Materials Science & Engineering Theses and Dissertations
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Item Flux Maps Obtained from Core Geometry Approximations: Monte Carlo Simulations and Benchmark Measurements for a 250 kW TRIGA Reactor(2009) Mohamed, Ali Bellou; Al-Sheikhly, Mohamad; Silverman, Joseph; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Two MCNP models (detailed and approximated) of University of Maryland Training Reactor were created. The detailed model attempted to simulate the reactor according to engineering specifications while the simplified model eliminated all structural materials above and below the core. Neutron flux spectrum calculations for both models within the core showed that the results obtained from both models agreed within less than 0.5%. It was concluded that reactors equipped with standard TRIGA fuels enriched to 20 percent in uranium-235 can be modeled with all structures above and below the core eliminated entirely from the model without increasing the error due to geometry modeling simplifications of the core. In TRIGA reactors supplied with standard TRIGA fuels enriched to 20 percent in U-235, the graphite reflectors above and below the fuel act as "neutron energy regulators." Neutrons reflected back into the core through the graphite reflectors quickly become thermalized even if their energies were altered due to the change in materials properties above and below the core. Both MCNP models results agree well with measured data. It was also found that simplification in the target geometry leads to substantial uncertainty in the calculated results. The neutron energy spectrum, thermal flux, and total flux were calculated at the thermal column access plug face; in the pneumatic transfer system rabbit, and on top and bottom sections of the most center fuel element. The thermal flux and the total flux at the thermal column access plug face both agreed with measured data within a 5% uncertainty. The thermal flux, fast flux, and the total flux in the rabbit differ by 18.8%, 35%, and 5.7% respectively, from the measured data. The relatively high uncertainty (in the neutron energy distribution but not the total neutron flux) was attributed to the use of air as the target irradiated inside the rabbit. For such a thin target (15 mg/cm2), a precise neutron balance between reflection and absorption events is difficult to obtain; that will alter the thermal or fast flux values. The contribution of this work to the reactor users is that a virtual reactor model that compared well with experiment is created. Experiments utilizing the reactor experimental facilities (thermal column, through tube, pneumatic transfer system rabbit, and beam ports) can now be optimized before they are executed. The contribution of this work to the research reactor community at is that research reactors equipped with standard TRIGA fuels can be modeled with core geometry approximations, such as these adopted in this work, without affecting the precision and accuracy of the Monte Carlo calculations.Item Modeling and validation of dosimetry measurement assumptions within the Armed Forces Radiobiology Research Institute TRIGA Mark F reactor and associated exposure facilities using Monte Carlo techniques(2009) Hall, Donald Edward; Modarres, Mohammad; Al-Sheikhly, Mohamad; Nuclear Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The TRIGA Mark F reactor at the Armed Forces Radiobiology Research Institute in Bethesda Maryland is a 1 megawatt steady state reactor which can also be operated in pulse mode at a power of up to 2500 megawatts. It is characterized by a moveable core and two large exposure rooms, rather than a thermal column or beam ports, as found in most research reactors. A detailed model of the reactor and the associated exposure facilities was developed using the Monte Carlo N-Particle (MCNP) and Monte Carlo N-Particle Extended (MCNPX) software programs. The model was benchmarked against operational data from the reactor, to include total core excess reactivity, control rod worths, and nominal fuel element worths. The model was then used to model burnup within individual fuel elements within the core to determine the effect of core movement within the reactor pool on individual element burnup. Movement of the core with respect to the two exposure rooms was modeled to determine the effect of movement of the core on the radiation fields and gamma and neutron fluxes within the exposure rooms. Additionally, the model was used to demonstrate the effectiveness of gadolinium paint used within the exposure rooms to reduce thermal neutron production and subsequent Ar41 production within the exposure rooms. The model showed a good approximation to measured benchmark data across all applied metrics, and additionally provided confirmation of data on dose rates within the exposure rooms. It also showed that, while there was some variation of burnup within individual fuel elements based on core position within the reactor pool, the overall effect was negligible for effective fuel management within the core. Finally, the model demonstrated explicitly that the use of gadolinium paint within the exposure rooms was, and remains, an effective way of reducing the thermal flux, and subsequent Ar-41 production within the exposure rooms. It also demonstrated that the gadolinium paint also resulted in a much steeper neutron flux gradient within the exposure rooms than would have been obtained had neutrons been allowed to thermalize within the wood walls lining the rooms and then reenter the exposure facilities.Item NANOSTRUCTURE INVESTIGATION OF POLYMER SOLUTIONS, POLYMER GELS, AND POLYMER THIN FILMS(2009) Lee, Wonjoo; Bribeer, Robert M; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis discusses two systems. One is structured hydrogels which are hydrogel systems based on crosslinked poly((2-dimethylamino)ethyl methacrylate) (PDMAEMA) containing micelles which form nanoscale pores within the PDMAEMA hydrogel. The other is nanoporous block copolymer thin films where solvent selectivity is exploited to create nanopores in PS-b-P4VP thin films. Both of these are multicomponent polymer systems which have nanoscale porous structures. 1. Small angle neutron scattering of micellization of anionic surfactants in water, polymer solutions and hydrogels Nanoporous materials have been broadly investigated due to the potential for a wide range of applications, including nano-reactors, low-K materials, and membranes. Among those, molecularly imprinted polymers (MIP) have attracted a large amount of interest because these materials resemble the "lock and key" paradigm of enzymes. MIPs are created by crosslinking either polymers or monomers in the presence of template molecules, usually in water. Initially, functional groups on the polymer or the monomer are bound either covalently or noncovalently to the template, and crosslinking results in a highly crosslinked hydrogel. The MIPs containing templates are immersed in a solvent (usually water), and the large difference in the osmotic pressure between the hydrogel and solvent removes the template molecules from the MIP, leaving pores in the polymer network containing functionalized groups. A broad range of different templates have been used ranging from molecules to nanoscale structures inclucing stereoisomers, virus, and micelles. When micelles are used as templates, the size and shape before and after crosslinking is an important variable as micelles are thermodynamic objects whose structure depends on the surfactant concentration of the solution, temperature, electrolyte concentration and polymer concentration. In our research, the first goal is to understand the micellization of anionic surfactants in polymer solutions and the corresponding hydrogels using small angle neutron scattering (SANS). SANS has been widely used to investigate structures ranging from sub-nanometer to sub-micrometer. Since the scattering lengths of H and D atoms are quite different, the scattering contrast can be enhanced (and varied) through isotopic labeling. It is possible to investigate the structure of micelles in polymer solutions and hydrogels using H/D contrast matching methods with SANS. For this aim, water-soluble and chemically crosslinkgable poly((2-dimethylamino)ethyl methacrylate) (PDMAEMA) was synthesized using group transfer polymerization. In order to control the size and shape of micelles, the degree of quaternization of the polymer was also controlled through the reaction of PDMAEMA with methyl iodide. The micellization of deuterated sodium dodecylsulfate (d-SDS) in (quaternized) PDMAEMA solutions and the corresponding hydrogels was then observed using SANS and the size and shape of d-SDS micelles was obtained by modeling. 2. Nanopatterning using block copolymer/homopolymer blends Block copolymers are well-known to self-assemble into meso- and nano-scale structures. The use of block copolymers for nanostructured patterns has attracted increasing attention due to their potential use as templates and scaffolds for the fabrication of functional nanostructures. In order to realize the potential of these materials, it is necessary to be able to control the orientation of the nanoscale pattern in a precise manner. Numerous methods such as manipulation of the interfacial surface energies, use of electric fields, and controlling the rate of solvent evaporation have developed to control orientation. In addition, it has been shown that nanopores within cylindrical domains oriented normal to the substrate can be generated by several methods. For example, one component can be degraded by UV exposure, or the homopolymer in a block copolymer/homopolymer blend can be extracted in a selective solvent. In our work, polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP)/poly(4-vinylpyridine) (P4VP) films on silicon substrates were prepared using spincoating. The homopolymer was then extracted in ethanol generating pores perpendicular to the substrate. It is noted that the pore size and density were readily controlled by the amount of P4VP homopolymer in the PS-b-P4VP/P4VP solutions, giving simple control of the film structure. It was also possible to make pores more uniform and ordered by annealing in solvent vapor before extracting the homopolymer.Item Wafer-scale process and materials optimization in cross-flow atomic layer deposition(2009) Lecordier, Laurent Christophe; Rubloff, Gary W; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The exceptional thickness control (atomic scale) and conformality (uniformity over nanoscale 3D features) of atomic layer deposition (ALD) has made it the process of choice for numerous applications from microelectronics to nanotechnology, and for a wide variety of ALD processes and resulting materials. While its benefits derive from self-terminated chemisorbed reactions of alternatively supplied gas precursors, identifying a suitable process window in which ALD's benefits are realized can be a challenge, even in favorable cases. In this work, a strategy exploiting in-situ gas phase sensing in conjunction with ex-situ measurements of the film properties at the wafer scale is employed to explore and optimize the prototypical Al2O3 ALD process. Downstream mass-spectrometry is first used to rapidly identify across the [H2O x Al(CH3)3] process space the exposure conditions leading to surface saturation. The impact of precursor doses outside as well as inside the parameter space outlined by mass-spectrometry is then investigated by characterizing film properties across 100 mm wafer using spectroscopic ellipsometry, CV and IV electrical characterization, XPS and SIMS. Under ideal dose conditions, excellent thickness uniformity was achieved (1sigma/mean<1%) in conjunction with a deposition rate and electrical properties in good agreement with best literature data. As expected, under-dosing of precursor results in depletion of film growth in the flow direction across the wafer surface. Since adsorbed species are reactive with respect to subsequent dose of the complementary precursor, such depletion magnifies non-uniformities as seen in the cross-flow reactor, thereby decorating deviations from a suitable ALD process recipe. Degradation of the permittivity and leakage current density across the wafer was observed though the film composition remained unchanged. Upon higher water dose in the over-exposure regime, deposition rates increased by up to 40% while the uniformity degraded. In contrast, overdosing of TMA and ozone (used for comparison to water) did not affect the process performances. These results point to complex saturation dynamics of water dependent on partial pressure and potential multilayer adsorption caused by hydrogen-bonding.Item Characterization of Electrodeposited Chitosan: an Interfacial Layer for Bio-assembly and Sensing(2009) Buckhout-White, Susan Lynn; Rubloff, Gary W; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Microfluidics and Lab-on-a-Chip devices have revolutionized the field of analytical biology. To fully optimize the potential of the microfluidic environment it is critical to be able to isolate reactions in specific locations within a channel. One solution is found using chitosan, an amine-rich biopolymer with pH responsive solubility. Induction of hydrolysis at patterned electrodes within the fluidic channel provides a means to spatially control the pH, thus enabling biochemical functionalization that is both spatially and temporally programmable. While chitosan electrodeposition has proven to be reliable at producing films, its growth characteristics are not well understood. In situ optical characterization methods of laser reflectivity, fluorescence microscopy and Raman spectroscopy have been employed to understand the growth rate inter diffusion and lateral resolution of the deposition process. These techniques have also been implemented in determining where a molecule bound to an amine site of the polymer is located within the film. Currently, electrodeposited chitosan films are primarily used for tethering of biomolecules in the recreation of metabolic pathways. Beyond just a biomolecular anchor, chitosan provides a way to incorporate inorganic nanoparticles. These composite structures enable site-specific sensors for the identification of small molecules, an important aspect to many Lab-on-a-Chip applications. New methods for creating spatially localized sites for surface enhanced Raman spectroscopy (SERS) has been developed. These methods have been optimized for particle density and SERS enhancement using TEM and Raman spectroscopy. Through optimization, a viable substrate with retained chitosan amine activity capable of integration into microfluidics has been developed.Item Bottom-Up Multiferroic Nanostructures(2009) Ren, Shenqiang; Wuttig, Manfred; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Multiferroic and especially magnetoelectric (ME) nanocomposites have received extensive attention due to their potential applications in spintronics, information storage and logic devices. The extrinsic ME coupling in composites is strain mediated via the interface between the piezoelectric and magnetostrictive components. However, the design and synthesis of controlled nanostructures with engineering enhanced coupling remain a significant challenge. The purpose of this thesis is to create nanostructures with very large interface densities and unique connectivities of the two phases in a controlled manner. Using inorganic solid state phase transformations and organic block copolymer self assembly methodologies, we present novel self assembly "bottom-up" techniques as a general protocol for the nanofabrication of multifunctional devices. First, Lead-Zirconium-Titanate/Nickel-Ferrite (PZT/NFO) vertical multilamellar nanostructures have been produced by crystallizing and decomposing a gel in a magnetic field below the Curie temperature of NFO. The ensuing microstructure is nanoscopically periodic and anisotropic. The wavelength of the PZT/NFO alternation, 25 nm, agrees within a factor of two with the theoretically estimated value. The macroscopic ferromagnetic and magnetoelectric responses correspond qualitatively and semi-quantitatively to the features of the nanostructure. The maximum of the field dependent magnetoelectric susceptibility equals 1.8 V/cm Oe. Second, a magnetoelectric composite with controlled nanostructures is synthesized using co-assembly of two inorganic precursors with a block copolymer. This solution processed material consists of hexagonally arranged ferromagnetic cobalt ferrite (CFO) nano-cylinders within a matrix of ferroelectric Lead-Zirconium-Titanate (PZT). The initial magnetic permeability of the self-assembled CFO/PZT nanocomposite changes by a factor of 5 through the application of 2.5 V. This work may have significant impact on the development of novel memory or logic devices through self assembly techniques. It also demonstrates a universal two-phase hard template application. Last, solid-state self assembly had been used recently to form pseudoperiodic chessboard-like nanoscale morphologies in a series of chemically homogeneous complex oxide systems. We improved on this approach by synthesizing a spontaneously phase separated nanolamellar BaTiO3-CoFe2O4 bi-crystal. The superlattice is magnetoelectric with a frequency dependent coupling. The BaTiO3 component is a ferroelectric relaxor with a Vogel-Fulcher temperature of 311 K. Since the material can be produced by standard ceramic processing methods, the discovery represents great potential for magnetoelectric devices.Item NANOPOROUS AAO: A PLATFORM FOR REGULAR HETEROGENEOUS NANOSTRUCTURES AND ENERGY STORAGE DEVICES(2009) Perez, Israel; Rubloff, Gary W; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Nanoporous anodic aluminum oxide (AAO) has vast implications as a tool for nanoscience research and as a nanostructure in which nanoscale devices can be fabricated because of its regular and ordered nanopores. Self-assembly plays a critical role in pore ordering, causing nanopores to grow parallel with one another in high density. The mild electrochemical conditions in which porous AAO grows along with its relatively cheap starting materials makes this nanomaterial a cost effective alternative to advanced photolithography techniques for forming high surface area nanostructures over large areas. In this research, atomic layer deposition (ALD) was used to deposit conformal films within in nanoporous AAO with hopes to 1) develop methodologies to characterize ALD depositions within its high aspect ratio nanopores and 2) to better understand how to use nanoporous AAO templates as a scaffold for energy devices, specifically Metal-Insulator-Metal (MIM) capacitors. Using the nanotube template synthesis method, ALD films were deposited onto nanoporous AAO, later removing the films deposited within the templates nanopores for characterization in TEM. This nanotube metrology characterization involves first obtaining images of full length ALD-AAO nanotubes, and then measuring wall thickness as a function of depth within the nanopore. MIM nanocapacitors were also constructed in vertical AAO nanopores by deposition of multilayer ALD films. MIM stacks were patterned into micro-scale capacitors for electrical characterization.Item THE PHOTOCHEMISTRY OF POLYENYL RADICALS AND ITS APPLICATION TO UHMWPE FOR USE IN ARTIFICIAL CARTILAGE(2009) Kasser, Michael Jacob; Al-Sheikhly, Mohamad; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The use of UV light as an alternative to thermal treatments above the melting point (150 °C) to remove free radicals in irradiated UHMWPE was explored. It was found that, in contrast to the allyl free radical which is converted by 258 nm light to alkyl free radicals, polyenyl radicals are not converted to alkyl radicals by UV light. None-the-less, by sandwiching UV light treatments between low temperature thermal anneals (100 °C), it was possible to reduce free radical concentrations by 30%. This reduction was achievable for depths up to one millimeter. However, this reduction did not have a significant effect on oxidation due to an increase in oxidation susceptibility because of the concurrent increase in concentration of easily abstracted allylic hydrogens. By photoirradiating for the optimal amount of time, it was possible, for the first time, to synthesize a polyethylene sample whose residual free radicals consisted of almost entirely dienyl free radicals. This allowed unambiguous identification and simulation of dienyl free radical's EPR spectra to be a singlet containing nine peaks separated by 9 G hyperfine separation. Detailed studies of photoirradiation of UHMWPE containing free radicals revealed that photoirradiation with a continuous spectrum above 200 nm causes the decay of diene unsaturations and allyl free radicals, a reduction in the overall amount of free radicals, and an increase in the degree of unsaturation of polyenyl free radicals. Upon longer photoirradiation times, polyenyl radicals were converted from lower to higher degrees of unsaturation. This effect was identical in the presence and absence of oxygen, but was suppressed by hydrogen gas. These results showed that the conversion does not occur by a linear alkyl radical addition mechanism wherein alkyl radicals migrate to stable polyene unsaturations and polyenyl radicals thereby increasing their order, as previously suggested. The valid mechanism appears to be the direct photoconversion of diene unsaturations to dienyl radicals and lower order polyenyl radicals to higher order polyenyl radicals.Item GAS PHASE SYNTHESIS OF ALUMINUM AND CORE-SHELL NICKEL-IRON OXIDE NANOPARTICLES(2009) Pines, Daniel; Zachariah, Michael; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this master's thesis I will address the design of two aluminum and one nickel-iron oxide core-shell nanoparticle reactors, as well as the selection of the chemical precursors' triethylaluminum (TEA), aluminum trichloride, nickel carbonyl, and iron pentacarbonyl. This research provides evidence for the generation of aluminum oxide passivated aluminum nanoparticles from TEA, the failure to completely dissociate aluminum trichloride, and the successful growth of iron oxide (shell) onto nickel (core) nanoparticles. Reactions and synthesis are carried out in gas phase allowing the use of specialized aerosol sampling and characterization techniques. In addition to studying the particle in situ, TEM and EDS measurements are preformed post collection. Motivation for this work is driven by the nanoparticle's enhanced performance when used in explosive and propellants.Item SYNTHESIS AND CHARACTERIZATION OF LOW FLAMMABILITY POLYMER/LAYERED SILICATE NANOCOMPOSITES(2009) Zhang, Xin; Briber, Robert M.; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)There has been significant interest in the applications of polymer nanocomposites in a variety of areas. Polymer/layered silicate nanocomposites have been of interest because of relatively low raw material cost and improved materials properties such as higher Young's modulus, higher thermal deformation temperature, lower small molecule permeability, lower density (compared to metals and traditional glass fiber reinforced composites) as well as low flammability. The relationships between the flammability and the dispersion of the layered silicate platelets inside the polymer matrix is just being established. The complete set of factors that affect the flammability of polymer/layered nanocomposites are not fully identified. In this thesis polymer/layered silicate nanocomposites with different degrees of platelet dispersion were synthesized. The structure of the nanocomposites was characterized by X-ray diffraction (XRD), small angle X-ray scattering (SAXS), and transmission electron microscopy (TEM). The flammability of these nanocomposites was characterized by TGA, cone calorimetry and gasification. By coupling the structural and flammability data it has been concluded that forming a nanometer scale dispersed structure significantly improves the flammability but the details of the degree of dispersion are not critical. The improvement in the flammability arises from the formation of a residue or char layer at the surface of the nanocomposite. This residue layer acts as a radiation shield and as a physical barrier preventing the polymer degradation products from escaping and acting as fuel. It is observed that the stability of the residue layer formed during combustion has major impact on the flammability. This thesis also describes work to improve the flammability of the polymer/layered silicate nanocomposites by enhancing char/residue formation in order to improve the residue layer stability.