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1
Tabera, Martin Luis. « Evolution and properties of planetary systems ». Thesis, Uppsala universitet, Observationell astrofysik, 2020. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-438128.
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Thelwall, Simon. « The properties of 4.1G : localisation, interactions and evolution ». Thesis, University of Kent, 2006. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.429656.
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Groenewegen, Martin Arnold Theodoor. « On the evolution and properties of AGB stars ». Amsterdam : Amsterdam : Sterrenkundig Instituut 'Anton Pannekoek' ; Universiteit van Amsterdam [Host], 1993. http://dare.uva.nl/document/91969.
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Pascut, Aurelia. « Evolution of X-ray properties of galaxy groups ». Thesis, University of Birmingham, 2015. http://etheses.bham.ac.uk//id/eprint/6235/.
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This thesis presents the results from the Chandra Deep Group Survey, a survey dedicated to find high redshift groups of galaxies in deepest observations available in the Chandra archive. The catalogue compiled from this survey contains 26 groups and 36 clusters with available redshifts, with largest redshift being 1.3. This sample has been used to investigate the evolution of the thermal state of the gas at the centre of groups and compare this evolution to that of clusters. Different parameters have been used to quantify the strength of cool cores and it has been showed that groups and clusters have similar evolution of their cool core properties. Both classes of systems have a wide spread in the cool core strength at low redshifts, which then narrows at high redshifts showing a lack of strong cool core systems.
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Pascut, Aurelia. « Evolution of X-ray Properties of Galaxy Groups ». Thesis, University of Birmingham (United Kingdom), 2017. http://pqdtopen.proquest.com/#viewpdf?dispub=10591255.
Texte intégralRésumé :
Studies of scaling relations in groups and clusters of galaxies have shown that the X-ray properties of groups deviate the most from the self-similar prediction. This is because groups are more affected by non-gravitational processes due to their shallower potential well, a behaviour which makes groups an ideal class of systems for the study of the impact of feedback. From the observational point of view, the study of the X-ray properties of groups, especially at high redshifts is hindered by their lower surface brightness compared to their more massive counterparts. We present the result from the Chandra Deep Group Survey, a survey dedicated to find high redshift groups in the deepest observations available in the Chandra archive. We found 26 groups and 36 clusters with available redshifts, with largest redshift being 1.3. We have used this sample to investigate the evolution of cool cores in these two classes of systems using cooling time divided by the age of the cluster as a parameter to describe the cooling state. We have found that groups and clusters have similar evolution in their cool core properties. Both classes of systems have a wide spread in the cool core parameter at low redshifts, which then narrows at high redshifts showing a lack of strong cool core systems.
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Durkalec, Anna. « Properties and evolution of galaxy clustering at 2 ». Thesis, Aix-Marseille, 2014. http://www.theses.fr/2014AIXM4758/document.
Texte intégralRésumé :
Cette thèse porte sur l'étude des propriétés et l'évolution de regroupement de galaxies pour les galaxies de la gamme de 22. Je ai pu mesurer la distribution spatiale d'une population générale de galaxie à redshift z~3 pour la première fois avec une grande précision. Je ai quantifié le regroupement de galaxie en estimation et la modélisation de la fonction de corrélation projetée (espace réel) à deux points, pour une population générale de 3022 galaxies. Je ai prolongé les mesures de regroupement à la luminosité et des sous-échantillons de masse sélectionné stellaires. Mes résultats montrent que la force de regroupement de la population générale de la galaxie ne change pas de redshift z~3,5 à z~2,5, mais dans les deux redshift va plus lumineux et des galaxies plus massives sont plus regroupées que les moins lumineux (massives). En utilisant la distribution d'occupation de halo (HOD) formalisme je mesuré une masse moyenne de halo hôte au redshift z~3 significativement plus faible que les masses halo moyens observés à faible redshift. Je ai conclu que la population de formation d'étoiles observé des galaxies à z~3 aurait évolué dans le massif et lumineux la population de galaxies au z=0. Aussi, je interpréter les mesures de regroupement en termes de biais de galaxies à grande échelle linéaire. Je trouve que ce est nettement plus élevé que le biais des galaxies redshift intermédiaire et faible. Enfin, je ai calculé le ratio-stellaire Halo masse (SHMR) et l'efficacité intégrée de formation d'étoiles (ISFE) pour étudier l'efficacité de la formation des étoiles et l'assemblage masse stellaireThis thesis focuses on the study of the properties and evolution of galaxy clustering for galaxies in the redshift range 22. I was able to measure the spatial distribution of a general galaxy population at redshift z~3 for the first time with a high accuracy. I quantified the galaxy clustering by estimating and modelling the projected (real-space) two-point correlation function, for a general population of 3022 galaxies. I extended the clustering measurements to the luminosity and stellar mass-selected sub-samples. My results show that the clustering strength of the general galaxy population does not change significantly from redshift z~3.5 to z~2.5, but in both redshift ranges more luminous and more massive galaxies are more clustered than less luminous (massive) ones. Using the halo occupation distribution (HOD) formalism I measured an average host halo mass at redshift z~3 significantly lower than the observed average halo masses at low redshift. I concluded that the observed star-forming population of galaxies at z~3 might have evolved into the massive and bright (Mr<-21.5) galaxy population at redshift z=0. Also, I interpret clustering measurements in terms of a linear large-scale galaxy bias. I find it to be significantly higher than the bias of intermediate and low redshift galaxies. Finally, I computed the stellar-to-halo mass ratio (SHMR) and the integrated star formation efficiency (ISFE) to study the efficiency of star formation and stellar mass assembly. I find that the integrated star formation efficiency is quite high at ~16% for the average galaxies at z~3
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Dighe, Manish D. « Quantitative characterization of damage evolution in an Al-Si-Mg base cast alloy ». Thesis, Georgia Institute of Technology, 1998. http://hdl.handle.net/1853/20219.
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Han, Jixiong. « Processing microstructure evolution and properties of nanoscale aluminum alloy ». Cincinnati, Ohio : University of Cincinnati, 2005. http://www.ohiolink.edu/etd/view.cgi?acc%5Fnum=ucin1121701078.
Texte intégralRésumé :
Thesis (Ph. D.)--University of Cincinnati, 2005.Title from electronic thesis title page (viewed Apr. 11, 2006). Includes abstract. Keywords: Al-Cu nanoparticle; Al nanoparticle; Al-Al2O3 composite; 2024Al-Al2O3 composite; nanocomposite; nanoparticle; phase transformation; precipitate; plasma ablation; inert gas condensation; exploding wire; consolidation; sinter; cold roll; hot roll; aging; thermal-soaking; mechanical properties; strengthening mechanism. Includes bibliographical references.
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Slijkhuis, Sander. « On the properties and evolution of proto-planetary nebulae ». Amsterdam : Amsterdam : Sterrenkundig Instituut 'Anton Pannekoek' ; Universiteit van Amsterdam [Host], 1991. http://dare.uva.nl/document/92056.
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Grigoriou, Georgios. « Structure evolving systems : model structure evolution and system properties ». Thesis, City University London, 2012. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.576948.
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This thesis deals with a new paradigm for Complex Systems of an evolving structure which is referred to as Structure Evolving Systems (SES). A wide range of challenging issues are concerned involving the evolution of structure and associated system properties through different forms of complexity and these are closely related to integrated system design. The thesis deals specifically with issues related to the representation of t.he different forms of structural evolut.ion. A number of new forms of evolution are introduced expressing differ- ent design problems, such as Design Time Evolution, Dimensional Graph Evolution, Life Cycle Structural Evolution, Cascade Design Evolution. The main objective is to develop a representation framework for the different types of structural evolution which will offer the proper set up for the study of evolution of structural characteristics and system prop- erties. The main part of the study is concerned with the development of representation of the System Structure Evolution in terms of a chain of models as a way to represent the different stages of Design Time Evolution which involves the modelling of a system from the very early stages up to the late stages when detailed descriptions are used for design. The development of nested representations for the case of Single-Input, Single-Output Systems is presented here based on the theory of continuous fraction expansion of rational functions and then examine property evolution within the naturally generated nests. A " natural" way for generating chains of models of variable complexity is introduced based on a frequency domain approach using the infinite Laurent expansion. This approach provides a natural link to the problem of partial realization, which in turn provides a useful set up for tpe study of evolution of system structure and properties. Within the framework of Cascade Design Evolution, we develop a generic representation describing the evolution from the aggregate system to the composite system. This involves the development of the new concept of COTY1- pleteness of composition. We also clarify the notion of deviations from completeness ,; , develop a representation of the noncomplete composite system in terms of standard control concepts and tools. This provides the fundamentals for the development of a control theory based approach for designing composite systems with desirable structural characteristics. Within this framework, completeness is expressed as output feedback and deviations from completeness as input and output decentralized squaring down. This representation pro- vides control concepts and tools which allow the structuring of linear composite systems to be addressed as a problem of generalised control design. The present work contributes to the development of the theory of the new system paradigm and studies the evolution of a number of system properties under different types of system structure evolution.
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Kokotanekova, Rosita. « Bulk properties and evolution of Jupiter-family comet nuclei ». Thesis, Open University, 2018. http://oro.open.ac.uk/57371/.
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This thesis presents the results from photometric time-series observations of Jupiter-family comets (JFCs). A method was developed that allows mutual absolute photometric calibration of data taken at different epochs with different instruments and results in absolute-calibration uncertainty of ∼0.02 mag. The method was applied to various datasets with the goal of studying the rotation rates and surface properties of ten comets. Previously published properties of JFCs were collected and complemented by new results. The resulting comprehensive sample was used to study the ensemble properties of JFC nuclei. It confirmed the cut-off in bulk density at ∼0.6 g cm−3 and provided evidence for a lower limit on the bulk tensile strength of 10-25 Pa. New lightcurves of three JFCs were used to look for spin changes over their last orbits. None of the observed comets had detectable period changes, and strict conservative upper limits were set. Comparing these results with all eight other JFCs with measured rotational changes suggests that large JFCs are less likely to undergo rotationally-driven mass-loss, and are therefore more likely to survive more perihelion passages than smaller nuclei. This conclusion is supported by evidence from the cumulative size distributions of JFCs and dormant comets, as well as from recent dynamical simulations. This work almost doubled the sample of JFCs with both albedos and phase-function slopes known. The extended sample shows a possible correlation of increasing phase-function slopes for larger geometric albedos, which can be interpreted as an evolutionary trend for JFCs. According to this hypothesis, newly activated JFCs have higher albedos and steeper phase functions, which gradually decrease due to sublimation-driven erosion. If confirmed, this correlation could be used to analyse surface erosion from the ground and to distinguish between dormant comets and asteroids.
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Han, Jixiong. « Processing Microstructure Evolution and Properties of Nanoscale Aluminum Alloys ». University of Cincinnati / OhioLINK, 2005. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1121701078.
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Väisänen, Petri. « Infrared properties of galaxies and constraints on galaxy evolution ». Helsinki : University of Helsinki, 2001. http://ethesis.helsinki.fi/julkaisut/mat/tahti/vk/vaisanen/.
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Gillone, Melissa. « The evolution of galaxies in groups : how galaxy properties are affected by their group properties ». Thesis, University of Birmingham, 2016. http://etheses.bham.ac.uk//id/eprint/6855/.
Texte intégralRésumé :
It has been long known that galaxy properties are strongly connected to their environment; however, a complete picture is still missing. This work's aim is to better understand the role of environment in shaping the galaxy properties, using a sample of 25 redshift-selected galaxy groups at 0.060 < z < 0.063, for which 30 multi-wavelength parameters are available. Given the wide variety of group dynamical states, it was fundamental to try and identify different classes of groups performing a statistical clustering analysis using all the available parameters independently of their physical meaning, which resulted in two classes distinct by their mass. To move beyond mass driven correlations, a new clustering analysis was performed removing the mass dependent properties, this approach provided a categorisation in four classes with distinctive group properties. Based on this, the galaxy properties were investigated and the classes interpreted as follows: a class of field-like galaxies in the early stage of structure formation; a class of low-mass groups either still in formation phase, or evolved, but small because they are isolated; a class of massive groups with no, or very little, ongoing star formation, likely in a more evolved stage of structure formation; and a class of massive groups possibly experiencing merger events. The result obtained have shown that it is possible to distinguish between classes of groups and thus be able to study the property of galaxies in systems with homogeneous properties. The method developed applied to data sets with larger statistics and good data quality could be a powerful tool to study galaxy evolution in galaxy groups.
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Clopet, Caroline Roberta. « Microstructural evolution and mechanical properties of deeply undercooled eutectic alloys ». Thesis, University of Leeds, 2016. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.713488.
Texte intégralRésumé :
An experimental investigation has been undertaken in an attempt to determine the mechanism of formation of anomalous eutectics. A melt fluxing method and a drop tube technique have been used to undercool eutectic Ag-Cu alloy. During the fluxing experiments, the growth fronts of the undercooled alloy samples have been monitored using high speed camera imaging. The evolved microstructures of the fluxed samples undercooled ≤ 60 K exhibit a trizonal structure consisting of mixed anomalous and lamellar eutectic. The high speed camera imaging reveals that the growth front propagates in a spasmodic manner, where periods of rapid growth are separated by significant intervals in which growth totally arrests. Depending upon undercooling, growth is either continuous or spasmodic. Continuous growth is characteristic of the advancement of a planar front, while during spasmodic growth a double recalescence occurs, the first of which is characteristic of the propagation of a dendritic front. The microstructure of drop tube processed Ag-Cu samples comprises of a mixture of lamellar and anomalous eutectic structures and a silver-rich phase, which appears as spherical inclusions at the eutectic cell boundaries. It is concluded that, during spasmodic growth, the propagation of eutectic dendrites is observed, which subsequently remelt to form the anomalous eutectic, while the lamellar eutectic grows during post-recalescence cooling. It is also postulated that this eutectic dendrite may be growing away from the eutectic point at high growth rates, which could lead to silver building up ahead of the interface that, in the drop tube samples, leads to the formation of the silver-rich phase observed at the cell boundaries. In the fluxed samples, as the silver builds up, the dendrite is no longer viable and growth arrests until sufficient silver atoms have diffused into the bulk liquid for the transient growth cycle to restart.
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Hilding, Tobias. « Evolution of coke properties while descending through a blast furnace ». Licentiate thesis, Luleå, 2005. http://epubl.luth.se/1402-1757/2005/19.
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Momeu, Iuliana Carmen [Verfasser]. « Improving glucose oxidase properties by directed evolution / Iuliana Carmen Momeu ». Bremen : IRC-Library, Information Resource Center der Jacobs University Bremen, 2008. http://d-nb.info/1034722700/34.
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Alshino, Abdulmonem. « Evolution of X-ray and optical properties of galaxy clusters ». Thesis, University of Birmingham, 2010. http://etheses.bham.ac.uk//id/eprint/1267/.
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This thesis is comprised of X-ray and optical studies of 27 X-ray selected galaxy clusters from the XMM-LSS survey. These systems are mostly groups and poor clusters, with temperatures 0.6-4.8 keV, spanning the redshift range 0.05 to 1.05, hence these are some of the highest redshift X-ray selected clusters to have been studied. In the X-ray study, the evolution in the X-ray surface brightness profiles of the hot intracluster plasma is studied. Comparing the profiles with a standard -model it is found that 54% of the sample possess cuspy (cool) cores. Trends with both temperature and redshift in the outer slope beta of the X-ray surface brightness and in the incidence of cuspy cores are investigated. Fits indicate that the incidence of cuspy cores does not decline at high redshifts, as has been reported in rich clusters. Rather such cores become more prominent with increasing redshift. It is also found that has a positive correlation with temperature. In the optical study, CFHTLS optical photometry has been used to study the galaxy luminosity functions of 14 members of the sample. Individual luminosity functions (LFs) as well as redshift-stacked and temperature-stacked LFs in three filters, g', r' and z', down to M = -14:5 are derived. All LFs were fitted by Schechter functions which well-constrained the faint-end slope, . Derived values of ranged from -:03 to as steep as -2:1. No evidence is found for upturns at faint magnitudes. Evolution in was apparent in all bands: it becomes shallower with increasing redshift. It is found that at z~0:3, alpha is steeper (-1.67) in the green (g') band than it is (-1.30) in the red (z') band. This colour trend disappears at low redshift, which is attributed to reddening of faint blue galaxies from z ~0.3 to z~0. Also, the total optical luminosity is calculated and is found to correlate strongly with X-ray luminosity and temperature, which is consistent with expectations for self-similar clusters with constant mass-to-light ratio.
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Hassani, Hamid. « Structure evolution and mechanical properties of ion-exchanged silicate glass ». Doctoral thesis, Università degli studi di Trento, 2019. http://hdl.handle.net/11572/243415.
Texte intégralRésumé :
Soda-lime silicate glass (SLS glass) is one of the most commercialized glasses with a huge extent of applications from homewares and kitchenware appliance to cover for touchscreen gadgets. The latter has gained notable attentions recently since the majority of available touchscreen gadgets have covers from other glass families such as aluminosilicate. Technically, these types of glasses are difficult to produce and very costly, therefore, the prices of articles which are made by these glasses are high. In comparison, soda-lime silicate glass has lower price because of globally growing production and promising future. However, due to the intrinsic weaker mechanical properties, the application of SLS glass as cover of touchscreen gadgets is restricted. Several techniques have been applied to improve the mechanical properties of SLS glass. Among them chemical tempering is one the most promising technique.
Typically, the chemical tempering is done by an ion-exchange process where sodium atoms contained in the glass are substituted by potassium ions diffusing from the molten salt. The effect of variables such as glass composition, molten bath composition, temperature, and time is crucial in the ion-exchange process. Particularly, selecting an unsuitable time and temperature of the process can affect mechanical properties of glass through a stress relaxation phenomenon. Therefore, optimization of the time and temperature can guarantee efficient reinforcement of glass.
In this PhD research, three different temperatures (430°C, 450°C, and 470°C) and five different times (4 h, 8 h, 24 h, 48 h and 168 h) selected for chemical tempering of glass samples in pure molten KNO3 and molten KNO3 systematically poisoned by NaNO3.
The compressive residual stress and case depth were determined by optical methods, the flexural strength was measured by a ring-on-ring test method and the surface chemical composition of the glass was analysed by Energy Dispersion X-ray Spectroscopy (EDXS). The resistivity of treated glass against forming surface cracks was studied by Vickers hardness and scratch test. To study the structural evolution, micro-Raman (μ-Raman) spectroscopy was used.
The results pointed out that below addition of 0.5 wt% NaNO3, the ion-exchange process is always effective. Indeed, compressive residual stress, flexural strength, surface concentration and potassium penetration in Na-containing baths are substantially identical to values recorded on glasses treated in “pure” KNO3. Actually, case depth and interdiffusion coefficient are invariant with respect to the sodium content at least up to 1 wt%. No significative difference between “tin” and “air” side are revealed.
Influence of time of tempering on Na-K exchange process showed that the concentration of K+ on the surface of glass was increased by increasing the duration of of the process. Compressive residual stress, on the other hand, was decreased by time due to the surface structural relaxation. A surface crack tendency under a Vickers indenter and scratch test gave the evident that K/Na ion-exchange process for more than 24 h is responsible for an indent mechanical reinforcement. A structural reorganization of the glass network occurred and a higher number of Q2 and Q3 species were present in the tempered glasses with respect to the pristine one. Such re-polymerization could account for a more plastic behavior under Vickers indentation and scratch test, making the material less susceptible to surface cracking.
The case depth of tempered samples was increased by temperature at the expense of the compressive residual stress due to the stress relaxation. Relative concentration of K+ on the surface, as well, increased by the temperature. The increase in the temperature of ion-exchange process led to increase in the tendency of glass against formation of crack under Vickers indenter and scratch test. The limit of this propensity is tempering at 470 °C for 48 h as the formation of radial cracks and shorter plastic deformation regions under scratch test observed. From structural point of view, Na-K exchange caused reorganization of the glass network which is responsible for a more plastic behavior under Vickers indentation and scratch test.
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Parikka, Anna. « Properties and evolution of dense structures in the interstellar medium ». Thesis, Paris 11, 2015. http://www.theses.fr/2015PA112221/document.
Texte intégralRésumé :
Mon travail de thèse présente deux types de structures denses : des sources froides compactes détectées par Planck et des condensations denses dans une région de photodissociation (PDR), à savoir la Barre d’Orion. Les deux types de structures sont étroitement liées à la formation des étoiles. Les sources froides sont étudiées comme objets potentiellement gravitationnellement liés, c’est-à-dire comme objets préstellaires. La Barre d’Orion est intéressante en tant que PDR à fort champ UV (G0 ∼104) prototypique, avec plusieurs disques protoplanétaires connus, éclairés par les jeunes étoiles du Trapèze.D’abord, je présente un article publié dans A&A: The Physical state of selected cold clumps. Dans cet article, nous avons comparé les observations du continuum de la poussière par Herschel provenant de l’open time key program Galactic Cold Cores (Herschel) aux observations de raies moléculaires par le radiotélescope de 20-m de l’Onsala Space Observatory en Suède. Les objets ont été sélectionnés en fonction de leur luminosités et faibles températures de couleur des poussières (T∼10−15 K). Nous avons calculé les masses du viriel et de Bonnor-Ebert et les avons comparées aux masses déduites à partir des observations. Les résultats indiquent que la plupart des objets froids observés ne sont pas nécessairement préstellaires.Dans mon étude de la Barre d’Orion, j’utilise des observations de l’instrumentPACS d’Herschel du programme Unveiling the origin and excitation mechanisms of the warm CO, OH and CH+ . Je présente des cartes de 110” ×110” du cation méthylidyne (CH+ J=3-2), des doublets de OH à 84 μm, et des raies de CO á hauts J (J=19-18). C’est la première fois que ces traceurs des PDR ont présentés avec une telle résolution spatiale et un tel rapport signal-sur-bruit.La répartition spatiale de CH+ et OH montre la même structure de la Barre qui a été vue dans d’autres observations. La morphologie du CH+ et H2 confirme que la formation et l’excitation de CH+ est fortement dépendante du H2 excité vibrationnellement. Le maximum d’émission de OH84 μm correspond à un objet brillant jeune, identifié comme le disque protoplanetaire 244-440.Je présente également des transitions rotationnelles de CO de moyenne (∼20 K) et haute (∼1000 K) énergie. La morphologie de l’émission du CO rotationnellement excité est corrélée avec la présence des petites structures denses irradiées. Nous établissons le lien entre le coeur de ces structures, tracé en CS J=2-1 par Lee et al. (2013) et H 13 CN par Lis and Schilke (2003) et le bord de la PDR, tracé en CO J=19-18 et le H 2 vibrationnellement excitée. Nous montrons également que le CO est principalement excité par le chauffage UVIn this thesis I present a study of two kinds of dense ISM structures: compact cold sources detected by Planck and dense condensations in a photodissociation region (PDR), namely the Orion Bar detected by ground-based and Herschel telescopes. Both kinds of structures are closely related to star formation. The cold sources are investigated as potentially gravitationally bound, prestellar, objects. The Orion Bar is a highly FUV-illuminated (G0=104) prototypical PDR, with several known protoplanetary disks, illuminated by the young Trapezium stars.First I introduce a paper published in A&A: The Physical state of selected cold clumps. In this paper we compared the Herschel dust continuum observations from the open time key program Galactic Cold Cores to ground based molecular line observations from the 20-m radio telescope of the Onsala Space Observatory in Sweden. The clumps were selected based on their brightness and low dust color temperatures (T=10-15 K). We calculated the virial and Bonnor-Ebert masses and compared them to the masses calculated from the observations. The results indicate that most of the observed cold clumps are not necessarily prestellar.Then I move on to the warm and dense condensations of the ISM. In my study of the Orion Bar, I use observations from PACS instrument on board Herschel from the open time program Unveiling the origin and excitation mechanisms of the warm CO, OH and CH+. I present maps of 110”x110” of the methylidyne cation (CH+ J=3-2), OH doublets at 84 µm, and high-J CO (J=19-18). This is the first time that these PDR tracers are presented in such a high spatial resolution and high signal-to-noise ratio. The CH+ and OH have critical densities (1010 cm-3) and upper level energy temperatures (250 K). In addition the endothermicity of the CH+ + H2 reaction (4300 K) that forms CH+ is comparable to the activation barrier of the O + H2 reaction (4800 K) forming OH. Given these similarities it is interesting to compare their emission. The spatial distribution of CH+ and OH shows the same clumpy structure of the Bar that has been seen in other observations. The morphology of CH+ and H2 confirms that CH+ formation and excitation is strongly dependent on the vibrationally excited H2, while OH is not. The peak in the OH 84 µm emission corresponds to a bright young object, identified as the externally illuminated protoplanetary disk 244-440.Finally, I study the high-J CO in the Orion Bar. I also introduce low- and mid-J CO observations of the area. The high-J CO morphology shows a clumpy structure in the Bar and we establish a link between the dense core of the clumps, traced in CS J=2-1 by Lee et al. (2013) and in H13CN by Lis and Schilke (2003). We also show that the high-J CO is mainly excited by the UV heating
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SALVALAGLIO, MARCO. « Continuum modeling of vertical heterostructures : elastic properties and morphological evolution ». Doctoral thesis, Università degli Studi di Milano-Bicocca, 2016. http://hdl.handle.net/10281/100682.
Texte intégralRésumé :
The development of cutting-edge opto- and micro-electronic devices requires novel fabrication techniques, able to deliver high-quality materials, monolithically integrable into Si-based technology. Theoretical models and simulations supporting the experimental activities are highly needed to fully understand the growth physics at the nano- and micro-scale and tailor semiconductor heterostructures for technological applications. In this work, the modeling of the plasticity onset and of the morphological evolution for Ge/Si vertical heterostructures is introduced, fostered by the peculiar features of such systems with respect to the standard heteroepitaxy. Indeed, the aim of this thesis is to understand the main properties of systems with large height-to-base aspect-ratios, in order to offer new solutions for the realization of heterostructures with unprecedented material quality. Continuum models are selected to describe length scales ranging from a few nanometers to microns, and time scales of minutes (or even more).
By means of the linear elasticity theory equations, solved by Finite Element Method (FEM) simulations, the competition between elastic and plastic relaxation in vertical Ge/Si systems is investigated. The critical parameters for the insertion of dislocations are determined for a single-layer structure, made of a SiGe layer on a Si pillar, and then generalized to multilayer configurations. Moreover, the possibility to achieve coherent structures at any size is demonstrated, provided that a proper grading of the Ge content during the growth is considered. A recipe for the calculation of such a grading of the Ge content is also introduced. Several comparisons with experiments show the generality of the proposed investigation for heterostructures at the nanoscale, and the versatility of the developed method. Moreover, thanks to dedicated experiments stimulated by the theoretical predictions, dislocation-free structures are proven to be feasible also at the micrometer scale.
The three-dimensional evolution in time of vertical microcrystals is investigated by means of a phase-field model and FEM simulations. In particular, the annealing of Ge on Si microcrystals is modeled by considering the surface diffusion driven by the tendency toward the minimization of the surface energy. This allows the evolution induced by annealing of single structures to be described. Moreover, the coalescence mechanism for crystal arrays, resulting in the formation of a suspended film, is predicted. Such an evolution is confirmed by dedicated experiments and leads to the fabrication of a promising system for the high-quality heterogeneous integration of semiconductors. The coalescence occurring for closely spaced crystals during high-temperature growth is also assessed. The original extensions of the PF model, required by the theoretical investigations of the morphological evolution, are illustrated in the details. Particular attention is devoted to the description of anisotropic surface energies responsible for crystal faceting in thermodynamic regimes. Moreover, further extensions of the method, dealing with an accurate description of the growth processes, are reported.
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Deshpande, Girish Nilkanth. « Effect of polysiloxane side groups on chemistry and kinetics of degradation and evolution of pores in the resultant polymer ». Diss., Georgia Institute of Technology, 2001. http://hdl.handle.net/1853/11845.
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Kurapov, Denis. « Structure evolution, properties, and application of alumina films deposited by PECVD / ». Aachen : Shaker, 2005. http://bvbr.bib-bvb.de:8991/F?func=service&doc_library=BVB01&doc_number=014643764&line_number=0001&func_code=DB_RECORDS&service_type=MEDIA.
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Wijers, Ralph Antoine Marie Joseph. « On the properties and evolution of binary and single neutron stars ». Amsterdam : Amsterdam : Sterrenkundig Instituut 'Anton Pannekoek' ; Universiteit van Amsterdam [Host], 1991. http://dare.uva.nl/document/92033.
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Chamanfar, Ahmad. « Evolution of microstructure and mechanical properties in linear friction welded waspaloy ». Thesis, McGill University, 2013. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=114429.
Texte intégralRésumé :
The Ni-base superalloy, Waspaloy, was linear friction welded (LFWed) under various processing conditions. Specifically, axial shortening, in which all linear friction welding (LFW) parameters such as amplitude, frequency, pressure, and time are included, was varied from 0.7 to 4.6 mm. In-situ temperature measurements during welding were carried out by placing thermocouples at different locations from the weld interface. Mechanical properties of the weldments in the as-welded and post-weld heat treated (PWHTed) conditions were evaluated using tensile testing, integrated with the optical image correlation Aramis® system, and microhardness measurements. To correlate the process parameters and microstructural changes that affect the mechanical properties, microstructure evolution across the weld in the as-welded and PWHTed conditions was investigated using optical microscopy, electron backscatter diffraction (EBSD), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), transmission electron microscopy (TEM), and micro computed tomography imaging. Tensile testing indicated that there is a critical axial shortening value (2 mm) below which LFWed and PWHTed Waspaloy exhibited weak integrity. At and above this critical shortening, the yield strength and ultimate tensile stress values were more or less the same as those of the parent material. However, total elongation continued to increase with axial shortening even above the critical value due to decrease in the width of the thermomechanically affected zone (TMAZ). The sample with the highest axial shortening (4.9 mm), exhibited an elongation equal to 91 % of the parent material elongation. Weak integrity at axial shortening below 2 mm was mainly due to lack of bonding and/or presence of oxides at the weld interface. Microstructure examination using EBSD mapping revealed that dynamic recrystallization (DRX) occurred in a region about 1 mm wide on either side of the weld interface. In the as-welded condition, SEM and TEM studies indicated that progressive dissolution of γ' precipitates took place from the parent material to the weld interface, where almost no γ' precipitates were observed. The γ' dissolution significantly influenced the hardness profile measured across the TMAZ. The applied post-weld heat treatment (PWHT) restored the hardness in the weld region.The data recorded by thermocouples indicated that during LFW the temperature in the weld area reached up to 50 C below the melting point of the bulk alloy (1330 C). This temperature is well above the liquation temperature of the low melting point components in the alloy (1245 C). The possible occurrence of liquation and consequent microcracking were investigated in this study. It was shown that the high pressure applied during the oscillation and forge phases of the LFW process and the resulting grain refinement contributed in preventing liquation and microcracking in the weldments.Finally, hot compression tests were conducted on Waspaloy in the 1060 to 1140 C temperature range and 0.001 to 1 s-1 strain rate range up to a strain of 0.83 to study the high temperature deformation behavior of the alloy. Flow softening and microstructure investigation indicated that DRX occurred during deformation. For the investigated conditions, the activation energy for hot deformation of Waspaloy was determined to be 462 kJ/mol. The equations relating the dynamic recrystallized (DRXed) grain size to temperature and strain rate were developed from the hot compression experiments. The developed equations were then used to predict the grain size and strain rate in LFWed Waspaloy. The prediction results were validated against experimental findings and data reported in the literature. It was found that the developed equations can reliably predict the grain size of LFWed Waspaloy. Moreover, the predicted strain rate (1520 s-1) was in agreement with the finite element modeling (FEM) data reported in the literature.Le superalliage base Nickel, Waspaloy, est soudé par friction linéaire sous diverses conditions. Précisément, la réduction axiale, dans laquelle tous les paramètres du soudage par friction linéaire (SFL) tels que l'amplitude, la fréquence, la pression et le temps sont inclus, varie de 0.7 à 4.6 mm. Les mesures de température durant le soudage ont été effectuées en plaçant des thermocouples à différents endroits à partir de l'interface soudée. Les propriétés mécaniques des soudures dans les conditions « tel que soudé » et « traité thermiquement après soudure » ont été évaluées par des essais de traction et par mesures de dureté. L'évolution microstructurale à travers la soudure dans les conditions précitées a été étudiée par microscopie optique, EBSD, SEM, TEM et par imagerie tomographique.Les essais de traction ont indiqué qu'il existe une valeur critique de réduction axiale (2 mm) en dessous de laquelle la soudure n'est pas totale. A ce niveau critique de réduction et au delà, les valeurs de limite élastique et de résistance en traction sont plus ou moins les mêmes que celles obtenues dans le matériau de base. Cependant, l'allongement total continue d'augmenter avec la réduction axiale même au delà de la valeur critique à cause de la diminution dans la largeur de la zone affectée thermo-mécaniquement (ZATM). L'échantillon avec la plus grande réduction axiale (4.9 mm) présente un allongement équivalent à 91% de l'allongement du matériau de base. La faible qualité de soudure pour la réduction axiale en-dessous de 2 mm est essentiellement due au manque de lien et/ou à la présence d'oxydes à l'interface de la soudure. L'examen de la microstructure par EBSD a révélé que la recristallisation dynamique (DRX) se produisait dans une région d'environ 1 mm de large de chaque côté de l'interface. Dans la condition « tel que soudé », les études par SEM et TEM ont indiquées que la dissolution progressive des précipités γ' avait lieu du matériau de base vers l'interface de la soudure. La dissolution des γ' influence grandement le profil de dureté mesuré à travers la ZATM. Le traitement thermique après soudure appliqué restaure la dureté dans la zone soudée.Les données enregistrées par les thermocouples ont indiqué que durant le procédé SFL, la température dans la zone soudé atteignait jusqu'à 50 C en dessous du point de fusion du matériau massif (1330 C). Cette température est bien au-dessus de la température de liquéfaction des constituants à faible point de fusion de l'alliage (1245 C). La possibilité de liquéfaction and la conséquente microfissuration a été examinée. Il a été démontré que la haute pression appliqué durant le procédé SFL ainsi que la réduction de taille de grains en résultant contribuent à limiter la liquéfaction et la microfissuration dans les soudures.Finalement, des tests de compression à chaud ont été conduit dans la gamme de température 1060-1140°C et dans la gamme de vitesse de déformation 0.001-1 s-1 pour étudier le comportement en déformation à chaud de cet alliage. Les courbes contrainte-déformation et l'étude de la microstructure ont révélé que la DRX se produisait pendant la déformation. Dans les conditions testées, l'énergie d'activation de déformation serait 462 kJ/mol. Les équations reliant la taille des grains recristallisés dynamiquement à la température et à la vitesse de déformation ont été développées à partir d'expériences de compression à chaud. Les équations développées permettent de prédire la taille de grain et la vitesse de déformation dans le Waspaloy soudé par friction linéaire. Les résultats prédits ont été validés avec les données expérimentales obtenues et celles provenant de la littérature. Il a été démontré que les équations développées peuvent prédire de manière fiable la taille de grain du Waspaloy soudé par friction linéaire. De plus, la vitesse de déformation prédite (1520 s-1) est en accord avec les données de modélisation par éléments finis rapportés dans la littérature.
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Hankin, G. L. « Radiation-induced evolution of microstructure and mechanical properties of stainless steels ». Thesis, Loughborough University, 1998. https://dspace.lboro.ac.uk/2134/7065.
Texte intégralRésumé :
Radiation-induced changes in microstructures often lead to significant changes in mechanical properties of alloys used in the construction of nuclear reactors. It is desirable to test small specimens to make efficient use of the small volumes available in test and commercial reactor cores and also because small specimens are less affected by the sometimes steep flux gradients experienced in reactor cores and the sometimes large temperature gradients developed in the specimens from gamma heating. (Continues...).
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Haines, Thomas J. « The evolution of petrophysical properties across carbonate hosted normal fault zones ». Thesis, University of Aberdeen, 2014. http://digitool.abdn.ac.uk:80/webclient/DeliveryManager?pid=225315.
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Chahda, Juan Sebastian. « Analysis of Scaling Properties of Embryonic Morphogen Gradients During Drosophila Evolution ». Case Western Reserve University School of Graduate Studies / OhioLINK, 2015. http://rave.ohiolink.edu/etdc/view?acc_num=case1437000710.
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Pracy, Michael Benjamin Physics Faculty of Science UNSW. « The properties and evolution of galaxy populations in the rich cluster environment ». Awarded by:University of New South Wales. Physics, 2006. http://handle.unsw.edu.au/1959.4/23033.
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This thesis is concerned with the role the rich cluster environment plays in the evolution of its galaxy population. We approach this issue from two angles, first we use deep wide-field imaging to investigate the effect of the cluster environment on the spatial and luminosity distribution of galaxies. Secondly, we focus on one particularly interesting class of galaxy, the enigmatic E+A galaxies, using a combination of state-of-the-art telescopes and novel instrumentation to elucidate the physical mechanisms and environmental influences causing the rapid change in star-formation activity in these galaxies. We present results from a deep photometric study of the rich galaxy cluster Abell 2218 (z=0.18) based on Hubble Space Telescope images. These have been used to derive the cluster luminosity function to extremely faint limits. We find the faint-end slope of the luminosity function to vary with environment within the cluster - in the sense that the ratio of `dwarf' galaxies to `giant' galaxies increases in the lower-density outskirt regions. Using imaging obtained with the Isaac Newton Telescope (INT) we confirm the presence of luminosity segregation in Abell 2218. However, luminosity segregation in clusters does not appear to be ubiquitous, with two other clusters studied with the INT (A119 at z=0.04 and A2443 at z=0.11) showing no sign of luminosity segregation of their galaxy populations. We use integral field spectroscopy of a sample of E+A galaxies in intermediate redshift clusters, obtained with the FLAMES system on the European Southern Observatory's VLT and the GMOS instrument on Gemini-North, to determine the radial variation in the strength of Hdelta absorption in these galaxies, and hence map out the distribution of the recently formed stellar population. We find a diversity of behaviour amongst these galaxies in terms of the radial variation in Hdelta absorption: with gradients that are either negative, flat, or positive. By comparing with numerical simulations we suggest that the first of these different types of radial behaviour provides evidence for a merger/interaction origin, whereas the latter two types of behaviour are more consistent with the truncation of star formation in normal disk galaxies.
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Buzit, Sebastien. « Evolution of crystallographic textures and TRIP effects in stainless steel AISI 304 ». Thesis, Georgia Institute of Technology, 2002. http://hdl.handle.net/1853/19584.
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Vora, Khalil Ur Rehman. « Mechanical properties evolution during cure for out-of-autoclave carbon-epoxy prepregs ». Thesis, Wichita State University, 2012. http://hdl.handle.net/10057/5979.
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Extent of cure and rheological properties were obtained for out-of autoclave materials, Cycom 5320-8HS and Cycom 5320-PW, for the manufacturer recommended cure cycle using Differential Scanning Calorimeter (DSC) and Encapsulated Sample Rheometer (ESR), respectively. Rheological properties from ESR were further used in designing the cure cycles to study the evolution of mechanical properties. Five panels were cured at different cure stages using the designed cure cycles and coupons were tested for short beam shear and combined loading compression properties at different cure stages. To correlate the mechanical properties with its respective glass transition temperature, Dynamic Mechanical Analyzer (DMA) was used to obtain the glass transition temperature for the coupons obtained from the respective panels. Statistical results showed significant difference in short beam shear and combined loading compression properties up to vitrification, however, no significant difference was observed on these mechanical properties after vitrification. The observed linear trend between degree of cure (DOC) and glass transition temperature (Tg) was validated using Dibenedetto relation. Linearly increasing trend between degree of cure (DOC) and glass transition temperature (Tg) for different cure states suggests that both DOC and Tg can be used interchangeably to define the state of material. A good correlation was observed between material cure state and the mechanical properties. A mathematical model was also proposed to determine the short beam shear and combined loading compression properties based on material cure state.Thesis (M.S.)--Wichita State University, College of Engineering, Dept. of Mechanical Engineering
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Torrey, Paul A. « Modeling the Evolution of Galaxy Properties across Cosmic Time with Numerical Simulations ». Thesis, Harvard University, 2014. http://dissertations.umi.com/gsas.harvard:11496.
Texte intégralRésumé :
We present a series of numerical galaxy formation studies which apply new numerical methods to produce increasingly realistic galaxy formation models. We first investigate the metallicity evolution of a large set of idealized hydrodynamical galaxy merger simulations of colliding galaxies. We find that inflows of metal--poor interstellar gas triggered by galaxy tidal interactions can account for the systematically lower central oxygen abundances observed in local interacting galaxies. We show the central metallicity evolution during merger events is determined by a competition between the inflow of low--metallicity gas and enrichment from star formation. We find a time-averaged depression in the galactic nuclear metallicity of ~0.07 dex for gas--poor disk--disk interactions, which explains the observed close pair mass-metallicity and separation-metallicity relationships.Astronomy
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Simonelli, Marco. « Microstructure evolution and mechanical properties of selective laser melted Ti-6Al-4V ». Thesis, Loughborough University, 2014. https://dspace.lboro.ac.uk/2134/15070.
Texte intégralRésumé :
Selective laser melting (SLM) has been shown to be an attractive manufacturing route for the production of ??/?? titanium alloys, and in particular Ti-6Al-4V. A thorough understanding of the relationship between the process, microstructure and mechanical properties of the components produced by this technology is however crucial for the establishment of SLM as an alternative manufacturing route. The purpose of the present study is thus to determine the microstructure evolution, crystallographic texture and the mechanical properties of SLM Ti-6Al-4V. The effect of several processing parameters on the density and the microstructure of the SLM samples were initially investigated. It was found that different sets of process parameters can be used to fabricate near fully dense components. It was found that the samples built using the optimised process window consist exclusively of ????? martensitic phase precipitated from prior ?? columnar grains. It was observed that the ?? grain solidification is influenced by the laser scan strategy and that the ?? phase has a strong <001> texture along its grain growth direction. The ????? martensitic laths that originate from the parent ?? grains precipitate according to the Burgers orientation relationship. It was found that ????? laths clusters from the same ?? grain have a specific misorientation that minimise the local shape strain. Texture inheritance across successive deposited layers was also observed and discussed in relation to various variant selection mechanisms. The mechanical properties of as-built and stress relieved SLM Ti-6Al-4V built using the same optimised process parameters were then investigated. It was found that the build orientation affects the tensile properties, and in particular the ductility of the samples. Samples built perpendicularly to the building direction showed higher ductility than those built in the vertical orientation. It was also observed that a stress relief heat treatment was beneficial to the mechanical properties of SLM Ti-6Al-4V. The ductility of the stress relieved samples was indeed higher than those found in the as-built condition. It was found that the predominant fracture mode during tensile testing is inter-granular. In terms of high-cycle fatigue, it was found that SLM Ti-6Al-4V is comparable to HIPed cast Ti-6Al-4V but it has a significantly lower fatigue resistance than that of wrought and annealed alloys. It was observed that porosity and the elongated prior ?? grain boundaries decrease substantially the fatigue life of the components. Cracks propagate either by fatigue striation or ductile tearing mechanisms. Using alternative laser scan strategies it was possible to control the microstructure of the as-built samples. It was observed that the laser scan vector length influences several microstructural features, such as the width of the prior ?? grains and the thickness of the ????? laths. It was found that re-melting the same layer has instead little effect on the microstructure. A novel laser scan strategy characterised by much lower laser power and scan speed than those typically used in SLM enabled finally to fabricate SLM Ti-6Al-4V with a microstructure close to that of conventionally manufactured Ti-6Al-4V. This study investigates for the first time the crystallographic texture evolution in Ti-6Al-4V manufactured by SLM. Further, this research presents for the first time the effect of the characteristic microstructure and crystallographic texture on the mechanical properties and fracture of SLM Ti-6Al-4V. Lastly, for the first time this research shows examples of microstructural control during the SLM fabrication of the same alloy using long laser dwell times.
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Leoz, Marie. « HIV-1 Group O Genetic Diversity : Characterization, Evolution and Associated Viral Properties ». Rouen, 2016. http://www.theses.fr/2016ROUENR14.
Texte intégralRésumé :
Les VIH-1 de groupe 0 (VIH-1/0) sont des variants rares, essentiellement retrouvés au Cameroun où ils représenteraient plusieurs milliers de cas. Les causes de leur faible diffusion restent méconnues: leur émergence serait aussi ancienne que les VIH-1 de groupe M, pourtant devenus pandémiques. En près d'un siècle, les VIII-1/0 ont développé une importante diversité génétique, dont la description reste ambiguë du fait de l'existence de plusieurs nomenclatures basées sur un nombre restreint de séquences génétiques. Peu de données sont disponibles quant aux causes de cette diversité et à son impact sur diverses propriétés virologiques. La mutation Y181C de la Transcriptase Inverse (TI) a toutefois été proposée comme critère de classification des VIH-1/0, et suspectée de conférer un avantage réplicatif à ceux chez qui elle serait naturellement présente. Ce travail a donc constitué à caractériser plus largement la diversité génétique des vm-vo, et à explorer la dynamique de leur évolution au cours du temps à l'aide de méthodes bayésiennes. Nous avons ainsi identifié deux phases de diversification successives, liées au développement de deux sous-groupes: T, minoritaire et ayant émergé dans les années 1960, et H, majoritaire et ayant émergé du sous-groupe T dans les années 1980. La présence de la mutation 181C a été associée au sous-groupe H, ce qui a conduit à la définition de trois populations virales: H/181C-like, H/181Y-like et T/181Y-like. Une forte conservation du motif signature K28, K103, 1142, D174, L178 au niveau de la poche non catalytique de la TI des virus H/181C-like a été mise en évidence, mais aucun de ces marqueurs (présence de la mutation 181C, appartenance à la population H/181C-like ou présence du motif associé) ne semble être lié à un plus haut niveau de réplication virale in vivo. Afin de développer les connaissances sur le tropisme des VIH-110, une propriété en lien avec l'évolution 1 clinique de l'infection par VIH-11M ou VIH-2, nous avons développé un outil de phénotropisme basé sur la production de pseudovirus présentant un clone de glycoprotéine d'Enveloppe V111-1/0. Nous avons montré que les 16 Enveloppes produites n'utilisaient que le corécepteur CCR5, malgré l'exploration d'échantillons séquentiels et/ou prélevés en phase d'infection tardive. Enfin, pour permettre l'analyse des propriétés phénotypiques liées aux différentes populations VIH-1/0, nous avons participé à la production de trois nouveaux de clones moléculaires infectieux représentant deux souches H/181C-like et une souche T/181Y-like. Ces clones ont contribué à démontrer comment les VIH-1/0 contrecarraient l'action de la Tetherine et modulaient l'expression de NFKB par le biais des protéines Nef et Vpu. En conclusion, nos résultats suggèrent une dynamique d'évolution des V111-1/0 plus complexe que suspectée jusqu'alors, ayant entraîné l'émergence de deux sous-groupes distincts. Nous avons unifié les 1 différents systèmes de nomenclature, décrit des caractéristiques distinguant trois populations virales, et développé de nouveaux outils pour contribuer à une meilleure connaissance des conséquences de leur diversité génétiqueHIV-1 group 0 viruses (HIV-1/0) are rare variants that are mostly found in Cameroon, where the y are thought to represent thousands of cases. The reasons for their limited diffusion are poorly understood: their emergence is estimated to be as ancient as that of the pandemic HIV-1 group M. Almost a century later, HIV-1/0 have developed a broad genetic diversity, which can only ambiguously be described due to the existence of several nomenclatures, each based on few genetic sequences. Little is known of the causes for this diversity and the impact it has on diverse viral properties. The Reverse Transcriptase (RT) mutation Y181C, however, has been proposed as classification criteria and suspected to confer a replicative advantage to the HIV-1/0 strains naturally presenting it. Hence, we aimed at better characterizing HIV-1/0 genetic diversity and exploring their diversification dynamics over tune using Bay Sian inférence. In so doing, we have identified two successive phases of HIV-1/0 diversification, each linked to the development of a particular subgroup: the minor subgroup T emerged in the 1960's, and the major subgroup H emerged from T in the 1980's. Natural presence of RT mutation 181C has been associated with subgroup H, which led to the udefinition of three viral populations: H1181C-like, H/181Y-like and T/181Y-like. A high degree of K28, K103, 1142, D174, L178 signature pattern conservation in the non-catalytic RT pocket of H/181C-like viruses has been observed, but neither presenting the 181C mutation, nor belonging to the H/181C-like cluster or presenting the associated pattern was related to higher replication level in vivo. In an attempt to developing knowledge of H1V-1/0 tropism — a property linked with the evolution of the natural course of HIV-1/M or HIV-2 infection — we have developed a phenotropism assay based on the production of clonai HIV-1/0 Env glycoprotein-presenting pseudotyped viruses. We have shown that the 16 Env we tested only used CCR5, despite the exploration of sequential and/or late stage samples. Finally, in order to allow for phenotypic investigation of diverse HIV-1/0 populations, we participated in the production of three new Infectious Molecular Clones deriving from two representatives of H/181C-like strains and one representative of T/181Y-like strains. These clones have contributed to unravelling the mechanisms for Tetherin counteraction and regulation of NFkB 111 expression by HIV-1/0 Nef and Vpu. In conclusion, our results suggest that HIV-1/O evolution dynamics were more complex than suspected and led to the emergence of two distinct subgroups. We have unified the diverse nomenclature systems, described some characteristics associated to three viral populations, and developed new tools to contribute in a better understanding of the consequences of HIV-1/O genetic diversity
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Reid, Jeffrey Spencer. « Emission, evolution, and radiative properties of particles from biomass burning in Brazil / ». Thesis, Connect to this title online ; UW restricted, 1998. http://hdl.handle.net/1773/10099.
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Lee, Eunha. « Microstructure evolution and microstructure/mechanical properties relationships in [alpha]+[beta] titanium alloys ». Connect to this title online, 2004. http://rave.ohiolink.edu/etdc/view?acc%5Fnum=osu1092756139.
Texte intégralRésumé :
Thesis (Ph. D.)--Ohio State University, 2004.Title from first page of PDF file. Document formatted into pages; contains xxiii, 229 p.; also includes graphics (some col.) Includes bibliographical references (p. 224-229). Available online via OhioLINK's ETD Center
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Struck, Travis Jared, et Travis Jared Struck. « Research Effort and Evolutionary Properties of Genes ». Thesis, The University of Arizona, 2016. http://hdl.handle.net/10150/621183.
Texte intégralRésumé :
Recent research effort (measured in number of publications) on genes is biased towards genes that have been studied heavily in the past. Some factors for why this occurs is that many of these historically studied genes are important for survival or there are more tools available that make genetic studies of them much more accessible. Studies of research effort on \textit{Saccharomyces cerevisiae} genes characterized with genetic or protein interactions found that there is an aversion to studying lesser-known genes in networks. As well, in a study of three human protein families, many of the genes that have recently been discovered to have association with complex disease, through methods such as genome wide association studies (GWAS), are understudied in the present compared to the small number of historically heavily studied genes. In this study we explore possible causes of and diversion from this preferential bias with gene conservation and human genes being disease-associated. We find there is some evidence of conservation driving biases in research effort for essential genes in \textit{Saccharomyces cerevisiae}, but inconclusive evidence in other organisms. We look for effects of disease association through Mendelian and complex diseases in a historical, pre-GWAS, and contemporary, post-GWAS, context. Within both contexts we find that Mendelian disease genes may drive preferential study bias. For contemporary research effort we utilize a model of publication rates and find that there are individual GWAS genes that tend to be investigated more than predicted compared to non-GWAS genes. It appears that the proportion of GWAS genes that had highly unexpected increases in publication rate compared to model predictions rose fairly quickly but has been declining. Our analysis suggests that GWAS has had a small impact on what genes some scientists study despite preferential study biases. However GWAS gene-disease association's impact on research effort appears to be declining, possibly due to scientists not being as interested in GWAS results as time goes on.
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Jiang, Kaiyun [Verfasser]. « Al2O3 thin films : relation between structural evolution, mechanical properties and stability / Kaiyun Jiang ». Aachen : Hochschulbibliothek der Rheinisch-Westfälischen Technischen Hochschule Aachen, 2012. http://d-nb.info/1019852542/34.
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Dalgård, Elvi Catherine. « Evolution of microstructure, microtexture, and mechanical properties in linear friction welded titanium alloys ». Thesis, McGill University, 2011. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=103485.
Texte intégralRésumé :
Two titanium alloys were welded using linear friction welding (LFW) techniques. The two alloys consisted of one α + β alloy, Ti-6Al-4V, and one near-β alloy, Ti-5553. The welding conditions were varied in order to assess the effect of each parameter on the mechanical properties, microstructure, and crystallographic texture of the materials. Axial pressures from 50 MPa to 150 MPa, oscillation frequencies from 30 Hz to 110 Hz, and oscillation amplitudes from 1.5 mm to 3 mm were employed.The linear friction welded (LFWed) samples of Ti-6Al-4V and Ti-5553 were examined using electron backscatter diffraction techniques (EBSD) to relate the texture and phase changes to the thermomechanical conditions. Characterization of the welds included analysis of the microstructural features of the weld region and thermomechanically affected zone (TMAZ) in relation to the parent material. Mechanical properties were evaluated using tensile tests and microhardness measurements.The maximum strains and strain rates experienced by the material during LFW for each set of welding parameters were estimated based on the process parameters and the measured time of oscillation. A heat input equation was developed in order to estimate the temperature at different points in the joint and temperature measurements were made during welding to corroborate the calculated temperatures. The strains, strain rates and temperatures measured and calculated for the welding conditions employed were found to be sufficient to initiate dynamic recrystallization in both alloys. This finding is in agreement with the microstructures and textures observed in the weld centres.The near-β Ti-5553 alloy was examined not only in the as-welded state but also in two post-weld-heat-treated (PWHT) conditions. The TMAZ and weld centre of this alloy were weakened by welding due to the reduction of the α phase volume fraction during rapid cooling from super-transus temperatures in and near the weld. With the restoration via PWHT plus aging of the α phase fraction, the UTS's for the welded samples were restored to the literature values for the heat-treated condition of this alloy.In addition to the two titanium alloys comprising the main focus of the study, a number of other materials were examined in the context of linear friction welding. These were: IMI-834, a near-α titanium alloy (nominally Ti-5.8Al-4Sn-3.5Zr-0.7Nb-0.5Mo-0.3Si); CMSX-4, a single crystal Ni-based superalloy (Ni‑9.5Co-6.4Cr-6.4Ta-6.4W-5.6Al-2.9Re-1Ti-0.6 Mo-0.1Hf wt %); stainless steel 316L; mild steel A42; as well as dissimilar pairs aluminum alloy 6063 to commercial purity Cu and stainless steel 316L to Zr alloy Zr702. All of these materials were successfully linear friction welded after some refinement of the welding parameters.Deux alliages de titane ont été soudés par friction linéaire (linear friction welding - LFW). Le premier alliage, Ti-6Al-4V, est biphasé α + β, alors que le second, Ti-5553, est principalement constitué d'une phase bêta. Différentes conditions de soudage (pression axiale variant de 50 à 150 MPa, fréquence d'oscillation entre 30 et 11 Hz et amplitude d'oscillation entre 1.5 et 3 mm) ont été utilisées afin d'étudier leur effet sur les propriétés mécaniques, la microstructure ainsi que la texture des deux alliages. Les cordons de soudure ainsi que les régions adjacentes ont été analysés par microscopie électronique, plus particulièrement au moyen de la technique de diffraction des électrons rétrodiffusés, afin de mieux comprendre l'effet qu'ont les paramètres de soudage sur la texture et la transformation de phase cristallographique. Des essais de microdureté et traction ont aussi été effectués afin de déterminer les propriétés mécaniques des soudures. Pour chaque combinaison de paramètres de soudage, les déformations et vitesses de déformation maximales obtenues durant la soudure par friction linéaire ont été estimées à partir des conditions de soudage et des temps d'oscillation. De plus, une équation paramétrique à été développée afin d'estimer la température aux différents endroits du cordon de soudure. La validité de cette équation à été confirmée à partir de mesures de la température effectuées lors de la soudure. Les déformations, les vitesses de déformation ainsi que les températures générées durant la soudure sont suffisamment élevées pour que la microstructure recristallise dynamiquement. L'analyse des microstructures et de la texture a en effet révélé des traces évidentes de recristallisation dynamique dans les deux alliages. Les zones centrales et TMAZ de la soudure sur l'alliage Ti-5553 sont affaiblies par la diminution du volume de la phase α qui se produit lors du refroidissement rapide suivant le soudage. L'application de traitements thermiques après le soudage permet la restauration complète de la microstructure. Finalement, des essais de soudure par friction linéaire ont été faits sur plusieurs autres matériaux, dont IMI-834 (Ti-5.8Al-4Sn-3.5Zr-0.7Nb-0.5Mo-0.3Si), mono cristal CMSX-4, (Ni‑9.5Co-6.4Cr-6.4Ta-6.4W-5.6Al-2.9Re-1Ti-0.6 Mo-0.1Hf wt %), l'acier austénitique 316L, l'acier doux A42, etc.
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Kurapov, Denis [Verfasser]. « Structure evolution, properties, and application of alumina films deposited by PECVD / Denis Kurapov ». Aachen : Shaker, 2005. http://d-nb.info/1186579722/34.
Texte intégral
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Guo, Guannan. « Aluminum microstructure evolution and effects on mechanical properties in quenching and aging process ». Digital WPI, 2017. https://digitalcommons.wpi.edu/etd-dissertations/334.
Texte intégralRésumé :
"High strength aluminum alloys are recently widely used in aircraft, automobile and construction industry fields. Typical T6 heat treatment process can be applied to improve the heat treatable aluminum alloy in order to facilitate the formation of prime strengthening precipitate phases. Critical steps in T6 heat treatment process include solution treatment, quenching and aging. Due to high thermal gradients in quenching process and aging process, large thermal stress will remain in the matrix and may bring unexpected deformation or distortion in further machining. Therefore, in order to predict the thermal stress effects, constitutive model and precipitate hardening model are needed to simulate the mechanical properties of alloy. In this dissertation, an optimized constitutive model, which is used to describe the mechanical behavior during quenching and intermediate period of quenching and aging process, was given based on constitutive models with Zenor-Holloman parameter. Modification for constitutive model is based on the microstructure model, which is developed for the quenching and aging processes. Quench factor analysis method was applied to describe the microstructure evolution and volume fraction of primary precipitate phases during quenching process. Some experimental phenomena are discussed and explained by precipitate distributions. Classical precipitate hardening models were reviewed and two models were selected for Al-Cu-Mn alloy aging treatment. Thermal growth model and Euler algorithm were used to improve the accuracy and the selected precipitate hardening models were validated by yield stress and microstructure observations of Al-Cu-Mn aging response experiments."
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Fan, Yue Ph D. Massachusetts Institute of Technology. « Atomistic simulation of defect structure evolution and mechanical properties at long time scales ». Thesis, Massachusetts Institute of Technology, 2013. http://hdl.handle.net/1721.1/82865.
Texte intégralRésumé :
Thesis (Ph. D.)--Massachusetts Institute of Technology, Department of Nuclear Science and Engineering, 2013.Cataloged from PDF version of thesis.
Includes bibliographical references (pages 127-146).
This thesis is a computational and theoretical investigation of the response of materials' mechanical properties to a wide range of environmental conditions, with a particular focus on the coupled effects of strain rate and temperature. The thesis provides original contributions to the fundamental understanding of how the materials mechanical properties change, as manifested by defect structure evolution, with temperature and strain rate conditions, as well as to the development of methodology used for enabling the investigation of dislocation-defect interactions over a much wider range of time scales than of reach to traditional techniques. This thesis advanced the capabilities of a recently developed activation-relaxation based atomistic method to enhance the accuracy of kinetic predictions, and to enable the investigation of dislocation-defect interactions dynamically at long time scales. We took the Autonomous Basin Climbing (ABC) method as a starting point, and incorporated the ability to sample multiple transition pathways associated with a given state. This new feature addresses the problem of overestimating the system evolution time due to the one-dimensional nature of the original ABC algorithm. The ABC method was further implemented in a dynamic framework, which makes it possible for the first time to directly simulate the dislocation-obstacle interactions at very low strain rates. This approach allows for a new way to connect the atomistic results to models at the meso-scale for simulating the plasticity of metals. We analytically derived how the applied strain rate couples with the thermal activation process, based on the framework of transition state theory informed by the atomistic approach described above. We demonstrated the coupling effect is a common mechanism behind many important phenomena, and provide three examples from the atomic level on the dislocation mobility and dislocation interactions with radiation induced defects. (i) A well-known universal flow stress upturn behavior in metals has been examined. We provide a simple physically based model to predict the flow stress at various strain rates, without invoking any assumed mechanisms or fitting parameters as in the traditional constitutional models. (ii) We implemented this new model in (i) to investigate the dislocation-obstacle interactions. The approach enabled us to map the interaction between an edge dislocation and a self interstitial atom (SIA) cluster in Zr in a two-parameter space consisting of temperature and strain rate. This approach allows the direct atomistic simulation of dislocation-obstacle interactions at experimental time scale, namely at low strain rates, which cannot be reached by traditional atomistic techniques. The dislocation is found to absorb the SIA cluster and climb at low strain rates and high temperatures, while it passes through the SIA cluster at high strain rates and low temperatures. The predicted mechanism map is able to reconcile the seeming controversy between previous experimental and computational findings. (iii) A dislocation-void interaction in bcc Fe at prescribed strain rate is also investigated. We demonstrated that different applied strain rates can affect the interaction mechanism and the defect microstructure, and eventually lead to a negative strain rate sensitivity (nSRS) of yield strength below a critical strain rate. This finding at the unit process level supplements the previous explanations of the nSRS with higher level constitutive relations. Beyond the specific cases analyzed in metals in this thesis, the insights gained on the coupling between strain rate and thermal activation can be used to explain the dependence on strain rate and temperature in other important classes of materials (e.g. colloids, cement) and phenomena (e.g. corrosion, creep).
by Yue Fan.
Ph.D.
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Cheung, E. Cheung Cora. « Evolution of optical gain properties through three generations of electroluminescent fluorene-based polymers ». Thesis, Imperial College London, 2010. http://hdl.handle.net/10044/1/6058.
Texte intégralRésumé :
Conjugated polymer semiconductors combine the processing and mechanical characteristics of plastics with the desirable optical and electronic properties of semiconductors. The aim of the research reported in this thesis was to investigate the evolution of the optical gain properties through three generations of electroluminescent fluorene-based polymers. Detailed optical, optoelectrical and gain characterisations were carried out on a range of different electroluminescent polyfluorene-based polymers. It was discovered that not all of the polymers were gain media as some were unable to give ASE. SC006 was found to be the most intriguing material among the rest of the tested polymers; this third generation polymer was found to be a non ASE material while achieving a high PLQE of 96% with 1.3ns-long excited state lifetime. Therefore it was evident that optimised highly efficient light emitting conjugate polymers for PLEDs are not necessarily effective optical gain media, and high steady state PLQE and long excited state lifetime are insufficient for good optical gain properties. Furthermore, in order to investigate the ASE quenching mechanism in SC006, a series of solvatochromism studies were carried out on this polymer. The time-resolved PL characteristics were compared between polymers of second and third generations. The combination of intermolecular and intramolecular energy transfer process was found to be responsible for the ASE quenching. Moreover, the effects of the differences in Yamamoto and Suzuki synthesis routes on optical gain properties of the first generation statistical and alternating copolymers were investigated and were found to be insignificant. Finally, the application of the gain quenching mechanism was demonstrated by an optical switching process performed on a polymer DFB laser. This enabled complete control over the laser emission from the polymer laser, thus achieving a minimum of a thirty fold reduction in the visible light output in the presence of a control pulse.
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Kim, Yongha. « Densification and properties evolution of stainless steel alloys fabricated by three-dimensional printing ». Pullman, Wash. : Washington State University, 2009. http://www.dissertations.wsu.edu/Thesis/Summer2009/y_kim_080509.pdf.
Texte intégralRésumé :
Thesis (M.S. in mechanical engineering)--Washington State University, August 2009.Title from PDF title page (viewed on Sept. 16, 2009). "School of Engineering and Computer Science." Includes bibliographical references (p. 77-83).
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Qian, Feng. « Microstructural evolution of Mn-based maraging steels and their influences on mechanical properties ». Thesis, University of Sheffield, 2015. http://etheses.whiterose.ac.uk/9459/.
Texte intégralRésumé :
A set of Mn-based maraging TRIP steels was designed by Max-Planck-Institut für Eisenforschung GmbH (MPIE) for light weight and safe automotive applications. According to their research, these Mn-based maraging TRIP steels exhibited a simultaneous increase in both strength and ductility upon aging. They attributed this surprising effect to the combination of precipitation strengthening mechanism and TRIP effect of reverted/retained austenite. This thesis carried out a further study on this type of steels with minor modification of chemical composition (7-12 wt.% Mn, with additional ~ 1 wt.% Al). The unknown precipitates were characterized as L21-ordered Ni2TiAl intermetallic phase for the first time. This type of precipitates is not only coherent but also coplanar with the martensite matrix. Their special orientation relationship together with the small lattice misfit (1.24%) led to the precipitates remaining coherent with the martensite matrix even after a long-term aging for 10080 min. Analyses on precipitate size revealed that the coarsening rate constants follows the diffusion-controlled coarsening kinetics form r ̅^3~Kt predicted by LSW theory, but the experimental precipitate size distributions (PSDs) is much broader than the theoretical PSD function. In addition, a core/shell structure was observed within the precipitates, but the exact structure of this structure is still not clear. The formation of reverted austenite nanolayers initiated at the onset of aging by a diffusionless shear mechanism since the critical Mn concentration for austenite reversion at the interface is very low. The accumulated Mn segregation at grain boundaries in the following aging led to the austenite nanolayers that grew to lath-like reverted austenite, which means the lateral growth of austenite was supported by the diffusion of Mn. Due to the low diffusion rate of Mn and the thermodynamic resistance to coalescence, the growth rate of lath-like reverted austenite is slow and thus the austenite maintained in the range 70-200 nm for a long time. The segregation of Ti and Mo on grain boundaries in the initial aging stage resulted in the Mn concentration of austenite nanolayers being far from that indicated by the equilibrium Fe-Mn phase diagram. The segregation of Ti and Mo gradually vanished with the enrichment of Mn during the succeeding aging process. The TEM-EDS analyses revealed the Mn concentration of lath-like austenite was at the level of ~24 at.% which is higher than that of retained austenite (8-12 at.%) reported in conventional Mn-based TRIP or Q&P steels. Nanoindentation testing revealed that the high stability of reverted austenite in Mn-based maraging steels was mainly attributed to the high Mn concentration of austenite. The nano-size of reverted austenite was also considered to be responsible for the high stability. Severe embrittlement occurred in samples aged at lower temperatures or for short times. Increasing aging temperatures and duration can significantly improve the embrittlement phenomena. An ultimate tensile strength (UTS) of 1120 MPa with total elongation (TE) of 18.4% was obtained in the 12% Mn alloy by aging at 500 °C for 5760 min. It was demonstrated that the dense precipitates contributed to the increase in yield strength whereas the work hardening of reverted austenite contributed to the enhanced strength and ductility after yielding. The TRIP effect of reverted austenite reported by Raabe et al. does not occur to any significant amount owing to the high stability of reverted austenite in Mn-based maraging steels.
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Ecay, Lionel. « Concrete transfer properties evolution and nuclear containment vessel tightness assessment during an accident ». Thesis, Pau, 2018. http://www.theses.fr/2018PAUU3022/document.
Texte intégralRésumé :
L’accident de Fukushima a démontré qu’aujourd’hui encore, malgré les progrès déjà réalisésdans le domaine de la sûreté nucléaire, une interruption prolongée du circuit de refroidissement primaire du réacteur (plusieurs semaines ici) était possible. La France s’est par conséquent vue contrainte de réévaluer le niveau de sûreté de ses centrales. Plus spécifiquement, le cas le plus défavorable qu’EDF considérait jusqu’alors, qui consistait en un arrêt total du système de refroidissement primaire de 24h, a été réévalué à deux semaines. Ce changement d’échelle temporelle a introduit des problématiques de fluage, d’évolution hygrométrique du béton ainsi que de flux de vapeur – celle-ci transportant les radionucléides – jusque-là laissées de côté. De ces considérations est né le projet ANR/RSNR MACENA (MAîtrise du Confinement d’une ENceinte en Accident), qui vise à évaluer le confinement d’une enceinte soumise à une température de 180°C et à une pression de 5 bar pendant deux semaines. Ce travail s’inscrit dans la lignée de celui entamé par Khaddour [2014] dans le but de parvenir à une meilleure prédiction des propriétés de transport des milieux poreux en se basant uniquement sur une caractérisation indirecte, la distribution des tailles de pore obtenue grâce à la technique de porosimétrie par intrusion de mercure. Si l’aptitude du modèle initial à estimer correctement les perméabilités intrinsèques a été confirmée sur des matériaux représentant une large gamme de perméabilités, il n’était pas, en l’état, capable de prédire les perméabilités relatives au gaz et au liquide. C’est pourquoi une approche dite redistributive a été développée, laquelle a montré de bons résultats sur diverses PSD. Enfin, un nouveau modèle a été développé pour essayer de s’affranchir de cette approche tout en améliorant la capacité à déterminer les perméabilités relatives. Bien qu’efficace sur des PSD monomodales, étroites ou larges, il a montré ses limites sur une PSD réelle bimodale. En parallèle, une étude de faisabilité a exhibé la possibilité de suivre un front de saturation à l’intérieur d’un matériau poreux, ce qui permettrait à terme de valider l’inclusion d’autres phénomènes dans le modèle, liés notamment à la pression de perçageThe Fukushima catastrophe that struck Japan in 2011 demonstrated that despite significant progress in the field of nuclear safety a prolonged reactor primary cooling circuit breakdown was possible (several weeks in this specific case). With 4 nuclear power plants located on its shoreline, France therefore needed to reassess the safety level of its facilities. More specifically, the worst case scenario considered up until that point by EdF — which consisted in a 24h breakdown of the primary cooling system — was revised up to two weeks. This time-scale shift induced creep, drying and vapour flow problems previously left aside. Thus came to be the ANR/RSNR MACENA (MAîtrise du Confinement d’une ENceinte en Accident) project, which aims at bettering the tightness assessment of a nuclear containment vessel submitted to a temperature of 180°C and to a pressure of 5 bar for two weeks. This work falls in line with that initiated by Khaddour [2014], who set out to better predict porous materials’ transport properties based solely on an indirect characterisation of their topology, namely Pore Size Distribution (PSD) which is obtained via an experimental technique called Mercury Intrusion Porosimetry (MIP). The initial model’s ability to correctly estimate intrinsic permeabilities was confirmed on several different materials whose intrinsic permeabilities span several orders of magnitude. However, it was not fit to accurately account for gas and liquid relative permeabilities. This led to the introduction of a so-called redistributive approach, which yielded better results on various PSD and corresponding experimental datasets. Finally, a new model was developed to try and avoid said approach while bettering relative permeability predictions. Although it behaved well with monomodal pore size distributions, be they wide or narrow, it fell short when applied to a real bimodal PSD. In parallel, an experimental feasibility study demonstrated the possibility of tracking a saturation front within a porous material, which should ultimately allow for the implementation of several other phenomena into the model, linked among others to breakthrough capillary pressure
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FERSINI, FRANCESCO. « TYPICAL AND ATYPICAL SH3 INTERACTIONS : EVOLUTION AND SPECIFICITY OF STRUCTURAL AND BIOCHEMICAL PROPERTIES ». Doctoral thesis, Università degli Studi di Milano, 2012. http://hdl.handle.net/2434/168397.
Texte intégralRésumé :
As part of PENELOPE network we focused on biophysical characterization of SH3 domains involved in actin independent polarization. In particular we were interested to characterize at the structural and biochemical level the interactions between the proteins Bem1, Cdc42 and Cdc24, which are partially mediated by canonical and non-canonical SH3 interactions. The ultimate goal was to compare in the four yeasts the nature of this small protein's interaction network, in order to shed light on the nature and evolution of this network in the four yeast organisms, which very likely evolved from a common ancestor.
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Gover, Rachel K. E. « Carbon nanostructures : formation and evolution in the laboratory and the ISM ». Thesis, University of Nottingham, 2016. http://eprints.nottingham.ac.uk/37810/.
Texte intégralRésumé :
The chemistry of large carbon-containing molecules in space is widely researched with notable advances including understanding of the aromatic infrared emission features in terms of their general origin in polycyclic aromatic hydrocarbon (PAH) molecules, identification of the fullerenes C60 and C70 as the largest molecules detected in the interstellar medium, and knowledge of the structure and composition of various solid-state interstellar grains. However, there remain major challenges. The work described in this Thesis addresses some longstanding, unanswered questions in astrochemistry, such as the origin of variation in the profiles of the aromatic infrared bands, the route to formation of C60, and the role of astrophysical grains in the reactions of carbon-containing species. This research was carried out using a range of laboratory and computational techniques, in combination with astronomical data, yielding new results that can be compared with existing work to find solutions to these problems. The varying profile of the 11.2 μm aromatic infrared emission band within two objects, NGC 7023 and the Red Rectangle, is investigated. The feature’s changing spectral shape, which depends on the position within the object, is interpreted in terms of variation in the distribution of molecular masses of the PAHs that give rise to the feature. This was achieved using a model that fits astronomical data which was obtained with the instrument MICHELLE on the UKIRT telescope in Hawaii and the Spitzer satellite observatory. Based on the calculated emission features of four neutral PAHs that are taken to represent ‘low-mass’ (fewer than 50 C atoms) or ‘high-mass’ (more than 50 C atoms), it is found that, with increasing distance from the star, there is an increase in the proportion of low-mass PAHs in NGC 7023, in good agreement with less specific previous studies that have been carried out using other approaches. The modelling results for the Red Rectangle indicate that the mass distribution does not change significantly with offset, although evidence from other sources appears in part to contradict this conclusion. A laboratory investigation is detailed in which formation of C60 from PAH precursors is explored. Transmission electron microscopy (TEM) and matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry (MALDI-TOF MS) are used to irradiate, using the electron beam (e-beam) in TEM and the UV laser in MALDI-TOF MS, samples of PAH molecules and to monitor the products of any resulting transformations within the samples. It is found that under both e-beam and UV irradiation, PAH molecules lose one or more H atoms and form larger PAH 'oligomers’. Based on the MALDI-TOF MS results, specific chemical structures of these oligomers are proposed, and it is found that the precursor molecules coronene and perylene give rise to formation of C60. A mechanism is given for this process, in which an oligomer containing 60 carbon atoms or more is formed, and then ‘shrinks’ via loss of H atoms, C-C bond formation, and Stone-Wales rearrangements. Important structural features required for these mechanisms, namely the presence of ‘bay’ and 'fjord’ carbon edge sites, are discussed in an astrophysical context. An experimental methodology was designed and implemented to use TEM and MALDI-TOF MS in conjunction with each other, in which a specific area of a PAH sample is viewed using low-dose TEM, is then irradiated using the UV laser in MALDI-TOF MS, and finally is viewed a second time using TEM to observe the effects of the UV irradiation. Experiments were carried out to investigate reactions of acetylene and CO in the presence of astrophysically relevant grains. It was found that CO undergoes a disproportionation reaction over olivine grains, forming gaseous CO2 and depositing solid-state carbon on the grain surface; this has implications for the formation of other carbonaceous species such as PAHs and even carbon nanotubes. The formation of PAHs from acetylene gas over grains of SiC is reported, in addition to the inhibiting impact of CO when mixed with acetylene. In one experiment olivine samples were irradiated by the e-beam in TEM, and decomposition of the olivine structure was seen to occur, resulting in the formation of metallic nanoparticles. This behaviour is discussed in terms of its likely occurrence in astrophysical environments such as the Red Rectangle, and its potential for catalysis that results from reactions of molecules on the metal nanoparticle surface.
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Kar, Sujoy Kumar. « Modeling of mechanical properties in alpha/beta-titanium alloys ». The Ohio State University, 2005. http://rave.ohiolink.edu/etdc/view?acc_num=osu1122570452.
Texte intégral
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Mridha, Sanghita. « Structure Evolution and Nano-Mechanical Behavior of Bulk Metallic Glasses and Multi-Principal Element Alloys ». Thesis, University of North Texas, 2017. https://digital.library.unt.edu/ark:/67531/metadc984260/.
Texte intégralRésumé :
Bulk metallic glasses and multi-principal element alloys represent relatively new classes of multi-component engineering materials designed for satisfying multiple functionalities simultaneously. Correlating the microstructure with mechanical behavior (at the microstructural length-scales) in these materials is key to understanding their performance. In this study, the structure evolution and nano-mechanical behavior of these two classes of materials was investigated with the objective of fundamental scientific understanding of their properties. The structure evolution, high temperature nano-mechanical behavior, and creep of two Zr-based alloys was studied: Zr41.2Ti13.8Cu12.5Ni10.0Be22 (Vitreloy1) and Zr52.5Ti5Cu17.9Ni14.6All0 (Vitreloy105). Devitrification was found to proceed via the formation of a metastable icosahedral phase with five-fold symmetry. The deformation mechanism changes from inhomogeneous or serrated flow to homogenous flow near 0.9Tg, where Tg is the glass transition temperature. The creep activation energy for Vitreloy1 and Vitreloy105 were 144 kJ/mol and 125 kJ/mol, respectively in the range of room temperature to 0.75Tg. The apparent activation energy increased drastically to 192 kJ/mol for Vitreloy1 and 215 kJ/mol for Vitreloy105 in the range of 0.9Tg to Tg, indicating a change in creep mechanism. Structure evolution in catalytic amorphous alloys, Pt57.5Cu14.7Ni5.3P22.5 and Pd43Cu27Ni10P20, was studied using 3D atom probe tomography and elemental segregation between different phases and the interface characteristics were identified.
The structure evolution of three multi-principal element alloys were investigated namely CoCrNi, CoCrFeMnNi, and Al0.1CoCrFeNi. All three alloys formed a single-phase FCC structure in as-cast, cold worked and recrystallized state. No secondary phases precipitated after prolonged heat treatment or mechanical working. The multi-principal element alloys showed less strain gradient plasticity compared to pure metals like Ni during nano-indentation. This was attributed to the highly distorted lattice which resulted in lesser density of geometrically necessary dislocations (GNDs). Dislocation nucleation was studied by low load indentation along with the evaluation of activation volume and activation energy. This was done using a statistical approach of analyzing the "pop-in" load marking incipient plasticity. The strain rate sensitivity of nanocrystalline Al0.1CoCrFeNi alloy was determined by in situ compression of nano-pillars in a Pico-indenter. The nanocrystalline alloy demonstrated a yield strength of ~ 2.4 GPa, ten times greater than its coarse grained counterpart. The nanocrystalline alloy exhibited high strain rate sensitivity index of 0.043 and activation volume of 5b3 suggesting grain boundary dislocation nucleation.