ABSTRACT
We measured the total thermal conductivity λ of four resorcinol-formalde-hyde (RF-) aerogel tiles in a guarded hot plate apparatus and a hot-wire device. The temperature was varied between 20 and 80°C, the gas pressure (air) from 1000 mbar down to 1 • 10−4 mbar.
All samples with bulk densities of ϱ = 158, 180, 205 and 236 kg/m 3 have been prepared using the same molar ratio of resorcinol to catalyst (R/C = 200).
The measured thermal conductivities are between 5 and 8 • 10−3 Wm −l K −l in the evacuated state and in the range of 11 to 13 • 10−3 Wm −l K −1 in air at room temperature. The thermal conductivity data derived from the hot plate and the hot-wire device agree within 10−3 Wm −l K −l or 10 to 20 % of the absolute value, respectively.
From the thermal conductivity measurements as a function of air pressure a typical pore size in the aerogels between 20 nm for ϱ = 236 kg/m 3 and 30 nm for ϱ = 158 kg/m 3 was derived.
From additional infrared-optical transmission and reflection measurements the mass specific extinction e and the temperature dependent radiative conductivity could be calculated. The solid conductivity was separated by subtracting the infrared-optically derived radiative conductivity from the total conductivity of the evacuated samples. The solid thermal conductivity was found to increase by about 20 % in the temperature range from 20°C to 80°C.
The total thermal conductivity in air as a function of the aerogel density shows a broad minimum. The optimal densitity for minimized thermal conductivity at room temperature was found to be about 180kg/m 3. To our knowledge the measured conductivities of the resorcinol-formaldehyde aerogels are within the lowest thermal conductivities ever measured for any solid body in air.
