01.06.2022

A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, steel, nickel, cobalt, etc. and attracts or repels other magnets. Ferromagnetic materials can be divided into magnetically "soft" materials like annealed iron, which can be magnetized but do not tend to stay magnetized, and magnetically "hard" materials, which do. Permanent magnets are made from "hard" ferromagnetic materials such as alnico and ferrite that are subjected to special processing in a strong magnetic field during manufacture to align their internal microcrystalline structure, making them very hard to demagnetize. To demagnetize a saturated magnet, a certain magnetic field must be applied, and this threshold depends on coercivity of the respective material. "Hard" materials have high coercivity, whereas "soft" materials have low coercivity. The overall strength of a magnet is measured by its magnetic moment or, alternatively, the total magnetic flux it produces. The local strength of magnetism in a material is measured by its magnetization.

Types of magnets: Shape magnets: square magnets, tile magnets, special-shaped magnets, cylindrical magnets, ring magnets, wafer magnets, bar magnets, magnetic frame magnets, attribute magnets: samarium cobalt magnets, ferrite magnets, AlNiCo magnets, FeCrCo magnets, industrial magnets: magnetic components, motor magnets, rubber magnets, plastic magnets, etc. Magnets are divided into permanent magnets and soft magnets. Permanent magnets are combined with strong magnetism, so that the spin of magnetic substances and electron angular momentum are arranged in a fixed direction. Soft magnetism is the addition of electricity. When the current is removed, the soft iron will gradually lose its magnetism. Magnetic materials have also been widely used in the military field. For example, ordinary mines or mines can only explode on contact with the target, and thus have limited effect. And if sensor magnets are installed on mines or mines, since tanks or warships are made of steel, when they are close (without touching the target), the sensors can detect changes in the magnetic field and cause the mines or mines to explode, increasing the lethality.

A neodymium magnet (also known as NdFeB, NIB or Neo magnet) is the most widely used type of rare-earth magnet. It is a permanent magnet made from an alloy of neodymium, iron, and boron to form the Nd2Fe14B tetragonal crystalline structure. Developed independently in 1984 by General Motors and Sumitomo Special Metals, neodymium magnets are the strongest type of permanent magnet commercially available. Because of different manufacturing processes, they are divided into two subcategories, namely sintered NdFeB magnets and bonded NdFeB magnets. They have replaced other types of magnets in many applications in modern products that require strong permanent magnets, such as electric motors in cordless tools, hard disk drives and magnetic fasteners. Neodymium is a metal that is magnetic, specifically it has antiferromagnetic properties, however in pure form its magnetic properties only appear at low temperatures, below 19 K (−425.5 °F). However, some compounds of neodymium with transition metals such as iron are ferromagnetic, with Curie temperatures well above room temperature, and these are used to make neodymium magnets. The strength of neodymium magnets is the result of several factors. The most important is that the tetragonal Nd2Fe14B crystal structure has exceptionally high uniaxial magnetocrystalline anisotropy (HA ≈ 7 T – magnetic field strength H in units of A/m versus magnetic moment in A·m2). This means a crystal of the material preferentially magnetizes along a specific crystal axis but is very difficult to magnetize in other directions. Like other magnets, the neodymium magnet alloy is composed of microcrystalline grains which are aligned in a powerful magnetic field during manufacture so their magnetic axes all point in the same direction. The resistance of the crystal lattice to turning its direction of magnetization gives the compound a very high coercivity, or resistance to being demagnetized.

There are two principal neodymium magnet manufacturing methods: 1) Classical powder metallurgy or sintered magnet process: Sintered Nd-magnets are prepared by the raw materials being melted in a furnace, cast into a mold and cooled to form ingots. The ingots are pulverized and milled; the powder is then sintered into dense blocks. The blocks are then heat-treated, cut to shape, surface treated and magnetized.
2) Rapid solidification or bonded magnet process: Bonded Neodymium magnets are prepared by melt spinning a thin ribbon of the NdFeB alloy. The ribbon contains randomly oriented Nd2Fe14B nano-scale grains. This ribbon is then pulverized into particles, mixed with a polymer, and either compression- or injection-molded into bonded magnets. In 2015, Nitto Denko Corporation of Japan announced their development of a new method of sintering neodymium magnet material. The method exploits an "organic/inorganic hybrid technology" to form a clay-like mixture that can be fashioned into various shapes for sintering. Most importantly, it is said to be possible to control a non-uniform orientation of the magnetic field in the sintered material to locally concentrate the field to, e.g., improve the performance of electric motors. Mass production is planned for 2017. As of 2012, 50,000 tons of neodymium magnets are produced officially each year in China, and 80,000 tons in a "company-by-company" build-up done in 2013.China produces more than 95% of rare earth elements and produces about 76% of the world's total rare-earth magnets, as well as most of the world's neodymium.

NdFeB bonded permanent magnet material is made by adding NdFeB magnetic powder to a binder. Since the successful development of this material in 1988, its development has been quite rapid, and its output has increased exponentially. As a high-performance permanent magnet material, it is in line with the development trend of contemporary electronic products in the direction of short-term, small, light and thin. The production and application development of bonded NdFeB permanent magnet materials is relatively late, the application is not wide, and the amount is small, mainly used in office automation equipment, electrical machinery, audio-visual equipment, instrumentation, small motors and measuring machinery, in mobile , CD-ROM, DVD-ROM drive motor, hard disk spindle motor HDD, other micro DC motors and automation instruments and other fields are widely used. In recent years, the application proportion of bonded NdFeB permanent magnet materials in my country is as follows: computers account for 62%, electronics industry accounts for 7%, office automation equipment accounts for 8%, automobiles account for 7%, appliances account for 7%, and others account for 9%.

Neodymium magnets are graded according to their maximum energy product, which relates to the magnetic flux output per unit volume. Higher values indicate stronger magnets. For sintered neodymium magnets, there is a widely recognized international classification. Their values range from 28 up to 52. The first letter N before the values is short for neodymium, meaning sintered NdFeB magnets. Letters following the values indicate intrinsic coercivity and maximum operating temperatures (positively correlated with the Curie temperature), which range from default (up to 80 °C or 176 °F) to TH (230 °C or 446 °F). Sintered Nd2Fe14B tends to be vulnerable to corrosion, especially along grain boundaries of a sintered magnet. This type of corrosion can cause serious deterioration, including crumbling of a magnet into a powder of small magnetic particles, or spalling of a surface layer. This vulnerability is addressed in many commercial products by adding a protective coating to prevent exposure to the atmosphere. Nickel plating or two-layered copper-nickel plating are the standard methods, although plating with other metals, or polymer and lacquer protective coatings, are also in use.

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