Analyse thermofiabiliste de matériaux poreux céramiques à haut taux de densification

National audienceSee http://hal.archives-ouvertes.fr/docs/00/59/27/90/ANNEX/r_IF8J8H1Q.pd

Thermal conductivity of porous materials

Incorporation of porosity into a monolithic material decreases the effective thermal conductivity. Porous ceramics were prepared by different methods to achieve pore volume fractions from 4 to 95%. A toolbox of analytical relations is proposed to describe the effective thermal conductivity as a function of solid phase thermal conductivity, pore thermal conductivity, and pore volume fraction (νp). For νp 0.65, the thermal conductivity of kaolin-based foams and calcium aluminate foams was well described by the Hashin Shtrikman upper bound and Russell's relation. Finally, numerical simulation on artificially generated microstructures yields accurate predictions of thermal conductivity when fine detail of the spatial distribution of the phases needs to be accounted for, as demonstrated with a bio-aggregate materia

Elaboration, caractérisation et modélisation de matériaux poreux (Influence de la structure poreuse sur la conductivité thermique effective)

Cette étude se propose de déterminer l'influence de certains paramètres microstructuraux sur la conductivité thermique effective des matériaux poreux dans la gamme de porosité : 45%-75%. Dans cet objectif, trois axes de travail ont été établis : élaboration, caractérisation et modélisation. Nous avons élaboré plusieurs types de matériaux poreux aux microstructures différentes (porosité aléatoire et organisée). Des échantillons de zircone présentant une structure cellulaire ont été préparés selon une méthode innovante basée sur un processus d'hétérocoagulation. La comparaison des résultats expérimentaux avec des prédictions par des modèles analytiques et par des simulations numériques nous a permis d'évaluer l'influence de paramètres tels que la taille et la forme des pores, l'organisation de la porosité. Le modèle de percolation, via l'expression de Landauer, permet des prédictions satisfaisantes de la conductivité thermique pour des taux de porosité inférieurs à 60%LIMOGES-BU Sciences (870852109) / SudocLIMOGES-ENSCI (870852305) / SudocSudocFranceF

Mechanical properties and thermal conductivity of porous alumina ceramics obtained from particle stabilized foams

International audienceIn this work, foams were obtained by direct foaming of aqueous alumina suspensions containing butyric acid. Butyric acid is incorporated in the suspensions in order to hydrophobize alumina particles, leading to their adsorption at the air/water interface. After setting and drying, wet foams were sintered at 1585 °C for 2 h. The resulting porous samples were characterized in terms of microstructure, mechanical properties and thermal conductivity. Microstructures of sintered alumina foams reveal closed pores surrounded by a thin alumina layer. The microstructure of these macroporous ceramics is related to (i) the composition of the initial suspension (alumina and butyric acid contents) and (ii) the stirring velocity during the foaming process. Macroporous ceramics with pore sizes (d50) ranging from 20 µm to 140 µm and porosities between 25% and 89% were obtained. Three-point bending tests were performed on foams with porosities between 65% and 89%. The results of mechanical tests were analyzed with Weibull statistic. The bending strength values are between 5 MPa and 42 MPa. The Young’s modulus, obtained from 3-point bending tests, decreases with the porosity level according to Gibson–Ashby model. Thermal diffusivity measurements were made with the laser flash technique in order to determine the effective thermal conductivity. Experimental values are significantly higher than the predictions with Landauer’s relation and almost close to Maxwell/Hashin–Shtrikman upper bound

Impact of perlite, vermiculite and cement on the thermal conductivity of a plaster composite material: Experimental and numerical approaches

International audienceThis work is dealing with the thermo mechanical behaviour of a new composite material used as thermal insulator and for fire passive protection in building construction. The composite panel results of combination of a calcium sulphate hydrates matrix mainly water and inorganic mineral additives. The effective thermal conductivity of the composite is measured by the hot-disc method. Analytical and numerical models are used to determine the composite thermal conductivity which can be expected vs the rate of additives. The experimental values are compared with analytical and numerical models. In particular, effective medium percolation theory (EMPT) for a two-phase system gives analytical values which are close to the experimental results. Moreover, the numerical model based on a simple description can allow the description of the thermal conductivity evolution with the rate of additives. An unexpected phenomenon which can be attributed to the confined water in additives can explain the gap between the numerical models and the experimental results

Comportement rhéologique de matières d’œuvre céramiques

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Shrinkage behaviour of ceramic bodies during drying: following extrusion or pressing

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Influence of the Microstructure for the Thermophysical Properties of Thick Coatings compared to Bulk Ceramics

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Effect of humidity on the dielectric constant and electrical impedance of mesoporous zirconia ceramics

International audienceThe effect of humidity on the dielectric constant and electrical impedance of porous 8 mol% yttria stabilized zirconia ceramic is examined. Samples with pore volume fractions from 33% to 55% were prepared from a fine powder by uniaxial pressing and firing. Measurements, in different relative humidities from <3% to 100%, showed a strong increase of dielectric constant with water content of the unfired and fired porous ceramics. The dielectric constant can be described with good agreement to experimental values using a 2 step model involving the Hashin-Shtrikman upper bound and Landauer’s effective medium relation, analogous to the behavior of thermal conductivity. Water is assumed to be located as a layer of variable thickness on the interior solid surfaces of the pores. A.c. impedance measurements also reveal a short circuit of insulating dielectric behavior at higher water contents, interpreted as surface conduction of protons

Micro-Computed Tomography Image Based Thermo-Elastic Properties Studies of Freeze-Cast MMCs.

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