THERMAL-EXPANSION OF CARBON AND SILICA AEROGELS ABOVE ROOM-TEMPERATURE

Thermal expansion of aerogels was studied from 30 to 130 degrees C for silica aerogels and up to 1000 degrees C for carbon aerogels. Measurements were performed in air and in vacuum with different heating/cooling rates. The upper temperature limit is determined by partially irreversible shrinkage of silica aerogels due to condensation reactions and by the pyrolysis temperature range for carbon aerogels (1050-2100 degrees C). The linear thermal expansion of silica aerogels is much higher than that of vitreous silica (4 x 10(-7) K-1). In vacuum, silica aerogels had values in the range 2.0-4.0 x 10(-6) K-1, with only a slight decrease with increasing temperature. Samples containing physisorbed water exhibited values up to 6 X 10(-6) K-1 at room temperature but a much steeper decrease with increasing temperature. In carbon aerogels, thermal expansion coefficients close to 10(-6) K-1 were measured at room temperature, increasing to (3.5-8) x 10(-6) K-1 at 1000 degrees C. This temperature dependence is discussed in comparison with the thermal expansion of graphite.