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Fifteen basic concepts of magnetic materials
Time:07/24/2020
Fifteen basic concepts of magnetic materials:
 
1. The magnetic
 
Experiments show that any substance can be more or less magnetized in an external magnetic field, just with different degrees of magnetization. According to the properties of substances in the external magnetic field, they can be classified into five classes: paramagnetic, diamagnetic, ferromagnetic, ferromagnetic, ferromagnetic and antiferromagnetic. We call paramagnetic material and diamagnetic material as weak magnetic material, ferromagnetic material, ferromagnetic material as strong magnetic material.
 
2. Magnetic materials
 
Soft magnetic materials: magnetic materials with low coerce and high permeability can achieve maximum magnetization with a minimum external magnetic field. Soft magnetic materials are easy to magnetize and easy to demagnetize. For example: soft ferrite, amorphous nanocrystalline alloy. Hard magnetic material: also known as permanent magnetic material, it refers to the material that is difficult to magnetize and difficult to demagnetize once magnetized. Its main characteristics are high coerce, including rare earth permanent magnetic material, metal permanent magnetic material and permanent magnetic ferrite. Functional magnetic materials: mainly magnetostrictive materials, magnetic recording materials, magnetoresistance materials, magnetic bubble materials, magneto-optical materials and magnetic film materials.
 
3. Ndfeb permanent magnet
 
Sintered NdFeb permanent magnet adopts powder metallurgy process. The smelted alloy is made into powder and pressed in magnetic field to make pressed embryo. Pressed embryo is sintered in inert gas or vacuum to achieve densification. Bonded NdFeb is a permanent magnet powder mixed with rubber with good windability or hard and light weight plastic, rubber and other bonding materials. It is directly molded into permanent magnet parts of various shapes according to customer requirements. Hot pressed NdFeb can achieve magnetic properties similar to sintered NdFEB without adding more rare earth elements. It has the advantages of high density, high orientation, good corrosion resistance, high coerce and near final molding. However, it has poor mechanical properties and high processing cost due to patent monopoly.
 
4. The remanence Br
 
It refers to the magnetic induction intensity of sintered NdFeb magnet when a magnet is magnetized by the outer magnetic field in closed circuit environment to the technology saturation and then the outer magnetic field is withdrawn. Colloquially, it can be briefly 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 a magnet is magnetized in reverse, the value of reverse magnetic field strength required to reduce the magnetic induction intensity to zero is called magnetic induction coerce. However, the magnetization of the magnet is not zero at this time, but the added reverse magnetic field cancels with the magnetization of the magnet. At this time, if the external magnetic field is withdrawn, the magnet still has certain magnetic properties. The units are oster (Oe) or an /m (A/m), 1A/m= 79.6OE.
 
6. Intrinsic coercivity Hcj
 
The reverse magnetic field strength required to reduce the magnetization of a magnet to zero is called intrinsic coercivity. Magnetic materials are classified according to the size of their intrinsic coercive force. Low coercive force N, medium coercive force M, high coercive force H, extra high coercive force UH, extremely high coercive force EH, maximum coercive force TH.
 
7. Maximum magnetic energy product (BH) Max
 
Represents the magnetic energy density established by the two magnetic poles of a magnet, that is, the magnetostatic energy per unit volume of the air gap, which is the maximum value of the product of Br and Hcj. Its size directly indicates the performance of the magnet. Under the same conditions, namely the same size, the same number of poles and the same magnetization voltage, the magnetization table of magnetic parts with high magnetic energy product is also high. However, at the same (BH) Max value, the height of Br and Hcj has the following effects on the magnetization:
 
Br high, Hcj low: under the same magnetizing voltage, a higher surface magnetism can be obtained; Br low, Hcj high: A higher magnetizing voltage is required to obtain the same surface magnetism. SI system and CGS system
 
The Si system of units and the Gaussian system of units are like the difference between meters and li in units of length. There is a complex conversion relationship between the International system of Units and the Gaussian system of units.
 
Curie temperature
 
The temperature at which a magnetic material changes between a ferromagnet and a paramagnetic substance, below the Curie temperature when the material becomes a ferromagnet, the magnetic field associated with the material is difficult to change. When the temperature is higher than the Curie temperature, the material becomes a paramagnetic field, and the magnetic field of the magnet is easily changed by changes in the surrounding magnetic field.
 
The Curie temperature represents the theoretical operating temperature limit of magnetic materials. The Curie temperature of Ndfeb is about 320-380 degrees Celsius. The Curie point is related to the crystal structure formed by sintering with magnets. If the temperature reaches Curie temperature, the molecules inside the magnet will move violently and demagnetize, which is irreversible. After demagnetization, the magnet can be magnetized again, but its magnetism will be greatly reduced, only to about 50% of its original strength.
 
10. Operating temperature
 
The highest working temperature of sintered NdFeb is far lower than its Curie temperature. The magnetic force will decrease if the temperature rises within the working temperature, but most of the magnetic force will recover after cooling.
 
Relationship between working temperature and Curie temperature: the higher the Curie temperature is, the higher the working temperature of magnetic materials is, and the better the temperature stability is. The addition of cobalt, terbium, dysprosium and other elements into sintered Ndfeb raw material can increase its Curies temperature. Dysprosium is ubiquitously contained.
 
The highest temperature of sintered Ndfeb depends on its magnetic properties and working point. For the same sintered NdFeb magnet, the more closed the working magnetic circuit, the higher the maximum operating temperature of the magnet and the more stable the performance of the magnet. Therefore, the maximum operating temperature of a magnet is not a certain value, but varies with the degree of closure of the magnetic circuit.
 
11. Magnetic field orientation
 
Magnetic materials can be divided into isotropic and anisotropic magnets. Isotropic magnets have the same magnetic properties in any direction and can be drawn together at will. 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 strongest magnetic field only in the orientation direction. The other two directions are much weaker. Magnetic material in the production process if there is a orientation process is an anisotropic magnet, sintered Ndfeb is generally formed by magnetic field orientation, then it is anisotropic, so before production need to determine the orientation, the coming direction of magnetization. Magnetic field orientation of powder is one of the key technologies to manufacture high performance NdFeb. (Bonded NdFeb is isotropic or anisotropic)
 
12. The magnetic table
 
It refers to the magnetic induction intensity at a point on the surface of the magnet (the surface magnetism at the center and edge of the magnet is different), and it is the value measured by the Gauss meter in contact with a surface of the magnet, rather than the overall magnetic property of the magnet.
 
13. Magnetic flux
 
Is located in the magnetic induction intensity for B and strong magnetic field, there is a area of S and a plane perpendicular to the direction of the magnetic field, the product of the magnetic induction intensity B and the area S, called through the planar magnetic flux, hereinafter referred to as flux, the symbol "Φ", the unit is webber (Wb). Magnetic flux is a physical quantity representing the distribution of magnetic field. It is a scalar quantity, but has positive and negative, which only represents its direction. Φ = B, S, when the vertical side of S and B is the Angle theta, Φ = b. s. cosine theta.
 
14. Electroplating
 
Sintered ndfeb permanent magnet materials, was produced by powder metallurgy technology is a kind of very strong chemical activity of powder materials, within tiny pore and holes, easy corrosion and oxidation in air, and so before use must be strict surface treatment, electroplating as a mature means of metal surface treatment which has been widely applied.
 
The most commonly used coatings for Ndfeb strong magnets are galvanized and nickel plated. There are obvious differences in appearance, corrosion resistance, service life and price.
 
Difference in polishing: nickel plating is superior to zinc plating on the polished surface. High requirements for the appearance of the product will generally choose nickel plating, and some magnets are not exposed, relatively low requirements for the appearance of the product of the general galvanized. Corrosion resistance difference: Zinc is active metal, can react with acid, so corrosion resistance is poor; After the surface treatment of nickel plating, 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, mainly manifested in the surface coating is easy to fall off after a long service time, leading to the oxidation of magnets, thus affecting the magnetic properties. Hardness difference: nickel plating is higher than galvanized, in the process of use, can avoid collision to a great extent, resulting in the ndFeb strong magnet Angle, fracture and other phenomena. Price difference: Galvanized in this respect is extremely advantageous, prices from low to high for galvanized, nickel plating, epoxy resin, etc.
 
15. Single-sided magnets
 
Magnets all have two poles, but in some working locations, single-pole magnets are needed, so it is necessary to cover one side of the magnet with an iron sheet to shield the magnetism of the side covered by the iron sheet. Such magnets are collectively called single-side magnets or single-side magnets. There are no real single-sided magnets.