# Review on the Electrical Resistance/Conductivity of Carbon Fiber Reinforced Polymer
## 2 Structure of CFRP and Its Electrical Conductivity
### Conductivity and Electromagnetic Shielding  
Electromagnetic interference (EMI) shielding is also widely concerned in communication and electronic industries [94; 95]. EMI can be defined as radiated and/or conducted electromagnetic signals emitted by electrical circuits. EMI refers to the ability to block electromagnetic radiation [96].  
As electromagnetic radiation tends to interfere with electronics, EMI shielding of both radiation and electronics source is widely required. The performance of a shielding material involves shielding effectiveness (SE). EMI shielding mainly involves a reflection mechanism [97; 98]. For the radiation reflection, the shield should have mobile charge carriers, which can interact with the electromagnetic fields. Therefore, the shield tends to be electrically conductive, although it does not require a high conductivity. Metals, such as steel, copper, and aluminum typically serve as excellent EMI shielding agents [99]. Since metal shielding is characterized by high cost, heavy weight and susceptibility to environmental degradation, polymer composites with discontinuous conductive fillers are widely employed in EMI shielding. So far, EMI remains a challenge due to the rapid development of electronic devices.  
CFRP composites, used in aeronautic industry, have a good shielding and absorbing effectiveness while presenting high mechanical and chemical resistance per unit weight [100]. The high electrical conductivity of carbon fibers allows for good electromagnetic shielding effect since the electric field lines are attracted to conductive fibers and result in surface electric currents [101; 102; 103]. Therefore, it is necessary to explore the structure and function of carbon fiber-based composites [104]. The orientation and distribution of carbon fibers largely influences the electrical conductivity and shielding effectiveness of the composites. The exposed clearance and the destroyed conductive network mainly led to the decrease in the shielding effectiveness [105].  
In summary, the SE of a composite material is mainly related to the filler's dielectric constant, intrinsic conductivity, and the aspect ratio [106]. Several approximations have been proposed for SE of composite materials in recent years. [PERSON] et al. proposed a far-field calculation equation of SE for electrically thin samples [107]. [PERSON] et al. adopted the \"Classical Good Conductor Approximation\" and \"Simon Formalism\" in SE calculations. However, various equations yielded significantly different results [108].  
Two common approaches are generally used to achieve EMI shielding of CFRP for plastics products. One is coating with the conductive metal, the other is blending with conductive fibers or particles [109]. The addition of metal powders into CFRP composites was a cost-effective method for improving shielding effectiveness [98].  
The incorporation of conductive fillers in the polymer matrix can improve the electrical conductivity of plastics and solve the EMI problem. Although adding more carbon fibers can yield satisfactory shielding effectiveness, it is difficult to produce composites with high fiber volume fraction through extrusion and injection molding. More carbon fibers also significantly increase the cost. The problem can be solved by adding a thin layer of metal on carbon fibers' surface. In addition, metal-coated carbon fibers also endow polymer composites with high modulus, light weight, and high strength [110; 111; 112].  
Another method [108; 113] mentioned that the polymer matrix in the composites was typically electrically insulating, so it could not provide shielding. However, electrically conducting fillers endow the composites with the shielding ability. Both discontinuous and continuous fillers are used for this purpose [114].