Application and Research Progress of Carbon Nanotubes in Electromagnetic Shielding of High-Speed Trains

In order to further improve the running speed of high-speed railway，lightweight design is often used on the train. The materials used in traditional lightweight design are mainly aluminum alloy. In recent years，with the increasing maturity of new material application technology，carbon fiber composites have been more and more widely used in high-speed train body and structural materials. Due to the low conductivity and compactness，the electromagnetic shielding performance of carbon fiber composites is far lower than that of aluminum alloy，steel and other metal materials. Therefore，a layer of electromagnetic shielding material needs to be added to the surface of carbon fiber vehicle body to protect the normal operation of the vehicle. Carbon nanotubes（CNTs）have excellent thermal conductivity，large specific surface area and high conductivity of about 1×106 S·cm-1. When added into the composite material or coating as a shield，it is easy to form a complex conductive network in the composite conductive coating and improve the conductivity of the material，resulting in impedance mismatch when the electromagnetic wave passes through the interface between the shield and other media. Finally，the electromagnetic shielding performance of the material is significantly improved in the reflection loss part. However，the direct use of CNTs as shielding material is not good. Composite use of CNTs with ferromagnetic materials or MXenes can achieve better shielding effect. In addition to this，direct chemical plating or deposition of ferromagnetic metals，such as iron，cobalt and nickel，on the surface of CNTs. Using CNTs loaded with ferromagnetic materials as fillers，excellent electromagnetic shielding efficiency could be obtained with a low addition amount. Polyvinylidene fluoride（PVDF）/CNTs/Ni@CNTs electromagnetic shielding materials were synthesized by solvothermal method，solution casting and molding. When the content of carbon nanotubes increased from 2% to 6%（mass fraction），the conductivity increased by two orders of magnitude. At the same time，in the K-band（18~26.5 GHz），the overall electromagnetic shielding efficiency of the coating increased from 14.4 to 36.6 dB. Further research showed that the shielding mechanism of the composite coating was mainly absorption mechanism. When the addition amount of CNTs was 10% and the addition amount of Ni@CNTs was 12%，the electromagnetic shielding performance of the composite was the best，with a total shielding efficiency of 46.6 dB，an absorption shielding efficiency of 37.9 dB and a reflection shielding efficiency of 8.7 dB. This change was mainly due to the significant increase of absorption loss rather than the increase of reflection loss. Adding rare earth elements to the bath during electroless plating could effectively improve the deposition rate of alloy elements，make the prepared electroless alloy coating more compact and significantly improve the surface quality of alloy coating. The process of electroless plating Co-Ni-B coating on the surface of copper alloy could provide a reference. With the increase of Ce addition in the plating solution，the deposition rate of the alloy first increased and then decreased，and the addition of Ce also made the alloy coating change from amorphous to crystalline，which would help the alloy coating maintain anisotropy and make the coating not easy to be too hard and brittle. The deposition of some magnetic rare earth elements such as samarium（Sm），neodymium（Nd）and lanthanum（La）on the surface of the shielding material could also significantly improve the number of magnetic domains in the shielding body，increase the magnetic loss caused by the polarization of magnetic dipole and improve the electromagnetic shielding ability of the material. The shielding efficiency of Co-Ni-P coated fabric with Nd in the range of 0~6000 MHz was 42.27~66.76 dB，which was about 11 dB higher than that of unmodified fabric. The results showed that adding an appropriate amount of lanthanum could also significantly improve the microwave absorbing properties of the coating. The effective improvement of shielding efficiency came from the CNTs electroless nickel cobalt lanthanum alloy. On the basis of its original high dielectric loss，the shield had obvious magnetic loss effect due to the addition of rare earth lanthanum. In order to further expand the application prospect of CNTs in the field of electromagnetic shielding，several innovative research directions were put forward：（1）The layer by layer coating method could be used in the process of CNTs electroless alloy coating，the composition and structure of metal coating on the surface of CNTs could be designed and controlled，and a new electroless CNTs shield with better performance could be designed and prepared.（2）Explore the mechanism of surface modification of CNTs by rare earth elements such as Sm，Nd and La，so as to provide guidance for further performance improvement.（3）The modified CNTs material was crystallized and densified by heat treatment，so as to improve the shielding performance of the material.（4）Software simulation was used to combine theoretical calculation with actual test data to reduce the cost of material design and test. © 2023 Editorial Office of Chinese Journal of Rare Metals. All rights reserved.