A simple scaling theory for the sintering of fractal aerogels is presented. The densification at small scales is described by an increase of the lower cut-off length a accompanied by a decrease of the upper cut-off length ξ, in order to conserve the total mass of the system. Scaling laws are derived which predict how a, ξ and the specific pore surface area Σ should depend on the density ρ.
The experimental density dependence of gas (argon and nitrogen) permeability of partially densified silica aerogels in the Knudsen regime is quantitatively accounted for by a computer model. The model simulates both the structure of the sintered material and the random ballistic motion of a point particle inside its voids. The same model is also able to account for the density dependence of the specific pore surface as measured from nitrogen adsorption experiments.
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