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Neodymium magnet

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Nickel plated neodymium magnet on a bracket from a hard drive.
Nickel-plated neodymium magnet cubes
Left: High-resolution transmission electron microscopy image of Nd2Fe14B; right: crystal structure with unit cell marked

A neodymium magnet (also known as NdFeB, NIB or Neo magnet), the most widely used[1] 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.[2] Developed in 1982 by General Motors and Sumitomo Special Metals, neodymium magnets are the strongest type of permanent magnet made.[2][3] They have replaced other types of magnet in the many applications in modern products that require strong permanent magnets, such as motors in cordless tools, hard disk drives and magnetic fasteners.

Contents

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Description [edit]

The tetragonal Nd2Fe14B crystal structure has exceptionally high uniaxial magnetocrystalline anisotropy (HA~7 teslas). 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) and typically 1.3 teslas. 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 MG·Oe), considerably more than samarium cobalt (SmCo) magnets, which were the first type of rare earth magnet to be commercialized. In practice, the magnetic properties of neodymium magnets depend on the alloy composition, microstructure, and manufacturing technique employed.

History and manufacturing techniques [edit]

In 1982, General Motors (GM) and Sumitomo Special Metals discovered the Nd2Fe14B compound. The effort was principally driven by the high material cost of the SmCo permanent magnets, which had been developed earlier. GM focused on the development of melt-spun nanocrystalline Nd2Fe14B magnets, while Sumitomo developed full-density sintered Nd2Fe14B magnets. GM commercialized its inventions of isotropic Neo powder, bonded Neo magnets, and the related production processes by founding Magnequench in 1986. Magnequench, now part of the Neo Materials Technology, Inc., 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. Hitachi holds more than 600 patents covering Neodymium magnets.[4]

Sintered Nd2Fe14B tends to be vulnerable to corrosion. In particular, corrosion along grain boundaries may cause deterioration of a sintered magnet. This problem is addressed in many commercial products by adding a protective coating. Nickel plating or two-layered copper-nickel plating are the standard methods, although plating with other metals or polymer and lacquer protective coatings is also in use.[5]

Production [edit]

There are two principal neodymium magnet manufacturing routes:

  • The classical powder metallurgy or sintered magnet process
  • The rapid solidification or bonded 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 to tiny particles. This undergoes a process of liquid-phase sintering whereby the powder is magnetically aligned into dense blocks which are then heat-treated, cut to shape, surface treated and magnetized. Currently,[when?] between 45,000 and 50,000 tons of sintered neodymium magnets are produced each year, mainly in China and Japan. As of 2011[update], China produces more than 95% of rare earth elements, and produces 76% of the world's total rare earth magnets.[4]

Bonded Nd-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 [edit]

Neodymium magnets (small cylinders) lifting steel spheres. Such magnets can easily lift thousands of times their own weight.

Grades [edit]

Grades of Neodymium magnets N35-N52 33M-48M 30H-45H 30SH-42SH 30UH-35UH 28EH-35EH [6]

Magnetic properties [edit]

Some important properties used to compare permanent magnets are: remanence (Br), 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.[7] The table below compares the magnetic performance of neodymium magnets with other types of permanent magnets.

MagnetBr (T)Hci (kA/m)BHmax (kJ/m3)TC (°C)
Nd2Fe14B (sintered)1.0–1.4750–2000200–440310–400
Nd2Fe14B (bonded)0.6–0.7600–120060–100310–400
SmCo5 (sintered)0.8–1.1600–2000120–200720
Sm(Co, Fe, Cu, Zr)7 (sintered)0.9–1.15450–1300150–240800
Alnico (sintered)0.6–1.427510–88700–860
Sr-ferrite (sintered)0.2–0.4100–30010–40450

Physical and mechanical properties [edit]

Comparison of physical properties of sintered neodymium and Sm-Co magnets[8]
PropertyNeodymiumSm-Co
Remanence (T)1–1.30.82–1.16
Coercivity (MA/m)0.875–1.990.493–1.59
Relative permeability1.051.05
Temperature coefficient of remanence (%/K)−0.12−0.03
Temperature coefficient of coercivity (%/K)−0.55..–0.65−0.15..–0.30
Curie temperature (°C)320800
Density (g/cm3)7.3–7.58.2–8.4
CTE, magnetizing direction (1/K)5.2×10−65.2×10−6
CTE, normal to magnetizing direction (1/K)−0.8×10−611×10−6
Flexural strength (N/mm2)250150
Compressive strength (N/mm2)1100800
Tensile strength (N/mm2)7535
Vickers hardness (HV)550–650500–550
Electrical resistivity (Ω·cm)(110–170)×10−686×10−6

 

Applications [edit]

In technology [edit]

Ring magnets
Hard disk drive

Neodymium magnets have replaced alnico and ferrite magnets in many of the myriad applications in modern technology where strong permanent magnets are required, because their greater strength allows the use of smaller, lighter magnets for a given application. Some examples are:

Neodymium content is estimated to be 31% of magnet weight.[4]

Other applications [edit]

In addition, the greater strength of neodymium magnets has inspired new applications in areas where magnets were not used before, such as magnetic jewelry clasps, children's magnetic building sets (and other neodymium magnet toys) and as part of the closing mechanism of modern sport parachute equipment.[12] The strength and magnetic field homogeneity on neodymium magnets has also opened new applications in the medical field with the introduction of open magnetic resonance imaging (MRI) scanners used to image the body in radiology departments as an alternative to superconducting magnets that use a coil of superconducting wire to produce the magnetic field. They also are the main metal in the formerly popular, but now banned,[13] desk-toy magnets "Bucky Balls".

See also [edit]

References [edit]

  1. ^ "What is a Strong Magnet?". The Magnetic Matters Blog. Adams Magnetic Products. October 5, 2012. Retrieved October 12, 2012. 
  2. ^ a b Fraden, Jacob (2010). Handbook of Modern Sensors: Physics, Designs, and Applications, 4th Ed.. USA: Springer. p. 73. ISBN 1441964657. 
  3. ^ "What are neodymium magnets?". wiseGEEK website. Conjecture Corp. 2011. Retrieved October 12, 2012. 
  4. ^ a b c Chu, Steven. Critical Materials Strategy United States Department of Energy, December 2011. Accessed: 23 December 2011.
  5. ^ Drak, M.; Dobrzanski, L.A. (2007). "Corrosion of Nd-Fe-B permanent magnets". Journal of Achievements in Materials and Manufacturing Engineering 20 (1–2). 
  6. ^ http://www.china-magnet.net/neodymium-magnet/Grade%20of%20neodymium%20magnet.pdf, Neodymium Grads.
  7. ^ a b As hybrid cars gobble rare metals, shortage looms, Reuters, August 31, 2009.
  8. ^ Juha Pyrhönen, Tapani Jokinen, Valéria Hrabovcová (2009). Design of Rotating Electrical Machines. John Wiley and Sons. p. 232. ISBN 0-470-69516-1. 
  9. ^ Swain, Frank (March 6, 2009). "How to remove a finger with two super magnets". The Sciencepunk Blog. Seed Media Group LLC. Retrieved 2009-06-28. 
  10. ^ "CPSC Safety Alert: Ingested Magnets Can Cause Serious Intestinal Injuries". U.S. Consumer Product Safety Commission. Retrieved 13 December 2012. 
  11. ^ Constantinides, Steve (2011). "Rare Earth Materials Update – May, 2011". SMMA Motor and Motion Association conference 2011. Arnold Magnetic. Retrieved February 11, 2013. 
  12. ^ United Parachute Technologies Options Guide: http://www.unitedparachutetechnologies.com/index.php?option=com_content&task=view&id=22
  13. ^ http://usatoday30.usatoday.com/money/industries/retail/story/2012-07-25/buckyballs-ban/56481316/1

Further reading [edit]

External links [edit]