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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.

Etude de la séparation de phase dans des verres silicatés par résonance magnétique nucléaire haute résolution solide et microscopie electronique / Study of phase separation in silicate glasses using high resolution solid state nuclear magnetic resonance and electron microscopy

Martel, Laura 05 December 2011 (has links)
La compréhension de la structure des verres est actuellement à l’origine de nombreuses recherches scientifiques. L’une des preuves expérimentale d’un certain ordre dans ceux-ci est la séparation de phase. En effet, ce phénomène est lié à la présence dans ces matériaux amorphes d’au moins deux phases de composition chimique différentes. Ainsi, une étude des prémices de la séparation de phase de type nucléation-croissance dans des verres de silicate de sodium a été menée. La RMN c’est révélée la plus efficace pour cette étude. L’utilisation des expériences de corrélation 29Si-29Si a permis de sonder le réseau silicaté à des échelles plus grandes que celles communément considérées. Nous avons donc pu identifier ce phénomène, caractériser la composition des phases qui apparaissaient et enfin établi un lien avec la cristallisation observée pour de long temps de recuits. A l’inverse, la séparation de phase dans les aluminosilicates de calcium étant visible à l’échelle macroscopique, nous voulions donc suivre la diminution de la séparation de phase avec l’ajout d’alumine. Ainsi, en élaborant un protocole de synthèse spécifique, nous avons pu synthétiser ces verres à hautes températures. Ils ont été étudiés par le biais de la microscopie électronique et de la RMN. Comme ces verres sont composés de nano-domaines vitreux intégrés dans une matrice de composition différente, nous avons pu obtenir des matériaux nano-structurés de taille contrôlée. L’utilisation de la RMN nous a permis de montrer que l’aluminium s’insérait sous forme de « clusters » dans le réseau silicaté. / The understanding of the vitreous state is actually a center of great interest in inorganic chemistry. In fact, even if a glass is often described as a totally disordered material, presence of structures at the atomic scale has to be considered. One of the first experimental proof against the random network theory is the observation of phase separation. In fact, in these systems, at least two glasses with different compositions are observed. In this way, the first steps of phase separation in sodium silicate glasses have been studied. This was afforded with the NMR technique using 29Si-29Si correlations experiments. Thus, the silicon network was probed at higher distances compared with which was commonly observed in these glasses. Therefore, we have been able to identify this phenomenon at the atomic scale, to define the composition of each glassy phase and to do a link with crystallization which happened after long heat treatment of the glass. At the contrary phase separation in calcium aluminosilicate glasses can be observed at the macroscopic scale, observation of its decrease with increasing amount of alumina has been under the scope of this study. The main challenge of this study was to synthesized high temperature glasses. Since an efficient protocol has been implemented, we have been able to obtained nano-structured materials with a controlled size of nano-domains. This caracterisation of the material has been possible using electron microscopy. Combined with NMR, we have been able to propose a mechanism to explain insertion of alumina in the silicon network.

Phase separation in carbon:transition metal nanocomposite thin films

Berndt, M. 16 September 2010 (has links) (PDF)
kein Abstract vorhanden

Structure-property relationships of chain-extended thermoplastic polyurethane elastomers

Sykes, Paul A. January 1999 (has links)
The effect of chain extender chemical structure on the physical and mechanical properties of thermoplastic polyurethane/urethaneurea elastomers was systematically investigated. Several series of materials were synthesised using 4,4' -diphenylmethane diisocyanate (MDI) and poly(tetramethylene oxide) glycol (PTMG), each series incorporating a particular class of chain extender compound. Elucidation of the influence of chain extender structural variations within each series was the principal objective of the investigation…

The tangential velocity profile and momentum transfer within a microgravity, vortex separator

Ellis, Michael Clay 15 May 2009 (has links)
Liquid and gas do not separate naturally in microgravity, presenting a problem for twophase space systems. Increased integration of multiphase systems requires a separation method adaptable to a variety of systems. Researchers at Texas A&M University (TAMU) have developed a microgravity vortex separator (MVS) capable of handling both a wide range of inlet conditions and changes in these conditions. To optimize the MVS design, the effects of nozzle area, separator geometry, and inlet flow rate must be understood. Computational fluid dynamics (CFD), in the form of Adapco’s Star-CD, is used, along with laboratory testing, to accomplish this goal. Furthermore, as analysis aids for the laboratory data and CFD results, relationships for radial pressure, bubble transit time, and momentum transfer were developed. Ground testing data showed a linear relationship between rotational speed and inlet flow rate. The CFD results compared well with the ground data and indicated that the majority of the rotational flow travels at nearly the same rotational speed. Examination of the tangential velocity profile also showed that a reduction of nozzle outlet area resulted in increased tangential velocities. Using dimensional analysis, a relationship between separator radius, inlet momentum rate, fluid properties, and rotational speed was found. Applying this relationship to the ground data and CFD results showed a strong correlation between the two dimensionless groups. Linear regression provided an equation linking rotational speed to the separator parameters. This equation was tested against the ground data and shown to predict average rotational speed well for all separator models. These results were used to calculate the radial and axial transit times of gas bubbles within the separation volume. Radial transit time was found to decrease more rapidly than axial transit time as gas volume increased, indicating axial and radial transit times are closest in value for the all liquid case and increasing gas core diameter improves the operational characteristics of the separator. From a design standpoint, the all liquid case provides a minimum flow rate for successful phase separation. Maximum flow rate depends on the pressure resources of the system.


Zha, Weibin January 2006 (has links)
No description available.

Computer modelling of multidimensional multiphase flow and application to T-junctions

Oliveira, Paulo Jorge Dos Santos Pimentel de January 1992 (has links)
No description available.

Phase Separation and Second Phase Precipitation in Beta Titanium Alloys

Devaraj, Arun 05 1900 (has links)
The current understanding of the atomic scale phenomenon associated with the influence of beta phase instabilities on the evolution of microstructure in titanium alloys is limited due to their complex nature. Such beta phase instabilities include phase separation and precipitation of nano-scale omega and alpha phases in the beta matrix. The initial part of the present study focuses on omega precipitation within the beta matrix of model binary titanium molybdenum (Ti-Mo) alloys. Direct atomic scale observation of pre-transition omega-like embryos in quenched alloys, using aberration-corrected high resolution scanning transmission electron microscopy and atom probe tomography (APT) was compared and contrasted with the results of first principles computations performed using the Vienna ab initio simulation package (VASP) to present a novel mechanism of these special class of phase transformation. Thereafter the beta phase separation and subsequent alpha phase nucleation in a Ti-Mo-Al ternary alloy was investigated by coupling in-situ high energy synchrotron x-ray diffraction with ex-situ characterization studies performed using aberration corrected transmission electron microscopy and APT to develop a deeper understanding of the mechanism of transformation. Subsequently the formation of the omega phase in the presence of simultaneous development of compositional phase separation within the beta matrix phase of a Ti-10V-6Cu (wt%) alloy during continuous cooling has been investigated using a combination of transmission electron microscopy and atom probe tomography. The results of these investigations provided novel insights into the mechanisms of solid-state transformations in metallic systems by capturing the earliest stages of nucleation at atomic to near atomic spatial and compositional resolution.

Thermoreversible Gelation, Crystallization and Phase Separation Kinetics in Polymer Solutions under High Pressure

Fang, Jian 13 October 2008 (has links)
This thesis is an experimental investigation of phase behavior, crystallization, gelation and phase separation kinetics of polymer solutions in dense fluids at high pressures. The miscibility and dynamics of phase separation were investigated in solutions of atactic polystyrene with low polydispersity (Mw = 129,200; PDI = 1.02) in acetone. Controlled pressure quench experiments were conducted at different polymer concentrations to determine both the binodal and the spinodal envelops using time- and angle resolved light scattering techniques. At each concentration, a series of rapid pressure quenches with different penetration depths in a range from 0.1 MPa to 3 MPa were imposed and the time evolution of the angular distribution of the scattered light intensities was monitored. The solution with 11.4 wt % polymer concentration underwent phase separation by spinodal decomposition mechanism for both shallow and deep quenches. Below this critical polymer concentration, phase separation was found to proceed by nucleation and growth mechanism for shallow quenches, but by spinodal decomposition for deeper quenches. Gelation and crystallization processes and the influence of pressure and the fluid [Cho et al. 1993]composition were investigated in solutions of poly(4-methyl-1-pentene) [P4MP1] in n-pentane + CO₂ and in solutions of syndiotactic polystyrene [sPS] in toluene + CO₂, and also in acetophenone + CO₂ fluid mixtures over a pressure range up to 55 MPa and carbon dioxide levels up to 50 wt %. In pure pentane, P4MP1 undergoes crystallization and leads to Form III polymorph at low pressures, but to Form II at high pressures. In n-pentane + CO₂ mixture fluids, the polymorphic state changes from a mixture of Forms III and II to Form II and eventually to Form I with increasing CO₂ content. High level of carbon dioxide (≥40 wt %) in the solution was found to lead to gelation instead of crystallization. No liquid-liquid phase boundaries could be observed in any of the P4MP1 solutions. In contrast to P4MP1 in n-pentane, syndiotactic polystyrene was found to undergo gelation in toluene or acetophenone forming a polymer-solvent compound with the δ crystal form. Also in contrast to P4MP1 systems, addition of carbon dioxide to sPS solutions alters the process from that of gelation to crystallization leading to the β crystal form. In solutions with high CO₂ level, in addition to the gelation or crystallization boundaries, a liquid-liquid phase separation boundary was also observed. The phase separation path followed was found to influence the eventual morphology and the crystal state of the polymer. In sPS solutions in toluene + CO₂, if the sol-gel boundary were crossed first by cooling the solution at a fixed pressure, the resulting morphology was found to be fibrillar and the polymer displayed the δ crystal form. If instead, the liquid—liquid phase boundary were crossed first by reducing pressure at a fixed temperature, the polymer-rich phase leads to a stacked-lamellar morphology with the β crystal form while the polymer-lean phase leads to a mixed morphology with lamellar layers connected by fibrils with the polymer displaying δ + β crystal forms. In solutions in acetophenone + carbon dioxide, when the gelation boundary is crossed first, the resulting structure is the δ form as in the toluene + CO₂ case. At comparable CO₂ levels, when the L-L phase boundary is crossed first, in the acetophenone system, polymer-rich phase was found to generate a mixture of δ + β forms while only the δ form was found in the polymer-lean phase, in contrast to the observations in the toluene + CO₂ systems. Based on crystallographic, spectral and microscopic data, a thermodynamic framework was developed which provides a mechanistic account for the formation of the different polymorphs. / Ph. D.


KULKARNI, AMIT S. 26 May 2005 (has links)
No description available.

Theoretical and finite-element investigation of the mechanical response of spinodal structures

Read, D.J., Teixeira, P.I., Duckett, R.A., Sweeney, John, McLeish, T.C.B. January 2002 (has links)
no / In recent years there have been major advances in our understanding of the mechanisms of phase separation in polymer and copolymer blends, to the extent that good control of phase-separated morphology is a real possibility. Many groups are studying the computational simulation of polymer phase separation. In the light of this, we are exploring methods which will give insight into the mechanical response of multiphase polymers. We present preliminary results from a process which allows the production of a two-dimensional finite-element mesh from the contouring of simulated composition data. We examine the stretching of two-phase structures obtained from a simulation of linear Cahn-Hilliard spinodal phase separation. In the simulations, we assume one phase to be hard, and the other soft, such that the shear modulus ratio ... is large (... ). We indicate the effect of varying composition on the material modulus and on the distribution of strains through the stretched material. We also examine in some detail the symmetric structures obtained at 50% composition, in which both phases are at a percolation threshold. Inspired by simulation results for the deformation of these structures, we construct a "scaling" theory, which reproduces the main features of the deformation. Of particular interest is the emergence of a lengthscale, below which the deformation is non-affine. This length is proportional to ... , and hence is still quite small for all reasonable values of this ratio. The same theory predicts that the effective composite modulus scales also as ..., which is supported by the simulations.

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