Today, it is almost impossible to find a technical industry that does not use hard magnetic materials and permanent magnets. These are acoustics, and radio electronics, and computer, and measuring equipment, and automation, and heat and power, and electric power, and construction, and metallurgy, and any kind of transport, and agriculture, and medicine, and ore processing, and even in the kitchen of everyone there is a microwave oven, it warms up the pizza. It is impossible to enumerate everything, magnetic materials accompany us at every step of our life. And all products with their help work according to completely different principles: engines and generators have their own functions, and braking devices have their own, the separator does one thing, and the flaw detector does another. Probably, there is no complete list of technical devices where hard magnetic materials are used, there are so many of them.
What are magnetic systems
Our planet itself is an exceptionally well-oiled magnetic system. All the rest are built on the same principle. Hard magnetic materials have very diverse functional properties. In the catalogs of suppliers, it is not in vain that not only their parameters are given, but also physical properties. In addition, it can be magnetically hard and magnetically soft materials. For example, take resonant tomographs, where systems with a highly uniform magnetic field are used, and compare with separators, where the field is sharply inhomogeneous. Quite a different principle! Magnetic systems have been mastered, where the field can be switched on and off. That's how grips are designed. And some systems even change the magnetic field in space. These are well-known klystrons and traveling wave lamps. The properties of soft and hard magnetic materials are truly magical. They are like catalysts, they almost always act as intermediaries, but without the slightest loss of their own energy, they are able to transform someone else's, turning one species into another.
For example, a magnetic impulse is converted into mechanical energy in the operation of couplings, separators, and the like. Mechanical energy is converted with the help of magnets into electrical energy, if we are dealing with microphones and generators. And vice versa happens! In speakers and motors, magnets convert electricity into mechanical energy, for example. And that is not all. Mechanical energy can even be converted into thermal energy, as does the magnetic system in the operation of a microwave oven or in a braking device. Are ablemagnetically hard and magnetically soft materials and on special effects - in Hall sensors, in magnetic resonance tomographs, in microwave communication. You can write a separate article about the catalytic effect on chemical processes, how gradient magnetic fields in water affect the structures of ions, protein molecules, and dissolved gases.
Magic from antiquity
Natural material - magnetite - was known to mankind several millennia ago. At that time, all the properties of hard magnetic materials were not yet known, and therefore they were not used in technical devices. And there were no technical devices yet. No one knew how to do calculations for the operation of magnetic systems. But the influence on biological objects has already been noticed. The use of hard magnetic materials at first went purely for medical purposes, until the Chinese invented the compass in the third century BC. However, treatment with the help of a magnet has not ceased until today, even though there are constant discussions about the harmfulness of such methods. The use of hard magnetic materials in medicine in the USA, China, and Japan is especially active. And in Russia there are adherents of alternative methods, although it is impossible to measure the magnitude of the impact on the body or plant with any instrument.
But back to history. In Asia Minor, many centuries ago, the ancient city of Magnesia already existed on the banks of the full-flowing Meander. And today you can visit its picturesque ruins in Turkey. It was there that the first magnetic iron ore was discovered, which was named aftercities. Quite quickly, it spread throughout the world, and five thousand years ago, with its help, the Chinese invented a navigation device that still does not die. Now mankind has learned to produce magnets artificially on an industrial scale. The basis for them are a variety of ferromagnets. The University of Tartu has the largest natural magnet, capable of lifting about forty kilograms, while itself weighs only thirteen. Today's powders are made of cob alt, iron and various other additives, they hold loads five thousand times more than they weigh.
Hysteresis loop
There are two types of artificial magnets. The first type is constants, which are made of hard magnetic materials, their properties are in no way associated with external sources or currents. The second type is electromagnets. They have a core made of iron - a soft magnetic material, and a current passes through the winding of this core, which creates a magnetic field. Now we need to consider the principles of its work. Characterizes the magnetic properties of the hysteresis loop for hard magnetic materials. There are quite complex technologies for the manufacture of magnetic systems, and therefore information is needed on magnetization, magnetic permeability, and energy losses when magnetization reversal occurs. If the change in intensity is cyclic, the remagnetization curve (changes in induction) will always look like a closed curve. This is the hysteresis loop. If the field is weak, then the loop is more like an ellipse.
When the tensionthe magnetic field increases, a whole series of such loops is obtained, enclosed in each other. In the process of magnetization, all vectors are oriented along, and at the end, a state of technical saturation will come, the material will be completely magnetized. The loop obtained during saturation is called the limit loop, it shows the maximum achieved value of the induction Bs (saturation induction). When tension decreases, residual induction remains. The area of the hysteresis loops in the limit and intermediate states shows the energy dissipation, that is, the hysteresis loss. It depends most of all on the magnetization reversal frequency, material properties, and geometric dimensions. The limiting hysteresis loop can determine the following characteristics of hard magnetic materials: saturation induction Bs, residual induction Bc and coercive force Hc.
Magnetization curve
This curve is the most important characteristic, because it shows the dependence of the magnetization and the strength of the external field. Magnetic induction is measured in Tesla and is related to magnetization. The switching curve is the main one, it is the location of the peaks on the hysteresis loops, which are obtained during the cyclic remagnetization. This reflects the change in magnetic induction, which depends on the field strength. When the magnetic circuit is closed, the field strength reflected in the form of a toroid is equal to the external field strength. If the magnetic circuit is open, poles appear at the ends of the magnet, which create demagnetization. Difference betweenthese tensions determines the internal tension of the material.
There are characteristic sections on the main curve that stand out when a single crystal of a ferromagnet is magnetized. The first section shows the process of shifting the boundaries of unfavorably tuned domains, and in the second, the magnetization vectors turn towards the external magnetic field. The third section is the paraprocess, which completes the magnetization stage; here the magnetic field is strong and directed. The application of soft and hard magnetic materials depends to a large extent on the characteristics obtained from the magnetization curve.
Permeability and energy loss
To characterize the behavior of a material in a field of tension, it is necessary to use such a concept as absolute magnetic permeability. There are definitions of impulse, differential, maximum, initial, normal magnetic permeability. The relative is traced along the main curve, so this definition is not used - for simplicity. Magnetic permeability under conditions when H=0 is called initial, and it can be determined only in weak fields, up to approximately 0.1 units. The maximum, on the contrary, characterizes the highest magnetic permeability. The normal and maximum values provide an opportunity to observe the normal course of the process in each particular case. In the saturation region in strong fields, the magnetic permeability always tends to unity. All these values are necessary for the use of hard magneticmaterials, always use them.
Energy loss during magnetization reversal is irreversible. Electricity is released in the material as heat, and its losses are made up of dynamic losses and hysteresis losses. The latter are obtained by displacing the domain walls when the magnetization process is just beginning. Since the magnetic material has an inhomogeneous structure, energy is necessarily expended on the alignment of the domain walls. And dynamic losses are obtained in connection with eddy currents that occur at the moment of changing the strength and direction of the magnetic field. Energy is dissipated in the same way. And the losses due to eddy currents exceed even the hysteresis losses at high frequencies. Also, dynamic losses are obtained due to residual changes in the state of the magnetic field after the intensity has changed. The amount of aftereffect losses depends on the composition, on the heat treatment of the material, they appear especially at high frequencies. The aftereffect is the magnetic viscosity, and these losses are always taken into account if ferromagnets are used in pulsed mode.
Classification of hard magnetic materials
The terms that speak of softness and hardness do not apply to mechanical properties at all. Many hard materials are in fact soft magnetic, and from a mechanical point of view, soft materials are quite hard magnetic. The process of magnetization in both groups of materials occurs in the same way. First, the domain boundaries are displaced, then the rotation begins inin the direction of an increasingly magnetizing field, and finally, the paraprocess begins. And this is where the difference comes in. The magnetization curve shows that it is easier to move the boundaries, less energy is expended, but the rotation process and the paraprocess are more energy intensive. Soft magnetic materials are magnetized by displacement of boundaries. Hard magnetic - due to rotation and paraprocess.
The shape of the hysteresis loop is approximately the same for both groups of materials, saturation and residual induction are also close to equal, but the difference exists in the coercive force, and it is very large. Hard magnetic materials have Hc=800 kA-m, while soft magnetic materials have only 0.4 A-m. In total, the difference is huge: 2106 times. That is why, based on these characteristics, such a division was adopted. Although, it must be admitted that it is rather conditional. Soft magnetic materials can saturate even in a weak magnetic field. They are used in low frequency fields. For example, in magnetic memory devices. Hard magnetic materials are difficult to magnetize, but they retain magnetization for a very long time. It is from them that good permanent magnets are obtained. The areas of application of hard magnetic materials are numerous and extensive, some of them are listed at the beginning of the article. There is another group - magnetic materials for special purposes, their scope is very narrow.
Details of hardness
As already mentioned, hard magnetic materials have a wide hysteresis loop and a large coercive force, low magnetic permeability. They are characterized by the maximum specific magnetic energy given off inspace. And the "harder" the magnetic material, the higher its strength, the lower the permeability. The specific magnetic energy is given the most important role in assessing the quality of the material. A permanent magnet practically does not give off energy to the outer space with a closed magnetic circuit, because all the lines of force are inside the core, and there is no magnetic field outside of it. In order to make the most of the energy of permanent magnets, an air gap of a strictly defined size and configuration is created inside a closed magnetic circuit.
Over time, the magnet "gets old", its magnetic flux decreases. However, such aging can be both irreversible and reversible. In the latter case, the causes of its aging are shocks, shocks, temperature fluctuations, constant external fields. The magnetic induction is reduced. But it can be magnetized again, thus restoring its excellent properties. But if the permanent magnet has undergone any structural changes, re-magnetization will not help, aging will not be eliminated. But they serve for a long time, and the purpose of hard magnetic materials is great. Examples are literally everywhere. It's not just permanent magnets. This is a material for storing information, for recording it - both sound, and digital, and video. But the above is only a small part of the application of hard magnetic materials.
Cast hard magnetic materials
According to the method of production and composition, hard magnetic materials can be cast, powder and others. They are based on alloys.iron, nickel, aluminum and iron, nickel, cob alt. These compositions are the most basic in order to get a permanent magnet. They belong to precision, since their number is determined by the strictest technological factors. Cast hard magnetic materials are obtained during precipitation hardening of the alloy, where cooling occurs at a calculated rate from melting to the onset of decomposition, which occurs in two phases.
The first - when the composition is close to pure iron with pronounced magnetic properties. As if plates of single-domain thickness appear. And the second phase is closer to the intermetallic compound in composition, where nickel and aluminum have low magnetic properties. It turns out a system where the non-magnetic phase is combined with strongly magnetic inclusions with a large coercive force. But this alloy is not good enough in magnetic properties. The most common is another composition, alloyed: iron, nickel, aluminum and copper with cob alt for alloying. Cob alt-free alloys have lower magnetic properties, but they are much cheaper.
Powder hard magnetic materials
Powder materials are used for miniature but complex permanent magnets. They are metal-ceramic, metal-plastic, oxide and micropowder. The cermet is especially good. In terms of magnetic properties, it is quite a bit inferior to cast ones, but somewhat more expensive than them. Ceramic-metal magnets are made by pressing metal powders without any binding material and sintering them at very high temperatures. Powders are usedwith the alloys described above, as well as those based on platinum and rare earth metals.
In terms of mechanical strength, powder metallurgy is superior to casting, but the magnetic properties of metal-ceramic magnets are still somewhat lower than those of cast ones. Platinum-based magnets have very high values of coercive force, and the parameters are highly stable. Alloys with uranium and rare earth metals have record values of maximum magnetic energy: the limit value is 112 kJ per square meter. Such alloys are obtained by cold pressing the powder to the highest degree of density, then the briquettes are sintered with the presence of a liquid phase and casting of a multicomponent composition. It is impossible to mix the components to such an extent by simple casting.
Other hard magnetic materials
Hard magnetic materials also include those with a highly specialized purpose. These are elastic magnets, plastically deformable alloys, materials for information carriers and liquid magnets. Deformable magnets have excellent plastic properties, they lend themselves perfectly to any kind of mechanical processing - stamping, cutting, machining. But these magnets are expensive. Kunife magnets made of copper, nickel and iron are anisotropic, that is, they are magnetized in the direction of rolling, they are used in the form of stamping and wire. Vikalloy magnets made of cob alt and vanadium are made in the form of a high-strength magnetic tape, as well as wire. This composition is good for very small magnets with the most complex configuration.
Elastic magnets - on a rubber base, in whichThe filler is a fine powder of a hard magnetic material. Most often it is barium ferrite. This method allows you to get products of absolutely any shape with high manufacturability. They are also perfectly cut with scissors, bent, stamped, twisted. They are much cheaper. Magnetic rubber is used as magnetic memory sheets for computers, in television, for corrective systems. As information carriers, magnetic materials meet many requirements. This is a high-level residual induction, a small effect of self-demagnetization (otherwise the information will be lost), a high value of the coercive force. And to facilitate the process of erasing records, just a small amount of this force is needed, but this contradiction is removed with the help of technology.
