Silica Glass from Aerogels

by Michel Prassas

Aerogel evolution during sintering

Aerogel to glass transformation   From Small Angle X-ray Scattering studies the structure of a silica aerogel  is viewed as a 3D network of  dense spherical nanoparticles of 2 to 4 nm in diameter (primary particles).

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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). 

Alcoxy groups are not only residuals from the hydrolysis and polycondensation reaction but also the product of an esterification  reaction 

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. 

Figure 9 shows the weight loss versus temperature in the range between room temperature and 1000°C of a typical silica aerogel 

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.

The silica aerogel structure is quite stable up to 1000°C without significant changes of its density and surface area as it is shown on the figure 10 below. 

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 heating rate. 
Consolidation  under primary vacuum is increasing the onset of sintering and the final densification temperature by 40 to 50°C. 

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. 
The aerogel was consolidated by a moving annular heating element at 1150°C. The material is constituted by two end structures; one fully dense (P=0 %) and the other fully porous (P=95 %).

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.