| Silica powder is a material of interest in several fields such as medicine, materials science or pharmaceutics. Silica aerogel powders have several applications, including: as additives for conventional foaming operations, chromatography packing materials, controlled release agents, encapsulation of products, catalysis and high surface-area adsorbents. The silica was prepared by chemical precipitation and sol-gel process, then drying under high temperature. But the silica powders have many limitations, for example, density is big, specific surface area is low, disperse is bad. So the studying of the new preparation technique is very necessarily. Now the supercritical fluid technique has been introduced into drying powders, and there are some researches of supercritical drying silica powders, but the gel was synthesized with TEOS and TMOS etc., the cost of silica powders are very high. So it is necessary to research preparation of silica powders with inorganic salt by supercritical fluid technique. This paper prepared the silica aerogel powders with waterglass by supercritical fluid technique. The influences of the nucleation, growth, reaction, supercritical media and supercritical drying parameters to size of particles, composing and structure of surface were investigated. The samples were investigated using TEM, IR, TG/DTA, XRD and BET etc. The preparation of silica aerogel powders by supercritical ethanol drying was investigated. The result showed that the volume of ethanol and aerogel was 1/2~3/5 volume of reactor if the request of supercritical conditions was the temperature is 265℃, the pressure was 7.5Mpa and the aerogel density was 0.8~1.2g/cm3. The effect was optimal when the rate of heat was 1~2℃/min. Its dispersibility was the best when pH was about 4.5, the particles were spherical, its diameter was about 20~30nm, the particles' fringe were clear and its diameter was not change after annealed 500℃ for 4h. There were little groups of –OH and -CH2-CH2- on the surface of the aerogel particles. The sample showed a 7% weight loss in the temperature range 0~400℃, weight loss was due to the pyrolysis of residual organics. The specific surface area of the aerogel was lower than the xerogel. The samples were amorphous.The extracting ethanol of supercritical CO2 from gel on static state was investigated. The result showed that the rate of extraction was higher under the supercritical conditions of the system of CO2+C2H5OH than that of subcritical conditions. The influence of temperature to extraction under subcritical conditions could be ignored. The sample under supercritical conditions was network in the TEM photo, its diameter were about 20~40nm, but its diameter became bigger and the network was cracked under subcritical conditions. The samples under supercritical conditions had an endothermic peak around 90℃, an exothermic peak around 300℃, but the samples under subcritical conditions still had an exothermic peak around 450℃, all the samples had 26% weight losses. The samples were amorphous.The preparation of silica aerogel powders by CO2 supercritical drying was investigated. The result showed that the cycle of extraction had better to be 15~20h, when the temperature was 40℃, the pressure was 10Mpa, the rate of fluid was 5~10kg/min. The influence of pH to the microstructure of the aerogel powders could be ignored, The samples were network floccules, the aperture structure were abundance, the particles' diameter were 10~20nm, the particles' fringe were clear and its diameter didn't change after annealed 500℃ for 4h. There are the groups of –OH and -CH2-CH2- on the surface of the aerogel particles before annealing. The sample showed a 10% weight loss in the temperature range 0~400℃, weight loss was due to the pyrolysis of residual organics. The specific surface area of the aerogel was higher than the xerogel. The samples were amorphous.
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