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Characteristics and classification of materials

2022-12-17 超级管理员 Read 184

Magnetic materials are mainly divided into permanent magnetic materials and soft magnetic materials. Permanent magnetic materials are also called hard magnetic materials. Magnets can retain their strong magnetism for a long time after being subjected to an external magnetic field, which is characterized by high coercivity (Hc). Generally, the coercivity Hc ≥ 10A4/m. The magnetic energy product (BH) max is large. Soft materials are magnetic materials that are easy to be magnetized and demagnetized after magnetic field is applied. They are characterized by low coercivity, generally Hc ≤ 10A3/m.


Four main magnetic characteristics of permanent magnetic materials


(1) High maximum magnetic energy product Maximum magnetic energy product (BH) max is a measure of the maximum magnetic energy density stored and available per unit volume of permanent magnetic materials.


(2) The high coercivity (Hc) is a measure of the permanent magnetism of permanent magnetic materials due to the magnetic and non-magnetic interference.


(3) High residual magnetic flux density (Br) and high residual magnetization (Mr) are measures of magnetic field strength in air gap.


(4) High stability refers to the high stability of the change of external interference magnetic field, temperature, vibration and other environmental factors.


Main classification of permanent magnetic materials


(1) Metal permanent magnetic material: it is an alloy permanent magnetic material with iron and iron elements (such as nickel, cobalt, etc.) as important elements that has been developed and applied earlier, mainly including rare earth permanent magnet (such as neodymium iron boron rare earth alloy permanent magnet), aluminum nickel cobalt (AINiCo) system and iron chromium cobalt (FeCrCo) system.


(2) Ferrite permanent magnetic material: It is a strong magnetic material composed of composite oxides with Fe2O3 as the main element. It is characterized by high resistivity, which is particularly conducive to frequency and microwave applications. Such as barium ferrite permanent magnetic materials, strontium ferrite permanent magnetic materials, etc.


(3) Other permanent magnetic materials: such as micro powder permanent magnetic materials, nano permanent magnetic materials, plastic permanent magnetic materials, etc.


Main characteristics of soft magnetic materials


(1) Low coercivity Hc: it shows that magnetic materials are easy to be magnetized by an external magnetic field and demagnetized by an applied magnetic field or other factors, and the magnetic loss is low.


(2) High saturation magnetic flux density Bs and high saturation magnetization Ms: In this way, high permeability µ and low coercivity Hc can be deliberately obtained, which can also improve the magnetic flux density.


(3) Low magnetic loss and electrical loss: this requires low coercivity Hc and high resistivity.


(4) High stability: It has high stability to the changes of environmental factors such as temperature and vibration.


Main classification of soft magnetic materials


(1) Ferrite soft magnetic materials: a series of composite oxide materials (or ceramic materials) containing iron oxide, characterized by low saturation magnetic induction (below 0.5T) but high permeability. The resistivity is also very high, which is generally used at high frequencies. Such as manganese zinc ferrite (Mn Zn Cores) and magnesium zinc iron (Mg Zn Cores).


(2) Metal soft magnetic materials: compared with ferrite soft magnetic materials, they have high saturation magnetic induction and low coercivity. It mainly includes iron series soft magnets, such as industrial pure iron and iron powder cores; Iron nickel alloy soft magnets, such as iron nickel molybdenum magnetic powder cores (MPP Cores), high flux iron nickel magnetic powder cores (High Flux Cores), etc; Ferrosilicon alloy soft magnets, such as ferrosilicon aluminum magnetic powder cores, silicon steel sheets, etc.


(3) Amorphous soft magnetic materials and nanocrystalline soft magnetic materials are new soft magnetic materials developed in the late 20th century.


Metal magnetic powder core is a kind of soft magnetic material, which is made of metal magnetic particles, insulated and coated on the surface, and mixed with insulating medium (organic or inorganic adhesive). Because the metal magnetic powder particles are very small and separated by non-magnetic insulating film materials, on the one hand, they can isolate eddy currents, and the materials are suitable for higher frequencies. On the other hand, due to the gap effect between particles, the materials have low permeability and constant magnetic conductivity. At the same time, the magnetic powder core has natural air gap distribution characteristics, which is extremely suitable for the use of energy storage inductors. Due to the small size of magnetic powder particles, skin effect will not occur basically, and the change of magnetic permeability with frequency is relatively stable. The magnetoelectric properties of the magnetic particle core mainly depend on the permeability, particle size and shape, their filling coefficient, the content of insulating medium, forming pressure and heat treatment process of the powder material.


Due to the existence of natural air gap distribution in the metal magnetic particle core, which has the characteristics of soft saturation, it has great advantages over ferrite. Its DC bias does not have the hard saturation of ferrite, but with the increase of DC bias force, its magnetic permeability slowly decreases in the expected way.


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