From Wikipedia, the free encyclopedia Samarium–cobalt magnet
Samarium–cobalt magnet
A samarium–cobalt magnet, a type of rare earth magnet, temperature increases within certain temperature
is a strong permanent magnet made of an alloy of samar- ranges. By combining samarium and gadolinium in the
ium and cobalt. They were developed in the early 1970s. alloy, the temperature coefficient can be reduced to
They are generally the second-strongest type of magnet nearly zero.
made, less strong than neodymium magnets, but have
higher temperature ratings and higher coercivity. They Coercivity mechanism
are brittle, and prone to cracking and chipping. Samar- SmCo5 magnets have a very high coercivity (coercive
ium–cobalt magnets have maximum energy products force); that is, they are not easily demagnetized. They are
(BHmax) that range from 16 megagauss-oersteds (MGOe) fabricated by packing wide-grain lone-domain magnetic
to 32 MGOe; their theoretical limit is 34 MGOe. They are powders. All of the motes are aligned with the easy ax-
available in two "series", namely Series 1:5 and Series is direction. In this case, all of the domain walls are at
2:17. 180 degrees. When there are no impurities, the rever-
sal process of the bulk magnet is equivalent to lone-do-
Series 1:5 main motes, where coherent rotation is the dominant
mechanism. However, due to the imperfection of fab-
These samarium–cobalt magnet alloys (generally written ricating, impurities may be introduced in the magnets,
as SmCo5, or SmCo Series 1:5) have one atom of rare which form nuclei. In this case, because the impurities
earth samarium and five atoms of cobalt. By weight this may have lower anisotropy or misaligned easy axes, their
magnet alloy will typically contain 36% samarium with directions of magnetization are easier to spin, which
the balance cobalt. The energy products of these samari- breaks the 180° domain wall configuration. In such mate-
um–cobalt alloys range from 16 MGOe to 25 MGOe. These rials, the coercivity is controlled by nucleation. To obtain
samarium–cobalt magnets generally have a reversible much coercivity, impurity control is critical in the fabri-
temperature coefficient of -0.05%/°C. Saturation magne- cation process.
tization can be achieved with a moderate magnetizing
field. This series of magnet is easier to calibrate to a spe-
cific magnetic field than the SmCo 2:17 series magnets. Series 2:17
In the presence of a moderately strong magnetic These alloys (written as Sm2Co17, or SmCo Series 2:17)
field, unmagnetized magnets of this series will try to are age-hardened with a composition of two atoms of
align its orientation axis to the magnetic field. Unmagne- rare-earth samarium and 13–17 atoms of transition met-
tized magnets of this series when exposed to moderate- als (TM). The TM content is rich in cobalt, but contains
ly strong fields will become slightly magnetized. This can other elements such as iron and copper. Other elements
be an issue if postprocessing requires that the magnet be like zirconium, hafnium, and such may be added in small
plated or coated. The slight field that the magnet picks up quantities to achieve better heat treatment response. By
can attract debris during the plating or coating process weight, the alloy will generally contain 25% of samarium.
causing for a potential plating or coating failure or a me- The maximum energy products of these alloys range
chanically out-tolerance condition. from 20 to 32 MGOe. These alloys have the best reversible
temperature coefficient of all rare-earth alloys, typically
Reversible temperature coefficient being -0.03%/°C. The "second generation" materials can
Br drifts with temperature and it is one of the important also be used at higher temperatures.[1]
characteristics of magnet performance. Some applica-
tions, such as inertial gyroscopes and travelling wave Coercivity mechanism
tubes (TWTs), need to have constant field over a wide In Sm2Co17 magnets, the coercivity mechanism is based
temperature range. The reversible temperature coeffi- on domain wall pinning. Impurities inside the magnets
cient (RTC) of Br is defined as impede the domain wall motion and thereby resist the
(∆Br/Br) x (1/∆ T) × 100%. magnetization reversal process. To increase the coercivi-
ty, impurities are intentionally added during the fabrica-
To address these requirements, temperature compensat-
tion process.
ed magnets were developed in the late 1970s[1]. For con-
ventional SmCo magnets, Br decreases as temperature in-
creases. Conversely, for GdCo magnets, Br increases as
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From Wikipedia, the free encyclopedia Samarium–cobalt magnet
Machining samarium–cobalt • Expensive and subject to price fluctuations (cobalt is
market price sensitive)
The alloys are typically machined in the unmagnetized
state. Samarium–cobalt should be ground using a wet Material properties
grinding process (water based coolants) and a diamond Some of the properties of samarium–cobalt magnets in-
grinding wheel. The same type of process is required if clude:[2]
drilling holes or other features that are confined. The • Density: 8.4 g/cm³
grinding waste produced must not be allowed to com- • Electrical resistivity 0.8×10−4 Ω·cm
pletely dry as samarium–cobalt has a low ignition point. • Coefficient of thermal expansion (perpendicular to
A small spark, such as that produced with static electric- axis): 12.5 µm/(m·K)
ity, can easily commence combustion. The fire produced • Flux density variation under 5% per 100°C change in
will be extremely hot and difficult to control. temperature (in the range of 25–250°C)
Production Uses
The reduction/melt method and reduction/diffusion Fender is using one of legendary designer Bill Lawrence’s
method are used to manufacture samarium–cobalt mag- latest designs named the Samarium Cobalt Noiseless se-
nets. The reduction/melt method will be described since ries of pickups (SCN) in Fender’s American Deluxe Series
it is used for both SmCo5 and Sm2Co17 production. The Guitars and Basses.[3]
raw materials are melted in an induction furnace filled Other uses include:
with argon gas. The mixture is cast into a mold and • High-end electric motors used in the more
cooled with water to form an ingot. The ingot is pulver- competitive classes in slotcar racing
ized and the particles are further milled to further re- • Turbomachinery
duce the particle size. The resulting powder is pressed • Traveling-wave tube field magnets
in a die of desired shape, in a magnetic field to orient • Applications that will require the system to function
the magnetic field of the particles. Sintering is applied at cryogenic temperatures or very hot temperatures
at a temperature of 1100˚C–1250˚C, followed by solution (over 180°C)
treatment at 1100˚C–1200˚C and tempering is finally per- • Applications in which performance is required to be
formed on the magnet at about 700˚C–900˚C. It then is consistent with temperature change
ground and further magnetized to increase its magnetic
properties. The finished product is tested, inspected and See also
packed.
• Neodymium magnet
Hazards References
• Samarium–cobalt magnets can easily chip; eye
[1] Nanocomposite Sm-Co melt spun ribbons
protection must be worn when handling them.
[2] "Standard Specifications for Permanent Magnet
• Allowing magnets to snap together can cause the
Materials". MMPA 0100-00. http://www.intl-
magnets to shatter, which can cause a potential
magnetics.org/pdfs/0100-00.pdf.
hazard.
[3] Smith, Dan. "THE HEART & SOUL OF THE NEW
• Samarium–cobalt is manufactured by a process
FENDER AMERICAN DELUXE SERIES". The Story of
called sintering, and as with all sintered materials,
the Samarium Cobalt Noiseless Pickups. Fender.
inherent cracks are very possible. The magnets do
http://billlawrence.com/Pages/SCNHistory.htm.
not provide mechanical integrity; instead the
Retrieved 2007-12-14.
magnet must be utilized for its magnetic functions
and other mechanical systems must be designed to
provide the mechanical reliability of the system.
Attributes
• Extremely resistant to demagnetization
• Good temperature stability (maximum use
temperatures between 250 and 550 °C; Curie
temperatures from 700 to 800 °C)
Retrieved from "http://en.wikipedia.org/w/index.php?title=Samarium%E2%80%93cobalt_magnet&oldid=428915975"
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From Wikipedia, the free encyclopedia Samarium–cobalt magnet
Categories:
• Ferromagnetic materials
• Types of magnets
• Cobalt alloys
• Magnetic alloys
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