A neodymium magnet (also known as NdFeB, NIB, or Neo magnet), a type of rare-earth magnet, is a permanent magnet made from an alloy of neodymium, iron, and boron to form the Nd2Fe14B tetragonal crystalline structure. This material is currently the strongest type of permanent magnet.
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Description
The tetragonal Nd2Fe14B crystal structure has exceptionally high uniaxial magnetocrystalline anisotropy (HA~7 Tesla). This gives the compound the potential to have high coercivity (i.e., resistance to being demagnetized). The compound also has a high saturation magnetization (Js ~1.6 T or 16 kG). Therefore, as the maximum energy density is proportional to Js2 this magnetic phase has the potential for storing large amounts of magnetic energy (BHmax ~ 512 kJ/m3 or 64 MGOe), considerably more than samarium cobalt (SmCo) magnets, which were the first type of rare earth magnet to be commercialized [1]. In practice, the magnetic properties of Neodymium magnets depend on the alloy composition, microstructure, and manufacturing technique employed.
History and manufacturing techniques
In 1982, General Motors corporation,Sumitomo Special Metals, and the China Academy of Science discovered the Nd2Fe14B compound. The effort was principally driven by the high material cost of the SmCo permanent magnets, which had been developed earlier. General Motors focused on the development of melt-spun nanocrystalline Nd2Fe14B magnets, while Sumitomo developed full density sintered Nd2Fe14B magnets. General Motors Corporation commercialized its inventions of isotropic Neo powder, bonded Neo magnets and the related production processes by founding Magnequench in 1986. Magnequench is now part of the Neo Materials Technology Inc. and supplies melt spun Nd2Fe14B powder to bonded magnet manufacturers. The Sumitomo facility has become part of the Hitachi corporation and currently manufactures and licenses other companies to produce sintered Nd2Fe14B magnets.
Production
There are two principal neodymium magnet manufacturing routes:
- The classical powder metallurgy or sintered magnet process
- The rapid solidification or bonded magnet process
Sintered Neo magnets are prepared by pulverizing an ingot precursor and liquid phase sintering the magnetically aligned powder into dense blocks which are then heat treated, cut to shape, surface treated and magnetized. Currently, between 45,000 and 50,000 tons of sintered neodymium magnets are produced each year, mainly from China and Japan.
Bonded Neo magnets are prepared by melt spinning a thin ribbon of the Nd-Fe-B 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. Bonded magnets offer less flux than sintered magnets but can be net-shape formed into intricately shaped parts and do not suffer significant eddy current losses. There are approximately 5,500 tons of Neo bonded magnets produced each year. In addition, it is possible to hot press the melt spun nanocrystalline particles into fully dense isotropic magnets, and then upset-forge/back-extrude these into high energy anisotropic magnets.
Properties
Magnetic properties
Some important properties used to compare permanent magnets are: remanence (Mr), which measures the strength of the magnetic field; coercivity (Hci), the material's resistance to becoming demagnetized; energy product (BHmax), the density of magnetic energy; and Curie temperature (TC), the temperature at which the material loses its magnetism. Neodymium magnets have higher remanence, much higher coercivity and energy product, but often lower Curie temperature than other types. Neodymium is alloyed with terbium and dysprosium in order to preserve its magnetic properties at high temperatures.[1] The table below compares the magnetic performance of neodymium magnets with other types of permanent magnets.
| Magnet | Mr (T) | Hci (kA/m) | BHmax (kJ/m3) | TC (°C) |
|---|---|---|---|---|
| Nd2Fe14B (sintered) | 1.0–1.4 | 750–2000 | 200–440 | 310–400 |
| Nd2Fe14B (bonded) | 0.6–0.7 | 600–1200 | 60–100 | 310–400 |
| SmCo5 (sintered) | 0.8–1.1 | 600–2000 | 120–200 | 720 |
| Sm(Co, Fe, Cu, Zr)7 (sintered) | 0.9–1.15 | 450–1300 | 150–240 | 800 |
| Alnico (sintered) | 0.6–1.4 | 275 | 10–88 | 700–860 |
| Sr-ferrite (sintered) | 0.2–0.4 | 100–300 | 10–40 | 450 |
Physical and mechanical properties
| Thermal conductivity | 7.7 kcal/(hm°C) |
| Young’s modulus | 1.7×104 Gg/m2 |
| Bending strength | 24 Gg/m2 |
| Compressive strength | 80 Gg/m2 |
| Electrical resistivity | 16 mΩm |
| Density | 7.4–7.5 Mg/m3 |
| Vickers hardness | 500–600 |
Applications
 Ring Magnets |
 Hard Disk Drive |
Permanent magnets have become indispensable components in modern technology (automotives, computers, telephones, medical equipment, music systems, etc.). They are found in all kinds of domestic and professional electromechanical and electronic devices. Some examples are
- head actuators for computer HDDs
- magnetic resonance imaging (MRI)
- magnetic guitar pickups
- magnetic bearings and couplings
- permanent magnet motors:
- servo motors
- lifting and compressor motors
- synchronous motors
- spindle and stepper motors
- electrical power steering
- drive motors for electric and hybrid vehicles (cars, bicycles)
- actuators
- consumer and office electrical/electronic devices such as CD and DVD players, mobile phones, digital cameras, clocks, watches, timer switches, speakers
The electric motors of each Toyota Prius require 1 kilogram (2.2 pounds) of neodymium.[2]
See also
- Lanthanide series of rare-earth magnets LnFeB
- Samarium-cobalt magnet
- Transition metal substitutions like NdCoB
References
- MMPA 0100-00, Standard Specifications for Permanent Magnet Materials
- K.H.J. Buscchow (1998) Permanent-Magnet Materials and their Applications, Trans Tech Publications Ltd., Switzerland, ISBN 0-87849-796-x
Further reading
- Campbell, Peter (1994). Permanent Magnet Materials and their Application. New York: Cambridge University Press. ISBN 0521249961.
- Furlani, Edward P. (2001). Permanent Magnet and Electromechanical Devices: Materials, Analysis and Applications. London: Academic Press. ISBN 0122699513.
- DN Brown, BM Ma, Z Chen, “Developments in the processing and properties of NdFeB-type permanent magnets.” Journal of Magnetism and Magnetic Materials 248 (2002) 432–440
- The Dependence of Magnetic Properties and Hot Workability of Rare Earth-Iron-Boride Magnets Upon Composition.
External links
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