## 4 Functionalization of Electrospun Nanofiber Mats for Electromagnetic Shielding
### Magnetic Nanofiber Mats  
In many cases, magnetic nanoparticles are embedded in the spinning solution to prepare magnetic nanofiber mats. [PERSON] et al. prepared core/shell nanofibers from PA6 and Fe\({}_{3}\)O\({}_{4}\) (magnetic) and found good electromagnetic shielding even for a low magnetite loading of only 1.7 wt% [67]. [PERSON] and [PERSON] used a higher magnetite content of 11 wt% in PVDF nanofibers and reported that more than 17 wt% magnetite resulted in aggregation of the nanoparticles on the nanofiber surfaces [68]. PAN nanofiber mats with even higher magnetite contents were produced by [PERSON] et al., e.g., 72 wt% magnetite in the final nanofiber mat with strong agglomerations [69], while smaller amounts or the limitation of the magnetite nanoparticles to the shell of coaxially electrospun nanofibers resulted in even nanofibers without many agglomerations [70; 69; 71]. Besides magnetite, many other magnetic nanoparticles have been embedded in polymeric nanofibers. [PERSON] et al. report on Ni nanoparticles in recycled poly(ethylene terephthalate) (PET) nanofiber mats for EMI shielding [72]. [PERSON] et al. prepared TaC/Fe\({}_{3}\)C/C electrospun nanofiber mats by pyrolysis and found a high conductivity of approx. 1.5 KS/m, high magnetization at saturation of 13.3 emu/g and a shielding of up to 46 dB in the range of 8.2-12.4 GHz [73]. NiZn ferrite (Ni\({}_{0.5}\)Zn\({}_{0.5}\)Fe\({}_{2}\)O\({}_{4}\)) nanofibers were prepared by [PERSON] et al., resulting in a large magnetization of up to \(\sim\)60 emu/g [74], while [PERSON] et al. discussed the influence of thermal stabilization and carbonization on the magnetic properties of magnetite- and nickel-ferrite (Fe\({}_{2}\)O\({}_{3}\)/NiO)-containing PAN nanofiber mats [75]. Magnetic coatings are another way to make a nanofiber mat magnetic. [PERSON] et al. used magnetron sputtering with Ni on SiC/C nanofiber mats and found strongly frequency-dependent reflection losses with peaks at 7 GHz, 11.8 GHz and 14.5 GHz which they attributed to interface polarization, as depicted in Figure 5[76]. They explained the importance of the conductive path in the network to allow transporting excited hopping electrons, in addition to the microwave dissipation due to the magnetic loss of the Ni particles. In  
Figure 4: Schematic of the EMI shielding mechanism in the electroless plated PAN membranes. From [62], originally published under a CC-BY license. addition, multiple interfaces between Ni, SiC and carbon were found to cause dielectric dipole interactions and related relaxations, so that more microwaves were attenuated. [PERSON] et al. also applied magnetron sputtering of FeCo coatings on carbon nanofibers to reach a magnetization at saturation of \(\sim\)3 emu/g and a reflection loss of 17-24 dB at 5-15 GHz [77]. [PERSON] et al. used electroless plating of Ni-Co instead to prepare magnetically coated PAN-poly(urethane) (PU) nanofiber mats [78]. They found a magnetization at saturation of approx. 50 emu/g and an electric conductivity of 7 \(\times\) 10\({}^{3}\)-11 \(\times\) 10\({}^{5}\) S/m, resulting in a shielding effectiveness of \(\sim\)30-90 dB for samples with different coatings and nanofiber-mat thicknesses in the frequency range of 8-26 GHz (X-band, Ku-band and K-band). A simple dip-coating procedure was used by [PERSON] et al. to receive magnetic nanofiber mats [79]. Starting with an electrospun thermoplastic polyurethane (TPU)/PAN/magnetic nanofiber mat, MXene was subsequently added by dip-coating, resulting in a strong adhesion of the MXene nano-sheets to PAN and magnetite. In this way, a magnetization at saturation around 20 emu/g was reached, combined with a conductivity of 6-20 S/m, resulting in a shielding effectiveness of 10-25 dB in the range of 8-12 GHz. Finally, it must be mentioned that even pure metallic nanofibers can be produced by electrospinning, followed by calcination of the polymer that is necessary as a spinning agent. [PERSON] et al.