Explanation of 15 concepts related to magnetic materials (Basic Edition)

Magnetism

Experiments show that any matter can be magnetized more or less in the external magnetic field, but the degree of magnetization is different. According to the characteristics of materials in the external magnetic field, materials can be divided into five categories: paramagnetic materials, diamagnetic materials, ferromagnetic materials, ferromagnetic materials and antiferromagnetic materials. We call paramagnetic material and diamagnetic material weak magnetic material, ferromagnetic material and ferromagnetic material strong magnetic material.

Magnetic materials

Soft magnetic material: can use the minimum external magnetic field to achieve the maximum magnetization, is a magnetic material with low coercivity and high permeability. Soft magnetic materials are easy to magnetize and demagnetize. For example: soft ferrite, amorphous nanocrystalline alloy.
Hard magnetic material: also known as permanent magnetic material, refers to the material which is difficult to magnetize and demagnetize once magnetized. Its main feature is high coercivity, including rare earth permanent magnetic material, metal permanent magnetic material and permanent ferrite.
Functional magnetic materials: mainly magnetostrictive materials, magnetic recording materials, magnetoresistance materials, magnetic bubble materials, magneto-optical materials and magnetic film materials.

NdFeB permanent magnet materials

Sintered Nd-Fe-B permanent magnet material adopts powder metallurgy process. The melted alloy is made into powder and pressed into briquette in magnetic field. The briquette is sintered in inert gas or vacuum to achieve densification. In order to improve the coercivity of magnet, aging heat treatment is usually required, and then the finished product is obtained after post-processing and surface treatment.
Bonded Nd-Fe-B is made of permanent magnet powder mixed with flexible rubber, hard and light plastic, rubber and other bonding materials, which can be directly molded into various shapes of permanent magnet parts according to the requirements of users.
The magnetic properties of hot pressed NdFeB are similar to that of sintered NdFeB without adding heavy rare earth elements. It has the advantages of high density, high degree of orientation, good corrosion resistance, high coercivity and near final forming. However, its mechanical properties are not good and its processing cost is high due to patent monopoly.

Remanence br

It refers to the magnetic induction intensity of sintered NdFeB magnets when a magnet is magnetized by an external magnetic field in a closed-circuit environment until the technology is saturated and then the external magnetic field is withdrawn. Generally speaking, it can be understood as the magnetic force of the magnetized magnet. The unit is Tesla (T) and Gauss (GS), 1gs = 0.0001t.

Coercivity HCB

When the magnet is magnetized reversely, the value of the reverse magnetic field strength required to reduce the magnetic induction to zero is called the magnetic induction coercivity. At this time, if the magnetization of the magnet is zero, the interaction between the magnetic field and the magnetization of the magnet does not necessarily cancel. 1A/m=(4π/1000)Oe, 1 Oe =(1000/4π) A/m。

Intrinsic coercivity Hcj

The intensity of the reverse magnetic field required to reduce the magnetization of the magnet to zero is called intrinsic coercivity. The classification of magnetic materials is based on their intrinsic coercivity. Low coercivity n, medium coercivity m, high coercivity h, very high coercivity uh, very high coercivity eh, and very high coercivity th.

Maximum magnetic energy product (BH) max

It represents the magnetic energy density established by the space between the two magnetic poles of the magnet, that is, the magnetostatic 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, i.e. the same size, the same number of poles and the same magnetization voltage, the surface magnetism of the magnetic parts with high magnetic energy product is also high. But at the same (BH) max value, the influence of Br and Hcj on magnetization is as follows:
BR is high, Hcj is low: under the same magnetizing voltage, higher surface magnetism can be obtained;
Br low, Hcj high: in order to get the same surface magnetism, higher magnetization voltage is needed.

SI system and CGS system

That is, the international system of units and the Gauss system of units, just like the difference between the “meter” and the “Mile” in the unit of length. There is a complex conversion relationship between the international system of units and the Gauss system of units.

Curie temperature

When the Curie temperature 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 paramagnetic, and the magnetic field of the magnet is easy to change with the change of the surrounding magnetic field.
The Curie temperature represents the theoretical working temperature limit of magnetic materials. The Curie temperature of NdFeB is about 320-380 ℃. The Curie point is related to the crystal structure of sintered magnets. If the temperature reaches Curie temperature, the molecules inside the magnet will move violently and demagnetize irreversibly; after demagnetization, the magnet can be magnetized again, but the magnetic force will drop sharply, reaching only about 50% of the original value.

Working temperature

The maximum working temperature of sintered NdFeB is much lower than its Curie temperature. The magnetic force will decrease with the increase of temperature within the working temperature, but most of the magnetic force will recover after cooling.
The relationship between working temperature and Curie temperature: the higher Curie temperature, the higher the working temperature of magnetic material, and the better the temperature stability. The Curie temperature of sintered NdFeB can be increased by adding cobalt, terbium, dysprosium and other elements It generally contains dysprosium.

The maximum operating temperature of sintered NdFeB depends on its magnetic properties and the selection of working point. For the same sintered NdFeB magnet, the more closed the 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 temperature of the magnet is not a certain value, but changes with the degree of closure of the magnetic circuit.

Magnetic field orientation

Magnetic materials can be divided into isotropic magnets and anisotropic magnets. The magnetic properties of isotropic magnets are the same in any direction and can be attracted together at will; the magnetic properties of anisotropic magnets are different in different directions, and the direction in which they can obtain the best magnetic properties is called the orientation direction of magnets.
For a square sintered NdFeB magnet, only the orientation direction has the highest magnetic field strength, and the other two directions have much lower magnetic field strength. In the production process of magnetic materials, if there is orientation process, it is anisotropic magnet. Sintered NdFeB is generally pressed by magnetic field orientation forming, so it is anisotropic. Therefore, before production, it is necessary to determine the orientation direction, the magnetization direction in the future. The magnetic field is one of the key technologies for the manufacture of NdFeB powder. (there are isotropic and anisotropic bonded NdFeB)

Surface magnetism

It refers to the magnetic induction intensity of a certain point on the surface of the magnet (the surface magnetism of the center and edge of the magnet is different). It is the value measured by the Gauss meter contacting with a certain surface of the magnet, not the magnetic property of the whole magnet.

Magnetic flux

In a uniform magnetic field with magnetic induction intensity B, there is a plane whose area is s and is perpendicular to the direction of the magnetic field. The product of magnetic induction intensity B and area s is called the magnetic flux passing through this plane. The symbol “Φ” for short, and the unit is Weber (WB). Magnetic flux is a physical quantity that represents the distribution of magnetic field. It is a scalar, but it has positive and negative, and positive and negative only represent its direction. When there is an angle θ between the vertical plane of S and B, Φ = B · s · cos θ.

Electroplating

Sintered Nd-Fe-B permanent magnet material is produced by powder metallurgy process. It is a kind of powder material with strong chemical activity. There are micro pores and cavities in it, which are easy to be corroded and oxidized in the air. Therefore, strict surface treatment must be carried out before use. Electroplating is widely used as a mature metal surface treatment method.
The most commonly used coatings of NdFeB magnets are zinc plating and nickel plating. There are obvious differences in appearance, corrosion resistance, service life and price
Polishing difference: nickel plating is superior to zinc plating in polishing, and it is brighter in appearance. Nickel plating is generally used for products with high appearance requirements, while some magnets are not exposed, and general zinc plating is used for products with relatively low appearance requirements.
Difference in corrosion resistance: zinc is an active metal, which can react with acid, so its corrosion resistance is poor; after nickel plating surface treatment, its corrosion resistance is higher.
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, resulting in the oxidation of magnets, thus affecting the magnetic properties.
Hardness difference: nickel plating is higher than zinc plating. In the process of use, it can avoid collision to a great extent, which makes the NdFeB strong magnet drop angle and break.
Price difference: in this respect, zinc plating is very advantageous, and the price from low to high is zinc plating, nickel plating, epoxy resin, etc.

Single sided magnet

Magnets have two poles, but in some working positions need a single pole magnet, so we need to use iron sheet to cover one side of the magnet, so that the side of the magnet covered by iron sheet is shielded. Such magnets are collectively referred to as single-sided magnet or single-sided magnet. There is no real one-sided magnet.
Source: China Permanent Magnet Manufacturer – www.ymagnet.com

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