SILICA-AEROGEL THERMAL-EXPANSION INDUCED BY SUBMONOLAYER HELIUM ADSORPTION

We present measurements of the thermal expansion of silica-aerogels of various densities in the two following situations: thermal expansion of evacuated samples and the additional expansion due to He-4 adsorpti on on the silica surface in the temperature range 2-80 K. At the same time we measured the number of He-4 atoms adsorbed at the surface. To analyze the adsorption data we use a Brunauer-Emmett-Teller model, whe re we introduce a distribution of binding energies to take into accoun t the surface heterogeneity. A simple model,involving the same distrib ution, allows us to relate the expansion to adsorption. We therefore d etermine some microscopic parameters for the adsorption energies and d erive the elastic properties at different scales of the fractal struct ure.