Interpretation of mercury porosimetry applied to aerogels
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The observation of aerogels submitted to a pressure of mercury indicates that this porous material is compacted and not intruded by the mercury. Consequently, the classical Washburn equation cannot be applied. A relation is established between the pressure P of compaction and the size L of the largest pores. The size of pores is estimated by using the nitrogen adsorption-desorption isotherms analysis and SEM measurements. A relation is found in which P is proportional to L~4. The new relation is applied to mercury porosimetry. Finally, a mechanical model is proposed that reproduces successfully the behavior of aerogels under high pressure of mercury.
I. INTRODUCTION Aerogels are porous materials obtained from the supercritical drying of highly cross-linked inorganic or organic gels. These materials exhibit ultrafine cell/pore sizes, continuous porosity, high specific surface area, and a microstructure composed of interconnected colloidallike particles or polymeric chains with characteristic diameters of about 10 nm. This microstructure is responsible for their unusual optical, acoustic, thermal, and mechanical properties. A variety of techniques have been used to investigate the continuous porosity of aerogels which can range from less than 1 nm to more than 1 fim.1 (i) Image analysis of TEM or SEM micrographs. The drawback of this technique is related to the fact that it provides only a two-dimensional projection of a three-dimensional structure, (ii) The nitrogen adsorption-desorption isotherm analysis that gives a distribution of specific surface area and pore volumes in a wide range of porosities from micropores to mesopores of radius smaller than 50 nm. (iii) Mercury porosimetry that gives information on the structure of mesopores and macropores. The analysis of the distribution of the specific surface area and of the specific pore volume in relation with the pore size from mercury porosimetry data is classically based on Washburn's equation. This equation has been derived on the assumption that the mercury intrudes the pore network. Unfortunately, it has been observed experimentally that the structure of aerogels is damaged by the high pressure required to intrude small cells. That can be explained by the small value of the Young modulus and of the ultimate compressive strength of aerogels.2 Consequently, Washburn's equation cannot be applied to aerogels. Given the complex nature of aerogels, it would be insufficient to base their description on only one method of characterization. With this in mind, we present 2114 http://journals.cambridge.org
J. Mater. Res., Vol. 10, No. 8, Aug 1995 Downloaded: 28 Aug 2014
in this paper a quantitative study of the effect of the pressure of mercury on the structure of ZrO2-SiO 2 aerogel using the mercury porosimetry method. This investigation leads us to establish a relation between the pressure and the size of the largest pores and to propose a mechanical model that reproduces successfully the behavior of this class of aerogels under high pressure of mercury. The choice
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