Effect of the pore volume fraction on the thermal conductivity and mechanical properties of kaolin-based foams

Candidate materials for thermal insulation combine a solid phase of low thermal conductivity with porosity. Very porous ceramics based on a commercial clay containing 79% kaolinite were made by a direct foaming method using methylcellulose, drying at 70 °C before a consolidation at 1100 °C. The pore volume fraction (vpvp) was varied in the kaolin-based foam by modifying the amount of clay incorporated in the starting mixture. Measurements of the thermal conductivity with the hot disk method revealed a decrease from 0.23 W m−1 K−1 at vpvp = 0.57 to 0.054 W m−1 K−1 at vp=0.95vp=0.95 in close agreement to predictions by the Hashin–Shtrikman upper bound, a cubic pore model and numerical simulation of an artificial microstructure representing the polyhedral pore shape with Voronoï mosaics. The effective Young's modulus, obtained from mechanical compression tests, also decreases with pore volume fraction as described by Ashby's relation. However the mechanical strength was sufficient for handling even the more porous kaolin-based foams.