Electromagnetic shielding conductive coatings development present situation

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Electromagnetic shielding conductive coatings development present situation Powered By Docstoc
					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
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 [1]. 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 [2], 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 [4] : 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 [5] 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 [6] 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 [7]. 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) [8]; (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 [9]. 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. [10] 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 [11]. 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 [12]. 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 [13]. 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. Sub-micron, nanometer material the
volume effect and surface effect make it has special physical and chemical properties,
the electromagnetic field under the radiation, the atoms, electronic movement speed,
prompting magnetization, make the electromagnetic energy into heat energy, thus adding
to the electromagnetic wave absorption performance, but also because of specific
surface area, high, easy to form conductive network. Therefore, the application of
nanomaterials is developed for shielding coating trends. 参考文献:
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1995材料屏蔽效能的测量方法[S]. [5]Paul CR.Introduction to Electromagnetic Compatibility
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[J].涂料技术,2000,(1):40. [7]高晓敏,刘际伟,刘永华.分散助剂对丙烯酸基电磁屏蔽导
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