THERMAL-CONDUCTIVITY OF SILICA AEROGEL POWDERS AT TEMPERATURES FROM 10 TO 275 K

Thermal conductivities of physical mixtures of evacuated silica aeroge l powders mixed with carbon black were measured as a function of tempe rature and external load. The measured thermal conductivity results fr om solid conduction and infrared radiative transport. The latter was q uantified with infrared-optical transmission and reflection measuremen ts in the wavelength range 2.3-200 mu m and by solving the radiative h eat transfer equation. Radiative contributions to the total thermal co nductivity are found to be negligible at temperatures < 100 K. The der ived contributions of solid thermal conduction via grain-to-grain cont acts depend on the external load and on the load history. The temperat ure dependence of the solid thermal conductivity of the aerogel powder is found to be the same as for monolithic silica aerogels in the meas ured temperature range. The temperature independent reduction factor, g' = lambda(mono)/lambda(pow) is between 3 and 10, depending on the po wder type and external pressure load. The quantity g = lambda(vs)/lamb da(pow), which compares the measured thermal conductivity of aerogel p owders with that of vitreous silica, is temperature-independent only f or temperatures > 50 K and is of the order of 500-1500. For lower temp eratures it increases strongly due to the limitation of the phonon mea n free path caused by the nanoporosity of the aerogel. Silica aerogel powders provide a thermal conductivity considerably lower than that of perlite, which is often used for low-temperature thermal insulation.