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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.
1081

Modélisation avancée du contact pneu-chaussée pour l'étude des dégradations des chaussées en surface / Advanced Modeling of Tire-Pavement Contact to Investigate Pavement Surface Degradation

Manyo, Edem Yawo 14 February 2019 (has links)
L'apparition récente de nouveaux matériaux dans les structures de chaussée associée à une diminution de l'épaisseur des couches de surface et une augmentation du chargement des poids lourds et de leur fréquence de passage a entrainé de nouvelles pathologies de dégradation. Outre les problèmes d'orniérage bien connus, apparaissent désormais des fissures descendantes (top down cracking) ainsi que des problèmes de décohésion aux interfaces. Ces nouvelles pathologies entrainent des dépenses considérables sur l'ensemble du réseau (environ 15 milliards d'euros par an), particulièrement en zones urbanisées plus sujettes aux dégradations de surface et ne permettent pas d'estimer convenablement les durées de vie de la chaussée, le plus souvent surestimée dans les méthodes de dimensionnement actuelles. Ce travail de doctorat propose une nouvelle approche du contact pneu-chaussée permettant de mieux appréhender les contraintes principales et résiduelles dans une structure de chaussée bitumineuse. A l'aide d'un outil numérique rapide de calcul basé sur une approche semi-analytique (« Semi-Analytical Methods » (SAM)), la géométrie précise du pneumatique est intégrée afin d'obtenir une répartition de pression de contact ainsi qu'un cisaillement surfacique réelle sur la chaussée. Dans un premier temps, un modèle de contact roulant tractif élastique est implémenté pour des cas théoriques simples et validé par des résultats analytiques et numériques de la littérature. Ensuite, ce modèle est étendu pour prendre en compte le comportement élasto-plastique des corps en contact. Ce dernier est comparé à un résultat numérique basé sur la méthode des éléments finis issu de la littérature. Les résultats, pour une application contact pneu-chaussée, montrent une répartition non homogène des contraintes dans la structure et principalement dans les premiers centimètres sous la surface avec des niveaux beaucoup plus importants que peuvent le prédire les modèles actuels qui utilisent une charge uniformément répartie. La pression de contact est comparée aux mesures effectuées par un système nommé TekScan et les champs mécaniques en sous couches sont comparés à ceux d'Alizé-LCPC dans le cas d'une structure simple. Les cisaillements surfaciques sont déterminés dans le cas du roulement tractif. Une application est effectuée sur la modélisation des dégradations des chaussées en surface. Dans un premier temps, des analyses sur le comportement de la chaussée en surface sont effectuées pour une couche de béton bitumineux semi grenu (BBSG) semi-infinie supposée élastique, homogène sous conditions d'accélération, de freinage et de virage. Pour des études sur le top down cracking, des déformations et directions principales sont déterminées et analysées. Ensuite, le modèle de contact élasto-plastique est appliqué sur une couche semi-infinie de grave bitume GB3. Des déformations et contraintes résiduelles générées dans la structure sont déterminées en vue d'une analyse sur les ornières d'instabilité. Une fois validés, ces résultats permettront d'estimer plus fidèlement la durée de vie résiduelle des chaussées mais également de comprendre et d'éviter les mécanismes de dégradation en surface ou proche de la surface. / The recent appearance of new materials in road structures associated with surface layers thickness decreasing and the increasing of trucks loading and their passage frequency has led to new pathologies of degradation. In addition to the well-known rutting problems, top down cracking is now appearing as well as problems of decohesion at the interfaces. These new pathologies led to considerable expenditure on the entire network (around 15 billion euros per year), particularly in urbanized areas that are more prone to surface damage and do not make it possible to adequately estimate the lifetimes of the roadway, most often overestimated in current design methods. This doctoral work proposes a new approach of the tire-road contact allowing for better apprehend of the main and residual stresses in a bituminous pavement structure. Using a fast numerical tool based on a semi-analytical approach ("Semi-Analytical Methods" (SAM)), the precise geometry of the tire is integrated in order to obtain a real contact pressure distribution as well as surface shear on the pavement surface. Initially, an elastic tractive rolling contact model is implemented for simple theoretical cases and validated by analytical and numerical results from the literature.Then, this model is extended to take into account the elastoplastic behavior of the bodies in contact. This is compared to a numerical result based on the nite element method from the literature. The application for tire-pavement contact results, show a non-uniform distribution of stresses in the structure and mainly in the rst centimeters below the surface with much higher levels than can be predicted by current models that use a uniformly distributed load. The contact pressure is compared to the measurements made by a system called TekScan and the mechanical elds in sublayers are compared to those of Alizé-LCPC in the case of a simple structure. The surface shears are determined in the case of tractive rolling. An application is carried out on the modeling of surface pavement damage. Firstly, analyzes of the behavior of the surface pavement are carried out for a semi-innite semi-grit asphalt concrete layer supposed to be elastic, homogeneous under conditions of acceleration, braking and turning. For studies on top down cracking, principals deformations and directions are determined and analyzed. Then, the elastoplastic contact model is applied on a semi-innite asphalt agragate layer. Deformations and residuals stresses generated in the structure are determined for an analysis on the instability ruts. Once validated, these results will make it possible to more accurately estimate the residual life of pavements but also to understand and avoid surface or near surfacedegradation mechanisms.
1082

我國上市公司股權集中度,薪酬設計與公司經營績效關係之研究 / A Study on the Relationship Among Top Executive Compensation Scheme, Ownership Structure and Firm Performance for Listed Companies in Taiwan

吳政穎, Wu, Cheng-Ying Unknown Date (has links)
高階主管薪酬計畫與股權結構是解決公司代理問題的重要機制,代理理論提出以績效決定酬勞的制度,可提供高階主管努力誘因,促使股東與高階主管之間的目標一致;同時股權結構也會影響高階主管的努力,進而對公司的。故本研究旨在探討我國上市公司股權集中度、酬勞計畫與公司經營績效之關係,並利用模式推導相關的命題與假說。 本研究利用混合迴歸就四年度主管薪酬資料加以分析,Scheffe檢定檢驗高階主管的持股比例與薪酬計畫的關係,實證結果整理如下: 1、董事長持股比例與薪酬無關,總經理持股比例與薪酬有關。 2、公司價值與董事會持股比例呈非線性的關係。 3、機構投資人偏向效率監督假說。 4、總經理薪酬計畫中的項目比薪酬水準的高低重要。 5、公司經營績效與高階主管薪酬呈正相關 / In agency theory, executive compensation contracts and ownership structures provide primary mechanisms for solving agency problems. Top executive compensation linked to performance can align the incentives of firms'' top executive with firms''stockholder. Ownership structure can influce the effort of the top executive. The main purpose of this study is to examine the relationship among top executive compensation scheme, ownership structure and firm performance for listed companies in Taiwan, and use modelling to develop related propositions and hypotheses. Pooling regressions using four years'' pool data and Scheffe test are performed to test hypothese, Empirical results are summarized as follows: 1.The holdings of chairman has nothing to do with the level of compsation and that of CEO has something to do with. 2.The non-linear relationship between firm value and holdings of board of directors. 3.The role of institution tends to be efficinet monitoring hypothesis. 4.Incentive component of CEO compensation scheme tends to be more important than level of that sheme. 5.Firm performance is positively associated with top executive''s cash compensation.
1083

審計機關與行政機關的決算互動:以邁向頂尖大學計畫為例 / The reacation beteween audit organization and administration organization: a case study of the aim for the top university project

陳柏志, Chen, Po Chih Unknown Date (has links)
政府隨時更新建設以因應各式公共建設及政策,但並非所有建設都能如預期 使用,審計機關適時扮演著監督者之角色,因此,行政機關與審計機關在決算時 的互動,成為可以適時督促政府的管道。在過去文獻中,鮮少有人討論審計機關 與行政機關在決算互動之情況,本研究先探討良好的決算為理論基礎,共分三層 次:決算目標、決算使用審計方式及審計人員能力為架構,以探討行政與審計兩 者間互動時之反應,並以我國頂尖大學計畫為例,運用訪談法及二手資料分析, 以解釋決算的過程、成果之重要性,及雙方互動後的結果。 本文藉由訪談頂尖大學及審計機關,針對研究問題提出下列實務上之政策建 議: 一、放寬法規上之規定:頂大計畫之經費之流用應加寬彈性,畢竟頂大計畫僅是 達成計畫的手段。 二、決算進行時,審計機關與行政機關開拓新溝通管道:利用電子化溝通平台減 少審計與行政機關溝通之時間。 三、審計機關與行政機關嘗試以和平互惠角度共存:本研究認為在往後之行政審 計互動關係應嘗試以和平互惠之關係進行,而此使兩者之間能夠以互惠面合 作的前提 四、重視頂大計畫工作圈意見:在未來工作圈會議時,將以頂大辦公室及會計人 員共同進行討論,有利於雙方之瞭解。 / The administrative organizations renew infrastructures based on public and policy, but now all infrastructures can be anticipated to use. The audit plays the supervisor to the administrative organizations. Therefore, the reaction of financial statement will be a good way to surprise the administrative organizations. The documents seldom discuss this issue in the past. This research discusses how to attain the theory of good financial statement with three levels: the goal of financial statement, the way audit organizations to use and the ability of the auditors. This research takes “The Aim for the Top University Project” for example to discuss the reactions of two organizations, and use In-depth interviews and researching secondary materials to explain the process, the importance and the outcome after reactions. First, broaden the rules: the expenditure should be more flexible. The project is just a method approach to the goal. Second, the two organizations should explore new communication channel: use E-communication medium to decrease time. Third, the two organizations should try to stay in peace for collaborating. Fourth, respect the opinion form the teams of top universities. Let staffs of both sides discuss, which will make them understand each other.
1084

在高度分散式環境下進行Top-k相似文件檢索 / Similar Top-k documents retrieval in highly distributed environments

王俊閎, Wang, Chun Hung Unknown Date (has links)
在文件資料庫的查詢處理上,Top-k相似文件查詢主要是協助使用者可以從龐大的文件集合中,檢索出和查詢文件具有高度相關性的文件集合。將資料庫內的文件依據和查詢文件之相似度程度,選擇出相似度最高的前k篇文件回傳給使用者。然而過去集中式資料庫,因其覆蓋性和可擴充性的不足,使得這種排名傾向的文件查詢處理,需耗費大量時間及運算成本。近年來,使用端對端(Peer-to-peer, P2P)架構解決相關的文件檢索問題已成為一種趨勢,但在高度分散式環境下,支援排名傾向的相似文件查詢是困難的,因為缺乏全域資訊和適當的系統協調者。 在本研究中,我們先針對各節點資料庫作分群前處理,並提出一個利用區域切割的作法[1],將P2P環境劃分成數個子區塊後,建立特徵索引表。因此在查詢處理時,可透過索引表加快挑選出Top-k相似群集的速度,並且確保有適當數量的回傳結果。最後在實驗中,我們提出的方法會與傳統集中式搜尋引擎以及SON-based [1] 做比較,在高度分散式環境下,我們的方法在執行Top-k相似文件查詢時,會比上述兩種作法有較為優異的表現。 / On query processing in a large database, similar top-k documents query is an important mechanism to retrieve the highly correlated document collection with query for users. It ranks documents with a similarity ranking function and reports the k documents with highest similarity. However, the former approach in web searching, i.e., centralized search engines, rises some issues such as lack of coverage and scalability, impact provides rank-based query become a costly operation. Recently, using Peer-to-peer (P2P) architectures to tackle above issues has emerged as a trend of solution, but due to the shortage of global knowledge and some appropriate central coordinators, support rank-based query in highly distributed environment has been difficulty. In this paper, we proposed a framework to solve these problems. First, we performed the local cluster pre-processing on each peer, followed by the zone creation process, forming sub-zones over P2P network, and then constructing the feature index table to improve the performance of selecting similar top-k cluster results. The experiments show that our approach performs similar top-k documents query outperforms than SON-based approach in highly distributed environment.
1085

Brottsskeden kring infästning av stålpelare

Endre, Robert January 2012 (has links)
En utfackningsvägg är en icke bärande väggkonstruktion, ofta av trä som har för syfte att minimera energiförbrukningen för byggnader i betong. Inuti konstruktionen används ibland stålpelare för att bära ovanliggande konstruktioner. Stålpelarna utsätts för laster som de måste dimensioneras för. Över och under pelarna svetsas plåtar fast för att öka den belastade arean och hindra brott i betongen. I detta examensarbete har därför en datormodell i programmet Excel framtagits. Modellen dimensionerar kantpelare i stål enligt Eurokoderna. De brottsfall som ingår i modellen är genomstansning, prägling, spjälkning, avstånd till betongkant, reducering vid håltagning i betong, tryckkraftskapacitet för fot respektive topplåtarna och böjmoment för plåtarna.   Genomstansning är ett koniskt sprött brott som sker i betongen, det spröda brottet sker direkt utan förvarning och betongplattas bärförmåga sjunker snabbt vilket kan leda till ras.  Prägling är en lokal krossning av betongen och spjälkningen innebär att på grund av tryckkraften så uppstår en horisontell dragkraft i betongen vilket kan leda till brott. Då det är svårt att uppfatta i Eurokoderna för hur avståndet mellan plåtarna och betongkanten påverkar hållfastheten har därför en beräkningsmetod framställts, likvärdigt gäller för påverkan av håltagning nära en pelare. Exempel på hål kan vara trappor, hissar eller håltagning för värmestammar till radiatorer. Plåtarna beräknas i tvärsnittsklass tre. Modellen är uppbygg så att olika dimensioner och kvalitéer väljs, hållfastheten för konstruktionen beräknas och sedan redovisa om lasten klaras eller inte. I och med det så kan till exempel olika dimensioner och kvalitéer testas fram för att få en så optimal konstruktionslösning som möjligt. Avgränsningar har gjorts. Modellen beräknar bara de olika brottsfall som ovan nämns och berör endast kvadratiska VKR- profiler och plåtar. Pelare har beräknats som ledat infäst i båda upplagen där endast tryckkrafter dimensionerat betongen och plåtarna. Under det första skedet av arbetet så har fakta inhämtning för det olika brottsfallen gjort. Fakta har hämtats från litteratur, rapporter, undersökningar, forskningar, tidigare lösningar, diskussion med branscherfarna och tidigare respektive nyare regelverk. Därefter har datormodellen gjorts, målet med modellen är att få ett snabbt resultat och en komplett redovisning. Därför är modellen uppbygg så att varje brottsfall har en egen flik och kan redovisas enskilt.  För att modellen ska kunna användas av utomstående har standardbeteckningar, bilder och kommentarer använts. Modellen har kontrollerats med diverse beräkningsexempel och program för att få ett trovärdigt/användbart resultat. Slutsats: En väl fungerande modell har tagits fram och som kan användas av utomstående byggnadsingenjörer med minst gymnasieingenjörsutbildning. / A infill wall is a non-bearing wall construction, often is made af wood, which has the aim of minimizing the energy consumption of buildings in concrete. The inside the construction is sometimes steel designed for supporting the overlyingstructures. Steel studs are subjected to loads that they must be dimensioned to support. Above and below the colums plates are welded to increase the loaded area and prevent damages to the concrete. In this thesis a computer program in Excel has been created, the program dimensions border pillars in steel that follows the Eurocodes. Included in the program is punching, local pressure, splitting, distance to concrete, reduction when drilling in concrete, compression load capacity of foot and top plates, and bending torque the plates. Punching is a conical damage that occurs in the concrete, the damage occurs without warning and the concrete’s bearing capacity decreases rapidly which can lead to collapse. Local pressure is a local damage of the concrete and splitting, meaning that because of the pressure force arising can lead to a horizontal thrust in the concrete, which can lead to injuries. Since it is difficult to perceive the distance between the plates and the concrete edge of structural strength in the Eurocodes a calculation method has been made, equally applies to the consequences when drilling near a pillar. Examples of holes can be stairs, elevators or holes for heat strain for radiators. The plates are calculated in cross-section class three. The program is built so that different dimensions and qualities is selected, then the program calculates the strength of the structure and reports if will support the load. Different dimensions and qualities can be tested to get the optimal structure sollution. Limits have been set, the program only calculates the various injuries mentioned above and applies only to square VKR-sections and plates. Pillar has been calculated as articulated in both secured repository where the only pressure forces dimensioned concrete and plates. During the first phase of the work, gathering facts for the various types of damages has been done. Facts have been gathered from literature, reports, studies, researches, previous solutions, discussions with industry experienced and previous and more recent regulations. Since then, the computer program has been made, the goal of the program is to get a quick result and a complete report. Therefore, the program is made so that all damages is on different tabs and can be reported separately. So that the program can used by third parties standard names, images and comments are used. The program has been checked with various calculation examples and applications to achieve a credible / useful result. Conclusion: A well-functioning model has been developed and can be used by structural engineers with a education at least of technical college.
1086

TOP-K AND SKYLINE QUERY PROCESSING OVER RELATIONAL DATABASE

Samara, Rafat January 2012 (has links)
Top-k and Skyline queries are a long study topic in database and information retrieval communities and they are two popular operations for preference retrieval. Top-k query returns a subset of the most relevant answers instead of all answers. Efficient top-k processing retrieves the k objects that have the highest overall score. In this paper, some algorithms that are used as a technique for efficient top-k processing for different scenarios have been represented. A framework based on existing algorithms with considering based cost optimization that works for these scenarios has been presented. This framework will be used when the user can determine the user ranking function. A real life scenario has been applied on this framework step by step. Skyline query returns a set of points that are not dominated (a record x dominates another record y if x is as good as y in all attributes and strictly better in at least one attribute) by other points in the given datasets. In this paper, some algorithms that are used for evaluating the skyline query have been introduced. One of the problems in the skyline query which is called curse of dimensionality has been presented. A new strategy that based on the skyline existing algorithms, skyline frequency and the binary tree strategy which gives a good solution for this problem has been presented. This new strategy will be used when the user cannot determine the user ranking function. A real life scenario is presented which apply this strategy step by step. Finally, the advantages of the top-k query have been applied on the skyline query in order to have a quickly and efficient retrieving results.
1087

Conformal Thermal Models for Optimal Loading and Elapsed Life Estimation of Power Transformers

Pradhan, Manoj Kumar 08 1900 (has links)
Power and Generator Transformers are important and expensive elements of a power system. Inadvertent failure of Power Transformers would cause long interruption in power supply with consequent loss of reliability and revenue to the supply utilities. The mineral oil impregnated paper, OIP, is an insulation of choice in large power transformers in view of its excellent dielectric and other properties, besides being relatively inexpensive. During the normal working regime of the transformer, the insulation thereof is subjected to various stresses, the more important among them are, electrical, thermal, mechanical and chemical. Each of these stresses, appearing singly, or in combination, would lead to a time variant deterioration in the properties of insulation, called Ageing. This normal and inevitable process of degradation in the several essential properties of the insulation is irreversible, is a non-Markov physico-chemical reaction kinetic process. The speed or the rapidity of insulation deterioration is a very strong function of the magnitude of the stresses and the duration over which they acted. This is further compounded, if the stresses are in synergy. During the processes of ageing, some, or all the vital properties undergo subtle changes, more often, not in step with the duration of time over which the damage has been accumulated. Often, these changes are non monotonic, thus presenting a random or a chaotic picture and understanding the processes leading to eventual failure becomes difficult. But, there is some order in this chaos, in that, the time average of the changes over short intervals of time, seems to indicate some degree of predictability. The status of insulation at any given point in time is assessed by measuring such of those properties as are sensitive to the amount of ageing and comparing it with earlier measurements. This procedure, called the Diagnostic or nondestructive Testing, has been in vogue for some time now. Of the many parameters used as sensitive indices of the dynamics of insulation degradation, temporal changes in temperatures at different locations in the body of the transformer, more precisely, the winding hot spots (HST) and top oil temperature (TOT) are believed to give a fairly accurate indication of the rate of degradation. Further, an accurate estimation of the temperatures would enable to determine the loading limit (loadability) of power transformer. To estimate the temperature rise reasonably accurately, one has to resort to classical mathematical techniques involving formulation and solution of boundary value problem of heat conduction under carefully prescribed boundary conditions. Several complications are encountered in the development of the governing equations for the emergent heat transfer problems. The more important among them are, the inhomogeneous composition of the insulation structure and of the conductor, divergent flow patterns of the oil phase and inordinately varying thermal properties of conductor and insulation. Validation and reconfirmation of the findings of the thermal models can be made using state of the art methods, such as, Artificial Intelligence (AI) techniques, Artificial Neural Network (ANN) and Genetic Algorithm (GA). Over the years, different criteria have been prescribed for the prediction of terminal or end of life (EOL) of equipment from the standpoint of its insulation. But, thus far, no straightforward and unequivocal criterion is forth coming. Calculation of elapsed life in line with the existing methodology, given by IEEE, IEC, introduces unacceptable degrees of uncertainty. It is needless to say that, any conformal procedure proposed in the accurate prediction of EOL, has to be based on a technically feasible and economically viable consideration. A systematic study for understanding the dynamical nature of ageing in transformers in actual service is precluded for reasons very well known. Laboratory experiments on prototypes or pro-rated units fabricated based on similarity studies, are performed under controlled conditions and at accelerated stress levels to reduce experimental time. The results thereof can then be judiciously extrapolated to normal operating conditions and for full size equipment. The terms of reference of the present work are as follows; 1. Computation of TOT and HST Theoretical model based on Boundary Value Problem of Heat Conduction Application of AI Techniques 2. Experimental Investigation for estimating the Elapsed Life of transformers Based on the experimental investigation a semi-empirical expression has been developed to estimate the loss of life of power and station transformer by analyzing gas content and furfural dissolved in oil without performing off-line and destructive tests.
1088

Linear Logic and Noncommutativity in the Calculus of Structures

Straßburger, Lutz 11 August 2003 (has links) (PDF)
In this thesis I study several deductive systems for linear logic, its fragments, and some noncommutative extensions. All systems will be designed within the calculus of structures, which is a proof theoretical formalism for specifying logical systems, in the tradition of Hilbert's formalism, natural deduction, and the sequent calculus. Systems in the calculus of structures are based on two simple principles: deep inference and top-down symmetry. Together they have remarkable consequences for the properties of the logical systems. For example, for linear logic it is possible to design a deductive system, in which all rules are local. In particular, the contraction rule is reduced to an atomic version, and there is no global promotion rule. I will also show an extension of multiplicative exponential linear logic by a noncommutative, self-dual connective which is not representable in the sequent calculus. All systems enjoy the cut elimination property. Moreover, this can be proved independently from the sequent calculus via techniques that are based on the new top-down symmetry. Furthermore, for all systems, I will present several decomposition theorems which constitute a new type of normal form for derivations.
1089

Mesure de la section efficace de production de quarks top en paires dans le canal lepton+jets à D0 et à ATLAS et interprétation en terme de boson de Higgs chargé dans ATLAS.

Chevallier, Florent 10 May 2007 (has links) (PDF)
La mise en évidence de nouvelle physique est l'un des enjeux des expériences auprès des grands collisionneurs TeVatron et LHC. Le quark top se place dans ce contexte de recherche. Des mesures de couplages et de propriétés du quark top non conformes aux prédictions théoriques seraient les premiers effets indirects d'une physique non décrite par le Modèle Standard.<br /><br />Le travail décrit dans cette thèse porte sur l'étude de la section efficace de production de quarks top par interaction forte au sein des deux collaborations D0 et ATLAS.<br /><br />Mon travail à D0 a tout d'abord consisté en l'amélioration de la reconstruction des électrons de faible impulsion au sein de jets. Ce nouvel algorithme peut désormais être utilisé pour identifier les jets de quarks b, produits dans les événements top-antitop recherchés. Ce travail s'est poursuivi par une analyse de mesure de la section efficace de production de paires de quarks top, avec 420 pb-1 de données de l'expérience D0. Les résultats de cette analyse sont en accord avec les prédictions du Modèle Standard.<br /><br />Dans l'expérience ATLAS, mon travail a consisté à élaborer une procédure de sélection des événements top-antitop, en adaptant les connaissances et les techniques issues de l'expérience D0 à l'environnement plus exigeant des collisions du LHC. Cette analyse a permis d'identifier les sources d'incertitudes qui auront un impact sur la mesure. Pour une année de fonctionnement du détecteur à basse luminosité, cette section efficace pourrait être mesurée avec une précision de quelques pourcents. Cette sensibilité permettrait de déceler des signes de nouvelle physique. La phénoménologie des événements top-antitop est modifiée par la présence de bosons de Higgs chargés apparaissant dans des modèles standard non minimaux. L'analyse effectuée dans ce nouveau cadre théorique a ainsi montré qu'une découverte serait possible pour certaines régions de l'espace des paramètres.
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Réseaux bidimensionnels d'agrégats magnétiques préformés en phase gazeuse

Hannour, Abdelkrim 08 February 2007 (has links) (PDF)
Ce travail de thèse porte sur l'organisation de nanoparticules sur des surfaces grâce à une approche originale basée sur le dépôt à faible énergie d'agrégats, préformés en phase gazeuse (approche Bottom-up), sur substrats de graphite HOPG, fonctionnalisés par des réseaux 2D de défauts créés par faisceau d'ions focalisés nano-FIB (approche Top-down). L'obtention de réseaux 2D d'amas d'agrégats est conditionnée par la diffusion des agrégats déposés et leur sensibilité aux défauts artificiels FIB. Le développement de cette approche passe tout d'abord par des simulations Monte-Carlo réalisées dans le cadre du modèle DDA (Dépôt-Diffusion-Agrégation). Ensuite, sa faisabilité expérimentale est testée avec succès en déposant des agrégats d'or. En particulier, cette étude montre la dépendance des propriétés morphologiques et structurales des défauts FIB en fonction de la dose d'ions. Par ailleurs, la sensibilité des agrégats aux défauts FIB, ainsi que la possibilité de contrôler la taille des amas, la période du réseau et sa géométrie sont analysées. Enfin, en se plaçant dans les conditions optimales permettant la meilleure organisation, notre approche est étendue aux agrégats magnétiques Co50Pt50, candidats potentiels au stockage d'information à ultra-haute densité. Une étude de l'influence du recuit sur les réseaux d'agrégats Co50Pt50 montre une amélioration de la qualité de l'organisation, et surtout une transition de phase des agrégats de la structure<br />A1 vers la structure L10.

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