Samarium-cobalt magnet

Samarium-cobalt magnets are primarily composed of samarium and cobalt. They have been available since the early 1970s. This type of rare-earth magnet is very powerful, however they are brittle and prone to cracking and chipping. Samarium-cobalt magnets have Maximum Energy Products (BHmax) that range from 16 Mega-Gauss Oersteds (MGOe) to 32 MGOe, their theoretical limit is 34 MGOe. Samarium Cobalt magnets are available in two "series", namely Series 1:5 and Series 2:17.

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Samarium Cobalt 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 samarium cobalt 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 Samarium Cobalt 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 orietation axis to the magnetic field. Unmagnetized magnets of this series when exposed to moderately strong fields will get slightly magnetized. This can be an issue if post processing 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 of tolerance condition.

Samarium Cobalt Series 2:17

These Samarium Cobalt magnet alloys (written as Sm₂Co₁₇, or SmCo Series 2:17) are an age hardening type with a composition of two atoms of rare earth Samarium Cobalt 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 zirconium, 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 MGOe to 32 MGOe. These alloys have the best reversible temperature coefficient of all rare earth alloys, typically being -0.03%/°C. The "second generation" Samarium Cobalt materials can also be used at higher temperatures.

Machining Samarium Cobalt

The Samarium Cobalt 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 since 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.

Hazards of Samarium Cobalt Magnets

• Samarium Cobalt magnets can easily chip, eye protection must be worn when handling them.
• Keep Samarium Cobalt magnets away from children.
• 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 sintering, and as with all sintered materials, inherent cracks are very possible. Design engineers must not expect the magnet to 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

• High resistance to demagnetization
• High energy (magnetic strength is strong for its size)
• Good temperature stability (maximum use temperatures between 250 and 350 °C; Curie temperatures from 700 to 800 °C)
• Expensive material (cobalt is market price sensitive)

Material properties

• Density: 8.4 g/cm³
• Electrical Resistivity 0.8×10⁻⁴ Ω·cm
• Coefficient of thermal expansion (perpendicular to axis): 12.5 µm/(m·K)

Uses of Samarium-Cobalt

• Fender is using one of legendary designer Bill Lawrence (guitar maker)'s latest designs named the Samarium Cobalt Noiseless series of pickups (SCN) in Fender's top of the line and highly acclaimed American Deluxe Series Guitars and Basses.^[1]
• High speed motors.
• Turbo-machinery.
• Traveling Wave Tubes.
• Applications that will require the system to function below cryogenic temperatures or very hot temperatures (over 180°C).
• Applications where performance is required to be consistent with temperature change. The flux density of a samarium cobalt magnet will vary under 5% per 100°C change in temperature (in the range of 25°C - 250°C).

References