Permanent magnetic materials are also called "hard magnetic materials". A material that can maintain constant magnetism once magnetized. It has wide hysteresis loop, high coercive force and high remanence. In practice, permanent magnetic materials work in the second quadrant demagnetization part of the hysteresis loop after deep magnetic saturation and magnetization. Commonly used permanent magnet materials are divided into aluminum nickel cobalt permanent magnet alloy, iron chromium cobalt permanent magnet alloy, permanent magnet ferrite, rare earth permanent magnet material and composite permanent magnet material, which are briefly described as follows:
(1) Al Ni Co permanent magnet alloy
It is mainly composed of iron, nickel and aluminum, as well as copper, cobalt and titanium. With high remanence and low temperature coefficient, it is magnetically stable. There are two kinds of casting alloy and powder sintered alloy. In the 1930s and 1960s, it was widely used in the instrument industry to manufacture magnetoelectric instruments, flowmeters, micro and special motors, relays, etc.
(2) Fe Cr Co permanent magnet alloy
It is mainly composed of iron, chromium and cobalt, and also contains molybdenum and a small amount of titanium and silicon. It has good processability and can be subjected to cold and hot plastic deformation. Its magnetism is similar to that of Al Ni Co permanent magnet alloy, and its magnetic properties can be improved through plastic deformation and heat treatment. It is used to manufacture various small magnet elements with small section and complex shape.
(3) Permanent ferrite
It mainly includes barium ferrite and strontium ferrite, which have high resistivity and high coercivity, and can be effectively used in the atmospheric gap magnetic circuit, especially for small generators and motors. The permanent magnet ferrite does not contain precious metals such as nickel and cobalt. The raw materials are abundant, the process is simple, and the cost is low. It can replace the aluminum nickel cobalt permanent magnets to manufacture magnetic separators, magnetic thrust bearings, loudspeakers, microwave devices, etc. However, its maximum magnetic energy product is low, its temperature stability is poor, its texture is brittle and fragile, and it is not resistant to shock and vibration, so it is not suitable for measuring instruments and magnetic devices with precise requirements.
(4) Rare earth material
They are mainly rare earth cobalt permanent magnetic materials and neodymium iron boron permanent magnetic materials. The former is an intermetallic compound formed by rare earth elements such as cerium, praseodymium, lanthanum, neodymium, etc. and cobalt. Its magnetic energy product can reach 150 times of carbon steel, 3-5 times of aluminum nickel cobalt permanent magnetic materials, and 8-10 times of permanent magnet ferrite. It has low temperature coefficient, stable magnetism, and coercive force up to 800 kA/m. It is mainly used for magnetic systems of low-speed torque motors, starting motors, sensors, magnetic thrust bearings, etc. NdFeB permanent magnetic material is the third generation rare earth permanent magnetic material. Its remanence, coercivity and maximum magnetic energy product are higher than those of the former, which are not fragile and have good mechanical properties. The alloy density is low, which is conducive to the lightness, thinness, miniaturization and super miniaturization of magnetic components. But its high magnetic temperature coefficient limits its application.
(5) Composite materials
It is composed of permanent magnetic material powder and plastic material as binder. Because it contains a certain proportion of binder, its magnetic properties are significantly lower than those of corresponding magnetic materials without binder. Except for metal composite permanent magnetic materials, other composite permanent magnetic materials are limited by the heat resistance of the binder, and the use temperature is low, generally not more than 150 ℃. However, the composite permanent magnet material has high dimensional accuracy, good mechanical properties, and good uniformity of all parts of the magnet, which makes it easy to conduct radial orientation and multipolar magnetization of the magnet. It is mainly used to manufacture instruments and meters, communication equipment, rotating machinery, magnetic therapy equipment, sports goods, etc.
