## 2 New EMI Shielding Materials: Biochars, Scaffolds, Rare Earth, and Ferrite-Based Materials
### Iron Oxide-Based Materials as EMI Shielding Material  
Iron oxides are inorganic compounds of iron and oxygen that exhibit very different stoichiometries due to the difference in the Fe oxidation state. Briefly, they can be classified into three groups: oxides of Fe\({}^{2+}\) (FeO, wustite), oxides of Fe\({}^{3+}\) (various forms of Fe\({}_{2}\)O\({}_{3}\), hematite, maghemite), and combined Fe\({}^{2+}\)/Fe\({}^{3+}\) oxides (Fe\({}_{3}\)O\({}_{4}\), magnetite, Fe\({}_{4}\)O\({}_{5}\), Fe\({}_{5}\)O\({}_{6}\), etc.). Apart from the difference in the Fe oxidation state, many existing crystal structures also enhance the diversity of iron oxides depending on the synthetic routes. For example, a variety of polymorphs of iron (III) oxide has been reported to date [112]. \(\alpha\)-Fe\({}_{2}\)O\({}_{3}\) is the most common form of iron (III) oxide and occurs as the mineral hematite. It has a rhombohedral structure and shows antiferromagnetic behavior below \(\sim\)260 K and weak ferromagnetism at temperatures of between 260 and 950 K [113]. Common methods to  
Figure 5: The EMI shielding efficiency of the Er-doped and Gd-doped \(\alpha\)-MnO\({}_{2}\) samples according to the (**a**) total, (**b**) absorption, (**c**) reflection, and (**d**) skin depth variation in the frequency range of 8 to 18 GHz. Schematic representation of (**e**) impact of hazardous EM waves radiated from various electronic sources on livelihood and demonstration of most possible EMI shielding mechanism to prevent it. The figure is adapted from reference [111]. Copyright 2023, Elsevier B.V.  
prepare \(\alpha\)-Fe\({}_{2}\)O\({}_{3}\) are thermal decomposition and precipitation in the liquid phase. The \(\beta\)-phase of Fe\({}_{2}\)O\({}_{3}\) is metastable, and at temperatures above 773 K, it converts to the \(\alpha\)-phase. It can be obtained by the thermal decomposition of iron (III) salts. Similar to the \(\beta\)-phase, \(\gamma\)-Fe\({}_{2}\)O\({}_{3}\) is also metastable and converts to the \(\alpha\)-phase at high temperatures [114]. It has a cubic structure, and in the bulk form, it is ferromagnetic, but \(\gamma\)-Fe\({}_{2}\)O\({}_{3}\) nanoparticles less than 10 nm in size show superparamagnetic behavior. The methods of \(\gamma\)-Fe\({}_{2}\)O\({}_{3}\) preparation involve the thermal dehydration of gamma iron (III) oxide-hydroxide, the modest oxidation of Fe\({}_{3}\)O\({}_{4}\), or the thermal decomposition of suitable iron salts [115]. The \(\varepsilon\)-phase is rhombic with properties between those of the alpha and gamma phases. It is metastable with a tendency to transform to the alpha phase at temperatures of between 773 and 1000 K. The synthesis of pure \(\varepsilon\)-Fe\({}_{2}\)O\({}_{3}\) is challenging and involves the oxidation of iron in an electric arc or sol-gel precipitation from iron (III) nitrate [116]. In addition, an amorphous form of Fe\({}_{2}\)O\({}_{3}\) also exists, which can be prepared under high pressure. Oxides of iron have become interesting candidates for electromagnetic interference (EMI) shielding due to several factors. Fe\({}_{2}\)O\({}_{3}\) is a magnetic material, while Fe\({}_{3}\)O\({}_{4}\) is a ferromagnetic. Magnetic materials can interact with and absorb electromagnetic waves, especially at lower frequencies (like radio waves), thus reducing EMI. Iron oxides exhibit a certain level of electrical conductivity, allowing them to contribute to EMI shielding by reflecting or absorbing the electrical components of electromagnetic waves. In addition, they can contribute to the absorption of electromagnetic energy through dielectric loss, a mechanism that involves converting electromagnetic energy into heat. This makes it effective in dissipating energy and preventing the transmission of EMI. Finally, iron oxides are abundant, inexpensive, and easy to combine in composite materials, such as polymers or carbon-based materials, and can maintain their properties over time.