Etude des sollicitations mécaniques aux interfaces d'un empilement de couches minces - Application à la mesure d'adhérence par la technique de flexion 4 points

Le CEA-LITEN utilise une méthode de mesure d'énergie interfaciale basée sur la technique de flexion 4 points proposée par Charalambides. La valeur de mixité modale doit être connue pour exploiter les résultats de ces essais. Dans cette étude, la simulation par la Méthode des Eléments Finis a été mise en oeuvre pour préciser la mixité modale en fonction des propriétés mécaniques des matériaux et de la configuration des éprouvettes. Dans un second temps, le rôle des contraintes d'élaboration, importantes dans le cas des cellules de SOFC, a été étudié. Ces calculs permettent de dimensionner les éprouvettes

Utilisation conjointe des méthodes des éléments discrets et des éléments finis pour modéliser la compaction de poudres céramiques uranifères agrégées

La simulation des procédés d'élaboration de pièces à partir de poudres passe par la prise en compte du caractère particulaire des matériaux mis en jeux. La méthode des éléments discrets (Discrete Element Method, DEM) est bien adaptée pour cette tâche. Elle permet de calculer le comportement d'un ensemble de particules à partir des forces de contact exercées sur chacune d'elles. Nous montrons comment la DEM peut être utilisée pour alimenter un code aux éléments finis en lois constitutives. Le code éléments finis simule quant à lui le comportement de la pièce entière lors de la compaction en matrice. Nous nous concentrons ici sur la génération des surfaces de charge et de rupture. L'application visée est la simulation de la compaction de poudres d'oxyde d'uranium qui se présentent sous forme d'agrégats poreux. Nous proposons une loi de contact adaptée pour décrire l'indentation, la décharge élastique et la décohésion de ces agrégats

Nd2_2Fe14_{14}B permanent magnets substituted with non-critical light rare earth elements (Ce, La): A review

International audienceThe growing demand in high performance Nd-Fe-B permanent magnets, driven by emerging key technologies, notably vehicles electrification and wind turbines, raises the question of the availability of critical raw materials, namely the Light Rare Earth Elements (LREEs) Nd and Pr. Co-extracted with these LREEs, Ce and La represent about 70 % of the total amount of rare earths in minerals and, for this reason, could constitute a cost-effective diversification in the raw materials sourcing for magnets. Largely unused and stored, these non-critical elements are progressively incorporated in the permanent magnet industry as a substitution of Nd and Pr, allowing a more balanced utilization of the resources. The restoration of high magnetic performances, after the substitution of Nd by Ce, constitutes, however, a considerable challenge that requires to re-design some steps of the conventional routes for producing dense anisotropic magnets. This objective has motivated a wide range of research activities during the last decade from which different approaches are now emerging: the Hot-Deformation technique followed by an infiltration of eutectic alloys, on the one hand, and, on the other hand, the powder pressing and sintering possibly completed with the Grain Boundary Restructuring or with the Grain Boundary Diffusion Process. The performances of Ce based magnets improved by these routes are analyzed for a wide range of substitution rates and compositions. The strategies specifically deployed in the powder metallurgy route for enhancing the coercivity and the remanence, based on grain boundary engineering, additives alloying and on dual-powder sintering, are discussed in detail

Thermal management design of transformers for Dual Active Bridge power converters

International audienceA methodology for the design of compact transformers operating at high frequency is presented in this paper. A particular emphasis is paid to the thermal management of the magnetic core and of the winding components. For a 7kW Dual Active Bridge DC/DC converter, the objective is to reduce the core volume ( 99 %) and integrate the serial inductance (8.7 µH) as being the leakage inductance of the transformer. A parametric study shows that the heating in the copper winding is very sensitive to the anisotropic thermal conduction behaviour of the wire. Due to this characteristic, a pot-core configuration is prone to a higher warming compared to the E-E core geometry, as the efficiency of the winding cooling is lower. In order to take part of the self-shielding ability of pot-cores, we studied new configurations in which internal thermal drains are inserted into voids specially designed to shorten the distance between the external cooled walls and the hottest points of the winding. The heating of internal components of the transformer and resulting thermal stress peak is reduced by 40 % paving the way for robust transformers with a power density that could theoretically reach up to 200 kW/dm3

Effect of residual stresses on the propagation of interface cracks between dissimilar brittle materials: contribution of two and three-dimensional analyses

International audienc

Study of magnetic properties of NiZnCu ferrite synthesized by Pechini method and solid-state reactions

Ni-Zn-Cu ferrites are exhibiting excellent magnetic properties at large frequencies and are consequently produced as low-loss magnetic materials used for magnetic components in power electronics. Here we present an experimental study of (Ni0.31Zn0.49Cu0.20)0.979Co0.021Fe1.9-dO4 ferrites synthesized by the Pechini type sol-gel method. Their structural and magnetic properties are characterized and compared to the ones of reference ferrites synthesized by classical solid-state reactions. Optimized parameters for Pechini method allow obtaining comparable low magnetic core-losses for both synthesis methods. The cores obtained by this sol-gel method present higher static permeability and lower coercive field than the ones obtained by classical solid-state reactions. Results suggest that the microstructure of ferrites synthesized by Pechini method can be considered as a promising alternative route for the design of low-loss magnetic components

Microstructural correlations for specific surface area and triple phase boundary length for composite electrodes of solid oxide cells

International audienceThe performances of Solid Oxide Cells (SOCs) are controlled by key microstructural properties such as the density of Triple Phase Boundary lengths (TPBl) and the interfacial specific surface areas (Si/j). These electrode properties are dependent on basic morphological parameters defined by the phase volume fractions and the Particle Size Distributions (PSD) of the percolated solid phases. The understanding of these relationships is of central importance for designing an optimum electrode microstructure. In this study, semi-analytical expressions for the density of TPBl and the interfacial specific surface areas are investigated. For this purpose, a large number of synthetic microstructures are generated by using validated models based on the sphere packing and the truncated Gaussian random field methods. The coefficients of the parametric equations for both investigated properties (TPBl density and Si/j) are fitted on the large database generated. The predictions of the microstructural correlations are in good agreement with the parameters directly computed on 3D reconstructions of typical LSCF-CGO and Ni-YSZ electrodes, thereby validating their reliability

A 20 kW, 3-level flying capacitor 1500 V inverter with characterized GaN devices for grid-tie applications

International audienceThis work presents the static and dynamic characterizations of high voltage GaN power devices (GaN FET 900 V and GaN HEMT 1200 V) in order to implement a 3-level flying capacitor 1500 VDC inverter for high power density grid-tie applications with renewable energy sources such as solar and hydrogen energy. In the first part, the static characterizations are shown for two selected GaN power devices. Then these GaN devices were placed in a double-pulse-test-bench dedicated to the dynamic characterizations intended to observe the switching behaviors of the devices under the nominal voltage and current. Finally, in order to demonstrate the compactness of the converter, these GaN devices were implemented in a 20 kW, 3-level flying capacitor 1500 VDC inverter with the full-custom suitable passive elements of the output filters connected to the 800 VAC 3-phase grid

Dimensionnement et mise en forme d'inductances pour convertisseurs de puissance

International audienceLes convertisseurs de puissance utilisent des composants magnétiques passifs qui interviennent à hauteur de 40 % dans le volume et le coût du convertisseur. La montée en fréquence des composants actifs, notamment via la technologie GaN, permet d'envisager une réduction importante de la taille des inductances dès lors que les pertes magnétiques seront contrôlées. Deux voies complémentaires peuvent permettre d'atteindre cet objectif : (i) la réduction des dissipations en fréquence au sein du matériau magnétique et (ii) la mise en forme d'architectures plus favorables pour la gestion thermique des inductances. Le procédé de coulage en bande a été mis en œuvre pour réaliser des noyaux magnétiques à partir de ferrite spinelle. Une méthodologie de dimensionnement spécifique a permis de déterminer les facteurs de forme optimaux. Elle s'appuie sur des caractérisations de pertes magnétiques

Dimensioning and shaping of inductors for power converters

International audienc