## 2 New EMI Shielding Materials: Biochars, Scaffolds, Rare Earth, and Ferrite-Based Materials
### Iron Oxide-Based Materials as EMI Shielding Material
For EMI shielding applications, iron oxides are often combined with other materials, such as various polymers, graphene oxide, carbon nanotubes, or similar, to achieve desirable EMI shielding, mechanical strength, and electrical and thermal conductivity. Using an in situ polymerization technique, [PERSON] et al. created iron oxide and polypyrrole nanocomposites and examined their EMI shielding properties in the 0.1-18 GHz frequency range [117]. The composites' intimate contact between the conducting and magnetic phases increased absorption by 10.10 dB at the instrument's highest frequency limit (17-18 GHz), while iron oxide nanoparticles only increased the absorption by 2.6 dB. As the possible cause of this improvement, they proposed a better match between dielectric loss, magnetic loss, and improved dispersion of the magnetic/conductive nanocomposites in the matrix. [PERSON] et al. [118] reported that the microwave shielding properties were affected by the various morphologies of iron oxides. Using a two-step sol-gel process, they produced various ferrite structures, including cubes, rods, and flakes, covered with multilayer rGO. Their EMI shielding capabilities were evaluated in the Ku-band frequency range. In comparison to the flake- and cube-shaped iron oxides, the rod-shaped iron oxide covered with rGO sheets had the highest shielding efficiency value of \(\sim\)33.30 dB (\(>\)99.9% attenuation). This resulted from the combined effect of magnetic and dielectric losses. Anisotropy energy in the composites, eddy current effects, and natural resonances were the sources of magnetic loss. The nanoferrite particle content of the composite was the main cause of eddy currents in the microwave ranges. The surface anisotropic field would result in higher anisotropy energy for the small materials due to the small-size effect. The higher anisotropy energy also contributed to greater microwave absorption. Moreover, the magnetic iron oxide was coated with an rGO layer, which increased the interfaces and surface polarization charges. Interfacial polarization is a significant polarization process, and the corresponding relaxation will lead to a loss mechanism. One of the main causes of dielectric loss could be interfacial polarization. It is well known that some heating-induced absorption losses are caused by the interaction of surface-formed molecular dipoles with the microwave field. [PERSON] et al. created a conducting ferrimagnetic PANI nanocomposite implanted with titanium dioxide (70-90 nm) and \(\gamma\)-Fe\({}_{2}\)O\({}_{3}\) (9-12 nm) nanoparticles using microemulsion polymerization [119]. They discovered that the high shielding effectiveness value of \(-45\) dB owing to absorption (SE\({}_{\rm A}\)) was caused by dielectric and magnetic losses that resulted from the combined action of \(\gamma\)-Fe\({}_{2}\)O\({}_{3}\) and TiO\({}_{2}\). In contrast, the SE\({}_{\rm A}\) of PANITiO\({}_{2}\) was around 22.4 dB, but that of PANI-\(\gamma\)-Fe\({}_{2}\)O\({}_{3}\) was approximately 8.8 dB. In another work, PANI tubes made of rGO coated with \(\gamma\)-Fe\({}_{2}\)O\({}_{3}\) nanoparticles were synthesized and characterized by [PERSON] et al. [120]. The intercalated iron oxide nanoparticles were produced by thermally breaking down ferric acetyl acetone in a reducing atmosphere. The \(\beta\)-naphthalene sulphonic acid-induced oxidative polymerization of aniline, which produced the core-shell shape, was also used to enclose those nanoparticles. At a thickness of 2.5 mm, the presence of rGO-\(\gamma\)-Fe\({}_{2}\)O\({}_{3}\) in the PANI core structures increased the composite's interfacial polarization and effective anisotropy energy, which increased scattering and produced a high shielding efficiency of about 51 dB. Iron oxides are frequently combined with different carbon nanomaterials. The composite material can effectively absorb, reflect, and dissipate electromagnetic radiation over a wider frequency range by fusing the electrical conductivity of carbon nanotubes with the magnetic qualities of iron oxides.