Diffusion During the Supercritical Drying of Silica Gels

Abstract

Drying is the most critical elaboration step of large monolithic and crack-free silica aerogel plates. In the present work, we are studying the supercritical CO₂ drying and more precisely the first step, here called the supercritical washing step. This phase consists of replacing the liquid phase contained in the nanopores with supercritical CO₂. Within this study, this step is governed by molecular diffusion through the gels. These phenomena were investigated experimentally in order to estimate the duration of the washing step. The experimental results were then fitted with an analytical mass transfer model to identify the effective diffusion coefficient.

Keywords:

• Effective diffusion coefficient
• Silica aerogels
• Supercritical CO₂ drying

Acknowledgments

The European Commission is highly acknowledged for its financial support. The authors thank all the HILIT + participants for their faithful collaboration: K.I. Jensen, J.M. Schultz, and F.H. Kristiansen (DTU, Denmark); P.A. Bonnardel and J. Slifirski (PCAS, France); B. Sunden and V. Andersson (LTH, Sweden); M.A. Einarsrud and R.A. Strøm (NTNU, Norway); G. Petermann and L. Gullberg (AIRGLASS AB, Sweden); Br. Chevalier (CSTB, France); and P. Nitz (FhG-ISE). A special thanks is adressed to Mr. R. Pirard (Liege University, Belgium) for mercury porosimetry measurements and fruitful discussions.

Notes

¹PCAS, BP 181-91 161 Longjumeau Cedex, France, http://www.pcas.fr/

²Airglass, BOX 150, 245 22 Staffanstorp, Sweden, http//www.airglass.se/