Evacuated insulation panels filled with pyrogenic silica powders: properties and applications
Roland Caps, Ulrich Heinemann, Michael Ehrmanntraut, Jochen Fricke
Thermal conductivity, l, and thermal diffusivity, a, of porous refractory oxide ceramic materials vary in a complicated manner with temperature, T, and gas pressure, p, which cannot be explained on the basis of known classical heat-transfer mechanisms in porous materials, which are conduction in solid and gas phases, gas convection, and heat radiation. This abnormal behaviour includes: (i) different T dependence of l at atmospheric and at low pressure; (ii) at high T, thermal diffusivity measured in vacuum can exceed this property measured at atmospheric pressure; (iii) l and a can depend on the measurement method. In order to explain the peculiarities of behaviour of l and a, two additional groups of heat-transfer mechanisms are reviewed, including heterogeneous heat and mass transfer processes occurring in pores and in cracks, and microstructural changes due to nonuniform thermal expansion of particles and grains.