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The latest developments in magnetic materials and device technology

1 Introduction 

Magnetic materials include permanent magnet materials used in motors, solenoids, speakers, sensors, etc., and soft ferrite core materials used in transformers, coils (inductors), sensors, and magnetic heads. Along with the miniaturization, light weight and high performance of electronic complete machines, there is an urgent need to develop magnetic materials with higher magnetic performance and good temperature characteristics.

2. Permanent magnet materials

Permanent magnet materials can be roughly divided into sintered permanent magnets and bonded permanent magnets, divided into ferrite permanent magnets and rare earth permanent magnets according to their types. Sintered permanent magnets are made by pouring magnetic powder into a mold for sintering and casting, and bonded permanent magnets are made by mixing a small amount of plastic or rubber into the magnetic powder and forming it. The most commonly used ferrite permanent magnet is because of its stable magnetic properties and low price.

Ferrite permanent magnets, the most used barium and strontium ferrite permanent magnets so far, but a new generation of lanthanum-cobalt ferrite permanent magnets is currently being developed, which is compared with the above two ferrite permanent magnets , With good magnetic performance and temperature characteristics, centering on the application on the motor, the demand is constantly expanding.

Rare earth permanent magnets have a strong magnetic energy product, mainly including samarium cobalt and neodymium iron boron permanent magnets.

Samarium-cobalt-based rare earth permanent magnets have good corrosion resistance and heat resistance, and are widely used in fields such as automobiles and sensors that are difficult to involve in neodymium-iron-boron permanent magnets.

NdFeB permanent magnets have the strongest magnetic energy product in permanent magnets. In recent years, the application field has become larger and larger, and it has made great contributions to the miniaturization of electronic complete machines. By vigorously developing the neodymium-iron-boron permanent magnet with high magnetic properties, stability and heat resistance, the current maximum magnetic energy product has reached 560KJ/m3 (57.8MGOe), the world's highest production level. In terms of bonded permanent magnets, we have developed isotropic NdFeB bonded permanent magnets with good corrosion resistance, high filling rate and high remanence. Nano composite magnetic powders for bonded permanent magnets are used to make generators and super The ring-shaped bonded neodymium iron boron permanent magnet for magnetic conductive bearings has a maximum diameter of 300mm. It uses anisotropic bonded permanent magnets. It is about 50% lighter than ferrite permanent magnets used in motors in the past. It is widely used For automotive DC motors.

Although the NdFeB bonded permanent magnet is not as strong as the magnetic field strength of the sintered permanent magnet, it is not prone to cracks and edge falling, which makes the demand continue to expand. The ferrite rubber-bonded permanent magnet for small motors that greatly improves the magnetization performance is currently being developed. It is easy to be made into a thin sheet and made into a cylindrical shape to be mounted on the stator or rotor housing of the motor. Regarding neodymium iron boron bonded permanent magnets, corresponding to the past isotropic bonded permanent magnets, anisotropic bonded permanent magnets with high magnetic properties have been developed and entered the practical stage.

2.1 Sintered permanent magnet

a. Ferrite permanent magnet

Ferrite permanent magnet is a permanent magnet made of iron oxide as the main component. Compared with other permanent magnets, it is widely used in motors, speakers, relays, sensors, toys and other fields because of its low price and good stability. In recent years, it has also been widely used in automotive electrical appliances and office automation equipment. The maximum energy product of ferrite permanent magnet is 42.9KJ (5.4MGOe). Its magnetic energy product has almost reached its limit, and it is currently seeking to improve the coercivity through improved technology.

Ferrite permanent magnets are made by mixing raw materials such as strontium or barium with iron oxide, and then pre-sintering to make ferrite into ultrafine powder through solid phase reaction. The ultra-fine powder is pressed into the required shape and sintered at a temperature of about 1200°C. Molding methods include dry anisotropy, dry anisotropy and wet anisotropy.

Recently, lanthanum-cobalt ferrite permanent magnets with good magnetic properties and temperature properties have been widely used in motors and solenoids. This kind of permanent magnet uses lanthanum to replace part of strontium and cobalt to replace part of iron, which greatly improves the magnetic properties and temperature characteristics. Compared with the past ferrite permanent magnet, the maximum energy product ([BH]max) has increased by 30-40%, the remanence (Br) has increased by 10-15%, and it also has high coercivity and temperature stability. It has also increased. For this reason, compared with the past strontium-based ferrite permanent magnets, it has achieved small size and light weight.

b. Rare earth permanent magnet

The representative rare earth permanent magnet is samarium-cobalt permanent magnet, which not only has a certain magnetic field strength, but also has good heat resistance. It is used in automotive sensors, ultra-small motors, solenoids, pickups, and high-quality headphones. In the product.

As a new type of samarium-cobalt magnetic steel, it is a product formed by powder molding. The product with a maximum energy product of 239KJ/m3 (30MGOe), a coercivity of 1194KA/m3 (15KOe), and a residual magnetic flux density of 11KG has been commercialized.

Due to the uniform density of the molded body after extrusion, even thin-shaped products will not have cracks and edge drop, and the magnetic flux density error of the upper and lower sides after cutting is small. It is suitable for making ring and flat products. Good price/performance ratio.

Japan's Shin-Etsu Chemical Industry Co., Ltd. used the technical know-how of developing neodymium iron boron permanent magnets and successfully applied it to samarium cobalt permanent magnets, greatly improving the magnetic performance. The products that achieve the highest magnetic performance include R33H with a maximum energy product of 230~270 KJ/m3 (29~34MGOe) and R26H with a maximum energy product of 191~239KJ/m3 (24~30MGOe), which can withstand high temperatures exceeding 250℃, and start batches produce.

c. NdFeB permanent magnet

Neodymium-Fe-B (Nd-Fe-B) has the highest magnetic energy product in permanent magnets. Compared with samarium cobalt permanent magnets, it is characterized by low price. For this reason, it has promoted the application of various miniaturized electronic machine motors, HDD (Hard Disk Drive) VCM (Voice Coil Motor), small speakers, sensors, solenoids, relays, and medical equipment.

The magnetic properties of NdFeB permanent magnets, taking mass-produced sintered products as an example, the maximum energy product is 278~438KJ/m3 (35~55MGOe), and the theoretical value reaches 512KJ/m3 (64MGOe). The coercivity of the permanent magnet is superior to that of the previous products.

The high-performance NdFeB permanent magnet produced by NEOMAX has a maximum magnetic energy product of 460KJ/m3 (57.8MGOe), the world's highest level. The coercive force of the permanent magnet can be maintained at a high level of 784KA/m3 (9.85KOe), and as a neodymium iron boron permanent magnet, it has the highest magnetic performance in the world. In addition, the NEOMAX AH series high temperature resistant neodymium iron boron permanent magnets were developed and mass production began. According to the required heat resistance, the permanent magnet can be selected from five materials in the temperature range of 140℃~240℃. It is expected to continue to solve the problem of heat resistance of the weak points of NdFeB permanent magnets in the future.

TDK has developed the NEOREC53 series of NdFeB permanent magnets with a maximum energy product of 420 KJ/m3 (53MGOe) by using the dry method. This product has increased the maximum magnetic energy product by 7~9% compared with the previous 50 series products.

The manufacturing methods of neodymium iron boron permanent magnets include a dry method in which a powdered material is formed in an inert gas and a wet method in which the material is formed in a liquid state. The dry process is a neodymium iron boron permanent magnet with single-sided, easily oxidizable rare earth elements as the main component. The oxygen content has a certain influence on the magnetic properties. TDK has introduced a unique new low-oxygen process technology in the dry process to ensure that the oxygen content from the crushing of the material to the molding process is almost zero, and the oxidation of the raw material powder is fully inhibited.

In addition, using the wet method with special oil as the solvent, a high-performance neodymium iron boron permanent magnet with low oxygen content has been developed and has been mass-produced. Compared with the wet method and the dry method, because the particle size of the micropowder has nothing to do with the oxygen content, the particle size of the micropowder can be made smaller in the range where the micropowder oriented in the magnetic field has little effect. The maximum energy product of this permanent magnet is 438 KJ/m3 (55MGOe).

NdFeB permanent magnets produced by Shin-Etsu Chemical Industry Co., Ltd., with a maximum magnetic energy product of 382~422KJ/m3 (48~53MGOe) and high magnetic performance N52 series products have also been mass-produced. Boron-based permanent magnets have also begun to be put on the market.

NdFeB permanent magnets are prone to rust due to iron as the main component and many components such as neodymium. For this reason, it is necessary to apply surface treatment to the product. Surface treatments include nickel and aluminum ion coating, nickel plating and resin coating. At present, nickel plating is the standard surface treatment method, which has good corrosion resistance, surface cleaning and hardness.

2.2 Bonded permanent magnet

The bonded permanent magnet is prepared by adding a small amount of plastic or rubber and other binders to the magnetic powder, and then using injection molding or extrusion molding methods. Various products with complex shapes can be manufactured. Because it can be produced in large quantities, the cost is low and the benefit is good. It has been widely used in recent years.

Bonded permanent magnets, like sintered permanent magnets, have ferrite system and rare earth system bonded permanent magnets. Ferrite system bonded permanent magnets are used in refrigerator door seals, motors, encoder rotors, sensors, magnetic films, magnetic rollers of copiers, ring magnets for color adjustment of CRTs, etc. Rare earth bonded permanent magnets are used in HDD (Hard Disk Drive) motors, CD-ROM spindle motors, vibration motors, small speakers, and various sensors.

a. Magnetic powder

The magnetic powders used for bonding permanent magnets are ferrite and samarium-cobalt magnetic powders made of sintered permanent magnets. The neodymium-iron-boron system is an amorphous ribbon made by liquid quenching. The isotropic permanent magnet is obtained by heat treatment. Magnetic powder.

As a new type of magnetic powder, NEOMAX has developed nano composite magnetic powder. The bonded permanent magnet made of the magnetic powder has a nano-composite structure in which a nano-crystalline phase surrounds a hard magnetic phase, and a small amount of rare earth elements is added to obtain high coercivity. In addition, the use of high-speed continuous casting method for mass production can achieve nano-crystallization. Because rare earth elements are small, no coating is required, and it has good corrosion resistance.

In order to achieve higher performance, the technology that converts isotropy into anisotropy should be researched and developed and put into practical use. As a magnetically anisotropic NdFeB bonded permanent magnet, Japan has developed the HDDR treatment method. This method is to react the NdFeB alloy with hydrogen and simultaneously perform magnetic anisotropy with reproduced magnetic properties.

b. Bonding material (adhesive)

The bonding materials used include various synthetic rubbers and nitrile rubbers such as polyvinyl chloride and chlorinated polyethylene rubber used in rubber permanent magnets, and nylon, PPS resin and epoxy resin used in plastic permanent magnets. As a molding method, the rubber permanent magnet is formed by calender roll or extrusion, and the plastic permanent magnet is compression formed (when using a thermoplastic adhesive). The anisotropic ring-shaped permanent magnet processed by hot extrusion has also entered the practical stage. The content of the bonding material usually used for bonding permanent magnets is 2 to 4% by weight for compression molding and 6 to 8% by weight for injection molding.

c. Development Trend

The magnetic properties obtained by bonded magnets, among them, the maximum energy product of the ferrite system is 17.5 KJ/m3 (2.2MGOe), and the samarium cobalt system is about 31.8~80 KJ/m3 (4~10MGOe: isotropic) and 119 KJ /m3 (15MGOe: anisotropy), NdFeB is about 80~88 KJ/m3 (10~11MGOe: isotropic).

In terms of anisotropic bonded permanent magnets, the HDDR (hydrogenation, phase decomposition, dehydrogenation, recombination) method has been used to develop anisotropic bonded permanent magnets. The production level is higher than the previous isotropic bonded permanent magnets. The magnetic performance is doubled to 159 KJ/m3 (20MGOe).

In addition, due to the poor heat resistance of NdFeB bonded permanent magnets, the development of bonded permanent magnets with high heat resistance is the main development goal in the future.

Japan’s Aichi Steel Corporation has successfully developed a new process (d-HDDR method) for NdFeB anisotropic magnetic powder without adding cobalt. The magnetic powder made by this process is used to produce plastic bonded permanent magnets. . The maximum magnetic energy product reaches 199KJ/m3 (25MGOe). The thin ring-shaped magnetic steel made is used in motors and has good heat resistance. In the past, the anisotropic magnetic powder needed to add a large amount of precious metal cobalt, which greatly increased the production cost. Using a new process, the reaction speed of neodymium iron boron alloy and hydrogen was used to control the reaction rate, and no cobalt was added.

The new type of isotropic bonded permanent magnets also include the samarium (Sm), iron (Fe) and nitrogen (N) isotropic bonded permanent magnets of Japan's Datong Special Steel and Datong Electronics. The maximum energy product of the permanent magnet reaches 110KJ/m3 (14MGOe), the world's highest magnetic performance, and has good corrosion resistance and heat resistance. It is widely used in ultra-small motors for office automation, communication equipment, sensors, small high-output motors for automobiles and water pumps, etc. The manufacturing method of this kind of bonded permanent magnet is to adjust the alloy of the specified composition and process it in nitrogen to produce Sm-Fe-N magnetic powder, which is then mixed with resin and molded. The magnetic powder manufacturing process is different from the past NdFeB bonded permanent magnets. Corresponding to the NdFeB magnetic powder, the samarium magnetic powder adopts the quenching method of special equipment and can be easily manufactured by using a vacuum melting furnace.

3. Ferrite core material

There are two types of magnetic core materials: ferrite system and metal system. In recent years, with the high frequency of electronic equipment, the most used ferrite material (soft ferrite) with high-quality high-frequency characteristics.

Ferrite materials are widely used in power transformers, communication transformers, CRT deflection coils, chokes, filters and magnetic cores used in noise suppression devices.

As a ferrite material, MnZn (manganese, zinc)-based ferrite materials are currently the most eye-catching. Compared with other ferrite materials, this material has low magnetic anisotropy and large natural magnetization. For this reason, it has the characteristics of high permeability, low loss and high magnetic flux density, and is widely used in communication transformers, power transformers, inverter transformers and chokes.

The current power supply is switching power supply as the mainstream. The transformer used in the switching power supply is manufactured by winding a coil on a ferrite core with good high frequency characteristics. In order to make the transformer smaller and thinner, a high-performance ferrite core is required. Especially in recent years, as the requirements for miniaturization and high efficiency of the whole machine have been further increased, as a ferrite core material for transformers, power loss is required to be small.

Power consumption is the heat generated by a part of the input energy, which brings about temperature rise and reduces efficiency. If the power consumption is reduced, the loss of the transformer can be reduced and the temperature rise can be reduced.

Compared with ferrites of the nickel zinc (NiZn) system, this power ferrite core material has the characteristics of small crystalline magnetic anisotropy constant and magnetostriction coefficient, and large natural magnetization. For this reason, the power loss is small and the saturation magnetic flux density is high, which is suitable for making switching power supply transformers.

Like the transformer, the choke coil also winds the ferrite core with a coil. The chokes used for DC converters in mobile communications and other equipment are required to be small and thin, and the core material used should have high magnetic flux density and low leakage flux.

Recently, electrical appliances such as computers have begun to develop in the direction of low voltage and large current. Corresponding to large current, choke coils are required to have low loss and small size. In addition, in recent years, based on computer simulations, various shapes of ferrite cores for transformers can be manufactured.

As a core material for communications, TDK has developed ferrite materials for pulse transformers for high-temperature LAN systems and high-speed LAN systems, and ferrite materials for xDSL module transformers that reduce high harmonic distortion.

Magnetic core materials for power transformers require low power loss. Recently, MnZn ferrite materials for power transformers have been developed that greatly reduce power loss at a wide temperature range of 25°C to 120°C. Not only as a general transformer for switching power supplies, but also particularly suitable for main transformers for next-generation automobiles. Recently, MnZn ferrite materials with high saturation magnetic flux density at high temperatures have also been developed and are suitable for liquid crystal inverter transformers and automotive transformers.

Communication transformers use high-permeability ferrite materials. Recently, a large number of pulse transformers are used in high-speed LAN systems. The pulse transformer has certain requirements for the inductance of the DC bias load, and its performance must be ensured. For this reason, not only must the inductance at the initial permeability level be guaranteed, the key is also good DC superposition characteristics. The development of ferrite materials with improved DC superposition characteristics is the future development direction.