Neodymium magnet

Neodymium -iron- boron alloy of neodymium, iron and boron with the composition Nd2Fe14B, are produced from the material as the currently strongest permanent magnets. It is one of the materials of the rare earth and was developed in 1982 by General Motors and Sumitomo Special Metals. The material can be used as a permanent magnet used where strong permanent magnetic fields are required. Example applications include speakers and headphones, DC motors in cordless power tools, in linear motors for hard disk drives to control the read-write heads to the electrical generators that are used in wind turbines. Besides trivial applications exist as holding magnets as toys and magnets for artistic design.

Properties

Permanent magnet materials are next to a high spontaneous polarization (iron) have a large uniaxial magnetic anisotropy. This refers to a preferred magnetic direction ( " easy direction " ), which is intended for permanent magnet rare earth base by the crystal structure and electronic structure. NdFeB has high magnetic anisotropy, as the " magnetic" 4f shell is shielded by the outer 5s25p6 shells from the ligand field of the crystal and thus the orbital moment of the shell remains fully effective. Due to the spin -orbit coupling, the spins are coupled to the anisotropic crystal field, and a rotation of the spins and thus the magnetic moments from the easy direction is connected with energy.

Nd2Fe14B has a Tetragonal crystal system with high magnetic anisotropy. Depending on the microstructure and the manufacturing process high coercivities 870-2750 kA / m can be achieved. The residual flux density is typically 1.3 T, under ideal conditions, at levels up to 1.6 T. The magnetic energy density is at the maximum (BH) max of 512 kJ/m3 in which information about the values ​​of the material samarium- cobalt ( SmCo ) is. The Curie temperature of Nd2Fe14B is 310 ° C and depends strongly on additional alloying constituents from.

Commercial Nd2Fe14B magnets are with an N followed by a number. The number represents the magnetic strength of the permanent magnet. Typical values ​​are in the range N35 to N50.

Large crystals of Nd2Fe14B can be relatively easy to demagnetize and are therefore unsuitable as a permanent magnet. NdFeB materials therefore have a fine crystalline structure. The Nd2Fe14B crystals are also surrounded by a thin layer in which the rare earth element is highly enriched. This structure is produced in a discovered by General Motors, and the Japanese company Sumitomo together and patented sintering process. According to this method, the magnets are alloyed, milled, pressed and sintered into powder. By pressing, but especially by applying an external magnetic field during the process, the crystals are aligned anisotropically. Only in this way, the magnetic properties are fully utilized. In addition, in the automotive industry, plastic-bonded isotropic NdFeB magnets are used. This one is in the shape of the magnets a little more flexible, can do without an additional surface protection and rely on proven injection molding process. The recoverable values ​​of these plastic-bonded magnets are usually inferior to that of NdFeB sintered magnets.

Magnets, which consist only of neodymium, iron and boron without further alloying additions, demagnetize at temperatures of 80 ° C partially and are very susceptible to corrosion. By other rare earth additives, in particular dysprosium, or terbium, the temperature stability can be increased to over 200 ° C. To increase the corrosion stability of other alloying components, such as cobalt, are often alloyed. This has major limitations for use of this material were removed. However, NdFeB materials are inferior to samarium -cobalt magnets in these two points. Therefore, improved NdFeB magnets for most application areas must be protected from corrosion by a protective layer. Most often this nickel or epoxy resin coatings are used.

NdFeB magnets are used today wherever you need a strong magnetic field at low volume. You have now replaced the more easily to be demagnetized AlNiCo magnets in many applications.

Environmental impact

The majority of NdFeB magnets is now produced in China. One of the starting materials, the neodymium, one of the rare earths and is mined and extracted with status 2011 to 97 % in China. The mining and processing of neodymium leads, as for all rare earths to locally impact on the environment. NdFeB magnets are used for permanent excitation of generators as of 2011 around 15% of the wind turbines, particularly in wind turbines with direct drive. The environmental aspects of Neodymgewinnung affect in the overall context a negative impact on the sustainability of these wind turbines.

Safety

Due to the high strength of the magnets otherwise arise rather unexpected dangers. This includes the case of larger NdFeB magnets in particular bruising if handled improperly and non-observance of safety distances to ferromagnetic materials such as iron or other magnets in the vicinity. The adhesive forces of NdFeB disc with approximately 10 cm diagonal and about 1.5 cm thickness can reach some 1000 N.

Smaller NdFeB magnets are the relevant commercially available in the form of balls or cubes and serve primarily play or decoration purposes. If more than a small NdFeB magnet is swallowed, danger to life due to a possible bowel perforation. On November 15, 2012, in Australia little, swallowed hard magnetic NdFeB magnets, which are marketed as a toy, is prohibited. On January 23, 2013, import and trade ban on this type of toy magnets was imposed by the New Zealand Parliament.

The strong magnetic field can already damage or erase magnetic recording ( magnetic tape, floppy disks, hard disks) from a distance. Similarly, distortion and color distortion may occur in picture tubes. In a machining operation, such as filing, sawing or drilling is NdFeB dust and chips can ignite due to the heat generated during machining, as well as the chips are not easily by the strong magnetization separate from the main body. The material tends to be sharp chipping, even if two magnets snap together, no brakes, so wear appropriate protective clothing and safety goggles should be worn when handling neodymium-iron- boron.