plated neodymium magnet on a bracket from a
hard drive.]]
A
neodymium magnet (also known as
NdFeB,
NIB, or
Neo magnet), the most widely-used type of
rare-earth magnet, is a
permanent magnet made from an
alloy of
neodymium,
iron, and
boron to form the Nd
2Fe
14B
tetragonal crystalline structure. Developed in 1982 by
General Motors and
Sumitomo Special Metals, neodymium magnets are the strongest type of permanent magnet made. 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
The
tetragonal Nd
2Fe
14B 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/m
3 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.
History and manufacturing techniques
In 1982,
General Motors Corporation and
Sumitomo Special Metals discovered the Nd
2Fe
14B 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 Nd
2Fe
14B magnets, while Sumitomo developed full density
sintered Nd
2Fe
14B 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 Nd
2Fe
14B 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 Nd
2Fe
14B 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.
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 (M
r), 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. The table below compares the magnetic performance of neodymium magnets with other types of permanent magnets.
{| class="wikitable" border="1"
!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
{| class="wikitable"
|+ Comparison of physical properties of sintered neodymium and Sm-Co magnets
!Property !! Neodymium !! Sm-Co
|-
|
Remanence (T) || 1–1.3 || 0.82–1.16
|-
|
Coercivity (MA/m) ||0.875–1.99 ||0.493–1.59
|-
|
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/cm
3) ||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/mm
2) ||250 ||150
|-
|
Compressive strength (N/mm
2) ||1100 ||800
|-
|
Tensile strength (N/mm
2) ||75 ||35
|-
|
Vickers hardness (HV) ||550–650 ||500–550
|-
|Electrical
resistivity (Ω·cm) ||(110–170)×10
–6 || 86×10
–6
|}
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. 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. The stronger magnetic fields can be hazardous to mechanical and electronic devices, as they can erase magnetic media such as
floppy disks and credit cards, and magnetize watches and other clockwork mechanisms and 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.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.
See also
Lanthanide series of rare-earth magnets LnFeB
Neodymium magnet toys
Samarium-cobalt magnet
Transition metal substitutions like NdCoB
References
Further reading
MMPA 0100-00, Standard Specifications for Permanent Magnet Materials
K.H.J. Buschow (1998) Permanent-Magnet Materials and their Applications, Trans Tech Publications Ltd., Switzerland, ISBN 0-87849-796-x
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
Geeky Rare-Earth Magnets Repel Sharks, Genevieve Rajewski, 05.15.07 , wired.com
Concern as China clamps down on rare earth exports, Cahal Milmo, 01.02.10, independent.co.uk
Category:Magnetic alloys
Category:Types of magnets
Category:Ferromagnetic materials
Category:Loudspeaker technology
![Largest neodymium magnet in the world? [HD]](http://web.archive.org./web/20110210134608im_/http://i.ytimg.com/vi/XVv8qWZ8xzw/2.jpg "Largest neodymium magnet in the world? [HD]")
