Samarium cobalt magnets are strong, permanent magnets with a high temperature limit. They are made from rare earth minerals and have a magnetic force stronger than iron-based magnets. The magnets are fragile and difficult to handle, but can be produced through sintering and pressing. The alloys used in samarium cobalt magnets are of two types, with magnetic strengths ranging from 16 to 32 MGOe.
Samarium cobalt magnets are a type of rare earth magnet. The alloy content ranges from about 25% to 36% samarium by weight. These strong, permanent magnets are especially useful for high-temperature applications.
The term rare earths refers not to the scarcity of minerals in the earth’s crust, but to the relatively low concentration of the atoms of interest in rare earth mineral deposits. Because they are less concentrated, extracting these compounds tends to be more expensive. Some rare earth minerals exhibit permanent magnetism, such as iron and iron compounds. Magnets made with rare earth minerals are superior to iron based magnets in that their magnetic force is stronger. Smaller parts can be made with the same magnetic field from rare earth minerals.
The magnetism is due to a slight net charge distribution which is not zero. At the atomic and subatomic level, the lowest and most stable energy state of electrons and other charged particles is in orbitals or other motions that are not geometrically symmetrical. This off-center feature is enough to attract you to the Earth’s magnetic pole. In the case of the lodestone, a naturally occurring magnetic mineral, the magnetism was introduced when the rock cooled from the state of molten lava, giving the atoms time to align with the earth’s magnetic field before being frozen into the solid rock.
Magnetic materials have an upper temperature limit, the Curie temperature, above which the material is no longer bound at the atomic or molecular level. Samarium cobalt magnets are second in strength only to neodymium magnets, but can be used in higher temperature operations. Curie temperatures of samarium cobalt magnets are between 1,100°F and 1,300°F (600°C and 710°C), with a favorable working range between 500°F and 1,000°F (250°C and 550° C).
The strength of magnets is typically measured in energy products in units of megagauss-oersted, or MGOe. The theoretical limit is 34MGOe. Samarium cobalt magnets range from 16 to 32 MGOe. Although they have a very high magnetic force, they are extremely fragile and prone to breaking and chipping. Handling is also difficult, so machine finishing with diamond cutting tools is a highly skilled operation.
The alloys used in samarium cobalt magnets are of two types. The first is known as the 1:5 Series, with a ratio of one atom of samarium to five atoms of cobalt. Their magnetic strength ranges from 16 to 25 MGOe. The 2:17 series designates a spectrum of compounds with ratios of approximately two samarium to 17 cobalt atoms. Some cobalt atoms can be replaced with other transition metal atoms such as iron and copper. The magnetic strength of these alloys ranges from 20 to 32 MGOe.
Samarium cobalt magnets are produced by a variety of sintering and pressing steps. Sintering fuses small particles of the samarium cobalt materials together. Pressing operations can involve mold pressing, with pressure applied from one direction, or isotatic pressing in rubber molds, where pressure is applied in all directions. Tradeoffs between tolerances, Curie temperature, and magnetic force are made in manufacturing processes.
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