Fuel cells are the direct energy conversion devices which convert the chemical energy of a fuel to electrical energy with much greater efficiency than conventional devices. Solid Oxide Fuel Cell (SOFC) is one of the various types of available fuel cells; wherein the major components are made of inherently brittle ceramics. Planar SOFC have the advantages of high power density and design flexibility over its counterpart tubular configuration. However, structural integrity, mechanical reliability, and durability are of great concern for commercial applications of these cells. The stress distribution in a cell is a function of geometry of fuel cell, temperature distribution, external mechanical loading and a mismatch of thermo-mechanical properties of the materials in contact. The mismatch of coefficient of thermal expansion and elastic moduli of the materials in direct contact results in the evolution of thermal stresses in the positive electrode/electrolyte/negative electrode (PEN) assembly during manufacturing and operating conditions (repeated start up and shut down steps) as well. It has long been realized and demonstrated that the durability and reliability of SOFCs is not only determined by the degradation in electrochemical performance but also by the ability of its component materials to withstand the thermal stresses. In the present work, an attempt has been made to evaluate the thermal stresses as a function of thermal and mechanical properties of the component materials assuming contribution from other factors such as thermal gradient, mechanical loading and in-service loading conditions is insignificant. Materials used in the present study include the state of art anode (Ni-YSZ), electrolyte(YSZ) and cathode materials(LM and LSM) of high temperature SOFC and also the ones being suggested for intermediate temperature SOFC Ni-SCZ as an anode, GDC and SCZ as electrolyte and LSCF as the cathode. Variation of thermo-mechanical properties namely coefficient of thermal expansion, and elastic and shear moduli were studied using thermo-mechanical analyzer and resonant ultrasound spectroscope respectively in 25-900°C temperature range. A non-linear variation in elastic and shear moduli- indicative of the structural changes in the studied temperature range was observed for most of the above mentioned materials. Coefficient of thermal expansion (CTE) was also found to increase non-linearly with temperature and sensitive to the phase transformations occurring in the materials. Above a certain temperature (high temperature region- above 600°C), a significant contribution from chemical expansion of the materials was also observed. In order to determine thermal stress distribution in the positive electrode, electrolyte, negative electrode (PEN) assembly, CTE and elastic and shear moduli of the component materials were incorporated in finite element analysis at temperature of concern. For the finite element analysis, anode supported configuration of PEN assembly (of 100mm x 100mm) was considered with 1mm thick anode, 10μm electrolyte and 30μm cathode. The results have indicated that cathode and anode layer adjacent to cathode/electrolyte and electrolyte/anode interface respectively are subjected to tensile stresses at the operating temperature of HT-SOFC (900°C) and IT-SOFC (600°C). However, the magnitude of stresses is much higher in the former case (500MPa tensile stress in cathode layer) when compared with the stress level in IT-SOFC (178MPa tensile stress in cathode layer). These high stresses might have been resulted from the higher CTE of cathode when compared with the adjacent electrolyte. However, it is worth mentioning here that in the present work, we have not considered any contribution from the residual stresses arising from fabrication and the stress relaxation from softening of the glass sealant.

Structural elements comprised of high strength concrete (HSCs) have gained popularity due to their high compressive strength, increased tensile strength, and low permeability that can be achieved with smaller placements relative to what would be needed with traditional ready mixed concretes. HSCs are also gaining interest for mass placements that are very large. Determining in-place properties of any of these structures is critical to the overall success of a project and elusive to determine prior to placement. In this dissertation, a laboratory based thermo-mechanical framework is outlined to predict in-place properties of modest to mass sized HSC structures using mostly existing and common laboratory testing methods with a few additional items on the same scale as existing equipment. Various curing protocols were evaluated in this study to determine a recommended set of protocols to reproduce thermal profiles of modest and mass sized structures on laboratory scale specimens. These specimens can then be tested following standard testing protocols to reasonably estimate in-place mechanical properties. This framework is envisioned to be a foundational piece of a standard test method in the future. Approximately 600 concrete specimens were tested for compressive strength, 300 specimens for elastic modulus, 100 for splitting tensile strength as well as 100 cement paste specimens for compressive strength. Additionally, approximately 400 time-temperature curves were recorded for both cement paste and HSC specimens.

<p>Gradient structured (GS) metallic materials have shown unique properties including the synergy of high strength and good ductility, improved fatigue and wear/friction resistance etc. One of the severe surface modification technique, surface mechanical grinding treatment (SMGT), has been proven an effective method for the generation of gradient structures in metallic materials. Most of Ni-based superalloys are precipitation strengthened and with an extraordinary combination of high strength, ductility and resistance to oxidation at high temperatures. The precipitation behaviors of these materials are sensitive to their initial microstructures. This thesis focuses on the microstructure evolution and mechanical behaviors of two types of gradient Ni alloys. </p> <p>GS Hastelloy C-22HS and Inconel 718 (IN718) Ni-based superalloys were fabricated through the SMGT technique. The gradient structures consist of nanograined (NG) or nanolaminate (NL) surface layer and the subsurface layers with deformation twins. <i>In situ </i>compression test results reveal that intergranular back stress may contributes to the high work hardening capability of the GS C-22HS alloy. Mo-rich thick grain boundaries (GBs) formed in the gradient C-22HS samples after heat treatment. <i>In situ</i> micropillar compression studies coupled with molecular dynamics (MD) simulations reveal that the Mo-rich thick GBs are stronger barriers than conventional thin GBs to the transmission of dislocations, leading to significant strengthening. Furthermore, the formation of thick GBs also contributes to the improvement of thermal stability of nanograins in the C-22HS alloys. The gradient microstructures in the IN718 alloy changed the precipitation behavior and thermal stability of nanograins in the alloy. The studies on precipitation behaviors of GS IN718 alloy reveal that η phase formed in the severely deformed surface NG layer after annealing at 700 ^oC. Thermal stability studies show that NG IN718 alloy with grain sizes smaller than the critical value of ~ 40 nm is thermally more stable than their coarse-grained counterpart. The underlying mechanisms of strengthening and improved thermal stability of the gradient Ni-based superalloys are discussed based on transmission electron microscopy studies and MD simulations. This work suggests that tailoring the gradient microstructures may lead to the discovery of metallic materials with novel mechanical and thermodynamic properties. </p>

In Chapter 1, a brief introduction of nanoindentation and finite element method is presented. General procedures have been developed based on FEM modeling of nanoindentation data to obtain the mechanical properties of thin films. Selected FEM models are illustrated in detail. In Chapter 2, nanoindentation test is simulated using finite element method based on contact mechanics approach. The relationship between load and indentation depth is obtained. The numerical results show good agreement with experimental data. It is shown that FEM is an effective tool for simulation of nanoindentation tests of metallic films. However, limitations caused by simplification of models and assumptions should not be neglected. In Chapter 3, finite element method is used to analyze bonded repair structure of aluminum plates with Multiple Site Damage (MSD). A 2-D 3-layer technique is used to deal with the damage area. A typical aluminum plate with multiple collinear twin cracks is taken as an example. The effects of relative position of two cracks, patch size, and patch thickness on stress intensity factors are studied in detail. The results reveal that the stress intensity factors at the tips of collinear twin cracks can be reduced greatly through bonded composite repair. In order to increase the performance of the patch repair, the adhesive properties, the patch length and thickness must be optimized. In Chapter 4, finite element method is used for thermo-mechanical analysis of porous coatings in steel micro channels used for catalysis. Thermal stresses in the coating due to temperature changes are obtained. The effects of micro channel geometry on thermal stresses are studied in detail. The results reveal that in order to increase the mechanical performance of the coatings, film thickness and profile geometry must be optimized. Chapter 5 summarizes major results and outlines future work.

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Vascular networks are required to support the formation and function of three-dimensional tissues. Biodegradable scaffolds are being considered in order to promote vascularization where natural regeneration of lost or destroyed vascular networks fails. Particularly; composite materials are expected to fulfill the complex demands of a patient's body to support wound healing. Microbial biopolyesters are being regarded as such second and third generation biomaterials. Methylobacterium extorquens is one of several microorganisms that should be considered for the production of advanced polyhydroxyalkanoates (PHAs). M. extorquens displays a distinct advantage in that it is able to utilize methanol as an inexpensive substrate for growth and biopolyester production. The design of functionalized PHAs, which would be made of both saturated short-chain-length (scl, C [less than or equal to] 5) and unsaturated medium-chain-length (mcl, 6 [less than or equal to] C [less than or equal to] 14) monomeric units, aimed at combining desirable material properties of inert scl/mcl-PHAs with those of functionalized mcl-PHAs. By independently inserting the phaC1 or the phaC2 gene from Pseudomonas fluorescens GK13, recombinant M. extorquens strains were obtained which were capable of producing PHAs containing C-C double bonds. A fermentation process was developed to obtain gram quantities of biopolyesters employing the recombinant M. extorquens ATCC 55366 strain which harbored the phaC2 gene of P. fluorescens GK13, the better one of the two strains at incorporating unsaturated monomeric units. The PHAs produced were found in a blend of scl-PHAs and functionalized scl/mcl-PHAs (4 [less than or equal to] C [less than or equal to] 6), which were the products of the native and of the recombinant PHA synthase, respectively. Thermo-mechanical analysis confirmed that the functionalized scl/mcl-PHAs exhibited the desirable material properties expected. This project contributed to current research on polyhydroxyalkanoates at different levels. The terminal double bonds of the functionalized scl/mcl-PHAs are amenable to chemical modifications and could be transformed into reactive functional groups for covalently linking other biomacromolecules. It is anticipated that these biopolyesters will be utilized as tissue engineering materials in the future, due to their functionality and thermo-mechanical properties.

Fundamental thermodynamic investigations have been carried out regarding the phase equilibria of La0.8Sr0.2MnO3±δ (LSM), a cathode of a solid oxide fuel cell (SOFC), utilizing the CALculation of PHAse Diagram (CALPHAD) approach. The assessed thermodynamic databases developed for LSM perovskite in contact with YSZ fluorite and the other species have been discussed. The application of computational thermodynamics to the cathode is comprehensively explained in detail, including the defect chemistry analysis as well as the quantitative Brouwer diagrams, electronic conductivity, cathode/electrolyte interface stability, thermomechanical properties of the cathode and the impact of gas impurities, such as CO2 as well as humidity, on the phase stability of the cathode. The quantitative Brouwer diagrams for LSM at different temperatures are developed and the detailed analysis of the Mn3+ charge disproportionation behavior and the electronic conductivity in the temperature range of 1000-1200°C revealed a good agreement with the available experimental observations. The effects of temperature, CO2 partial pressure, O2 partial pressure, humidity level and the cathode composition on the formation of secondary phases have been investigated and correlated with the available experimental results found in the literature. It has been indicated that the CO2 exposure does not change the electronic/ionic carriers’ concentration in the perovskite phase. The observed electrical conductivity drop is predicted to occur due to the formation of secondary phases such as LaZr2O7, SrZrO3, SrCO3 and Mn oxides at the LSM/YSZ interface, resulting in the blocking of the electron/ion transfer paths. For the thermo-mechanical properties of LSM, a new weight loss Mechanism for (La0.8Sr0.2)0.98MnO3±δ using the La-Sr-Mn-O thermodynamic database is modeled with respect to the compound energy formalism model. This newly proposed mechanism comprehensively explains the defect formation as a result of volume/weight change during the thermal cycles. According to the proposed mechanism the impact of cation vacancies regarding the volume change of cathode was explained.

The preliminary goal of this study is to determine the effects of processing conditions, compositions and microstructural morphologies of the constituents on the physical and thermo-mechanical properties of alumina (Al_2O_3) reinforced aluminum (Al) composites. Composites with 0, 5, 10, 20 and 25 vol% Al_2O_3 were manufactured using powder metallurgy method. The elastic properties (Young's and shear modulus) and the coefficient of thermal expansion (CTE) of the composites were determined using Resonant Ultrasound Spectroscopy (RUS) and Thermo Mechanical Analyzer (TMA) respectively at various temperatures. Increasing compacting pressure improved relative density (or lowered porosity) of the composites. Furthermore, increasing the Al_2O_3 vol% in the composite increased the elastic moduli and reduced the CTE of the composites. Increasing the testing temperature from 25 to 450 oC, significantly reduced the elastic moduli of the composites, while the CTE of the composites changed only slightly with temperatures. Secondly, the goal of this study is to determine the effect of microstructures on the effective thermo-mechanical properties of the manufactured Al-Al_2O_3 composites using finite element (FE) method. Software OOF was used to convert the SEM micrographs of the manufactured composites to FE meshed models, which were then used to determine the effective elastic modulus and CTE. It was observed that, effective modulus dropped by 19.7% when porosity increased by 2.3%; while the effective CTE was mildly affected by the porosity. Additionally, the effect of residual stress on the effective thermo-mechanical properties was studied, and the stress free temperature of the composites was determined. Another objective of this study is to examine the stress-strain response of Al-Al_2O_3 composites due to compressive loads at various temperatures. Elastic modulus, yield stress and strain hardening parameters were determined from the stress-strain curves and their dependency on temperature, porosity and volume fraction were studied. The experimental results were compared with the numerical results. It was observed that high-localized stresses were present near the pores and at the interfaces between Al and Al_2O_3 constituents. Finally, functionally graded materials (FGMs) with varying Al_2O_3 concentration (0, 5and 10 vol%) in Al were manufactured; and their stress-strain response and CTE were determined at various temperatures.

L'objectif de cette thèse a été de démontrer la pertinence du procédé sol-gel photoinduit pour la synthèse de films nanocomposites originaux à partir de précurseurs hybrides mono- et bis-silylés. Bien que mentionné dans la littérature, ce procédé basé sur une catalyse via des super-acides de Bronsted créés par la photolyse d'un photogénérateur d'acides reste à ce jour très marginal. Selon la nature des précurseurs mis en oeuvre, la photopolymérisation sol-gel a parfois été combinée à une photopolymérisation organique pour mener à l'obtention en une seule étape de réseaux hybrides organique-inorganique. Au cours de ces travaux, l'accent a tout d'abord été mis sur la caractérisation structurale des matériaux synthétisés. Pour cela, des mesures par spectroscopie infrarouge à transformée de Fourier, résonance magnétique nucléaire du 29Si et du 13C en phase solide et diffraction des rayons X ont été réalisées. Ces études ont permis de révéler les nombreux avantages liés à la voie photochimique tels que la forte réactivité, le caractère vivant de la réaction, l'absence d'eau ou de solvant et l'obtention de matériaux organisés à l'échelle locale. Enfin, l'originalité de cette étude a également résidé dans la mise en œuvre de nombreuses techniques de caractérisation thermo-mécanique des films : analyse mécanique dynamique, calorimétrie différentielle à balayage, nanoindentation, tribologie et tests de résistance à la rayure. L'influence de la nature chimique de la fonctionnalité organique du précurseur employé a ainsi pu être soulignée et des corrélations entre microstructure et propriétés finales des matériaux photopolymérisés ont été établies. / The present work questioned the interest of sol-gel photopolymerization as a novel route to synthesize nanocomposite films from hybrid mono- and bis-silylated precursors. The potentialities of this process that is based on a catalysis promoted by photogenerated Bronsted superacids have been poorly investigated so far. Depending on the precursor nature, the sol-gel photopolymerization was possibly combined with an organic photopolymerization in a view to generate the dual crosslinking of the organic and inorganic networks in a single step. A first aspect of this work concemed the structural properties of the hybrid films: Fourier transformed IR spectroscopy, 29Si and 13C solid state Nuclear Magnetic Resonance spectroscopy and X-rays analysis were thus implemented. These experiments highlighted the numerous advantages of the photoinduced sol-gel process: its high reactivity, its living character, the absence of water or solvent and the local organization in the resulting films. Finally, the thermo-mechanical properties of the UV­ cured materials were assessed by using a wide range of characterization techniques: dynamic mechanical analysis, differentia!scanning calorimetry, nanoindentation, tribological and scratch tests. The effect of the organic moiety functionality of the precursors was thus evidenced and relationships between microstructure and properties ofthe hybrid films were established.

Electrically conductive polymer composites are suitable for use in the manufacture of antistatic products and components for electronic interconnects, fuel cells and electromagnetic shielding. The most widely used processing techniques for producing electrically conductive polymer composites place an inherent constraint on the geometry and architecture of the part that can be fabricated. Hence, this thesis investigates selective laser sintering (SLS), a rapid prototyping technique, to fabricate and characterize electrically conductive nanocomposites of Nylon-12 filled with 4% by weight of carbon black. The objective of the dissertation was to study the effects of the SLS process on the microstructure and properties of the nanocomposite. The effect of laser power and the scan speed on the flexural modulus and part density of the nanocomposite was studied. The set of parameters that yielded the maximum flexural modulus and part density were used to fabricate specimens to study the tensile, impact, rheological and viscoelastic properties. The electrical conductivity of the nanocomposite was also investigated. The thermo-mechanical properties and electrical conductivity of the nanocomposites produced by SLS were compared with those produced by extrusion-injection molding. The structure and morphology of the SLS-processed and extrusion-injection molded nanocomposites were characterized using gas pycnometry, gel permeation chromatography, differential scanning calorimetry, electron microscopy, polarized light microscopy and x-ray diffraction. Physical models were developed to explain the effects of the processing technique on the structure and properties of the nanocomposites. Finally, a one-dimensional heat transfer model of the SLS process that accounted for sintering-induced densification and thermal degradation of the polymer was implemented in order to study the variation in part density with respect to the energy density of the laser beam. This dissertation demonstrated that SLS can be successfully used to fabricate electrically conductive polymer nanocomposites with a relatively low percolation threshold. This capability combined with the ability of SLS to fabricate complicated three-dimensional objects without part-specific tooling could open up several new opportunities.