> Brief Introduction of Permanent Magnets
Ceramic-based and other permanent magnets are used in many industrial applications for their high strength, demagnetization resistance and corrosion resistance. Permanent magnets are a vital part of modern life. They are found in or used to produce almost every modern convenience today, from air conditioners and washing machines to speakers in mobile phones and electric motors in hybrid cars. Permanent magnets are used increasingly in technological applications, including traveling wave tubes, Hall effect sensors, high-temperature-resistant permanent magnets, thin-film coating equipment and flywheel storage systems.
Types of Permanent Magnet
Permanent or hard magnetic materials retain a large amount of residual magnetism after exposure to a strong magnetic field. These materials typically have coercive force (HC) values of several hundred to several thousand oersteds (Oe).
A number of major families of permanent magnets are available for designers, ranging from ferrite, known for its low cost and low energy, to rare earth (RE) materials, which are more expensive and offer higher performance. Designers need to analyze magnetizing field strength and magnetic output of magnetic materials prior to deciding on the appropriate one.
Four major families of permanent magnet materials available commercially include ferrite, alnico, samarium cobalt (SmCo) and neodymium-iron-boron (NdFeB). Other factors also affect the choice of magnetic materials, such as operating temperature, size and weight constraints, environmental concerns, and required magnetic energy. Each family of materials has several grades with a range of magnetic properties.
Ceramic magnets, often called “ferrites” or “hard ferrites” are predominantly complex oxides. These magnets are made of a sintered composite of powdered iron oxide and barium/strontium carbonate ceramic. Introduced in the 1950s, ceramic magnets have become the most widely used permanent magnet materials. Due to their low cost and excellent resistance to demagnetization and corrosion, ceramic magnets are the most popular permanent magnets today. Ferrite magnets are brittle and must be treated like other ceramics.
The ceramic hard ferrites have the advantage of being made of less expensive raw materials than their metal counterparts, such as Alnico and rare earth cobalt. They also have coercive fields, unlike the mass-produced metal magnet types. This allows them to be made into shapes involving high demagnetizing fields.
Alnico magnets are made by casting or sintering a combination of aluminum, nickel and cobalt with iron and small amounts of other elements added to enhance the properties of the magnet. Sintering offers superior mechanical characteristics, whereas casting delivers higher magnetic fields and allows for the design of intricate shapes. Alnico magnets resist corrosion and have physical properties more forgiving than ferrite, but not quite as desirable as a metal.
Injection-molded magnets are a composite of various types of resin and magnetic powders, allowing parts of complex shapes to be manufactured by injection molding. The physical and magnetic properties of the product depend on the raw materials, but are generally lower in magnetic strength and resemble plastics in their physical properties.
Flexible magnets are similar to injection-molded magnets, using a flexible resin or binder such as vinyl. Produced in flat strips, shapes or sheets, these magnets are lower in magnetic strength but can be very flexible depending on the binder used. Flexible magnets can be used in industrial printers.
Rare earth magnets are strong permanent magnets made from alloys of RE elements. Developed in the 1970s and 1980s, RE magnets are the strongest type of permanent magnets, producing significantly stronger magnetic fields than other types. The typical magnetic field of these magnets can be in excess of 1.4 teslas, whereas ferrite or ceramic magnets typically exhibit fields of 0.5-1.0 tesla. There are two types: neodymium magnets and samarium-cobalt magnets. Rare earth magnets are extremely brittle and vulnerable to corrosion, so they are usually plated or coated to protect them from breaking and chipping.