| Summary: | 碩士 === 國立清華大學 === 化學工程學系 === 94 === Monolithic silica aerogels with thermal conductivities as low as 0.036 W/m-K and porosities as high as 97% were successfully prepared with ambient pressure drying through a multiple surface modification approach. The tetraethoxysilane (TEOS)-derived wet gel was made hydrophobic with multiple surface treatments of trimethylchlorosilane (TMCS) and dried under ambient pressure. It was found that the contact angle could reach 143o after multiple surface modifications, which was higher than the products prepared with the single surface modification procedure. This multiple surface modification procedure led to lower volume shrinkages during the ambient pressure drying and the silica aerogels thus obtained possessed higher porosities. As to the effect of the solution pH, it was found that higher pH resulted in higher pore volumes and lower thermal conductivities. When the pH was controlled at 10~11, silica aerogels with thermal conductivities as low as 0.03W/m-K were obtained.
In this research, metal oxide-SiO2 composite aerogels were also prepared and studied. Monolithic TiO2-SiO2 composite aerogels with thermal conductivities as low as 0.04W/m-K together with better mechanical strengths were successfully prepared with the multiple surface modification procedure at ambient pressure drying.
SnO2-SiO2 composite aerogels were also prepared and studied for their photoluminescence and photocatalysis performance. In the sol-gel reaction procedure, it was found that higher incorporation concentrations of SnO2 precursor produced SnO2-SiO2 composite aerogels with higher quantum yields of PL because of the more well crystallized SnO2 in the aerogels. The SnO2-SiO2 composite aerogels heat-treated at higher temperatures exhibited higher quantum yields of PL and higher photocatalytic conversions, because the crystal growth achieved in heat treatment of higher temperatures was more complete than in lower temperatures. For SnO2-SiO2 composite aerogels produced from the liquid deposition procedure, products of heat treatment at 500oC gave higher quantum yields of PL and photocatalytic efficiency than produced of 400oC, due again to the better crystallinity. For products obtained at heat treatment temperature of 700oC, the quantum yields of PL was however lower than those prepared at 500oC, because the grain size of tin oxide became larger and the specific surface area thus was lower.
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