Pore Morphology Study of Silica Aerogels
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ultra-small andbesmall angleHowever, x-ray scattering methods,onthedryentire spatial of multiple aerogels scattering ( 5 I.tm) may probed. light scattering aerogels canscale cause which gives errors in data analysis. Even though efforts have been made to utilize this multiple scattering [7,8] or to correct the intensity at all angles [9,10], we think this problem still exists in most dry gel studies and is especially serious when the gel has microcracks. Generally speaking, refractive index matching is necessary for light scattering. Due to the nature of supercritical drying, the resulting aerogel structure should be the same as the wet gel. Therefore, one can presumably study the wet gel structure and properties of the aerogel-precursor as an indication of the resulting aerogel. With the combination of ultra-small and small angle x-ray scattering techniques, a clearer picture of aerogel structure should be obtained. Most studies of aerogel structure have concentrated on smaller length scales which probe sizes less than 100 nm [1,9,10]. A considerable effort has been made on the comparison of both polymeric and colloidal aerogels with various precursor sizes and the utility of colloidal gels as model systems has been demonstrated [11]. In this study, simple experiments on colloidal aerogels are performed in order to refine this approach of studying larger scale structure with SALS on wet gels which have been contrast matched followed by USAXS and SAXS on aerogels produced via supercritical CO2 drying. By changing the pH value used for sol destabilization, we hope to change the gel time and large scale structure in a systematic fashion. Also, by studying structural changes during isostatic compaction, we hope to observe the pore structure changes that occur during drying from solvents of finite surface tension. EXPERIMENTAL Silica gels were made following Pelous, Foret, and Vacher [11] by destablizing silica sols with HC1 until the desired pH value is obtained. The pH of the sols for the wet gels were 1.42, 2.26, 3.32, 4.65, and 5.50, while pH of the sols for the CO2 dried aerogel rods were 1.47, 3.45, 4.12, and 5.25. All samples were aged for 24 hours at 80°C after gelation. The wet gels were kept in ethanol until light scattering measurement and CO2 supercritical drying. An alkoxide-derived gel was also used and was made using a two-step acid/base catalyzed hydrolysis and condensation of TEOS denoted as B2 and which is described in more detail elsewhere [12]. A vertical solid state light scattering setup with a 10 mW He-Ne laser and a CCD detector (512x512 pixels) was used. Different q ranges can be obtained by moving the samples on the setup, circular averaging takes care of the speckle problem. Basically, the q range covered is from 0.2 to 6 •tm-1 (2x10-5 to 6xl0-4 A-l). A Tracor Northern linear photo diode array with 2K resolution was also attempted. Samples were placed between two plates of micro slides, methanol was blended into the sample in order to minimize refractive index differences. Benzoyl alcohol wi
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