Rare-earth sintered magnet and method of producing the same

1. A rare-earth sintered magnet of a composition of (R1 x +R2 y )T 100-x-y-z Q z , where R1 is at least one element selected from the group consisting of all rare-earth elements excluding La, Y and Sc, R2 is Y and may optionally include La and/or Sc, T is at least one element selected from the group consisting of all transition elements, and Q is B and may optionally include C, and comprising a crystal grain of an Nd 2 Fe 14 B type compound as a main phase, wherein:

molar fractions x, y and z satisfy

8≦x≦18 at %,

0.1≦y≦3.5 at % and

3≦z≦20 at %, respectively; and

a concentration of R2 is higher in at least a part of a grain boundary phase than in the crystal grain, and

wherein an amount of oxygen is in a range of 2000 ppm to 8000 ppm by weight.

2. The rare-earth sintered magnet according to claim 1 , wherein the molar fractions x and y satisfy 0.01≦y/(x+y)≦0.23.

3. A method of producing a rare-earth sintered magnet, comprising the steps of:

preparing a powder of a rare-earth alloy having a composition of (R1 x +R2 y )T 100-x-y-z Q z where R1 is at least one element selected from the group consisting of all rare-earth elements excluding La, Y and Sc; R2 is Y and may optionally include La and/or Sc; T is at least one element selected from the group consisting of all transition elements; and Q is B and may optionally include C, wherein molar fractions x, y and z satisfy 8≦x≦18 at %, 0.1≦y≦3.5 at % and 3≦z≦20 at %, respectively, and wherein an amount of oxygen included in the rare-earth alloy powder is in a range of 2000 ppm by weight to 8000 ppm by weight; and

sintering the rare-earth alloy powder,

wherein R2 existing in a main phase crystal grain of an Nd 2 Fe 14 B crystalline structure in the rare-earth alloy before sintering is diffused into a grain boundary phase in the sintering step, whereby a concentration of R2 is higher in at least a part of the grain boundary phase than in the crystal grain.

4. The method of producing a rare-earth sintered magnet according to claim 3 , wherein R1 existing in the grain boundary phase in the rare-earth alloy before sintering is diffused into the main phase crystal grain during the sintering step.

5. The method of producing a rare-earth sintered magnet according to claim 3 , wherein an oxide of R2 is formed in the grain boundary phase during the sintering step.

6. The method of producing a rare-earth sintered magnet according to claim 3 , wherein the sintering step comprises a first step of maintaining the rare-earth alloy powder at a temperature in a range of 650 to 1000° C. for 10 to 240 minutes, and a second step of further sintering the rare-earth alloy powder at a temperature higher than that used in the first step.

7. The method of producing a rare-earth sintered magnet according to claim 3 , wherein the rare-earth alloy powder is obtained through pulverization in a gas whose oxygen concentration is controlled.

8. The method of producing a rare-earth sintered magnet according to claim 3 , wherein the rare-earth alloy powder is obtained through pulverization in a gas whose oxygen concentration is controlled to be 20000 ppm or less.

9. The method of producing a rare-earth sintered magnet according to claim 3 , wherein an average particle diameter (FSSS particle size) of the rare-earth alloy powder is 5 μm or less.

10. A rare-earth sintered magnet, having a composition of (R1 x +R2 y )(T1 p +T2 q ) 100-x-y-z-r Q z M r where R1 is at least one element selected from the group consisting of all rare-earth elements excluding La, Y and Sc, R2 is Y and may optionally include La and/or Sc: T1 is Fe, T2 is at least one element selected from the group consisting of all transition elements excluding Fe, Q is B and may optionally include C, and M is at least one element selected from the group consisting of Al, Ga, Sn and In, and comprising a crystal grain of an Nd 2 Fe 14 B type compound as a main phase, wherein:

molar fractions x, y, z, p, q and r satisfy

8≦x+y≦18 at %,

0<y≦4 at %,

3≦z≦20 at %,

0<q≦20 at %,

0<q/(p+q)≦0.3 at % and

0≦r≦3 at %, respectively; and

wherein an amount of oxygen is in a range of 2000 ppm to 8000 ppm by weight and a concentration of R2 is higher in at least a part of a grain boundary phase than in the crystal grain.

11. The rare-earth sintered magnet according to claim 10 , wherein the molar fraction y satisfies 0.5<y≦3 at %.

12. The rare-earth sintered magnet according to claim 10 , wherein T2 includes at least Co.

13. A method of producing a rare-earth sintered magnet, comprising the steps of:

preparing a powder of a rare-earth alloy having a composition of (R1 x +R2 y )(T1 p +T2 q ) 100-x-y-z-r Q z M r where R1 is at least one element selected from the group consisting of all rare-earth elements excluding La), Y and Sc, R2 is Y and may optionally include La and/or Sc: T1 is Fe, T2 is at least one element selected from the group consisting of all transition elements excluding Fe, Q is B and may optionally include C, and M is at least one element selected from the group consisting of Al, Ga, Sn and In), and comprising, as a main phase, a crystal grain of an Nd 2 Fe 14 B crystalline structure, wherein:

molar fractions x, y, z, p, q and r satisfy

8≦x+y≦18 at %,

0<y≦4 at %,

3≦z≦20 at %,

0<q≦20 at %,

0<q/(p+q)≦0.3 at % and

0≦r≦3 at %, respectively, and

wherein an amount of oxygen included in the rare-earth alloy powder is in a range of 2000 ppm by weight to 8000 ppm by weight; and

sintering the rare-earth alloy powder,

wherein R2 existing in the main phase crystal grain of the Nd 2 Fe 14 B crystalline structure in the rare-earth alloy before sintering is diffused into a grain boundary phase in the sintering step, whereby a concentration of R2 is higher in at least a part of the grain boundary phase than in the crystal grain.

14. The method of producing a rare-earth sintered magnet according to claim 13 , wherein R1 existing in the grain boundary phase in the rare-earth alloy before sintering is diffused into the main phase crystal grain during the sintering step.

15. The method of producing a rare-earth sintered magnet according to claim 13 , wherein an oxide of R2 is formed in the grain boundary phase in the sintering step.

16. The method of producing a rare-earth sintered magnet according to claim 13 , wherein the sintering step comprises a first step of maintaining the rare-earth alloy powder at a temperature in a range of 650 to 1000° C. for 10 to 240 minutes, and a second step of further sintering the rare-earth alloy powder at a temperature higher than that used in the first step.

17. The method of producing a rare-earth sintered magnet according to claim 13 , wherein the rare-earth alloy powder is obtained through pulverization in a gas whose oxygen concentration is controlled.

18. The method of producing a rare-earth sintered magnet according to claim 13 , wherein the rare-earth alloy powder is obtained through pulverization in a gas whose oxygen concentration is controlled to be 20000 ppm or less.

19. The method of producing a rare-earth sintered magnet according to claim 13 , wherein an average particle diameter (FSSS particle size) of the rare-earth alloy powder is 5 μm or less.