High frequency electromagnetic interference shielding response of mixtures and multilayer films based on conducting polymers

The electromagnetic interference (EMI) shielding efficiency (SE) is measured for conductive polyaniline and its mixtures containing conducting powders. The frequency ranges of the EMI SE is 10 MHz-1 GHz by using the ASTM D4935-89 technique. The EMI SE of hydrochloric acid doped polyaniline containing silver (Ag) powder is similar to 46 dB at room temperature, which indicates that the materials can be commercially applied to shielding against electromagnetic (EM) radiation. The theoretical SE is calculated by using measured dc conductivity, which is compared to the experimental data. We observe that the SE increases with increasing dc conductivity. A model to account for the increase of the SE through the chemical doping and mixing process is presented. The theory for the calculation of the EMI SE of multilayer films based on conducting polymers is discussed in terms of boundary conditions for electric and magnetic fields of EM plane waves. The high frequency (greater than or equal to 300 MHz) EMI shielding response of the monolayer and multilayer films with the same thickness is theoretically compared. The frequency dependence of SE of monolayer films shows a constant behavior at low frequency and an oscillating behavior with a single period at high frequency. The SE of multilayer films, consisting of the high H and low L conducting layers with the same thickness as that of monolayer films in total, shows a constant behavior at low frequency (less than or equal to 10 GHz) and an oscillating behavior with two major periods at high frequency (> 10 GHz). The SE of H-L-H layers is smaller than that of H-H-H layers at low frequency. However, there exist frequency ranges where the SE of H-L-H layers is larger than that of H-H-H layers (for example, from 35-70 GHz). The result suggests that the coherent multiple reflections at the internal interfaces of H-L-H layers contribute to an increase of SE. We observe that the effect of coherent multiple reflections decreases as the thickness of the layer increases. (C) 2000 American Institute of Physics. [S0021-8979(00)08013-0].