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The experimental results presented in this paper show the influence of theelastic deformation of porous solids on the adsorption process. With p+-typeporous silicon formed on highly boron doped (100) Si single crystal, we canmake identical porous layers, either supported by or detached from thesubstrate. The pores are perpendicular to the substrate. The adsorptionisotherms corresponding to these two layers are distinct. In the regionpreceding capillary condensation, the adsorbed amount is lower for the membranethan for the supported layer and the hysteresis loop is observed at higherpressure. We attribute this phenomenon to different elastic strains undergoneby the two layers during the adsorption process. For the supported layer, theplanes perpendicular to the substrate are constrained to have the sameinteratomic spacing as that of the substrate so that the elastic deformation isunilateral, at an atomic scale, and along the pore axis. When the substrate isremoved, tridimensional deformations occur and the porous system can find a newconfiguration for the solid atoms which decreases the free energy of the systemadsorbate-solid. This results in a decrease of the adsorbed amount and in anincrease of the condensation pressure. The isotherms for the supported porouslayers shift toward that of the membrane when the layer thickness is increasedfrom 30 to 100 microns. This is due to the relaxation of the stress exerted bythe substrate as a result of the breaking of Si-Si bonds at the interfacebetween the substrate and the porous layer. The membrane is the relaxed stateof the supported layer.

Influence of Elastic Strains on the Adsorption Process in Porous Materials. An Experimental Approach