Influence of the sorbed atmospheric humidity on the microwave conductivity of a composite material: potential applications in EMI shielding and absorbers

An experiment to extract water from wet air has been realized with a particular dielectric composite material (CM). The experimental results indicate that the typical S-shaped isotherms of CM are analogous to the data obtained by adsorption of water vapour on various mineral adsorbents, showing the fundamental nature of the phenomena involved. We have found that for 80% relative humidity ratio, for example, and for 1 g of CM, the mass of sorbed water at thermodynamic equilibrium is 2 g. To characterize the CM by an electrical method, the microwave conductivity is measured at 8.62 GHz as a function of the mass of water sorbed from atmospheric humidity, using the cavity perturbation technique applicable to thin films in a cylindrical cavity tuned on the TM210 magnetic mode. The experimental results show that microwave conductivity presents a maximum value at the formation of the first layer of monomolecular sorbed water on the surface of CM (BET mass). On the other hand, the microwave reflectivity (MR) is examined against mass of sorbed water. The CM has MR less than or equal to -28 dB (4% of reflection) at the BET mass. The conductive CM has the potential for lightweight shielding in an environment containing high-speed electronics circuits.