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Les fontes usuelles sont des alliages de fer à hautes teneurs en C et Si dans lesquels le carbone est précipité sous forme de graphite. La plupart des fontes industrielles reçoivent un traitement de sphéroïdisation du graphite qui précipite alors sous forme de nodules. Toutefois, la présence dans le métal de certaines impuretés ou éléments d’addition (en particulier les terres rares), de même que de lentes vitesses de refroidissement, peuvent conduire à l’apparition de formes dégénérées du graphite sphéroïdal. Une de celles-ci est le graphite "chunky" dont l’apparition réduit de façon importante les propriétés mécaniques des pièces coulées. Le but de ce travail a été l’étude de l’influence de certains éléments chimiques sur la forme de croissance du graphite dans la fonte sphéroïdale. Une nouvelle méthode d’analyse chimique a été mise au point pour quantifier les éléments minoritaires dans ce type d’alliages. Elle a permis d’abaisser les limites traditionnelles de quantification par rapport à d’autres techniques, et a été appliquée pour démontrer l’absence de macro-ségrégations dans des pièces épaisses. La nature eutectique des cellules du graphite Chunky a aussi pu être mise en évidence. L’étude de prélèvements sur pièces épaisses a montré la présence de nombreuses inclusions, qui ont pu être comptées et analysées dans les zones à graphite chunky et dans celles qui en sont exemptes. L’analyse de ces inclusions a montré qu’elles ne contiennent pas d’oxygène pour la plupart d’entre elles et doivent avoir une formulation stœchiométrique du type (Ce,La)xSy. Ce type d’inclusions a été étudié par analyse thermique différentielle afin de caractériser les conditions de leur formation et de leur dissolution. Les domaines de température correspondants à leur précipitation et leur dissolution ont été proposés. Prenant comme départ une composition chimique et une gamme de températures de travail typiques des procédés de fonderie de ce type d’alliages, la précipitation des sulfures binaires de cérium et la séquence de formation la plus probable des inclusions ont été simulées par calculs thermodynamiques. Compte tenu de la variation en composition chimique avec l’évolution de la fraction liquide durant la solidification eutectique des fontes, la possibilité de précipitation secondaire de chaque type de sulfure de cérium a été modélisée. / Usual grey cast irons are Fe-rich alloys with high C and Si content in which carbon precipitates as graphite. For many applications, a so-called spheroidizing treatment is applied to industrial cast irons so that graphite precipitates as spherical nodules. However, the presence in the melt of impurities or alloying elements such as rare earths, as well as very low cooling rates, may lead to the formation of degenerate spheroidal graphite. One of these degenerate shapes is known as "chunky" graphite which strongly reduces the mechanical properties of the castings. The goal of this work was studying the effect of some chemical elements on the growth shape of graphite in spheroidal graphite cast irons. A new chemical analysis methodology has been established for quantifying minor elements in this kind of alloys. This technique allowed reducing the quantification limits with respect to usual techniques, thus allowing to check for the absence of macrosegregation in thick cast parts. The eutectic nature of chunky graphite cells could also be established. Study of samples machined out from heavy-section cast parts showed the presence of numerous inclusions which could be quantified in areas with and without chunky graphite. EDS analysis of these inclusions showed most of them do not contain oxygen and should have a formula close to (Ce,La)xSy. These inclusions have been studied by differential thermal analysis (DTA) in order to demonstrate the conditions for their formation and dissolution. Temperature ranges for their precipitation upon cooling and dissolution upon heating could be suggested. Taking into consideration standard ranges for chemical composition and temperature range for melt processing, precipitation of binary cerium sulfides and the most probable sequence of precipitation have been simulated using thermodynamic data. Considering the change of chemical composition of the liquid during the eutectic solidification of cast irons, the possibility of precipitation of each type of cerium sulfide has been simulated

Thesis (Ph.D.)--University of Delaware, 2006. / Principal faculty advisor: Pei C. Chiu, Dept. of Civil & Environmental Engineering. Includes bibliographical references.

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Includes bibliographical references (p. 87-95).

In this dissertation several aspects on one-dimensional edge states in grapheme are studied. First, a background in the history and development of graphitic forms is presented. Then some novel features found in two-dimensional bulk graphene are presented. Here, some focus is given to the chiral nature of the Dirac equation and the symmetries found in the grahene. Magnetism and interactions in graphene is also briefly discussed. Finally, the graphene nanoribbon with its two typical edges: armchair and zigzag is introduced. Gaps due to finite-size effects are studied. Next, the problem of determining the zigzag ground state is presented. Later, we develop this in an attempt to add the Coulomb interaction to the zigzag flat-band states. These nanoribbons can be stimulated with a tight-binding code on a lattice model in which many different effects can be added, including an A/B sublattice asymmetry, spin-orbit coupling and external fields. The lowest Landau level solutions in the different ribbon orientations is of particular current interest. This is done in the context of understanding new physics and developing novel applications of graphene nanoribbon devices. Adding spin-orbit to a graphene ribbon Hamiltonian leads to current carrying electronic states localized on the sample edges. These states can appear on both zigzag and armchair edges in the semi-finite limit and differ qualitatively in dispersion and spin-polarization from the well known zigzag edge states that occur in models that do not include spin-orbit coupling. We investigate the properties of these states both analytically and numerically using lattice and continuum models with intrinsic and Rashba spin-orbit coupling and spin-independent gap producing terms. A brief discussion of the Berry curvature and topological numbers of graphene with spin-orbit coupling also follows.

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No description available.

Thesis (Ph. D.)--University of Texas at Austin, 2007. / Vita. Includes bibliographical references.

Thesis (M. App. Sc.) -- University of Adelaide, Department of Chemical, Engineering, 1966.