## 2 New EMI Shielding Materials: Biochars, Scaffolds, Rare Earth, and Ferrite-Based Materials
### Rare-Earth-Based Materials as EMI Shielding Material
The effects of the 4f-3d interaction on electronic locality, magnetism, and charge migration during hybridization between Er and Fe/Co were studied [22; 76]. Due to enhanced EMI shielding properties, La, Ce, Pr, Gd texaphyrin, Tb, and Er were studied and selected as potential new shielding materials, as well as Co-Mg-La ferrite/graphene composites [77; 78; 79]. Gd3+-doped MnFe2O4 synthesized by the sol-gel method expressing very low dielectric loss, saturation, and minimum coercivity can be a potential material with applications in storage and microwave absorption devices [80]. Gd-doped Fe3O4 oxide (GdxFe3-xO4) samples with different percentages of Gd contents were prepared by the hydrothermal method [81]. The substitution of smaller Fe ions with RE ions leads to significant changes in physical properties such as the structural distortion of crystal units, the formation of a new phase, and changes in the morphological, optical, electrical, and dielectric properties due to the different ionic radii between ions. The partial substitution of RE ions, such as with Sm, Ce, Er, Dy, La, and Nd for Fe3+, leads to a structural bend in the spinel structure which induces strain and considerably modifies the electrical and dielectric properties [82]. All ferrites exhibit a cubic spinel structure with the Fd-3m space group. The ionic radius of RE3+ ions in this environment (Gd3+ = 0.94 A, Eu3+ = 0.95 A, Sm3+ = 0.96 A, Nd3+ = 0.96 A, Pr3+ = 0.99 A) is larger than that of Fe2+ ions (0.65 A) [83]. The substitution of Fe2+ by RE3+ ions limits solubility in the lattice and inhibits grain growth. The shifting of the positions of diffraction peaks to smaller two-theta angles with an increase in the RE3+ concentration results from the increased lattice constant [84; 85; 86]. RE substitution has been reported to immensely affect cobalt ferrite's structural, morphological, and physical properties [87; 88]. It induces strain, causes the distortion of the crystal lattice, promotes the creation of vacancies, reduces crystallite sizes, increases grain grain boundary interfaces, and promotes nanoscale magnetic effects such as spin canting and the core-shell effect. The insertion of RE in place of Fe promotes 3d-4f coupling, leading to variation in the magnetic, dielectric, and conduction mechanisms, and an enhancement in the SE can be obtained. RE substitution can also decrease the grain size, which is an important factor in low-noise media. [PERSON] et al. obtained an increase in the polar Kerr rotation for Er- and Tm-doped cobalt ferrite films, while no significant changes in the magneto-optical response were observed for the Ho, Yb, and Lu ion substitutions [89]. For example, different Gd\({}^{3+}\) doping concentrations in the ferrite structure led to noticeable variations in the magnetic saturation values. This can be explained by the finite size effects of nanoparticles, indirect interactions of Fe-Gd and Co-Gd and relatively weaker Fe-Fe interactions, or surface dipole disorder induced by lattice distortions upon dopant substitution with different ionic radii [90]. These interactions can significantly influence the electromagnetic properties of spinel ferrites [91]. A low-density polyethylene/MWCNT/graphene/LaFe\({}_{2}\)O\({}_{3}\) composite and La\({}^{3+}\) ions substituted into CoLa\({}_{3}\)Fe\({}_{2-x}\)O\({}_{4}\) and NiLa\({}_{x}\)Fe\({}_{2-x}\)O\({}_{4}\) resulted in enhanced microwave absorption at an X-band frequency range of 8-12 GHz [92; 93; 94]. The nanocomposite Gd-doped MoS\({}_{2}\)/reduced graphene oxide Gd-MoS\({}_{2}\)/(rGO) for high-performance EMI shielding applications was prepared using a hydrothermal method with a varying percentage of Gd doping. The MoS\({}_{2}\)/rGO nanocomposite without dopant showed a low total SE, -16.18 dB, and the one with 20% Gd-doped MoS\({}_{2}\)/rGO nanocomposite showed a higher total SE, -20.47 dB, in the frequency range of 8.0-12.0 GHz due to enhanced electrical conductivity, defect dipole polarization, and interfacial polarization [95].