Magnet
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Magnets are a commonly-encountered form of material that can be useful to the amateur chemist.
Contents
General magnetic properties
Curie temperature
The Curie temperature represents the temperature at which all magnetic crystal domains will realign randomly, effectively causing the magnet to lose all traces of magnetic force. This temperature is useful for removing the dangerous magnetic fields of stronger magnets.
Magnetic field strength
The magnetic field strength is how much force the magnet can exert at a given size and magnetization. For example, a 1-pound 2"x2"x1" block of neodymium-iron-boron alloy, when fully magnetized, can lift over 500 pounds (227 kg) of weight.
Types of magnets
Neodymium-iron-boron magnets
Neodymium magnets are composed of an alloy of neodymium, iron and boron surrounded by a protective, oxygen-excluding metallic shell. See Neodymium iron boron magnet for more information. These magnets are approximately 27% Nd by weight. The typical range of Curie temperature for this alloy is 310 to 340 degrees C. These alloys may contain significant (>10%) amounts of cerium, gadolinium, praseodymium, and other rare earths, as substituting these for Nd does not significantly impact the magnetic properties.
Samarium-cobalt magnets
Samarium-cobalt magnets are composed of an alloy of samarium and cobalt, typically surrounded by a protective metallic or Teflon shell. Some laboratory stir bars may use the SmCo alloy as the magnetic material owing to its higher temperature resistance than neodymium magnets. Two types of SmCo alloys are commonly made: SmCo5 and Sm2Co17. The first is approximately 34% Sm by weight, and the second is approximately 23% Sm by weight. The typical range of Curie temperature for this alloy is 700 to 800 degrees C.
Alnico magnets
Alnico magnets are composed of an alloy of aluminium, nickel and cobalt, hence the name. These magnets' composition is not as tightly controlled as are rare-earth magnets', and as such the magnet composition can consist of a range of 8-12% Al, 15-26% Ni, 5-24% Co, 0-6% Cu, 0-1% Ti, and the remainder is Fe.
Ferrite magnets
Ferrite magnets are chiefly composed of a ceramic magnetic crystal form of iron oxide, with trace additions of nickel, manganese, zinc, strontium, and/or barium to create a spinel crystal structure consisting of the compound AB2O4, where A is Fe, B is some 2+ cation such as zinc or barium, and O is oxygen.
Availability
Magnets can be cheaply purchased online or from the electronic stores. You can also salvage them from old machines, toys, electronics, though old ferrite and alnico magnets tends to become weaker with age, and they aren't very useful.
Projects
- Isolation of neodymium from magnets
- Samarium from SmCo5 magnets
- Make a magnetic stirrer
- Demonstration of diamagnetism (bismuth or pyrolytic carbon levitation)
- Make a metal storage holder to hold ferromagnetic items
- Make electricity from movement
Handling
Safety
Rare earths can cause heavy metal poisoning if ingested, but are otherwise safe to handle with basic PPE. Magnetized materials, however, can exert strong enough forces to injure the human body, and should thus be either demagnetized at their Curie temperature or kept away from other magnetic/ferrous materials.
Storage
Stronger magnets such as neodymium/samarium alloy magnets should be stored in soft foam padding, with spacers between individual magnets to avoid injury. Weaker varieties such as Alnico and ferrites can be stored anywhere.
Disposal
Stronger magnets should always be demagnetized by heating before disposal to prevent injury.
