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The dust particles generated due to mechanical wear of iron ore pellets and clusters formed in molten stainless steel alloyed with rare earth metals (REM) are considered in this study. Firstly, the influence of the characteristics of iron ore pellets, applied load on a pellet bed and partial reduction of the pellets on the size distribution of the generated dust was investigated. Secondly, REM clusters are investigated to evaluate the size distribution of the clusters. Also, an extreme value distribution (EVD) analysis has been applied for the observed REM clusters. The large sized pellets showed 10-20% higher wear rate than small sized pellets during wear in a planetary mill. Moreover, an increase of ~67% was observed in the friction and dust generation in the pellet bed as the applied load increased from 1 to 3 kg. Also, it was observed that a higher friction in the pellet bed can lead to an increased amount of airborne particles. The mechanical wear experiments of pellets reduced at 500 °C (P500) and 850 °C (P850) showed that P500 pellets exhibit ~16-35% higher wear rate than unreduced pellets. For the P850 pellets, the wear is inhibited by formation of a metallic layer at the outer surface of the pellets. The mechanism of dust generation has been explained using the obtained results. A reliable cluster size distribution of REM clusters was obtained by improving the observation method and it was used to explicate the formation and growth mechanism of REM clusters. The results show that the growth of clusters is governed by different types of collisions depending on the size of the clusters. For EVD analysis three different size parameters were considered. Moreover, using the maximum length of clusters results in a better correlation of EVD regression lines compared to other size parameters. Moreover, a comparison of predicted and observed maximum lengths of clusters showed that further work is required for the application of EVD analyses for REM clusters. / Studien fokuserar på två olika typer av mikropartiklar som är valda från olika delar av järn- och ståltillverkningsprocessen. Dessa partiklar är dels stoft som genereras på grund av mekanisk nötning av partiklar och dels klusters som bildas i flytande rostfria stål legerade med sällsynta jordartsmetaller (REM). Inledningsvis så undersöktes inverkan av tre faktorer på storleksfördelningen hos stoft som bildas vid hantering av järnoxidpellets. De undersökta faktorerna inkluderade karakteristiken hos järnoxidpellets, det applicerade trycket på pelletsbädden och den partiella reduktionen av järnoxidpellets. Därefter så utfördes tredimensionella undersökningar av REM kluster som extraherats med hjälp av elektrolytisk extraction för att bestämma storleksfördelningen hos klustren. Dessutom så utfördes en extremvärdesdistribution (EVD) studie för de studerade klustren. En planetkvarn användes för att undersöka inverkan of karakeristiken hos pellets på stoftbildningen. Resultaten visade att storleken på pellets kan påverka nötningshastigheten under dessa försöksförhållanden. Pellets som hade en större storlek (13.5&lt; Deq &lt;15.0 mm) uppvisade en 10 till 20% högre nötningshastighet i jämförelse med mindre pellets (9.5&lt; Deq &lt;12.5 mm). Baserat på analyserna av stoftet som genererades under nötningsexperimenten så konstaterades att nötningsmekanismerna för dessa pellets var abrasions- och kollisionsnötning. En pelletsbädd skapades för att möjliggöra studier av inverkan av ett applicerat tryck på stoftbildningen och friktionskrafterna i en pelletsbädd. Ett varierat tryck på mellan 1 till 3 kg applicerades på pelletsbädden. Resultaten visade att en ökning på ~67% av friktionskraften och stoftbildningen ägde rum när det applicerade trycket ökades från 1 till 3kg. Dessutom så visade resultaten att en högre friktionskraft i pelletsbädden kan resultera in en ökad mängd luftburna partiklar. Den mekaniska nötningen av pellets som reducerats vid 500 °C (P500) och 850 °C (P850) studerades också genom användande av en planetkvarn. Resultaten visade att P500 pellets uppvisade en ~ 16 till 35% högre nötningshastighet i jämförelse med oreducerade referenspellets.  Resultaten för P850 pellets visade att den mekaniska nötningen motverkades genom bildningen av ett metalliskt skikt på den yttre delen av pelletsen. Resultaten visade också att stoftet som bildats pga mekanisk nötning av reducerade pellets innehöll 3 till 6 gånger mer grova partiklar  (&gt;20µm) i jämförelse med stoft som bildats från oreducerade pellets. Slutligen så diskuterades hur dessa resultat kan relateras till industriella förhållanden med avseende på mekanismerna som är involverade i den mekaniska nötningen av pellets samt med avseende på relationen mellan hastigheten av de utgående gaserna och storlken och morfologin hos stoftpartiklarna. Klusters innehållande REM-oxider som extraherats från en 253MA rostfri stålsort undersöktes med användande av en tredimensionell teknik. En trovärdig storleksfördelning av klusters (CSD) erhölls genom att förbättra undersökningsmetoden och denna användes för att studera bildningen och tillväxten av REM oxider. Dessutom så användes cirkularitetsfaktorn hos klusters för att dela in klustren i två olika grupper, vilka bildas och tillväxer enligt olika mekanismer. Resultaten visade också att tillväxten av klusters gynnas av olika typer av kollisioner som beror av av storleken på klusters. För REM-klusters så drogs slutsatsen att turbulenta kollisioner är den huvudsakliga mekanismen som påverkar tillväxten. Avhandlingen behandlar även problemet om hur det är möjligt att hantera synfält där det inte förekommer kluster vid en extremvärdesdistribution (EVD) analys. Tre olika parametrar undersöktes i EVD analysen. Resultaten visar att om den maximala längden på kluster (LC) används i analysen så erhålls den bästa korrelationen gällande regressionslinjen för en EVD analys. Specifikt så var R2 värdet upp till 0.9876 i jämförelse med de andra storleksparametrarna som har värden i intervallet 0.9656 – 0.9774. Slutligen så visar resultaten från en jämförelse mellan beräknade och observerade maximala klusterlängder att EVD analyser för studier av REM kluster behöver undersökas ytterligare i framtiden. / <p>QC 20161128</p>

Ce travail concerne l'ajout de GeH4 au système de précurseurs gazeux classique SiH4+C3H8 pour la croissance épitaxiale de SiC par dépôt chimique en phase vapeur. L'objectif principal était d'explorer l'influence de la présence de l'élément Ge (impureté isoélectronique à SiC), dans la matrice SiC ou à sa surface, sur les mécanismes de croissance et sur la qualité et les propriétés des couches minces déposées. La croissance épitaxiale a été réalisée dans la gamme de température 1450-1600°C sur des substrats 4H-SiC(0001) désorientés fortement (4° et 8°) ou faiblement (0° et 1°). Sur les germes désorientés, nous avons exploré l'impact des atomes de Ge sur la qualité des couches homoépitaxiales, d'un point de vue morphologique et structural. Les mécanismes d'incorporation de cette impureté ont été étudiés en fonctions des paramètres de croissance. Il a été montré que l'incorporation de cet élément peut être contrôlée dans la gamme 1x1016 - 7x1018 at.cm-3. De plus, cette incorporation de Ge s'accompagne d'une augmentation du dopage de type n. Les caractérisations électriques de ces couches montrent une amélioration de la mobilité et de la conductivité électrique du matériau 4H-SiC sans aucun impact négatif sur les caractéristiques de contact Schottky. Sur les substrats faiblement désorientés, GeH4 a été ajouté à la phase gazeuse uniquement pendant l'étape de préparation de la surface, c’est-à-dire avant d'initier la croissance de SiC. Il a été montré que des couches hétéroépitaxiales de 3C-SiC exemptes de macles peuvent être déposées dans une fenêtre de conditions expérimentales (température et flux de GeH4). Un mécanisme permettant l'élimination des macles a été proposé. Il implique une étape transitoire de croissance homoépitaxiale, favorisée par la présence de Ge liquide à la surface, suivie de la nucléation de 3C-SiC sur les larges terrasses résultant du facettage des marches. Ces couches de 3C-SiC ont été caractérisées électriquement par microscopie à force atomique en mode conduction / In this work, addition of GeH4 gas to the classical SiH4+C3H8 precursor system is reported for the epitaxial growth of SiC by chemical vapor deposition. The main objective of this fundamental study is to explore the influence of Ge presence within SiC lattice or at its surface on the overall growth mechanism and the grown layer quality and properties. Epitaxial growth was performed either on high off axis (8 and 4°) or low off-axis (1° and on-axis) 4H-SiC substrate in the temperature range 1450-1600°C. On high off-axis seeds, we discussed the impact of Ge atoms on the homoepitaxial layer quality from surface morphological and structural point of view. Ge incorporation mechanism in these layers as a function of growth parameters was also investigated. The Ge incorporation can be controlled from 1x1016 - 7x1018 at.cm-3. Moreover, a clear link between n-type doping and Ge incorporation was found. Electrical characterizations of these layers show an improvement of electron mobility and conductivity of 4H-SiC material while the performances of Schottky contacts were not negatively impacted. On low off-axis seeds, GeH4 was added to the gas phase only during the surface preparation step, i.e. before starting the SiC growth. It was found that there is a conditions window (temperature and GeH4 flux) for which heteroepitaxial 3C-SiC twin free layers can be grown. Interpretation of the results allowed proposing a mechanism leading to twin boundary elimination. It involves a transient homoepitaxial growth step, favored by the presence of liquid Ge at the surface, followed by 3C nucleation when large terraces are formed by step faceting. Electrical characteristics of the twin free 3C-SiC layers were studied using conductive atomic force microscopy (c-AFM)

Shape and size control as well as the control of the assembly of nanostructures are current challenges in nano sciences. Focussing on metal nanostructures all of these aspects have been addressed in the frame of the present work. It was possible to develop a new aqueous seeded growth method that produces gold nanoparticles with adjustable diameters over a large range of sizes. The spherical particles obtained show very low polydispersities and a good long term stability. Furthermore it was possible to reveal the growth mechanism of these particles utilizing electron microscopy and optical investigations coupled with theoretical calculations. It was found that there is a formation of small nucleation sites on the surface of the seeds in the beginning of the growth process. These sites then subsequently grow into "blackberry-like" intermediate particles. A final intraparticle ripening step leads to smooth and uniform spherical gold nanoparticles. By correcting the dielectric function of gold for charging and the free mean path effect and taking into account the particle size distribution it was possible to accurately model the optical properties of the gold sols obtained using Mie theory. By controlling the concentration of chloride ions it was possible to influence both the ripening of the "blackberry-like" shaped particles and the morphology of gold nanoparticles. An increased concentration of the chloride ions in the standard citrate reduction procedure leads to larger and elongated particles, whereas the complete removal of the chloride ions made it possible to obtain star shaped, decahedral and \"desert-rose\" shaped particle morphologies. Using the layer-by-layer technique gold nanoparticles of different sizes could be immobilized on glass substrates. The surface-enhanced Raman scattering intensity of these mixed films were about 60% higher than compared to a film made of a single particle size. The optical properties were further investigated by comparing experimentally obtained UV/Vis spectra with generalized Mie theory simulations. Additionally it could be shown that tetrazole and its derivatives are suitable stabilizing agents in the aqueous synthesis of silver nanoparticles. It was found that depending on the tetrazole derivative used the tendencies of the nanoparticles to agglomerate vary significantly. Different agglomeration stages have been investigated by UV/Vis and Raman spectroscopy. The removal of the ligands used and a resulting improvement of the applicability of the silver nanostructures as SERS substrates is still a challenge. In the last part of this work the focus was changed from the optical properties of noble metal nanoparticles to their catalytic properties. Therefore gold and palladium nanoparticles have been successfully immobilized on highly porous zinc oxide aerogels. It was possible to synthesize sponge-, flake-, and ribbon-like zinc oxide gels with high specific surface areas. The facile approach of generating mixed metal oxide/noble metal aerogels is very promising for the preparation of highly selective and highly active heterogenous catalysts. First catalytic investigations of a sponge-like palladium loaded zinc oxide aerogel toward the semi-hydrogenation of acetylene showed very high selectivities of up to 85%.

Quasi one-dimensional (1D) nanostructures show great promise for many applications, including in solar cells, nanogenerators and chemical sensors, due to the high surface-to-volume ratio and unique properties of nanostructures. The growth of these nanostructures is commonly catalyzed by metal nanoparticles and generally attributed to the vapor-liquid-solid (VLS) mechanism. The purpose of this research is to better understand the role of the catalyst nanoparticles in the growth of 1D nanostructures, in order to allow improved control of the synthesis process. To this end, nanostructures were grown with a variety of compositions, including Au- and Sn-catalyzed ZnO, Au-catalyzed FexOy and Au-catalyzed Si nanostructures. The morphology of the nanostructures was characterized with electron microscopy, and the crystallographic orientation with X-ray texture analysis. The catalyst particles were further characterized with both in-situ and post-growth X-ray diffraction. The types of bonding in the source material and catalyst play a significant role in the diffusion path of the source material to the growth front and in the catalyst particle state during growth. Dissimilar bonding types in the source material and catalyst prevent bulk diffusion of the source material through the catalyst, thereby preventing eutectic melting of the catalyst. These results bring new insight into the catalyzed growth of 1D nanostructures and assist in the informed choice of appropriate catalyst materials, which may aid the utilization of 1D nanostructures in energy-related and other applications.

Magister Scientiae - MSc (Physics) / Zinc oxide (ZnO) is a wide and direct semiconductor with a wurtzite crystal structure. Its multifunctionality as the ideal candidate in applications such as blue-UV light emitting diodes, transparent conducting oxide, selective gas sensor and efficient catalyst support among others, has attracted a significant interest worldwide. Nano-scaled ZnO has been synthesized in a plethora of shapes. A rich variety of physical and chemical methodologies have been used in the synthesis of undoped or doped ZnO. However, such methods either necessitate relatively high vacuum infrastructures, elevated temperatures, or the use of toxic reagents. The "green chemistry" synthesis of metal oxide nanoparticles which is based on using natural plant extract as an effective 'reducing agent' of metal precursor, has been reported to be a cleaner and environment-friendly alternative to the physical and chemical methods. The thesis is based on the synthesis and the main physical properties of pure ZnO nanoparticles synthesized by a completely green chemistry process using the natural extract of Aspalathus Linearis to bio-reduce the zinc acetate precursor. The obtained ZnO nanopowdered samples were annealed at different temperatures from 300 °C to 600 °C. The samples were characterized using Scanning Electron Microscopy, Energy Dispersive Spectroscopy, Transmission Electron Microscopy, X-ray Diffraction, Differential Scanning Calorimetry, Thermogravimetric Analysis and Fourier Transform Infrared. Highly pure quasi-spherical ZnO nanoparticles with an average crystallite size of 24.6 nm (at 300 °C), 27.2 nm (at 400 °C), 27.6 nm (at 500 °C), and 28.5 nm (at 600 °C) were found. The results also showed that the average crystallite size increased with an increase in annealing temperature. It was successfully demonstrated that the natural plant extract of Aspalathus Linearis can be used in the bio-reduction of zinc acetate dihydrate to prepare highly pure ZnO nanoparticles.

abstract: Chemical Vapor Deposition (CVD) is the most widely used method to grow large-scale single layer graphene. However, a systematic experimental study of the relationship between growth parameters and graphene film morphology, especially in the industrially preferred cold wall CVD, has not been undertaken previously. This research endeavored to address this and provide comprehensive insight into the growth physics of graphene on supported solid and liquid Cu films using cold wall CVD. A multi-chamber UHV system was customized and transformed into a cold wall CVD system to perform experiments. The versatile growth process was completely custom-automated by controlling the process parameters with LabVIEW. Graphene growth was explored on solid electrodeposited, recrystallized and thin sputter deposited Cu films as well as on liquid Cu supported on W/Mo refractory substrates under ambient pressure using Ar, H₂ and CH₄ mixtures. The results indicate that graphene grown on Cu films using cold wall CVD follows a classical two-dimensional nucleation and growth mechanism. The nucleation density decreases and average size of graphene crystallites increases with increasing dilution of the CH₄/H₂ mixture by Ar, decrease in total flow rate and decrease in CH₄:H₂ ratio at a fixed substrate temperature and chamber pressure. Thus, the resulting morphological changes correspond with those that would be expected if the precursor deposition rate was varied at a fixed substrate temperature for physical deposition using thermal evaporation. The evolution of graphene crystallite boundary morphology with decreasing effective C deposition rate indicates the effect of edge diffusion of C atoms along the crystallite boundaries, in addition to H₂ etching, on graphene crystallite shape. The roles of temperature gradient, chamber pressure and rapid thermal heating in C precursor-rich environment on graphene growth morphology on thin sputtered Cu films were explained. The growth mechanisms of graphene on substrates annealed under reducing and non-reducing environment were explained from the scaling functions of graphene island size distribution in the pre-coalescence regime. It is anticipated that applying the pre-coalescence size distribution method presented in this work to other 2D material systems may be useful for elucidating atomistic mechanisms of film growth that are otherwise difficult to obtain. / Dissertation/Thesis / Doctoral Dissertation Materials Science and Engineering 2018

Ce travail de thèse s’est intéressé au dépôt électrochimique de filmsde palladium ultra-minces sur Au(111), à leur caractérisation et àl’insertion d'hydrogène dans ceux-ci. La caractérisation des nanofilmsen milieu sulfurique montre des signatures bien définies, qui évoluentavec l’épaisseur des dépôts. Nous avons pu attribuer à chaque pic uneréaction spécifique, en accord avec les mécanismes de croissancerévélés par les mesures SXRD in situ. La croissance pseudomorphede la 1ère couche se fait avec une première étape d'adsorption, suiviepar un mécanisme de nucléation et croissance. La croissance 3D de latroisième couche démarre avant la fin de la deuxième couchepseudomorphe.L'absorption d'hydrogène dans les nanofilms a été étudiée en milieusulfurique. L’isotherme d’insertion présente un élargissement dudomaine de la solution solide, un plateau avec une pente dans ledomaine bi-phasique et une diminution du taux maximal d'insertion del’hydrogène par rapport au Pd massif. Ce taux diminue avecl’épaisseur mai approche celui de Pd massif au déla delà de 15 MC.Deux éléments ont été considérés pour expliquer le comportement desisothermes: les deux premières couches pseudomorphes sontcontraintes par le support et des « tours » tridimensionnelles relaxéesse forment au delà de la 2ème couche. / This thesis focused on electro-deposition, characterization andhydrogen strorage of ultrathin palladium film over Au(111). Theelectrochemical characterization of the nanofilms in sulphuric mediumshows well-defined features evolving with the deposit thickness. Wecould assign each peak to a specific reaction, in agreement with thegrowth mechanisms revealed by in situ SXRD measurements. Thepseudomorphic growth of the 1st layer firstly undergoes an adsorptionstep, followed by nucleation and growth mechanism, as shown bycurrent transient measurements. 3D growth of the 3rd layer beginsbefore the completion of the second pseudomorphic one.Hydrogen absorption in the nanofilms was studied in sulphuric mediumas well. Isotherms show an enlargement of the solid solution domain, asloppy plateau in the two-phase region, a decrease of maximuminsertion ratio (H/Pd)max compared to bulk Pd. This last valuedecreases with film thickness, approaching bulk Pd beyond about15 ML. Two contributions were considered to explain the isothermbehaviour: the two first Pd layers heavily constraint by the substrateand the 3D “towers like” relaxed structures growing on the secondpseudomorphic Pd layer.

La croissance cristalline de KH2PO4(KDP)and K(H1-xDx)2PO4(DKDP)a été fortement étudiée depuis de nombreuses années. Les propriétés optiques nonlinéaires (conversion de fréquence: doublage pour le KDP et triplage pour le DKDP)et les études fondamentales sur les mécanismes de croissance sont à la base du développement important de la croissance de ces cristaux. Au début des années 90, un fort intérêt s'est porté sur le KDP et DKDP pour les dispositifs optiques à large ouverture pour les applications industrielles de fusion inertielle comme au NAtional Ignition Facility (NIF) aux USA ou pour le laser MégaJoule en France. La dimension de ces optiques (40*40 cm²) nécessite des cristaux géants crûs en solution. Une technique de croissance rapide a été développé par abaissement de température dans un réacteur de 1000L et par une filtration en continu afin d'éviter la nucléation spontanée. Cette méthode est très robuste et fiable pour la croissance rapide de cristaux géants de KDP mais néanmoins montre des limitations inhérentes à cette méthode. En effet, cela mène à des cristaux inhomogènes (défauts, inhomogénéités isotopiques)ce qui est rédhibitoire pour des solutions solides intermédiaires comme le DKDP : la composition en début de croissance peut varier significativement de celle en fin de croissance. Très récemment, des méthodes par circulation de solution en conditions stationnaires ont été développées pour palier à ce problèmeet sont considérées comme les plus pertinentes. C'est pourquoi nous avons développé un système par circulation en conditions stationnaires avec un traitement original de la solution. Dans un premier temps, le système a été testé sur un composé modèle KDP puis dans un deuxième sur le composé utilisé pour l'application DKDP. / Crystal Growth of KH2PO4(KDP)and K(H1-xDx)2PO4(DKDP)has been extensively covered over the years. For decades KDP and DKDP crystals have been grown either for their nonlinear optical properties (frequency conversion : doubling for KDP and tripling for DKDP) or for fundamental studies on crystal growth mechanisms. At the beginning of the 90's, a special interest arose for KDP for large aperture optical elements for laser fusion facilities such as the National Ignition Facility (NIF)in the USA or for the laser MegaJoule in France. The size of such optics(40*40 cm²)requires giant crystals to be grown in solution. A rapid growth technique has been developed based on the temperature lowering of a 1000L solution and its continuous filtration to avoid spurious nucleation. While this method is very robust and fully mature for the rapid growth of giant KDPs it nonetheless suffers from the limitations inherent to the Temperature Lowering Method(TLM).It does not provide stable growth conditions(temperature and supersaturation change).This can lead to inhomogeneous crystals (defects, isotopic inhomogeneity)and this is critical for intermediate of a solid solutions as the DKDP : the composition grown at the beginning can differ significantly from the one crystallizing later. Very early, transport methods growing crystals in stationary conditions, were considered to be "the most pertinent ones". That's why we have developed a growth system in stationary conditions with an original treatment of the solution. The grown compound selected was firstly KDP (model compound)then DKDP (KDP deuterated) for the desired application.

Shape and size control as well as the control of the assembly of nanostructures are current challenges in nano sciences. Focussing on metal nanostructures all of these aspects have been addressed in the frame of the present work. It was possible to develop a new aqueous seeded growth method that produces gold nanoparticles with adjustable diameters over a large range of sizes. The spherical particles obtained show very low polydispersities and a good long term stability. Furthermore it was possible to reveal the growth mechanism of these particles utilizing electron microscopy and optical investigations coupled with theoretical calculations. It was found that there is a formation of small nucleation sites on the surface of the seeds in the beginning of the growth process. These sites then subsequently grow into "blackberry-like" intermediate particles. A final intraparticle ripening step leads to smooth and uniform spherical gold nanoparticles. By correcting the dielectric function of gold for charging and the free mean path effect and taking into account the particle size distribution it was possible to accurately model the optical properties of the gold sols obtained using Mie theory. By controlling the concentration of chloride ions it was possible to influence both the ripening of the "blackberry-like" shaped particles and the morphology of gold nanoparticles. An increased concentration of the chloride ions in the standard citrate reduction procedure leads to larger and elongated particles, whereas the complete removal of the chloride ions made it possible to obtain star shaped, decahedral and \"desert-rose\" shaped particle morphologies. Using the layer-by-layer technique gold nanoparticles of different sizes could be immobilized on glass substrates. The surface-enhanced Raman scattering intensity of these mixed films were about 60% higher than compared to a film made of a single particle size. The optical properties were further investigated by comparing experimentally obtained UV/Vis spectra with generalized Mie theory simulations. Additionally it could be shown that tetrazole and its derivatives are suitable stabilizing agents in the aqueous synthesis of silver nanoparticles. It was found that depending on the tetrazole derivative used the tendencies of the nanoparticles to agglomerate vary significantly. Different agglomeration stages have been investigated by UV/Vis and Raman spectroscopy. The removal of the ligands used and a resulting improvement of the applicability of the silver nanostructures as SERS substrates is still a challenge. In the last part of this work the focus was changed from the optical properties of noble metal nanoparticles to their catalytic properties. Therefore gold and palladium nanoparticles have been successfully immobilized on highly porous zinc oxide aerogels. It was possible to synthesize sponge-, flake-, and ribbon-like zinc oxide gels with high specific surface areas. The facile approach of generating mixed metal oxide/noble metal aerogels is very promising for the preparation of highly selective and highly active heterogenous catalysts. First catalytic investigations of a sponge-like palladium loaded zinc oxide aerogel toward the semi-hydrogenation of acetylene showed very high selectivities of up to 85%.