Rare-earth magnet: Strong permanent magnets made from alloys of rare earth elements. Developed in 1970s and 80s, rare-earth magnets are strongest type of permanent magnets made, Producing stronger magnetic fields than other types such as ferrite or alnico magnets. 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. Magnets are brittle and vulnerable to corrosion, They are usually plated or coated to protect them from breaking and chipping. These metals are not particularly rare or precious. They are as abundant as tin or lead. Development of rare earth magnets began around 1966, when US Air Force Materials Laboratory discovered alloy of yttrium and cobalt, YCo5, had largest magnetic anisotropy constant of any material then known. Rare earth metals are ferromagnetic, meaning 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 metals such as iron, nickel, and cobalt, some of these have Curie temperatures above room temperature. Advantage of rare earth compounds over other magnets is crystalline structures have very high magnetic anisotropy. This means crystal of material is easy to magnetize in one particular direction, but resists being magnetized in any other direction. Neodymium magnet with nickel plate, invented in 1980s, are strongest and most affordable rare-earth magnet. Made of alloy of neodymium, iron and boron, Neodymium magnets are used in applications requiring strong, compact permanent magnets, such as electric motors for cordless tools, hard drives, and magnetic hold downs and jewelry clasps. Use of protective surface treatments such as gold, nickel, zinc and tin plating and epoxy resin coating can provide corrosion protection. High cost limited use to applications requiring compactness together with high field strength. Both raw materials and patent licenses were expensive. In 1990s, NIB magnets became steadily less expensive, and inspired new uses such as magnetic building toys. Hazards: Greater force exerted by rare earth magnets creates hazards. Magnets larger than few centimeters are strong enough to cause injuries to body parts pinched between two magnets, or magnet and metal, even causing broken bones. Magnets allowed to get too near each other can strike with enough force to chip and shatter brittle material, flying chips can cause injuries. There have been cases where children who swallowed several magnets have had fold of digestive tract pinched between magnets, causing injury and in one case intestine perforations, sepsis and death.
Posted on: Wed, 26 Jun 2013 03:52:05 +0000
Trending Topics
Recently Viewed Topics
© 2015