Thermal Transport Properties of Functionally Graded Carbon Aerogels

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 kg center dot m(-3) to 370 kg center dot m(-3) and the effective pore diameter determined via small angle X-ray scattering and SEM increase systematically from 70 nm up to 11,000 nm. One specimen was cut perpendicular to the cylinder axis into disc-shaped samples; their thermal conductivities in an argon atmosphere, as determined via a standard laser-flash technique, range from 0.06W center dot m(-1)center dot K(-1) to 0.12W center dot m(-1)center dot K(-1) at 600 A degrees 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.