Browsing by Author "Farkas, Diana"
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- Alkali attack of coal gasifier refractory liningGentile, Maria (Virginia Tech, 1987-05-15)An experimental test system was designed to simulate the operating conditions found in nonslagging coal gasifiers. The reaction products that form when refractory linings in coal gasifiers are exposed to alkali impurities (sodium or potassium) were experimentally determined. Analysis of selected physical and chemical properties of the reaction products, which typically form between the alkali and the refractory will lead to a better understanding of the mechanisms behind refractory failures associated with alkali attack. The reaction products sodium aluminate (Na₂O·Al₂O₃), N₂C₃A₅ (2Na₂O·3CaO·5A1₂O₃), nepheline (Na₂0·Al₂0₃·2SiO₂), potassium aluminate, (K₂Oâ·Al₂0₃), and kaliophilite (K₂O·Al₂0₃·2Si0₂) were synthesized and their solubility in water and coefficients of linear thermal expansion were: measured. Of the compounds tested, the formation of potassium aluminate would be the most detrimental to the gasifier lining. The linear thermal expansion of potassium aluminate was 2.05% from room temperature to 800°C, which was twice as large as the other compounds. Potassium aluminate also possessed the highest solubility in water which was 8.893/L at 90°C.
- Annealing twins in nanocrystalline fcc metals: A molecular dynamics simulationFarkas, Diana; Bringa, Eduardo M.; Caro, Alfredo (American Physical Society, 2007-05-23)We report fully three-dimensional atomistic molecular dynamics studies of grain growth kinetics in nanocrystalline Cu of 5 nm average grain size. We observe the formation of annealing twins as part of the grain growth process. The grain size and energy evolution was monitored as a function of time for various temperatures, yielding an activation energy for the process. The atomistic mechanism of annealing twin formation from the moving boundaries is described.
- Atomistic Modeling of Defect Energetics and Kinetics at Interfaces and Surfaces in Metals and AlloysAlcocer Seoane, Axel Emanuel (Virginia Tech, 2024-01-02)Planar defects such as free surfaces and grain boundaries in metals and alloys play important roles affecting many material properties such as fracture toughness, corrosion resistance, wetting, and catalysis. Their interactions with point defects and solute elements also play critical roles on governing the microstructural evolution and associated property changes in materials. This work seeks to use atomistic modeling to obtain a fundamental understanding of many surface and interface related properties and phenomena, namely: orientation-dependent surface energy of elemental metals and alloys, segregation of solute elements at grain boundaries and their impact on grain boundary cohesive strength, and the controversial sluggish diffusion in both the bulk and grain boundaries of high entropy alloys. First, an analytical formula is derived, which can predict the surface energy of any arbitrary (h k l) crystallographic orientation in both body-centered-cubic (BCC) and face-centered-cubic (FCC) pure metals, using only two or three low-index (e.g., (100), (110), (111)) surface energies as input. This analytical formula is validated against 4357 independent single element surface energies reported in literature or calculated by the present author, and it proves to be highly accurate but easy to use. This formula is then expanded to include the simple-cubic (SC) structure and tested against 4542 surface energies of metallic alloys of different cubic structures, and good agreement is achieved for most cases. Second, the effect of segregation of substitutional solute elements on grain boundary cohesive strength in BCC Fe is studied. It is found that the bulk substitution energy can be used as an effective indicator to predict the embrittlement or strengthening potency induced by the solute segregation at grain boundaries. Third, the controversial vacancy-mediated sluggish diffusion in an equiatomic FeNiCrCoCu FCC high entropy alloy is studied. Many literature studies have postulated that the compositional complexity in high entropy alloys could lead to sluggish diffusion. To test this hypothesis, this work compares the vacancy-mediated self-diffusion in this model high entropy alloy with a hypothetical single-element material (called average-atom material) that has similar average properties as the high entropy alloy but without the compositional complexity. The results show that the self-diffusivities in the two bulk systems are very similar, suggesting that the compositional complexity in the high entropy alloy may not be sufficient to induce sluggish diffusion in bulk high entropy alloys. Based on the knowledge learned from the bulk alloy, the exploration of the possible sluggish diffusion has been extended to grain boundaries, using a similar approach as in the study of self-diffusion in bulk. Interestingly, the results show that sluggish diffusion is evident at a Σ5(210) grain boundary in the high entropy alloy due to the compositional complexity, especially in the low temperature regime, which is different from the bulk diffusion. The underlying mechanisms for the sluggish diffusion at this grain boundary is discussed.
- Atomistic Molecular Dynamics Studies of Grain Boundary Structure and Deformation Response in Metallic NanostructuresSmith, Laura Anne Patrick (Virginia Tech, 2014-05-06)The research reported in this dissertation focuses on the response of grain boundaries in polycrystalline metallic nanostructures to applied strain using molecular dynamics simulations and empirical interatomic force laws. The specific goals of the work include establishing how local grain boundary structure affects deformation behavior through the quantitative estimation of various plasticity mechanisms, such as dislocation emission and grain boundary sliding. The effects of strain rate and temperature on the plastic deformation process were also investigated. To achieve this, molecular dynamics simulations were performed on both thin-film and quasi-2D virtual samples constructed using a Voronoi tessellation technique. The samples were subjected to virtual mechanical testing using uniaxial strain at strain rates ranging from 105s-1 to 109s-1. Seven different interatomic embedded atom method potentials were used in this work. The model potentials describe different metals with fcc or bcc crystal structures. The model was validated against experimental results from studying the tensile deformation of irradiated austenitic stainless steels performed by collaborators at the University of Michigan. The results from the model validation include a novel technique for detecting strain localization through adherence of gold nanoparticles to the surface of an experimental sample prior to deformation. Similar trends with respect to intergranular crack initiation were observed between the model and the experiments. Simulations of deformation in the virtual samples revealed for the first time that equilibrium grain boundary structures can be non-planar for model potentials representing fcc materials with low stacking fault energy. Non-planar grain boundary features promote dislocation as deformation mechanisms, and hinder grain boundary sliding. This dissertation also reports the effects of temperature and strain rate on deformation behavior and correlates specific deformation mechanisms that originate from grain boundaries with controlling material properties, deformation temperature and strain rate.
- Atomistic simulation of dislocation core structures in B2 NiAlXie, Zhao-Yang (Virginia Tech, 1993)A systematic study of the core structures of (100), (110), and (111) dislocations in B2 NiAI has been conducted using atomistic simulations with an embedded atom method (EAM) potential. New flexible boundary conditions and a new method of graphic representation of dislocation core structure have been employed. The main findings are the following: Core structures: There are no planar core structures of the dislocations found in B2 NiAl. The core spreading of (100) dislocations in NiAl can occur along a variety of planes depending on dislocation slip plane and line orientation. Discrete lattice effects reduced the high strain levels from anisotropic elasticity solution at the dislocation core considerably and resulted in asymmetrical core structures. The core structure of the (110) dislocations is mutilayered with spreading on the {110} plane. The extent of the same strain level comparing with (100) and (111) dislocations is much larger. The complete (111) dislocations in NiAl are also highly non-planar and are stable with respect to splitting into exact 1/2(111) partials as well as to alternative splittings that correspond to the stable fault in the vicinity of the antiphase boundary (APB), in both {110} and {112} planes. Peierls stresses: Peierls stresses of the dislocations have been calculated and have been compared for their relative ease of motion. Local disordering effects: The local disordering effects on the core structure are found to be significant only in the immediate vicinity of the point defect. Compositional deviation from stoichiometry: The simulation results of (100), (110), and (111)dislocations in off stoichiometric NiAl show that the core structures became more extended than the ones in the stoichiometric NiAl. The core structures are not only dependent on the overall composition but also on their local atomic arrangement near the core region. When compositional deviation from stoichiometry is introduced, the response to the applied stress is different for the various slip systems. The Peierls stresses for the usually easiest moving (100){110} dislocations increased and for the (100){100} dislocations decreased, and the latter are expected to be more active in the deformation processes. The practical implications of these results are that it seems very difficult to modify the alloy behaviors through local changes in stoichiometry and ordering state. The best way to improve the ductility of B2 NiAl is to stabilize (111) slip through the addition of alloying elements that can lower the APB energy.
- Characterization and modeling of dry etch processes for titanium nitride and titanium films in Cl₂/N₂ and BCl₃ plasmasMuthukrishnan, N. Moorthy (Virginia Tech, 1996-11-04)In the past few years, the demands for high speed semiconductor integrated circuits have warranted new techniques in their fabrication process which will meet the ever-shrinking dimensions. The gaseous plasma assisted etching is one of these revolutionary processes. However, the plasma and the etch process are very complex in nature. It has been very difficult to understand various species present in the plasma and their role in the etch reaction. In addition, the submicron geometries also require interconnect materials which will satisfy the necessary properties such as thermal stability and low electrical resistance. Titanium (Ti) and titanium nitride (TiN) are widely used as barriers between aluminum (Al) and silicon (Si) to prevent the destructive intermixing of these two materials. The process of patterning of the interconnect containing Ti and TiN along with Al has been a challenge to the semiconductor process engineers. Therefore, complete characterization of the plasma etch process of Ti and TiN films and development of mathematical models to represent the responses such as the etch rate and uniformity is necessary for a good understanding of the etching process. A robust and well controlled metal etch process usually results in good die yield per wafer and hence can translate into higher profits for the semiconductor manufacturer. The objective of this dissertation is to characterize the plasma etch processes of Ti and TiN films in chlorine containing plasmas such as BCl₃ and Cl₂/N₂ and to develop mathematical models for the etch processes using statistical experimental design and analysis technique known as Response Surface Methodology (RSM). In this work, classical experiments are conducted on the plasma etch process of Ti and TiN films by varying the process parameters, such as gas flow, radio frequency (RF) power, reaction pressure, and temperature, one parameter at a time, while maintaining the other parameters constant. The variation in the etch rate with the change in the process parameter of the film is studied and the results were explained in terms of the concepts of plasma. These experiments, while providing very good understanding of the main effects of the parameters, yield little or no information on the higher order effects or interaction between the process parameters. Therefore, modern experimental design and analysis techniques using computerized statistical methods need to be employed for developing mathematical models for these complex plasma etch processes. The second part of this dissertation concentrates on the Design and Analysis of Experiments using Response Surface Methodology (RSM) and development of models for the etch rate and the etch uniformity of the Ti and TiN films in chlorine-containing plasmas such as Cl₂/N₂ and Cl₂/N₂/BCl₃. A complete characterization of the plasma etch process of Ti and TiN films is achieved with the RSM technique and a well fitting and statistically significant models have been developed for the process responses, such as the etch rate and the etch uniformity. These models also provide a means for quantitative comparison of main effects, which are also known as first order effects, second order effects and two factor interactions. The models, thus developed, can be effectively used for an etch process optimization, prediction of the responses without actually conducting the experiments, and the determination of process window. This dissertation work has achieved a finite study of the plasma etch process of Ti and TiN films. There is tremendous potential and scope for further research in this area, limited only by the available resources for wafer processing. A few of the possibilities for further research is discussed in the next few sentences. The optimized process derived from the RSM technique needs to be implemented in the actual production process of the semiconductor ICs and its effects on the wafer topography, etch residue and the resulting die yield have to be studied. More research studies are needed to examine the effect of process parameters such as temperature, the size and shape of the etch chamber, the quality of the film being etched, among other parameters. It is worth emphasizing in this respect that this dissertation marks beginning of research work into the ever-increasing complexities of gas plasma.
- Computational Studies of the Mechanical Response of Nano-Structured MaterialsBeets, Nathan James (Virginia Tech, 2020-05-18)In this dissertation, simulation techniques are used to understand the role of surfaces, interfaces, and capillary forces on the deformation response of bicontinuous metallic composites and porous materials. This research utilizes atomistic scale modeling to study nanoscale deformation phenomena with time and spatial resolution not available in experimental testing. Molecular dynamics techniques are used to understand plastic deformation of metallic bicontinuous lattices with varying solid volume fraction, connectivity, size, surface stress, loading procedures, and solid density. Strain localization and yield response on nanoporous gold lattices as a function of their solid volume fraction are investigated in axially strained periodic samples with constant average ligament diameter. Simulation stress results revealed that yield response was significantly lower than what can be expected form the Gibson-Ashby formalism for predicting the yield response of macro scale foams. It was found that the number of fully connected ligaments contributing to the overall load bearing structure decreased as a function of solid volume fraction. Correcting for this with a scaling factor that corrects the total volume fraction to "connected, load bearing" solid fraction makes the predictions from the scaling equations more realistic. The effects of ligament diameter in nanoporous lattices on yield and elastic response in both compressive and tensile loading states are reported. Yield response in compression and tension is found to converge for the two deformation modes with increasing ligament diameter, with the samples consistently being stronger in tension, but weaker in compression. The plastic response results are fit to a predictive model that depends on ligament size and surface parameter (f). A modification is made to the model to be in terms of surface area to volume ratio (S/V) rather than ligament diameter (1/d) and the response from capillary forces seems to be more closely modeled with the full surface stress parameter rather than surface energy. Fracture response of a nanoporous gold structure is also studied, using the stress intensity-controlled equations for deformation from linear elastic fracture mechanics in combination with a box of atoms, whose interior is governed by the molecular dynamics formalism. Mechanisms of failure and propagation, propagation rate, and ligament-by-ligament deformation mechanisms such as dislocations and twin boundaries are studied and compared to a corresponding experimental nanoporous gold sample investigated via HRTEM microscopy. Stress state and deformation behavior of individual ligaments are compared to tensile tests of cylinder and hyperboloid nanowires with varying orientations. The information gathered here is used to successfully predict when and how ligaments ahead of the crack tip will fracture. The effects of the addition of silver on the mechanical response of a nanoporous lattice in uniaxial tension and compression is also reported. Samples with identical morphology to the study of the effects of ligament diameter are used, with varying random placement concentrations of silver atoms. A Monte Carlo scheme is used to study the degree of surface segregation after equilibration in a mixed lattice. Dislocation behavior and deformation response for all samples in compression and tension are studied, and yield response specifically is put in the context of a surface effect model. Finally, a novel bicontiuous fully phase separated Cu-Mo structure is investigated, and compared to a morphologically similar experimental sample. Composite interfacial energy and interface orientation structure are studied and compared to corresponding experimental results. The effect of ligament diameter on mechanical response in compressive stress is investigated for a singular morphology, stress distribution by phase is investigated in the context of elastic moduli calculated from the full elastic tensor and pure elemental deformation tests. Dislocation evolution and its effects on strain hardening are put in the context of elastic strain, and plastic response is investigated in the context of a confined layer slip model for emission of a glide loop. The structure is shown to be an excellent, low interface energy model that can arrest slip plane formation while maintaining strength close to the theoretical prediction. Dislocation content in all samples was quantified via the dislocation extraction algorithm. All visualization, phase dependent stress analysis, and structural/property analysis was conducted with the OVITO software package, and its included python editor. All simulations were conducted using the LAMMPS molecular dynamics simulation package. Overall, this dissertation presents insights into plastic deformation phenomena for nano-scale bicontinuous metallic lattices using a combination of experimentation and simulation. A more holistic understanding of the mechanical response of these materials is obtained and an addition to the theory concerning their mechanical response is presented.
- Computer simulation of grain boundary multiplicity in Ni₃AlCardozo, Antonio Fernando Cabral (Virginia Tech, 1991-04-08)not OCRd
- Computer simulation of high fluence ion beam surface modification processesRangaswamy, Mukundhan (Virginia Polytechnic Institute and State University, 1989)Various processes that participate in ion beam surface modification are studied using phenomenological, analytical and first principle models. The processes that are modelled phenomenologically include preferential sputtering, radiation-damage induced migration and second phase precipitation. The models are based on numerical solutions of the transport equation and include the processes of ion collection, sputtering, lattice dilation or accommodation and diffusion as well. The model for preferential sputtering takes into account the depletion of the preferentially sputtered element at the surface and the atomic transport process that results from the concentration gradients caused by the depletion. Results are presented for the case of Ta implantation into Fe. ln the radiation-damage induced migration the flux of the solute atoms is coupled to the concentration gradient of the continuously introduced defects. Examples of implantation of Sn into Fe and N into Fe are modeled to demonstrate the influence of radiation-damage induced migration. The precipitation of second phases during irradiation is modelled using thermodynamic considerations but with solubility values under irradiation obtained from experiment. In the model the solute atoms in excess of the solubility limit are assumed to precipitate out. Calculations are presented for the case of N implantation into Nb. Using first principle calculation for binary collisions in solids a computer simulation code was developed to study the collisional mixing occurring during high fluence ion implantation. It is based on the Monte Carlo code TRIM, and is capable of updating the target composition as the implantation process proceeds to high fluences. The physical basis for the dynamic simulation as well as a detailed analysis on the statistics required for obtaining the profiles with a given accuracy are presented. Vectorized results in a high computational efficiency. The predicted collisional broadening of the implantation profiles is presented for Ar bombardment into a Sn-Fe target as well as Ti implantation into C-Fe. The results are compared to those of the diffusion approximation. A semi-empiricaI model based on an analytical evaluation of ion mixing at low temperatures was developed taking into account collisional mixing and thermal spike effects, as well as the thermal spike shape. The ion beam mixing parameter for the thermal spike is derived as being proportional to different powers of the damage parameter, i.e. the damage energy scaled by the cohesive energy of the matrix, dependent on the thermal spike shape and point defect density in the thermal spike regions. Three different regions of ion beam induced mixing were recognized according to different density levels of the damage parameter. An experiment was conducted to determine the effect of chemical or thermodynamic factors in the migration of C in the presence of Fe and Ti atoms. A marker layer of C in a Fe-Ti matrix was ion beam mixed using Ar. The large mixing effect is tentatively attributed to a favorable heat of mixing values.
- Corrosion resistance of modified β-EucryptiteBattu, Laurent P. (Virginia Tech, 1991-04-01)The corrosion resistance of chemically modified β-eucryptite (Li0.41Mg0.035AlP0.52Si0.480₄) having low expansion anisotropy and a near zero coefficient of thennal expansion was evaluated. Samples were exposed to aqueous hydrochloride acid at temperatures up to 100°C and environments containing sodium sulfate up to l000°C. The corrosion resistance was characterized by dilatometry, scanning electron microscopy, X-ray diffraction, energy dispersive x-ray analysis, weight variations, and mechanical properties variations. The results show that modified β-eucryptite is more severely corroded than commercial lithium-alumina-silicate glass-ceramics when exposed to these environments. Aqueous HCI removes AIP04 from modified β-eucryptite leaving a very porous structure. Molten salt corrodes modified β-eucryptite by penetration of sodium and sulfur which form an alkali melt under the surface. The modulus of rupture and the Young's modulus are reduced by both types of corrosion.
- Defect Structures in Ordered Intermetallics; Grain Boundaries and Surfaces in FeAl, NiAl, CoAl and TiAlMutasa, Batsirai M. (Virginia Tech, 1997-05-16)Ordered intermetallics based on transition metal aluminides have been proposed as structural materials for advanced aerospace applications. The development of these materials, which have the advantages of low density and high operating temperatures, have been focused on the aluminides of titanium, nickel and iron. Though these materials exhibit attractive properties at elevated temperatures, their utilization is limited due to their propensity for low temperature fracture and susceptibility to decreased ductility due to environmental effects. A major embrittlement mechanism at ambient temperatures in these aluminides has been by the loss of cohesive strength at the interfaces (intergranular failure). This study focuses on this mechanism of failure, by undertaking a systematic study of the energies and structures of specific grain boundaries in some of these compounds. The relaxed atomistic grain boundary structures in B2 aluminides, FeAl, NiAl and CoAl and L1₀ γ-TiAl were investigated using molecular statics and embedded atom potentials in order to explore general trends for a series of B2 compounds as well as TiAl. The potentials used correctly predict the proper mechanism of compositional disorder of these compounds. Using these potentials, point defects, free surface energies and various grain boundary structures of similar energies in three B2 compounds, FeAl, NiAl and CoAl were studied. These B2 alloys exhibited increasing anti-phase boundary energies respectively. The misorientations chosen for detailed study correspond to the Σ5(310) and Σ5(210) boundaries. These boundaries were investigated with consideration given to possible variations in the local chemical composition. The effects of both boundary stoichiometry and bulk stoichiometry on grain boundary energetics were also considered. Defect energies were calculated for boundaries contained in both stoichiometric and off-stoichiometric bulk. The surface energies for these aluminides were also calculated so that trends concerning the cohesive energy of the boundaries could be studied. The implications of stoichiometry, the multiplicity of the boundary structures and possible transformations between them for grain boundary brittleness are also discussed.
- Deformation mechanisms in B2 aluminides: shear faults and dislocation core structures in FeAl, NiAl, CoAl and FeNiAlVailhé, Christophe N. P. (Virginia Tech, 1996)Although aluminides with the B2 crystal structures have good properties for high temperature applications, the strong ordered bonds that make them durable at high temperature also make them too brittle at room temperature for industrial fabrication. In order to better understand this lack of ductility, molecular statics simulations of planar fault defects and dislocation core structures were conducted in a series of B2 aluminides with increasing ordering energy (FeAl, NiAl, CoAl). The simulation results in NiAl were compared with in-situ straining observations of dislocation motion. The dislocations simulated were of (100) and (111) types. The simulations results obtained indicate a strong influence of the planar fault energies on the mobility of the dislocations. As the cohesive energy increases from FeAl to CoAl, antiphase boundary and unstable stacking fault energies increase resulting in more constricted dislocation core spreadings. This constriction of the cores decreases the mobility of dislocation with planar core structures and increases the mobility of dislocations with non-planar cores. The (100) screw dislocations were found with planar cores in {110} planes for FeAl, NiAl and CoAl. For very high APB values, the cores were very compact, as predicted by the Peierls- Nabarro model. As the APB energies decrease, increasingly two dimensional spreading of the cores was observed and ultimately dislocation dissociation into partials. As a result of the deviation of the stable planar fault energy from the APB fault, the partials were not exact 1/2(111) but deviate to the point corresponding to the actual minima of the γ-surfaces for these compounds. Alloying NiAl with Fe was found to promote the dissociation of the (100) dislocation. The in-situ straining of a single crystal of NiAl only revealed the motion of (100) dislocations. Both in-situ observations and atomistic simulations agreed on the zig-zag shape of the (100) dislocation with an average screw orientation. In this configuration, the mobility of the dislocation is severely reduced.
- Development of measurement techniques for evaluation of inhibitors for controlling rebar corrosion in concreteGuerin, Pascal Claude Raymond (Virginia Tech, 1990-04-25)Concrete provides a nearly perfect environment for corrosion protection of steel; However, the use of de-icing salts on the highway system has accelerated the deterioration rate of bridge decks in the snow belt. In 1981, over 100,000 bridges were reported to be structurally deficient, and the estimated cost of repair was placed at $93 billion. Concrete specimens, 1 ft. x 1 ft. x 4 in., containing four pieces of steel reinforcing bars, were prepared. In a first time, five specimens with different rebar networks were cast in order to study the effects of the rebar network on corrosion. Half-cell potential measurements were used to monitor the corrosion behavior of each specimen. In a second time, calcium nitrite, monofluorophosphate and sodium borate were evaluated for their capacity to control corrosion. The various corrosion inhibitors were tested either externally (in the test solution), internally (as a concrete admixture). The specimens were exposed to alternate complete immersion in a 6 wt% sodium chloride solution, plus eventual addition of corrosion inhibitor, for 3 days and 12 hours of drying at 110°F. The effects of corrosion inhibitors were evaluated using half-cell potential measurements and Electrochemical Impedance Spectroscopy (EIS) measurements. The EIS data were analyzed through a computer assisted EIS data analysis system. This allowed for circuit modeling of the corrosion mechanisms and evaluation of polarization resistance values for the different specimens. In both phases of this work, chloride concentration profiles as function of depth were determined. The half-cell potential measurements in complete immersion were found to give an average over the length of bar or electrically connected bars. The corrosion inhibitors applied internally were found to reduce corrosion better than corrosion inhibitors applied externally. It was shown that chloride concentration is not the only parameter controlling corrosion initiation.
- Dislocation pinning effects on fracture behavior: Atomistic and dislocation dynamics simulationsNoronha, S. J.; Farkas, Diana (American Physical Society, 2002-10-01)We introduce an approach in which results from atomistic simulations are combined with discrete dislocation dynamics simulations of crack-tip plasticity. The method is used to study the effects of dislocation pinning due to grain boundaries or secondary particles on the fracture behavior of aluminum. We find that the fracture resistance is reduced with decreasing pinning distance. The results show that the pinning of the dislocations causes a net decrease in the shear stress projected on the slip plane, preventing further dislocation emission. Semibrittle cleavage occurs after a certain number of dislocations is emitted.
- The effect of demand uncertainty on planning: the steel industry in ArgentinaFarkas, Diana (Virginia Polytechnic Institute and State University, 1985)The traditional method for conducting sensitivity analysis is to repeatedly solve a model while varying the parameters. The solution is then obtained as some average of these optimal solutions under those different conditions or states of the world. The present work presents results of conducting sensitivity analysis using a method more firmly ground in mathematical programming theory. The present analysis models the investment decisions in a case with large uncertainty in demand: the steel industry in Argentina. Special emphasis is devoted to the recent history, where a recent shift in economic policy (1976-1981) towards allowing free competition with imported products resulted in a severe crisis for the steel industry and its trading partners. An increase in exports was observed during this period which is not likely to continue if there is a recovery process. In the first sections, the relation of steel production and economic growth is analyzed in the context of the world situation of the industry, setting the background for the analysis of the Argentinian industry as a case study. The results of the present model adequately describe the existence of unutilized capacity observed in the industry, as well as the recent increase in exports. The most important conclusion of the model is that the traditional method of conducting sensitivity analysis results in significant inefficiency of the reached decisions, involving large losses for a case such as the steel industry considered here.
- Effects of Carbon on Fracture Mechanisms in Nanocrystalline BCC Iron - Atomistic SimulationsHyde, Brian (Virginia Tech, 2004-04-20)Atomistic computer simulations were performed using embedded atom method interatomic potentials in α-Fe with impurities and defects. The effects of intergranular carbon on fracture toughness and the mechanisms of fracture were investigated. It was found that as the average grain size changes the dominant energy release mechanism also changes. Because of this the role of the intergranular carbon changes and these mechanisms compete affecting the fracture toughness differently with changing grain size. Grain boundary accommodation mechanisms are seen to be dominant in the fracture of nanocrystalline α-Fe. To supplement this work we investigate grain boundary sliding using the Σ = 5,(310)[001] symmetrical tilt grain boundary. We observe that in this special boundary sliding is governed by grain boundary dislocation activity with Burgers vectors belonging to the DSC lattice. The sliding process was found to occur through the nucleation and glide of partial grain boundary dislocations, with a secondary grain boundary structure playing an important role in the sliding process. Interstitial impurities and vacancies were introduced in the grain boundary to study their role as nucleation sites for the grain boundary dislocations. While vacancies and H interstitials act as preferred nucleation sites, C interstitials do not.
- Electro-optic Properties of Semiconductor Nano-crystals And Electro-optic Polymers And Their ApplicationsZhang, Fajian (Virginia Tech, 2002-09-26)In recent years, electro-optic polymers have been used to make various optical devices in the telecommunication field due to several advantages, such as large and fast electro-optic (EO) response. Semiconductor nano-crystals promise even higher response speed due to the unique quantum confinement mechanism, and they also show very high EO response because of surface and quantum size effects. Many investigative efforts have been made in the area of semiconductor nano-clusters. These efforts mainly focus on synthesizing high quality particles, and their physical and chemistry properties (luminescence spectra, nonlinear optical, and other effects), but their electro-optic properties and potential uses in devices have not been fully investigated, so there is still much work to do in this aspect. For application of electro-optic polymers in electro-optic devices, the challenges are to develop more stable electro-optic polymers with higher electro-optic coefficients. The electrostatic self-assembly (ESA) technique has many advantages over traditional polymer electro-optic film synthesis processes, such as spin coating. For ESA-generated EO films, no poling field is needed, high orientation of the EO polymer can be obtained which does not degrade with time, so the films can be very stable, and this processing is easily compatible with semiconductor VLSI technology. This is a very attractive technique. The goal of this research is to develop new electro-optic materials by means of ESA techniques and to use them to form improved performance next generation electro-optic devices, with emphasis on two kinds of electro-optic materials: nano-sized II-VI semiconductors (CdS, CdSe), and electro-optic active polymers (chromophores), and their potential use in electro-optic devices. In this research work, II-VI semiconductor nano-clusters have been synthesized, with particle diameters ranging from 4 nm to several tens of nanometers. There is a difference in peak positions of absorption and photo luminescence spectra, related to defects in nano-crystals. Larger CdS particles have larger differences than small CdSe particles. Particle sizes measured by absorption spectrum and by HRTEM methods are very close. Based on quantum mechanical theory, peak spectral shifts as a function of particle size can be predicted, but the theoretical results are typically far from the experimental results, because many complicating factors should be considered. Films fabricated by ESA have much stronger absorption than spin coated films, and exhibit a slight blue shift in peak position wavelength. Photo luminescence spectra also show a blue shift for ESA films with respect to spun films. Polymeric electro-optic films were also fabricated by the ESA technique. Effects due to applying an external electrical field during the ESA process on film growth and properties have also been investigated. Peak position, optical density and wavelength at maximum absorption, all increase with the number of bilayers, and films made under external fields have lower absorption and peak wavelength than those of films fabricated without an external field. These results are related to the order parameter, and indicate that molecule alignment can be improved by the application of an external field during the process of ESA film growth. CdSe nano-clusters have a much higher electro-optic coefficient than their bulk crystal counterparts. In comparison with polymers, they have totally different origins in their electro-optic effects. For both nano-cluster-and chromophore based ESA films, electro-optic coefficients are hi gher than those of spin-coated films, and no poling voltage is needed. The reasons have been fully discussed. This result means that the ESA technique is effective to align and hold the dipoles in films and to intensify the electro-optic effect. CdSe quantum dots need 17. 5 ms to complete their physical orientation due to a rotation of the permanent dipole moment. Therefore, at lower frequencies (<100Hz), electro-optic modulation mainly stems from the orientation of the permanent dipole moment. At frequencies higher than 100 Hz, the electro-optic modulation mainly arises from the induced dipole moment orientation and pure electron movement. The ratio of the electro-optic coefficients r333/r113 > 3. This means that ESA films cannot be treated as an ideal isotropic system with the C v symmetry, and interactions should be considered. Quadratic Kerr electro-optic coefficients have a similar frequency dependence to that of the linear electro-optic coefficients r333 and r113. This indicates that the orientational distribution of the CdSe quantum dots particularly contributes to the quadratic electro-optic modulation. From the FT-IR measurement of the films, proton irradiation can break the N=N double bonding in pi-conjugated bridges, leading to damage of the conjugating structure, so causing a decrease of the EO coefficient. But the thermal and temporal stability of ESA films are much better than those of spin coated films; this is a significant feature of ESA technique. The effect of an external field and film thickness on the optical and electro-optic properties of ESA films has been investigated. Electro-optic coefficient decreases with thickness. Electrical field influences the electronic states of the chromophores. Based on the properties of electro-optic films, the applications of polymer and nano-cluster electro-optic films are discussed. A nano-cluster CdSe electro-optic film has a higher refractive index than the PS-119 polymer film, and these values they are much lower than that of semiconductor wafers, but slightly higher than optical silica glasses. Accordingly optical silica glasses are the ideal substrates for those films. By analysis, the cutoff thickness was determined, which defines the minimum film thickness required for light propagation. For channel waveguides, the aspect ratio w/t, w, and t are determined versus the refractive index of the electro-optic films. Modulator beam length and modulation index were discussed, for high speed operation. Modulator beam length should be carefully chosen to obtain high modulation index; similarly important is the refractive index match between core, substrate, and cladding layers. For high speed operation, traveling wave electrode designs were considered, based on effective refractive index and impedance matching. The effective dielectric constant and characteristic impedance as a function of electrode configuration (sizes) were diagramed, and this served as a basic design suggestion for traveling wave electrodes.
- Embedded-atom interatomic potentials for hydrogen in metals and intermetallic alloysRuda, M.; Farkas, Diana; Abriata, J. (American Physical Society, 1996-10-01)Interatomic potentials of the embedded-atom type have been developed for H in metals. The potentials are constructed through a fitting procedure involving the thermodynamic heat of solution of H in the various metals and the volume expansion of the host lattice upon the dissolution of H. The potentials have been developed in such a way that the same functions for hydrogen are used for all the metals considered and the resulting set of potentials is suitable for the study of hydrogen effects in alloys. The pure metals considered are Ni, Al, Ti, Zr, and Fe. The heats of solution of hydrogen in various intermetallic alloys formed by these metals (TiAl, Ti3Al, NiAl, Ni3Al, NiTi, and FeAl) have been studied using the developed potentials in conjunction with existing potentials for the intermetallic systems. The effects of increasing hydrogen absorbed in the host are also simulated for the case of Ni.
- Empirical many-body interatomic potential for bcc transition metalsPasianot, R.; Farkas, Diana; Savino, E. J. (American Physical Society, 1991-03-01)A simple many-body interatomic potential is proposed. This is an empirical extension of the embedded-atom method (EAM). The EAM models the lattice energy and elastic compressibility using a pair interaction plus a many-body term. It does not include any contribution of many-body terms to the crystal elastic shear. This contribution is included in the model developed here. It implies a simplified treatment of the angularity inherent to covalent bonding in transition metals. A set of interatomic potentials is deduced for bcc Nb, Fe, and Cr. While in previous works in the literature the EAM has already been successfully applied to the fitting of interatomic potentials for Nb and Fe, this was not the case for Cr, for which the elastic-constant values implied a negative Cauchy pressure.
- Energetics and Deformation Response of Random Grain Boundaries in FCC NickelFloyd, Niklas Paul (Virginia Tech, 2010-05-13)Molecular dynamics simulations are use to study the energetics and deformation response of random grain boundaries in polycrystalline Nickel. Computer generated samples of defect-free Ni were created, plastically deformed, and examined as a baseline understanding to the underlying mechanisms of deformation and intergranular fracture in FCC metals. Two types of samples were utilized: a sample with columnar grains consisting of pure <110> tilt boundaries and a thin-film sample with 3D grain orientations modeled after an experimental sample of austenitic steel. The structure and energies of these random boundaries under stress and temperature was analyzed. Heterogeneous displacement maps were made for a side-by-side comparison of the dislocation activity and interactions with the grain boundaries. The dislocation behavior was found to be consistent between the two digital sample types and further comparison with experimental samples was made. The intergranular cracking behavior was also studied and various factors were examined to generate general trends. Crack initiation was observed to typically occur in random high-angle boundaries close to a triple junction where the cracks have high angles with respect to the tensile loading direction. The cracking results from the simulations agree well with current preliminary results of experimentally deformed austenitic steel samples. Furthermore, the behavior and failure of the thin-film sample is compared with its corresponding experimental sample.
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