# Electromagnetic Interference Shielding Anisotropy of Unidirectional CFRP Composites
## 1 Introduction  
Recently, with the rapid development in communication and electronic devices, electromagnetic (EM) pollution has become a serious concern that can lead to electromagnetic compatibility (EMC) issues in electronic equipment and the resulting EM radiation of specific frequencies could be harmful to human health [1; 2; 3; 4; 5]. This is especially true as the market for 5G technologies and autonomous vehicles are growing rapidly around the world [6; 7]. EM radiation usually causes electromagnetic interference (EMI) with other electronics, which can lead to electronic device malfunction [8]. Hence, research focused on effectively solving and avoiding EM pollution has attracted considerable attention. Nowadays, shielding materials play a key role in protecting electronic equipment from EMI and prevent such radiation sources from emitting radio waves. EMI shielding refers to the absorption or reflection of EM waves with efficient shielding materials, which are composed of either conductive or magnetic-based materials [9; 10]. In the past few decades, metal materials have been widely used as EMI shielding materials for excellent shielding performance, but they have disadvantages of low corrosion resistance, poor mechanical flexibility, etc. [11; 12]. MXenes have been studied as EMI shielding materials due to their outstanding EMI shielding performance, low density and special metallic features [13; 14]. The MXenes composites with different structures have been fabricated to improve its EMI shielding performance [15]. Besides, carbon materials have been investigated as alternatives to metal-based composites to provide great EMI shielding performance. Polymer composites reinforced with carbon-based fillers, such as carbon fibers, graphite, carbon nanotube(CNT), graphene and carbon black have been widely studied for use as shielding materials to provide great shielding abilities to EM waves [16; 17; 18; 19; 20; 21; 22]. Conventional carbon fiber reinforced polymer (CFRP) composites show not only good shielding properties, but also achieve excellent mechanical performance, which can expand their applications in the aeronautic industry [23; 24; 25]. Other research works [26; 27] also observed excellent shielding performance of short carbon fiber reinforced composites, but its mechanical properties and EMI shielding performance were inferior to continuous carbon fiber composite materials [28; 29]. Besides, the EMI shielding performance of CFRP could also be improved by being coated with conductive paints filled with metallic particles or nanowires [30], but it still meets the challenge of poor dispersion of conductive fillers and poor wear. In recent years, intrinsically conducting polymers (ICPs) have also been used as fillers for EMI shielding [31; 32; 33], but their poor mechanical and thermal properties limit their practical applications [34]. The shielding materials should obtain excellent EMI shielding performance in the specified frequency range to meet different practical applications. Many studies have achieved the purpose of improving the EMI shielding performance of composite materials in a specific frequency band by controlling the filler's size. [PERSON] et al. [35] found that the composite with high carbon fiber aspect ratio (L/D = 100) shows higher shielding effectiveness in the frequency range of 8-12 GHz (X-band). [PERSON] et al. [36] reported that shielding performance of CFRP was enhanced with the decrease in the array spacing and the increased number of layers in the frequency range of 30-750 MHz, but the number of layers exhibited little effect on the EMI shielding of CFRP at the frequency range of 750 MHz-1.5 GHz. The shielding property of carbon nano-fiber-based composites is also improved in the X-band by increasing the magnetic particle size introduced into the matrix [37]. As above all, shielding materials with various filler sizes and structures exhibit different shielding performances in various frequency bands.