T. Rettelbach et al., THERMAL-CONDUCTIVITY OF SILICA AEROGEL POWDERS AT TEMPERATURES FROM 10 TO 275 K, Journal of non-crystalline solids, 186, 1995, pp. 278-284
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.