<p> Adhesive bonding is a common, robust, and inexpensive method of joining materials. Of particular interest is the behavior under shear loading, where adhesive bonding excels compared to alternative joining methods. However, while the quasi-static response of these joints is well understood, the dynamic behavior is largely unknown. </p><p> To this end, a series of experiments were devised and performed where two bars are adhesively bonded using a simple lap joint and subjected to a high-speed impact from a steel slug. These tests were configured to, as much as possible, isolate the type of wave that generates adhesive shear and minimize the effect of reflected and induced waves. While keeping the overall geometry constant, the adhesive material, substrate material, and projectile velocity were varied. </p><p> The wave behavior was recorded using surface-mounted strain gages. Also, digital image correlation techniques were developed to analyze high-speed video of the impact event. From these experiments, a number of useful measures can be extracted, including the critical input (projectile) kinetic energy and the specific energy absorbed by the adhesive. </p><p> The techniques developed in this thesis allow for the suitability of different substrate/adhesive combinations under ballistic shear impact to be quantitatively evaluated. </p><p> Additionally, dynamic plate theory is used to derive an analytical model of the substrate/adhesive system. Several solutions to this model which were solved using a Finite Difference approach are included. These solutions were then compared to the strain histories recorded in the physical experiments. </p><p>

<p> Cadmium Telluride (CdTe) thin film photovoltaic technology has shown favorable progress due to inexpensive and efficient processing techniques. However, efficiencies have yet to reach the overall projected CdTe device efficiency, with the back contact being a main source of CdTe performance limitations. Tellurium (Te) applied as a back contact has led to significant increases in fill factor and an overall progress in device efficiency. Devices deposited with Te show significant improvement in uniformity, even without intentional Cu doping, when compared to devices without Te. In current - density measurements, Te shows stability even at low temperatures, which is indicative of a low barrier developed at the CdTe/Te interface. X-ray and ultra-violet photoelectron spectroscopy were carried out to examine the valence band offset at the CdTe/Te back contact interface. The valence band offset was shown to be highly dependent on the Te thickness and was largely affected by oxidation and contamination at the surface. Capacitance measurements were carried out to study the effect Te has on the absorber depletion width. Data indicate a decreased depletion width with Te applied at the back of thin film CdTe devices, which agrees with increased device performance. Te thickness was varied in all studies to understand the effect of application thickness on device performance and material characteristics. With a thicker Te layer leading to overall improvement in device performance and favorable device characteristics. </p><p>

<p> Quasi static and intermediate strain rate (5 s^&ndash;1 and 500 s^&ndash;1) tests are conducted on various aluminum and steel ASTM E8 subsize tensile specimens to validate a newly developed testing method which combines a previously developed serpentine bar for load monitoring and a newly described high-speed actuator. This new actuator is controlled by a semi-passive piezoelectrically actuated brake system mounted to a standard actuator, which allows for the actuator to produce high loads and quick response times (&ap; 100 &micro;s). Limitations of this experimental method are that tests must be monotonic (tension or compression but not cyclic loading) and strain rate rise times limit this method to the intermediate strain rate regime (below 500 s^&ndash;1).</p><p>

<p> Due to their lightweight and high specific strength, magnesium and its alloys have a great potential for a variety of applications. However, compared to face-centered cubic (FCC) metals, magnesium has a limited number of easy slip modes which cannot accommodate the strain along the <i>c</i>-axis, and thus twinning in Mg is an important mechanism of plastic deformation. Although numerous theoretical and experimental studies have been conducted on twinning in magnesium for decades, the mechanisms have yet been understood clearly. There are major discrepancies between theoretical and experimental results. The mainstream models for deformation twinning cannot describe the twinning behavior correctly. </p><p> In deformation twinning, there is a one-to-one lattice correspondence between the parent and the twin lattices. Thus, this concept of lattice correspondence can be used to resolve the twinning mechanisms in hexagonal close-packed (HCP) metals, but the study of lattice correspondence in deformation twinning in HCP metals only remains on the mathematic level. The concept of lattice correspondence is difficult to be verified by experiments. However, molecular dynamics (MD) simulations can be used to examine lattice correspondence, by tracking the transformation of crystallographic planes of the parent lattice to the corresponding plane of the twin. Thus, twinning mechanisms can be better understood. </p><p> Although lattice correspondence is a key concept in deformation twinning, as of now, it has often been neglected in the studies of twinning. Consequently, the existing models are unable to account for the phenomena observed in experiments or precisely predict the twinning behavior in Mg, and the twinning mechanisms remain controversial. The purpose of this dissertation is to use molecular dynamics simulations to investigate, in great detail, lattice correspondence in the two major twinning modes, i.e. {101&macr;2}{101&macr;1&macr;} and {101&macr;1}{101&macr;2&macr;}, in pure Mg. Specific crystallographic planes of the parent are pre-selected and tracked during twinning, and the results are compared with the calculations based on the classical twinning theory. </p><p> The results obtained from the simulations indicate that, indeed, a unique lattice correspondence exists for each twinning mode. For {101&macr;2}{101&macr;1&macr;} twinning mode, the corresponding planes obtained in the simulations well agree with the crystallography-based calculations. However, in the simulations, no shear deformation is observed and the twin boundary is extremely incoherent. Lattice transformation is solely achieved by atomic shuffling and no twinning dislocations are involved. For {101&macr;1}{101&macr;2&macr;} twinning mode, although the twinning elements were correctly predicted by the classical theory, discrepancies are revealed between atomistic simulation results and the crystallography-based calculations. These results unambiguously demonstrate how important lattice correspondence analysis is in resolving twinning mechanisms in complex crystal structures.</p><p>

<p> Interest in titanium for mass-scale applications has driven an exploration for rapid, cost-effective consolidation of titanium powders. The effects of electric current and pressure on binary diffusion in beta phase titanium niobium are studied to enhance understanding of Spark Plasma Sintering (SPS), an advanced powder consolidation technology. Binary diffusion couples were annealed in the SPS system, as well as a custom-fabricated load-free furnace, for one hour to elucidate the influences of pressure and current at 1000, 1100, and 1150 &deg;C. The results show Ti-Nb interdiffusion coefficient dependence on composition, temperature, current, and pressure. Compared to published results, the activation energy for low concentration Nb, 10-22 at%, has shown to be reduced in the SPS by an average 38kJ/mol at 15MPa and 70kJ/mol at 80MPa. The effect on activation energy of direct current without pressure, at a similar current density as the SPS, shows an average decrease of 105kJ/mol. The possible mechanisms for these changes are discussed, and concepts for subsequent studies are provided.</p><p>

<p> Thermoplastic elastomers are a class of rubbery polymeric materials that exhibit solidlike properties due to physically associating moieties. Block copolymers are often used as the network forming component of thermoplastic elastomers. Additionally, block copolymers can be modified with block selective solvents that contribute a specific functionality to the system; these solvent modified systems will be referred to throughout as thermoplastic elastomer gels. Thermoplastic elastomers and their gels have a long history of applications as specialty materials for passive systems where traditional rubbers cannot meet the required design criteria&mdash;often properties of softness, toughness and low hysteresis are of interest. Herein, we discuss the use of thermoplastic elastomer gels as active materials that respond to external stimuli to change their mechanical and thermal properties.</p><p> First, the text will introduce concepts of phase behavior and resultant physical behavior of block copolymers in the presence of a selective solvent. Included are specific details pertinent to materials used in experimental discussions presented in this work. Following this broad discussion, the introduction of a specific class of smart and responsive materials, known as dielectric elastomer actuators, is detailed in a survey of recent technological developments in the field.</p><p> The main body of the text describes multiple applications of thermoplastic elastomer gels. It begins with an entirely novel use of a semi-crystalline olefin block polymer gel as a dielectric elastomer actuator exhibiting programmable anisotropy and promising actuation behavior. The subsequent study uses specific control over the architecture of a polydimethylsiloxane elastomer to make ultra-soft films for exceptional dielectric elastomers. These so-called bottlebrush elastomers are formed from heavily grafted polymer backbones that reduce entanglements resulting in incredibly soft elastomers. As dielectric elastomers, these materials operate with no mechanical prestrain and achieve strains greater than 300% by area. This is followed by the use of a traditional ABA triblock copolymer (poly[styrene-bethylene- co-butylene-b-styrene]) with a crystallizing selective solvent to impart shape memory behavior. This is the first demonstration of a dielectric elastomer utilizing crystallization for electroactive strain fixation. Finally, we conclude with the discussion of thermoplastic copolyester based gels as form-stable phase change materials. These phase change gels have applications in passive thermal energy management systems and compete with existing commercial technologies.</p><p>

<p> Materials interface properties bring both opportunities and challenges for materials design. Especially for applications at extreme conditions, the metastable nature of interface inevitably leads to the big question of structural stability. In this thesis, the nanopore surface in mesoporous MCM-41 and the grain boundaries in nanocrystalline yttria-stabilized zirconia (YSZ) were selected as the two presentative interface types. They were studied with atomistic simulation methods to help understand and improve their structure stability at different conditions, and other related properties for relevant applications. </p><p> For MCM-41, its thermostability and pore structure transformation mechanisms subjected to temperatures from 300 K up to 2885 K was studied by the combination of molecular dynamics (MD) and Monte Carlo simulations. Silica was experimentally characterized to inform the models and enable prediction of changes in gas adsorption/separation properties. MD simulations suggest that the pore closure process is activated by a collective diffusion of matrix atoms into the porous region, accompanied by bond reformation at the surface. Degradation is kinetically limited, such that complete pore closure is postponed at high heating rates. Applying the Kissinger equation, a strong correlation between the simulated pore collapse temperatures and the experimental values was found, which implies an activation energy of 416 &plusmn; 17 kJ/mol for pore closure. MC simulations give the adsorption and selectivity for thermally treated MCM-41, for N₂, Ar, Kr and Xe at room temperature within the 1&ndash;10000 kPa pressure range. Relative to pristine MCM-41, it was observed that increased surface roughness due to decreasing pore size amplifies the difference of the absolute adsorption amount differently for different adsorbate molecules. In particular, it was found that adsorption of strongly-interacting molecules can be enhanced in the low-pressure region while adsorption of weakly-interacting molecules is inhibited. This then results in higher selectivity in binary mixture adsorption in mesoporous silica. </p><p> For YSZ, the stabilization effect of La³⁺ doping on the grain boundary structure of YSZ was studied using MC simulation. It reveals the segregation of La³⁺ at eight tilt grain boundary (GB) structures and predicted an average grain boundary (GB) energy decrease of 0.25 J/m², which is close to experimental values reported in the literature. Cation stabilization was found to be the main reason for the GB energy decrease, and energy fluctuations near the grain boundary are smoothed out with La³⁺ segregation. Then the segregation effect on materials ionic conductivity was studied with MD simulation. Both dynamic and energetic analysis on &Sigma;13 (510)/[001] GB structure revealed La³⁺ doping hinders O^2&ndash; diffusion in the GB region, where the diffusion coefficient monotonically decreases with increasing La³⁺ doping concentration. The effect was attributed to the increase in the site-dependent migration barriers for O^2&ndash; hopping caused by segregated La³⁺, which also leads to anisotropic diffusion at the GB. </p><p>

<p> Recently, increased awareness of the significance of developing sustainable materials for construction has renewed the interest in exploring Alkali activated concrete (AAC), a concrete that contains no cement, but only industrial by-products such as fly ash and slag, as a low energy alternative to the conventional concrete. Although the feasibility of making alkali&ndash;activated concrete with acceptable strength and mechanical properties is well documented, the information regarding the long-term durability, including resistance to acid attack and alkali silica reaction (ASR), is far from comprehensive and there is a need to increase the understanding of these durability issues. In this dissertation, these durability issues are addressed, and improvements in this novel technology will increase acceptance in industry. This dissertation presents a comprehensive evaluation into the acid leaching resistance of Alkali-Activated Concrete (AAC) and Ordinary Portland Cement (OPC). The deterioration in AAC and OPC when exposed to different types of acid laden (organic and inorganic) environments are quantified by characterizing the strength degradation, mass change and visual appearances. The changes in microstructure development and chemical composition are examined and analyzed in order to determine the mechanism of deterioration. Additionally, the effect of the addition of nanoparticles on the mechanical properties and resistance to sulfuric leaching of Alkali Activated Slag concrete (AAS) are also explored in this study. </p><p> Furthermore, this dissertation summarizes the findings of an experimental evaluation of alkali silica reaction (ASR) in cement free alkali activated concrete (AAC). The susceptibility of AAC to deleterious ASR was evaluated in this study in accordance with relevant ASTM standards. This study also compares the resistance of AAC with ordinary portland cement concrete (OPC) while exposed to ASR under ASTM C 1293 and ASTM C1567 tests. In particular, the focus of this investigation is to assess the effectiveness of existing ASTM test methods in identifying the occurrence of ASR in alkali activated slag cement (AAS) concrete. In addition to that, influences of activator parameters including the effect of binder type, activator concentration, activator type and water content to the resistance of ASR in AAC were also evaluated. Finally, a scanning electron microscopic study coupled with EDX analyses was used to explain the mechanism of ASR occurrence in AAC and OPC.</p><p>

<p> HNBR is a widely used oil resistant polymer with good tear strength. Due to these properties, HNBR is used in oil wells. However, harsh working environments require high equipment maintenance fees and HNBR will be degraded when contacted with H₂S. This study aims to improve the mechanical properties and H₂S resistance of HNBR through molecular dynamics simulations. Some of the simulation results are compared with experimental results and literature values. In this study, the solubility parameters and densities of pure HNBR with varying acrylonitrile content, FKM and three surfactants (KBM503, a trimethoxysilane methacrylate, A10, a perfluoroalkoxy bis(alkylamide), and Capstone-62MA, a semifluorinated methacrylate) are calculated by molecular dynamics simulation. The cohesive energy densities of 50/50 HNBR/FKM blends with different kinds and content of surfactants are calculated. The diffusion of H₂S and CO₂ are predicted by molecular dynamics simulation. The solubility coefficients of H₂S and CO₂ are predicted by Grand Canonical Monte Carlo (GCMC) simulations. A series of NPT simulations (constant of number of atoms, pressure and temperature) are used to estimate the glass-transition temperature of Capstone-62MA grafted HNBR. Dissipative Particles Dynamics (DPD) simulations are used to obtain the micro phase separation of Capstone-62MA grafted HNBR. The results shows that the solubility parameter values and densities we obtained from molecular dynamics simulations are fitted very well with literature values. According to our calculation of energy of mixing for HNBR/FKM blends with three surfactant (KBM503, A10 and Capstone-62MA), KBM503 has the largest effect. Based on the experiment results for HNBR/FKM blends with different mass fractions of KBM503, the tensile stress at break and elongation at break increases with the increases of KBM503 content until the mass fraction KBM503 is equal to 5%. When the mass fraction of KBM503 is 5%, adding more KBM503 decreases both mechanical properties. However, the tear strength keeps increasing when the mass fraction of KBM503 increases. The conclusion obtained from these experiments and simulations indicates that mixing HNBR with FKM can improve some mechanical properties but this method has disadvantages due the large discrepancy between the solubility parameters of HNBR and FKM. Gas diffusion and solubility calculations indicate that the diffusion and solubility of H₂S decrease with the content of Capstone-62MA increases. The gas diffusion of H₂S also decreases with increasing content of acrylonitrile in HNBR. However, the solubility of H₂S also increases with the content of acrylonitrile in HNBR. For comparison with H₂S, the diffusivity and solubility of CO₂ are calculated. The diffusion of CO₂ increases with the increase of Capstone-62MA content. The solubility of CO₂ decreases with increases of Capstone-62MA in HNBR with 17 wt% acrylonitrile content. For HNBR with 36 wt% acrylonitrile content, increasing the content of Capstone-62MA first increases the solubility of CO₂ and then reduces it when the content of Capstone-62MA is larger than 2%. The calculation also indicates that diffusion and solubility coefficient are reduced when the content of acrylonitrile increases in HNBR. Calculations for the glass-transition temperature of HNBR with different numbers of Capstone-62MA chains suggest that the glass-transition temperature is not changed by grafting Capstone-62MA onto the backbone of HNBR. These results are compared with experimental results. Although the glass-transition temperatures obtained from simulations are higher than those obtained from experiment, they have the same trend as the content of Capstone-62MA is changed. DPD simulations suggest that micro phase separation exists in the Capstone-62MA grafted HNBR and this phenomenon improves the mechanical performance of polymers. In summary, we have used computer modeling to design new polymer materials and perform molecular dynamics simulations, Monte Carlo simulations and DPD simulations to predict some properties of these new materials. Some simulation results are compared with experimental results indicating that we indeed obtain a newly a polymer material with improved properties with the help of computer simulations.</p><p>

<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.