Dissertations / Theses: 'Nucleation mechanism'

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Jawor-Baczynska, Anna. "Nucleation mechanism of crystal formation during antisolvent or cooling induced crystallisation." Thesis, University of Strathclyde, 2010. http://oleg.lib.strath.ac.uk:80/R/?func=dbin-jump-full&object_id=22626.

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This project studied the nucleation mechanism of crystal formation during antisolvent or cooling crystallisation of simple amino acids: D,L-valine and glycine. These amino acids can co-precipitate with proteins to form Protein Coated Microcrystals (PCMCs) in which the crystals create a solid support and the biomacromolecules cover their surface while remaining in a native state. The understanding of the formation mechanism of small microcrystals would help to better control and manage the process which leads to ordered attachment of biomacromolecules on their surfaces. Spectrophotometry, 1H nuclear magnetic resonance (NMR), dynamic light scattering (DLS) and optical microscopy were used to probe the evolution of the system from the transparent solution to a suspension of microcrystals. The nucleation mechanism of antisolvent crystallisation was found to involve formation of a transparent nanoemulsion composied of sub-micron valine-rich liquid nanodroplets with an average size and size distribution depending on supersaturation and the mixing conditions used during sample preparation. The supersaturated solutions prepared by cooling crystallisation, without agitation produced smaller nanodroplets and resulted in formation of only a few large crystals with an extremely slow crystallisation rate compared to samples with identical composition prepared by antisolvent crystallisation. The following nucleation mechanism of amino acids crystals is proposed: dissolution of amino acid into an aqueous/2-propanol mixture at concentration close to saturation results in spontaneous formation of a thermodynamically stable system consisting of amino acid rich liquid nanodroplets dispersed in amino acid solution; above a particular amino acid composition (consistent with the crystal solubility limit) the dispersed nanodroplets become metastable and shear induced coalescence of nanodroplets can provide access to a fast crystallisation pathway (non-classical); in the absence of shear the nanodroplets are colloidally-stable and crystallisation follows a much slower pathway (classical). The spontaneous formation of solute-rich nanodroplets below the crystalline saturation limit as well as formation of metastable solute-rich nanodroplets above this limit provides a paradigm shift which can be potentially used to develop fundamental understanding of nonclassical crystallisation phenomena. It will be crucial for better design and control of crystallisation processes in pharmaceutical applications.

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Liu, Yao. "Experimental investigation of the mechanism for non-photochemical laser induced nucleation." Thesis, University of Edinburgh, 2017. http://hdl.handle.net/1842/28723.

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The aim of this thesis was to discover the mechanism for non-photochemical laser-induced nucleation (NPLIN), which is a technique for inducing nucleation of crystals with laser light without absorption. The mechanism of the optical Kerr effect (OKE) was suggested by Garetz et al. [Physical Review Letters 77, 3475–3476 (1996)] to give an explanation for NPLIN. Since the feasibility of the OKE mechanism for NPLIN has been questioned, a series of experiments on NPLIN of aqueous supersaturated urea were carried out to quantify the relationship between crystal alignment and laser polarization. Digital imaging of crystal growth during laser irradiation showed that nascent needle-shaped crystals of urea were not aligned with the direction of the electric field of the laser. Additionally, work on glycine was aimed at verifying the possibility to control the polymorph of the obtained crystal via the laser polarization. However, our finding shows that the probability of γ-glycine is more likely to increase with increasing supersaturation; and the influence of laser polarization on the resulting morphologies is not strong as reported by Sun et al. [Crystal Growth & Design 6, 684–689 (2006)]. Furthermore, in another work on NPLIN of L-histidine, based on Sun et al. [Crystal Growth & Design 8, 1720–1722 (2008)], we were unable to reproduce the results as stated in Sun’s published paper. We find their results exhibit a large uncertainty when recalculated through the Wilson score interval for binomial distributions. On account of these revised uncertainties, it is unlikely that laser polarization gives polymorphism control. Comparison with the nucleation probability of unfiltered samples in the work of urea and glycine shows that the number of filtered samples nucleated as a result of NPLIN was largely decreased. Moreover, experiments on NPLIN of NaCl and KCl also exhibited that the number of filtered samples nucleated was significantly lower than that of unfiltered samples. This downward tendency in nucleation probability after filtration cannot be explained by Garetz’s OKE mechanism. On account of this, an alternative mechanism named particle-heating mechanism was proposed, and supported by experiments on NPLIN of sodium acetate. Sodium acetate experiments showed that the crystallization of sodium acetate can be induced by a single pulse of a nanosecond laser (1064 nm) with minimum power of 0.1 J cm−2. As discovered by Oliver et al. [D. Oliver, PhD Thesis, Edinburgh University, 2014], anhydrous or trihydrate sodium acetate can be formed under the effects of different organic and inorganic additives, such as poly- (methacrylic acid) and disodium hydrogen phosphate. We demonstrate that crystalline growth velocities and crystal morphology can be influenced by these additives. We find that high levels of additive cause only nucleation of bubbles. By counting the number of crystals, which is approximately consistent to the number of bubbles observed, video microscopy of laser-induced crystallization of sodium acetate has revealed that the general mechanism of NPLIN is most likely to be caused by a particle-heating mechanism. Chapter 8 of the thesis describes a number of solute molecules that were tested using NPLIN, but failed. In terms of improvements for future work or a perspective on NPLIN, detailed suggestions have been described in Chapter 9, which also gives a summary of all work presented.

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Mahony, Michael F. "Investigation into the mechanism of acicular ferrite nucleation in steel weld metal." Thesis, Monterey, Calif. : Springfield, Va. : Naval Postgraduate School ; Available from National Technical Information Service, 1999. http://handle.dtic.mil/100.2/ADA367290.

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Moors, Matthieu. "Study of the nucleation mechanism of carbon nanotubes by field emission techniques." Doctoral thesis, Universite Libre de Bruxelles, 2010. http://hdl.handle.net/2013/ULB-DIPOT:oai:dipot.ulb.ac.be:2013/210109.

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The present work is focused on the nucleation and growth mechanism of carbon nanotubes (CNT) that we have studied through different field emission techniques (FEM, FIM and atom-probe (PFDMS)). Reaction conditions associated with the CVD synthesis method were modeled inside the microscope aiming at studying nucleation phenomena at high resolution. The interaction between different metals (Fe, Co, Ni, conditioned as sharp tips) and gases (acetylene, ethylene and ethanol) was analyzed operando at high temperatures (500–900K), with the aim of reproducing growth conditions during the imaging process.

Ni was, in the end, the only metal studied, due to the poor quality of images acquired from Co and Fe. Aimed at reproducing the conditioning step of the catalyst often observed in CVD protocols, a first study showed that the crystal adopts a polyhedral morphology at the working temperature (873K) in an hydrogen atmosphere or under Ultra-High-Vacuum conditions, by the extension of dense crystal planes like {111} or {100}. The presence of hydrogen in the chamber does not seem to present any influence on the final crystal morphology at temperatures above 600K.

When exposed to a carbon-containing gas, nickel crystals present two distinct behaviors following the temperature region that is explored. At temperatures below ~623K, exposing Ni to ethylene or acetylene leads to the formation of a stable and poorly structured nickel carbide layer. The superficiality of this carbide is proven by the ease of its physical (by increasing the electrical field) or chemical (exposure to hydrogen or oxygen) evacuation. These three treatments initiate a clean-off phenomenon that evacuates the carbide layer. Reproducing these experiments in the atom-probe confirmed the carbidic nature of the surface as NiCy compounds were collected.

At temperatures above 623K, the carbide layer (formed by exposing Ni to the same gases) becomes unstable. Its formation is related to a transition period that precedes the nucleation of graphenes on the surface. The Ni crystal undergoes a massive morphological transformation when acetylene is introduced in the chamber at 873K. This phenomenon is induced by the presence of carbon on the surface which adsorbs so strongly on step sites that it provokes their creation. Carbon also induces a considerable enhancement of Ni atoms mobility that allows for this transition to occur. Once the new morphology is attained, nucleation of graphenes is observed to start on the extended and carbon-enriched step-containing crystal planes. By reproducing these experiments in the atom-probe, a high surface concentration of carbon dimers and trimers was observed. A kinetic study of their formation was thus achieved and showed that they were formed on the surface by the recombination of Cad. Their potential role as building-blocks of the CNT growth process (which had previously been proposed following theoretical considerations) is thus suggested on the basis of experimental results for the first time.

Two critical surface concentrations are highlighted in the present work. The first one is needed for the formation of carbon dimers and trimers and the second one has to be attained, during the morphological transformation, before the onset of graphene nucleation, probably providing a sufficient growth rate of the graphitic nuclei and allowing them to attain their critical size before their decomposition.

Finally, the observation of rotational circular patterns, most probably related to carbon nanotubes, suggests that CNT growth (and not only graphene nucleation) occurred episodically in our conditions, confirming the validity of our model.

Doctorat en Sciences
info:eu-repo/semantics/nonPublished

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Kim, HoKwon. "A study of the nucleation and growth mechanism of graphene on copper." Thesis, Imperial College London, 2013. http://hdl.handle.net/10044/1/15159.

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Two approaches for the large scale synthesis of graphene were investigated with the objective of achieving high-quality graphene for practical applications: (1) non-covalent solution based exfoliation of graphite, and (2) chemical vapour deposition of graphene on copper. As the processing conditions, structure, and properties of graphene are inherently connected, the aim of the study was to gain fundamental insights on the critical mechanisms that govern the properties and device performance of graphene. The first part of this thesis describes the efficient non-covalent exfoliation of graphite to produce few-layer-graphene dispersion in N-methylpyrrolidone and large-scale thin film deposition using the Langmuir-Blodgett assembly. In the second part, the chemical vapour deposition of polycrystalline graphene films on copper was investigated as it has emerged as the most promising route toward large scale synthesis of monolayer graphene for optoelectronics applications due to its properties approaching that of ideal graphene and the relatively low cost of copper. An extensive range of growth parameters was employed to develop a model of two-dimensional nucleation and self-limited growth of graphene on the surface of copper. The analysis of the nucleation and growth kinetics has revealed the relationship between atomic processes that impart two distinct temperature regimes of nucleation, whereas the growth of the individual nuclei was shown to be rate-limited by the carbon attachment at the nuclei edges. Moreover, the growth on high index copper surfaces has shown that graphene nanostructures of controlled shapes, density, and dimensions can be produced, depending on the Cu crystal orientation. Interestingly, few-layer-graphene grown on Cu frequently exhibits AA stacking with interlayer spacing of ~3.6 Å , with a preserved linear dispersion relationship. This work thus provides practical guidelines for achieving wafer scale single crystal graphene as well as the control over the mesoscale structure by the careful selection of the growth parameters.

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Callahan, Craig James. "The influence of hydrodynamic environment on the nucleation mechanism of a chiral crystallization." Thesis, Heriot-Watt University, 2014. http://hdl.handle.net/10399/2785.

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This thesis presents the results and discussions of an investigation into nucleation mechanisms in a cooling crystallization of sodium chlorate in both a stirred tank crystallizer (STC) and an oscillatory baffled crystallizer (OBC) under various crystallizer configurations and operational conditions. The key question to be addressed was why nucleation took place in an OBC without seeds, while seeds were essential in an STC for the same chemistry and at the same process conditions. Various hypotheses have been initiated, tested and verified in both primary and secondary nucleation experiments, and new scientific insights and better understanding have been achieved on the parameters that have influenced the nucleation mechanisms and some explanations as to why seeds were not necessary in the OBC are put forward. For the seeded nucleation, the fluid dynamic environment and mixing mechanics were responsible for dictating the nature of the nucleation mechanism. The unique scraping action of the baffles against the crystallizer wall in the OBC enabled a different enantiomorphism of the product crystals compared to the seed crystal. Removing such a motion in the OBC provided product crystals similar to that seen in the STC. The degree and the means of mixing near the single seed crystal also affected the crystal handedness. For the primary nucleation experiments, it was found that the handedness of the product crystals in the STC was strongly orientated towards a single enantiomorph, while both enantiomorphs were formed in the OBC. The results were suggestive of a lower free energy barrier in the OBC, in turn resulting in primary nucleation being favoured in the OBC as opposed to rapid secondary nucleation being dominant in the STC.

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Leswin, Joost Sieger Kaspar. "Particle Formation in RAFT-mediated Emulsion Polymerization." University of Sydney, 2007. http://hdl.handle.net/2123/2176.

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Doctor of Philosophy(PhD)
Particle formation in RAFT-mediated emulsion polymerization has been studied using reaction calorimetry. By measuring the heat flow during controlled feed ab-initio emulsion polymerization in the presence of amphipathic RAFT agents, particle formation by self-assembly of these species could be observed. Two different monomer systems, i.e. styrene and n-butyl acrylate, and various degrees of hydrophobicity of the initial macro-RAFT agents have been studied and compared. The different macro-RAFT agents were synthesized by first forming a hydrophilic block of poly(acrylic acid) that would later on act as the electrosteric stabilizing group for the particles. Subsequently, different lengths of hydrophobic blocks were grown at the reactive end of the poly(acrylic acid) hydrophilic block via the RAFT-mediated controlled radical polymerization, either comprised of n-butyl acrylate or styrene. Two processes govern particle formation: adsorption of macro-RAFT agents onto growing particles and formation of new particles by initiation of micellar aggregates or by homogeneous nucleation. Competition between these processes could be observed when monomers with a relatively high (n-butyl acrylate) or low (styrene) propagation rate coefficient were used. A model describing particle formation has been developed and the results of model calculations are compared with experimental observations. Preliminary modeling results based on a set of reasonable physico-chemical parameters already showed good agreement with the experimental results. Most parameters used have been verified experimentally. The development of the molecular weight distribution of the macro-RAFT agents has been analyzed by different techniques. Quantification of the particle formation process by analytical techniques was difficult, but qualitative insights into the fundamental steps governing the nucleation process have been obtained. The amount of macro-RAFT agents initially involved in particle formation could be determined from the increase of molecular weight. The particle size distribution has been measured by capillary hydrodynamic fractionation, transmission electron microscopy and dynamic light scattering. From the data obtained from these particle-sizing techniques, the number of particles during the reaction could be monitored, leading to an accurate estimate for the particle formation time. Upon implementation of the experimental data obtained for the surface active macro-RAFT systems, the model demonstrated to be very sensitive towards the “headgroup” area of the macro-RAFT species. Three nucleation cases based on the initial surface activity of the macro-RAFT species in the aqueous phase are proposed to explain the deviations from the assumptions of the nucleation model. Even though the macro-RAFT species have a narrow molecular weight distribution, they are nevertheless made up of a distribution of block lengths of polystyrene upon a distribution of block lengths of poly(acrylic acid). The resulting differences in initial surface activity are the most probable reason for the observed differences between model calculations and experimental results for the nucleation time and particle size distribution of the final latex product. With the procedure described above, latexes have been synthesized without using conventional surfactants and the mechanisms involved in the particle formation for these systems have been elucidated. The results of this work enable production of latex systems with well defined molecular mass distributions and narrow particle size distributions. Furthermore, the technique based on the application of amphipathic RAFT agents is promising for the production of complex polymeric materials in emulsion polymerization on a technical scale.

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Nagaraj, Madhu [Verfasser], and Christiane [Akademischer Betreuer] Ritter. "Structural Analysis of Nucleation Mechanism in Curli Biogenesis using Nuclear Magnetic Resonance Spectroscopy / Madhu Nagaraj ; Betreuer: Christiane Ritter." Braunschweig : Technische Universität Braunschweig, 2012. http://d-nb.info/1175822051/34.

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Rabizadeh, Taher. "The nucleation, growth kinetics and mechanism of sulfate scale minerals in the presence and absence of additives as inhibitors." Thesis, University of Leeds, 2016. http://etheses.whiterose.ac.uk/16652/.

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In this research, I focused on elucidating the crystallisation kinetics and formation mechanism of calcium sulfate phases, specifically gypsum, in the presence and absence of additives as inhibitors. In many industries such as oil production and water desalination, calcium sulfate formation is a problem that causes pipeline and membrane clogging and reduces system efficiency. Thus, different types of additives are added to these systems as inhibitors to tackle the scaling problem. However, their efficiency or effectiveness in terms of calcium sulfate inhibition has not been fully tested and the processes are not well-understood at the mechanistic level. In this thesis, therefore, I investigated the effects of various carboxylic acids, alkali / alkaline earth metal cations, polycarboxylic acids and phosphonates as gypsum inhibitors to fill this knowledge gap. My results revealed that all additives delayed the crystallisation of gypsum to various degrees and by various pathways. I monitored the change in the time needed for turbidity in a reacting solution to start (induction time) and develop (crystallisation kinetics). I analysed the changes in both solution chemistry and solid characteristics, including surface properties, morphologies as well as composition, to derive a mechanistic understanding of how these additives affect gypsum formation. The results illustrated that among the tested carboxylic acids (tartaric, maleic and citric), citric acid performed far better than tartaric and maleic (at equivalent concentrations) and using citric acid dramatically increased the induction time (~ 4 fold, to ~ 25 minutes). Among the tested alkali / alkaline earth metal cations (Li+, K+, Na+ and Mg2+), Mg2+ decreased the nucleation and growth kinetics ~ 5 to ~ 10 fold more than Li+, Na+ and K+ even at low concentrations. Mg2+, Li+ and K+ only adsorbed to the gypsum crystals surfaces, while ~ 25% of associated Na+ became incorporated into the synthesised crystals and Li+ and Mg2+ also acted as shape and size modifiers. When I tested the effects of biodegradable polycarboxylic additives (polyaspartic acid; PASP and polyepoxysuccinic acid; PESA) and compared their efficiency with a traditionally used non-biodegradable (polyacrylic acid; PAA) antiscalant, I showed that PASP and PESA had a profound effect on gypsum crystallisation, with PASP having the highest inhibition efficiency. The PAA conformation and molecular weight both played important roles in affecting the crystallisation kinetics because of changes in surface adsorption mechanisms. Finally, I tested some industrial phosphonate inhibitors and demonstrated that they are indeed strong gypsum inhibitors, but I also showed for the first time that an increase in the number of functional groups affected the efficiency of the additive. Among the tested phosphonates with five phosphonate functional groups, the one with longer molecular chains was the better inhibitor. It is unclear how these additives interacted with the growing gypsum crystals (surface adsorption and / or structural incorporation), but I clearly showed that they affected gypsum growth kinetics and morphologies.

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Ikni, Aziza. "Nucléation non-photochimique induite par laser (NPLIN) : Application à la carbamazépine et résultats préliminaires sur le sulfathiazole." Thesis, Châtenay-Malabry, Ecole centrale de Paris, 2015. http://www.theses.fr/2015ECAP0022/document.

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L’étude du polymorphisme de composés pharmaceutiques est un domaine en plein essor. Cela est dû d’une part aux pressions économiques exercées par l’industrie pharmaceutique et d’autre part pour une meilleure compréhension des conséquences du polymorphisme sur les propriétés des médicaments (stabilité physique et chimique, solubilité, vitesse de dissolution,biodisponibilité, propriétés mécaniques, etc.). Cette thèse étudie la cristallisation par la technique de nucléation non-photochimique induite par laser (NPLIN) des molécules pharmaceutiques organiques. Dans ce contexte, un nouveau montage expérimental semi-automatisé adapté aux exigences des études NPLIN a été conçu et réalisé, permettant le contrôle de nombreux paramètres et le traitement simultané d’un grand nombre d'échantillons et ainsi réalisé un gain de temps considérable. Une méthode de travail a été également établie afin de limiter l’aléa expérimental. L’étude de l’impact des paramètres du laser sur la cristallisation de la carbamazepine et du sulfathiazole dans différents solvants(méthanol, éthanol, acétonitrile) a été menée. Les résultats obtenus ont permis de démontrer pour la première fois dans la littérature que la NPLIN permet d’obtenir des cristaux de CBZ et de sulfathiazole dont les formes polymorphiques dépendent de la polarisation du laser et du type de solvant. Une approche permettant la prédiction de la forme polymorphique des molécules organiques de la base CSD, dans le cas de l’utilisation de la NPLIN, a été proposée et appliquée pour les molécules CBZ et la sulfathiazole étudiées expérimentalement. A partir de nos résultats expérimentaux, des calculs d’énergie d’interactions ab initio de dimères des molécules de carbamazépine et de sulfathiazole et sur les résultats observés dans la littérature nous avons pu émettre des hypothèses quant au mécanismes impliqués dans la nucléation NPLIN
Polymorphism study of pharmaceutical compounds is a growing field. This is due, on one hand, to the economic pressure of the pharmaceutical industry and on the other hand, to the more awareness of the polymorphism consequences on the drug properties (chemical and physical stability, solubility, dissolution rate, bioavailibality, mechanical properties,manufacturing process, etc).This thesis studies the crystallisation of organic pharmaceutical molecules by using the Non Photochemical Light Induced technique. Within this context, a new semi-automatic experimental setup suitable to the requirements of NPLIN studies was devised and realised,allowing the control of many parameters and a simultaneous processing of a great number of samples and thus saving a considerable amount of time. A strict working methodology is also established to limit experimental errors. The study of the impact of laser parameters on the crystallisation of carbamazepine and sulfathiazole within different solvents (methanol,ethanol, acetonitrile) was carried-out. The obtained results allowed demonstrating, for the first time in literature, that NPLIN allows to obtain CBZ and sulfathiazole crystals for which the polymorphic forms depend upon the laser polarisation and solvent type. An approach for predicting the polymorphic form of organic molecules in the CSD database, for the case of NPLIN, was proposed and applied for the case of CBZ and sulfathiazole that were studied experimentally. We have also provided hypotheses to explain the mechanisms involved in NPLIN nucleation

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Koss, Hans [Verfasser], and Michael [Akademischer Betreuer] Schleicher. "Mechanism of actin polymerization with yeast formin Bni1p : effect of the KCI concentration on the FH2 mediated actin nucleation / Hans Koss. Betreuer: Michael Schleicher." München : Universitätsbibliothek der Ludwig-Maximilians-Universität, 2012. http://d-nb.info/102966160X/34.

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Congdon, Erin Elizabeth. "Insights into the mechanism of Tau polymerization and the effects of small molecules." The Ohio State University, 2007. http://rave.ohiolink.edu/etdc/view?acc_num=osu1185397658.

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Scrofani, Jacopo 1984. "Mechanism of RanGTP dependent microtubule assembly during mitosis." Doctoral thesis, Universitat Pompeu Fabra, 2014. http://hdl.handle.net/10803/289621.

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During mitosis, spindle assembly involves different sources of microtubules including centrosomes and chromosomes. While the role of centrosomes has been extensively studied, we still do not fully understand how chromosomes trigger microtubule assembly thereby contributing to the formation of the mitotic spindle. The chromosomal pathway is largely determined by a RanGTP gradient centered on the chromosomes that induces the local activation of spindle assembly factors. To get a better understanding on the RanGTP-dependent microtubule assembly during mitosis we aimed at: i) Identifying new RanGTP regulated proteins involved in spindle assembly. Our results pointed to three novel proteins with a putative mitotic role in the RanGTP pathway. ii) Understanding how the RanGTP pathway regulates microtubule nucleation during mitosis. We found that TPX2 together with Aurora-A and RHAMM are part of a RanGTP-dependent complex that binds and strongly stimulates the -TuRC nucleation activity. iii) Investigating the contribution of the RanGTP pathway to spindle assembly. Our data provided novel evidences that MTs nucleated close to the chromosomes participate in the k-fibers assembly process.
Durante la mitosis, el proceso de ensamblaje del huso mitótico implica diferentes fuentes de microtúbulos incluyendo centrosomas y cromosomas. Mientras que el rol de los centrosomas ha sido extensamente estudiado, no se entiende en su totalidad como los cromosomas inducen la formación de microtúbulos contribuyendo de esta manera al ensamblaje del huso mitótico. La vía de los cromosomas está mayormente determinada por un gradiente de RanGTP, centrado en los cromosomas, que induce la activación local de factores mitóticos. Para entender el mecanismo que promueve el ensamblaje de los microtúbulos vía RanGTP durante la mitosis, este trabajo tuvo los siguientes objetivos: i) La identificación de nuevas proteínas reguladas por RanGTP que participan en el ensamblaje del huso. Nuestros resultados apuntan a tres nuevas proteínas con un posible papel mitótico en la vía RanGTP de ensamblaje de los microtúbulos. ii) El estudio de el mecanismo para el cual RanGTP regula la nucleación de microtúbulos durante la mitosis. Hemos descubierto que TPX2 junto con Aurora-A y RHAMM son parte de un complejo RanGTP dependiente que estimula la actividad de nucleación de el TuRC. iii) El estudio del papel de la vía de RanGTP en el ensamblaje del huso. En particular, nuestros datos proporcionan nuevas evidencias sobre la participación de los MTs nucleados alrededor de los cromosomas en el proceso del ensamble de fibras cinetocóricas.

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Young, Adam Hamilton. "Study of the Nucleation Mechanism of a-Ti in Bimodal and Fully Lamellar Microstructures Developed in the High-Strength, Near-Beta Titanium Alloy, TIMETAL®18." The Ohio State University, 2013. http://rave.ohiolink.edu/etdc/view?acc_num=osu1374159274.

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Trinkle, Dallas Rhea III. "A theoretical study of the HCP to omega martensitic phase transition in titanium." Connect to this title online, 2003. http://rave.ohiolink.edu/etdc/view?acc%5Fnum=osu1070481734.

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Thesis (Ph. D.)--Ohio State University, 2003.
Title from first page of PDF file. Document formatted into pages; contains xviii, 201 p.; also includes graphics. Includes bibliographical references (p. 195-201).

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Zhang, Steven. "Formation Mechanism and Computational Modelling of Isle of Rum Plagioclase Stellates." Thèse, Université d'Ottawa / University of Ottawa, 2013. http://hdl.handle.net/10393/24068.

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We propose a hypothesis and a numerical model for the formation of branching plagioclase textures visible at both macroscopic (∼cm to ∼m) and microscopic scale within melagabbro of the Isle of Rum, Scotland, based on macroscopic, microscopic observations and relevant geological history. The plagioclase crystals are typically linked as twins and form meshes of planar stellate structures (m-scale) with a large range in geometrical organization from patchy to radiating. Evidence of macroscopic crystal aggregation and alignment is attributed to interfacial free energy minimization at the microscopic scale during growth. Accordingly, a binary immiscible Lattice Boltzmann model was developed to simulate diffusion of simplified plagioclase in the melt phase. Isothermal phase transitions modelled via first order chemical reactions are subsequently coupled with stochastic dynamics at the crystal growth front to simulate energy minimization processes including twinning during crystallization in an igneous environment. The solid phase and the liquid phase are coupled with a temporal flexibility that sets the overall ratio between the rate of diffusion and chemical enrichment in the liquid state and the rate of crystallization. The parameter space of the model is explored extensively, followed by a reasonable transcription of physical parameters and an estimation of other parameters to construct realistic simulation scenarios yielding synthetic plagioclase stellates. The results are presented, analyzed and discussed. They appear to be in reasonable qualitative agreement with observations, and several aspects of the natural stellates such as the stellate spacing and long continuous stretches of plagioclase with epitaxial junctions seem to be in reasonable quantitative agreement with observations.

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Ouyang, Liyan. "Synthèse de nanoparticules de cuivre par dismutation de complexes de cuivre (I) et développement de nouvelles voies d’accès par stratégie organométallique." Thesis, Montpellier, Ecole nationale supérieure de chimie, 2022. http://theses.enscm.fr/interne/ENSCM_2022_OUYANG.pdf.

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Le sujet de cette thèse concerne la synthèse de nanoparticules de cuivre (CuNPs) de morphologie et de taille contrôlées via des réactions de dismutation de complexes de Cu(I) ou de décomposition thermique d’organocuivres ainsi que l’étude de leurs propriétés physico-chimiques.Ce manuscrit est composé de quatre chapitres et d’une partie expérimentale.Le premier chapitre est une introduction bibliographique décrivant les méthodes et stratégies de synthèse des nanoparticules métalliques.Le deuxième chapitre décrit les résultats que nous avons obtenu lors de l’étude de la synthèse de nanoparticules de cuivre par réaction de dismutation de complexes cuivre/amines/phosphine. Il commence par une introduction où sont présentés et commentés les résultats de la littérature concernant les réactions de dismutation des sels de cuivre. Cette première sous partie se termine par la description des objectifs que nous nous étions fixés au commencement de ce travail, à savoir mesurer les effets des différents paramètres de cette réaction (la concentration, la température de réaction, l’ordre d’introduction des réactifs, le nombre d’équivalents et la nature des phosphines employées comme ligands…) sur la morphologie des nanoparticules pour tenter d’établir un modèle prédictif.Les conclusions tirées de l’ensemble de cette étude sont qu’il n’est pas possible de trouver une corrélation générale entre les caractéristiques physico-chimiques des ligands phosphorés étudiés (pKa, encombrement stérique, énergies d’affinité avec le sel de cuivre (I) utilisé) et la morphologie des nanoparticules de cuivre obtenues.Le troisième chapitre décrit les résultats que nous avons obtenu dans les réactions de dismutation de complexes de Cuivre ligandés par des carbènes de la famille des CAACs.Dans le dernier chapitre de cette thèse nous avons mis au point une nouvelle voie d’accès au nanoparticules de cuivre (0) qui n’utilise pas d’amines à longues chaines comme solvant et qui se fait dans des conditions réactionnelles beaucoup plus douces que celles décrites pour les réactions de dismutation. Nous avons tout d’abord mis au point un premier protocole permettant l’obtention de particules qui ont été entièrement caractérisées par l’ensemble des méthodes d’analyses disponibles, à savoir l’UV, le TEM, le HRTEM, le XRD et l’XPS. Ces analyses ont démontré que la réaction de thermolyse de complexes organocuivre-PPh3 produisait des nanoparticules de cuivre (0) de forme sphérique et de taille contrôlée
The synthesis of copper nanoparticles (CuNPs) with controlled shape and size have received increasing interest for a few years. The ability to control the shape and size of CuNPs have found enormous potential for altering their plasmonic, catalytic and optoelectronic properties.During these 3 years of thesis study, we have successfully synthetized CuNPs with tunable size and shape through several chemical methods.The first chapter of this work presented a brief overview of the most recent reports showing how noble metal nucleate and grow in solution. We thus focused on the reaction factors that are able to influence the final size and shape of the nanoparticles, and also the specific optical property of CuNPs. We also presented a few examples of methods reported in the literature for synthetizing CuNPs through the chemical reduction route.The Chapter II focused on the synthesis of designable size and morphology of CuNPs by using a range of phosphorus ligands (TOP, TOPO, PPh3, P(PhMe3)3, P(OMe)3, P(OC7H14)3, P(NMe2)3, P(O)((NMe2)3) via disproportionation route. From the use of these phosphorus ligands, we are able to form CuNPs with various shapes: cubes, spheres, octahedra, tetrahedra, rectangles, hexagons, wires and the mixed shapesIn Chapter III, we have investigated the preparation of CAACs-stabilized copper nanoparticles through a simple “one-pot” disproportionation reaction using only two reactants: CAACs-Cu(I) precursors and primary amine as solvent. We have been able to synthetize tunable size and shape-controlled CAACs-stabilized CuNPs by tuning the nature of CAACs ligands or solvent, and the amount of precursor or solvent.In Chapter IV, we reported a simple, innovative and robust thermal decomposition of organocopper reagents for the synthesis of well-defined CuNPs with narrow size distribution. Using this more atom-economic method, CuNPs were synthetized with tunable size ranging from 3 to 20 nm by modifying several reaction parameters (e.g. the amount of phosphine ligand, the temperature, the injection time and amount of DDT, the nature of the phosphorous ligand, the nature and amount of the Grignard reagent). This method involves organocopper (RCu) and organocuprate (R2Cu•MgX) compounds, both prepared in situ from commercially available copper halides and Grignard reagents. Both organocopper or organocuprate reagents were shown to undergo thermal decomposition under mild conditions to synthetize CuNPs

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Bajul, Audrey. "Influence des carboxymethylcelluloses sur la cristallisation des sels tartriques dans les solutions hydroalcooliques et les vins." Thesis, Toulouse, INPT, 2018. http://www.theses.fr/2018INPT0048.

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Un problème majeur de l’industrie vinicole est la difficulté à contrôler la cristallisation des sels tartriques. Ce phénomène confère une image négative chez les consommateurs, en particulier pour les vins effervescents dont l’instabilité tartrique provoque le débordement (ou gerbage) de la bouteille. La cristallisation de ces sels ioniques modifie l’équilibre acido-basique des vins qui influence leurs qualités organoleptiques. Parmi les méthodes de contrôle préventif, l’utilisation de carboxyméthylcellulose (CMC) a été autorisée dans les vins depuis 2009. Les CMC sont des polyélectrolytes que l’on retrouve sous forme de sels de sodium. Cette famille de macromolécules a démontré son pouvoir inhibiteur de la cristallisation des sels tartriques, mais le mécanisme précis reste encore incompris, ce qui empêche de fait l’optimisation des propriétés de ces CMC à travers les processus de fabrication développés par la société CELODEV. Les travaux de cette thèse visent à expliciter les mécanismes en jeu. La première partie est consacrée à la discrimination des propriétés des CMC pour améliorer l'effet inhibiteur des sels tartriques dans les solutions hydroalcooliques. La caractérisation des CMC concerne un large éventail de propriétés physico-chimiques de la CMC notamment le degré de substitution (DS) déterminé par RMN, la répartition des substituts le long de la chaine du polymère par méthode RMN et HPLC, le degré de polymérisation (DP) déterminé par viscosimétrie intrinsèque, le poids moléculaire déterminé par SEC-MALLS, le taux de cristallinité (Ic) et la stabilité du polymère déterminée par analyse thermique. D’autre part, les propriétés de charges du polyélectrolyte dans son milieu solvant ont été étudiées. Une méthode a été développée dans le cadre de ces travaux pour évaluer la densité de charges des CMC afin de comprendre le comportement qu’elles peuvent avoir en fonction de leurs propriétés physico-chimiques et des conditions du milieu. La deuxième partie de l’étude distingue les effets thermodynamiques et cinétiques de l’ajout de CMC dans des solutions modèles hydro-alcooliques – sels – CMC. La présence d'additifs dans une solution sursaturée peut influencer tous les paramètres de la cristallisation notamment la solubilité et la nucléation/croissance des cristaux. Dans cette étude, les solubilités des sels tartriques ont été déterminées expérimentalement, puis modélisée par un modèle thermodynamique permettant de prendre en compte la complexité du système composé de sels, alcool, et macromolécule. Les additifs dans ces travaux ne modifient pas la solubilité. Dans un second temps l’étude cinétique de cristallisation de ces sels en milieu hydro-alcoolique est réalisée avec la détermination de fréquences de nucléation des sels en milieu sursaturé (en présence ou non d’additifs) à travers des mesures de temps d’induction. Ces études de nucléation confirment l’inhibition importante des sels tartriques en présence des CMC et donne des informations concrètes sur le mécanisme de formation des cristaux en solution. Enfin, une analyse morphologique des cristaux a permis de montrer comment ces additifs modifient le faciès cristallin notamment sur la face {010}. La troisième et dernière partie utilise les deux précédentes parties pour définir les mécanismes hypothétiques sur l’inhibition tartrique par les CMC. En recoupant l’ensemble des propriétés des CMC, il a été démontré que les plus efficaces avaient un DS compris entre 0,90 et 0,95, un Ic faible pour avoir une flexibilité suffisante, un DP faible pour assurer une certaine accessibilité des charges pouvant s’adsorber sur les cristaux de KHT. Les approches phénoménologiques développées dans cette thèse ont permis de lever les verrous scientifiques concernant les modes d’actions des additifs de cristallisation sur les sels tartriques en vue de mettre en place des critères permettant le choix des CMC les plus performantes parmi toute la gamme pour le milieu vinicole
A major problem in the wine industry is the difficulty to control tartaric salts crystallization, which causes a negative feeling among consumers particularly in sparkling wine which causes excessive gushing. The crystallization of these ionic salts also changes the acid-base balance of the wine, which influences the organoleptic qualities of wines. Amongst the methods of control, the use of cellulose gums or carboxymethyl cellulose (CMC) was authorized for wine production since 2009. These CMCs are macromolecular polyelectrolytes in the form of sodium salts. This family of macromolecules has demonstrated its inhibitory power, but the precise mechanism is not fully understood yet. This currently prevents the optimization of the manufacturing process of these gums by the CELODEV Company. The aim work of this thesis is based on three main parts. The first part is devoted to the discrimination of the properties of carboxymethylcelluloses to improve the inhibitory effect face to tartaric salts in hydro alcoholic solutions. A whole study on the characterization of CMCs has been made. First, It deals with the physicochemical properties of CMC, which are the degree of substitution (DS) determined by NMR, the distribution of substitutes along the polymer chain by NMR and HPLC method, the degree of polymerization (DP) determined by viscometry, the molecular weight determined by SEC-MALLS and GPC, the degree of crystallinity (Ic) and the stability of the polymer determined by thermal analysis. Secondly, the charge properties of this polyelectrolyte depending on the medium has been studied. A method has been developed as part of this work to evaluate the charge density (q) of CMCs in order to understand the behaviour that these can have according to their physico-chemical properties and environmental conditions. The second part of the study is devoted to the study of the thermodynamics of hydro-alcoholic solutions - salts - CMC. The presence of additives in a supersaturated solution can influence all the parameters of the crystallization, in particular the solubility and the nucleation / growth of the crystals. In this study, the solubilities of tartaric salts were determined experimentally by a classical approach, then modelled by a thermodynamic model allowing to take into account the complexity of the system composed of salts, alcohol, and macromolecule. The additives do not change the solubility. In a second part, the kinetic study of crystallization of these salts in a hydro-alcoholic medium was approached with the determination of the frequency of nucleation in a supersaturated salts medium (with or without additives), determined from a measurement of induction time. These nucleation studies have provided concrete information on the crystal formation mechanism in solution. Finally, a morphological analysis of the crystals made it possible to show how these additives modify the crystalline facies especially on the {010} face. The third and last part defines the hypothetical mechanisms that CMCs have on tartaric inhibition. By cross-checking all the properties of the CMCs, it has been shown that the most effective CMCs have a DS between 0.90 and 0.95, a weak Ic to have sufficient flexibility and a low DP to ensure a certain accessibility of the crystal lattice for adsorption on THK crystals. Phenomenological approaches developed in this thesis will remove the barriers on the knowledge of the action mechanisms of additives on the crystallization of tartaric salts. Results will also help establish criteria for the selection of the most efficient CMC across the range for the wine-making industry

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19

Sebastián, Paula. "Surface Influence on the First Stages of Metal Electrodeposition in Ionic Liquids." Doctoral thesis, Universidad de Alicante, 2018. http://hdl.handle.net/10045/79870.

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Esta tesis es un estudio profundo de las propiedades interfaciales Metal|líquido iónico (LI), y como estas influyen en las primeras etapas de formación de electrodepósito metálico. Dos líquidos iónicos, de dos familias diferentes, se han escogido para el análisis: un Room Temperature Ionic Liquid (RTIL): [Emmim][Tf2N]; y un Deep Eutectic Solvent (DES): 1ChCl:2urea. Para estudiar la influencia superficial en los distintos procesos, se empleó monocristales de platino (Pt(111)) y de oro Au(hkl), principalmente. El análisis inicial de las propiedades interfaciales M(hkl)|LI se realizó utilizando, entre otras técnicas, la técnica de salto de temperatura con láser pulsante, técnica que además permitió estimar el valor de potencial de carga (valor característico de la interfase metal|electrolito) en cada caso. Una vez caracterizados electroquímicamente estos sistemas, ambos LIs se utilizaron para estudiar el depósito metálico en distintas superficies tanto orientadas como poliorientadas. En concreto, se investigó el depósito de Ag y Cu en DES y sobre carbono vítreo, obteniendo que el DES influenciaba el mecanismo de nucleación y crecimiento y permitía modular el tamaño de grano. Se analizó la formación de ad-capas UPD de Cu en Au(hkl) y en DES, observándose dependencia del proceso con la orientación del sustrato y el tipo de electrolito. Finalmente, se evaluó la aplicabilidad de estos dos solventes para modificar un sustrato orientado de platino (Pt(111)) con Ni, y la sensibilidad superficial del proceso, para ello combinando técnicas clásicas como voltametría cíclica y cronoamperometría con técnicas ex-situ como SEM y AFM (para recubrimientos de baja cobertura). El presente trabajo doctoral muestra el potencial de estos solventes para modificar una superficie con distinto metales y de forma sencilla, a través de la técnica formación de depósito electroquímico, abriendo la posibilidad de utilizar estos novedosos solventes para el diseño de nuevos materiales.

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20

Streiff, Dorothée. "Etude de la nucléation et de la propagation dynamique d'une rupture par la méthode des nombres d'ondes discrets." Université Joseph Fourier (Grenoble), 1995. http://www.theses.fr/1995GRE10242.

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Cette these etudie la nucleation et la propagation dynamique d'une rupture par une methode d'equations integrales aux frontieres ou les fonctions de green sont calculees par la methode des nombres d'ondes discrets. Pour cela un modele simple de crack en mode plan dans un milieu homogene, lineaire et elastique est considere. Pour caracteriser la propagation, les conditions de rupture proviennent du critere numeriquement defini par hamano. A partir de ce concept, plusieurs travaux ont ete effectues. Les resultats montrent que la vitesse de rupture obtenue depend de la resistance du milieu. Celle-ci est caracterisee par le coefficient s. Suivant les valeurs de s, la vitesse de rupture obtenue est sub-rayleigh ou super-cisaillante. Il a alors ete possible de montrer que lors du seisme d'imperial valley en californie, la vitesse de rupture pouvait atteindre des valeurs proches de la vitesse des ondes p dans le milieu. Nous avons ensuite considere le seisme de parkfield en californie pour introduire dans nos modeles le cas des sauts de faille. Les zones de nucleation possibles pour un second segment sont tout d'abord determinees. La propagation sur le second segment est realisee en supposant la presence d'une zone de faible resistance a la rupture. Deux cas de propagation ont ete modelise: dans le premier cas, la rupture s'arrete sur le premier segment avant de commencer a se propager sur le second segment, et dans le second cas, la rupture sur le deuxieme segment commence avant la fin de la propagation sur le premier segment. Nous avons alors observe que la nucleation et la propagation sur le second segment dependent de la valeur du coefficient s

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21

Borra, Venkata Shesha Vamsi. "Whiskers: The Role of Electric Fields in the Formation Mechanism and Methods for Whisker Growth Mitigation." University of Toledo / OhioLINK, 2017. http://rave.ohiolink.edu/etdc/view?acc_num=toledo1513381893591481.

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22

Scheck, Johanna [Verfasser]. "The Mechanisms of Iron(III) (Oxyhdr)oxide Nucleation / Johanna Scheck." Konstanz : Bibliothek der Universität Konstanz, 2017. http://d-nb.info/1135573220/34.

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23

Zheng, Yufeng. "Nucleation Mechanisms of Refined Alpha Microstructure in Beta Titanium Alloys." The Ohio State University, 2013. http://rave.ohiolink.edu/etdc/view?acc_num=osu1366296464.

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24

Selzer, Daniel Ruben [Verfasser]. "On Primary Crystal Nucleation in Electrolyte Solutions of Well-Soluble Substances Primary Nucleation Kinetics and Nucleation Mechanisms at Low and High Supersaturation / Daniel Ruben Selzer." München : Verlag Dr. Hut, 2021. http://d-nb.info/1232846686/34.

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25

Berhan, M. N. "Fatigue crack nucleation at second phase particles." Thesis, University of Newcastle Upon Tyne, 1992. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.357296.

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26

Ewing, Helen Clifford. "The nucleation and growth mechanisms of amorphous Si-N-O fibres." Thesis, University of Strathclyde, 1999. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.366923.

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27

Knight, P. A. "Nucleation and development of radiation damage in ceramics." Thesis, University of Oxford, 1989. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.258166.

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28

Husowitz, Barry Charles. "Effect of Confinement and Heterogeneity on Phase Behavior: A Density Functional Approach." Diss., The University of Arizona, 2007. http://hdl.handle.net/10150/196124.

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Density functional theory of statistical mechanics in a square gradient approximation was used to study nucleation in confined systems such as a cylindrical pore and in-between two cylindrical disks. This approximation was further applied to study the evaporation and condensation in nanopores with finite lengths. Confinement effects induced nucleation phenomena that are not observed in more open systems. Density functional theory was also used to explore the solvation properties of a spherical solute immersed in a supercritical diatomic fluid. The solute was modeled as a hard core Yukawa particle surrounded by a diatomic Lennard-Jones fluid represented by two fused tangent spheres using an interaction site approximation. The results of this study indicate that local density augmentation and the solvation free energies are particularly sensitive to changes in solute and solvent particle geometry and solute/solvent anisotropic interactions. Density functional theory allowed us to systematically study the effect of a variety of geometric and interaction parameters on the properties and behavior of all the systems. Although more sophisticated, but computationally more demanding, theoretical approaches can be used, our results provide fundamental physical insights into the behavior of real systems and create a solid basis for the development of more realistic models.

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29

Cui, Yuqing Ph D. Massachusetts Institute of Technology. "Study of nucleation mechanisms and rational design of small-scale continuous crystallizers." Thesis, Massachusetts Institute of Technology, 2016. http://hdl.handle.net/1721.1/104202.

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Thesis: Ph. D., Massachusetts Institute of Technology, Department of Chemical Engineering, 2016.
Cataloged from PDF version of thesis.
Includes bibliographical references.
Crystallization is a separation process widely used in several important industries such as food, pharmaceuticals, and specialty chemicals, where crystallization serves as the final step of purification and isolation. Crystallization is a two-step process: nucleation (birth of crystals) and crystal growth. Nucleation can occur via several mechanisms, and the most industrially relevant ones are heterogeneous nucleation (nucleation on a foreign surface), and contact secondary nucleation (nucleation from an existing crystal due to a contact force). Despite the importance of the nucleation process and decades of research, nucleation mechanisms remain enigmatic. This thesis advanced the understanding of the mechanisms of two types of nucleation: contact secondary nucleation, and contact-induced heterogeneous nucleation. By taking advantage of the thermodynamic and kinetic characteristics of glycine system, it is concluded that contact secondary nuclei could originate from both the semi-ordered solute molecules at the interface layer on the surface of existing crystals and directly from parent crystals themselves via the mechanism of micro-attrition depending on the magnitude of the contact force. The contact experiments on functionalized gold plates further confirmed that micro-attrition is not the exclusive mechanism for contact secondary nucleation-contact can induce nucleation even in the presence of only appropriate functional groups. For contact-induced heterogeneous nucleation, by relating nucleation frequency and polymorph with the strength and type of interactions between solute molecules and the templates, it is concluded that contact-induced heterogeneous nucleation share similarities with undisturbed heterogeneous nucleation in terms of attracting and stabilizing pre-nucleation clusters, but they are also fundamentally different in that crystals generated by the former are not chemically interacting with the templates. This thesis also describes the rational design of two small-scale continuous crystallizers, which are taking on increasingly important roles in the next-generation pharmaceutical manufacturing. The first design is a tubular crystallizer that generates crystals of narrow sizes through contact secondary nucleation. The second design is a multi-stage mixed-suspension mixed-product removal (MSMPR) system with a small footprint that works robustly at suspension handling. When optimizing this design with antisolvent crystallization, it was discovered that crystallization kinetic parameters are affected by antisolvent composition independent from supersaturation levels. This thesis highlights, for the first time, that optimizing continuous antisolvent crystallization in MSMPR crystallizers must take into account the effects of antisolvent composition on crystallization kinetics.
by Yuqing Cui.
Ph. D.

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30

Goyal, Vinay K. "Analytical Modeling of the Mechanics of Nucleation and Growth of Cracks." Diss., Virginia Tech, 2002. http://hdl.handle.net/10919/30006.

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With the traditional fracture mechanics approaches, an initial crack and self-similar progression of cracks are assumed. In this treatise, theoretical and numerical tools are developed to mathematically describe non-self-similar progression of cracks without specifying an initial crack. A cohesive-decohesive zone model, similar to the cohesive zone model known in fracture mechanics as Dugdale-Barenblatt model, is adopted to represent the degradation of the material ahead of the crack tip. This model unifies strength-based crack initiation and fracture based crack progression. The cohesive-decohesive zone model is implemented with an interfacial surface material that consists of an upper and lower surface connected by a continuous distribution of normal and tangential nonlinear elastic springs that act to resist either Mode I opening, Mode II sliding, Mode III sliding, or mixed mode. The initiation of fracture is determined by the interfacial strength and the progression of fracture is determined by the critical energy release rate. The material between two adjacent laminae of a laminated composite structure or the material between the adherend and the adhesive is idealized with an interfacial surface material to predict interfacial fracture. The interfacial surface material is positioned within the bulk material to predict discrete cohesive cracks. The proper work-conjugacy relations between the stress and deformation measures are identified for the interfacial surface theory. In the principle of virtual work, the interfacial cohesive-decohesive tractions are conjugate to the displacement jumps across the upper and lower surfaces. A finite deformation kinematics theory is developed for the description of the upper and lower surface such that the deformation measures are invariant with respect to superposed rigid body translation and rotation. Various mechanical softening constitutive laws thermodynamically consistent with damage mechanics are postulated that relate the interfacial tractions to the displacement jump. An exponential function is used for the constitutive law such that it satisfies a multi-axial stress criterion for the onset of delamination, and satisfies a mixed mode fracture criterion for the progression of delamination. A damage parameter is included to prevent the restoration of the previous cohesive state between the interfacial surfaces. In addition, interfacial constitutive laws are developed to describe the contact-friction behavior. Interface elements applicable to two dimensional and three dimensional analyses are formulated for the analyses of contact, friction, and delamination problems. The consistent form of the interface element internal force vector and the tangent stiffness matrix are considered in the formulation. We investigate computational issues related to interfacial interpenetration, mesh sensitivity, the number of integrations points and the integration scheme, mathematical form of the softening constitutive law, and the convergence characteristics of the nonlinear solution procedure when cohesive-decohesive constitutive laws are used. To demonstrate the predictive capability of the interface finite element formulation, steadystate crack growth is simulated for quasi-static loading of various fracture test configurations loaded under Mode I, Mode II, Mode III, and mixed-mode loading. The finite element results are in agreement with the analytical results available in the literature and those developed in this work. A progressive failure methodology is developed and demonstrated to simulate the initiation and material degradation of a laminated panel due to intralaminar and interlaminar failures. Initiation of intralaminar failure can be by a matrix-cracking mode, a fiber-matrix shear mode, and a fiber failure mode. Subsequent material degradation is modeled using damage parameters for each mode to selectively reduce lamina material properties. The interlaminar failure mechanism such as delamination is simulated by positioning interface elements between adjacent sublaminates. The methodology is validated with respect to experimental data available in the literature on the response and failure of quasi-isotropic panels with centrally located circular cutouts. Very good agreement between the progressive failure analysis and the experiments is achieved if the failure analyses includes the interaction of intralaminar and interlaminar failures in the postbuckling response of the panels. In addition, ideas concerning the implementation of a fatigue model incorporated with a cohesive zone model are discussed.
Ph. D.

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31

Guilleumas, Montserrat. "Nucleation and cavitation in liquid helium." Doctoral thesis, Universitat de Barcelona, 1995. http://hdl.handle.net/10803/667593.

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Las transiciones de fase no siempre ocurren bajo condiciones de equilibrio. Un fluido puede ser conducido a un estado metastable. A pesar de ser internamente estable, existe otra configuración con un potencial termodinámico menor, por tanto energéticamente más favorable y a la cual tenderá el sistema. Esta transición está bloqueada por una barrera termodinámica que puede ser superada con resultado de la formación y crecimiento de pequeños nucleos de la nueva fase estable en la fase metastable. Este proceso se llama nucleación (formación de gotas en un vapor sub-enfriado) o cavitacion (burbujas en un líquido sobre-calentado). En esta tesis hemos estudiado los fenómenos de cavitación y nucleación en helio dentro del marco de la teoria del funcional de la densidad. Primero hemos estudiado la activación térmica en 3He y 4He puros y en las mezclas 3He-4He, estimando la presión a partir de la cual tiene lugar la transición de fase. Para los sistemas puros hemos estudiado también la activación cuántica a muy bajas temperaturas, estimando la temperatura de transición entre ambos regímenes.

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32

Socrate, Simona. "Mechanics of microvoid nucleation and growth in high-strength metastable austenitic steels." Thesis, Massachusetts Institute of Technology, 1995. http://hdl.handle.net/1721.1/11257.

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Mohanty, Pravansu Sekhar. "Studies on the mechanisms of heterogeneous nucleation of grains and pores in aluminum castings." Thesis, McGill University, 1994. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=41722.

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In the present study, a fundamental theoretical and experimental investigation has been carried out on the mechanisms of heterogeneous nucleation of grains and pores in aluminum castings. A direct addition technique has been developed to introduce known types and quantities of inoculants into liquid aluminum alloys, irrespective of their wettability and chemical reactivity while preserving the surface characteristics and melt chemistry. Many different types of inoculants such as: $ rm Al sb2O sb3$, SiC, MgO, $ rm Mg sb2AlO sb4$, TiB$ sb2$, TiC, SrO and Sr(OH)$ sb2$ have been successfully added into liquid aluminum alloys, yielding single particulate distributions while avoiding incorporation of naturally occuring oxide films.
The commercial grain refining practice of Al and its alloys has been experimentally simulated by introducing synthetic TiB$ sb2$ and TiC crystals into melts containing dissolved Ti. Experimental findings indicate that in the absence of dissolved Ti, TiB$ sb2$ crystallites alone do not nucleate $ alpha$-Al. TiC particles which are generally believed to be the nucleating substrate are unstable and form various complex carbides. In the presence of dissolved Ti even below the peritectic level, an interfacial layer of TiAl$ sb3$ is formed at the TiB$ sb2$/melt interface which subsequently nucleates the $ alpha$-Al. A 'duplex' nucleation mechanism is proposed based on the solute segregation phenomenon to the substrate/melt interface. In the case of hypoeutectic Al-Si alloy, this interfacial layer was found to be a ternary compound of Al-Si-Ti, however, a drastic drop in the peritectic solidification temperature presumably reduces its grain refining potency at higher Si content.
Particles which do not nucleate the solid phase and/or do not get engulfed by the growing solid, are continuously rejected by the solid/liquid (S/L) interface until the end of local solidification. These substrates act as a barrier to the fluid flow as well as to the diffusion field at the S/L interface, giving rise to enhanced gas segregation and viscous pressure drop. A novel theoretical mechanism for the heterogeneous nucleation of pores has been proposed, based on this behaviour of foreign particles at the advancing S/L interface. Mathematical analyses have been employed to predict the gas segregation and pressure drop in the gap between the particle and the S/L interface. An order of magnitude analysis is done, and it is shown that pressures in the range of the activation barrier can be obtained in normal castings. To substantiate the mechanism further, experimental studies were carried out by introducing various possible inclusions into liquid aluminum. The experimental findings are in line with the theoretical predictions.

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34

Priante, Giacomo. "Formation mechanisms of heterostructures and polytypes in III-V nanowires." Thesis, Paris 6, 2016. http://www.theses.fr/2016PA066283/document.

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Ce travail examine des nanofils III-V synthétisés en mode vapeur-liquide-solide, une goutte liquide catalysant la croissance unidimensionnelle. En conjuguant expériences d’épitaxie par jets moléculaires, caractérisations structurales et analyses théoriques, j'étudie et clarifie plusieurs questions cruciales. L’une d'elles est le contrôle de la phase cristalline, qui s’avère souvent un mélange de segments cubiques et hexagonaux. Au moyen d’une analyse probabiliste de la séquence d'empilement de nanofils d’InP, je montre que la sélection de phase est déterminée non seulement par les conditions de croissance mais aussi par des interactions entre monocouches. Je souligne et discute le rôle de l'énergie de bord du germe qui médie la formation de chaque monocouche. On sait par ailleurs que le caractère abrupt des interfaces dans les hétérostructures axiales est limité par l’accumulation de matière dans la goutte (effet réservoir). J'étudie la formation de telles hétérostructures dans des nanofils de GaAs auto-catalysés en utilisant un second élément des groupes V (P) ou III (Al). Les profils compositionnels des insertions ternaires sont analysés à la résolution atomique. Les interfaces se révèlent soit plus larges (GaAsP) soit plus étroites (AlGaAs) qu’attendu et la morphologie du front de croissance dépend de la supersaturation. Je montre que, dans les deux cas, la largeur d’interface peut être réduite à quelques monocouches et je suggère d’autres améliorations. Enfin, je présente mes tentatives pour réaliser des nanofils ultraminces de GaAs et GaP et je discute du contrôle de la croissance à l’échelle d’une monocouche par réduction du caractère stochastique de la nucléation
This work investigates III-V nanowires synthesized via the vapor-liquid-solid method, whereby a catalyst droplet promotes one-dimensional growth. By combining molecular beam epitaxy experiments, structural characterization and theoretical analyses, I study and clarify several critical issues. One of them is the control of the crystal phase, which is frequently found to be a mix of cubic and hexagonal segments. By performing a probabilistic analysis of the stacking sequence of InP nanowires, I show that phase selection is determined not only by growth conditions but also by interactions between layers. I highlight and discuss the role of the edge energy of the nucleus that mediates the formation of each monolayer. Another important problem is the formation of axial heterostructures, which interface sharpness is severely limited by material accumulation in the droplet (‘reservoir effect’). To this end, I study the formation of such heterostructures in Ga-catalyzed GaAs nanowires using either a second group V element (P) or a second group III element (Al). The composition profiles of the ternary insertions are analyzed with monolayer resolution. The interface widths are found to be larger (GaAsP) or narrower (AlGaAs) than expected, and the morphology of the growth front depends on supersaturation. In both cases, I demonstrate that the interface width can be reduced to a few monolayers and suggest further improvements. Attempts to achieve ultrathin GaAs and GaP nanowires that would permit lateral quantum confinement are presented. Finally, I consider the possibility of minimizing the stochastic character of nucleation ultimately to control the growth of single monolayers

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Wigginton, Sarah S. "The Influence of Mechanical Stratigraphy on Thrust-Ramp Nucleation and Propagation of Thrust Faults." DigitalCommons@USU, 2018. https://digitalcommons.usu.edu/etd/7344.

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Our current understanding of thrust fault kinematics predicts that thrust faults nucleate on low angle, weak surfaces before they propagate upward and forms a higher angle ramp. While this classic kinematic and geometric model serves well in some settings, it does not fully consider the observations of footwall deformation beneath some thrust faults. We examine an alternative end-member model of thrust fault formation called “ramp-first” fault formation. This model hypothesizes that in mechanically layered rocks, thrust ramps nucleate in the structurally strong units, and that faults can propagate both upward and downward into weaker units forming folds at both fault tips. To explore this model, we integrate traditional structural geology field methods, two dimensional cross section reconstructions, and finite element modeling. Field data and retro-deformable cross sections suggest that thrust faults at the Ketobe Knob, in Utah nucleated in strong layers and propagated upward and downward creating folds in weak layers. These findings support the hypothesis that thrust faults and associated folds at the Ketobe Knob developed in accordance with the ramp-first kinematic model.We can apply this understanding of the mechanics behind thrust fault nucleation and propagation in mechanically layered stratigraphy to a wide range of geological disciplines like structural geology and tectonics, seismology, and petroleum geology. By incorporating our knowledge of lithology into fault models, geologists are more likely to correctly interpret structures with limited data sets.

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Tsivoulas, Dimitrios. "Effects of combined Zr and Mn additions on the microstructure and properties of AA2198 sheet." Thesis, University of Manchester, 2011. https://www.research.manchester.ac.uk/portal/en/theses/effects-of-combined-zr-and-mn-additions-on-the-microstructure-and-properties-of-aa2198-sheet(6bb2c9db-7584-464b-8064-bab0cc2d397c).html.

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The effect of individual and combined zirconium and manganese additions have been compared for an AA2198 6 mm thick sheet in T351 temper regarding their influence primarily on recrystallisation resistance and secondly on fracture toughness and overageing resistance. A complete characterisation of the dispersoid distributions was carried out for a deeper understanding of the effects of the Al3Zr and Al20Cu2Mn3 particles, involving studying their formation from the as-cast and homogenised stage.The most important finding in this work was the lower recrystallisation resistance in the alloy containing 0.1 wt%Zr + 0.3 wt%Mn compared to that containing only 0.1 wt%Zr. This result was rather unexpected, if one considers the opposite microsegregation patterns of Zr and Mn during casting, which leads to dispersoids occupying the majority of the grains’ volume and minimising dispersoid-free zones that could be potential sites for nucleation of recrystallisation. The other two alloys with dispersoid additions 0.05 wt%Zr + 0.3 wt%Mn and 0.4 wt%Mn, were partially and fully recrystallised respectively in the rolled T351 condition.Equally important in this work, was the observation that the opposite microsegregation trend of Zr and Mn sufficed to restrict grain growth in unrecrystallised areas. The 0.1Zr-0.3Mn alloy exhibited the lowest grain size of all alloys, both in the T351 temper and after annealing at 535oC for up to 144 hours. The reason for this was the combined action of Al20Cu2Mn3 dispersoids and Mn solute in the regions where the Zr concentration was low (i.e. near the grain boundaries), which offered additional pinning pressure to those areas compared to the 0.1Zr alloy.The lower recrystallisation resistance of the 0.1Zr-0.3Mn alloy was explained on the grounds of two main factors. The first was the lower subgrain size and hence stored energy within bands of Al20Cu2Mn3 dispersoids, which increased the driving force for recrystallisation in these regions. The second was the interaction between Zr and Mn that led to a decrease in the Al3Zr number density and pinning pressure. Since Zr was the dominant dispersoid family in terms of inhibiting recrystallisation, inevitably this alloy became more prone to recrystallisation. The Al3Zr pinning pressure was found to be much lower especially within bands of Al20Cu2Mn3 dispersoids. The detrimental effect of the Mn addition on the Al3Zr distribution was proven not to result from the dissolution of Zr within Mn-containing phases, and several other phases, at the grain interior and also in grain boundaries. The observed effect could not be precisely explained at this stage.Concerning mechanical properties, the 0.1Zr alloy exhibited the best combination of properties in the Kahn tear tests for fracture toughness. Further, it had a higher overageing resistance compared to the 0.1Zr-0.3Mn alloy.As an overall conclusion from this work, considering all the studied properties here that are essential for damage tolerant applications, the addition of 0.1 wt%Zr to the AA2198 6 mm thick sheet was found to be superior to that of the combined addition of 0.1 wt%Zr + 0.3 wt%Mn.

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37

Jullian, Christelle Francoise. "Investigation of polarization switching over broad time and field domains in various ferroelectrics." Thesis, Virginia Tech, 2003. http://hdl.handle.net/10919/9853.

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Investigations of polarization switching over broad time and electric field domains, in various modified Pb-based perovskite ferroelectrics, were systematically performed by ferroelectric switching current transient and bipolar drive P-E responses. Studies were performed from E«Ec to E»Ec, where Ec is the coercive field These investigations have shown the presence of broad relaxation time distributions for the switching process, which can extend over several decades in order of magnitude in time, and where the distribution is strongly dependent on the applied electric field. By performing the study of domain dynamics and polarization switching over extremely broad time domains (10⁻⁸ t < 10² sec), more complete information has been obtained that allows for development of a better mechanistic understanding. Prior polarization kinetics studies have focused on relatively narrow time ranges, and were fit to the Avarami equation, which contains a single relaxation time. However, our broad band width polarization dynamics and frequency relaxation studies have been fit to multiple stretched exponential functions extending over decades of order of magnitude in the time domain. Stretched exponential functions for domain nuclei formation, and for domain variant growth have been found. For example, [001]c, [110]c, and [111]c oriented PZN-4.5%PT crystals, nucleation was found to be a volume process (n=3) rather than just a domain wall restricted process. Consequently, nucleation is heterogeneous. And, growth of a domain variant with reversed polarization was found to be a boundary process (n=2), involving diffuse or rough domain walls. We have extended these studies to various types of ferroelectrics including hard, soft and relaxor types.
Master of Science

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38

González, Oyarce Aníbal Lautaro. "360° domain walls : nucleation mechanisms during thin film switching, and their application to high density non-volatile memory." Thesis, University of Cambridge, 2014. https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.708186.

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39

Boulfelfel, Salah Eddine. "Atomic Scale Investigation of Pressure Induced Phase Transitions in the solid State." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2009. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-25283.

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In this work, atomic scale investigation of pressure-induced transformations in the solid state have been carried out. A series of compounds including GaN, ZnO, CaF2, and AgI, in addition to elemental phosphorus have been studied. The corresponding transition mechanisms have been elucidated with a clear description of atomic displacements and intermediate structures involved therein. In the first group of compounds, the long standing debate on the transition path of the wurtzite(WZ)-to-rocksalt(RS) transition in semiconductors, GaN and ZnO was resolved using geometrical modeling combined with molecular dynamics (MD) simulations conducted in the frame of transition path sampling (TPS) method. In GaN, a two-step mechanism through a metastable intermediate phase with a tetragonal structure iT has been revealed from simulations. In ZnO, the tetragonal intermediate structure was kinetically less stable, although still part of the real transition mechanism. It appeared at the interface between WZ and RS as consequence of a layers shearing. The transition regime in ZnO was characterized by a competition between iT structure and another hexagonal intermediate with hexagonal symmetry iH. Although possible, the latter is not functional for the transition. In both cases, GaN and ZnO, two points of agreement with experiments have been revealed. The tilting of structures after transition, and the phonon mode softening associated with atomic displacements leading to the tetragonal structure iT In the second group of compounds, the investigation of transitions in superionic conductors, CaF2 and AgI, demonstrated a different and particular behavior of atomic motion under pressure. The solid-solid reconstruction of CaF2 structure was shown to be initiated and precedented by high disorder of the anionic sublattice. The percolation of fluoride ions through voids in the fluorite structure created a thin interface of liquid like state. The sparce regions caused by the departure of anions facilitates the cation sublattice reconstruction. In AgI, ion diffusion during the wurtzite/zincnlende(ZB)$rocksalt transition was more pronounced due to the extended stacking disorder WZ/ZB. The Ag+ ions profited not only from the structure of the interface but used the combination of interstitial voids offered by both phases, WZ and ZB, to achieve long diffusion paths and cause the cation sublattice to melt. Clearly, a proper account for such phenomena cannot be provided by geometry-designed mechanisms based on symmetry arguments. In phosphorus, the question of how the stereochemically active lone pairs are reorganized during the orthorhombic (PI) to trigonal (PV) structural transition was answered by means of simulations. Computation was performed at different levels theory. First, the mechanism of the transition was obtained from TPS MD simulations. MD runs were performed within density functional tight binding method (DFTB). The analysis of atomic displacements along the real transformation path indicated a fast bond switching mechanism. In a second step, the nature of the interplay between orbitals of phosphorus during the bond switching was investigated. A simultaneous deformation of lone pair and P−P bond showed a mutual switching of roles during the transformation. This interplay caused a low dimensional polymerization of phosphorus under pressure. The corresponding structure formed as zigzag linear chain of fourfold coordinated phosphorus atoms (· · ·(P(P2))n · · ·) at the interface between PI and PV phases. A further result of this work was the development of a simulation strategy to incorporate defects and chemical doping to structural transformations. On top of the transition path sampling iterations, a Monte Carlo like procedure is added to stepwise substitute atoms in the transforming system. Introducing a chemically different dopant to a pure system represents a perturbation to the energy landscape where the walk between different phases is performed. Therefore, any change in the transition regime reflects the kinetic preference of a given structural motif at times of phase formation. This method was applied to the elucidation of WZ-RS transition mechanism in the series of semiconducting compounds AlN, GaN, and InN. Simulations showed that In atoms adopt the same transformation mechanism as in GaN and favor it, while Al atoms demonstrated a significant reluctance to the path going through tetragonal intermediate iT. The difference between transition regime in mixed systems InxGa1−xN and AlxGa1−xN is in agreement with experiments on high pressure behavior of AlN, GaN, and InN. While transitions in GaN and InN are reversible down to ambient conditions, AlN is stable. The work presented in this thesis constitutes the seed of new perspectives in the understanding of pressure-induced phase transformations in the solid state, where the physics and the chemistry are brought together by means of computer simulations.

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40

Carvalho, Ana Teresa Carrilho. "The mechanism of the Legionella VipA protein in altering actin dynamics during infection." Master's thesis, Universidade Nova de Lisboa, Instituto de Tecnologiaa Química e Biológica António Xavier, 2019. http://hdl.handle.net/10362/120872.

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Dissertation presented to obtain the Master Degree in Biochemistry for Health
"Legionella pneumophila is a facultative intracellular bacterium that is commonly found in fresh water and soils, parasitizing a wide range of protozoa hosts. Co-evolution mechanisms allowed this bacterium to parasitize other phagocytic cells and rendered it the ability to become a potential human pathogen. For instance, after inhalation of contaminated aerosols by the host, it can survive and replicate inside human macrophages in the lungs by creating a specialized phagosome that is not degraded, possibly resulting in the severe pneumonia known as Legionnaires’ disease. L. pneumophila relies on a type IV secretion system to inject effector proteins into the host cell to disrupt or take advantage of several cellular processes.(...)"
N/A

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41

Tucker, Garritt J. "Atomistic simulations of defect nucleation and free volume in nanocrystalline materials." Diss., Georgia Institute of Technology, 2011. http://hdl.handle.net/1853/41107.

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Atomistic simulations are employed in this thesis to investigate defect nucleation and free volume of grain boundaries and nanocrystalline materials. Nanocrystalline materials are of particular interest due to their improved mechanical properties and alternative strain accommodation processes at the nanoscale. These processes, or deformation mechanisms, within nanocrystalline materials are strongly dictated by the larger volume fraction of grain boundaries and interfaces due to smaller average grain sizes. The behavior of grain boundaries within nanocrystalline materials is still largely unknown. One reason is that experimental investigation at this scale is often difficult, time consuming, expensive, or impossible with current resources. Atomistic simulations have shown the potential to probe fundamental behavior at these length scales and provide vital insight into material mechanisms. Therefore, work conducted in this thesis will utilize atomistic simulations to explore structure-property relationships of face-centered-cubic grain boundaries, and investigate the deformation of nanocrystalline copper as a function of average grain size. Volume-averaged kinematic metrics are formulated from continuum mechanics theory to estimate nonlocal deformation fields and probe the nanoscale features unique to strain accommodation mechanisms in nanocrystalline metals. The kinematic metrics are also leveraged to explore the tensile deformation of nanocrystalline copper at 10K. The distribution of different deformation mechanisms is calculated and we are able to partition the role of competing mechanisms in the overall strain of the nanocrystalline structure as a function of grain size. Grain boundaries are observed to be influential in smaller grained structures, while dislocation glide is more influential as grain size increases. Under compression, however, the resolved compressive normal stress on interfaces hinders grain boundary plasticity, leading to a tension-compression asymmetry in the strength of nanocrystalline copper. The mechanisms responsible for the asymmetry are probed with atomistic simulations and the volume-averaged metrics. Finally, the utility of the metrics in capturing nonlocal nanoscale deformation behavior and their potential to inform higher-scaled models is discussed.

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42

Khan, Mushtaq Ahmad. "A study of fatigue crack nucleation and propagation properties of a B.S. 4360 - 50D structural steel by ultrasonic methods." Thesis, Brunel University, 1993. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.261124.

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43

Decker, Franziska [Verfasser], Stephan [Gutachter] Grill, and Jan [Akademischer Betreuer] Brugués. "Mechanisms of microtubule nucleation in metaphase spindles and how they set spindle size / Franziska Decker ; Gutachter: Stephan Grill ; Betreuer: Jan Brugués." Dresden : Technische Universität Dresden, 2018. http://d-nb.info/1226813518/34.

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44

Pendse, Siddhi. "Morphological properties of poly (ethylene terephthalate) (PET) nanocomposites in relation to fracture toughness." Thesis, University of North Texas, 2005. https://digital.library.unt.edu/ark:/67531/metadc4845/.

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The effect of incorporation of montmorillonite layered silicate (MLS) on poly (ethylene terephthalate) (PET) matrix was investigated. MLS was added in varying concentration of 1 to 5 weight percent in the PET matrix. DSC and polarized optical microscopy were used to determine the crystallization effects of MLS addition. Non isothermal crystallization kinetics showed that the melting temperature and crystallization temperature decrease as the MLS percent increases. This delayed crystallization along with the irregular spherulitic shape indicates hindered crystallization in the presence of MLS platelets. The influence of this morphology was related with the fracture toughness of PET nanocomposites using essential work of fracture coupled with the infra red (IR) thermography. Both the essential as well as non essential work of fracture decreased on addition of MLS with nanocomposite showing reduced toughness.

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45

Jin, Yan. "THREE-DIMENSIONAL MICROSTRUCTURAL EFFECTS ON MULTI-SITE FATIGUE CRACK NUCLEATION BEHAVIORS OF HIGH STRENGTH ALUMINUM ALLOYS." UKnowledge, 2016. http://uknowledge.uky.edu/cme_etds/63.

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An experimental method was further developed to quantify the anisotropy of multi-site fatigue crack initiation behaviors in high strength Al alloys by four-point bend fatigue testing under stress control. In this method, fatigue crack initiation sites (fatigue weak-links, FWLs) were measured on the sample surface at different cyclic stress levels. The FWL density in an alloy could be best described using a three-parameter Weibull function of stress, though other types of sigmoidal functions might also be used to quantify the relationship between FWL density and stress. The strength distribution of the FWLs was derived from the Weibull function determined by fitting the FWLs vs. stress curve experimentally obtained. As materials properties, the FWL density and strength distribution could be used to evaluate the fatigue crack nucleation behaviors of engineering alloys quantitatively and the alloy quality in terms of FWL density and strength distribution. In this work, the effects of environment, types of microstructural heterogeneities and loading direction on FWLs were all studied in detail in AA7075-T651, AA2026-T3511, and A713 Al alloys, etc. It was also found that FWLs should be quantified as a Weibull-type function of strain instead of stress, when the applied maximum cyclic stress exceeded the yield strength of the tested alloys. In this work, four-point bend fatigue tests were conducted on the L-T (Rolling-Transverse), L-S (Rolling-Short transverse) and T-S planes of an AA7075-T651 alloy plate, respectively, at room temperature, 20 Hz, R=0.1, in air. The FWL populations, measured on these surfaces, were a Weibull-type function of the applied maximum cyclic stress, from which FWL density and strength distribution could be determined. The alloy showed a significant anisotropy of FWLs with the weak-link density being 11 mm-2, 15 mm-2 and 4 mm-2 on the L-T, L-S and T-S planes, respectively. Fatigue cracks were predominantly initiated at Fe-containing particles on the L-T and L-S planes, but only at Si-bearing particles on the T-S plane, profoundly demonstrating that the pre-fractured Fe-containing particles were responsible for crack initiation on the L-T and L-S planes, since the pre-fracture of these particles due to extensive deformation in the L direction during the prior rolling operation could only promote crack initiation when the sample was cyclically stressed in the L direction on both the L-T and L-S planes. The fatigue strengths of the L-T, L-S and T-S planes of the AA7075 alloy were measured to be 243.6, 273.0 and 280.6 MPa, respectively. The differences in grain and particle structures between these planes were responsible for the anisotropy of fatigue strength and FWLs on these planes. Three types of fatigue cracks from particles, type-I: the micro-cracks in the particles could not propagate into the matrix, i.e., type-II: the micro-cracks were fully arrested soon after they propagated into the matrix, and type-III: the micro-cracks became long cracks, were observed in the AA7075-T651 alloy after fatigue testing. By cross-sectioning these three-types of particles using Focused Ion Beam (FIB), it was found that the thickness of the particles was the dominant factor controlling fatigue crack initiation at the particles, namely, the thicker a pre-fractured Fe-containing particle, the easier it became a type-III crack on the L-T and L-S planes. On the T-S plane, there were only types-I and III Si-bearing particles at which crack were initiated. The type-I particles were less than 6.5 μm in thickness and type-III particles were thicker than 8.3 μm. Cross-sectioning of these particles using FIB revealed that these particles all contained gas pores which promoted crack initiation at the particles because of higher stress concentration at the pores in connection with the particles. It was also found that fatigue cracks did not always follow the any specific crystallographic planes within each grain, based on the Electron Backscatter Diffraction (EBSD) measurement. Also, the grain orientation did not show a strong influence on crack initiation at particles which were located within the grains. The topography measurements with an Atomic Force Microscope (AFM) revealed that Fe-containing particles were protruded on the mechanically polished surface, while the Si-bearing particles were intruded on the surface, which was consistent with hardness measurements showing that Si-bearing particles were softer (4.030.92 GPa) than Fe-containing ones (8.9 0.87 GPa) in the alloy. To verify the 3-D effects of the pre-fractured particles on fatigue crack initiation in high strength Al alloys, rectangular micro-notches of three different types of dimensions were fabricated using FIB in the selected grains on the T-S planes of both AA2024-T351 and AA7075-T651 Al alloys, to mimic the three types of pre-fractured particles found in these alloys. Fatigue testing on these samples with the micro-notches verified that the wider and deeper the micro-notches, the easier fatigue cracks could be initiated at the notches. In the AA2024-T351 samples, cracks preferred to propagate along the {111} slip plane with the smallest twist angle and relatively large Schmid factor. These experimental data obtained in this work could pave a way to building a 3-D quantitative model for quantification of fatigue crack initiation behaviors by taking into account the driving force and resistance to short crack growth at the particles in the surface of these alloys.

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46

Colombi, Ciacchi Lucio. "Growth of Platinum Clusters in Solution and on Biopolymers: The Microscopic Mechanisms." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2002. http://nbn-resolving.de/urn:nbn:de:swb:14-1026811686718-71695.

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Thema der vorgelegten Dissertation ist der Mechanismus der Keimbildung und des Wachstums von Platinclustern in Lösung und auf Biopolymeren nach der Reduktion von Platin-Salzen. Die Untersuchung wird auf atomarer Skala durch ab-initio Molekulardynamik mit der Methode von Car und Parrinello durchgeführt. In einem klassischen, generell akzeptierten Mechanismus erfolgt die Aggregation von Pt-Atomen nur nach kompletter Reduktion der Pt(II)-Komplexen zum metallischen Pt(0)-Zustand. Im Gegensatz dazu, in der hier beobachteten Reaktionsablauf entstehen stabile Pt-Pt-Bindungen schon nach einem einzigen Reduktionsschritt. Darüber hinaus wird es gefunden, dass kleine Pt-Cluster durch Addition von unreduzierten PtCl2(H2O)2-Komplexen wachsen können. Das stimmt mit einem experimentell beocbachteten autokatalytischen Clusterwachstumsmechanismus überein. Es wird weiterhin gefunden, dass Pt(II)-Komplexe, die kovalent an DNA oder an Proteine gebunden sind, als sehr effiziente Nukleationszentren für das weitere Metallclusterwachstum wirken können. Das ist eine Konsequenz des starken Donor-Charakters der organischen Liganden, in derer Anwesenheit stärkere Metall-Metall-Bindungen als frei in der Lösung gebildet werden können. In der Tat, in Metallisierungsexperimenten können 5 Nanometer dünne, mehrere Mikrometer lange, regelmässige Clusterkette erzeugt werden, die rein heterogen auf das Biomolekulare Templat gewachsen sind
In this thesis we investigate the molecular mechanisms of platinum cluster nucleation and growth in solution and on biopolymers by means of first-principles molecular dynamics. In contrast with a classical picture where clusters nucleate by aggregation of metallic Pt(0) atoms, we find that Pt--Pt bonds can form between dissolved Pt(II) complexes already after a single reduction step. Furthermore, we observe that small clusters grow by addition of unreduced PtCl2(H2O)2 complexes, consistently with an autocatalytic growth mechanism. Moreover Pt(II) ions covalently bound to biopolymers are found to act as preferred nucleation sites for the formation of clusters. This is a consequence of the strong donor character of the organic ligands which induce the formation of stronger metal-metal bonds than those obtained in solution. In fact, in metallization experiments we obtain a clean and purely heterogeneous metallization of single DNA molecules leading to thin and uniform Pt cluster chains extended over several microns

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47

Lay, Philippe. "Elaboration et caracterisation de lingots de silicium polycristallin polix." Caen, 1987. http://www.theses.fr/1987CAEN2001.

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Le but de cette etude est le developpement du procede d'elaboration de lingots de si polycristallin polix, par caracterisation de ces lingots et correlation avec l'arrangement experimental. Les proprietes du materiau dependent des conditons de germination et de croissance. L'encapsulation du lingot et les conditons thermiques limitent le nombre de cristaux est contribuent a leur developpement. Les plans de macle (111) jouent un role predominant dans le mecanisme de germination. La croissance est controlee par les transferts thermiques et la qualite chimique. Un modele thermique a permis d'isoler les parametres principaux influencant la vitesse de solidification et l'extrapolation a de grands lingots. Mise en place de methodes de caracterisation pour optimiser les conditions d'elaboration

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48

Lid, Steffen [Verfasser], Ciacchi Lucio [Akademischer Betreuer] Colombi, Ciacchi Lucio [Gutachter] Colombi, and Dominik [Gutachter] Horinek. "Molecular mechanisms of crystal nucleation and growth at ferritin/oxide Interfaces : a theoretical investigation / Steffen Lid ; Gutachter: Lucio Colombi Ciacchi, Dominik Horinek ; Betreuer: Lucio Colombi Ciacchi." Bremen : Staats- und Universitätsbibliothek Bremen, 2018. http://d-nb.info/1166849732/34.

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49

Colombi, Ciacchi Lucio. "Growth of Platinum Clusters in Solution and on Biopolymers: The Microscopic Mechanisms." Doctoral thesis, [S.l.] : [s.n.], 2001. http://deposit.ddb.de/cgi-bin/dokserv?idn=964998998.

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50

Lazar, Paul. "Transport mechanisms and wetting dynamics in molecularly thin films of long-chain alkanes at solid/vapour interface : relation to the solid-liquid phase transition." Phd thesis, Universität Potsdam, 2005. http://opus.kobv.de/ubp/volltexte/2005/527/.

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Wetting and phase transitions play a very important role our daily life. Molecularly thin films of long-chain alkanes at solid/vapour interfaces (e.g. C30H62 on silicon wafers) are very good model systems for studying the relation between wetting behaviour and (bulk) phase transitions. Immediately above the bulk melting temperature the alkanes wet partially the surface (drops). In this temperature range the substrate surface is covered with a molecularly thin ordered, solid-like alkane film ("surface freezing"). Thus, the alkane melt wets its own solid only partially which is a quite rare phenomenon in nature. The thesis treats about how the alkane melt wets its own solid surface above and below the bulk melting temperature and about the corresponding melting and solidification processes.
Liquid alkane drops can be undercooled to few degrees below the bulk melting temperature without immediate solidification. This undercooling behaviour is quite frequent and theoretical quite well understood. In some cases, slightly undercooled drops start to build two-dimensional solid terraces without bulk solidification. The terraces grow radially from the liquid drops on the substrate surface. They consist of few molecular layers with the thickness multiple of all-trans length of the molecule. By analyzing the terrace growth process one can find that, both below and above the melting point, the entire substrate surface is covered with a thin film of mobile alkane molecules. The presence of this film explains how the solid terrace growth is feeded: the alkane molecules flow through it from the undercooled drops to the periphery of the terrace.
The study shows for the first time the coexistence of a molecularly thin film ("precursor") with partially wetting bulk phase. The formation and growth of the terraces is observed only in a small temperature interval in which the 2D nucleation of terraces is more likely than the bulk solidification. The nucleation mechanisms for 2D solidification are also analyzed in this work. More surprising is the terrace behaviour above bulk the melting temperature. The terraces can be slightly overheated before they melt. The melting does not occur all over the surface as a single event; instead small drops form at the terrace edge. Subsequently these drops move on the surface "eating" the solid terraces on their way. By this they grow in size leaving behind paths from were the material was collected. Both overheating and droplet movement can be explained by the fact that the alkane melt wets only partially its own solid. For the first time, these results explicitly confirm the supposed connection between the absence of overheating in solid and "surface melting": the solids usually start to melt without an energetic barrier from the surface at temperatures below the bulk melting point. Accordingly, the surface freezing of alkanes give rise of an energetic barrier which leads to overheating.
Sowohl Benetzung als auch Phasenübergänge spielen eine sehr wichtige Rolle im täglichen Leben. Molekular dünne Filme langkettiger Alkane an Festkörper/Gas-Grenzflächen (z. B. C30H62 an Silizium-Waferoberflächen) sind sehr gute Modellsysteme um die Wechselbeziehung zwischen Benetzungsverhalten und (Volumen-)Phasenübergängen zu untersuchen. In einem Temperaturbereich knapp oberhalb der Volumenschmelztemperatur benetzt die Alkanschmelze die Substratoberfläche nur partiell (Alkantropfen). In diesem Temperaturbereich ist die Substratoberfläche mit einer molekular dünnen, festkörperartig geordneten Alkanschicht bedeckt ("Oberflächengefrieren" ). Die Alkanschmelze benetzt also die eigene Festkörperoberfläche nur partiell, ein in der Natur ziemlich seltenes Phänomen. Die Dissertation beschäftigt sich damit wie die Alkanschmelze ihre eigene Festkörperoberfläche über und unter dem Volumenschmelzpunkt benetzt und mit den entsprechenden Vorgängen beim Schmelzen bzw. Erstarren. Flüssige Alkantropfen lassen sich einige Grad unter ihren Schmelzpunkt unterkühlen ohne sich sofort zu verfestigen. Dieses "Unterkühlungsverhalten" ist üblich und es ist theoretisch qualitativ gut verstanden. Allerdings beobachtet man bei den Alkanen bei leichter Unterkühlung statt einer eventuellen Volumenverfestigung oft die Ausbildung von zweidimensionalen Terrassen aus erstarrtem Alkanen. Die Terrassen wachsen auf der Substratoberfläche radial aus den flüssigen Tropfen. Sie bestehen aus wenigen Alkanlagen mit jeweils der Dicke einer Moleküllänge. Die Analyse der Terrassen-Wachstumsprozesse zeigt, dass die gesamte Substratoberfläche einschliesslich der Terrassen sowohl oberhalb als auch unterhalb der Volumenschmelztemperatur mit einer dünnen Schicht mobiler Alkanmoleküle bedeckt ist. Durch diese Schicht fliessen bei Unterkühlung die Alkane vom unterkühlten Tropfen zur Terrassenkante und liefern den Nachschub für deren Wachstum. Die Untersuchungen zeigen damit erstmalig die Koexistenz eines molekular dünnen Films ("Precursor") mit einer partiell benetzenden Volumenphase. Die Entstehung und das Wachstum der Terrassen wird nur in einem engen Temperaturfenster beobachtet in dem die Keimbildung zweidimensionaler Terrassen wahrscheinlicher ist als die dreidimensionale Volumenverfestigung. Auch dieses Keimbildungsverhalten wird in der Dissertation genauer analysiert. Noch erstaunlicher als das Terrassenwachstum, d. h. das Verfestigungsverhalten ist das Schmelzverhalten der Terrassen. Sie lassen sich bis zu einer gewissen Temperatur überhitzen bevor sie schmelzen! Weiterhin findet bei genügender Überhitzung das Schmelzen nicht gleichzeitig überall statt sondern es entstehen zuerst kleine Alkantropfen an den Terrassenrändern. Diese bewegen sich dann über die Substratoberfläche und "fressen" sich durch die festen Terrassen. Dabei wachsen sie weil sie das geschmolzene Material aufnehmen und hinterlassen eine alkanfreie Spur. Sowohl die Überhitzung als auch die Tropfenbewegung lassen sich damit erklären dass die flüssige Alkanschmelze ihren eigenen Festkörper nur partiell benetzt. Die Ergebnisse bestätigen erstmals explizit den seit vielen Jahrzehnten vermuteten Zusammenhang zwischen der üblicherweise nicht beobachtbaren Überhitzung von Festkörpern und Oberflächenschmelzen: Festkörper beginnen normalerweise ohne Energiebarriere von der Oberfläche an zu schmelzen. Entsprechend bildet das Oberflächengefrieren der Alkane eine Energiebarriere und erlaubt damit deren Überhitzen.

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Dissertations / Theses on the topic 'Nucleation mechanism'

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