With the development of modern high-tech, electromagnetic interference (EMI) and electromagnetic compatibility (EMC) problems caused by electromagnetic waves are becoming more and more serious. They not only cause interference and damage to electronic instruments and equipment, affect their normal operation, and seriously restrict our country’s international competitiveness in electronic products and equipments, and also pollute the environment and endanger human health; in addition, the leakage of electromagnetic waves will also endanger the national information security and the security of military core secrets. In particular, electromagnetic pulse weapons, which are new-concept weapons, have made substantial breakthroughs, which can directly attack electronic equipment, power systems, etc., causing temporary failure or permanent damage to information systems, etc.
Therefore, exploring efficient electromagnetic shielding materials to prevent electromagnetic interference and electromagnetic compatibility problems caused by electromagnetic waves will improve the safety and reliability of electronic products and equipment, enhance international competitiveness, prevent electromagnetic pulse weapons, and ensure safety of information communication systems and network system, transmission systems, weapon platforms, etc. are of great significance.
1. Principle of electromagnetic shielding(EMI)
Electromagnetic shielding is the use of shielding materials to block or attenuate the propagation of electromagnetic energy between the shielded area and the outside world. The principle of electromagnetic shielding is to use the shielding body to reflect, absorb and guide the electromagnetic energy flow, which is closely related to the charges, currents and polarization induced on the surface of the shielding structure and inside the shielding body. Shielding is divided into electric field shielding (electrostatic shielding and alternating electric field shielding), magnetic field shielding (low-frequency magnetic field and high-frequency magnetic field shielding) and electromagnetic field shielding (electromagnetic wave shielding) according to its principle. Generally speaking, electromagnetic shielding refers to the latter, that is, shielding the electric and magnetic fields at the same time.
2. Electromagnetic shielding material
At present, composite electromagnetic shielding coatings are widely used. Their main compositions are film-forming resin, conductive filler, diluent, coupling agent and other additives. Conductive filler is an important part of it. The common one are silver(Ag) powder and copper(Cu) powder., nickel(Ni) powder, silver-coated copper powder, carbon nanotubes, graphene, nano ATO, etc.
2.1 Carbon nanotubes(CNTs)
Carbon nanotubes have a great aspect ratio, excellent electrical, magnetic properties, and have shown excellent performance in conductivity, absorbing and shielding. Therefore, the research and development of carbon nanotubes as conductive fillers for electromagnetic shielding coatings has been more and more popular. This places high requirements on the purity, productivity, and cost of carbon nanotubes. The carbon nanotubes produced by Hongwu Nano, including single-walled and multi-walled, have a purity of up to 99%. Whether the carbon nanotubes are dispersed in the matrix resin and whether they have good affinity with the matrix resin becomes a direct factor affecting the shielding performance. Hongwu Nano also supplies dispersed carbon nanotube dispersion solution.
2.2 Flake silver powder with low apparent density
The earliest published conductive coating was a patent issued by the United States in 1948 that made silver and epoxy resin into conductive adhesive. The electromagnetic shielding paint prepared with the ball milled flake silver powders produced by Hongwu Nano has the characteristics of low resistance, good conductivity, high shielding efficiency, strong environmental tolerance, and convenient construction. They’re widely used in communication, electronics, medical, aerospace, nuclear facilities and other fields. Shielding paint is also suitable for surface coating of ABS, PC, ABS-PCPS and other engineering plastics. The performance indicators including wear resistance, high and low temperature resistance, humidity and heat resistance, adhesion, electrical resistivity, electromagnetic compatibility, etc. can reach the standard.
2.3 Copper powder and nickel powder
Copper powder conductive paint has low cost and it is easy to paint, also has good electromagnetic shielding effect, and thus it is widely used. It is especially suitable for the anti-electromagnetic wave interference of electronic products with engineering plastics as the shell, because the copper powder conductive paint can be sprayed or brushed easily. Plastic surfaces of various shapes are metalized to form an electromagnetic shielding conductive layer, so that the plastic can achieve the purpose of shielding electromagnetic waves. The morphology and amount of copper powder have a great influence on the conductivity of the coating. Copper powder has spherical, dendritic, and flake-like shapes. The flake shape has a much larger contact area than the spherical shape and shows better conductivity. In addition, the copper powder (silver-coated copper powder) is coated with inactive metallic silver powder, which is not easy to oxidize, and the content of silver is generally 5-30%. Copper powder conductive coating is used to solve the electromagnetic shielding of ABS, PPO, PS and other engineering plastics and wood And electrical conductivity, has a wide range of application and promotion value.
In addition, the electromagnetic shielding effectiveness measurement results of nano nickel powder and electromagnetic shielding coatings mixed with nano and micron nickel powder show that the addition of nano Ni particle can reduce the electromagnetic shielding effectiveness, but can increase the absorption loss. The magnetic loss tangent is reduced, as well as the damage to the environment, equipment and human health caused by electromagnetic waves.
2.4 Nano Tin Antimony Oxide (ATO)
Nano ATO powder, as a unique filler, has both high transparency and conductivity, and a wide range of applications in the fields of display coating materials, conductive antistatic coatings, and transparent thermal insulation coatings. Among the display coating materials for optoelectronic devices, nano ATO materials have anti-static, anti-glare and anti-radiation functions, and were first used as display electromagnetic shielding coating materials. ATO nano coating materials have good light-color transparency, good electrical conductivity, mechanical strength and stability, and their application to display devices is one of the most important industrial applications of ATO materials at present. Electrochromic devices (such as displays or smart windows) are currently an important aspect of nano-ATO applications in the display field.
2.5 Graphene
As a new type of carbon material, graphene is more likely to become a new type of effective electromagnetic shielding or microwave absorbing material than carbon nanotubes. The main reasons include the following aspects:
①Graphene is a hexagonal flat film composed of carbon atoms, a two-dimensional material with the thickness of only one carbon atom;
②Graphene is the thinnest and hardest nanomaterial in the world;
③The thermal conductivity is higher than that of carbon nanotubes and diamonds, reaching about 5 300W/m•K;
④Graphene is the material with the smallest resistivity in the world, only 10-6Ω•cm;
⑤The electron mobility of graphene at room temperature is higher than that of carbon nanotubes or silicon crystals, exceeding 15 000 cm2/V•s. Compared with traditional materials, graphene can break through the original limitations and become an effective new wave absorber to meet the requirements of absorption. Wave materials have the requirements of “thin, light, wide and strong”.
The improvement of electromagnetic shielding and absorbing material performance depends on the content of the absorbing agent, the performance of the absorbing agent and the good impedance matching of the absorbing substrate. Graphene not only has a unique physical structure and excellent mechanical and electromagnetic properties, but also has good microwave absorption properties. After it is combined with magnetic nanoparticles, a new type of absorbing material can be obtained, which has both magnetic and electrical losses. And it has good application prospects in the field of electromagnetic shielding and microwave absorption.
For the above common electromagnetic shielding materials nano powders, both are all available by Hongwu Nano with stable and good quality.