## 5 Electromagnetic Shielding by Nanofiber Mats in the X-Band and Ku-Band  
As could already be expected from the previous sections, most studies working on electromagnetic shielding concentrate on the X-band. Here, some papers from the last years are discussed. A special composite, suggested by [PERSON] et al., is ZrC/SiC which was prepared by electrospinning a polycarbosilane (PCS) solution with zirconium acetylacetone (Zr(acac)), followed by pyrolysis of the corresponding nanofiber mat [85]. In this way, the average nanofiber diameter was significantly reduced from 2.6 \(\upmu\)m of SiC nanofibers to 330 nm, and the conductivity was increased by five orders of magnitude to 13 S/m, resulting in a shielding effectiveness of 19 dB in the X-band. Interestingly, in high-temperature measurements up to 600 \({}^{\circ}\)C, this value was even slightly increased to 20 dB, making this material suitable for high-temperature shielding applications. The shielding process is attributed to a combination of high incidence surface area, highly conductive ZrC nanoparticles resulting in micro-currents on the nanofiber network which lead to conductivity loss, and, finally,  
Figure 6: Relationship between diameter versus return loss of electrospun ceramic fibers from Si-based preceramic polymers. From [84], copyright (2022), with permission from Elsevier. “This work” refers to [84]. grain boundaries and material interfaces between ZrC and SiC which lead to interface polarization and dielectric loss (Figure 7) [85]. Combining conductive and magnetic properties, [PERSON] and [PERSON] prepared PVP nanofiber mats with different numbers of embedded magnetite nanoparticles up to 5 wt%, with the last value resulting in undesired agglomerations [86]. They found an increase in the average nanofiber diameter with increasing numbers of nanoparticles and magnetizations at saturation of 4-10 emu/g, in comparison with the value for pure magnetite nanoparticles of approx. 70 emu/g. In addition, the electric conductivity was decreased by four orders of magnitude from the pure PVP go PVP with 4 wt% magnetite. Correspondingly, the shielding effectiveness in the X-band increased from approx. 5 dB of the pure PVP nanofiber mat to 20-24 dB of PVP with 4 wt% magnetite. A shielding effectiveness between 30 dB and nearly 100 dB was reached by electrospun carbon nanofiber films, combined with silicone in alternating layers of a hot-pressed multilayer stack [87]. [PERSON] et al. tested different numbers of layers and showed that this alternating structure was superior to pure carbon nanofiber films, although a pure silicone film did not show any shielding effect. With a composite of PAN/SiO\({}_{2}\) nanofiber mats, coated by Ag nanoparticles in different amounts and dip-coated by 1H,1H,2H,2H-perfluorodecanethiol (PFDT), [PERSON] et al. reached conductivities around 2-18 kS/m and a shielding effectiveness of up to 80 dB, depending on AG and PFDT content [88]. Most of the shielding effectiveness was based on absorption, with only a small addition due to reflection. Many other groups reported new approaches to reaching high electromagnetic-shie lding effectiveness by nanofiber mats, e.g., by adding silver nanowires [89], using conductive polymers such as poly(aniline) (PANi) as a coating for cellulose nanofibers [90], coating poly(lactic acid) (PLA) nanofiber mats with MXene [91], multilayer structures with magnetic and conductive properties [92], or combining PVP with Ag nanoparticles to reach a shielding effectiveness of nearly 100 dB [93]. Too many papers are published per year, dealing with electromagnetic shielding in the X-band, to mention more than a small excerpt of them. This is, however, different for other frequency ranges; they will be discussed in the next sections. Electromagnetic shielding in the Ku band, i.e., in the frequency range of 12-18 GHz, is usually performed together with the X-band, sometimes also combined with the K-band (18-27 GHz) [62].