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A neodymium magnet (also known as NdFeB, NIB or Neo magnet) is the most widely used type of rare-earth magnet.
It is a permanent magnet made from an alloy of neodymium,
iron
Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in ...
, and boron to form the Nd
2Fe
14B tetragonal crystal system, tetragonal crystalline structure.
Developed independently in 1984 by
General Motors
The General Motors Company (GM) is an American Multinational corporation, multinational Automotive industry, automotive manufacturing company headquartered in Detroit, Michigan, United States. It is the largest automaker in the United States and ...
and
Sumitomo Special Metals,
neodymium magnets are the strongest type of permanent magnet available commercially.
NdFeB magnets can be classified as sintered or bonded, depending on the manufacturing process used.
[Sintered NdFeB Magnets](_blank)
What are Sintered NdFeB Magnets?[Bonded NdFeB Magnets](_blank)
What are Bonded NdFeB Magnets? They have replaced other types of magnets in many applications in modern products that require strong permanent magnets, such as
electric motors in cordless tools,
hard disk drive
A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device that stores and retrieves digital data using magnetic storage with one or more rigid rapidly rotating platters coated with mag ...
s and magnetic fasteners.
History
General Motors (GM) and Sumitomo Special Metals independently discovered the Nd
2Fe
14B compound almost simultaneously in 1984.
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 Nd
2Fe
14B magnets, while Sumitomo developed full-density
sintered
Clinker nodules produced by sintering
Sintering or frittage is the process of compacting and forming a solid mass of material by pressure or heat without melting it to the point of liquefaction.
Sintering happens as part of a manufacturing ...
Nd
2Fe
14B magnets.
[ Chu, Steven]
Critical Materials Strategy
''United States Department of Energy
The United States Department of Energy (DOE) is an executive department of the U.S. federal government that oversees U.S. national energy policy and manages the research and development of nuclear power and nuclear weapons in the United States ...
'', December 2011. Accessed: 23 December 2011.
GM commercialized its inventions of
isotropic
Isotropy is uniformity in all orientations; it is derived . Precise definitions depend on the subject area. Exceptions, or inequalities, are frequently indicated by the prefix ' or ', hence '' anisotropy''. ''Anisotropy'' is also used to describ ...
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 Nd
2Fe
14B powder to bonded magnet manufacturers. The
Sumitomo
The is one of the largest Japanese ''keiretsu'', or business groups, founded by Masatomo Sumitomo (1585-1652) around 1615 during the early Edo period.
History
The Sumitomo Group traces its roots to a bookshop in Kyoto founded circa 1615 by Mas ...
facility became part of the
Hitachi Corporation, and has manufactured but also licensed other companies to produce sintered Nd
2Fe
14B magnets. Hitachi has held more than 600 patents covering neodymium magnets.
[ Chu, Steven]
Critical Materials Strategy
''United States Department of Energy
The United States Department of Energy (DOE) is an executive department of the U.S. federal government that oversees U.S. national energy policy and manages the research and development of nuclear power and nuclear weapons in the United States ...
'', December 2011. Accessed: 23 December 2011.
Chinese manufacturers have become a dominant force in neodymium magnet production, based on their control of much of the world's rare-earth mines.
The
United States Department of Energy
The United States Department of Energy (DOE) is an executive department of the U.S. federal government that oversees U.S. national energy policy and manages the research and development of nuclear power and nuclear weapons in the United States ...
has identified a need to find substitutes for rare-earth metals in permanent magnet technology and has funded 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. Because of its role in permanent magnets used for
wind turbines, it has been argued that neodymium will be one of the main objects of geopolitical competition in a world running on
renewable energy
Renewable energy is energy that is collected from renewable resources that are naturally replenished on a human timescale. It includes sources such as sunlight, wind, the movement of water, and geothermal heat. Although most renewable energy ...
. This perspective has been criticized for failing to recognize that most wind turbines do not use permanent magnets and for underestimating the power of economic incentives for expanded production.
Explanation of strength
In its pure form, neodymium has magnetic properties—specifically, it is
antiferromagnetic—but only at temperatures, below . Neodymium magnets are made from compounds of neodymium with
transition metal
In chemistry, a transition metal (or transition element) is a chemical element in the d-block of the periodic table (groups 3 to 12), though the elements of group 12 (and less often group 3) are sometimes excluded. They are the elements that can ...
s such as
iron
Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in ...
that are
ferromagnetic
Ferromagnetism is a property of certain materials (such as iron) which results in a large observed magnetic permeability, and in many cases a large magnetic coercivity allowing the material to form a permanent magnet. Ferromagnetic materials ...
, with
Curie temperatures well above room temperature.
The strength of neodymium magnets is the result of several factors. The most important is that the
tetragonal Nd
2Fe
14B crystal structure has exceptionally high uniaxial
magnetocrystalline anisotropy (''H''
A ≈ 7
T –
magnetic field strength H in units of A/m versus
magnetic moment
In electromagnetism, the magnetic moment is the magnetic strength and orientation of a magnet or other object that produces a magnetic field. Examples of objects that have magnetic moments include loops of electric current (such as electroma ...
in A·m
2).
This means a crystal of the material preferentially magnetizes along a specific
crystal axis but is very difficult to magnetize in other directions. Like other magnets, the neodymium magnet alloy is composed of
microcrystalline
A microcrystalline material is a crystallized substance or rock that contains small crystals visible only through microscopic examination. There is little agreement on the range of crystal sizes that should be regarded as microcrystalline, but the ...
grains which are aligned in a powerful magnetic field during manufacture so their magnetic axes all point in the same direction. The resistance of the crystal lattice to turning its direction of magnetization gives the compound a very high
coercivity
Coercivity, also called the magnetic coercivity, coercive field or coercive force, is a measure of the ability of a ferromagnetic material to withstand an external magnetic field without becoming demagnetized. Coercivity is usually measured in ...
, or resistance to being demagnetized.
The neodymium atom can have a large
magnetic dipole moment
In electromagnetism, the magnetic moment is the magnetic strength and orientation of a magnet or other object that produces a magnetic field. Examples of objects that have magnetic moments include loops of electric current (such as electromagnet ...
because it has 4
unpaired electrons in its electron structure
as opposed to (on average) 3 in iron. In a magnet it is the unpaired electrons, aligned so that their spin is in the same direction, which generate the magnetic field. This gives the Nd
2Fe
14B compound a high
saturation magnetization (''J''
s ≈ 1.6
T or 16
kG) and a remnant magnetization of typically 1.3 teslas. Therefore, as the maximum energy density is proportional to ''J''
s2, this magnetic phase has the potential for storing large amounts of magnetic energy (''BH''
max ≈ 512kJ/m
3 or 64
MG·Oe).
This magnetic energy value is about 18 times greater than "ordinary" ferrite magnets by volume and 12 times by mass. This magnetic energy property is 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.
The Nd
2Fe
14B crystal structure can be described as alternating layers of iron atoms and a neodymium-boron compound.
The
diamagnetic boron atoms do not contribute directly to the magnetism but improve cohesion by strong covalent bonding.
The relatively low rare earth content (12% by volume, 26.7% by mass) and the relative abundance of neodymium and iron compared with
samarium and
cobalt
Cobalt is a chemical element with the symbol Co and atomic number 27. As with nickel, cobalt is found in the Earth's crust only in a chemically combined form, save for small deposits found in alloys of natural meteoric iron. The free element, p ...
makes neodymium magnets lower in price than
samarium-cobalt magnets.
Properties
Grades
Neodymium magnets are graded according to their
maximum energy product, which relates to the
magnetic flux
In physics, specifically electromagnetism, the magnetic flux through a surface is the surface integral of the normal component of the magnetic field B over that surface. It is usually denoted or . The SI unit of magnetic flux is the weber ...
output per unit volume. Higher values indicate stronger magnets. For sintered NdFeB magnets, there is a widely recognized international classification. Their values range from 28 up to 52. The first letter N before the values is short for neodymium, meaning sintered NdFeB magnets. Letters following the values indicate intrinsic coercivity and maximum operating temperatures (positively correlated with the
Curie temperature), which range from default (up to ) to TH ().
[How to Understand the Grade of Sintered NdFeB Magnet?](_blank)
Grades of Sintered NdFeB Magnets
Grades of sintered NdFeB magnets:
["Grades of Neodymium magnets" (PDF)](_blank)
Everbeen Magnet. Retrieved December 6, 2015.
* N30 – N55
* N30M – N50M
* N30H – N50H
* N30SH – N48SH
* N30UH – N42UH
* N28EH – N40EH
* N28TH – N35TH
Magnetic properties
Some important properties used to compare permanent magnets are:
*
Remanence
Remanence or remanent magnetization or residual magnetism is the magnetization left behind in a ferromagnetic material (such as iron) after an external magnetic field is removed. Colloquially, when a magnet is "magnetized", it has remanence. The ...
(''B''
r), which measures the strength of the magnetic field.
*
Coercivity
Coercivity, also called the magnetic coercivity, coercive field or coercive force, is a measure of the ability of a ferromagnetic material to withstand an external magnetic field without becoming demagnetized. Coercivity is usually measured in ...
(''H''
ci), the material's resistance to becoming demagnetized.
*
Maximum energy product (''BH''
max), the density of magnetic energy, characterized by the maximum value of
magnetic flux density(B) times
magnetic field strength (H).
*
Curie temperature (''T''
C), 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 of magnets. Special neodymium magnet alloys that include
terbium and
dysprosium
Dysprosium is the chemical element with the symbol Dy and atomic number 66. It is a rare-earth element in the lanthanide series with a metallic silver luster. Dysprosium is never found in nature as a free element, though, like other lanthanide ...
have been developed that have higher Curie temperature, allowing them to tolerate higher temperatures.
[As hybrid cars gobble rare metals, shortage looms]
Reuters, August 31, 2009. The table below compares the magnetic performance of neodymium magnets with other types of permanent magnets.
Physical and mechanical properties
Corrosion problems
Sintered Nd
2Fe
14B tends to be vulnerable to
corrosion
Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engi ...
, especially along
grain boundaries
In materials science, a grain boundary is the interface between two grains, or crystallites, in a polycrystalline material. Grain boundaries are two-dimensional defects in the crystal structure, and tend to decrease the electrical and therma ...
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
Spall are fragments of a material that are broken off a larger solid body. It can be produced by a variety of mechanisms, including as a result of projectile impact, corrosion, weathering, cavitation, or excessive rolling pressure (as in a ba ...
of a surface layer.
This vulnerability is addressed in many commercial products by adding a protective coating to prevent exposure to the atmosphere. Nickel, nickel-copper-nickel and zinc platings are the standard methods, although plating with other metals, or polymer and lacquer protective coatings, are also in use.
Temperature effects
Neodymium has a negative coefficient, meaning the coercivity along with the magnetic energy density (''BH''
max) decreases with temperature. Neodymium-iron-boron magnets have high coercivity at room temperature, but as the temperature rises above , the coercivity decreases drastically until the Curie temperature (around ). This fall in coercivity limits the efficiency of the magnet under high-temperature conditions such as in wind turbines, hybrid motors, etc.
Dysprosium
Dysprosium is the chemical element with the symbol Dy and atomic number 66. It is a rare-earth element in the lanthanide series with a metallic silver luster. Dysprosium is never found in nature as a free element, though, like other lanthanide ...
(Dy) or
terbium (Tb) is added to curb the fall in performance from temperature changes, making the magnet even more expensive.
Hazards
The greater forces exerted by rare-earth magnets create hazards that may not occur 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 ferrous metal surface, even causing broken bones.
Magnets that get too near each other can strike each other with enough force to chip and shatter the brittle magnets, and the flying chips can cause various injuries, especially
eye injuries. There have even been cases where young children who have swallowed several magnets have had sections of the
digestive tract pinched between two magnets, causing injury or death. Also this could be a serious health risk if working with machines that have magnets in or attached to them.
The stronger magnetic fields can be hazardous to mechanical and electronic devices, as they can erase magnetic media such as
floppy disk
A floppy disk or floppy diskette (casually referred to as a floppy, or a diskette) is an obsolescent type of disk storage composed of a thin and flexible disk of a magnetic storage medium in a square or nearly square plastic enclosure lined ...
s and
credit card
A credit card is a payment card issued to users (cardholders) to enable the cardholder to pay a merchant for goods and services based on the cardholder's accrued debt (i.e., promise to the card issuer to pay them for the amounts plus the ...
s, and magnetize watches and the
shadow masks of
CRT type monitors at a greater distance than other types of magnet. In some cases, chipped magnets can act as a fire hazard as they come together, sending sparks flying as if they were a lighter
flint
Flint, occasionally flintstone, is a sedimentary cryptocrystalline form of the mineral quartz, categorized as the variety of chert that occurs in chalk or marly limestone. Flint was widely used historically to make stone tools and start ...
, because some neodymium magnets contain
ferrocerium
Ferrocerium (also known in Europe as Auermetall) is a synthetic pyrophoric alloy of mischmetal (cerium, lanthanum, neodymium, other trace lanthanides and some iron – about 95% lanthanides and 5% iron) hardened by blending in oxides ...
.
Production
There are two principal neodymium magnet manufacturing methods:
* Classical powder metallurgy or
sintered
Clinker nodules produced by sintering
Sintering or frittage is the process of compacting and forming a solid mass of material by pressure or heat without melting it to the point of liquefaction.
Sintering happens as part of a manufacturing ...
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; the powder is then sintered into dense blocks. The blocks are then heat-treated, cut to shape, surface treated and magnetized.
* Rapid solidification or bonded magnet process
**Bonded Nd-magnets are prepared by
melt spinning a thin ribbon of the NdFeB alloy. The ribbon contains randomly oriented Nd
2Fe
14B nano-scale grains. This ribbon is then pulverized into particles, mixed with a
polymer
A polymer (; Greek '' poly-'', "many" + '' -mer'', "part")
is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...
, and either
compression- or
injection-molded into bonded magnets.
In 2015,
Nitto Denko Corporation of Japan announced their development of a new method of sintering neodymium magnet material. The method exploits an "organic/inorganic hybrid technology" to form a clay-like mixture that can be fashioned into various shapes for sintering. Most importantly, it is said to be possible to control a non-uniform orientation of the magnetic field in the sintered material to locally concentrate the field to, e.g., improve the performance of electric motors. Mass production is planned for 2017.
As of 2012, 50,000
tons
Tons can refer to:
* Tons River, a major river in India
* Tamsa River, locally called Tons in its lower parts (Allahabad district, Uttar pradesh, India).
* the plural of ton, a unit of mass, force, volume, energy or power
:* short ton, 2,000 poun ...
of neodymium magnets are produced officially each year in China, and 80,000tons in a "company-by-company" build-up done in 2013.
China produces more than 95% of rare earth elements and produces about 76% of the world's total rare-earth magnets, as well as most of the world's neodymium.
[
]
Applications
Existing magnet 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 for a given application. Some examples are:
* Head actuators for computer hard disk
A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device that stores and retrieves digital data using magnetic storage with one or more rigid rapidly rotating platters coated with magn ...
s
* Mechanical e-cigarette
An electronic cigarette is an electronic device that simulates tobacco smoking. It consists of an atomizer, a power source such as a battery, and a container such as a cartridge or tank. Instead of smoke, the user inhales vapor. As such ...
firing switches
* Locks for doors
* Loudspeaker
A loudspeaker (commonly referred to as a speaker or speaker driver) is an electroacoustic transducer that converts an electrical audio signal into a corresponding sound. A ''speaker system'', also often simply referred to as a "speaker" or ...
s and headphone
Headphones are a pair of small loudspeaker drivers worn on or around the head over a user's ears. They are electroacoustic transducers, which convert an electrical signal to a corresponding sound. Headphones let a single user listen to an a ...
s
* Mobile phon
speakers, taptic feedback and auto focus actuators
* Magnetic bearings and couplings
* Benchtop NMR spectrometers
* Electric motors
** Cordless tools
** Servomotors
** Lifting and compressor
A compressor is a mechanical device that increases the pressure of a gas by reducing its volume. An air compressor is a specific type of gas compressor.
Compressors are similar to pumps: both increase the pressure on a fluid and both can tr ...
motors
** Synchronous motors
** Spindle and stepper motors
** Electrical power steering
A power steering is a mechanical device equipped on a motor vehicle that helps drivers steer the vehicle by reducing steering effort needed to turn the steering wheel, making it easier for the vehicle to turn or maneuver at lower speeds.
Hydra ...
** Drive motors for hybrid and electric vehicle
An electric vehicle (EV) is a vehicle that uses one or more electric motors for propulsion. It can be powered by a collector system, with electricity from extravehicular sources, or it can be powered autonomously by a battery (sometimes ch ...
s. The electric motors of each Toyota Prius
The is a car built by Toyota which has a hybrid drivetrain, combining an internal combustion engine with an electric motor. Initially offered as a four-door sedan, it has been produced only as a five-door liftback since 2003.
In 2007, ...
require of neodymium.[
** Actuators
* ]Electric generator
In electricity generation, a generator is a device that converts motive power ( mechanical energy) or fuel-based power (chemical energy) into electric power for use in an external circuit. Sources of mechanical energy include steam turbines, g ...
s for wind turbine
A wind turbine is a device that converts the kinetic energy of wind into electrical energy. Hundreds of thousands of large turbines, in installations known as wind farms, now generate over 650 gigawatts of power, with 60 GW added each year. ...
s (only those with permanent magnet excitation)
* Voice coil
* Retail media case decouplers
* In process industries, powerful neodymium magnets are used to catch foreign bodies and protect product and processes
New applications
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
Neodymium magnets, usually small spheres, have been manufactured as educational toys, stress relief products, and an artistic medium.
In the United States, as a result of an estimated 2,900 emergency room visits between 2009 and 2013 due t ...
) and as part of the closing mechanism of modern sport parachute equipment. They are the main metal in the formerly popular desk-toy magnets, "Buckyballs" and "Buckycubes", though some U.S. retailers have chosen not to sell them because of child-safety concerns, and they have been banned in Canada for the same reason.
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
Magnetic resonance imaging (MRI) is a medical imaging technique used in radiology to form pictures of the anatomy and the physiological processes of the body. MRI scanners use strong magnetic fields, magnetic field gradients, and radio wave ...
(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.
Neodymium magnets are used as a surgically placed anti-reflux system which is a band of magnets surgically implanted around the lower esophageal sphincter to treat gastroesophageal reflux disease (GERD). They have also been implanted in the fingertips in order to provide sensory perception
Perception () is the organization, identification, and interpretation of sensory information in order to represent and understand the presented information or environment. All perception involves signals that go through the nervous system ...
of magnetic fields, though this is an experimental procedure only popular among biohackers and grinders.[I.Harrison, K.Warwick and V.Ruiz (2018), "Subdermal Magnetic Implants: An Experimental Study", Cybernetics and Systems, 49(2), 122-150.]
See also
*
*
*
*
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,
*
*
*
The Dependence of Magnetic Properties and Hot Workability of Rare Earth-Iron-Boride Magnets Upon Composition
External links
* {{cite web, last=Bowley, first=Roger, title=More with Magnets, url=http://www.sixtysymbols.com/videos/magnet_pipe.htm, website=Sixty Symbols, publisher= Brady Haran for the University of Nottingham
, mottoeng = A city is built on wisdom
, established = 1798 – teacher training college1881 – University College Nottingham1948 – university status
, type = Public
, chancellor ...
Geeky Rare-Earth Magnets Repel Sharks
What Are Neodymium Magnets?
Ferromagnetic materials
Loudspeaker technology
Magnetic alloys
Magnetic levitation
Rare earth alloys
Types of magnets
Borides
Neodymium compounds
Ferrous alloys