Study On Preparation Processes Of Silica Aerogels At Ambient Pressure

Silica aerogels were prepared by a 2-step acid-base catalyzed sol-gel technique using tetraethyoxysiliane (TEOS) as a precursor. The hydrophobic and monolithic silica aerogels could be made after aging in H₂O/EtOH and TEOS/EtOH solutions respectively and surface modifying in IPA/TMCS/n-Hexane solutions. The main properties of the aerogels were also studied.Aging in a certain solution will cause some shrinkage, which will be increased as the increasing of temperature. With the extension of aging time, the shrinkage of the aerogels will firstly be increased and then decreased after a few hours and increased again if the time is long enough. The lowest shrinkage will be obtained when the alcogels are firstly aging at 30vol%H₂O/EtOH solutions for 24 hours at 60℃, followed by at 70vol%TEOS/EtOH solutions for 16 hours at 70℃. The total shrinkage will be decreased after surface modification at IPA/TMCS/n-Hexane solutions for 48 hours at 60℃subsequently.The monolithic silica aerogels prepared by the above methods have an excellent hydrophobic property, with a contact angle nearly about 180°. The mass variation ratio of aerogel is only 7.69% when immersed into water for a week. The hydrophobic property will deteriorate after 300℃heat-treatment, as its mass is sharply increased almost 2 times at the initial three days after immersed into water. But it will not change at the following 4 days. So it is indicated that the pores structure in the aerogels has been changed after heat-treatment, which is in favor of the absorption of water as the connectivity of pores is become better. The hydrophilic groups are increased during heat-treatment. Moreover, the Porod Scattering and Fractal Structure are also investigated via Small Angle X-ray Scattering testing of aerogels. A distinct interface can be found between the gas-solid states of them whether DCCA be doped or not. All of them obey the Porod law, but the kinds of DCCA can affect the Porod constant. Its kinds also have a great effect on the volume fractal dimension, but their effect on the surface fractal dimension is very little. The fractal phenomena scopes are also studied in this experiment.