Magnet basics

Overview of magnetic materials:

Magnetic is one of the basic properties of matter. The magnetic phenomenon is associated with various forms of charge motion, and magnetic properties are generated due to the electron motion and spin inside the material, which generate a magnetic field of a certain magnitude. Everything is magnetic. The nature of the magnetic can be divided into paramagnetic substances, diamagnetic substances, ferromagnetic substances, antiferromagnetic substances, and ferrimagnetic substances, of which ferromagnetic substances and ferrimagnetic substances are ferromagnetic substances, usually these two The substances are collectively referred to as magnetic materials.

Classification, performance characteristics and uses of magnetic materials:

1 Ferrite magnetic material: generally refers to the oxide of oxide and other metal oxides. Most of them have ferrimagnetic properties. Features: The resistivity is much higher than that of metal, about 1-10 (12th power) ohms/cm, so the vortex loss and skin effect are small, suitable for high frequency use. The saturation magnetization is low and it is not suitable for high magnetic density applications. The Curie temperature is relatively low.

2 Ferromagnetic material: refers to a material with ferromagnetism. For example, iron-nickel-cobalt and its alloys, alloys of certain rare earth elements. Below the Curie temperature, the material has a large magnetization when external magnetic is applied.

3 Ferrous magnetic material: refers to materials with ferrimagnetic properties, such as various ferrites. Below the Neel temperature, the material has a large magnetization when external magnetic is applied.

4 Permanent magnet material: The magnet is magnetized to remove the external magnetic field and still has strong magnetic properties, characterized by high coercive force and large magnetic energy product. It can be divided into aluminum nickel cobalt, rare earth cobalt, neodymium iron boron and the like. Ferrite Permanent magnets, for example, barium ferrite, barium ferrite, other permanent magnets, such as plastic magnetic.

5 soft magnetic material: material that is easy to magnetize and demagnetize. Mn-Zn ferrite soft magnetic material, its working frequency is between 1K-10M. Nickel-zinc ferrite soft magnetic material, the working frequency is generally 1-300MHZ.

6 Metal soft magnetic material: Compared with ferrite, it has high saturation magnetic induction and low coercivity, such as engineering pure iron, iron-aluminum alloy, iron-cobalt alloy, iron-nickel alloy, etc., which are commonly used in transformers.

Magnet common sense

The terminology of magnets:

1 Saturation magnetic induction: (saturation magnetic flux density) The magnetic induction when the magnetic body is magnetized to saturation. In practical applications, the saturation magnetic induction often refers to the magnetic induction under a given magnetic field (substantially reaching the magnetic field of magnetic saturation).

2 Residual magnetic induction: From the saturation state of the magnetic body, the magnetic field (including the self-retracting magnetic field) is monotonously reduced to a magnetic induction of zero.

3 Magnetic flux density coercive force, which is the magnetic induction intensity when the magnetic induction intensity B is reduced to zero by monotonously changing the magnetic field strength from the saturation magnetization state of the magnetic body.

4 Intrinsic coercive force: The magnetic field strength which reduces the magnetization M to 0 from the saturation magnetization state of the magnetic body.

5 Magnetic energy product: The product of the magnetic induction and the magnetic field strength at any point on the demagnetization curve of the permanent magnet.

6 Initial permeability: The limit of the magnetic permeability of a magnetic body in a magnetic neutral state.

7 Loss tangent: It is the ratio of the imaginary part to the real part of the series complex permeability. The physical meaning is the ratio of the loss energy to the stored energy in each cycle of the alternating magnetic field.

8 Specific loss tangent: This is the ratio of the loss tangent of the material to the initial permeability.

9 Temperature coefficient: The amount of change measured between two given temperatures divided by the amount of temperature change.

10 Specific temperature coefficient of magnetic permeability: the ratio of the temperature coefficient of magnetic permeability to the magnetic permeability.

11 Curie temperature: At this temperature, the spontaneous magnetization is zero, that is, the critical temperature at which the ferromagnetic material (or submagnetic material) changes from a ferromagnetic state (or a ferrimagnetic state) to a paramagnetic state.

Common terms for magnets

Properties of NdFeB

The permanent magnet materials mainly include AlNiCo metal permanent magnets, the first generation SmCo5 permanent magnets (referred to as 1:5 type samarium cobalt alloy), and the second generation Sm2Co17 (referred to as 2:17 type samarium cobalt alloys). Magnet, third-generation rare earth permanent magnet alloy NdFeB (called neodymium iron boron alloy). With the development of science and technology, the performance of NdFeB permanent magnet materials continues to increase, and the application fields continue to expand. High magnetic energy product (50 mega high ≈ 400 kJ/m3), high coercive force (28EH, 32EH) and high service temperature (240C) sintered NdFeB have been industrially produced. The main raw materials of NdFeB permanent magnets are rare earth metal lanthanum (Nd) 32%, metallic element iron (Fe) 64% and non-metallic element boron (B) 1% (a small amount of yttrium (Dy), lanthanum (Tb), cobalt (Co), niobium (Nb), gallium (Ga), aluminum (Al), copper (Cu) and other elements). The NdFeB ternary permanent magnet material is based on a Nd2Fe14B compound, and its composition should be similar to the compound Nd2Fe14B. However, when the Nd2Fe14B is completely distributed, the magnetic properties of the magnet are very low, even without magnetism. It is only in the actual magnet that the content of niobium and boron is higher than that of the Nd2Fe14B compound to obtain better permanent magnet properties.

1. Processing technology of NdFeB

Sintering: batching (recipe) → smelting → milling → pressing (forming orientation) → sintering and timely effect → magnetic property inspection → machining → surface coating treatment (electroplating) → finished product inspection

Bonding: raw material → particle size adjustment → kneading with binder → forming (compression, extrusion, injection molding) → firing treatment (compression) → reprocessing → finished product inspection

2, the quality standard of NdFeB

There are three main parameters: Residual Induction, unit Gauss, after removing the magnetic field from saturation, the remaining magnetic flux density represents the magnetic field strength of the magnet; Coccive Force, unit Oersteds puts the magnet in a reverse applied magnetic field. When the applied magnetic field increases to a certain intensity, the magnet's magnetism disappears. The ability to resist the applied magnetic field is called coercivity, which represents the measurement of anti-demagnetization ability; magnetic energy. The product BHmax, unit Gauss-Oersteds, is the magnetic field energy produced by a unit volume of material, which is a physical quantity of energy that a magnet can store.

3. Application and use of NdFeB

At present, the main application fields are: permanent magnet motor, generator, nuclear magnetic resonance imager, magnetic separator, acoustic speaker, magnetic suspension system, magnetic drive, magnetic lifting, instrumentation, liquid magnetization, magnetic therapy equipment, etc. Manufacturing materials, general machinery, petrochemicals, electronic information industry and cutting-edge technology are indispensable functional materials.

4. Comparison of NdFeB and other permanent magnet materials

NdFeB permanent magnet material is the most magnetic permanent magnet material in the world. Its magnetic energy product is ten times higher than that of widely used ferrite, and is about one higher than the first and second generation Rare earth magnets (samarium cobalt permanent magnet). Double, known as the "king of permanent magnets." Using him instead of other permanent magnet materials can double the volume and weight of the device. Due to the abundant resources of strontium, the replacement of expensive cobalt with iron compared with samarium-cobalt permanent magnets makes the products cost-effective and thus has been widely used.

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