ISOTHERMAL SINTERING OF SIO2-AEROGELS

Isothermal sintering has been studied for two base-catalyzed SiO2-aerogels with initial densities ρ0 = 122 kg/m3 and 256 kg/m3, covering a temperature range from 750 to 950 °C and time periods up to 200 h; the highest density achieved was 339 kg/m3. The change in density with time at a given temperature is compared with predictions of sintering theories. The elastic constants determined by ultrasonic measurements reveal scaling with the density achieved upon sintering. Microscopic structural changes during sintering have been studied by means of small angle X-ray scattering (SAXS). Characteristic structural parameters are the mean diameter of the particles, i.e. the building blocks of the gel network, the mean size of the macropores within the tenuous structure and the specific surface area. The major finding of the investigations is the observation of two processes with different time scales: a fast reduction of the size of the macropores accompanied by a slow decrease in specific surface area as well as a slight increase in particle diameter. For both samples investigated all parameters can be unequivocally correlated with the achieved density upon sintering - independent of the initial density. © 1990.