International audienceIncorporation 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 Maxwell–Eucken relation for closed porosity and Landauer relation for open porosity give good agreement to experimental data on tin oxide, alumina, and zirconia ceramics. 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 material

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International audiencePorous potassium based geopolymers with a mutli-scale porosity were synthesized. Silica fume is introduced as an additive to the geopolymer formulation. The free silicon contained inside this silica fume is oxidized in alkaline solution, releasing molecular hydrogen which generates the porosity. Previous work has shown how the porosity can be controlled with temperature, repeated temperature cycles and the mass introduced. Using this protocol, homogeneous foams were made and then studied with scanning electron microscopy. In particular the foam expansion has been followed with time in relation to the microstructure. The thermal conductivity values of the foams were evaluated using a fluxmeter method. The effective thermal conductivities are comprised between 0.12 and 0.35 W m−1 K−1 for apparent densities ranging from 0.40 to 0.85 g cm−3. The corresponding calculated pore volume fractions are in the range of 65–85%. The interest of this material is that it combines the advantages of low bulk density and insulating properties with the characteristics of a geopolymer skeleton. Literature reports a very good fire and acid/base resistance, a low cost of production and the possibility of recycling industrial waste in the form of silica fume

International audienceA clay-based material exhibiting high pore volume fraction andlow thermal conductivity suitable for thermal insulation isdescribed. Starting with a commercial clay containing >75% kaolinite,foams were made by mixing in water and methyl celluloseas a surfactant then beating. After drying at 70°C, the pore volumefraction >94% remains almost constant for treatments upto 1150°C. In contrast, the phases constituting the solid skeletonevolve strongly with removal of surfactant, dehydroxylation ofkaolinite, and formation of mullite. The latter leads to greatermechanical strength but also an increase in thermal conductivity.Thermal treatment of the kaolin foam at 1100°C yields a suitablecompromise between low thermal conductivity of 0.054W.(m.K) – 1 at room temperature with a compressive yield stressof 0.04 MPa. The radiation component in the effective thermalconductivity is <10% at 20°C increasing to >50% at 500°