## 2 New EMI Shielding Materials: Biochars, Scaffolds, Rare Earth, and Ferrite-Based Materials
### Rare-Earth-Based Materials as EMI Shielding Material
#### Synthesis, Morphology, and EMI Shielding Efficiency of RE-Based Materials  
Synthesis of Gd\({}_{3}\)Fe\({}_{3-x}\)O\({}_{4}\), x = 0, 0.02, 0.04, 0.06, 0.08, and 0.1  
A series of Gd\({}_{x}\)Fe\({}_{3-x}\)O\({}_{4}\) materials with different Gd contents, x = 0, 0.02, 0.04, 0.06, 0.08, and 0.1, was obtained by using the hydrothermal method at a temperature of T = 180 \({}^{\circ}\)C and high vapor pressure. Gd(NO\({}_{3}\))\(\cdot\)6H\({}_{2}\)O, FeCl\({}_{3}\)\(\cdot\)6H\({}_{2}\)O, and FeCl\({}_{2}\)\(\cdot\)4H\({}_{2}\)O, as precursors of Gd\({}^{3+}\), Fe\({}^{3+}\), and Fe\({}^{2+}\) ions, were dissolved in water separately and then mixed in the corresponding ratio, and the pH was adjusted up to the value of 12.8 using 10 M NaOH. The mixture was transferred to Teflon-lined stainless steel sealed containers and heated for 2 h at a temperature of 180 \({}^{\circ}\)C. After cooling, the obtained samples were washed several times with deionized water. For specific absorption rate (SAR) measurement, the obtained samples were functionalized by stirring overnight in a PEG solution [81]. The first series of CoFe\({}_{2-x}\)Dy\({}_{x}\)O\({}_{4}\) nanoparticles with a stoichiometric ratio of Co(NO\({}_{3}\))\({}_{2}\)\(\cdot\)6H\({}_{2}\)O, Fe(NO\({}_{3}\))\({}_{3}\)\(\cdot\)9H\({}_{2}\)O, and Dy(NO\({}_{3}\))\({}_{3}\)\(\cdot\)H\({}_{2}\)O precursor solutions in distilled water was synthesized by the co-precipitation approach. Oleic acid, followed by a liquid ammonia solution, was added dropwise at pH values of between 10 and 12. The obtained precipitates were washed and sintered at 500 \({}^{\circ}\)C for 2 h to form a fine powder. The nanocomposites of polypyrrole and 10 wt% CoFe\({}_{2-x}\)Dy\({}_{x}\)O\({}_{4}\) were obtained via in situ chemical oxidative polymerization, and the synthesis is presented in detail in Figure 3[110]. The obtained nanocomposites are promising candidates for applications in the 12.4 GHz to 18 GHz range, particularly in satellite communication systems, due to their impressive shielding performance, with an SE of approximately 16.3 dB at a shield thickness of 1.5 mm. Synthesis of Gd- and Er-Doped \(\alpha\)-MnO\({}_{2}\) Nanorods  
Gd- and Er-doped \(\alpha\)-MnO\({}_{2}\) with a homogenously distributed rod-shaped morphology, as can be seen in Figure 4, was prepared by a modified chemical-route-assisted hydrothermal method. Er(CH\({}_{3}\)COO)\({}_{2}\), Gd(CH\({}_{3}\)COO)\({}_{2}\), and Mn(CH\({}_{3}\)COO)\({}_{2}\), as precursors of Er, Gd, and Mn ions, were dissolved in deionized water separately, according to the stoichiometry ratio of 7 wt.% to PVA; KMnO\({}_{4}\) was used as a reaction catalyst. The PVA was dissolved in hot (T = 80 \({}^{\circ}\)C), double-distilled water with continuous stirring. When it was dissolved in water, it became transparent. When the PVA became a transparent solution and cooled down to room temperature, the solution of Er/Gd and Mn ions precursors was added to it very slowly with constant stirring, followed by the addition of KMnO\({}_{4}\). The pH value was adjusted to \(\sim\)10 by adding KOH solution drop by drop, and the mixture of solutions was left for aging. The brownish-black precipitate was centrifuged and washed several times with distilled water, put into the hydrothermal reactor, and heated at 160 \({}^{\circ}\)C for 8 h. The brownish-black precipitate was again centrifuged, washed, dried additionally at 80 \({}^{\circ}\)C for 18 h, mortared, and annealed at 450 \({}^{\circ}\)C for 2 h to acquire a fine powder of Gd- and Er-doped \(\alpha\)-MnO\({}_{2}\) samples with good crystallinity. The EMI SE (presented in Figure 5a-d,showing total absorption and reflection and skin depth variation in the frequency range of 8 to 18 GHz) achieved a maximum of \(-43\) dB at 15.3 GHz and \(-46\) dB at 15.3 GHz for thin-layer Gd- and Er-doped \(\alpha\)-MnO\({}_{2}\) at a of thickness \(\sim\)600 \(\upmu\)m, respectively.