Method for preparing high-performance sintered NdFeB magnets and sintered NdFeB magnets

Abstract

The present disclosure relates to a method for preparing high-performance sintered NdFeB magnets. The method comprises the steps of: a) attaching a multi-element alloy powder onto a surface of the sintered NdFeB magnet, wherein the multi-element alloy is of formula (1) Pr.sub.aRH.sub.bGa.sub.cCu.sub.d (1) with RH being at least one element selected from Dy and Tb and a, b, c, and d satisfying the conditions 0.30≤(a+b)/(a+b+c+d)≤0.65, 0.20≤d/(c+d)≤0.50, and 0.23≤b/(a+b)≤0.60; and b) performing a diffusion process.

Claims

1. A method for preparing high-performance sintered NdFeB magnets comprising the steps of: a) attaching a multi-element alloy powder onto a surface of the sintered NdFeB magnet, wherein the multi-element alloy is of formula (1)
Pr.sub.aRH.sub.bGa.sub.cCu.sub.d  (1) with RH being at least one element selected from Dy and Tb and a, b, c, and d satisfying the conditions 0.30≤(a+b)/(a+b+c+d)≤0.65, 0.20≤d/(c+d)≤0.50, and 0.23≤b/(a+b)≤0.60; and b) performing a diffusion process.

2. The method of claim 1, wherein in the diffusion process of step b) a diffusion temperature is in the range of 720° C. to 980° C. for a period of 5 to 25 hours.

3. The method of claim 1, wherein step b) is followed by step c) of performing an aging process.

4. The method of claim 2, wherein step b) is followed by step c) of performing an aging process.

5. The method of claim 3, wherein in the aging process of step c) an aging temperature is in the range of 480° C. to 680° C. for a period of 1 to 10 hours.

6. The method of claim 4, wherein in the aging process of step c) an aging temperature is in the range of 480° C. to 680° C. for a period of 1 to 10 hours.

7. The method of claim 1, wherein an average particle size of the multi-element alloy powder is in the range of 10 μm to 1000 μm.

8. The method of claim 7, wherein the average particle size of the powder is 50 μm to 600 μm.

9. The method of claim 2, wherein an average particle size of the multi-element alloy powder is in the range of 10 μm to 1000 μm.

10. The method of claim 9, wherein the average particle size of the powder is 50 μm to 600 μm.

11. The method of claim 3, wherein an average particle size of the multi-element alloy powder is in the range of 10 μm to 1000 μm.

12. The method of claim 11, wherein the average particle size of the powder is 50 μm to 600 μm.

13. The method of claim 4, wherein an average particle size of the multi-element alloy powder is in the range of 10 μm to 1000 μm.

14. The method of claim 13, wherein the average particle size of the powder is 50 μm to 600 μm.

15. The method of claim 1, wherein in step a) the multi-element alloy powder is attached to a surface which perpendicular to the orientation direction of the sintered NdFeB magnet.

16. The method of claim 2, wherein in step a) the multi-element alloy powder is attached to a surface which perpendicular to the orientation direction of the sintered NdFeB magnet.

17. The method of claim 3, wherein in step a) the multi-element alloy powder is attached to a surface which perpendicular to the orientation direction of the sintered NdFeB magnet.

18. The method of claim 4, wherein in step a) the multi-element alloy powder is attached to a surface which perpendicular to the orientation direction of the sintered NdFeB magnet.

19. The method of claim 5, wherein in step a) the multi-element alloy powder is attached to a surface which perpendicular to the orientation direction of the sintered NdFeB magnet.

20. A high-performance sintered NdFeB magnet produced by the method according to claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Features will become apparent to those of ordinary skill in the art by describing in detail exemplary embodiments with reference to the attached drawings in which:

[0020] FIG. 1-1 is Tb element EDS mapping in example 1;

[0021] FIG. 1-2 is Pr element EDS mapping in example 1;

[0022] FIG. 2-1 is Tb element EDS mapping in example 2;

[0023] FIG. 2-2 is Pr element EDS mapping in example 2;

[0024] FIG. 3-1 is Tb element EDS mapping in example 3;

[0025] FIG. 3-2 is Pr element EDS mapping in example 3;

[0026] FIG. 4-1 is Dy element EDS mapping in example 4;

[0027] FIG. 4-2 is Pr element EDS mapping in example 4;

[0028] FIG. 5-1 is Tb+Dy element EDS mapping in example 5;

[0029] FIG. 5-2 is Pr element EDS mapping in example 5;

[0030] FIG. 6-1 is Tb element EDS mapping in comparative example 3;

[0031] FIG. 6-2 is Pr element EDS mapping in comparative example 3;

DETAILED DESCRIPTION

[0032] Reference will now be made in detail to embodiments, examples of which are illustrated in the accompanying drawings. Effects and features of the exemplary embodiments, and implementation methods thereof will be described with reference to the accompanying drawings. In the drawings, like reference numerals denote like elements, and redundant descriptions are omitted. The present disclosure, however, may be embodied in various different forms, and should not be construed as being limited to only the illustrated embodiments herein. Rather, these embodiments are provided as examples so that this disclosure will be thorough and complete, and will fully convey the aspects and features of the present disclosure to those skilled in the art.

[0033] Generally, there is provided a method for preparing high-performance sintered NdFeB magnets comprising the steps of:

a) attaching a multi-element alloy powder onto a surface of the sintered NdFeB magnet, wherein the multi-element alloy is of formula (1)

Pr.sub.aRH.sub.bGa.sub.cCu.sub.d  (1)

with RH being at least one element selected from dysprosium Dy and terbium Tb and a, b, c, and d satisfying the conditions 0.30≤(a+b)/(a+b+c+d)≤0.65, 0.20≤d/(c+d)≤0.50, and 0.23≤b/(a+b)≤0.60; and
b) performing a diffusion process.

[0034] The multi-element alloy powder may be prepared by melting the raw material according to the atomic ratio of the composition in, for example, a vacuum induction furnace. By vacuum spinning multi-element alloy flakes ca be produced. The multi-element alloy flakes are crushed into powders and then attached onto the surface of the neodymium iron boron sintered magnet as diffusion source. Crushing is performed such that an average particle size of the powders is 10 μm to 1000 μm, in particular 50 μm to 600 μm.

[0035] The average particle diameter of the particles may be for example measured by a laser diffraction device using appropriate particle size standards. Specifically, the laser diffraction device is used to determine the particle diameter distribution of the particles, and this particle distribution is used to calculate the arithmetic average of particle diameters.

[0036] The multi-element alloy powder is preferably attached onto a surface of the magnet which perpendicular to the (magnetic) orientation direction.

[0037] Then, a high-temperature diffusion treatment and low-temperature aging treatment is performed in a furnace under vacuum or inert conditions to obtain a diffused neodymium iron boron sintered magnet. Said step of high-temperature diffusion is characterized by a diffusion temperature in the range of 720° C. to 980° C. with a duration time of 5 of 25 hours. Directly following the high-temperature treatment or after a short timely delay of cooling down the magnet to a temperature in the range of 20° C. to 400° C., the low-temperature aging treatment is performed at an aging temperature in the range of 480° C. to 680° C. with a duration time of 1 to 10 hours.

[0038] To have a better understanding of the present disclosure, the examples set forth below provide illustrations of the present disclosure. The examples are only used to illustrate the present disclosure and do not limit the scope of the present disclosure.

Example 1

[0039] A vacuum induction furnace is charged with a raw material consisting of Pr50Tb15Ga28Cu7 (atomic ratio) and the molten alloy is made into alloy flakes by a vacuum spinning. The alloy flakes are crushed into a powder with an average particle size of 1000 μm. 2.0 wt. % of the powder is attached to a surface of a sintered NdFeB magnet which perpendicular to the orientation direction. The sintered NdFeB magnet is a N55 grade magnet prepared by a conventional process. The thickness of magnet sample in the diffusion direction is 4.0 mm. The initial performance is Br 1.505 T, Hcj 756.0 kA/m, squareness (Hk/Hcj) 0.95, and the magnet contains Nd, Fe, B, Cu, Co and other elements.

[0040] A vacuum heating furnace is used for heat treatment of the powder coated magnet, wherein diffusion is performed at a temperature of 720° C. for 25 hours and subsequently aging is performed at a temperature of 480° C. for 10 hours.

[0041] The magnetic properties of the diffused samples are measured, and the element distribution in the depth of 400 to 411 μm from the diffused surface is detected using EDS (X-ray energy spectrometer).

Example 2

[0042] The procedure was carried out as in Example 1, but with the following differences:

[0043] The powder consists of Pr12Tb18Ga35Cu35 having an average particle size of 10 μm. Diffusion is performed at a temperature of 980° C. for 5 hours and aging is performed at a temperature of 680° C. for 1 hour.

Example 3

[0044] The procedure was carried out as in Example 1, but with the following differences:

[0045] The powder consists of Pr30Tb20Ga35Cu15 having an average particle size of 50 μm. Diffusion is performed at a temperature of 900° C. for 10 hours and aging is performed at a temperature of 520° C. for 3 hours.

Example 4

[0046] The procedure was carried out as in Example 1, but with the following differences:

[0047] The powder consists of Pr30Dy20Ga35Cu15 having an average particle size of 600 μm. Diffusion is performed at a temperature of 900° C. for 10 hours and aging is performed at a temperature of 520° C. for 3 hours.

Example 5

[0048] The procedure was carried out as in Example 1, but with the following differences:

[0049] The powder consists of Pr30Tb10Dy10Ga35Cu15 having an average particle size of 300 μm. Diffusion is performed at a temperature of 900° C. for 10 hours and aging is performed at a temperature of 520° C. for 3 hours.

[0050] Table 1 summarizes the compositions and heavy rare earth contents of the diffusion powders used in Examples 1-5.

TABLE-US-00001 TABLE 1 Pr Tb Cu Ga Dy Pr + Tb + Dy (Tb + Dy)/ example (at. %) (at. %) (at. %) (at. %) (at. %) (at. %) (Pr + Tb + Dy) Cu/(Ga + Cu) 1 50.00 15.00 7.00 28.00 0.00 65.00 0.23 0.20 2 12.00 18.00 35.00 35.00 0.00 30.00 0.60 0.50 3 30.00 20.00 15.00 35.00 0.00 50.00 0.40 0.30 4 30.00 0.00 15.00 35.00 20.00 50.00 0.40 0.30 5 30.00 10.00 15.00 35.00 10.00 50.00 0.40 0.30

[0051] Table 2 lists the magnetic performance of the treated magnets according to Example 1-5.

TABLE-US-00002 TABLE 2 example Br (T) Hcj(kA/m) Hk/Hcj ΔHcj(kA/m) ΔBr(T) Dy + Tb(wt. %) 1 1.484 1846.2 0.94 1090.2 −0.021 0.40 2 1.475 1928.2 0.95 1172.2 −0.030 0.62 3 1.476 1921.8 0.95 1165.8 −0.029 0.59 4 1.475 1460.2 0.93 704.3 −0.030 0.60 5 1.482 1636.1 0.94 880.1 −0.023 0.59

Comparative Example 1

[0052] The procedure was carried out as in Example 1, but with the following differences:

[0053] The powder consists of Tb70Cu30 having an average particle size of 300 μm. Diffusion is performed at a temperature of 900° C. for 10 hours and aging is performed at a temperature of 520° C. for 3 hours.

Comparative Example 2

[0054] The procedure was carried out as in Example 1, but with the following differences:

[0055] The powder consists of Pr70Ga20Cu10 having an average particle size of 300 μm. Diffusion is performed at a temperature of 900° C. for 10 hours and aging is performed at a temperature of 520° C. for 3 hours.

Comparative Example 3

[0056] The procedure was carried out as in Example 1, but with the following differences:

[0057] The powder consists of Pr20Tb5Ga35Cu40 having an average particle size of 300 μm. Diffusion is performed at a temperature of 900° C. for 10 hours and aging is performed at a temperature of 520° C. for 3 hours.

[0058] Table 3 summarizes the compositions and heavy rare earth contents of the diffusion powders used in Comparative Examples 1-3.

TABLE-US-00003 TABLE 3 Comparative Pr Tb Cu Ga Dy Pr + Tb + Dy (Tb + Dy)/ example (at. %) (at. %) (at. %) (at. %) (at. %) (at. %) (Pr + Tb + Dy) Cu/(Ga + Cu) 1 0.00 70.00 30.00 0.00 0.00 70.00 1.00 1.00 2 70.00 0.00 10.00 20.00 0.00 70.00 0.00 0.33 3 20.00 5.00 40.00 35.00 0.00 25.00 0.20 0.53

[0059] Table 4 lists the magnetic performance of the treated magnets of Comparative Examples 1-3.

TABLE-US-00004 TABLE 4 comparative example Br (T) Hcj(kA/m) Hk/Hcj ΔHcj(kA/m) ΔBr(T) Dy + Tb(wt. %) 1 1.420 1691.0 0.87 935.0 −0.085 1.71 2 1.461 1136.4 0.94 380.4 −0.044 0.00 3 1.475 1235.8 0.93 479.9 −0.030 0.18

[0060] According to the results of Examples 1 to 5, it can be concluded that with the infiltration amount of heavy rare earth no more than 0.62% by weight, the coercivity increased over 704.3 kA/m after diffusion, and the remanence is not less than 1.475 T. Even when a low amount of heavy rare earth is used, a significant increase in coercivity is achieved without causing a significant decrease in remanence.

[0061] EDS (X-ray energy spectrometer) results showed that the diffusion depth of heavy rare earth elements exceeds 400 μm. Praseodymium and heavy rare earth elements formed a shell structure on the periphery of the main phase grains. In said shell structure, the distribution range of heavy rare earth elements does not exceed the distribution range of praseodymium. This structure not only increases the magnetocrystalline anisotropy field of the main phase grains, but also avoids heavy rare earth elements infiltrating into the centre of the main phase grains. That means, the coercivity increases obviously without large loss of remanence after diffusion.

[0062] Comparative Example 1 uses a terbium-copper binary alloy to diffuse into the base magnet. Although the coercivity is greatly improved after diffusion, the infiltration amount of heavy rare earth is too high and exceeds 1.7% by weight. At the same time, the remanence reduction value is as high as 0.085 T. The method of Comparative Example 1 therefore has low comprehensive performance and high raw material costs.

[0063] Comparative Example 2 uses a praseodymium-copper-gallium ternary alloy as a diffusion source. The low melting point makes the diffusion depth of each element in the diffusion process larger and the microstructure is more uniform. But because the diffusion source does not contain heavy rare earth elements, a shell structure with higher magnetocrystalline anisotropy fields in the grain boundaries is not formed. That results in only a small increase of coercivity.

[0064] In Comparative Example 3 a praseodymium-terbium-copper-gallium quaternary alloy is used, wherein the proportion of praseodymium and terbium in the alloy is relatively low, which however decreases the driving energy for diffusion. In particular, terbium cannot be detected in a depth of 400 μm and more according to the EDS mapping result. As a consequence, coercivity increase is limited.

[0065] In summary, the present invention provided a method for preparing NdFeB magnets magnet with higher magnetic performance and improved microstructure.