| Popis: |
The experimental results on the physical properties such as monolithicity, optical transmission, thermal conductivity and porosity of silica aerogels processed by three different precursors: (a) tetraethoxysilane (TEOS) (b) polyethoxydisiloxane (PEDS) (c) tetramethoxysilane (TMOS) are reported. The aerogels have been prepared by sol–gel polymerization of the parent solvent diluted precursor in the presence of a catalyst, followed by supercritical CO2 solvent exchange and drying. It has been found that the monolithicity of the aerogels is strongly dependent on the type of catalyst used for each precursor. For TEOS and PEDS precursors, acid catalysts and for TMOS precursor base catalysts resulted in monolithic aerogels. It has been found that the optical transmission at 900 nm for 1 cm thick sample of the TMOS and PEDS precursor aerogels are the highest (>92%) and far lower (∼70%) for the TEOS precursor aerogels. The thermal conductivities of the PEDS and TMOS aerogels have been found to be lower (0.015 and 0.020 W m−1 K−1, respectively) compared to the TEOS (0.060 W m−1 K−1) aerogels. The pore sizes obtained from the N2 adsorption measurements varied from 30 to 180 nm, 60 to 190 nm and 80 to 200 nm in the TEOS, TMOS and PEDS precursor aerogels, respectively. The scanning electron microscopy studies of the aerogels indicated that the PEDS and TMOS aerogels show narrow and uniform pores while particles of the SiO2 network are very small. On the other hand, TEOS aerogels show non-uniform pores such that the number of smaller size pores are less compared to the pores of larger size while SiO2 particles of the network are larger compared to both the PEDS and TMOS aerogels. Hence, the surface area of the aerogels prepared using TEOS precursor has been found to be the lowest (∼800 m2 g−1) compared to the PEDS (∼1100 m2 g−1) and TMOS (∼1000 m2 g−1) aerogels. |
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