Modelling the plastic deformation during high-temperature creep of a powder-metallurgy coarse-grained superalloy

The study of creep deformation in a coarse-grained Udimet 720 superalloy obtained by powder-metallurgy reveals a good resistance associated to a dislocational deformation mechanism. A model is proposed for simulating creep and tensile curves. This model is used to understand the effect of microstructural changes on the deformation mechanisms

Doped TiO2 aerogels as alternative catalyst supports for proton exchange membrane fuel cells: A comparative study of Nb, V and Ta dopants

International audienceNb, Ta and V-doped TiO2 aerogels and xerogels have been synthesized as possible new alternatives to carbon blacks for Proton Exchange Membrane Fuel Cells catalyst supports. A comparative study of different dopants was realized in a single study. Nb, Ta and V showed different behaviors with respect to the final material structure and morphology, composition and electronic conductivity. They are all prone to surface segregation, to different extents. V-doped TiO2 apart, the rutile structure could only be obtained after calcination in a reducing atmosphere at 800 °C for Nb or Ta-doped TiO2. The electronic conductivity exhibited a maximum at 10 at.% for Nb and Ta, 5 at.% for V. Nb revealed to be the most appropriate dopant to increase the electronic conductivity of TiO2, followed by Ta and V. 4 to 5 orders of magnitude were gained after Nb doping for xerogels conductivity to reach almost 0.1 S cm−1. The role of point defects was discussed to account for phase transition and evolution of conductivity

Electron microscopy to study MEA materials and structure degradation

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Modelling the plastic deformation during high-temperature creep of a powder-metallurgy coarse-grained superalloy

International audienceThe study of creep deformation in a coarse-grained Udimet 720 superalloy obtained by powder-metallurgy reveals a good resistance associated to a dislocational deformation mechanism. A model is proposed for simulating creep and tensile curves. This model is used to understand the effect of microstructural changes on the deformation mechanisms

Post Mortem Analysis of Degraded DMFC After Extended Operation

This work presents an ex-situ analysis of long-term (>1000 h) aged DMFC. Specifically, the data comprise a comparison of pristine, activated and aged MEA components in order to assess the observed physical changes with respect to their impact on permanent performance loss. The components analyzed by means of X-ray photoelectron spectroscopy and electron microscopy are GDLs, MPLs, CLs and membranes. Our results clearly show that Ru cross-over from anode CL to cathode CL and MPL, associated with observation of catalyst precipitates in the membrane, occurs mainly at the beginning of the operation (first 30 h) and proceeds only slowly upon long term aging. Hence, this effect seems to represent a minor contribution to the overall performance loss. In the MPL and CL of the anode and the cathode side a loss of polymers is observed which occurs predominantly at the beginning of operation too. An effect attributed to long-time operation is the gradual chemical decomposition of PTFE (fluorine depletion) especially in the anodic GDL

Study of the role played by nitrogen on the deep‐drawing properties of aluminium killed steel sheets obtained after a continuous annealing

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Optimization of transports in a PEMFC catalyst layer at high current densities: coupled modeling/imaging approach

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Optimization of transports in a PEMFC catalyst layer at high current densities: coupled modeling/imaging approach

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Optimization of Transports in a Proton-Exchange Membrane Fuel Cell Cathode Catalyst Layer at High Current Densities: A Coupled Modeling/Imaging Approach

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Effect of the structure of Pt–Ru/C particles on CO_ad monolayer vibrational properties and electrooxidation kinetics

International audienceIn this paper, we combined FTIR spectroscopy and COad stripping voltammetry to investigate COad adsorption and electrooxidation on Pt–Ru/C nanoparticles. The Pt:Ru elemental composition and the metal loading were determined by ICP-AES. The X-ray diffraction patterns of the Pt–Ru/C indicated formation of a Pt–Ru (fcc) alloy. HREM images revealed an increase in the fraction of agglomerated Pt–Ru/C particles with increasing the metal loading and showed that agglomerated Pt–Ru/C nanoparticles present structural defects such as twins or grain boundaries. In addition, isolated Pt–Ru/C nanoparticles have similar mean particle size (ca. 2.5 nm) and particle size distributions whatever the metal loading. Therefore, we could determine precisely the effect of particle agglomeration on the COad vibrational properties and electrooxidation kinetics. FTIR measurements revealed a main COad stretching band at ca. ν(COL) = 2030 cm-1, which we ascribed to a-top COad on Pt domains electronically modified by the presence of Ru. As the metal loading increased, the position of this band was blue shifted by ca. 5 cm−1 and a shoulder around 2005 cm−1 developed, which was ascribed to a-top COad on Ru domains. The reason for this was suggested to be the increasing size of Ru domains on agglomerated Pt–Ru/C particles, which lifts dipole–dipole coupling and allows two vibrational features to be observed (COad/Ru, COad/Pt). This is evidence that FTIR spectroscopy can be used to probe small chemical fluctuations of the Pt–Ru/C surface. Finally, we comment on the COad electrooxidation kinetics. We observed that COad was converted more easily into CO2 as the metal loading, i.e. the fraction of agglomerated Pt–Ru/C nanoparticles, increased