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A tutorial review on cellular as well as nanoporous carbides covering their structure, synthesis and potential applications. Especially new carbide materials with a hierarchical pore structure are in focus. As a central theme silicon carbide based materials are picked out, but also titanium, tungsten and boron carbides, as well as carbide-derived carbons, are part of this review. / Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG-geförderten) Allianz- bzw. Nationallizenz frei zugänglich.

L’objet de cette thèse est d’étudier un procédé de fraisage par microélectroérosion (μEE), qui est un procédé sans contact permettant d’usiner tous les matériaux durs conducteurs d’électricité à l’aide d’un micro-outil cylindrique ultrafin. Le principe consiste à créer des micro-décharges électriques entre le micro-outil et une pièce conductrice immergés dans un diélectrique liquide. En faisant parcourir à l’outil un parcours 3D, il est possible de creuser une forme complexe dans la pièce avec des détails à fort rapport d’aspect. Dans ce travail, nous avons tout d’abord amélioré un procédé d’élaboration de microoutils cylindriques ultrafins par gravure électrochimique de barreaux de tungstène. Des outils de diamètre 32,6 ± 0,3 μm sur une longueur de 3 mm ont été obtenus de manière automatique et reproductible. L’écart type a été divisé par 2 par rapport à l’état de l’art antérieur. Des outils de diamètre inférieur ont été obtenus avec une intervention de l’opérateur, et ce jusqu’à 3 μm de diamètre. Puis ces micro-outils ont été mis en oeuvre pour usiner des pièces avec le procédé de fraisage par microélectroérosion. Pour ce faire, une machine de 2ème génération a été entièrement développée sur la base de travaux antérieurs. Il a été possible d’usiner de l’acier inoxydable dans de l’eau déionisée avec des micro-outils de 3 μm de diamètre sans détérioration de l’outil. Par ailleurs, Le procédé de μEE a été caractérisé en termes de résolution d’usinage, taux d’enlèvement de matière et usure de l’outil. Un générateur de décharges original a permis d’usiner avec des micro-décharges de 1 à 10 nJ / étincelle avec une diminution très sensible de l’usure de l’outil par rapport à l’état de l’art. Un procédé original de caractérisation en ligne des décharges et de cartographie dans l’espace a aussi été développé / This work aims at studying Micro Electrical Discharge Milling (μEDM milling), which is a non-contact process allowing machining all hard and electrically conductive materials with a cylindrical ultrathin tool. The principle is based on the creation of electrical micro discharges between the tool and an electrically conductive part immersed in a liquid dielectric. By means of a 3D path, the tool machines a complex shape in the part with high aspect ratio details. In this work, we have firstly improved a process for making cylindrical ultrathin micro-tools by electrochemical etching of tungsten rods. Tools with a diameter of 32.6 ± 0.3 μm and a length of 3 mm have been obtained with an automated and reproducible process. Standard deviation has been divided by 2 by comparison with the previous state of the art. Tools with diameter as low as 3 μm have been fabricated with the help of the machine operator Then these micro-tools have been used for machining parts with the μEDM milling process. To do so, a second generation machine has been entirely developed on the basis of previous work. It has been possible to machine stainless steel in deionized water with 3 μm micro-tools without damaging the tools. In other respects, the μEDM milling process has been characterized in terms of machining resolution, material removal rate and tool wear. An innovative generator of discharges allow machining with 1 to 10 nJ / spark with a reduced tool wear by comparison to the state of the art. An innovative process for the on line characterization of discharges with spatial distribution capability has been developed

Cette thèse s’inscrit dans le cadre du développement d’un combustible nucléaire faiblement enrichi pour les réacteurs de recherche, constitué de particules d’uranium-molybdène mélangées à une matrice d’aluminium. Dans certaines conditions sous irradiations, les particules d’U(Mo) interagissent avec la matrice d’aluminium, provoquant un gonflement rédhibitoire de la plaque combustible. Pour inhiber ce phénomène, une solution consiste à déposer, à la surface des particules d’U(Mo), une fine couche de silicium, pour créer un effet barrière. Cette thèse a concerné l’étude du procédé de dépôt chimique à partir d’une phase vapeur (CVD) en lit fluidisé à partir de silane pour déposer le silicium sur la poudre d’U(Mo), qui a une densité exceptionnelle de 17,5. Pour atteindre cet objectif, deux axes d’études ont été traités au cours de la thèse : l’étude et l’optimisation de la mise en fluidisation d’une poudre aussi dense, puis celles du procédé de dépôt de silicium. Pour le premier axe, une campagne d’essais a été réalisée sur poudre simulante de tungstène dans différentes colonnes de fluidisation en verre et en acier avec des diamètres internes compris entre 2 et 5 cm, à température ambiante et à haute température (650°C), proche de celle des dépôts. Cette campagne a permis d’identifier des phénomènes d’effets de bord au sein du lit fluidisé, pouvant conduire à des dépôts hétérogènes ou à des prises en masse. Des dimensions de colonnes de fluidisation et des conditions opératoires permettant une fluidisation satisfaisante de la poudre ont pu être identifiées, ouvrant la voie à l’étude du dépôt de silicium. Plusieurs campagnes d’essais de dépôt sur poudre simulante, puis sur poudre U(Mo), ont ensuite été menées dans le cadre du second axe d’étude. L’influence de la température du lit, de la fraction molaire d’entrée en silane dilué dans l’argon, et du débit total de fluidisation, a été étudiée pour différents diamètres de réacteur et pour diverses masses de poudre. Des analyses de caractérisation morphologique et structurale (MEB, DRX...) ont révélé un dépôt de silicium uniforme sur toute la poudre et autour de chaque grain, majoritairement cristallisé et dont l’épaisseur atteint les objectifs visés. Des recommandations précises ont ainsi pu être émises pour optimiser les caractéristiques du dépôt de silicium sur la poudre combustible U(Mo) par le procédé de CVD en lit fluidisé. / This thesis is part of the development of low-enriched nuclear fuel, for the Materials Test Reactors (MTRs), constituted of uranium-molybdenum particles mixed with an aluminum matrix. Under certain conditions under irradiations, the U(Mo) particles interact with the aluminum matrix, causing unacceptable swelling of the fuel plate. To inhibit this phenomenon, one solution consists in depositing on the surface of the U(Mo) particles, a thin silicon layer to create a barrier effect. This thesis has concerned the study of the fluidized bed chemical vapor deposition (CVD) process to deposit silicon from silane, on the U(Mo) powder, which has an exceptional density of 17,500 kg/m3. To achieve this goal, two axes were treated during the thesis: the study and the optimization of the fluidization of a so dense powder, and then those of the silicon deposition process. For the first axis, a series of tests was performed on a surrogate tungsten powder in different columns made of glass and made of steel with internal diameters ranging from 2 to 5 cm, at room temperature and at high temperature (650°C) close to that of the deposits. These experiments helped to identify wall effects phenomena within the fluidized bed, which can lead to heterogeneous deposits or particles agglomeration. Some dimensions of the fluidization columns and operating conditions allowing a satisfactory fluidization of the powder were identified, paving the way for the study of silicon deposition. Several campaigns of deposition experiments on the surrogate powder and then on the U(Mo) powder were carried out in the second axis of the study. The influence of the bed temperature, the inlet molar fraction of silane diluted in argon, and the total gas flow of fluidization, was examined for different diameters of reactor and for various masses of powder. Morphological and structural characterization analyses (SEM, XRD…) revealed a uniform silicon deposition on all the powder and around each particle, mostly crystallized and whose thickness reached the objectives. Specific recommendations were proposed to optimize the characteristics of the silicon deposit on the U(Mo) powder by the fluidized bed CVD process.

Made available in DSpace on 2014-10-09T12:51:30Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:02:10Z (GMT). No. of bitstreams: 1 11328.pdf: 13067508 bytes, checksum: 886d399df69a8146f0353092e99aa16a (MD5) / Tese (Doutoramento) / IPEN/T / Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP

Made available in DSpace on 2014-10-09T12:51:30Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:02:10Z (GMT). No. of bitstreams: 1 11328.pdf: 13067508 bytes, checksum: 886d399df69a8146f0353092e99aa16a (MD5) / O objetivo deste trabalho foi desenvolver potenciais materiais para aplicações automotivas, em particular como insertos para assento de válvula em motores de combustão interna à gasolina. O desenvolvimento compreendeu as etapas de processamento dos materiais via metalurgia do pó, tratamento térmico, caracterização microestrutural e mecânica, usinagem e desgaste dos mesmos. O desenvolvimento objetivou principalmente a redução de custo e a aplicação de elementos menos poluentes, excluindo-se, por exemplo, a aplicação do Co devido ao seu alto custo e do Pb devido aos seus efeitos toxicológicos e prejudiciais ao meio ambiente. A realização de uma pesquisa minuciosa na busca de patentes relacionadas indicou que os materiais estudados apresentam composições particulares, e, portanto inéditas. Os resultados encontrados nos ensaios de dureza e de resistência à compressão radial nas amostras tratadas termicamente apresentaram resultados superiores aos da liga comercial atualmente em uso. Testes de usinabilidade em amostras sem tratamento térmico apresentaram comportamento similar ao da liga comercial, indicando que a usinabilidade do material praticamente não foi afetada com a mudança na composição química. Após a etapa de tratamento térmico, as ligas obtidas apresentaram valores de esforços de corte superiores aos do material comercial. Os resultados dos testes de desgaste abrasivo em amostras tratadas termicamente apresentaram menores valores de coeficientes de atrito e perda de massa em todos os casos em relação ao material comercial. Este comportamento é devido aos benefícios introduzidos pelo tratamento térmico executado e pela adição de alguns elementos resistentes ao desgaste na forma de carbonetos de Nb e Ti/W. Os resultados dos esforços de corte apresentaram boa concordância com os resultados dos ensaios de desgaste. Os materiais produzidos apresentam-se como potenciais candidatos para substituir, com vantagens, inserto para assento de válvula a base de Fe-Co em motores de combustão interna a gasolina. / Tese (Doutoramento) / IPEN/T / Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP

Submitted by Claudinei Pracidelli (cpracide@ipen.br) on 2016-11-11T09:40:30Z No. of bitstreams: 0 / Made available in DSpace on 2016-11-11T09:40:30Z (GMT). No. of bitstreams: 0 / A soldagem tem grande importância no setor metroferroviário, pois é empregada na fabricação de componentes estruturais e no acabamento de vagões de passageiros, que em sua maior parte são de aço inoxidável. As juntas soldadas podem apresentar descontinuidades que são interrupções que afetam as propriedades mecânicas e metalúrgicas da junta soldada. A presença destas descontinuidades, dependendo do seu tamanho, natureza ou efeito combinado, pode ocasionar a reprovação da junta soldada, quer pela redução de propriedades mecânicas ou pela não aceitação, segundo critérios estabelecidos em normas. Uma estrutura que tenha uma solda reprovada durante sua qualificação ou inspeção, pela presença de descontinuidades pode ser recuperada, por meio de um retrabalho a ser realizado nesta junta. A refusão do cordão de solda por meio do processo TIG (Tungsten Inert Gas), sem a utilização do material de adição, é uma técnica de retrabalho que pode ser empregada, em especial pela viabilidade técnica e econômica do processo. Neste estudo analisou-se a influência que o processo de reparo por soldagem TIG exerceu no comportamento mecânico e microestrutural das juntas soldadas pelo processo MIG, por meio de: ensaios mecânicos (tração, fadiga e microdureza Vickers), ensaios não destrutivos (inspeção visual e líquidos penetrantes) e caracterização microestrutural do cordão de solda. Resultados das amostras de ensaio de tração e fadiga indicaram que o reparo dos cordões de solda não alterou o comportamento mecânico das juntas. As juntas submetidas ao reparo nas quais foram retirados os reforços dos cordões apresentaram modificações nas propriedades mecânicas, mas também apresentaram resultados satisfatórios. / Dissertação (Mestrado em Tecnologia Nuclear) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

The knowledge of diffusion parameters provides important understanding of many physical and mechanical properties of materials. In most of the applications silicides are grown by a diffusion controlled process mainly in thin film condition. Because of this reason, most of the studies till date are available in thin film condition. Although more than one phase is present in all these systems, mainly disilicides were found at the interface. In this thesis bulk interdiffusion studies are conducted by coupling pure refractory metals (group IVB, VB and VIB elements) with single crystal Si. Several phase layers grow between binary refractory metal and Si systems. The layer thicknesses of the phases are measured from the microstructures. Composition profiles were measured in electron probe microanalyzer. Different diffusion parameters are estimated such as parabolic growth constants, integrated diffusion coefficients, activation energy for diffusion and ratio of tracer diffusivities of the components are estimated. Growth mechanisms of the phases are discussed with the help of diffusion parameters. The atomic mechanism of the diffusion is discussed considering crystal structure of the phases along with possible defects present. Solid diffusion couple experiments are conducted to analyse the growth mechanism of the phases and the diffusion mechanism of the components in the Ti-Si system. The calculation of the parabolic growth constant and of the integrated diffusion coefficients substantiates that the analysis is intrinsically prone to erroneous conclusions if it is based just on the parabolic growth constants determined for a multiphase interdiffusion zone. The location of the marker plane is detected based on the uniform grain morphology in the TiSi2 phase, which indicates that this phase grows mainly because of Si diffusion. The growth mechanism of the phases and morphological evolution in the interdiffusion zone are explained with the help of imaginary diffusion couples. The activation enthalpies for the integrated diffusion coefficient of TiSi2 and the Si tracer diffusion are calculated as 190±9 and 170±12 kJ/mol, respectively. The crystal structure, details on the nearest neighbours of the elements and the relative mobilities of the components indicate that the vacancies are mainly present on the Si sublattice. Diffusion controlled growth of the phases in the Hf-Si and Zr-Si are studied by bulk diffusion couple technique. Only two phases grow in the interdiffusion zone, although several phases are present in both the systems. The location of the Kirkendall marker plane detected based on the grain morphology indicates that the disilicides grow by the diffusion of Si. Diffusion of the metal species in these phases is negligible. This indicates that vacancies are present mainly on the Si sublattice. The activation energies for integrated diffusion coefficients in the HfSi2 and ZrSi2 are estimated as 394 ± 37 and 346 ± 34 kJ/mol, respectively. The same is calculated for the HfSi phase as 485±42 kJ/mol. The activation energies for Si tracer diffusion in the HfSi2 and ZrSi2 phases are estimated as 430 ± 36 and 348 ± 34 kJ/mol, respectively. We conducted interdiffusion studies to understand the atomic mechanism of the diffusing species and the growth mechanism of the phases. Integrated diffusion coefficients and the ratio of tracer diffusion coefficients were estimated for these analyses. The activation energies for the integrated diffusion coefficients were calculated as 550 ± 70 and 410 ± 39 kJ/mol in the TaSi2 and the Ta5Si3 phases, respectively. In the TaSi2 phase, Ta has a slightly lower but comparable diffusion rate with respect to Si, although no TaTa bonds are present in the crystal. In the Ta5Si3 phase, Si has higher diffusion rate, which is rather unusual, if we consider the atoms in the nearest-neighbor positions for both the elements. The ratio of Si to Ta tracer diffusion coefficients is found to be lower in the Si-rich phase, TaSi2, compared to the Si-lean phase, Ta5Si3, which is also unusual. This indicates the type of structural defects present. An analysis on the growth mechanism of the phases indicates that duplex morphology and the Kirkendall marker plane should only be present in the TaSi2 phase. This is not present in the Ta5Si3 phase because of the very high growth rate of the TaSi2 phase, which consumes most of the Ta5Si3 phase layer. The problems in the calculation method used previously by others in this system are also explained. Experiments are conducted in the W-Si system to understand the diffusion mechanism of the species. The activation energies for integrated diffusion are found to be 152±7 and 301±40 kJ/mol in the WSi2 and W5Si3 phases, respectively. In both the phases, Si has a much higher diffusion rate compared to W. The result found in the WSi2 phase is not surprising, if we consider the nearest neighbors in the crystal. However, it is rather unusual to find that Si has higher diffusion rate in the W5Si3 phase, indicating the presence of high concentration of Si antisites in this phase. In the group IVB, VB and VIB M-Si systems are considered to show an interesting pattern in diffusion of components with the change in atomic number in a particular group. MSi2 and M5Si3 are considered for this discussion. Except in the Ta-Si system, activation energy for integrated diffusion of MSi2 is always lower than M5Si3. Interestingly, in both the phases, the relative mobilities measured by the ratio of tracer D* diffusion coefficients, S i decreases with the increase in atomic number in both the DM* groups. Both the phases have similar crystal structures in a particular group in which these parameters are calculated. In both the phases Si has higher diffusion rate compared to M. Absence of any M-M bonds in MSi2 and increase in the diffusivities of M with the increase in atomic number substantiates the increasing concentration of M anti-sites and higher interactions of M with vacancies. Only one or two Si-Si bonds are present in M5Si3, however, the higher diffusion rate of Si indicates the presence of vacancies mainly D* on its sublattice. On the other hand, increase in S i with increasing atomic number in DM* Both the groups substantiates increasing interactions of M and vacancies.