<p>Accessing improved elastomeric composites filled with functionalized graphene sheets (FGSs) requires an understanding of how the FGSs aggregate and how the position of FGSs affects the mechanical properties of the final composite material. In this thesis, I study both effects by devising models for 2-D particles in the 10s of microns scale and comparing my results with experiments. These models enable an understanding of the effect of the particles in a level that is hard to be studied experimentally or by molecular models. In the first part, I present a model for aggregation of 2-D particles and apply it to study the aggregation of FGS in water with varying concentrations of sodium dodecyl sulfate (SDS). The model produces clusters of similar sizes and structures as a function of SDS concentration in agreement with experiments and predicts the existence of a critical surfactant concentration beyond which thermodynamically stable FGS suspensions form. Around the critical surfactant concentration, particles form dense clusters and rapidly sediment. At surfactant concentrations lower than the critical concentration, a contiguous ramified network of FGS gel forms which also densifies, but at a lower rate, and sediments with time. This densification leads to graphite-like structures. In the second part, I present a model for the prediction of the mechanical properties of elastomers filled with 2-D particles. I apply this model to the Poly-dimethylsiloxane (PDMS)-FGS system. For a perfect polymer matrix and when inter-particle forces are ignored the strength of the composite can be increased with the addition of particles but elongation at failure decreases relative to neat PDMS. Maximum load transfer to the particles is achieved when particles are covalently linked to span the whole polymer matrix. Minimum drop in elongation at failure can be achieved by maximizing the distance between the covalently linked particles. When the assumption of a perfect polymer matrix is relaxed, it can be shown that there is a certain particle concentration range for which elongation at failure can be increased as the particles can protect the polymer by redistributing high stresses created by inherent polymer defects that would lead to early failure.

<p>Understanding the curing kinetics of a thermoset resin has a significant importance in developing and optimizing curing cycles in various industrial manufacturing processes. This can assist in improving the quality of final product and minimizing the manufacturing-associated costs. One approach towards developing such an understanding is to formulate kinetic models that can be used to optimize curing time and temperature to reach a full cure state or to determine time to apply pressure in an autoclave process. Various phenomenological reaction models have been used in the literature to successfully predict the kinetic behavior of a thermoset system. The current research work was designed to investigate the cure kinetics of Bisphenol-A based Benzoxazine (BZ-a) and Cycloaliphatic epoxy resin (CER) system under isothermal and nonisothermal conditions by Differential Scanning Calorimetry (DSC). The cure characteristics of BZ-a/CER copolymer systems with 75/25 wt% and 50/50 wt% have been studied and compared to that of pure benzoxazine under nonisothermal conditions. The DSC thermograms exhibited by these BZ-a/CER copolymer systems showed a single exothermic peak, indicating that the reactions between benzoxazine-benzoxazine monomers and benzoxazine-cycloaliphatic epoxy resin were interactive and occurred simultaneously. The Kissinger method and isoconversional methods including Ozawa-Flynn-Wall and Freidman were employed to obtain the activation energy values and determine the nature of the reaction. The cure behavior and the kinetic parameters were determined by adopting a single step autocatalytic model based on Kamal and Sourour phenomenological reaction model. The model was found to suitably describe the cure kinetics of copolymer system prior to the diffusion-control reaction. Analyzing and understanding the thermoset resin system under isothermal conditions is also important since it is the most common practice in the industry. The BZ-a/CER copolymer system with 75/25 wt% ratio which exhibited high glass transition temperature compared to polybenzoxazine was investigated under isothermal conditions. The copolymer system exhibited the maximum reaction rate at an intermediate degree of cure (20 to 40%), indicating that the reaction was autocatalytic. Similar to the nonisothermal cure kinetics, Kamal and Sourour phenomenological reaction model was adopted to determine the kinetic behavior of the system. The theoretical values based on the developed model showed a deviation from the obtained experimental values, which indicated the change in kinetics from a reaction-controlled mechanism to a diffusion-controlled mechanism with increasing reaction conversion. To substantiate the hypothesis, Fournier et al?s diffusion factor was introduced into the model, resulting in an agreement between the theoretical and experimental values. The changes in cross-linking density and the glass transition temperature (Tg) with increasing epoxy concentration were investigated under Dynamic Mechanical Analyzer (DMA). The BZ-a/CER copolymer system with the epoxy content of less than 40 wt% exhibited the greatest Tg and cross-linking density compared to benzoxazine homopolymer and other ratios.

<p> In today&rsquo;s industry, engineering materials are continuously pushed to the limits. Often, the application only demands high-specification properties in a narrowly-defined region of the material, such as the outermost surface. This, in combination with the economic benefits, makes case hardening an attractive solution to meet industry demands. While case hardening has been in use for decades, applications demanding high hardness, deep case depth, and high corrosion resistance are often under-served by this process. Instead, new solutions are required.</p><p> The goal of this study is to develop and characterize a new borochromizing process applied to a pre-carburized AISI 8620 alloy steel. The process was successfully developed using a combination of computational simulations, calculations, and experimental testing. Process kinetics were studied by fitting case depth measurement data to Fick&rsquo;s Second Law of Diffusion and an Arrhenius equation. Results indicate that the kinetics of the co-diffusion method are unaffected by the addition of chromium to the powder pack. The results also show that significant structural degradation of the case occurs when chromizing is applied sequentially to an existing boronized case. The amount of degradation is proportional to the chromizing parameters.</p><p> Microstructural evolution was studied using metallographic methods, simulation and computational calculations, and analytical techniques. While the co-diffusion process failed to enrich the substrate with chromium, significant enrichment is obtained with the sequential diffusion process. The amount of enrichment is directly proportional to the chromizing parameters with higher parameters resulting in more enrichment. The case consists of M₇C₃ and M₂₃C₆ carbides nearest the surface, minor amounts of CrB, and a balance of M₂B.</p><p> Corrosion resistance was measured with salt spray and electrochemical methods. These methods confirm the benefit of surface enrichment by chromium in the sequential diffusion method with corrosion resistance increasing directly with chromium concentration. The results also confirm the deleterious effect of surface-breaking case defects and the need to reduce or eliminate them. </p><p> The best combination of microstructural integrity, mean surface hardness, effective case depth, and corrosion resistance is obtained in samples sequentially boronized and chromized at 870&deg;C for 6hrs. Additional work is required to further optimize process parameters and case properties.</p><p>

<p> Electron Beam Additive Manufacturing (EBAM) is a relatively new additive manufacturing (AM) technology that uses a high-energy electron beam to melt and fuse powders to build full-density parts in a layer by layer fashion. EBAM can fabricate metallic components, particularly, of complex shapes, in an efficient and cost-effective manner compared to conventional manufacturing means. EBAM is an enabling technology for rapid manufacturing (RM) of metallic components, and thus, can efficiently integrate the design and manufacturing of aerospace components. However, EBAM for aerospace-related applications remain limited because the effect of the EBAM process on part characteristics is not fully understood. In this study, various techniques including microhardness, optical microscopy (OM), X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), and electron backscatter diffraction (EBSD) were used to characterize Ti-6Al-4V components processed using EBAM. The results were compared to Ti-6Al-4V components processed using conventional techniques. In this study it is shown that EBAM built Ti-64 components have increased hardness, elastic modulus, and yield strength compared to wrought Ti-6Al-4V. Further, it is also shown in this study that the horizontal build EBAM Ti-6Al-4V has increased hardness, elastic modulus, and yield strength compared to vertical build EBAM due to a preferential growth of the &beta; phase.</p><p>

The micro-structural approach, which relates the mechanical behavior of a material to its micro-fabric and the properties of the constituent particles, is a more rational way of modeling the mechanics of granular materials. Within this approach is the numerical simulation method in the framework of the Discrete Element Method (DEM) of analysis. Instead of a continuum, DEM treats granular material as an assemblage of distinct particles, each governed by the laws of classical mechanics. Deformation analysis of inter-particle contacts does not imply continuity at particle boundaries. As this technique has evolved, it has been used in a wide variety of research applications in mechanics and Geotechnical engineering. However, there are some drawbacks to its use especially in the simulation and interpretation of real granular material behavior. Inadequate understanding of the micro-kinematics of particle rotation and contact rolling have rendered most DEM models ineffective in translating its usefulness to the overall study of the mechanics of granular assemblies. This study evaluated DEM analysis for the purpose of improving computer simulation models of granular materials in order to enhance the capability of predicting real granular behavior and its usefulness as an alternative to full-scale modeling. Implicit and explicit numerical integration algorithms are discussed on the basis of a generalized collocation formulation. In relation to DEM, it is shown that the explicit velocity Verlet method improves convergence, stability and accuracy. Using the concept of rolling friction, closed-form expressions were derived for contact rolling stiffness for both 2-D and 3-D problems. The developed DEM simulation model shows that the effects of rolling friction on the stress-strain behavior, shear strength and the development of shear bands are very significant. The study proves that simulation of granular media is greatly enhanced and the microstructure and micro-mechanisms are better revealed. Validation tests showed good agreement between DEM simulation results and available experimental tests on rod assemblies. Comparisons of heterogeneous deformation fields and the uniform strain fields indicate the need to incorporate a high gradient of strain theory in predicting the constitutive law of granular materials.

Zeolite NaA films and membranes have been prepared using both in situ and seeded growth preparation processes. Films prepared using in situ preparations have shown this technique to be unsuitable for further development due to its inability to control the film microstructure, poor reproducibility, and dissolution of the substrate resulting in amorphous material incorporated in the film. Seeded growth, however, shows the ability to prepare highly oriented zeolite NaA films, the first zeolite films reported with this high degree of orientation. For the seeded growth preparation, nanometer sized zeolite particles are used in suspension to cast seed films. These films are prepared using dip coating, film casting, and electrostatic deposition. The seed films show a high degree of orientation with the [h00] planes of the seed crystals aligned parallel to the substrate surface. A higher degree of orientation where the particles are deposited in a hexagonal packed array can be achieved using dip coating with extremely slow withdrawal rates (∼1 cm/hr). These seed films are then subjected to a secondary growth process to eliminate the interzeolitic pores and form continuous zeolite layers. This has been achieved with clear solutions or gels resulting in continuous films 0.5 to 7μm thick with a high degree of orientation. The regrowth mechanism was investigated and results indicate that the growth of zeolite A films proceeds by multiple processes including epitaxial growth of the seeds and deposition of particles from solution. The membranes have been used for alcohol/water pervaporation. The membranes are highly selective for water and show selectivities >3,100 for water using (90/10) ethanol/water feed systems. In permeation measurements, these membranes show no selectivities other than Knudsen for permanent gases. Unlike Knudsen diffusion, these membranes show increasing permeation with increasing temperature. This indicates the probability of small defects in the films around 10Å. The defects shown by the gas permeation measurements indicate that cracks have formed in the membranes, possibly upon drying. It is believed that these are caused by the contraction of the zeolite NaA structure upon removal of water, which gives a 0.16% contraction in the dimensions of the unit cell.

Microphase separated block copolymers have been subject of investigation for past two decades. While most of the work is focused on classical phases of lamellae or cylinders, spherical phases have received less attention. The present study deals with the self-assembly in spherical phases of block copolymers that results into formation of a three-dimensional cubic lattice. A model triblock copolymer with several transition temperatures is chosen. Solidification in this model system results from either the arrangement of nanospheres of minor block on a BCC lattice or by formation of physical network where the nanospheres act as crosslinks. The solid-like behavior is characterized by extremely slow relaxation modes. Long time stress relaxation of the model material was examined to distinguish between the solid and liquid behavior. Stress relaxation data from a conventional rheometer was extended to very long times by using a newly built instrument, Relaxometer. The BCC lattice structure of the material behaves as liquid over long time except at low temperatures where an equilibrium modulus is observed. This long time behavior was extended to low shear rate behavior using steady shear rheology. The zero shear viscosity observed at extremely low shear rates has a very high value that is close to the viscosity calculated from stress relaxation experiments. The steady shear viscosity decreases by several orders of magnitude over a small range of shear rates. SAXS experiments on samples sheared even at very low rates indicated loss of the BCC order that was present in the annealed samples before shearing. In the second part, response of the BCC microstructure to large stress was explored. Shearing at constant rate and with LAOS at low frequencies lead to destruction of BCC lattice. The structure recovers upon cessation of the shear with kinetics similar to the one following thermal quench. Under certain conditions, LAOS leads to formation of monodomain textures. At low frequencies, there exists an upper and lower bound on strain amplitude where mono-domain textures can be obtained. Upon alignment, the modulus drops by about 30%. Measurement of rheological properties offers an indirect method to distinguish between polycrystalline structure and monodomain texture.

Continuing pressures for higher performance and efficiency in energy conversion and propulsion systems are driving ever more demanding needs for new materials which can survive high stresses at the elevated temperatures. In such severe environments, the characterization of creep properties becomes indispensable. Conventional techniques for the measurement of creep are limited to about 1,700°C. A new method which can be applied at temperatures higher than 2,000°C is strongly demanded. This research presents a non-contact method for the measurements of creep resistance of ultra-high-temperature materials. Using the electrostatic levitation (ESL) facility at NASA MSFC, a spherical sample was rotated quickly enough to cause creep deformation due to the centripetal acceleration. The deformation of the sample was captured with a digital camera, and the images were then analyzed to measure creep deformation and to estimate the stress exponent in the constitutive equation of the power-law creep. To compare experimental results, numerical and analytical analyses on creep deformation of a rotating sphere have been conducted. The experimental, numerical, and analytical results showed a good agreement with one another.

Nanometer-scale-thick films of metals and semiconductor heterostructures are used increasingly in modern technologies, from microelectronics to various areas of nanofabrication. Processing of such ultrathin-film materials generates structural defects, including voids and cracks, and may induce structural transformations. Furthermore, the mechanical behavior of these small-volume structures is very different from that of bulk materials. Improvement of the reliability, functionality, and performance of nano-scale devices requires a fundamental understanding of the atomistic mechanisms that govern the thin-film response to mechanical loading in order to establish links between the films’ structural evolution and their mechanical behavior. Toward this end, a significant part of this study is focused on the analysis of atomic-scale mechanisms of plastic deformation in freestanding, ultrathin films of face-centered cubic (fcc) copper (Cu) that are subjected to biaxial tensile strain. The analysis is based on large-scale molecular-dynamics simulations. Elementary mechanisms of dislocation nucleation are studied and several problems involving the structural evolution of the thin films due to the glide of and interactions between dislocations are addressed. These problems include void nucleation, martensitic transformation, and the role of stacking faults in facilitating dislocation depletion in ultrathin films and other small-volume structures of fcc metals. Void nucleation is analyzed as a mechanism of strain relaxation in Cu thin films. The glide of multiple dislocations causes shearing of atomic planes and leads to formation of surface pits, while vacancies are generated due to the glide motion of jogged dislocations. Coalescence of vacancy clusters with surface pits leads to formation of voids. In addition, the phase transformation of fcc Cu films to hexagonal-close packed (hcp) ones is studied. The resulting martensite phase nucleates at the film’s free surface and grows into the bulk of the film due to dislocation glide. The role of surface orientation in the strain relaxation of these strained thin films under biaxial tension is discussed and the stability of the fcc crystalline phase is analyzed. Finally, the mechanical response during dynamic tensile straining of pre-treated fcc metallic thin films with varying propensities for formation of stacking faults is analyzed. Interactions between dislocations and stacking faults play a significant role in the cross-slip and eventual annihilation of dislocations in films of fcc metals with low-to-medium values of the stable-to-unstable stacking-fault energy ratio, γs/γu. Stacking-fault-mediated mechanisms of dislocation depletion in these ultrathin fcc metallic films are identified and analyzed. Additionally, a theoretical analysis for the kinetics of strain relaxation in Si1-xGex (0 ≤ x ≤ 1) thin films grown epitaxially on Si(001) substrates is conducted. The analysis is based on a properly parameterized dislocation mean-field theoretical model that describes plastic-deformation dynamics due to threading dislocation propagation; the analysis addresses strain relaxation kinetics during both epitaxial growth and thermal annealing, including post-implantation annealing. The theoretical predictions for strain relaxation as a function of film thickness in Si0.80Ge0.20 /Si(001) samples annealed after growth, either unimplanted or after He+ implantation, are in excellent agreement with reported experimental measurements.

The separation of hydrogen from other light gases is of particular importance to the chemical process industry. Membrane based processes offer a cost effective alternative to traditional processing while allowing the combination of separation and reaction in a single unit. Dense palladium or palladium alloy films are a natural choice for hydrogen separation due to their potential infinite selectivity for hydrogen. In this dissertation we investigated the construction of palladium-based supported hydrogen separation membranes using Supercritical Fluid Deposition (SFD). Compared to other deposition methods, SFD offers an effective metal deposition approach for porous materials due to its high precursor solubility, rapid mass transfer, and lack of surface tension. Three palladium precursors were evaluated for membrane construction in terms of thermal stability, reactivity and surface selectivity. Pd-X (X = Ag, Ni, or Cu) co-depositions were studied to determine the potential of SFD for direct alloy deposition. Intrinsic to effective membrane construction is the control of membrane location and thickness. Several different reactor and reactants geometries were utilized to control membrane location. An opposed reactants geometry was used to produce sub-surface membranes at controlled depths (80–600 μm) in porous α-alumina. A same-sided reactants geometry was used to produce surface films ranging in thickness from 100 nm to 5 μm on numerous support materials. Membranes were characterized using a variety of techniques including: SEM, XPS, XRD, EPMA, and gas permeation.