Abstract: this paper comprehensively expounded electromagnetism radiate harms and the necessity of implementing electromagnetic shielding electromagnetic shielding, expounds the conductive coatings of shielding mechanism, composition, types and the influence of various factors, shielding electromagnetic shielding conductive coatings are introduced the research status, and pointed out the existing main problems and new development tendency. Keywords: electromagnetic shielding, Conductive coatings, Shielding effectiveness. Chinese library classification number: TQ63TM27 literature identification code: A articles Numbers: 155-9731 (2004) supplement 0921-04-1 introduction with the rapid development of electronic industry, all sorts of household appliances, computer, office equipment and other electronic products are widely popularization and application. However their work to people bring great convenience also release electromagnetic radiation and give off a lot of endangering people's health . Studies show that people long-term exposure to the strong electromagnetic radiation can cause abnormal function of cells and cells, causing neurasthenia abnormal condition and neural function disorder, endocrine disorders, serious and even induce cancerous. In addition, the electromagnetic radiation in the body form thermal-effect can lead to cataract generation, declining eyesight, and even blindness. At present, the electromagnetic radiation on human caused damage mechanism is unceasingly thorough, electromagnetic pollution prevention and control are also increasingly have widely received concerned. In addition, the electromagnetic radiation of electronic product itself also exist great harm. On the one hand, engineering plastic with its low price, qualitative light, easy to process the advantages of electronic products are widely used for the shell, but its electromagnetic wave is almost completely transparent, no shield effect of electromagnetic interference. On the other hand, electronic products internal micro electric current control signal power and the external electromagnetic noise power close, vulnerable to interference and appear misoperation, system paralyze etc serious consequences, at the same time these electronic product itself of electromagnetic will also launch foreign to adjacent electronic equipment disturbance. Therefore have world- wide issued anti-electromagnetism interference of relevant regulations. China also in 1998, pushing the electromagnetic compatibility (EMC) standard, already products will be the electromagnetic compatibility requirements in the national mandatory product certification scope from 2003, the regulation on May 1, every listed in the national mandatory product certification directory products have not undergone authentication shall not leave the factory, import and sales. Radiation back to information security brings a lot of hidden trouble. Computer etc scientific equipment in will through electromagnetic wave will be useful information leak out , leading to the information disclosure, may cause major political, economic and military loss, endangering state security. Research shows that ordinary without adopting shielding processing computer, while working with the electromagnetic wave radiation that information, can be in a mile outside be accepted and reiteration, this is the foreign intelligence door intelligence gathering the principal way. The war in Iraq to reveal the information electronic warfare has become an important decision today's war twilight factors. On the one hand, military electronic equipment and fighting weapons platform must possess reliable anti-jamming and stealth performance; On the other hand, its own signal radiation, leakage, should be as low as possible, in order to ensure the safety of military information and themselves. Electromagnetic radiation on normal operation constitutes the harm of society has aroused people's great attention, for electronic equipment to adopt effective shielded can effective to prevent the outside incident electromagnetic interference, and can prevent electric equipment, instrument at work generates electromagnetic wave radiation hazards homework personnel and other equipment, and avoid interference information disclosure. Therefore have world-wide vigorously develop electromagnetic shielding techniques and shielding material research, such as: shielding coating, electroplating, vacuum metalizing aluminum, etc. Including electromagnetic shielding coating with its material widely, construction is convenient flexible, practical strong and high performance-cost ratio and extremely popular. Statistics show that in 1998, in the United States use electromagnetic shielding coating methods of various shielding ways of 78]. 2 the basic principle of electromagnetic shielding electromagnetic shielding, as shown in figure 1 shows the basic principle is: using low resistance of conductive materials, and using electromagnetic wave in shielding conductor surface reflection in the absorption and internal conductor transmission process in loss and make the electromagnetic wave energy pass on that hindered reduced to won't disturb the extent of instrumental working, rise to shield effect. Good shielding materials can be reflected off most of the incident wave, using internal absorption and multiple reflection losses off part into the material of electromagnetic waves, the only allowed a small bit of electromagnetic wave through materials continued spread. Waves through shielding materials of total shielding represented by shielding effectiveness for a given, defined as sources outside shielding, in a certain point the shield returns before installing electric field or magnetic field intensity than, namely the shield returns to electromagnetic signal attenuation extent, the value of the said materials size shielding, good or bad, usually measured in decibels. Namely  : among them, PI, EI respectively incident power density and incident electric intensity, PT, ET for through power density and field strength. As shown in figure 1 shows, electromagnetic shielding effectiveness by reflection losses, absorption loss and multiple reflection losses composition. Namely  SE = A R B (dB) (2) type: A attenuation, R for the reflection surface in shielding attenuation, B for internal multiple reflections attenuation (when in A greater than when 15dB negligible). The electromagnetic radiation area with field source by for the center, one wavelength in radius circle could be divided into area near field and distant district field. Area near field of electromagnetic field intensity than distant district field. From this point, the emphasis will be on electromagnetic protection area near field. For our most often contact from short-wave section 30MHz to microwave segment of the 3000MHz band range, their wavelengths from 10 ~ 1-m. Type, t for shields thickness (cm); F for frequency (Hz); R for material than magnetic conductance, sigma r for material than conductivity. So, A with t, fr sigma r increases while the increase, R with sigma R/rf increases with increase so materials on the conductivity of good, namely material resistivity is lower, the absorption and reflection losses will increase shielding performance is good. The national security agency (NSA) regulation, shield effectively lower limit for 40dB, corresponding volume resistivity about 10-4 Ω m., surface resistivity of around 0.5 Ω / /. In practical application, decide the best shielding scheme shall comprehensive consideration of the required shielding grades and paint coating substrate after the screen effect, physical and mechanical properties, satisfying the requirements of shield premise and lower costs, while good physical and mechanical properties. 3 electromagnetic shielding coating of conductive mechanism of electromagnetic shielding coating according to its composition and conductive mechanism can be divided into this size and add type two kinds big. This shape electromagnetic shielding coating is refers to the coating film where high-molecular compounds itself has conductivity, no need to add other conductivity components. Conductive polymer with light quality, but by molecular design resistivity adjusting etc advantages, but at this stage the synthesis and processing difficulties, high cost and application is restricted, at present is limited to laboratory studies, there has been commercialized products appear. At present, said electromagnetic shielding coating usually refers to add type electromagnetic shielding coatings. Using insulation polymer film content, join for conductive filler provide free charge carriers and make it has electrical conductivity. The charge of conduction modes have two kinds, one is particle interaction bypass formation pathways, The distance between the two is when particles 10nm extremely hours (within), due to produce "tunneling effect", although between have polymer film, carriers have enough energy through this energy barriers in particle transmission between. Therefore, shielding coating on the conductivity of mainly depends on the packing in polymer dispersing states makings, i.e., conductive circuits on how to form and conductive how the circuitry conductive two aspects. Four electromagnetic shielding coating research status by adding type electromagnetic shielding coating base material, electric fillers, additives and solvent composition. Makings is an important part of the conductive coatings, shall, in accordance with the first to coating (or insulation) the substrate properties selection, to get good physical and mechanical performance, Secondly shall make added conducting filler in them fully formed conductive circuits, dispersive, produce shielding effect. For the same kind of conductive fillers, polymer of critical wet surface tension is lesser, be helpful for polymer filler particle wetting and decentralizing, make the conductive particles and contact number increase, hence it is easy to conduction, reduce the conductive filler of seepage critical point. Because of conductive particles and contact number increased, and the film's highest conductivity increases. In addition, polymer molecular weight also affects the dispersion and conductive between particles clearance thickness, thereby affecting the coating electrical conductivity. In satisfying coating physical and mechanical properties of the precondition of and the relative molecular mass with small is preferred. Commonly used synthetic resin has acrylic resin, epoxy, polyurethane, phenolic resin etc. With polyimide According to the type of the conductive filler, mainly divided into silver department, carbon department, ni-based and copper is electromagnetic shielding coatings, etc. Common metal conductivity shown in table 1. 4.1 silver is conducting filler silver is the earliest development of conductive filler. Silver is best, oxidation of coating conductivity bit slower and oxidation product can be conductive, thermal conductivity, good medium function is admirable, shielding effect is admirable (surface resistivity below 0.010 / cm, can be in as much as the scope of 10GHz 75dB above). But the silver as conductive filler also exists some problems: the expensive price tag, the limit its only in shielding demands extremely strict environment use; Silver migration will give electronic products miniaturization brings difficulties. So-called silver migration, is when silver as electrode and dc current flows from the anode silver in the cathode is dentrites growth, cause short- circuit phenomenon. Prevent silver migration are the most effective ways to reduce coatings of moisture. Recently, ZhaoBin etc  by hydrazine AgNO3 solution phase reduction with legal prepared ultrafine silver used in conductive paint, can reduce the fabic dosage and keep coating good electrical conductivity. GaoXiaoMin was studied by introducing dispersant and coupling agent of conductive coatings shielding performance influence . 4.2 copper is conducting filler copper powder of price moderate, conductivity is also extremely good, but in the air for easy oxidation and oxide nonconductive and restricted. Current copper powder oxidation technology has made great progress, is currently the most commercial value of conductive filler. Current copper powder oxidation technology mainly include: (1) the surface plating inert metals (Ag, Al, Ni etc) ; (2) join reductant will copper powder on the surface of the copper oxide reduction, (3) organophosphorus hydrates, processing, (4) polymer dilute solution treatment; (5) with coupling agent handling . Practical application, in order to achieve better antioxidant effect, can be integrated with the proposed method. Surface coated with silver for the currently the most widely used method, its surface resistivity can reach 0.075 ~ 0.10 / cm, in more than 1GHz within the scope of the shield performance can reach 75dB.  4.3 ni-based conductive filler nickel powder prices also comparison modest, the oxidation resistance between silver and copper powder, therefore between in use process without as antioxidant processing, good hardness to film provides excellent durability performance. Nickel electrical conductive silver, copper low, its surface resistivity is 1 cm /, shielding tiptop 65dB (5 ~ 60 ~ 1800MHz), ni-based coatings in the low frequency area (below 30MHz) shielding effect than copper department coatings. But ni-based coating, magnetic permeability of high attenuation, large range of vector absorb electromagnetic interference ability. Nickel has excellent oxidation resistance, anti chemical corrosivenese etc can meet the application requirements, so ni-based electromagnetic shielding coatings in coating class in the greater importance . American Acheson company produces the Electrodag 550 type ni-based shielding coatings have been applied for 20 years. In recent years, domestic sichuan university on ni-based conductive coatings have done a lot of research and made great progress . 4.4 carbon is conducting filler carbon conductive filler mainly includes the carbon black and graphite. According to the shape of the division, basically have powder and fiber two kinds big. In manufacturing conductive coatings, often be mixed with carbon graphite use or performance of different carbon black mixed used to meet different requirements of operation. Because carbon conductive filler belong to semiconductor, by the formation of the coating is far less than the conductivity filler metal kind formation of conductive paint, its surface resistivity is 30 ~ 50 / cm, screen shadowing effect maximum 30 ~ 50dB (50 ~ 450MHz), only in pairs shield less demanding environment is used to save cost. But in recent years, the ultrafine carbon black achieved a series of achievements, has greatly improved its shield performance. The United States has developed a ultrafine carbon black, can be used in the manufacture of electromagnetic shielding materials. Such as Cabot company's "Super - Conductive carbon black and the chemical company" Columbia "Conductex40-220" carbon black. This mitsubishi rayon, company research of ultrafine carbon black/PP, its density for 118g/cm3, its shielding effect to 40dB, known as the world's lightest electromagnetic shielding materials. 4.5 composite conductive filler in order to reduce the cost and improve the conductive filler conductivity, often USES composite conductive filler. Composite conductive filler according to shape can be divided into composite powder and composite fiber. According to the core of nuclear material is different, metal-coated apply type composite powder can be divided into the metal to metal (such as es-ag/Cu), metal - non-metal (such as Cu/graphite) and metal - ceramic (such as es-ag/SiO2) 3 kinds. Besides a metal oxide include apply type composite powder. Composite fiber are various, such as nylon, spun glass, carbon fiber etc plating deposited metal or metal oxide, etc. This kind of conducting general as the above several main packing add ingredients used on conductive coatings, the shield performance fine-tune, flexible and adapt to the needs of various situations. Now the domestic and foreign several typical electromagnetic shielding coating listed in table 2 . 5 conclusion and prospect with rapid development of electronic industry, people to electromagnetic radiation hazards rise ceaselessly, the understanding of high- performance shielding materials demand is growing. But the present stage high shield performance coatings are mainly focused on silver department, silver plated copper powder two categories, its cost is relative taller, adding amount is larger also, so the current needs to solve the main problem is: (1) the guarantee coating shield performance and improve the dispersion of packing in base material to reduce packing dosage, (2) developing recycling sex shielding coatings, raise the utilization rate of conductive filler. Abroad in recent years has been in this aspect has made great progress, and a series of product appearance, domestic is still in the blank. (3) developing water-borne conductive paint in order to adapt to the need of environment, especially suitable for shielding indoor wall use conductive paint to replace the existing generally with sheet metal building shielding enclosures of technology. In the domestic and foreign difference is very big also: at present domestic conductive paint is normally for solvent, Foreign company 008A Acheson ElectrodagSP - namely for water- borne coatings, and can be used in the construction and utilization. (4) preparation of low cost efficient shielding materials. Conductive polymer such as: polyaniline, poly [b] thiophene, polypyrrole intercalative, etc., which has close to metal conductivity and the conductivity by changing the number of molecules conjugate p-bond to adjust, if can solve their industrialization, issues in the prospect of electromagnetic interference (emi) shielding nots allow to ignore. 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