RFeB SYSTEM SINTERED MAGNET

Abstract

An RFeB system sintered magnet which does not contain a heavy rare-earth element R.sup.H (Dy, Tb and Ho) in a practically effective amount and yet is suited for applications in which the magnet undergoes a temperature increase during its use. The RFeB system sintered magnet contains at least one element selected from the group consisting of Nd and Pr as a rare-earth element R in addition to Fe and B while containing none of Dy, Tb and Ho, the magnet having a temperature characteristic value t.sub.(100-23) which satisfies −0.58<t.sub.(100-23)<0, where t.sub.(100-23) is defined by the following equation:

[00001]$t_{\left(100-23\right)}=\frac{H_{\mathrm{cj}}\left(100\right)-H_{\mathrm{cj}}\left(23\right)}{\left(100-23\right)\times H_{\mathrm{cj}}\left(23\right)}\times 100$

using H.sub.cj(23) which is the value of the coercivity at a temperature of 23° C. and H.sub.cj(100) which is the value of the coercivity at a temperature of 100° C.

Claims

1. An RFeB system sintered magnet containing at least one element selected from a group consisting of Nd and Pr as a rare-earth element R in addition to Fe and B while containing none of Dy, Tb and Ho, wherein: a temperature coefficient of coercivity t.sub.(100-23) satisfies −0.58<t.sub.(100-23)<0, where t.sub.(100-23) is defined by a following equation: $t_{\left(100-23\right)}=\frac{H_{\mathrm{cj}}\left(100\right)-H_{\mathrm{cj}}\left(23\right)}{\left(100-23\right)\times H_{\mathrm{cj}}\left(23\right)}\times 100$ using H.sub.cj(23) which is a value of a coercivity at a temperature of 23° C. and H.sub.cj(100) which is a value of the coercivity at a temperature of 100° C.

2. The RFeB system sintered magnet according to claim 1, wherein the temperature coefficient of coercivity t.sub.(100-23) is within a range of −0.58<t.sub.(100-23)≦−0.48.

3. The RFeB system sintered magnet according to claim 1 wherein a 50% cumulative diameter in the particle size distribution on an area basis D.sub.ave—S calculated from a circle-equivalent diameters D of crystal grains determined from a microscopic image of a section of the RFeB system sintered magnet is equal to or smaller than 1 μm.

4. A method for producing the RFeB system sintered magnet according to claim 1, comprising steps of: preparing a shaped body oriented by a magnetic field and subsequently sintering the shaped body, using an RFeB system alloy powder having a 50% cumulative diameter in the particle size distribution on an area basis D.sub.ave—s of equal to or smaller than 0.7 μm.

5. The method for producing the RFeB system sintered magnet according to claim 4, wherein the RFeB system alloy powder is prepared by performing an HDDR on a coarse powder of the raw material alloy to prepare coarse particles each having fine grains, pulverizing these coarse particles having fine grains by hydrogen decrepitation, and subsequently further pulverizing the same powder by a jet milling method using helium gas.

6. The RFeB system sintered magnet according to claim 2, wherein a 50% cumulative diameter in the particle size distribution on an area basis D.sub.ave—S calculated from a circle-equivalent diameters D of crystal grains determined from a microscopic image of a section of the RFeB system sintered magnet is equal to or smaller than 1 μm.

7. A method for producing the RFeB system sintered magnet according to claim 2, comprising steps of: preparing a shaped body oriented by a magnetic field and subsequently sintering the shaped body, using an RFeB system alloy powder having a 50% cumulative diameter in the particle size distribution on an area basis D.sub.ave—S of equal to or smaller than 0.7 μm.

8. A method for producing the RFeB system sintered magnet according to claim 3, comprising steps of: preparing a shaped body oriented by a magnetic field and subsequently sintering the shaped body, using an RFeB system alloy powder having a 50% cumulative diameter in the particle size distribution on an area basis D.sub.ave S of equal to or smaller than 0.7 μm.

9. A method for producing the RFeB system sintered magnet according to claim 6, comprising steps of: preparing a shaped body oriented by a magnetic field and subsequently sintering the shaped body, using an RFeB system alloy powder having a 50% cumulative diameter in the particle size distribution on an area basis D.sub.ave—S of equal to or smaller than 0.7 μm.

10. The method for producing the RFeB system sintered magnet according to claim 7, wherein the RFeB system alloy powder is prepared by performing an HDDR on a coarse powder of the raw material alloy to prepare coarse particles each having fine grains, pulverizing these coarse particles having fine grains by hydrogen decrepitation, and subsequently further pulverizing the same powder by a jet milling method using helium gas.

11. The method for producing the RFeB system sintered magnet according to claim 8, wherein the RFeB system alloy powder is prepared by performing an HDDR on a coarse powder of the raw material alloy to prepare coarse particles each having fine grains, pulverizing these coarse particles having fine grains by hydrogen decrepitation, and subsequently further pulverizing the same powder by a jet milling method using helium gas.

12. The method for producing the RFeB system sintered magnet according to claim 9, wherein the RFeB system alloy powder is prepared by performing an HDDR on a coarse powder of the raw material alloy to prepare coarse particles each having fine grains, pulverizing these coarse particles having fine grains by hydrogen decrepitation, and subsequently further pulverizing the same powder by a jet milling method using helium gas.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0024] FIG. 1 is a chart illustrating one example of the method for producing an RFeB system sintered magnet according to the present invention.

[0025] FIG. 2 is a graph showing the grain diameter distribution for RFeB system sintered magnets of Present Example 1 as well as Comparative Examples 1 and 2, determined from the circle-equivalent diameters of the sectional areas of the crystal grains based on a microscopic image at a plane perpendicular to the axis of orientation.

[0026] FIG. 3 is a graph showing the temperature coefficient of coercivity t.sub.(T-23) (including t.sub.(100-23) at T=100° C.) of RFeB system sintered magnets of Present Example 1 and Comparative Examples 1 and 2.

[0027] FIG. 4 is a graph showing the temperature coefficient of coercivity t.sub.(T-23) (including t.sub.(100-23) at T=100° C.) of RFeB system sintered magnets of Present Example 2 and Comparative Examples 3-5.

DESCRIPTION OF EMBODIMENTS

[0028] An embodiment of the RFeB system sintered magnet according to the present invention is described using FIGS. 1-4.

EXAMPLE

[0029] Initially, one example of the method for producing the RFeB system sintered magnet according to the present invention is described using FIG. 1. The present production method includes five processes, i.e. the HDDR (Hydrogenation Disproportionation Desorption Recombination) process (Step S1), pulverizing process (Step S2), filling process (Step S3), orienting process (Step S4) and sintering process (Step S5). A lump of SC alloy prepared by a strip casting (SC) method is used as the raw material. The SC alloy lump is normally in the form of flakes with each side measuring a few millimeters. In the present embodiment, two kinds of SC alloy lumps, labeled “1” and “2”, with different compositions were used. Table 1 shows the composition of the SC alloy lumps 1 and 2. Neither the SC alloy lump 1 nor 2 contains heavy rare-earth elements R.sup.H.

TABLE-US-00001 TABLE 1 Composition of SC Alloy Lumps (Unit: mass %) Nd Pr B Cu Al Co Fe SC Alloy Lump 1 27.5 4.15 1.00 0.50 0.23 0.96 bal. SC Alloy Lump 2 30.51 0.07 0.98 0.10 0.22 0 bal.

[0030] In the HDDR process, the SC alloy lump is initially heat treated under hydrogen gas pressure (“Hydrogenation”) to decompose the R.sub.2Fe.sub.14B compound (main phase) in the SC alloy lump into the three phases of RH.sub.2, Fe.sub.2B and Fe (“Disproportionation”). In the present example, the hydrogen gas pressure was set at 100 kPa, and the heat treatment was performed at a temperature of 950° C. (first heat treatment temperature) for 60 minutes. In the subsequent steps, while the temperature is maintained at a second heat treatment temperature which is lower than the first heat treatment temperature, the atmosphere is changed to vacuum to desorb hydrogen from the RH.sub.2 phase (“Desorption”) and make this phase recombine with the Fe.sub.2B phase and Fe phase (“Recombination”). In the present example, the second heat treatment temperature was set at 800° C., and the vacuum was maintained for 60 minutes. As a result, an RFeB system polycrystalline body with a 50% cumulative diameter in the particle size distribution on an area basis D.sub.ave—s of approximately 0.6 μm is obtained.

[0031] In the pulverizing process, the RFeB system polycrystalline body is initially exposed to hydrogen gas without being heated from the outside. Then, the RFeB system polycrystalline body automatically generates heat and becomes brittle by occluding hydrogen. Next, the RFeB system polycrystalline body is coarsely pulverized with a mechanical crusher to obtain coarse powder. This coarse powder is subsequently introduced into a complete jet mill plant with helium gas circulation system (manufactured by Nippon Pneumatic Mfg. Co., Ltd., which is hereinafter called the “He jet mill”) and further pulverized. The He jet mill can generate a high-speed gas stream which is approximately three times as fast as the gas stream generated by an N.sub.2 jet mill which uses nitrogen gas. The gas stream accelerates the material to high speeds, making the material collide repeatedly, whereby the material can be pulverized to a 50% cumulative diameter in the particle size distribution on an area basis D.sub.ave—s of less than 1 μm, which cannot be achieved by the N.sub.2 jet mill. In this manner, two samples of RFeB system alloy powder whose 50% cumulative diameter in the particle size distribution on an area basis D.sub.ave—s did not exceed 0.7 μm were prepared, with D.sub.ave—S being approximately 0.6 μm for the SC alloy lump 1 and approximately 0.67 μm for the SC alloy lump 2.

[0032] In the filling process, a mold having a cavity whose shape corresponds to that of the RFeB system sintered magnet as the final product is filled with the RFeB system alloy powder at a predetermined filling density (in the present example, 3.6 g/cm.sup.3). Subsequently, in the orienting process, a magnetic field (in the present example, a pulsed direct-current magnetic field of 5 T) is applied to the RFeB system alloy powder in the mold to orient the alloy powder. In the sintering process, the oriented alloy powder held in the mold is contained in a sintering furnace and heated under vacuum (in the present embodiment, at 880° C. for two hours) to sinter the powder. No mechanical pressure for molding the alloy powder is applied throughout the filling, orienting and sintering processes. By following the procedure described to this point, the RFeB system sintered magnet of the present embodiment is obtained. Hereinafter, the RFeB system sintered magnet created from the SC alloy lump 1 is called “Present Example 1”, while the one created from the SC alloy lump 2 is called “Present Example 2”.

[0033] As the comparative examples, RFeB system sintered magnets were additionally created using the RFeB system alloy powder prepared by pulverizing the same lot of the SC alloy lumps 1 and 2 as used for the present examples. The pulverization of the SC alloy lump was performed in such a manner that the 50% cumulative diameter in the particle size distribution on an area basis D.sub.ave —s would be 1.4 μm (Comparative Example 1) and 3.1 μm (Comparative Example 2) for the SC alloy lump 1, as well as 1.32 μm (Comparative Example 3), 3.30 μm (Comparative Example 4) and 4.10 μm (Comparative Example 5) for the SC alloy lump 2. In these comparative examples, the HDDR process was omitted. In the pulverization process, the SC alloy was embrittled by the hydrogen occlusion method and then coarsely pulverized to prepare a coarse powder, which was further pulverized with the He jet mill to obtain the alloy powder. The filling, orienting, and sintering processes were performed by the same method as used for Present Examples 1 and 2.

[0034] The graph in FIG. 2 shows the grain diameter distribution for the RFeB system sintered magnet of Present Example 1 as well as those of Comparative Examples 1 and 2, determined from the circle-equivalent diameters of the sectional areas of the crystal grains based on a microscopic image at a plane perpendicular to the axis of orientation. The 50% cumulative diameter in the particle size distribution on an area basis D.sub.ave—S calculated from this graph was 0.83 μm in Present Example 1, 1.78 μm in Comparative Example 1, and 3.65 μm in Comparative Example 2.

[0035] The graph in FIG. 3 shows the temperature coefficient of coercivities t.sub.(T-23) at T=60° C., 100° C., 140° C. and 180° C. determined on the basis of the data of the coercivity H.sub.cj acquired for the RFeB system sintered magnet of Present Example 1 as well as those of Comparative Examples 1 and 2. The data lying on the vertical broken line in this graph are the temperature coefficient of coercivities t.sub.(100-23) at T=100° C. defined in the present invention. Although the coercivity H.sub.cj changes with the temperature, the vertical relationship (i.e. order) of the data of the Present Example 1 as well as the Comparative Examples 1 and 2 represented by the temperature coefficient of coercivity t.sub.(T-23) is always the same and independent of T. The temperature coefficient of coercivity t.sub.(100-23) in Present Example 1 was −0.53, which is higher than −0.66 in Comparative Example 1, −0.73 in Comparative Example 2, and −0.58, i.e. the highest value mentioned in Non Patent Literature 1. This result confirms that the RFeB system sintered magnet of Present Example 1 has better temperature characteristics than those of the Comparative Examples 1 and 2 as well as the one described in Non Patent Literature 1.

[0036] The graph in FIG. 4 shows the temperature coefficient of coercivities t.sub.(T-23) similarly determined for Present Example 2 and Comparative Examples 3-5. The temperature coefficient of coercivity t.sub.(100-23) in Present Example 2 was −0.48, which is higher than the values obtained in Comparative Examples 3-5 (−0.66 to −0.60) as well as the highest value mentioned in Non Patent Literature 1, −0.58. Furthermore, the temperature coefficient of coercivity t.sub.(100-23) in Present Example 2 is higher than the value in Present Example 1. The reason for this is because the content of Pr in Present Example 2 was 0.07 mass % and lower than the value in Present Example 1 (4.15 mass %).