Samarium-cobalt magnet
Encyclopedia
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 magnet
s, but have higher temperature ratings and higher coercivity
. They are brittle, and prone to cracking and chipping. Samarium–cobalt magnets have maximum energy products (BHmax) that range from 16 megagauss-oersteds (MGOe)
to 32 MGOe; their theoretical limit is 34 MGOe. They are available in two "series", namely Series 1:5 and Series 2:17.
In the presence of a moderately strong magnetic field, unmagnetized magnets of this series will try to align its orientation axis to the magnetic field. Unmagnetized magnets of this series when exposed to moderately strong fields will become slightly magnetized. This can be an issue if postprocessing requires that the magnet be plated or coated. The slight field that the magnet picks up can attract debris during the plating or coating process causing for a potential plating or coating failure or a mechanically out-tolerance condition.
To address these requirements, temperature compensated magnets were developed in the late 1970shttp://www.electronenergy.com/about_us/company_info.htm. For conventional SmCo magnets, Br decreases as temperature increases. Conversely, for GdCo magnets, Br increases as temperature increases within certain temperature ranges. By combining samarium and gadolinium in the alloy, the temperature coefficient can be reduced to nearly zero.
(coercive force); that is, they are not easily demagnetized. They are fabricated by packing wide-grain lone-domain magnetic powders. All of the motes are aligned with the easy axis direction. In this case, all of the domain walls are at 180 degrees. When there are no impurities, the reversal process of the bulk magnet is equivalent to lone-domain motes, where coherent rotation is the dominant mechanism. However, due to the imperfection of fabricating, impurities may be introduced in the magnets, which form nuclei. In this case, because the impurities may have lower anisotropy or misaligned easy axes, their directions of magnetization are easier to spin, which breaks the 180° domain wall configuration. In such materials, the coercivity is controlled by nucleation. To obtain much coercivity, impurity control is critical in the fabrication process.
, hafnium
, and such may be added in small quantities to achieve better heat treatment response. By weight, the alloy will generally contain 25% of samarium. The maximum energy products of these alloys range from 20 to 32 MGOe. These alloys have the best reversible temperature coefficient of all rare-earth alloys, typically being -0.03%/°C. The "second generation" materials can also be used at higher temperatures.
process. To increase the coercivity, impurities are intentionally added during the fabrication process.
's latest designs named the Samarium Cobalt Noiseless series of pickups (SCN) in Fender's American Deluxe Series Guitars and Basses.
Other uses include:
Alloy
An 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 samarium
Samarium
Samarium is a chemical element with the symbol Sm, atomic number 62 and atomic weight 150.36. It is a moderately hard silvery metal which readily oxidizes in air. Being a typical member of the lanthanide series, samarium usually assumes the oxidation state +3...
and cobalt
Cobalt
Cobalt is a chemical element with symbol Co and atomic number 27. It is found naturally only in chemically combined form. The free element, produced by reductive smelting, is a hard, lustrous, silver-gray metal....
. They were developed in the early 1970s. They are generally the second-strongest type of magnet made, less strong than neodymium magnet
Neodymium magnet
A neodymium magnet , the most widely-used type of rare-earth magnet, is a permanent magnet made from an alloy of neodymium, iron, and boron to form the Nd2Fe14B tetragonal crystalline structure. Developed in 1982 by General Motors and Sumitomo Special Metals, neodymium magnets are the strongest...
s, but have higher temperature ratings and higher coercivity
Coercivity
In 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...
. They are brittle, and prone to cracking and chipping. Samarium–cobalt magnets have maximum energy products (BHmax) that range from 16 megagauss-oersteds (MGOe)
Oersted
Oersted is the unit of magnetizing field in the CGS system of units.-Difference between cgs and SI systems:...
to 32 MGOe; their theoretical limit is 34 MGOe. They are available in two "series", namely Series 1:5 and Series 2:17.
Series 1:5
These samarium–cobalt magnet alloys (generally written as SmCo5, or SmCo Series 1:5) have one atom of rare earth samarium and five atoms of cobalt. By weight this magnet alloy will typically contain 36% samarium with the balance cobalt. The energy products of these samarium–cobalt alloys range from 16 MGOe to 25 MGOe. These samarium–cobalt magnets generally have a reversible temperature coefficient of -0.05%/°C. Saturation magnetization can be achieved with a moderate magnetizing field. This series of magnet is easier to calibrate to a specific magnetic field than the SmCo 2:17 series magnets.In the presence of a moderately strong magnetic field, unmagnetized magnets of this series will try to align its orientation axis to the magnetic field. Unmagnetized magnets of this series when exposed to moderately strong fields will become slightly magnetized. This can be an issue if postprocessing requires that the magnet be plated or coated. The slight field that the magnet picks up can attract debris during the plating or coating process causing for a potential plating or coating failure or a mechanically out-tolerance condition.
Reversible temperature coefficient
Br drifts with temperature and it is one of the important characteristics of magnet performance. Some applications, such as inertial gyroscopes and travelling wave tubes (TWTs), need to have constant field over a wide temperature range. The reversible temperature coefficient (RTC) of Br is defined as x (1/∆ T) × 100%.To address these requirements, temperature compensated magnets were developed in the late 1970shttp://www.electronenergy.com/about_us/company_info.htm. For conventional SmCo magnets, Br decreases as temperature increases. Conversely, for GdCo magnets, Br increases as temperature increases within certain temperature ranges. By combining samarium and gadolinium in the alloy, the temperature coefficient can be reduced to nearly zero.
Coercivity mechanism
SmCo5 magnets have a very high coercivityCoercivity
In 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...
(coercive force); that is, they are not easily demagnetized. They are fabricated by packing wide-grain lone-domain magnetic powders. All of the motes are aligned with the easy axis direction. In this case, all of the domain walls are at 180 degrees. When there are no impurities, the reversal process of the bulk magnet is equivalent to lone-domain motes, where coherent rotation is the dominant mechanism. However, due to the imperfection of fabricating, impurities may be introduced in the magnets, which form nuclei. In this case, because the impurities may have lower anisotropy or misaligned easy axes, their directions of magnetization are easier to spin, which breaks the 180° domain wall configuration. In such materials, the coercivity is controlled by nucleation. To obtain much coercivity, impurity control is critical in the fabrication process.
Series 2:17
These alloys (written as Sm2Co17, or SmCo Series 2:17) are age-hardened with a composition of two atoms of rare-earth samarium and 13–17 atoms of transition metals (TM). The TM content is rich in cobalt, but contains other elements such as iron and copper. Other elements like zirconiumZirconium
Zirconium is a chemical element with the symbol Zr and atomic number 40. The name of zirconium is taken from the mineral zircon. Its atomic mass is 91.224. It is a lustrous, grey-white, strong transition metal that resembles titanium...
, hafnium
Hafnium
Hafnium is a chemical element with the symbol Hf and atomic number 72. A lustrous, silvery gray, tetravalent transition metal, hafnium chemically resembles zirconium and is found in zirconium minerals. Its existence was predicted by Dmitri Mendeleev in 1869. Hafnium was the penultimate stable...
, and such may be added in small quantities to achieve better heat treatment response. By weight, the alloy will generally contain 25% of samarium. The maximum energy products of these alloys range from 20 to 32 MGOe. These alloys have the best reversible temperature coefficient of all rare-earth alloys, typically being -0.03%/°C. The "second generation" materials can also be used at higher temperatures.
Coercivity mechanism
In Sm2Co17 magnets, the coercivity mechanism is based on domain wall pinning. Impurities inside the magnets impede the domain wall motion and thereby resist the magnetization reversalMagnetization reversal
Magnetization reversal, or switching, represents the process that leads to a 180° reorientation of the magnetization vector with respect to its initial direction, from one stable orientation to the opposite one. Technologically, this is one of the most important processes in magnetism that is...
process. To increase the coercivity, impurities are intentionally added during the fabrication process.
Machining samarium–cobalt
The alloys are typically machined in the unmagnetized state. Samarium–cobalt should be ground using a wet grinding process (water based coolants) and a diamond grinding wheel. The same type of process is required if drilling holes or other features that are confined. The grinding waste produced must not be allowed to completely dry as samarium–cobalt has a low ignition point. A small spark, such as that produced with static electricity, can easily commence combustion. The fire produced will be extremely hot and difficult to control.Production
The reduction/melt method and reduction/diffusion method are used to manufacture samarium–cobalt magnets. The reduction/melt method will be described since it is used for both SmCo5 and Sm2Co17 production. The raw materials are melted in an induction furnace filled with argon gas. The mixture is cast into a mold and cooled with water to form an ingot. The ingot is pulverized and the particles are further milled to further reduce the particle size. The resulting powder is pressed in a die of desired shape, in a magnetic field to orient the magnetic field of the particles. Sintering is applied at a temperature of 1100˚C–1250˚C, followed by solution treatment at 1100˚C–1200˚C and tempering is finally performed on the magnet at about 700˚C–900˚C. It then is ground and further magnetized to increase its magnetic properties. The finished product is tested, inspected and packed.Hazards
- Samarium–cobalt magnets can easily chip; eye protection must be worn when handling them.
- Allowing magnets to snap together can cause the magnets to shatter, which can cause a potential hazard.
- Samarium–cobalt is manufactured by a process called sinteringSinteringSintering 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...
, and as with all sintered materials, inherent cracks are very possible. The magnets do not provide mechanical integrity; instead the 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)
- Expensive and subject to price fluctuations (cobalt is market price sensitive)
Material properties
Some of the properties of samarium–cobalt magnets include:- DensityDensityThe mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
: 8.4 g/cm³ - Electrical resistivityResistivityElectrical 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...
0.8×10−4 Ω·cm - Coefficient of thermal expansion (perpendicular to axis): 12.5 µm/(m·K)
- Flux density variation under 5% per 100°C change in temperature (in the range of 25–250°C)
Uses
Fender is using one of legendary designer Bill LawrenceBill Lawrence (guitar maker)
Bill Lawrence is a recording musician and an electric guitar pickup designer/maker and guitar designer/maker in the musical instrument industry, designing pickups and guitars for Fender, Gibson, Peavey and other companies from the 1950s to the present.MusicianThe Bill Lawrence story began...
's latest designs named the Samarium Cobalt Noiseless series of pickups (SCN) in Fender's American Deluxe Series Guitars and Basses.
Other uses include:
- High-end electric motors used in the more competitive classes in slotcar racingSlotcar racingNote: this article concerns slot car racing competition. For information on the general slot car hobby, including more detailed coverage of history, scales, track-types and mechanical functioning, see slot car....
- TurbomachineryTurbomachineryTurbomachinery, in mechanical engineering, describes machines that transfer energy between a rotor and a fluid, including both turbines and compressors. While a turbine transfers energy from a fluid to a rotor, a compressor transfers energy from a rotor to a fluid...
- Traveling-wave tube field magnets
- Applications that will require the system to function at cryogenicCryogenicsIn physics, cryogenics is the study of the production of very low temperature and the behavior of materials at those temperatures. A person who studies elements under extremely cold temperature is called a cryogenicist. Rather than the relative temperature scales of Celsius and Fahrenheit,...
temperatures or very hot temperatures (over 180°C) - Applications in which performance is required to be consistent with temperature change