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Nd2Fe14B magnet

Neodymium magnet

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Nickel plated neodymium magnet on a bracket from a hard drive.

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 known type of permanent magnet.

 Description

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). 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 [1]. In practice, the magnetic properties of neodymium magnets depend on the alloy composition, microstructure, and manufacturing technique employed.

[edit] History and manufacturing techniques

In 1982, General Motors Corporation 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. 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.

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 providing a protective coating. Nickel plating or two layered copper nickel plating is used as a standard method, although plating with other metals or polymer and lacquer protective coatings are also in use.[1]

 Production

There are two principal neodymium magnet manufacturing routes:

  1. The classical powder metallurgy or sintered magnet process
  2. 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.[2] 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 160 mΩ·cm
Density 7.4–7.5 Mg/m3
Vickers hardness 500–600

 Hazards

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 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.[3] 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 that have swallowed several magnets have had a fold of the digestive tract pinched between the magnets, causing injury or death.[4] The stronger magnetic fields can be hazardous also, and can erase magnetic media such as hard disks and credit cards, and magnetize the shadow masks of CRT type monitors at a significant distance.

 Applications

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. Some examples are

  • head actuators for computer hard disks
  • magnetic resonance imaging (MRI)
  • magnetic guitar pickups
  • loudspeakers and headphones
  • magnetic bearings and couplings
  • permanent magnet motors:
    • cordless tools
    • servo motors
    • lifting and compressor motors
    • synchronous motors
    • spindle and stepper motors
    • electrical power steering
    • drive motors for hybrid and electric vehicles. The electric motors of each Toyota Prius require 1 kilogram (2.2 pounds) of neodymium.[2]
    • actuators

In addition, the greater strength of neodymium magnets has inspired a few new applications in areas where magnets weren't used before, such as magnetic jewelry clasps and children's magnetic building sets.

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