Radiative properties of tannin-based, glasslike, carbon foams

Abstract

This work deals with the measurement of radiative properties of lightweight rigid foams derived from tannins. Such cellular solids derived from renewable resources were investigated before and after carbonisation, the latter leading to glasslike carbon foams. Spectral quantities such as transmittance and reflectance were measured within the range of infrared wavelengths 1.5–12μm, from which optical thickness, albedo, scattering and absorption coefficients, and absorptance were deduced. Temperature dependence of emissivity of carbon foams was also derived from measurements and from calculated blackbody emissive power. Both organic and carbon foams were shown to be poor thermal conductors, whose radiative transfer properties increase when the porosity decreases, due to both bigger cells, and to thinner and to less pore walls and struts. The radiative conductivity of carbon foams was deduced from the measured overall conductivity and from a simple analytical model accounting for the conduction conductivity.