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Introduction to five common permanent magnetic materials

Aluminum-nickel-cobalt permanent magnet


The permanent magnet AlNiCo was developed in the 1930s. At that time, it had the best magnetic properties and low temperature coefficient, so it was widely used. After the 1960s, with the advent of ferrite permanent magnet and rare earth permanent magnet, al-Ni-Co permanent magnet was gradually replaced. At present, in the use of motor, it is only used on individual motors for detection, such as speed measuring generators.


Advantages:


1. The magnetic property of casting mold is high, and the process of powder sintering type is simple, which can be directly pressed into the desired shape.


2. A small temperature coefficient means that the magnetic properties change little with temperature. Products with high temperature stability requirements are still a good choice.


Disadvantages:


1. Low coerced force, so it should be avoided in the use of any magnetic material to avoid local irreversible demagnetization or magnetic flux distribution deformity.


2. Hard and brittle, poor processing performance.


Ferrite permanent magnet material


Ferrite permanent magnetic materials belong to non-metallic permanent magnetic materials. In the use of motors, there are two commonly used types: Barium ferrite (BaO·6Fe2O3) and strontium ferrite (SrO·Fe2O3).


Advantages:


1. High cost performance, excluding rare earth elements, cobalt, nickel and other precious metals;


2. Simple production process;


3. Strong coerce and demagnetization resistance;


4. A large part of the demagnetization curve is close to the straight line, and the recovery line basically coincides with the straight line part of the demagnetization curve, which can be used widely without the need for magnetic stabilization.


Disadvantages:


1. The remanence density is not high, and the maximum magnetic energy product is not high, so the cross-sectional area providing magnetic flux needs to be increased, leading to the large volume of the motor;


2. Ambient temperature has a great influence on magnetic properties. Therefore, it is necessary to carry out the checking calculation of the lowest temperature and maximum demagnetization working point in use to prevent irreversible demagnetization at low temperature;


3. Hard and brittle, and cannot be processed by electricity;


Rare earth cobalt permanent magnetic material


Rare earth cobalt permanent magnet (RARE-earth Co) emerged in the mid-1960s as a magnet with excellent magnetic properties.


Advantages:


1. The residual magnetic induction intensity and the maximum magnetic energy product are very high, among which the residual magnetic induction intensity can be close to the level of al-Ni-Co material, and the magnetic induction coerce is about 3 times that of ferrite permanent magnet material;


2. Demagnetization curve is basically a straight line, and the recovery line basically coincides with the demagnetization curve, with strong demagnetization resistance;


3. Due to the high Curie temperature, this rare earth cobalt permanent magnet material has the best magnetic stability and is suitable for manufacturing high-performance permanent magnet motors;


Disadvantages:


1. It is expensive, so it is usually only used in military industry or special occasions;


2. It is hard and brittle, with low tensile and flexural strength;


Ndfeb permanent magnet material


NdFeB (NdFeB) permanent magnetic material was introduced in 1983.


Advantages:


1. The magnetic property is higher than the rare earth cobalt permanent magnet. The residual magnetic induction strength, magnetic induction coerce and maximum magnetic energy product are all very high.


2. Cost-effective. Since neodymium contains more than ten times as much rare earth as samarium, iron and boron are cheap, and it contains no cobalt, a strategic material. This is why ndFeb has been widely used and promoted.


Disadvantages:


1. Low Curie temperature, high temperature coefficient, so when used at high temperature, the magnetic loss is large and the thermal stability of magnetic properties is poor;


2. Because it contains a large amount of iron and neodymium, it is easy to rust;


Bonded permanent magnet material


It should be noted that the binding permanent magnet material is not a specific component, but a composite permanent magnet material made by using branches, plastics, low-melting alloy and other materials as adhesives, uniformly mixing with the permanent magnet material powder, and then compression, injection or extrusion.


Advantages:


1. With large degree of freedom in shape, it is easy to make magnetic rings or thin-walled rings with complex shapes;


2. In the case of injection molding, it can be embedded into other parts for molding;


3. High dimensional precision, not easy to deformation, and does not need secondary processing to make high-precision magnets;


4. High resistivity, easy to realize multipole magnetization;


5. Low performance dispersion, high qualification rate, suitable for mass production;


6. High mechanical strength, not easy to break, can be machined;


Disadvantages:


Because of the adhesive, the magnetic properties are slightly poor.


Among the consumer products, ferrite permanent magnet material, NdFeb permanent magnet material and binding permanent magnet material are the most widely used. Generally speaking, ferrite is enough for the product. For the strong performance, neodymium iron boron is needed. In special cases, permanent magnet material may need to be bonded.


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