## 5 Solid vs.Porous EMI Shielding Materials  
Electromagnetic shielding is directly correlated with the electrical properties of the nanocomposites. Several conducting nanoinclusions have been incorporated into polymer matrices, such as carbon nanotubes (CNTs), carbon nanofibers (CNFs), graphene, MXenes, magnetic NPs and ferrites, to fabricate electrically conductive polymer nanocomposites for EMI shielding and microwave absorption. The formation of a long-order interconnectivity of these nanof fillers within the polymer matrix is of paramount importance to achieve the desired electrical conductivity and EMI shielding performance. In polymer nanocomposites, at a particular filler concentration there exists a critical point where a conductive filler network is formed, and the DC conductivity substantially increases. This point at which a nanocomposite becomes capable of conducting direct current is termed the percolation threshold. It is also well-established that below this percolation threshold, polymer composites act as mere insulators. As per the classical percolation theory, the relationship between the DC conductivity of a polymer/filler composite (\(\sigma_{\text{dc}}\)) and the volume fraction of the conducting filler (v\({}_{\text{f}}\)) can be expressed as follows [98]:  
\[\sigma_{\text{DC}}=\sigma_{0}\Big{(}\varnothing_{f}-\varnothing_{c}\Big{)}^{ \text{t}} \tag{14}\]  
where \(\sigma_{0}\) = conductivity of the filler, \(\varnothing_{f}\) = volume fraction of the filler, \(\varnothing_{c}\) = percolation threshold and t = critical exponent. This equation is valid for concentrations higher than the percolation threshold, i.e., \(\varnothing_{f}>\varnothing_{c}\), and the critical exponent value depends on the dimension and aspect ratio of the filler used in the system. In this region, the overall electrical conductivity is enhanced due to possible charge transport mechanisms, viz. conduction, tunneling or hopping. Conduction occurs when there is physical contact between neighboring fillers so that electrons can be transferred from one filler to another. However, for hopping and tunneling mechanisms, physical contact is not required for charge transport. Several strategies have been adopted to reduce the percolation threshold and achieve higher conductivity at extremely low filler concentrations. Fabricating porous polymer nanocomposites such as foams/aerogels is the mostly widely used technique to reduce the percolation threshold and enhance microwave absorption. A schematic representation of how the percolation threshold varies in foam and bulk polymer nanocomposites is depicted in Figure 4. Foamed polymer nanocomposites showcase similar electrical conductivity at lower volume fractions of filler (\(\phi_{\text{foam}}<\phi_{\text{Bulk}}\)) when compared to their bulk counterparts. In this context, [PERSON] et al. [95] fabricated lightweight and porous polyetherimide (PEI)/graphene composite foams and examined the dependence of electrical conductivity and microwave absorption with solid nanocomposites. Interestingly, it was observed that via the foaming process, the percolation threshold significantly reduced from 0.21 to 0.18 vol.% for solid and foam nanocomposites, respectively. It is also worth mentioning that the percentage of microwave absorption in foam nanocomposites was found to be 8.1-14.4% more than in the solid shields [95]. [PERSON] et al. [99] performed extensive studies to understand the effect of foaming on the electrical conductivity and EMI shielding performance of HDPE-graphene nanoplatelet (GnP) composite foams. These composite foams were fabricated by physical foaming using the injection molding process. The degree of foaming greatly affected the electrical conductivity of these nanocomposites. The percolation threshold drastically decreased from 19 to 9.1 vol.% GnP at a 7% degree of foaming and due to the volume exclusion effect induced by the gas phase.