Density and refractive index of silica aerogels after low- and high-temperature supercritical drying and thermal treatment

This paper reports on the density of silica aerogels after lowand high-temperature supercritical drying as well as on the corresponding optical refractive index. The influence of adsorbed water or methanol has also been investigated. Aerogel samples are prepared via soI3el process of pH 11 and pH 13 tetramethoxysilanehnethanolfwater solutions using ammonia as catalyst. After high-temperature drying, the pH 11 aerogels show volume shrinkage of less than lo%, while pH 13 aerogels and aerogels dried at low temperature with carbon dioxide shrink by about 50%. A linear relationship between refractive index and density is observed. Its slope is (0.221 f 0.002) x m3 kg-' and (0.193 f 0.002) x m3 kg-' before and after thermal treatment at 500°C for 14 h in air, respectively. The larger value is due to hydroxyl or methoxy groups in the aerogel. Adsorbed water or methanol cause the index to increase, as do molecules. The distribution of Si02 on the nanoscoDic level does not influence the index.