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 near-field with a dual-chamber box and in the far-field 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 near-field and far-field shielding which are prevalent in the literature do not accurately describe this region of cross-over. 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.