A Nonisothermal Study of the Kinetics of the Nanoporosity Elimination in Sonogels-derived Silica Xerogels

Abstract

A nonisothermal study of the kinetics of the nanoporosity elimination in monolithic silica xerogels, prepared from acid and ultrasound catalyzed hydrolysis of tetraethylortosilicate (TEOS), has been carried out by means of in situ linear shrinkage measurements performed with different heating rates. The study could be applied up to almost alpha approximately 0.6 of the volume fraction alpha of eliminated pores. The activation energy was found increasing from about 3.2 x 10(2) kJ/mol for alpha approximately 0.06 up to about 4.4 x 10(2) kJ/mol for alpha approximately 0.44. The sintering process accompanying the nanopore elimination in this set of xerogels is in agreement with a viscous flux sintering process with the hydroxyl content diminishing with the volume fraction of eliminated pores. All the volume fraction of eliminated pores versus temperature (T) curves can be matched onto a unique curve with an appropriate rescaling of the T axis, independent of the heating rate. This scaling property suggests that the path of sintering seems the same, regardless of the heating rate; the difference is that the rate is faster at higher temperature.

Structural evolution up to 1100°C of xerogels prepared from TEOS sonohydrolysis and liquid phase exchanged by acetone

“…The sharp diminution of the bulk density together with the rapid thickness shrinkage above 800°C (Fig. 4(a) and (b)) is due to the setting of a viscous flux sintering process in which the hydroxyl group content diminishes with the fraction of eliminated pores [20]. A non-isothermal kinetic study of the rapid pore elimination at high temperature has been carried out for nonexchanged xerogels [20].…”

“…4(a) and (b)) is due to the setting of a viscous flux sintering process in which the hydroxyl group content diminishes with the fraction of eliminated pores [20]. A non-isothermal kinetic study of the rapid pore elimination at high temperature has been carried out for nonexchanged xerogels [20]. It was suggested there is a scaling property between temperature and fraction of eliminated pores so the path of sintering seems the same, regardless of the heating rate [20].…”

“…A non-isothermal kinetic study of the rapid pore elimination at high temperature has been carried out for nonexchanged xerogels [20]. It was suggested there is a scaling property between temperature and fraction of eliminated pores so the path of sintering seems the same, regardless of the heating rate [20]. Fig.…”

“…In Fig. 6 (DL), we also included a DL test carried out under a close heating rate (2.0°C/min) on an analogous cylindrical monolithic non-exchanged xerogel, also previously hold for 10 h at 800°C [20]. Clearly, the rapid pore elimination process starts at a high temperature in the case of acetone-exchanged xerogels.…”

“…DTA and TG 0 were carried out under a heating rate of 10°C/min. The DL test was carried out under a heating rate of 2.5°C/min in a cylindrical monolithic acetone-exchanged sample previously hold at 800°C for 10 h. A DL test with a close heating rate of 2.0°C/min, carried out in an analogous cylindrical monolithic non-exchanged sample also previously hold at 800°C for 10 h (after[20]), is drawn in the figure for direct comparison.…”