Neodymium magnet

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Neodymium magnet on a bracket from a hard drive.

A neodymium magnet or NIB magnet, a variety of rare-earth magnet, is a permanent magnet made of an alloy of neodymium, iron, and boron — Nd2Fe14B. They are the strongest type of permanent magnets made.


[edit] Description

Neodymium magnets are very strong relative to their mass, but are also mechanically fragile. Like other ferromagnetic materials, neodymium magnets lose their magnetism above a temperature known as the Curie point. The most powerful grades have a maximum working temperature at a relatively low temperature: below 80 degrees Celsius (176 degrees Fahrenheit). High-temperature grades will operate at up to 200 and even 230°C but their strength is only marginally greater than that of a samarium-cobalt magnet.

As of 2008 neodymium magnets cost about $44/kg, $1.40 per BHmax.[1]

Most neodymium magnets are anisotropic, and hence can only be magnetised along one direction although B10N material is isotropic. During manufacture fields of 30-40 kOe are required to saturate the material.[2]

Neodymium magnets have a coercivity (required demagnetisation field from saturation) of about 10,000-12,000 Oersted.

[edit] Range of strength

Neodymium magnets (or “neo” as they are known in the industry) are graded in strength from N24 to the strongest, N55. The theoretical limit for neodymium magnets is grade N64. The number after the N represents the magnetic energy product, in megagauss-oersteds (MGOe) (1 MG·Oe = 7,958·10³ T·A/m = 7,958 kJ/m³). N48 has a remnant static magnetic field of 1.38 teslas and an H (magnetic field intensity) of 13,800 Oersteds (1.098 MA/m). By volume, one requires about 18 times as much ceramic magnetic material for the equivalent magnet lifting strength, and about 3 to 5 times as much for the equivalent dipole moment.

A neodymium magnet holding up 1300 times its own weight

[edit] Uses

They have replaced marginally weaker and significantly more heat-resistant samarium-cobalt magnets in most applications, due mainly to their lower cost.

Used for stabilization and angular head motors in computer hard drives, neodymium magnets are also popular with hobbyists, and a small magnet can have amazing properties — it exhibits magnetic braking when moved near a non-magnetic metal due to induced eddy currents. An excellent demonstration for students to see the effects of Lenz's Law in non-ferrous metals may be performed by dropping a strong neodymium magnet through a copper pipe. The magnet will travel through the pipe remarkably slowly as it falls. The effect may be greatly enhanced by immersing the pipe in liquid nitrogen (thus increasing its conductivity even further) prior to dropping the magnet through.

A somewhat larger magnet interacts strongly enough with the magnetic field of the Earth to allow its tendency to align with that field to be perceived directly when holding it, essentially forming a compass. Cylinder- and disc-shaped neodymium magnets are especially responsive to the Earth's magnetic fields. Neodymium magnets are used for the transducers in many headphones. Neodymium magnets are becoming increasingly common in loudspeakers for high-volume sound reinforcement applications.

A toy containing dozens of NIB magnets.

[edit] Toys

As NIB magnets produced in China have become less expensive in the last few years, the toy industry has used millions of them in magnetic building sets and other products including magnetic jewelry. Geomag manufactures a popular line of toys containing neodymium magnets. The small cylindrical magnets are used at the ends of corners of plastic pieces in order to allow connections of multiple pieces. The Magnetix brand was the subject of a March, 2006 recall notice by the Consumer Product Safety Commission as well as numerous consumer lawsuits due to product safety concerns. In defective kits the NIB magnets became dislodged from their plastic housing, and many children of varying ages consumed the small magnets; see health hazards below. Not all neodymium toys have been taken off of the market, some, like the NeoCube, have added more stringent warning labels.[3]

[edit] Music Industry

Neodymium magnets are now used in the music industry. Some manufacturers are using them as speaker magnets due to their strong magnetic field, stability, and weight. This makes for equipment that is lighter and more portable.

Neodymium magnets have also proved useful as replacements for springs, such as those in bass drum pedals[3]. Magnetic repulsion replaces the spring of the pedal, ensuring a consistent feel that won't change over time.

[edit] Model Aircraft & Radio control models

Until 1995, the dominant power source for model aircraft were either rubber bands for smaller models, or internal combustion engines. Scale Jet Engines do exist, but are the preserve of wealthy modellers. With the advent of the NIB, more electromotive force was available when applied to an electric motor than had ever been experienced previously. The earliest electric motor-powered models had either been dependent on fixed power sources, or very expensive silver-sodium-chloride batteries, and none had been suitable for radio control.

In 1996, a combination of events saw the NIB become a dominant in model aircraft : The development of inexpensive lightweight NiMH batteries, Samarium cobalt and NIB brushless motors, and simultaneous use of NIB in increasingly smaller servo motors for aircraft control. By 1999, these were widespread and inexpensive.

The use of NIB have made electric model aircraft become as significant as internal combustion aircraft have been for 3 decades since the 1950s. Today, manufacturers are producing electric motor systems that approximate the large internal combustion engines, and electric models now dominate radio control modelling, in the air, land, and water.

[edit] Further development

The neodymium magnet industry is continually working to push the maximum energy product (strength) closer to the theoretical maximum of 64 MGOe.[4] Scientists are also working hard to improve the maximum operating temperature for any given strength.[5]

[edit] Health hazards

Neodymium magnets should always be handled carefully. Some that are only slightly larger than a two-centimetre diameter disc are powerful enough to lift over 10 kilograms. While most solid state electronic devices are not affected by magnetic fields, some medical devices such as electronic pacemakers are not manufactured to withstand the effects of strong magnetic fields. These design limitations can be hazardous to patients using these devices.[6]If more than one magnet is swallowed, then they may trap sections of gut between them, leading to gastrointestinal perforation and a risk of peritonitis.[7]

Larger neodymium magnets can severely pinch skin or fingers, or even break bones [8] when suddenly attracted to a ferromagnetic object. Operating a large neodymium magnet close to smaller ferromagnetic objects (steel or iron keys, pens, etc.) and larger ferromagnetic surfaces can cause injury if flesh is trapped between the magnet and the ferromagnetic object or surface.

Neodymium magnets are very fragile, so they are often plated with a protective coating of metals such as nickel. The magnets can fracture at temperatures over 150 °C, or under impact as a result of their own acceleration. The magnets may then break apart so suddenly that flying pieces cause injury. Some magnet suppliers now supply these magnets encased in plastic or rubber to lessen the risk of injury and breakage on impact.

[edit] Other dangers

A neodymium magnet is powerful enough to damage the contents of magnetic media such as a floppy disk sufficiently that the information is unrecoverable. In addition, neodymium magnets are among the few materials that can erase the information on the magnetic stripes of credit cards [9]

Neodymium magnets are often strong enough not only to magnetize the shadow mask on the front of a CRT television or computer monitor, but also to physically deform the mask itself, in a way that is not repairable by degaussing[citation needed].

[edit] Physical and mechanical properties

Thermal conductivity 7.7 kcal/m-h-°C
Young’s modulus 1.7 x 104 kg/mm2
Bending strength 24 kg/mm2
Compressive strength 80 kg/mm2
Electrical resistivity 160 µ-ohm-cm/cm2
Density 7.4-7.5 g/cm3
Vickers hardness 500 - 600

[edit] See also

[edit] References

[edit] Further reading

  • Campbell, Peter (1994). Permanent Magnet Materials and their Application. New York: Cambridge University Press. ISBN 0521249961. 
  • Furlani, Edward P. (2001). Permanent Magnet and Electromechanical Devices: Materials, Analysis and Applications. London: Academic Press. ISBN 0122699513. 

[edit] External links

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