Dissertations / Theses on the topic 'Granular materials High pressure (Science)'

Thesis (Ph.D.)--University of Delaware, 2006.
Principal faculty advisors: Suresh G. Advani, Dept. of Mechanical Engineering, and Eric D. Wetzel, Army Research Laboratory. Includes bibliographical references.

AB SOMAS Ventiler manufactures valves for different applications. A valve of type DN VSSL 400, PN 100, used in high-temperature and high-pressure applications was investigated in this thesis. This type of valve is coated with high cobalt alloys to achieve the tribological properties needed for this severe condition. However there is a request from AB Somas Ventiler to find another solution. This request is based on the fact that demands on higher temperatures, from customers, yields higher requirements on the material. It is also a price issue since cobalt is quite expensive. Materials investigated were high-nitrogen steel, Vanax 75, nickel-based superalloy Inconel 718 and hardened steels, EN 1.4903 and EN 1.4923 presently used as base material in the valve. Calculation of contact pressure that arises when the valve is closed was first approached by using finite element method (FEM). Several models were constructed to show the behavior of the valve during closing in terms of deformation. Hot wear tests, in which a specimen was pressed against a rotating cylinder, were performed to be able to compare the materials to the solution of today and among each other. Data extracted from the tests were compiled in the form of coefficients of friction. Profilometer examinations were used to reveal the volumes of worn and adhered material and together with scanning electron microscopy (SEM) the wear situation for each material couple could be assessed. Wear mechanisms detected in SEM were adhesive and abrasive and the results clearly showed that the steels were not a good solution because of severe adhesive wear due to the similarity of mating materials creating a more efficient bonding between the asperities. Vanax 75 showed much better performance but there was still an obvious difference between the steels and the superalloy in terms of both coefficient of friction and amount of wear. On this basis, Inconel 718 was selected as the most suitable material to replace the high cobalt alloys used in the valves today.
AB Somas ventiler är ett företag som tillverkar ventiler för ett brett spann av applikationer. I det här examensarbetet har undersökningar genomförts på en ventil av modell DN VSSL 400, PN 100, som normalt används i applikationer för höga tryck och höga temperaturer. Ventilen beläggs i dagsläget med höghaltiga koboltlegeringar för att uppnå de tribologiska egenskaper som krävs i de påfrestande arbetsförhållanden som råder. AB Somas Ventiler har dock framfört en förfrågan om att hitta en alternativ lösning, en förfrågan som grundar sig i att kundernas ständiga önskemål på att ventilerna ska klara högre arbetstemperaturer också medför högre krav på ventilmaterialen. Det är även en prisfråga, då kobolt är en dyr legering att använda sig av. De material som inkluderades i undersökningen var det kvävelegerade stålet Vanax 75, nickelbaserade superlegeringen Inconel 718 samt de två stålen EN 1.4903 och EN 1.4923 i härdat tillstånd. De två sistnämnda används idag som basmaterial i ventilen. Genom att använda den finita element metoden (FEM) kunde en första beräkning göras av det kontakttryck som uppstår då ventilen stängs. Flera modeller konstruerades för att simulera ventilens deformation vid stängning. Där efter utfördes nötningstester i hög temperatur på de alternativa materialen, genom att låta en provbit pressas mot en roterande cylinder, för att sedan kunna göra en jämförelse mellan materialen och även med den nuvarande lösningen. Från nötningstesterna erhölls data som kunde användas för att ta fram friktionskoefficienter för de olika materialparen. Med hjälp av undersökningar med profilometer och svepelektronmikroskop (SEM) kunde värden på nötta och vidhäfta volymer erhållas tillsammans med information om nötningssituationer för ytorna mellan de olika materialparen. De nötningsmekanismer som påvisades med hjälp av SEM-undersökningen var adhesiv och abrasiv nötning, och resultaten visade tydligt att nötningen av stålen var omfattande, på grund av att lika material i kontakt med varandra skapar starkare band mellan ytorna, och att de därför inte var en intressant lösning. Det kvävelegerade Vanax 75 uppförde sig visserligen bättre men en tydlig skillnad mot superlegeringarna kunde dock fortfarande konstateras, sett till både friktionskoefficient och mängden slitage. Baserat på dessa resultat valdes Inconel 718 som det bäst lämpade materialet att ersätta de höghaltiga koboltlegeringarna som idag används i ventilen.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2004.
Includes bibliographical references (leaf 30).
Packer elements are traditionally rubber seals that can operate under specified downhole conditions and provide a seal for either a short-term, retrievable, or a long-term, permanent, completion. In this case a retrievable 19.7cm (7-3/4") packer element for a high-pressure high-temperature (HPHT) environment was designed and tested. The element created a seal between the mandrel, or tubing, and the casing. At high temperature and pressure rubber needs to be contained so that it will create and maintain an energized seal. In this study only Aflas rubber was tested. Various backup systems were examined; some more traditional designs such as the carbon steel foldback ring were compared to more experimental ideas. Results of theoretical simulations were compared to actual test results in order to gain a greater understanding of element behavior. Experiments were also performed to study the process of element setting, which is difficult to observe due to the high pressures and temperatures required. In a related study alternative materials to rubber such as annealed high-conductivity oxygen-free copper were tested to determine if the properties could be applied for packer element applications. The most successful design was the foldback ring with an anti-extrusion PEEK ring under the gage ring. This design passed a liquid test at 134 MPa (19.5k psi) differential pressure and a gas test at 87.6 MPa (12.7k psi) differential pressure. New designs such as the split ring with mesh and the garter spring with mesh did not pass fixture tests but could be successful with further modifications. FEA was used as an analytical tool to create simulations of the element after a setting force is applied. The modeling was shown to correlate to the actual test results and therefore it would be a good tool to use in future studies.
by Stephanie Berger.
S.M.

Cold chamber high pressure die casting, (HPDC), is an important commercial process for the production of complex near net shape aluminium and magnesium alloy castings. The work presented in the thesis was aimed at investigating the microstructure formation in this type of casting. The solidification characteristics related to the process and the alloys control the formation of grains and defects. This again has a significant impact on the mechanical properties of the castings.

The investigations were carried out mainly using the AM60 magnesium alloy and the A356 aluminium alloy. Two different casting arrangements were used: the cold chamber HPDC and the gravity die casting methods, which allowed for different flow and solidification conditions. The microstructures in the castings were investigated using optical microscopy, image analysis, scanning electron microscopy, electron back scatter diffraction measurements and electron probe microanalysis.

In the HPDC experiments, the shot sleeve solidification conditions were investigated primarily by changing the melt superheat on pouring. This significantly affected the microstructures in the castings. The fraction of externally solidified crystals (ESCs) was consistently found to be largest near the gate in both the AM60 and the A356 die castings. This was attributed to the inherent shot sleeve solidification conditions and the flow set up by the plunger movement. When the superheat was increased, a lower fraction of ESCs was found in the castings. Furthermore, a high superheat gave ESCs with branched dendritic/elongated trunk morphology whilst a low superheat generated coarser and more globular ESCs, both in the AM60 and the A356 castings. The ESCs typically segregated towards the central region of the cross sections at further distances from the gate in the die castings.

When a thin layer of thermal insulating coating was applied on the shot sleeve wall in the production of AM60 die castings, it nearly removed all ESCs in the castings. Using an A356 alloy, (and no shot sleeve coating), with no Ti in solution gave a significantly lower fraction of ESCs, whereas AlTi5B1 grain refiner additions induced an increase in the fraction of ESCs and a significantly finer grain size in the castings. The formation of globular ESCs was enhanced when AlTi5B1 grain refiner was added to the A356 alloy.

In controlled laboratory gravity die casting experiments, typical HPDC microstructures were created by pouring semi-solid metal into a steel die: The ESCs were found to segregate/migrate to the central region during flow, until a maximum packing, (fraction of ESCs of ~35-40%), was reached. The extent of segregation is determined by the fraction of ESCs, and the die temperature affects the position of the ESCs. The segregation of ESCs was explained to occur during flow as a result of lift forces.

The formation of banded defects has also been studied: the position of the bands was affected by the die temperature and the fraction of ESCs. Based on the nature of the bands and their occurrence, a new theory on the formation of defect bands was proposed: During flow the solid distribution from the die wall consists of three regions: 1) a solid fraction gradient at the wall; 2) a low solid fraction region which carries (3) a network of ESCs. A critical fraction solid exists where the deformation rate exceeds the interdendritic flow rate. When the induced stress exceeds the network strength, deformation can occur by slip, followed by liquid flow. The liquid flow is caused by solidification shrinkage, hydrostatic pressure on the interior ESC network, and gaps forming which draw in liquid.

The present work describes the use of atomistic computer simulations in the area of Condensed Matter Physics, and speci cally its application to the study of two problems: the dynamics of the melting phase transition and the properties of materials at extremely high pressures and temperatures, problems which defy experimental measurements and purely analytical calculations. A good sampling of techniques including classical and rst-principles Molecular Dynamics, and Metropolis Monte Carlo simulation have been applied in this study. It includes the calculation of melting curves for a wide range of pressures for elements such as Xe and H2, the comparison of two di erent models for molecular interactions in ZrO2 with respect to their ability to reproduce the melting point of the stable cubic phase, the study of the elastic constants of Fe at the extreme conditions of the Earth's inner core, and the stability of its crystalline phases. One of the most interesting results in this work is the characterization of di usion and defects formation in generic models of crystalline solids (namely Lennard-Jones and Embedded-atom) at the limit of superheating, including the role they play in the triggering of the melting process itself.
QC 20100708

High-nitrogen-content energetic compounds containing multiple N-N bonds are an attractive alternative towards developing new generation of environmentally friendly, and more powerful energetic materials. High-N content translates into much higher heat of formation resulting in much larger energy output, detonation pressure and velocity upon conversion to large amounts of non-toxic, strongly bonded N2 gas. This thesis describes recent advances in the computational discovery of group-I alkali and hydrogen polynitrogen materials at high pressures using powerful first-principles evolutionary crystal structure prediction methods. This is highlighted by the discovery of a new family of materials that consist of long-sought after all-nitrogen N􀀀 5 anions and metal or hydrogen cations. The work has inspired a resurgence in the efforts to synthesize the N􀀀 5 anion. After describing the methodology of first-principles crystal structure prediction, several new high-nitrogen-content energetic compounds are described. In addition to providing information on structure and chemical composition, theory/simulations also suggests specific precursors, and experimental conditions that are required for experimental synthesis of high-N pentazolate EMs. To aid in experimental detection of newly synthesized compounds, XRD patterns and corresponding Raman spectra are calculated for several candidate structures. The ultimate success was achieved in joint theoretical and experimental discovery of cesium pentazolate, which was synthesized by compressing and heating cesium azide CsN3 and N2 precursors in a diamond anvil cell. This success highlights the key role of first-principles structure prediction simulations in guiding experimental exploration of new high-N energetic materials.

The present thesis is concerned to the application of first-principles self-consistent total-energy calculations within the density functional theory on different topics in materials science.

Crystallographic phase-transitions under high-pressure has been study for TiO2, FeI2, Fe3O4, Ti, the heavy alkali metals Cs and Rb, and C3N4. A new high-pressure polymorph of TiO2 has been discovered, this new polymorph has an orthorhombic OI (Pbca) crystal structure, which is predicted theoretically for the pressure range 50 to 100 GPa. Also, the crystal structures of Cs and Rb metals have been studied under high compressions. Our results confirm the recent high-pressure experimental observations of new complex crystal structures for the Cs-III and Rb-III phases. Thus, it is now certain that the famous isostructural phase transition in Cs is rather a new crystallographic phase transition.

The elastic properties of the new superconductor MgB2 and Al-doped MgB2 have been investigated. Values of all independent elastic constants (c11, c12, c13, c33, and c55) as well as bulk moduli in the a and c directions (Ba and Bc respectively) are predicted. Our analysis suggests that the high anisotropy of the calculated elastic moduli is a strong indication that MgB2 should be rather brittle. Al doping decreases the elastic anisotropy of MgB2 in the a and c directions, but, it will not change the brittle behaviour of the material considerably.

The three most relevant battery properties, namely average voltage, energy density and specific energy, as well as the electronic structure of the Li/LixMPO4 systems, where M is either Fe, Mn, or Co have been calculated. The mixing between Fe and Mn in these materials is also examined. Our calculated values for these properties are in good agreement with recent experimental values. Further insight is gained from the electronic density of states of these materials, through which conclusions about the physical properties of the various phases are made.

The electronic and magnetic properties of the dilute magnetic semiconductor Mn-doped ZnO has been calculated. We have found that for an Mn concentration of 5.6%, the ferromagnetic configuration is energetically stable in comparison to the antiferromgnetic one. A half-metallic electronic structure is calculated by the GGA approximation, where Mn ions are in a divalent state leading to a total magnetic moment of 5 μB per Mn atom.

En metod för dispersiv analys av försök med delad hopkinsonstång tillämpad på provning av granved vid hög töjningshastighet

Syftet var att etablera en metod för att studera sambandet mellan spänning och töjning för granved vid hög töjningshastighet. Detta åstadkoms genom att anpassa och något vidareutveckla tekniken med delad hopkinsonstång ("Split Hopkinson Pressure Bar", SHPB).

Vanligtvis har hopkinsonstavar cirkulärt tvärsnitt och en diameter som är mycket mindre än de verksamma våglängderna. Under sådana förhållanden är vågutbredningen i stängerna approximativt ickedispersiv, och en endimensionell (1D) vågutbredningsmodell kan användas. När det, som är fallet i denna studie, däremot inte kan säkerställas att stängernas tvärdimensioner är små i förhållande till våglängderna, är en helt igenom 1D vågutbredningsmodell otillräcklig, och tvärsnittets geometri, vilken var kvadratisk i denna studie, måste beaktas. Därför utvecklades med hjälp av Hamiltons princip en approximativ 3D vågutbredningsmodell för stänger med godtyckligt tvärsnitt. Modellen ger ett dispersionssamband (vågtal som funktion av vinkelfrekvens) samt medelvärden för förskjutningar och spänningar över gränsytorna mellan stänger och provstav. En kalibreringsprocedur utvecklades också.

Provning av granved genomfördes vid hög töjningshastighet (omkring 103 s-1) med den anpassade SHPB-tekniken, samt för jämförelse vid låg (8×10-3 s-1) och måttlig (17 s-1) töjningshastighet med en servohydraulisk provningsmaskin. Fukthalterna i veden motsvarade ugnstorr, fibermättnad och fullständig mättnad, och proven utfördes i radiell, tangentiell och axiell riktning i förhållande till trädets stam. För vart fall utfördes fem försök vid rumstemperatur. Resultaten visar töjningshastighetsberoendet för sambandet mellan spänning och töjning för granved under alla studerade förhållanden.

The aim was to establish a method for studying the relation between stress and strain in spruce wood at high strain rate. This was achieved by adapting and somewhat further developing the split Hopkinson pressure bar (SHPB) technique.

Hopkinson bars usually have a circular cross-section and a diameter much smaller than the operative wavelengths. The wave propagation in the bar is then approximately non-dispersive and a one-dimensional (1D) wave propagation model can be used. When, as in this study, it is not certain that the transverse dimensions of the bars are small in relation to the wavelengths, a solely 1D wave propagation model is insufficient and the geometry of the cross-section, which was square in this study, must be taken into account. Therefore, an approximate 3D wave propagation model for bars with arbitrary cross-section was developed using Hamilton's principle. The model provides a dispersion relation (wavenumber vs. angular frequency) and average values for displacements and stresses over the bar/specimen interfaces. A calibration procedure was also developed.

Tests on spruce wood specimens were carried out at a high strain rate (about 10³ s^-1) using the adapted SHPB technique, and for comparison at low (8×10^-3 s^-1) and medium (17 s^-1) strain rates using a servohydraulic testing machine. The moisture contents of the wood specimens corresponded to oven dry, fibre saturated and fully saturated, and the testing was performed in the radial, tangential and axial directions relative to the stem of the tree. In each case, five tests were run at room temperature. The results show the strain rate dependence of the relation between stress and strain for spruce wood under all conditions studied.

Hydrogen has been considered as a potentially efficient and environmentally friendly alternative energy solution. However, one of the most important scientific and technical challenges that the “hydrogen economy” faces is the development of safe and economically viable on-board hydrogen storage for fuel cell applications, especially to the transportation sector. Ammonia borane (BH3NH3), a solid state hydrogen storage material, possesses exceptionally high hydrogen content (19.6 wt%).However, a fairly high temperature is required to release all the hydrogen atoms, along with the emission of toxic borazine. Recently research interests are focusing on the improvement of H2 discharge from ammonia borane (AB) including lowering the dehydrogenation temperature and enhancing hydrogen release rate using different techniques. Till now the detailed information about the bonding characteristics of AB is not sufficient to understand details about its phases and structures. Elemental substitution of ammonia borane produces metal amidoboranes. Introduction of metal atoms to the ammonia borane structure may alter the bonding characteristics. Lithium amidoborane is synthesized by ball milling of ammonia borane and lithium hydride. High pressure study of molecular crystal provides unique insight into the intermolecular bonding forces and phase stability. During this dissertation, Raman spectroscopic study of lithium amidoborane has been carried out at high pressure in a diamond anvil cell. It has been identified that there is no dihydrogen bond in the lithium amidoborane structure, whereas dihydrogen bond is the characteristic bond of the parent compound ammonia borane. It has also been identified that the B-H bond becomes weaker, whereas B-N and N-H bonds become stronger than those in the parent compound ammonia borane. At high pressure up to 15 GPa, Raman spectroscopic study indicates two phase transformations of lithium amidoborane, whereas synchrotron X-ray diffraction data indicates only one phase transformation of this material. Pressure and temperature has a significant effect on the structural stability of ammonia borane. This dissertation explored the phase transformation behavior of ammonia borane at high pressure and low temperature using in situ Raman spectroscopy. The P-T phase boundary between the tetragonal (I4mm) and orthorhombic (Pmn21) phases of ammonia borane has been determined. The transition has a positive Clapeyron slope which indicates the transition is of exothermic in nature. Influence of nanoconfinemment on the I4mm to Pmn21 phase transition of ammonia borane was also investigated. Mesoporus silica scaffolds SBA-15 with pore size of ~8 nm and MCM-41 with pore size of 2.1-2.7 nm, were used to nanoconfine ammonia borane. During cooling down, the I4mm to Pmn21 phase transition was not observed in MCM-41 nanoconfined ammonia borane, whereas the SBA-15 nanocondfined ammonia borane shows the phase transition at ~195 K. Four new phases of ammonia borane were also identified at high pressure up to 15 GPa and low temperature down to 90 K.

The mixing of polymers and nanoparticles makes it possible to give advantageous macroscopic material performance by tailoring the microstructure of composites. In this thesis, five combinations of nano inclusion and polymer matrix have been investigated. The first type of composites is titanium dioxide/ polyaniline combination. The effects of 4 different doping-acids on the microstructure, morphology, thermal stability and thermoelectric properties were discussed, showing that the sample with HCl and sulfosalicylic dual acids gave a better thermoelectric property. The second combination is titanium dioxide/polystyrene composite. Avrami equation was used to investigate the crystallization process. The best fit of the mass derivative dependence on temperature has been obtained using the double Gaussian dependence. The third combination is titanium dioxide/polyaniline/ polystyrene. In the titanium dioxide/polyaniline/ polystyrene ternary system, polystyrene provides the mechanical strength supporting the whole structure; TiO2 nanoparticles are the thermoelectric component; Polyaniline (PANI) gives the additional boost to the electrical conductivity. We also did some investigations on Polyethylene odide-TiO2 composite. The cubic anatase TiO2 with an average size of 13nm was mixed with Polyethylene-oxide using Nano Debee equipment from BEE international; Single wall carbon nanotubes were introduced into the vinyl acetate-ethylene copolymer (VAE) to form a connecting network, using high pressure homogenizer (HPH). The processing time has been reduced to 1/60 of sonication for HPH to give better sample quality. Theoretical percolation was derived according to the excluded volume theory in the expression of the threshold as a function of aspect ratio.

The aim of this thesis is to provide microstructural and mechanical characterisation of α-β titanium alloys exposed to a range of thermo-mechanical conditions, in particular under-going high rate deformation at elevated temperatures, representative of the Linear Friction Welding (LFW) manufacturing process. Three α-β titanium alloys provided by Rolls-Royce are studied: Ti-64 blade, disc and Ti-6246 disc. Ti-64 and Ti-6246 show complex deformation behaviour with strain, strain rate and temperature, especially near the transus temperature, where the low temperature α phase is transformed into the high temperature β phase. The microstructure and mechanical properties evolve in an interconnected fashion, and understanding this mutual influence is necessary to better predict the behaviour of these alloys. Characterisation of the mechanical properties was performed through uniaxial compression tests at strain rates from 0.001 to 3000 s^-1, using an Instron screw-driven machine at quasi-static rates, a servo-hydraulic machine at medium rates and a Split-Hopkinson Pressure Bar and a drop-weight tower at high strain rates. The tests were performed over a range of temperatures from room temperature to 1300 °C. The main focus was on high strain rate and high temperature tests, with the development of a gravity driven direct impact Hopkinson bar, referred as a drop-weight system, which is intended to evaluate the mechanical response of metals to high strain rate loading at temperatures up to c. 1300 °C. The design and principles of operation of the system are presented, along with calibration and validation data. Preliminary tests were performed on stock Ti-64, heated at two rates: 1 and 20 °C s^-1. The evolution of the mechanical properties was analysed, focussing on the strain rate, temperature and phases dependencies. Characterisation of the microstructure was realised by performing interrupted compression tests, first at room temperature, three plastic strains, 4%, 10% and 20%, and two different strain rates, 0.001 and 2000 s^-1; then at 4% plastic strain, a strain rate of 2000 s^-1 and three elevated temperatures, 700, 900 and 1100 °C. A better understanding of the microstructure evolution with strain, strain rates and temperature, including the macrotexture and microtexture of the specimens, was obtained using Electron Backscatter Diffraction (EBSD) to characterise the texture of the undeformed and deformed materials. The better understanding of the flow stress and microstructural evolution of both Ti-64 and its individual α and β phases with various strain rates and temperatures is intended to be used in the development of more accurate models representing the behaviour of these alloys. Predicting the microstructure evolution and then the mechanical properties of a material is essential to optimise the final mechanical properties of the alloys when welded by manufacturing processes such as the LFW process.

Recently, ammonia borane has increasingly attracted researchers’ attention because of its merging applications, such as organic synthesis, boron nitride compounds synthesis, and hydrogen storage. This dissertation presents the results from several studies related to ammonia borane. The pressure-induced tetragonal to orthorhombic phase transition in ammonia borane was studied in a diamond anvil cell using in situ Raman spectroscopy. We found a positive Clapeyron-slope for this phase transformation in the experiment, which implies that the phase transition from tetragonal to orthorhombic is exothermic. The result of this study indicates that the rehydrogenation of the high pressure orthorhombic phase is expected to be easier than that of the ambient pressure tetragonal phase due to its lower enthalpy. The high pressure behavior of ammonia borane after thermal decomposition was studied by in situ Raman spectroscopy at high pressures up to 10 GPa. The sample of ammonia borane was first decomposed at ~140 degree Celcius and ~0.7 GPa and then compessed step wise in an isolated sample chamber of a diamond anvil cell for Raman spectroscopy measurement. We did not observe the characteristic shift of Raman mode under high pressure due to dihydrogen bonding, indicating that the dihydrogen bonding disappears in the decomposed ammonia borane. Although no chemical rehydrogenation was detected in this study, the decomposed ammonia borane could store extra hydrogen by physical absorption. The effect of nanoconfinement on ammonia borane at high pressures and different temperatures was studied. Ammonia borane was mixed with a type of mesoporous silica, SBA-15, and restricted within a small space of nanometer scale. The nano-scale ammonia borane was decomposed at ~125 degree Celcius in a diamond anvil cell and rehydrogenated after applying high pressures up to ~13 GPa at room temperature. The successful rehydrogenation of decomposed nano-scale ammonia borane gives guidance to further investigations on hydrogen storage. In addition, the high pressure behavior of lithium amidoborane, one derivative of ammonia borane, was studied at different temperatures. Lithium amidoborane (LAB) was decomposed and recompressed in a diamond anvil cell. After applying high pressures on the decomposed lithium amidoborane, its recovery peaks were discovered by Raman spectroscopy. This result suggests that the decomposition of LAB is reversible at high pressures.

TiAlN and TiAlN-based coatings that are used of relevance as protection of cutting tool inserts used in metal machining have been studied. All coatings were deposited by reactive cathodic arc evaporation using industrial scale deposition systems. The metal content of the coatings was varied by using different combinations of compound cathodes. The as-deposited coatings were temperature annealed at ambient pressure and in some cases also at high pressure. The resulting microstructure was first evaluated through a combination of x-ray diffraction and transmission electron microscopy. In addition, mechanical properties such as hardness by nanoindentation were also reported. TiAlN coatings with two different compositions were deposited on polycrystalline boron nitride substrates and then high pressure high temperature treated in a BELT press at constant 5.35 GPa and at 1050 and 1300 °C for different times. For high pressure high temperature treated TiAlN it has been shown that the decomposition is slower at higher pressure compared to ambeint pressure and that no chemical interaction takes place between TiAlN and polycrystalline cubic boron nitride during the experiments. It is concluded that this film has the potential to protect a polycrystalline cubic boron nitride substrate during metal machining due to a high chemical integrity. TiZrAlN coatings with different predicted driving forces for spinodal decomposition were furthermore annealed at different temperatures. For this material system it has been shown that for Zr-poor compositions the tendency for phase separation between ZrN and AlN is strong at elevated temperatures and that after spinodal decomposition stable TiZrN is formed.

L’obtention de très faible pression (UHV) est une condition essentielle pour les accélérateurs de particules de haute énergie et de hautes performances. Par conséquent, la compréhension de l'évolution de la pression dynamique pendant le fonctionnement des accélérateurs est fondamentale afin de trouver des solutions qui permettent de minimiser les hausses de pression induites par de multiples phénomènes présents dans les lignes faisceaux. Pour le LHC, l'apparition d'instabilités peut être due à la succession de plusieurs processus. Tout d’abord, le faisceau de protons de haute intensité ionise le gaz résiduel, produisant des ions positifs (principalement H₂⁺ et CO⁺) ainsi que des électrons qui sont accélérés et qui impactent la paroi en cuivre des tubes de faisceaux. Ensuite, ces interactions induisent : (i) une désorption des gaz absorbés sur les parois, conduisant à des élévations de pression ; (ii) la création de particules secondaires (ions et électrons). Dans ce dernier cas, la production d'électrons secondaires entraîne, par effet d’avalanche, la formation de nuages d’électrons, dont la limitation est l'un des enjeux majeurs de l'anneau de stockage du LHC. Ces nuages génèrent des montées de pression et des dépôts de chaleur sur les parois du collisionneur pouvant conduire à des « quench » d’aimants supraconducteurs. Tous ces phénomènes limitent l'intensité maximale et augmentent l’émittance des faisceaux et donc la luminosité ultime atteignable dans un accélérateur de protons. Ce travail de thèse a pour but d’étudier certains phénomènes fondamentaux qui contrôlent la pression dynamique dans le LHC, à savoir les effets induits par les électrons et les ions, d’une part, et l'influence de la chimie de surface du cuivre constituant les écrans faisceaux, d’autre part. Dans un premier temps, les courants d’électrons et d’ions ainsi que la pression ont été mesurés in situ dans le Secteur Pilote Vide (VPS) situé sur l'anneau du LHC pendant la deuxième période d’exploitation du collisionneur. En analysant ces résultats, une quantité d’ion plus importante que prévu a été détectée et la relation entre les électrons, les ions et les variations de pression a été étudiée. D’autre part, la désorption stimulée par les ions a été mesurée au laboratoire au CERN en utilisant un bâti expérimental dédié. L'influence de la nature, de la masse et de l'énergie des ions incidents interagissant avec les surfaces sur les rendements de désorption ionique a été discutée. De plus, des analyses approfondies de la surface de cuivre constituant l'écran faisceau ont été réalisées dans le laboratoire IJCLab pour identifier le rôle joué par la chimie de surface du cuivre sur le rendement d’émission électronique, les processus de conditionnement de surface et la désorption de gaz stimulée. Le rôle fondamental de composés chimiques sur la surface (contaminants, présence de carbone et d'oxydes natifs) sur le rendement de production des électrons secondaires a été mis en évidence. Enfin, nous avons proposé un code de simulation permettant de prédire les profils de pression dans les chambres à vide des accélérateurs de particules ainsi que leur évolution temporelle. Ce nouveau code de simulation appelé DYVACS (DYnamic VACuum Simulation) est une amélioration du code VASCO développé par le CERN. Il a été appliqué pour simuler la pression dynamique dans le VPS. L'évolution du nuage d'électrons a été implémentée dans le code via des « maps » permettant de calculer l'évolution de la densité des nuages d'électrons. L'ionisation du gaz résiduel par les électrons a également été prise en compte. Finalement, les résultats obtenus avec DYVACS ont été comparés aux mesures de pression enregistrées dans le LHC. Les résultats obtenus à l’issu de ces travaux de thèse, ainsi que les développements expérimentaux et de simulation réalisés, pourront permettre l’étude de la stabilité du vide de futurs accélérateurs de particules tels que HL-LHC ou FCC(ee et hh)
Ultra-High Vacuum is an essential requirement to achieve design performances and high luminosities in high-energy particle colliders. Consequently, the understanding of the dynamic pressure evolution during accelerator operation is fundamental to provide solutions to mitigate pressure rises induced by multiple-effects occurring in the vacuum chambers and leading to beam instabilities. For the LHC, the appearance of instabilities may be due to the succession of several phenomena. First, the high intensity proton beams ionize the residual gas producing positive ions (mainly H₂⁺ or CO⁺) as well as accelerated electrons which impinge the copper wall of the beam pipe. Then, these interactions induce: (i) the desorption of gases adsorbed on the surfaces leading to pressure rises; (ii) the creation of secondary particles (ions, electrons). In this latter case, the production of secondary electrons leads to the so-called “Electron Cloud” build-up by multipacting effect, the mitigation of which being one of the major challenges of the LHC storage ring. Electron clouds generate beam instabilities, pressure rises and heat loads on the walls of beam pipe and can lead to “quench” of the superconducting magnets. All these phenomena limit the maximum intensity of the beams and thus the ultimate luminosity achievable in a proton accelerator. This work aims to investigate some fundamental phenomena which drive the dynamic pressure in the LHC, namely the effects induced by electrons and ions interacting with the copper surface of the beam screens on the one hand and the influence of the surface chemistry of copper on the other hand. First, in-situ measurements were performed. Electron and ion currents as well as pressure were recorded in situ in the Vacuum Pilot Sector (VPS) located on the LHC ring during the RUN II. By analyzing the results, more ions than expected were detected and the interplay between electrons, ions and pressure changes was investigated. Then, the ion-stimulated desorption was studied, using a devoted experimental set-up at the CERN vacuum Lab. The influence of the nature, mass, and energy of the incident ions interacting with the copper surface on the ion-desorption yields was discussed. In addition, extensive surface analyses were performed in the IJCLab laboratory to identify the role played by the surface chemistry on the electron emission yield, surface conditioning processes and the stimulated gas desorption. The fundamental role of the surface chemical components (contaminants, presence of carbon and native oxide layers) on the secondary electron yield was evidenced. Finally, we proposed a simulation code allowing to predict the pressure profiles in the vacuum chambers of particle accelerators as well as their evolution under dynamic conditions (i.e. as a function of time). This new simulation code called DYVACS (DYnamic VACuum Simulation) is an upgrade of the VASCO code developed at CERN. It was applied to simulate the dynamic pressure in the VPS when proton beams circulate into the ring. The electron cloud build-up was implemented in the code via electron cloud maps. The ionization of the residual gas by electrons was also considered. Results obtained with the DYVACS code are compared to pressure measurements recorded during typical fills for physics and a good agreement is obtained. This PhD study has provided interesting results and has allowed the development of new experimental and simulation tools that will be useful for further investigations on the vacuum stability of future particle accelerators such as HL-LHC or FCC (ee and hh)

Quality of the castings is affected by several factors which the designer should take into consideration during the product development process. Although residual stress is one of those, it is often not considered in practical computations. Hence residual stresses are one of the forgotten areas in designing of machine parts. This master thesis is focused on the investigation of residual stresses in a high pressure die casted component, with the aim of extending its service life, by taking results from the study as a feedback.

The investigation of residual stresses was done on a variety of specimens, cast aluminum flywheel, provided by Husqvarna AB. This flywheel is a component in a product of the same company.In evaluating the residual stresses in the part, two tools-simulation and physical measurement were used. Moreover, comparison with these two methods is also done at an area of interest on the flywheel. The simulation was carried out by using MAGMAhpdc-a module for high pressure die casting process, from the commercial software package MAGMAsoft; while for the physical measurements, the hole drilling method was used, a method believed to be less accurate at low stresses areas.

The findings obtained from this study show that the results from both procedures are close, with small deviations observed, which reveals the reliability of the hole drilling method even when the stress levels are low. It is also found that the compressive residual stresses dominate in the component-a preferred phenomenon with regards to residual stress.

Les méthodes d'imagerie et de vélocimétrie ultrasonores ont prouvé leur efficacité pour étudier des matériaux divers. À haute intensité, il est connu que les ultrasons exercent des forces stationnaires dans les fluides newtoniens, par le biais d'effets non linéaires comme la pression de radiation acoustique. Néanmoins, ces effets n'ont encore jamais été exploités d'un point de vue fondamental dans le contexte de la physique des matériaux mous. L'objet de cette thèse est d'exploiter l'interaction d'ultrasons de puissance avec des matériaux bloqués afin de sonder activement, voire d'influencer leurs propriétés mécaniques. Nous proposons tout d'abord une méthode de microrhéologie active : la « mésorhéologie acoustique ». En analysant le mouvement d'un intrus sous l'effet de la pression de radiation acoustique, nous caractérisons localement la rhéologie du matériau étudié. Nous mettons cette technique en œuvre avec un fluide à seuil simple : un microgel de carbopol. Nous exploitons les résultats obtenus à la lumière d'une caractérisation rhéologique poussée du comportement de ce matériau en dessous de son seuil d'écoulement et proposons diverses pistes d'amélioration du dispositif.Ensuite, nous décrivons la mise en écoulement d'un empilement granulaire immergé par des ultrasons intenses focalisés et comparons les observations aux résultats de simulations de dynamique moléculaire. La transition de fluidification observée car l'injection d'énergie y est discontinue. Elle est intermittente et hystérétique, propriétés reproduites par des simulations numériques et dont un modèle phénoménologique simple permet de rendre compte.Enfin, en remplaçant le plan d'un rhéomètre classique par un transducteur ultrasonore, nous mesurons l'effet de vibrations à haute fréquence sur les propriétés mécaniques d'un gel colloïdal fragile de noir de carbone. Nous observons un effet significatif et potentiellement irréversible des ultrasons sur le module élastique et sur la mise en écoulement de ce système. Les vibrations semblent favoriser le glissement du gel aux parois mais il semble toutefois qu'elles induisent également des changements en volume dans l'échantillon
Ultrasonic imaging and velocimetry has been proved to be very efficient methods to study various materials. At high intensity, ultrasonic waves are known to exert steady forces in newtonian fluid through nonlinear effects like the acoustic radiation pressure. However those effects have never been used in fundamental studies of the physics of soft materials. This thesis aims at exploiting the interaction between high intensity ultrasound and soft jammed materials to probe actively and even modify their mechanical properties.We first introduce an alternative technique for active microrheology we called « acoustic mesorheology ». By analyzing the motion of an intruder under the acoustic radiation pressure we characterize locally the rheology of the system under study. We test this technique on a simple yield stress fluid, namely a carbopol microgel. We compare the results with those obtained by standard rheology measurements of the behaviour of this gel under its yield stress.Then we describe the fluidization of an immersed granular packing by high intensity focused ultrasound. We compare our observations with the results of molecular dynamics simulations. The obtained fluidization is original as the injection of energy is discontinuous in time. It is hysteretic and intermittent and those properties are well captures by both simulations and a phenomenological model.Finally, we replace the plane of a standard cone-plate rheometer by an ultrasonic transducer. This allows us to characterize the effect of high frequency vibrations on the rheology of a fragile carbon black gel. We observe a significant and eventually irreversible effect of ultrasound on the elastic modulus and on the yielding of the system. Vibrations are shown to favor wall slip but seem to induce changes in the volume of the sample though

In 1844 Charles Goodyear obtained U.S. Patent #3,633 for his “Gum Elastic Composition”. In a published circular, which describes his patent for the sulfur vulcanization of gum elastic composition, he stated: “No degree of heat, without blaze, can melt it (rubber)… It resists the most powerful chemical reagents. Aquafortis (nitric acid), sulphuric acid, essential and common oils, turpentine and other solvents… …” Goodyear's sulfur vulcanization of rubber fueled much of the industrial revolution and made transportation possible, as it exists today. In doing so, Goodyear created one of the most difficult materials to recycle. Rubber will not melt, dissolve, or lend itself to the usual methods of chemical decomposition. Ironically, Goodyear recognized this problem and in 1853 he patented the process of adding ground rubber to virgin material, now currently known as regrind blending. Today, scrap tires represent one of the most serious sources of pollution in the world. Studies estimate that there are roughly 2 billion scrap tires in U.S. landfills and more are being added at a rate of over 273 million tires per year. Current methods of recycling waste tires are crude, ineffective, and use rubber powder as a low cost filler instead of a new rubber. The groundwork for a very simple and effective method of producing high-quality rubber goods using 100% scrap rubber was discovered in 1944 by A. V. Tobolsky et al. This application, however, was not recognized until recently in our laboratory. The process as studied to date represents a method of creating quality, high-value added rubber goods with nothing other than heat and pressure. High pressure is required to obtain a void-free compaction of the rubber particles by forcing all of the free surfaces into intimate contact. High temperature then activates the chemical rearrangement, scission, and reformation of the chemical bonds thus providing new bridges between the once fractured interfaces. This occurs both within and between particles. The technique of high-pressure high-temperature sintering has worked on all types of thermoset materials. Typical mechanical properties for sintered SBR powder rubber are as follows: 1.3 MPa 100% Modulus, 12.0 MPa Tensile Strength and 300% Elongation at Break. The goal of this research is two-fold. First, to gain an understanding of the variables that control the process of high-pressure high-temperature sintering. Second, to study the factors governing the mechanism of fusion with the hope of controlling and exploiting this process so that tires can be recycled to produce high quality and high-value added products.

Diblock copolymers have enormous potential use as templating materials for the fabrication of highly ordered materials on a nanometer scale. This derives from the complexity of phase behavior in microphase-separated block copolymer systems. In the ordered state block copolymers commonly self-organize into ordered lamellae, cylinders or spheres. Recently, high-pressure diluents have been used as a processing aid when using block copolymers as a templating material. In this study several aspects of polymer-diluent phase behavior have been investigated using high-pressure carbon dioxide as a diluent. It is well established that dilation of a copolymer changes the characteristic interdomain spacing of a phase-separated system. The interdomain spacing decreases with dilation by non-selective diluents and increases with selective diluents. The characteristic spacing of poly(styrene-block-dimethyl siloxane) has been measured using small angle neutron scattering (SANS) in high-pressure CO2. A new power-law scaling for the change in interdomain spacing induced by dilation, which accounts for selectivity of the diluent, is presented. The effect of high-pressure carbon dioxide on the order-to-disorder transition (ODT) of poly(styrene-block-2-vinyl pyridine) has been investigated using static birefringence. The results of these experiments indicate that in addition to the suppression of the ODT, dilation of a copolymer by carbon dioxide can induce the appearance of a second morphology. Further the appearance of this second morphology is strongly dependant on the sample history. Methods for measuring the swelling of homopolymers in carbon dioxide have also been investigated. High-pressure spectroscopic ellipsometry and laser interferometry have been developed as methods to measure the swelling of homopolymer films. It is shown that spectroscopic ellipsometry and laser interferometry are both capable of measuring the swelling of homopolymer films at high pressure.

A novel manufacturing process called high-temperature high-pressure sintering was studied and explored. Solid fiber reinforced composites are produced by consolidating and compacting layers of polymeric fabrics near their melting temperature under high pressure. There is no need to use an additional matrix as a bonding material. Partial melting and recrystallization of the fibers effectively fuse the material together. The product is called a “matrix free” fiber reinforced composite and essentially a one-polymer composite in which the fiber and the matrix have the same chemical composition. Since the matrix is eliminated in the process, it is possible to achieve a high fiber volume fraction and light weight composite. Interfacial adhesion between fibers and matrix is very good due to the molecular continuity throughout the system and the material is thermally shapeable. Plain woven Spectra ® cloth made of Spectra® fiber was used to comprehensively study the process. The intrinsic properties of the material demonstrate that matrix free Spectra® fiber reinforced composites have the potential to make ballistic shields such as body armor and helmets. The properties and structure of the original fiber and the cloth were carefully examined. Optimization of the processing conditions started with the probing of sintering temperatures by Differential Scanning Calorimetry. Coupled with the information from structural, morphological and mechanical investigations on the samples sintered at different processing conditions, the optimal processing windows were determined to ensure that the outstanding original properties of the fibers translate into high ballistic performance of the composites. Matrix free Spectra® composites exhibit excellent ballistic resistance in the V50 tests conducted by the US Army. In the research, process-structure-property relationship is established and correlations between various properties and structures are understood. Thorough knowledge is obtained for this creative process regarding the procedures, outcomes, advantages and capabilities. Two other ultra high molecular weight polyethylene fiber containing materials, Dyneema Fraglight® nonwoven felt and Spectra Shield® Plus PCR prepreg, were also carefully studied using the process of high-temperature high-pressure sintering. Their structures, morphologies and thermo-mechanical properties were compared with consolidated Spectra® cloth. The results clearly demonstrate that Spectra® cloth is the best candidate for making ballistic protective shields.

This work describes the first chemical method for the preparation of quartz nanocrystals. Microscopic characterization to evaluate phase purity, size distribution and extent of aggregation is performed. In addition, pressure dependent thermodynamic properties are also assessed. Submicron quartz powders are initially produced in hydrothermal reactions where soluble silica precursors reprecipitate as pure crystalline silica. To yield nanocrystalline material these particles can be purified and size selected by dialysis, filtration and centrifugation. Transmission electron microscopy and x-ray diffraction illustrate that the product is phase-pure alpha-quartz, consisting of isolated (i.e. non-aggregated) nanocrystals. Depending on the size selection method, crystallites with average sizes of 10 to 100 nanometers can be recovered. The high-pressure behavior of nanocrystalline alpha-quartz is studied by synchrotron x-ray diffraction up to 8.6 GPa. The unit cell volume change with pressure is fitted with the third order Birch-Murnaghan equation of state giving a bulk modulus, K0, of 40 +/- 1.3 GPa. The reported value of compressibility for nanoquartz is, with reasonable agreement, the same as previously published values for bulk quartz. However, quartz nanoparticles exhibit an increased pressure dependence of the bulk modulus. In addition, a size dependent lattice expansion at ambient as well as elevated pressure is observed.

Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2006.
Source: Dissertation Abstracts International, Volume: 67-07, Section: B, page: 3976. Adviser: G. Mesri. Includes bibliographical references (leaves 895-906) Available on microfilm from Pro Quest Information and Learning.

Thermosetting materials have long been considered impossible to reuse since they do not melt or dissolve. Few technologies have been developed to recycle waste thermosets compared to those available for the recycling of metals, glasses, and thermoplastics (meltable polymers). As rubbers are a sub-category of thermosetting materials, they also suffer from these limitations to recycling. Currently, scrap rubber tires and waste polyurethanes represent two of the largest recycling dilemmas facing our society. The work herein offers two potential solutions to this problem of recycling thermosets. The first technique, High-Pressure High-Temperature Sintering (HPHTS), allows parts to be produced from 100% recycled material (current techniques typically use less than 10% recycled content). Several thermosetting systems were investigated in efforts to understand why certain thermosets are more recyclable via HPHTS than others. The goal of this work was to understand the mechanism of HPHTS and design and/or synthesize thermosets that are more easily recycled when they reach the waste stream. During this study, it was realized that Chemical Stress Relaxation (CSR) techniques offered excellent insight into the HPHTS process. As such, a section of the thesis is focused on Chemical Stress Relaxation and its correlation with HPHTS. The last sections devoted to HPHTS involve the utilization of additives in the HPHTS process as a means of increasing mechanical properties and engineering the backbone of thermosets in efforts to enhance recyclability. These chapters target the end uses of these materials; their purpose being to increase properties so that the materials can be utilized for real world products. The second technique, degradation/devulcanization of thermosets (specifically rubber), is carried out at high temperatures (>280°C) under a variety of conditions (in a melt press while under pressure, in a Parr-reactor, etc.) The resulting viscous liquid-like material has numerous uses that include revulcanization, asphalt modification, and oil replacement in the compounding and molding of virgin rubber. Many of these uses not only increased the amount of rubber recycled, but also offered potential improvements over prior art. Materials incorporating over 35% recycled content were produced and maintained all of the original mechanical properties of the control (oil compounded) specimen.

The earth's core consists of a solid metallic center surrounded by a liquid metallic outer layer. Understanding the compositions of the inner and outer cores allows us to better understand the dynamics of the earth's core, as well as the dynamics of the cores of other terrestrial planets and moons. The density and size of the earth's core indicate that it is approximately 90% metallic, predominantly iron, with about 10% light elements. Iron meteorites, believed to be the remnants of planetary cores, provide further constraints on the composition of the earth's core, indicating a composition of 86% iron, 4% nickel, and 10% light elements. Any potential candidate for the major light element core component must meet two criteria: first, it must have high cosmic abundances and second, it must be compatible with Fe. Given these two constraints there are five plausible elements that could be the major light element in the core: H, O, C, S, and Si. Of these five possible candidates this thesis focuses on S and C as well exploring the effect of minor amounts of O and H on the eutectic temperature in a Fe-FeS core. We look at two specific aspects of the Fe-FeS system: first, the shape of the liquidus as a function of pressure, second, a possible cause for the reported variations in the eutectic temperature, which draws on the effect of H and O. Finally we look at the effect of S and C on partitioning behavior of Ni, Pt, Re,Co, Os and W between cohenite and metallic liquid. We are interested in constraining the shape of the Fe-FeS liquidus because as a planet with a S-enriched core cools, the thermal and compositional evolution of its core is constrained by this liquidus. In Chapter 1 I present an equation that allows for calculation of the temperature along the liquidus as a function of pressure and composition for Fe-rich compositions and pressures from 1 bar to 10 GPa. One particularly interesting feature of the Fe -rich side of the Fe-FeS eutectic is the sigmoidal shape of the liquidus. This morphology indicates non-ideal liquid solution behavior and suggests the presence of a metastable solvus beneath the liquidus. An important consequence of such curved liquidi is that isobaric, uniform cooling requires substantial variations in the solidification rate of the core. Additionally, in bodies large enough for P variation within the core to be significant, solidification behavior is further complicated by the P dependence of the liquidus shape. Brett and Bell (1969) show that at 3 GPa, the liquidus curvature relaxes, implying that the liquid solution becomes more ideal. By 10 GPa, the liquidus approaches nearly ideal behavior (Chen et al., 2008b). However, at 14 GPa, the liquidus again assumes a sigmoidal curvature (Chen et al., 2008a; Chen et al., 2008b), suggesting a fundamental change in the thermodynamic behavior of the liquid. Chapter 1 of this thesis accounts for the observed complexity in the liquidus up to 10 GPa thus enabling more accurate modeling of the evolution of the cores of small planets (Buono and Walker, 2011). Accurately knowing the eutectic temperature for the Fe-FeS system is important because it places a minimum bound on the temperature of a S-enriched core that has a solid and liquid component which are in equilibrium. Unfortunately literature values for the 1 bar to 10 GPa eutectic temperature in the Fe-FeS system are highly variable making the estimation of core temperature, an important geodynamic parameter, very difficult. In Chapter 2 we look at a possible cause of this observed variation by experimentally investigating the effects of H on the eutectic temperature in the Fe-FeS system at 6 and 8 GPa. We find that H causes a decrease in the eutectic temperature (but that O does not) and that this decrease can explain some of the observed scatter in the available data. The effect of H on the eutectic temperature increases with increasing pressure (i.e. the eutectic temperature is more depressed at higher pressures), matching the trend reported for the Fe-FeS system (Fei et al., 1997). Our work suggests a significantly higher eutectic temperature than is commonly used in the Fe-S system and explains the lower observed eutectic temperatures by employing the ternary Fe-S-H system. Additionally, we report an equation which allows for accurate prediction of the composition of the eutectic in the Fe-FeS system. The constraints presented here (eutectic temperature in the Fe-FeS system are 990 °C up to at least 8 GPa in conjunction with the equation presented in Chapter 1, allows for complete prediction of the Fe-rich liquidus in the Fe-FeS system to 8 GPa. It is important to understand the partitioning behavior of trace elements between the solid and liquid components of a system because it fundamentally informs our understanding of that systems chemical evolution. In light of this, we investigate partitioning behavior in the context of the Fe-S-Ni-C system in Chapter 3. Choice of this system was motivated by work outside the scope of this thesis investigating the liquidus relationships in the Fe-S-C system (Dasgupta et al., 2009). In these experiments, cohenite (Fe₃C) is the stable solid phase, instead of Fe-metal and we find that the partition coefficients between cohenite and Fe-C-S liquids are significantly lower than those between Fe-metal and Fe-S liquids. There are two potential situations to which this work can be applied. With respect to core formation, although it is unlikely that any planets entire inner core is carbide, it is possible that in a C-rich planet, as the Fe core crystallizes, C in the liquid phase could be enriched to the point where cohenite is a stable crystalizing phase. Under these circumstances, we would predict smaller depletions of the elements studied in the outer core than would be the case for Fe-metal crystallization. This work can also be applied to the earth's upper mantle which is thought to become Fe-Ni metal-saturated as shallow as 250 km. Under these circumstances, the sub-system Fe-Ni-C (diamond) -S (sulfide) becomes relevant and Fe-Ni carbide rather than metallic Fe-Ni alloy could become the crystalline phase of interest. Our study implies that if cohenite and Fe-C-S melt are present in the mantle, the mantle budget of Ni, Co, and Pt may be dominated by Fe-C-S liquid. Additionally, in the case of a S-free system, W, Re, and Os will also be slightly enriched in Fe-Ni-C liquid over cohenite. In total this body of work better constrains several key aspects of the compositional and thermal evolution of cores in small planetary bodies and has potential implications for the earth's mantle.

abstract: Carbon lacks an extended polyanionic chemistry which appears restricted to carbides with C4-, C22-, and C34- moieties. The most common dimeric anion of carbon atoms is C22- with a triple bond between the two carbon atoms. Compounds containing the dicarbide anion can be regarded as salts of acetylene C2H2 (ethyne) and hence are also called acetylides or ethynides. Inspired by the fact that molecular acetylene undergoes pressure induced polymerization to polyacetylene above 3.5 GPa, it is of particular interest to study the effect of pressure on the crystal structures of acetylides as well. In this work, pressure induced polymerization was attempted with two simple metal acetylides, Li2C2 and CaC2. Li2C2 and CaC2 have been synthesized by a direct reaction of the elements at 800ºC and 1200ºC, respectively. Initial high pressure investigations were performed inside Diamond anvil cell (DAC) at room temperature and in situ Raman spectroscopic measurement were carried out up to 30 GPa. Near 15 GPa, Li2C2 undergoes a transition into a high pressure acetylide phase and around 25 GPa this phase turns amorphous. CaC2 is polymorphic at ambient pressure. Monoclinic CaC2-II does not show stability at pressures above 1 GPa. Tetragonal CaC2-I is stable up to at least 12 GPa above which possibly a pressure-induced distortion occurs. At around 18 GPa, CaC2 turns amorphous. In a subsequent series of experiments both Li2C2 and CaC2 were compressed to 10 GPa in a multi anvil (MA) device and heated to temperatures between 300 and 1100oC for Li2C2, and 300°C to 900°C for CaC2. The recovered products were analyzed by PXRD and Raman spectroscopy. It has been observed that reactions at temperature higher than 900°C were very difficult to control and hitherto only short reaction times could be applied. For Li2C2, a new phase, free of starting material was found at 1100°C. Both the PXRD patterns and Raman spectra of products at 1100oC could not be matched to known forms of carbon or carbides. For CaC2 new reflections in PXRD were visible at 900ºC with the starting material phase.
Dissertation/Thesis
M.S. Chemistry 2012