15 concepts related to magnetic materials

- Sep 07, 2019-

1. The magnetic


Experiments show that any material can be more or less magnetized in an external magnetic field, but only to different degrees. According to their characteristics in the external magnetic field, materials can be divided into five categories: paramagnetic materials, diamagnetic materials, ferromagnetic materials, ferromagnetic materials, ferromagnetic materials and antiferromagnetic materials. We call paramagnetic and diamagnetic materials as weak magnetic materials, ferromagnetic materials, ferromagnetic materials as strong magnetic materials.


2. Magnetic materials


Soft magnetic material: can use the smallest external magnetic field to achieve the maximum magnetization intensity, is a magnetic material with low coercivity and high permeability. 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 magnet material, refers to the material that is difficult to magnetize and difficult to demagnetize once magnetized. Its main characteristics are high coercivity, including rare earth permanent magnet material, metal permanent magnet material and permanent magnet ferrite.


Functional magnetic materials: mainly magnetostrictive materials, magnetic recording materials, magnetoresistive materials, magnetic bubble materials, magneto-optical materials and magnetic thin film materials.


3. Ndfeb permanent magnet material


Sintered ndfeb permanent magnet material is used in the powder metallurgy process, after melting the alloy into powder and in the magnetic field pressure made into pressing blank, pressing blank in inert gas or vacuum sintering to densification, in order to improve the coercivity of the magnet, usually need to carry out aging heat treatment, after processing and surface treatment to obtain the finished product.


Bonded ndfeb is made of permanent magnet powder mixed with flexible rubber or hard and light plastic, rubber and other bonding materials.


Hot pressure ndfeb without adding additional rare earth elements can achieve similar magnetic properties and sintered ndfeb, with high density, high degree of orientation, good corrosion resistance, high coercing force and near final molding advantages, but mechanical performance is not good and due to patent monopoly, high processing cost.


4. The remanence Br


It refers to the magnetic induction intensity of sintered ndfeb magnet when a magnet is magnetized to the outside magnetic field in a closed circuit and the outside magnetic field is cancelled after the technology is saturated. Generally speaking, it can be briefly understood as the magnetic force of magnet after magnetization. The units are tesla (T) and gauss (Gs), 1Gs= 0.0001t.



5. Coercive force Hcb


When the magnet is reverse magnetized, the value of the required reverse magnetic field strength to reduce the magnetic induction intensity to zero is called magnetic inductive coercivity. However, at this time, the magnetization intensity of the magnet is not zero, but the interaction between the added reverse magnetic field and the magnetization intensity of the magnet cancels each other. At this time, if the external magnetic field is withdrawn, the magnet still has certain magnetic properties. The unit is oster (Oe) or ampere/meter (A/m), 1A/m=79.6Oe.


6. Inherent coercivity Hcj


The strength of the reverse magnetic field needed to reduce the magnetization of the magnet to zero is called the intrinsic coercivity force. The classification of magnetic material brands is based on the size of its inherent coercivity. Low coercivity N, medium coercivity M, high coercivity H, very high coercivity UH, very high coercivity EH, very high coercivity TH.



7. Maximum magnetic energy product (BH) Max


Represents the magnetic energy density established in the space of two magnetic poles of magnet, that is, the static magnetic energy per unit volume of air gap, is the maximum value of product of Br and Hcj, and its size directly indicates the performance of magnet. Under the same conditions, that is, under the same size, the same number of poles and the same magnetizing voltage, the surface magnetic field obtained by magnetic components with high magnetic energy product is also high. However, when the Max value of (BH) is the same, the height of Br and Hcj has the following effects on magnetization:



Br high, Hcj low: under the same magnetizing voltage, higher surface magnetism can be obtained;


Br low, Hcj high: to get the same surface magnetism, need higher magnetizing voltage.


8.SI system and CGS system



The international system of units and gauss system of units, like the unit of length in the "meter" and "mile" difference. There are some complex conversion relations between si system and gauss system.




9. Curie temperature


It is the temperature that the magnetic material changes between ferromagnet and paramagnetic. When the temperature is lower than Curie, the material becomes a ferromagnet. At this time, the magnetic field related to the material is difficult to change. When the temperature is higher than the Curie temperature, the material becomes a paramagnetic field. 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 Curie point is related to the crystal structure formed by magnet sintering. If the temperature reaches Curie temperature, the molecules inside the magnet move violently and demagnetization occurs, which is irreversible. After demagnetization, the magnet can be magnetized again, but the magnetic force will drop sharply, only reaching about 50% of the original force.



10. Operating temperature



The maximum working temperature of sintered ndfeb is much lower than its Curie temperature. When the temperature rises, the magnetic force will drop, but after cooling, most of the magnetic force will recover.



The relation between working temperature and Curie temperature: Curie temperature is higher, the working temperature of magnetic material is higher, and the temperature stability is better. Cobalt, terbium, dysprosium and other elements in sintered ndfeb raw materials can improve the Curie temperature, so high coercivity products (H, SH...) Dysprosium is common.



The maximum 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 highest use temperature of the magnet, the more stable the performance of the magnet. Therefore, the maximum temperature of the magnet is not a fixed value, but varies with the closure of the magnetic circuit.



11. Magnetic field orientation



Magnetic materials are classified into 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 it 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 and much less in the other two directions. Magnetic material in the production process of orientation process is anisotropic magnets, sintered ndfeb generally with magnetic field orientation molding press, then it is anisotropic, so before the production needs to determine the orientation direction, the direction of magnetization to come. The powder magnetic field orientation is one of the key technologies to manufacture high performance ndfeb. (bonding ndfeb isotropic and anisotropic)



12. The magnetic table



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), which is the value measured by the gauss meter when it contacts with a certain surface of the magnet, but not the overall magnetic properties 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 it has plus or minus, 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.


Neodymium iron boron strong magnet coating is most commonly used for galvanizing and nickel plating these two kinds, they in the appearance, corrosion resistance, service life, price and other aspects have a very obvious difference:


Polishing difference: nickel plating in polishing is superior to galvanized on the surface, more bright. High requirements for the appearance of products will generally choose nickel plating, and some magnets are not exposed, relatively low requirements for the appearance of products of general galvanized.


Corrosion resistance: zinc is a reactive 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 galvanized is lower than that of nickel plating, mainly in the use of a long time surface coating easy to fall off, leading to oxidation of the magnet, thus affecting the magnetic properties.


Hardness difference: nickel plating than galvanized high, in the use of the process, can greatly avoid collision and other conditions, make ndfeb strong magnet appear Angle, broken and other phenomena.


Price difference: galvanized in this respect is extremely advantageous, the price from low to high order for galvanized, nickel plating, epoxy resin, etc.


15. Single-sided magnet


Magnets have two poles, but in some working positions need a single pole of the magnet, so you need to use an iron sheet to cover one side of the magnet, so that the iron sheet to cover the side of the magnetic shielding, such magnets are collectively known as single-sided or single-sided magnets. There are no real single-sided magnets.