Magnetic materials can be divided into hard magnetic materials and soft magnetic materials. Among them, hard magnetic materials refer to materials that are magnetized to saturation in an external magnetic field, but after removing the external magnetic field, they can still maintain high remanence and provide a stable magnetic field. , Also called permanent magnet material. Taking advantage of this feature, permanent magnet materials are widely used in many industries such as energy, information and communication, transportation, computers, and medical equipment. In recent years, the superior performance of permanent magnet materials in the fields of energy-saving home appliances, hybrid electric vehicles/pure electric vehicles, wind power and hydropower generation has attracted more and more attention.

The application and research of permanent magnet materials began at the end of the nineteenth century. With the in-depth study of material magnetism and the improvement of various manufacturing processes, the research of permanent magnet materials mainly includes three stages: metal alloy magnets, ferrite magnetic materials and rare earth permanent magnet materials. Among them, although metal alloy magnets and ferrite magnetic materials have the advantages of low cost and abundant raw materials, their maximum magnetic energy product (BH)max is generally less than 10MGOe, and their magnetic properties are poor, so they are gradually replaced by rare earth permanent magnet materials.

Since its appearance in the early 1960s, after decades of development, three generations of rare earth permanent magnet materials with practical value have been formed: the first generation rare earth permanent magnet material (SmCo5), the second generation rare earth permanent magnet material (Sm2Co17) And the third generation rare earth permanent magnet material (Nd2Fe14B).

Classification menu:

1.1 AlNiCo Magnets

AlNiCo (AlNiCo) is the earliest developed permanent magnet material, which is an alloy composed of aluminum, nickel, cobalt, iron and other trace metal elements. Alnico permanent magnet material was successfully developed in the 1930s. At that time, it had the best magnetic properties and a small temperature coefficient, so it was the most widely used in permanent magnet motors. After the 1960s, with the advent of ferrite permanent magnets and rare earth permanent magnets, the application of alnico permanent magnets in motors was gradually replaced, and the proportion showed a downward trend.

Permanent magnet Alnico (Alnico) is an iron alloy, in addition to iron, also added aluminum (Al), nickel (Ni), cobalt (Co) and a small amount of other ingredients to enhance magnetic properties. The English term name "Alnico" is formed by merging the element symbols of the three main additions.

Alnico alloy has high coercivity and high Curie temperature. Alnico alloy is hard and brittle, and cannot be cold worked (cold work). It must be made by casting or sintering (Sintering) procedures. Alnico can generate magnetic fields up to 0.15 Tesla. To give an example of an anisotropic cast Alnico alloy with intermediate properties, the composition of Alnico-6 is 8% Al, 16% Ni, 24% Co, 3% Cu, 1% Ti, and the others are Fe. Alnico-6 has a maximum magnetic energy product (BHmax) of 3.9 megagauss-oesteds (MG Oe), a coercivity of 780 oersted, a Curie temperature of 860 °C, and a maximum operating temperature of 525 °C.

Classification

According to different production processes, it is divided into sintered AlNiCo (Sintered AlNiCo) and cast AlNiCo (Cast AlNiCo). The product shapes are mostly round and square. The casting process can be processed into different sizes and shapes; compared with the casting process, the sintered product is limited to a small size, and the dimensional tolerance of the blank produced by it is better than that of the casting product, and the magnetic property is slightly lower than that of the casting product, but it can be Processability is better. Among the permanent magnet materials, the cast AlNiCo permanent magnet has the lowest reversible temperature coefficient, and the working temperature can be as high as 600 degrees Celsius. Alnico permanent magnet products are widely used in various instrumentation and other application fields.

Advantages

The advantages of AlNiCo magnets are high remanence (up to 1.35T) and low temperature coefficient. When the temperature coefficient is -0.02%/℃, the maximum operating temperature can reach about 520℃. The disadvantage is that the coercive force is very low (usually less than 160kA/m), and the demagnetization curve is nonlinear. Therefore, although AlNiCo magnets are easily magnetized, they are also easily demagnetized.

Applications

Many industrial and consumer products require strong permanent magnets. For example, electric motors, electric guitar pickups, microphones, sensors, speakers, traveling wave tubes, cow magnets, etc., all use alnico magnets. But now, many products use rare earth magnets instead, because this type of material can give a stronger magnetic field (Br) and a higher maximum energy product (BHmax), allowing to reduce the size of the product.

1.2 Fe-chromium-cobalt permanent magnet alloy

The main components are iron, chromium and cobalt, and it also contains molybdenum and a small amount of titanium and silicon. Its processing performance is good, and it can undergo cold and hot plastic deformation. Its magnetic properties are similar to AlNiCo permanent magnet alloys, and its magnetic properties can be improved through plastic deformation and heat treatment. It is used to manufacture various small magnet components with small cross-section and complex shape.

2.1 Ferrite magnets

Ferrite magnet is a sintered permanent magnet material, which is composed of barium and strontium ferrite. This kind of magnetic material not only has strong anti-demagnetization performance, but also has the advantage of low cost. Ferrite magnets are rigid and brittle and require special machining processes. Because the opposite magnet is oriented along the manufacturing direction, it must be magnetized in the direction taken, while the same-sex magnet can be magnetized in any direction because it is not oriented, although a slightly stronger magnetic induction will be found on the side where the pressure is often the smallest. The magnetic energy product ranges from 1.1MGOe to 4.0MGOe. Due to its low cost, ferrite magnets have a wide range of applications, from motors, speakers to toys and handicrafts, so they are currently the most widely used permanent magnet materials.

Material characteristics

Produced by powder metallurgy method, the residual magnetism is low, and the recovery magnetic permeability is small. Large coercive force, strong anti-demagnetization ability, especially suitable for magnetic circuit structure under dynamic working conditions. The material is hard and brittle and can be used for cutting with diamond tools. The main raw material is oxide, so it is not easy to corrode. Working temperature: -40°C to +200°C.

Ferrite magnets are further divided into anisotropy (anisotropy) and isotropy (isotropy). The isotropic sintered ferrite permanent magnet material has weak magnetic properties, but it can be magnetized in different directions of the magnet; the anisotropic sintered ferrite permanent magnet material has strong magnetic properties, but it can only be magnetized along the direction of the magnet. Predetermined magnetization direction magnetization.

Differences from NdFeB magnets

A ferrite magnet is a metal oxide with ferromagnetic properties. In terms of electrical properties, the resistivity of ferrite is much larger than that of metal and alloy magnetic materials, and it also has higher dielectric properties. The magnetic properties of ferrite are also shown to have higher magnetic permeability at high frequencies. Therefore, ferrite has become a widely used non-metallic magnetic material in the field of high frequency and weak current. Belonging to non-metallic magnetic materials, it is a composite oxide (or ferrite) of magnetic ferric oxide and one or more other metal oxides. The magnetic force is usually 800-1000 gauss, and it is often used in speakers, speakers and other equipment.

The advantages of NdFeB magnets are high cost performance and good mechanical properties; the disadvantages are that the Curie temperature point is low, the temperature characteristics are poor, and it is easy to be pulverized and corroded. It must be adjusted by adjusting its chemical composition and adopting surface treatment methods. Improvement can meet the requirements of practical application. NdFeB belongs to the third generation of rare earth permanent magnet materials. It has the characteristics of small size, light weight and strong magnetism. It is the magnet with the best performance and price ratio at present. The advantages of high energy density make NdFeB permanent magnet materials widely used in modern industry and electronic technology. In the state of bare magnets, the magnetic force can reach about 3500 Gauss.

2.2 Rubber Magnets

Rubber Magnet is a kind of ferrite magnetic material series, which is made of bonded ferrite magnetic powder and synthetic rubber, and is made by extrusion molding, calendering molding, injection molding and other processes. It has softness, elasticity and twistability. the magnet. It can be processed into strips, rolls, sheets, blocks, rings and various complex shapes.

Original features

It has flexibility, elasticity and bendability, and can be produced into rolls, sheets, strips, blocks, rings and various complex shapes through extrusion, calendering, injection, mold forming and other processes. Its surface can also be covered with PVC sheet, coated paper, double-sided tape, coated with UV oil, or color printed and die-cut into various shapes.

Processing features

Rubber magnets are composed of magnetic powder (SrO6, Fe2O3), chlorinated polyethylene (CPE) and other additives (EBSO, DOP), etc., and are manufactured by extrusion and calendering. Rubber magnets can be homosexual or heterosexual, and can be bent, twisted, or rolled. It can be used without further machining, and the shape can be trimmed according to the required size, and it can also be covered with PVC, adhesive, UV oil, etc. according to customer requirements. Its magnetic energy product is 0.60-1.50 MGOe.

Production Process

Ingredients→mixing→extrusion/calendering/injection molding→processing→magnetization→inspection→packaging

performance test

Appearance, size, magnetic properties, magnetic polarity, hardness, specific gravity, tensile strength, aging resistance, rotation performance

Industry application field

Application fields of rubber magnets: refrigerators, message notice racks, fasteners for fixing objects to metal bodies for advertising, etc., magnetic sheets for toys, teaching instruments, switches and sensors. Mainly used in industries such as micro motors, refrigerators, disinfection cabinets, kitchen cabinets, toys, stationery, and advertisements.

3.1 Samarium cobalt magnets

Samarium cobalt (SmCo), as the second-generation rare earth permanent magnet, not only has a high magnetic energy product (14-32MGOe) and reliable coercive force, but also shows good temperature characteristics in the rare earth permanent magnet series. Compared with NdFeB, SmCo is more suitable for working in high temperature environment.

SmCo5 Sm2Co17

Remanence Br>1.05T (>10.5kGs)

Magnetic induction coercivity HcB>676kA/m (>8.5kOe)

Intrinsic coercivity Hcj>1194kA/m (>15kOe)

Maximum energy product (BH) max>209.96kJ/m3(26~30MGs.Oe)

Br temperature coefficient -0.03%/℃

Reversible magnetic permeability μ 1.03H/m

Curie temperature Tc 670~850℃

3.2 Neodymium magnets

Neodymium magnet, also known as NdFeB magnet (NdFeB magnet), is a tetragonal crystal formed by neodymium, iron, and boron (Nd2Fe14B). In 1982, Masato Sagawa of Sumitomo Special Metals discovered neodymium magnets. The magnetic energy product (BHmax) of this magnet is greater than that of the samarium cobalt magnet, and it was the material with the largest magnetic energy product in the world at that time. Later, Sumitomo Special Metals successfully developed the powder metallurgy process, and General Motors successfully developed the melt-spinning process, which was able to prepare NdFeB magnets. This kind of magnet is the second most magnetic permanent magnet after the absolute zero holmium magnet, and it is also the most commonly used rare earth magnet. NdFeB magnets are widely used in electronic products, such as hard drives, mobile phones, earphones, and battery-powered tools.

Classification

NdFeB is divided into sintered NdFeB and bonded NdFeB. Bonded NdFeB is magnetic in all directions and is corrosion-resistant; and sintered NdFeB is easy to corrode, and the surface needs to be coated. Generally, there are galvanized, Nickel, environment-friendly zinc, environment-friendly nickel, nickel-copper-nickel, environment-friendly nickel-copper-nickel, etc. Sintered NdFeB is generally divided into axial magnetization and radial magnetization, depending on the required working surface.

Chemical composition

NdFeB permanent magnet material is a permanent magnet material based on the intermetallic compound Nd2Fe14B. The main components are rare earth elements neodymium (Nd), iron (Fe), boron (B). Among them, the rare earth element is mainly neodymium (Nd). In order to obtain different properties, it can be partially replaced by other rare earth metals such as dysprosium (Dy) and praseodymium (Pr). Iron can also be partially replaced by other metals such as cobalt (Co) and aluminum (Al). The content of boron is small, but it plays an important role in the formation of tetragonal crystal structure intermetallic compounds, making the compounds have high saturation magnetization, high uniaxial anisotropy and high Curie temperature.

The third-generation rare earth permanent magnet NdFeB is the most powerful permanent magnet among contemporary magnets. Its main raw materials are rare earth metal neodymium 29%-32.5%, metal element iron 63.95-68.65%, non-metal element boron 1.1-1.2% and dysprosium 0.6-8% niobium 0.3-0.5% aluminum 0.3-0.5% copper 0.05-0.15% and other elements.

Process flow

Technological process: batching → smelting ingot/spinning → powder making → pressing → sintering and tempering → magnetic testing → grinding → pin cutting → electroplating → finished product. The ingredients are the basis, and the sintering and tempering is the key.

NdFeB magnet blank production tools and performance testing tools: smelting furnace, strip furnace, jaw crusher, jet mill, compression molding machine, vacuum packaging machine, isostatic pressing machine, sintering furnace, heat treatment vacuum furnace, magnetic performance test instrument, Gauss meter.

NdFeB magnet machining tools: centerless grinder, rounding machine, double-end grinder, flat grinder, slicer, double-sided grinder, wire cutting, bench drill, special-shaped grinder, etc.

Application

Sintered NdFeB permanent magnet materials have excellent magnetic properties and are widely used in electronics, electrical machinery, medical equipment, toys, packaging, hardware machinery, aerospace and other fields. The more common ones are permanent magnet motors, speakers, magnetic separators, Computer disk drives, magnetic resonance imaging equipment instruments, etc.