ANISOTROPIC NANOCRYSTALLINE RARE EARTH PERMANENT MAGNET AND PREPARATION METHOD THEREOF

Abstract

Disclosed are an anisotropic nanocrystalline rare earth permanent magnet and a preparation method thereof. The rare earth permanent magnet includes an REFeB matrix phase and a second phase, wherein the REFeB matrix phase includes main phase RE.sub.2Fe.sub.14B flaky nanocrystallines regularly arranged and an RE-rich phase around main phase grains, the main phase RE.sub.2Fe.sub.14B flaky nanocrystallines having an average grain size in a length direction of 70 nm to 800 nm and an average grain size in a thickness direction of 30 nm to 200 nm; and the second phase includes at least one selected from the group consisting of an M-Cu phase and an MCuO phase, M being at least one selected from the group consisting of Ca and Mg.

Claims

1. An anisotropic nanocrystalline rare earth permanent magnet, comprising an REFeB matrix phase and a second phase, wherein the REFeB matrix phase comprises main phase RE.sub.2Fe.sub.14B flaky nanocrystallines regularly arranged and an RE-rich phase around main phase grains, the main phase RE.sub.2Fe.sub.14B flaky nanocrystallines having an average grain size in a length direction of 70 nm to 800 nm, and an average grain size in a thickness direction of 30 nm to 200 nm, and each having a length-to-thickness ratio of greater than 1.2; the REFeB matrix phase comprises a coarse-grained region having a volume fraction of not more than 10%, which is calculated based on a volume of the coarse-grained region and a volume of the rare earth permanent magnet, the coarse-grained region being an equiaxed grain region with a grain size of greater than 500 nm; and the second phase comprises at least one selected from the group consisting of an MCu phase and an MCuO phase, M being at least one selected from the group consisting of Ca and Mg.

2. The anisotropic nanocrystalline rare earth permanent magnet as claimed in claim 1, wherein the rare earth permanent magnet is prepared from a raw material powder obtained by mixing an REFeB magnetic powder and an MCu alloy powder, M being at least one selected from the group consisting of Ca and Mg; and the REFeB magnetic powder has a formula of RE.sub.xFe.sub.100-x-y-zTM.sub.yB.sub.z, wherein RE is at least one selected from the group consisting of La, Ce, Pr, Nd, Y, Dy, Tb, and Ho; TM is at least one selected from the group consisting of Co, Zr, Cr, V, Nb, Si, Ti, Mo, Mn, W, Ga, Cu, Al, and Zn; and x, y, and z each represent a mass fraction of an element, and satisfy inequalities: 26.0x36.0,0.14y8.0, and 0.8z1.36.

3. The anisotropic nanocrystalline rare earth permanent magnet as claimed in claim 2, wherein the MCu alloy powder is in an amount of 0.1 wt. % to 5.0 wt. % based on a mass of the REFeB magnetic powder; and the CaCu alloy powder has a Cu mass percentage of 10 wt. % to 60 wt. %; and the MgCu alloy powder has a Cu mass percentage of 1 wt. % to 50 wt. %; and the CaMgCu alloy powder has a Cu mass percentage of 10 wt. % to 50 wt. %.

4. (canceled)

5. The anisotropic nanocrystalline rare earth permanent magnet as claimed in claim 1, wherein the coarse-grained region in the REFeB matrix phase has the volume fraction of not more than 1%, which is calculated based on a volume of the coarse-grained region and a volume of the rare earth permanent magnet.

6. A method for preparing the anisotropic nanocrystalline rare earth permanent magnet as claimed in claim 1, comprising the steps of S1. mixing the REFeB magnetic powder with the MCu alloy powder to obtain a raw material powder, wherein M is at least one selected from the group consisting of Ca and Mg, the REFeB magnetic powder comprises at least one selected from the group consisting of an anisotropic magnetic powder and an isotropic magnetic powder, and the raw material powder obtained by mixing comprises at least one selected from the group consisting of an anisotropic raw material powder and an isotropic raw material powder; and S2. subjecting the raw material powder obtained in S1 to thermal processing, such that the main phase RE.sub.2Fe.sub.14B flaky nanocrystallines are regularly arranged, to obtain the anisotropic nanocrystalline rare earth permanent magnet.

7. (canceled)

8. (canceled)

9. The method as claimed in claim 6, wherein the thermal processing in S2 comprises at least one selected from the group consisting of thermal deformation, thermal extrusion deformation, magnetic field-oriented sintering molding, and mechanically-oriented sintering molding.

10. The method as claimed in claim 9, wherein the thermal deformation comprises the steps of S2.1. preparing the raw material powder at a temperature ranging from room temperature to 800 C. into a green body with a density of 50% to 99.99%; and S2.2. subjecting the green body to the thermal deformation at a temperature of 600 C. to 850 C. to reach a deformation amount of 30% to 90% at a deformation rate of 0.01 mm/s to 3.5 mm/s such that the main phase RE.sub.2Fe.sub.14B flaky nanocrystallines are regularly arranged.

11. The method as claimed in claim 9, wherein the thermal extrusion deformation comprises the steps of S2.1. preparing the raw material powder at a temperature ranging from room temperature to 800 C. into a green body with a density of 50% to 99.99%; and S2.2. subjecting the green body to the thermal extrusion deformation at a temperature of 600 C. to 850 C. such that the main phase RE.sub.2Fe.sub.14B flaky nanocrystallines are regularly arranged; or a step of S2 subjecting the raw material powder to the thermal extrusion deformation at a temperature of 600 C. to 850 C. such that the main phase RE.sub.2Fe.sub.14B flaky nanocrystallines are regularly arranged.

12. (canceled)

13. The method as claimed in claim 9, wherein the magnetic field-oriented sintering molding comprises the steps of S2.1. subjecting the anisotropic raw material powder to oriented molding under an external magnetic field of 1 T to 3 T, to obtain a molded green body; and S2.2. subjecting the molded green body to hot pressing sintering, wherein the hot pressing sintering is conducted at a temperature of 450 C. to 850 C. and a pressure of 50 MPa to 500 MPa for 1 min to 60 min, or S2.2. subjecting the molded green body to pressureless sintering, wherein the pressureless sintering is conducted at a temperature of 600 C. to 850 C. for 10 min to 120 min.

14. (canceled)

15. (canceled)

16. The method as claimed in claim 9, wherein the mechanically-oriented sintering molding comprises the steps of S2. subjecting the anisotropic raw material powder to hot-pressing oriented sintering at a temperature of 450 C. to 850 C. under a uniaxial pressure of 50 MPa to 500 MPa for 1 min to 60 min, wherein the REFeB magnetic powder of the anisotropic raw material powder is a flaky magnetic powder with a length-to-thickness ratio of 1.2 to 30; and the flaky magnetic powder has an easy magnetization direction parallel to a thickness direction of the flaky magnetic powder; or the steps of S2.1. subjecting the anisotropic raw material powder to cold pressing molding at room temperature under a uniaxial pressure to obtain a molded green body, wherein the REFeB magnetic powder of the anisotropic raw material powder is a flaky magnetic powder with a length-to-thickness ratio of 1.2 to 30; the flaky magnetic powder has an easy magnetization direction parallel to a thickness direction of the flaky magnetic powder; and the molded green body has a density of 40% to 70%; and S2.2. subjecting the molded green body to pressureless sintering at a temperature of at 600 C. to 850 C. for 10 min to 120 min.

17. (canceled)

18. The anisotropic nanocrystalline rare earth permanent magnet as claimed in claim 2, wherein the coarse-grained region in the REFeB matrix phase has the volume fraction of not more than 1%, which is calculated based on a volume of the coarse-grained region and a volume of the rare earth permanent magnet.

19. The anisotropic nanocrystalline rare earth permanent magnet as claimed in claim 3, wherein the coarse-grained region in the REFeB matrix phase has the volume fraction of not more than 1%, which is calculated based on a volume of the coarse-grained region and a volume of the rare earth permanent magnet.

20. The method as claimed in claim 6, wherein the rare earth permanent magnet is prepared from a raw material powder obtained by mixing an REFeB magnetic powder and an MCu alloy powder, M being at least one selected from the group consisting of Ca and Mg; and the REFeB magnetic powder has a formula of RE.sub.xFe.sub.100-x-y-zTM.sub.yB.sub.z, wherein RE is at least one selected from the group consisting of La, Ce, Pr, Nd, Y, Dy, Tb, and Ho; TM is at least one selected from the group consisting of Co, Zr, Cr, V, Nb, Si, Ti, Mo, Mn, W, Ga, Cu, Al, and Zn; and x, y, and z each represent a mass fraction of an element, and satisfy inequalities: 26.0x36.0,0.14y8.0, and 0.8z1.36.

21. The method as claimed in claim 6, wherein the MCu alloy powder is in an amount of 0.1 wt. % to 5.0 wt. % based on a mass of the REFeB magnetic powder; and the CaCu alloy powder has a Cu mass percentage of 10 wt. % to 60 wt. %; and the MgCu alloy powder has a Cu mass percentage of 1 wt. % to 50 wt. %; and the CaMgCu alloy powder has a Cu mass percentage of 10 wt. % to 50 wt. %.

22. The method as claimed in claim 6, wherein the coarse-grained region in the REFeB matrix phase has the volume fraction of not more than 1%, which is calculated based on a volume of the coarse-grained region and a volume of the rare earth permanent magnet.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0046] FIG. 1 shows an X-ray diffraction (XRD) pattern of a CaCu alloy powder prepared from Ca.sub.70Cu.sub.30 alloy in Example 1.

[0047] FIGS. 2A, 2B, and, 2C show scanning electron microscopy (SEM) images of a cross-section of the anisotropic nanocrystalline rare earth permanent magnet prepared in Comparative Example 1.

[0048] FIGS. 3A, 3B, and 3C show SEM images of cross-sections of anisotropic nanocrystalline rare earth permanent magnets prepared in Example 1 with 0.5 wt. % of CaCu alloy powder.

[0049] FIGS. 4A, 4B, and 4C show SEM images of cross-sections of anisotropic nanocrystalline rare earth permanent magnets prepared in Example 1 with 1.0 wt. % of CaCu alloy powder.

[0050] FIGS. 5A, 5B, and 5C show SEM images of cross-sections of anisotropic nanocrystalline rare earth permanent magnets prepared in Example 1 with 2.0 wt. % of CaCu alloy powder.

[0051] FIGS. 6A, 6B, 6C, 6D, and 6E show SEM-energy dispersive spectroscopy (EDS) spectra at an interface of the magnetic powder inside the anisotropic nanocrystalline rare earth permanent magnet prepared in Comparative Example 1, and FIGS. 6F, 6G, 6H, 6I, 6J, 6K, and 6L show SEM-EDS spectra at an interface of the magnetic powder inside the anisotropic nanocrystalline rare earth permanent magnet prepared by adding 1.0 wt. % of CaCu alloy powder in Example 1.

[0052] FIGS. 7A, 7B, 7C, 7D, 7E, and 7F show transmission electron microscope (TEM)-EDS spectra of the CaCuO phase region at an interface of the magnetic powder inside the anisotropic nanocrystalline rare earth permanent magnet prepared by adding 1.0 wt. % of CaCu alloy powder in Example 1.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0053] The present disclosure provides an anisotropic nanocrystalline rare earth permanent magnet and a preparation method thereof. To make the objects, technical solutions, and effects of the present disclosure clearer, the following further describes the present disclosure in detail. It should be understood that the specific embodiments described herein are merely intended to explain the present disclosure, but not to limit the present disclosure.

[0054] At present, there are two main methods for preparing the anisotropic nanocrystalline REFeB rare earth permanent magnet. The first method is to densify nanocrystalline or amorphous magnetic powder into a green body, and then thermoplastic processing is conducted on the green body, such that the main phase RE.sub.2Fe.sub.14B flaky nanocrystallines in the green body are regularly arranged through thermal coupling, that is to say, c-axes of the main phase RE.sub.2Fe.sub.14B flaky nanocrystallines are arranged parallel, to prepare the anisotropic nanocrystalline rare earth permanent magnet. The second method is to subject anisotropic magnetic powder with a nanocrystalline structure to magnetic field orientation and sintering density to prepare the anisotropic nanocrystalline rare earth permanent magnet.

[0055] After long-term research, the applicant has found that although nanocrystal grains with a single domain size is realized in the anisotropic nanocrystalline REFeB rare earth permanent magnet through preparation process control, its coercive force is far lower than an empirically expected value. This is mainly related to the microstructure of the magnet. Anisotropic nanocrystalline rare earth permanent magnets include main phase RE.sub.2Fe.sub.14B flaky nanocrystallines and an RE-rich phase distributed around the main phase grains. The former imparts the magnet with high saturation magnetization, and the latter's demagnetizing coupling ensures that the magnet shows a high coercive force. As a result, the grain size of the main phase and the composition and distribution of the RE-rich phase both are the determinants of the coercive force of rare earth permanent magnets. After extensive research, the applicant has also found that there are three main reasons for the decrease in coercive force of anisotropic nanocrystalline REFeB rare earth permanent magnet. On one hand, during the preparation, stress concentration and local overheating occur at the magnetic powder interface, and the grains in the interface region are prone to abnormal growth, forming coarse grains and resulting in a reduction in coercive force. On the other hand, there is a high ferromagnetic element content in the grain boundary phase between the main phase nano-grains, and exchange coupling occurs between the main phase grains, resulting in a reduction in the coercive force. Finally, the rare earth-rich phase is highly active and could be rapidly oxidized at room temperature into rare earth oxides, which reduce a demagnetizing coupling effect of the rare earth-rich phase, resulting in a reduction in the coercive force.

[0056] During the research, the applicant has surprisingly discovered that by alloying Ca and/or Mg with Cu, an MCu alloy with low melting points could be generated, namely a CaCu alloy (490 C.), an MgCu alloy (485 C.), or a CaMgCu alloy. The added MCu alloy does not react with the main phase RE.sub.2Fe.sub.14B, and low-melting MCu alloy could flow and diffuse during the magnet preparation process to achieve uniform distribution without breaking into extremely fine powder, thus avoiding serious oxidation of Ca or Mg. Compared with elemental Ca and Mg powders, the MCu alloy has better oxidation resistance, thus retaining the reducing activity of Ca and Mg, which could reduce rare earth oxides on the surface of the REFeB magnetic powder during the thermal processing, thereby increasing the fluidity of the rare earth-rich phase, inhibiting the enrichment of the rare earth-rich phase at the interface, and thereby better diffusing onto the surface of the main phase grains inside the magnet. In view of this, a pinning effect of grain boundaries relative to magnetic domain walls is enhanced to improve the coercive force of the magnet. In addition, the MCu phase or the MCuO phase generated in situ exists at the REFeB magnetic powder interface inside the magnet, and could inhibit the formation of coarse grains during thermal processing and is conducive to refining the main phase grains. This mechanism further improves the coercive force of anisotropic nanocrystalline rare earth permanent magnet.

[0057] The present disclosure is proposed based on the above researches.

[0058] The present disclosure provides an anisotropic nanocrystalline rare earth permanent magnet. The rare earth permanent magnet includes an REFeB matrix phase and a second phase, wherein the REFeB matrix phase includes main phase RE.sub.2Fe.sub.14B flaky nanocrystallines regularly arranged and an RE-rich phase around main phase grains: c-axes of the main phase RE.sub.2Fe.sub.14B flaky nanocrystallines are arranged in parallel: the main phase RE.sub.2Fe.sub.14B flaky nanocrystallines have an average grain size in a length direction of 70 nm to 800 nm, preferably 100 nm to 600 nm, more preferably 150 nm to 400 nm, even more preferably 200 nm to 400 nm, even more preferably 210 nm to 350 nm, and even more preferably 230 nm to 300 nm; and have an average grain size in a thickness direction of 30 nm to 200 nm, preferably 50 nm to 150 nm, and more preferably 70 nm to 120 nm; and the flaky nanocrystallines each have a length-to-thickness ratio of greater than 1.2, preferably not less than 1.5, and more preferably not less than 2. The second phase includes at least one selected from the group consisting of an MCu phase and an MCuO phase, and M is at least one selected from the group consisting of Ca and Mg.

[0059] In the present disclosure, the rare earth permanent magnet is prepared from a raw material powder obtained by mixing an REFeB magnetic powder and an MCu alloy powder, wherein M is at least one of Ca and Mg, the REFeB magnetic powder has a formula of RE.sub.xFe.sub.100-x-y-zTM.sub.yB.sub.z, wherein RE is one or more of La, Ce, Pr, Nd, Y, Dy, Tb, and Ho, preferably at least one of La, Ce, Pr, Nd, and Y: TM is one or more of Co, Zr, Cr, V, Nb, Si, Ti, Mo, Mn, W, Ga, Cu, Al, and Zn, preferably at least one of Co, Zr, Cr, V, Nb, Ga, and Mo; and x, y, and z each represent a mass fraction of an element, and satisfy inequalities: 26.0x36.0, 0.14y8.0, and 0.8z1.36.

[0060] Based on a mass of the REFeB magnetic powder, the MCu alloy powder is added in an amount of 0.1-5.0 wt. %, preferably 0.5-3 wt. %, more preferably 0.5-2.0 wt. %, even more preferably 0.5-1.5 wt. %, and still even more preferably 0.5-1.0 wt. %. The MCu alloy powder is distributed at the magnetic powder interface inside the REFeB magnet. An insufficient addition amount is not conducive to improving grain boundary phase distribution and suppressing grain coarsening: while an excessive addition amount leads to agglomeration of the MCu alloy powder, which is not conducive to improving the performance of nanocrystalline rare earth permanent magnet. In some embodiments, the CaCu alloy powder has a Cu mass percentage of 10-60 wt. %, preferably 15-55 wt. %, more preferably 20-50 wt. %. In some embodiments, the MgCu alloy powder has a Cu mass percentage of 1-50 wt. %, preferably 5-45 wt. %, and more preferably 10-45 wt. %. In some embodiments, the CaMgCu alloy powder has a Cu mass percentage of 10-50 wt. %. In some embodiments, the MCu alloy powder has an average particle size of not more than 300 m, preferably not more than 200 m, and more preferably not more than 100 m. The MCu alloy powder partially reduces the rare earth oxides, forming the MCuO phase, and the unreacted part remains as the MCu phase, which is distributed at the magnetic powder interface, inhibiting the growth of grains at the REFeB interface to form coarse grains. The REFeB matrix phase includes a coarse-grained region having a volume fraction of not more than 10%, which is calculated based on a volume of the coarse-grained region and a volume of the rare earth permanent magnet, the volume fraction is more preferably not more than 8%, even more preferably not more than 6%, even more preferably not more than 4%, even yet more preferably not more than 3%, even more preferably not more than 2%, even more preferably not more than 1% and not less than 0, or not less than 0.5%. The coarse-grained region refers to an equiaxed grain region with a grain size of greater than 500 nm, and a volume fraction of the coarse-grained region is calculated based on the volume of the coarse-grained region and the volume of the rare earth permanent magnet. Determination of the volume fraction of the coarse-grained region: a cross-section parallel to a c-axis (easy magnetization axis) of the magnet is tested by SEM, and then an average value of a ratio of the sum of the coarse-grained region area to a total area of the magnet in 5 low-magnification SEM images is averaged. In some embodiments, the low-magnification SEM images have a magnification of 500 times or other magnifications, whichever could achieve the measurement of the area of the coarse-grained region.

[0061] The present disclosure provides a method for preparing the anisotropic nanocrystalline rare earth permanent magnet, including the following steps: [0062] S1, mixing the REFeB magnetic powder with the MCu alloy powder to obtain the raw material powder, wherein M is at least one selected from the group consisting of Ca and Mg, the REFeB magnetic powder includes at least one selected from the group consisting of an anisotropic magnetic powder and an isotropic magnetic powder, and the raw material powder obtained by mixing includes at least one selected from the group consisting of an anisotropic raw material powder and an isotropic raw material powder; and [0063] S2, subjecting the raw material powder obtained in step S1 to thermal processing, such that the main phase RE.sub.2Fe.sub.14B flaky nanocrystallines are regularly arranged to obtain the anisotropic nanocrystalline rare earth permanent magnet.

[0064] In some embodiments, the REFeB magnetic powder adopts a nanocrystalline magnetic powder or an amorphous magnetic powder: the nanocrystalline magnetic powder has a structure of an equiaxed crystal structure or a flaky crystal structure: in the equiaxed crystal structure, an average grain size is in a range of 10 nm to 400 nm, preferably 30 nm to 200 nm, and more preferably 35 nm to 150 nm; and in the flaky crystal structure, an average grain size in a length direction of grains is in a range of 20 nm to 500 nm, preferably 50 nm to 400 nm, and more preferably 100 nm to 300 nm; and an average grain size in a thickness direction of the grains is in a range of 10 nm to 200 nm, preferably 30 nm to 150 nm, and more preferably 50 nm to 100 nm. In some embodiments, the REFeB magnetic powder is a flaky magnetic powder, a granular magnetic powder, or an irregular magnetic powder, wherein the flaky magnetic powder has a length-to-thickness ratio of 1.2 to 30, preferably 1.5 to 30, more preferably 2 to 20, even more preferably 3 to 15, and even more preferably 5 to 10. In some embodiments, an easy magnetization direction of the anisotropic flaky magnetic powder is parallel to a thickness direction of the magnetic powder.

[0065] In some embodiments, in step S2, the thermal processing includes at least one of thermal deformation, thermal extrusion deformation, magnetic field-oriented sintering molding, and mechanically-oriented sintering molding.

[0066] In some specific embodiments, the thermal deformation includes the following steps: [0067] S2.1, preparing the raw material powder at a temperature ranging from room temperature to 800 C. into a green body with a density of 50% to 99.99%, preferably 70% to 99.99%, more preferably 80% to 99.99%, and even more preferably 90% to 99.99%; and [0068] S2.2, subjecting the green body to the thermal deformation at a temperature of 600 C. to 850 C., preferably 650 C. to 800 C. to reach a deformation amount of 30% to 90%, preferably 50% to 80%, and more preferably 55% to 75% at a deformation rate of 0.01 mm/s to 3.5 mm/s, preferably 0.1 mm/s to 2 mm/s, such that the main phase RE.sub.2Fe.sub.14B flaky nanocrystallines are regularly arranged.

[0069] In some specific embodiments, the thermal extrusion deformation includes the following steps: [0070] S2.1, preparing the raw material powder at a temperature ranging from room temperature to 800 C. into a green body: preferably, preparing the nanocrystalline mixed magnetic powder into the green body at a temperature ranging from room temperature to 700 C., or preparing the amorphous mixed magnetic powder into the green body at a temperature of 600 C. to 750 C., wherein the green body has a density of 50% to 99.99%, preferably 70% to 99.99%, more preferably 80% to 99.99%, and even more preferably 90% to 99.99%; and [0071] S2.2, subjecting the green body to the thermal extrusion deformation at a temperature of 600 C. to 850 C., preferably 650 C. to 800 C. such that main phase RE.sub.2Fe.sub.14B flaky nanocrystallines are regularly arranged: or the following steps: [0072] S2, subjecting the raw material powder to the thermal extrusion deformation at a temperature of 600 C. to 850 C., preferably 650 C. to 800 C. such that the main phase RE.sub.2Fe.sub.14B flaky nanocrystallines are regularly arranged.

[0073] In some embodiments, the magnetic field-oriented sintering molding includes the following steps: [0074] S2.1, subjecting the anisotropic raw material powder to oriented molding under an external magnetic field of 1 T to 3 T, preferably 1.5 T to 3 T to obtain a molded green body; and [0075] S2.2, subjecting the molded green body to hot pressing sintering at a temperature of 450 C. to 850 C., preferably 650 C. to 800 C. and a pressure of 50 MPa to 500 MPa, preferably 50 MPa to 300 MPa for 1 min to 60 min, preferably 5 min to 30 min; or [0076] S2.2, subjecting the molded green body to pressureless sintering at a temperature of 600 C. to 850 C., preferably 650 C. to 800 C. for 10 min to 120 min, preferably 10 min to 60 min.

[0077] In some embodiments, the mechanically-oriented sintering molding includes the following steps: [0078] S2, subjecting the anisotropic raw material powder to hot-pressing oriented sintering at a temperature of 450 C. to 850 C., preferably 650 C. to 800 C. under a uniaxial pressure of 50 MPa to 500 MPa, preferably 50 MPa to 230 MPa for 1 min to 60 min, preferably 5 min to 30 min, wherein the REFeB magnetic powder of the anisotropic raw material powder is a flaky magnetic powder with a length-to-thickness ratio of 1.2 to 30, preferably 1.5 to 30; and the flaky magnetic powder has an easy magnetization direction parallel to a thickness direction of the magnetic powder: or the following steps: [0079] S2.1, subjecting the anisotropic raw material powder to cold-pressing molding at room temperature under a uniaxial pressure to obtain a molded green body, wherein the REFeB magnetic powder of the anisotropic raw material powder is a flaky magnetic powder with a length-to-thickness ratio of 1.2 to 30, preferably 1.5 to 30; the flaky magnetic powder has an easy magnetization direction parallel to a thickness direction of the magnetic powder; and the molded green body has a density of 40% to 70%, preferably 50% to 70%; and [0080] S2.2, subjecting the molded green body to pressureless sintering at a temperature of 600 C. to 850 C., preferably 650 C. to 800 C. for 10 min to 120 min, preferably 10 min to 60 min.

[0081] The present disclosure is further described below through specific examples.

EXAMPLE 1

[0082] This example provided an anisotropic nanocrystalline rare earth permanent magnet and a preparation method thereof, wherein the preparation method was as follows.

[0083] S1. An alloy ingot with a chemical formula of Nd.sub.29.89Fe.sub.62.62Co.sub.5.93Ga.sub.0.64B.sub.0.92 (wt. %) was subjected to melt-spinning at a linear speed of a fast quenching roller of 28 m/s, to obtain a nanocrystalline rapidly-quenched magnetic powder with a particle size of 50 m to 450 m, wherein the nanocrystalline rapidly-quenched magnetic powder was an isotropic magnetic powder with an equiaxed crystal structure, and in the equiaxed crystal structure, an average grain size was 40 nm. An alloy ingot with a chemical formula of Ca.sub.70Cu.sub.30 (wt. %) was mechanically crushed to obtain an alloy powder with an average particle size of 100 m, and a phase of the alloy powder was determined by XRD. As shown in FIG. 1, the alloy powder included a Ca phase and a Ca.sub.2Cu phase, and the crushed alloy powder was not oxidized. The Ca.sub.70Cu.sub.30 alloy powder and the nanocrystalline rapidly-quenched magnetic powder were mixed three-dimensionally for 2 h under a protective atmosphere of argon to obtain a raw material powder, wherein the raw material powder was an isotropic raw material powder. In the prepared raw material powder, the Ca.sub.70Cu.sub.30 alloy powder was in an amount of 0.5 wt. %, 1.0 wt. %, 1.5 wt. %, and 2.0 wt. % of the nanocrystalline rapidly-quenched magnetic powder, respectively.

[0084] S2. The raw material powder prepared in S1 was subjected to thermal processing, specifically as follows:

[0085] S2.1, the raw material powders of different components were separately prepared into a green body under a vacuum degree of 110.sup.2 Pa and at a temperature of 650 C. and a pressure of 200 MPa, with a density of 99%; and

[0086] S2.2, the green body was subjected to thermal deformation at 780 C. under a vacuum degree of 110.sup.2 Pa at a thermal deformation rate of 0.1 mm/s, a deformation amount in a height direction of the green body being 70%, to obtain an anisotropic nanocrystalline magnet.

Comparative Example 1

[0087] Comparative Example 1 was different from Example 1 in that the Ca.sub.70Cu.sub.30 alloy powder was not added to the nanocrystalline rapidly-quenched magnetic powder in Comparative Example 1, while the remaining preparation steps were the same as those in Example 1.

Comparative Example 2

[0088] Comparative Example 2 was different from Example 1 in that the Al.sub.70Cu.sub.30 alloy powder was added to the nanocrystalline rapidly-quenched magnetic powder in Comparative Example 2, and the Al.sub.70Cu.sub.30 alloy powder was added in an amount of 1 wt. % of a mass of the nanocrystalline rapidly-quenched magnetic powder: while the remaining preparation steps were the same as those in Example 1.

[0089] Table 1 shows the magnetic properties of different magnets prepared in Example 1, Comparative Example 1, and Comparative Example 2.

TABLE-US-00001 TABLE 1 Magnetic properties of anisotropic nanocrystalline magnets Alloy added B.sub.r (kGs) H.sub.cj (kOe) (BH).sub.max (MGOe) 0 wt. % Ca.sub.70Cu.sub.30 13.60 14.15 42.5 0.5 wt. % Ca.sub.70Cu.sub.30 13.55 16.5 41.52 1.0 wt. % Ca.sub.70Cu.sub.30 13.49 17.70 41.41 1.5 wt. % Ca.sub.70Cu.sub.30 13.2 16.58 40.0 2.0 wt. % Ca.sub.70Cu.sub.30 13.12 15.8 38.2 1.0 wt. % Al.sub.70Cu.sub.30 13.12 16.54 39.95

[0090] As shown in Table 1, a certain amount of the Ca.sub.70Cu.sub.30 alloy could significantly improve a coercive force of the anisotropic nanocrystalline rare earth permanent magnet, and the amount was preferably 0.5-1.5 wt. %, and optimally 0.5-1.0 wt. %. Combined with FIG. 2A, it can be seen that in the thermal deformation-based magnet without adding CaCu alloy in Comparative Example 1, the white Nd-rich phase was enriched in the form of strips at the magnetic powder interface, and the independent point-like phases in the Nd-rich phase are rare earth oxides. As shown in FIG. 2B, there are coarse grains at the magnetic powder interface in the magnet. The coarse grains are equiaxed crystals, and an average grain size of the equiaxed crystals is 550 nm. A volume fraction of the coarse-grained region in the NdFeB matrix phase is 13.5%, and the volume fraction of the coarse-grained region was calculated based on a volume of the coarse-grained region and a volume of the rare earth permanent magnet. Determination of the volume fraction of the coarse-grained region: a cross-section parallel to the c-axis (easy magnetization axis) of the magnet was tested by SEM, and then a ratio of the sum of the coarse-grained region area to a total area of the magnet in 5 low-magnification (500 times) SEM images was averaged using Image-Pro Plus software. As shown in FIG. 2C, the NdFeB matrix phase includes regularly arranged main phase Nd.sub.2Fe.sub.14B flaky nanocrystallines and an Nd-rich phase around main phase grains. C-axes (grain thickness direction) of the main phase Nd.sub.2Fe.sub.14B flaky nanocrystallines are arranged in parallel. The main phase Nd.sub.2Fe.sub.14B flaky nanocrystallines have an average grain size in the length direction of 300 nm and an average grain size in the thickness direction of 100 nm, and the flaky nanocrystallines each have a length-to-thickness ratio of 3.

[0091] As shown in FIG. 3A, in the thermal deformation-based magnet with 0.5 wt. % of CaCu alloy, the white Nd-rich phase enrichs at the magnetic powder interface decreased. As shown in FIG. 3B, there were coarse grains at some magnetic powder interfaces. The coarse grains are equiaxed crystals, and an average grain size of the equiaxed crystals is 512 nm. Statistics show that a volume fraction of the coarse-grained region is 8%, and a black particle phase of CaCu alloy is distributed at the magnetic powder interface. As shown in FIG. 3C, the NdFeB matrix phase includes regularly arranged main phase Nd.sub.2Fe.sub.14B flaky nanocrystallines and an Nd-rich phase around main phase grains. C-axes of the main phase Nd.sub.2Fe.sub.14B flaky nanocrystallines (grain thickness direction) are arranged in parallel. The main phase Nd.sub.2Fe.sub.14B flaky nanocrystallines have an average grain size in the length direction of 290 nm and an average grain size in the thickness direction of 95 nm, and the flaky nanocrystallines each have a length-to-thickness ratio of 3.05.

[0092] As shown in FIG. 4A, in the thermal deformation-based magnet with 1.0 wt. % of CaCu alloy, there is only a small quantity of enriched white Nd-rich phase at the magnetic powder interface, and the Nd-rich phase at the magnetic powder interface is substantially completely suppressed, and the black particle phase of CaCu alloy is distributed at the magnetic powder interface. As shown in FIG. 4B, the coarse grains at the magnetic powder interface are significantly reduced. Statistics show that the volume fraction of the coarse-grained region is not more than 1% and not less than 0.5%, indicating that the coarse grains of the thermal deformation-based magnet are substantially completely suppressed and the coarse-grained region substantially disappears. As shown in FIG. 4C, the NdFeB matrix phase includes regularly arranged main phase Nd.sub.2Fe.sub.14B flaky nanocrystallines and an Nd-rich phase around main phase grains. C-axes (grain thickness direction) of the main phase Nd.sub.2Fe.sub.14B flaky nanocrystallines are arranged in parallel. The main phase Nd.sub.2Fe.sub.14B flaky nanocrystallines have an average grain size in the length direction of 280 nm and an average grain size in the thickness direction of 80 nm, and the flaky nanocrystallines each have a length-to-thickness ratio of 3.5.

[0093] As shown in FIG. 5A, in the thermal deformation-based magnet with 2.0 wt. % of CaCu alloy, the black particle phase is enriched at the magnetic powder interface, while there is no enriched white Nd-rich phase at the magnetic powder interface. As shown in FIG. 5B, further increase in the CaCu alloy content leads to the agglomeration of CaCu alloy particles at the magnetic powder interface inside the magnet, resulting in uneven microstructure inside the magnet and decreased magnetic properties of the magnet. Statistics show that a volume fraction of the coarse-grained region is not more than 1%. As shown in FIG. 5C, the NdFeB matrix phase includes regularly arranged main phase Nd.sub.2Fe.sub.14B flaky nanocrystallines and a Nd-rich phase around main phase grains. C-axes (grain thickness direction) of the main phase Nd.sub.2Fe.sub.14B flaky nanocrystallines are arranged in parallel. The main phase Nd.sub.2Fe.sub.14B flaky nanocrystallines have an average grain size in the length direction of 281 nm and an average grain size in the thickness direction of 79 nm, and the flaky nanocrystallines each have a length-to-thickness ratio of 3.56.

[0094] Combining the above analysis, it can be seen that: on one hand, an appropriate amount of CaCu alloy added to the anisotropic nanocrystalline rare earth permanent magnet could inhibit the formation of coarse grains at the magnet interface: on the other hand, the CaCu alloy could also refine the grains of the main phase RE.sub.2Fe.sub.14B flaky nanocrystallines. In addition, the CaCu alloy could inhibit the enrichment of the Nd-rich phase at the magnetic powder interface, increase the content of the Nd-rich phase between the main phase grains inside the magnet, enhance the pinning effect of the grain boundaries on the magnetic domain walls, and improve the coercive force of the magnet.

[0095] Further, as shown in FIG. 6A to FIG. 6E, in the thermal deformation-based magnet without adding CaCu alloy in Comparative Example 1, there is a large quantity of granular rare earth oxide NdO phase in the Nd-rich phase at the magnetic powder interface inside the magnet. As shown in FIG. 6F to FIG. 6L, in the thermal deformation-based magnet with 1.0 wt. % of CaCu alloy in Example 1, the rare earth oxide NdO phase at the magnetic powder interface reacts with part of the CaCu alloy to form a CaCuO phase, which could be further confirmed by the TEM-EDS spectrum of the thermal deformation-based magnet. Combined with FIG. 7A to FIG. 7F, the EDS element mapping of the particle phase in the framed area in FIG. 7A is shown in FIG. 7B to FIG. 7F, indicating that the particle phase in the framed area is composed of three elements: Ca, Cu, and O. The unreacted CaCu alloy remains as the CaCu phase in the thermal deformation-based magnet, indicating that the CaCu alloy does not react with the main phase Nd.sub.2Fe.sub.14B. FIG. 7A, FIG. 7B, and FIG. 7C also intuitively show the main phase Nd.sub.2Fe.sub.14B flaky nanocrystallines and the grain boundary Nd-rich phase, and the grain boundary Nd-rich phase is continuously distributed in the grain boundaries to improve the pinning effect. Therefore, the coercive force of the thermal deformation-based magnet was significantly improved.

[0096] Compared with the thermal deformation-based magnet of Example 1 with 1.0 wt. % of Ca.sub.70Cu.sub.30 alloy, the thermal deformation-based magnet of Comparative Example 2 with 1.0 wt. % of Al.sub.70Cu.sub.30 alloy, a residual magnetism of the thermal deformation-based magnet prepared in Comparative Example 2 is significantly reduced, and there is a slight coercive force increase. This was mainly due to the fact that there are more coarse grains at the magnetic powder interface of the thermal deformation-based magnet, and statistics show that the volume fraction of the coarse-grained region is 12%. Although Al and Cu elements could improve the grain boundary composition of nanocrystalline magnets through diffusion, the coarse-grained region at the interface of nanocrystalline magnets is not effectively suppressed, resulting in a significant reduction in residual magnetism and a low coercive force increase of the magnet.

Example 2

[0097] This example provided an anisotropic nanocrystalline rare earth permanent magnet and a preparation method thereof, and the preparation method was as follows:

[0098] S1. An alloy ingot with a chemical formula of Nd.sub.29.39Ce.sub.0.5Fe.sub.62.76Co.sub.5.93 Ga.sub.0.5B.sub.0.92 (wt. %) was subjected to melt-spinning at a linear speed of a fast quenching roller of 35 m/s, to obtain an amorphous rapidly-quenched magnetic powder with a particle size of 50 m to 450 m, and the amorphous rapidly-quenched magnetic powder was an isotropic magnetic powder. An alloy ingot with a chemical formula of Ca.sub.65Cu.sub.35 (wt. %) was mechanically crushed to obtain an alloy powder with an average particle size of 100 m. The Ca.sub.65Cu.sub.35 alloy powder and the amorphous rapidly-quenched magnetic powder were mixed three-dimensionally for 2 h under a protective atmosphere of argon to obtain a raw material powder, wherein the raw material powder was an isotropic raw material powder. In the prepared raw material powder, the Ca.sub.65Cu.sub.35 alloy powder was added in an amount of 1.0 wt. % of the amorphous rapidly-quenched magnetic powder.

[0099] S2. The raw material powder prepared in S1 was subjected to thermal processing, specifically as follows:

[0100] S2.1, the raw material powder was prepared into a hot-pressed green body having a diameter of 30 mm under a vacuum degree of 110.sup.2 Pa at a temperature of 680 C. and a pressure of 200 MPa, with a density of 99%; and

[0101] S2.2, the hot-pressed green body was subjected to thermal extrusion deformation at 750 C. at a thermal extrusion rate of 0.1 mm/s under a vacuum degree of 110.sup.2 Pa, to obtain an anisotropic nanocrystalline magnetic ring with a wall thickness of 2 mm and an outer diameter of 30 mm.

Comparative Example 3

[0102] Comparative Example 3 was different from Example 2 in that the Ca.sub.65Cu.sub.35 alloy powder was not added in Comparative Example 3, while the remaining preparation steps were the same as those in Example 2. Table 2 shows the magnetic properties of the anisotropic nanocrystalline magnetic rings prepared in Example 2 and Comparative Example 3.

TABLE-US-00002 TABLE 2 Magnetic properties of anisotropic nanocrystalline magnetic ring B.sub.r (kGs) H.sub.cj (kOe) (BH).sub.max (MGOe) Example 2 13.42 17.93 40.1 Example 3 13.54 15.4 40.3

[0103] By comparing the magnetic properties of the anisotropic nanocrystalline rare earth permanent magnets prepared in Example 2 and Comparative Example 3, it can be seen that the coercive force of the thermal extrusion-based magnetic ring is greatly improved after adding 1.0 wt. % of Ca.sub.65Cu.sub.35 alloy in Example 2. SEM results show that the main phase Nd.sub.2Fe.sub.14B flaky nanocrystallines in the non-doped NdFeB thermal extrusion-based magnetic ring have an average grain size in the length direction of 240 nm and an average grain size in the thickness direction of 110 nm. There are a large number of coarse equiaxed crystals at the magnetic powder interface inside the magnetic ring. According to statistics, the volume fraction of the coarse crystal region is 11.5%. In addition, there is also a large quantity of rare earth oxide NdO phase at the magnetic powder interface. At the magnetic powder interface inside the thermal extrusion-based magnetic ring with 1.0 wt. % Ca.sub.65Cu.sub.35 alloy, the rare earth oxide NdO phase substantially disappears, and the enrichment of the Nd-rich phase at the magnetic powder interface is inhibited. The CaCu phase and CaCuO phase are evenly distributed at the interface. The matrix phase includes regularly arranged main phase Nd.sub.2Fe.sub.14B flaky nanocrystallines and a Nd-rich phase around main phase grains. C-axis (grain thickness direction) of the main phase Nd.sub.2Fe.sub.14B flaky nanocrystallines are arranged in parallel. In addition, the grains of the main phase Nd.sub.2Fe.sub.14B flaky nanocrystallines of the magnetic ring have also been refined. The main phase Nd.sub.2Fe.sub.14B flaky nanocrystallines have an average grain size in the length direction of 235 nm and an average grain size in the thickness direction of 105 nm, and the flaky nanocrystallines each have a length-to-thickness ratio of 2.24. Statistics show that the volume fraction of the coarse-grained region is less than 1%, and the coarse-grained region at the interface is inhibited.

Example 3

[0104] This example provided an anisotropic nanocrystalline rare earth permanent magnet and a preparation method thereof, and the preparation method was as follows:

[0105] S1. An alloy ingot with a chemical formula of Nd.sub.30.2Fe.sub.62.45Co.sub.5.93Ga.sub.0.5B.sub.0.92 (wt. %) was subjected to melt-spinning at a linear speed of a fast quenching roller of 25 m/s, to obtain a nanocrystalline rapidly-quenched magnetic powder with a particle size of 50 m to 450 m, and the nanocrystalline rapidly-quenched magnetic powder was an isotropic magnetic powder. The nanocrystalline rapidly-quenched magnetic powder was subjected to heat preservation at 730 C. for 30 min under a vacuum degree of 510.sup.3 Pa to obtain an isotropic nanocrystalline magnetic powder with a flaky crystal structure, where the flaky crystal structure had an average grain size in the length direction of 240 nm and an average grain size in the thickness direction of 95 nm. An alloy ingot with a chemical formula of Ca.sub.70Cu.sub.30 (wt. %) was mechanically crushed to obtain an alloy powder with an average particle size of 100 m. The Ca.sub.70Cu.sub.30 alloy powder and the isotropic nanocrystalline magnetic powder with a flaky crystal structure were mixed three-dimensionally for 2 h under a protective atmosphere of argon to obtain an isotropic raw material powder. In the prepared raw material powder, the Ca.sub.70Cu.sub.30 alloy powder was added in an amount of 1.5 wt. % and 2.0 wt. % of the isotropic nanocrystalline magnetic powder with a flaky crystal structure, respectively.

[0106] S2. The raw material powder prepared in S1 was subjected to thermal processing, specifically as follows:

[0107] S2, the raw material powders of different components were directly extruded with an extruder under a vacuum degree of 110.sup.2 Pa at 680 C. to obtain an anisotropic nanocrystalline magnetic ring, wherein the magnetic ring had a diameter of 20 mm and a wall thickness of 2.5 mm. Table 3 shows the magnetic properties of the anisotropic magnetic ring.

TABLE-US-00003 TABLE 3 Magnetic properties of anisotropic magnetic ring Ca.sub.70Cu.sub.30 content B.sub.r (kGs) H.sub.cj (kOe) (BH).sub.max (MGOe) 1.5 wt. % 12.7 16.12 34.2 2.0 wt. % 12.2 16.09 32.5

[0108] It can be seen from the magnetic properties in Table 3 that by adding the low-melting Ca.sub.70Cu.sub.30 alloy the magnetic powder could be directly extruded into an anisotropic magnetic ring. SEM results show that the CaCu alloy is distributed at the magnetic powder interface, part of the CaCu alloy reacts with rare earth oxides to form the CaCuO phase, and the unreacted CaCu alloy remains as the CaCu phase. The REFeB matrix phase inside the thermal extrusion-based magnetic ring with 1.5 wt. % of Ca.sub.70Cu.sub.30 alloy includes regularly arranged main phase Nd.sub.2Fe.sub.14B flaky nanocrystallines and an Nd-rich phase around main phase grains: the main phase Nd.sub.2Fe.sub.14B flaky nanocrystallines have an average grain size in a length direction of 280 nm and an average grain size in a thickness direction of 100 nm; and the flaky nanocrystallines each have a length-to-thickness ratio of 2.8. Statistics show that a volume fraction of the coarse-grained region is not more than 1%. Inside the thermal extrusion-based magnetic ring with 2.0 wt. % of Ca.sub.70Cu.sub.30 alloy, the main phase Nd.sub.2Fe.sub.14B flaky nanocrystallines have an average grain size in a length direction of 279 nm and an average grain size in a thickness direction of 100 nm; and the flaky nanocrystallines each have a length-to-thickness ratio of 2.79. Statistics show that a volume fraction of the coarse-grained region is not more than 1%, indicating that interface coarse grains are inhibited.

Example 4

[0109] This example provided an anisotropic nanocrystalline rare earth permanent magnet and a preparation method thereof, and the preparation method was as follows:

[0110] S1. An alloy ingot with a chemical formula of Nd.sub.30.2Fe.sub.62.45Co.sub.5.93Ga.sub.0.5B.sub.0.92 (wt. %) was subjected to melt-spinning at a linear speed of a fast quenching roller of 25 m/s, to obtain a nanocrystalline rapidly-quenched magnetic powder with a particle size of 50 m to 450 m. The nanocrystalline rapidly-quenched magnetic powder was subjected to heat preservation at 730 C. for 30 min under a vacuum degree of 510.sup.3 Pa to obtain an isotropic nanocrystalline magnetic powder with a flaky crystal structure, wherein the flaky crystal structure had an average grain size in the length direction of 240 nm and an average grain size in the thickness direction of 95 nm. The isotropic nanocrystalline magnetic powder with a flaky crystal structure was subjected to hot-rolled deformation at 750 C., and a thickness of the magnetic powder was reduced by 65%, thereby obtaining the flaky anisotropic nanocrystalline magnetic powder. The flaky magnetic powder had a length-to-thickness ratio of 1.5 to 30, and an easy magnetization direction of the flaky magnetic powder was parallel to a thickness direction of the magnetic powder. The prepared magnetic powder properties included: Br=13.6 kGs, Hci=14.5 kOe, (BH) max=43.8 MGOe. An alloy ingot with a chemical formula of Ca.sub.70Cu.sub.30 (wt. %) was mechanically crushed to obtain an alloy powder with an average particle size of 100 m. The Ca.sub.70Cu.sub.30 alloy powder and the anisotropic nanocrystalline magnetic powder with a flaky crystal structure prepared were mixed three-dimensionally for 2 h under a protective atmosphere of argon to obtain an anisotropic raw material powder. In the prepared raw material powder, the Ca.sub.70Cu.sub.30 alloy powder was in an amount of 1.0 wt. % of a mass of the anisotropic nanocrystalline magnetic powder.

[0111] S2. The raw material powder prepared in S1 was subjected to thermal processing, specifically as follows:

[0112] S2, the raw material powder was subjected to hot pressing sintering under a vacuum degree of 110.sup.2 Pa at 680 C. and a pressure of 200 MPa for 5 min, and the parallel arrangement of the flaky anisotropic magnetic powder was promoted through the uniaxial pressure to obtain the anisotropic magnet.

Comparative Example 4

[0113] Comparative Example 4 was different from Example 4 in that the Ca.sub.70Cu.sub.30 alloy powder was not added in Comparative Example 4, while the remaining preparation steps were the same as those in Example 4. Table 4 shows the magnetic properties of the anisotropic magnets prepared in Example 4 and Comparative Example 4.

TABLE-US-00004 TABLE 4 Magnetic properties of anisotropic magnet B.sub.r (kGs) H.sub.cj (kOe) (BH).sub.max (MGOe) Example 4 12.9 14.1 37.5 Comparative Example 4 13.1 12.3 38.2

[0114] It can be seen from Table 4 that: an appropriate amount of CaCu alloy in Example 4 could significantly improve the coercive force of the mechanically-oriented anisotropic magnet. SEM results show that there is a large quantity of rare earth oxides at the magnetic powder interface inside the anisotropic magnet without adding CaCu alloy, and the volume fraction of the coarse-grained region is 14%; the main phase Nd.sub.2Fe.sub.14B flaky nanocrystallines have an average grain size in the length direction of 280 nm and an average grain size in the thickness direction of 120 nm, and the flaky nanocrystallines each have a length-to-thickness ratio of 2.33. In the anisotropic magnet with CaCu alloy in Example 4, the NdFeB matrix phase includes regularly arranged main phase Nd.sub.2Fe.sub.14B flaky nanocrystallines and an Nd-rich phase around main phase grains. The main phase Nd.sub.2Fe.sub.14B flaky nanocrystallines have an average grain size in the length direction of 275 nm and an average grain size in the thickness direction of 115 nm, and the flaky nanocrystallines each have a length-to-thickness ratio of 2.39. The rare earth oxides at the magnetic powder interface substantially disappear, and the CaCu phase and CaCuO phase are evenly distributed at the interface. In addition, the coarse-grained region at the magnetic powder interface substantially disappears, and a volume fraction of the coarse-grained region is calculated to be 2%.

Example 5

[0115] This example provided an anisotropic nanocrystalline rare earth permanent magnet and a preparation method thereof, where the preparation method was as follows:

[0116] S1. An alloy ingot with a chemical formula of Nd.sub.30.2Fe.sub.62.45Co.sub.5.93Ga.sub.0.5B.sub.0.92 (wt. %) was subjected to melt-spinning at a linear speed of a fast quenching roller of 25 m/s, to obtain a nanocrystalline rapidly-quenched magnetic powder with a particle size of 50 m to 450 m. The nanocrystalline rapidly-quenched magnetic powder was subjected to heat preservation at 730 C. for 30 min under a vacuum degree of 510.sup.3 Pa to obtain an isotropic nanocrystalline magnetic powder with a flaky crystal structure, wherein an average grain size in the length direction of the flaky crystal structure was 240 nm, and an average grain size in the thickness direction thereof was 95 nm. The isotropic nanocrystalline magnetic powder with a flaky crystal structure was subjected to hot-rolled deformation at 750 C., and a thickness of the magnetic powder was reduced by 65%, thereby obtaining the flaky anisotropic nanocrystalline magnetic powder. The flaky magnetic powder had a length-to-thickness ratio of 1.5 to 30, and an easy magnetization direction of the flaky magnetic powder was parallel to a thickness direction of the magnetic powder. The prepared magnetic powder properties included: Br=13.6 kGs, Hci=14.5 kOe, (BH) max=43.8 MGOe. An alloy ingot with a chemical formula of Mg.sub.85.5Cu.sub.14.5 (wt. %) was mechanically crushed to obtain an alloy powder with an average particle size of 100 m. The Mg.sub.85.5Cu.sub.14.5 alloy powder and the anisotropic nanocrystalline magnetic powder with a flaky crystal structure prepared were mixed three-dimensionally for 2 h under a protective atmosphere of argon to obtain an anisotropic raw material powder. In the prepared raw material powder, the Mg.sub.85.5Cu.sub.14.5 alloy powder was added in an amount of 0.5 wt. %, 1.0 wt. %, and 2.0 wt. % of the flaky anisotropic nanocrystalline magnetic powder, respectively.

[0117] S2. The raw material powder prepared in S1 was subjected to thermal processing, specifically as follows:

[0118] S2.1, the raw material powder was subjected to oriented molding under an external magnetic field of 2 T under a pressure of 50 MPa to obtain a molded green body; and

[0119] S2.2, the molded green body was vacuum-sintered under a vacuum degree of 110.sup.2 Pa at 680 C. for 20 min to obtain the anisotropic nanocrystalline permanent magnet.

Comparative Example 5

[0120] Comparative Example 5 was different from Example 5 in that the Mg.sub.85.5Cu.sub.14.5 alloy powder was not added in Comparative Example 5, while the remaining preparation steps were the same as those in Example 5. Table 5 shows the magnetic properties of the anisotropic magnets prepared in Example 5 and Comparative Example 5.

TABLE-US-00005 TABLE 5 Magnetic properties of anisotropic magnets B.sub.r (kGs) H.sub.cj (kOe) (BH).sub.max (MGOe) 0 wt. % 13.41 12.1 40.09 0.5 wt. % 13.31 12.9 39.91 1.0 wt. % 13.22 14.1 39.82 2.0 wt. % 13.02 13.2 38.84

[0121] As shown in Table 5, the addition of a certain amount of the Mg.sub.85.5Cu.sub.14.5 alloy could significantly improve a coercive force of anisotropic nanocrystalline magnet, and the amount is optimally 0.5-1.0 wt. %. The SEM results show that in the sintered nanocrystalline magnet without MgCu alloy added in Comparative Example 5, the main phase Nd.sub.2Fe.sub.14B flaky nanocrystallines have an average grain size in the length direction of 280 nm and an average grain size in the thickness direction of 105 nm, and there are coarse grains in the magnetic powder interface, statistics show that the volume fraction of the coarse-grained region is 14.5%. Further, there is a large quantity of rare earth oxides at the magnetic powder interface. In Example 5, in the sintered nanocrystalline magnet with 0.5 wt. % MgCu alloy, the NdFeB matrix phase includes regularly arranged main phase Nd.sub.2Fe.sub.14B flaky nanocrystallines and an Nd-rich phase around main phase grains. C-axis (grain thickness direction) of the main phase Nd.sub.2Fe.sub.14B flaky nanocrystallines are arranged in parallel, the main phase Nd.sub.2Fe.sub.14B flaky nanocrystallines have an average grain size in the length direction of 278 nm and an average grain size in the thickness direction of 102 nm, and the flaky nanocrystallines each have a length-to-thickness ratio of 2.73, there are coarse grains at some magnetic powder interfaces. Statistics show that the volume fraction of the coarse-grained region is 10%. After further increasing the MgCu alloy content to 1.0 wt. %, the main phase Nd.sub.2Fe.sub.14B flaky nanocrystallines have an average grain size in the length direction of 275 nm and an average grain size in the thickness direction of 101 nm, and the flaky nanocrystallines each have a length-to-thickness ratio of 2.72. Statistics show that the volume fraction of the coarse-grained region is 2.5%. When the MgCu alloy content reaches 2.0 wt. %, the main phase Nd.sub.2Fe.sub.14B flaky nanocrystallines have an average grain size in the length direction of 275 nm and an average grain size in the thickness direction of 100 nm, and the flaky nanocrystallines each have a length-to-thickness ratio of 2.75. Statistics show that the volume fraction of the coarse-grained region is 2.5%. Therefore, an appropriate amount of MgCu alloy could inhibit the formation of coarse grains at the interface. In addition, after the addition amount of MgCu alloy powder reaches 1.0 wt. %, on one hand, the rare earth oxide at the interface of the sintered magnet substantially disappears, and the MgCu phase and MgCuO phase are evenly distributed at the interface. On the other hand, the enrichment of the Nd-rich phase at the magnetic powder interface is also inhibited, thereby increasing the Nd-rich phase content between the main phase grains inside the magnetic powder, enhancing the pinning effect of the grain boundaries on the magnetic domain walls, and improving the coercive force of the magnet.

[0122] It should be understood that those of ordinary skill in the art can make improvements or transformations based on the above description, and all these improvements and transformations should fall within the protection scope of the appended claims of the present disclosure.