## 2 New EMI Shielding Materials: Biochars, Scaffolds, Rare Earth, and Ferrite-Based Materials
### Biochar as EMI Shielding Material
During synthesis, Fe(III) ions promoted the graphitic  
Figure 2: Different starting materials (_Ricinus communis_ outer shell, bamboo, wood, apple, and quince stillage, olive tree pruning, eucalyptus) in biochar production, at various temperatures, usually in an oxygen-controlled atmosphere, lead to the production of porous materials with domains with graphene and amorphous carbon. zation of the amorphous carbon in biomass and contributed to biochar surface stabilization. The results of EMI SE measurements showed that the reflection component of the EMI SE of pure PMMA was near \(-10\) dB at frequencies of 7.8 and 10.1 GHz, while it was reduced by \(\sim\)60% when the content of BC-Fe in the polymer was 40 wt.% in the frequency range of 8.5-12.9 GHz. [PERSON] et al. successfully developed a mortar/brick structure using wood-derived porous carbon (WPC) as a skeleton and conductive 3D MXene aerogel [1]. The MXene aerogel/WPC composite was composed of highly ordered honeycomb cells inside WPC as a microreactor. Herein, a high graphitization level of natural wood was achieved by applying a high carbonization temperature of 1500 \({}^{\circ}\)C. The MXene aerogel/WPC composite showed an excellent EMI SE value of 71.3 dB in a frequency range of between 8.5 GHz and 12.5 GHz, while the sample density was only 0.197 g/cm\({}^{3}\). In the study, a wall-like mortar brick structure (WPC as mortar and MXene aerogel as brick) solved the instability of the MXene aerogel network as well as prolonged the transmission path of EM waves, dissipating the incident EM waves in the form of heat and electrical energy. The material's superior EMI shielding performance was achieved thanks to a specific design. The shielding efficiency of a gypsum-biochar drywall-like composite was investigated by [PERSON] et al. [43]. They used wooden chips and eucalyptus biochar and combined them with gypsum. An enhancement in the shielding efficiency was recorded with an increased biochar content in the composites. Namely, the EMI SE values of the drywall-like plates with 10%, 20%, and 40% \(w/w\) biochar contents were 11.65 \(\pm\) 1.6, 19.2 \(\pm\) 5.7, and 19.25 \(\pm\) 1.8 dB at a frequency of 6 GHz. This investigation contributes to expanding innovative bio-based sustainable materials with EM shielding properties in the microwave region. [PERSON] et al. used sewage sludge to obtain biochar and investigated its EMI-shielding features [44; 45]. The biochar was mixed with epoxy polymer (20 wt%) and cast into a film 4 mm thick. The composite showed a promising EMI SE of \(-10\) dB [44]. [PERSON] et al. reported an outstanding electrical conductivity of 300 S/m [44]. The same group compared the EMI SE properties of biochar produced from carbonized sludge with graphene nanoplatelets [46]. Herein, the authors produced polymer composites with polyvinylidene fluoride (PVDF), while fillers (biochar and graphene) were added at 90 wt%. The biochar-PVDF composite showed transmission scattering of less than \(-30\) dB. [PERSON] et al. also studied commercial biochar produced from wood biomass to coat several layers of common building components, such as drywall panels [47]. They recorded EMI SE values of 17 dB at 1 GHz and 25 dB at 18 GHz. Drywall panels coated with several layers of biochar are easy to fabricate and a low-cost solution to realize a protected surrounding for healthcare applications (chemotherapy and tomography) to minimize the intensity of the EM field close to electronic equipment. The same research group produced a cement-based composite with commercial lignin-based biochar and polyvinyl chloride (PVC) [48]. They introduced biochar powder into cement paste to improve its shielding properties. PVC, which was used as a filler, was obtained by decommissioning old electrical cables that would have ended up in a landfill. A combination of 10 wt.% biochar and 6 wt.% PVC revealed the best shielding capacity, around 16 dB in the 5.4-8 GHz frequency range.