Thesis (Ph.D.)--Lehigh University, 2009. / Adviser: Robert P. Wei.

<p> Inspired by the Sandcastle Worm, biomimetic of the water-borne adhesive was developed by complex coacervation of the synthetic copolyelectrolytes, mimicking the chemistries of the worm glue. The developed underwater adhesive was designed for sealing fetal membranes after fetoscopic surgery in twin-to-twin transfusion syndrome (TTTS) and sealing neural tissue of a fetus in aminiotic sac for spina bifida condition.</p><p> Complex coacervate with increased bond strength was created by entrapping polyethylene glycol diacrylate (PEG-dA) monomer within the cross-linked coacervate network. Maximum shear bond strength of ~ 1.2 MPa on aluminum substrates was reached. The monomer-filled coacervate had complex flow behavior, thickening at low shear rates and then thinning suddenly with a 16-fold drop in viscosity at shear rates near 6 s^-1. The microscale structure of the complex coacervates resembled a three-dimensional porous network of interconnected tubules. This complex coacervate adhesive was used in vitro studies to mimic the uterine wall-fetal membrane interface using a water column with one end and sealed with human fetal membranes and poultry breast, and a defect was created with an 11 French trocar. The coacervate adhesive in conjunction with the multiphase adhesive was used to seal the defect. The sealant withstood an additional traction of 12 g for 30&minus;60 minutes and turbulence of the water column without leakage of fluid or slippage. The adhesive is nontoxic when in direct contact with human fetal membranes in an organ culture setting. </p><p> A stable complex coacervate adhesive for long-term use in TTTS and spina bifida application was developed by methacrylating the copolyelectrolytes. The methacrylated coacervate was crosslinked chemically for TTTS and by photopolymerization for spina bifida. Tunable mechanical properties of the adhesive were achieved by varying the methacrylation of the polymers. Varying the amine to phosphate (A/P) ratio in the coacervate formation generated a range of viscosities. The chemically cured complex coacervate, with sodium (meta) periodate crosslinker, was tested in pig animal studies, showing promising results. The adhesive adhered to the fetal membrane tissue, with maximum strength of 473 &plusmn; 82 KPa on aluminum substrates. The elastic modulus increased with increasing methacrylation on both the polyphosphate and polyamine within the coacervate. Photopolymerized complex coacervate adhesive was photocured using Eosin-Y and treiethanolamine photoinitiators, using a green laser diode. Soft substrate bond strength increased with increasing PEG-dA concentration to a maximum of ~90 kPa. The crosslinked complex coacervate adhesives with PEG networks swelled less than 5% over 30 days in physiological conditions. The sterile glue was nontoxic, deliverable through a fine cannula, and stable over a long time period. Preliminary animal studies show a novel innovative method to seal fetal membrane defects in humans, <i>in utero</i>. </p>

<p> Cement based brittle matrix composites that show deflection hardening called high performance fiber reinforced cementitious composites (HPFRCC) have the potential of offering high resiliency and environmentally sustainable benefits in numerous applications. However, more information is needed to fully understand, predict the behavior, and add functionality to HPFRCCs. This experimental research program aims to develop and characterize a new type of HPFRCC. This new HPFRCC is composed of polyvinyl alcohol (PVA) microfibers, carbon nanofibers (CNF), and a high volume of fly ash (HVA) to form a self-consolidating and self-sensing HPRFCC. The multi-functionality of the CNFs allow for increased mechanical properties and strain and damage sensing capabilities. The hybrid fiber reinforced cement composite developed is environmentally sound, due to the large amounts of recycled fly ash, with enhanced stiffness, and tensile strain capacity. This research considers the determination of fresh properties, hardened mechanical properties (elastic modulus, first cracking stress, ultimate stress, and maximum plastic strain), and electrical conductivity of the composite in response to strain, which is measured simultaneously through uniaxial tension tests. Digital Image Correlation (DIC) is used extensively to capture the tensile strain and provide a visualization of the behavior of the composite under increasing displacements. Results from this research program provide a preliminary understanding of the behavior of CNF HPFRCCs, which will aid in future research of similar composites. A standard mixing procedure is established that can be adopted in large scale processing of CNF HPFRCC. Increased mechanical properties and damage detection offers engineers with the ability to quantify structural health and optimize designs. The use of multi-functional self-sensing HPFRCCs is a step towards providing the public with a resilient and sustainable infrastructure for their communities.</p>

<p> Developing light metals that have desirable mechanical properties is always the object of the endeavor of materials scientists. Magnesium (Mg), one of the lightest metals, had been used widely in military and other applications. Yet, its relatively poor formability, as well as its relatively low absolute strength, in comparison with other metals such as aluminum and steels, caused the use of Mg to be discontinued after World War II. Owing to the subsequent energy crisis of the seventies, recently, interest in Mg development has been rekindled in the materials community. The main focus of research has been quite straight-forward: increasing the strength and formability such that Mg and its alloys may replace aluminum alloys and steels to become yet another choice for structural materials. This dissertation work is mainly focused on fundamental issues related to Mg and its alloys. More specifically, it investigates the mechanical behavior of different Mg-based materials and the corresponding underlying deformation mechanisms. In this context, we examine the factors that affect the microstructure and mechanical properties of pure Mg, binary Mg-alloy (with addition of yttrium), more complex Mg-based alloys with and without the addition of lanthanum, and finally Mg-based metal matrix composites (MMCs) reinforced with ex-situ ceramic particles. More specifically, the effects of the following factors on the mechanical properties of Mg-based materials will be investigated: addition of rare earths (yttrium and lanthanum), in-situ/ex-situ formed particles, particle size or volume fraction and materials processing, effect of thermal-mechanical treatment (severe plastic deformation and warm extrusion), and so on and so forth. </p><p> A few interesting results have been found from this dissertation work: (i) although rare earths may improve the room temperature ductility of well-annealed Mg, the addition of yttrium results in ultrafine and un-recrystallized grains in the Mg-Y alloy subjected to equal channel angular pressing (ECAP); (ii) the reverse volume fraction effect arises as the volume fraction of nano-sized ex-situ formed reinforcements is beyond 10%; (iii) nano-particles are more effective in strengthening Mg than micro-particles when the volume fraction is below 10%; (iv) complete dynamic recovery and/or recrystallization is required to accomplish the moderate ductility in Mg, together with a strong matrix-particle bonding if it is a Mg-based composite; and (v) localized shear failure is observed in all Mg samples, recrystallized completely, which is attributed to the reduced strain hardening rate as a result of the exhaustion of twinning and/or dislocation multiplication.</p>

Intermolecular interactions govern molecular self-recognition and self-assembly, giving rise to 3-dimensional solids. Thus, independent intermolecular interactions should result in a predictable crystal packing. Our working hypothesis is the crystal packing of symmetrically substituted tetra-n-alkoxy-bis-indane-piperazinediones of H-shaped topography will be governed by three factors: First, assembly of molecules into "one-dimensional" tapes through establishment of reciprocal amide hydrogen bonds. Second, assembly of the tapes into "two-dimensional" grooved sheets through establishment of edge-to-face arene interactions. Third, assembly of the sheets into "three-dimensional" solids by "tongue-in-groove" interdigitation of the n-alkyl "tails" in extended conformations independent of chain length. Liquid crystal properties are anticipated for molecules with hydrocarbon chains of sufficient length. Rupture of one or more of the three structure-determining interactions may be independent of the others and observable. "Dissection" of the melting process by correlation of crystallographic data with thermochemical data would permit formulation of a mechanism for melting. Compounds of this homologous series with methoxy, ethoxy, butoxy, hexyloxy, octyloxy and nonyloxy substituents have been synthesized, crystallized, and studied by X-ray crystallography, differential scanning calorimetry (DSC) and optical microscopy. The solid-state structures for the first three in the series were determined and each engaged in intermolecular amide-to-amide hydrogen bonding interactions that established "ladder-like" parallel tapes. Intertape organization was governed by the development of arene and/or van der Waals contact interactions. The melting behaviors of the series were remarkable in that these were not only high melting solids but most also demonstrate transitions by DSC at lower temperatures and magnitude consistent with liquid crystalline behavior. Also remarkable and consistent with liquid crystalline behavior are the optical properties of the longer alkoxy substituted compounds of the series, demonstrating birefringence of cross-polarized fight. In the case of the tetra-n-hexyloxy compound a transition was observed by optical microscopy that correlates with DSC data. Design, syntheses and characterizations of this new family of compounds, which are highly ordered at the molecular level, supporting our hypothesis of liquid crystalline behavior, are discussed. Using these data a mechanism of melting is postulated and discussed.

Thin ceramic substrates are widely used in engineering applications in modern industry. For example, they are used as molecular filters in fuel cells and solid oxide electrolyzers for oxygen generation. Development of high-reliability substrate materials inevitably requires the accurate characterization of their mechanical properties. The loading conditions in service on the ceramic substrates, such as the solid oxide electrolytes with a thickness of much less than 2 mm, often involve multiaxial bending instead of simple tension or bending. In this dissertation, the ASTM standard piston-on-3-ball experimental technique at ambient temperature is employed to investigate the quasi-static biaxial flexural strength of pure 8YSZ and Al₂O₃ or 3YSZ doped 8YSZ ceramic substrates. Furthermore, this piston-on-3-ball experimental technique is developed into a dynamic piston-on-3-ball technique at ambient temperature and a quasi-static piston-on-3-ball technique at elevated temperatures. Stress distribution functions in the tensile surface of a specimen under piston-on-3-ball loading condition are formulated and used to develop statistical models, which are proven to be in the form of a Weibull distribution function, to describe the biaxial flexural strength behavior of ceramic substrates under piston-on-3-ball loading condition. Analytical modeling was conducted on the dynamic piston-on-3-ball loading configuration. This analytical model can be used to guide the experimental design and judge the validity of experimental results. A new material constitutive model is developed to give a good description of the dynamic strength behavior of ceramic materials under constant stress-rate loading. Quasi-static experiments under piston-on-3-ball loading are conducted at both ambient temperature and elevated temperatures, while dynamic experiments are conducted at ambient temperature. Experimental results, as well as observations from SEM microstructure images and values of fracture toughness measured using a newly developed Vickers micro-indentation toughness technique, lead to a conclusion that no obvious overall improvement to the SYSZ ceramic substrates in the biaxial flexural strength can be observed by adding Al₂O₃ additive with amount up to 3 mol% or 3YSZ additive with amount up to 30 wt%.

The optical properties of planar semiconductor microstructures and three-dimensional nanostructures containing narrow linewidth In₀.₀₄Ga₀.₉₆As quantum wells are studied in this dissertation. The interaction of quantum-well excitons with light in environments different from free space gives a pronounced effect on the optical response. N periodically arranged quantum wells are coupled to each other by light leading to N exciton-polariton eigenmodes. Each eigenmode is characterized by a distinct energy and radiative lifetime depending on the periodicity of the quantum wells. For a period of about half the excitonic transition wavelength, linear measurements of reflection, transmission, and absorption show significant features of the light-coupled eigenmodes. At Bragg periodicity, the oscillator strengths of all quantum well excitons are concentrated into one superradiant mode resulting in an N times increased radiative decay rate. The slope of the reflectivity linewidths versus N gives the radiative linewidth of the quantum well exciton. For off-Bragg periodicity, however, other eigenmodes become optically active and show their features in reflection and absorption spectra. Oxide-aperture three-dimensional nanocavities containing a single quantum well are investigated. The discrete transverse modes due to the lateral confinement of the optical field are observed in empty cavities with various aperture sizes. The linewidth measurements of the cavity modes show quality-factor values around 2000 for aperture diameters down to 2 μm. This is high enough to give a strong light-coupling effect in the nonperturbative regime, named normal mode coupling or vacuum Rabi splitting. The anti-crossing behavior of exciton and cavity modes for a 2 μm diameter aperture cavity is measured in transmission by temperature tuning of the exciton resonance through the lowest transverse cavity mode. A minimum splitting value of 3.9 meV and a splitting-to-linewidth ratio of 4.9 are obtained. Then, nonlinear pump-probe measurements on nanocavities with several aperture sizes are performed. The transition from the nonperturbative regime to the weak coupling regime is observed as the pump power increases. From the measured saturation powers for various aperture diameters, a photon density of 90 photons/μm² is found necessary to saturate the normal-mode peaks. The effect of quantum fluctuations of the light field in the nonperturbative regime of planar semiconductor microcavities containing quantum wells is studied. A pronounced third transmission peak lying spectrally between the two normal modes is observed in resonant single-beam-transmission and pump-probe measurements. Measurements on three-dimensional nanocavities confirm the important role of guided modes for this intriguing effect.

Surface micromachining using deposited polysilicon films is a technology that is widely used for the fabrication of micro-electromechanical structures. One of the biggest yield and reliability problems in the fabrication of such structures is "stiction" or adhesion to the substrate. This may occur during the drying step that is required after wet processing and/or during use of a device. Deposition of self-assembled monolayer coatings is one of the most successful approaches to chemical modification of silicon surfaces to reduce stiction. This approach involves making the surfaces of pre-oxidized polysilicon highly hydrophobic. As a result, microstructures come out of the final water rinse extremely dry without being broken or adhered to the substrate. Available technology requires that these coatings are applied from organic media . However, increasing pressure on semiconductor companies to reduce the generation of organic wastes has sparked interest in the feasibility of applying these coatings from aqueous media. The objective of this research was to develop the chemistry and techniques for the application of hydrophobic coatings on polysilicon from aqueous media. The results obtained from three commercially available water dispersible silanes and cationic alkoxysilanes are discussed. Key experimental variables that were investigated are concentration of reactive silane, type of oxidation pretreatment of polysilicon, pH and temperature of the silane dispersion and curing temperature of the coating. The stability of the dispersions was characterized by viscosity measurements. The formation and quality of the films were studied using atomic force microscopy (AFM), ellipsometry, dynamic contact angle measurements and electrochemical impedance spectroscopy (EIS). The coatings showed contact angles greater than 100&deg;. It was found using AFM that the structure of these films is a continuous film with some particulates attributed to bulk polymerization of the precursor molecule in water. EIS results indicated that the coatings had low porosity as well as high charge transfer resistance across the silicon/HF interface. Ellipsometric analysis showed that thickness of these coatings is roughly a (statistical) monolayer. The stability improvement of the dispersions by the addition of quaternary ammonium cationic surfactants is also discussed.

Effects of pore, secondary dendrite arm spacings (SDAS ), hot isostatic pressing (Hipping), and strontium-modification on fatigue behavior were studied in an aluminum casting alloy (A356.2). Microstructures were revealed by X-ray radiography, light microscopy and scanning electron microscopy. Small-cracks were monitored by taking replicas of the surfaces with which the cracks intersected. As the SDAS increases from 15 to 55 μm, fatigue life decreases by a factor of 3 in low-cycle fatigue, and 100 in high-cycle fatigue. When SDAS is less than 30 mum, the pore size is below a critical size of ∼80 μm and large eutectic constituents initiate cracks; and the initiation life is as high as 70% of the fatigue life. As the SDAS increases beyond 30 μm, pores are the main crack-initiation sites; the initiation life is as low as 5% of the fatigue life. Near-surface oxides initiate the fatigue crack regardless of SDAS. When crack initiated at pore and oxides, fatigue life is well correlated with the size of the initiation site and the effect of SDAS is overshadowed by the effect of pore. Non-hipped A356.2 without Sr shows better fatigue life and the deleterious effect of pores overshadowed the beneficial effect that Sr-modification might have had. Hipping significantly increased the initiation life and small-crack propagation life of A356.2 with Sr as a result of the elimination of the porosity. However, hipping did not significantly improve the fatigue life of A356.2 without Sr. After hipping, Sr-modification is beneficial in improving the crack initiation life, and increasing both small-crack and long-crack propagation lives. Fracture mechanics models (Newman-Raju, and Trantina-Barishpolsky models) yielded similar results on the crack-propagation rate against the effective stress-intensity factor range. In the micro-mechanics model, the theory of continuously distributed dislocations was applied to represent crack and crack-tip plastic zone, and the propagation rate was related to the length of the crack-tip plastic zone. When the grain size is used as the characteristic length of the microstructures, the model predicts the oscillations of the propagation rates and the predicted rates agreed reasonably well with those from experiments.

This dissertation describes research conducted to apply the Fractional Step Method to finite-element simulations of directional solidification. The Fractional Step Method (FSM) is also referred to as a projection method and as a splitting method, and has been applied commonly to high Reynolds number flow simulations. However, it is less common for low Reynolds number flows, such as occur in an alloy undergoing directional solidification (DS). The FSM offers increased speed and reduced memory requirements by allowing non-coupled solution of the pressure and velocity components. The FSM provides significant benefits for predicting flows in a DS alloy, since other methods presently employed are not computationally efficient. Previously, the most suitable finite-elements based methods for predicting flow in a DS alloy has been the penalty method for two-dimensional simulations and Galerkin least-squares (GLS) for three-dimensional simulations. The penalty method and GLS have the disadvantage that they require the coupled solution of the velocity components. The FSM allows decoupled iterative solution of the finite element equations, thereby greatly increasing the efficiency of the method, both in terms of memory and CPU requirements. Numerical simulations are now commonly used to predict macrosegregation in directionally solidified (DS) castings, which are used in jet and spacecraft engines. In particular, the finite-element simulations can predict the existence of "channels" within the processing mushy zone and subsequently "freckles" within the fully processed solid, which are known to result from macrosegregation. This macrosegregation is a direct result of thermosolutal convection of the melt during the solidification process. Freckles cause strong material non-uniformities in the castings that are therefore scrapped. The phenomenon of thermosolutal convection in an alloy undergoing DS is explained, along with applications for DS alloys. Next, the momentum and continuity equations for a binary alloy undergoing DS, and the application of the FSM to these equations are presented, along with characteristics of the FSM that make its application to DS challenging. Finally, results of applying the FSM to simulations of DS in a binary alloy are given for two-dimensional and three-dimensional geometries, including performance improvements over methods previously applied.