Neodymium Iron Boron Multipole Magnet
NdFeB magnets have a higher Maximum Energy Product, (BH)max, than Sm-Co magnets. (BH) max of Nd-Fe-B can easily reach 30 MGOe and even goes up to 48 MGOe. Nd-Fe-B magnets can replace Sm-Co magnets in most cases, especially where operating temperature is less than 80 degrees Centigrade.
The temperature stability of Nd-Fe-B is not as good as Sm-Co magnets. Magnetic performance of Nd-Fe-B magnets will deteriorate rapidly above about 180 degrees Centigrade. Compared to Sm-Co magnets, the corrosion and oxidation resistance of Nd-Fe-B is relatively low. Sintered Nd-Fe-B permanent magnets are made with several steps. At first, a Nd-Fe-B alloy is formulated based on the properties of final permanent magnets supposed to reach. The alloy is produced in a vacuum furnace.
Then the alloy is crushed into a powder form. Sintered Nd-Fe-B permanent magnets are formed by powder metallurgical process. These magnets can be die pressed or isostatically pressed. During the pressing process, magnetic fields are applied with assistance of specially designed fixture to align magnetic “domains” and optimize the magnetic performance of these magnets.
Then pressed magnets are placed into a furnace under protective atmosphere for sintering. After sintering the magnet shape is rough, and need to be machined and ground to achieve desired shape and size. A surface coating is usually applied on Nd-Fe-B magnets. Zinc or nickel coating is common used as a protective layer. Other materials such as cadmium chromate, aluminum chromate, tin or polymer (epoxy) are also used for this purpose.
Both Nd-Fe-B and Sm-Co magnets can be made either in sintered or polymer-bonded magnets. The polymer (such as epoxy)-bonded magnets can be produced with close tolerances off tool, with little or no finishing required. Stamford Magnets supplies polymer-bonded Nd-Fe-B permanent magnets made by both compression moulding and injection moulding.
The sintered magnets usually require some finishing operations in order to hold close mechanical tolerances. The sintered magnets, however, provide better magnetic properties than bonded magnets.