Method for producing magnetic powder and magnetic powder

Abstract

A method for producing a magnetic powder includes the steps of: mixing neodymium oxide, boron, and iron to prepare a first mixture; adding and mixing calcium to the first mixture to prepare a second mixture; mixing an alkali metal with the second mixture to prepare a third mixture; and placing a carbon sheet on the third mixture, placing silica sand (SiO.sub.2 sand) thereon, and then heating the same to a temperature of 800° C. to 1100° C.

Claims

1. A method for producing a magnetic powder, comprising: mixing neodymium oxide, boron, and iron to prepare a first mixture; adding calcium to the first mixture and mixing to prepare a second mixture; mixing an alkali metal with the second mixture to prepare a third mixture; and placing a carbon sheet on the third mixture, placing silica sand thereon, and then heating at a temperature of 800° C. to 1100° C.

2. The method for producing a magnetic powder of claim 1, wherein: the alkali metal is one or more selected from the group consisting of Li, Na, K, Rb, and Cs.

3. The method for producing a magnetic powder of claim 1, wherein: in the mixing of an alkali metal with the second mixture to prepare a third mixture, a content of the alkali metal is 1 wt % to 20 wt %.

4. The method for producing a magnetic powder of claim 1, wherein: the produced magnetic powder is Nd.sub.2Fe.sub.14B.

5. The method for producing a magnetic powder of claim 1, wherein: in the heating of the third mixture at temperature of 800° C. to 1100° C., a heating time is 10 minutes to 6 hours.

6. The method for producing a magnetic powder of claim 1, wherein: in the mixing of neodymium oxide, boron, and iron to prepare a first mixture, the first mixture further includes a metal fluoride.

7. The method for producing a magnetic powder of claim 6, wherein: the metal fluoride is one or more selected from the group consisting of fluorides of alkali metals, alkaline-earth metals, and transition metals.

8. The method for producing a magnetic powder of claim 7, wherein: the metal fluoride includes one or more metal fluorides selected from the group consisting of CaF.sub.2, LiF, AlF.sub.3, CoF.sub.2, CuF.sub.2, CrF.sub.3, FeF.sub.2, NiF.sub.2, GaF.sub.3, and ZrF.sub.4.

9. The method for producing a magnetic powder of claim 1, wherein: in the mixing of the neodymium oxide, the boron, and the iron to prepare the first mixture, one or more selected from the group consisting of Group 1 elements, Group 2 elements, and transition metals are further included.

10. The method for producing a magnetic powder of claim 1, wherein: the produced magnetic powder includes anisotropic crystal grains.

Description

DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows XRD patterns of magnetic powders produced in Examples 1 to 7 of the present invention.

(2) FIG. 2 shows magnetization hysteresis loops of magnetic powders produced in Examples 1 to 7.

(3) FIG. 3 shows magnetization hysteresis loops of magnetic powders produced in Examples 1 to 7.

(4) FIG. 4 is scanning electron microscope images of magnetic powders produced in Examples 1 to 7.

(5) FIG. 5 is particle size analysis (PSA) data of magnetic powders produced in Examples 1, 2, and 4.

(6) FIG. 6 shows the results of measuring B—H of a sintered magnet produced in Example 8.

(7) FIG. 7 is scanning electron microscope images of a magnetic powder produced in Comparative Example 1.

(8) FIG. 8 shows the results of measuring B—H of a sintered magnet produced in Comparative Example 2.

MODE FOR INVENTION

(9) Hereinafter, a method for producing a magnetic powder according to an exemplary embodiment of the present disclosure will be described in detail. The method for producing a magnetic powder according to the present exemplary embodiment may be a method for producing a Nd.sub.2Fe.sub.14B magnetic powder. That is, the method for producing a magnetic powder according to the present exemplary embodiment may be a method for producing a Nd.sub.2Fe.sub.14B-based alloy powder. The Nd.sub.2Fe.sub.14B alloy powder is a permanent magnet and may be referred to as a neodymium magnet.

(10) A method for producing a magnetic powder according to an exemplary embodiment of the present invention includes: mixing neodymium oxide, boron, and iron to prepare a first mixture; adding calcium to the first mixture and mixing them to prepare a second mixture; mixing an alkali metal with the second mixture to prepare a third mixture; and placing a carbon sheet on the third mixture, placing silica sand (SiO.sub.2 sand) thereon, and then heating the mixture at a temperature of 800° C. to 1100° C.

(11) The production method is a method of mixing raw materials such as neodymium oxide, boron, and iron, and reducing and diffusing the raw materials at a temperature of 800° C. to 1100° C. to form a Nd.sub.2Fe.sub.14B alloy powder. Specifically, a mole ratio of neodymium oxide, boron, and iron in the mixture of neodymium oxide, boron, and iron may be between 1:14:1 and 1.5:14:1. Neodymium oxide, boron, and iron are raw materials for producing a Nd.sub.2Fe.sub.14B magnetic powder, and when they satisfy the mole ratio, a Nd.sub.2Fe.sub.14B alloy powder may be produced at a high yield. When the mole ratio is 1:14:1 or less, there may be a problem that formation of the composition of a Nd.sub.2Fe.sub.14B main phase is failed or a Nd-rich grain boundary phase is not formed, and when the mole ratio is 1.5:14:1 or more, there may be a problem that reduced Nd remains due to an excessive amount of Nd and Nd remaining in a later-stage treatment process changes into Nd (OH).sub.3 or NdH.sub.2.

(12) In the step of mixing neodymium oxide, boron, and iron to prepare a first mixture, a step of mixing a metal fluoride may be further included. Here, a content of the fluoride may be 0.1 to 0.2 mol %, based on the entire first mixture. The metal fluoride may be one or more selected from the group consisting of fluorides of alkali metals, alkaline-earth metals, transition metals, and other metals

(13) Specifically, the metal fluoride may be one or more metal fluorides selected from the group consisting of CaF.sub.2, LiF, AlF.sub.3, CoF.sub.2, CuF.sub.2, CrF.sub.3, FeF.sub.2, NiF.sub.2, GaF.sub.3, and ZrF.sub.4.

(14) In addition, in the step of mixing neodymium oxide, boron, and iron to prepare a first mixture, the first mixture may further include one or more selected from the group consisting of Group 1 elements, Group 2 elements, and transition metals. As an example, copper or aluminum may be further added.

(15) Next, calcium is added to the first mixture and mixed to prepare a second mixture. Here, the calcium may be a reducing agent.

(16) An alkali metal is mixed with the second mixture to prepare a third mixture. The alkali metal may be one or more selected from the group consisting of Li, Na, K, Rb, and Cs. The alkali metal induces formation of anisotropic crystal grains inside a sintered magnet when the magnetic powder is sintered. Accordingly, magnetic crystal anisotropy of the sintered magnet may be optimized. When the magnetic powder is produced by a reduction-diffusion method in the state of not containing an alkali metal, the thus-produced magnetic powder has an irregular or isotropic shape. Accordingly, it is difficult to induce anisotropic crystal grains inside the sintered magnet, which acts as a limitation on optimizing the magnetic crystal anisotropy of the sintered magnet. However, the method for producing a magnetic powder according to the present exemplary embodiment may induce the anisotropic crystal grains of a magnetic powder by the alkali metal and control particle size and agglomeration.

(17) In addition, agglomeration of a Fe powder occurs locally due to limitation of dry mixing at the time of synthesis of Nd.sub.2O.sub.3, B, and Fe powder which are raw materials. In addition, agglomeration and particle growth occur due to atom transfer between Fe powders at the time of synthesis at a high temperature. However, when the alkali metal having a low melting point is used together as in an exemplary embodiment of the present invention, the alkali metal blocks atom transfer, so that particle separation becomes easy. Accordingly, the magnetic powder may be produced into fine particles.

(18) That is, when the alkali metal is added, the size of powder particles is decreased, spherical particles may be formed, and it is possible to produce spherical particles having a powder size of 1 to 2 μm.

(19) Here, a content of the alkali metal may be 1 wt % to 20 wt %. Preferably, when the content is 3 wt % to 7 wt %, shape and agglomeration are controlled well. When the content of the alkali metal is less than 1 wt %, shape and agglomeration may not be controlled well, and when the content is 20 wt % or more, vapor of the alkali metal occurs in the process and treatment before and after the process may be difficult.

(20) A carbon sheet is placed on the third mixture, silica sand (SiO.sub.2 sand) is placed thereon, and the mixture is heated at a temperature of 800° C. to 1100° C. Alkali metal vapor is adsorbed (captured) by the use of silica sand and contamination of process equipment with the alkali metal may be controlled.

(21) The step of heating the mixture to a temperature of 800° C. to 1100° C. may be performed for 10 minutes to 6 hours under an inert gas atmosphere. When a heating time is 10 minutes or less, the metal powder is not sufficiently synthesized, and when a heating time is 6 hours or more, a metal powder size becomes coarse and agglomeration between primary particles may occur.

(22) The thus-produced magnetic powder may be Nd.sub.2Fe.sub.14B. In addition, the size of the produced magnetic powder may be 0.5 μm to 10 μm. In addition, the size of the magnetic powder produced according to an exemplary embodiment may be 0.5 μm to 5 μm. In addition, the thus-produced magnetic powder includes anisotropic crystal grains. Accordingly, when the magnetic powder is sintered, magnetic crystal anisotropy of the sintered magnet may be optimized.

(23) Usually, in order to form a Nd.sub.2Fe.sub.14B alloy powder, raw materials are melted at a high temperature of 1500° C. to 2000° C. and then quenched to form a raw material mass, and the mass are subjected to coarse grinding, hydrogen crushing, and the like to obtain a Nd.sub.2Fe.sub.14B alloy powder.

(24) However, this method requires a high temperature for melting the raw materials and a process of cooling and grinding the raw materials again, and thus, a process time is long and the process is complicated.

(25) However, when the NdFeB-based powder is produced by the reduction-diffusion method as in the present exemplary embodiment, the Nd.sub.2Fe.sub.14B alloy powder is formed by reduction and diffusion of the raw materials at a temperature of 800° C. to 1100° C. In this step, since the size of the alloy powder is formed in a unit of several micrometers, a separate grinding process is not needed. More specifically, the size of the magnetic powder produced in the present exemplary embodiment may be 0.5 μm to 10 μm. Particularly, the size of the alloy powder may be adjusted by adjusting the size of an iron powder used as the raw material.

(26) In addition, since the alkali metal is included in the production process, formation of anisotropic crystal grains of the magnetic powder is induced by the alkali metal. Accordingly, magnetic crystal anisotropy of the sintered magnet may be optimized.

(27) Then, hereinafter, the magnetic powder according to an exemplary embodiment will be described. The magnetic powder according to the present exemplary embodiment may be produced by the produced method described above. In addition, the magnetic powder according to the present exemplary embodiment may include Nd.sub.2Fe.sub.14B, have a size of 0.5 μm to 10 μm, and include anisotropic crystal grains.

(28) Then, hereinafter, the method for producing a magnetic powder according to the present disclosure will be described by the specific examples.

EXAMPLE 1

Addition of Li

(29) To a sample in which 6.8682 g of Nd.sub.2O.sub.3, 0.2101 g of B, and 13.6742 g of Fe were uniformly mixed using a ball-mill and a paint shaker, 3.6742 g of Ca was further added and the materials were remixed using a Turbula mixer. The mixture was placed in a SUS tube having an optional shape, 0.1416 g of Li was added to the mixture, a carbon sheet was placed on the tapped mixture, silica sand (SiO.sub.2 sand) was placed thereon, and the reaction was performed in a tube electric furnace at 920° C. for 1 hour under an inert gas (Ar, He) atmosphere. After the reaction is completed, the sample was ground to form a powder, CaO which is a byproduct was removed using a NH.sub.4NO.sub.3-MeOH solution (or a NH.sub.4Cl-MeOH solution, NH.sub.4Ac-MeOH solution), and the powder was washed with acetone to finish a primary cleaning process and then vacuum dried. Thereafter, 0.2 g of SbF.sub.3 was dissolved in methanol to form a solution, which was placed in a vessel having an optional shape together with the synthesized powder, a balls for a ball mill was added to the vessel to grind the powder using a Turbula mixer, and the powder was secondarily cleaned with methanol and washed with acetone, and vacuum dried.

EXAMPLE 2

Addition of Na

(30) To a sample in which 6.8682 g of Nd.sub.2O.sub.3, 0.2101 g of B, and 13.6742 g of Fe were uniformly mixed using a paint shaker, 3.6742 g of Ca was further added, and the materials were remixed using a Turbula mixer. The mixture was placed in a SUS tube, 0.4691 g of Na was added to the mixture, and the mixture was tapped, reacted as presented in Example 1), and subjected to post-treatment.

EXAMPLE 3

Addition of NaK Mixture

(31) To a sample in which 6.8682 g of Nd.sub.2O.sub.3, 0.2101 g of B, and 13.6742 g of Fe were uniformly mixed using a paint shaker, a powder in which 0.7230 g of NaK and 3.6742 g of Ca were mixed was further added, and the materials were remixed using a paint shaker again. The mixture was placed in SUS, tapped, reacted by the method presented in Example 1), and subjected to post-treatment. NaK used in the present exemplary embodiment is an alloy of Na:K=20:80 and is in a liquid state at room temperature, and thus, uniform mixing is possible.

EXAMPLE 4

Addition of CaF.SUB.2.+Li

(32) To a sample in which 6.8682 g of Nd.sub.2O.sub.3, 0.2101 g of B, 13.6742 g of Fe, and 0.3035 g of CaF.sub.2 were uniformly mixed using a paint shaker, 3.6742 g of Ca was further added, and the materials were remixed using a Turbula mixer. The mixture was placed in a SUS tube, 0.1416 g of Li was added to the mixture, and the mixture was tapped, reacted by the method presented in Example 1), and subjected to post-treatment.

EXAMPLE 5

Addition of CaF.SUB.2.+Na

(33) To a sample in which 6.8682 g of Nd.sub.2O.sub.3, 0.2101 g of B, 13.6742 g of Fe, and 0.3035 g of CaF.sub.2 were uniformly mixed using a paint shaker, 3.6742 g of Ca was further added, and the materials were remixed using a Turbula mixer. The mixture was placed in a SUS tube, 0.4691 g of Na was added to the mixture, and the mixture was tapped, reacted by the method presented in Example 1), and subjected to post-treatment.

EXAMPLE 6

Addition of CaF.SUB.2.+NaK Mixture

(34) To a sample in which 6.8682 g of Nd.sub.2O.sub.3, 0.2101 g of B, 13.6742 g of Fe, and 0.3035 g of CaF.sub.2 were uniformly mixed using a paint shaker, a powder in which 0.7230 g of NaK and 3.6742 g of Ca were mixed was further added, and the materials were remixed using a paint shaker again. The mixture was placed in a SUS tube, tapped, reacted by the method presented in Example 1), and subjected to post-treatment.

EXAMPLE 7

Addition of LiF+NaK Mixture

(35) To a sample in which 6.8682 g of Nd.sub.2O.sub.3, 0.2101 g of B, 13.6742 g of Fe, and 0.2065 g of LiF were uniformly mixed using a paint shaker, a powder in which 0.7230 g of NaK and 3.6742 g of Ca were mixed was further added, and the materials were remixed using a paint shaker again. The mixture was placed in a SUS tube, tapped, reacted by the method presented in Example 1), and subjected to post-treatment.

EXAMPLE 8

Addition of Al+NaK Mixture+Sintering (NdH.SUB.2.)

(36) To a sample in which 6.8682 g of Nd.sub.2O.sub.3, 0.2101 g of B, 13.6742 g of Fe, 0.0617 g of Cu, and 0.042 g of Al were uniformly mixed using a paint shaker, a powder in which 0.7230 g of NaK and 3.6742 g of Ca were mixed was further added, and the materials were remixed using a paint shaker again. The mixture was placed in a SUS tube, tapped, reacted by the method presented in Example 1), and primarily cleaned. Thereafter, the powder was added to a NH.sub.4NO.sub.3-MeOH solution, ground-cleaned using a Turbula mixer, secondarily cleaned with methanol, washed with acetone, and vacuum dried. 8 g of NdFeBCu.sub.0.05Al.sub.0.08 powder particles and a NdH.sub.2 powder of a mass ratio of 12% were mixed, butanol as a lubricant was added thereto, and the mixture was molded in a magnetic field and then sintered at 1040° C. for 2 hours using a vacuum sintering furnace.

COMPARATIVE EXAMPLE 1

No Addition of Alkali Metal

(37) To a sample in which 6.8682 g of Nd.sub.2O.sub.3, 0.2101 g of B, and 13.6742 g of Fe were uniformly mixed using a paint shaker, 3.6742 g of Ca was further added, and the materials were remixed using a Turbula mixer. The mixture was placed in a SUS tube, tapped, reacted by the method presented by Example 1), and subjected to post-treatment. The scanning electron microscope image of the magnetic powder produced in Comparative Example 1 is illustrated in FIG. 7.

EXAMPLE 9

Sintering of Powder Produced in Example 3

(38) The powder produced in Example 3) was used to orient 3 g of the powder and sintered at 1040 C for 2 hours using a vacuum sintering furnace.

COMPARATIVE EXAMPLE 2

Sintering of Powder Produced in Comparative Example 1

(39) The powder produced in Comparative Example 1) was used to orient 3 g of the powder and sintered at 1040 C for 2 hours using a vacuum sintering furnace.

EVALUATION EXAMPLE 1

XRD Pattern

(40) XRD patterns of the magnetic powders produced in Examples 1 to 7 are shown in FIG. 1. FIG. 1 shows that a Nd.sub.2Fe.sub.14B main phase was formed well.

EVALUATION EXAMPLE 2

Magnetic Hysteresis Loop Data

(41) Magnetic hysteresis loops of the magnetic powders produced in Examples 1 to 7 are shown in FIG. 2, and the partially enlarged magnetic hysteresis loops of FIG. 2 are shown in FIG. 3. The magnetic hysteresis loop of the resulting magnetic powder may be confirmed from the results.

EVALUATION EXAMPLE 3

Scanning Electron Microscope Image

(42) Scanning electron microscope images of the magnetic powders produced in Examples 1 to 7 are shown in FIG. 4. It was confirmed from the results that the produced magnetic powder had an anisotropic shape and a size in a micro level.

EVALUATION EXAMPLE 4

PSA Data

(43) PSA data of the magnetic powders produced in Examples 1, 2, and 4 is shown in FIG. 5. The size distribution of the produced magnetic powder may be confirmed from the results.

EVALUATION EXAMPLE 5

B—H Data

(44) B—H of the sintered magnet produced in Example 8 was measured and the results are shown in FIG. 6. The magnetic characteristics of the produced sintered magnet may be confirmed from the results.

EVALUATION EXAMPLE 6

B—H Data

(45) B—H of the sintered magnet produced in Example 9 was measured and the results are shown in FIG. 8. In addition, B—H of the sintered magnet produced in Comparative Example 2 was measured and the results are shown together in FIG. 8. It may be confirmed therefrom that the sintered magnet produced in Example 9 had more improved characteristics than the sintered magnet produced in Comparative Example 2.

(46) While this invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. On the contrary, it is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.