Patent Grant
7244318
The present invention relates to a method of producing a rare-earth-iron-boron based permanent magnet with a high performance, and more particularly to a method of producing a magnet with excellent heat resistance which is used in a rotating machine such as a motor, an actuator, or the like.
Dysprosium (Dy) is conventionally added to a material alloy for the purposes of improving heat resistance of a rare-earth-iron-boron based (R-T-B) sintered magnet, and of maintaining the coercive force high even in a high temperature condition. The Dy is a kind of rare earth element exhibiting an effect of enhancing an anisotropic magnetic field of R2T14B phase as a main phase of the R-T-B sintered magnet. The Dy is a rare element. For this reason, if the practical use of electric vehicles is advanced, and the demand for magnets with high heat resistance used in motors for the electric vehicles is increased, an increase in material cost is a matter of concern as a result of tightening of the Dy source. Therefore, the development of technology for reducing the use of Dy in magnets with high coercive force is strongly required.
Conventionally, Dy is added in such a manner that the Dy is blended and melted together with the other elements in material casting. According to such a conventional method, Dy is uniformly distributed in a main phase of a magnet. However, the mechanism for generating the coercive force of the R-T-B sintered magnet is nucleation type, so that, in order to increase the coercive force, it is important to suppress the generation of opposing magnetic domain in the vicinity of the surface of R2Fe14B crystal grains as a main phase. For this reason, as shown in
As methods of reducing the use amount of Dy, thereby obtaining a structure shown in
However, the above-mentioned method of adding the oxide involves a problem that the magnetization is disadvantageously deteriorated as a result of the increase in the amount of oxygen as an impurity. The method of adding the hydride involves a problem that the degree of sintering is deteriorated.
In order to avoid such problems, many suggestions such as the followings are made for structure control by means of multi-alloy method in which a main phase alloy having a composition closer to the stoichiometric composition of Nd2Fe14B and a liquid-phase alloy of Dy-rich are blended.
However, all of the compositions of the Dy alloys used in the above-identified prior arts are rare-earth rich, so that they are easily oxidized during the pulverization or the like. As a result, the amount of oxygen included in the final magnet is increased, so that there exists a problem that the magnetic properties are deteriorated. In addition, since the embrittlement by means of hydrogen occlusion process cannot be efficiently performed for any of the alloys, the degree arid the efficiency of pulverization are bad, and it is difficult to finally obtain fine particles. In addition, in the case where the Dy—Cu alloy or the Dy—Co alloy is used, there exists a problem that the degree of sintering is significantly deteriorated.
A main object of the present invention is to provide a method of suppressing the oxidation of non main-phase alloy, and of improving the ease of pulverization, in a method of producing a permanent magnet obtained by blending a powder of main phase alloy with a powder of non main-phase alloy including a rare-earth element such as Dy which contributes to the improvement of coercive force.
The method of producing a permanent magnet according to the present invention includes the steps of: preparing a blended powder including a first powder and a second powder, the first powder containing an R2T14Q phase (R is at least one element selected from the group consisting of all rare-earth elements and Y (yttrium), T is at least one element selected from the group consisting of all transition elements, and Q is at least one element selected from the group consisting of B (boron) and C (carbon)) as a main phase, the second powder containing an R2T17 phase at 25 wt % or more of the whole; and sintering the blended powder.
In a preferred embodiment, a ratio of the second powder to the blended powder is in a range of 1 to 30 wt %.
In a preferred embodiment, the second powder contains Cu in the range of 0.1 to 10 at % (atom %).
In a preferred embodiment, the sintering step includes a step of melting the R2T17 phase contained in the second powder by way of eutectic reaction.
In a preferred embodiment, the first powder is a powder of alloy represented by a composition formula of RxT100−x−yQy, and x and y for defining molar fractions satisfy the following relationships, respectively: 12.5≦x≦18 at %); and 5.5≦y≦20 (at %).
The second powder may be a powder of alloy represented by a composition formula of (R1pR2q)CurT100−p−q−r (R1 is at least one element selected from the group consisting of Dy and Tb, and R2 is at least one element selected from the group consisting of rare-earth elements excluding Dy and Tb, and Y), and p, q, and r for defining molar fractions satisfy the following relationships respectively: 10≦(p+q)≦20 (at %); 0.2≦p/(p+q)≦1.0; and 0.1≦r≦10 (at %).
The method of producing a permanent magnet according to the present invention includes the steps of: preparing a blended powder including a first powder and a second powder, the first powder containing an R2T14Q phase (R is at least one element selected from the group consisting of all rare-earth elements and Y (yttrium), T is at least one element selected from the group consisting of all transition elements, and Q is at least one element selected from the group consisting of B (boron) and C (carbon)) as a main phase, the second powder being a powder of alloy represented by a composition formula of (R1pR2q)CurT100−p−q−r (R1 is at least one element selected from the group consisting of Dy and Tb, and R2 is at least one element selected from the group consisting of rare-earth elements excluding Dy and Tb, and Y); and sintering the blended powder.
The method of producing a permanent magnet according to the present invention includes the steps of: preparing a blended powder including a first powder and a second powder, the first powder containing an R2T14Q phase (R is at least one element selected from the group consisting of all rare-earth elements and Y (yttrium), T is at least one element selected from the group consisting of all transition elements, and Q is at least one element selected from the group consisting of B (boron) and C (carbon)) as a main phase, the second powder containing an RmTn phase (m and n are positive numbers, and satisfy the relationship of m/n≦(⅙)) at 25 wt % or more of the whole; and sintering the blended powder.
In a preferred embodiment, the RmTn phase is an R2T17 phase.
In a preferred embodiment, the step of preparing the blended powder may include a step of performing a hydrogen embrittlement process to the alloy for the second powder, thereby obtaining an average particle diameter of the second powder of 100 μm or less.
An average particle size (FSSS particle size) of the blended powder may be made to be 5 μm or less in a stage before the sintering.
The inventors of the present invention found that to a first powder containing an R2T14B phase as a main phase, a second powder containing an R2T17 phase including a rare-earth element with a lower molar fraction at 25 wt % or more of the whole was added and mixed, and then they were sintered, so that R in the R2T17 phase could be unevenly distributed in a grain boundary portion of the main phase crystal grains. Herein, R is at least one element selected from the group consisting of all rare-earth elements and yttrium, and T is at least one element selected from the group consisting of all transition elements. Preferably, T includes 50 at % or more Fe, and more preferably, T includes Co in addition to Fe for the purpose of improving the heat resistance.
Carbon (C) may be substituted for part of or all of boron (B), so that the R2T14B phase can also be represented as R2T14Q phase (Q is at least one element selected from the group of boron (B) and carbon (C)).
If a rare-earth element such as Dy is included in the R2T17 phase of the second powder as R, the rare-earth element such as Dy can be locally distributed in a grain surface region of a main phase of relatively high concentration, i.e., can be concentrated.
The second powder can be easily obtained by performing hydrogen embrittlement process to a material alloy mainly including R2T17 phase. This is because in a structure in which the R2T17 phase exists together with another phase, the lattice constant of the R2T17 phase is enlarged by hydrogen occlusion, and breakage easily occurs in the grain boundary portion. Such an alloy for the second powder includes a relatively small amount of rare-earth element, as compared with the main phase alloy including the R2T14B phase. Specifically, the alloy for the second powder is mainly constituted by the R2T17 phase, and the residual portion is constituted by RT2 phase, RT3 phase, RT5 phase, and/or other phases.
If the existent ratio of R2T17 phase in the alloy for the second powder is low, the degree of pulverization of the alloy for the second powder is degraded, and the amount of rare-earth element is relatively increased. As a result, oxidation disadvantageously occurs. Accordingly, the content ratio of the R2T17 phase in the alloy for the second powder is preferably 25 wt % or more, and more preferably 40 wt % or more. Such a material alloy can be prepared by a quenching method such as strip casting, instead of the ingot casting. As for the above-mentioned material alloy, the content of rare-earth element is relatively low as compared with a prior-art liquid phase alloy. For this reason, the material alloy can hardly be oxidized during the pulverization, so that an oxide which badly affects the magnetic properties is hardly generated.
On the other hand, the main phase alloy used in the present invention as the material for the first powder is desired to have a composition of rare earth rich, as compared with the stoichiometric composition of the R2Fe14Q compound. Because the composition is rare-earth rich, the rare-earth rich phase included in the main phase alloy is reacted with the R2T17 phase of the second powder in sintering, thereby generating a molten liquid. Thus, liquid phase sintering appropriately progresses.
The R2T17 phase dissolves by the reaction with the R-rich phase as described above. If the composition after the blending of powders is short of B (boron), the R2T17 phase is formed again in a cooling process. The R2T17 phase is a soft magnetic phase. For this reason, if the R2T17 phase remains in the sintered magnet, the coercive force is disadvantageously deteriorated. In order to prevent the R2T17 phase from remaining, the composition of the main phase alloy is preferably B rich, as compared with the stoichiometric composition of the R2T14B compound.
In order to attain the effect of increasing the coercive force, it is preferred that Dy be added to the material alloy for the second powder. Since Tb exhibits the same effects as those of Dy, Tb may be added together with Dy or instead of Dy.
Dy and/or Tb may be added to the material alloy for the first powder. However, in order to effectively attain the object of the present invention of increasing the coercive force while the amount of Dy and/or Tb to be used is reduced, it is preferred that Dy and Tb be not added to the material alloy for the first powder.
The addition of an appropriate amount of Cu to the first powder and/or the second powder, especially to the second powder is preferable, because it is possible to decrease the Dy concentration in the grain boundary phase, and the effect of further increasing the concentration of Dy which is concentrated in the grain surface region of the main phase can be attained. Based on experiments, a preferable range of the Cu content in the second powder is 0.1 to 10 at %.
The element T included in the first powder and the second powder is at least one element selected from the group consisting of all transition elements. Practically, the element T is desired to be selected from the group consisting of Fe, Co, Al, Ni, Mn, Sn, In, and Ga. The element T is preferably formed mainly from Fe and/or Co. For various purposes, other elements are added. For example, Al is added to the material alloy, a superior degree of sintering can be attained even in a relatively lower temperature region (about 800° C.).
The addition of Al to the second powder is preferably performed in a range of not less than 1 at % nor more than 15 at %.
From the above-described view, when the material alloy for the first powder is represented by a composition formula of RxT100−x−yQy, x and y for defining molar fractions preferably satisfy the relationships of 12.5≦x≦18 (at %), and 5.5≦y≦20 (at %), respectively.
The material alloy for the second powder can be represented by a composition formula of (R1pR2q)CurT100−p−q−r (R1 is at least one element selected from the group consisting of Dy and Tb, R2 is at least one element selected from the group consisting of rare-earth elements excluding Dy and Tb, and Y, and T is at least one element selected from the group consisting of all transition elements). According to experiments, p, q, and r for defining molar fractions preferably satisfy the relationships of 10≦(p+q)≦20 (at %), 0.2≦p/(p+q)≦1.0, and 0.1≦r≦10 (at %), respectively.
The material alloy for the second powder is prepared so as to mainly contain the R2T17 phase. Alternatively, the material alloy may contain an RmTn phase which includes a relatively small amount of rare-earth element (m and n are positive numbers, and satisfy the relationship of m/n≦(⅙)) at 25 wt % or more of the whole.
The mixing of the first powder and the second powder prepared by coarsely pulverizing the material alloys having the above-described compositions may be performed before a pulverization process or after the pulverization process. In the case where the mixing of the first powder with the second powder is performed before the pulverization, the pulverization of the alloy for the first powder and the pulverization of the alloy for the second powder are simultaneously performed. On the contrary, the alloy for the first powder and the alloy for the second powder which were coarsely pulverized separately may be further pulverized separately, and then the powders may be mixed at a predetermined ratio. Alternatively, the alloy for the first powder and the alloy for the second powder which are separately pulverized may be merchandized, and they may be mixed at an appropriate ratio. The ratio of the second powder to the whole of the blended powder is preferably set in the range of 1 to 30 wt %.
As for the second powder, before the mixing with the first powder, the material alloy may be coarsely pulverized by hydrogen embrittlement process, and an average particle diameter is preferably 100 μm or less. The alloy for the second powder used in the present invention contains R2T17 phase, so as to have an advantage that the alloy is easily hydrogen-embrittled. In addition, the average particle size (FSSS particle size) of the mixed powder after the first powder and the second powder are mixed is preferably 5 μm or less in a stage before sintering. A more preferable average particle size of the mixed powder is 2 μm or more and 4 μm or less. As compared with the prior art, the alloy for the second powder contains a smaller amount of rare-earth element, so that the oxidation in pulverization is suppressed. As a result, the oxygen concentration in the sintered magnet which is finally obtained can be suppressed to be 8000 ppm or less by weight. More preferably, the oxygen concentration in the sintered magnet is 6000 ppm by weight.
As described above, as for the alloy for the second powder used in the present invention, poor degree of pulverization which is a problem in the case of the liquid phase alloy of rare-earth rich which has been proposed and the activity to the oxygen caused by the high rare-earth composition can be suppressed. In addition, the degree of sintering is superior. As described above, according to the present invention, a magnet with high coercive force can be produced with good productivity.
In these examples, alloys A1 to A6 shown in Table 1 are used as material alloys A for the first powder, and alloys B1 to B5 ed as material alloys B for the second powder.
In order to investigate the variation in constituent phase of the material alloys B caused by the difference of casting methods, the alloy B2 containing 15.5 at % Dy was cast by using three methods, i.e., strip casting, centrifugal casting, and ingot casting, and the constituent phases were examined. The results are shown in
As is seen from
In order to investigate how the constituent phase of the alloy B was affected when the content of rare-earth element in the alloy B was varied, X-ray diffraction measurement was performed for the alloys B1 to B5 with different contents of rare-earth elements. The results are shown in
From the above-described results, it is understood that the upper limit of the preferable range of the amount of Dy (the amount of rare-earth element) in the alloy B is 20 at % or less. When the amount of Dy (the amount of rare-earth element) in the alloy B is smaller than 10 at %, the magnetic properties are deteriorated. Therefore, the amount of Dy (the amount of rare-earth element) in the alloy B is preferably 10 at % or more and 20 at % or less.
Hereinafter, the production methods of the examples and the comparative examples will be described.
First, the hydrogen occlusion and dehydrogenation processes were performed for the respective alloys A and B having the compositions shown in Table 1, thereby performing coarse pulverization (hydrogen embrittlement process). In the alloy B4 and the alloy B5 containing a large amount of Dy, the degree of pulverization by the hydrogen process was poor. For this reason, after the hydrogen embrittlement treatment process, mechanical pulverization was performed, until the particle diameter became 420 μm or less by using a stamp mill.
Next, after the alloy A and the alloy B were mixed at a blend ratios shown in respective boxes of Examples 1 to 4 and Comparative Examples 1 to 2 in Table 1, pulverization was performed by using a jet mill of N2 gas atmosphere. An average particle size (FSSS particle size) of the blended powder after the pulverization was about 3 to 3.5 μm. The variation in Dy amount before and after the pulverization is shown in Table 2.
The “remaining proportion” in the most right column in Table 2 is an amount indicated by (Dy amount after pulverization/Dy amount before pulverization)×100. A larger amount indicates superior degree of pulverization of the alloy B. As is seen from Table 2, in the comparative examples 1 and 2, the degree of pulverization of the alloy B is poor.
Next, after a compaction process in an aligned magnetic field was performed by using the thus-obtained fine powder, a sintering process was performed, thereby manufacturing a permanent magnet. Evaluated results of magnetic properties of the magnet are shown in Table 3, and
From the results, in the cases of Examples 1 to 4, it is seen that a high coercive force can be obtained with a smaller Dy amount, as compared with a one-alloy method. In addition, in Comparative Examples 1 to 2, even though the Dy amount in the alloy B is large, the effect of increasing a coercive force caused by the addition of Dy is not observed. Moreover, since the Dy remaining proportion in pulverization is low, Dy is wastefully used, and the Dy reducing effect cannot be sufficiently attained.
According to the present invention, two kinds of alloy powders with excellent degree of pulverization and oxidation resistance are appropriately mixed, so that a structure in which the concentration of a specific rare-earth element such as Dy in a grain surface region of a main phase is made higher than that of the other portions can be produced with good production yield. Accordingly, as compared with a method in which Dy is added at the point of melting the material alloy and Dy is uniformly diffused, the present invention can inexpensively produce a sintered magnet exhibiting high coercive force with a reduced amount of Dy with good productivity. In addition, according to the present invention, Dy can be efficiently concentrated in a grain surface region of a main phase, so that the saturation magnetization in the main phase inner portion of the sintered magnet is maintained to be high, and the reduction in residual magnetic flux density Br due to the addition of Dy can be suppressed.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2001-021226 | Jan 2001 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP02/00442 | 1/22/2002 | WO | 00 | 7/30/2003 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO02/061769 | 8/8/2002 | WO | A |
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| Number | Date | Country | |
|---|---|---|---|
| 20040050454 A1 | Mar 2004 | US |
