SUPERCRITICAL CO_2 DRYING OF SILICA AEROGELS SYNTHESIZED IN 2-PROPANOL

摘要

Drying of silica gels is a critical step to obtain monolithic and crack-free aerogels. It is known that drying under supercritical (SC) conditions of CO_2 avoids densification and occurrence of cracks resulting from the capillary stresses appearing during the "conventional" sub-critical evaporative drying. However, one of the limitation of this process is the SC "washing" phase which consists in replacing the interstitial solvent (here 2-propanol) by the SC CO_2. In fact, this phase is generally governed by molecular diffusion mechanisms of the interstitial solvent in the surrounding CO_2 and is limited by the low permeability of the nanostructured gel network. Furthermore, an incomplete washing can damage the final properties of the aerogels (because of re-condensation during depressurization). The present paper focuses on our first experimental investigations for the determination of the criterion of the end of the SC washing step. The evolution of the propanol concentration in the autoclave was followed "on-line" with a "micro" gas chromatograph and compared with analytical estimations obtained from a bibliographic model. Current work is using the experimental results obtained in order to estimate the effective coefficient of diffusion of the propanol. The research is partly funded by the European Commission within the frameworks of the Energy, Environment and Sustainable Development Program - EESD through the contract ENK6-CT-2002-000648 (acronym : HILIT+).