a large number of nanoparticles require a fast and reliable method for 3-D characterization. In this dissertation, a direct measurement method for thicknesses, volumes or compositions of nanomaterials by quantitative atomic number contrast in High-Angle Annular Dark-Field (HAADF) Scanning Transmission Electron Microscopy (STEM) is presented. A HAADF detector collects electrons scattered incoherently to high angles. The HAADF signal intensity is in first-order approximation proportional to the sample thickness and increases with atomic number. However, for larger sample thicknesses this approach fails. A simple description for the thickness dependence of the HAADF-STEM contrast has been developed in this dissertation. A new method for the calibration of the sensitivity of the HAADF detector for a FEI F30 transmission electron microscope (TEM) is developed in this dissertation. A nearly linear relationship of the HAADF signal with the electron current is confirmed. Cross sections of multilayered samples provided by TriQuint Semiconductors in Apopka, FL, for contrast calibration were obtained by focused ion-beam (FIB) preparation yielding data on the interaction cross section per atom.To obtain an absolute intensity calibration of the HAADF-STEM intensity, Convergent Beam Electron Diffraction (CBED) was performed on Si single crystals. However, for samples prepared by the focused ion beam technique, CBED often significantly underestimates the sample thickness. Multislice simulations from Dr. Kirkland's C codes are used for comparison with experimental results. TEM offers high lateral resolution, but contains little or no information on the thickness of samples. Thickness maps in energy-filtered TEM (EFTEM), CBED and tilt series are so far the only methods to determine thicknesses of particles in TEM. In this work I have introduced the use of wedge-shaped multilayer samples prepared by FIB for the calibration of HAADF-STEM contrasts. This method yields quantitative contrast data as a function of sample thickness. A database with several pure elements and compounds has been compiled, containing experimental data on the fraction of electrons scattered onto the HAADF detector for each nanometer of sample thickness. The use of thick samples reveals an increased signal at the interfaces of high- and low-density materials. This effect can be explained by the transfer of scattered electrons from the high density material across the interface into the less-absorbing low-density material. The calibrations were used to determine concentration gradients in nanoscale Fe-Pt multilayers as well as thicknesses and volumes of individual Au-Fe, Pt, and Ag nanoparticles. Volumes of nanoparticles with known composition can be determined with accuracy better than 15%. Porosity determination of materials becomes available with this method as shown in an example of porous Silicon.?
Show less
Date Issued: 2012
Identifier: CFE0004464, ucf:49355
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0004464

Title: BST-Inspired Smart Flexible Electronics.
Creator: Shen, Ya, Gong, Xun, Wahid, Parveen, Sundaram, Kalpathy, Coffey, Kevin, University of Central Florida
Abstract / Description: The advances in modern communication systems have brought about devices with more functionality, better performance, smaller size, lighter weight and lower cost. Meanwhile, the requirement for newer devices has become more demanding than ever. Tunability and flexibility are both long-desired features. Tunable devices are 'smart' in the sense that they can adapt to the dynamic environment or varying user demand as well as correct the minor deviations due to manufacturing fluctuations,...
Show moreThe advances in modern communication systems have brought about devices with more functionality, better performance, smaller size, lighter weight and lower cost. Meanwhile, the requirement for newer devices has become more demanding than ever. Tunability and flexibility are both long-desired features. Tunable devices are 'smart' in the sense that they can adapt to the dynamic environment or varying user demand as well as correct the minor deviations due to manufacturing fluctuations, therefore making it possible to reduce system complexity and overall cost. It is also desired that electronics be flexible to provide conformability and portability.Previously, tunable devices on flexible substrates have been realized mainly by dicing and assembling. This approach is straightforward and easy to carry out. However, it will become a (")mission impossible(") when it comes to assembling a large amount of rigid devices on a flexible substrate. Moreover, the operating frequency is often limited by the parasitic effect of the interconnection between the diced device and the rest of the circuit on the flexible substrate. A recent effort utilized a strain-sharing Si/SiGe/Si nanomembrane to transfer a device onto a flexible substrate. This approach works very well for silicon based devices with small dimensions, such as transistors and varactor diodes. Large-scale fabrication capability is still under investigation.A new transfer technique is proposed and studied in this research. Tunable BST (Barium Strontium Titanate) IDCs (inter-digital capacitors) are first fabricated on a silicon substrate. The devices are then transferred onto a flexible LCP (liquid crystalline polymer) substrate using wafer bonding of the silicon substrate to the LCP substrate, followed by silicon etching. This approach allows for monolithic fabrication so that the transferred devices can operate in millimeter wave frequency. The tunability, capacitance, Q factor and equivalent circuit are studied. The simulated and measured performances are compared. BST capacitors on LCP substrates are also compared with those on sapphire substrates to prove that this transfer process does not impair the performance.A primary study of a reflectarray antenna unit cell is also conducted for loss and phase swing performance. The BST thin film layout and bias line positions are studied in order to reduce the total loss. Transferring a full-size BST-based reflectarray antenna onto an LCP substrate is the ultimate goal, and this work is ongoing at the University of Central Florida (UCF).HFSS is used to simulate the devices and to prove the concept. All of the devices are fabricated in the clean room at UCF. Probe station measurements and waveguide measurements are performed on the capacitors and reflectarray antenna unit cells respectively.This work is the first comprehensive demonstration of this novel transfer technique.
Show less
Date Issued: 2012
Identifier: CFE0004439, ucf:49339
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0004439

Title: Thermally annealled plasmonic nanostructures.
Creator: Wang, Chaoming, Su, Ming, Coffey, Kevin, Chai, Xinqing, Schelling, Patrick, University of Central Florida
Abstract / Description: Localized surface plasmon resonance (LSPR) is induced in metal nanoparticles by resonance between incident photons and conduction electrons in nanoparticles. For noble metal nanoparticles, LSPR can lead to strong absorbance of ultraviolet-violet light. Although it is well known that LSPR depends on the size and shape of nanoparticles, the inter-particle spacing, the dielectric properties of metal and the surrounding medium, the temperature dependence of LSPR is not well understood. By...
Show moreLocalized surface plasmon resonance (LSPR) is induced in metal nanoparticles by resonance between incident photons and conduction electrons in nanoparticles. For noble metal nanoparticles, LSPR can lead to strong absorbance of ultraviolet-violet light. Although it is well known that LSPR depends on the size and shape of nanoparticles, the inter-particle spacing, the dielectric properties of metal and the surrounding medium, the temperature dependence of LSPR is not well understood. By thermally annealing gold nanoparticle arrays formed by nanosphere lithography, a shift of LSPR peak upon heating has been shown. The thermal characteristics of the plasmonic nanoparticles have been further used to detect chemicals such as explosive and mercury vapors, which allow direct visual observation of the presence of mercury vapor, as well as thermal desorption measurements.
Show less
Date Issued: 2012
Identifier: CFE0004454, ucf:49322
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0004454

Title: Corrosion behaviour of AISI 304 stainless steel in contact with eutectic salt for concentrated solar power plant applications.
Creator: Ahmed, Omar, Sohn, Yongho, Mohajeri, Nahid, Coffey, Kevin, Fang, Jiyu, University of Central Florida
Abstract / Description: In response to the extensive energy demands on national and global levels, concentrated solar power (CSP) plants are designed to harness and convert solar energy to electricity. For such green energy application, robust, reliable and durable materials for CSP constructions are required. The corrosion resistance is among many parameters to consider in these thermal-electrical stations such as for pipes and storage tanks in CSP. In this investigation, the corrosion behavior of AISI 304...
Show moreIn response to the extensive energy demands on national and global levels, concentrated solar power (CSP) plants are designed to harness and convert solar energy to electricity. For such green energy application, robust, reliable and durable materials for CSP constructions are required. The corrosion resistance is among many parameters to consider in these thermal-electrical stations such as for pipes and storage tanks in CSP. In this investigation, the corrosion behavior of AISI 304 stainless steel (18 wt. % Cr, 8 wt. % Ni) with the heat transfer fluid, also known as solar salt, has been examined. The ternary eutectic salt mixture with the composition, 53 wt. % KNO3, 40 wt. % NaNO2, and 7 wt. % NaNO3, that melts at 142(&)deg;C, has a potential use in CSP as a heat transfer fluid. The solar salt was prepared for this corrosion study from reagent grades of high purity nitrites and nitrates. Samples of AISI 304 stainless steel were sectioned from a sheet stock of the alloy and exposed to solar salt at 530(&)deg;C in air at 1 atmospheric pressure. After test intervals of 250, 500, and 750 hours in total immersion condition, AISI 304 stainless steel samples have developed a scale of corrosion products made up of multiple oxides. X-ray diffraction and scanning electron microscopy with X-ray energy-dispersive spectroscopy were employed to examine the extent of corrosion and identify the corrosion products. Transmission electron microscopy was used to verify the corrosion products identity via electron diffraction patterns. Oxides of iron were found to be the primary corrosion products in the presence of the molten alkali nitrates-nitrite salt mixture because of the dissolution of the protective chromium oxide (Cr2O3) scale formed on AISI 304 stainless steel coupons. The corrosion scale was uniform in thickness and made up of sodium iron oxide (NaFeO2), iron oxide, hematite (Fe2O3), and chromium-iron oxide (Cr,Fe)2O3 solid solution. The latter was found near the AISI 304 stainless steel. This indicates that the scale formed, particularly on the upper layers with presence of sodium iron oxide and iron oxide, hematite, is protective, and forms an effective barrier against penetration of fused solar salt. At the alloy interface with the bulk corrosion scale, the corrosion process induced a compositional modification in the grains located at the interface. There are iron rich and iron depleted grains at the interface if compared to the nominal iron content of the alloy. The mode of attack is identified as uniform at the test temperature of 530(&)deg;C, showing a parabolic behavior with a parabolic rate constant (Kp) equals to 1.08(&)#215;10^(-17)(m2/sec). By extrapolation, annual corrosion rate is estimated to reach 0.784 mils per year. Corrosion behavior of AISI 304 stainless steel is discussed in terms of thermodynamics and reaction paths.
Show less
Date Issued: 2013
Identifier: CFE0004969, ucf:49587
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0004969

Title: Size, Shape, Composition and Chemical state effects in nanocatalysis.
Creator: Ahmadi, Mahdi, Roldan Cuenya, Beatriz, Rahman, Talat, Kara, Abdelkader, Coffey, Kevin, University of Central Florida
Abstract / Description: The field of nanocatalysis has gained significant attention in the last decades due to the numerous industrial applications of nanosized catalysts. Size, shape, structure, and composition of the nanoparticles (NPs) are the parameters that can affect the reactivity, selectivity and stability of nanocatalysts. Therefore, understanding how these parameters affect the catalytic properties of these systems is required in order to engineer them with a given desired performance. It is also important...
Show moreThe field of nanocatalysis has gained significant attention in the last decades due to the numerous industrial applications of nanosized catalysts. Size, shape, structure, and composition of the nanoparticles (NPs) are the parameters that can affect the reactivity, selectivity and stability of nanocatalysts. Therefore, understanding how these parameters affect the catalytic properties of these systems is required in order to engineer them with a given desired performance. It is also important to gain insight into the structural evolution of the NP catalysts under different reaction conditions to design catalysts with long durability under reaction condition. In this dissertation a synergistic combination of in situ, ex situ and operando state-of-the art techniques have allowed me to explore a variety of parameters and phenomena relevant to nanocatalysts by systematically tuning the NP size, chemical state, composition and chemical environment.
Show less
Date Issued: 2016
Identifier: CFE0006243, ucf:51084
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0006243

Diffusion (5)	+ -
TEM (5)	+ -
MMC (3)	+ -
interdiffusion (3)	+ -
Aluminum (2)	+ -

Characterization (2)	+ -
Cu (2)	+ -
HAADF (2)	+ -
Interdiffusion (2)	+ -
Intermetallics (2)	+ -
Magnesium (2)	+ -
Nanocomposite (2)	+ -
Nanoindentation (2)	+ -
STEM (2)	+ -
Uranium (2)	+ -
XPS (2)	+ -
aluminum (2)	+ -
diffusion (2)	+ -
metal matrix composite (2)	+ -
zirconium (2)	+ -
15-5 PH (1)	+ -
3D printing (1)	+ -
AA5083 (1)	+ -
AISI 304 stainless steel (1)	+ -
Aerosol (1)	+ -
Al (1)	+ -
Al/CuO (1)	+ -
Alloy (1)	+ -
Aluminium (1)	+ -
Ammonium Perchlorate (1)	+ -

Coffey, Kevin (52)	+ -
University of Central Florida (52)	+ -
Sohn, Yongho (23)	+ -
Heinrich, Helge (9)	+ -
Fang, Jiyu (6)	+ -

Suryanarayana, Challapalli (6)	+ -
An, Linan (4)	+ -
Challapalli, Suryanarayana (4)	+ -
Gaume, Romain (4)	+ -
Campiglia, Andres (3)	+ -
Harper, James (3)	+ -
Kara, Abdelkader (3)	+ -
Orlovskaya, Nina (3)	+ -
Schelling, Patrick (3)	+ -
Sundaram, Kalpathy (3)	+ -
Frazer, Andrew (2)	+ -
Gordon, Ali (2)	+ -
Hickman, James (2)	+ -
Hofmeister, Clara (2)	+ -
Ishigami, Marsahir (2)	+ -
Kammerer, Catherine (2)	+ -
Kar, Aravinda (2)	+ -
Park, Young Joo (2)	+ -
Peale, Robert (2)	+ -
Rahman, Talat (2)	+ -
Roldan Cuenya, Beatriz (2)	+ -
Schoenfeld, Winston (2)	+ -
Seal, Sudipta (2)	+ -
Shoulders, William (2)	+ -
Stolbov, Sergey (2)	+ -

Search In

Pages

Pages

Sort Results By:

Narrow results by:

Topical Subject

Collection

Type of Resource

Language

Creator

Format

Owner