Comparison between sintered and compressed aerogels

Aerogels can be densified either by thermal sintering, or at room temperature by isostatic compression. We report here a comparative analysis of silica aerogel densified by these two methods. To better follow their structural evolutions we use SAXS measurements performed on aerogels exhibiting a fractal geometry. This fractal geometry specially gives information about the way the solid network is firstly established and how it evolves with densification. The structural features such as particle and cluster sizes are observed to change differently according to the densification method. While the specific surface area of sintered aerogels decreases with densification, it does not change when densification is performed under isostatic compression. Furthermore the pore size distribution analysis evidences that the pressure induces the collapse of the largest pores while sintering acts on all pores. A microscopic model is proposed. It allows to explain the structural changes observed both by isostatic compression and by thermal sintering. Moreover, it agrees well with the evolution of elastic constant and internal friction with densification. (C) 2003 Elsevier B.V. All rights reserved.