Browsing by Subject "density functional theory"
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Item Atomistic Simulations of Bonding, Thermodynamics, and Surface Passivation in Nanoscale Solid Propellant Materials(2012-10-19) Williams, KristenEngineering new solid propellant materials requires optimization of several factors, to include energy density, burn rate, sensitivity, and environmental impact. Equally important is the need for materials that will maintain their mechanical properties and thermal stability during long periods of storage. The nanoscale materials considered in this dissertation are proposed metal additives that may enhance energy density and improve combustion in a composite rocket motor. Density Functional Theory methods are used to determine cluster geometries, bond strengths, and energy densities. The ground-state geometries and electron affinities (EAs) for MnxO?: x = 3, 4, y = 1, 2 clusters were calculated with GGA, and estimates for the vertical detachment energies compare well with experimental results. It was found that the presence of oxygen influences the overall cluster moment and spin configuration, stabilizing ferrimagnetic and antiferromagnetic isomers. The calculated EAs range from 1.29-1.84 eV, which is considerably lower than the 3.0-5.0 eV EAs characteristic of current propellant oxidizers. Their use as solid propellant additives is limited. The structures and bonding of a range of Al-cyclopentadienyl cluster compounds were studied with multilayer quantum mechanics/molecular mechanics (QM:MM) methods. The organometallic Al-ligand bonds are generally 55-85 kcal/mol and are much stronger than Al-Al interactions. This suggests that thermal decomposition in these clusters will proceed via the loss of surface metal-ligand units. The energy density of the large clusters is calculated to be nearly 60% that of pure aluminum. These organometallic cluster systems may provide a route to extremely rapid Al combustion in solid rocket motors. Lastly, the properties of COOH-terminated passivating agents were modeled with the GPW method. It is confirmed that fluorinated polymers bind to both Al(111) and Al(100) at two Al surface sites. The oligomers HCOOH, CH3CH2COOH, and CF3CF2COOH chemisorb onto Al(111) with adsorption energies of 10-45 kcal/mol. The preferred contact angle for the organic chains is 65-85 degrees, and adsorption energy weakens slightly with increasing chain length. Despite their relatively weak adsorption energies, fluorinated polymers have elevated melting temperatures, making them good passivation materials for micron-scale Al fuel particles.Item Environmetally Assisted Cracking in Metals under Extreme Conditions(2012-10-19) Pham, HieuEnvironmentally Assisted cracking (EAC) is a very critical materials science problem that concerns many technological areas such as petrochemical engineering, aerospace operations and nuclear power generation, in which cracking or sudden failure of materials may happen at stress far below the tensile strength. This type of corrosion is initiated at the microscopic level and is complicated due to the combination of chemistry (reaction caused by corrosive agents) and mechanics (varying load). As EAC is generally related to the segregation of impurity elements to defects (mainly grain boundaries), the symptoms of risk may not be apparent from the exterior of the metal components: hence EAC remains latent and gives no sign of warning until the failure occurs. Due to its intricate nature, conducting experiments on this phenomenon involves difficulties and requires much effort. In this work, we employed advanced molecular simulation techniques to study EAC in order to give insight into its atomistic behavior. First, Density-Functional Theory (DFT) method was used to investigate the fundamental processes and mechanism of EAC-related issues at the nanoscale level, with two case studies concerning the stress corrosion in iron and hydrogen embrittlement in palladium. When segregating to the grain boundary (GB) of iron, different impurity elements such as sulfur, phosphorus and nitrogen raise corrosion failures in a variety of ways. Hydrogen atoms, due to their mobility and small atomic size, are able to form high occupation at crystal defects, but show different interactions to vacancy and GB. Then, we used the classical Molecular Dynamics (MD) method to gain an understanding of the dynamic response of materials to mechanical load and the effects of temperature, strain and extreme conditions (high pressure shock compression) on structural properties. The MD simulations show that hydrogen maintains the highest localization at grain boundaries in the vicinity of ambient temperatures, and grain boundaries are the preferred nucleation sites for dislocations and voids. This computational work, using DFT and MD techniques, is expected to contribute to the better understanding on chemistry and mechanisms of complex environment-assisted cracking phenomenon at a fundamental level in order to beneficially complement conventional laboratory approaches.Item Surface Oxidation and Dissolution of Metal Nanocatalysts in Acid Medium(2012-10-19) Callejas-Tovar, JuanOne of the most important challenges in low-temperature fuel cell technology is improving the catalytic efficiency at the electrode-catalyst where the oxygen reduction reaction (ORR) occurs. Platinum is the best pure catalyst for this reaction but its high cost and scarcity hinder the commercial implementation of fuel cells in automobiles. Pt-based alloys are promising alternatives to substitute platinum while maintaining the efficiency and life-time of the pure catalyst. However, the acid medium and the oxidation of the surface reduce the activity and durability of the alloy catalyst through changes in its local composition and structure. Molecular simulation techniques are applied to characterize the thermodynamics and dynamic evolution of the surface of platinum-based alloy catalysts under reaction conditions.1-10 A simulation scheme of the surface oxidation is proposed which combines classical molecular dynamics (MD) and density functional theory (DFT). This approach is able to reproduce the main features of the oxidation phenomena observed experimentally, it is concluded that the dissolution mechanism of metal atoms involves: 1) Surface segregation of alloy atoms, 2) oxygen absorption into the subsurface of the catalyst, and 3) metal detachment through the interaction with ions in the solvent. Therefore, to improve the durability of platinum-based alloy catalysts, the steps of the dissolution mechanism must be prevented. A versatile 3-D kinetic Monte Carlo (KMC) code is developed to study the degradation and dealloying in nanocatalysts. The results on the degradation of Pt nanoparticles under different potential regimes demonstrate that the dissolution depends on the potential path to which the nanocatalyst is exposed. Metal atoms detach from the boundaries of (111) facets expecting a reduction in the activity of the nanoparticle. Also, the formation of Pt hollow nanoparticles by the Kirkendall effect is addressed, the role of vacancies is crucial in the removal of the non-noble core that yields to hollow nanoparticles. To investigate the reasons for the experimentally found enhanced ORR activity in porous/hollow nanoparticles, the effect of subsurface vacancies on the main ORR activity descriptors is studied with DFT. It is found that an optimum amount of vacancies may enhance the ORR activity of Pt-monolayer catalysts over certain alloy cores by changing the binding energies of O and OH.