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Amorphous carbon samples with a total porosity of about 85% were synthesized via pyrolysis of sol-gel derived resin precursors. Since the pores in the samples investigated have dimensions of a few tens of nanometers only, the gaseous contribution to the thermal conductivity is largely suppressed at ambient pressure. Values for the total thermal conductivity as low as 0.054 W·m −1 ·K −1 at 300 • C are detected. However, the pyrolysis temperature has a great impact on the contribution of the solid backbone to the total thermal conductivity. From the same precursor a series of samples was prepared via pyrolysis at temperatures ranging from 800 to 2500 • C. The thermal conductivity of this series of carbons at 300 • C under vacuum increases by a factor of about 8 if the pyrolysis temperature is shifted from 800 to 2500 • C. To elucidate the reason for this strong increase, the infrared radiative properties, the electrical conductivity, the macroscopic density, the microcrystallite size, the sound velocity, and the inner surface of the samples were determined. Evaluation of the experimental data yields only a negligible contribution from radiative heat transfer and electronic transport to the total thermal conductivity. The main part of the increasing thermal conductivity therefore has to be attributed to an increasing phonon mean free path in the carbons prepared at higher pyrolysis temperatures. However, the phonon mean free path does not match directly the in-plane microcrystallite size of the amorphous carbon. Rather, the in-plane microcrystallite size represents an upper limit for the phonon mean free path. Hence, the limiting factor for the heat transport via phonons has to be defects within the carbon microcrystallites which are partially cured at higher temperatures.

Novel graded carbon aerogels were synthesized to study the impact of different synthesis parameters on the material properties on a single sample and to test a new, locally resolved thermal conductivity measurement technique. Two identical cylindrical aerogels with a graded structure along the main cylindrical axis were synthesized. Along the gradient with an extension of about 20 mm the densities range from 240 to 370 kg·m -3 and the effective pore diameter determined via small angle X-ray scattering and SEM increase systematically from 70 up to 11000 nm. One specimen was cut perpendicular to the cylinder axis into disc shaped samples; their thermal conductivities in argon atmosphere as determined via standard laser flash range from 0.06 to 0.12 W·m -1 ·K -1 at 600°C. The second specimen, cut to obtain a sample with the gradient in plane, was investigated with a spatially resolved laser flash technique at ambient conditions. The results of the two different techniques are compared and discussed in detail.

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