EMI Shielding Measurements of Conductive Polymer Blends

Shielding efficiencies for a number of blends containing a variety of conductive fillers including intrinsically conductive polymers have been measured in the nearfield with a dual-chamber box and in the farfield with a transmission line fixture. Although all samples tested satisfied the classical good-conductor approximation, most of them exhibited a crossover from being electrically thin (thickness < skin depth) to being electrically thick (thickness > skin depth) over the frequency range of interest, 1 MHz to 3 GHz. The theoretical relations for both nearfield and farfield shielding which are prevalent in the literature do not accurately describe this region of crossover. We have derived expressions which describe the behavior accurately over the entire range of interest. Far-field shielding efficiencies as high as 70 dB at 1 GHz were measured for purely organic composites consisting of an intrinsically conductive polymer, polyaniline, dispersed in a thermoplastic matrix.