What is hard magnetic material

What is hard magnetic material?

Hard magnetic materials generally refer to permanent magnetic materials, which can maintain constant magnetic once magnetized. In application, the permanent magnet material works on the second quadrant demagnetization part of the deep magnetic saturation and hysteresis loop after magnetization. The permanent magnet materials commonly used are divided into Alnico permanent magnet alloy, Ferrochromium-cobalt permanent magnet alloy, ferrite permanent magnet, rare earth permanent magnet materials and composite permanent magnet materials.
Hard magnetic materials are characterized by high anisotropy field, high coercive force, large hysteresis loop surface area, and large magnetization field required for magnetization to saturation, after removing the external magnetic field, it can still maintain strong magnetism for a long time. In application, the permanent magnet material works on the second quadrant demagnetization part of the deep magnetic saturation and hysteresis loop after magnetization. As an important basic magnetic functional material, permanent magnet material has a very broad application field. China’s permanent magnet material industry plays an important role in the world. Many research work of enterprises not only engaged in production and application have been in the ascendant. The following introduces the types, main properties, application precautions and selection principles of permanent magnet materials.

Types of permanent magnet materials

1. Ferrite magnet

• 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 performance of ferrite is not high. At present, the magnetic energy product (one of the parameters to measure the performance of magnet) can only be slightly higher than 4mgoe. The biggest advantage of this material is that it is low in price. At present, it is still widely used in many fields.
• 3. Ferrite is Ceramic. Therefore, its processing performance is similar to that of ferrite magnet. It is formed by mold and sintered. If it needs processing, it can only be simply ground. Due to the difficulty of mechanical processing, most ferrite products are simple in shape and have large dimensional tolerances. The square shape product is good and can be ground. Circular, generally grinding only two planes. Other dimensional tolerances are given according to the percentage of nominal dimensions.
• 4. Due to the wide application of ferrite and low prices, many manufacturers will have ready-made products such as ring blocks of conventional shapes and sizes to choose from. Since ferrite is made of ceramic material, there is basically no corrosion problem. The finished product does not need surface treatment such as electroplating or coating.

2. Rubber magnet

• 1. Rubber magnet is a kind of ferrite magnetic material Series. It is made of composite ferrite magnetic powder and synthetic rubber, and is made of softness through extrusion molding, calendering molding, injection molding and other processes, elastic and twistable magnets. It can be processed into strips, rolls, sheets, blocks, rings and various complex shapes.
• 2. Its magnetic energy product is 0.60~1.50 the application fields of MGOe rubber magnetic material: refrigerator, message display rack, fasteners that fix objects in metal bodies for advertising, etc. are used in toys, teaching instruments, magnetic plates for switches and sensors.
• 3. It is mainly used in micro-special motors, refrigerators, disinfection cabinets, kitchen cabinets, toys, stationery, advertising and other industries.

3. Samarium and cobalt magnet

• 1. Samarium and cobalt are the main components of samarium and cobalt. Since the two materials are expensive in themselves, samarium cobalt magnet is also the most expensive one among several magnets. The magnetic energy product of samarium-cobalt magnet can currently reach 30mgoe or even higher.
• 2. In addition, samarium cobalt magnet has high coercive force and high temperature resistance, which can be applied to high temperature of 350 degrees Celsius, so it cannot be replaced in many applications. Samarium-cobalt magnet belongs to powder metallurgy products. General manufacturers according to the size and shape of the finished product needs, sintered into the square blank, and then use diamond blade cut into the size of the finished product. Because samarium cobalt has electrical conductivity, wire cutting processing can be performed.
• 3. In theory, samarium cobalt can be cut into shapes that can be cut into wire cutting. If the problem of magnetization and larger size is not considered. Samarium magnet, good corrosion resistance generally does not need anti-corrosion plating or coating. In addition, samarium cobalt magnet is very brittle, so it is more difficult to process small size or thin-walled products.

4. Neodymium iron boron magnet

• 1. NdFeB is a magnet product that is widely used and developing rapidly. Neodymium iron boron from the invention to the present widely used, but also more than 20 years. Due to its high magnetic properties and workability, the price is not very high, so the application field is expanding rapidly.
• 2. At present, the magnetic energy product of commercialized NdFeB can reach 50mgoe, which is 10 times that of ferrite. NdFeB also belongs to powder metallurgy products and the processing method is similar to samurite.
• 3. At present, the highest working temperature of NdFeB is about 180 degrees Celsius. If it is a harsh environment, it is generally recommended not to exceed 140 degrees Celsius. Neodymium iron boron is very easily corroded. Therefore, most finished products should be electroplated or painted.
• 4. The conventional surface treatment includes: nickel plating (nickel copper nickel), zinc plating, aluminum plating, electrophoresis, etc. If you work in a closed environment, phosphation can also be used. Due to the high magnetic properties of NdFeB, in many occasions, it is used to replace other magnetic materials to reduce product volume. If you use the current mobile phone size of the ferrite magnet, I am afraid it will not be less than half a brick.
• 5. Both Samarium Cobalt Magnet and NdFeB magnets have better processing performance. Therefore, the dimensional tolerance of the product is much better than ferrite. General product size tolerance can be (+/-)0.05mm.

5. Aluminum-nickel-cobalt magnet

• 1. Aluminum nickel cobalt magnet has two processes: casting and sintering. More domestic casting. The magnetic energy product of alnico can reach 9MGOe, and it has a biggest characteristic that the working temperature of high temperature can reach 550 degrees Celsius. However, Alnico is very easy to demagnetize under the reverse magnetic field. If you push the same pole direction (two N or two S) of two aluminum nickel cobalt together, the magnetic field of one magnet will be reversed or reversed. Therefore, it is not suitable to work in reverse magnetic field (such as motor).
• 2. The hardness of aluminum nickel cobalt is very high. Although grinding and wire cutting can be carried out, the cost is relatively high. General supply of finished products, there are two kinds of grinding or not grinding. Alnico is widely used in the field of sensors.

Main properties of permanent magnet materials

1. Residual magnetic induction strength

The magnetic induction intensity value of the permanent magnet material when it is magnetized to saturation in the external magnetic field and the external magnetic field is zero. This index data is directly related to the level of air gap magnetic density in the motor. The higher the value of magnetic induction intensity is, the higher the air gap magnetic density of the motor will be, and the main indexes of the motor such as torque constant and back-potential coefficient will reach the best value, the value relationship between electric load and magnetic load of the motor may be the most reasonable, and the efficiency is the best.

2. Coercive force Hc,(magnetic induction coercive force HcB)

In the case of permanent magnetization, the reverse magnetic field strength required when the remanence Br drops to zero. This index is related to the anti-demagnetization ability of the motor, that is, the overload multiple and the air gap magnetic density. The larger the Hc value, the stronger the anti-demagnetization ability of the motor. The larger the overload multiple, the stronger the adaptability to the dynamic working environment of strong demagnetization. At the same time, the air gap flux density of the motor will be improved.

3. Maximum magnetic energy product (BH)max

The maximum energy of the magnetic field provided by the permanent magnet material to the outer magnetic circuit. This index is directly related to the amount of permanent magnet material in motor. The larger (BH)max indicates that the permanent magnet material can provide greater magnetic energy for external magnetic circuit, that is, the less permanent magnet materials are used in the motor under the same power.

4. Internal power of speech

This index refers to the value of magnetic field intensity when the residual magnetization M drops to zero. When B is equal to 0 on the demagnetization curve, the corresponding Hc B value only indicates that the permanent magnet cannot provide energy with the outer magnetic circuit at this time, and it 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 demagnetized at this time and has completely saved no magnetic field energy. Although Hc I is not directly related to the working point of the motor, it is the real coercive force of permanent magnet materials, representing that the permanent magnet materials have the magnetic field energy and the ability to resist the magnetic field. The size of internal coercive force is closely related to the temperature stability of permanent magnet materials. The higher the internal coercive force, the higher the working temperature of permanent magnet material.

5. Temperature coefficient α

Temperature is one of the main factors affecting the magnetic properties of permanent magnet materials. When the temperature changes by 1 ℃, the percentage of reversible changes in magnetic properties is called the temperature coefficient of magnetic materials. Temperature coefficient can be divided into Residual induction temperature coefficient and coercive force 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 use temperature range of the motor. Indirectly affect the power volume ratio of the motor.

Precautions for application of permanent magnet materials

1. The actual magnetic properties of permanent magnet materials are related to the specific manufacturing process of the production plant, and there is often a certain deviation between its value and the data specified in the standard. As for the permanent magnet material of the same brand, there will be certain magnetic energy differences in different factories or in the same factory and in different batch numbers. For the shape and size of the permanent magnet actually used in the motor, there will be some differences between its magnetic properties and the standard data.

In addition, the capacity of the magnetizer and the magnetizing method will affect the uniformity of the magnetizing state of the permanent magnet and affect the magnetic performance. Therefore, in order to improve the accuracy of motor design calculation, we need to ask the manufacturer for the measured demagnetization curve of the permanent magnet with the actual size of the batch number at room temperature and working temperature, when conditions are available, it is best to sample and directly measure the demagnetization curve, which is relatively safe. For motors with high consistency requirements, it is more necessary to detect permanent magnet materials piece by piece.

 

2. The magnetic property of permanent magnet material is not only related to the alloy composition and manufacturing process, but also related to the magnetic field heat treatment process. The so-called magnetic field heat treatment is the external magnetic field applied by permanent magnetic materials during the decomposition reaction. After the magnetic field heat treatment, the magnetic properties of the permanent magnet material is improved, and with directionality, and the direction of the paramagnetic field is the largest, and the direction of the vertical magnetic field is the smallest, which is called anisotropy. For the permanent magnetic material that has not been heat treated by magnetic field, the magnetic property has no directionality, which is called isoorientation. It should be noted that the magnetic field direction of the anisotropic permanent magnet during magnetization should be consistent with the magnetic field direction during heat treatment, otherwise the magnetic energy will be reduced to some extent.

3. The permanent magnet material will rise from room temperature to the highest working temperature and keep it for a certain period of time before cooling to room temperature, and its open-circuit magnetic flux allows no more than 5% of the irreversible loss. Therefore, in order to ensure stable performance of permanent magnet motor during operation and no obvious irreversible demagnetization, magnetic stabilization treatment should be performed before use, the method is to heat the magnetized permanent magnet material to the expected maximum working temperature and keep it warm for 2 to 4 hours to eliminate this part of irreversible loss in advance.

Selection principle of permanent magnet material

There are many kinds of permanent magnet materials and their performances vary greatly. Therefore, when designing permanent magnet motors, we must first choose suitable permanent magnet materials and specific performance indexes.

• (1) it should be able to ensure that there is 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 the specified environmental conditions, working temperature and service conditions.
• (3) have good mechanical properties to facilitate processing and assembly.
• (4) the economy is better and the price is appropriate.

At present, Ba and Sr ferrite are still the most widely used permanent magnetic materials, but many applications are gradually being replaced by Nd-Fe-B kinds of materials. Moreover, the current output 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 major industry.

Source: China Permanent Magnet Manufacturer – www.rizinia.com