Synthesis of an alumina enriched Al2O3-SiO2 aerogel: Reinforcement and ambient pressure drying

- Al2O3-SiO2 aerogel can be obtained only by reinforce with tetraethoxysilane.
- Al2O3-SiO2 aerogel was synthesized by ambient pressure drying.
- The apparent density and surface area were 0.35 g/cm3 and 481.79 m2·g, respectively.
- The specific surface area of aerogel maintains 360.01 m2·g after 1000 °C treatment.
- The alcogel skeleton was strengthened by newly formed AlOSiOSiOAl chains.

An alumina enriched Al2O3-SiO2 aerogel was successfully synthesized using an inexpensive inorganic aluminum salt Al(H2O)9(NO3)3 as the precursor and tetraethoxysilane (TEOS) as the framework reinforcing agent via a sol-gel process followed by ambient pressure drying. In order to show the enhanced effect of TEOS on aerogel, its structure and properties were detected by FESEM, TEM and BET. Furthermore, FTIR and Raman spectra were collected to explain the enhancement mechanism of TEOS to Al2O3-SiO2 aerogel at the molecular level. The FTIR spectra showed that some AlOSi and AlOSiOSiOAl bonds gradually formed during the reinforcement process. These long AlOSiOSiOAl chains supported and strengthened the alcogel skeleton and effectively avoided potential shrinkage and cracking during the ambient pressure drying process. In addition, Raman spectra supported this conclusion by verifying that AlOSi and SiOSi bonds were present. Moreover, the Raman spectra indicated that with the increase of the Si/Al mole ratio from 0 to 3.0, the primary gel skeleton of AlOAl bonds was gradually transformed to SiOSi bonds. When the Si/Al mole ratio was greater 3.0, SiOSi bonds became the primary bonds in the final aerogel.

Reinforcing process was the key step to successfully prepare the alumina riched Al2O3-SiO2 aerogels. And without this process, only some dried gels been obtained after the ambient pressure drying. Actually, with reinforced by teraethoxysilane, hydroxyls of the alumina's skeleton were almost fully eliminated and some silicon alkyls were introduced, leading to the form of SiOAl and SiOSi bonds. Moreover, these SiOAl and SiOSi bonds also would react with each other, and some AlOSiOSiOAl bonds were formed, which supported the aerogels' structure and ensured the aerogel obtained after the ambient pressure drying process.109