Neodymium-iron-boron

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Two neodymium magnets (each ∅ 20 mm × 10 mm, surface nickel-plated ), which can hardly be separated with bare hands

Neodymium magnet is an alloy of neodymium , iron and boron having the composition of Nd 2 Fe 14 B, from which the material which is currently the strongest permanent magnets can be produced. It is one of the materials used for rare earth magnets and was developed independently in 1982 by General Motors Research Laboratories and Sumitomo Special Metals ( Masato Sagawa ). The material is used where strong permanent magnetic fields are required. Example applications are loudspeakers and headphones , DC motors in battery tools, in linear motors for hard drives to control the read / write heads to the electrical generators that are used in wind turbines . There are also trivial applications such as holding magnets, toys and magnets for artistic design.

properties

Left: Transmission electron microscope (TEM) image of Nd 2 Fe 14 B; On the right the schematic crystal structure . The unit cell is marked with a square
A neodymium-iron-boron magnet (small disc between massive steel balls) carries 1300 times its own weight

In addition to a high spontaneous polarization (iron), permanent magnetic materials should have a large uniaxial magnetic anisotropy . This denotes a preferred magnetic direction (“easy direction”) which, in the case of rare earth- based permanent magnets , is determined by the crystal structure and electronic structure. NdFeB has a high magnetic anisotropy, since the “magnetic” 4f-shell is shielded from the ligand field of the crystal by the outer 5s 2 5p 6 -shells and so the orbital moment of the shell remains fully effective. Through the spin-orbit coupling , the spins are coupled to the anisotropic crystal field, and twisting the spins and thus the magnetic moments from the easy direction is associated with energy expenditure.

Nd 2 Fe 14 B has a tetragonal crystal system with high magnetic anisotropy . Depending on the microstructure and manufacturing process, high coercive field strengths of 870 to 2750 kA / m can be achieved. The remanent flux density is typically 1.3  T , under ideal conditions values ​​up to 1.6 T. The magnetic energy density is at its maximum (BH) max at 512 kJ / m 3 , which is above the values ​​of the material samarium-cobalt (SmCo) lies. The Curie temperature of Nd 2 Fe 14 B is 310  ° C and is far below the 700 ° C-800 ° C of samarium cobalt.

Commercially available Nd 2 Fe 14 B magnets are designated with an N followed by a number. The number stands for the magnetic strength of the permanent magnet. Usual values ​​are in the range N35 to N50. A following letter indicates an increased permissible operating temperature (gradations M – H – UH or similar are common), which corresponds to a higher coercive field strength at the same temperature.

Large crystals of Nd 2 Fe 14 B can be demagnetized relatively easily and are therefore unsuitable as permanent magnets. NdFeB materials therefore have a finely crystalline structure. The Nd 2 Fe 14 B crystals are also surrounded by a fine layer in which the rare earth element is highly enriched. This structure is manufactured using a sintering process jointly discovered and patented by General Motors and the Japanese company Sumitomo Special Metals . After this process the magnets are alloyed, ground into powder, pressed and sintered. The crystals are aligned anisotropically through the pressure, but above all through the application of an external magnetic field during the process. Only then are the magnetic properties fully used. In addition, plastic-bonded isotropic NdFeB magnets are also used in the automotive industry. You are a bit more flexible in shaping the magnets, you can do without additional surface protection and you can fall back on tried and tested injection molding processes. The achievable values ​​of these plastic-bonded magnets are, however, worse than with sintered NdFeB magnets.

Magnets that only consist of neodymium, iron and boron without any other alloy additives, partially demagnetize themselves at temperatures of 80 ° C and are very sensitive to corrosion. By adding other rare earth elements, in particular dysprosium or terbium , the temperature stability can be increased to over 200 ° C. To increase the corrosion stability, other alloy components such as cobalt are often added. This removed major restrictions on the use of this material. Nevertheless, NdFeB materials are inferior to samarium-cobalt magnets in these two respects. Therefore, improved NdFeB magnets for most areas of application must be protected from corrosion by a protective layer. Nickel or epoxy resin coatings are most commonly used for this .

NdFeB magnets are used today wherever strong magnetic fields are needed in a small volume. In the meantime they have replaced the easier to demagnetize AlNiCo magnets in many applications.

Environmental impact

Most of the NdFeB magnets are produced in China today. One of the raw materials, neodymium, is a rare earth element and as of 2011, 97% of it is mined and extracted in China. The mining and processing of neodymium leads, as for all rare earths, to pollution on site for the environment. As of 2011, NdFeB magnets are used for permanent excitation of the generators in around 15% of wind turbines, especially in wind turbines with direct drive . When viewed as a whole, the environmental aspects of neodymium production have a negative effect on the sustainability of these wind turbines.

safety instructions

A toy of 216 spherical NdFeB magnets, diameter per sphere approx. 5 mm

The high strength of the magnets would otherwise result in unexpected dangers. In the case of larger NdFeB magnets, this includes, in particular, crushing if handled improperly and if the safety distance to ferromagnetic materials such as iron or other magnets in the vicinity is not observed. The holding power of an NdFeB disc occurring at around 10 cm diagonal and 1.5 cm thick, some 1000  N reach.

Smaller NdFeB magnets are available in the relevant trade in the form of spheres or as cuboids and serve u. a. Play or decoration purposes. If more than one small NdFeB magnet is swallowed, there is a risk of death due to possible intestinal perforation . On November 15, 2012, small, swallowable, highly magnetic NdFeB magnets sold as toys were banned in Australia. On January 23, 2013, the New Zealand Parliament issued an import and trade ban on this type of toy magnet.

The strong magnetic field can damage or delete magnetic recordings (magnetic tape, floppy disks) from a distance. Distortions and color falsifications can also occur in picture tubes . During machining such as filing, sawing or drilling, NdFeB dusts and chips can ignite due to the heat generated during machining, and the chips cannot easily be separated from the base body (and possibly the tool).

The material tends to splinter with sharp edges, which is why appropriate protective clothing and goggles should be worn when working with neodymium-iron-boron. Even if two magnets snap together without braking, this breaking behavior can lead to dangerous situations.

Web links

Commons : Neodymium-iron-boron magnets  - Collection of images, videos and audio files

Individual evidence

  1. ↑ JJ Croat, JF Herbst, RW Lee, FE Pinkerton: High-energy product Nd-Fe-B permanent magnets . In: Applied Physics Letters . tape 44 , no. 1 , January 1984, pp. 148-149 , doi : 10.1063 / 1.94584 ( PDF ).
  2. ↑ Hitachi Metals, Ltd. - The Magnet Industry Newsmaker
  3. ↑ Jacob Fraden: Handbook of Modern Sensors: Physics, Designs, and Applications, 4th Ed. . Springer, USA 2010, ISBN 1-44196465-7 , p. 73.
  4. ↑ http://www.china-magnet.net/neodymium-magnet/Grade%20of%20neodymium%20magnet.pdf , Neodymium Grads.
  5. ↑ Patent EP0265413 : Process for the manufacture of rare-earth metals and of alloys containing rare-earth metals. Registered on August 18, 1987 , published on February 26, 1992 , applicant: Treibacher Chemische Werke , inventor: Hans Zeiringer. ‌
  6. ^ Spiegel Online, April 10, 2009: The new gold
  7. ↑ NDR: Neodym: The dirty secret of clean wind turbines
  8. ^ Nicole Vormann / Murphy & Spitz: Murphy & Spitz Research: Position on neodymium and wind turbines. (Background paper; PDF; 358 kB) June 2011, accessed on June 27, 2011 .
  9. ↑ Not an everyday use: hand squeezed between two magnets ( memento from April 2, 2012 in the Internet Archive ) on feuerwehr.de
  10. ↑ JA Cauchi, RN Shawis: Multiple magnet ingestion and gastrointestinal morbidity , In: Arch Dis Child , 2002, 87, pp 539-540; doi : 10.1136 / adc.87.6.539 .
  11. ↑ VIC: Update: Permanent ban on small, high powered magnets . Product Safety Australia. Retrieved October 14, 2013.
  12. ^ Ban on the sale of high powered magnet sets . New Zealand Government. January 23, 2013. Retrieved October 14, 2013.