Cobalt compounds used
Wind turbines, hard disk drives, motors, sensors, actuators, magnetic resonance imaging etc.
Since being utilised for its magnetic properties in magnetic steels in 1917, cobalt is present in many important types of magnet which cover a range of applications.
In general, there are two classes of magnetic substances, hard magnets and soft magnets. Soft magnets have a low coercivity and therefore do not retain magnetism induced by a magnetic field if removed. In contrast, hard magnets have a high coercivity and can be permanently magnetised by applying a magnetic field.
Cobalt is predominantly used in hard magnets however some soft magnetic cobalt alloys are also in use. These alloys contain cobalt and iron with vanadium added to improve ductility.
Cobalt based soft magnet alloys have advantages over other soft magnets in that they have a high saturation point, have a good permeability in magnetic fields and have a high Curie Point of 950-990oC resulting in magnetic properties not being altered under temperatures of 500oC.
One of the first modern permanent magnets created was the aluminium-nickel-iron (Al-Ni-Fe) magnets in 1932, Tokyo. Twenty years of development followed whereby adding cobalt was found to significantly enhance the properties hence the aluminium-nickel-cobalt (Al-Ni-Co) series of alloys. Since then the series of alloys has been developed with processing changes further adding beneficial properties. Although stronger magnets are now available in the rare-earth classes, Alnico magnets are still utilised in sensors and in motors.
In the 1960’s cobalt - rare-earth compounds were postulated as being promising hard magnet materials. In 1970, samarium-cobalt (Sm-Co) magnets were produced from sintering and soon came into prominence in the market place. The improvement in performance was great and similar to that seen from the leap between steel magnets and Al-Ni-Co. The new technology led to further innovations in instrumentation, telephones, electronics and motors. Although still in use, the neodymium-iron-boron (Nd-Fe-B) magnets took their place due to being more powerful, less costly, and more versatile.
The Nd-Fe-B magnets were developed in 1983 and represented a tenfold increase in magnetic energy compared to Sm-Co. Again, the improved properties of the magnetic material allowed for large technological advances in other sectors including motors, disc drives, magnetic resonance imaging and high efficiency motors. Despite the materials strong properties, weaknesses were still present in thermal instability and poor corrosion resistance. Coating techniques have been employed to overcome these characteristics which has been largely achieved due to the addition of cobalt. Compared to Al-Ni-Co the cobalt content is still low however with Nd-Fe-B magnets only containing around 1-5%.