Rare-earth magnet
Rare-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, producing significantly stronger magnetic fields than other types such as ferrite or alnico magnets. The magnetic field typically produced by rare-earth magnets can be in excess of 1.4 teslas, whereas ferrite or ceramic magnets typically exhibit fields of 0.5 to 1 tesla. There are two types: neodymium magnets and samarium-cobalt magnets. Rare earth magnets are extremely brittle and also vulnerable to corrosion, so they are usually plated or coated to protect them from breaking and chipping.
The term "rare earth" can be misleading as these metals are not particularly rare or precious;[1][2] they are about as abundant as tin or lead.[3] Interest in rare earth compounds as permanent magnets began in 1966, when K. J. Strnat and G. Hoffer of the US Air Force Materials Laboratory discovered that an alloy of yttrium and cobalt, YCo5, had by far the largest magnetic anisotropy constant of any material then known.[4]
Explanation of strength
The rare earth (lanthanide) elements are metals that are ferromagnetic, meaning that like iron they can be magnetized, but their Curie temperatures are below room temperature, so in pure form their magnetism only appears at low temperatures. However, they form compounds with the transition metals such as iron, nickel, and cobalt, and some of these have Curie temperatures well above room temperature. Rare earth magnets are made from these compounds.
The advantage of the rare earth compounds over other magnets is that their crystalline structures have very high magnetic anisotropy. This means that a crystal of the material is easy to magnetize in one particular direction, but resists being magnetized in any other direction.
Atoms of rare earth elements can retain high magnetic moments in the solid state. This is a consequence of incomplete filling of the f-shell, which can contain up to 7 unpaired electrons with aligned spins. Electrons in such orbitals are strongly localized and therefore easily retain their magnetic moments and function as paramagnetic centers. Magnetic moments in other orbitals are often lost due to strong overlap with the neighbors; for example, electrons participating in covalent bonds form pairs with zero net spin.
High magnetic moments at the atomic level in combination with a stable alignment (high anisotropy) results in high strength.
Magnetic properties
Some important properties used to compare permanent magnets are: remanence (Br), 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. Rare earth magnets have higher remanence, much higher coercivity and energy product, but (for neodymium) lower Curie temperature than other types. The table below compares the magnetic performance of the two types of rare earth magnet, neodymium (Nd2Fe14B) and samarium-cobalt (SmCo5), with other types of permanent magnets.
| Magnet | Br (T) | Hci (kA/m) | (BH)max (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 |
Types
Samarium-cobalt
Samarium-cobalt magnets (chemical formula: SmCo5), the first family of rare earth magnets invented, are less used than neodymium magnets because of their higher cost and weaker magnetic field strength. However, samarium-cobalt has a higher Curie temperature, creating a niche for these magnets in applications where high field strength is needed at high operating temperatures. They are highly resistant to oxidation, but sintered samarium-cobalt magnets are brittle and prone to chipping and cracking and may fracture when subjected to thermal shock.
Neodymium
Neodymium magnets, invented in the 1980s, are the strongest and most affordable type of rare-earth magnet. They are made of an alloy of neodymium, iron and boron: (Nd2Fe14B) Neodymium magnets are used in numerous applications requiring strong, compact permanent magnets, such as electric motors for cordless tools, hard drives, and magnetic holddowns and jewelry clasps. They have the highest magnetic field strength, but have lower Curie temperature and are more vulnerable to oxidation than samarium-cobalt magnets. Use of protective surface treatments such as gold, nickel, zinc and tin plating and epoxy resin coating can provide corrosion protection where required.
Originally, the high cost of these magnets limited their use to applications requiring compactness together with high field strength. Both raw materials and patent licenses were expensive. Beginning in the 1990s, NIB magnets have become steadily less expensive, and the low cost has inspired new uses such as magnetic building toys.
Hazards
The greater force exerted by rare earth magnets creates hazards that are not seen with other types of magnet. 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.[5] 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.[6] 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
Since their prices became competitive in the 1990s, neodymium magnets have been replacing Alnico and ferrite magnets in the many applications in modern technology requiring powerful magnets. Their greater strength allows smaller and lighter magnets to be used for a given application.
Common applications
Common applications of rare-earth magnets include:
- computer hard drives
- wind turbine generators
- audio speakers / headphones
- bicycle dynamos
- fishing reel brakes
- permanent magnet motors in cordless tools
- self-powered flashlights, employing rare earth magnets for generating electricity in a shaking motion
Other applications
Other applications of rare-earth magnets include:
- Linear motors (used in Mag-lev trains, etc.)
- Stop motion animation as tie-downs when the use of traditional screw and nut tie-downs is impractical[clarification needed]
- Diamagnetic levitation experimentation, the study of magnetic field dynamics and superconductor levitation.
- Electrodynamic bearings.
- Launched roller coaster technology found on roller coaster and other thrill rides.
- LED Throwies An LED throwie is a small LED attached to a coin battery and a rare earth magnet (usually with conductive epoxy or electrical tape), used for the purpose of creating non-destructive graffiti and light displays.
- Electric guitar pickups.
- Miniature figure (gaming) in particular Warhammer 40,000 and Warhammer Fantasy Battle Rare Earth magnets have gained popularity in the miniatures gaming community for their small size and relative strength assisting in swapping weapons between models to adhere to WYSIWYG conversions.
- Windbelt for electricity generation through electromagnetic induction and aeroelastic flutter principles.
References
- ^ McCaig, Malcolm (1977). Permanent Magnets in Theory and Practice. USA: Wiley. p. 123. ISBN 0727316044.
- ^ Sigel, Astrid (2003). The lanthanides and their interrelations with biosystems. USA: CRC Press. pp. v. ISBN 0824742451.
{{cite book}}: Unknown parameter|coauthors=ignored (|author=suggested) (help) - ^ Bobber, Robert J. (1981). "New types of transducers". Underwater acoustics and signal processing: proceedings of the NATO Advanced Study Institute held at Kollekolle, Copenhagen, Denmark, August 18–29, 1980. USA: Springer. pp. 251–252.
{{cite conference}}:|access-date=requires|url=(help); Check date values in:|accessdate=(help); Unknown parameter|booktitle=ignored (|book-title=suggested) (help) - ^ Cullity, B. D. (2008). Introduction to Magnetic Materials. Wiley-IEEE. p. 489. ISBN 0471477419.
{{cite book}}: Unknown parameter|coauthors=ignored (|author=suggested) (help) - ^ Swain, Frank (March 6, 2009). "How to remove a finger with two super magnets". The Sciencepunk Blog. Seed Media Group LLC. Retrieved 2009-06-28.
{{cite web}}: Cite has empty unknown parameter:|coauthors=(help) - ^ "Magnet Safety Alert" (PDF). U.S. Consumer Product Safety Commission. Retrieved 7 August 2009.
Further reading
- Edward P. Furlani, "Permanent Magnet and Electromechanical Devices: Materials, Analysis and Applications", Academic Press Series in Electromagnetism (2001). ISBN 0-12-269951-3.
- Peter Campbell, "Permanent Magnet Materials and their Application" (Cambridge Studies in Magnetism)(1996). ISBN 978-0521566889.
- Brown, D.N. (2004). "The Dependence of Magnetic Properties and Hot Workability of Rare Earth-Iron-Boride Magnets Upon Composition" (PDF). IEEE Transactions on Magnetics. 40 (4): 2895–2897. Bibcode:2004ITM....40.2895B. doi:10.1109/TMAG.2004.832240. ISSN 0018-9464.
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External links
- MMPA 0100-00, Standard Specifications for Permanent Magnet Materials
- Edwards, Lin (22 March 2010). "Iron-nitrogen compound forms strongest magnet known". PhysOrg.