Nd based sintered magnet
A sintered Nd base magnet segment has a coercive force high at the periphery and lower toward the inside. A method for preparing the magnet includes the steps of: (a) providing a sintered Nd base magnet block having surfaces and a magnetization direction, (b) coating the surfaces of the magnet block excluding the surface perpendicular to the magnetization direction with a Dy or Tb oxide powder, a Dy or Tb fluoride powder, or a Dy or Tb-containing alloy powder, (c) treating the coated block at a high temperature for causing Dy or Tb to diffuse into the block, and (d) cutting the block in a plane perpendicular to the magnetization direction into a magnet segment having a coercive force distribution on the cut section that the coercive force is high at the periphery and lower toward the inside and a constant coercive force distribution in the magnetization direction.
1. A magnet segment for a rotor of a permanent magnet rotating machine, comprising:
a sintered Nd base magnet, having a diffused layer containing Dy or Tb diffused into the sintered Nd base magnet, the diffused layer covering the periphery of the sintered Nd base magnet excluding surfaces perpendicular to the magnetization direction, the magnet segment having a cross-section along the magnetization direction of a C or D shape,
wherein the magnet segment has a coercive force distribution where the coercive force is higher at the periphery and lower in a central portion along the surfaces perpendicular to the magnetization direction, and a constant coercive force distribution in the magnetization direction, wherein the sintered Nd base magnet is obtained by a method comprising the steps of:
providing a rectangular box shaped sintered Nd base magnet block having two surfaces perpendicular to a magnetization direction and four surfaces extending to the magnetization direction;
coating the four surfaces of the magnet block excluding the two surface perpendicular to the magnetization direction with powder of material selected from the group consisting of Dy oxide, Tb oxide, Dy fluoride, Tb fluoride, Dy-containing alloy, and Tb-containing alloy, thereby obtaining a coated block;
heating the coated block so that the Dy or Tb in the Dy or Tb oxide powder, the Dy or Tb fluoride powder, or the Dy or Tb-containing alloy powder, diffuses into the block, thereby forming the diffused layer covering the four surfaces extending to the magnetization direction;
cutting the coated block in a plane perpendicular to the magnetization direction to form a cut section into the magnet segment having the coercive force distribution on the cut section so that the coercive force is high at a periphery of the cut section and lower toward an inside of the cut section and the constant coercive force distribution in the magnetization direction; and then
machining the magnet segment on the surface perpendicular to the magnetization direction so as to make the cross-section along the magnetization direction into the C or D shape,
wherein the four surfaces extending to the magnetization direction is covered by the diffused layer, and
wherein the two surfaces opposing to each other, which is perpendicular to a magnetization direction or obtained by machining at least one of the two surfaces into a curved surface to form the cross-section along the magnetization direction into the C or D shape, are not covered by the diffused layer.
2. A sintered Nd base magnet segment according to claim 1 which is used as permanent magnet in a permanent magnet rotary machine.