When it comes to magnetic materials, you are probably no stranger to them - the sticky notes attached to refrigerator doors, the magnetic buckles in bags, the speakers in mobile phones, and even the nuclear magnetic resonance equipment in hospitals, all cannot do without their "presence". These materials that can be attracted by magnets or generate magnetic fields themselves may seem ordinary, but they hide many interesting scientific principles. Today, we will start from the most basic knowledge and walk into the world of magnetic materials together.
1. Magnetism
Experiments show that any substance can be more or less magnetized in an external magnetic field, but the degree of magnetization varies. According to the characteristics of substances in an external magnetic field, substances can be divided into five categories: paramagnetic substances, diamagnetic substances, ferromagnetic substances, ferrimagnetic substances, and antiferromagnetic substances. We call paramagnetic substances and diamagnetic substances weak magnetic substances, and ferromagnetic substances and ferrimagnetic substances strong magnetic substances.
2. Magnetic Materials
Soft magnetic materials: They can achieve the maximum magnetization with the minimum external magnetic field, and are magnetic materials with low coercivity and high magnetic permeability.
Soft magnetic materials are easy to magnetize and demagnetize. For example: soft ferrite, amorphous nanocrystalline alloy.
Hard magnetic materials: Also called permanent magnetic materials, they refer to materials that are difficult to magnetize and once magnetized, are difficult to demagnetize. Their main feature is high coercivity, including rare earth permanent magnetic materials, metal permanent magnetic materials and permanent ferrite.
Functional magnetic materials: mainly include magnetostrictive materials, magnetic recording materials, magnetoresistive materials, magnetic bubble materials, magneto-optical materials and magnetic thin film materials, etc.
3. NdFeB Permanent
Magnetic Materials Sintered NdFeB permanent magnetic materials adopt powder metallurgy technology. The alloy after smelting is made into powder and pressed into a compact in a magnetic field. The compact is sintered in an inert gas or vacuum to achieve densification. In order to improve the coercivity 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 flexible rubber or hard and lightweight plastic, rubber and other bonding materials, which can be directly formed into various shapes of permanent magnet components 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 coercivity and near-net shape, but its mechanical properties are poor and the processing cost is high due to patent monopoly.
4. Remanence (Br)
Remanence refers to the magnetic induction intensity exhibited by a sintered NdFeB magnet when it is magnetized to technical saturation in a closed-circuit environment by an external magnetic field and then the external magnetic field is removed. Generally speaking, it can be temporarily understood as the magnetic force of the magnet after magnetization. The units are Tesla (T) and Gauss (Gs), and 1Gs = 0.0001T.
5. Coercivity (Hcb)
The value of the reverse magnetic field strength required to reduce the magnetic induction intensity to zero when the magnet is reversely magnetized is called the磁感 coercivity. However, at this time, the magnetization intensity of the magnet is not zero, but the applied reverse magnetic field and the magnetization intensity of the magnet cancel each other out. If the external magnetic field is removed at this time, the magnet still has a certain magnetic property. 1A/m = (4π/1000)Oe, 1 Oe = (1000/4π) A/m.
6. Intrinsic Coercivity (Hcj)
The intrinsic coercivity is the reverse magnetic field strength required to reduce the magnetization intensity of the magnet to zero, which we call intrinsic coercivity. The classification of magnetic material grades is based on the size of their intrinsic coercivity. Low coercivity N, medium coercivity M, high coercivity H, ultra-high coercivity UH, extremely high coercivity EH, and very high coercivity TH.
7. Maximum Energy Product ((BH)max)
The maximum energy product represents the magnetic energy density established in the space between the two magnetic poles of the magnet, that is, the static magnetic energy per unit volume of the air gap. It is the maximum value of the product of B and H, and 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 magnetic parts with high energy product can obtain higher surface magnetic flux.
However, when the (BH)max value is the same, the levels of Br and Hcj have the following effects on magnetization: High Br and low Hcj: Under the same magnetizing voltage, higher surface magnetic flux can be obtained; Low Br and high Hcj: To obtain the same surface magnetic flux, a higher magnetizing voltage is required.
8. SI System and CGS System
The SI system and CGS system are the International System of Units and the Gaussian system of units, which are like the difference between "meter" and "li" in length units. There are certain complex conversion relationships between the International System of Units and the Gaussian system of units.
9. Curie Temperature
The Curie temperature is the temperature at which a magnetic material changes between a ferromagnet and a paramagnet. Below the Curie temperature, the substance 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 substance 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 working temperature limit of magnetic materials. The Curie temperature of NdFeB is about 320-380 degrees Celsius. The level of the Curie point is related to the crystal structure formed by magnet sintering. If the temperature reaches the Curie temperature, the molecules inside the magnet move violently and demagnetization occurs, which is irreversible; the magnet can be re-magnetized after demagnetization, but the magnetic force will drop significantly, only reaching about 50% of the original.
10. Working Temperature
The maximum working temperature of sintered NdFeB is much lower than its Curie temperature. Within the working temperature, the magnetic force will decrease when the temperature rises, 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 cobalt, terbium, dysprosium and other elements to the raw materials of sintered NdFeB can increase its Curie temperature, so high coercivity products (H, SH, ...) generally contain dysprosium.
The maximum service 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, the higher the maximum service temperature of the magnet, and the more stable the performance of the magnet. Therefore, the maximum service temperature of the magnet is not a fixed value, but changes 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 to each other arbitrarily; anisotropic magnets have different magnetic properties in different directions, and 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 maximum magnetic field strength only in the orientation direction, and the magnetic field strength in the other two directions is much smaller. If the magnetic material has an orientation process in the production process, it is an anisotropic magnet. Sintered NdFeB is generally pressed by magnetic field orientation molding, so it is anisotropic. Therefore, it is necessary to determine the orientation direction before production, that is, the future magnetizing direction. Powder magnetic field orientation is one of the key technologies for manufacturing high-performance NdFeB. (Bonded NdFeB can be isotropic or anisotropic)
12. Surface Magnetic Flux
Surface magnetic flux refers to the magnetic induction intensity at a certain point on the surface of the magnet (the surface magnetic flux at the center and edge of the magnet is different). It is the value measured by a gaussmeter in contact with a certain surface of the magnet, not the overall magnetic performance of the magnet.
13. Magnetic Flux
Magnetic flux is set 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 direction of the magnetic field. The product of the magnetic induction intensity B and the area S is called the magnetic flux through this plane, referred to as magnetic flux, with the symbol "Φ" and the unit is Weber (Wb). Magnetic flux is a physical quantity that represents the distribution of the magnetic field. It is a scalar, but has positive and negative values, which only represent its direction. Φ = B·S, when there is an angle θ between S and the vertical plane of B, Φ = B·S·cosθ.
14. Electroplating
Sintered NdFeB permanent magnetic materials are produced by powder metallurgy technology. They are a kind of powder materials with very strong chemical activity, with tiny pores and cavities inside, 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 for NdFeB strong magnets are galvanizing and nickel plating. They have obvious differences in appearance, corrosion resistance, service life, price and other aspects:
Difference in polishability: Nickel plating is superior to galvanizing in polishing and has a brighter appearance. Those with high requirements on product appearance generally choose nickel plating, while some magnets that are not exposed and have relatively low requirements on product appearance generally choose galvanizing.
Difference in corrosion resistance: Zinc is an active metal and 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 the different corrosion resistance, the service life of galvanizing is lower than that of nickel plating, which is mainly manifested in that the surface coating is easy to fall off after long-term use, leading to oxidation of the magnet, thus affecting the magnetic performance.
Difference in hardness: Nickel plating is higher than galvanizing. In the use process, it can greatly avoid collisions and other situations, causing NdFeB strong magnets to have corner drops, cracks and other phenomena.
Difference in price: In this aspect, galvanizing has great advantages. The price from low to high is galvanizing, nickel plating, epoxy resin, etc.
15. Single-sided Magnets
All magnets have two poles, but in some working positions, single-sided magnets are needed. Therefore, it is necessary to wrap one side of the magnet with an iron sheet to shield the magnetism of the side wrapped by the iron sheet. Such magnets are collectively called single-sided magnets or one-sided magnets. There is no real single-sided magnet.
