Preparation and characterization of ceramic sol–gel composite coatings - densification temperature optimisation -

Thick (~ 25 µm) ceramic coatings on porous ceramic substrates were elaborated by means of dispersing alumina and rutile powders in a silica sol-gel solution. Resulting coatings, which present a composite structure consisting in Al2O3 and TiO2 grains embedded in an amorphous SiO2 matrix, demonstrate a good adhesion to the substrate and a real improvement of its surface by closing the porosity and also reducing the roughness. Mechanical characterization by micro-indentation showed an increase of the coating hardness when the thermal treatment temperature increases. This hardness increase is attributed to the densification of the coating that proceeds by grain rearrangement thanks to the sol-gel derived silica phase viscosity decrease during the thermal treatment. In order to decrease the coating densification temperature, the silica intergranular phase was modified by incorporating Na+ ions in its structure. This was done by adding NaCl salt in the sol-gel solution and leads to the decrease of the densification temperature

Transport de charges dans les alumines polycristallines - Application à l'optimisation de la rigidité diélectrique

Dielectric breakdown constitutes an important limitation in the use of insulating materials under high-tension since it leads to the local fusion and the sublimation of material. The microstructure (average grain size, intergranular phase) has a great influence on the ability of material to resist this catastrophic phenomenon. Indeed, the interfaces between the various phases constitute potential sites of trapping for the charges. The optimization of the dielectric breakdown strength of a polycrystalline alumina sintered with a liquid phase passes necessarily through the control of the microstructural parameters. Thus, it is shown that by controlling the conditions of the process (rate of sintering aids, powder grain size and thermal cycle), it is possible to control the density (by the average grain size) but also the nature (by the crystallization or not of anorthite) of the grain boundaries. The study of the influence of these two parameters as well temperature on the properties of charge transport and storage was carried out by methods ICM and SEMME. The results, interpreted in light of the numerical simulation of the charge transport in bulk alumina sample during electron beam irradiation, allowed to highlight behaviors, and the corresponding microstructures, favourable to the dielectric breakdown resistance according to the considered temperature. Thus, at room temperature a high density of interfaces (low grain size and crystallized intergranular phase) makes it possible material to durably trap a great amount of charges, which leads to a high dielectric strength. On the other hand, at higher temperature, the presence of shallow traps (vitreous intergranular phase) supports the cherge diffusion and makes it possible to delay breakdown.Le claquage diélectrique constitue une limitation importante dans l'utilisation des matériaux isolants sous haute-tension puisqu'il conduit à la fusion et la sublimation locales du matériau. La microstructure (taille de grains, phase intergranulaire) joue un rôle important sur la capacité du matériau à résister à ce phénomène catastrophique. En effet, les interfaces entre les différentes phases constituent des sites potentiels de piégeage pour les charges. L'optimisation de la rigidité diélectrique passe donc nécessairement par le contrôle des paramètres microstructuraux. Ainsi, est montré qu'en maîtrisant les conditions d'élaboration (taux d'ajouts, granulométrie de la poudre et cycle thermique), il est possible de contrôler la densité (par la taille moyenne de grains) mais également la nature (par la cristallisation ou non de l'anorthite) des joints de grains. L'étude de l'influence de ces deux paramètres ainsi que de la température sur les propriétés de transport et de piégeage des charges a été réalisée par les méthode ICM et SEMME. Les résultats ainsi obtenus, interprétés à la lumière de la simulation numérique du comportement d'un isolant soumis à une irradiation électronique, ont permis de mettre en évidence des comportements, et les microstructures correspondantes, favorables à une bonne tenue au claquage diélectrique en fonction de la température considérée. Ainsi, à température ambiante une densité d'interfaces élevée (taille de grains faible et phase vitreuse cristallisée) permet au matériau de piéger durablement une quantité importante de charges, ce qui conduit à une rigidité diélectrique élevée. En revanche, à plus haute température, la présence de pièges de faible profondeur (phase intergranulaire vitreuse) favorise la diffusion des charges et permet de retarder le claquage

Calendering of Li(Ni0.33Mn0.33Co0.33)O2‐based cathodes: analyzing the link between process parameters and electrode properties by advanced statistics

International audienceThe optimization of the calendering process represents one of the key tasks for tuning the lithium‐ion battery performance. In this study we present a systematic statistical‐based study of the three main calendering parameters (namely, the applied pressure, roll temperature and line speed) effect on the porosity, electrode mechanical properties and electronic conductivity. Our work main goal is to understand how by changing the calendering parameters, the electrode properties can be tuned and up to which degree they determine the electrode capacity of Li(Ni0.33Mn0.33Co0.33)O2‐based cathodes. The statistical tools used for the analysis were the analysis of the covariance (ANCOVA), the principal components analysis (PCA) and the unsupervised machine learning k‐means clustering algorithm. Our results showed that while porosity and the mechanical properties depend mainly on the applied pressure, the electrode’s conductivity correlates mainly with the temperature. All of them were found to influence the cathode’s capacity (at a rate equal to C), being the best condition applied pressures between 60 and 120 MPa and roll temperatures between 60 and 75 °C

Understanding the calendering processability of Li(Ni0.33Mn0.33Co0.33)O2-based cathodes

International audienceThe calendering process aims at enhancing the electrode energy density, and improving the electronic conductivity, and determines the final porous electrode micro/mesostructure. In this sense, one of the main parameters of interest is its impact in the electrode porosity (ε) and the electrochemical performance. Here, we present a systematic study of the calendering conditions (applied calender pressure and roll temperature) effect on the final NMC-based electrodes ε in terms of the active material/carbon additive/binder composition and the amount of solvent used during the preparation of the slurries. The calendering processability was assessed through the cathodes compressibility resistance and minimal attainable ε, the electrode mechanical properties (hardness and elastic deformability), the pore size distribution, the electrode film mesostructure and the C-rate cathode electrochemical performance. Based on our results, it was found that the distribution and organization of the inactive carbon black (CB)/PVdF phase and the electrode mesostructure are the key parameters that control the cathode processability through calendering. Electrodes with high CB/PVdF content and prepared with higher amounts of solvent in the slurry ensure a good electronic conductivity and a film-like structure of the electronic conducting phase around the NMC particles which upon calendering outputs a better electrochemical performance

Phase analysis and mechanical properties of the new Al-Cr-Fe-Mn-Mo high entropy alloy family prepared by powder metallurgy route

International audienc

Biocompatible silica-based magnesium composites

National audienc

Powder metallurgy processing of a novel massive AlCrFeMnMo High entropy Alloy: a comparative study of the mechanical alloying techniques.

International audienc

Alumina based ceramics for high-voltage insulation

International audienceDielectric breakdown constitutes an important limitation in the use of insulating materials under high-voltage since it can lead to the local fusion and sublimation of the insulator. The role of electrical charge transport and trapping in alumina ceramics on their resistance to this catastrophic phenomenon is studied in this work. In polycrystalline materials, the interfaces between the various phases play a main role because they constitute potential sites for the trapping of electrical charges. The density and the nature of these interfaces can be controlled by the way of the microstructure parameters. So, the aim of the present paper is to highlight the influence of average grain size and intergranular phase crystallization rate on the ability of polycrystalline alumina materials to resist to dielectric breakdown. Thus, it is shown that the control of the process conditions (sintering aids content, powder grain size and thermal cycle) makes it possible to change not only the density (by the average grain size) but also the nature (by the crystallization or not of anorthite) of the grain boundaries. On one hand, at room temperature a high density of interfaces, due to low grain size and highly crystallized intergranular phase, leads to a high dielectric strength. On the other hand, at higher temperature (250 degrees C), the presence of vitreous intergranular phase makes it possible to delay breakdown. That behaviour is explained thanks to charge transport and trapping characterizations