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 commercially available.[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.

Description[edit]

The tetragonal Nd2Fe14B crystal structure has exceptionally high uniaxial magnetocrystalline anisotropy (HA~7 teslas - magnetic field strength H in A/m versus magnetic moment in A.m2).[4] 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). This property is considerably higher in NdFeB alloys than in 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[edit]

In 1982, General Motors (GM) and Sumitomo Special Metals discovered the Nd2Fe14B compound. The research was initially driven by the high raw materials cost of 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 has since become part of Neo Materials Technology, Inc., which later merged into Molycorp). The company supplied melt-spun Nd2Fe14B powder to bonded magnet manufacturers.

The Sumitomo facility became 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.[5]

Chinese manufacturers have become a dominant force in neodymium magnet production, based on their control of much of the world's sources of rare earth ores.[6]

The United States Department of Energy has identified a need to find substitutes for rare earth metals in permanent magnet technology, and has begun funding such research. The Advanced Research Projects Agency-Energy has sponsored a Rare Earth Alternatives in Critical Technologies (REACT) program, to develop alternative materials. In 2011, ARPA-E awarded 31.6 million dollars to fund Rare-Earth Substitute projects.[7]

Production[edit]

There are two principal neodymium magnet manufacturing methods:

  • Classical powder metallurgy or sintered magnet process
  • 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, which then undergo a process of liquid-phase sintering in which the powder is magnetically aligned into dense blocks. The blocks are then heat-treated, cut to shape, surface treated and magnetized.

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.[8] China produces more than 95% of rare earth elements, and produces about 76% of the world's total rare-earth magnets.[5]

Bonded Nd-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. Bonded magnets offer less flux intensity than sintered magnets, but can be net-shape formed into intricately shaped parts, as is typical with Halbach arrays or arcs, trapezoids and other shapes and assemblies (e.g. Pot Magnets, Separator Grids, etc.),[9] and do not suffer significant eddy current losses in rapidly varying magnetic fields. 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 or 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]

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 and range from N35 up to N52. Letters following the grade indicate maximum operating temperatures (often the Curie temperature), which range from M (up to 100 degrees Celsius) to EH (200 degrees Celsius).[10]

Grades of Neodymium magnets:[11]

  • N35-N52
  • 33M-48M
  • 30H-45H
  • 30SH-42SH
  • 30UH-35UH
  • 28EH-35EH

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.[12] The table below compares the magnetic performance of neodymium magnets with other types of permanent magnets.

Magnet Br (T) Hci (kA/m) BHmax (kJ/m3) TC (°C) TC (°F)
Nd2Fe14B (sintered) 1.0–1.4 750–2000 200–440 310–400 590–752
Nd2Fe14B (bonded) 0.6–0.7 600–1200 60–100 310–400 590–752
SmCo5 (sintered) 0.8–1.1 600–2000 120–200 720 1328
Sm(Co, Fe, Cu, Zr)7 (sintered) 0.9–1.15 450–1300 150–240 800 1472
Alnico (sintered) 0.6–1.4 275 10–88 700–860 1292–1580
Sr-ferrite (sintered) 0.2–0.4 100–300 10–40 450 842

Physical and mechanical properties[edit]

Comparison of physical properties of sintered neodymium and Sm-Co magnets[13]
Property Neodymium Sm-Co
Remanence (T) 1–1.3 0.82–1.16
Coercivity (MA/m) 0.875–1.99 0.493–1.59
Relative permeability 1.05 1.05
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) 320 800
Density (g/cm3) 7.3–7.5 8.2–8.4
CTE, magnetizing direction (1/K) 5.2×10−6 5.2×10−6
CTE, normal to magnetizing direction (1/K) −0.8×10−6 11×10−6
Flexural strength (N/mm2) 250 150
Compressive strength (N/mm2) 1100 800
Tensile strength (N/mm2) 75 35
Vickers hardness (HV) 550–650 500–650
Electrical resistivity (Ω·cm) (110–170)×10−6 86×10−6

Corrosion problems[edit]

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.[14]

Hazards[edit]

The greater force exerted by rare-earth magnets creates hazards that are not seen with other types of magnet. Neodymium magnets larger than a few cubic centimeters are strong enough to cause injuries to body parts pinched between two magnets, or a magnet and a metal surface, even causing broken bones.[15]

Magnets allowed to get too near each other can strike each other with enough force to chip and shatter the brittle material, and the flying chips can cause injuries. There have even been cases where young children who have swallowed several magnets have had a fold of the digestive tract pinched between the magnets, causing injury or death.[16] The stronger magnetic fields can be hazardous to mechanical and electronic devices; they can erase magnetic media such as floppy disks and credit cards, and magnetize watches and the shadow masks of CRT type monitors at a greater distance than other types of magnet.

Applications[edit]

Existing magnet applications[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.[5]

New 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.[18] They also are the main metal in the formerly popular desk-toy magnets, "Buckyballs", though some retailers have chosen not to sell them due to child-safety concerns.[19]

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.[citation needed]

Neodymium magnets are used as a surgically placed anti-reflux system which is a band of magnets[20] surgically implanted around the lower esophagael sphincter to treat gastroesophageal reflux disease (GERD).[21]

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. ^ "Magnetic Anisotropy". Hitchhiker's Guide to Magnetism. Retrieved 2 March 2014. 
  5. ^ a b c Chu, Steven. Critical Materials Strategy United States Department of Energy, December 2011. Accessed: 23 December 2011.
  6. ^ Peter Robison and Gopal Ratnam (29 September 2010). "Pentagon Loses Control of Bombs to China Metal Monopoly". Bloomberg News. Retrieved 24 March 2014. 
  7. ^ "Research Funding for Rare Earth Free Permanent Magnets". ARPA-E. Retrieved 23 April 2013. 
  8. ^ "The Permanent Magnet Market - 2015" (PDF). Magnetics 2013 Conference. Magnetics 2013 Conference. February, 7-8, 2013. Retrieved November 28, 2013.  Check date values in: |date= (help)
  9. ^ "An Introduction to Neodymium Magnets". NdFeB-Info website. e-Magnets UK. Retrieved November 28, 2013. 
  10. ^ "Magnet Grade Chart". Magnet Grade Chart. Amazing Magnets, LLC. Retrieved December 4, 2013. 
  11. ^ http://www.china-magnet.net/neodymium-magnet/Grade%20of%20neodymium%20magnet.pdf, Neodymium Grades
  12. ^ a b As hybrid cars gobble rare metals, shortage looms, Reuters, August 31, 2009.
  13. ^ Juha Pyrhönen, Tapani Jokinen, Valéria Hrabovcová (2009). Design of Rotating Electrical Machines. John Wiley and Sons. p. 232. ISBN 0-470-69516-1. 
  14. ^ Drak, M.; Dobrzanski, L.A. (2007). "Corrosion of Nd-Fe-B permanent magnets". Journal of Achievements in Materials and Manufacturing Engineering 20 (1–2). 
  15. ^ 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. 
  16. ^ "CPSC Safety Alert: Ingested Magnets Can Cause Serious Intestinal Injuries". U.S. Consumer Product Safety Commission. Retrieved 13 December 2012. 
  17. ^ Constantinides, Steve (2011). "Rare Earth Materials Update – May, 2011". SMMA Motor and Motion Association conference 2011. Arnold Magnetic. Retrieved February 11, 2013. 
  18. ^ United Parachute Technologies Options Guide: http://www.unitedparachutetechnologies.com/index.php?option=com_content&task=view&id=22
  19. ^ http://usatoday30.usatoday.com/money/industries/retail/story/2012-07-25/buckyballs-ban/56481316/1
  20. ^ "TAVAC Safety and Effectiveness Analysis: LINX® Reflux Management System". 
  21. ^ "THE LINX REFLUX MANAGEMENT SYSTEM: STOP REFLUX AT ITS SOURCE". Torax Medical Inc. 

Further reading[edit]

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