## 4 Functionalization of Electrospun Nanofiber Mats for Electromagnetic Shielding
### Conductive Nanofiber Mats  
Conductive nanofiber mats can be prepared by electrospinning from conductive solutions or melts, by carbonization after electrospinning, or by applying a conductive coating [58]. [PERSON] et al. prepared core-shell nanofiber mats with polyvinylidene fluoride (PVDF) as the core and poly (3_A_-ethylenedioxythiophene) (PEDOT) as the shell, without additional conductive fillers, and found an increased absorption of electromagnetic waves and multi-reflections due to the porous nanostructure [59]. They reported an SE of approx. 40 dB in the X-band, i.e., in the range of 8-12 GHz.  
Combining a conductive precursor and carbonization, [PERSON] et al. used a PAN/TaCl\({}_{5}\) solution for electrospinning [60]. After thermal stabilization and carbonization at 1200 \({}^{\circ}\)C, they received TaC/C composite nanofiber mats with conductivities around 5-10 \(\times\) 10\({}^{2}\) S/m and a shielding effectiveness of 24-38 dB, depending on the nanofiber mat thickness and the production parameters.  
A more complicated process of gaining conductive nanofibers was described by [PERSON] et al., who started with an electrospun poly(acrylonitrile) (PAN) nanofiber mat, which was treated with NaOH and dispersed [61]. This dispersion was mixed with graphene oxide (GO), resulting in a GO-PAN film by filtration, which was afterwards thermally stabilized and carbonized at 2000 \({}^{\circ}\)C to create a graphene/carbon fiber film. They found conductivities around 6 \(\times\) 10\({}^{4}\) S/m-1.7 \(\times\) 10\({}^{5}\) S/m and a shielding effectiveness around 48-56 dB in the X-band for samples prepared with varying production parameters.  
[PERSON] et al. also started with a PAN nanofiber mat which was aminated and immersed in a silver-ammonia solution to obtain Ag seeds, before Ag, Cu or Ni coatings were added by electroless deposition of the respective metallic nanoparticles [62]. They found conductivities in the range of 1.3-57 \(\times\) 10\({}^{5}\) S/m, depending on the coating material, and a shielding effectiveness of around 50-60 dB for Co and Ni, as well as around 75-90 dB for Ag coating in the range of 8-26 GHz (X-band, Ku-band and K-band), which they also attributed to the high absorption upon multi-reflection, as depicted in Figure 4[62].  
In a similar approach, [PERSON] et al. used electron beam evaporation to coat electrospun nylon (PA66) nanofiber mats with 50 nm of silver [63]. These metallized nanofiber mats, containing oriented nanofibers, were stacked with varying nanofiber orientations and hot-pressed to prepare composites. Conductivities of 2 \(\times\) 10\({}^{6}\) S/m and 6 \(\times\) 10\({}^{6}\) S/m were reached perpendicular and parallel to the nanofiber orientation, respectively, resulting in a shielding effectiveness of up to 60 dB in the range of 8.2-18 GHz.  
A dip-coating method was applied by [PERSON] et al. to prepare a conductive nanofiber network [64]. Starting from glycerol-grafted PAN nanofiber mats, these membranes were immersed into PEDOT:PSS and dried in the air in a stretched state. The authors described that, in this way, the capillary effect during the evaporation of water upon drying resulted in collapsing of the larger pores and thus increased connections between adjacent nanofibers, in this way, increasing the conductivity. They reported conductivities around \(10^{2}\)-\(10^{3}\) S/m, depending on the preparation parameters, which were fully maintained during 1000 bending cycles. The shielding effectiveness in the range of 8-12.5 GHz was around 22-55 dB, again explained by multi-reflection inside the nanofiber mat.  
As these few examples show, there are various possibilities to make nanofiber mats conductive. In addition, the shielding effectiveness can often be further enhanced by adding dielectric fillers, such as ZnO [65]. Similarly, many ways exist to create magnetic nanofiber mats [66].