Iron-cobalt alloys have been extensively studied as potential hard disk drive write head materials due to their potentially high saturation flux densities (~2.4T), low coercivities and ease of deposition. Iron-cobalt plating solutions have, however, been shown to have stability issues, necessitating that they be used at low pH or that a stabilizing agent be added to the solution. The purpose of this thesis is to evaluate the stability of a dibasic ammonium citrate plating solution and to characterize the deposits which result from its use. The plating solutions are found to be less stable than previously claimed. The solutions are oxidized by dissolved oxygen, which leads to a valence change in the iron ions and eventually the formation of iron oxide/hydroxide precipitates. These effects are exacerbated by heating or the application of a voltage across the solution. Deposits plated from the solution are fine grained (<40nm) and compact through their thickness. While normally deposited as the equilibrium BCC phase, metastable phases are deposited at elevated temperatures, high pH or in the absence of a stabilizing agent. A metastable phase which is isomorphous to α-Mn is deposited at elevated temperatures. This phase transforms to the BCC phase when annealed at >174ºC and is highly textured. Its presence is detrimental to deposit coercivity. / Materials Engineering

The ability to detect and quantify presence and concentration of unknown gasses is sought for applications ranging from environmental monitoring to medical analysis. Metal oxide based chemical sensing technology currently exists but the ability to provide a compositional gas breakdown reliably within a short time frame is not readily available. A very small sensor that can differentially identify the type and concentration of a gas is required. Novel methods of creating low cost and easily tuned one and two-dimensional gas sensing elements are explored. Tungsten trioxide has been thoroughly documented as an electrochromic coating, but highly sensitive WO3 elements with beam and nanowire structures have yet to be explored. Research of WO3 as a gas sensor encompasses three major components: A suitable sensing chamber with accurate analyte gas flow control and temperature control, a reliable method for WO3 deposition, and a high yield fabrication process. This thesis explores all three of these technologies. Chapter two starts with a summary of existing tungsten trioxide fabrication methods. An overview of WO3 processing follows. A comprehensive setup was designed and created to test the gas sensing response of a series of metal oxide based resistive elements through conductimetric analysis. Chapter three provides an in depth account of gas sensor test chamber design and testing. Critical test chamber aspects such as temperature control, precise gas flow control, highly efficient analyte gas switching and ease of use are presented. Chapter four outlines WO3 electrodeposition and the fabrication of beam structures for testing, while chapter five explores the templated electrodeposition of WO3 segments intercalated between gold nanowire segments. Finally, chapter six provides a summary of the research presented in this thesis as well as future directions and options available for further exploration of WO3 gas sensing elements. / Micro-Electro-Mechanical Systems (MEMS) and Nanosystems

Demands for high energy-density batteries have sharpened with the increased use of portable electronic devices, as has the focus global warming is now placing on the need for electric and electric-hybrid vehicles. Li-ion battery technology is superior to other rechargeable battery technologies in both energy- and power-density. A remaining challenge, however, is to find an alternative candidate to graphite as the commercial anode. Several metals can store more lithium than graphite, e.g., Al, Sn, Si and Sb. The main problem is the large volume changes that these metals undergo during the lithiation process, leading to degradation and pulverization of the anode with resulting limitations in cycle-life. The Li-ion battery is studied in this thesis with the goal of better understanding the critical parameters determining high and stable electrochemical performance when using a metal or a metal-alloy anode. Various antimony-containing systems will be presented. These represent different routes to circumvent the problems caused by volume change. Sb-compounds exhibit a high lithium storage capability. At most, three Li-ions can be stored per Sb atom, leading to a theoretical gravimetric capacity of 660 mAh/g. Model systems with stepwise increasing complexity have been designed to better understand the factors influencing lithium insertion/extraction. It is demonstrated that the microstructure of the anode material is crucial to stable cycling performance and high reversibility. The relative importance of the various factors controlling stability, such as particle-size, oxide content and morphology, varies strongly with the type of system studied. The cycling performance of pure Sb is improved dramatically by incorporating a second component, Sb2O3. With a critical oxide concentration of ~25%, a stable capacity close to the theoretical value of 770 mAh/g is obtained for over 50 cycles. Cu2Sb shows stable cycling performance in the absence of oxide. Cu9Sb2 has been presented for the first time as an anode material in a Li-ion battery context. Studies of the Solid Electrolyte Interphase (SEI) formed on AlSb composite electrodes show an SEI layer thinner than graphite, and with a clearly dynamic character.

This work outlines a series of developments and discoveries related to surface chemistry of controlled near-surface architectures. Through a combination of various X-ray spectroscopy techniques and innovative electrochemical fabrication techniques, valuable knowledge has been added to the fields of electrochemical fabrication, electrocatalysis, and fundamental surface chemistry. Described here is a specific new development in the technique of surface limited redox replacement (SLRR). This work, along with an accompanying journal publication1, reports the first-ever use of nickel as an intermediary for SLRR. In addition, this work identifies specific deviations from the nominal reaction stoichiometry for SLRR-grown films. This led to the proposal of a new reaction mechanism for the initial stages of the SLRR process, which will assist future fabrication attempts in this field. This work also discovered fundamental changes in Pt overlayer systems as the thickness of the overlayer on a gold support is increased from less than a single atomic monolayer to multilayer thicknesses. It was found that Pt overlayers below a certain threshold thickness exhibited increased affinity for hydroxyl groups, along with an increased propensity for formation of oxide and chloride species. These films were also studied for methanol, carbon monoxide, and ethylene glycol electro-oxidation. Finally, this work reports controlled surface architectures of Pt and Cu deposits on application-oriented TiO₂ nanotube arrays and Au-carbon supports.

Ce travail de thèse concerne l'étude de l'influence de la nanostructuration du système Cu-Co sur ses propriétés magnétiques et magnétorésistives. Dans un premier temps, l'alliage granulaire Cu 80 Co 20 a été synthétisé sous différentes formes : poudres, couches minces et nanofils. Les poudres d'alliage ont été obtenues par broyage mécanique et les couches minces et nanofils par électrodépôt. Cela a permis d'étudier, d'une part, l'influence de la forme de l'échantillon et, d'autre part, l'influence de la technique d'élaboration sur la nanostructure et les propriétés magnétiques et magnétorésistives des échantillons. Dans un second temps, des nanofils multicouches Cu/Co ont été réalisés par électrodépôt. Des protocoles expérimentaux pour l'analyse à l'échelle nanométrique par microscopie électronique à transmission et par sonde atomique tomographique ont été mis en place. De telles analyses se sont avérées indispensables à la compréhension et à la corrélation complète des propriétés magnétiques et magnétorésistives. Contrairement aux nombreuses études publiées, qui ont souvent conclu à l'obtention de solutions solides sur la base de caractérisations microstructurales, les analyses à l'échelle nanométrique par sonde atomique tomographique et par microscopie électronique à transmission de nos alliages granulaires ont montré qu'aucune solution solide Cu-Co n'a pu être obtenue. De plus, un effet positif de magnétorésistance sous faible champ magnétique appliqué a été observé, et corrélé à la présence d'oxydes.

Nanocrystalline materials have generated immense scientific interest, primarily due to observations of significantly enhanced strength and hardness resulting from Hall-Petch grain size strengthening into the nano-regime. Unfortunately, however, most previous studies have been unable to present material strength measurements using established tensile tests because the most commonly accepted tensile test protocols call for specimen geometries that exceeded the capabilities of most nanocrystalline material synthesis processes. This has led to the development of non-standard mechanical test methodologies for the evaluation of miniature specimens, and/or the persistent use of hardness indentation as a proxy for tensile testing. This study explored why such alternative approaches can be misleading and revealed how reliable tensile ductility measurements and material strength information from hardness indentation may be obtained. To do so, an electrodeposition-based synthesis method to produce artifact-reduced specimens large enough for testing in accordance with ASTM E8 was developed. A large number of 161 samples were produced, tested, and the resultant data evaluated using Weibull statistical analysis. It was found that the impact of electroforming process control on both the absolute value and variability of achievable tensile elongation was strong. Tensile necking was found to obey similar processing quality and geometrical dependencies as in conventional engineering metals. However, unlike conventional engineering metals, intrinsic ductility (as measured by maximum uniform plastic strain) was unexpectedly observed to be independent of microstructure over the grain size range 10-80nm. This indicated that the underlying physical processes of grain boundary-mediated damage development are strain-oriented phenomena that can be best defined by a critical plastic strain regardless of the strength of the material as a whole. It was further shown that the HV = 3•σUTS expression is a reliable predictor of the relationship between hardness and strength for electrodeposited nanocrystalline materials, provided the material is ductile enough to sustain tensile deformation until the onset of necking instability. The widely used relationship HV = 3•σY was found to be inapplicable to this class of materials owing to the fact that they do not deform in an “ideally plastic” manner and instead exhibit plastic deformation that is characteristic of strain hardening behaviour.

High strength-to-weight nanocrystalline alloy enabled conductor (NEC) prototypes were successfully developed by reinforcing an oxygen-free copper core material with electrodeposited cobalt phosphorus (CoP) coatings. A rule of mixtures approach was utilized to design the NEC prototypes to meet materials performance indices. Three unique NEC prototypes were produced with CoP coatings composed of alternating nanocrystalline (11 nm) and coarse-grained layers. The tensile properties were dependant on the coating microstructures, with tensile strengths of 1000 MPa, 970 MPa, and 900 MPa, respectively, and corresponding tensile elongations of 4.6%, 6.1%, and 10%, respectively. The electrical conductivity of the NEC prototypes was ~58 %IACS (resistivity of ~2.96 µΩ-cm). The rule of mixtures approach effectively predicted the tensile strength and conductivity. The NEC samples were significantly stronger than the incumbent high-strength aerospace conductor material, Be-Cu alloy CS95, which exhibits a tensile strength of only 655 MPa and conductivity of 63 %IACS.

High strength-to-weight nanocrystalline alloy enabled conductor (NEC) prototypes were successfully developed by reinforcing an oxygen-free copper core material with electrodeposited cobalt phosphorus (CoP) coatings. A rule of mixtures approach was utilized to design the NEC prototypes to meet materials performance indices. Three unique NEC prototypes were produced with CoP coatings composed of alternating nanocrystalline (11 nm) and coarse-grained layers. The tensile properties were dependant on the coating microstructures, with tensile strengths of 1000 MPa, 970 MPa, and 900 MPa, respectively, and corresponding tensile elongations of 4.6%, 6.1%, and 10%, respectively. The electrical conductivity of the NEC prototypes was ~58 %IACS (resistivity of ~2.96 µΩ-cm). The rule of mixtures approach effectively predicted the tensile strength and conductivity. The NEC samples were significantly stronger than the incumbent high-strength aerospace conductor material, Be-Cu alloy CS95, which exhibits a tensile strength of only 655 MPa and conductivity of 63 %IACS.

Nanocrystalline materials have generated immense scientific interest, primarily due to observations of significantly enhanced strength and hardness resulting from Hall-Petch grain size strengthening into the nano-regime. Unfortunately, however, most previous studies have been unable to present material strength measurements using established tensile tests because the most commonly accepted tensile test protocols call for specimen geometries that exceeded the capabilities of most nanocrystalline material synthesis processes. This has led to the development of non-standard mechanical test methodologies for the evaluation of miniature specimens, and/or the persistent use of hardness indentation as a proxy for tensile testing. This study explored why such alternative approaches can be misleading and revealed how reliable tensile ductility measurements and material strength information from hardness indentation may be obtained. To do so, an electrodeposition-based synthesis method to produce artifact-reduced specimens large enough for testing in accordance with ASTM E8 was developed. A large number of 161 samples were produced, tested, and the resultant data evaluated using Weibull statistical analysis. It was found that the impact of electroforming process control on both the absolute value and variability of achievable tensile elongation was strong. Tensile necking was found to obey similar processing quality and geometrical dependencies as in conventional engineering metals. However, unlike conventional engineering metals, intrinsic ductility (as measured by maximum uniform plastic strain) was unexpectedly observed to be independent of microstructure over the grain size range 10-80nm. This indicated that the underlying physical processes of grain boundary-mediated damage development are strain-oriented phenomena that can be best defined by a critical plastic strain regardless of the strength of the material as a whole. It was further shown that the HV = 3•σUTS expression is a reliable predictor of the relationship between hardness and strength for electrodeposited nanocrystalline materials, provided the material is ductile enough to sustain tensile deformation until the onset of necking instability. The widely used relationship HV = 3•σY was found to be inapplicable to this class of materials owing to the fact that they do not deform in an “ideally plastic” manner and instead exhibit plastic deformation that is characteristic of strain hardening behaviour.

Nowadays, the development of novel materials involves diverse branches of science as a consequence of the new requirements imposed by modern society. This includes aspects ranging from the optimization of the manufacturing processes to the durability of the materials themselves. Ideally, some synergism should exist between the durability, the properties of interest in the material. Although metals in their pure state are often desired, the best properties or combination of properties often cannot be satisfactorily achieved with a single metal. In these situations, the desired properties can be attained by the formation of alloys of these metals with others. Ni-Co alloys are no exceptions and so have received considerable attention especially in microsystem technology due to the magnetic properties of cobalt and the corrosion and wear resistance of nickel. Moreover, this interest has been further stimulated by its use in the manufacture of sensors, magnetic devices, microrelays, inductors, actuators, memory devices and hard drives. The fabrication of these alloys (particularly coatings) via electroplating has been shown to be techno-economically feasible in comparison with other processes: capability of high volume production, low cost and the ability to coat thin layers on non-planar substrates. In addition, the materials fabricated by this technology exhibit excellent characteristics such as refined grain structure, smoothness, low residual stress and coercivity, etc., making them advantageous to materials produced by other physical methods of deposition. Nevertheless, one of the biggest problems faced during the formation of Ni-Co alloys is its anomalous behavior whereby cobalt preferentially deposits over nickel under most conditions, even when the Ni(II) concentration is significantly higher than that of Co(II). This problem has complicated the prediction and control of the metal composition in these alloys during their production and as a consequence the ability to obtain the desirable properties associated with high nickel content. Although this problem is not recent, the studies that have been carried out so far to analyze this system have not always been as comprehensive as they could be in terms of the experimental conditions investigated or the reaction mechanisms and mathematical models developed to describe its behavior. Consequently, the origin of this behavior is still not completely understood. Thus, this work presents a contribution in terms of the analysis of the reaction mechanisms for single metal deposition of nickel and cobalt and for the formation of Ni-Co alloys in sulphate media with the intention of gaining a better understanding of the phenomena controlling the anomalous behavior of this system. Analyses of the single metal deposition of nickel and cobalt are first carried out to better understand their reaction mechanisms. Such an approach should allow the contributions of the reduction of each metal ion and interactions between the two systems during alloy co-deposition to be more clearly understood. In order to analyse the aforementioned systems, both steady state and transient techniques are employed. Among these techniques, electrochemical impedance spectroscopy (EIS) is employed since it is a robust and powerful method to quantitatively characterize the various relaxation phenomena occurring during the electrodeposition of metals. The experimental data acquired from this technique are analyzed with comprehensive physicochemical models and the electrochemical processes are quantified by fitting the models to these data to determine the kinetic parameters. During the development of the physicochemical models, several assumptions (e.g. neglect of convection, homogeneous reactions and single electron-transfer steps) made in former models are relaxed in order to investigate their combined impact on the predicted response of the system. Estimates of the kinetic parameters determined by EIS for the deposition of the single metals reveals that the first step of Co(II) reduction is much faaster tha the corresponding step of Ni(II) reduction. Some limitations of the EIS technique (i.e. analysis at high overpotentials) are exposed and compared in the case of the nickel deposition using linear sweep voltammetry (LSV). Likewise, physicochemical models accounting for most of the important phenomena are derived and fitted to experimental data. Ni-Co alloy formation is analyzed using LSV and steady state polarization experiments for different pH, current density and electrolyte composition. Current efficiencies for metal depsoition and alloy composition are also evaluated. To date, no experimental study considering all these variables has been reported in the literature. Then a steady state model is presented to describe the electrode response during alloy formation and used to provide insight into the anomalous behavior of this system. This model is based on information obtained from previous studies reported in the literature and from the current research. After being fitted to the experimental data, the model reveals that the anomalous behavior observed for this alloy is likely caused by the much faster charge-transfer of Co(II) reduction than that of Ni(II) reduction and not by other previously proposed mechanisms such as competition between adsorbed species for surface sites, formation of aqueous hydroxides (MeOH+) or mixed intermediate species (NiCo(III)ads) on the surface of the electrode.