The electromagnetic wave is inevitable in electronic information equipment at work, and this electromagnetic radiation is like a double-edged sword. On the one hand, it realizes the wireless, high-speed and long-distance transmission of information. On the other hand, it also has negative effects, such as electromagnetic interference, electromagnetic environmental pollution, and electromagnetic information leakage and so on.
What can we do with electromagnetic waves? The most famous military example is the capture of Saddam Hussein. At that time, it was because Saddam Hussein and his wife’s mobile phone conversation produced electromagnetic leakage, resulting in electromagnetic information by the United States intelligence surveillance and capture, and was locked in the hiding place.
In order to prevent the double-edged sword from pointing at the user, it is necessary to reduce the harm caused by electronic radiation and conduct electromagnetic shielding. That is to say, low-resistance conductor materials are used to shield the surface reflection of electromagnetic waves on the conductor, the absorption of the conductor and the loss in the transmission process.
The traditional method of shielding electromagnetic wave is to use metal and metal complex materials, but they have many shortcomings such as not easy to process, cumbersome, poor environmental stability, shielding band is not suitable for control, and so on, the application scope is limited. In contrast, electromagnetic shielding coating has the advantages of easy processing, convenient construction, and low cost, and has become the most widely used electromagnetic shielding material at home and abroad.
Classification of doped conductive coatings
Generally speaking, the conductive property of metal filler conductive coating is stronger than that of carbon filler conductive coating. The selection of conductive filler is mainly based on the need to choose the appropriate type of conductive filler, shape, and dosage, and the properties of the fillers selected also play an important role in the properties of the final conductive coating.
For example, if the particle size of conductive filler particles is too large, the gap between filler particles will be large, and it is not easy to form a conductive three-dimensional grid structure, and it is easy to be affected by gravity and precipitate; On the contrary, if the particle size of conductive filler is too small, flocculation of filler particles will occur, and the conductivity will be reduced, and the oxidation of metallic conductive filler will be accelerated at the same time.
- Metal powder conductive filler
Metal powder conductive fillers mainly include copper, silver, or nickel powder. Silver has the best conductivity, slow oxidation speed, and the oxide can conduct electricity; Copper has excellent conductivity, and its low-frequency shielding effect is better than that of nickel; Nickel has the advantages of low price, moderate price, good oxidation resistance, and stable electrical conductivity.
Metal oxide conductive filler mainly includes tin oxide, titanium dioxide, zinc oxide. Metal oxides have poor electrical conductivity but good heat resistance. They have stable electrical conductivity in the range of hundreds to thousands of temperatures and do not change with time and air humidity, which have permanent electrical conductivity.
Besides, metallic flakes such as aluminum foil have bright colors and good electrical conductivity; Metal fibers such as aluminum and copper have good electrical conductivity but are expensive and difficult to process.
- Carbon conductive filler
Carbon black conductive fillers include acetylene carbon black, oil furnace carbon black, tank carbon black and thermal cracking carbon black. The resistivity of carbon black is about 10-2 Ω CM, it has good electrical conductivity but poor dispersion.
Carbon fibers such as polyacrylonitrile and asphaltene have good conductivity, but the production cost is high and the processing difficulty is worse than PAN-based carbon fibers.
Graphite includes both natural and artificial graphite, and its electrical conductivity varies from the place of origin, while carbon nanotubes have good conductivity and large aspect ratio.
Various conductive fillers have their advantages and disadvantages. For example, the structure of carbon black is stable and the surface is not easy to oxidize. It is easy to form a conductive structural network in the polymer, which is greatly affected by the structure, specific surface area, and surface chemical properties. In this way, although the conductivity has been improved, the coating adhesion, appearance and service stability are poor, and it is easy to cause cracks and peeling. Precious metals can be used to obtain materials with excellent conductivity, but it will greatly increase the cost of products, and the migration of metal ions in the polymer will reduce the surface conductivity, resulting in unstable conductivity of the material.
In addition, if the shielding coating prepared by using a single conductive filler is not ideal, the conductive filler is modified. Generally, it is a kind of conductive material with high conductivity, good environmental stability and corrosion resistance that is coated with a layer or layers of conductive material on the surface of cheap and light material particles by various methods, so as to make the conductive filler of core-shell structure and develop a composite conductive filler with better conductivity.
Applications of conductive coatings
- Antistatic coating
Nowadays, polymer materials have been applied more and more widely. However, due to its poor conductivity, the surface of the polymer material shell will produce a static charge or even discharge phenomenon, which affects the use. Conductive coating can make itself does not have the conductivity of the polymer material surface metallization, so as to timely eliminate static charge and avoid harm.
Antistatic coatings are also widely used in pipeline transportation. In the process of oil transportation, the oil will constantly friction with the pipeline transporting oil or the storage tank to produce static charge, and there is a danger of explosion If the accumulated static charge is not removed. Conductive coating on their surface can play a good protective role.
- The electromagnetic wave shield conductive coating
Electromagnetic wave shielding of conductive coatings is mainly reflected in two aspects, one is to prevent the internal electromagnetic radiation out so as to prevent information leakage, and the other is to shield the interference of external electromagnetic waves.
The electronic equipment has a small internal current, and it is easy to be interfered by electromagnetic waves. If their shells are made of plastic stools, conductive coatings are needed to shield them from electromagnetic waves. Conductive coating can absorb, reflect and refract electromagnetic wave, constantly weaken electromagnetic wave, so that the energy of the electromagnetic wave cannot interfere with the normal operation of the instrument. Militarily, when the conductive coating is applied to the surface of aircraft radome, it can well solve the problems of electromagnetic interference in the sky and enhance the comprehensive performance of aircraft.
- Electric heating coating
Conductive coating can be used to achieve the conversion of electrical energy to thermal energy, which can replace the traditional metal electric heating wire production of a variety of electrical heating equipment, so it is widely used in aircraft, ships, motor vehicles, automotive glass fog removal, defrosting and cold regions, etc.. The thermal properties of conductive coatings can change the electromagnetic radiation characteristics of coated objects and resist the detection of infrared detection technology, so they are widely used in modern military fields.
- Application in electronic circuits
Using conductive coating directly on the circuit board to coat the corresponding circuit to replace the traditional metal sheet conduction current can greatly save metal materials. Besides, the conductive coating takes up less space, so it can save space to meet the requirements of smaller space use. For example, the use of carbon conductive coating to make the ultra-thin switch can meet the needs of placing the switch in a narrow space.
- High temperature resistant conductive anticorrosive coating
The metal grounding pole of the grounding network of the power system is directly buried underground, and the long-term contract between the grounding pole and soil moisture and microorganisms is bound to lead to oxidation corrosion. By coating the surface of the grounding pole with the corrosion-resistant conductive coating, the direct contact between the grounding pole and the soil can be avoided and the corrosion problem can be solved.
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