Everyone held a magnet in their hands and played with it as a child. Magnets can be very different in shape and size, but all magnets have a common property - they attract iron. It seems that they themselves are made of iron, at least of some kind of metal for sure. There are, however, “black magnets” or “stones”; they also strongly attract pieces of iron, and especially each other.
But they don’t look like metal; they break easily, like glass. Magnets have many useful uses, for example, it is convenient to “pin” paper sheets to iron surfaces with their help. A magnet is convenient for collecting lost needles, so, as we can see, this is a completely useful thing.
Science 2.0 - The Great Leap Forward - Magnets
Magnet in the past
More than 2000 years ago, the ancient Chinese knew about magnets, at least that this phenomenon could be used to choose a direction when traveling. That is, they came up with a compass. Philosophers in ancient Greece, curious people, collecting various amazing facts, encountered magnets in the vicinity of the city of Magnessa in Asia Minor. There they discovered strange stones that could attract iron. At that time, this was no less amazing than aliens could become in our time.
It seemed even more surprising that magnets do not attract all metals, but only iron, and iron itself can become a magnet, although not so strong. We can say that the magnet attracted not only iron, but also the curiosity of scientists, and greatly moved forward such a science as physics. Thales of Miletus wrote about the “soul of a magnet,” and the Roman Titus Lucretius Carus wrote about the “raging movement of iron filings and rings” in his essay “On the Nature of Things.” He could already notice the presence of two poles of the magnet, which later, when sailors began to use the compass, were named after the cardinal points.
What is a magnet? In simple words. A magnetic field
We took the magnet seriously
The nature of magnets could not be explained for a long time. With the help of magnets, new continents were discovered (sailors still treat the compass with great respect), but no one still knew anything about the very nature of magnetism. Work was carried out only to improve the compass, which was also done by the geographer and navigator Christopher Columbus.
In 1820, the Danish scientist Hans Christian Oersted made a major discovery. He established the action of a wire with an electric current on a magnetic needle, and as a scientist, he found out through experiments how this happens under different conditions. In the same year, the French physicist Henri Ampere came up with a hypothesis about elementary circular currents flowing in the molecules of magnetic matter. In 1831, the Englishman Michael Faraday, using a coil of insulated wire and a magnet, conducted experiments showing that mechanical work can be converted into electric current. He also established the law of electromagnetic induction and introduced the concept of “magnetic field”.
Faraday's law establishes the rule: for a closed loop, the electromotive force is equal to the rate of change of the magnetic flux passing through this loop. All electrical machines operate on this principle - generators, electric motors, transformers.
In 1873, Scottish scientist James C. Maxwell combines magnetic and electrical phenomena into one theory, classical electrodynamics.
Substances that can be magnetized are called ferromagnets. This name associates magnets with iron, but besides it, the ability to magnetize is also found in nickel, cobalt, and some other metals. Since the magnetic field has already entered the field of practical use, magnetic materials have become the subject of great attention.
Experiments began with alloys of magnetic metals and various additives in them. The resulting materials were very expensive, and if Werner Siemens had not come up with the idea of replacing the magnet with steel magnetized by a relatively small current, the world would never have seen the electric tram and the Siemens company. Siemens also worked on telegraph devices, but here he had many competitors, and the electric tram gave the company a lot of money, and ultimately pulled everything else along with it.
Electromagnetic induction
Basic quantities associated with magnets in technology
We will be interested mainly in magnets, that is, ferromagnets, and will leave a little aside the remaining, very vast area of magnetic (better said, electromagnetic, in memory of Maxwell) phenomena. Our units of measurement will be those accepted in SI (kilogram, meter, second, ampere) and their derivatives:
l Field strength, H, A/m (amps per meter).
This quantity characterizes the field strength between parallel conductors, the distance between which is 1 m, and the current flowing through them is 1 A. The field strength is a vector quantity.
l Magnetic induction, B, Tesla, magnetic flux density (Weber/m2)
This is the ratio of the current through the conductor to the length of the circle, at the radius at which we are interested in the magnitude of induction. The circle lies in the plane that the wire intersects perpendicularly. This also includes a factor called magnetic permeability. This is a vector quantity. If you mentally look at the end of the wire and assume that the current flows in the direction away from us, then the magnetic force circles “rotate” clockwise, and the induction vector is applied to the tangent and coincides with them in direction.
l Magnetic permeability, μ (relative value)
If we take the magnetic permeability of vacuum as 1, then for other materials we will obtain the corresponding values. So, for example, for air we get a value that is almost the same as for vacuum. For iron we get significantly larger values, so we can figuratively (and very accurately) say that iron “pulls” magnetic lines of force into itself. If the field strength in a coil without a core is equal to H, then with a core we get μH.
l Coercive force, A/m.
Coercive force measures how much a magnetic material resists demagnetization and remagnetization. If the current in the coil is completely removed, then there will be residual induction in the core. To make it equal to zero, you need to create a field of some intensity, but in reverse, that is, let the current flow in the opposite direction. This tension is called coercive force.
Since magnets in practice are always used in some connection with electricity, it should not be surprising that such an electrical quantity as ampere is used to describe their properties.
From what has been said, it follows that it is possible, for example, for a nail that has been acted upon by a magnet to become a magnet itself, albeit a weaker one. In practice, it turns out that even children who play with magnets know this.
There are different requirements for magnets in technology, depending on where these materials go. Ferromagnetic materials are divided into “soft” and “hard”. The first ones are used to make cores for devices where the magnetic flux is constant or variable. You cannot make a good independent magnet from soft materials. They demagnetize too easily, and this is precisely their valuable property, since the relay must “release” if the current is turned off, and the electric motor should not heat up - excess energy is spent on magnetization reversal, which is released in the form of heat.
WHAT DOES A MAGNETIC FIELD REALLY LOOK LIKE? Igor Beletsky
Permanent magnets, that is, those that are called magnets, require hard materials for their manufacture. Rigidity refers to magnetic, that is, a large residual induction and a large coercive force, since, as we have seen, these quantities are closely related to each other. Such magnets are used in carbon, tungsten, chromium and cobalt steels. Their coercivity reaches values of about 6500 A/m.
There are special alloys called alni, alnisi, alnico and many others, as you might guess they include aluminum, nickel, silicon, cobalt in different combinations, which have a greater coercive force - up to 20,000...60,000 A/m. Such a magnet is not so easy to tear off from iron.
There are magnets specifically designed to operate at higher frequencies. This is the well-known “round magnet”. It is “mined” from an unusable speaker from a stereo system, or a car radio, or even a TV of yesteryear. This magnet is made by sintering iron oxides and special additives. This material is called ferrite, but not every ferrite is specifically magnetized this way. And in speakers it is used for reasons of reducing useless losses.
Magnets. Discovery. How it works?
What happens inside a magnet?
Due to the fact that atoms of a substance are peculiar “clumps” of electricity, they can create their own magnetic field, but only in some metals that have a similar atomic structure is this ability very strongly expressed. Iron, cobalt, and nickel are located next to each other in Mendeleev’s periodic table, and have similar structures of electronic shells, which turns the atoms of these elements into microscopic magnets.
Since metals can be called a frozen mixture of various very small crystals, it is clear that such alloys can have a lot of magnetic properties. Many groups of atoms can “unfold” their own magnets under the influence of neighbors and external fields. Such “communities” are called magnetic domains, and form very bizarre structures that are still being studied with interest by physicists. This is of great practical importance.
As already mentioned, magnets can be almost atomic in size, so the smallest size of a magnetic domain is limited by the size of the crystal in which the magnetic metal atoms are embedded. This explains, for example, the almost fantastic recording density on modern computer hard drives, which, apparently, will continue to grow until the drives have more serious competitors.
Gravity, magnetism and electricity
Where are magnets used?
The cores of which are magnets made from magnets, although usually simply called cores, magnets have many more uses. There are stationery magnets, magnets for latching furniture doors, and chess magnets for travelers. These are magnets known to everyone.
Rarer types include magnets for charged particle accelerators; these are very impressive structures that can weigh tens of tons or more. Although now experimental physics is overgrown with grass, with the exception of that part that immediately brings super-profits on the market, but itself costs almost nothing.
Another interesting magnet is installed in a fancy medical device called a magnetic resonance imaging scanner. (Actually, the method is called NMR, nuclear magnetic resonance, but in order not to frighten people who are generally not strong in physics, it was renamed.) The device requires placing the observed object (the patient) in a strong magnetic field, and the corresponding magnet has frightening dimensions and the shape of the devil's coffin.
A person is placed on a couch and rolled through a tunnel in this magnet while sensors scan the area of interest to doctors. In general, it’s not a big deal, but some people experience claustrophobia to the point of panic. Such people will willingly allow themselves to be cut alive, but will not agree to an MRI examination. However, who knows how a person feels in an unusually strong magnetic field with an induction of up to 3 Tesla, after having paid good money for it.
To achieve such a strong field, superconductivity is often used by cooling a magnet coil with liquid hydrogen. This makes it possible to “pump up” the field without fear that heating the wires with a strong current will limit the capabilities of the magnet. This is not a cheap setup at all. But magnets made of special alloys that do not require current biasing are much more expensive.
Our Earth is also a large, although not very strong, magnet. It helps not only the owners of the magnetic compass, but also saves us from death. Without it, we would be killed by solar radiation. The picture of the Earth's magnetic field, simulated by computers based on observations from space, looks very impressive.
Here is a short answer to the question about what a magnet is in physics and technology.
At home, at work, in our own car or on public transport, we are surrounded by various types of magnets. They power motors, sensors, microphones and many other common things. Moreover, in each area, devices with different characteristics and features are used. In general, the following types of magnets are distinguished:
What types of magnets are there?
Electromagnets. The design of such products consists of an iron core on which turns of wire are wound. By applying electric current with different parameters of magnitude and direction, it is possible to obtain magnetic fields of the required strength and polarity.
The name of this group of magnets is an abbreviation of the names of its components: aluminum, nickel and cobalt. The main advantage of alnico alloy is the material’s unsurpassed temperature stability. Other types of magnets cannot boast of being able to be used at temperatures up to +550 ⁰ C. At the same time, this lightweight material is characterized by a weak coercive force. This means that it can be completely demagnetized when exposed to a strong external magnetic field. At the same time, due to its affordable price, alnico is an indispensable solution in many scientific and industrial sectors.
Modern magnetic products
So, we sorted out the alloys. Now let's move on to what types of magnets there are and what uses they can find in everyday life. In fact, there is a huge variety of options for such products:
3) Office magnets. Magnetic boards are used for presentations and planning meetings, which allow you to clearly and in detail present any information. They also prove extremely useful in school classrooms and university classrooms.
Where magnetite deposits were discovered in ancient times.
The simplest and smallest magnet can be considered an electron. The magnetic properties of all other magnets are due to the magnetic moments of the electrons inside them. From the point of view of quantum field theory, electromagnetic interaction is carried by a massless boson - a photon (a particle that can be represented as a quantum excitation of the electromagnetic field).
Weber- magnetic flux, when it decreases to zero, an amount of electricity of 1 coulomb passes through a circuit connected to it with a resistance of 1 ohm.
Henry- international unit of inductance and mutual induction. If a conductor has an inductance of 1 H and the current in it varies uniformly by 1 A per second, then an emf of 1 volt is induced at its ends. 1 henry = 1.00052 10 9 absolute electromagnetic units of inductance.
Tesla- a unit of measurement of magnetic field induction in SI, numerically equal to the induction of such a uniform magnetic field in which a force of 1 newton acts on 1 meter of length of a straight conductor perpendicular to the magnetic induction vector with a current of 1 ampere.
Use of magnets
- Magnetic storage media: VHS cassettes contain reels of magnetic tape. Video and audio information is encoded onto a magnetic coating on the tape. Also, in computer floppy disks and hard drives, data is recorded on a thin magnetic coating. However, storage media are not magnets in the strict sense, since they do not attract objects. Magnets in hard drives are used in drive and positioning motors.
- Credit, debit, and ATM cards all have a magnetic stripe on one side. This band encodes the information needed to connect to a financial institution and link to their accounts.
- Conventional TVs and Computer Monitors: TVs and computer monitors containing a cathode ray tube use an electromagnet to control a beam of electrons and form an image on the screen. Plasma panels and LCD displays use different technologies.
- Loudspeakers and Microphones: Most loudspeakers use a permanent magnet and a current coil to convert electrical energy (the signal) into mechanical energy (the movement that creates sound). The winding is wound on a coil, attached to the diffuser, and an alternating current flows through it, which interacts with the field of a permanent magnet.
- Another example of the use of magnets in audio engineering is in the pickup head of an electrophone and in cassette recorders as an economical erasing head.
Magnetic heavy mineral separator
- Electric motors and generators: Some electric motors (as well as loudspeakers) rely on a combination of an electromagnet and a permanent magnet. They convert electrical energy into mechanical energy. A generator, on the other hand, converts mechanical energy into electrical energy by moving a conductor through a magnetic field.
- Transformers: Devices that transfer electrical energy between two windings of wire that are electrically insulated but coupled magnetically.
- Magnets are used in polarized relays. Such devices remember their state when the power is turned off.
- Compasses: A compass (or marine compass) is a magnetized pointer that can rotate freely and aligns itself with the direction of a magnetic field, most commonly the Earth's magnetic field.
- Art: Vinyl magnetic sheets can be attached to paintings, photographs and other decorative items, allowing them to be attached to refrigerators and other metal surfaces.
Magnets are often used in toys. M-TIC uses magnetic bars connected to metal spheres
Egg-shaped rare earth magnets that attract each other
- Toys: Given their ability to resist gravity at close range, magnets are often used in children's toys with fun effects.
- Magnets can be used to make jewelry. Necklaces and bracelets can have a magnetic clasp, or can be made entirely from a series of linked magnets and black beads.
- Magnets can pick up magnetic objects (iron nails, staples, tacks, paper clips) that are either too small, difficult to reach, or too thin to handle with your fingers. Some screwdrivers are specially magnetized for this purpose.
- Magnets can be used in scrap metal processing to separate magnetic metals (iron, steel and nickel) from non-magnetic ones (aluminum, non-ferrous alloys, etc.). The same idea can be used in what is called a "Magnetic Test", in which the car body is examined with a magnet to identify areas repaired using fiberglass or plastic putty.
- Maglev: Magnetic levitation train driven and controlled by magnetic forces. Such a train, unlike traditional trains, does not touch the rail surface during movement. Since there is a gap between the train and the moving surface, friction is eliminated, and the only braking force is the force of aerodynamic drag.
- Magnets are used in furniture door latches.
- If magnets are placed in sponges, then these sponges can be used to wash thin sheets of non-magnetic materials on both sides at once, with one side being difficult to reach. This could be, for example, the glass of an aquarium or balcony.
- Magnets are used to transmit torque “through” a wall, which could be, for example, a sealed container of an electric motor. This is how the GDR toy “Submarine” was designed. In the same way, in household water flow meters, rotation is transmitted from the sensor blades to the counting unit.
- Magnets together with a reed switch are used in special position sensors. For example, in refrigerator door sensors and security alarms.
- Magnets together with a Hall sensor are used to determine the angular position or angular velocity of the shaft.
- Magnets are used in spark gaps to speed up arc extinction.
- Magnets are used for non-destructive testing using the magnetic particle method (MPC)
- Magnets are used to deflect beams of radioactive and ionizing radiation, such as in surveillance cameras.
- Magnets are used in indicating instruments with a deflecting needle, such as an ammeter. Such devices are very sensitive and linear.
- Magnets are used in microwave valves and circulators.
- Magnets are used as part of a deflection system of cathode ray tubes to adjust the trajectory of the electron beam.
- Before the discovery of the law of conservation of energy, there were many attempts to use magnets to build a “perpetual motion machine”. People were attracted by the seemingly inexhaustible energy of the magnetic field of permanent magnets, which have been known for a very long time. But the working model was never built.
- Magnets are used in non-contact brake designs consisting of two plates, one is a magnet and the other is made of aluminum. One of them is rigidly fixed to the frame, the other rotates with the shaft. Braking is controlled by the gap between them.
Magnetic toys
- Uberorbs
- Magnetic constructor
- Magnetic drawing board
- Magnetic letters and numbers
- Magnetic checkers and chess
Medicine and safety issues
Due to the fact that human tissue has a very low level of susceptibility to static magnetic fields, there is no scientific evidence of its effectiveness for use in the treatment of any disease. For the same reason, there is no scientific evidence of a risk to human health associated with exposure to this field. However, if a ferromagnetic foreign body is in human tissue, the magnetic field will interact with it, which can pose a serious danger.
Magnetization
Demagnetization
Sometimes the magnetization of materials becomes undesirable and it becomes necessary to demagnetize them. Demagnetization of materials is achieved in various ways:
- heating a magnet above the Curie temperature always leads to demagnetization;
- place a magnet in an alternating magnetic field that exceeds the coercive force of the material, and then gradually reduce the effect of the magnetic field or remove the magnet from it.
The latter method is used in industry for demagnetizing tools, hard drives, erasing information on magnetic cards, and so on.
Partial demagnetization of materials occurs as a result of impacts, since a sharp mechanical impact leads to disorder of domains.
Notes
Literature
- Savelyev I. V. General physics course. - M.: Nauka, 1998. - T. 3. - 336 p. - ISBN 9785020150003
see also
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To create elements and devices of control and automation systems, they are used magnetic material s, which mainly set the following requirements:
1. The material should be easily magnetized under the action of a constant field or a unipolar field pulse and easily remagnetized in an alternating field; the hysteresis loop should be quite narrow with a small value of H C and a large value of m. Such requirements make it possible to increase the sensitivity of electromagnetic elements.
2. The materials must have a high saturation induction value B S, i.e. ensure the penetration of a large magnetic flux into a core with an appropriate cross-section. Fulfilling this requirement allows us to obtain the smallest dimensions and weight of the device, and if the dimensions are specified, then the greatest power or voltage at the output of the device.
3. When working in an alternating magnetic field, the material should have the lowest costs, which form eddy currents, magnetic viscosity and hysteresis, because they determine the operating temperature of the core and device. Reducing them not only increases the efficiency of the device, but also makes it possible to create elements that operate at higher frequencies (400, 500, 1000 Hz and more) and have significantly higher speed and smaller dimensions and weight than elements that are powered by an industrial frequency voltage of 50 Hz .
In addition to the listed basic requirements for magnetic materials used in certain electromagnetic devices, specific requirements are set.
Thus, to improve temperature stability (consistency of magnetic properties when the ambient temperature changes), it is important that the Curie point of the material is as high as possible.
The closer to unity the squareness coefficient of the material is, the linear dependence of the output signal on the input signal is, the easier it is to recognize signals in digital devices.
The clearly discovered magnetic anisotropy improves the quality of devices based on thin magnetic films, and the high purity of the crystalline structure of the material is a necessary condition for creating devices based on cylindrical magnetic domains.
Magnetic materials can be divided into hard magnetic materials, for which the intensity Hc is tens and hundreds of amperes per centimeter and soft magnetic with the intensity Hc in tenths and hundredths of an ampere per centimeter. Hard magnetic materials are used to make permanent magnets, soft magnetic - for the manufacture of elements in which the field is created by currents passing through the windings.
To create ACS elements and devices, they are mainly used soft magnetic materials. Magnetic-hard powder materials are included in ferolacs that cover magnetic tapes and disks.
Soft magnetic materials can be divided into three groups: electrical steels, alloys based on iron with other metals (nickel, cobalt, aluminum) and ferrites (non-metallic ferromagnets).
Electrical steels are the cheapest materials, having high saturation inductions (about 1.8 ... 2.3 T), and this makes it possible to create compact and cheap electromagnetic elements from them. But due to the relatively large (compared to iron-nickel alloys) coercive force of electrical steel (about 0.1 ¸ 0.5 A / cm), the sensitivity of steel elements to changes in the external field generated by the windings is low.
Zalizonickel alloys (permalloy) are 15-20 times more expensive than steel alloys, have a lower saturation induction, but make it possible to obtain highly sensitive magnetic elements due to their low coercive force and high initial magnetic permeability. Zalizonickel alloys are manufactured in the form of sheets or strips. The thickness of the tape sometimes reaches several micrometers.
Zalizoaluminium alloys 16YUKH and 16YUM, which contain 16% aluminum, are not inferior in magnetic properties to permalloy, but have increased (10 ... 20 times more than in permalloy) wear resistance. They are widely used for the manufacture of magnetic heads in magnetic recording devices, where during operation the head continuously rubs against the surface of the tape.
Ferrites are non-metallic magnetic materials (solid solutions) made from a mixture of iron oxides with oxides of magnesium, copper, manganese, nickel and other metals. The general formula of ferrites is MeO × Fe2 Oz, where Me is any metal.
The oxides are crushed into small pieces and mixed in a certain proportion. Magnetic cores of the required sizes and configurations are pressed from the resulting mixture at a pressure of 10-30 kN/cm2 (1-3 t/cm2) and burned at a temperature of 1200-1400 ° C. The finished gray-black magnetic cores have high hardness, but are quite fragile . The windings are usually wound directly onto ferrite magnetic cores without additional insulation of the latter. Specific
The electrical resistance of ferrites is millions of times greater than that of metal ferromagnets, which practically eliminates eddy currents. This allows magnetization reversal ferrites with a frequency of hundreds of kilohertz and ensure high speed of operations of modern control and computing machines. The most common magnesium-manganese ferrites are VT grades (1.3VT, 0.16 VT, etc.). They have a relatively low Curie point (140 - 300 ° C), which causes a significant change in their magnetic parameters when heated. Lithium-based ferrites, with a Curie point of 630 ° C, have significantly better temperature characteristics. Biferites are widely used for magnetic circuits of digital devices; there are ferrites with two metals, for example, magnesium-manganese or lithium-sodium ferrites, as well as polypherites, which are solid solutions of three or more ferrites.
Magnetic hard materials. Magnetic-hard materials, as already noted, are used:
— For the manufacture of permanent magnets;
— To record information (for example, for sound recording).
When assessing the properties of magnetically hard materials, mechanical properties (strength), workability of the material during the production process, as well as density, electrical resistivity, etc. may be significant. It is especially important in some cases of stability of magnetic properties.
The most important materials for permanent magnets are Fe-Ni-Al alloys. The mechanism of dispersion hardening plays a major role in the formation of the highly coercive state of these alloys.
Such materials have a high coercivity value because their magnetization occurs mainly due to rotation processes.
Fe-Ni-Al alloys without alloying elements are not used due to their relatively low magnetic properties. The most common alloys are those alloyed with copper and cobalt. High-cobalt alloys containing more than 15% Co are typically used with a magnetic or magnetic and crystalline texture.
The magnetic texture is the result of thermomagnetic treatment, which consists of cooling the alloy in a magnetic field with a strength of 160-280 kA/m from high temperatures (1250-1300 0 C) to approximately 500 0 C. In this case, an increase in magnetic characteristics occurs only in the direction of the field action, those. the material becomes magnetically anisotropic.
A further significant increase in the magnetic properties of Fe-Ni-Al-(Co) alloys is possible by creating magnets from a macrostructure in the form of columnar crystals. The crystalline structure is obtained through special cooling conditions of the alloy.
Here are brief recommendations for choosing alloy grades. Cobalt-free alloys (UND, etc.). There are cheap ones, their properties are relatively low. Alloys YUNDK15 and YUNDK18 are used when relatively high magnetic properties are required and the material should not have magnetic anisotropy. Alloys containing 24% Co (YuN13DK24, etc.) have high magnetic properties in the direction of the magnetic texture, are well technologically developed and are widely used.
Alloys with directional crystallization, for example YuN13DK25BA, etc., which have the highest W max and, therefore, can provide the smallest mass and dimensions of magnetic systems.
In cases where the system is open, alloys with the highest Hc are used, for example titanium alloy YUNDK35T5.
Alloys with a single-crystal structure (YUNDK35T5AA and YUNDK40T8AA) have the following advantages compared to alloys with directional crystallization: higher magnetic properties due to further improvement of the structure, the presence of three mutually perpendicular directions in which the properties are optimal; better mechanical properties.
The main disadvantages of Fe-Ni-Al-(Co) alloys are poor mechanical properties (high hardness and brittleness), which significantly complicates their mechanical processing.
Powder magnets. Magnets produced by powder metallurgy methods can be divided into metal-ceramic, metal-plastic and oxide.
For the first two groups, the physical processes of the formation of a high-coercive state depend on the same reasons as for monolithic magnets; for the other two groups, a necessary condition for obtaining high-coercive properties is a state ground to a certain degree of dispersion, which corresponds to a single-domain structure.
Ceramic-metal magnets are produced from metal powders by pressing them without any material that binds them and sintering them at high temperatures. In terms of magnetic properties, they are only slightly inferior to cast magnets, but more expensive than others.
Metal-plastic magnets are produced, like metal-ceramic magnets, from metal powders, but they are pressed together with an insulating binder and heated to a low temperature necessary for the polymerization of the substance that binds them. Compared to cast magnets, they have reduced magnetic properties, but have high electrical resistance, low density and are relatively cheap.
Among oxidizing magnets, magnets based on barium and cobalt ferrites are of practical importance.
Barium magnets. The industry produces two groups of barium magnets: isotropic (BI) and anisotropic (BA).
Compared to cast magnets, barium magnets have a very high coercive force and low residual induction. The electrical resistivity r of barium magnets is millions of times higher than that of metallic materials, allowing barium magnets to be used in magnetic circuits that are exposed to high frequency fields. Barium magnets do not contain scarce and expensive materials; they are approximately 10 times cheaper than magnets with UNDC24.
The disadvantages of barium magnets include poor mechanical properties (high fragility and hardness) and, most importantly, a greater dependence of magnetic properties on temperature. The temperature coefficient of residual magnetic induction TC B r of barium magnets is approximately 10 times greater than TC B r of cast magnets. In addition, barium magnets are irreversible properties during cooling, i.e. have higher temperature stability than barium. However, they also have temperature hysteresis, but it does not appear in the region of negative temperatures, as in barium magnets, but at positive temperatures (when heated above 80 ° C).
Other materials for permanent magnets.
Martensitic steels. Martensite is the name given to the type of microstructure of steel obtained when it is hardened. The formation of martensite is accompanied by significant volumetric changes, the creation of large internal lattice stress and the appearance of large coercive force values.
Martensitic steels began to be used for the manufacture of permanent magnets earlier than other materials. Currently, they are used relatively little due to their low magnetic properties. However, they have not yet been completely abandoned, because they are inexpensive and can be machined on metal-cutting machines.
Alloys are plastically deformed. These alloys have high machinability properties. They are well stamped, cut with scissors, and processed on metal-cutting machines. Alloys that can be plastically deformed can be used to make tapes, plates, sheets, and wire. In some cases (when producing small magnets of complex configuration), it is advisable to use metal-ceramic technology. There are many grades of alloys that are plastically deformed, and the physical processes due to which they have high magnetic properties are varied. The most common alloys are kunife (Cu-Ni-Fe) and vikaloy (Co-V). Kunife alloys are anisotropic, magnetized in the rolling direction, and are often used in the form of thin wire and stamping. Vikaloy is used for the manufacture of the smallest magnets of complex or openwork configuration and as high-strength magnetic tapes or wire.
Alloys based on noble metals. These include alloys of silver with manganese and aluminum (silmanal) and alloys of platinum with iron (77.8% Pt; 22.2% Fe) or platinum with cobalt (76.7% Pt; 23.3% Co). Materials in this group, especially those containing platinum, are very expensive, so they are used only for subminiature magnets weighing a few milligrams. Metal-ceramic technology is widely used in the manufacture of magnets from all alloys of this group.
Elastic magnets. As noted, the most important disadvantage of the main groups of materials for permanent magnets - cast alloys and hard magnetic ferrites - is their poor mechanical properties (high hardness and brittleness). The use of plastically deformable alloys is limited by their high cost. Recently, rubber-based magnets have appeared. They can be of any shape that rubber technology allows - in the form of cords, long strips, sheets, etc. Such material is easily cut with scissors, stamped, bent, and twisted. It is known to use “magnetic rubber” as magnetic memory letters for computers, magnets for deflection systems in television, magnets for correcting, etc.
Elastic magnets are made of rubber and fine powder of hard magnetic materials (filler). Barium ferrite is most often used as a filler.
Materials for magnetic tapes. Magnetic tapes mean magnetic recording media. The most common are solid metal tapes made of stainless steel, bimetallic tapes and plastic-based tapes with a powder working layer. Solid metal tapes are used mainly for special purposes and when working in a wide temperature range; Plastic-based tapes are more widely used. The main purpose of a magnetic recording medium is to create a magnetic field on the surface of the reproduced head, the strength of which changes (as the tape is pulled) over time in the same way as the signal that is being recorded. The properties of tapes coated with magnetic powders significantly depend not only on the properties of the source materials, but also on the degree of particle grinding, the volumetric density of the magnetic material in the working layer, the orientation of the particles if they have shape anisotropy, etc.
The working layer (or thickness of the metal tape) should be as thin as possible, and the tape itself should be smooth and flexible to ensure maximum interaction (magnetic contact) between the magnetic materials of the tape and the head. The residual magnetization of the material should be as high as possible.
Contradictory requirements are placed on the coercive force: to reduce self-demagnetization, a higher possible value of H c is necessary (at least 24 kA / m), and to facilitate the process of erasing a record, a small H c is desirable. The requirements of high residual magnetization and minimal sensitivity to self-demagnetization are best satisfied with a rectangular section of the demagnetization hysteresis loop, i.e. It is desirable to have a maximum value of the convexity coefficient. Temperature and other changes in the magnetic properties of the tape material should be minimal.
The industry produces magnetic tapes made of non-rusting alloy EP-31A and bimetal EP-352/353. The tapes have a thickness of 0.005-0.01 mm, N c = 24 - 40 kA / m; B r = 0.08 T.
Domestic plastic-based tapes are made mainly of types A2601-6 (type 6 - for studio tape recorders) and A4402 - 6 (type 10 - for household and reportage). In accordance with GOST, the following is used in the designations of tapes: the first element - a letter index - indicates the purpose of the tape: A - sound recording, T - video recording, B - computer technology, I - exact recording: the second element - digital index (from 0 to 9), indicates the material bases: 2 - diacetylcellulose, 3 - triacetylcellulose, 4 - polyethylene terephthalag (lavsan), the third element is a digital index (from 0 to 9), indicating the thickness of the tape:
2 - 18 microns, 3 - 27 microns, 4 - 36 microns, 6 - 55 microns, 9 - more than 100 microns, the fourth element is a digital index (from 01 to 99), means the technological development number; the fifth element is the numerical value of the nominal width of the tape in millimeters. After the fifth element there must be an additional letter index: P - for perforated tapes; P - for tapes used in radio broadcasting B - for tapes from household tape recorders.
The following materials are used for magnetic powders: iron ferrite (magnetite), cobalt ferrite, chromium dioxide, etc. Each of them has its own advantages and disadvantages. The most widely used is gamma iron oxide (g-Fe 2 O 3), which is needle-shaped with a particle length of about 0.4 μm and a length-to-diameter ratio of approximately three. Powder (g-Fe 2 O 3) is obtained by oxidizing magnetite (iron ferrite) FeO × Fe 2 O 3 by heating it in air at a temperature of about 150 o C.
The production of magnetic tapes can be varied. More often, the working layer (magnetic varnish) is applied to the finished base, for example, by pouring varnish from a die. Magnetic varnish is prepared in advance and consists of magnetic powder, a binder, a solvent, a plasticizer and various additives that promote wetting and separation of powder particles and reduce the abrasiveness of the working layer.
When using powders with anisotropy of particle shape (for example, needle-shaped g-Fe) in the production process of the tape, the lobes are oriented in a certain way as a result of the influence of a magnetic field on them. The final processing of the belt consists of calendering and polishing to improve the quality of its surface.
Type 6 tape provides high quality sound recording and playback when used in professional equipment at a speed of 19.05 cm/s and in household tape recorders at a speed of 9.53 and 4.75 cm/s.
Tapes must be stored at a temperature of 10-25 ° C and a relative humidity of 50-60%; Temperatures above 30°C are unacceptable, temperatures below 10°C are not recommended.
In addition to types 6 and 10, the domestic industry produces other types of tapes, for example, the T4402-50 tape with a width of 50.8 mm for cross-line recording of black and white images.
Alloys based on rare earth metals (REM). A number of compounds and alloys with rare-earth metals have very high values of coercive force and maximum specific energy. Of this group of materials, the most interesting are intermetallic compounds of the RCo 5 type, where R is a rare earth metal.
In addition to the main groups of magnetic materials considered, some others are also used in technology, which have a limited scope of application.
Thermomagnetic materials. Thermomagnetic are materials with a significant dependence of magnetic induction (more precisely, saturation magnetization, because usually thermomagnetic material operates in saturation mode) on temperature in a certain range (in most cases +60 ¸ -60 0 C). Thermomagnetic materials are used mainly as magnetic shunts or additional supports. The inclusion of such elements in magnetic circuits makes it possible to compensate for temperature errors or to ensure a change in magnetic induction in the air gap according to a given law (thermal regulation).
Magnetostrictive materials. Magnetostriction has direct technical application in magnetostrictive vibrators (generators) of sound and ultrasonic vibrations, as well as in some radio engineering circuits and devices (instead of quartz for frequency stabilization, in electromechanical filters, etc.).
Nickel, permendur (Fe-Co alloys characterized by high saturation magnetization), Alfer (Fe-Al alloys), nickel and nickel-cobalt ferrites, etc. are used as magnetostrictive materials.
Nickel has a large absolute value of the saturation magnetostriction coefficient l S = D l / l = -35 × 10 -6 (l is the length of the plate to the field, D l is the change in length as a result of the field; the minus sign means a decrease in length). Typically, grade H nickel is used with a thickness of 0.1 mm in the form of a rigid, unfired strip. After cutting, the plates are oxidized by heating in air to 800 o C for 15-25 minutes. The oxide film thus formed serves to electrically insulate the plates when making up the stack. Nickel has high anti-corrosion properties and a low temperature coefficient of elastic modulus.
Recently, magnetostrictive ferrites have been used more widely, especially in precision filters.
Alloys with high saturation induction. Of the common materials, iron has the highest induction (»2.1 T).
In cases where the highest requirements are placed on the dimensions of the device, its mass and flow size, high-alcobalt alloys are used, in which the saturation induction reaches 2.43 T, which allows for savings in mass and volume compared to iron by 15 - 20% . In practice, alloys containing 30-51% Co and 1.5-2.0% V are used, which improves the technological properties of the alloys and the ability to process them in a cold state. These alloys are called permendur.
The saturation induction of alloys with high and low cobalt content is approximately the same. High-cobalt alloys in weak and medium fields have higher magnetic permeability values than low-cobalt alloys, but the latter are cheaper.
In addition to its high saturation induction value, permendur has significant reversible permeability, which makes it especially valuable as a material for telephone membranes. Disadvantages of permendur: low electrical resistivity r, high cost and scarcity of cobalt and vanadium. Permendur is used in constant magnetic fields or in weak alternating fields with strong magnetization by a constant field. Of the materials in this group, the standardized alloy is 50 KF (49.0-51% Co; 1.5-2.0% V). The alloy has a saturation induction of at least 2.35 T and q = 980 °C.
The advantage of high-cobalt alloys over technically pure iron is felt at magnetic induction above 1.0 Tesla. The difference in magnetic permeability values reaches a maximum at a magnetic induction value of about 1.8 T, while the permeability of cobalt alloys is tens of times greater than the permeability of soft iron varieties.
Vasyura A.S. — Book “Elements and devices of automation control systems”
Even in ancient times, people discovered the unique properties of certain stones - attracting metal. Nowadays, we often come across objects that have these qualities. What is a magnet? What is his strength? We will talk about this in this article.
An example of a temporary magnet is paper clips, buttons, nails, a knife and other household items made of iron. Their strength lies in the fact that they are attracted to a permanent magnet, and when the magnetic field disappears, they lose their properties.
The field of an electromagnet can be controlled using electric current. How does this happen? A wire wound in turns on an iron core changes the strength of the magnetic field and its polarity when a current is supplied and changed.
Types of permanent magnets
Ferrite magnets are the most famous and actively used in everyday life. This black material can be used as fasteners for various items, such as posters, wall boards used in the office or school. They do not lose their attractive properties at temperatures not lower than 250 o C.
Alnico is a magnet consisting of an alloy of aluminum, nickel and cobalt. This gave it its name. It is very resistant to high temperatures and can be used at 550 o C. The material is lightweight, but completely loses its properties when exposed to a stronger magnetic field. Mainly used in the scientific industry.
Samarium magnetic alloys are high performance materials. The reliability of its properties allows the material to be used in military developments. It is resistant to aggressive environments, high temperatures, oxidation and corrosion.
What is a neodymium magnet? It is the most popular alloy of iron, boron and neodymium. It is also called a supermagnet, as it has a powerful magnetic field with high coercive force. By observing certain conditions during operation, a neodymium magnet can retain its properties for 100 years.
Use of neodymium magnets
It is worth taking a closer look at what a neodymium magnet is? This is a material that is capable of recording the consumption of water, electricity and gas in meters, and not only. This type of magnet belongs to permanent and rare earth materials. It is resistant to fields of other alloys and is not subject to demagnetization.
Neodymium products are used in the medical and industrial industries. Also in domestic conditions they are used for attaching curtains, decorative elements, and souvenirs. They are used in search instruments and electronics.
To extend their service life, magnets of this type are coated with zinc or nickel. In the first case, spraying is more reliable, as it is resistant to aggressive agents and can withstand temperatures above 100 o C. The strength of the magnet depends on its shape, size and the amount of neodymium included in the alloy.
Applications of Ferrite Magnets
Ferrites are considered the most popular permanent magnets. Thanks to strontium included in the composition, the material does not corrode. So what is a ferrite magnet? Where is it used? This alloy is quite fragile. That's why it is also called ceramic. Ferrite magnet is used in automotive and industrial applications. It is used in various equipment and electrical appliances, as well as household installations, generators, and acoustic systems. In automobile manufacturing, magnets are used in cooling systems, window lifters, and fans.
The purpose of ferrite is to protect equipment from external interference and prevent damage to the signal received via the cable. Due to this, they are used in the production of navigators, monitors, printers and other equipment where it is important to obtain a clean signal or image.
Magnetotherapy
A procedure called magnetic therapy is often used and is carried out for medicinal purposes. The effect of this method is to influence the patient's body using magnetic fields under low-frequency alternating or direct current. This treatment method helps get rid of many diseases, relieve pain, strengthen the immune system, and improve blood flow.
It is believed that diseases are caused by disturbances in the human magnetic field. Thanks to physiotherapy, the body returns to normal and the general condition improves.
From this article you learned what a magnet is, and also studied its properties and applications.