A research group at Japan’s National Institute of Advanced Industrial Science and Technology (AIST) Materials Research Institute for Sustainable Development, has developed a technology to produce sintered isotropic Sm-Fe-N (samarium-iron-nitrogen)-based magnets which will have magnetic properties close to those of Nd-Fe-B (neodymium-iron-boron)-based permanent magnets.
This is expected to lessen the reliance on NdFeB magnets for electric motors used in hybrid and electric vehicles. Additionally, NdFeB magnets contain a small amount of Dy (dysprosium), a rare earth which is becoming increasing expensive and which has to be imported into Japan.
The researchers report that the new technology developed at AIST allows the high performance Sm-Fe-N powder to be consolidated and sintered at 400C to a relative density of 90% or higher. Previously, the high magnetic properties of Sm-Fe-N were lost when sintered at a temperature of 500C or higher, restricting this type of material to resin bonded magnets.
The new technology involves the use of a pulse electric current sintering method, whereby current pulse is applied to a metal mould that contains the Sm-Fe-N powder. The metal mould and powder are electrically resistant so when an electric current is applied direct heat is very quickly generated – holding time is only 1 minute.
Because this is a direct-heating method the crystal structure is prevented from changing by a rapid rise in temperature. In addition, by using pulse current method it becomes possible to promote bonding of the powder particle boundary surfaces without raising the temperature of the powder. As a result, Sm-Fe-N-based magnet powder can be sintered without lowering its properties.
The researchers also report that density was enhanced by using servo pressing with programmable control of loading.
AIST used isotropic Sm-Fe-N-based magnet powder made by Daido Steel Co Ltd and formed an isotropic sintered magnet with the following properties: remanent flux density, coercive force and maximum energy product of the formed isotropic sintered magnet are 0.91T (9.1kG), 770kA/m (9.68kOe) and 129kJ/m3 (16.2MGOe), respectively. These are about 70%, 80% and 40% those of a normal Nd-Fe-B-based magnet.
The high-performance sintered isotropic magnet does not contain resin and has higher heat resistance and oxidation resistance than Nd-Fe-B-based magnets. Therefore, it is expected to be used in a high-temperature, high-humidity environment. Also, the performance of the new isotopic Sm-Fe-N-based sintered magnet is expected to be further enhanced by improving the material characteristics and controlling crystals.