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Permanent magnetic material

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permanent magnetic materials introduction, classification, development history

permanent magnetic materials are divided into aluminum nickel and cobalt permanent magnet alloy, iron chromium cobalt permanent magnetic alloys, permanent magnet ferrite, rare earth permanent magnet materials and composite permanent magnetic materials.

Permanent magnetic material introduction

Permanent magnetic material is a wide hysteresis loop, high coercive force, high remanence, once it is magnetized it can keep constant magnetic force, also known as hard magnetic materials. Practically, permanent magnet is working in the second quadrant demagnetization part of the hysteresis loop and deep magnetic saturation. The commonly used permanent magnet materials are divided into aluminum nickel and cobalt permanent magnet alloy, iron chromium cobalt permanent magnetic alloys, permanent magnet ferrite, rare earth permanent magnet materials and composite permanent magnetic materials.

1. Aluminum nickel and cobalt permanent magnetic alloys.

Iron, nickel, aluminum as the main component, also contains the elements such as copper, cobalt, titanium with high remanence and low temperature coefficient, magnetic stability. There are  two kinds of alloy casting and powder sintering alloy. It is widely used in 1930 ~ 1960s, now mainly used in the instrument, meter, micro motors, relays, etc.

2. Iron chromium cobalt permanent magnetic alloys 

With iron, chromium, cobalt element as the main ingredient, also contain molybdenum and a small amount of titanium, silicon. The processing performance is good, can be cold thermoplastic deformation. Its magnetic property is similar to aluminum nickel and cobalt permanent magnet alloy, and can improve the magnetic performance through plastic deformation and heat treatment. Used in the manufacture of all kinds of small cross section, complex shape of small magnet components.

3. Permanent magnetic ferrite

Mainly include barium ferrite, strontium ferrite, and it has high resistivity, big coercive force, can be effectively applied in the air gap magnetic circuit, especially suitable for small generator and permanent magnet motors. Permanent magnetic ferrite do not contain precious metals such as nickel, cobalt, has rich source of raw materials, simple process, low cost, can replace aluminum nickel and cobalt permanent magnet in system of magnetic separator, magnetic bearing, speakers, microwave devices, etc. But the maximum magnetic energy is low, temperature stability is low, quality of a material is brittle, fragile, low resistance to impact and vibration, unfavorable for measuring instrument and precision requirements of the magnetic device.

4. Rare earth permanent magnet materials

Mainly include Cobalt rare earth permanent magnet materials and ndfeb permanent magnet materials. The former is a rare earth element cerium, praseodymium, lanthanum, neodymium and cobalt formation of intermetallic compounds, its magnetic energy can be up to 150 times that of carbon steel, 3 ~ 5 times of aluminum nickel and cobalt permanent magnet, 8 ~ 10 times of permanent magnet ferrite, low temperature coefficient, stable magnetic performance, coercive force up to 800000 / m. It is mainly used for low speed torque motors, start motors, sensors, magnetic system of magnetic bearing, etc. Ndfeb permanent magnet material is the third generation of rare earth permanent magnet materials, the remanence, coercive force and the maximum magnetic energy higher than the former, less fragile, has good mechanical properties, alloy density is low, is advantageous to the magnetic components of light-duty, thin, small and microminiaturization. But its magnetic temperature coefficiency is high which limit its application.

5. Composite permanent magnetic materials

made from permanent magnetic material and powder compounded plastic material as a binder. Due to its certain percentage of binder content, so the magnetic than corresponding no binder magnetic materials significantly reduced. In addition to the metal composite permanent magnetic materials, other composite permanent magnetic materials due to the binder and the limit of the heat resistance, temperature is low, generally less than 150 ℃. But composite permanent magnetic materials high dimensional accuracy, good mechanical performance, magnet performance uniformity is good, the parts easy to radial orientation and multi-polar magnet magnetization. Mainly used in the manufacture of instruments and meters, communication equipment, rotating machinery, such as magnetic therapy equipment and sporting goods.

Permanent magnetic material --Classification

The first major categories: permanent magnet alloy materials, including rare earth permanent magnet (ndfeb Nd2Fe14B), samarium cobalt (SmCo), aluminum nickel and cobalt (AlNiCo) 

The second major category is: Ferrite permanent magnet (Ferrite) 

According to different production process it can be divided into: sintered ferrite, bonded ferrite, plastic injection ferrite, furthermore, on the basis of the three process, as per the magnetic crystal orientation it can be  divided into isotropic and anisotropic magnets.

These are the major permanent magnet material currently on the market, and someothers because of the production process or cost reasons, are not widely used even washed out such as Cu - Ni - Fe nickel iron (copper), Fe - Co - Mo iron, cobalt, molybdenum), Fe - Co - V (iron cobalt vanadium), MnBi manganese (bi)

Rare earth permanent magnet (ndfeb Nd2Fe14B) :

according to the different production process can be divided into the following three:

Sintered neodymium magnets (Sintered NdFeB) - after airflow grinding powder and melting, Sintered NdFeB permanent magnets’ coercive force value is very high, and has extremely high magnetic energy, the maximum magnetic energy product (BHmax)is more than 10 times of Ferrite (Ferrite). Its mechanical properties are also very good, can cut into different shape and drilling. High-performance products of the highest working temperature can reach 220 degrees Celsius. Because of its material content is easy to cause corrosion, so according to different requirements it must be carried out surface coating treatment (such as galvanized zinc, nickel, environmental friendly zinc, environmental friendly nickle, nickel-copper-nickel). It is very hard and brittle, with high demagnetization resistance, high cost/performance ratio, is not suitable for high temperature working (> 220 ℃).

Bonded NdFeB (Bonded NdFeB) - Bonded NdFeB is NdFeB powder and resin, plastic or low melting point metal binder such as mixing, and then use methods such as compression, extrusion and injection molding compound made of NdFeB permanent magnets. Product forming at a time, without secondary processing, can be directly to make all kinds of complicated shape. Every direction of bonded ndfeb magnetic, can be processed into ndfeb compression mould and injection mould. Precision, magnetic is admirable, good corrosion resistance, high temperature stability is good.

Plastic injection NdFeB- made anisotropy has high precision, easy to shape complicated thin-walled rings or thin magnet

The main raw material of Sintered Ferrite (Sintered Ferrite) including BaFe12O19 and SrFe12O19, according to the orientation of the magnetic crystal divided into isotropic and different sex magnets. Because of its low price and moderate magnetic can become a magnet which has been widely applied at present. Ferrite magnets are made by ceramic technology, the quality of a material is solid, also is a brittle material, because of ferrite magnets have good temperature resistance and low prices, has become more widely applied permanent magnets.

Rubber magnetic (Rubber magnets)  is one of the series of ferrite magnets, by binding with synthetic Rubber compound soft ferrite powder by extrusion molding, rolling molding, injection molding process and made of a soft, elastic and can distort the magnets. Can be processed into strips, roll, sheet and various kinds of complicated shape. Rubber magnet by magnetic powder (SrO6Fe2O3), polyethylene (CPE) and other additives (EBSO, DOP) etc, manufactured by extrusion, calendering. Rubber magnets can be the same or opposite sex, it is composed of ferrite magnetic powder, CPE, and some trace elements, can bend, twist, roll. It can be used without more mechanical processing, can also according to the required size trim shape, rubber magnetic, can also according to customer's requirement after PVC back glue, UV oil, etc. The magnetic energy product is between 0.60 to 1.50 MGOe. Rubber magnets application fields: refrigerators, message sign, will object fixed on the metal body of fasteners, such as used for advertising used in toys, teaching instrument, switch and sensor of the disk. Mainly used in micro motors, refrigerator, disinfection cabinet, kitchen cabinets, toys, stationery, advertising and other industries.

Aluminum nickel and cobalt (AlNiCo) is the earliest developed a kind of permanent magnetic material, is made from aluminum, nickel, cobalt, iron and other trace metal elements of an alloy. According to different production process can be divided into Sintered AlNiCo, Sintered AlNiCo) and casting aluminum nickel and cobalt (Cast AlNiCo). The product shape are round and square. Casting process can be processed into different size and shape; Compared with casting process, the sintering products is limited to small size, its production of blank size tolerance is better than blank casting products, magnetic is slightly less than the casting products, but it is better machinability. In the permanent magnetic material, casting aluminum nickel and cobalt permanent magnet has the lowest temperature coefficient is reversible, working temperature can be as high as 600 degrees Celsius above. Alnico permanent magnet products are widely used in all kinds of instruments and meters and other applications

Samarium cobalt (SmCo) according to the composition of different divided into SmCo5 and Sm2Co17, respectively was just generation and second generation rare earth permanent magnet materials. Due to its raw material is very scarce, expensive and make its development was restricted. Samarium cobalt (SmCo) as the second generation of rare earth permanent magnets, not only have higher magnetic energy product (14 to 28 mgoe) and reliable coercive force, and in the rare earth permanent magnet series shows good temperature characteristic. Compared with the ndfeb, samarium cobalt is more suitable for work in the environment of high temperature (> 200 ℃).

Permanent magnetic material -- History of development

With the development of the society, the application of the magnet is becoming more and more widely, from high-tech products to the most simple packaging, the most widely used or ndfeb strong magnetic and ferrite magnets.

From the development history of permanent magnetic material, the end of the 19th century the use of carbon steel, magnetic energy product (BH) Max (measure the quantity of the stored magnetic energy density permanent magnet) less than 1 mgoe high Mr (megabytes), and the foreign mass-produced Nd - Fe - B permanent magnet, magnetic energy product has reached more than 50 mgoe. This century, the material of remanent magnetism Br increase is very small, can improve due to product improvement of coercive force Hc. And coercive force increase, mainly thanks to the understanding of its nature and high magnetocrystalline anisotropy compounds are found, and the progress of the preparation technology. At the beginning of the 20th century, people mainly use carbon steel, tungsten, cobalt chromium steel and steel for permanent magnetic material. Developed in the late 1930 s, AlNiCo permanent magnet materials, makes the large-scale application of permanent magnet material possible. 50 s, the emergence of barium ferrite, reduces the cost of permanent magnets, and to broaden the application scope of permanent magnet materials to high frequency field. The emergence of cobalt to 60 s, rare earth permanent magnet, is for the application of permanent magnet has opened up a new era. In 1967, the United States at the university of Dayton Strnat etc., made by powder method successfully SmCo5 permanent magnets, marks the arrival of the era of rare earth permanent magnet. So far, diluted 10 permanent magnet has experienced the first generation of SmCo5, the second generation Sm2Co17 precipitation hardening type, development to the third generation of Nd - Fe - B permanent magnet materials. In addition, in history it is also used as a permanent magnetic material like I - Ni - Fe Cu, Fe, Co - Mo, Fe - Co - V, MnBi, A1MnC alloy, etc. These alloys due to performance is not high, the cost is not low, in most cases has been rarely used. And AlNiCo, FeCrCo and PtCo alloy is applied in some special occasions. At present, Ba, Sr ferrite still is one of the biggest amount of permanent magnetic material, but its many applications is gradually replaced by the Nd - Fe - B type material. And now the output value of the rare earth permanent magnet materials have greatly more than ferrite permanent magnet, rare earth permanent magnet material production has developed into a big industry.

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