Method for producing rare-earth magnet

Abstract

The present invention is a method capable of producing a rare-earth magnet with excellent magnetization and coercivity. The method includes producing a sintered body including a main phase and grain boundary phase and represented by (R1.sub.1-xR2.sub.x).sub.aTM.sub.bB.sub.cM.sub.d (where R1 represents one or more rare-earth elements including Y, R2 represents a rare-earth element different than R1, TM represents transition metal including at least one of Fe, Ni, or Co, B represents boron, M represents at least one of Ti, Ga, Zn, Si, Al, etc., 0.01x1, 12a20, b=100acd, 5c20, and 0d3 (all at %)); applying hot deformation processing to the sintered body to produce a precursor of the magnet; and diffusing/infiltrating melt of a R3-M modifying alloy (rare-earth element where R3 includes R1 and R2) into the grain boundary phase of the precursor.

Claims

1. A method for producing a rare-earth magnet, comprising: a first step of producing a sintered body with a structure including a main phase and a grain boundary phase, the sintered body consists of Nd, Pr, Fe, B, and M wherein the sintered body has a composition expressed by a formula: (Nd.sub.1-xPr.sub.x).sub.aFe.sub.bB.sub.cM.sub.d where B represents boron, M is at least one selected from the group consisting of Ti, Ga, Zn, Si, Al, Nb, Zr, Ni, Co, Mn, V, W, Ta, Ge, Cu, Cr, Hf, Mo, P, C, Mg, Hg, Ag, and Au, 0<x <0.5, 12a20, b=100acd , 5c 20, and 0d 20, and 0d 3 all by at %; a second step of applying hot deformation processing to the sintered body to produce a precursor of a rare-earth magnet; and a third step of providing a Nd-Cu alloy consisting of Nd and Cu on a surface of the precursor of the rare-earth magnet and then heat treating the precursor of the rare-earth magnet to diffuse and infiltrate a melt of the Nd-Cu alloy into the grain boundary phase of the precursor of the rare-earth magnet to produce a rare-earth magnet, wherein the rare-earth magnet has a main phase with a core-shell structure, wherein a composition of a shell formed around the core is a (NdPr)FeB phase, in which a content of Nd is more than a content of Pr, wherein a proportion of the main phase to the entire structure of the rare-earth magnet being is 95% or greater by volume percent, and the rare-earth magnet has a coercivity at 200 C. of higher than 4.8 kOe and less than 5.6 kOe.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIGS. 1A and B are schematic views sequentially illustrating a first step of a method for producing a rare-earth magnet of the present invention, and FIG. 1C is a schematic view illustrating a second step thereof.

(2) FIG. 2A is a view illustrating the micro-structure of a sintered body shown in FIG. 1B, and FIG. 2B is a view illustrating the micro-structure of a precursor of a rare-earth magnet shown in FIG. 1C.

(3) FIG. 3 is a schematic view illustrating a third step of the method for producing the rare-earth magnet of the present invention.

(4) FIG. 4 is a view showing the micro-structure of the crystal structure of the produced rare-earth magnet.

(5) FIG. 5 is a further enlarged view of the main phase and the grain boundary phase in FIG. 4.

(6) FIG. 6 is a diagram illustrating the heating path in the third step in producing a specimen.

(7) FIG. 7 is a diagram showing the relationship between the infiltration temperature of a modifying alloy and the coercivity of the produced rare-earth magnet in experiments, for each amount of substitution of Pr.

(8) FIG. 8 is a diagram showing the relationship between the amount of substitution of Pr and the amount of increase of coercivity in an experiment at an infiltration temperature of 580 C.

(9) FIG. 9 is a diagram showing the relationship between the temperature and the coercivity of each of a rare-earth magnet that contains Pr in the main phase and does not contain a modifying alloy diffused in the grain boundaries and a rare-earth magnet that contains Pr in the main phase and also contains a modifying alloy diffused in the grain boundaries.

(10) FIG. 10 is a diagram showing the relationship between the amount of Pr in the main phase and the coercivity at room temperature.

(11) FIG. 11 is a diagram showing the relationship between the amount of Pr in the main phase and the coercivity under an atmosphere of 200 C.

(12) FIG. 12 is a TEM photograph of a rare-earth magnet.

(13) FIG. 13 is a diagram showing the analysis results of EDX lines.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(14) (Method for Producing Rare-Earth Magnet)

(15) FIGS. 1A and 1B are schematic views sequentially illustrating a first step of a method for producing a rare-earth magnet of the present invention, and FIG. 1C is a schematic view illustrating a second step thereof. FIG. 3 is a schematic view illustrating a third step of the method for producing the rare-earth magnet of the present invention. In addition, FIG. 2A is a view illustrating the micro-structure of a sintered body shown in FIG. 1B, and FIG. 2B is a view illustrating the micro-structure of a precursor of a rare-earth magnet shown in FIG. 1C. Further, FIG. 4 is a view showing the micro-structure of the crystal structure of the produced rare-earth magnet. FIG. 5 is a further enlarged view of the main phase and the grain boundary phase in FIG. 4.

(16) As shown in FIG. 1A, an alloy ingot is melted at high frequency through single-roller melt-spinning in a furnace (not shown) with an Ar gas atmosphere whose pressure has been reduced to 50 kPa or less, for example, and then the molten metal with a composition that will provide a rare-earth magnet is sprayed at a copper roll R to produce a quenched thin strip (i.e., a quenched ribbon) B. Then, the quenched thin strip B is coarsely ground.

(17) A cavity, which is defined by a carbide die D and a carbide punch P that slides within a hollow space therein, is filled with coarse powder produced from the quenched thin strip B as shown in FIG. 1B, and then, pressure is applied thereto with the carbide punch P, and electrical heating is performed with current made to flow in the pressure application direction (i.e., the X-direction), whereby a sintered body S is produced that has a structure including a main phase and a grain boundary phase and represented by the compositional formula: (R1.sub.1-xR2.sub.x).sub.aTM.sub.bB.sub.cM.sub.d (where R1 represents one or more rare-earth elements including Y, R2 represents a rare-earth element different than R1, TM represents transition metal including at least one of Fe, Ni, or Co, B represents boron, M represents at least one of Ti, Ga, Zn, Si, Al, Nb, Zr, Ni, Co, Mn, V, W, Ta, Ge, Cu, Cr, Hf, Mo, P, C, Mg, Hg, Ag, or Au, 0.01x1, 12a20, b=100acd, 5c20, and 0d3 (all at %)). The main phase has a crystal grain size of about 50 to 300 nm (hereinabove, a first step).

(18) As shown in FIG. 2A, the sintered body S has an isotropic crystal structure in which gaps between nanocrystal grains MP (i.e., main phase) are filled with a grain boundary phase BP. Herein, in order to impart magnetic anisotropy to the sintered body S, the carbide punch P is made to abut the end faces of the sintered body S in the longitudinal direction thereof (in FIG. 1B, the horizontal direction is the longitudinal direction) as shown in FIG. 1C, and hot deformation processing is applied thereto while pressure is applied with the carbide punch P (in the X-direction), whereby a precursor C of a rare-earth magnet with a crystal structure that contains anisotropic nanocrystal grains MP is produced as shown in FIG. 2B (hereinabove, a second step).

(19) It should be noted that when the degree of processing (i.e., compressibility) of the hot deformation processing is high, for example, when the compressibility is greater than or equal to about 10%, the hot deformation processing can also be called hot high-strength processing or be simply called high-strength processing. However, processing is preferably performed at a degree of processing of about 60 to 80%.

(20) In the crystal structure of the precursor C of the rare-earth magnet shown in FIG. 2B, the nanocrystal grains MP have flat shapes, and an interface that is substantially parallel with the anisotropy axis is curved or bent, and is not formed by a particular plane.

(21) Next, as shown in FIG. 3, as a third step, modifying alloy powder SL is sprayed at the surface of the precursor C of the rare-earth magnet, and then, the precursor C is put in a high-temperature furnace H, and is kept therein under a high-temperature atmosphere for a predetermined retention time, whereby a melt of the modifying alloy SL is diffused and infiltrated into the grain boundary phase of the precursor C of the rare-earth magnet. It should be noted that the modifying alloy powder SL may be either processed into a plate shape so as to be placed on the surface of the precursor of the rare-earth magnet or be made into slurry so as to be applied to the surface of the precursor of the rare-earth magnet.

(22) For the modifying alloy powder SL herein, a modifying alloy is used that contains a transition metal element and a light rare-earth element and has a eutectic point as low as 450 to 700 C. For example, it is preferable to use one of a NdCu alloy (eutectic point: 520 C.), PrCu alloy (eutectic point: 480 C.), NdPrCu alloy, NdAl alloy (eutectic point: 640 C.), PrAl alloy (eutectic point: 650 C.), NdPrAl alloy, NdCo alloy (eutectic point: 566 C.), PrCo alloy (eutectic point: 540 C.), or NdPrCo alloy. Above all, it is more preferable to use an alloy with an eutectic point of less than or equal to 580 C., which is relatively low, such as a NdCu alloy (eutectic point: 520 C.), PrCu alloy (eutectic point: 480 C.), NdCo alloy (eutectic point: 566 C.), or PrCo alloy (eutectic point: 540 C.).

(23) When the melt of the modifying alloy SL is diffused and infiltrated into the grain boundary phase BP of the precursor C of the rare-earth magnet, the crystal structure of the precursor C of the rare-earth magnet shown in FIG. 2B changes, and the interfaces of the crystal grains MP become clear as shown in FIG. 4. Thus, magnetic separation between crystal grains MP, MP progresses, and a rare-earth magnet RM with improved coercivity is produced (i.e., a third step). It should be noted that while the crystal structure is being modified by the modifying alloy shown in FIG. 4, an interface that is substantially parallel with the anisotropy axis is not formed yet (i.e., not formed by a particular plane), but in the stage where modification by the modifying alloy has sufficiently progressed, an interface that is substantially parallel with the anisotropy axis (i.e., a particular plane) is formed. Thus, a rare-earth magnet whose crystal grains MP exhibit rectangular shapes or shapes close to rectangular shapes, when seen from the direction orthogonal to the anisotropy axis, is formed.

(24) As the main phase MP that partially constitutes the precursor C of the rare-earth magnet contains Pr that is the R2 element in addition to Nd that is the R1 element, for example, a substitution phenomenon occurs between the modifying alloy SL and the R2 element at the interface of the main phase, so that infiltration of the modifying alloy SL into the inside of the magnet is promoted.

(25) For example, when an NdCu alloy is used as the modifying alloy SL, as the main phase contains Pr with a lower melting point than Nd, the outer side of the main phase (i.e., an interface region between the main phase and the grain boundary phase) dissolves due to heat that is generated while the NdCu alloy is diffused in the grain boundaries, so that the dissolved region expands with the grain boundary phase BB in the molten state.

(26) Consequently, although the proportion of the grain boundary phase BP, which serves as an infiltration path for the NdCu alloy, has been low due to the high proportion of the main phase, it becomes possible to increase the efficiency of infiltration of the NdCu alloy with the expanded infiltration path. Consequently, the NdCu alloy can sufficiently infiltrate the inside of the magnet.

(27) After the NdCu alloy is diffused in the grain boundaries by the heat treatment in the third step, the temperature is returned to the room temperature. Thus, the outer region of the main phase MP, which has dissolved so far, is recrystallized, whereby a main phase with a core-shell structure is formed that includes a core phase in the center region of the main phase and a shell phase in the recrystallized outer region (see FIG. 5).

(28) The thus formed main phase with the core-shell structure can maintain the initial high proportion of the main phase. Thus, it is possible to obtain a rare-earth magnet with excellent magnetization performance as well as excellent coercivity performance as the NdCu alloy is sufficiently diffused in the grain boundaries of the grain boundary phase. As an example of such a core-shell structure, a (PrNd)FeB phase, which is a Pr-rich phase, can be used for the composition of the core that forms the main phase, and a (NdPr)FeB phase, which is a relatively Nd-rich phase, can be used for the composition of the shell around the core.

(29) [Experiments of Verifying the Magnetic Properties of Rare-Earth Magnets Produced with the Production Method of the Present Invention and the Results Thereof]

(30) The inventors produced a plurality of rare-earth magnets by applying the production method of the present invention and variously changing the concentration of Pr in the magnetic materials, and then conducted experiments of identifying the relationship between the infiltration temperature of the modifying alloy and the coercivity of the rare-earth magnets. In addition, the inventors also conducted experiments of identifying the temperature dependence of the coercivity of each rare-earth magnet. Further, the inventors conducted experiments of identifying the relationship between the substitution rate of Pr and the coercivity at room temperature and under a high-temperature atmosphere. Furthermore, the inventors conducted EDX analysis and confirmed that the main phase has a core-shell structure.

(31) (Experimental Method)

(32) A liquid quenched ribbon with a composition: (Nd.sub.(100-x) Pr.sub.x).sub.13.2Fe.sub.balB.sub.5.6Co.sub.4.7Ga.sub.0.5 (at %) was produced with a single-roller furnace (X=0, 1.35, 25, 50, or 100), and the obtained quenched ribbon was sintered to produce a sintered body (at a sintering temperature of 650 C. at 400 MPa). Then, high-strength processing was applied to the sintered body (at a processing temperature of 780 C. and a degree of processing of 75%) to produce a precursor of a rare-earth magnet. Then, heat treatment was applied to the obtained precursor of the rare-earth magnet in accordance with a heating path diagram shown in FIG. 6 to perform a process of infiltrating a NdCu alloy, thereby producing a rare-earth magnet (the modifying alloy used was a Nd.sub.70Cu.sub.30 material: 5%, and the thickness of the magnet before diffusion was 2 mm). The magnetic properties of each of the produced rare-earth magnets was evaluated with VSM and TPM. FIG. 7 shows the measurement results regarding the relationship between the infiltration temperature of the modifying alloy and the coercivity of the produced rare-earth magnet. FIG. 8 shows the experimental results regarding the relationship between the amount of substitution of Pr and the amount of increase of coercivity at an infiltration temperature of 580 C. FIG. 9 shows the experimental results regarding the temperature dependence of coercivity. Further, FIGS. 10 and 11 show the experimental results regarding the relationship between the amount of substitution of Pr and the coercivity at room temperature and under a high-temperature atmosphere (200 C.), respectively.

(33) From FIG. 7, it is found that each composition experiences little change even when the infiltration temperature is changed from 580 to 700 C. Herein, from the relationship between the concentration of Pr and the rate of change of coercivity at an infiltration temperature of 580 C. shown in FIG. 8, it is found that infiltration does not occur efficiently when the concentration of Pr is 0%, resulting in decreased coercivity, whereas the coercivity greatly improves at concentrations other than 0%.

(34) This is considered to be due to the fact that when the main phase has a small amount of Pr added thereto, the efficiency of infiltration of the NdCu alloy will increase, and thus, the NdCu alloy can sufficiently infiltrate the inside of the magnet.

(35) Next, from FIG. 9, it is found that a rare-earth magnet that contains Pr in the main phase and also contains a NdCu alloy infiltrated therein has higher coercivity than a rare-earth magnet without a NdCu alloy infiltrated therein by about as large as 5 kOe.

(36) In addition, from FIGS. 10 and 11, it is found that after a NdCu alloy is infiltrated at room temperature, the coercivity tends to increase in a parallel translation manner in the range in which the coercivity improves even when the concentration of Pr is changed, while at 200 C., the coercivity tends to increase not in a parallel translation manner but by the amount of parallel translation+ in the range in which the coercivity improves.

(37) This is considered to be due to the fact that at room temperature, the effect of improving the separation property of the crystal grains of the main phase by the NdCu alloy has a great influence, while at 200 C., not only is there the effect of improving the separation property but also the average magnetocrystalline anisotropy at high temperature is improved by the formation of the core-shell structure upon occurrence of the substitution of elements at the interface of the main phase.

(38) To be more specific, in the range in which the amount of substitution of Pr is 1 to 50%, an amount of increase of coercivity by a gain of + is observed, while at a substitution rate of 100%, it is considered that the gain is lost under the strong influence of the deterioration of the magnetocrystalline anisotropy of the core phase under a high-temperature atmosphere.

(39) FIG. 12 shows a TEM photograph of the structure of the rare-earth magnet, and FIG. 13 shows the analysis results of EDX lines.

(40) In FIG. 13, zero at the abscissa axis represents the starting point of the arrow in FIG. 12, and the abscissa axis represents the length of the structure from the starting point. A main phase 1 is the core phase and a main phase 2 is the shell phase. The total length of the main phases 1 and 2 is about 23 nm, and the grain boundary phase is located on the outer side thereof.

(41) The present analysis of the EDX lines can confirm that according to the magnet composition used in the experiments, the main phase 1 has a high Pr content and the main phase 2 has a high Nd content, and thus that a main phase with a core-shell structure with different compositions is formed.

(42) The main phase 1 that forms the core phase is a phase with high coercivity at room temperature, while the main phase 2 that forms the shell phase on the outer side of the core phase is a phase with high coercivity at high temperature. With the production method of the present invention, it is possible to produce a magnet with high coercivity as the separation property is improved as a result of a NdCu alloy having been sufficiently infiltrated. It should be noted that as the produced rare-earth magnet has a proportion of the main phase as high as 96 to 97%, such a magnet has high magnetization in addition to high coercivity.

(43) The present experiments have verified that the method for producing the rare-earth magnet in accordance with the present invention is an innovative production method that can increase not only the magnetization but also the coercivity of a rare-earth magnet that has a high proportion of a main phase and thus can otherwise frequently have a grain boundary phase in which a melt of a modifying alloy is not sufficiently infiltrated.

(44) Although the embodiments of the present invention have been described in detail with reference to the drawings, specific structures thereof are not limited thereto. Any design changes that may occur within the spirit and scope of the present invention fall within the present invention.

DESCRIPTION OF SYMBOLS

(45) R Copper roll B Quenched thin strip (Quenched ribbon) D Carbide die P Carbide punch S Sintered body C Precursor of rare-earth magnet H High-temperature furnace SL Modifying alloy powder (Modifying alloy) M Modifying alloy powder MP Main phase (nanocrystal grains, crystal grains) BP Grain boundary phase RM Rare-earth magnet