Patent Grant
10424426
This application is a National Stage of International Application No. PCT/JP2014/064995 filed Jun. 5, 2014, claiming priority based on Japanese Patent Application No. 2013-118893, filed Jun. 5, 2013, the contents of all of which are incorporated herein by reference in their entirety.
The present invention relates to a rare earth magnet and a method for manufacturing the same.
A rare earth magnet using a rare earth element is also called a permanent magnet and is used for a motor making up a hard disk or an MRI as well as for a driving motor of a hybrid vehicle, an electric vehicle, etc.,
The index indicative of the magnet performance of the rare earth magnet includes residual magnetization (residual flux density) and coercive force. Meanwhile, as the amount of heat generation grows due to the trend to a more compact motor and a higher current density, heat resistance is more increasingly required also of the rare earth magnet used therein, and how the coercive force of a magnet can be maintained in use at high temperatures is one of important research themes in this technical field. Considering an Nd—Fe—B magnet that is one of rare earth magnets often used in a vehicle driving motor, attempts are made to increase the coercive force, for example, by achieving refinement of a crystal grain, using an alloy of a composition having a large Nd amount, or adding a heavy rare earth element having a high coercivity performance, such as Dy and Tb.
As the rare earth element, there are not only a general sintered magnet in which the crystal grain constituting the structure is on a scale of approximately from 3 to 5 μm, but also a nanocrystalline magnet in which the crystal grain is refined to a nanoscale of 50 to 300 nm.
The microstructure of an Nd—Fe—B general rare earth magnet consists of an Nd-rich crystal grain and a grain boundary intervening between crystal grains. Since Nd constituting the crystal grain is an expensive rare earth element, how the amount of the element used can be reduced while ensuring the magnet performance is one of important development challenges in this technical field.
As the measure regarding the reduction in the amount of Nd used, it is conceivable to use a light rare earth element such as Ce and La or use an element such as Gd, Y, Sc, Sm and Lu.
However, as well as in the case of applying such an element in place of Nd, even when most of Nd is substituted by such an element, significant deterioration of the magnetic properties of the rare earth magnet is envisaged. Therefore, the amount of such an element used must be limited, and an effect of sufficiently reducing the material cost cannot be expected. Furthermore, when such an element having low magnetic properties is used, there is generally a very strong tendency that the use form thereof is limited to an isotropic form.
In the case where anisotropization of a rare earth magnet using the above-described light rare earth element or an element such as Gd and Y is attempted, the coercive force of the rare earth magnet decreases significantly, for example, in the working process such as hot plastic working, and the magnetic properties are inevitably deteriorated.
Here, Patent Document 1 discloses a magnetic material produced through a rapid solidification process and the subsequent heat annealing process, wherein the magnetic material has, by atomic percentage, the following composition: (R1-aR′a)uFe100-u-v-w-x-yCovMwTxBy (wherein R is Nd, Pr, didymium (a natural mixture of Nd and Pr, having a composition of Nd0.75Pr0.25), or a combination thereof, R′ is La, Ce, Y, or a combination thereof, M is one or more of Zr, Nb, Ti, Cr, V, Mo, W and Hf, T is one or more of Al, Mn, Cu and Si, 0.01≤a≤0.8, 7≤u≤13, 0≤v≤20, 0.01≤w≤1, 0.1≤x≤5, and 4≤y≤12) and exhibits a residual magnetism (Br) value of about 6.5 kG to about 8.5 kG and an intrinsic coercive force of about 6.0 kOe to about 9.9 kOe.
While the magnetic material disclosed is a magnetic material where part of Nd is substituted by La or Ce, this is a compositional material for a rare earth-lean nano-composite magnet or a compositional material close thereto, and such a compositional material is composed of not an anisotropic but isotropic magnetic powder. Because, in the case of a compositional material for a nano-composite magnet or a compositional material close thereto, even when hot plastic working is performed in the plastic state with an attempt to form an oriented magnet, only a magnet having insufficient magnet performance can be formed.
In this way, the magnet material disclosed in Patent Document 1 may be an isotropic magnet material and is improper as a magnet material for the manufacture of an anisotropic rare earth magnet. Furthermore, Patent Document 1 is absolutely silent as to taking a measure for imparting anisotropy to such an isotropic magnet material.
Patent Document 1: Kohyo (National Publication of Translated Version) No. 2007-524986
The present invention has been made by taking into account the problems above, and an object of the present invention is to provide a rare earth magnet ensuring excellent magnetic anisotropy while reducing the amount of a rare earth magnet such as Nd, and a manufacturing method thereof.
In order to attain the above-described object, the rare earth magnet according to a first aspect comprises a crystal grain having an overall composition of (R2(1-x)R1x)yFe100-y-w-z-vCowBzTMv (wherein R2 is at least one of Nd, Pr, Dy and Tb, R1 is an alloy of at least one or two or more of Ce, La, Gd, Y and Sc, TM is at least one of Ga, Al, Cu, Au, Ag, Zn, In and Mn, 0<x<1, y=12 to 20, z=5.6 to 6.5, w=0 to 8, and v=0 to 2), wherein the average grain size of the crystal grain is 1,000 nm or less, the crystal grain consists of a core and an outer shell, the core has a composition of R1 that is richer than R2, and the outer shell has a composition of R2 that is richer than R1.
In order to attain the above-described object, the rare earth magnet according to a second aspect comprises a crystal grain having an overall composition of (Nd(1-x)Cex)yFe100-y-w-z-vCowBzTMv (wherein TM is at least one of Ga, Al, Cu, Au, Ag, Zn, In and Mn, 0<x<1, y=12 to 20, z=5.6 to 6.5, w=0 to 8, and v=0 to 2), wherein the crystal grain consists of a core and an outer shell and has a composition of x in the core that is larger than x in the outer shell.
In the rare earth magnet of the present invention, the crystal grain thereof consists of a core and an outer sell and the core is richer in the light rare earth element such as Ce and La or in the element such as Gd and Y than in Nd, etc., so that the material cost can be greatly reduced, compared with a rare earth magnet composed of a crystal grain having an Nd-rich core. In this way, the core is in the state of that is rich in an inexpensive element with low magnetic properties, nevertheless, thanks to a configuration where an outer shell in the state of that is rich in Nd, etc., is present, magnetic decoupling between crystal grains is achieved while suppressing reduction in the magnetic properties, as a result, a rare earth magnet excellent in the magnetic anisotropy is formed.
Incidentally, the core of the crystal grain is a semi-hard phase having a relatively low coercive force because of a small amount of Nd, etc., whereas the outer shell of the crystal grain is a hard phase having a high coercive force due to a large amount of Nd, etc., and therefore, the crystal grain constituting the rare earth magnet can be the to have a composite structure of a semi-hard phase and a hard phase. Thus, the crystal grain has, as an outer shell, a hard phase with a high coercive force and in turn, magnetic decoupling between crystal grains is achieved, leading to enhancement of the magnetic properties.
Furthermore, in the rare earth magnet of the present invention, the average grain size of the crystal grain is adjusted to 1,000 nm or less, so that a given demagnetization resistance, i.e., a given coercive force, can be ensured. The reason therefor is as follows. That is, unlike a pure Nd2Fe14B magnet (neodymium magnet), the crystal grain constituting the rare earth magnet of the present invention has a core in the state of that is rich in Ce, La, etc., with low magnetic properties. Here, the relationship between the average grain size of the crystal grain and the coercive force of the material generally has a tendency that as shown in
The conditions necessary to form a magnet without causing demagnetization of a semi-hard phase are described by using the Kronmuller formula. The Kronmuller formula can be represented by the following formula 1:
Hc=αHa−NMs (formula 1)
wherein Hc: a coercive force, α: a factor to which decoupling property between crystal grain contributes, Ha: a crystal magnetic anisotropy (specific to the crystal grain material), N: a factor to which the grain size of the crystal grain contributes, and Ms: a saturation magnetization (specific to the crystal grain material).
When N (Neff) at α=1 is determined, the following formula is established:
Neff=(Ha−Hc)/Ms (formula 2)
The relationship between Neff and the crystal grain size D, when experimentally determined on a rare earth magnet, is as shown in
Neff=0.25Ln(D)−0.475 (formula 3)
The relationship between the required coercive force Hc and the crystal grain size D is obtained from formula 2 and formula 3 and can be represented by the following formula 4:
D≤exp(4(Ha−Hc)/Ms+1.7) (formula 4)
According to the present inventors, it is specified that in formula 1 established as a premise for completing a magnet, Hc≥0 and for sufficiently ensuring the magnet performance, Hc≥13.
For example, when Ce is selected as the semi-hard phase and Hc=13 kOe (=about Ha/2), since Ha=26 kOe and Ms=12 kG, D≤417 nm is led and it is understood that the average grain size D is preferably about 500 nm or less. On the other hand, in the case of use for applications requiring Hc of about 10 kOe, D is about 1,133 nm with Hc=10 kOe and therefore, when the crystal grain is used in an average grain size D region satisfying D<1,133 nm, i.e., in the range of about 1,000 nm or less, magnetic domain reversal resulting from reduction in the magnetic force due to self-magnetization of the grain itself does not occur, and a rare earth magnet ensuring the magnetic properties is formed.
For these reasons, in the rare earth magnet of the present invention, the average grain size of the crystal grain thereof is specified to be 1,000 nm or less, preferably 500 nm.
According to the rare earth magnet of the present invention, the amount of an expensive element as a compositional component of the crystal grain, such as Nd, Pr, Dy and Tb, is reduced and instead, a relatively inexpensive Ce, La, etc., is applied, so that the material cost can be far lower than that of conventional rare earth magnets. Moreover, the crystal grain has a structure where an outer shell rich in Nd, Pr, Dy, Tb, etc., is present around a core rich in Ce, La, etc., and therefore, a rare earth magnet composed of a crystal grain with excellent magnetic anisotropy is obtained.
The present invention also provides a method for manufacturing a rare earth magnet, and the manufacturing method includes a first step of performing hot pressing by using a magnetic powder containing a crystal grain having a composition of (R2(1-x)R1x)yFe100-y-w-z-vCowBzTMv (wherein R2 is at least one of Nd, Pr, Dy and Tb, R1 is an alloy of at least one or two or more of Ce, La, Gd, Y and Sc, TM is at least one of Ga, Al, Cu, Au, Ag, Zn, In and Mn, 0<x≤1, y=12 to 20, z=5.6 to 6.5, w=0 to 8, and v=0 to 2) to produce a rare earth magnet precursor, and a second step of diffusing and impregnating a modifying metal composed of an R2 element or an R2-TM alloy into the rare earth magnet precursor to manufacture a rare earth magnet provided with a crystal grain having an average grain size of 1,000 nm or less and consisting of a core and an outer shell, the core having a composition of R1 that is richer than R2, and the outer shell having a composition of R2 that is richer than R1.
In the manufacturing method of the present invention, a rare earth magnet precursor is produced in the first step by using a crystal grain in which part of Nd, etc., is substituted by a light rare earth element, etc., and thereafter, a modifying metal composed of an R2 element or an R2-TM alloy is diffused and impregnated into the rare earth magnet precursor in the second step, whereby a rare earth magnet excellent in the magnetic anisotropy and composed of a crystal grain consisting of a core in the state of that is rich in a light rare earth element, etc., and an outer shell in the state of that is rich in Nd, etc., can be manufactured. In the method, the first step may be a step where hot press working is performed to produce a compact and this compact is subjected to hot plastic working to produce a rare earth magnet precursor.
In the first step of manufacturing a rare earth magnet precursor, the precursor can be produced by various methods, and specific examples thereof include four production methods.
A first production method is a method where a magnetic powder pulverized to about 10 μm or less is subjected to magnetic field orientation and then to liquid phase sintering to produce an anisotropic rare earth magnet precursor. A second production method is a method where an isotropic magnetic powder of a nanocrystalline structure is produced by a liquid quenching method and the powder is subjected to hot press working to produce an isotropic rare earth magnet precursor. A third production method is a method where after the hot press working in the second production method, hot plastic working is applied to produce an anisotropic rare earth magnet precursor. A fourth production method is a method where an isotropic or anisotropic magnetic powder prepared by an HDDR method (Hydrogenation Decomposition Desorption Recombination) is subjected to hot press working to produce an isotropic or anisotropic rare earth magnet precursor.
In any of these methods, the crystal grain constituting the rare earth magnet precursor produced in the first step is a crystal grain containing Nd, etc., in a small amount and having low magnetic properties (composed of only the above-described semi-hard phase). In order to form an outer shell working out to a hard phase on the crystal grain above, in the second step, a modifying metal composed of an R2 element or an R2-TM alloy (R2 element: at least one of Nd, Pr, Dy and Tb, and TM: at least one of Ga, Al, Cu, Au, Ag, Zn, In and Mn) is diffused and impregnated into the rare earth magnet precursor. The method for diffusing and impregnating the modifying metal also includes various methods, and specific examples thereof include three methods.
A first method is a method of applying a vapor phase method where an R2 element is vaporized in a vacuum at around 850° C. to penetrate into the grain boundary of the rare earth magnet precursor. A second method is a method of applying a liquid phase method where a melt of an R2-TM alloy with a low melting point is liquid-phase impregnated into the grain boundary of the rare earth magnet precursor. A third method is a method of applying a solid phase method where an R2 element, an R2-TM alloy, or a solid of its compound with oxygen, fluorine, etc., is brought into contact with the rare earth magnet precursor and heated at approximately from 500 to 900° C. to cause an exchange reaction of an R1 solid solution remaining in the grain boundary between crystal grains with the R2 element and thereby diffuse and impregnate a modifying metal through the grain boundary.
Another embodiment of the method for manufacturing a rare earth magnet of the present invention includes a step of heating an Re-M alloy (wherein Re is a rare earth element and M is an element capable of reducing the melting point of the rare earth element by that is alloyed) at a temperature not lower than the melting point thereof to melt the alloy, and a step of bringing the molten Re-M alloy into contact with a magnetic particle containing a transition metal element to diffuse Re in the Re-M alloy into the magnetic particle. Here, Re is preferably at least either one of Nd and Sm, the melting point of the Re-M alloy is preferably 800° C. or less, M is preferably at least one of Cu, Fe, Al and Ga, and the transition metal is preferably at least one of Fe, Co and Ni.
In the manufacturing method of the present invention, the average grain size of the crystal grain in the rare earth magnet manufactured is also adjusted to 1,000 nm or less and is preferably adjusted to an average grain size of 500 nm or less.
As understood from the description in the foregoing pages, according to the rare earth magnet of the present invention and the manufacturing method thereof, the amount of an expensive element as a compositional component of the crystal grain, such as Nd, Pr, Dy and Tb, is reduced and instead, a relatively inexpensive Ce, La, etc., is applied, so that in addition to reduction in the material cost, by virtue of the crystal grain having a structure where an outer shell rich in Nd, Pr, Dy, Tb, etc., is present around a core rich in Ce, La, etc., magnetic decoupling between crystal grains can be achieved and a rare earth magnet composed of a crystal grain with excellent magnetic anisotropy can be provided.
The mode for carrying out the rare earth magnet of the present invention and the manufacturing method thereof are described below by referring to the drawings.
(Rare Earth Magnet)
The crystal grain 10 has a so-called core-shell structure consisting of a core 1 and an outer shell 2.
The crystal grain 10 has an overall composition of (R2(1-x)R1x)yFe100-y-w-z-vCowBzTMv (wherein R2 is at least one of Nd, Pr, Dy and Tb, R1 is an alloy of at least one or two or more of Ce, La, Gd, Y and Sc, TM is at least one of Ga, Al, Cu, Au, Ag, Zn, In and Mn, 0<x<1, y=12 to 20, z=5.6 to 6.5, w=0 to 8, and v=0 to 2), and this crystal grain consists of a core and an outer shell, where the core has a composition of R1 that is richer than R2 and the outer shell have a composition of R2 that is richer than R1.
Here, the crystal grain preferably has a composition where R2 is Nd, R1 is Ce, and x in the core is larger than x in the outer shell.
The core 1 is in the state where R1 is richer than R2 or x in the core is larger than x in the outer shell, more specifically, for example, an element rendering the material cost far lower than Nd, etc., such as Ce or La, is richer than Nd, etc., so that the material cost can be greatly reduced, compared with a rare earth magnet composed of a magnetic material having an Nd-rich core, i.e., a general Nd2Fe14B magnet (neodymium). The “in the core 1, R1 is richer than R2” as used herein encompasses a case where the concentrations of R1 and R2 are the same.
However, since the core 1 constituting the crystal grain 10 is in the state of that is rich in Ce, La, etc., the magnetic properties are inevitably reduced, compared with a general Nd2Fe14B magnet.
In order to suppress this reduction in the magnetic properties, the crystal grain 10 illustrated has, around the core 1, an outer shell 2 in which R2 is richer than R1, i.e., an outer shell 2 in which Nd, etc., is richer than Ce, La, etc., and the magnetic decoupling between adjacent crystal grains 10 can be thereby achieved, providing for magnetic anisotropy, as a result, reduction in the magnetic properties such as coercive force and residual magnetization is suppressed.
This is easily understood from
In this way, the crystal grain 10 is configured to have an outer shell 2 rich in Nd, etc., while greatly reducing the amount of Nd, etc., by forming a core 1 in the state of that is rich in an inexpensive element, and be thereby prevented from reduction in the magnetic properties. More specifically, the coercive force is enhanced when the magnetic anisotropy of the outer shell is higher than the magnetic anisotropy of the core, and therefore, it is considered that the rare earth magnet of the present invention, by virtue of having a core-shell structure, is insusceptible to an external magnetic field and is less likely to allow a magnetization reversal on the periphery of a crystal, as a result, a magnetization reversal of the entire magnet phase is suppressed. Accordingly, the rare earth magnet 100 composed of such a crystal grain 10 comes to have magnetic anisotropy and excellent magnetic properties while achieving a reduction in the material cost of the rare earth magnet and a consequent reduction in the production cost of the rare earth magnet.
In addition, in the rare earth magnet of a core-shell structure of the present invention, the coercive force at a temperature of 160° C. or less is enhanced, compared to conventional magnets in which a boundary is not present between the core and the outer shell and magnet phases Nd2Fe14B and Ce2Fe14B are mixed. This is considered to be achieved because while the temperature characteristic is enhanced due to Ce2Fe14B in the core, the magnetization is less likely to reverse due to Nd2Fe14B in the outer shell and the ratio of decrease in the coercive force at a high temperature is thereby suppressed.
The average grain size of the crystal grain 10 illustrated in
The “average grain size” as used herein indicates an average value of longitudinal lengths t of crystal grains 10, for example, shown in
The reason why the average grain size of the crystal grain 10 is set to 1,000 nm or less, preferably 500 nm or less, is as described above.
In the rare earth magnet of the present invention, the core of the crystal grain is a central portion of the crystal grain, and the outer shell is a surface portion of the crystal grain.
(Method for Manufacturing Rare Earth Magnet)
The method for manufacturing the rare earth magnet 100 shown in
First, a magnetic power containing a crystal grain having a composition of (R2(1-x)R1x)yFe100-y-w-z-vCowBzTMv (wherein R2 is at least one of Nd, Pr, Dy and Tb, R1 is an alloy of at least one or two or more of Ce, La, Gd, Y and Sc, TM is at least one of Ga, Al, Cu, Au, Ag, Zn, In and Mn, 0<x≤1, y=12 to 20, z=5.6 to 6.5, w=0 to 8, and v=0 to 2) is produced.
As the method for producing the magnetic powder, for example, a method of producing an isotropic magnetic powder of a nanocrystalline structure by a liquid quenching method, or a method of producing an isotropic or anisotropic magnetic powder by an HDDR method, can be applied.
Describing the method by a liquid quenching method, for example, an alloy ingot is high-frequency melted by a melt spinning method using a single roll in a furnace (not shown) in an Ar gas atmosphere at a pressure reduced to 50 kPa or less, and a molten metal having the composition of the core 1 is sprayed on a copper roll to prepare a quenched thin strip B (quenched ribbon), which is then coarsely pulverized, whereby the magnetic powder can be produced.
A magnetic powder pulverized, for example, to about 10 μm or less is subjected to magnetic field orientation and then to liquid phase sintering to produce an anisotropic rare earth magnet precursor. Alternatively, an isotropic magnetic powder of a nanocrystalline structure produced by a liquid quenching method is subjected to hot press working to produce an isotropic rare earth magnet precursor. Alternatively, an isotropic magnetic powder of a nanocrystalline structure is subjected to hot press working and then to hot plastic working to produce an anisotropic rare earth magnet precursor. Alternatively, an isotropic or anisotropic magnetic powder prepared by an HDDR method is subjected to hot press working to produce an isotropic or anisotropic rare earth magnet precursor.
An isotropic or anisotropic rare earth magnet precursor is produced by the method above (up to this is the first step of the manufacturing method).
The crystal grain constituting the rare earth magnet precursor produced in the first step is a crystal grain containing Nd, etc., in a small amount and having low magnetic properties (composed of only the semi-hard phase described above). In order to form an outer shell working out to a hard phase on the crystal grain above, a modifying metal composed of an R2 element or an R2-TM alloy (R2 element: at least one of Nd, Pr, Dy and Tb, and TM: Ga or an element obtained by substituting part of Ga with at least one of Al, Cu, Au, Ag, Zn, In, Mn and Fe) is diffused and impregnated into the rare earth magnet precursor (the second step of the manufacturing method).
For example, a vapor phase method where an R2 element is vaporized in a vacuum at around 850° C. to penetrate into the grain boundary of the rare earth magnet precursor is applied. Alternatively, a liquid phase method where a melt of an R2-TM alloy with a low melting point is liquid-phase impregnated into the grain boundary of the rare earth magnet precursor is applied. Alternatively, a solid phase method where an R2 element, an R2-TM alloy, or a solid of its compound with oxygen, fluorine, etc., is brought into contact with the rare earth magnet precursor and heated at approximately from 500 to 900° C. to cause an exchange reaction of an R1 solid solution remaining in the grain boundary between crystal grains with the R2 element and thereby diffuse and impregnate a modifying metal through the grain boundary is applied.
Here, a heavy rare earth element such as Dy and Tb may be used as the R2 element or R2-TM alloy, but it is preferable to use either Nd or Pr as the R2 element or use a transition metal element or typical metal element as the TM element of the R2-TM alloy, without using a heavy rare earth element. Any one of Cu, Mn, In, Zn, Al, Ag, Ga and Fe is preferably used. Specific examples of the R2-TM alloy include an Nd—Cu alloy (eutectic point: 520° C.), a Pr—Cu alloy (eutectic point: 480° C.), an Nd—Pr—Cu alloy, an Nd—Al alloy (eutectic point: 650° C.), and an Nd—Pr—Al alloy, and in all of these alloys, the eutectic point is a very low temperature of about 650° C. or less. Incidentally, even in the case of using a heavy rare earth element or an alloy thereof as the modifying alloy, an alloy having a eutectic point of about 900° C. or less should be used.
In the case of not using a heavy rare earth element for the R2 element or R2-TM alloy, the material cost can be further reduced. In addition, since an R2-TM alloy having a low eutectic point as described above is used and its diffusion and impregnation at a low temperature is achieved, the manufacturing method of the present invention is suitable for a nanocrystalline magnet (crystal grain size is approximately from 50 to 300 nm) that encounters a problem of coarsening of the crystal grain when placed, for example, in a high-temperature atmosphere of about 800° C. or more.
In another embodiment of the method for manufacturing a magnet of the present invention, an Re-M alloy (wherein Re is a rare earth element and M is an element capable of reducing the melting point of the rare earth element by that is alloyed) is heated at a temperature not lower than the melting point thereof to melt the alloy (first step). As the alloy containing a rare earth element, the above-described R2-TM alloy may be used, and an alloy of a difficultly reducible metal element, i.e., a metal having a redox potential of −1 eV or less, and an element capable of reducing the melting point of the metal, is preferred. Re is preferably at least either one of Nd and Sm, M is at least one of Cu, Fe, Al and Ga, and Re-M is mot preferably SmCu or SmFe. The melting point of the Re-M alloy is preferably 800° C. or less. In order to have a melting point of 800° C. or less, in the case of Nd—Cu, the Nd content is set to be from 40 to 90%; in the case of Nd—Fe, the Nd content is set to be from 60 to 80%; and in the case of Sm—Cu, the Sm content is set to be from 40 to 90%.
Next, the molten Re-M alloy is brought into contact with a magnetic particle containing a transition metal element to diffuse Re in the Re-M alloy into the magnetic particle (second step). The transition metal element is preferably at least one of Fe, Co and Ni.
This method is described by referring to the drawings. As shown in
Control of a difficulty reducible element-containing nano-level structure that is difficult to synthesize by a chemical technique, i.e., nanoparticle formation or formation of a core-shell structure, has been conventionally performed by rapid cooling/solidification or pulverization, but there is a problem in the anisotropization or oxidizability. Among others, an alloy containing a rare earth magnetic material is highly active and in order to impart high magnetic properties, nanostructure formation is supposed to be necessary, but a production method capable of providing an ideal structure has not been heretofore known.
The difficultly reducible element is readily oxidized and since the surface area is increased at the time of nanoparticle formation by pulverization, oxidation proceeds, leading to breaking from the originally targeted structure. In the rapid solidification method, when anisotropization by unidirectional solidification is attempted, the particle is coarsened to few micro meters or more. Nanostructure formation may be achieved by increasing the rapid cooling rate, but anisotropization cannot be effected.
On the other hand, according to the method of the present invention, as to an alloy containing a difficultly reducible element, a particle having a core-shell structure can be easily and simply produced, structure control at the nanostructure level becomes possible, and in the case of a magnetic material, a structure not more than the single domain particle size can be produced, leading to enhancement of the coercive force.
An alloy having a composition of (Nd(1-x)Cex)yFe100-y-w-z-vCowBzGav (x=1, y=13.5, z=5.8, w=4, and v=0.5) was nanocrystallized by liquid quenching (amorphous may be heat-treated). The conditions for quenching conducted here are a molten metal temperature of 1,450° C., an inert atmosphere (reduced-pressure Ar atmosphere) and a peripheral velocity of 20 to 40 m/s. The resulting ribbon having a nanocrystalline structure was packed in a die and subjected to pressurization/heating to produce a compact. The conditions for molding conducted here are a molding pressure of 200 MPa, a temperature of 650° C., and a holding time of 180 s. The obtained compact was subjected to hot plastic working (strong working) to form an oriented nanocrystalline structure. The conditions for strong working conducted here are a working temperature of 750° C., and a strain rate of 0.1 to 10/s, and the working method is swaging. The rare earth magnet precursor (core) produced by the swaging work is Ce2Fe14B and is in a semi-hard state lower in the coercive force than Nd2Fe14B. Then a low-melting-point alloy of Nd70Cu30 was brought into contact with the rare earth magnet precursor in the semi-hard state and heat-treated at a temperature high enough to melt the alloy. The conditions for heat treatment conducted here are a heat treatment temperature of 700° C., a treatment time of 165 to 360 min, and a contact amount of alloy of 10 wt % (relative to the rare earth magnet precursor). Here, the Nd70Cu30 alloy was produced by weighing Nd (produced by Kojundo Chemical Laboratory Co., Ltd.) and Cu (produced by Kojundo Chemical Laboratory Co., Ltd.), arc melting these elements, and liquid-quenching the melt.
Through these steps, a core-shell type magnet having a structure where the core is a Ce2Fe14B phase and the outer shell is a (Nd0.5Ce0.5)2Fe14B phase, and having an overall composition of (Nd0.2Ce0.8)2Fe14B was obtained. Here, although Co and Ga are contained in the starting alloy, these Co and Ga are not contained in the core and outer shell of the obtained magnet, because Co and Ga are actually contained in the core and out shell but since their contents are very small, these are ignored. The same applies to Examples 2 and 3 and Comparative Examples 1 to 3 below. Part of the thus-obtained magnet was sampled by FIB imaging, and a grain having an average grain size (250×500 nm) was extracted.
A core-shell type magnet was obtained in the same manner as in Example 1 except that the contact amount of Nd70Cu30 alloy was changed to 20 wt %. The measurement results of volume fraction and Nd concentration are shown below.
Overall composition: (Nd0.31Ce0.69)2Fe14B
Core: Ce2Fe14B, 53.6%
Outer shell: (Nd0.669Ce0.331)2Fe14B, 46.4%
Nd Concentration in outer shell: 66.9%
Nd Concentration in the entirety: 31.0%
A core-shell type magnet was obtained in the same manner as in Example 1 except that the contact amount of Nd70Cu30 alloy was changed to 40 wt %. The measurement results of volume fraction and Nd concentration are shown below.
Overall composition: (Nd0.337Ce0.663)2Fe14B
Core: Ce2Fe14B, 53.5%
Outer shell: (Nd0.726Ce0.274)2Fe14B, 46.5%
Nd Concentration in outer shell: 72.6%
Nd Concentration in the entirety: 33.7%
An alloy where in the formula of Example 1, x=0.75 ((Nd0.25Ce0.75)13.5Fe76.2Co4B5.8Ga0.5), was used as the starting material and after production of a magnet, by not contacting Nd70Cu30 therewith, a magnet (strongly worked body, (Nd0.25Ce0.75)2Fe14B) in which a boundary is not present between the core and the outer shell and magnet phases Nd2Fe14B and Ce2Fe14B are mixed, was manufactured.
An alloy where in the formula of Example 1, x=0.5 ((Nd0.5Ce0.5)13.5Fe76.2Co4B5.8Ga0.5), was used as the starting material and after production of a magnet, by not contacting Nd70Cu30 therewith, a magnet (strongly worked body, (Nd0.5Ce0.5)2Fe14B) in which a boundary is not present between the core and the outer shell and magnet phases Nd2Fe14B and Ce2Fe14B are mixed, was manufactured.
An alloy where in the formula of Example 1, x=0.25 ((Nd0.75Ce0.25)13.5Fe76.2Co4B5.8Ga0.5), was used as the starting material and after production of a magnet, by not contacting Nd70Cu30 therewith, a magnet (strongly worked body, (Nd0.75Ce0.25)2Fe14B) in which a boundary is not present between the core and the outer shell and magnet phases Nd2Fe14B and Ce2Fe14B are mixed, was manufactured.
An alloy where in the formula of Example 1, x=1 (Ce13.5Fe76.2Co4B5.8Ga0.5), was used as the starting material and after production of a magnet, by not contacting Nd70Cu30 therewith, a magnet (strongly worked body, (Ce13.5Fe76.2Co4B5.8Ga0.5) in which a boundary is not present between the core and the outer shell, was manufactured.
An alloy where in the formula of Example 1, x=0 (Nd13.5Fe76.2Co4B5.8Ga0.5), was used as the starting material and after production of a magnet, by not contacting Nd70Cu30 therewith, a magnet (strongly worked body, (Nd13.5Fe76.2Co4B5.8Ga0.5) in which a boundary is not present between the core and the outer shell, was manufactured.
With respect to the obtained magnets, after pulse magnetization of 10 T, the coercive force was measured at room temperature by VSM (Lake Shore). Subsequently, the hysteresis curve was measured at respective temperatures (room temperature, 60, 80, 100, 140, 160, 180 and 200° C.) ranging from room temperature to 200° C., and the coercive force was determined. The results at ordinary temperature are shown in Table 1 below.
Similarly to Examples and Comparative Examples above, the coercive force was measured on each of rare earth magnets manufactured by using, as the starting material, respective alloys where in the formula of Example 1, x=1, 0.5 and 0.25, and performing only hot plastic working without diffusing and impregnating a modifying metal (corresponding to Comparative Examples 4, 2 and 3), and three kinds of rare earth magnets manufactured by setting the heat treatment temperature at the time of diffusion and impregnation of Nd70Cu30 to 580, 650 and 700° C., and the results are shown in Table 2 below and
As seen from
In addition, it is demonstrated that the coercive force is saturated at a heat treatment temperature of about 580° C. and even when heat-treated at a higher temperature, the coercive force shows no change in its value.
On the other hand, as seen from
Furthermore, as to the rare earth magnet manufactured by performing only hot plastic working without diffusing and impregnating a modifying metal and the rare earth magnet manufactured by diffusing and impregnating Nd70Cu30, respective TEM images were photographed and EDX analysis of each rare earth magnet was conducted.
It can be confirmed from
It is demonstrated by this experiment that a crystal grain consisting of a core as a semi-hard phase and an outer shell as a hard phase and having a composite structure of a semi-hard phase and a hard phase, enabling magnetic decoupling between crystal grains, is formed and in turn, a high-performance rare earth magnet utilizing Ce is obtained.
A quenched thin strip of Sm71Cu29 alloy was mixed with chemically synthesized Fe nanoparticles (particle diameter: about 100 nm), and the mixture was heat-treated at 800° C. for 30 minutes to obtain a core-shell type magnet. The magnetic properties of the obtained particle were measured by VSM, and the structure was observed by SEM.
A quenched thin strip of Sm72.5Fe27.5 alloy was mixed with chemically synthesized Fe nanoparticles (particle diameter: about 100 nm), and the mixture was heat-treated at 800° C. for 30 minutes. The magnetic properties of the obtained particle were measured by VSM, and the structure was observed by SEM.
A quenched thin strip of Sm71Cu29 alloy was mixed with Fe3N particles (particle diameter: about 3 μm), and the mixture was heat-treated at 800° C. for 30 minutes. The magnetic properties of the obtained particle were measured by VSM, and the structure was observed by SEM.
A quenched thin strip of Sm72.5Fe27.5 alloy was mixed with Fe3N particles (particle diameter: about 3 μm), and the mixture was heat-treated at 800° C. for 30 minutes. The magnetic properties of the obtained particle were measured by VSM, and the structure was observed by SEM.
A quenched thin strip of Sm71Cu29 alloy was mixed with Fe4N particles (particle diameter: about 3 μm), and the mixture was heat-treated at 800° C. for 30 minutes. The magnetic properties of the obtained particle were measured by VSM, and the structure was observed by SEM.
A quenched thin strip of Sm72.5Fe27.5 alloy was mixed with Fe4N particles (particle diameter: about 3 μm), and the mixture was heat-treated at 800° C. for 30 minutes. The magnetic properties of the obtained particle were measured by VSM, and the structure was observed by SEM.
A quenched thin strip of Sm71Cu29 alloy was mixed with Fe particles (particle diameter: about 50 μm), and the mixture was heat-treated at 800° C. for 30 minutes. The magnetic properties of the obtained particle were measured by VSM, and the structure was observed by SEM.
A quenched thin strip of Sm72.5Fe27.5 alloy was mixed with Fe particles (particle diameter: about 50 μm), and the mixture was heat-treated at 800° C. for 30 minutes. The magnetic properties of the obtained particle were measured by VSM, and the structure was observed by SEM.
A quenched thin strip of Sm71Cu29 alloy was mixed with Co particles (particle diameter: about 50 μm), and the mixture was heat-treated at 800° C. for 30 minutes. The magnetic properties of the obtained particle were measured by VSM, and the structure was observed by SEM.
A quenched thin strip of Sm72.5Fe27.5 alloy was mixed with Co particles (particle diameter: about 50 μm), and the mixture was heat-treated at 800° C. for 30 minutes. The magnetic properties of the obtained particle were measured by VSM, and the structure was observed by SEM.
As shown in
A quenched thin strip of Nd70Cu30 alloy was mixed with Fe92B8 particles, and the mixture was heat-treated at 580° C. for 30 minutes. The magnetic properties of the obtained particle were measured by VSM, and the structure was observed by SEM.
A quenched thin strip of Nd70Cu30 alloy was mixed with Fe83B17 particles, and the mixture was heat-treated at 580° C. for 30 minutes. The magnetic properties of the obtained particle were measured by VSM, and the structure was observed by SEM.
A quenched thin strip of Nd70Cu30 alloy was mixed with Fe67B33 particles, and the mixture was heat-treated at 580° C. for 30 minutes. The magnetic properties of the obtained particle were measured by VSM, and the structure was observed by SEM.
The evaluations results of magnetic properties are shown together in Table 3 below. All starting substances were a soft magnetic material having no coercive force (0 kOe) before impregnation, but those brought into contact with an impregnation material became a hard magnetic phase by more or less developing a coercive force.
1: Core, 2: outer shell, 10: crystal grain: 20: grain boundary, and 100: rare earth magnet.
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