Design of lossy dielectric polymer nanocomposite alternating A-B multilayers for absorption-dominated EMI shielding in the X-band regime

The role of permittivity mismatch and of individual layer thickness on the electromagnetic interference (EMI) shielding performance of lossy dielectric polymer nanocomposite multilayer shields is explored via a combined theoretical and experimental approach. The A-B multilayer shields comprise of two or more alternating layers of low and high permittivity facilitated by alternating the filler concentration. A parametric study based on a transfer matrix model is conducted making use of permittivity measurement data on nanocomposites of poly(methyl methacryclate) (PMMA) with carbon nanotubes (CNTs). Theoretical insights are confirmed via an experimental and numerical finite element case study on PMMA-CNT A-B multilayer stacks in a waveguide. The case study highlights the trade-off between absorption-based shielding and high shielding effectiveness. For a given total shield thickness, the shielding performance becomes independent of the number of layers when the individual layer thickness is less than the skin depth of the composite material in the high permittivity layer. Moreover, to obtain absorption-based shielding, less but thicker layers, like a bilayer, can be advantageous. For instance, we demonstrate that a 4 mm thick bilayer with a 1 wt% to 7 wt% CNT concentration mismatch exhibits absorption-based shielding with a shielding effectiveness close to 40 dB within the X-band frequency regime.