1. Magnetism
Experiments show that any material can be magnetized to a greater or lesser extent in an external magnetic field, but the degree of magnetization is different. According to the characteristics of the material in the external magnetic field, the material can be divided into five categories: paramagnetic material, diamagnetic material, ferromagnetic material, ferrimagnetic material, and antiferromagnetic material. We call paramagnetic and diamagnetic materials weak magnetic materials, and ferromagnetic and ferrimagnetic materials strong magnetic materials.
2. Magnetic materials
Soft magnetic materials: can achieve the maximum magnetization intensity with the smallest external magnetic field, and are magnetic materials with low coercivity and high magnetic permeability. Soft magnetic materials are easy to magnetize and easy to demagnetize. For example, soft ferrites and amorphous nanocrystalline alloys.
Hard magnetic materials: also called permanent magnetic materials, refer to materials that are difficult to magnetize and difficult to demagnetize once magnetized. Their main feature is high coercivity, including rare earth permanent magnetic materials, metal permanent magnetic materials, and permanent magnetic ferrites.
Functional magnetic materials: mainly magnetostrictive materials, magnetic recording materials, magnetoresistance materials, magnetic bubble materials, magneto-optical materials magnetic film materials, etc.
3. NdFeB permanent magnet materials
Sintered NdFeB permanent magnet materials adopt a powder metallurgy process. The alloy after smelting is made into powder and pressed into pressed embryos in a magnetic field. The pressed embryos are sintered in an inert gas or vacuum to achieve densification
To improve the coercive force of the magnet, aging heat treatment is usually required, and then the finished product is obtained after post-processing and surface treatment.
Bonded NdFeB is a mixture of permanent magnet powder and rubber with good winding properties or hard and light plastics, rubber, and other bonding materials, which are directly formed into permanent magnet parts of various shapes according to user requirements.
Hot-pressed NdFeB can achieve magnetic properties similar to sintered NdFeB without adding heavy rare earth elements. It has the advantages of high density, high orientation, good corrosion resistance, high coercive force, and near-final forming, but the mechanical properties are not good and the processing cost is high due to patent monopoly.
4. Remanence(Br)
refers to the magnetic induction intensity of a sintered NdFeB magnet after a magnet is magnetized to technical saturation in a closed-circuit environment and the external magnetic field is removed. In layman's terms, it can be temporarily understood as the magnetic force of the magnet after magnetization. The units are Tesla (T) and Gauss (Gs), 1GS=0.0001T.
5. Coercive force(Hcb)
When the magnet is reversely magnetized, the value of the reverse magnetic field strength required to make the magnetic induction intensity zero is called the magnetic coercive force. However, the magnetization intensity of the magnet is not zero at this time, but the applied reverse magnetic field and the magnetization intensity of the magnet cancel each other out. At this time, if the external magnetic field is removed, the magnet still has certain magnetic properties. 1A/m=(4T/1000)0e,1 0e =(1000/4T)A/m.
6. Intrinsic coercive force(Hcj)
The reverse magnetic field strength required to reduce the magnetization intensity of the magnet to zero is called the intrinsic coercive force. The classification of magnetic material grades is based on the size of their intrinsic coercive force. Low coercive force N, medium coercive force M, high coercive force H, ultra-high coercive force UH, extremely high coercive force EH, and highest coercive force TH.
7. Maximum magnetic energy product (BH)max
Represents the magnetic energy density established by the space between the two magnetic poles of the magnet, that is, the static magnetic energy per unit volume of the air gap, which is the maximum value of the product of B and H. Its size directly indicates the performance of the magnet. Under the same conditions, that is, the same size, the same number of poles and the same magnetizing voltage, the surface magnetism obtained by the magnetic parts with high magnetic energy product is also high, but at the same (BH)max value, the level of B. and Hcj has the following effects on magnetization:
Br is high, Hcj is low: under the same magnetizing voltage, a higher surface magnetism can be obtained;
Br is low, Hcj is high: to obtain the same surface magnetism, a higher magnetizing voltage is required.
8. Sl system and CGS system
That is, the International System of Units and the Gaussian System of Units, just like the difference between "meter" and "mile" in the length unit. There is a certain complex conversion relationship between the International System of Units and the Gaussian System of Units.
9. Curie temperature
It is the temperature at which the magnetic material changes between ferromagnets and paramagnets. When it is lower than the Curie temperature, the material becomes a ferromagnet, and the magnetic field related to the material is difficult to change. When the temperature is higher than the Curie temperature, the material becomes a paramagnet, and the magnetic field of the magnet can easily change with the change of the surrounding magnetic field.
The Curie temperature represents the theoretical operating temperature limit of the magnetic material. The Curie temperature of NdFeB is about 320-380 degrees Celsius. The height of the Curie point is related to the crystal structure formed by the sintering of the magnet.
If the temperature reaches the Curie temperature, some molecules in the magnet move violently and demagnetization occurs, and it is irreversible; the magnet can be magnetized again after demagnetization, but the magnetic force will drop significantly and can only reach about 50% of the original.
10. Working temperature
The maximum working temperature of sintered NdFeB is much lower than its Curie temperature. When the temperature rises within the working temperature range, the magnetic force will decrease, but most of the magnetic force will recover after cooling.
The relationship between working temperature and Curie temperature: The higher the Curie temperature, the higher the working temperature of the magnetic material, and the better the temperature stability. Adding elements such as cobalt, terbium, and dysprosium to the raw materials of sintered NdFeB can increase its Curie temperature, so high coercive force products (H, SH, ...) generally contain dysprosium.
The maximum operating temperature of sintered NdFeB depends on its own magnetic properties and the selection of working points. For the same sintered NdFeB magnet, the more closed the working magnetic circuit is, the higher the maximum operating temperature of the magnet is, and the more stable the performance of the magnet is. Therefore, the maximum operating temperature of the magnet is not a fixed value, but varies with the degree of closure of the magnetic circuit.
11. Magnetic field orientation
Magnetic materials are divided into two categories: isotropic magnets and anisotropic magnets. Isotropic magnets have the same magnetic properties in any direction and can be attracted together at will; anisotropic magnets have different magnetic properties in different directions. The direction in which they can obtain the best magnetic properties is called the orientation direction of the magnet.
A square sintered NdFeB magnet has the largest magnetic field intensity only in the orientation direction, and the magnetic field intensity in the other two directions is much smaller. If there is an orientation process in the production process of magnetic materials, it is anisotropic magnets. Sintered NdFeB is generally formed and pressed by magnetic field orientation, so it is anisotropic. Therefore, it is necessary to determine the orientation direction before production, that is, the future magnetization direction. Powder magnetic field orientation is one of the key technologies for manufacturing high-performance NdFeB. , (Bonded NdFeB has both isotropic and anisotropic)
12. Surface magnetism
Refers to the magnetic induction intensity at a certain point on the surface of the magnet (the surface magnetism at the center and edge of the magnet is different). It is the teaching value measured by the contact between the Gauss meter and a certain surface of the magnet, not the overall magnetic properties of the magnet.
13. Magnetic flux
Assume that in a uniform magnetic field with a magnetic induction intensity of B, there is a plane with an area of S and perpendicular to the magnetic field direction. The product of the magnetic induction intensity B and the area S is called the magnetic flux passing through this plane, referred to as magnetic flux, with the symbol "$" and the unit being Weber (Wb). Magnetic flux is a physical quantity that represents the distribution of the magnetic field. It is a scalar, but it has positive and negative values, which only represent its direction. 中=B·S. When there is an angle between the vertical planes of S and B, 中=B:S:cos0.
14. Electroplating
Sintered NdFeB permanent magnet material is produced by powder metallurgy process. It is a powder material with very strong chemical activity. There are tiny pores and voids inside. It is easily corroded and oxidized in the air. Therefore, strict surface treatment must be carried out before use. Electroplating is a mature metal surface treatment method and is widely used.
The most commonly used coatings for NdFeB strong magnets are zinc plating and nickel plating. They have obvious differences in appearance, corrosion resistance, service life, price, etc.:
Difference in polishing: Nickel plating is superior to zinc plating in polishing, and it looks brighter. Those who have high requirements for product appearance generally choose nickel plating, while some magnets are not exposed, and those who have relatively low requirements for product appearance generally choose zinc plating.
Difference in corrosion resistance: Zinc is an active metal that can react with acid, so its corrosion resistance is poor; after nickel plating surface treatment, its corrosion resistance is higher, and the difference in service life: Due to different corrosion resistance, the service life of zinc plating is lower than that of nickel plating, which is mainly manifested in that the surface coating is easy to fall off after a long time of use, causing the magnet to oxidize, thereby affecting the magnetic performance.
Difference in hardness: Nickel plating is higher than zinc plating. During use, it can greatly avoid collisions and other situations, causing the NdFeB strong magnet to fall off and break. Price difference: In this regard, zinc plating is extremely advantageous, and the prices are arranged from low to high as zinc plating, nickel plating, epoxy resin, etc.
15. Single-sided magnet
So it is necessary to wrap one side of the magnet with an iron sheet so that the magnetism of the side wrapped by the iron sheet is shielded. Such magnets have two poles, but magnets with single-sided poles are required in certain working positions. They are collectively called single-sided magnets or single-sided magnets. There is no real single-sided magnet.