Absorption Dominated Directional Electromagnetic Interference Shielding through Asymmetry in a Multilayered Construct with an Exceptionally High Green Index

Abstract

Fabricating green electromagnetic interference (EMI) shields is the need of the hour because strong secondary reflections in the vicinity of the shield adversely affect the environment and the reliability of the neighboring devices. To this end, the present work aims to maximize the absorption-based EMI shielding through a multilayered construct comprising a porous structure (pore size less than λ/5), a highly conducting entity, and a layer to match the impedance. The elements of this construct were positioned so that the incoming electromagnetic (EM) radiation interacts with the other layers of the construct before the conducting entity. This positioning of the layers in the construct offers a high green shielding index (g s) and low reflection coefficient (R ∼ 0.1) with an exceptionally high percent absorption (up to 99%). Polyurethane (PU) foams were fabricated using the salt-leaching technique and strategically positioned with carbon nanotube (CNT) papers and polycarbonate (PC)-based films to obtain symmetric and asymmetric constructs. These structures were then employed to gain mechanistic insight into the directional dependency of shielding performance, g s, and heat dissipation ability. Interestingly, maximum total shielding effectiveness (SET) of −52 dB (88% absorption @8.2 GHz) and specific shielding effectiveness/thickness (SSEt) of −373 dB/cm2g were achieved for a symmetric construct whereas, for the asymmetric construct, the SET and SSEt were −37 dB and −280 dB/cm2g, respectively, with an exceptionally high g s of 8.6, the highest reported so far. The asymmetricity in the construct led to directional dependence of the absorption component (% SEA, shielding effectiveness due to absorption) and heat dissipation, primarily governed by the electrical and thermal conductivity gradient, respectively. This study opens new avenues in this field and reports constructs with an exceptionally high green index.

A universal approach to ‘host’ carbon nanotubes on a charge triggered ‘guest’ interpenetrating polymer network for excellent ‘green’ electromagnetic interference shielding

“…The part of the incident EM waves passing through the shield material can further be absorbed and dissipated. 37,38 The multiple internal reflections M is considered as a correction factor for thin shields in finite-dimensional media. The total contributions of these three contributing factors result in the total EMI SE (SE T ) following the equation below:SE T = SE R + SE A + SE M where SE A , SE R , and SE M refer to the shielding via absorption, reflection, and multiple reflections, respectively.…”

“…A portion of the incident EM waves is reflected from the surface of the EM shields and the non-reflected part that transmits into the shield is referred to as the transmission coefficient or transmissivity, T. The part of the incident EM waves passing through the shield material can further be absorbed and dissipated. 37,38 The multiple internal reflections M is considered as a correction factor for thin shields in finite-dimensional media. The total contributions of these three contributing factors result in the total EMI SE (SE T ) following the equation below:…”

“…However, the role of SE M is noticeable in thin metals and when used at low EM band frequencies, i.e., ∼kHz range. 37,38 In practice, the two-port network model of Vector Network Analyzer (VNA) is used to measure the scattering parameters (S 1 , S 12 , S 21 , and S 22 ) wherein the reflection R and transmission T coefficients are obtained from these scattering parameters using the following equations.…”

“…S 21 refers to the forward transmission, and S 12 refers to the reverse transmission. 37 If the shield material is uniform, S 11 = S 22 and S 21 = S 12 . 37,38 The absorption (SE A ) and (surface) reflections (SE R ) are also estimated from the measured S parameters.…”

“…37 If the shield material is uniform, S 11 = S 22 and S 21 = S 12 . 37,38 The absorption (SE A ) and (surface) reflections (SE R ) are also estimated from the measured S parameters. If SE A ≫ 10 dB, then the SE M can be neglected and there is an effective absorption A eff , as described by the following equation: 38…”