Carbon nanotubes and silver flakes filled epoxy resin for new hybrid conductive adhesives

Combining conductive micro and nanofillers is a new way to improve electrical conductivity. Micrometric silver flakes and nanometric carbon nanotubes (CNTs) exhibit high electrical conductivity. A new type of hybrid conductive adhesives filled with silver flakes and carbon nanotubes (DWCNTs or MWCNTs) were investigated. High electrical conductivity is measured as well as improved mechanical properties at room temperature. Small agglomerates and free MWCNTs dispersed in the silver/epoxy composites improve the electrical conductivity and a synergistic effect between MWCNTs and micro sized silver flakes is observed in hybrid composites. Glassy and rubbery storage moduli of the hybrid composites increase with increasing silver loading at fixed CNTs volume fraction. High value of the storage modulus, measured in DWCNTs/μAg hybrid composites at rubbery state, is caused by strong agglomeration of DWCNTs bundles. The electrical and mechanical properties are consistent with the morphologies of the hybrid composites characterized by SEM

Trente ans après : choses revues

Né en 1945 à Charleville-Mézières dans les Ardennes, Jean Jamin est directeur d’études à l’École des hautes études en sciences sociales (EHESS) où, jusqu’en 2013, il a enseigné l’épistémologie et l’histoire de l’anthropologie. Il a fondé en 1986, avec Michel Leiris, la revue Gradhiva (d’abord sous-titrée Revue d’histoire et d’archives de l’anthropologie) aujourd’hui publiée par le musée du quai Branly, avec comme sous-titre Revue d’anthropologie et d’histoire des arts. De 1997 à 2015, il a di..

Observations of the effect of earth tides on groundwater fluxes variations at the scale of a borehole

For a long time, characterization of aquifers has been mainly based on the monitoring of groundwater heads variations. This approach allowed to demonstrate that pressure changes induced by earth tides have a significant and measurable impact on groundwater heads monitored in confined aquifers. Nowadays efficient methods provide a direct estimation of groundwater fluxes. This is the case of the Finite Volume Point Dilution Method (FVPDM), a single-well tracer experiment that allows continuously monitoring and quantifying groundwater flux variations over time. Yet, the potential effect of earth tides on local groundwater flow has never been investigated. In this context, FVPDM tests have been performed in a confined aquifer in order to monitor groundwater fluxes over several tidal cycles. Results show significant groundwater flux variations over time (around 20% of the flux value), clearly correlated with pressure changes induced by earth tides. Subsurface heterogeneities could explain the fact that earth tides induce groundwater flow variations. Indeed, groundwater heads variations induced by earth tides depend on the local specific storage (in confined conditions) of aquifer. Any spatial variation of this parameter could induce variations of the hydraulic gradient and thus of groundwater fluxes. Therefore, these preliminary observations seem to open new perspectives for subsurface characterization by showing how groundwater flow variations measured in confined aquifer and induced by earth tides can be used as a marker of subsurface heterogeneities

Experimental characterization of mechanical properties of the cement-aggregate interface in concrete

International audienceThe microstructure of the Interfacial Transition Zone (ITZ) between the aggregates and the cement paste is characterized by a higher porosity than that of the bulk paste. The particular properties of this zone strongly influence the mechanical behavior of concrete. Microscopic cracks, which develop during subsequent loading, appear either in the matrix (cement paste or mortar) or along the cement-aggregates interface. Cracks could be caused by either tensile, shear strengths or by combinations of both. In this work, the mechanical properties of the cement paste – aggregate sample are experimentally studied. The experimental tests are performed on parallelepipedic samples at classical aggregate scale (one centimeter sections). These samples are composed of limestone aggregates and Portland cement paste, hereafter named ''composite ". The cement paste is prepared with a water/cement ratio of 0.5. The shape of the prepared composites makes them convenient for direct tensile and shear tests. At different stages of hydration, we performed direct tensile and shear tests on the composites by means of specific devices. The same tests were carried out on the cement paste in order to compare with the composite results. The analysis of the experimental results showed that the tensile strength of the cement-aggregate interface was about 30% lower than that of the cement paste tensile strength. Also, the shear strength of the cement-aggregate interface was smaller than the shear strength of the cement paste. In the same way as macroscopic Mohr–Coulomb criterion, we observed an increase of shear strength when normal stress increased. It provides access to a local cohesion (c) and a local friction angle ðUÞ at classical aggregate scale

Étude statistique de la réponse mécanique lors d'essais de traction directe à l'échelle locale

National audienceLa dispersion de la réponse mécanique d'un essai de traction directe sur une pâte de ciment à l'échelle locale, ou échelle de l'interface pâte de ciment/granulat, dépend, d'une part, de la dispersion des caractéristiques mécaniques de la pâte de ciment et, d'autre part, du dispositif expérimental. Une étude statistique numérique quantifie la dépendance de la dispersion des réponses mécaniques d'un essai de traction directe à la position de fixations en acier servant à la traction de l'échantillon. Les résultats de cette étude sont comparés à des observations expérimentales

Étude statistique de la réponse mécanique lors d’essais de traction directe à l’échelle locale

National audienc

Étude statistique de la réponse mécanique lors d'essais de traction directe à l'échelle locale

National audienceLa modélisation de la fissuration du béton nécessite de considérer la liaison entre la pâte de ciment et les granulats du béton, communément appelée interface, auréole de transition. Ce constat a motivé la mise en place d'essais mécaniques spécifiques en laboratoire à l'échelle dite locale afin de caractériser et d'identifier le comportement mécanique d'une pâte de ciment pure et de l'interface pâte de ciment/granulat à partir d'un échantillon composite. Des essais de traction directe sur des éprouvettes de ciment ont été mis en place dans le but d'identifier le comportement mécanique à la rupture de la pâte de ciment pure à l'échelle locale. Pour un type de ciment donné, la dispersion de la réponse mécanique des essais dépend, d'une part, de la dispersion des caractéristiques mécaniques de l'échantillon et, d'autre part, du dispositif expérimental. Cet article s'intéresse à la dispersion de la réponse mécanique sur plusieurs essais qui est grandement influencée par la préparation des échantillons. Une étude statistique numérique met en exergue la forte dépendance de la statistique des réponses mécaniques de l'essai de traction à la position des fixations en acier servant à la traction de l'échantillon. Les résultats de cette étude sont comparés à des observations expérimentales