Browsing by Subject "thermodynamics"
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Item A Computational-based Approach for the Design of Trip Steels(2013-08-06) Li, Sheng-YenThe purpose of this work is to optimize the chemical composition as well as the heat treatment for improving the mechanical performance of the TRIP steel by employing the theoretical models. TRIP steel consists of the microstructure with ferrite, bainite, retained austenite and minor martensite. Austenite contributes directly to the TRIP effect as its transformation to martensite under the external stress. In order to stabilize austenite against the martensitic transformation through the heat treatment, the two-step heat treatment is broadly applied to enrich the carbon and stabilize the austenite. During the first step of the heat treatment, intercritical annealing (IA), a dual phase structure (ferrite+austenite) is achieved. The austenite can be initially stabilized because of the low carbon solubility of ferrite. The bainite isothermal treatment (BIT) leads to the further carbon enrichment of IA-austenite by the formation of carbon-free ferrite. Comparing to the experiments, the thermodynamic and kinetic models are the lower and upper bounds of the carbon content of retained austenite. The mechanical properties are predicted using the swift model based on the predicted microstructure. In this work, a theoretical approach is coupled to a Genetic Algorithm-based optimization procedure to design (1) the heat treated temperatures to maximize the volume fraction of retained austenite in a Fe-0.32C-1.42Mn-1.56Si alloy and the chemical composition of (2) Fe-C-Mn-Si and (3) Fe-C-Mn-Si-Al-Cr-Ni alloy. The results recommend the optimum conditions of chemical composition and the heat treatment for maximizing the TRIP effect. Comparing to the experimental results, this designing strategy can be utilized to explore the potential materials of the novel alloys.Item A thermodynamic approach for compaction of asphaltic composites(2009-05-15) Koneru, SaradhiThis thesis studies the mechanics which can be associated with asphalt concrete compaction and develops continuum models in a general thermo-mechanical setting which can be used in future work to corroborate experimental compaction experiment results. Modeling asphalt concrete compaction, and also the ability to thereby predict response of mixes, is of great importance to the pavement industry. Asphalt concrete exhibits nonlinear response even at small strains and the response of asphalt concrete to different types of loading is quite different. The properties of asphalt concrete are highly influenced by the type and amount of the aggregates and the asphalt used. The internal structure of asphalt concrete continues to evolve during the loading process. This is due to the influence of different kinds of activities at the micro-structure level and to the interactions with the environment. The properties of asphalt concrete depend on its internal structure. Hence, we need to take into account the evolution of the internal structure in modeling the response of asphalt concrete. A theoretical model has been developed using the notion of multiple natural configurations to study a variety of non-linear dissipative responses of real materials. By specifying the forms for the stored energy and the rate of dissipation function of the material, a specific model was developed using this framework to model asphalt compaction. A compressible model is developed by choosing appropriate forms of stored energy and rate of dissipation function. Finally, a parametric study of the model is presented for a simple compression deformation. It is anticipated that the present work will aid in the development of better constitutive equations which in turn will accurately model asphalt compaction both in laboratory and in field. Distinct numerical approaches have been used to demonstrate the applicability of the theoretical framework to model material response of asphalt.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 Conversion of CO2 to Polycarbonates and Other Materials: Insights through Computational Chemistry(2014-09-25) Yeung, Andrew DThe use of carbon dioxide as a chemical feedstock for the copolymerization with epoxides to give polycarbonates, and for coupling with hydrocarbons to give carboxylic acids, was probed using computational chemistry. Metal-free systems were modeled at high levels using composite methods that give ?chemical accuracy?, whereas metal-bound systems were studied using density functional theory, benchmarked to these high-accuracy results for confidence. The thermodynamics of polymer vs. cyclic carbonate formation was calculated, and polymer is the exothermic product, whereas cyclic carbonate is the entropic product. The barriers for the metal-free carbonate and alkoxide backbiting reactions were also determined, carbonate backbiting having a higher barrier than alkoxide backbiting. The base degradation of polymers to epoxide co-monomers, and the acid- and base-catalyzed degradation of glycerol carbonate to glycidol were investigated too. Poly(cyclopentene carbonate) preferentially degrades to epoxide co-monomer instead of cyclic carbonate due to angle strain for alkoxide backbiting. Conversely, glycerol carbonate only yields glycidol instead of the isomeric 3-hydroxyoxetane because formation of the latter has a higher barrier. The (salen)Cr(III)- and (salen)Co(III)-catalyzed copolymerization reactions were studied for a variety of epoxides, and the overall displacement of a polymeric carbonate by an epoxide, followed by ring-opening, was found to be rate limiting. Chromium(III)-catalyzed systems have higher free energy barriers than cobalt(III) systems due to enthalpy, which explains why such polymerization reactions have to be run at higher temperatures. The metal-bound polymer carbonate and alkoxide backbiting reactions generally have higher barriers than when unbound, due to the terminal oxygen atoms? reduced nucleophilicity. The carboxylation of metal-hydride and metal-carbon bonds was studied for a series of trans-ML2XY complexes, and thermodynamics of carboxylation of the M-X bond are influenced by M, L, and Y, in decreasing order. Similar cis-complexes did not exhibit as clear trends. Examination of these complexes indicated that the three steps for the overall conversion of hydrocarbons to carboxylic acids (oxidative addition of hydrocarbon, carboxylation, and reductive elimination of the carboxylic acid) must be optimized in parallel for the successful direct synthesis of carboxylic acids.Item Fundamental Properties and Processes of Energetic Materials(2012-10-19) Ojeda Mota, Oscar UlisesEnergetic materials comprise a set of systems of tremendous technological importance. Besides helping shape landscapes to establish communications, they have been used to reach fuel reservoirs, deploy safety bags and prevent heart strokes. Understanding its behavior can help in attaining strategic and tactical superiority, and importantly, preserve lives of people who handle these materials. The large discrepancy in length and time scales at which characteristic processes of energetic materials are of relevance pose a major challenge for current simulation techniques. We present a systematic study of crystalline energetic materials of different sensitivity and analyze their properties at different theoretical levels. Equilibrium structures, vibrational frequencies, conformational rearrangement and mechanical properties can be calculated within the density functional theory and molecular dynamics at finite temperatures. We have found marked differences in the calculated properties in systems with ranging sensitivities. Reactions at elevated temperatures have been studied using ab initio molecular dynamics method for crystals of nitroethane. Furthermore, while presenting the state of the art of energetic materials modeling, the limitations of each methodology are also discussed. Prospective systems and an elasticity driven approach that can be applied to other type of materials is also presented.Item The impact of protein fluctuations on molecular recognition(2008-12-05) anthony C manson; Dr. Wlodek Bujalowski; Dr. Werner Braun; Dr. Montgomery Pettitt; Dr. Mary MoslenThe effect of protein fluctuations on molecular recognition is poorly understood. Prediction of useful properties such as binding affinity using rigid structures has produced sporadic success. Although attempts have been made to model the effect of\r\nconformational fluctuations, capturing the impact of backbone relaxation has remained\r\nparticularly elusive. In order to investigate these effects, a series of surface exposed\r\nAla/Gly mutants were designed in the flexible RT loop of the C-terminal SH3 domain of\r\nSEM5. One set of mutations was designed to perturb the ensemble of accessible\r\nconformations in the unbound ensemble while leaving the interaction surface with the\r\nligand unchanged. The other set was designed to perturb both the interaction surface as\r\nwell as the ensembles of bound and free conformations. The effects of these mutations\r\nwere investigated by generating random conformations of the RT loop and performing\r\nprincipal component analysis to organize the randomly generated conformational states\r\ninto a coherent landscape. To predict the effect of these mutations, we developed a\r\nstatistical mechanical technique using a simplified energy function that only applied the\r\neffects of excluded volume and implicit solvation. This energy function was utilized to\r\nweight an ensemble of conformational states from which aggregate thermodynamic\r\nproperties could be derived. The computed effects of the mutations on the binding\r\naffinity agreed with experimentally determined values (R= 0.97) from isothermal titration\r\ncalorimetry. The results indicate that the bound state of SEM5 SH3 domain contains a\r\nconsiderable repertoire of conformational variants of the high-resolution structure and\r\nthat the determinants of binding cannot be elucidated from the static structure of the\r\nbound complex.\r\nItem Modelling Flow through Porous Media under Large Pressure Gradients(2013-11-01) Srinivasan, ShriramThe most interesting and technologically important problems in the study of flow through porous media involve very high pressures and pressure gradients in the flow do- main such as enhanced oil recovery and carbon dioxide sequestration. The popular Darcy or Brinkman models do not take into account the changes in the fluid properties (like viscosity) due to high pressures and temperatures, or the deformation of the solid itself as the fluid flows through it. We focus on the pressure dependence of viscosity and show that its significance in these problems cannot be neglected. Mixture theory is employed as the tool to develop models for this task. The popular models due to Darcy and Brinkman (and their generalizations) are derived using a general thermodynamic framework which appeals to the criterion of maximal rate of entropy production. Such a thermodynamic approach has been used with great success to describe various classes of material response and here we demonstrate its use within the context of mixture theory. One such generalization of the Brinkman model takes into account the variation of the viscosity and the drag coefficient with the pressure and is used in the problems studied subsequently. We then consider the steady flow of a fluid through a porous slab, driven by a large pressure gradient, and show that the traditional approach that ignores the variation of the viscosity and drag with the pressure greatly over-predicts the mass flux taking place through the porous structure. While incorporating the pressure dependence of viscosity and drag leads to a ceiling flux, the traditional approaches lead to a continued increase in the flux with the pressure difference. The effect of inhomogeneities and anisotropy of the porous medium is investigated by modifying the previous problem to prescribe the drag coefficient as a piecewise constant, positive definite second order tensor. Finally, we allow for the possibility that the flow is unsteady, the viscosity and drag are dependent on the pressure and consider the flow of a fluid due to a pulsatile forcing pressure at one end of a rigid, homogenoues, isotropic solid while the other end is open to the atmosphere. In contrast to certain non-Newtonian fluids where the volumetric flux is enhanced by pulsating the pressure gradient about a non-zero mean value, we find that pulsations in the pressure diminish the volumetric flux in case of the flow through a porous medium when the fluid viscosity is considered to be pressure dependent.Item A thermodynamic definition of protein folds(2008-05-01) Jason Vertrees; Robert Fox; Wlodek Bujalowski; Vincent Hilser; Montgomery Pettitt; Henry EpsteinModern techniques in structural biology, like homology modeling, protein threading, protein fold classification, and homology detection have proven extremely useful. For example, they have provided us with evolutionary information about protein homology which has in some many cases lead directly to therapeutics. Due to the importance of these methods, augmenting or improving them may lead to significant advances in understanding proteins. These methods treat the high-resolution structure as a static entity upon which they operate, however we know that proteins are not static entities---they are polymers that exist in an enormous array of conformational states. Therefore, we propose to model the proteins from a statistical thermodynamic viewpoint based upon their average energetic properties. We show that this model can be used to (1) better characterize the partial unfolding process of proteins, and (2) reclassify the protein fold space from a new perspective.Item Thermomechanical modeling of a shape memory polymer(2009-05-15) Ghosh, Pritha B.The aim of this work is to demonstrate a Helmholtz potential based approach for the development of the constitutive equations for a shape memory polymer undergoing a thermomechanical cycle. The approach is motivated by the use of a simple spring-dashpot type analogy and the resulting equations are classified as state-equations and suitable kinetic equations for the recoverable-energy elements and the dissipative elements in the model respectively. These elements have mechanical properties which vary with temperature. The governing equations of the model are developed starting from the basic conservation laws together with the laws of thermodynamics. The entire set of equations are written in a state-evolution form as a set of ordinary differential equations to be solved using Matlab. It is shown that the results of the simulation in Matlab are in qualitative and quantitative agreement with experiments performed on polyurethane. Subsequently, we study the dependence of the yield-stress on temperature to be similar and different functions of heating or cooling processes.