## 4 Tuning Microwave Absorption for Superior EMI Shielding in Porous Materials  
For any EMI shielding material to absorb excess EM radiation impinging on its surface, one should keep the dielectric constant equal to one or close to that of air. The reflection of EM radiation is primarily due to the impedance mismatch between the wave impedances of the signal propagating in air and those of the absorber material. A facile approach for this impedance matching is fabricating porous architectures of conductive polymer nanocomposites. The relative volume of air in these porous polymer nanocomposites is indeed very high, and thus facilitates better impedance matching at the shield/air interface. Nevertheless, the presence of air bubbles in the cells can reflect and scatter EM radiation and make it difficult to escape from the porous structure, and it eventually becomes absorbed and transferred as heat energy, as shown in Figure 3. The EMI SE value of these porous nanocomposites is mostly absorption-dominant due to this phenomenon. In addition, porous structures create multiple filler-polymer interfaces, where large amounts of charges would accumulate, thereby increasing the interfacial/space charge polarization and thus the EM attenuating capacity [95]. \begin{table}
\begin{tabular}{c c c c} \hline
**EMI Shielding Effectiveness (dB)** & **Shielding Efficiency (\%)** & **Reflection Loss (\(\mathbf{R_{L}}\))** & **Microwave Absorption (\%)** \\ \hline
0 & 0 & 0 & 0 \\ \(-10\) & 90 & \(-10\) & 90 \\ \(-20\) & 99 & \(-20\) & 99 \\ \(-30\) & 99.9 & \(-30\) & 99.9 \\ \(-40\) & 99.99 & \(-40\) & 99.99 \\ \(-50\) & 99.999 & \(-50\) & 99.999 \\ \(-60\) & 99.9999 & \(-60\) & 99.9999 \\ \(-70\) & 99.99999 & \(-70\) & 99.99999 \\ \(-80\) & 99.999999 & \(-80\) & 99.999999 \\ \(-90\) & 99.9999999 & \(-90\) & 99.9999999 \\ \(-92\) & 99.99999994 & \(-92\) & 99.99999994 \\ \hline \end{tabular}
\end{table} 
Table 1: The correlation between the EMI shielding effectiveness value (dB) with shielding efficiency (%) and the reflection loss (\(\mathbf{R_{L}}\)) value with microwave absorption (%). Reprinted/adapted with permission from Ref [90]. Copyright 2016, The American Association for the Advancement of Science. Figure 3: Schematic representation of microwave propagation and scattering across PEI/graphene nanocomposite foam. Reproduced with permission from [95]. Copyright 2013, American Chemical Society. In general, porous materials are usually considered to be composites that are composed of solid and air components, where the Maxwell Garnett (MG) theory is an emblematic mechanism for understanding the variation of the effective permittivity. As per the MG theory, the effective permittivity in any porous material can be expressed as in the equation below [96]:  
\[\varepsilon_{\text{eff}}^{\text{MG}}=\varepsilon_{1}\bigg{[}\frac{(\varepsilon_ {2}+2\varepsilon_{1})+2\text{f}(\varepsilon_{2}-\varepsilon_{1})}{( \varepsilon_{2}+2\varepsilon_{1})-\text{f}(\varepsilon_{2}+\varepsilon_{1})} \bigg{]} \tag{11}\]  
where \(\varepsilon_{1}\) = permittivity of the solid component, \(\varepsilon_{2}\) = permittivity of air and f = volume fraction of air. The permittivity of air is 1, and the solid component possesses a value higher than 1. From the above equation, one can deduce that the effective permittivity decreases with the increasing pore volume or void content. This will now facilitate towards better impedance matching and also promote incoming EM waves to enter into the absorber more easily. The presence of a porous architecture will therefore reduce complex permittivity. In general, EM attenuation capacity and good impedance matching are the fundamental design principles for high-performance microwave absorption.