magnetic material
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magnetic material
A substance whose magnetization can be induced or changed by a magnetic field. According to the strength of magnetism, substances can be divided into diamagnetism, paramagnetism, ferromagnetism, antiferromagnetism and ferrimagnetism. Ferromagnetic and ferrimagnetic substances are strong magnetic substances, and the rest are weak magnetic substances. Most of the practical magnetic materials in modern engineering belong to ferromagnetic materials, which are usually called ferromagnetic materials.
Magnetic materials are widely used. Mainly using its various magnetic characteristics and special effects to make components or devices; Used for storing, transmitting and converting electromagnetic energy and information, or generating a magnetic field with a certain intensity and distribution in a specific space; Sometimes it is directly used in the natural form of matter (such as magnetic liquid). Magnetic materials play an important role in electronic technology and other scientific and technological fields.
A Brief History China is the first country in the world to discover the magnetic phenomena of matter and apply magnetic materials. As early as the Warring States period, there were records about natural magnetic substances (such as magnetite). 165438+ The method of manufacturing artificial permanent magnet materials was invented in the 20th century. 1086 Meng Qian recorded the production and use of compass. From 1099 to 1 102, the phenomenon of geomagnetic declination was also found by navigation with a compass. Since modern times, the development of electric power industry has promoted the development of metal magnetic material-silicon steel sheet (ferrosilicon alloy). Permanent magnet metal developed from carbon steel in19th century to later rare earth permanent magnet alloy, and its performance was improved by more than 200 times. With the development of communication technology, soft magnetic metal materials, from flake to filament to powder, still cannot meet the requirements of frequency expansion. In the 1940s, J.L. Si Nuo Ike of the Netherlands invented ferrite soft magnetic materials with high resistivity and good high frequency characteristics, and then low-cost permanent ferrite appeared. In the early 1950s, with the development of electronic computers, Wang An, an American Chinese, first used rectangular magnetic alloy components as the internal memory of computers, which was quickly replaced by rectangular magnetic ferrite storage cores, and played an important role in the development of computers in the 1960s and 1970s. In the early 1950s, ferrite was found to have unique microwave characteristics, and a series of microwave ferrite devices were made. Piezoelectric materials have been used in sonar technology since World War I, but their usage has decreased due to the appearance of piezoelectric ceramics. Later, rare earths with strong piezomagnetism appeared.
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Alloys. Amorphous (amorphous) magnetic materials are the results of modern magnetic research. After the invention of rapid quenching technology, 1967 solved the problem of belt making technology and has been put into practical use.
Classification magnetic materials are classified according to their magnetic functions, including permanent magnets, soft magnets, torque magnets, rotating magnets and piezomagnetic materials; According to chemical composition, there are metal magnets and ferrite; According to the structure, there are single crystal, polycrystalline and amorphous magnets; According to the form, there are magnetic films, plastic magnets, magnetic liquids and magnetic blocks. Magnetic materials are usually classified by function.
After being magnetized by an external magnetic field, the permanent magnetic material can still maintain part or most of its original magnetization direction even under the action of a considerable reverse magnetic field. The requirements for such materials are high remanence induction Br, strong coercivity BHC (i.e. anti-demagnetization ability) and large magnetic energy product (BH)max (i.e. magnetic field energy provided to space). Compared with soft magnetic materials, it is also called hard magnetic materials.
Permanent magnet materials include alloys, ferrites and intermetallic compounds. ① Alloys: including casting, sintering and machinable alloys. The main varieties of casting alloys are: AlNi(Co), FeCr(Co), FeCrMo, FeAlC, FeCo (V) (W); Sintered alloy has: Re-Co (RE stands for rare earth element), Re-Fe, AlNi(Co), FeCrCo, etc. Machinable alloys include FeCrCo, PtCo, MnAlC, CuNiFe and AlMnAg, and the lower BHC of the latter two is also called semi-permanent magnet material. ② Ferrite: the main component is Mo 6Fe2O3, and m represents Ba, Sr, Pb or composite components such as SrCa and LaCa. ③ Intermetallic compounds: mainly represented by MnBi.
Permanent magnet materials have many uses. ① Applications based on the principle of electromagnetic force mainly include: speakers, microphones, electric meters, buttons, motors, relays, sensors, switches, etc. ② The applications based on magnetoelectric principle mainly include magnetron, traveling wave tube and other microwave tubes, picture tubes, titanium pumps, microwave ferrite devices, magnetoresistance devices, Hall devices and so on. ③ Applications based on the principle of magnetism mainly include: magnetic bearings, concentrators, magnetic separators, magnetic suction cups, magnetic seals, magnetic blackboards, toys, signs, coded locks, photocopiers, temperature controllers, etc. Other applications include: magnetic therapy, magnetized water, magnetic anesthesia, etc.
Permanent magnet materials can have different structures and shapes according to the needs of use. There are also differences between isotropic and anisotropic materials.
The main functions of soft magnetic materials are magnetic conductivity and electromagnetic energy transfer. Therefore, this kind of material is required to have high permeability and magnetic induction strength, and at the same time, the area or magnetic loss of hysteresis loop is required to be small. Contrary to permanent magnet materials, the smaller the Br and BHC, the better, but the larger the saturation magnetic induction intensity Bs. Soft magnetic materials can be generally divided into four categories. ① Alloy strip or sheet: FeNi(Mo), FeSi, FeAl, etc. ② Amorphous alloy belt: iron-based, cobalt-based, FeNi-based or FeNiCo-based, with appropriate doping elements such as Si, B and P added, also called magnetic glass. ③ Magnetic medium (iron powder core): powder materials such as FeNi(Mo), FeSiAl, carbonyl iron, ferrite, etc., which are coated and bonded with electrical insulating medium and pressed into shape as required. ④ Iron and oxygen
Main body: including spinel type-M++-M++O Fe (M+++
2O3 represents NiZn, MnZn, MgZn, Li 1/2Fe 1/2Zn, CaZn, etc. ), magnesite type ── Ba3me24O4 1 (Me stands for cobalt, nickel, magnesium, zinc, copper and their composite components).
Soft magnetic materials are widely used in magnetic antennas, inductors, transformers, magnetic heads, earphones, relays, vibrators, TV deflection coils, cables, delay lines, sensors, microwave absorbing materials, electromagnets, high-frequency accelerating cavities of accelerators, magnetic field probes, magnetic substrates, magnetic shielding, high-frequency quenching energy gathering, electromagnetic suction cups, magnetic sensitive elements (such as magneto-thermal materials as switches) and so on.
Magnetic moment magnetic materials and magnetic recording materials are mainly used for information recording, contactless switching, logical operation and information amplification. This material is characterized by a rectangular hysteresis loop.
Gyromagnetic materials have unique microwave magnetism, such as tensor characteristics of permeability, Faraday rotation, * * * * vibration absorption, field shift, phase shift, birefringence and spin wave effect. The devices designed based on this are mainly used for the transmission and conversion of microwave energy, such as isolators, circulators, filters (fixed or electrically adjustable), attenuators, phase shifters, modulators, switches, limiters and delay lines, as well as magnetic surface wave and magnetostatic wave devices still under development (see microwave ferrite).
Batch equipment). A series of commonly used materials have been formed, including ferrite materials such as Ni series, Mg series, Li series, YlG series and BiCaV series. It can be made into different structures and shapes such as single crystal, polycrystalline, amorphous or thin film according to the needs of devices.
Piezoelectric materials are characterized by mechanical deformation under the action of external magnetic field, so they are also called magnetostrictive materials. Their function is to convert magnetic sound or magnetic energy. It is often used in vibration head of ultrasonic generator, mechanical filter of communication machine and delay line of electric pulse signal. Combined with microwave technology, micro-acoustic (or rotary acoustic) devices can be made. Ni-based and NiCo-based alloys are often used in vibration heads because of their high mechanical strength, vibration resistance and non-bursting. Nickel-based and nickel-based iron are mainly used for small signals.
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Oxygen. A new amorphous alloy with strong piezomagnetism is suitable for making delay lines. The production and application of piezomagnetic materials are far less than the first four materials. It is expected that the basic law of magnetoelectricity will lead to the mutual promotion and development of magnetic materials and electronic technology. For example, optoelectronic technology has promoted the development of magneto-optical materials and magneto-optical materials. Magnetic semiconductor materials and magnetic sensitive materials and devices can be applied to remote sensing, remote sensing technology and robots. People are studying new amorphous and rare earth magnetic materials (such as FeNa alloy). Magnetic liquid has entered the practical stage. The discovery of some new physical and chemical effects (such as topological effects) also provides conditions for the development and application of new materials (such as the application of magnetoacoustics and magnetocaloric effect). philology
Dai Lizhi, Metal Magnetic Materials, Shanghai People's Publishing House, Shanghai, 1973. Zhou Zhigang et al., Ferrite Magnetic Materials, Science Press, Beijing.
Li, Ferrite Physics, 2nd Edition, Science Press, 1983.
Materials with ferromagnetic properties. Magnetic materials commonly used in electrical technology can be divided into two categories: soft magnetic materials with high permeability, low coercivity and low remanence, and permanent magnetic materials with high coercivity and high remanence. Permanent magnetic materials are also called hard magnetic materials.
Magnetism is the basic property of matter. According to its internal structure and properties in external magnetic field, substances can be divided into diamagnetism, paramagnetism, ferromagnetism, antiferromagnetism and ferrimagnetism. Ferromagnetic and ferrimagnetic substances are strong magnetic substances, while others are weak magnetic substances.
Magnetic materials are isotropic and anisotropic.
Gender distinction. The magnetism of anisotropic materials is different in different directions. Therefore, when using anisotropic materials, we must pay attention to its magnetic direction. Magnetic materials commonly used in the field of electrical engineering belong to strong magnetic substances. The basic magnetism of magnetic materials is magnetization curve, hysteresis loop and magnetic loss. The magnetization curve and hysteresis loop reflect the magnetization characteristics of magnetic materials. It can be used to determine some basic characteristic parameters of magnetic materials, such as permeability μ, saturation magnetic flux density Bs, residual magnetic field strength (coercive force Hc), residual magnetic flux density (remanence Br), hysteresis loss P, etc. The basic magnetization curve is the curve of B changing with H in the process of repeated magnetization, which is called magnetization curve for short (Figure 1). It is the basis for determining the working point of soft magnetic materials. The relationship between b and h is as follows: b = μ 0 (h+m)
Where μ0 is the permeability (also called magnetic constant) of vacuum, and in the International System of Units (SI), its value is
μ=4π× 10-7
0 heng/m; H is the magnetic field strength in ampere/meter (a/m); M is the magnetization in ampere/meter (A/m). In the figure, the value of B when magnetized to saturation is called saturated magnetic flux density Bs, and the corresponding magnetic field intensity is Hs. Generally, magnetic materials are required to have a high Bs value.
The ratio of b to h at any point on the magnetization curve is the permeability μ, that is, for isotropic permeability.
Material μ = b/h, and relative permeability μ r is commonly used.
=μ/μ0, which is a dimensionless pure number used to express
Represents the magnetization ability of a substance. So according to the size of μ r, all kinds of substances are divided into: μ r.
Diamagnetism of< 1
Matter, μ r > 1, μr paramagnetic material
1 ferromagnetic substance. According to the B-H curve, μ-H can be described.
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The curve, micron and μi in the figure are called maximum permeability and initial permeability respectively. μi is an important parameter for using soft magnetic materials in low magnetic field.
Fig. 2 shows the closed curve formed by the change of B with H when the external magnetic field H changes once.
Because the change of b lags behind the change of h, this phenomenon is called hysteresis. The closed curve is called hysteresis loop. As can be seen from the figure, when Hs is reduced to zero, B does not return to zero, but only reaches point B. This value (Br) is called residual magnetic flux density, or remanence for short. In order to reduce Br to zero, an anti-magnetic field needs to be added. The absolute value of this diamagnetic field strength is called
Magnetic induction coercivity, abbreviated as coercivity Hrr.
The ratio of C. B to Bs is called remanence ratio or switching rectangle ratio (B/Bs), which indicates the degree to which the hysteresis loop of rectangular magnetic material is close to rectangle. The shape and area of hysteresis loop directly characterize the main magnetic properties of magnetic materials.
Soft magnetic material has narrow hysteresis loop, so its coercivity and hysteresis loss are low (Figure 3a), and it is often used in iron core magnetic circuits of motors, transformers and relays. If the hysteresis loop is narrow and nearly rectangular (called rectangular magnetic material)
Material) (fig. 3c), which has not only low coercivity, but also Br.
The value of /Bs is also very high, and it is suitable as a storage element.
And a switching element. The hysteresis loop area of permanent magnet material is very wide (Figure 3b), Br
After saturation magnetization, the stored magnetic field energy is large. It is usually used as a permanent magnetic pole of generators and motors, as well as a permanent magnet in measuring instruments and speakers.
Magnetic loss The magnetic material per unit weight is magnetized in an alternating magnetic field, and the power absorbed from the changing magnetic field and dissipated in the form of heat is called magnetic loss or iron loss P, which is mainly caused by hysteresis loss and eddy current loss. Among them, the energy loss caused by hysteresis is called hysteresis loss, which is proportional to the area surrounded by hysteresis loop. The hysteresis loss power Ph can be calculated by the following formula: Ph=кhBmnV.
Where is the frequency (Hz); Bm is the maximum magnetic flux density (t); Exponent n is an empirical parameter, which is related to the size of Bm; V is the volume of the magnetic substance; кh is the coefficient related to the properties of ferromagnetic materials. In the alternating magnetic field, conductive substances (including ferromagnetic substances) will induce eddy currents, and the resistance loss caused by eddy currents is called eddy current loss. The power Pe of eddy current loss can be calculated by the following formula.
e=кeBmnV
Where e is the coefficient related to the resistivity, cross-sectional size and shape of the material. Ph and Pe are important parameters to measure the quality of electrical equipment and instruments.
Materials with strong magnetism. The microscopic feature of this kind of materials is that the magnetic moments of adjacent atoms or ions are arranged in order, thus showing ferromagnetism or ferrimagnetism. The macroscopic feature is that it has obvious magnetization under the action of external magnetic field. 4 / 17
According to chemical composition, it can be basically divided into two categories: metallic magnetic materials and ferrite. ① Metal magnetic materials. Mainly iron, nickel, cobalt and their alloys, such as iron-silicon alloy, iron-nickel alloy, iron-cobalt alloy, samarium-cobalt alloy, platinum-cobalt alloy, manganese-aluminum alloy and so on. They have metallic conductivity, usually ferromagnetic, high saturation magnetization, high Curie temperature and low temperature coefficient, and have large eddy current loss and skin effect in alternating electromagnetic fields, so metallic soft magnetic materials are usually suitable for low-frequency and high-power power power and electronics industries. For example, the saturation magnetic induction intensity of silicon steel sheet is about 2T (Tesla), which is five times larger than that of ordinary ferrite, and it is widely used as power transformer. At present, metal permanent magnet materials have high magnetic energy product, which can be used to make small and light permanent magnet devices, especially in aerospace and other aerospace science and technology fields. Its disadvantage is that nickel, cobalt and rare earth metals are expensive and the material sources are few. ② Ferrite. Refers to the magnetic oxide with iron oxide as the main component, which was translated as "iron oxide magnet" in the early stage, referred to as "iron oxide" for short. Because its preparation process follows the process of ceramics and powder metallurgy, it is sometimes called magnetic porcelain. Most of them are ferrimagnetic, so the saturation magnetization is low, but their resistivity is higher than that.
Metal magnetic materials are high 106.
It has low loss in alternating electromagnetic field and shows unique advantages in high frequency, microwave and optical frequency band applications. Considering the crystal structure, ferrites are mainly divided into: spinel type (like natural MgAl2O4 spinel), such as manganese-zinc ferrite, nickel-zinc ferrite and so on; Garnet type [isomorphous with natural (Fe, Mn)3Al2(SiO4)3 garnet], such as yttrium iron garnet ferrite (Y3Fe5O 12), etc. ; Hexagonal ferrite, such as barium, is isomorphic to natural Pb (Fe 7.5) Mn 3.5 Al 0.5 Ti 0.5) O19 magnesite.
Ferrite (BaFe)O2+
12 19), Y-ferrite with easy magnetization axis in hexagonal plane (Ba2MeFe 12)O22), etc. According to the classification of applications, it can be divided into six categories (because of the wide variety and wide application of magnetic materials, these six categories can not be completely summarized).
(1) permanent magnetic material, also known as hard magnetic material. It has high coercivity and remanence. The quality factor of permanent magnet materials is usually measured by the maximum magnetic energy product (BH) m, such as Al-Ni-Co alloy, Sm-Co alloy, Mn-Al alloy, Fe-Cr-Co alloy, Ba ferrite and Sr ferrite.
② Soft magnetic materials. It has low coercivity and narrow hysteresis loop. Usually at initial permeability,
The equivalent values of saturated magnetic induction intensity and AC loss mark its main performance. The main materials are pure iron, iron-silicon alloy system, iron-nickel alloy system, manganese-zinc ferrite, nickel-zinc ferrite and so on. Soft magnetic materials are the most diverse and widely used magnetic materials, which are mainly used as transformers in the power industry.
Motor and
The magnetic materials of generators are made into various magnetic components in the electronic industry, which are widely used in television, broadcasting, communication and other fields.
③ Rectangular magnetic material. The hysteresis loop is rectangular, but the coercivity is very small. Generally, the static characteristics of soft magnetic materials are marked by the rectangular ratio Br/Bm of the ratio of remanence BR to maximum magnetic induction intensity BM. The main materials are lithium manganese ferrite, manganese magnesium ferrite and so on. Used in electronic computers, automatic control and other technologies, it is often used as a material for memory elements, switches and logic elements.
④ gyromagnetic materials. Magnetic materials using gyromagnetic effect are usually used in microwave frequency band, and their main properties are marked by complex tensor permeability and saturation magnetization. Commonly used materials are garnet ferrite and lithium ferrite. Various types of microwave devices can be manufactured, such as isolators, circulators, phase shifters, etc. Since 1952, the application of ferrite in microwave field has promoted the revolutionary change of microwave technology. A series of non-reciprocal microwave devices can be manufactured only by using the tensor permeability characteristics of ferrite. Using the nonlinear effect of ferrite, we can design a series of active devices, such as frequency multiplier and oscillator. ⑤ Piezoelectric materials. Magnetic materials using magnetostrictive effect are usually used for the mutual conversion between mechanical energy and electrical energy, and their main performance is marked by magnetostrictive coefficient. For example, it can be made into various ultrasonic devices, filters, magnetic twisted wire memories, vibrometers, etc. Commonly used materials are nickel sheet, nickel ferrite and so on. At present, people are deeply studying the magneto-acoustic coupling effect to open up new application fields.
⑥ Magnetic recording materials. It mainly includes magnetic head materials and magnetic recording media. The former belongs to soft magnetic materials and the latter belongs to permanent magnetic materials. Because of its importance in application and special requirements in performance, it is classified as another category. In addition to the general characteristics of soft magnetic materials, magnetic head materials usually require high recording density and low wear. Commonly used are hot-pressed polycrystalline ferrite, single crystal ferrite, Al-Si-Fe alloy, cemented carbide and so on. Magnetic recording media need a large remanence value and a suitably high coercivity value.
So as to transmit electrical information through the magnetic head.
Some residual tracks are recorded on the magnetic tape. The commonly used material is γ-iron oxide. Materials with high recording density include chromium dioxide metal films. At present, magnetic recording has been widely used in various fields, such as recording, coding and video recording. Therefore, the output of magnetic recording materials has increased dramatically in recent years. Broadly speaking, foam materials also belong to this category.
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Magnetic materials are constantly developing. Such as amorphous magnetic materials, magnetic semiconductors, etc. , is a very active research field at present. The use of magnetic materials is more and more extensive.
philology
Li and co-editor. Physics of ferrite, revised edition, Science Press, Beijing, 1978. Guo Yicheng: Ferromagnetism, Higher Education Press, 1965. R.S. Teber and D.J. craik, translated by Beijing Metallurgical Research Institute: Magnetic Materials, Science Press, Beijing, 1979. (R.S.Tebble and D.J.Craik, Magnetic Materials, Willy Inters cience, London, 1969. )
In a broad sense, ferromagnetic substances with magnetic sequence also include weak magnetic and antiferromagnetic substances that can apply their magnetic and magnetic effects. Magnetism is the basic property of matter. According to its internal structure and properties in external magnetic field, substances can be divided into diamagnetism, paramagnetism, ferromagnetism, antiferromagnetism and ferrimagnetism. Ferromagnetic and ferrimagnetic substances are strong magnetic substances, while diamagnetic and paramagnetic substances are weak magnetic substances. Magnetic materials can be divided into metal and nonmetal according to their properties. The former mainly includes electrical steel, nickel-based alloy and rare earth alloy, while the latter is mainly ferrite material. It can be divided into soft magnetic materials, permanent magnetic materials and functional magnetic materials according to their uses. Functional magnetic materials mainly include magnetostrictive materials, magnetic recording materials, [[magnetoresistance materials], bubble materials, magneto-optical materials, gyromagnetic materials and magnetic thin film materials. , reflecting the basic magnetic properties of magnetic materials, such as magnetization curve, hysteresis loop, magnetic loss, etc.
magnetite
The power absorbed by magnetic material per unit mass in alternating magnetic field and dissipated in the form of heat is called magnetic loss, or iron loss, including hysteresis loss and eddy current loss. Among them, the energy loss caused by hysteresis is hysteresis loss, which is proportional to the area surrounded by hysteresis loop. In an alternating magnetic field, conductive substances will induce eddy currents.
The resistance loss caused by eddy current is called eddy current loss.
Chapter 2: Selection and design points of electromagnetic shielding materials.
Selection and design points of electromagnetic shielding materials
Shielding is the metal isolation between two spatial areas to control the induction and radiation of electric field, magnetic field and electromagnetic wave from one area to the other. Specifically, shielding is used to surround the interference sources of components, circuits, components, cables or the whole system to prevent the interference electromagnetic field from spreading outward; Surround the receiving circuit, equipment or system with a shield to prevent them from being affected by external electromagnetic fields. Because the shielding body can absorb energy (eddy current loss), reflect energy (interface reflection of electromagnetic waves on the shielding body) and cancel energy (electromagnetic induction generates a reverse electromagnetic field on the shielding body layer, which can cancel some interference electromagnetic waves) to external interference electromagnetic waves and internal electromagnetic waves from wires, cables, components, circuits or systems, the shielding body has the function of reducing interference.
(1) When the frequency of the interference electromagnetic field is high, the eddy current generated in the metal material with low resistivity is used to cancel the external electromagnetic wave, so as to achieve the shielding effect.
(2) When the frequency of interfering electromagnetic waves is low, materials with high magnetic permeability should be used to confine the magnetic field lines in the shielding body to prevent them from spreading into the shielding space.
(3) In some cases, if both high-frequency and low-frequency electromagnetic fields are needed.
When it has good shielding effect, different metal materials are often used to form multi-layer shielding bodies.
Many people don't understand the principle of electromagnetic shielding, and think that as long as a box is made of metal and then grounded, it can play the role of electromagnetic shielding. Under the guidance of this concept, the result is failure. Because electromagnetic shielding has nothing to do with whether the shield is grounded or not. There are only two factors that really affect the shielding effectiveness of the shielding body: one is that the whole surface of the shielding body must be conductive and continuous, and the other is that there can be no conductor that directly penetrates the shielding body. There are many conductive discontinuities on the shield, and the most important one is the non-conductive gap formed at the joint of different parts of the shield. These non-conductive gaps will produce electromagnetic leakage, just as fluid will leak from gaps in containers. One way to solve this leakage is to fill the gap with conductive elastic materials to eliminate non-conductive points. It's like filling rubber in the gap of a fluid container. This elastic conductive filling material is the electromagnetic gasket.
In many documents, electromagnetic shielding is compared to a liquid-tight container. It seems that only by sealing the gap with conductive elastic material to waterproof level can electromagnetic wave leakage be prevented. In fact, this is not accurate. Because whether the gap or hole will leak electromagnetic waves depends on the size of the gap or hole relative to the wavelength of electromagnetic waves. When the wavelength is larger than the opening size, there will be no obvious leakage. Therefore, when the frequency of interference is high and the wave length is short, electromagnetism should be used.
Sealing washer. Specifically, when the interference frequency exceeds 10MHz, electromagnetic sealing gasket should be considered.
Any elastic and conductive material can be used as the electromagnetic gasket. The electromagnetic gasket manufactured according to this principle includes:
Conductive rubber: Silicone rubber is filled with metal particles accounting for 70 ~ 80% of the total weight, such as silver powder, copper powder, aluminum powder, silver-plated copper powder, silver-plated aluminum powder and silver-plated glass balls. This material retains some good elasticity of silicone rubber and has good conductivity.
Metal woven mesh: it is a long tubular strip made of beryllium copper wire, monel wire or stainless steel wire, which looks like the shielding layer of shielded cable. But its weaving method is different from that of cable shielding layer. The cable shielding layer is woven with multiple wires, and this shielding gasket is woven with one wire. Figuratively speaking, it's like the sleeve of a sweater. In order to enhance the elasticity of metal mesh, rubber core is sometimes added to the network management.
Finger spring: a reed made of beryllium copper, which has good elasticity and conductivity. Electrical conductivity and elasticity.
Multi-conductive rubber: consists of two layers of rubber, the inner layer is ordinary silicone rubber and the outer layer is conductive rubber. This material overcomes the disadvantage of poor elasticity of traditional conductive rubber.
Point, so that the elasticity of rubber is fully reflected. Its principle is a bit like a wire mesh strip with a rubber core.
When choosing what kind of electromagnetic sealing gasket to use, four factors should be considered: shielding efficiency requirements, environmental sealing requirements, installation structure requirements and cost requirements. The comparison of characteristics of different gasket materials is shown in the following table.
According to the mechanism, shielding can be divided into electric field shielding, magnetic field shielding and electromagnetic field shielding.
1 shielding mechanism of electric field: electric field induction is regarded as the coupling between distributed capacitors.
Design points:
A, shielding plate should be close to the protected object, shielding plate grounding must be good! ! !
B, the shape of the shielding plate has obvious influence on the shielding efficiency. A fully enclosed metal box is the best, but it is difficult to do in engineering!
C the shielding plate is made of a good conductor, but there is no requirement for its thickness, as long as it has sufficient strength.
2 magnetic field shielding magnetic field shielding usually refers to the shielding of DC or low frequency magnetic field
Shielding, its effect is much worse than electric field shielding and electromagnetic field shielding Shielding mechanism: It mainly relies on the low reluctance of high permeability materials to shunt the magnetic flux, which greatly weakens the magnetic field in the shield.
Design points:
A, selecting materials with high magnetic permeability, such as permalloy;
B, increasing the thickness of the shield; All of the above is to reduce the reluctance of the shield; C, the shield should not be arranged near the shield, and the magnetic flux passing through the shield should be reduced as much as possible;
D, pay attention to the structural design of the shield, joints, ventilation, etc. It may increase the magnetic resistance of the shield, thus reducing the shielding effect.
E, for the shielding of strong magnetic field, double-layer magnetic shielding structure can be adopted. In order to shield the external strong magnetic field, the outer layer of the shield is made of materials that are not easily saturated, such as silicon steel; The interior can be made of high permeability materials that are easy to reach saturation, such as permalloy. On the other hand, if the internal strong magnetic field is to be shielded, the arrangement order of materials should be reversed. When installing internal and external shields, pay attention to the insulation between them. When there is no grounding requirement, it can be supported by insulating materials. If grounding is needed, non-ferromagnetic materials (such as copper and aluminum) can be selected as the support.
3 electromagnetic field shielding electromagnetic field shielding is a measure to prevent electromagnetic field from spreading in space by using shielding body.
Chapter 3: Characteristics of ferromagnetic materials.
Characteristics of ferromagnetic materials
Ferromagnetic material has strong magnetization characteristics, and its slip ring can generate additional magnetic field much larger than the external magnetic field under the action of external magnetic field. The magnetic field of coils with iron core is much stronger than that of coils without iron core, so motors, electrical appliances and other equipment should use iron core. This carbon brush sample can generate a strong magnetic field with a small current, which greatly reduces the volume and weight of the coil.
Iron materials mainly include constant pressure springs, which have the following magnetism:
① High magnetic permeability. In the case of -throw, the permeability μ of ferromagnetic material is much larger than that of non-ferromagnetic material.
② remanence. After the ferromagnetic material is started by a magnetic brushless loop-less starter, if the excitation current is reduced to O, some remanence can still be retained in the ferromagnetic material.
3 magnetic saturation. When the magnetic field in ferromagnetic materials increases to a certain value, the magnetic field enhancement becomes extremely slow and reaches the saturation value.
4 lag. In the process of alternating magnetization of ferromagnetic materials, the change of magnetic induction intensity lags behind the change of magnetic field intensity and hysteresis loss per mu.
Ferromagnetic materials are usually divided into two categories, soft materials and magnetic materials. The remanence and hysteresis loss of soft magnetic materials are small, and the commonly used soft magnetic materials are silicon steel sheet (electrical steel sheet), cast steel and cast iron. Hard magnetic materials have great remanence and hysteresis loss. Hard magnetic materials can obtain strong remanence after magnetization and are not easy to demagnetize. Commonly used hard magnetic materials are tungsten steel, Al-Ni-diamond alloy and so on. , mainly used for manufacturing permanent magnets.