Silica Glass from Aerogels
The surface of particles is covered by residual OH and alcoxy groups OR which presence is clearly visible on the IR spectra around 3600 cm-1 and 2900 cm-1 respectively (figure 6. Click on to zoom the picture).
between surface Si-OH groups and alcoholic vapors inside the autoclave. The Differential Thermal Analysis (DTA) curve (figure 7), indicates the presence of two well resolved exothermic peaks around 278 °C and 315°C. They are related to the oxidation of the alcoxy groups.
Figure 8 shows the evolution of IR spectrum of a silica aerogel versus temperature. It clearly shows that OR groups are not fully oxidized at 300°C. Their presence is still visible on the IR spectrum up to 650° C.
The first significant loss appears at around 278°C in agreement with DTA and concerns loss due to oxidation of alcoxy groups and their replacement by less heavy OH. This loss represents about 2 wt %. Then from 300 to 1000°C the gel lose another 3 wt % due primarily to OH condensation and water removal. The total weight loss from room temperature to 1000°C is around 5 wt %. It can changes in respect to the initial chemistry of the gel from 4 to 7 wt%. This is a considerable amount of gas which if not totally evacuated by a careful heat treatment leads to super saturation and subsequent bloating at higher temperatures.
Sintering which occurs by viscous flow
above the transition temperature of the corresponding glass, starts around
1050 °C and end at
about 1150°C. These characteristic temperatures can be shifted by 10 to 50°C depending
on the initial silica gel chemistry and the
Chlorination treatment used to fully dehydrate the aerogel before sintering can move this temperatures by more than 100°C.
In any case, the temperature at which the silica glass is obtained is almost 900°C less than the temperature used conventionally to obtain the same material by melting the quartz. The total linear shrinkage is close to 50 %
It should be stressed here that whatever the heating rate and the sintering
atmosphere, 80 % of the aerogel density change take place in a narrow
temperature window, approximately less than 50 °C. By applying an appropriate thermal gradient across
the length of the aerogel, monolithic material with a porosity gradient
from 0 to 90 % can be obtained. An extreme case where a biphasic
material (glass/porous) with a small transition porosity gradient
exist is illustrated on the picture at the left.
Heat treatment at any fixed temperature between 950 and 1150°C can be applied to partially consolidate the structure, and achieve a pre-selected porosity value. Partially densified aerogel can be used as host, with improved mechanical strength than the initial aerogels, for preparing a variety of compositions and nanocomposites by impregnation methods and subsequent treatments.