Assessment of pressure-assisted sintering models through sinter-forging tests: A case study of alumina incorporating capillarity forces

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Issues and opportunities from Peltier effect in functionally-graded colusites: from SPS temperature modeling to enhanced thermoelectric performances

International audienceThe quaternary sulphide V-Sn colusite, Cu 26 V 2 Sn 6 S 32 , is one of the most promising costefficient thermoelectric materials to date because of the low toxicity, wide availability and low cost of the composing elements. Recent studies have demonstrated the potential of this environmentally-friendly material and its transport properties are now well understood. In the present work, we take the next step of producing large quantities of optimised V-Sn colusite using industrial-grade precursors and investigating the effect of Spark Plasma Sintering (SPS) in the production of large cylindrical pucks of up to 30 mm in diameter and 10 mm in thickness. In the process, we identified and solved several key issues including the generation of temperature gradients during SPS, porosity and defect formation. The generation of radial and axial temperature gradients within the sample during SPS has been modeled using modified Fourier and Ohm laws and confirmed experimentally thanks to the T SPS-dependent transport properties of V-Sn colusite and EDS analysis. We demonstrate that large pucks of colusite with enhanced thermoelectric properties can be produced using a combination of SPS and High-temperature Isostatic Press (HIP). Overall, our work experimentally and theoretically demonstrates that the production of both homogeneous and functionally-graded bulk materials can be easily up-scaled through a careful control of the SPS conditions

Issues and opportunities from Peltier effect in functionally-graded colusites: from SPS temperature modeling to enhanced thermoelectric performances

International audienceThe quaternary sulphide V-Sn colusite, Cu 26 V 2 Sn 6 S 32 , is one of the most promising costefficient thermoelectric materials to date because of the low toxicity, wide availability and low cost of the composing elements. Recent studies have demonstrated the potential of this environmentally-friendly material and its transport properties are now well understood. In the present work, we take the next step of producing large quantities of optimised V-Sn colusite using industrial-grade precursors and investigating the effect of Spark Plasma Sintering (SPS) in the production of large cylindrical pucks of up to 30 mm in diameter and 10 mm in thickness. In the process, we identified and solved several key issues including the generation of temperature gradients during SPS, porosity and defect formation. The generation of radial and axial temperature gradients within the sample during SPS has been modeled using modified Fourier and Ohm laws and confirmed experimentally thanks to the T SPS-dependent transport properties of V-Sn colusite and EDS analysis. We demonstrate that large pucks of colusite with enhanced thermoelectric properties can be produced using a combination of SPS and High-temperature Isostatic Press (HIP). Overall, our work experimentally and theoretically demonstrates that the production of both homogeneous and functionally-graded bulk materials can be easily up-scaled through a careful control of the SPS conditions

Modeling spark plasma sintering of zirconia with prediction of final stage high densification rate

International audienceIn this study, the SPS of 0.1 µm zirconia has been performed and shows high densification rate behavior in the final stage. This work consists of modeling the densification behavior of this powder especially at the intermediate/final stage transition. To model this, a Master Sintering Curve (MSC) is combined with an analytic model to identify the activation energy and the sintering moduli. The modeling of the behavior of the powder during the sintering tests is done via the Skorohod-Olevsky model. It appears from this study that the modeling of final stage sintering requires a regime transition to model the exceptionally high densification rate of the powder. The final stage grain growth seems not to decrease the final stage sintering kinetics

Spark plasma sintering grain growth assessment by densification kinetics analysis

International audienceModeling the spark plasma sintering of ceramics can be a very time-consuming task requiring high temperature sintering tests with fully dense/porous specimens in different configurations. Final stage sintering requires a particular attention as the densification kinetic is closely related to the grain growth phenomenon. In this work, we describe a combined master sintering curve and regression method which is able to determine all sintering model parameters. The original approach of this study lies in the possibility to estimate the grain size curve with the grain growth disturbance on the densification kinetics at the final stage. The model and grain size curve estimation reproduce well the experimental data points

Modeling spark plasma sintering of zirconia with prediction of final stage high densification rate

International audienceIn this study, the SPS of 0.1 µm zirconia has been performed and shows high densification rate behavior in the final stage. This work consists of modeling the densification behavior of this powder especially at the intermediate/final stage transition. To model this, a Master Sintering Curve (MSC) is combined with an analytic model to identify the activation energy and the sintering moduli. The modeling of the behavior of the powder during the sintering tests is done via the Skorohod-Olevsky model. It appears from this study that the modeling of final stage sintering requires a regime transition to model the exceptionally high densification rate of the powder. The final stage grain growth seems not to decrease the final stage sintering kinetics

Spark plasma sintering grain growth assessment by densification kinetics analysis

International audienceModeling the spark plasma sintering of ceramics can be a very time-consuming task requiring high temperature sintering tests with fully dense/porous specimens in different configurations. Final stage sintering requires a particular attention as the densification kinetic is closely related to the grain growth phenomenon. In this work, we describe a combined master sintering curve and regression method which is able to determine all sintering model parameters. The original approach of this study lies in the possibility to estimate the grain size curve with the grain growth disturbance on the densification kinetics at the final stage. The model and grain size curve estimation reproduce well the experimental data points

Pressure assisted sintering stress exponent assessment methods: Accuracy analysis and effect of sintering stress

International audiencePressure assisted sintering models involve creep based mechanisms having different stress exponent values. The later evolve from linear viscous diffusional creep mechanisms to highly non-linear mechanisms involving dislocation motion. Consequently, the determination of the stress exponent is of key importance to define the sintering mechanisms and one of the first parameters to identify for the assessment of sintering model. Different methods exist in the literature involving tests at different pressures or with a stepwise pressure profile to extract the creep stress sensitivity. Open questions remain on the accuracy of these methods with the impact of sintering stress (high in ceramics nano-powders) or transient microstructure evolution for the stepwise approaches. This accuracy issue is investigated on an alumina submicronic powder by comparing different methods based on sinter-forging and Spark Plasma Sintering (SPS) at 1200°C. We show that the sintering stress has a high influence on the identified value of stress exponent. Otherwise, the combination of sintering stress and transient behavior at the level of the pressure "jump" of the stepwise method can lead to high disturbance in the identified values

Nappes, trous, ligaments et gouttes

Ce manuscrit aborde le problème de l'atomisation de films liquides à travers diverses expériences portant sur leur formation, déstabilisation et fragmentation. Une première partie identifie et décrit deux mécanismes de désintégration à l'œuvre lors de la rétractation d'un film de savon. Lorsque le film se détache sur une frontière, le bord de Plateau est entrainé et son accélération explique la déstabilisation. Lorsque le perçage a lieu au centre, le battement du film résulte de l'interaction avec l'atmosphère au repos, ce qui conduit à sa fragmentation. Une seconde étude explique le plissement singulier de certaines cloches liquides par un ressaut hydraulique à interfaces libres assurant la transition d'un écoulement supercritique vis-à-vis des ondes capillaires à un écoulement subcritique. La courbure marquée associée à cette transition conduit à une accélération centripète du liquide susceptible de déstabiliser la cloche en aval. La formation des embruns par l'intermédiaire de l'éclatement des bulles de surface est abordée dans une troisième partie. La géométrie des bulles, le mécanisme de drainage de leurs films et les événements de perçage sont étudiés afin de rendre compte de l'épaisseur de la bulle au début de l'éclatement. La dynamique ultérieure de fragmentation est détaillée conduisant à une caractérisation du spray produit. Une dernière partie étudie l'atomisation dite effervescentesur une expérience modèle : une nappe liquide plane ensemencée en fines bulles d'air favorisant la nucléation de trous. Sur la base de mesures des taux de nucléation et croissance des trous, un modèle statistique du réseau de ligaments liquides formés par la réunion des bourrelets ceinturant les trous permet un description du spray issu de la fragmentation du réseau.This thesis tackles the problem of liquid films atomization through various experimental configurations dealing with their formation, destabilization and fragmentation. In a first part, two mechanisms of receding soap film disintegration are identified and described. When the film is released from a border, the acceleration sustained by the Plateau border explains the destabilization. When puncture occurs in the center, the film flapping is promoted by the interaction with the surrounding atmosphere which leads to its fragmentation. In a second study, the singular folding of liquid bells is explained by an original free surfaces hydraulic jump insuring transition from a subcritical to a supercritical flow with respect to capillary waves. The strong curvatures associated with this transition, also implying strong centripetal acceleration of the flowing liquid, may induce the downstream sheet destabilization. The question of marine aerosols formation via surface bubble bursting is investigated in a third part. Bubbles geometry, their film drainage mechanism and puncture events are studied in order to account for the film thickness at the onset of bursting. The subsequent bursting dynamics is detailed to characterize the spray production. In a last part, effervescentatomization is studied on a model experimental set up: a flat liquid sheet seeded with small air bubbles acting as heterogeneous hole nuclei. On the basis of holes nucleation and growth measurements, a statistical model of the liquid ligament red resulting from the holes rims combination offers a description of the spray generated by the red fragmentation.AIX-MARSEILLE1-Bib.electronique (130559902) / SudocSudocFranceF

A scalable synthesis route for multiscale defect engineering in the sustainable thermoelectric quaternary sulfide Cu26V2Sn6S32

International audienceIn recent years, thermoelectric materials inspired from the natural mineral colusite have emerged as a new class of environmentally-friendly copper-based sulfides composed of abundant elements. Herein, high performance bulk colusite Cu26V2Sn6S32 materials were synthesized using mechanical alloying and spark plasma sintering of low-cost industrial-grade metal sulfides. This new synthesis route has led to the formation of various types of nano-to-microscale defects, from local Sn-site structural disorder to nano-inclusions and vanadium-rich core-shell microstructures. These multiscale defects have a strong impact over phonon scattering, making it possible to reach ultra-low lattice thermal conductivity. Simultaneously, the electrical transport properties are impacted through variations in charge carrier concentration and effective mass, leading to a synergistical improvement of both electrical and thermal properties. The resulting power factor, over 1 mW m−1 K−2 above 623 K with an average value of 0.86 mW m−1 K−2 over the temperature range 300 ≤ T / K ≤ 650 K, is the highest reported for a germanium-free colusite to date. Our optimization strategy based on defect engineering in bulk materials is an exciting prospect for new low-cost thermoelectric systems