Effect of Thermocyclic Treatment on Coercivity of Nd-Fe-B Sintered Magnets.

Abstract

Nd-Fe-B sintered magnets based on the compound Nd2Fe14B have been widely studied. Even though the Nd-Fe-B pennanent magnet exhibits excellent magnetic properties, its applications are limited because of its poor thermal stability. It is well-known that magnetic properties such as coercivity and remanence decrease with increasing temperture，and disappear permanently over a certain temperature limit. For magnetic applications requiring high temperatures (over 200oC), such as electric vehicle driving motors, a coercivity of at least 2390 kA/m at room temperature is required to overcome the coercivity losses due to thermal effects. For possible applications in high performance permanent magnets, some efforts have been made to improve the thermal stability of Nd-Fe-B sintered magnets. ln this study, a thermocyclic treatment was repeatedly performed in certain temperature regions in order to control the microstructures of Nd-rich phases and to reduce thermal stress on grain boundaries which could be caused during expansion and shrinkage of Nd2Fe14B phases. The diffrence of thermal expansion coefficient between Nd2Fe14B and Nd-rich phases are the mechanism for thermocyclic treatment. The Nd-rich phases which were located in junctions could penetrate into the grainboundaries between Nd2Fe14B phases due to the difference of thermal expansion coefficient. An increase of coercive force was observed, and the microcracks that could be formed during thermocyclic treatment by repeated expansion and shrinkage were not observed.