DENSITY AND REFRACTIVE-INDEX OF SILICA AEROGELS AFTER LOW-TEMPERATUREAND HIGH-TEMPERATURE SUPERCRITICAL DRYING AND THERMAL-TREATMENT

This paper reports on the density of silica aerogels after low- and hi gh-temperature supercritical drying as well as on the corresponding op tical refractive index. The influence of adsorbed water or methanol ha s also been investigated. Aerogel samples are prepared via sol-gel pro cess of pH 11 and pH 13 tetramethoxysilane/methanol/water solutions us ing ammonia as catalyst. After high-temperature drying, the pH 11 aero gels show volume shrinkage of less than 10%, 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 obs erved. Its slope is (0.221 +/- 0.002) X 10(-3) m3 kg-1 and (0.193 +/- 0.002) x 10(-3) m3 kg-1 before and after thermal treatment at 500-degr ees-C for 14 h in air, respectively. The larger value is due to hydrox yl or methoxy groups in the aerogel. Adsorbed water or methanol cause the index to increase, as do molecules. The distribution of SiO2 on th e nanoscopic level does not influence the index.