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This thesis describes an ion irradiation study of ordered Cu<sub>3M/sub>Au. Argon, Copper and Krypton ions of 50 and 100keV were implanted to nominal doses of 10¹¹ ions cm^-2 both at room temperature and at liquid helium temperature. The damage induced at low temperature was observed in-situ and after subsequent warming to room temperature. The collapse of collision cascades to vacancy dislocation loops was observed to occur with moderately high probability under all irradiation conditions at low temperature, with no subsequent increase in the number of loops on warming. Comparison of the collapsed fraction of cascades at low temperature with that produced by room temperature irradiation revealed that the collapse process is more efficient in the latter case. An increase in collapse probability with ion mass was also observed, although no dependence on ion energy was found over the range investigated. Quantitative analysis of the disordered zones which result in ordered Cu₃Au at individual cascade sites revealed that cascades which had collapsed to loops generally produced larger disordered zones. Additionally, increases in disordered zone size with ion mass, ion energy and irradiation temperature were identified. The observation that disordered zone sizes exceed the theoretically predicted cascade dimensions for the case of the highest energy density in this study (50keV Kr) is interpreted as evidence for leakage of energy from the cascade region to the surrounding lattice. A model is proposed which attributes the observed cascade collapse behaviour to vacancy migration during the cooling of the cascade region being systematically directed toward its centre, and accounts for the observed disordered zones in terms of extensive atomic rearrangement during the cooling. The major factors identified as controlling this vacancy and atomic motion are the initial spatial distribution of energy and vacancies within the region, and the rate at which the region cools.

An investigation was conducted of the corrosion of copper-gold alloys in ammonia solutions under oxygen pressure. The reaction, which took place in an autoclave, was followed by sampling and analysis of the solutions. It was found that only copper was dissolved from the alloys, the gold being left behind in a film on the surface of the corroding specimen. The reaction of pure copper as well as of four alloys ranging in gold content from 2 to 15 atomic percent were studied. Other variables examined include the concentrations of NH₃ and NH⁺₄ in the solution, the oxygen pressure and the temperature. It was found that while the rate curves for the dissolution of pure copper were linear, those for the alloys were generally parabolic in shape. The rate of dissolution of copper from the alloys appears to be determined by the transport of reactants and products through the gold rich films. Some copper oxide may also be precipitated in the pores of the film or in the region between the film and the underlying metal, further impeding the transport processes and contributing to the lowering of the rate. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

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Thesis (M.S.)--University of Wisconsin--Madison, 1963. / Typescript. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves 51-53).

Thesis (Ph. D.)--University of Hong Kong, 2002. / Includes bibliographical references (leaves 324-353).

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Le but de cette thèse est l’étude du système électrochimique or-cuivre en milieucyanure, la finalité étant la réalisation de microballons en or-cuivre de 800 μm de diamètre etd’épaisseur 20 à 40 μm. La composition, la microstructure ainsi que la rugosité doivent êtreparfaitement maîtrisées. La technique utilisée pour réaliser ce type d’objet est le dépôtélectrolytique en milieu cyanure.Dans un premier temps, l’influence des principaux paramètres d’élaboration(température de l’électrolyte, agitation, etc.) a été étudiée. Cette première étude a permis deréaliser des alliages d’or-cuivre de 5 μm d’épaisseur sur substrat plan de différentescompositions. En effet, il a été montré que la teneur en cuivre des dépôts augmente lorsque lepotentiel de réduction appliqué croît. Une augmentation du taux de cuivre modifie lesmécanismes de germination et de croissance des dépôts, ce qui a pour effet de diminuer lataille de grains et de modifier la microstructure. Ainsi, plus le dépôt est riche en cuivre, plus lataille de grains est faible et plus la structure est colonnaire, nodulaire voire dendritique.Des dépôts de 20 μm d’épaisseur ont ensuite été effectués sur substrat plan.Conformément à ce qui est décrit dans la littérature, ces dépôts se sont avérés très difficiles àréaliser en raison de l’apparition de nodules et de dendrites lorsque l’épaisseur augmente. Deplus, pour des épaisseurs supérieures à 10 μm, les dépôts sont alors constitués uniquementd’or, le cuivre n’étant plus réduit. Le changement de structure en cours de dépôt s’expliquepar l’inhibition de croissance engendrée par le cyanure libre. En effet, au cours de la réductionde l’aurocyanure et du cuprocyanure, du cyanure libre est libéré à la cathode. Ce cyanure libreinhibe la croissance latérale et promeut la croissance tridimensionnelle dite « instantanée »,provoquant l’apparition de nodules et de dendrites. Quant à l’appauvrissement en cuivre dudépôt, il s’explique également par la présence de cyanure libre à la cathode qui génère descomplexes cuprocyanure d’ordre 4. Les complexes d’ordre 4 possèdent une énergied’activation supérieure et un coefficient de diffusion plus élevé que les complexes d’ordre 3,d’où l’appauvrissement en cuivre du dépôt.Suite aux études électrochimiques, un modèle a été établi permettant d’expliquerl’influence du cyanure libre sur l’électrocristallisation des alliages d’or-cuivre. Ce modèle apermis de mettre en place des solutions visant à limiter l’inhibition électrochimique et ainsioptimiser les propriétés des alliages or-cuivre obtenus.L’une des solutions mises en place est l’application d’un champ ultrasonore pendant ledépôt. La cavitation générée par les ultrasons permet en effet d’évacuer le cyanure libre de lasurface de la cathode et d’optimiser le processus d’électrocristallisation. Ainsi, des dépôtsd’or-cuivre sur microballons ont été réalisés en présence d’ultrasons. Les analyses MEB etEDX de ces microballons montrent qu’il est possible d’obtenir des dépôts de 20 à 40 μmd’épaisseur de composition maitrisée. Les dépôts analysés ne présentent aucun gradient deconcentration dans l’épaisseur et il est ainsi possible de réaliser des alliages d’or-cuivrecontenant jusqu’à 45 %m de cuivre. Les dépôts réalisés présentent une structure lisse(80 ≤ Ra ≤ 230 nm) et compacte, et cela quelle que soit la concentration en cuivre. Quant à lamicrodureté de ces dépôts sur microballons, elle est fonction de la taille de grains (relation deHall-Petch) et donc de la concentration en cuivre du dépôt. / The aim of this PhD Thesis is to study the gold-copper cyanide electrochemicalsystem and finally to realize gold-copper microspheres with a diameter of 800 μm and athickness between 20 and 40 μm. The composition, the microstructure and the roughness ofthese shells must be perfectly controlled. To synthesise such a material, electrodepositionfrom a gold-copper alkaline cyanide bath has been chosen.Initially, the influence of the principal electrochemical parameters (temperature of theplating bath, stirring, etc.) was studied. This study showed that it is possible to realize5 μm thick gold-copper alloys with various compositions. Actually, it was shown that thecopper content of deposits varies with the applied potential. When increasing the coppercontent of coatings, the nucleation and growth mechanisms change. As a result, the grain sizeand the microhardness of the coatings are modified. An increase in the copper content reducesthe grain size witch increases the microhardness until a critical grain size of 6 nm. Thisincrease of copper content also affects the microstructure: columnar, nodular even dendriticalstructures were observed.Then, 20 μm thick gold-copper coatings were realized using the same electrochemicalparameters. As expected, these coatings were very difficult to plate because of the instabilityof the electrocrystallization process resulting in the development of columnar and nodularstructures. Moreover, for thicknesses above 10 μm, all deposits are free from copper. Themicrostructure change of deposits can be explained by inhibition phenomena generated byfree cyanide. Actually, the reduction of gold-copper generates free cyanide at the cathodesurface which inhibits the electrocrystallization and promotes instantaneous nucleation. Thisproduction of free cyanide also modifies the electrolyte chemistry promoting the formation ofCu(CN)43- instead of Cu(CN)32-. Cu(CN)43- complexes have lower diffusion coefficients andhigher activation energy witch explains why copper content reduces when increasing thethickness of deposits.Then a model was established which explains the influence of free cyanide on thegold-copper electrocrystallization. This model permitted to develop solutions in order to limitthe inhibition phenomena and to optimize the electrocrystallization of gold-copper.One of the solutions developed is the application of an ultrasonic field. The cavitationgenerated by the ultrasonic field eliminates the free cyanide from the cathode surface andoptimize the electrocrystallization process. Gold-copper deposits on shells were then platedunder sonication. SEM and EDS results show that it is possible to make 20 to 40 μm thickcoatings with a controlled composition. All the coatings plated under sonication were smooth(80 ≤ Ra ≤ 230 nm) and compact for various copper contents. The microhardness of thesecoatings varies with grain size (Hall-Petch relation) which depends of copper content.

Investigations on size dependent phase stability and transformations in isolated nanoparticles have gained momentum in recent times. Size dependent phase stability generates size specific particle microstructure which consequently yields size specific functionality. One important prerequisite for conducting studies on nanoparticles is their synthesis. A substantial amount of research effort has therefore been focused on devising methodologies for synthesizing nanoparticles with controlled shapes and sizes. The present thesis deals with both these two aspects: (a) synthesis of nanoparticles and (b) phase transformations in nanoparticles. The system chosen in this study is AuCu intermetallic nanoparticles. The choice of AuCu nanoparticle was due to the fact that the literature contains abundance of structural and thermodynamic data on Au–Cu system which makes it a model system for investigating size dependence of phase transformations. With respect to synthesis, the present thesis provides methodologies for synthesizing alloyed Au–Cu nanoparticles of different sizes, Au–Cu nano-chain network structures and uniform Au–Cu2S hybrid nanoparticles. For every type, results are obtained from a detailed investigation of their formation mechanisms which are also presented in the thesis. With respect to phase transformation, the thesis presents results on the size dependence of fcc to L10 transformation onset in Au–Cu nanoparticles under isothermal annealing conditions. The present thesis is divided into eight chapters. A summary of results and key conclusions of work presented in each chapter are as follows. The ‘introduction’ chapter (chapter I) describes the organization of the thesis. Chapter II (literature study) presents a review of the research work reported in the literature on the various methodologies used for synthesizing Au–Cu based nanoparticles of different shapes and sizes and on ordering transformation in AuCu nanoparticles. The chapter also presents a brief discussion on the reaction variables that control the process of nucleation and growth of the nanoparticles in solution. Chapter III titled ‘experimental details and instrumentation’ describes the synthesis procedures that were used for producing various nanoparticles in the present work. The chapter also briefly describes the various characterization techniques that were used to investigate the nanoparticles. The fourth chapter titled ‘synthesis and mechanistic study of different sizes of AuCu nanoparticles’ provides two different methodologies for synthesis, referred as ‘two-stage process’ and ‘two-step process’ that have been used for producing alloyed AuCu nanoparticles of different sizes (5, 7, 10, 14, 17, 25 nm). The ‘two-stage’ process involved sequential reduction of Au and Cu precursors in a one pot synthesis process. Whereas, the ‘two-step’ process involved a two-pot synthesis in which separately synthesized Au nanoparticles were coated with Cu to generate alloyed AuCu nanoparticles. In the two-stage synthesis process it was observed that by changing the total surfactant-to-metal precursor molar ratio, sizes of the alloyed AuCu nanoparticles can be varied. ‘Total surfactants’ here include equal molar amounts of oleic acid and oleylamine surfactants. Interestingly, it was observed that there exists a limitation with respect to the minimum nanoparticle size that can be achieved by using the two-stage process. The minimum AuCu nanoparticle size achieved using the two-stage synthesis process was 14 nm. Mechanism of formation of AuCu nanoparticles in the two-stage synthesis process was investigated to find out the reason for this size limitation and also to determine how the synthesis process can be engineered to synthesize alloyed AuCu nanoparticles with smaller (<14nm) sizes. Studies to evaluate mechanism of synthesis were conducted by investigating phase and size of nanoparticles present in the reaction mixture extracted at various stages of the synthesis process. Their studies revealed that (a) the nanoparticle formation mechanism in the two-stage synthesis process involves initial formation of Au nanoparticles followed by a heterogeneous nucleation and diffusion of Cu atoms into these Au rich seeds to form Au–Cu intermetallic nanoparticles and (b) by increasing the relative molar amount of the oleylamine surfactant, size of the initial Au seed nanoparticles can be further reduced from the minimum size that can be achieved in the case when equal molar amounts of oleylamine and oleic acid surfactants are used. The information obtained from the mechanistic study was then utilized to design the two-step synthesis process. In the two-step process, Au nanoparticles were synthesized in a reaction mixture containing only the oleylamine surfactant. Use of only oleylamine resulted in production of pure Au nanoparticles with sizes that were well below 10 nm. These Au nanoparticles were washed and dispersed in a solution containing Cu precursor. Introduction of a reducing agent into this reaction mixture led to the heterogeneous nucleation of Cu onto the Au seed particles and their subsequent diffusion into them to form alloyed AuCu nanoparticles with sizes of ~5, 7 and 10 nm. The study present in this chapter essentially signified that the surfactants used in the reaction mixture not only prevent nanoparticles from agglomerating in the final dispersion but also control their nucleation and growth and therefore can be used as a tool to tune nanoparticle sizes. The fifth chapter titled ‘size dependent onset of FCC-to-L10 transformations in AuCu alloy nanoparticles’ illustrates the effect of AuCu nanoparticle size on the onset of ordering under isothermal annealing conditions. Nanoparticles in this study were annealed in-situ in a transmission electron microscope. Samples were prepared by drop drying a highly dilute dispersion of as-synthesized nanoparticles onto an electron transparent TEM grid. Nanoparticles sitting on the TEM grid were well separated from each other to minimize particle sintering during the annealing operation. It was however observed that during the isothermal annealing, particle coarsening due to atomic diffusion was appreciable for 5 nm particles but negligible for 7 and 10 nm particles. Therefore for this study only 7 nm and 10 nm sized particles were considered. Onset of ordering was determined from the time when first sign of the diffraction spot, corresponding to the ordered phase, appears in the selected area electron diffraction pattern from a region containing large number of AuCu nanoparticles. Through a series of isothermal experiments it was observed that the time for onset of ordering increased with decrease in size of the nanoparticles. It is speculated that the delay in onset of ordering may be due to the fact that with a decrease in nanoparticle size the probability of a nanoparticle containing a fluctuation that shall generate a thermodynamically stable nuclei of the ordered phase decreases. A sharp interface between the ordered and the disordered phase inside the particle was also observed which suggested that the ordering transformation in as-synthesized fcc AuCu nanoparticles is a first order transformation. The sixth chapter titled ‘synthesis and characterization of Au1-xCux–Cu2S hybrid nanostructures: morphology control by reaction engineering’ provides a modified polyol method based synthesis strategy for producing uniform Au–Cu2S hybrid nanoparticles. Detailed compositional and structural characterization revealed that the hybrid nanoparticles are composed of cube shaped Au-rich, Au–Cu solid solution phase and hemispherical shaped Cu2S phase. Interestingly, the hemispherical Cu2S phase was attached to only one facet of the cube shaped phase. A study on the formation mechanism of hybrid nanoparticles was also conducted by characterizing specimens extracted from the reaction mixture at different stages of the synthesis process. The study revealed that the mechanism of formation of hybrid nanoparticles involved initial formation of isolated cube shaped pure Au nanoparticles and Cu–thiolate complex with a sheet morphology. With increase in time at 180°C, the Cu–thiolate complex decomposed and one part of the Cu atoms that were produced from the decomposition were utilized in forming the spherical Cu2S and other part diffused into the Au nanoparticles to form Au–Cu solid solution phase. The chapter also presents a study on the effect of dodecanethiol (DDT) on achieving the hemisphere-on-cube hybrid morphology. In this study it is illustrated that an optimum concentration of dodecanethiol is required both for achieving size and morphological uniformity of the participating phases and for their attachment to form a hybrid nanoparticle. The seventh chapter titled ‘synthesis of Au–Cu nano-chains network and effect of temperature on morphological evolution’ provides methodology for synthesizing fcc Au– Cu nano-chain network structures using polyvinylprrolidone (PVP) surfactant. It was observed that with increase in the molar amount of PVP in the reaction mixture, morphology of the as-synthesized product gradually changed from isolated nanoparticles to branched nano-chain like. The nano-chains contained twins which indicated an absence of continuous growth and possibility of growth by oriented attachment of initially formed Au–Cu nanoparticles. Both in-situ and ex-situ annealing of the nano-chains led to their decomposition into isolated nanoparticles of varying sizes. Annealing also caused fcc-to¬L10 phase transformation. Investigation of the wave length of perturbation leading to breaking of a nano-chain into particles indicated that the surface energy anisotropy affects the splitting of nano-chain network structure into nano-sized particles. The thesis ends with a last chapter where we have presented possible future extension of current work.