## 2 New EMI Shielding Materials: Biochars, Scaffolds, Rare Earth, and Ferrite-Based Materials
### Biochar as EMI Shielding Material
This investigation shows great importance in the EMI shielding application and from the aspect of the circular economy. [PERSON] et al. fabricated a conductive EBC@CNF@MWCNT composite aerogel by the freeze-drying process, mixing ferrite chloride with electrically conductive bamboo charcoal (EBC), with cellulose nanofibrils (CNFs) as a skeleton and multi-walled carbon nanotubes (MWCNTs) as a conductive enhancer in the freeze-drying process [36]. Afterward, the aerogel was soaked with polydimethylsiloxane and hot-pressed into the membrane. In the composite, EBC and MWCNTs were arranged uniformly in the CNF skeleton to form a 3 Dconducting network. The composite displayed exceptional electrical conductivity (47 S/m) and a high EMI shielding effectiveness of 39.5 dB, with an adsorption loss of \(\sim\)75%. Recently, [PERSON] et al. used biochar prepared from olive tree pruning to fabricate composite samples with carbon black and polytetrafluoroethylene as binders [49]. The EMI shielding capacity was measured in the 1-3 GHz frequency range. The results demonstrated that the raw pure biochar had a low EMI SE, between 1.5 and 4 dB, which was enhanced with an increase in the thickness from 0.1 to 0.5 mm. On the contrary, the composite significantly improved the EMI SE, reaching a value of 39 dB. This work indicates that biochar could be used as a basis for developing composites with high EMI SE values. [PERSON] et al. recently reported that agricultural biowaste collected after apple and quince processing could be converted into EMI shielding material [50]. The starting materials were chopped, homogenized, dried, and mixed with an equal mass of KOH. The mixtures were carbonized at 850 \({}^{\circ}\)C, under nitrogen flow. The resulting biochars were mixed with sodium silicate at 40 wt.%. The film of only 0.2 mm thickness showed an EMI SE of 15.5 dB in the 8-12 GHz frequency range. Although moderate EMI SE values were found, the study demonstrates the outstanding potential of selected biowaste in the production of sustainable EMI shielding materials. Another study by [PERSON] et al. proposed a new organic waste for biochar production [37]. Within the study, the composite built from _Ricinus communis_ outer shell-based biochar and epoxy was fabricated by slow pyrolysis at 400-700 \({}^{\circ}\)C. The results showed that the biochar pyrolysis at 700 \({}^{\circ}\)C led to a maximal electrical conductivity of 95 S/m due to the presence of graphitic carbon. A maximum EMI SE of 26.5 dB was found in the X-band frequency range, at 40 wt.% biochar to epoxy matrix. According to the reviewed literature, biochar is a promising, eco-friendly, sustainable, and renewable material that could be used as a filter for EMI shielding applications in electronic devices and construction materials. The biochar shielding effectiveness is associated with the electrically conductive graphene region in biochars and porous structure [50]. Namely, EMW attenuation is the result of wave reflection from the graphene region and multiple reflections inside the pores. Biochar features rely on two main factors:  
1. The experimental conditions (temperature, duration of heating, addition of chemical agent for pore formation or activtivation of graphitization, selected gas);
2. The starting material's chemical composition [39]. With an increased temperature, the porosity, specific free space, and carbon content increase too, while the selection of source materials affects both the properties and yields of the resulting biochar [39].