A
neodymium magnet the most widely-used type of
rare-earth magnetRare-earth magnets are strong permanent magnets made from alloys of rare earth elements. Developed in the 1970s and 80s, rare-earth magnets are the strongest type of permanent magnets made and have significant performance advantages over ferrite or alnico magnets...
, is a
permanent magnetA magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, and attracts or repels other magnets.A permanent magnet is an object...
made from an
alloyAn alloy is a mixture or metallic solid solution composed of two or more elements. Complete solid solution alloys give single solid phase microstructure, while partial solutions give two or more phases that may or may not be homogeneous in distribution, depending on thermal history...
of
neodymiumNeodymium is a chemical element with the symbol Nd and atomic number 60. It is a soft silvery metal that tarnishes in air. Neodymium was discovered in 1885 by the Austrian chemist Carl Auer von Welsbach. It is present in significant quantities in the ore minerals monazite and bastnäsite...
,
ironIron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
, and
boronBoron is the chemical element with atomic number 5 and the chemical symbol B. Boron is a metalloid. Because boron is not produced by stellar nucleosynthesis, it is a low-abundance element in both the solar system and the Earth's crust. However, boron is concentrated on Earth by the...
to form the Nd
2Fe
14B
tetragonalIn crystallography, the tetragonal crystal system is one of the 7 lattice point groups. Tetragonal crystal lattices result from stretching a cubic lattice along one of its lattice vectors, so that the cube becomes a rectangular prism with a square base and height .There are two tetragonal Bravais...
crystalline structure. Developed in 1982 by
General MotorsGeneral Motors Company , commonly known as GM, formerly incorporated as General Motors Corporation, is an American multinational automotive corporation headquartered in Detroit, Michigan and the world's second-largest automaker in 2010...
and
Sumitomo Special Metalsis a company with seat in Osaka, Japan. )It is the third largest integrated steel manufacturer in Japan withthree integrated steelworks and several other manufacturing plants.It is one of the largest manufacturers of Seamless Pipes and Tubes,...
, 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
cordlessThe term cordless literally means without a cord and is generally used to refer to electrical or electronic devices that are powered by a battery or battery pack and can operate without a power cord or cable attached to a fixed electricity supply such as an outlet, generator, or other centralized...
tools,
hard diskA hard disk drive is a non-volatile, random access digital magnetic data storage device. It features rotating rigid platters on a motor-driven spindle within a protective enclosure. Data is magnetically read from and written to the platter by read/write heads that float on a film of air above the...
drives, and magnetic fasteners.
Description
The tetragonal Nd
2Fe
14B crystal structure has exceptionally high uniaxial
magnetocrystalline anisotropyMagnetic anisotropy is the direction dependence of a material's magnetic properties. In the absence of an applied magnetic field, a magnetically isotropic material has no preferential direction for its magnetic moment while a magnetically anisotropic material will align its moment with one of the...
(HA~7
teslasThe tesla is the SI derived unit of magnetic field B . One tesla is equal to one weber per square meter, and it was defined in 1960 in honour of the inventor, physicist, and electrical engineer Nikola Tesla...
). This gives the compound the potential to have high
coercivityIn materials science, the coercivity, also called the coercive field or coercive force, of a ferromagnetic material is the intensity of the applied magnetic field required to reduce the magnetization of that material to zero after the magnetization of the sample has been driven to saturation...
(i.e., resistance to being demagnetized). The compound also has a high saturation magnetization (
Js ~1.6
TThe tesla is the SI derived unit of magnetic field B . One tesla is equal to one weber per square meter, and it was defined in 1960 in honour of the inventor, physicist, and electrical engineer Nikola Tesla...
or 16
kGThe gauss, abbreviated as G, is the cgs unit of measurement of a magnetic field B , named after the German mathematician and physicist Carl Friedrich Gauss. One gauss is defined as one maxwell per square centimeter; it equals 1 tesla...
) and typically 1.3 tesla. 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 cobaltA samarium–cobalt magnet, a type of rare earth magnet, is a strong permanent magnet made of an alloy of samarium and cobalt. They were developed in the early 1970s. They are generally the second-strongest type of magnet made, less strong than neodymium magnets, but have higher temperature ratings...
(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
In 1982,
General MotorsGeneral Motors Company , commonly known as GM, formerly incorporated as General Motors Corporation, is an American multinational automotive corporation headquartered in Detroit, Michigan and the world's second-largest automaker in 2010...
and
Sumitomo Special Metalsis a company with seat in Osaka, Japan. )It is the third largest integrated steel manufacturer in Japan withthree integrated steelworks and several other manufacturing plants.It is one of the largest manufacturers of Seamless Pipes and Tubes,...
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
sinteredSintering is a method used to create objects from powders. It is based on atomic diffusion. Diffusion occurs in any material above absolute zero, but it occurs much faster at higher temperatures. In most sintering processes, the powdered material is held in a mold and then heated to a temperature...
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 spunMelt spinning is a technique used for rapid cooling of liquids. A wheel is cooled internally, usually by water or liquid nitrogen, and rotated. A thin stream of liquid is then dripped onto the wheel and cooled, causing rapid solidification. This technique is used to develop materials that require...
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 Nd
2Fe
14B 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 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, between 45,000 and 50,000
tonsThe short ton is a unit of mass equal to . In the United States it is often called simply ton without distinguishing it from the metric ton or the long ton ; rather, the other two are specifically noted. There are, however, some U.S...
of sintered neodymium magnets are produced each year, mainly in China and Japan.
Bonded Nd-magnets are prepared by
melt spinningMelt spinning is a technique used for rapid cooling of liquids. A wheel is cooled internally, usually by water or liquid nitrogen, and rotated. A thin stream of liquid is then dripped onto the wheel and cooled, causing rapid solidification. This technique is used to develop materials that require...
a thin ribbon of the Nd-Fe-B alloy. The ribbon contains randomly oriented Nd
2Fe
14B nano-scale grains. This ribbon is then pulverized into particles, mixed with a
polymerA polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
and either compression or injection molded into bonded magnets. Bonded magnets offer less flux than do sintered magnets but can be net-shape formed into intricately shaped parts and do not suffer significant
eddy currentEddy currents are electric currents induced in conductors when a conductor is exposed to a changing magnetic field; due to relative motion of the field source and conductor or due to variations of the field with time. This can cause a circulating flow of electrons, or current, within the body of...
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.
Magnetic properties
Some important properties used to compare permanent magnets are:
remanenceRemanence or remanent magnetization is the magnetization left behind in a ferromagnetic material after an external magnetic field is removed. It is also the measure of that magnetization. Colloquially, when a magnet is "magnetized" it has remanence...
(M
r), which measures the strength of the magnetic field;
coercivityIn materials science, the coercivity, also called the coercive field or coercive force, of a ferromagnetic material is the intensity of the applied magnetic field required to reduce the magnetization of that material to zero after the magnetization of the sample has been driven to saturation...
(
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
terbiumTerbium is a chemical element with the symbol Tb and atomic number 65. It is a silvery-white rare earth metal that is malleable, ductile and soft enough to be cut with a knife...
and
dysprosiumDysprosium is a chemical element with the symbol Dy and atomic number 66. It is a rare earth element with a metallic silver luster. Dysprosium is never found in nature as a free element, though it is found in various minerals, such as xenotime...
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.
| Magnet | MrRemanence or remanent magnetization is the magnetization left behind in a ferromagnetic material after an external magnetic field is removed. It is also the measure of that magnetization. Colloquially, when a magnet is "magnetized" it has remanence...
(T) | HciIn materials science, the coercivity, also called the coercive field or coercive force, of a ferromagnetic material is the intensity of the applied magnetic field required to reduce the magnetization of that material to zero after the magnetization of the sample has been driven to saturation...
(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
Comparison of physical properties of sintered neodymium and Sm-Co magnets
| Property | Neodymium | Sm-Co |
RemanenceRemanence or remanent magnetization is the magnetization left behind in a ferromagnetic material after an external magnetic field is removed. It is also the measure of that magnetization. Colloquially, when a magnet is "magnetized" it has remanence...
(T) |
1–1.3 |
0.82–1.16 |
CoercivityIn materials science, the coercivity, also called the coercive field or coercive force, of a ferromagnetic material is the intensity of the applied magnetic field required to reduce the magnetization of that material to zero after the magnetization of the sample has been driven to saturation...
(MA/m) |
0.875–1.99 |
0.493–1.59 |
| Permeability In electromagnetism, permeability is the measure of the ability of a material to support the formation of a magnetic field within itself. In other words, it is the degree of magnetization that a material obtains in response to an applied magnetic field. Magnetic permeability is typically...
|
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 Flexural strength, also known as modulus of rupture, bend strength, or fracture strength, a mechanical parameter for brittle material, is defined as a material's ability to resist deformation under load...
(N/mm2) |
250 |
150 |
Compressive strengthCompressive strength is the capacity of a material or structure to withstand axially directed pushing forces. When the limit of compressive strength is reached, materials are crushed. Concrete can be made to have high compressive strength, e.g...
(N/mm2) |
1100 |
800 |
Tensile strengthUltimate tensile strength , often shortened to tensile strength or ultimate strength, is the maximum stress that a material can withstand while being stretched or pulled before necking, which is when the specimen's cross-section starts to significantly contract...
(N/mm2) |
75 |
35 |
| Vickers hardness (HV) |
550–650 |
500–550 |
Electrical resistivityElectrical resistivity is a measure of how strongly a material opposes the flow of electric current. A low resistivity indicates a material that readily allows the movement of electric charge. The SI unit of electrical resistivity is the ohm metre...
(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. If one accidentally becomes pinched between two magnets, or between a magnet and some magnetic material, such as steel, the natural reaction is to pull free. However, doing so makes the space between the magnets become thinner, and the pinch force increases dangerously. If possible, one should bear the pain and do whatever one can to keep the distance from closing up; only then should one pull free. Professionals who work with such magnets might include brass wedges as part of their tools.
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 tractThe human gastrointestinal tract refers to the stomach and intestine, and sometimes to all the structures from the mouth to the anus. ....
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 diskA floppy disk is a disk storage medium composed of a disk of thin and flexible magnetic storage medium, sealed in a rectangular plastic carrier lined with fabric that removes dust particles...
s and credit cards, and magnetize watches and other clockwork mechanisms and the
shadow maskThe shadow mask is one of two major technologies used to manufacture cathode ray tube televisions and computer displays that produce color images. The other approach is aperture grille, better known by its trade name, Trinitron. All early color televisions and the majority of CRT computer monitors...
s of
CRTThe cathode ray tube is a vacuum tube containing an electron gun and a fluorescent screen used to view images. It has a means to accelerate and deflect the electron beam onto the fluorescent screen to create the images. The image may represent electrical waveforms , pictures , radar targets and...
type monitors at a significant distance.
In technology
Neodymium magnets have replaced
alnicoAlnico is an acronym referring to iron alloys which in addition to iron are composed primarily of aluminium , nickel and cobalt , hence al-ni-co, with the addition of copper, and sometimes titanium. Alnico alloys are ferromagnetic, with a high coercivity and are used to make permanent magnets...
and
ferrite magnetsFerrites are chemical compounds consisting of ceramic materials with iron oxide as their principal component. Many of them are magnetic materials and they are used to make permanent magnets, ferrite cores for transformers, and in various other applications.Many ferrites are spinels with the...
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
- head actuators for computer hard disk
A hard disk drive is a non-volatile, random access digital magnetic data storage device. It features rotating rigid platters on a motor-driven spindle within a protective enclosure. Data is magnetically read from and written to the platter by read/write heads that float on a film of air above the...
s
- magnetic resonance imaging
Magnetic resonance imaging , nuclear magnetic resonance imaging , or magnetic resonance tomography is a medical imaging technique used in radiology to visualize detailed internal structures...
(MRIMagnetic resonance imaging , nuclear magnetic resonance imaging , or magnetic resonance tomography is a medical imaging technique used in radiology to visualize detailed internal structures...
)
- magnetic guitar pickups
- loudspeaker
A loudspeaker is an electroacoustic transducer that produces sound in response to an electrical audio signal input. Non-electrical loudspeakers were developed as accessories to telephone systems, but electronic amplification by vacuum tube made loudspeakers more generally useful...
s and headphones
- magnetic bearing
A magnetic bearing is a bearing which supports a load using magnetic levitation. Magnetic bearings support moving machinery without physical contact; for example, they can levitate a rotating shaft and permit relative motion with very low friction and no mechanical wear...
s and couplings
- electric motors:
- cordless tools
- servo motor
thumb|right|200px|Industrial servomotorThe grey/green cylinder is the [[Brush |brush-type]] [[DC motor]]. The black section at the bottom contains the [[Epicyclic gearing|planetary]] [[Reduction drive|reduction gear]], and the black object on top of the motor is the optical [[rotary encoder]] for...
s
- lifting and compressor
A gas compressor is a mechanical device that increases the pressure of a gas by reducing its volume.Compressors are similar to pumps: both increase the pressure on a fluid and both can transport the fluid through a pipe. As gases are compressible, the compressor also reduces the volume of a gas...
motors
- synchronous motor
A synchronous electric motor is an AC motor distinguished by a rotor spinning with coils passing magnets at the same rate as the power supply frequency and resulting rotating magnetic field which drives it....
s
- spindle and stepper motor
A stepper motor is a brushless, electric motor that can divide a full rotation into a large number of steps. The motor's position can be controlled precisely without any feedback mechanism , as long as the motor is carefully sized to the application...
s
- electrical power steering
Power steering helps drivers steer vehicles by augmenting steering effort of the steering wheel.Hydraulic or electric actuators add controlled energy to the steering mechanism, so the driver needs to provide only modest effort regardless of conditions. Power steering helps considerably when a...
- drive motors for hybrid
A hybrid vehicle is a vehicle that uses two or more distinct power sources to move the vehicle. The term most commonly refers to hybrid electric vehicles , which combine an internal combustion engine and one or more electric motors.-Power:...
and electric vehicleAn electric vehicle , also referred to as an electric drive vehicle, uses one or more electric motors or traction motors for propulsion...
s. The electric motors of each Toyota PriusThe Toyota Prius is a full hybrid electric mid-size hatchback, formerly a compact sedan developed and manufactured by the Toyota Motor Corporation...
require 1 kilogram (2.2 pounds) of neodymium.
- actuators
Other applications
In addition, the greater strength of neodymium magnets has inspired a new applications in areas where magnets were not used before, such as magnetic jewelry clasps, children's magnetic building sets (and other
neodymium magnet toysNeodymium magnet toys are toys consisting of or incorporating neodymium magnets. Most commonly they are magnetic construction sets or magnetic sphere building toys. They are sometimes marketed as executive toys. The first neodymium magnet toy set was Neocube, developed by Chris Reda from...
) and as part of the closing mechanism of modern sport parachute equipment.
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 imagingMagnetic resonance imaging , nuclear magnetic resonance imaging , or magnetic resonance tomography is a medical imaging technique used in radiology to visualize detailed internal structures...
(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. As with most solid-based magnets, the magnetic field gradient of neodymium magnets decreases towards the centers of their surfaces, thus there is a force that attracts metallic objects to the edges.
See also
- Lanthanide
The lanthanide or lanthanoid series comprises the fifteen metallic chemical elements with atomic numbers 57 through 71, from lanthanum through lutetium...
metals
- Neodymium magnet toys
Neodymium magnet toys are toys consisting of or incorporating neodymium magnets. Most commonly they are magnetic construction sets or magnetic sphere building toys. They are sometimes marketed as executive toys. The first neodymium magnet toy set was Neocube, developed by Chris Reda from...
- Samarium-cobalt magnet
A samarium–cobalt magnet, a type of rare earth magnet, is a strong permanent magnet made of an alloy of samarium and cobalt. They were developed in the early 1970s. They are generally the second-strongest type of magnet made, less strong than neodymium magnets, but have higher temperature ratings...
- Transition metal
The term transition metal has two possible meanings:*The IUPAC definition states that a transition metal is "an element whose atom has an incomplete d sub-shell, or which can give rise to cations with an incomplete d sub-shell." Group 12 elements are not transition metals in this definition.*Some...
substitutions like NdCoB
Further reading
External links