LOW-TEMPERATURE THERMAL TRANSPORT IN SILICA AEROGELS

Solid thermal conductivities of silica aerogels with densities between rho = 62 and 262 kg m-3 were measured within the temperature range T = 5 to 300 K. Instead of the usual four-point bar method, a flat plate geometry was employed. Radiative contributions could thus be reliably controlled. Determination of the spectral infrared optical absorption coefficient and use of suitable approximations for the equation of heat transfer allowed separation of the contributions from radiative transport and solid conduction. Thermal contact resistances between the specimen and the boundaries could be eliminated by a variation of the pressure load onto the specimens. Between 10 and 100 K the measured solid thermal conductivity scales with temperature according to lambda infinity T-alpha, with alpha = 1.7 +/- 0.2. Above 100 K the temperature dependence agrees well with that for non-porous SiO2 (vitreous silica). The absolute value of lambda, however, is reduced by a geometrical factor g, which strongly depends on density; g is found to be between 10(2) and 10(3) for aerogels.