Functionally graded rare earth permanent magnet
A functionally graded rare earth permanent magnet having a reduced eddy current loss in the form of a sintered magnet body having a composition R a E b T c A d F e O f M g is obtained by causing E and fluorine atoms to be absorbed in a R—Fe—B sintered magnet body from its surface. F is distributed such that its concentration increases on the average from the center toward the surface of the magnet body, the concentration of E/(R+E) contained in grain boundaries surrounding primary phase grains of (R,E) 2 T 14 A tetragonal system is on the average higher than the concentration of E/(R+E) contained in the primary phase grains, the oxyfluoride of (R,E) is present at grain boundaries in a grain boundary region that extends from the magnet body surface to a depth of at least 20 μm, particles of the oxyfluoride having an equivalent circle diameter of at least 1 μm are distributed in the grain boundary region at a population of at least 2,000 particles/mm 2 , the oxyfluoride is present in an area fraction of at least 1%. The magnet body includes a surface layer having a higher electric resistance than in the interior. In the permanent magnet, the generation of eddy current within a magnetic circuit is restrained.
1. A functionally graded rare earth permanent magnet having a reduced eddy current loss in the form of a sintered magnet body having an alloy composition of formula (1):
R a E b T c A d F e O f M g (1)
wherein R is at least one element selected from rare earth elements inclusive of Sc and Y, and E is at least one element selected from alkaline earth metal elements and rare earth elements, R and E do not contain the same element(s), T is one or both of iron and cobalt, A is one or both of boron and carbon, F is fluorine, O is oxygen, and M is at least one element selected from the group consisting of Al, Cu, Zn, In, Si, P, S, Ti, V, Cr, Mn, Ni, Ga, Ge, Zr, Nb, Mo, Pd, Ag, Cd, Sn, Sb, Hf, Ta, and W, a through g indicative of atom percents of the corresponding elements in the alloy have values in the range: 10≦a≦15 and 0.005≦b≦2, 3≦d≦15, 0.01≦e≦4, 0.04≦f≦4, 0.01 ≦g≦11, the balance being c, said magnet body having a center and a surface and being obtained by causing E and fluorine atoms to be absorbed in a R—Fe—B sintered magnet body from its surface,
wherein said sintered magnet body is obtained by heat treating the magnet body packed with a powder containing E and fluorine atoms to absorb and infiltrate E and fluorine atoms into the magnet body, and
wherein constituent element F is distributed such that its concentration increases on the average from the center toward the surface of the magnet body, grain boundaries surround primary phase grains of (R,E) 2 T 14 A tetragonal system within the sintered magnet body, the concentration of E/(R+E) contained in the grain boundaries is on the average higher than the concentration of E/(R+E) contained in the primary phase grains, an oxyfluoride of (R,E) is present at grain boundaries in a grain boundary region that extends from the magnet body surface to a depth of at least 20 μm, particles of said oxyfluoride having an equivalent circle diameter of at least 1 μm are distributed in said grain boundary region at a population of at least 2,000 particles/mm 2 , said oxyfluoride is present in an area fraction of at least 1%, and said magnet body includes a surface layer having a higher electric resistance than in the magnet body interior.
2. The rare earth permanent magnet of claim 1 wherein R comprises at least 10 atom % of Nd and/or Pr.
3. The rare earth permanent magnet of claim 1 wherein T comprises at least 60 atom % of iron.
4. The rare earth permanent magnet of claim 1 wherein A comprises at least 80 atom % of boron.
5. A functionally graded rare earth permanent magnet having a reduced eddy current loss in the form of a sintered magnet body having an alloy composition of formula (2):
(R•E) a+b T c A d F e O f M g (2)
wherein R is at least one element selected from rare earth elements inclusive of Sc and Y, and R and E contain the same element or elements, T is one or both of iron and cobalt, A is one or both of boron and carbon, F is fluorine, O is oxygen, and M is at least one element selected from the group consisting of Al, Cu, Zn, In, Si, P, S, Ti, V, Cr, Mn, Ni, Ga, Ge, Zr, Nb, Mo, Pd, Ag, Cd, Sn, Sb, Hf, Ta, and W, a through g indicative of atom percents of the corresponding elements in the alloy have values in the range: 10.005≦a+b≦17, 3≦d≦15, 0.01≦e≦4, 0.04≦f≦4, 0.01≦g≦11, the balance being c, said magnet body having a center and a surface and being obtained by causing E and fluorine atoms to be absorbed in a R-Fe-B sintered magnet body from its surface,
wherein said sintered magnet body is obtained by heat treating the magnet body packed with a powder containing E and fluorine atoms to absorb and infiltrate E and fluorine atoms into the magnet body, and
wherein constituent element F is distributed such that its concentration increases on the average from the center toward the surface of the magnet body, grain boundaries surround primary phase grains of (R,E) 2 T 14 A tetragonal system within the sintered magnet body, the E component is enriched adjacent to the grain boundaries, an oxyfluoride of (R,E) is present at grain boundaries in a grain boundary region that extends from the magnet body surface to a depth of at least 20 μm, particles of said oxyfluoride having an equivalent circle diameter of at least 1 μm are distributed in said grain boundary region at a population of at least 2,000 particles/mm 2 , said oxyfluoride is present in an area fraction of at least 1%, and said magnet body includes a surface layer having a higher electric resistance than in the magnet body interior.
6. The rare earth permanent magnet of claim 5 wherein R comprises at least 10 atom % of Nd and/or Pr.
7. The rare earth permanent magnet of claim 5 wherein T comprises at least 60 atom % of iron.
8. The rare earth permanent magnet of claim 5 wherein A comprises at least 80 atom % of boron.