Manufacturing and studying the properties of magnetic screens

The purpose of the work is the study of screening methods using HTSC devices, the production of volumetric and thick-film screens, and the study of their field attenuation coefficients.

General information

Shielding represents protection of the volume from the effects of external electric, magnetic or electromagnetic fields. As a rule, this volume contains a device that needs protection from this field. Depending on the type and orientation of the screened field, the material and design of the screen are selected. For example, magnetic fields are traditionally shielded using ferromagnetic structures, and electromagnetic fields are traditionally shielded using conductor structures. The design can be in the shape of a sphere, a glass with a bottom, a long cylinder, etc.

Application superconducting materials made it possible to significantly improve the weight and size parameters of shielding structures, however, the need to use liquid helium limits the use of such shields.

The use of HTSC electromagnetic screens at frequencies on the order of sound seems quite promising, since the use of ordinary metals, such as copper or aluminum, requires a large screen thickness (the corresponding skin layer thicknesses are several centimeters). Permalloy and other screens with high magnetic permeability are also characterized by large dimensions and weight.

For single-crystal HTSC samples, the penetration depths are fractions of a micrometer. For polycrystalline samples it is significantly larger (10 μm), however, the use of HTSC screens, shielding packages of integrated circuits, etc. is promising in comparison with other methods. The physical basis of the screen's operation is the Meissner-Ochsenfeld effect. The external magnetic field in a superconductor decreases with depth:

B(x) = B(0) exp(- x / λ L), (4.9)

Where x– distance from the surface,

λ L– London penetration depth.

For low temperature superconductors λ L=10 -7 m, therefore weak fields practically do not penetrate into the bulk superconductor. For real HTSCs, as already noted, this value is much larger. If the magnitude of the external magnetic field becomes comparable to the value of the lower critical field, a type II superconductor can transition to an intermediate state. In this case, the sample is divided into alternating superconducting and normal regions (Shubnikov state) and a magnetic field penetrates into it. The field induction at which the sample goes into the Shubnikov state is determined by its shape and the critical properties of the material. For a screen in the form of a cylinder with a flat bottom and the ratio of the internal to external diameter is not more than 0.7, this field (perpendicular to the cylinder axis) can be determined from the expression

B ││ = B C 1 [(1-d/D)/2] 1/2 , (4.10)

Where B C 1 – induction of the first critical field of the material;

D, d– external and internal diameters of the screen.

The axial field induction, at which the screen material passes into an intermediate state, is approximately equal to the critical field induction.

For HTSC materials the picture becomes more complicated due to the fact that they are granular conglomerates, where there are Josephson junctions between the SP granules. In this case, the shielding properties are associated with the value of the critical field of intergranular bonds, at which the field begins to penetrate into the HTSC.

Typically, HTSC magnetic screens are made by single-sided, double-sided or hydrostatic pressing of HTSC powder and subsequent firing. This method is suitable for making small screens. However, this method is not suitable for the manufacture of long cylinders or screens of more complex shapes (spheres). In this case, discrete screens consisting of ring fragments are used. In previous work, such fragment rings were made that can be assembled into a long cylinder. Such fragments can be made by applying thin or thick films to a ceramic base.

Shielding factor (field weakening) TO is defined as the ratio of the internal field magnitude B i to the outside - B e:

TO= B i / B e. (4.11)

The measurement is carried out as follows. A screen with a field sensor is placed inside a solenoid that sets the external field. The sensor used is a fluxgate sensor or, as in our case, a Hall sensor. The solenoid on the rod is lowered into a Dewar flask containing liquid nitrogen. The entire system is located inside a vertically installed two-layer ferromagnetic screen with a weakening coefficient of the Earth's magnetic field of about 100.

A resistor is connected in series with the solenoid winding. The voltage drop across the resistor is proportional to the magnitude of the external magnetic field of the solenoid, the Hall EMF is proportional to the magnitude of the internal field. From the graph U x= f( I c) you can estimate the field weakening coefficient for a given screen.

Rice. 4.8. Thick film fragment-ring of a magnetic screen:
1 – ceramics, 2 – film

Rice. 4.9. Temperature regime for burning HTSC film: T 1 =120°C (30 min) V 1 =30ºC/h; T 2 =910-915°C (10-20 min); T 3 =895°С, V 2 =6ºС/h; T 4 =860°C

Tasks

1). Obtain thick-film ring fragments.

1.1. Apply paste (Bi-2212 powder and 10–15% organic binder) to the ceramic base (Fig. 4.8).

1.2. Burn the paste in an electric oven (Fig. 4.9).

Rice. 4.10. Magnetic screen: Ф – ring-fragments of the screen; D – Hall sensor;
a – distance between fragment rings; L – solenoid winding

2). Assemble magnetic shields.

2.1. Assemble a screen from volumetric fragment rings.

2.2. Assemble a screen from film ring fragments.

3). Measure the shielding coefficient of volumetric and film screens.

3.1. Assemble a circuit to measure the shielding coefficient (Fig. 4.11).

Rice. 4.11. Installation diagram for measuring the shielding coefficient: IP - power supplies, D - Hall sensor, C - two-axis recorder; L – solenoid;
R – resistor

3.2. Get charts B i= f( B e).

3.3. By changing the distance between the rings, get graphs K=B i/B e= f( a).

4). Prepare a report containing graphs and their comparative assessment.

Control questions

1. How is shielding carried out?

2. What screens are there?

3. Which devices require shielding?

4. Describe and explain the Meissner effect.

5. Describe Shubnikov’s condition.

6. What are Abrikosov vortices?

7. Explain the nature of dependence x=f( a).

8. How does the attenuation coefficient measuring device work?

Literature

1. Krasov V.G. and others. Thick film technology in microwave microelectronics / Krasov V.G., Petratskas G.B., Chernozubov Yu.S. – M.: Radio and Communications, 1985.- 168 p.

2. Bondarenko S.I., Sheremet V.I. Application of superconductivity in magnetic measurements - L.: Energoatomizdat, 1982.-132 p.

Conclusion

In this book we examined the main issues of design and technology of high-temperature cryoelectronics. Due to the limited volume of the manual and the desire to save the reader’s time, the most important theoretical and practical issues were considered. Many essential points, not sufficiently “advanced” in practical terms, remained out of sight.

Recently it was 90 years since the discovery of superconductivity and 40 years since low-temperature superconducting technologies, including cryoelectronics, were born on the basis of superconducting materials and cryogenic technology at helium temperatures. One of its first elements was a wire cryotron. Over the past years, low-temperature cryoelectronics has undergone significant development: digital devices based on cryotrons (first film, and then Josephson) were invented; receivers and converters of microwave signals, devices based on SQUIDs, etc.

More than 15 years have passed since the discovery of high-temperature superconductivity - an event that should have stimulated work in the field of superconductivity in general and cryoelectronics in particular. And so it happened: the number and scope of research in this area increased sharply in 1996 and is now quite significant.

However, despite obvious successes, high-temperature cryoelectronics is still in its infancy, for various reasons.

Today, a dramatic and tense state of affairs in the field of HTSC research remains. Expectations remain high in this regard. The government and industrial firms, which have invested and continue to invest large amounts of money in HTSC research, closely monitor the applied aspects of research, fearing to miss the moment of a breakthrough into the knowledge-intensive (and therefore promising, prestigious and profitable) HTSC market. Great expectations force us to scrupulously evaluate both the current state of research and its market potential.

The reasons hindering the development of cryoelectronics also include:

· poor knowledge of cryoelectronic processes in cooled structures and films,

· insufficiency of real design and technological ideas for creating integrated cryoelectronic devices and especially reliable, reproducible, multi-element, multi-layer integrated circuits with submicron gaps.

There are practically no methods for reducing the energy intensity and weight and size indicators of cryostats, or increasing the period of their continuous operation.

In other words, it is necessary to find solutions that will make the results obtained cheap, reproducible, and accessible. We hope that the knowledge and skills you acquire will help you solve your problems.

Principles of magnetic field shielding

Two methods are used to shield the magnetic field:

Bypass method;

Screen magnetic field method.

Let's take a closer look at each of these methods.

Method of shunting a magnetic field with a screen.

The method of shunting a magnetic field with a screen is used to protect against a constant and slowly changing alternating magnetic field. Screens are made of ferromagnetic materials with high relative magnetic penetration (steel, permalloy). If there is a screen, the lines of magnetic induction pass mainly along its walls (Figure 8.15), which have low magnetic resistance compared to the air space inside the screen. The quality of shielding depends on the magnetic permeability of the shield and the resistance of the magnetic circuit, i.e. The thicker the screen and the fewer seams and joints running across the direction of the magnetic induction lines, the shielding efficiency will be higher.

Method of displacement of a magnetic field by a screen.

The method of displacement of a magnetic field by a screen is used to screen alternating high-frequency magnetic fields. In this case, screens made of non-magnetic metals are used. Shielding is based on the phenomenon of induction. Here the phenomenon of induction is useful.

Let's place a copper cylinder in the path of a uniform alternating magnetic field (Figure 8.16a). Variable EDs will be excited in it, which, in turn, will create alternating inductive eddy currents (Foucault currents). The magnetic field of these currents (Figure 8.16b) will be closed; inside the cylinder it will be directed towards the exciting field, and outside it - in the same direction as the exciting field. The resulting field (Figure 8.16, c) turns out to be weakened near the cylinder and strengthened outside it, i.e. the field is displaced from the space occupied by the cylinder, which is its shielding effect, which will be the more effective, the lower the electrical resistance of the cylinder, i.e. the greater the eddy currents flowing through it.

Thanks to the surface effect (“skin effect”), the density of eddy currents and the intensity of the alternating magnetic field decreases exponentially as one goes deeper into the metal

, (8.5)

Where (8.6)

– indicator of the decrease in field and current, which is called equivalent penetration depth.

Here is the relative magnetic permeability of the material;

– magnetic permeability of vacuum, equal to 1.25*10 8 g*cm -1;

– resistivity of the material, Ohm*cm;

- frequency Hz.

The value of the equivalent penetration depth is convenient to characterize the shielding effect of eddy currents. The smaller x0, the greater the magnetic field they create, which displaces the external field of the pickup source from the space occupied by the screen.

For a non-magnetic material in formula (8.6) =1, the shielding effect is determined only by and . What if the screen is made of ferromagnetic material?

If they are equal, the effect will be better, since >1 (50..100) and x 0 will be less.

So, x 0 is a criterion for the shielding effect of eddy currents. It is of interest to estimate how many times the current density and magnetic field strength become lower at depth x 0 compared to what they are at the surface. To do this, we substitute x = x 0 into formula (8.5), then

from which it can be seen that at a depth of x 0, the current density and magnetic field strength drop by e times, i.e. to a value of 1/2.72, which is 0.37 of the density and tension on the surface. Since the field weakening is only 2.72 times at depth x 0 not enough to characterize the shielding material, then use two more values of penetration depth x 0.1 and x 0.01, which characterize the drop in current density and field voltage by 10 and 100 times from their values on the surface.

Let's express the values x 0.1 and x 0.01 through the value x 0; for this, based on expression (8.5), we create the equation

AND ,

having decided which we get

x 0.1 = x 0 ln10 = 2.3x 0 ; (8.7)

x 0.01 = x 0 ln100 = 4.6x 0

Based on formulas (8.6) and (8.7) for various shielding materials, the values of penetration depths are given in the literature. For clarity purposes, we present the same data in the form of table 8.1.

The table shows that for all high frequencies, starting from the medium wave range, a screen made of any metal with a thickness of 0.5..1.5 mm is very effective. When choosing the thickness and material of the screen, you should not proceed from the electrical properties of the material, but be guided by considerations of mechanical strength, rigidity, resistance to corrosion, convenience of joining individual parts and making transition contacts with low resistance between them, convenience of soldering, welding, etc.

From the table data it follows that for frequencies greater than 10 MHz, a film of copper, and even more so of silver, with a thickness of less than 0.1 mm gives a significant shielding effect. Therefore, at frequencies above 10 MHz, it is quite acceptable to use screens made of foil getinax or other insulating material with a copper or silver coating applied to it.

Steel can be used as screens, but you need to remember that due to the high resistivity and hysteresis phenomenon, a steel screen can introduce significant losses into the shielding circuits.

Filtration

Filtration is the main means of reducing constructive interference created in the power supply and switching circuits of direct and alternating current ES. Noise suppression filters designed for this purpose make it possible to reduce conducted noise from both external and internal sources. Filtration efficiency is determined by the attenuation introduced by the filter:

dB,

The following basic requirements are imposed on the filter:

Ensuring the specified efficiency S in the required frequency range (taking into account the internal resistance and load of the electrical circuit);

Limitation of the permissible drop in direct or alternating voltage across the filter at maximum load current;

Ensuring acceptable nonlinear distortions of the supply voltage, which determine the requirements for filter linearity;

Design requirements - shielding efficiency, minimum overall dimensions and weight, ensuring normal thermal conditions, resistance to mechanical and climatic influences, manufacturability of the design, etc.;

Filter elements must be selected taking into account the rated currents and voltages of the electrical circuit, as well as the voltage and current surges caused in them, caused by instability of the electrical regime and transient processes.

Capacitors. They are used as independent noise suppression elements and as parallel filter units. Structurally, noise suppression capacitors are divided into:

Two-pole type K50-6, K52-1B, ETO, K53-1A;

Support type KO, KO-E, KDO;

Feed-through non-coaxial type K73-21;

Feedthrough coaxial type KTP-44, K10-44, K73-18, K53-17;

Capacitor units;

The main characteristic of a noise suppression capacitor is the dependence of its impedance on frequency. To reduce interference in the frequency range up to approximately 10 MHz, two-pole capacitors can be used, taking into account the short length of their leads. Reference noise suppression capacitors are used up to frequencies of 30-50 MHz. Symmetrical pass capacitors are used in a two-wire circuit up to frequencies of the order of 100 MHz. Pass capacitors operate over a wide frequency range up to approximately 1000 MHz.

Inductive elements. They are used as independent noise suppression elements and as sequential links of noise suppression filters. Structurally, the most common types of chokes are:

Turning on a ferromagnetic core;

Turn-free.

The main characteristic of a noise suppression choke is the dependence of its impedance on frequency. At low frequencies, it is recommended to use magnetodielectric cores of the PP90 and PP250 brands, made on the basis of m-permalloy. To suppress interference in equipment circuits with currents up to 3A, it is recommended to use HF chokes of the DM type, and for higher rated currents - chokes of the D200 series.

Filters. Ceramic pass-through filters of type B7, B14, B23 are designed to suppress interference in circuits of direct, pulsating and alternating currents in the frequency range from 10 MHz to 10 GHz. The designs of such filters are shown in Figure 8.17

The attenuation introduced by filters B7, B14, B23 in the frequency range 10..100 MHz increases from approximately 20..30 to 50..60 dB and in the frequency range above 100 MHz exceeds 50 dB.

Ceramic feed-through filters of the B23B type are built on the basis of ceramic disk capacitors and turn-free ferromagnetic chokes (Figure 8.18).

Turn-free chokes are a tubular ferromagnetic core made of grade 50 VCh-2 ferrite, mounted on a feed-through terminal. The inductance of the inductor is 0.08…0.13 μH. The filter housing is made of UV-61 ceramic material, which has high mechanical strength. The housing is metalized with a layer of silver to ensure low contact resistance between the outer lining of the capacitor and the grounding threaded bushing, which is used to secure the filter. The capacitor is soldered along the outer perimeter to the filter housing, and along the inner perimeter to the feed-through terminal. Sealing of the filter is ensured by filling the ends of the housing with a compound.

For B23B filters:

nominal filter capacitances – from 0.01 to 6.8 µF,

rated voltage 50 and 250V,

rated current up to 20A,

Overall dimensions of the filter:

L=25mm, D= 12mm

The attenuation introduced by B23B filters in the frequency range from 10 kHz to 10 MHz increases from approximately 30..50 to 60..70 dB and in the frequency range above 10 MHz exceeds 70 dB.

For onboard ES, the use of special noise-suppressing wires with ferrofillers having high magnetic permeability and high specific losses is promising. So, for PPE brand wires, the insertion attenuation in the frequency range 1...1000 MHz increases from 6 to 128 dB/m.

The design of multi-pin connectors is known, in which one U-shaped noise suppression filter is installed on each contact.

Overall dimensions of the built-in filter:

length 9.5 mm,

diameter 3.2 mm.

The attenuation introduced by the filter in a 50-ohm circuit is 20 dB at a frequency of 10 MHz and up to 80 dB at a frequency of 100 MHz.

Filtering of power supply circuits of digital electronic devices.

Pulse noise in power buses that occurs during the switching of digital integrated circuits (DIC), as well as penetrating externally, can lead to malfunctions in the operation of digital information processing devices.

To reduce the level of noise in power buses, circuit design methods are used:

Reducing the inductance of the “power” buses, taking into account the mutual magnetic coupling of the direct and return conductors;

Reducing the lengths of sections of “power” buses, which are common for currents for various digital information systems;

Slowing down the edges of pulse currents in the “power” buses using noise-suppressing capacitors;

Rational topology of power circuits on a printed circuit board.

Increasing the cross-sectional dimensions of the conductors leads to a decrease in the intrinsic inductance of the buses, and also reduces their active resistance. The latter is especially important in the case of the ground bus, which is the return conductor for signal circuits. Therefore, in multilayer printed circuit boards, it is desirable to make “power” buses in the form of conducting planes located in adjacent layers (Figure 8.19).

The overhead power buses used in printed circuit assemblies on digital ICs have larger transverse dimensions compared to busbars made in the form of printed conductors, and therefore have lower inductance and resistance. Additional advantages of mounted power buses are:

Simplified routing of signal circuits;

Increasing the rigidity of the PP by creating additional ribs that act as limiters that protect the IC with mounted ERE from mechanical damage during installation and configuration of the product (Figure 8.20).

The “power” bars, manufactured by printing and mounted vertically on the PCB, are highly technologically advanced (Figure 6.12c).

There are known designs of mounted busbars installed under the IC body, which are located on the board in rows (Figure 8.22).

The considered designs of the “supply” buses also provide a large linear capacitance, which leads to a decrease in the wave impedance of the “supply” line and, consequently, a decrease in the level of impulse noise.

The power distribution of the IC to the PCB should not be carried out in series (Figure 8.23a), but in parallel (Figure 8.23b)

It is necessary to use power distribution in the form of closed circuits (Fig. 8.23c). This design is close in its electrical parameters to solid power planes. To protect against the influence of an external interference-carrying magnetic field, an external closed loop should be provided along the perimeter of the PP.

Grounding

The grounding system is an electrical circuit that has the property of maintaining a minimum potential, which is the reference level in a particular product. The grounding system in the electrical system must provide signal and power return circuits, protect people and equipment from faults in power source circuits, and remove static charges.

The following basic requirements apply to grounding systems:

1) minimizing the overall impedance of the ground bus;

2) the absence of closed grounding loops sensitive to magnetic fields.

The ES requires at least three separate grounding circuits:

For signal circuits with low currents and voltages;

For power circuits with high power consumption (power supplies, ES output stages, etc.)

For body circuits (chassis, panels, screens and metallization).

Electrical circuits in the ES are grounded in the following ways: at one point and at several points closest to the grounding reference point (Figure 8.24)

Accordingly, grounding systems can be called single-point and multi-point.

The highest level of interference occurs in a single-point grounding system with a common series-connected ground bus (Figure 8.24 a).

The further away the grounding point is, the higher its potential. It should not be used for circuits with a large spread of power consumption, since high-power FUs create large return ground currents that can affect small-signal FUs. If necessary, the most critical FU should be connected as close as possible to the reference ground point.

A multipoint grounding system (Figure 8.24 c) should be used for high-frequency circuits (f≥10 MHz), connecting the RES FU at the points closest to the reference grounding point.

For sensitive circuits, a floating ground circuit is used (Figure 8.25). This grounding system requires complete isolation of the circuit from the chassis (high resistance and low capacitance), otherwise it is ineffective. The circuits can be powered by solar cells or batteries, and signals must enter and leave the circuit through transformers or optocouplers.

An example of the implementation of the considered grounding principles for a nine-track digital tape drive is shown in Figure 8.26.

There are the following ground buses: three signal, one power and one body. The analog FUs most susceptible to interference (nine sense amplifiers) are grounded using two separated ground buses. Nine write amplifiers, which operate at higher signal levels than the read amplifiers, as well as control ICs and interface circuits with data products are connected to the third signal bus, ground. The three DC motors and their control circuits, relays and solenoids are connected to the power bus ground. The most sensitive driveshaft motor control circuit is connected closest to the ground reference point. The chassis ground bus is used to connect the chassis and casing. The signal, power, and chassis ground buses are connected together at one point in the secondary power supply. It should be noted that it is advisable to draw up structural wiring diagrams when designing RES.

Usage: to obtain a space without a magnetic field, providing improved shielding quality. The magnetic screen is made in the form of a shell with a hatch, and the shell is located coaxially with a vertically installed ring of a permanent magnet with axial magnetization or two shells located movably relative to the ring, made of a composite or diamagnetic material. The invention can be used in medicine to relieve stress during magnetic storms, and in technology in the production of homogeneous permanent magnets, semiconductors, in the production and configuration of radio-electronic equipment. 3 salary positions, 2 ill.

The invention relates to measuring technology and can be used to obtain a space without a magnetic field in which adjustment and testing, for example, of flux-gate type sensors of radio measuring equipment are carried out. Magnetic screens made of ferromagnetic materials are known, for effective operation of which they use, for example, a demagnetizing inductor coil wound on the housing and a power source. Relatively often, to reduce the residual magnetic field, a screen made of several layers of ferromagnetic material is equipped with an additional demagnetizing winding. The disadvantage of such systems is the mandatory connection of the screen with a source of electrical energy, which is used with low efficiency. To reduce energy costs, screens made of superconducting material or containing superconducting windings are used. At the same time, the design becomes significantly more complicated and the use of the screen in field conditions is excluded. As a prototype, a shielded room device for magnetic measurements was used, which contains a frame with a multilayer ferromagnetic screen attached to it, support columns on the foundation, and lighting devices. However, in this case, as necessary, when the layers are magnetized by the Earth's magnetic field, a power source is connected to the tops of the corners of each layer. Thus, in this as in other cases, the Earth's magnetic field plays a negative role. To protect against it, various screens are created. The purpose of the invention is to improve the quality of shielding. This is achieved by the fact that the magnetic screen is made in the form of a shell with a hatch, and the shell is located coaxially with the ring of a permanent magnet with axial magnetization or two shells located movably relative to the ring, made of a composite or diamagnetic material, for example, copper. In fig. 1 shows a permanent ring magnet; Fig.2 - topography of the magnetic field of the ring. The permanent ring magnet 1 is made with an internal radius R and thickness , shells 3 with hatches 4 for access inside the shell are installed on the support 2 at a distance S from the ring. Figure 2 shows the topography of the magnetic field of a ring with axial magnetization, having specific regions l and k. The personnel places the devices used for work (setting) in the shell 3 (if necessary, they place it themselves) and installs it at a distance S, determined by the characteristic dimensions of the ring R and . In this case, the location of the shells coincides with the regions l and k, in which the magnetic field of the ring is zero. These areas are magnetic vacuum. The vertical placement of the ring simplifies its installation and the relative position of the shells. In this case, the magnetic field of the ring can either coincide or, in the general case, not coincide with the Earth’s magnetic field. The shell material is selected taking into account its neutrality to the magnetic field. In particular, such a material can be either a composite or a diamagnetic material. The use of a permanent magnet with axial magnetization in the form of a ring and a given arrangement of shells makes it possible to create volumes without a magnetic field, which, for example, can be used in medicine to relieve stress during magnetic storms, in technology for the production and configuration of radio-electronic equipment, in advanced industries ( production of homogeneous, permanent magnets, semiconductors, LSIs, etc.).

Claim

1. MAGNETIC SCREEN, made in the form of a shell with a hatch, characterized in that the shell is located coaxially with the ring of a permanent magnet with axial magnetization at a distance s = (2-2.5), where s is the distance from the plane of symmetry of the ring to the center of natural delamination the magnetic field of the said ring, in which the magnetic screen is located with its center aligned with the center of the natural stratification of the magnetic field from the magnetic source (ring); - the thickness of the ring, and the radius of the shell is commensurate with the radius of the central hole of the annular permanent magnet. 2. The screen according to claim 1, characterized in that it contains two shells located coaxially and movably relative to the permanent magnet. 3. Screen according to claims 1 and 2, characterized in that the shells are made of composite material. 4. Screen according to claims 1 and 2, characterized in that the shells are made of diamagnetic material, for example copper.

MAGNETIC SHIELDING

(magnetic) - protection of an object from magnetic influences. fields (constant and variable). Modern Research in a number of fields of science (physics, geology, paleontology, biomagnetism) and technology (space research, nuclear energy, materials science) is often associated with measurements of very weak magnetic fields. fields ~10 -14 -10 -9 T in a wide frequency range. External magnetic fields (for example, the Earth's T with noise T, magnets from electrical networks and urban transport) create strong interference with the operation of highly sensitive devices. magnetometric equipment. Reducing the influence of magnetic fields strongly determines the possibility of conducting magnetic fields. measurements (see, for example, Magnetic fields of biological objects). Among the methods of M. e. the most common are the following.

Shielding hollow cylinder made of ferromagnetic substance with ( 1 - ext. cylinder, 2 -internal surface). Residual magnetic field inside the cylinder

Ferromagnetic screen- sheet, cylinder, sphere (or any other shape) made of material with high magnetic permeability m low residual induction In r and small coercive force N s. The principle of operation of such a screen can be illustrated using the example of a hollow cylinder placed in a homogeneous magnetic field. field (fig.). External induction lines mag. fields B when passing from the medium into the screen material, the external fields become noticeably denser, and in the cavity of the cylinder the density of the induction lines decreases, i.e., the field inside the cylinder turns out to be weakened. Field weakening is described by f-loy

Where D- cylinder diameter, d- the thickness of its wall is mag. permeability of the wall material. To calculate the effectiveness of M. e. volumes decom. configurations often use file

where is the radius of the equivalent sphere (almost the average value of the screen dimensions in three mutually perpendicular directions, since the shape of the screen has little effect on the efficiency of the magnetoelectric system).

From formulas (1) and (2) it follows that the use of materials with high magnetic field. permeability [such as permalloy (36-85% Ni, rest Fe and alloying additives) or mu-metal (72-76% Ni, 5% Cu, 2% Cr, 1% Mn, rest Fe)] significantly improves the quality of screens (at iron). The seemingly obvious method of improving shielding by thickening the wall is not optimal. Multilayer screens with gaps between layers work more efficiently, for which the coefficients are shielding is equal to the product of coefficient. for dept. layers. It is multilayer screens (external layers of magnetic materials that are saturated at high values IN, internal ones - made of permalloy or mu-metal) form the basis of the designs of magnetically protected rooms for biomagnetic, paleomagnetic, etc. studies. It should be noted that the use of protective materials such as permalloy is associated with a number of difficulties, in particular with the fact that their magnesium. properties under deformation and that means. heat deteriorate, they practically do not allow welding, which means. bends and other mechanical loads In modern mag. Ferromagnets are widely used in screens. metal glasses(metglasses), close in magnetism. properties to permalloy, but not so sensitive to mechanical influences. The fabric, woven from metglass strips, allows the production of soft magnets. screens of arbitrary shape, and multilayer shielding with this material is much simpler and cheaper.

Screens made of material with high electrical conductivity(Cu, A1, etc.) serve to protect against alternating magnetic fields. fields. When changing external mag. fields in the walls of the screen arise inductively. currents that cover the shielded volume. Magn. the field of these currents is directed opposite to the external one. indignation and partially compensates for it. For frequencies above 1 Hz coefficient. shielding TO increases in proportion to frequency:

Where - magnetic constant, - electrical conductivity of the wall material, L- screen size, - wall thickness, f- circular frequency.

Magn. screens made of Cu and A1 are less effective than ferromagnetic ones, especially in the case of low-frequency electromagnetic. fields, but ease of manufacture and low cost often make them more preferable for use.

Superconducting screens. The action of this type of screens is based on Meissner effect - complete displacement of magnet. fields from a superconductor. With any change in external mag. flow in superconductors, currents arise, which, in accordance with Lenz's rule compensate for these changes. Unlike ordinary conductors, inductive superconductors. the currents do not fade and therefore compensate for the change in flux during the entire period of existence of the external current. fields. The fact that superconducting screens can operate at very low temps and fields not exceeding critical. values (see Critical magnetic field), leads to significant difficulties in the design of large magnetically protected “warm” volumes. However, the discovery oxide high temperature superconductors(OBC), made by J. Bednorz and K. Müller (J. G. Bednorz, K. A. Miiller, 1986), creates new opportunities in the use of superconducting magnets. screens. Apparently, after overcoming the technological difficulties in the manufacture of SBCs, superconducting screens will be used from materials that become superconductors at the boiling point of nitrogen (and in the future, possibly at room temperatures).

It should be noted that inside the volume magnetically protected by the superconductor, the residual field that existed in it at the moment of the transition of the screen material to the superconducting state is preserved. To reduce this residual field it is necessary to take a special . For example, transfer the screen to a superconducting state at a low magnetic field compared to the earth's. field in the protected volume or use the “inflating screens” method, in which the folded shell of the screen is transferred to a superconducting state and then expanded. Such measures make it possible, for now, to reduce residual fields to a value of T in small volumes limited by superconducting screens.

Active interference protection carried out using compensating coils that create a magnetic field. a field equal in magnitude and opposite in direction to the interference field. When added algebraically, these fields cancel each other out. Naib. Helmholtz coils are known, which are two identical coaxial circular coils with current, separated by a distance equal to the radius of the coils. Fairly homogeneous mag. the field is created in the center between them. To compensate for three spaces. components require a minimum of three pairs of coils. There are many options for such systems, and their choice is determined by specific requirements.

An active protection system is typically used to suppress low-frequency interference (in the frequency range 0-50 Hz). One of its purposes is post compensation. mag. Earth's fields, which require highly stable and powerful current sources; the second is compensation for magnetic variations. fields, for which weaker current sources controlled by magnetic sensors can be used. fields, e.g. magnetometers high sensitivity - squids or fluxgates. To a large extent, the completeness of compensation is determined by these sensors.

There is an important difference between active magnetic protection. screens. Magn. screens eliminate noise throughout the entire volume limited by the screen, while active protection eliminates interference only in a local area.

All magnetic suppression systems interference need anti-vibration. protection. Vibration of screens and magnetic sensors. The field itself can become a source of additions. interference

Lit.: Rose-Ince A., Roderick E., Introduction to the physics of superconductivity, trans. from English, M., 1972; Stamberger G. A., Devices for creating weak constant magnetic fields, Novosibirsk, 1972; Vvedensky V.L., Ozhogin V.I., Ultrasensitive magnetometry and biomagnetism, M., 1986; Bednorz J. G., Muller K. A., Possible high Tc superconductivity in the Ba-La-Cr-O system, "Z. Phys.", 1986, Bd 64, S. 189. S. P. Naurzakov.

Physical encyclopedia. In 5 volumes. - M.: Soviet Encyclopedia. Editor-in-Chief A. M. Prokhorov. 1988 .

See what "MAGNETIC SHIELDING" is in other dictionaries:

magnetic shielding- A fence made of magnetic materials that surrounds the installation site of the magnetic compass and significantly reduces the magnetic field in this area. [GOST R 52682 2006] Topics of navigation, surveillance, control equipment EN magnetic screening DE... ... Technical Translator's Guide

magnetic shielding

Protection from magnetic fields using screens made of ferromagnetic materials with low values of residual induction and coercive force, but with high magnetic permeability... Big Encyclopedic Dictionary

Protection from magnetic fields using screens made of ferromagnetic materials with low values of residual induction and coercive force, but with high magnetic permeability. * * * MAGNETIC SHIELDING MAGNETIC SHIELDING, protection from... ... encyclopedic Dictionary

Magnetic protection fields using ferromagnetic screens. materials with low values of residual induction and coercive force, but with high magnetic field. permeability... Natural science. encyclopedic Dictionary

The term moment in relation to atoms and atomic nuclei can mean the following: 1) spin moment, or spin, 2) magnetic dipole moment, 3) electric quadrupole moment, 4) other electric and magnetic moments. Different types… … Collier's Encyclopedia

- (biomagnetism m). The vital activity of any organism is accompanied by the flow of very weak electricity inside it. currents of biocurrents (they arise as a consequence of the electrical activity of cells, mainly muscle and nerve cells). Biocurrents generate magnetism. field… … Physical encyclopedia

blindage magnetique- magnetinis ekranavimas statusas T sritis fizika atitikmenys: engl. magnetic screening vok. magnetische Abschirmung, f rus. magnetic shielding, n pranc. blindage magnétique, m … Fizikos terminų žodynas

magnetic screening- magnetinis ekranavimas statusas T sritis fizika atitikmenys: engl. magnetic screening vok. magnetische Abschirmung, f rus. magnetic shielding, n pranc. blindage magnétique, m … Fizikos terminų žodynas

magnetinis ekranavimas- statusas T sritis fizika atitikmenys: engl. magnetic screening vok. magnetische Abschirmung, f rus. magnetic shielding, n pranc. blindage magnétique, m … Fizikos terminų žodynas

The operating principle of most measuring instrument converters is based on the conversion of electrical and magnetic energies; therefore, electric and magnetic fields induced inside measuring instruments by nearby sources distort the nature of the conversion of electrical and magnetic energy in the measuring device. To protect sensitive elements of devices from the influence of internal and external electric and magnetic fields, shielding is used.

By magnetic shielding of any region of space we mean the weakening of the magnetic field inside this region by limiting it with a shell made of soft magnetic materials. In practice, another shielding method is also used, when a source of magnetic field is placed in the shell, thereby limiting the spread of the latter into the environment.

The basics of shielding are based on the theory of propagation of electric and magnetic fields. The emitted energy is transmitted by an electromagnetic field. When a field changes over time, its electric and magnetic components exist simultaneously, and one of them may be greater than the other. If the electrical component is greater, then the field is considered electric; if the magnetic component is greater, then the field is considered magnetic. Typically the field has a pronounced character near its source at a wavelength distance. In free space, at a large distance from the energy source (relative to the wavelength), both components of the field have an equal amount of energy. In addition, any conductor located in an electromagnetic field necessarily absorbs and emits energy again, therefore, even at small distances from such a conductor, the relative distribution of energy differs from the distribution of energy in free space.

The electric (electrostatic) component of the field corresponds to the voltage on the conductor, and the magnetic (electromagnetic) component corresponds to the current. Determining the need for one or another degree of shielding of a given electrical circuit, as well as determining the sufficiency of one or another type of shield, is almost beyond technical calculation, because theoretical solutions to individual simple problems turn out to be unacceptable for complex electrical circuits consisting of arbitrarily located space of elements emitting electromagnetic energy in a wide variety of directions. To calculate the screen, one would have to take into account the influence of all these individual radiations, which is impossible. Therefore, a designer working in this area is required to have a clear understanding of the physical action of each shielding part, its relative importance in the complex of shield parts, and the ability to perform approximate calculations of the effectiveness of the shield.

Based on the principle of operation, electrostatic, magnetostatic and electromagnetic screens are distinguished.

The shielding effect of a metal screen is determined by two reasons: the reflection of the field from the screen and the attenuation of the field when passing through the metal. Each of these phenomena is independent of one another and must be considered separately, although the overall shielding effect is the result of both.

Electrostatic shielding consists of closing an electric field on the surface of the metal mass of the screen and transferring electrical charges to the device body (Fig. 1.).

If between structural element A, which creates an electric field, and element B, for which the influence of this field is harmful, a screen B is placed, connected to the body (ground) of the product, then it will intercept the electric power lines, protecting element B from the harmful influence of element A. Consequently, the electric field can be reliably shielded even by a very thin layer of metal.

The induced charges are located on the outer surface of the screen so that the electric field inside the screen is zero.

Magnetostatic shielding is based on the closure of the magnetic field in the thickness of the screen, which has increased magnetic permeability. The screen material must have a magnetic permeability significantly greater than the magnetic permeability of the environment. The principle of operation of the magnetostatic screen is shown in Fig. 2.

The magnetic flux created by a structural element (in this case, a wire) is closed in the walls of the magnetic shield due to its low magnetic resistance. The greater the magnetic permeability and thickness of such a screen, the greater the effectiveness of such a screen.

A magnetostatic screen is used only with a constant field or in the range of low frequencies of change in the field.

Electromagnetic shielding is based on the interaction of an alternating magnetic field with eddy currents induced by it in the thickness and on the surface of the conductive material of the shield. The principle of electromagnetic shielding is illustrated in Fig. 3. If a copper cylinder (screen) is placed in the path of a uniform magnetic flux, then alternating E.M.F. will be excited in it, which, in turn, will create alternating induced eddy currents. The magnetic field of these currents will be closed (Figure 3b); inside the cylinder it will be directed towards the exciting field, and outside it - in the same direction as the exciting field. The resulting field turns out to be weakened (Fig. 3c) inside the cylinder and strengthened outside it, i.e. displacement occurs from the space occupied by the cylinder, which is its shielding effect.

The efficiency of electromagnetic shielding increases with increasing reverse field, which will be greater the greater the eddy currents flowing through the cylinder, i.e. the greater the electrical conductivity of the cylinder.

The attenuation of a magnetic field by a metal can be calculated. It is proportional to the thickness of the screen, the eddy current coefficient and the square root of the product of the field frequency, magnetic permeability and conductivity of the screen material.

When shielding product elements with magnetostatic and electromagnetic shields, it should be taken into account that they will also be effective as electrostatic shields if they are securely connected to the device body.

Equipment, instruments and tools

When performing the work, the following are used: installation for creating an electromagnetic field; special form signal generator G6-26; measuring coil for estimating the electromagnetic field strength; oscilloscope S1-64; voltmeter; a set of screens made from various materials.

The sine wave signal is supplied from the installation's signal generator through a step-down transformer. To connect the measuring coil 5 to the oscilloscope and the electromagnetic field excitation coil 1 to the signal generators, terminal sockets 6 and 7 are fixed on the base 3 of the installation. The installation is turned on by toggle switch 8.

To characterize the shielding material, two more values of penetration depth are used: x 0.1, x 0.01, which characterize the drop in field strength density (hole) by 10 and 100 times from the value on its surface

which are given in reference tables for various materials. Table 2 shows the values of x0, x0.1, x0.01, for copper, aluminum, steel and permalloy.

When choosing a shield material, it is convenient to use the shielding efficiency curves shown in the graphs in Fig. 4.

Characteristics of alloys for magnetic shields

Alloys with high magnetic permeability are used as materials for magnetic screens in weak fields. Permalloys, which belong to the group of malleable alloys with high magnetic permeability, are well processed by cutting and stamping. Based on their composition, permalloys are usually divided into low-nickel (40-50% Ni) and high-nickel (72-80% Ni). To improve electromagnetic and technological properties, permalloys are often alloyed with molybdenum, chromium, silicon, cobalt, copper and other elements. The main indicators of the electromagnetic quality of these alloys are the values of the initial µ initial and maximum µ max magnetic permeability. The coercive force H c of permalloys should be as small as possible, and the electrical resistivity ρ and saturation magnetization M s should be as high as possible. The dependence of these parameters for the Fe-Ni binary alloy on the percentage of nickel is shown in Fig. 5.

The characteristic µ initial (Fig. 5) has two maxima, relative (1) and absolute (2). The region of relative minimum, limited by a nickel content of 40-50%, corresponds to low-nickel permalloy, and the region of absolute maximum, limited by a nickel content of 72-80%, corresponds to high-nickel permalloy. The latter also has the largest value µ max. The flow of characteristics µ 0 M s and ρ (Fig. 5) indicates that the magnetic saturation and electrical resistivity of low-nickel permalloy are significantly higher than those of high-nickel permalloy. These circumstances differentiate the areas of application of low-nickel and high-nickel permalloys

Low-nickel permalloy is used for the manufacture of magnetic screens operating in weak constant magnetic fields. Alloyed with silicon and chromium, low-nickel permalloy is used at higher frequencies.

Alloys 79НМ, 80НХС, 81НМА, 83НФ with the highest magnetic permeability in weak magnetic fields and saturation induction of 0.5 -0.75 Tesla for magnetic screens, magnetic amplifier cores and contactless relays. Alloys 27KH, 49KH, 49K2F and 49K2FA, which have a high technical saturation induction (2.1 - 2.25 T), are used for magnetic shields that protect equipment from the effects of strong magnetic fields

Safety requirements

Before starting work

Understand the location and purpose of laboratory controls and measuring equipment.
Prepare the workplace for safe work: remove unnecessary objects from the table and installation.
Check: the presence and serviceability of the grounding system, the integrity of the installation body, power cords, plug connectors. Do not start work if the protective panels of the laboratory installation (stand) are removed.

During work

Work can only be carried out on working equipment.
It is not allowed to block the ventilation openings (louvres) in the buildings of laboratory installations with foreign objects.
Do not leave the unit turned on when you are away even for a short time.
In the event of a power outage, the installation must be turned off.

In emergency situations

The laboratory unit must be switched off immediately in the following cases:

accident or threat to human health;
the appearance of a smell characteristic of burning insulation, plastics, paint;
the appearance of crackling, clicking, sparking;
damage to the plug connection or electrical cable supplying the installation.

After finishing work

Turn off the laboratory unit and measuring instruments.
Disconnect the installation and measuring instruments from the network. Tidy up your workspace.
Remove foreign objects and clear any possible debris (unnecessary paper).

Task and research methodology

To experimentally determine areas of effective use of various materials for electromagnetic materials when the frequencies of the electromagnetic field change from 102 to 104 Hz.

Connect the installation for creating an electromagnetic field to the signal generator. Connect the measuring coil to the input of the oscilloscope and to the voltmeter. Measure the amplitude U of the signal, proportional to the strength of the electromagnetic field inside the cylindrical frame of the field excitation coil. Cover the measuring coil with a screen

Measure the amplitude U' of the signal from the measuring coil. Determine shielding effectiveness

at a given frequency and write it down in the table (see appendix).

Take measurements according to clause 5.1.1. for frequencies 100, 500, 1000, 5000, 104 Hz. Determine the shielding effectiveness at each frequency.

Tested screen samples. An experimental study of the properties of materials for magnetic screens is carried out using samples in

in the form of cylindrical glasses 9 (Fig. 6), the main parameters of which are given in Table 3.

Screens can be either single-layer or multi-layer with an air gap between them, cylindrical or with a rectangular cross-section. Calculation of the number of shield layers can be carried out using rather cumbersome formulas, so it is recommended to select the number of layers according to the shielding efficiency curves given in reference books.

1 - electromagnetic field excitation coil;

2 - non-magnetic frame;

3 - non-magnetic base;

4 - step-down transformer;

5 - measuring coil;

6 and 7 - terminal sockets;

8 - toggle switch;

9 - magnetic screen;

10 - signal generator;

11 - oscilloscope;

12 - voltmeter.

Carry out measurements for screens made of ordinary quality steel, permalloy, aluminum, copper, brass.

Based on the measurement results, construct shielding efficiency curves for various materials similar to Fig. 4. Analyze the results of the experiment. Compare the results of the experiment with reference data and draw conclusions.

To experimentally determine the influence of the thickness of the screen wall (glass) on the shielding efficiency.

For materials with high magnetic permeability (steel, permalloy), conduct the experiment in an electromagnetic field at frequencies of 100 Hz, 500 Hz, 1000 Hz, 5000 Hz, 10000 Hz according to the method outlined for screens with different wall thicknesses.

For materials with electrical conductivity (copper, aluminum), conduct the experiment at frequencies of 100 Hz, 500 Hz, 1000 Hz, 5000 Hz, 10000 Hz according to the described method.

Analyze the results of the experiment. Compare the results of the experiment with the data given in Table 1. Draw conclusions

LITERATURE

1. Grodnev I. I. Electromagnetic shielding in a wide range of frequencies. M.: Communication. 1972. - 275 p.

2. Design of devices. In 2 books. / Ed. V. Krause; Per. with him. V.N. Palyanova; Ed. O.F. Tishchenko. - Book 1-M.: Mechanical Engineering, 1987.

3. Materials in instrument making and automation: Directory / pod. ed. Yu.M. Pyatina. - 2nd ed. Reworked And additional - M.: Mechanical Engineering, 1982.

4. Obergan A.N. Design and technology of measuring instruments. Tutorial. - Tomsk, Rotaprint TPI. 1987. - 95 p.

5. Govorkov V.A. Electric and magnetic fields. - M. Svyazizdat, 1968.

6. Generator of sinusoidal signals G6-26. Technical description and operating instructions. 1980 - 88s.

7. Oscilloscope S1-64. Technical description and operating instructions.

Educational and methodological manual

Compiled by: Gormakov A. N., Martemyanov V. M.

Computer typing and layout by V. S. Ivanova