Electromagnetic properties of model vitreous carbon foams

Abstract

This paper addresses the relationship between structural and electromagnetic (EM) properties of model vitreous carbon foams, i.e., presenting different porous structures in terms of bulk density, cell size and connectivity, while having the same composition and the same carbon texture. EM properties were investigated over a wide frequency range, from 20 Hz up to 250 THz. The bulk density is the main parameter controlling the EM behaviour up to ∼50 GHz, as no change was found by varying other structural parameters such as cell size or interconnectivity in such frequency range. At low frequency, foams behave similarly to metals and, when the density increases, the reflection increases and the absorption decreases. The behaviour changes above ∼50 GHz, absorption becoming the main mechanism. For cellular foams, transmission and reflection tend to be negligible in the infrared region, and behave like black bodies. However, reticulated foams present non-negligible transmission that increases with cell size. Resonance phenomena were observed for reticulated foams between about 0.2 and 3 THz. A simple model considering the fundamental mode TE10 of a rectangular waveguide whose largest dimension was the average cell diameter was proposed to predict minima and maxima of these resonances.