Permanent magnet materials, also known as hard magnetic materials, are characterized by high anisotropy field, high coercivity, large hysteresis loop area, large magnetization field required for magnetization to saturation, and strong long-term retention after removal of external magnetic fields. Magnetic. In practice, the permanent magnet material operates in the second quadrant demagnetization portion of the deep magnetic saturation and hysteresis loop after magnetization. As an important basic magnetic functional material, permanent magnet materials are widely used. China’s permanent magnet materials industry plays an important role in the world. Not only are there many enterprises engaged in production and application, but research work has also been in the ascendant. The types, main performances, application precautions, and selection principles of permanent magnet materials are described below.

Permanent Magnet Materials

Permanent Magnet Materials

Type of permanent magnet material

ferrite magnet

ferrite magnet

First, Ferrite

1. Ferrite is a non-metallic magnetic material, also known as magnetic ceramic. We took apart the traditional radio, and the horn magnet inside was ferrite.
2, the magnetic properties of ferrite is not high, the current magnetic energy product (one of the parameters to measure the performance of the magnet) can only achieve 4MGOe slightly higher. One of the biggest advantages of this material is its low price. Currently, it is still widely used in many fields.
3. Ferrite is ceramic. Therefore, the processing performance is similar to that of ceramics. Ferrite magnets are formed by molding and sintered. If machining is required, only simple grinding is required. Due to the difficulty in machining, ferrite products are mostly simple in shape and have large dimensional tolerances. The square shape product is fine and can be ground. Circular, generally only grinding two planes. Other dimensional tolerances are given as a percentage of the nominal size.
4, due to the wide range of low-cost ferrite applications, therefore, many manufacturers have ready-made rings and squares of conventional shapes and sizes to choose from. Since the ferrite is made of ceramic material, there is basically no corrosion problem. The finished product does not require surface treatment or coating such as plating.

rubber magnet

rubber magnet

Second, Rubber Magnet

1. Rubber magnet is one of the series of ferrite magnetic materials. It is made of bonded ferrite magnetic powder and synthetic rubber. It is made of extrusion, calendering and injection molding. It is soft and elastic. A twistable magnet. It can be processed into strips, rolls, sheets, blocks, rings and various complex shapes.
2. Its magnetic energy product is 0.60~1.50. Application fields of MGOe rubber magnetic material: refrigerator, message signage, fasteners for fixing objects to metal body for advertising, etc., for toys, teaching instruments, switches and induction The magnetic disk of the device.
3, mainly used in micro-motors, refrigerators, disinfection cabinets, kitchen cabinets, toys, stationery, advertising and other industries.

smco magnet

smco magnet

Third, SmCo – Samarium Cobalt

1. Samarium-cobalt magnets, the main components are bismuth and cobalt. Since the two materials themselves are expensive, samarium-cobalt magnets are also the most expensive of several types of magnets. The magnetic energy product of samarium-cobalt magnets can now be 30MGOe, or even higher.
2. In addition, the samarium-cobalt magnet has high coercive force and high temperature resistance, and can be applied to high temperatures of 350 degrees Celsius, so it cannot be replaced in many applications. Samarium cobalt magnets are powder metallurgy products. The general manufacturer sinters into a block blank according to the size and shape of the finished product, and then cuts it into a finished size using a diamond blade. Since samarium cobalt is electrically conductive, wire cutting processing can be performed.
3. Theoretically, samarium cobalt can be cut into a shape that can be cut by wire cutting, if the problem of magnetization and larger size is not considered. Samarium-cobalt magnets have good corrosion resistance and generally do not require anti-corrosion plating or coating. In addition, the samarium-cobalt magnet is very brittle, so it is difficult to process small-sized or thin-walled products.

NdFeB magnets

NdFeB magnets

Fourth, NdFeB

1. NdFeB is a magnet product that is widely used and developed rapidly. NdFeB has been widely used since its invention, but it has been more than 20 years. Due to its high magnetic performance and ease of processing, the price is not very high, so the application field is expanding rapidly.
2. At present, commercial NdFeB, the magnetic energy product can reach 50MGOe, which is 10 times that of ferrite. NdFeB is also a powder metallurgy product, and its processing method is similar to that of samarium cobalt.
3. At present, the maximum working temperature of NdFeB is around 180 degrees Celsius. If it is a harsh environment, it is generally recommended not to exceed 140 degrees Celsius. NdFeB is very susceptible to corrosion. Therefore, most of the finished products are electroplated or painted.
4. Conventional surface treatments include: nickel plating (nickel copper nickel), galvanizing, aluminum plating, electrophoresis, etc. If working in a closed environment, phosphating can also be used. Due to the high magnetic properties of NdFeB, in many cases, it is used to replace other magnetic materials to reduce the volume of the product. If you use ferrite magnets, the size of your current mobile phone is probably not less than half a brick.
5. Both the samarium-cobalt magnet and the neodymium-iron-boron magnet have better processing properties. Therefore, the dimensional tolerance of the product is much better than that of ferrite. For general products, dimensional tolerances can be achieved (+/-) 0.05mm.

alnico magnet

alnico magnet

Five, AlNiCo  – Aluminum Nickel Cobalt

1. AlNiCo magnets are available in both casting and sintering processes. There are more castings in China. Aluminium-nickel-cobalt has a magnetic energy product of up to 9MGOe, and has the biggest feature, that is, high temperature resistance, working temperature can reach 550 degrees Celsius. However, AlNiCo is very susceptible to demagnetization under a reverse magnetic field. If you force the two poles of the two AlNiCos (two N or two S) together, the magnetic field of one of the magnets will be reversed or reversed. Therefore it is not suitable for working in a reverse magnetic field (such as a motor).
2, AlNiCo is very hard, although it can also be used for grinding and wire cutting, but the cost is higher. The finished products are generally available in either ground or not. Aluminium-nickel-cobalt is widely used in the field of sensors.

Main properties of permanent magnet materials

1, residual magnetic induction
The magnetic induction value of the permanent magnet material when the permanent magnetic material is magnetized to saturation in the external magnetic field and when the external magnetic field is zero. The data of this indicator is directly related to the magnetic gap of the air gap in the motor. The higher the magnetic induction value, the higher the air gap magnetic density of the motor will be. The main indexes of the motor such as the torque constant and the back EMF coefficient will reach the optimal value. The relationship between the electric load and the magnetic load of the motor may be the most reasonable. In order to achieve the best efficiency.

2, coercive force Hc, (magnetic induction coercive force Hc b )
The reverse magnetic field strength required for the permanent magnet material in the case of saturation magnetization when the residual magnetic induction Br decreases to zero. This index is related to the anti-demagnetization capability of the motor, that is, the overload factor and the air gap magnetic density. The larger the Hc value, the stronger the anti-demagnetization capability of the motor, and the greater the overload factor, the stronger the adaptability to the dynamic demagnetization dynamic working environment. At the same time, the air gap magnetic density of the motor will also be improved.

3. Maximum magnetic energy product BHmax
The maximum value of the magnetic field energy provided by the permanent magnet material to the outer magnetic circuit. This index is directly related to the amount of permanent magnet material in the motor. The larger the BHmax, the greater the magnetic field energy provided by the permanent magnet material to the external magnetic circuit, that is, the more permanent magnet material used in the motor under the same power condition. less.

4, intrinsic coercivity Hc i
This indicator is the value of the magnetic field strength when the residual magnetization M drops to zero. The corresponding Hc b value at B=0 on the demagnetization curve only indicates that the permanent magnet cannot provide energy to the outer magnetic circuit at this time, and does not mean that the permanent magnet itself does not have energy. However, when M=0, the corresponding Hc i value indicates that the permanent magnet has been truly demagnetized, and it has no magnetic field energy storage. Although Hc i is not directly related to the operating point of the motor, it is the true coercive force of the permanent magnet material, which represents the ability of the permanent magnet material to have magnetic field energy and anti-demagnetization. The magnitude of the intrinsic coercivity is closely related to the temperature stability of the permanent magnet material. The higher the internal coercivity, the higher the operating temperature of the permanent magnet material.

5, temperature coefficient α
Temperature is one of the main factors affecting the magnetic properties of permanent magnet materials. The percentage of reversible change in magnetic properties when the temperature changes by 1 ° C is called the temperature coefficient of the magnetic material. The temperature coefficient can be divided into the residual magnetic induction temperature coefficient and the coercive temperature coefficient. This index has a great influence on the performance stability of the motor. The higher the temperature coefficient, the greater the change of the index when the motor runs from cold to hot, which directly limits the operating temperature range of the motor. Indirectly affects the power to volume ratio of the motor.

Application Notes OF Permanent magnetic material

1. The actual magnetic properties of permanent magnet materials are related to the specific manufacturing process of the manufacturer, and there is often a certain deviation between the values ​​and the data specified by the standard. For the same type of permanent magnet material, there will be some difference in magnetic properties between different batches of different factories or the same factory. There are also some differences between the magnetic properties and the standard data for the shape and size of the permanent magnets actually used in the motor.
In addition, the capacity of the magnetizer and the magnetization method affect the uniformity of the magnetization state of the permanent magnet and affect the magnetic properties. Therefore, in order to improve the accuracy of the motor design calculation, it is necessary to obtain the measured demagnetization curve of the actual size of the permanent magnet of the batch at room temperature and working temperature. It is better to directly measure the demagnetization curve when sampling. safe. For motors with high consistency requirements, permanent magnet materials need to be tested piece by piece.

Application Notes OF Permanent magnetic material

Application Notes OF Permanent magnetic material

2. The magnetic properties of permanent magnet materials are related to the alloy composition and manufacturing process, and also related to the magnetic field heat treatment process. The so-called magnetic field heat treatment is that the permanent magnetic material exerts an applied magnetic field during the decomposition reaction. After the magnetic field heat treatment, the magnetic properties of the permanent magnet material are improved, and the directionality is the largest, the direction of the magnetic field is the largest, and the direction of the vertical magnetic field is the smallest. This is called anisotropy. For permanent magnet materials that have not been subjected to magnetic field heat treatment, the magnetic properties have no directionality and are called isotropic. It should be noted that for anisotropic permanent magnets, the direction of the magnetic field during magnetization should be the same as the direction of the magnetic field during the heat treatment of the magnetic field, otherwise the magnetic properties will be reduced.

3. The permanent magnet material is raised from room temperature to the highest working temperature and then cooled to room temperature after being kept for a certain period of time. The open magnetic flux is allowed to have an irreversible loss of not more than 5%. Therefore, in order to ensure the stability of the permanent magnet motor during operation, no obvious irreversible demagnetization occurs, and the magnetization treatment should be performed before use. The method is to raise the magnetized permanent magnet material to the expected maximum working temperature and keep warm 2 It takes 4 hours to eliminate this part of the irreversible loss in advance.

Principle of selection of permanent magnet materials

There are many kinds of permanent magnet materials, and the performance is very different. Therefore, when designing a permanent magnet motor, we must first select a suitable variety of permanent magnet materials and specific performance indicators.
(1) It should be ensured that there is a large enough air gap magnetic field in the air gap of the motor and the specified motor performance index.
(2) The stability of magnetic properties should be ensured under specified environmental conditions, operating temperatures and conditions of use.
(3) Good mechanical properties for easy processing and assembly.
(4) The economy is better and the price is appropriate.

All of the major national science and technology plans, including the “973” plan, the “863” plan, and the natural science fund key projects, as well as the recent national key R&D plans, have permanent magnet materials. With the launch of the “Made in China 2025” program of action, permanent magnet materials will play an increasingly important role in the new round of technological revolution and industrial transformation. At present, Ba and Sr ferrite are still the most used permanent magnet materials, but many applications are gradually being replaced by Nd-Fe-B materials. Moreover, the current production value of rare earth permanent magnet materials has greatly exceeded that of ferrite permanent magnet materials, and the production of rare earth permanent magnet materials has developed into a large industry.

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