## 2 New EMI Shielding Materials: Biochars, Scaffolds, Rare Earth, and Ferrite-Based Materials
### Rare-Earth-Based Materials as EMI Shielding Material
A 5% Gd-doped MoS\({}_{2}\) electrode compound synthesized using cost-effective one-step hydrothermal methods can find applications in high-performance energy storage systems [96]. The importance and application of MoS\({}_{2}\)-based microwave-absorbing materials are in current social needs, including military radar stealth and civil electronic communication [97]. Flower-shaped Gd-doped FeNi\({}_{3}\) and a novel raspberry-like absorbent based on a biomimetic design with high electromagnetic wave (EMW) absorption performance for an ultra-wide bandwidth of 12.24 GHz were designed by thermal catalysis [98]. These materials were investigated in the 2-18 GHz frequency range, while EMW attenuation was assigned to dipole polarization, multiple wave scattering, and reflections. According to free electron theory, closely related to electrical conductivity, the hierarchical flower-like and NiO/rGO composites were designed using the hydrothermal method. The NiO/rGO composites with a thickness of 2 mm showed an improved reflection loss of \(-\)60 dB at 9.8 GHz in comparison with a reflection loss of \(-\)40 dB at 10.8 GHz for NiO [99]. On the other hand, Pr\({}^{3+}\)-doped Co\({}_{0.5}\)Zn\({}_{0.5}\)Bi\({}_{0.4-x}\)Pr\({}_{0.1}\)Fe\({}_{1.5+x}\)O\({}_{4}\) spinel ferrite series with different contents of Bi synthesized by the micro-emulsion method and Pr-doped (Bi\({}_{0.5}\)Na\({}_{0.5}\))\({}_{1-x}\)Pr\({}_{x}\)TiO\({}_{3}\) ceramics obtained by using the conventional solid-state method have remarkable potential for application in EMI shielding in the X-band frequency range of 8.2-12.4 GHz [100; 101]. Needymium (Nd)-doped spinel ferrites induce an increase in the polarization intensity, improving their saturation magnetization, electrical resistivity, thermal stability, and ability to absorb microwave radiation, making them suitable for advanced electronics, telecommunications, and power systems [102; 103; 104; 105]. Classes of Ce-doped W-type barium ferrites, Ba\({}_{1-x}\)Ce\({}_{x}\)Ni\({}_{2}\)Fe\({}_{15.4}\)O\({}_{27}\), with varying concentrations of Ce were successfully prepared by a sol-gel approach, whereby a Ce mole fraction of 0.3 gave the minimum reflection loss (RL) value of \(-\)52 dB, while a ferrite with Ce mole fraction of 0.4 had an RL value of \(-\)38 dB at 1.36 GHz frequency, suggesting the Ce enhancing EMW absorption of ferrites [106]. RE-metal-based MOFs have large specific surface areas. Their composites exhibit good dielectric loss, making them efficient EMW absorbers. The minimum RL value of the Ce(IV)-MOF is \(-\)32.12 dB at a frequency of 4.56 GHz at a thickness of 5.5 mm [107; 108]. Excellent EMW absorption performance and mechanisms have been unveiled for the first time for Gd\({}_{2}\)O\({}_{2}\)S/rGO composites with an absorption capacity of \(-\)65 dB and an absorption bandwidth of 5.6 GHz. Gd\({}_{2}\)O\({}_{2}\)S nanosheets with a 1 nm thickness were produced via a facile hot injection method and mixed with rGO [109]. The EMW absorption performances (minimum reflection loss, RL, effective absorption bandwidth, **EAB**, thickness, and frequency range used) of various RE-ferrite composites, RE-transition metal intermetallics, REO composites, RE-MOFs, and other RE compounds are presented in [64].