Browsing by Subject "nanostructure"
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Item Comprehensive Investigation of the Uranium-Zirconium Alloy System: Thermophysical Properties, Phase Characterization and Ion Implantation Effects(2013-07-31) Ahn, SangjoonUranium-zirconium (U-Zr) alloys comprise a class of metallic nuclear fuel that is regularly considered for application in fast nuclear energy systems. The U-10wt%Zr alloy has been demonstrated to very high burnup without cladding breach in the Experimental Breeder Reactor-II (EBR-II). This was accomplished by successfully accommodating gaseous fission products with low smear density fuel and an enlarged cladding plenum. Fission gas swelling behavior of the fuel has been experimentally revealed to be significantly affected by the temperature gradient within a fuel pin and the multiple phase morphologies that exist across the fuel pin. However, the phase effects on swelling behavior have not been yet fully accounted for in existing fuel performance models which tend to assume the fuel exists as a homogeneous single phase medium across the entire fuel pin. Phase effects on gas bubble nucleation and growth in the alloy were investigated using transmission electron microscopy (TEM). To achieve this end, a comprehensive examination of the alloy system was carried out. This included the fabrication of uranium alloys containing 0.1, 2, 5, 10, 20, 30, 40, and 50 wt% zirconium by melt-casting. These alloys were characterized using electron probe micro-analysis (EPMA), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). Once the alloys were satisfactorily characterized, selected U-Zr alloys were irradiated with 140 keV He^(+) ions at fluences ranging from 1 ? 10^(14) to 5 ? 10^(16) ions/cm^(2). Metallographic and micro-chemical analysis of the alloys indicated that annealing at 600 ?C equilibrates the alloys within 168 h to have stable ?-U and ?-UZr_(2) phase morphologies. This was in contrast to some reported data that showed kinetically sluggish ?-UZr_(2) phase formation. Phase transformation temperatures and enthalpies were measured using DSC-TGA for each of the alloys. Measured temperatures from different time annealed alloys have shown consistent matches with most of the features in the current U-Zr phase diagram which further augmented the EPMA observed microstructural equilibrium. Nevertheless, quantitative transformation enthalpy analysis also suggests potential errors in the existing U-Zr binary phase diagram. More specifically, the (?-U, ?2) phase region does not appear to be present in Zr-rich (> 15 wt%) U-Zr alloys and so further investigation may be required. To prepare TEM specimens, characterized U-Zr alloys were mechanically thinned to a thickness of ~150 ?m, and then electropolished using a 5% perchloric acid/95% methanol electrolyte. Uranium-rich phase was preferentially thinned in two phase alloys, giving saw-tooth shaped perforated boundaries; the alloy images were very clear and alloy characterization was accomplished. During in-situ heating U-10Zr and U-20Zr alloys up to 810 ?C, selected area diffraction (SAD) patterns were observed as the structure evolved up to ~690 ?C and the expected ?-U ? ?-U phase transformation at 662 ?C was never observed. For the temperature range of the (?-U, ?2) phase region, phase transformation driven diffusion was observed as uranium moved into Zr-rich phase matrix in U-20Zr alloy; this was noted as nonuniform bridging of adjacent phase lamellae in the alloy. From the irradiation tests, nano-scale voids were discovered to be evenly distributed over several micrometers in U-40Zr alloys. For the alloys irradiated at the fluences of 1 ? 10^(16) and 5 ? 10^(16) ions/cm^(2), estimated void densities were proportional to the irradiation doses, (250 ? 40) and (1460 ? 30) /?m^(2), while void sizes were fairly constant, (6.0 ? 1.5) and (5.2 ? 1.2) nm, respectively. Measured data could be foundational inputs to the further development of a semi-empirical metal fuel performance model.Item Dendrimer-encapsulated metal nanoparticles: synthesis, characterization, and applications to catalysis(Texas A&M University, 2004-09-30) Niu, YanhuiThe research in this dissertation examines the chemistry and applications of dendrimers in homogeneous catalysis. We examined interactions between dendrimers and charged probe molecules, prepared dendrimer-encapsulated metal nanoparticles in organic solvents, studied size-selectivity of dendrimer-encapsulted catalysts, and designed molecular rulers as in-situ probes to measure the location of dendrimer-encapsulted metal nanoparticles. The intrinsic proton binding constant and a constant that characterizes the strength of electrostatic interactions among occupied binding sites in poly(amidoamine) (PAMAM) dendrimers have been obtained by studying the effect of solution pH on the protonation of the dendrimers. The significant finding is that these two factors are greatly modulated by the unique and hydrophobic microenvironment in the dendrimer interior. Hydrophilic poly(propylene imine) (PPI) dendrimers were modified with various hydrophobic alkyl chains through an amide linkage and were then used as templates for preparing intradendrimer copper nanoclusters. The main driving force for encapsulating metal-ions was found to be the differences in metal-ion solubility between the solvent and the interior of the dendrimer. Nanometer-sized metal particles are synthesized and encapsulated into the interior of dendrimers by first mixing together the dendrimer and metal ion solution and then reducing the composite chemically, and the resulting dendrimer-encapsulated metal nanoparticles can then be used as catalysts. By controlling the packing density on the dendrimer periphery using either different dendrimer generations or dendrimer surface functionalities, it is possible to control access of substrates to the encapsulated catalytic nanoparticle. Molecular rulers consisting of a large molecular "stopper", a reactive probe and a linker were designed as in-situ probes for determining the average distance between the surface of dendrimer-encapsulated palladium nanoparticles and the periphery of their fourth-generation, hydroxyl-terminated PAMAM dendrimer hosts. By doing so, we avoid having to make assumptions about the nanoparticle size and shape. The results suggest that the surface of the encapsulated nanoparticle is situated 0.7 ? 0.2 nm from the surface of the dendrimer.Item Experimental and Numerical Investigation of Pool Boiling Heat Transfer on Engineered Nano-Finned Surfaces(2014-08-10) Yang, HongjooPool boiling experiments for nanocoatings (or nanostructured surfaces) show that despite the lower thermal conductivity values than carbon, silicon yielded higher values of CHF (critical heat flux). Subsequently numerical studies showed that the interfacial thermal resistance (Kapitza resistance or ?R_(k)?) between a nanofin and fluid molecules is the dominant component of the thermal impedance network. The values of R_(k) for silicon were predicted to be ~1000 times smaller than that of carbon in these numerical simulations. Since the total thermal impedance of silicon nanofins is lower than that of carbon they cause higher levels of enhancement of CHF. Surface adsorption of the liquid molecules on a nanofin results in the formation of dense ?compressed phase? which in turn induces thermal capacitance and diodic behavior. This is termed as the ?nanofin effect?: which implies that CHF is more sensitive to R_(k) than the thermal conductivity of the nanofin. Hence, the objective of this study was to verify the nanofin effect. Experimental and numerical investigation of transport phenomena during pool boiling were performed in this study for liquid subcooling of 0 ?, 5 ? and 10 ? on horizontal planar heater configuration. Surface temperature was measured using nanosensor (Thin Film thermocouple or ?TFT?) arrays. Heater surfaces (with or without nanofins of different heights) were composed of ceramic, oxide and metal surfaces. The nanofins were fabricated using Step and Flash Imprint Lithography (SFIL). Contact angle was measured both before and after the experiments. Nucleate pool boiling heat transfer was enhanced with increase in pillar height. Numerical predictions for R_(k) obtained from Molecular Dynamics (MD) simulations were found to be consistent with the level of heat flux enhancement observed in the experiments for the different nanofin configurations. Hence this study demonstrates that R_(k) is the more dominant parameter for heat transfer enhancement during pool boiling ? compared to the thermal conduction resistance (or material properties) of the nanofin itself. As an outcome of these investigations future topics of research are also proposed (such as, using temperature nano-sensors for the investigation of controlled fouling on pool boiling phenomena for heaters with micro/nano-structured surfaces).Item Radiation Damage in Nanostructured Metallic Films(2013-04-15) Yu, KaiyuanHigh energy neutron and charged particle radiation cause microstructural and mechanical degradation in structural metals and alloys, such as phase segregation, void swelling, embrittlement and creep. Radiation induced damages typically limit nuclear materials to a lifetime of about 40 years. Next generation nuclear reactors require materials that can sustain over 60 - 80 years. Therefore it is of great significance to explore new materials with better radiation resistance, to design metals with favorable microstructures and to investigate their response to radiation. The goals of this thesis are to study the radiation responses of several nanostructured metallic thin film systems, including Ag/Ni multilayers, nanotwinned Ag and nanocrystalline Fe. Such systems obtain high volume fraction of boundaries, which are considered sinks to radiation induced defects. From the viewpoint of nanomechanics, it is of interest to investigate the plastic deformation mechanisms of nanostructured films, which typically show strong size dependence. By controlling the feature size (layer thickness, twin spacing and grain size), it is applicable to picture a deformation mechanism map which also provides prerequisite information for subsequent radiation hardening study. And from the viewpoint of radiation effects, it is of interest to explore the fundamentals of radiation response, to examine the microstructural and mechanical variations of irradiated nanometals and to enrich the design database. More importantly, with the assistance of in situ techniques, it is appealing to examine the defect generation, evolution, annihilation, absorption and interaction with internal interfaces (layer interfaces, twin boundaries and grain boundaries). Moreover, well-designed nanostructures can also verify the speculation that radiation induced defect density and hardening show clear size dependence. The focus of this thesis lies in the radiation response of Ag/Ni multilayers and nanotwinned Ag subjected to charged particles. The radiation effects in irradiated nanograined Fe are also investigated for comparison. Radiation responses in these nanostructured metallic films suggest that immiscible incoherent Ag/Ni multilayers are more resistant to radiation in comparison to their monolithic counterparts. Their mechanical properties and radiation response show strong layer thickness dependence in terms of radiation hardening and defect density. Coherent twin boundaries can interact with stacking fault tetrahedral and remove them effectively. Twin boundaries can actively absorb radiation induced defects and defect clusters resulting in boundary migration. Size dependence is also found in nanograins where fewer defects exhibit in films with smaller grains.