THERMAL-CONDUCTIVITY OF FOAMS .2. THE THERMAL-CONDUCTIVITY OF A LAYER MINERAL FOAM

Thermal conductivity measurements have been carried out on samples of a foam made from a slurry of delaminated vermiculite, The foam consists of cells filled with air, surrounded by walls made up of a film material built from overlapping vermiculite lamellae. The foam is isotropic. Humidity, while affecting the adsorbed water content, does not have a noticeable effect on the thermal conductivity. Heat transfer by convection and by radiation seems negligible, largely due to the small cell diameters, around 100 mum. The dependence of conductivity on foam density can be quite well represented as a straight line extrapolating to the conductivity of still air at zero foam density. This result is compared with the predictions of theoretical calculations based on conduction by the air and the solid, modelling the latter using samples of a vermiculite paper. Good agreement is found, and this confirms both the theoretical approach and the view of the foam structure.