M. Letellier

Electromagnetic properties of polyurethane template-based carbon foams in Ka-band

The electromagnetic (EM) properties of polyurethane template-based reticulated carbon foams were investigated in the 26–37 GHz microwave frequency range (Ka-band). It was experimentally proved that carbon foams of a thickness of 2 mm and a density of 22–55 mg cm−3 are almost not transparent to microwave radiation, and this is especially true for the densest ones. Depending on bulk density, the EM response of carbon foams in the microwave region can be mainly accounted for by either reflection or absorption. EM shielding efficiency of more dilute samples is due to absorption mechanisms, whereas denser foams provide up to 80% reflection of EM signals. EM properties of carbon foams in the Ka-band can be accurately predicted by a very simple model based on Fresnel formulae developed in this communication.

Tannin-Based Carbon Foams for Electromagnetic Applications

Broadband dielectric analysis of tannin-based carbon foams produced at nine different densities, ranging from 0.036 to 0.114 g/cm3, was carried out in wide frequency (20 Hz-35 GHz) and temperature (25-500 K) ranges. Both dielectric constant and electrical conductivity of carbon foams in quasi-static regime, at 129 Hz, were high (more than 106 and close to 70 S/m, respectively), and increased with carbon foams density according to power laws. Significant electromagnetic interference shielding efficiency (EMI SE) was found at microwave (MW) frequencies for all produced samples. The highest value of electromagnetic attenuation at the level of 20 dB for 2-mm thick sample was observed for the foam having the highest density and the lowest mean cell size. For thicker samples (4 mm and 1 cm), EMI SE reached 39 and 73 dB, respectively. On cooling, dc conductivity decreased with temperature according to Motts law but foams still remained opaque to the MW radiation. MW performances of carbon foams reported here, along with other valuable properties such as “green” origin, low price, lightness, chemical inertness, thermal stability and high conductivity, open new routes for producing effective EMI shields from tannin-based carbonaceous porous structures.

Microwave Dielectric Properties of Tannin-Based Carbon Foams

Dielectric analysis of tannin-based carbon foams with different pore sizes, produced at different densities, was carried out in wide microwave frequency range (8 GHz – 35 GHz). Both dielectric permittivity and electrical conductivity of carbon foams at 8 GHz, were high, i.e. more than 70 and close to 10 S/m respectively, and increased with carbon foams density according to power laws. High microwave complex dielectric permittivity values of carbon foams reported here, along with other valuable properties such as “green” origin, low price, lightness, chemical inertness, thermal stability and high conductivity in static regime, open new routes for producing effective electromagnetic shields and filters from tannin-based carbonaceous porous structures.

Tannin-based carbon foams in microwave frequency range: Toward fully carbon photonic crystal

The electromagnetic (EM) properties of tannin-based rigid carbon foams were investigated in the 26-37 GHz microwave frequency range (Ka-band). It was experimentally proved that 2 mm thick carbon foams of 36-61 mg/cm3 density are almost not transparent to microwave radiation, and this is especially true for the densest foams having the smallest mean cell size. Depending on both cell size and bulk density, the electromagnetic response of carbon foams in the microwave region can be mainly accounted for by either reflection or absorption. In this communication, we demonstrate that the model based on long wave approximation for photonic crystals can be used to describe precisely the obtained experimental EM spectra, and to explain the observed features of the electromagnetic behavior.

Modelling the physical properties of glasslike carbon foams

In this work, model alveolar materials – carbon cellular and/or carbon reticulated foams – were produced in order to study and to model their physical properties. It was shown that very different morphologies could be obtained whereas the constituting vitreous carbon from which they were made remained exactly the same. Doing so, the physical properties of these foams were expected to depend neither on the composition nor on the carbonaceous texture but only on the porous structure, which could be tuned for the first time for having a constant pore size in a range of porosities, or a range of pore sizes at fixed porosity. The physical properties were then investigated through mechanical, acoustic, thermal and electromagnetic measurements. The results demonstrate the roles played by bulk density and cell size on all physical properties. Whereas some of the latter strongly depend on porosity and/or pore size, others are independent of pore size. It is expected that these results apply to many other kinds of rigid foams used in a broad range of different applications. The present results therefore open the route to their optimisation.