No description available.

The ability to fabricate structures and engineer materials on the nanoscale leads to the development of new devices and the study of exciting phenomena. Nanostructures attached to the surface of a substrate, in a manner that renders them immobile, have numerous potential applications in a diverse number of areas. Substrate-supported nanostructures can be fabricated using numerous modalities; however the easiest and most inexpensive technique to create a large area of randomly distributed particles is by the technique of thermal dewetting. In this process a metastable thin film is deposited at room temperature and heated, causing the film to lower its surface energy by agglomerating into droplet-like nanostructures. The main drawbacks of nanostructure fabrication via this technique are the substantial size distributions realized and the lack of control over nanostructure placement. In this doctoral dissertation, a new pathway for imposing order onto the thermal dewetting process and for manipulating the size, placement, shape and composition of preformed templates is described. It sees the confinement of substrate-supported thin films or nanostructure templates by the free surface of a metal film or a second substrate surface. Confining the templates in this manner and heating them to elevated temperatures leads to changes in the characteristics of the nanostructures formed. Three different modalities are demonstrated which alters the preformed structures by: (i) subtracting atoms from the templates, (ii) adding atoms to the template or (iii) simultaneously adding and subtracting atoms. The ability to carry out such processes depends on the choice of the confining surface and the nanostructured templates used. A subtractive process occurs when an electroformed nickel mesh is placed in conformal contact with a continuous gold film while it dewets, resulting in the formation of a periodic array of gold microstructures on an oxide substrate surface. When heated the gold beneath the grid selectively attaches to it due to the surface energy gradient which drives gold from the low surface energy oxide surface to the higher surface energy nickel mesh. With this process being confined to areas adjacent to and in contact with the grid surface the film ruptures at well-defined locations to form isolated islands of gold and subsequently, a periodic array of microstructures. The process can be carried out on substrates of different crystallographic orientations leading to nanostructures which are formed epitaxially and have orientations based on underlying substrate orientations. The process can be extended by placing a metallic foil of Pt or Ni over preformed templates, in which case a reduction in the size of the initial structures is observed. Placing a foil on structures with random placement and a wide size distribution results, not only in a size reduction, but also a narrowed size distribution. Additive processes are carried out by using materials which possess high vapor pressures much below the sublimation temperature of the template materials. In this case a germanium substrate was used as a source of germanium adatoms while gold or silver nanostructures were used as heterogeneous nucleation sites. At elevated temperatures the adatoms collect in sufficient quantities to transform each site into a liquid alloy which, upon cooling, phase separates into elemental components sharing a common interface and, hence, resulting in the formation of heterodimers and hollowed metal nanocrescents upon etching away the Ge. A process which combined aspects of the additive and subtractive process was carried out by using a metallic foil with a high vapor pressure and higher surface energy than the substrate surface (in this case Pd foil). This process resulted in the initial preformed gold templates being annihilated and replaced by nanostructures of palladium, thereby altering their chemical composition. The assembly process relies on the concurrent sublimation of palladium and gold which results in the complete transfer of the templated gold from the substrate to the foil, but not before the templates act as heterogeneous nucleation sites for palladium adatoms arriving to the substrate surface. Thus, the process is not only subtractive, but also additive due to the addition of palladium and removal of gold. / Mechanical Engineering

Efficient reconfigurable antennas are highly sought after in all communication applications for the ability to reduce space cost while maintaining the ability to control the frequency, gain, and polarization. The ability to control these parameters allows a single antenna to maximize its performance in a wide range of scenarios to satisfy changing operating requirements. This thesis will describe reconfigurable antennas using liquid-metal alloys that give the system this ability by injecting or retracting the liquid metal from various channels. After simulations were performed in an electromagnetic simulation software, proof-of-concept models were built, tested, and compared to the simulations to verify the results. / Master of Science / Antennas that can change the tuned center frequency and/or the direction they are pointing are needed in many different applications. Antenna adaptability allows the system to maximize the physical dimensions of the antenna to satisfy a wide range of situations without losing performance. This thesis describes antennas using a liquid-metal alloy that can make physical adaptations for the need at hand. After simulations were performed using computer software, proof-of-concept models were constructed and empirically validated to verify the simulation models.

Yes / We report formation of polymorphs and new eutectics and cocrystals of curcumin, a sparingly water-soluble active component in turmeric, structurally similar to cinnamic acid. The curcumin polymorphs were formed using liquid antisolvent precipitation, where acetone acted as a solvent and water was used as the antisolvent. The metastable form 2 of curcumin was successfully prepared in varied morphology over a wide range of solvent-to-antisolvent ratio and under acidic pH conditions. We also report formation of new eutectics and cocrystals of curcumin with cinnamic acid acting as a coformer. The binary phase diagrams were studied using differential scanning calorimetry and predicted formation of the eutectics at the curcumin mole fraction of 0.15 and 0.33, whereas a cocrystal was formed at 0.3 mole fraction of curcumin in the curcumin–cinnamic acid mixture. The formation of the cocrystal was supported with X-ray powder diffraction, the enthalpy of fusion values, Fourier-transform infrared spectroscopy, and scanning electron microscopy. The hydrogen bond interaction between curcumin and cinnamic acid was predicted from Fourier-transform infrared spectra, individually optimized curcumin and cinnamic acid structures by quantum mechanical calculations using Gaussian-09 and their respective unit cell packing structures.

During solidification of non-eutectic alloys, non-isothermal phase change causes dendritic growth of solid front with liquid phase entrapped within the dendritic network producing the mushy region. Solidification causes rejection of solute at the solid-liquid interface and within the mushy zone, causing a sharp concentration gradient to build up across the mushy region. At the same time, a temperature gradient is present as a result of externally imposed boundary conditions as well as due to evolution of latent heat, giving rise to the so-called “double-diffusive” or thermo-solutal convection. Depending on the relative density of the solute being rejected in the liquid phase during solidification process, thermal and solutal buoyancy can either aid or oppose each other. Rejection of a heavier solute leads to aiding thermo-solutal convection situation whereas the rejection of lighter solute causes the thermal and solutal buoyancy to oppose each other. If the thermal and solutal buoyancies oppose each other, flow instability arises adjacent to the mush-bulk liquid interface regions. Thus, there may be a wide variety of convection situations present in the solidifying domain for different combinations of solution concentrations and externally imposed boundary conditions. The situation becomes even more complex if the solid phase movement along with the bulk flow is involved in the process, leading to multiphase convection. Detachment of solid phase from the solid/liquid interface can be caused by remelting (solutal and/or thermal) and shearing action of a convecting liquid adjacent to the interface. Depending on the drag of the bulk flow and the density of the solid phase relative to that of the bulk liquid, these detached particles can either float or sediment. The redistribution of the rejected solute by means of diffusion (at a local scale) and thermo-solutal convection (at system level length scales) causes heterogeneous orientation of mixture constituents over the solidifying domain popularly known as macro-segregation. From the point of view of manufacturing, severe form of macro-segregation or heterogeneous species distribution is an undesirable phenomenon and hence, a thorough understanding of the species redistribution by means of diffusion and convection during solidification process is very important. Most of the earlier studies on double diffusive convection during solidification involved fixed dendrites. However, the advection of solid particles during the solidification process can generate major instability in the flow pattern while modifying the solid front growth, and hence the macro-segregation pattern considerably. With this viewpoint in mind, the overall objective of the present work is to address these wide-varieties of single phase and multi phase flow situations and their effect on solid front growth and macro-segregation during directional solidification of non-eutectic binary alloys, numerically as well as experimentally. Different configurations of directional solidification processes involving double diffusive convection have been studied for two different kinds of non-eutectic solutions. While solidification of hypoeutectic solutions leads to aiding type double diffusive convection, the solidification of hyper-eutectic solutions is characterized by opposing type double diffusive convection. Solidification of hypo-eutectic solution generally involves single phase flow, while most of the hyper-eutectic solidification involves movement of solid phase (i.e. multiphase flow). As far as the modeling part is concerned, transport phenomena during solidification with multiphase convection are not common in existing literature. This work is a first attempt to develop a solidification model with multiphase flow based entirely on macroscopic parameters. As a first step, a generalized macroscopic framework has been developed for mathematical modeling of multiphase flow during solidification of binary alloy systems. The complete set of equivalent single-domain governing equations (mass, momentum, energy and species conservation) are coupled with the phase (solid and liquid) velocities. A generalized algorithm has been developed to determine solid detachment and solid advection phenomena, based on two critical parameters, namely: critical solid fraction and critical velocity. While the first of these two parameters (critical solid fraction) represents the strength of the dendritic bond, the second (critical velocity) stands for the intensity of flow to create drag force and solutal remelting at the dendrite roots. A new approach for evaluating liquid/solid fraction by using fixed grid enthalpy updating scheme, that accounts for multiphase flow and, at the same time, handles equilibrium and non equilibrium solidification mechanisms, has been proposed. The newly developed model has been validated with existing literatures as well as with experimental observations performed in the present work. The experimental results were obtained by using PIV as well as laser scattering techniques. Side cooled as well as top cooled configurations are studied. Single phase convection is observed for the case of hypo-eutectic solution, whereas hyper-eutectic solutions involve convection with movement of solid phase. For the case of bottom cooled hyper-eutectic solution, finger-like convection leading to freckle formation is observed. For all the hyper-eutectic cases, solid phase movement is found to alter the convection pattern and final macrosegregation significantly. The numerical results are compared with experimental observations both qualitatively as well as quantitatively.

The thesis introduces a novel alloy system based on submicron distributions of intermetallic phases realised through eutectic solidification in the ternary system Ni-Al-Zr. Various compositions in this system comprising of intermetallic phases distributed in different eutectic structures show ultra-high strength at temperatures upto 700°C combined with reasonable tensile plasticity, exceptional oxidation resistance and high temperature structural stability. Intermetallics have long been used in high temperature alloys systems such as in the classical Ni-base superalloys that derive their strength from nanoscale dispersions of the aluminide, Ni3Al(γ’) in a matrix of disordered fcc Ni (γ), alloyed with expensive, high density refractory elements such as Re and Ru. The high temperature applications of intermetallics derive from their strength retention to high temperatures, creep resistance enabled by low diffusion rates, and attractive oxidation resistance based on high concentration of elements such as Al that forms stable oxides. Several decades of effort on the development of new generation of intermetallic alloys through the 80’s and 90’s have gone unrewarded, with the exception of TiAl based alloys that are now used in recent generation aircraft engines. The promise of intermetallics as high temperature candidate materials is limited by their poor ductility or toughness arising from several intrinsic properties such as low grain boundary cohesive strength (in the case of Ni3Al) or an insufficient number of slip systems (as in NiAl) or extrinsic effects such as embrittlement by hydrogen (Fe3Al) that derive fundamentally from the existence of directionality in bonding. However, low ductility or toughness can often be alleviated by limiting the length scale for slip. We have therefore examined the possibility of combining intermetallics in the form of eutectic structures, potentially limiting slip lengths within each intermetallic constituent. Eutectic structures in binary systems limit the choice of intermetallic combinations so that finding such combinations with engineering potential is difficult. On the other hand combinations of three elements or more would enable a significantly larger set of permutations of eutectic intermetallics, provided the constituent binary phase diagrams contain either eutectic or peritectic reactions involving intermetallic phases, as well as intermediate intermetallic phases. The ternary Ni-Al-Zr system met our criterion in several ways. The Ni-Al binary phase diagram shows a peritectic reaction from liquid and NiAl (Pm 3m, B2 with a lattice parameter of 0.288nm) to form Ni3Al (Pm 3m, L12 with a lattice parameter of 0.356 nm), intermetallics that have been extensively investigated in earlier literature. The Ni-Zr system shows a peritectic reaction between liquid and the Ni7Zr2 (C12/m1 with a lattice parameters a=0.469nm, b=0.823nm, c=1.219nm) phase to form the intermetallic Ni5Zr (F 43m with a lattice parameter of 0.670nm). Further the NiAl and Ni7Zr2 are both intermediate phases and should therefore form a mono-variant eutectic on the composition line joining these two phases in the ternary system. We note that Zr participates in many glass forming systems. In the Ni-Zr system, for example, glass forming ability has been associated with the structure of the liquid phase and associated low diffusivity. As a consequence, a fine scale eutectic structure may be expected. Zr has also been reported to strengthen and ductilise Ni3Al and NiAl. Finally, both Al and Zr form stable oxides and might promote oxidation resistance. After introducing the thesis in Chapter 1, the experimental details are outlined in the Chapter 2. The experimental results and subsequent discussions are presented in three subsequent chapters. Chapter 3 reports the microstructural aspects of as cast alloys in this ternary system Previous literature and our analysis of phase equilibria in the Ni-Al-Zr system based on Thermo-Calc, suggested that solidification from the liquid to form the Ni3Al + Ni5Zr, Ni3Al + Ni7Zr2 and NiAl+ Ni7Zr2 eutectics is possible. We obtained eutectic structures involving combinations of these intermetallic phases along a constant zirconium section at 11 at. %. The alloy A (Ni-77 at.%, Zr-11at.% and rest Al) contains eutectic structures containing the Ni3Al and Ni5Zr phases in two morphologies, a planar, lamellar structure and a more irregular form. The alloys B (Ni-74 at.%, Zr-11at.% and rest Al) and C (Ni-71 at.%, Zr-11at.% and rest Al) contain two different eutectic structures that combine the Ni3Al and Ni7Zr2 phases, and the NiAl and Ni7Zr2 phases. These phases were identified by a combination of X-ray diffraction, transmission electron microscopy coupled with energy dispersive spectroscopy and electron probe microanalysis. The volume fraction of each eutectic constituent is different in the two compositions in that alloy B(Ni-74 at.%, Zr-11at.% and rest Al) contains significantly higher volume fractions of the eutectic containing the Ni3Al and Ni7Zr2 phases than the alloy C (Ni-71 at.%, Zr¬11at.% and rest Al). In order to understand effect of individual phases we have melted several other alloys (alloy D to I) bounding these eutectic alloys (7-25 at.% Al, 5-15 at.% Zr and rest Ni) that form primary solidification phases of the intermetallic structures that constitute the eutectics. Chapter 4 discusses the mechanical behaviour of the fully eutectic alloys alloys as well as alloys with a combination of primary phases along with a eutectic. Mechanical behaviour was assessed in vacuum arc melted and suction cast material. The compressive strength of eutectic and off-eutectic compositions has been evaluated as a function of temperature. Very high strength levels of around 2 GPa could be achieved accompanied by reasonable room temperature tensile plasticity in the range 3-4%. The introduction of the respective primary phases of NiAl, Ni3Al, Ni5Zr and Ni7Zr2 results in decrease of strength. We have explored the origins of strength and tensile plasticity in alloys through micro and pico indentation (hardness) measurements and an examination of slip lines and crack initiation on pre-polished surface of the tensile tested samples as well as by transmission electron microscopy. Chapter 5 explores the oxidation resistance of these alloys in isothermal tests. The oxidation resistance of alloys compares well with recently developed cast single crystal alloys. Clearly, the oxide scale is extremely adherent and no spalling occurs. Electron microprobe analysis shows the presence of a fine scale, layered oxide structures and reaction zones within the substrate. The oxidation behaviour has been characterized using TGA, XRD and EPMA. We have attempted to understand the mechanism of oxidation through analysis of rate constants and activation energy coupled with microstructural observations. Chapter 6 presents a summary of the current work and present the scope for further work.

Cette thèse a pour but de caractériser le fonctionnement de supercondensateurs asymétriques composés de dioxyde de manganèse de structure birnessite et de carbone activé dans différents électrolytes. Les électrolytes aqueux neutres à base de sels inorganiques montrent les meilleures performances électrochimiques. La nature et la structure des cations et des anions du sel semblent impacter les performances électrochimiques et la stabilité de la structure du matériau d’oxyde de manganèse. Lors de cyclage en milieu aqueux avec de large de fenêtre de tension de fonctionnement appliquée, un mécanisme de dégradation du dispositif a été avancé tenant compte de la nature des anions ou des cations des sels utilisés. Quelques voies de modification du matériau MnO2, afin d’améliorer ces performances électrochimiques, ont été étudiés. Des électrolytes non aqueux originaux ont été également caractérisés et plus particulièrement, les solvants « Deep Eutectic » à base de N-méthylacétamide et de sels de Lithium. Ces derniers semblent prometteurs comme électrolytes pour des applications en température sur carbone activé ou matériaux d’insertion tels que le ferrophosphate de lithium. Cependant ils semblent non adaptés aux oxydes de manganèse, mais donnent de bons résultats en cyclage avec le carbone activé. / The aim of this thesis was to investigate the performances of asymmetric supercapacitors based on manganese dioxide (birnessite) and activated carbon electrode materials using various electrolytes. From this work, it appears that neutral aqueous electrolytes containing inorganic salts have the best electrochemical performances. Furthermore, the nature and the structure of both ions (cations and anions) in solution seem to impact strongly the electrochemical performances of the supercapacitors, as well as, the MnO2’s structure stability and affinity. In the case of aqueous-based electrolyte, a device degradation mechanism has been proposed as a function of salt ions structure and nature to further understand the supercapacitor’s life-cycling when a large potential window is applied. Some novel synthesis ways and/or modifications were investigated to further improve the electrochemical properties of MnO2 material. Additionaly, original non-aqueous electrolytes has been also formulated and then characterized, particularly the ‘Deep Eutectic’ Solvents, based on the N-methylacetamide mixed with a lithium salt. However, these electrolytes don’t have a good affinity with manganese oxide-based materials. Interestingly, these Deep Eutectic Solvents show good cycling results with activated carbon. In fact, these electrolytes seem to be promising for high temperature energy storage applications, especially using activated carbon or insertion electrode material like the lithium ferrophosphate.

The control of interfacial microstructural stability is of utmost importance to the reliability of liquid solder interconnects in high temperature electronic assemblies. This is primarily due to excessive intermetallic compounds (IMCs) that can form and continuously grow during high temperature operation, which practically renders conventional barrier metallisations inadequate. In this study, electrically conducting, NbOx containing Ni coatings were developed using electrodeposition. Their suitability as a solder diffusion barrier layer was assessed in terms of the electrical conductivity and barrier property. The present work explores a novel electrochemical route to produce Ni-NbOx composite coatings of good uniformity, compactness and purity, from non-aqueous glycol-based electrolytes consisting of NiCl2 and NbCl5 as metal precursors. The effects of cathodic current density and NaBH4 concentrations on the surface morphology, composition and thickness of the coatings were examined. A combined study of Scanning Transmission Electron Microscopy (STEM) and Electrochemical Quartz Crystal Microbalance (EQCM) was conducted to understand the fundamental aspects of this novel electrodeposition process. The composite coatings generally exhibited good electrical conductivity. The reaction behaviour between a liquid 52In-48Sn solder and Ni-NbOx, with Nb contents up to 6 at.%, were studied at 200°C. The results indicate that, Ni-NbOx with sufficient layer thickness and higher Nb content, offered longer service lifetime. Nb enrichment was generally observed at or close to the reaction front after high temperature storage, which suggests evident effectiveness of the enhanced diffusion barrier characteristics.

The excellent high temperature microstructure stability, high strength, and oxidation resistance of intermetallics has for long driven the development of intermetallic based alloys. More recent studies demonstrated attractive properties of eutectic intermetallic in the Ni-Al-Zr systems. This thesis deals with study of binary Ni3Al+Ni7Zr2, NiAl+Ni7Zr2 and Ni3Al+NiAl+Ni7Zr2 ternary intermetallic eutectic alloys in this system and includes the identification of compositions that would yield each eutectic structure and their microstructural characterization, mechanical and oxidation behavior. The thesis is divided into six chapters. Chapter 1 reviews the study on high temperature materials development and presents the objectives of work in the current thesis. Various experimental techniques used for alloy preparation (vacuum arc melting and vacuum suction casting), microstructural characterization (optical microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), x-ray Diffraction (XRD), electron probe micro analyzer (EPMA), differential scanning calorimetry (DSC)), compression tests, microhardness tests and thermo gravimetric analysis (TGA) are described in Chapter 2. The specific background of work related to each chapter together with experimental results and discussion are given in next three chapters. Chapter 3 reports the method of identification of the composition for each of the eutectic alloys referred to above. The identification of alloy compositions of binary eutectics Ni3Al+Ni7Zr2 (Ni-13.5Al-11Zr), NiAl+Ni7Zr2 (Ni-19Al-12Zr) and Ni3Al+NiAl+Ni7Zr2 ternary eutectic (Ni-18.4Al-11.6Zr) is carried out with the help of available liquidus projection of Ni-Al-Zr system, and the iterative melting of numerous compositions that were refined to define the critical compositions for each eutectic. The microstructural features of these alloys have been characterized using optical and electron microscopy. Phase identification is confirmed by X ray diffraction, EPMA and TEM. The microstructure of Ni3Al+Ni7Zr2 and Ni3Al+NiAl+Ni7Zr2 ternary eutectic alloy shows similar eutectic morphologies. The eutectic colony consists of lamellar plates at center and intermixed lamellar-rod irregular morphologies towards the boundaries of the colonies. However, the NiAl+Ni7Zr2 eutectic alloy shows a fine, lamellar plate morphology throughout the microstructure. The orientation relationship between eutectic phases is determined using TEM technique for each alloy composition. Onsets of melting and liquidus temperatures have been identified by Differential Scanning Calorimetry. Modified liquidus projections of Ni-Al-Zr system near the Ni3Al+NiAl+Ni7Zr2 ternary eutectic region have been derived from present experimental work. Chapter 4 focuses on understanding the mechanical behaviour of these individual eutectics at room temperature and high temperature. An attempt has been made to correlate the microstructure and mechanical properties of eutectics by measuring room temperature hardness, compressive yield strength at various temperatures, and examination of slip bands, crack initiation and fractography. It is observed that NiAl+Ni7Zr2 eutectic possesses the highest yield strength and hardness followed by ternary eutectic and then the Ni3Al+Ni7Zr2 eutectic. The yield strength of these eutectics decreases rapidly beyond 700oC and this decrease is accompanied by substantial increase in compressive ductility and steady state flow, with little work hardening. Chapter 5 explores the isothermal oxidation behavior at high temperatures of these eutectic alloys. Oxidation kinetics have been measured at various temperatures (900oC, 1000oC, 1050oC and 1100oC) are carried out using the thermo gravimetric analysis technique (TGA). The oxidation behavior has been characterized using TGA, X ray diffraction and EPMA. The Top surface of oxide layer shows compact, NiO layer with a fine grain size. The cross section of oxide samples shows five distinct microstructural and compositional layers at steady state. Attempt has been made to understand the oxidation mechanism, sequence of layer formation in correlation with microstructure and weight gains, rate constants and activation energy analysis. Finally Chapter 6 presents a summary of the current work and suggests for further work.

Well-logging and aerospace applications require electronics capable of withstanding elevated temperature operation. A key element of high temperature packaging technology is the Si die attach material, and a comparative study of two die attach systems for use in harsh environment has been performed. Die bond sample packages, using commercial adhesives and an Au-Si eutectic solder, have been manufactured and were subsequently thermally exposed for various times at 250 and 300°C respectively. The adhesive die bond packages comprised a high temperature co-fired ceramic (HTCC) substrate with W, Ni and Au metallisations whereas the Au-Si die bond packages used thick film Au metallised on a Al₂O₃ substrate. Optimisation of the eutectic die bonding parameters was successfully performed for the Au-Si system by an experimental design method, which improved mean and spread of maximum bonded areas and consequently, the shear load to failure. Bonded area was systematically assessed by scanning acoustic microscopy (SAM) followed by digital image analysis (DIA). Accelerated testing comprised thermal cycling and thermal shock and although showing some degradation, Au-2wt%Si die bonds were surprisingly robust, showing excellent subsequent stability during industrial device testing investigations.