Dissertations / Theses on the topic 'Elements de transition'

Methanol oxidation to formaldehyde is one of the most important industries in our lives; the reaction occurs on catalyst surface in heterogeneous catalysis. Iron molybdate is the current selective catalyst. However, molybdenum volatilises during methanol oxidation and leaving the catalyst with a low molybdenum ratio, which deactivates the catalyst, a 2.2 Mo: 1Fe iron molybdate catalyst was used instead the stoichiometric catalyst, while yield of formaldehyde cannot be 100%. The goal of this study is to find more selective and more productive catalyst than iron molybdate catalyst, the first step is to find another transition element as selective as molybdenum, because molybdenum is the selective part, and iron is the active part, the resulting iron molybdate catalyst is a selective catalyst to formaldehyde near molybdenum and active near iron. Experimentally, catalysts were prepared using co-precipitation method, however, some doped catalysts were papered by incipient wetness impregnation, also sol-immobilization was used to prepare nano-gold particles on the surfaces of few supports. Catalysts characterizations were carried out within several techniques for the surface analysis (XPS) and bulk analysis (XRD), also the surface area was measured by BET equipment. Raman too was used in this study, while micro-reactor was the reactor to determine selectivity and activity of each catalyst. When molybdenum replaced by vanadium, the catalyst yielded 100% formaldehyde at 200 oC; moreover, tungsten was selective. Likewise, iron was replaced by other active metals such as manganese, copper and bismuth, which are active. Nano-gold improved activity when doped on molybdenum oxide and iron molybdate supports.

Within the family of transition metal oxochlorides/bromides containing lone pair elements, the transition metal cations often adopt a low-dimensional arrangement such as 2D layers, 1D chains or 0D clusters. The reduced dimensionality is attributed to the presence of stereochemically active lone pairs which are positioned in the non-bonding orbital and will not participate in bond formation and instead act as structural spacers that help to separate coordination polyhedra around transition metal cations from forming three dimensional networks. On the other hand, the chlorine and bromine ions also play an important role to open up the crystal structure because of their low coordination number. However, fluorine has been rarely used in this concept due to the difficulties in synthesis. This thesis is focused on finding new compounds in the M-L-O-F system (M = transition metal cation, L= p-block lone pair elements such as Te4+, Se4+, or Sb3+) in order to study the structural character of fluorine. Hydrothermal reactions have been adopted instead of conventional chemical transport reactions that are commonly used for synthesizing compounds in the M-L-O-(Cl, Br) family. A total of 8 new transition metal oxofluorides containing lone pair elements have been synthesized and their structures have been determined via single crystal X-ray diffraction. Bond valence sum calculations are used to distinguish in between fluorine and oxygen due to their very similar X-ray scattering factors.

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 4: Manuscript. Paper 5: Manuscript.

To understand the mechanism of a pressure-induced structural phase transition, it is important to know which bonding changes lead to the stabilization of the new structure. A useful approach in this regard is the quantum chemical topology, which provides a large variety of indicators for the characterization of interatomic interactions. In this work, a number of topological indicators are used to analyze the bonding changes during the pressure-induced phase transition from the cubic diamond (cd) to the β-Sn-type structure of the elements of the 14th group of the periodic table. The ability of these indicators to reflect the presence of the cd → β-Sn transition in experiment for Si, Ge and Sn and its absence for carbon is investigated. Furthermore, the effect of pressure on the interatomic interactions in the cd- and β-Sn-type structures is examined. It is observed that the energy change along the cd → β-Sn transformation pathway correlates with the evolution of certain parameters of the electron density and the electron localizability indicator (ELI-D). Accordingly, criteria of structural stability were formulated based on characteristics of interatomic interactions. These results can serve as guidelines for the investigation of other solid-state phase transformations by the topological methods.

This thesis describes the synthesis, structural and reaction chemistry of a number of novel gallium and indium containing species, including metal complexes featuring previously unreported gallium and indium ligand systems. The synthesis, spectroscopic and structural characterisation of the asymmetric haloindyl (n5-C5R5)Fe(CO)2In(Mes*)Br (R = H, Me) and halogallyl Cp*Fe(CO)2Ga(Mes)I complexes are reported herein, with Cp*Fe(CO)2ln(Mes*)Br representing the first example of structurally characterised asymmetric bromoindyl complex. Cp*Fe(CO)2In(Mes*)Br is a versatile substrate for the synthesis of asymmetric indyl complexes via substitution chemistry, thereby allowing the synthesis and characterisation of Cp*Fe(CO)2In(Mes*)ERn (ERn = OC6H4/Bu-4, SPh). The synthesis, spectroscopic and structural characterisation of the dihalogallyl complex Cp*Fe(dppe)Gal2 is also reported this synthesis was accomplished via photolytic displacement of the carbonyl ligands in Cp*Fe(CO)2Gal2 2 by dppe 1,2-bis(diphenylphosphino)ethane. Cp*Fe(dppe)Gal2 has proven to be versatile reagent in the generation of the asymmetric halogallyl species via substitution chemistry with retention of the iron-gallium bond, thereby allowing the synthesis and characterisation of Cp*Fe(dppe)Ga(Mes)I. The diiodogallyl complex has also been implicated in the synthesis of the first structurally characterised base-free cationic gallylene complex Cp*Fe(CO)2Gal + BAr/4 bearing a terminally bound Gal ligand which is valence isoelectronic with CO and N2. Investigations into the synthesis of molybdenum- and ruthenium-phosphine containing systems are also reported herein. Insertion of 'Gal' into metal-halogen bonds has proved to a viable synthetic route, for example in the 7 7 formation of (n-C7H7)Mo(CO)2Gal2 2. Further reaction of (n-C7H7)Mo(CO)2Gal2 2 with dppe, however, has been shown to yield (r - C7H7)Mo(CO)2Gai2 2(u-dppe). Investigation of 'Gal' insertion reactions involving ruthenium-halogen bonds have shown to yield the tetraiodogallate species CpRu(PPh3)2(u-I)Gal3 and CpRu(dppe) Gal4. The synthesis, spectroscopic and structural characterisation of the bridging halo-indanediyl complexes Cp*Fe(CO)2 2lnX (X = Br, I) are reported. The bromo-substituted species has proven to be a useful precursor in the synthesis of the cationic trimetallic system {Cp*Fe(CO)2}2(-In) + BA/4, the formation of which has been shown to be strongly dependant on the nature of abstracting agent and on the identity of the halide. Reactivity studies of {Cp*Fe(CO)2}2( i-E) + (E = Ga, In) involving the addition of nucleophiles has allowed the synthesis and characterisation of the three-coordinate cationic complexes {Cp*Fe(CO)2}2(u-E L) + BAr4 (E = Ga, In L = thf, 4-picoline, PPh3). The syntheses and metallation of sterically bulky carbazol-9-yl ligands derived from 1,8-diaryl-3,6-dimethylcarbazole are described herein. Furthermore, the synthesis and structural characterisation of the amidogally l complexes (1,8-diphenyl-3,6-dimethylcarbazol-9-yl)gallium dichloride and (1,8-dimesity 1-3,6-dimethylcarbazol-9-yl)gallium dichloride are reported by salt metathesis reactions involving gallium trichloride. Investigations of subsequent reduction chemistry using potassium metal yielded the potassium salt 1,8-diphenyl-3,6-dimethylcarbazol-9-yl-potassium. Quantum chemical investigations using Density Functional Theory have been explored to probe the electronic structure in the novel bond types. In particular, studies targeted factors affecting the degree of 7c back-bonding (and hence multiple bond character) within the TM-ER bond.

Dans les alumines, les proprietes physiques peuvent etre modifiees par le dopage de ce materiau avec des elements de transition. La comprehension de l'influence des dopants sur la structure reelle de ces materiaux est donc un enjeu fondamental pour mieux controler leurs proprietes physiques. Deux facteurs conditionnent fortement l'influence du dopage. Le premier est l'interaction entre le dopant et le reseau cristallographique au sein des grains de corindon. Le second facteur concerne la localisation et la repartition des dopants dans les ceramiques alumines. L'objectif de ce travail etait d'analyser quantitativement l'influence du dopage par les ions de transition fer, chrome et manganese sur la structure reelle des ceramiques d'alumine. L'analyse de l'environnement local autour des ions de transitions et de la microstructure des alumines dopees a necessite l'utilisation combinee de techniques cristallographiques et spectroscopiques telles que la diffraction de rayons x (drx), la resonance paramagnetique electronique (rpe) et la spectroscopie optique en reflectance diffuse (drs). L'analyse quantitative rpe a ete effectuee grace au developpement d'un code de simulation des spectres rpe de poudre. Ce traitement a permis d'affiner les parametres de l'hamiltonien de spin pour des ions en champ cristallin intermediaire et de determiner l'environnement local des ions fe#3#+, cr#3#+ et mn#2#+ substitues a al#3#+ dans le corindon. L'affinement rietveld des diffractogrammes de rayons x et l'analyse des spectres de drs d'alumines dopees nous ont permis de mieux quantifier l'influence de procedes de synthese sur la localisation des dopants chrome et manganese au sein des ceramiques: le chrome est majoritairement insere dans le grain de corindon alors que le manganese est precipite sous la forme d'une phase spinelle aux joints de grain des ceramiques. Nous avons montre egalement dans cette etude la presence de contraintes interne residuelles dans les ceramiques quelles que soient les conditions de synthese

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Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)
Tese (Doutoramento)
IPEN/T
Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP
FAPESP:93/03991-2

This project fOllows on from work carried out at Imperial College London to develop aluminium alloys with enhanced superplastic properties for automotive applications. In this prior work it was found that AI-Mg-Zralloys exhibited superior superplastic properties to other 5xxx alloys including the widely used superplastic variant of5083. The aim of the current work was to fully characterise the AI-Mg-:Zr alloys at each stage of the production route in order to optimise the processing conditions and exploit alternative production routes. It is generally accepted that there are two main requirements for superplasticity in ..J aluminium alloys. The first is the addition of a solute element to reduce the high stacking fault energy associated with pure aluminium energy and hence favour recrystallisation over recovery during thermomechanical processing. The second is a homogeneous dispersion of fine, thermally stable second phase particles to control grain growth via Zener pinning. For this reason the characterisation of the AI-Mg-Zr alloys focussed on the individual roles of the solute (magnesium) and transition element (zirconium) additions. In order to' better understand the role of zirconium a binary AI-Zr alloy was also prepared and investigated. Furthermore, some preliminary casting trials' where zirconium was either partially or totally replaced by vanadium were conducted. The purpose of these experiments was to' produce a more economical and practical grain control methodology. The implication of the work presented in this thesis is that although it is well recognised that solute elements together with high amounts of stored energy (as a result of complex thermomechanical processing) are both necessary to achieve superplasticitY in aluminium alloys, under certain circumstances the presence of either may be sufficient Furthermore, useful conclusions about the pinning capability of the second phase particles were also drawn. The findings described in this thesis can have a significant impact on the design not of)ly of the processing route of the AIMg- Zr alloys but also of other established superplastic aluminium alloys.

In this work the effect of isolated surface roughness on the behaviour of a hypersonic boundary layer is investigated, with a particular focus on the effect of the three-dimensional roughness shape on the instability of the roughness wake and the subsequent transition process. The analysis is performed computationally using direct numerical simulations, which solve the compressible Navier-Stokes equations, and a new code, developed in the scope of the current work, to analyse the linear stability of these equations. The full three-stage roughness-induced transition process has been investigated: firstly, the receptivity process and generation of boundary layer instabilities from freestream disturbances; secondly, the generation of a roughness wake and its initial linear instability; and finally the non-linear breakdown to turbulence of the roughness wake. In particular the effect of the three-dimensional roughness shape on these processes has been studied, looking at the roughness height, frontal profile, planform shape and upward/downward ramps. Also the effect of freestream disturbance amplitude andwall cooling has been investigated. It has been found that the roughness height and frontal profile have a large influence on the stability characteristics of the resulting wake and the subsequent transition. The roughness planform shape has a marginal effect, although cylindrical and diamond-shaped elements yield more unstable wakes than a square roughness element. Bi-local stability analysis can be used in most cases to predict the most unstable wake mode, but it under-predicts the instability growth rates due to non-parallel effects. The roughness shape has been observed to affect the transition onset location. The criteria commonly used to predict roughness-induced transition, do not take into account the three-dimensional shape, and an alternative transition prediction, based on the amplitude of the roughness-induced streamwise streak, is considered.

Cette thèse vise à mettre en évidence les limites du contrôle passif en utilisant des éléments de rugosité miniaturisés. La topologie des écoulements induite par la présence d’une rugosité cylindrique et des générateurs de tourbillons miniaturisés a été étudiée pour analyser la dynamique pour des temps court et long. Différentes bifurcations supercritiques ont été examinées au moyen d’une analyse de stabilité globale. La bifurcation souscritique est déclenchée par des mécanismes de croissance transitoire de l'énergie ou par la réceptivité de modes globaux stables. Des structures deforçage optimal 3D sont extraites pour comprendre la distribution spatiale liée à la fréquence de résonance du système. La simulation numérique directe perturbée révèle le rôle central du mode global le moins stable dans les instationnarités non-linéaires observées. Une analyse détaillée des structures tourbillonnaires montre qu’elles sont principalement liées aux mécanismes linéaires sous-jacents. La principale caractéristique globale du mode propre est liée à la présence d’une zone de séparation en aval de la rugosité cylindrique. En utilisant des générateurs de tourbillon miniaturisés, cette zone de séparation est fortement diminuée et aucun mode global isolé est alors présent. La dynamique de l’écoulement se révèle être conduit non seulement par le nombre de Reynolds de rugosité et par son rapport d'aspect géométrique, mais aussi par le rapport entre la hauteur de la rugosité et l’epaisseur de couche limite
This thesis aims at highlighting the limits of passive control by usingminiaturized roughness elements. The flow topology induced by the presence ofcylindrical roughness and miniaturized vertex generators has been studied to uncover asymptotic and short time dynamics. Supercritical bifurcations has been investigated by means of global stability analysis. Subcritical bifurcation are induced by transient growth of the energy or receptivity of stable global modes. 3D optimal forcing structures are extracted to figure out the spatial distribution linked to the resonant pulsation. Perturbed direct numerical simulation reveals the pivotal role of the less steady global mode in the non-linear unsteadiness. A detailed analysis of the flowstructures is provided and linked to the involved linear mechanisms. Global feature of the eigenmode are linked to the presence of the separation zone behind the cylindrical roughness. By using miniaturized vortex generators the separation zone is suppressed and no isolated global modes are present. The flow dynamics turns out to be driven not only by roughness Reynolds number and geometrical aspect ratio but also by the ratio between the roughness height and the boundary layer thickens

Les travaux décrits dans ce mémoire portent sur le calcul de la structure électronique de clusters homo- ou hétéro-nucléaires de métaux du groupe 11, afin d’en interpréter leur stabilité, leur structure et éventuellement leurs propriétés. Nous nous sommes tout d’abord intéressés au fait que, contrairement à leurs homologues de l’or et de l’argent, les superatomes de cuivre sont très rares. Nos calculs montrent que ces derniers sont plus stables que les superatomes d’argent. Néanmoins, les méthodes de synthèse de superatomes par réduction de complexes de Cu(I) par le borohydrure conduisent préférentiellement à la formation de polyhydrures de Cu(I) en raison de leur grande stabilité. Nous nous sommes de plus intéressés à la stabilité de clusters contenant un cœur tétraédrique M16, analogue à celui contenu dans le cluster emblématique [Au₂₀]. Notre étude des clusters organométalliques à 20 électrons. [M₁₆Ni₂₄(CO)₄₀]⁴⁻ (M = groupe 11) indiquent que les quatre entités périphériques Ni₆(CO)₁₀ font partie intégrante du superatome, suggérant que [M₁₆]⁴⁻ n’est pas viable. Des calculs sur plusieurs séries de systèmes homo- ou hétéro-nucléaires nus proposent de contourner cet écueil soit en réduisant le nombre d’électrons à 18, soit en incorporant un élément encapsulé au centre de l’entité tétraédrique. Dans une autre étude, nous avons exploré la possibilité de dopage du cluster icosaèdrique à 18 électrons [WAu₁₂] par des atomes de platine (donneurs de 0 électron), soit [WAu₁₂Pdₓ] (x = 1-4). Le calcul indique que certains isomères sont viables et présentent un large spectre d’absorption UV-vis leur conférant des applications potentielles. Enfin, nous avons étudié la structure électronique de clusters organométalliques apparemment très déficitaires en électrons, [Cu₃Zn₄Cp*5] et [Cu₂Zn₅Cp*₅]⁺ et montré que ce déficit n’est aussi important qu’il n’apparaît
The work described in this manuscript concerns electronic structure calculations of homo- and hetero-nuclear clusters made of group 11 metals, in order to rationalize their stability, structure and in some cases properties. We have first looked at the fact that copper superatoms are very scarce, contrarily to their gold and silver counterparts. Our calculations indicate that copper superatoms are more stable than silver superatoms. However, the synthetic process based on the reduction of Cu(I) complexes by borohydride leads preferentially to the formation of very stable Cu(I) polyhydrides. On the other hand, we have looked at the stability of clusters containing a tetrahedral M₁₆ core similar to the one contained in the emblematic [Au₂₀] cluster. Our investigation of the 20-electron organometallic clusters [M₁₆Ni₂₄(CO)₄₀]⁴⁻ (M = group 11) showed that the four peripheral Ni₆(CO)₁₀ units are full part of the superatom entity, suggesting that the [M₁₆]⁴⁻ entity is not viable. Calculations on several homo- and hetero-nuclear series of bare species indicate that this instability can be avoided either in reducing the electron count to 18, or in incorporating a supplementary element in cluster center. In another investigation, we explored the possibility of doping the icosahedral 18-electron [WAu₁₂] cluster by 0-electron donor platinum atoms, namely [WAu₁₂Pdₓ] (x = 1-4). Calculations indicate that some isomers are stable and have a large spectrum of UV-vis absorption, providing them potential applications. Finally, we have investigated the electronic structure of organometallic clusters, [Cu₃Zn₄Cp*5] and [Cu₂Zn₅Cp*₅]⁺, which are apparently extremely electron-deficient and showed that this deficiency is not as large as it appears

This habilitation thesis describes the syntheses and characterizations of mostly new metal-rich cluster compounds of early transition elements. It is devided into 4 parts. The first part describes the syntheses and solid-state X-ray structures of a total of 14 mixed-halide (iodide-chloride) zirconium cluster phases, which crystallize in 9 different (some novel) structure types. All these cluster phases contain octahedral Zr6Z-units that are centered by an interstitial atom Z. Phase widths and relations between the differend structure types are discussed. The second part deals with molecular (soluble) zirconium cluster phases which are excised from solid-state precursors. It is shown that liquid mixtures of 1,3-dialkylimidazolium bromide and AlBr3 as well as molten 18-crown-6 are useful solvents for molecular zirconium cluster compounds. The syntheses and single crystal X-ray structures of 3 new materials are detailed. Furthermore, from the reaction of an iron containing Zr-cluster phase with KSCN crystals have been obtained from which a single crystal X-ray structure analysis shows that they contain a high symmetry Fe-cubane cluster. The third part of this thesis is concerned with theoretical investigations (Extended-Hckel band structure calculations) of a series of monoclinic phases which contain double-octahedral chains of rare earth metal clusters of the general formula RE3I3Z (RE = rare-earth metal atoms). The results of the electronic band structure calculations show that the observed structural variations within this series depend largely on the difference of the orbital energies of the rare earth metals and the interstitials. In the last part the phase pure synthesis of Nb21S8 and measurements of physical properties (electrical conductivity and magnetical susceptibility) as well as results from electronic band structure calculations (LMTO) are described. The property measurements show that this metal rich sulfide becomes superconducting below 3.7(2) K. The results from the electronic band structure calculations indicate the existence of an arrangement of electronic levels ("fingerprint") which favors the formation of superconducting Cooper pairs.

I investigated changes in biogeochemical cycling during the Precambrian-Cambrian transition on the Yangtze Platform in South China by analyzing about 350 predominantly black shale samples from several sedimentary successions deposited during the interval from the Late Cryogenian to the Lower Cambrian. I focused on redox-sensitive trace-metal concentrations in these sediments, whereby special attention is paid to molybdenum, vanadium and uranium to try to pinpoint the onset of increasing atmospheric oxygen levels and the transition from widespread euxinia to a pervasively oxygenated deep ocean. The measurements have been carried out using X-ray fluorescence analysis (XRF) and inductively coupled plasma mass spectrometry (ICP-MS). Besides that, total organic carbon and total sulphur contents have been measured for all samples. This approach is completed by iron speciation analysis which is considered to be a reliable redox proxy. I also conducted extended literature research on trace-metal enrichments in anoxic sediments throughout Earth history as well as a major review (which will be included as an extended introduction) on all currently available lines of evidence for a major Neoproterozoic Oxygenation Event, including carbon, sulphur, strontium, molybdenum and chromium isotope studies. I show that the well preserved sedimentary succession from the Precambrian-Cambrian transition on the Yangtze Platform might represent a unique archive of ancient geochemical conditions on the Earth’s surface, based on a significant increase of Mo, V and U enrichment in black shales across the Pc-C boundary. The evidence for predominantly anoxic-ferruginous and even intermittently euxinic conditions in the water column across the Pc-C boundary and significant regional variations in geochemical parameters unravel complex interactions between ocean chemistry, platformal configuration and paleontology.

L'élucidation du mécanisme de formation de divers verres naturels peut aider à comprendre la dynamique de la Terre car ces matériaux retiennent l'état de formation dans leurs structures. La spectroscopie XAFS appliquée aux matériaux amorphes issue des météorites fournit des informations sur les changements structuraux locaux des éléments présents. En effectuant des analyses dans la phase de verre des météorites et dans les verres issus de l'impact météoritiques des informations sur les conditions de formation des ces matériaux peuvent être obtenues. Dans cette thèse, l'analyse de la structure locale d’éléments de transition (Ti, Zr) dans les minéraux et les verres naturels a été réalisée par la XAFS. Des expériences de chauffage au laser à haute température et de flux gazeux sur zircon ont également été effectuées pour observer les changements structuraux à haute température dans les phases cristallines qui accompagnent le verre météoritique. La structure locale de ces matériaux est semblable à celle du Zr dans les sédiments de la limite Crétacé – Paléogène (K-Pg). Ce dernier ne montre aucune influence de diagenèse ou d'altération, ce qui suggère que les sédiments Cependant, l'abondance de Zr dans les sédiments n'est pas toujours suffisante pour obtenir des informations fiables sur la structure locale du Zr. Nous nous sommes ainsi intéressés à la structure local du Ti, qui appartient au même groupe que le Zr. Les conditions d'impact des météorites sur Mars et du passage des météorites à travers l'atmosphère peuvent être estimées par l'analyse de la structure locale de Zr dans la partie vitrifiée des météorites. L'analyse de la structure locale du Zr dans la météorite martienne de Tissint montre la croûte de fusion de cette météorite est semblable à celle des tectites et du verre d'impact qui les accompagne, alors que la partie intérieure vitreuse de Tissint est similaire à celle de la baddeleyite et de la fulgurite. Cette dernière aurait subi une vitrification pendant le passage à travers l'atmosphère. Afin de confirmer le changement structurel de Zr à haute température, nous avons effectué des expériences de chauffage du zircon, ainsi qu’une expérience de diffraction des rayons X sur poudre pendant le chauffage. L'analyse chimique dans le MEB a montré l'évaporation de la composante SiO2. Nous avons aussi obtenu un nouveau digramme de phase ZrO2-SiO2 dans lequel la région d’immiscibilité liquide proposée auparavant n’a pas pu être confirmée
Elucidation of the formation mechanism of various natural glasses can help to understand the dynamics of the Earth because these materials record the formation condition in their structures. XAFS applied to natural meteorite-related glasses provides information about the local structural changes of elements therein. By performing high-accuracy XAFS analyses for trace elements and their local structure in sedimentary rocks, in the glass phase of meteorites and in the meteorite impact-related glasses such as tektite and impact glass, information on the formation condition of these materials can be obtained. We have performed local structure analysis of IVb transition metal elements (Ti, Zr) in natural minerals and glasses K-edge XANES and EXAFS as well as high-temperature laser heating and gas flow experiments on zircon, to observe structural changes at high temperature in the crystalline phases that accompany the meteoric glass. The local structure of these materials is similar to that of of Zr in K-Pg sediments, which do not show any influence from diagenesis and weathering, suggesting that K-Pg sediments maintain the thermal quenching history of the meteorite impact. However, the low abundance of Zr in sediments prompted us to study Ti, which belongs to the same group as Zr and is much more abundant. Combination of the information about the local structures of Ti and Zr allows a higher reliability for the estimation of meteorite impact event. The local structure of Zr in the fusion crust and inner glassy part of the Martian meteorite of Tissint are similar, respectively, to that in tektite and accompanying impact glass and to that in baddeleyite and fulgurite. The latter was likely vitrified part during the passage through the atmosphere. It should be possible to estimate the conditions of meteorite impact on Mars and the passage of meteorites through the Earth’s atmosphere through the analysis of the local structure of Zr in the glassy part of meteorites. In order to confirm this hypothesis, we performed heating experiments of zircon with in-situ powder X-ray diffraction experiment was performed for ZrSiO4 and ZrO2. SEM analysis confirmed evaporation of the SiO2 component. We have obtained a new ZrO2-SiO2 phase diagram where the liquid-immiscible region proposed earlier does not appear

ETUDE DE TROIS ELEMENTS DE TRANSITION DANS LES SEMICONDUCTEURS PAR PHOTOCONDUCTIVITE. 1)GaAs: NOUS PROPOSONS UN NIVEAU SCHEMA D'ENERGIE. UN NIVEAU SIMPLE ACCEPTEUR (V)**(O)/(V)**(-) A E::(C)-0,16 EV ET UN NIVEAU DOUBLE ACCEPTEUR (V)-/(V), A Ec- 0,65 eV. 2)Si:Au NOUS CONFIRMONS LES POSITIONS RESPECTIVES DES NIVEAUX DONNEUR ET ACCEPTEUR LIES A L'OR A Ev + 0,35 eV ET Ev + 0,63 eV. 3)Si:Pt LES EXPERIENCES REALISEES ONT MIS EN EVIDENCE LE NIVEAU ACCEPTEUR (o/-) DU PLATINE SUBSTITUTIONNEL A UNE ENERGIE DE 0,915 eV PAR RAPPORT A LA BANDE DE VALENCE

The President's Emergency Plan for AIDS Relief (PEPFAR) has operated in South Africa since 2003 and invested over $4.2 billion towards South Africa's HIV/AIDS response. South Africa was the largest recipient of PEPFAR funding until 2012, when it was announced PEPFAR's budget would decrease from $550 million to $250 million by 2017. During this time agreements (Partnership Framework and Partnership Framework Implementation Plan (PFIP) were signed, laying out PEPFAR's plans to continue to support the South African National Strategic Plan for HIV and AIDS and improve the effectiveness of the HIV/AIDS response through health systems strengthening. The PFIP also outlined the initiatives which fell under the umbrella of "transition": The transition of PEPFAR's reduced financial support for HIV and AIDS in South Africa and the shift in focus from direct service delivery to health systems strengthening and capacity building. The following case study is a description of the Transition Management experience in South Africa in the PFIF context. The primary interest of this research is to understand the extent to which the South African National HIV/AIDS Programme is ready to take up full local ownership of the gains supported by the PEPFAR partnership. The research will also explore lessons concerning the transition to local ownership and address possible Public-Private Partnership engagement, in order to sustain gains and develop efficiencies within the programme. In order to respond the objectives of the research, this study will be qualitative in nature, more specifically it will take a developmental approach. This research will also aim to generate lessons concerning the transition to local ownership, so as to guide implementation strategies throughout the transition process, and inform similar processes elsewhere.
Mini Dissertation (MBA)--University of Pretoria, 2017.
ms2017
Gordon Institute of Business Science (GIBS)
MBA
Unrestricted

Secondary Al-Si-Cu-Mg based foundry alloys are widely used in automotive industry to particularly produce powertrain cast components mainly due to their good ratio between weight and mechanical properties, and excellent casting characteristics. Presence of impurity elements, such as Fe, Mn, Cr, Ti, V and Zr, in secondary Al-Si alloys is one of the critical issues since these elements tend to reduce alloy mechanical properties. There is an ongoing effort to control the formation of intermetallic phases containing transition metals, during alloy solidification. Although phases formation involving these transition metal impurities in non-grain-refined Al-Si alloys is well documented in the literature, the role of grain refinement in microstructural evolution of secondary Al-Si-Cu-Mg alloys needs further experimental investigations since chemical grain refinement is one of the critical melt treatment operations in foundries. The primary aim of this PhD work is thus defined to characterize the formation of intermetallic phases containing transition metals in secondary Al-7Si-3Cu-0.3Mg alloy before and after grain refinement by different master alloys and contribute to the understanding of the mechanisms underlying the microstructural changes occurring with the addition of grain refiner. Another critical issue related to Al-Si-Cu-Mg alloys is their limited thermal stability at temperatures above 200 oC. The operating temperature in engine combustion chamber is reported to often exceed 200 oC during service. Moreover, a further increase of operating temperature is anticipated due to the expected engine power enhancement in near future, which indicates the necessity for the development of a new creep-resistant Al alloys. Deliberate addition of transition metals is believed to yield a new heat-resistant alloy by promoting the formation of thermally stable dispersoids inside α-Al grains. This study thus also attempted to investigate the effect of adding transition metals Zr, V and Ni on the solidification processing, microstructural evolution and room/high-temperature tensile properties of secondary Al-7Si-3Cu-0.3Mg alloy, one of the most used alloys in automotive engine manufacturing. The influence of transition metal impurities on microstructural evolution of secondary Al-7Si-3Cu-0.3Mg alloy was investigated before and after chemical treatment with different master alloys: Al-10Sr, Al-5Ti-1B, Al-10Ti and Al-5B. The Al-10Zr, Al-10V and Al-25Ni master alloys were used for the experimental investigations of the effects of deliberate additions of transition metals on the solidification path, microstructure and mechanical properties of secondary Al-7Si-3Cu-0.3Mg alloy. Solidification path of the alloys was characterized by the traditional thermal analysis technique and differential scanning calorimetry (DSC). Optical microscope (OM), scanning electron microscope (SEM) equipped with energy-dispersive (EDS), wavelength-dispersive spectrometers (WDS) and electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM) equipped with EDS were used to characterize the type, morphology and distribution of the phases precipitated during solidification and heat treatment of the studied alloys. The static tensile properties of the alloys were characterized at room (20 oC) and high temperatures (200 and 300 ºC). Experimental findings indicate that the Sr-modification and grain refinement of secondary Al-7Si-3Cu-0.3Mg alloy with Al-Ti-B can be enough effective despite the presence of transition metal impurities in the material and the variation of pouring temperature. However, the V and Zr (~100 ppm each) available in secondary Al-7Si-3Cu-0.3Mg alloy tended to promote the precipitation of harmful, primary AlSiTi intermetallics during solidification of grain-refined alloy. This implies that more effective optimization of grain refiner addition level in secondary Al foundry alloys can be achieved by considering the role of transition metal impurities, Ti, V and Zr, since the formation of primary AlSiTi particles causes (1) the depletion of Ti needed for effective α-Al grains growth restriction and (2) the formation of casting defects, such as shrinkage, due to their flaky morphology. Iron available in secondary Al-7Si-3Cu-0.3Mg alloy as impurity only formed more desirable α-Al15(FeMn)3Si2 phase in non-grain refined state. After grain refinement by Al-5Ti-1B, Fe was also involved in the formation of more deleterious β-Al5FeSi phase. The TiB2 particles acted as nucleation site for β-Al5FeSi phase. Both higher cooling rate and higher Al-5Ti-1B addition levels tended to promote the formation of deleterious β-Al5FeSi at the expense of α-Al15(FeMn)3Si2 in the alloy refined by Al-5Ti-1B. This implies that rather than the ratio between Mn and Fe, the nucleation kinetics of Fe-rich intermetallics play a decisive role in the selection of competing α-Al15(FeMn)3Si2 and β-Al5FeSi intermetallic phases for the precipitation during alloy solidification. Moreover, grain refinement of secondary Al-7Si-3Cu-0.3Mg alloy by Al-5B showed comparable performance to that of Al-5Ti-1B master alloy, however, without any deleterious influence on the precipitation sequence of Fe-rich phases, i.e. deleterious β-Al5FeSi reaction remained unfavourable during alloy solidification. Experimental findings from the investigations of the effect of deliberate Zr and V addition revealed that Zr and V addition can induce the grain refinement of secondary Al-7Si-3Cu-0.3Mg alloy. While Zr addition yielded the formation of pro-peritectic Zr-rich particles, which are found to nucleate primary α-Al at low undercooling, the effect of adding V can be characterized by the enhancement of the degree of constitutional undercooling. Combined Zr and V addition showed more effective grain refinement level than their individual additions. Majority of both Zr and V added to the alloy were retained inside α-Al matrix during solidification. As a result, limited amounts of Zr and V were rejected to the interdendritic liquid by the growing α-Al dendrites, then forming small-sized and rarely distributed intermetallics. Owing to its low solid solubility in α-Al, nickel available as impurity (~ 200 ppm) or after deliberate addition (0.25 wt.%) in secondary Al-7Si-3Cu-0.3Mg alloy was mainly bound to interdendritic, insoluble intermetallics, such as Al6Cu3Ni and Al9(FeCu)Ni phases. The presence of ~ 200 ppm Ni was sufficient to diminish to a certain extent the precipitation hardening effect of Cu. Interdendritic Zr/V/Ni-rich phases remained undissolved into the α-Al matrix during solution heat treatment. Therefore, the supersaturated transition metals in α-Al solid solution obtained during solidification was only involved in the solid-state precipitation occurring during heat treatment. Unlike Cu/Mg-rich strengthening precipitates that commonly form during aging, the Zr/V-rich precipitates tended to form during solution heat treatment. Other transition metals, such as Mn, Fe, Cr and Ti, which were present as impurities in secondary Al-7Si-3Cu-0.3Mg alloy significantly promoted the formation of nano-sized Zr/V-rich precipitates inside α-Al grains. These thermally more stable precipitates, including novel α-Al(MnVFe)Si, were credited for the enhanced high-temperature strength properties of Al-7Si-3Cu-0.3Mg alloy by ~ 20 %.
Secondary Al-Si-Cu-Mg based foundry alloys are widely used in automotive industry to particularly produce powertrain cast components mainly due to their good ratio between weight and mechanical properties, and excellent casting characteristics. Presence of impurity elements, such as Fe, Mn, Cr, Ti, V and Zr, in secondary Al-Si alloys is one of the critical issues since these elements tend to reduce alloy mechanical properties. There is an ongoing effort to control the formation of intermetallic phases containing transition metals, during alloy solidification. Although phases formation involving these transition metal impurities in non-grain-refined Al-Si alloys is well documented in the literature, the role of grain refinement in microstructural evolution of secondary Al-Si-Cu-Mg alloys needs further experimental investigations since chemical grain refinement is one of the critical melt treatment operations in foundries. The primary aim of this PhD work is thus defined to characterize the formation of intermetallic phases containing transition metals in secondary Al-7Si-3Cu-0.3Mg alloy before and after grain refinement by different master alloys and contribute to the understanding of the mechanisms underlying the microstructural changes occurring with the addition of grain refiner. Another critical issue related to Al-Si-Cu-Mg alloys is their limited thermal stability at temperatures above 200 oC. The operating temperature in engine combustion chamber is reported to often exceed 200 oC during service. Moreover, a further increase of operating temperature is anticipated due to the expected engine power enhancement in near future, which indicates the necessity for the development of a new creep-resistant Al alloys. Deliberate addition of transition metals is believed to yield a new heat-resistant alloy by promoting the formation of thermally stable dispersoids inside α-Al grains. This study thus also attempted to investigate the effect of adding transition metals Zr, V and Ni on the solidification processing, microstructural evolution and room/high-temperature tensile properties of secondary Al-7Si-3Cu-0.3Mg alloy, one of the most used alloys in automotive engine manufacturing. The influence of transition metal impurities on microstructural evolution of secondary Al-7Si-3Cu-0.3Mg alloy was investigated before and after chemical treatment with different master alloys: Al-10Sr, Al-5Ti-1B, Al-10Ti and Al-5B. The Al-10Zr, Al-10V and Al-25Ni master alloys were used for the experimental investigations of the effects of deliberate additions of transition metals on the solidification path, microstructure and mechanical properties of secondary Al-7Si-3Cu-0.3Mg alloy. Solidification path of the alloys was characterized by the traditional thermal analysis technique and differential scanning calorimetry (DSC). Optical microscope (OM), scanning electron microscope (SEM) equipped with energy-dispersive (EDS), wavelength-dispersive spectrometers (WDS) and electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM) equipped with EDS were used to characterize the type, morphology and distribution of the phases precipitated during solidification and heat treatment of the studied alloys. The static tensile properties of the alloys were characterized at room (20 oC) and high temperatures (200 and 300 ºC). Experimental findings indicate that the Sr-modification and grain refinement of secondary Al-7Si-3Cu-0.3Mg alloy with Al-Ti-B can be enough effective despite the presence of transition metal impurities in the material and the variation of pouring temperature. However, the V and Zr (~100 ppm each) available in secondary Al-7Si-3Cu-0.3Mg alloy tended to promote the precipitation of harmful, primary AlSiTi intermetallics during solidification of grain-refined alloy. This implies that more effective optimization of grain refiner addition level in secondary Al foundry alloys can be achieved by considering the role of transition metal impurities, Ti, V and Zr, since the formation of primary AlSiTi particles causes (1) the depletion of Ti needed for effective α-Al grains growth restriction and (2) the formation of casting defects, such as shrinkage, due to their flaky morphology. Iron available in secondary Al-7Si-3Cu-0.3Mg alloy as impurity only formed more desirable α-Al15(FeMn)3Si2 phase in non-grain refined state. After grain refinement by Al-5Ti-1B, Fe was also involved in the formation of more deleterious β-Al5FeSi phase. The TiB2 particles acted as nucleation site for β-Al5FeSi phase. Both higher cooling rate and higher Al-5Ti-1B addition levels tended to promote the formation of deleterious β-Al5FeSi at the expense of α-Al15(FeMn)3Si2 in the alloy refined by Al-5Ti-1B. This implies that rather than the ratio between Mn and Fe, the nucleation kinetics of Fe-rich intermetallics play a decisive role in the selection of competing α-Al15(FeMn)3Si2 and β-Al5FeSi intermetallic phases for the precipitation during alloy solidification. Moreover, grain refinement of secondary Al-7Si-3Cu-0.3Mg alloy by Al-5B showed comparable performance to that of Al-5Ti-1B master alloy, however, without any deleterious influence on the precipitation sequence of Fe-rich phases, i.e. deleterious β-Al5FeSi reaction remained unfavourable during alloy solidification. Experimental findings from the investigations of the effect of deliberate Zr and V addition revealed that Zr and V addition can induce the grain refinement of secondary Al-7Si-3Cu-0.3Mg alloy. While Zr addition yielded the formation of pro-peritectic Zr-rich particles, which are found to nucleate primary α-Al at low undercooling, the effect of adding V can be characterized by the enhancement of the degree of constitutional undercooling. Combined Zr and V addition showed more effective grain refinement level than their individual additions. Majority of both Zr and V added to the alloy were retained inside α-Al matrix during solidification. As a result, limited amounts of Zr and V were rejected to the interdendritic liquid by the growing α-Al dendrites, then forming small-sized and rarely distributed intermetallics. Owing to its low solid solubility in α-Al, nickel available as impurity (~ 200 ppm) or after deliberate addition (0.25 wt.%) in secondary Al-7Si-3Cu-0.3Mg alloy was mainly bound to interdendritic, insoluble intermetallics, such as Al6Cu3Ni and Al9(FeCu)Ni phases. The presence of ~ 200 ppm Ni was sufficient to diminish to a certain extent the precipitation hardening effect of Cu. Interdendritic Zr/V/Ni-rich phases remained undissolved into the α-Al matrix during solution heat treatment. Therefore, the supersaturated transition metals in α-Al solid solution obtained during solidification was only involved in the solid-state precipitation occurring during heat treatment. Unlike Cu/Mg-rich strengthening precipitates that commonly form during aging, the Zr/V-rich precipitates tended to form during solution heat treatment. Other transition metals, such as Mn, Fe, Cr and Ti, which were present as impurities in secondary Al-7Si-3Cu-0.3Mg alloy significantly promoted the formation of nano-sized Zr/V-rich precipitates inside α-Al grains. These thermally more stable precipitates, including novel α-Al(MnVFe)Si, were credited for the enhanced high-temperature strength properties of Al-7Si-3Cu-0.3Mg alloy by ~ 20 %.

Transition metals (Ti, Zr, Hf, Mo, W, V, Nb, Ta, Pd, Pt, Cu, Ag, and Au) are essential building units of many materials and have important industrial applications. Therefore, it is important to understand their thermal and physical behavior when they are subjected to extreme conditions of pressure and temperature. This dissertation presents: An improved experimental technique to use lasers for the measurement of thermal conductivity of materials under conditions of very high pressure (P, up to 50 GPa) and temperature (T up to 2500 K). An experimental study of the phase relationship and physical properties of selected transition metals, which revealed new and unexpected physical effects of thermal conductivity in Zr, and Hf under high P-T. New phase diagrams created for Hf, Ti and Zr from experimental data. P-T dependence of the lattice parameters in α-hafnium. Contrary to prior reports, the α-ω phase transition in hafnium has a negative dT/dP slope. New data on thermodynamic and physical properties of several transition metals and their respective high P-T phase diagrams. First complete thermodynamic database for solid phases of 13 common transition metals was created. This database has: All the thermochemical data on these elements in their standard state (mostly available and compiled). All the equations of state (EoS) formulated from pressure-volume-temperature data (measured as a part of this study and from literature). Complete thermodynamic data for selected elements from standard to extreme conditions. The thermodynamic database provided by this study can be used with available thermodynamic software to calculate all thermophysical properties and phase diagrams at high P-T conditions. For readers who do not have access to this software, tabulated values of all thermodynamic and volume data for the 13 metals at high P-T are included in the APPENDIX. In the APPENDIX, a description of several other high-pressure studies of selected oxide systems is also included. Thermophysical properties (Cp, H, S, G) of the high P-T ω-phase of Ti, Zr and Hf were determined during the optimization of the EoS parameters and are presented in this study for the first time. These results should have important implications in understanding hexagonal-close-packed to simple-hexagonal phase transitions in transition metals and other materials.

This work was launched with a desire to develop novel versatile amido ligands for transition and main group metals. Of utmost importance was building ligands capable of forming strong and robust metal-ligand interactions. Bidentate mono- and di-anionic amido ligands were targeted due to the advantageous effects of chelation. 1,8-Diaminonaphthalene (1,8-DAN) and guanidines were identified as target ligand platforms. The majority of this work deals with the preparation and use of 1,8-DAN based dianionic ligands for transition (Ta, Zn) and main group (B, Al, C, Ge, Sn) metals. The synthesis and reactivity of half-sandwich zirconium guanidinato compounds is also presented. Chapter 1 presents the fundamental concepts surrounding ligand design and presents our rationale for choosing to investigate diamidonaphthalene and guanidinate ligands. Chapter 2 discusses the synthetic routes utilized for the preparation of both classes of ligands used further in this work. Reductive alkylation was used to introduce bulky secondary alkyl groups on the 1,8-DAN frame. The preparation of a guanidine ligand tethered to a silsesquioxane is also reported. Chapter 3 presents our efforts in preparing high-valent Ta(V) complexes supported by 1,8-DAN based ligands. Prevalent in this work is the tendency for Ta alkyl groups to further react with the ligand via C-H activation forming a metallaaziridine. Chapter 4 deals with the 1,8-DAN framework's ability to stabilize singlet carbenes. This new class of N-Heterocyclic Carbene (NHC) possessing a perimidine core demonstrates spectroscopic and reactive characteristics resembling those of the more reactive imidazolidine based NHCs. Chapter 5 describes the preparation of heavier group 14 analogues of perimidine based carbenes. The solid-state structure of the stannylene analogue indicated the presence of intermolecular Sn···arene interactions which were investigated using DFT calculations. Chapter 6 investigates the Lewis basic character of carbenes and germylenes supported by diamidonaphthalene and their potential as ligands for late transition metals. Chapter 7 discusses the use of diisopropyl-1,8-DAN as a ligand for the stabilization of coordinatively unsaturated group 13 elements. Chapter 8 presents a spectroscopic and structural study of mono(cyclopentadienyl)zirconium guanidinato species. Chapter 9 summarizes the work presented in this thesis and discusses how the work reveals but a sliver of the enormous potential of diamidonaphthalene ligands. (Abstract shortened by UMI.)

This thesis explores the development of material approaches in archaeology to discuss the relationship between material engagements in the world and cosmological schemes. In particular, the role of key materials in cosmologies is considered in light of ideas about elements and their fundamental role in the composition of the world. Three materials (wood, fire and water) are then considered in detail using anthropological illustrations to highlight the range of ways wood, fire and water are understood in traditional societies. Archaeological examples from the late Mesolithic and earliest Neolithic are examined in light of these ideas to suggest new possibilities for interpretations of the material remains discussed. These discussions of materials and cosmologies are set against a background of the debates surrounding the transition to the Neolithic. In particular, recent challenges to perceptions of the Mesolithic are considered to highlight the problematic discourses that have traditionally dominated studies of the period. It is amongst these critical reappraisals that this thesis contributes to emerging narratives of lives in the Mesolithic. The use of wood, fire and water is examined in contexts of the late Mesolithic and earliest Neolithic and is suggested to show much continuity in the practices that used wood, fire and water. Similar properties were actively called upon in these continued practices however, their deployment in new contexts and practices is suggested to indicate a shift in the importance of certain properties or characteristics. The implications of this in relation to cosmological schemes suggests aspects of continuity such as connections between materials and the landscape but accompanied by a stronger emphasis on certain relationships and processes within the world such as the transformation of bodies and articulation of community.

L'alliage intermetallique tial, de structure l1#0, ordonnee sur un reseau cubique a faces centrees, presente une transition fragile-ductile et une anomalie de limite d'elasticite. Ces proprietes ont ete etudiees pour les alliages tial dopes au manganese, au chrome ou au niobium, par des essais de rupture et par microscopie electronique en transmission (technique du faisceau faible). Les elements d'addition modifient largement les proprietes mecaniques macroscopiques. L'existence d'une transition fragile-ductile marquee a ete mise en evidence pour l'alliage dope au chrome. Pour les trois compositions, deux modes de rupture a la temperature ambiante coexistent: la rupture intergranulaire et le clivage. A haute temperature la rupture intergranulaire est predominante. Par ailleurs, le manganese et le chrome augmentent la limite d'elasticite. Le niobium fait disparaitre le pic. Les structures fines des dislocations ont ete examinees. Bien que les systemes de glissement actives evoluent avec la temperature, les microstructures de deformation generales sont semblables dans les trois alliages. En revanche, un effet de composition est apparu au niveau des structures internes des dislocations. Les parois d'antiphase sont favorisees par la presence de niobium. D'autre part, dans les nuances au manganese ou au chrome, aux configurations sessiles formees d'une paroi d'antiphase sur un plan cubique, viennent s'ajouter des structures formees de deux fautes d'empilement sur deux plans octaedriques en zone

Nous avons étudié des magmas ferrifères silicatés magnésiens à la pression du manteau terrestre en utilisant la dynamique moléculaire (First Principles Molecular Dynamics). Les résultats de l’équation d’état que nous avons obtenus à partir de nos simulations ont été utilisés pour créer un modèle chimique et minéralogique pour les zones de très basse vitesse sismique (ULVZ, anomalies régionales dans le manteau proche de la limite noyau-manteau). De plus, nous avons étudié le comportement du Ni, du Co et du Fe dans ces magmas et établi la dépendance du spin en fonction de la concentration, de la pression, de la température et du degré de polymérisation du magma silicaté. Nous avons montré qu’une baisse du spin moyen peut être corrélée au changement de pente (kink) observé précédemment pour les coefficients de partage du Ni et du Co. Nous avons analysé la structure du magma pour toutes les compositions étudiées en fonction de la pression. Nos résultats donnent un nouvel aperçu de la coordination des éléments majeurs et traces dans les magmas silicatés de différents degrés de polymérisation. Nous interprétons l’anomalie de coordination Ni-O en fonction de la pression comme un changement d’état de spin. L’effet de la polymérisation du magma silicaté sur les coefficients de partage du Co, du Ni et du W entre le métal et le magma silicaté a été étudié par expériences multi-enclumes en conditions isobares et isothermes. Nous avons réalisé des simulations FPMD de magmas à des degrés de polymérisation similaires aux expériences afin d’expliquer le caractère de plus en plus lithophile du W lorsque le degré de polymérisation du magma silicaté diminue. Nous proposons une explication structurale pour expliquer l’affinité décroissante apparente du W dans les magmas silicatés dépolymérisés
We explore Fe-bearing Mg-silicate melts through the pressure regime of the Earth’s mantle using First Principles Molecular Dynamics (FPMD). The equation of state results we obtained from our simulations are used to create a chemical and mineralogical model for Ultra-Low Velocity Zones (anomalous region on the mantle side of the core-mantle boundary). Furthermore we study the behaviour of Ni, Co, and Fe in these melts, and asses their spin-crossover dependencies on their concentration, pressure, temperature, and the degree of polymerization of the silicate melts. We show that a decrease in the average spin can be correlated with the previously observed kink in the partitioning coefficient of Ni and Co. We investigate the melt structure of all the compositions studied as a function of pressure. Our results provide new insight into the coordination of major and trace elements in silicate melts with different degrees of polymerization. We interpret the anomalous Ni-O coordination trend with pressure as the result of the spin state change. The effect of silicate melt polymerization on the partitioning of Co, Ni, and W between a metal and silicate melt, is investigated at isobaric and isothermic conditions using multi-anvil experiments. We have performed FPMD simulations of melts with similar degrees of polymerization as the experiments in order to explain the increasing lithophile character of W with the decrease in polymerization of the silicate melt. We propose a structural explanation for tungsten’s apparent increased affinity for depolymerized silicate melts

La croissance des cristaux de cdte et cd#0#,#9#6zn#0#,#0#4te dopes aux elements de transition (v, co, ni, ti) a ete effectuee par la technique de bridgman asymetrique sous controle de la pression de cd. La solubilite de ces elements dans la matrice cdte a ete etudiee. Les coefficients de segregation de v, as et ti ont ete determines par determination sims des profils de concentration de ces elements le long de lingots. L'imagerie sims revele la presence de precipites de vanadium d'environ 20 micro-metre de diametre dans les zones les plus riches en v. Les cristaux presentent une structure cristallographique mosaique, mise en evidence par un spectre multipic en double diffraction de rayons x et par cathodoluminescence en mode imagerie, due a la concentration elevee en vanadium. Une augmentation de la microdurete vickers, vis a vis de celle de cdte, est constatee. Elle est interpretee par des considerations liees aux energies de liaison chimique. Les mesures electriques revelent un fort degre de compensation. La spectroscopie de photoluminescence a permis d'identifier la presence d'impuretes donnant des niveaux superficiels. De nombreux niveaux ont ete identifies par spectroscopie dlts et dlos. Un niveau a 0,95 ev lie a la presence de v semble un bon candidat pour etre responsable de l'effet photorefractif. Une etude systematique de spectroscopie d'absorption optique permet d'aboutir a un diagramme de configuration. Les mesures du gain photorefractif sont satisfaisante. Un gain de 12 cm#-#1 a ete mesure pour des champs de 12 a 15 kv/cm a la frequence optimale de 40 hz, le gain depend enormement de la frequence du champ. La comparaison des coefficients de competition electrons trous a demontre que les proprietes photorefractives sont liees aux parametres de croissance. Tous les resultats de cristallogenese et de spectroscopie electrique et optique sont discutes en fonction des proprietes photorefractives du materiau et des perspectives d'applications qu'elles offrent

The purpose of this work is the development of a numerical tool devoted to the study of the flow field in the components of aerospace propulsion systems. The goal is to obtain a code which can efficiently deal with both steady and unsteady problems, even in the presence of complex geometries. Several physical models have been implemented and tested, starting from Euler equations up to a three equations RANS model. Numerical results have been compared with experimental data for several real life applications in order to understand the range of applicability of the code. Performance optimization has been considered with particular care thanks to the participation to two international Workshops in which the results were compared with other groups from all over the world. As far as the numerical aspect is concerned, state-of-art algorithms have been implemented in order to make the tool competitive with respect to existing softwares. The features of the chosen discretization have been exploited to develop adaptive algorithms (p, h and hp adaptivity) which can automatically refine the discretization. Furthermore, two new algorithms have been developed during the research activity. In particular, a new technique (Feedback filtering [1]) for shock capturing in the framework of Discontinuous Galerkin methods has been introduced. It is based on an adaptive filter and can be efficiently used with explicit time integration schemes. Furthermore, a new method (Enhance Stability Recovery [2]) for the computation of diffusive fluxes in Discontinuous Galerkin discretizations has been developed. It derives from the original recovery approach proposed by van Leer and Nomura [3] in 2005 but it uses a different recovery basis and a different approach for the imposition of Dirichlet boundary conditions. The performed numerical comparisons showed that the ESR method has a larger stability limit in explicit time integration with respect to other existing methods (BR2 [4] and original recovery [3]). In conclusion, several well known test cases were studied in order to evaluate the behavior of the implemented physical models and the performance of the developed numerical schemes.

Rare earth elements (REE) are known as the lanthanide series (La-Lu) plus yttrium (Y) and scandium (Sc). REE are essential materials for modern industries and especially for green technologies (wind turbines, batteries, lasers, catalysts, etc.). However, despite their high global demand, their supply is limited such that the EU has cataloged it as critical raw materials. In order to ensure the supply of REE in the future, the search for alternative sources of these elements worldwide has been promoted in recent years. Acid mine drainage (AMD) produced by the Fe-sulphide weathering can effectively leach Fe, Al, SO4, and REE from the host rock. This can lead to high concentrations of these liberated species in the affected waters. Thus, the REE concentrations in AMD can be between two and three orders of magnitude higher than natural waters, as such it can be considered as a complementary source of REE recovery. The increase of pH in AMD by mixing neutral waters results in the precipitation of iron oxy-hydroxysulfate (schwertmannite) from pH 3-3.5, and aluminum (basaluminite) from pH 4-4.5 in the river channels. This process may be accompanied by REE scavenging. Due to its acidity and high metal load, acid mine drainage presents a major environmental problem worldwide, therefore, different treatment systems have been developed to minimize its impact. Disperse Alkaline Substrate (DAS) passive remediation system neutralizes AMD by dissolving calcite, and allowing the sequential precipitation of schwertmannite and basaluminite in separated layers, where REE are preferably retained in the basaluminite-enriched waste. Despite this, there are still no studies describing the adsorption of REE on both basaluminite and schwertmannite in these environments. The REE scavenging mechanism is studied by adsorption on synthetic minerals of basaluminite and schwertmannite as a result of variation to the both the pH and sulfate concentration. A thermodynamic adsorption model is proposed based on experimental results in order to predict and explain the REE mobility in AMD mixtures with neutral waters and in a passive treatment system. Basaluminite and schwertmannite have a nanocrystalline character. Further, schwertmannite has been observed to transform into goethite on weekly timescales, resulting in sulfate release. However, there is a gap of knowledge about basaluminite stability at variable sulfate concentration and pH and its possible transformation to other more crystalline Al-minerals. In this study, basaluminite local order at different pH values and dissolved sulfate concentrations was characterized. Results demonstrate that basaluminite can transform to nanoboehmite in weeks under circumneutral pH. However, the presence of sulfate can inhibit this transformation. Separate adsorption experiments on both basaluminite and schwertmannite were performed with two different concentrations of SO4 while varying the pH (3-7). Results show that the adsorption is strongly dependent on pH, and to a lesser extent on sulfate concentration. Lanthanide and yttrium adsorption is most effective near pH 5 and higher, while that of scandium begins around pH 4. Due to the high concentrations of sulfate in acidic waters, the predominant aqueous REE species are sulfate complexes (MSO4+). Notably, Sc(OH)2+ represents a significant proportion of aqueous Sc. , A surface complexation model is proposed in which predominant aqueous species (Mz+) adsorb on the mineral surface, XOH, following the reaction: The adsorption of the lanthanides and yttrium occurs through the exchange of one and two protons from the basaluminite and schwertmannite surface, respectively, with the aqueous sulfate complexes. The sorbed species form monodentate surface complexes with the aluminum mineral and bidentate with the iron mineral. In the case of Sc, the aqueous species ScSO4+ and Sc(OH)2+ form bidentate surface complexes with both minerals. EXAFS analysis of the YSO4+ complex adsorbed on the basaluminite surface suggests the formation of a monodentate inner sphere complex, in agreement with the proposed thermodynamic model. Once the surface complexation model was validated, it was used to asses and predict the REE mobility in passive remediation systems and acidic water mixing zones with alkaline inputs from the field. The REE are preferentially retained in basaluminite-rich waste during passive remediation due to its sorption capacity between pH 5-6. In contrast, schwertmannite waste contains very little REE because the formation of this mineral occurs at pH lower than 4, which prevents REE adsorption. Further, Sc may be scavenged during schwertmannite precipitation as a result of this low pH The model correctly predicts the absence of REE in schwertmannite precipitates and the enrichment of the heavy and intermediate REE with respect to the light REE in basaluminite precipitates collected in the water mixing zones. However, there is a systematic overestimation of the fractionation of rare earths in basaluminite precipitate. This inaccuracy is mainly due to the fact that the mineral precipitation and adsorption are not synchronous process, while basaluminite precipitates from pH 4, REE adsorption occurs at higher pH values, between 5 and 7, when the water mixture reaches these values and a fraction of the particles have been dispersed.
Las tierras raras (en inglés rare earth elements, REE) son conocidas como el conjunto de la serie de los lantánidos (La-Lu), itrio (Y) y escandio(Sc). Las tierras raras son materiales indispensables para las industrias modernas y en especial para las tecnologías verdes (aerogeneradores, baterías, láseres, catalizadores, etc.). Sin embargo a pesar de su gran demanda mundial, su abastecimiento es limitado, por lo que han sido catalogadas por la UE como materias primas críticas (Critical Raw Materials). Con el objetivo de asegurar el abastecimiento de REE en el futuro, en los últimos años se ha promovido la búsqueda de fuentes alternativas de estos elementos en todo el mundo. El drenaje ácido de mina (en inglés acid mine drainage, AMD) producido por la meteorización de sulfuros de Fe, tiene un alto poder de lixiviación de las rocas, por lo que las aguas afectadas adquieren elevadas concentraciones en disolución de Fe, Al, SO4 y otros metales, como las REE. Así, las concentraciones de REE en AMD son entre dos y tres órdenes de magnitud superiores al resto de las aguas naturales y pueden suponer una fuente complementaria de recuperación de REE. El aumento de pH del AMD por mezcla con aguas neutras da lugar a la precipitación en los cauces de los ríos de oxy-hidroxisulfatos de hierro (schwertmannita), a partir de pH 3-3.5, y de aluminio (basaluminita), a partir de pH 4-4.5; acompañado de la eliminación de las tierras raras. Debido a su acidez y carga metálica, el drenaje ácido de mina presenta un problema medioambiental de primera magnitud, por lo que se han desarrollado diferentes sistemas de tratamiento para minimizar su impacto. El sistema de tratamiento pasivo Disperse Alkaline Substrate (DAS) produce la neutralización de las aguas ácidas por la disolución de la calcita presente en el sistema, permitiendo la precipitación secuencial, de schwertmannita y basaluminita. Las tierras raras quedan retenidas preferentemente en el residuo enriquecido en basaluminita. A pesar de ello, aún no existen estudios que describan la adsorción de tierras raras tanto en basaluminita como schwertmannita en estos ambientes. En esta tesis se estudia el mecanismo de retención de las tierras raras mediante adsorción en minerales sintéticos de basaluminita y schwertmannita, en función del pH y del contenido de sulfato disuelto. Con los resultados experimentales obtenidos, se propone un modelo termodinámico de adsorción para predecir y explicar la movilidad de las tierras raras observada en mezclas de AMD con aguas neutras y en un sistema de tratamiento pasivo. La basaluminita y la schwertmannita presentan un carácter nanocristalino. Es conocido que la schwertmannita se transforma en goethita en semanas, liberando sulfato. Sin embargo, nada se sabe de la basaluminita y su posible transformación a otros minerales de Al más cristalinos. De este modo, la caracterización del orden local de la basaluminita a diferentes valores de pH y sulfato se expone en primer lugar. Dependiendo del pH y el sulfato en disolución, la basaluminita se transforma en diferentes grados a nanoboehmita en semanas, pero tiende a estabilizarse con la presencia de sulfato en solución. Los experimentos de adsorción en basaluminita y schwertmannita con diferentes concentraciones de SO4 realizados para cada mineral y en rangos de 3-7 de pH han demostrado que la adsorción es fuertemente dependiente del pH, y en menor medida del sulfato. La adsorción de los lantánidos y del itrio es efectiva a pH 5, mientras que la del escandio comienza a pH 4. Debido a las altas concentraciones de sulfato en aguas ácidas, las especies acuosas predominantes de las tierras raras son los complejos con sulfato, MSO4+. Además del complejo sulfato, el Sc presenta importantes proporciones de Sc(OH)2+ en solución. En función de la dependencia del pH y de la importancia de la especiación acuosa, se propone un modelo de complejación superficial donde la especie acuosa predominante (Mz+) se adsorbe a la superficie libre el mineral, XOH, cumpliendo la siguiente reacción: La adsorción de los lantánidos y del itrio se produce a través del intercambio de uno o dos protones de la superficie de la basaluminita o de la schwertmannita, respectivamente, con los complejos sulfato acuoso, formando complejos superficiales monodentados con el mineral de aluminio y bidentados con el de hierro. En el caso del Sc, las especies acuosas ScSO4+ y Sc(OH)2+ forman complejos superficiales bidentados con ambos minerales. Complementando el modelo propuesto, el análisis de EXAFS del complejo YSO4+ adsorbido en la superficie basaluminita sugiere la formación de un complejo monodentado de esfera interna, coincidiendo con el modelo termodinámico propuesto. El modelo de complejación superficial, una vez validado, ha permitido evaluar y predecir la movilidad de REE en los sistemas de tratamiento pasivos y en zonas de mezcla de aguas ácidas con aportes alcalinos estudiados en el campo. La preferente retención de las tierras raras en la zona de la basaluminita precipitada en los sistemas de tratamiento pasivo ocurre por adsorción de las mismas a pH entre 5-6. La ausencia de tierras raras en la zona de schwertmannita se debe al bajo pH de su formación, inferior a 4, que impide la adsorción de las mismas. Sin embargo, debido a su menor pH de adsorción, una fracción de Sc puede quedar retenida en la schwertmannita. El modelo también predice correctamente la ausencia de REE en los precipitados de schwertmannita y el enriquecimiento de las tierras raras pesadas e intermedias respecto a las ligeras en los precipitados de basaluminita recogidos en el campo en las zonas de mezcla de aguas. Sin embargo, se ha observado una sistemática sobreestimación del fraccionamiento de las tierras raras en los precipitados de basaluminita. Este hecho se debe principalmente a que la precipitación del mineral no ocurre de forma síncrona con la adsorción, precipitando la basaluminita a partir de pH 4 y adsorbiendo tierras raras a pH más altos, entre 5 y 7, cuando las partículas sólidas han sido parcialmente dispersadas.

Cette thèse est consacrée à l'étude des séléniures ternaires connus sous le nom de phases de Chevrel. Ces composés séléniés cristallisent dans un système triclinique. Leur grande originalité réside dans une occupation particulière des ions 3d dans la cavité 2 et non dans la cavité 1. Cette occupation préférentielle entraîne une interaction métal-molybdène (d-d) importante avec des électrons d localisés et donc un transfert moindre sur le cluster donnant ainsi des propriétés physiques spécifiques. Grâce à une technique de cristallogénèse par fusion à haute température, des cristaux de taille convenable ont été isolés. Des études de susceptibilité magnétique ont mis en évidence l'existence d'un ordre magnétique à basse température. La nature de cet ordre magnétique a été confirmée par diffraction neutronique et études de chaleur spécifique. Des études de résistivité ont montré un comportement de type semi-métal. De nombreuses solutions solides de substitution sur le site chalcogène ont été isolées et leur structure cristalline déterminée. Leur étude révèle des propriétés originales (système rhomboédrique et changement du comportement magnétique).

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Dans la premiere partie de cette etude, nous avons mesure les coefficients de partage k de mn, fe, co et ni entre les deux phases majeures du manteau inferieur: la magnesiowustite et la perovskite, a 40, 50 et 70 gpa et a haute temperature. Les echantillons ont ete synthetises avec une cellule a enclumes de diamant chauffee par laser, puis etudies et analyses par microscopie electronique analytique en transmission. Nous avons trouve que: k#m#n < 1 < k#f#e < k#c#o << k#n#i, a 40, 50 et 70 gpa. Ces valeurs experimentales sont compatibles avec un modele pyrolitique du manteau inferieur ainsi qu'un modele de champ cristallin, montrant ainsi l'importance des phenomenes electroniques a haute pression et haute temperature. L'interet principal d'une telle modelisation est la prediction d'un changement spectaculaire dans la distribution de fe et de co si ces elements subissaient une modification de configuration electronique dans les conditions terrestres. Un deuxieme resultat important a ete l'observation d'une diminution du caractere siderophile de ni et de co avec la pression (40, 50 et 70 gpa). Jusqu'a present, ce comportement n'avait ete caracterise que pour des pressions ne depassant pas 30 gpa. Ainsi, le rapport ni/co observe actuellement dans le manteau peut s'expliquer par un processus d'equilibre entre metal et silicate solide pour des pressions de l'ordre de 70 gpa. Enfin, des simulations experimentales de la frontiere noyau-manteau ont ete realisees a partir de melanges fe-fesi-olivine, fe-fesi-fes-olivine et olivine-(fe,ni,si). Nos resultats confirment les reactions observees par goarant et al (1992) ecartant ainsi la proposition de boehler (1995). Ensuite, nous observons que les silicates au contact d'alliages (fe,si) sont enrichis en feo tant a pression ambiante qu'a haute pression. La connaissance de l'etat redox, mais aussi de la cinetique des echanges metal silicates apparaissent essentielles pour savoir si le silicium a pu etre significativement incorpore au metal du noyau lors de sa formation

Pour comprendre le lien entre les modulations d'absorption x et la geometrie d'un compose, nous avons calcule les sections efficaces en diffusion multiple (d. M. ) sur quelques composes de structure rutile. Les differents parametres du probleme ont ete etudies. Plusieurs analyses ont ete pratiquees sur le dioxyde de titane: une analyse atomique: creation de une ou plusieurs lacunes dans le cluster, effet des modes de vibration ; une etude en couches a l'aide du programme icxanes afin de comprendre l'influence de la dm entre et dans les couches, une analyse par chemin avec le programme feff6. D'autre part, nous avons etudie l'effet du potentiel d'echange: le type hedin-ludqvist rend mieux compte des modulations (amplitude et position) pour la region xanes (structures exafs peu prononcees ou absentes), alors que le type dirac-hara montre de meilleurs contrastes pour l'exafs (structures xanes trop amplifiees). Au cours de nos differentes analyses, nous avons montre l'importance de la dm dans la premiere couche pour la premiere structure du seuil nommee c et la dm entre les autres couches pour la seconde (d). De plus, nous avons pu confirmer que la separation des modulations c et d est caracteristique de la geometrie rutile (empilement d'octaedres). La variation des longueurs de liaisons apicales, equatoriales ou encore de l'angle equatorial modifie les amplitudes des structures. Nous avons aussi etudie vo#2 et mnf#2 rutiles et degage ainsi des traits communs a la structure rutile. Nous avons examine enfin deux autres formes du dioxyde de titane et de vanadium: l'anatase (temperature ambiante) et le vo#2 monoclinique

Des modeles thermodynamiques ont ete developpes dans le but d'etudier la stabilite a 25#oc des goethites et des hematites substituees par des elements de transition (me=sc, ti, v, cr, mn, co, ni, cu, zn). Les ions ni#2#+, sc#3#+, v#3#+, cr#3#+, mn#3#+, et ti#4#+ deplacent l'equilibre goethite pure-hematite pure en faveur de la goethite-me, tandis que cu#2#+, zn#2#+, et co#3#+, deplacent l'equilibre au profit de l'hematite-me. Les diagrammes d'equilibre a 25#oc des goethites-me et des hematites-me ont ete etablis pour differentes cristallinites et po#2. Les domaines de stabilite, a l'exception de celui du chrome, sont limites quand l'activite des ions augmente par l'apparition d'autres phases minerales dans le systeme fe-me-h#2o. Les taux de substitution maximums restent ainsi inferieurs a 1

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