Thermal Conductivity of Very Porous Kaolin- Based Ceramics

A clay-based material exhibiting high pore volume fraction and low thermal conductivity suitable for thermal insulation is described. Starting with a commercial clay containing >75% kaolinite, foams were made by mixing in water and methyl cellulose as a surfactant then beating. After drying at 70 degrees C, the pore volume fraction >94% remains almost constant for treatments up to 1150 degrees C. In contrast, the phases constituting the solid skeleton evolve strongly with removal of surfactant, dehydroxylation of kaolinite, and formation of mullite. The latter leads to greater mechanical strength but also an increase in thermal conductivity. Thermal treatment of the kaolin foam at 1100 degrees C yields a suitable compromise between low thermal conductivity of 0.054 W.(m.K)(-1) at room temperature with a compressive yield stress of 0.04MPa. The radiation component in the effective thermal conductivity is <10% at 20 degrees C increasing to >50% at 500 degrees C.