NdFeB system sintered magnet and method for producing the same
Provided is a NdFeB sintered magnet which can be used in the grain boundary diffusion method as a base material in which R H can be easily diffused through the rare-earth rich phase and which itself has a high coercive force, a high maximum energy product and a high squareness ratio, as well as a method for producing such a magnet. A NdFeB system sintered has an average grain size of the main-phase grains magnet is equal to or smaller than 4.5 μm, the carbon content of the entire NdFeB system sintered magnet is equal to or lower than 1000 ppm, and the percentage of the total volume of a carbon rich phase in a rare-earth rich phase at a grain-boundary triple point in the NdFeB system sintered magnet to the total volume of the rare-earth rich phase is equal to or lower than 50%.
1. A NdFeB system sintered magnet, wherein:
a) an average grain size of a main-phase grains in the NdFeB system sintered magnet is equal to or smaller than 4.5 μm;
b) a carbon content of the entire NdFeB system sintered magnet is greater than 0 ppm and equal to or lower than 1000 ppm; and
c) a percentage of a total volume of a carbon rich phase in a rare-earth rich phase at a grain-boundary triple point in the NdFeB system sintered magnet to a total volume of the rare-earth rich phase is greater than 0% and equal to or lower than 50%.
2. A method for producing the NdFeB system sintered magnet according to claim 1 , comprising:
a) a hydrogen pulverization process for coarsely pulverizing a NdFeB system alloy by making the NdFeB system alloy occlude hydrogen;
b) a fine pulverization process for finely pulverizing the coarsely pulverized NdFeB system alloy so that a grain size of the alloy will be equal to or smaller than 3.2 μm in terms of a median D 50 of a grain size distribution measured by a laser diffraction method; and
c) a press-less magnet-production process including a step of putting fine powder of the NdFeB alloy into a filling container and a subsequent step of orienting and sintering the fine powder as held in the filling container,
wherein:
the fine pulverization process and the press-less magnet-production process are performed without thermal dehydrogenation for desorbing the hydrogen occluded in the hydrogen pulverization process; and
the processes from the hydrogen pulverization process through the press-less magnet-production process are performed in an oxygen-free atmosphere.