The total thermal conductivity lambda of resorcinol-formaldehyde aerogel monoliths has been measured as a function of density rho in the range from rho = 80 to 300 kilograms per cubic meter. A record-low conductivity value in air at 300 K of lambda approximately 0.012 watt per meter per kelvin was found for rho approximately 157 kilograms per cubic meter. Caloric measurements under variation of gas pressure as well as spectral infrared transmission measurements allowed the determination of solid conductivity, gaseous conductivity, and radiative conductivity as a function of density. The development of such low conductivity materials is of great interest with respect to the substitution of environmentally harmful insulating foams made from chlorofluorocarbons.

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The thermal conductivity of electrochemically etched porous silicon (PS) layers was determined over a wide temperature range (T = 35 - 320 K) using the dynamic technique. Both the doping level of the silicon wafers (p and ) and the porosity P of the porous layers (P = 64 - 89%) were varied. The measured thermal conductivities were three to five orders of magnitude smaller than the values for bulk silicon. Furthermore, they increase with increasing the wafer doping level and with decreasing the porosity P of the layers. For all investigated PS layers the thermal conductivity increases with temperature. The results are discussed in terms of a simple model for heat conduction in PS based on the phonon diffusion model.

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Ultrasonic and static compression experiments were performed in the frequency range 50 kHz to 1 MHz in order to investigate the mechanical behaviour of silica aerogels as a function of internal gas pressure pG and external stress Pext. The measurement of longitudinal and transverse sound velocities allows the Young's modulus and the Poisson's ratio for aerogels of different densities to be determined upon variation of pG and Pext. For low-density aerogels ( rho approximately=100 kgm-3) the authors found a decrease in sound velocity upon evacuation. Surprisingly, the sound velocity decreases upon uniaxial compression with small loads. The most important finding from the static compression experiments is that low-density aerogels display creeping with a time constant of about 45 min.

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The thermal and electrical conductivity of monolithic carbon aerogels was investigated at room temperature. Results showed both the solid thermal conductivity and the electrical conductivity scale with the density in the range between 60 and 650 kg m−3. The scaling exponents for the two conductivities have identical values of 1.5. For a density of 82 kg m−3 a thermal conductivity of 0.029 W m−1 K−1 in air and 0.018 W m−1 K−1 after evacuation was found.