Finite element analysis of hot spots in flash sintering

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Use of phase resolved partial discharges for studying the incubation period of room temperature flash sintering of zinc oxide52

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In situ measurements of partial discharge patterns on porous YSZ pellets pressed between planar platinum electrodes used for flash sintering

Flash sintering of prismatic or cylindrical samples placed between planar electrodes allows processing larger volumes of material, compared to dogbone-shaped samples. The contact surface with platinum electrodes are usually coated with Pt paste in this setup, in order to improve the galvanic contact with the sample, and obtain a better current homogeneity. This coating is however too expensive for industrial applications whenever large surfaces are involved. Nevertheless, a homogeneous contact resistance remains in all cases essential to control the uniformity of the electrical current through the sample, hence the importance of an accurate characterization of the quality of the contact between electrodes and sample in the non-coated case. We propose to analyse the Partial Discharges (PDs) patterns obtained by Phase Resolved Partial Discharge (PRPD) method as an in situ measurement technique to study this problem. The setup consists in placing a ceramic sample in a furnace in the same configuration as flash sintering experiments but at a temperature lower than the onset temperature of the material. In this work, porous cylindrical samples of stabilised zirconia (average dimensions of 9 mm in diameter and 5 mm in height) are studied between 200°C and 400°C. A 50 Hz AC voltage is therefore applied to the sample and increased gradually up to 1.7 kV. The sample is connected in parallel with a 1 nF coupling capacitor in series with a quadrupole used to separate the high frequency current of the partial discharge signals from the AC current at 50 Hz. At a given amplitude of the applied voltage, we record for 60 s the patterns of partial discharges, their number and their phase with respect to the applied voltage. We compare the partial discharge signals to the net AC current at the power frequency measured simultaneously with a current probe at the primary winding of the low noise transformer. At a given temperature, we notice that well-defined patterns of low amplitude partial discharges appear when the applied voltage is increased above 300 V. At much higher voltages, partial discharges of larger amplitudes initiate and grow rapidly as well as the net 50 Hz current which increases non-linearly resulting from the temperature rise of the sample and its associated resistivity drop (similarly to what happens in (pre-)flash sintering conditions). Cross examination of the sample surface by optical microscopy shows physical traces related to the occurrence of partial discharges. For comparison, a YSZ sample coated with Pt paste is also analysed in the same conditions. In that case, at the same temperature and for applied voltages up to 1 kV, partial discharge patterns are almost inexistent. These results show that the partial discharge measurement is an efficient technique that can be used in situ prior to flash sintering experiments in order to assess the quality of the contact between the electrodes and the sample when no Pt paste is used. This study is supported by ERDF and the Walloon Region, in the frame of IMAWA-FLASHSINT research project (program 2014-2020)

Mécanismes de photoionisation et de phototransfert d'électron en solution

Nous avons étudié l'apport de la spectrométrie Raman de résonance résolue dans le temps à la compréhension des processus de transfert d'électron photoinduit en solution pour deux types de réactions modèles : la photoionisation de deux amines aromatiques et la photoréduction de la 4,4'-bipyridine par deux amines aliphatiques. L'objectif majeur de ce travail était la caractérisation expérimentale de paires d'ions intermédiaires dont la présence est prédite par les cinétiques obtenues par spectroscopies électroniques. L'étude de la photoionisation des amines aromatiques n'a conduit à aucune caractérisation spectroscopique de la présence de paires d'ions, les différences structurales entre l'ion libre et l'ion engagé dans la paire étant trop faibles pour être discernables. Cependant, l'ensemble des données obtenues par Raman de résonance et absorption transitoire a permis de mieux comprendre les processus de photoionisation en solution et notamment les phénomènes observables à temps courts. De plus, l'analyse structurale du premier état excité singulet des deux amines a été réalisées à partir des spectres Raman de résonnance enregistrés pour plusieurs dérivés isotopiques. Les résultats suggèrent une structure plane symétrique, ne présentant pas de distorsion quinoïdale, l'excitation étant localisée sur la partie cyclique des molécules. L'étude de la photoréduction de 44BPY a permis de mettre en évidence de manière directe la présence de paires d'ions. L'anion de 44BPY présent dans le CIP possède des fréquences Raman différentes de celles de l'anion libre, au contraire de l'anion engagé dans la SSIP. La réactivité de ces paires d'ions a été étudiée afin de mieux comprendre l'influence du solvant et notamment des liaisons hydrogène entre solutés et solvant sur la dynamique réactionnelle de la paire.LILLE1-BU (590092102) / SudocSudocFranceF

Combination of indirect stereolithography and gel casting methods to shape ceramic dental crowns

International audienceDental crowns are among the modern tools used in dentistry to restore physiological function such as phonation or mastication. They are currently mainly manufactured by machining pre-sintered disk but this technique has some drawbacks such as risks of damage to the part or some geometric limitations. To overcome this drawback some alternative techniques have been studied such as LASER machining, stereolithography (SLA) or selective LASER melting (SLM). But these alternative methods also suffer from defects and are therefore little used.This work is therefore part of a kind of continuity by exploring a new shaping method for dental crowns. In this project, two different techniques are combined, the first one is polymer stereolithography which is used to produce complex molds and the latter one is gel casting which is a process commonly used for producing parts with complex geometries. It was initially developed to overcome the limitations of other shaping processes such as injection molding or machining. Designed by Janney & Omatete in 1991, this process is very versatile, both in terms of its compatibility with different classes of materials and its compatibility with different molds manufacturing techniques. In practice, gel casting is based on the production of a slurry in an aqueous medium in the presence of a dispersant and a gelling agent. The slurry is poured into a mold where gelation can occur and form a three-dimensional network that ensures shape retention and maintenance of homogeneity within the part. A wide range of gelling agents can be used for gel casting with different families such as synthetic polymers, proteins or polysaccharides. This study is more focused on bio-sourced products and more specifically on the polysaccharide type. Two of them were selected: agarose and sodium alginate they have different gelling mechanisms which means two different gel casting processes. The presentation is more focused on formulations, processes, and whole characterisation of the parts

Processing and properties of calcium phosphates bioceramics by hot isostatic pressing

Stoichiometric β-tricalcium phosphate (β-TCP), hydroxyapatite (HA) and biphasic calcium phosphate (TCP/HA 60/40 %wt, BCP40) powders were synthesized by chemical precipitation of aqueous solutions of diammonium phosphate and calcium nitrate. After a calcination treatment and a milling step, powders were shaped by slip-casting. The sintering temperature effect on the density and the average grain size was investigated. By natural sintering, densities between 98 and 99.8% were obtained. Hot Isostatic Pressing (HIP) treatment was carried out after a pre-sintering of these materials. Transparent or translucent samples were obtained, indicating a relative density very close to the theoretical value (>99.9%). Mechanical properties (three-point bending strength, fracture toughness, Young's modulus and Vickers hardness) were measured on hipped materials with similar grain size (∼0.7μm)

Elaboration de prothèses dentaires par fabrication additive indirecte combinant stéréolithographie et gel casting

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