Effects of solid-gas coupling and pore and particle microstructures on the effective gaseous thermal conductivity in aerogels

An analytical model was developed to predict the pressure-dependent gaseous thermal conductivity in aerogels based on the spherical porous secondary particle aggregate structure. The model includes the effects of particle size, pore and particle microstructures, and solid-gas coupling including the quasi lattice vibrations for solid-like vibrating gas molecules in the gaps between adjacent secondary particles that are not included in previous models. The results show that the pressure-dependent effective gaseous thermal conductivities of RF and silica aerogels predicted by the present model agree well with experimental results. The solid-gas coupling significantly increases the effective gaseous thermal conductivity in the aerogels as the quasi lattice vibrating gas molecules in the gaps more effectively bridge adjacent particles. The effects of solid-gas coupling and pore and particle microstructures are significant for particle aggregate structures with mean pore and particle diameters in the range of 100 nm-10 mu m while the Knudsen formula and the Zeng's model have limited applicability in this size range. Micron and millimeter-scale pores that can occur in nanoporous silica aerogel samples due to the mechanical fragility of these nanostructures can be well represented by the present three pore size model.