Samarium Cobalt (SmCo) magnets
The abbreviation SmCo stands for an alloy of samarium and cobalt. The silvery shiny element samarium was discovered by the German mineralogist Heinrich Rose, who in turn named it after the Russian mining engineer Vasily Samarski-Bychowez. The metal is one of the rare earths and is still only extracted in China.
Cobalt belongs to the element category of transition metals, more precisely it is a ferromagnetic transition metal with a Curie temperature of 1150 ° C. The steel-gray, extremely tough heavy metal is suitable as a good conductor for heat and electricity.
Magnets play an indispensable role in many areas of the economy. This applies both to the magnetism in electromagnets caused by electrical current and to the permanent magnetism in permanent magnets. Among other things, the alloys made of samarium-cobalt are important basic materials for the production of permanent magnets. For this purpose, the alloys SmCo5 without iron content, which was developed in 1966, and the alloy Sm2Co17 developed in 1972 with approx. 20-25 percent iron content are used. Until the 1970s, the alloy of samarium and cobalt was the material with the highest known magnetic energy density.
Fundamentals of magnetism
In general, magnetic fields are induced by moving electrically charged particles. This is why a magnetic is created with every current flow in an electrical conductor (metals such as copper, aluminum and others). But also rotating electrically charged particles (particles with spin), such as electrons, generate a magnetic moment and thus represent a small magnet. Since all electrons have a so-called spin, it can be assumed that the entire matter is endowed with certain magnetic properties is. In most cases, however, the magnetic moments cancel each other out in such a way that the material appears non-magnetic to the outside. When exposed to an external magnetic field, the magnetic properties of each substance are changed. Depending on the behavior under the influence of an external strong magnetic field, a distinction is made between diamagnetic, paramagnetic, ferromagnetic, antiferromagnetic and ferrimagnetic materials. Basically, all substances are initially diamagnetic, since they all contain paired electrons with opposite spin. When an external magnetic field acts, a weak opposing field is formed in the material, which forces it out of the external field. However, the effect is so small that it is usually not noticed. If unpaired electrons are also present, they are aligned by an external magnetic field, which usually results in an unstable magnetization (paramagnetism) that easily pulls the material into the magnetic field. After removing or switching off the external magnetic field, the generated magnetic field collapses again. Paramagnetism is usually so weak that it cannot be observed without technical aids. However, if the same spin alignment of unpaired electrons can be stabilized, one speaks of ferromagnetism. These materials are then suitable for the production of permanent magnets.
Materials with ferromagnetic properties form the basis for the manufacture of permanent magnets. These are substances that have unpaired electrons with parallel spin in their atoms for quantum mechanical reasons and can therefore form their own magnetic field. Due to the action of an external magnetic field, the districts align themselves with the same electron spin and thus generate a permanent magnetic field. These materials include the metals iron, cobalt, nickel, some lanthanoids and certain alloys such as samarium-cobalt, AlNiCo neodymium or ferrite.
The properties of ferromagnetic materials
Inside, ferromagnetic materials contain so-called elementary magnets, which are generated by unpaired rectified electrons with parallel spin. An external magnetic field only aligns these elementary magnets and thus magnetizes the material, whereby its magnetic field strength is independent of the external field. When magnetizing ferromagnetic materials, there are so-called exchange interactions between the aligned electrons due to complicated quantum physical processes, which stabilize the alignment and thus build up a stable magnetic field. Magnetic energy is supplied to the material, the size of which is referred to as the energy product. However, soft magnetic materials break down their magnetization immediately after removal of the external magnetic field. In the case of ferromagnetic materials, residual magnetization remains (remanence). The strength of the remanence now determines the stability of the generated magnetic field. With some materials it can be so large that a permanent magnet with a strong magnetic field is created. The magnetic field can then only be reduced by heating (Curie temperature) or mechanical treatment of the material as well as by applying an external strong opposing field (coercive field). The challenge in developing permanent magnets is to find materials that can build up a strong magnetic field that can withstand high temperatures, high mechanical stress and strong external magnetic fields. The alloys of SmCo, neodymium, ferrite and AlNiCo have proven particularly successful here.
The properties of the samarium and cobalt alloys
Both alloys can build up a very high magnetic energy density, which can reach 130 to 200 kJ / m3 with SmCo5 and 160 to 260 kJ / m3 with Sm2Co17. Your energy product is therefore very high. Furthermore, their magnetic field is extremely stable and insensitive to external influences. The samarium-cobalt alloys are difficult to demagnetize. The Curie temperature is 450 degrees. The magnetization disappears above this temperature. The magnet can be used up to approx. 350 degrees without any loss of magnetic field. The temperature coefficient of the residual flux density is very low and lies between 0.03 and 0.04 percent per degree Celsius. This means that the magnetic field only decreases very slightly per degree of temperature rise. Furthermore, the coercive field strength is extremely large and sometimes exceeds other permanent magnets several times. Samarium-cobalt alloys also have the advantage that they are very corrosion-resistant. However, they are attacked by inorganic acids and alkalis.
Manufacture of samarium-cobalt magnets
The starting materials samarium and cobalt are melted from argon under a protective gas atmosphere and cast in ingot form because they would react with oxygen in air. Various alloy additives in the melt improve its thermal properties. During the solidification of the melt, crystal structures are created that prevent the magnetic field from stabilizing. The resulting alloy can be easily demagnetized and is not suitable as a magnetic material. Further processing is therefore necessary. The alloy obtained is then again pulverized under protective gas and the powder is subjected to a sintering process at temperatures from 1150 to 1250 degrees. The individual powder particles bake together. Pressing with plastics is also possible. Magnetization must take place in parallel during this process. The material is brought into the desired initial shape. Subsequent processing of the sintered magnetized materials is no longer possible because ferromagnetic tools are magnetized and chips that are produced can no longer be easily separated from the base body by magnetization. In addition, the material may splinter and self-ignite the finely divided powder. Post-processing of samarium-cobalt powder pressed in plastic is easier, but the magnets made from it have worse properties than the sintered magnetic materials. Since the elements used rarely occur on Earth, the price for strong SmCo magnets is relatively high. You also have to be careful with them, as this brittle material causes flaking relatively quickly. In addition, there is a relatively complex manufacturing process.
The use of permanent magnets
Permanent magnets, which also include the alloys of samarium with cobalt, find a variety of applications in everyday life, e.g. B. in locking systems, refrigerators, for attaching objects and much more. However, they are of particular importance in power generation and energy conversion. The generation of electricity takes advantage of the fact that a changing magnetic field sets mobile electrical particles, such as electrons, in motion. It is irrelevant whether the magnetic field is real or whether it appears to change due to a relative movement in relation to an electrical conductor. In generators, for example, the rotor consists of rotating permanent magnets, the magnetic fields of which generate an electrical current in the static coils (stator) of a current conductor made of wire (copper or silver-plated copper wire). Conversely, the permanent magnets can also be used in electric motors for converting electrical into mechanical energy.
The main areas of application of SmCoMagnets made of samarium-cobalt alloys are used where very strong magnetic fields are required under extreme conditions (temperatures in the temperature range -40 to 350 degrees). The main areas of application include:
- Numerous measuring devices
Two crystal structures can be used for the application: SmCo5 or Sm2Co17 (with iron, copper or zirconium as additional alloying elements)
The two forms of the alloy were developed in 1966 and 1972 and were the materials with the highest magnetic energy density known until the discovery of neodymium-iron-boron in 1982.
Due to their properties, permanent magnets made of SmCo are difficult to demagnetize and retain their magnetic forces up to an operating temperature of 450 ° C. Since the temperature coefficient of remanence is very low, the magnetic field loses its effect only slightly per degree of temperature rise. Furthermore, thanks to their high coercivity, they are extremely resistant to demagnetizing fields and have an enormous corrosion resistance.
Today, the Sm2Co17 alloy is still used for permanent magnets for the most part because it is less expensive due to the lower use of samarium. However, special areas of application require the use of SmCo5, especially when very strong magnetic field strengths are required.
Comparative examination of other magnet types with samarium-cobalt magnets
Samarium cobalt magnets, like other permanent magnets, have a high energy density, good temperature resistance and extremely high coercive field strengths. Nevertheless, there are some differences that can be advantageous to us and partly disadvantageous.
Comparison with neodymium magnets
Neodymium magnets consist of an alloy of neodymium, iron and boron. This material has been used since the 1970s. Its energy density is significantly higher than that of samarium cobalt. However, the neodymium magnet can only be used up to temperatures of 80 degrees. It is also more susceptible to corrosion than the samarium-cobalt magnet. Certain additives to the alloy can increase the temperature and corrosion resistance, but they cannot match the values of the samarium-cobalt alloy. Because of this, neodymium magnets must still be replaced by samarium-cobalt magnets in applications under extreme conditions.
Comparison with ferrite magnets
Ferrite magnets are made from iron oxide and barium or strontium carbonate. The resulting material is ceramic-like. Ferrite magnets are very popular worldwide because they are inexpensive and at the same time very corrosion-resistant. They can also be used in a temperature range from -40 degrees to 250 degrees. However, if very strong magnetic field strengths are required, neodymium or samarium-cobalt magnets must be used again.
Comparison with AlNiCo magnets
AlNiCo magnets consist of an alloy of aluminum, cobalt and nickel. They can be used up to 550 degrees, are very corrosion-resistant and show a high remanence. However, AlNiCo magnets have a very low coercive field strength and can therefore be easily demagnetized by external magnetic fields. In many areas of application, it has already been replaced by ferrite magnets. However, if high field strengths are required in combination with high temperatures, the use of samarium-cobalt magnets has proven itself.
SmCo magnets have a high magnetic energy density, can be used at high temperatures up to 350 degrees, are hardly influenced by external magnetic fields and are also very corrosion-resistant. However, their production is very expensive because samarium is one of the rare elements. Their use has proven particularly useful in areas where high magnetic field strengths at high temperatures are required. This affects generators, motors, sensors and also measuring devices.