BACKGROUND OF THE INVENTION
Field of the Invention
[0001] The present invention relates to a method of producing a rare-earth magnet.
Related Background Art
[0002] Rare-earth bonded magnets have been known as a type of rare-earth magnet containing
rare-earth elements. Such rare-earth bonded magnets have excellent magnetic characteristics,
and can be relatively easily applied to a complex shape. From this viewpoint, the
rare-earth bonded magnets have been used for various apparatuses such as motors. Recently,
in various apparatuses, size reduction and efficiency increases have been sought after,
and it is necessary to improve magnetic characteristics even further.
[0003] The following method has been proposed as a method of producing a rare-earth bonded
magnet. First, magnet powder produced according to the HDDR method (hydrogenation
disproportionation desorption recombination) is mixed with diffusion powder including
rare-earth elements such as Tb and Dy, and the mixed powder is subjected to a diffusion
heat treatment, thereby preparing anisotropic magnet powder in which the rare-earth
elements are diffused on the surface of the magnet powder or therein. The anisotropic
magnet powder is mixed with resin, coupling agent, antifriction, or the like to produce
a rare-earth bonded magnet (e.g., Japanese Patent Publication No.
3452254). In this method of producing the rare-earth bonded magnet, the anisotropic magnet
powder in which the rare-earth elements such as Tb and Dy are diffused, is used. Accordingly,
it is possible to improve magnetic coercive force and the like.
[0004] However, in the method of producing the rare-earth bonded magnet described in Japanese
Patent Publication No.
3452254, the diffusion state of the rare-earth elements in the magnet powder may easily become
non-uniform, and the magnetic coercive force or squareness ratio of the obtained rare-earth
bonded magnet is not particularly high. In the diffusion heat treatment, it is necessary
to heat the magnetic powder at about 700 to 1000°C, and the magnetic powder may be
fused to form a mass. For this reason, in this method of producing the rare-earth
bonded magnet, even when the magnetic powder having the magnetic anisotropy is used,
orientation of the magnetic powder included in the finally obtained rare-earth bonded
magnet is disordered and magnetic characteristics such as the squareness ratio deteriorate.
SUMMARY OF THE INVENTION
[0005] An advantage of some aspects of the invention is to provide a method capable of producing
a rare-earth magnet having sufficiently excellent magnetic characteristics.
[0006] According to an aspect of the invention, there is provided a method of producing
a rare-earth magnet containing a rare-earth compound having a first rare-earth element
and a second rare-earth element, including: a mixing step of mixing rare-earth compound
powder including the first rare-earth element and subjected to a process based on
hydrogenation disproportionation desorption recombination with a diffusion material
including the second rare-earth element different from the first rare-earth element;
a molding step of molding the mixed powder into a compact in a magnetic field; and
a heating step of heating the compact to diffuse the second rare-earth element into
the rare-earth compound powder.
[0007] According to the producing method, since the rare-earth compound powder and the diffusion
material are heated in close contact with each other, the second rare-earth element
included in the diffusion material can be uniformly diffused by outer peripheral portions
of the rare-earth compound particles, as compared with a case in which the mixture
of the rare-earth compound and the diffusion material mixed in a powder state is subjected
to a diffusion process. For this reason, it is possible to produce the rare-earth
magnet having the excellent magnetic coercive force (HcJ) and squareness ratio.
[0008] Since the molding is performed in the magnetic field before performing the heating
step, it is possible to perform magnetic field orientation in a state where the magnetic
powder is not fused. For this reason, the diffusion process is performed in a state
where the orientation of the particles of the rare-earth compound powder subjected
to the process based on the hydrogenation disproportionation desorption recombination
is sufficiently high. Accordingly, it is possible to produce the rare-earth magnet
having an excellent residual magnetic flux density (Br) with a high degree of orientation.
[0009] In the producing method of the invention, it is preferable to include an impregnating
step of impregnating the compact with resin and curing the resin to obtain the rare-earth
bonded magnet after the heating step. Since the rare-earth bonded magnet obtained
according to such a producing method is produced using the compact with the high degree
of orientation in which the diffusion material is uniformly diffused as described
above, the magnetic characteristics thereof are excellent. Since the compact is impregnated
with the resin after the producing the compact, it is possible to produce the rare-earth
bonded magnet having a high degree of orientation and excellent magnetic characteristics
without damaging the orientation of the rare-earth compound powder (HDDR powder) having
the magnetic anisotropy, as compared with the case of mixing the resin before producing
the compact. According to the method, it is possible to sufficiently reduce variation
in size before and after curing the resin, that is, a reduction ratio. For this reason,
it is possible to produce the rare-earth bonded magnet with high dimensional precision.
[0010] The producing method of the invention may include, after the heating step, a pulverizing
step of pulverizing the compact to prepare pulverized powder and a magnet producing
step of molding the mixture of the pulverized powder and resin in the magnetic field
and curing the resin to obtain the rare-earth bonded magnet. In the producing method,
since the mixture of the pulverized powder and the resin is shaped in the magnetic
field, it is possible to easily produce a complex-shaped rare-earth bonded magnet.
[0011] According to the invention, it is possible to provide the method capable of producing
the rare-earth magnet having sufficiently excellent magnetic characteristics.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] Fig. 1 is a perspective view illustrating a rare-earth bonded magnet obtained according
to a method of producing a rare-earth magnet according to an embodiment of the invention.
Fig. 2 is a diagram illustrating a magnetic hysteresis loop of the rare-earth bonded
magnet obtained according to the method of producing the rare-earth magnet according
to the embodiment of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0013] Hereinafter, exemplary embodiments of the invention will be described with reference
to the accompanying drawings.
[0014] A method of producing a rare-earth magnet includes an HDDR step of performing a process
based on hydrogenation disproportionation desorption recombination on a raw compound
including a first rare-earth element to prepare rare-earth compound powder, a preparing
step of preparing a diffusion material including a second rare-earth element, a mixing
step of the rare-earth compound powder including the first rare-earth element and
subjected to the process based on the hydrogenation disproportionation desorption
recombination with the diffusion material including the second rare-earth element
to prepare the mixed powder, a molding step of molding the mixed powder into a compact
in a magnetic field, a heating step of heating the compact to diffuse the second rare-earth
element into outer peripheral portions of the rare-earth compound powder, and an impregnating
step of impregnating the compact with resin and curing the resin to obtain a rare-earth
bonded magnet. Hereinafter, the processes will be described in detail.
[0015] In the HDDR step, first, the raw compound including the first rare-earth element
is prepared. A compound or alloy obtained by a general casting method, for example,
a strip casting method, a book molding method, or a centrifugal casting method may
be used as the raw compound. A homogenization heat treatment may be further performed.
The raw compound may include raw metal, or inevitable impurities derived from the
raw compound or the producing method.
[0016] As the first rare-earth element, any rare-earth element may be used, preferably light
rare-earth element is used, and more preferably at least one ofNd and Pr is used.
[0017] In the specification, the rare-earth elements are scandium (Sc), yttrium (Y), and
lanthanoids belonging to the third group of the long-period form periodic table. Lanthanoids
include, for example, lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd),
samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium
(Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu). The rare-earth
elements can be classified into light rare-earth elements and heavy rare-earth elements.
In the specification, the "heavy rare-earth elements" are Gd, Tb, Dy, Ho, Er, Tm,
Yb, and Lu, and the "light rare-earth elements" are Sc, Y, La, Ce, Pr, Nd, Sm, and
Eu.
[0018] A preferable composition of the raw compound may be a R-Fe-B based composition including
at least one of Nd and Pr as rare-earth elements, 0.5 to 4.5 mass% of B, and the remainders
which are Fe and inevitable impurities. The raw compound may further include another
element such as Co, Ni, Mn, Al, Cu, Nb, Zr, Ti, W, Mo, V, Ga, Zn, and Si, as necessary.
[0019] The raw compound having the composition is prepared, and then a process based on
the HDDR method is performed. The HDDR method is a process of sequentially performing
hydrogenation, disproportionation, desorption, and recombination. The HDDR method
will be described in detail hereinafter.
[0020] First, a homogenization heat treatment of keeping the raw compound in the depressurized
atmosphere (1 kPa or lower) or the atmosphere of inert gas such as argon and nitrogen
at a temperature of 1000 to 1200°C for 5 to 48 hours is performed.
[0021] It is preferable that the homogenized raw compound is pulverized by means such as
a stamp mill and a jaw crusher, and then is allowed to pass through a sieve. Accordingly,
it is possible to prepare the raw compound in a powder form with a particle size of
10 mm or less.
[0022] In the hydrogen storing step, the raw compound in the powder form is kept in the
atmosphere of hydrogen with a hydrogen partial pressure of 100 to 300 kPa at 100 to
200°C for 0.5 to 2 hours. Accordingly, hydrogen is stored in a crystal lattice of
the raw material compound.
[0023] Next, the raw compound in which hydrogen is stored, is kept in the atmosphere of
hydrogen at a predetermined temperature, thereby performing hydrogenation disproportionation
to obtain a decomposition product. It is preferable that the hydrogen partial pressure
is 10 to 100 kPa and the temperature is 700 to 850°C at the time of the hydrogenation
disproportionation. It is possible to obtain rare-earth compound powder formed from
the particles having the magnetic anisotropy by performing the hydrogenation disproportionation
under such conditions.
[0024] The decomposition product obtained by the hydrogenation disproportionation includes
hydride such as RH
x and iron compounds such as α-Fe and Fe
2B. In this step, the decomposition product has a minute matrix formed therein in the
order of 100 nm.
[0025] Subsequently, by reducing the hydrogen partial pressure, hydrogen is discharged from
the decomposition product, and anisotropic rare-earth compound powder containing the
first rare-earth element is obtained. The rare-earth compound powder has the composition
equivalent to that of the raw compound. The particle size of the rare-earth compound
powder is preferably 350 µm or less, more preferably 250 µm or less, and even more
preferably 212 µm or less. The lower limit of the particle size of the rare-earth
compound powder is not particularly limited, but preferably, for example, 1 µm or
more.
[0026] The rare-earth compound powder obtained by the HDDR method may be further pulverized
using a pulverizing mill such as a jet mill, a ball mill, a vibration mill, and a
wet attritor. The rare-earth compound powder subjected to the HDDR method has crystal
particles of a small size and is anisotropic. Accordingly, it is possible to easily
obtain the rare-earth magnet having a sufficiently high density and excellent magnetic
characteristics.
[0027] In the preparing step, the diffusion material in a powder form including the second
rare-earth element is prepared. As far as the second rare-earth element is different
from the first rare-earth element, the second rare-earth element is not particularly
limited. From the viewpoint of obtaining a rare-earth magnet having further high magnetic
coercive force, the second rare-earth element is preferably a heavy rare-earth element,
and more preferably Dy or Tb. The diffusion material may be a general rare-earth compound
such as hydride, oxide, halide, and hydroxide of rare-earth elements, or rare-earth
metal. From the view point of further improving the magnetic characteristics of the
rare earth magnet, it is preferable to use a heavy rare-earth compound having a heavy
rare-earth element as a constituent element.
[0028] The heavy rare-earth compound may include elements other than heavy rare-earth metal
elements, and may be alloy of heavy rare-earth metal and metal other than rare-earth
metal. From the viewpoint of producing a rare-earth magnet having further excellent
magnetic characteristics, the heavy rare-earth compound is preferably hydride and
fluoride, and more preferably hydride. When using the heavy rare-earth compound, it
is possible to sufficiently reduce the amount of impurities remaining in the rare-earth
magnet. Since hydride and fluoride are easily decomposed, it is possible to sufficiently
uniformly diffuse the second rare-earth element into the rare-earth compound powder
with a minute structure obtained by the HDDR method. By such factors, it is possible
to obtain the rare-earth magnet having the further excellent magnetic characteristics.
As preferable heavy rare-earth compounds, there are DyH
2, DyF
3, and TbH
2.
[0029] The rare-earth compound or the rare-earth metal can be produced by a general method.
Rare-earth compound powder or rare-earth metal powder can be prepared from the rare-earth
compound or the rare-earth metal, which is produced by the general method, by a dry
pulverizing method using a jet mill, or by mixing with an organic solvent according
to a wet pulverizing method using a ball mill or the like.
[0030] An average grain diameter of the diffusion material is preferably 100 nm to 30 µm,
more preferably 0.5 to 10 µm, and even more preferably 1 to 5 µm. When the average
grain diameter of the diffusion material is more than 30 µm, the second rare-earth
element is hardly diffused into the rare-earth compound powder, and the effect of
improving the sufficiently large HcJ and squareness ratio may be damaged. Meanwhile,
when the average grain diameter of the diffusion material is less than 100 nm, the
rare-earth element tends to be easily oxidized. As described above, when the rare-earth
oxide is created, the amount of diffusion of the second rare-earth element into the
rare-earth compound including the first rare-earth element becomes small, and the
improvement of the magnetic coercive force caused by the diffusion tends to decrease.
In the specification, an average grain diameter of the diffusion material is a volume
average grain diameter (d(50)) measured using a commercially available grain size
distribution calculator.
[0031] In the mixing step, the rare-earth compound powder including the first rare-earth
element subjected to the HDDR method as described above is mixed with the diffusion
material including the second rare-earth element to prepare mixed powder. The mixed
powder can be obtained by putting the rare-earth compound powder and the diffusion
material into a container, for example, at a predetermined mixing ratio, and then
mixing them for 1 to 30 minutes using a SPEX mixer. From the viewpoint of suppressing
oxidization of the diffusion material and the rare-earth compound powder, it is preferable
to perform the mixing in the atmosphere of inert gas such as argon gas. The mixing
method is not particularly limited, and may be a method using, for example, a V mixer,
a ball mill, or a Leica device may be used. Antifriction such as zinc stearate being
binder for molding at the time of mixing may be added. In this case, the amount of
addition thereof may be 0.01 to 0.5 mass%.
[0032] A combination ratio of the rare-earth compound powder and the diffusion material
is a ratio in which the content of the diffusion material in the mixed powder is preferably
0.5 to 5 mass%, more preferably 1 to 4 mass%, and even more preferably 1.5 to 3.5
mass%. When the content is less than 0.5 mass%, the amount of diffusion of the second
rare-earth element becomes small, and the effect of improving the sufficiently large
HcJ and squareness ratio tends to be hardly obtained. Meanwhile, when the content
is more than 5 mass%, the second rare-earth element diffused even into the rare-earth
compound powder, Br tends to decrease, and costs of materials tend to increase.
[0033] In the molding step, a compact having a desired shape is produced by molding the
mixed powder in the magnetic field. The molding in the magnetic field is performed
while applying the magnetic field, and the rare-earth compound powder having anisotropy
is fixed in a state where it is oriented in a predetermined direction. The molding
can be performed, for example, by compression molding using a compression molding
machine such as a mechanical press and a hydraulic press. Specifically, a mold cavity
is filled therein with the mixed powder, the filled powder is put between an upper
punch and a lower punch, and pressure is applied, thereby molding the mixed powder
to have a predetermined shape.
[0034] The shape of the compact obtained by the molding is not particularly limited, and
is determined according to a shape of a desirable rare-earth bonded magnet such as
a pillar shape, a plate shape, and a ring shape. At the time of the molding in the
magnetic field, it is preferable to apply the pressure with 580 to 1400 MPa. The orientation
magnetic field is preferably 800 to 2000 kA/m. As the molding method, in addition
to the dry molding of molding the mixed powder as described above, wet molding using
slurry in which the mixed powder is distributed in a solvent such as oil may be applied.
[0035] In the embodiment, the rare-earth compound powder subjected to the HDDR method is
used, and the molding of the mixed powder in the magnetic field is performed before
the heating step for diffusion without mixing with resin. For this reason, it is possible
to make the orientation of the rare-earth compound powder having the magnetic anisotropy
sufficiently uniform. Accordingly, it is possible to obtain the rare-earth bonded
magnet having particularly an excellent residual magnetic flux density. That is, it
is possible to sufficiently exhibit the magnetic characteristics of the rare-earth
compound powder having high anisotropy obtained by the HDDR method.
[0036] In the heating step, the compact obtained by the forming in the magnetic field is
heated enough to diffuse the second rare-earth element included in the diffusion material
into the outer peripheral portions of the rare-earth compound powder. Specifically,
the compact is kept under the depressurized condition or under the atmosphere of inert
gas such as argon gas, preferably at 700 to 1100 °C, more preferably 700 to 950°C,
and even more preferably 800 to 900°C, for 10 minutes to 12 hours. By the heating
under such conditions, the second rare-earth element is diffused into the outer peripheral
portions of the rare-earth compound powder, and particles having an inner layer in
which the first rare-earth element is abundant, and an outer layer in which the second
rare-earth element is abundant to coat the inner layer are formed. Accordingly, it
is possible to form the rare-earth bonded magnet having sufficiently high magnetic
coercive force. In the rare-earth compound powder subjected to the HDDR method, minute
cracks exist, but the diffusion material infiltrates the cracks and thus it is possible
to fill the cracks with the diffusion material. For this reason, it is possible to
improve the oxidization resistance and strength of the finally obtained rare-earth
bonded magnet.
[0037] In the heating step, when the heating temperature of the compact is too high or the
heating time is too long, the rare-earth compound powder is sintered. In an impregnating
step to be performed later, the compact tends to be hardly impregnated therein with
resin. Phase decomposition of the anisotropic magnetic powder obtained by performing
the HDDR method occurs, and the high magnetic characteristics may be damaged. Meanwhile,
when the heating temperature of the compact is too low or the heating time is too
short, the diffusion of the second rare-earth element tends not to sufficiently proceed.
Accordingly, it is preferable to set the heating temperature and the heating time
according to the kinds of the first and second rare-earth elements or the particle
size of the rare-earth compound powder.
[0038] In the producing method of the embodiment, since the heating step is performed in
the state where rare-earth compound powder is made into the compact, adhesion of the
rare-earth compound powder and the diffusion material is satisfactory and it is possible
to more uniformly diffuse the second rare-earth element into the outer peripheral
portions of the rare-earth compound powder. Accordingly, it is possible to obtain
the rare-earth bonded magnet with the sufficiently high squareness ratio and magnetic
coercive force.
[0039] In the impregnating step, the compact is impregnated with resin and is heated to
harden the resin, and a rare-earth bonded magnet containing the rare-earth compound
particles and the resin filling the spaces among the rare-earth compound particles
is obtained. Specifically, first, the compact subjected to the heating step is immersed
in a resin-contained solution prepared in advance, and is defoamed by depressurizing
an air-tight container, thereby putting the resin-contained solution into the voids
of the compact. Then, the compact is taken out of the resin-contained solution, and
the residual resin-contained solution attached to the surface of the compact is removed.
A centrifugal separator or the like may be used to remove the residual resin-contained
solution. Before the compact is immersed in the resin-contained solution, the compact
is put into the air-tight container and is immersed in a solvent such as toluene while
keeping the container in the depressurized atmosphere, thereby promoting defoaming.
Accordingly, it is possible to increase the amount of impregnation, and it is possible
to reduce the voids in the compact.
[0040] The resin contained in the resin-contained solution may be thermosetting resin such
as epoxy resin and phenol resin, and thermoplastic resin such as styrene-based, olefin-based,
urethane-based, polyester-based, polyamide-based elastomer such as nylon, ionomer,
ethylene propylene copolymer (EPM), and ethylene-ethyl acrylate copolymer. The thermosetting
resin is preferable among them, and the epoxy resin or phenol resin is more preferable.
[0041] The resin-contained solution can be prepared by dissolving the resin into a solvent.
As the solvent, a general organic solvent such as toluene, acetone, and ethyl alcohol
may be used. It is preferable to select the solvent according to the kinds of used
resin to sufficiently dissolve the resin. The resin content of the resin-contained
solution is not particularly limited, and it is preferable that the resin content
is large to obtain a rare-earth bonded magnet with high density and few voids.
[0042] The compact in which the resin-contained solution is infiltrated into the voids is
kept for example, in a thermostatic chamber in the depressurized atmosphere (1 kPa
or less) or the atmosphere of inert gas such as argon gas and nitrogen gas at 120
to 230°C for 1 to 5 hours. Thus, the solvent included in the resin-contained solution
is evaporated and the resin is cured. Then, a surface treating is performed as necessary,
and it is possible to obtain the anisotropic rare-earth bonded magnet.
[0043] The resin content of the rare-earth bonded magnet is preferably 0.5 to 10 mass%,
and more preferably 1 to 5 mass% from the viewpoint of achieving both excellent magnetic
characteristics and an excellent shape-keeping property. The amount of impregnated
resin can be controlled by changing the concentration of the resin in the resin-contained
solution or the molding pressure at the time of producing the compact.
[0044] Fig. 1 is a perspective view illustrating the rare-earth bonded magnet obtained according
to the producing method of the embodiment. The rare-earth bonded magnet 10 obtained
according to the producing method of the embodiment contains the particles having
the rare-earth compound as a main component and the resin filling the spaces among
the particles. The rare-earth compound includes the first rare-earth element and the
second rare-earth element as constituent elements. In the rare-earth bonded magnet
10, the second rare-earth element included in the diffusion material is uniformly
diffused into the outer peripheral portions of the rare-earth compound particles as
compared with the known magnet, and thus the rare-earth bonded magnet 10 has the sufficiently
high HcJ and squareness ratio. The anisotropic rare-earth compound powder is shaped
in the magnetic field without the heating step and mixing with resin. Accordingly,
it is possible to make the orientation of the particles of the rare-earth compound
more uniform than the known technique, the degree of orientation becomes high, and
it is possible to produce the rare-earth bonded magnet with excellent magnetic characteristics.
[0045] A modified example of the embodiment will be described. A method of producing a rare-earth
magnet according to the modified example includes a pulverizing step of pulverizing
the compact obtained in the heating step and formed of the rare-earth compound powder
in which the second rare-earth element is diffused into the outer peripheral portions
to prepare pulverized powder, and a magnet producing step of molding a mixture of
the pulverized powder and resin in the magnetic field and curing the resin to obtain
the rare-earth bonded magnet.
[0046] The producing method according to the modified example is the same as the embodiment
from the HDDR step of producing the compact to the heating step of diffusing the second
rare-earth element included in the diffusion material into the rare-earth compound
powder.
[0047] In the pulverizing step, the compact is pulverized by a stamp mill, and is allowed
to pass through a sieve as necessary, thereby preparing pulverized powder with an
average particle size of 100 to 300 µm. The pulverizing method is not particularly
limited, and a jaw crusher or various kinds of the known pulverizing means may be
used.
[0048] In the magnet producing step, first, a bonded magnet compound that is a mixture of
the pulverized powder and the resin is prepared. Specifically, the pulverized powder
is mixed with the same resin-contained solution as that of the embodiment and is heated
to volatilize at least a part of the solvent of the resin-contained solution, thereby
obtaining the bonded magnet compound. The resin content of the bonded magnet compound
is preferably 0.5 to 10 mass%, more preferably 1 to 5 mass%, and even more preferably
1 to 3 mass%.
[0049] Next, the bonded magnet compound is shaped in the magnetic field to produce a compact.
The molding in the magnetic field may be performed in the same manner as the embodiment.
Thereafter, the produced compact is kept for example, in a thermostatic chamber in
the depressurized atmosphere (1 kPa or less) or the atmosphere of inert gas such as
argon gas and nitrogen gas at 120 to 230°C for 1 to 5 hours. Thus, the solvent is
evaporated and the resin is cured. Then, a surface treating is performed as necessary,
and it is possible to obtain the rare-earth bonded magnet 10 shown in Fig. 1.
[0050] In the producing method according to the modified example, since the heating step
is performed on the compact in the state after the molding in the magnetic field in
the same manner as the embodiment, the second rare-earth element included in the diffusion
material is more uniformly diffused than the known method. Accordingly, it is possible
to obtain the rare-earth bonded magnet having the sufficiently high HcJ and squareness
ratio. Since the molding can be performed after mixing the pulverized powder including
the rare-earth compound powder and the diffusion material with the resin-contained
solution, it is possible to easily produce a rare-earth bonded magnet with a complex
shape.
[0051] Preferable embodiments have been described above, but the invention is not limited
to the embodiments. In the embodiment, the rare-earth bonded magnet is exemplified
as the rare-earth magnet, but the rare-earth magnet obtained according to the producing
method of the invention may have an aspect in which it is not impregnated with resin.
Such a rare-earth magnet may be, for example, anything which can be obtained by the
heating step of the embodiment.
Examples
[0052] The contents of the invention will be described hereinafter with reference to Examples
and Comparative Examples, but the invention is not limited to the following examples.
(Example 1)
<Production of Rare-Earth Bonded Magnet>
[0053] A raw compound containing Nd
2Fe
14B as a main component and having the following composition was prepared according
to a strip casting method.
[0054]
Nd: 28.0 mass%
B: 1.1 mass%
Ga: 0.35 mass%
Nb: 0.30 mass%
Cu: 0.03 mass%
Co: 3.8 mass%
Fe and Inevitable Impurities: Remainder
[0055] The raw compound included a small amount of inevitable impurities (0.5 mass% or less
in the whole raw compound). The raw compound was kept in the depressurized atmosphere
(1 kPa or lower) in the temperature range of 1000 to 1200°C for 24 hours (homogenization
heat treatment process). The product (Nd
2Fe
14B) obtained by the homogenization heat treatment was pulverized using a stamp mill
and was allowed to pass through a sieve, and raw powder (particle size: 1 to 2 mm)
was obtained.
[0056] The raw powder was put into a container made of molybdenum, and a tubular heating
furnace with an infrared heating manner was loaded with the container. Then, a process
based on the hydrogenation disproportionation desorption recombination (HDDR method)
was performed.
[0057] First, a hydrogen storing step of keeping the raw powder in the atmosphere of hydrogen
gas under a hydrogen partial pressure of 100 to 300 kPa at a temperature of 100° for
2 hours was performed. Subsequently, the hydrogen partial pressure in the furnace
was lowered while the temperature in the furnace was raised, and a hydrogenation disproportionation
step of keeping the raw powder in which hydrogen gas is stored, under the conditions
of a hydrogen partial pressure of 40 kPa and a temperature of 850°C for 1.5 hours
was performed.
[0058] Thereafter, a desorption recombination step was performed by lowering the hydrogen
pressure while the inside of the furnace was kept at 850°C. Thus, anisotropic magnetic
powder subjected to the HDDR method was obtained. The obtained magnetic powder was
pulverized using a stamp mill in the atmosphere of nitrogen gas, and was allowed to
pass through a sieve, thereby obtaining Nd
2Fe
14B powder with a particle size of 300 µm or less.
[0059] Subsequently, a diffusion material was prepared as follows, separately from the Nd
2Fe
14B powder. First, hydrogen was stored in Dy powder in the atmosphere of hydrogen at
350°C for 1 hour, and then the Dy powder was processed in the atmosphere of Ar at
600°C for 1 hour, thereby obtaining Dy hydride. The obtained Dy hydride was confirmed
as DyH
2 by X-ray diffraction measurement. The obtained DyH
2 powder was put into an ethanol solution and was subjected to a ball mill pulverizing
process, and it was made into fine DyH
2 powder with an average particle size (d(50)) of 3 µm.
[0060] The Nd
2Fe
14B powder obtained according to the above-described method was mixed with the fine
DyH
2 powder as the diffusion material using a V mixer, thereby preparing mixed powder.
A mixing ratio of the Nd
2Fe
14B powder and the diffusion material was a ratio in which the diffusion material was
3 mass% when the whole obtained powder was a reference. Zinc stearate was added at
a concentration of 0.1 mass % and was mixed into the whole amount of the mixed powder.
The mixed powder was shaped in the magnetic field under the conditions of a molding
pressure of 980 MPa and an orientation magnetic field of 1.2 T, and a rectangular
parallelepiped compact 10 shown in Fig. 1 was obtained. A magnetic field applying
direction was a direction indicated by the arrow a shown in Fig. 1. The dimension
and density of the compact 10 are as shown in Table 1.
[0061] The compact was subjected to a diffusion process of diffusing the Dy included in
the diffusion material into the outer peripheral portions of the Nd
2Fe
14B powder by a heat treatment of heating the compact in the atmosphere of argon gas
at 900°C for 30 minutes. A relative density of the compact after the diffusion process
was about 80%.
[0062] Next, the compact was put into a vacuum bell jar with the container in which toluene
was put, the compact was deposited into the toluene, a defoaming process of keeping
it in a state where the pressure in the container is 10 kPa or less for 30 minutes
was performed, and the pressure was returned to a normal pressure.
[0063] Epoxy resin was dissolved into the toluene to prepare an epoxy resin solution (the
content of epoxy resin: 50 mass%), separately from the compact. The epoxy resin solution
and the compact subjected to the defoaming process by performing the diffusion process
are sequentially put into the vacuum bell jar. The inside of the vacuum bell jar was
depressurized to 10 kPa or lower and was kept for 60 minutes, and thus the epoxy resin
solution was infiltrated into the compact.
[0064] The compact was taken out of the epoxy resin solution, and the epoxy resin solution
attached to the surface of the compact was removed by a centrifugal separator. Thereafter,
the compact impregnated with the epoxy resin solution was kept in the thermostatic
chamber at a temperature of 150°C (atmosphere: nitrogen gas) for 5 hours, and the
epoxy resin in the compact was cured, thereby obtaining the rare-earth bonded magnet
10. The dimension and mass of the obtained rare-earth bonded magnet 10 were measured
to calculate the density of the rare-earth bonded magnet. The dimensions and density
of the rare-earth bonded magnet are shown in Table 1 and Table 2.
[0065]
[Table 1]
|
Length of One Side (mm) |
Density |
Direction (Note 1) |
a |
b |
c |
(g/cm3) |
Compact α before Diffusion Process |
11.102 |
11.367 |
10.114 |
5.900 |
Rare-Earth Bonded Magnet β |
11.065 |
11.315 |
10.019 |
5.987 |
Variation in Size (β-α)/β |
-0.33% |
-0.46% |
-0.94% |
|
Note 1: Directions are the directions a, b, and c shown in Fig. 1
Note 2: Magnetic field applying direction at the time of shaping in the magnetic field
is the direction a shown in Fig. 1 |
<Measurement of Magnetic Characteristics>
[0066] The magnetic characteristics of the rare-earth bonded magnet produced as described
above were measured by a BH tracer. A residual magnetic flux density (Br), a coercive
magnetic force (HcJ), and a squareness ratio (Hk/iHc) were calculated from the obtained
result. Hk and bHc were calculated from a magnetic hysteresis loop, and the squareness
ratio was calculated using HcJ and Hk on the basis of the following formula (1). The
measurement result is shown in Table 2.
[0067] The squareness ratio is an indicator of magnetic performance, and denotes an angular
degree in the second quadrant of the magnetic hysteresis loop measured using the BH
tracer. The Hk of the formula (1) denotes an external magnetic intensity when a ratio
of magnetization to the residual magnetic flux density becomes 90% with respect to
the second quadrant of the magnetic hysteresis loop.
[0068]
(Example 2)
[0069] The rare-earth bonded magnet was prepared and measured in the same manner as Example
1, except for changing the mixing ratio of the diffusion material (fine DyH
2 powder) from 3 mass% to 1 mass% when the whole mixed powder was a reference. The
measurement result is shown in Table 2.
(Example 3)
[0070] The diffusion process of the compact was performed in the same manner as Example
1. After the diffusion process, the compact was pulverized using a stamp mill in the
atmosphere of nitrogen gas, to prepare the pulverized powder with a particle size
of 250 µm or less. The pulverized powder was mixed with the epoxy resin solution prepared
in the same manner as Example 1 at a ratio in which the content of the epoxy resin
of the compact was 3 mass%, toluene was evaporated, and a bonded magnet compound formed
of magnetic powder and resin was prepared. The bonded magnet compound was shaped in
the magnetic field under the conditions of a molding pressure of 980 MPa and an orientation
magnetic field of 1.2 T, and a rectangular parallelepiped compact was obtained.
[0071] The compact was kept in the thermostatic chamber at a temperature of 150°C for 5
hours, and the epoxy resin in the compact was cured, thereby obtaining a rare-earth
bonded magnet. The obtained rare-earth bonded magnet was measured in the same manner
as Example 1. The measurement result is shown in Table 2.
(Example 4)
[0072] A rare-earth bonded magnet was prepared and measured in the same manner as Example
1, except for using fine DyH
2 powder with an average particle size of 1 µm instead of the fine DyH
2 powder with an average particle size of 3 µm. The measurement result is shown in
Table 2. The average particle size of the fine DyH
2 powder was controlled by changing the conditions of the ball mill pulverization.
(Example 5)
[0073] A rare-earth bonded magnet was prepared and measured in the same manner as Example
1, except for using fine powder of Dy-Fe compound (Dy:Fe = 80:20 (mole ratio)) with
an average particle size of 3 µm instead of the fine DyH
2 powder as the diffusion material. The measurement result is shown in Table 2. The
fine powder of the Dy-Fe compound was prepared as follows. First, DyFe alloy and Fe
(electrolytic iron) were weighed and combined to be a desirable composition and were
dissolved by high-frequency dissolution, and a Dy-Fe compound was obtained according
to a strip casting method. The obtained Dy-Fe compound was pulverized using a jaw
crusher, and then was pulverized using a ball mill in an ethanol solution at 100 rpm
for 120 hours, thereby obtaining fine powder of the Dy-Fe compound with an average
particle size of 3 µm.
(Example 6)
[0074] A rare-earth bonded magnet was prepared and measured in the same manner as Example
1, except for using fine powder of a Dy-Fe-Co compound (Dy:Fe:Co = 80:10:10 (mole
ratio)) with an average particle size of 3 µm instead of the fine DyH
2 powder as the diffusion material. The measurement result is shown in Table 2. The
fine powder of the Dy-Fe-Co compound was prepared as follows. First, DyFe alloy, Fe
(electrolytic iron), and Co were weighed and combined to be a desirable composition
and were melted by arc melting and solidified, thereby obtaining a Dy-Fe-Co compound.
The Dy-Fe-Co compound was coarsely pulverized using a stamp mill in the atmosphere
of nitrogen, and then was pulverized using a ball mill in an ethanol solution at 100
rpm for 60 hours, thereby obtaining fine powder of the Dy-Fe-Co compound with an average
particle size of 3 µm.
(Example 7)
[0075] A rare-earth bonded magnet was prepared and measured in the same manner as Example
1, except for using fine DyF
3 powder with an average particle size of 3 µm instead of the fine DyH
2 powder as the diffusion material. The measurement result is shown in Table 2. The
fine DyF
3 powder was prepared by pulverizing DyF
3 powder made by Nihon Yttrium Co., Ltd. using a ball mill in an ethanol solution at
100 rpm for 12 hours.
(Example 8)
[0076] The rare-earth bonded magnet was prepared and measured in the same manner as Example
7, except for changing the mixing ratio of the diffusion material (fine DyF
3 powder) from 3 mass% to 1 mass% when the whole mixed powder was a reference. The
measurement result is shown in Table 2.
(Comparative Example 1)
[0077] Mixed powder of Nd
2Fe
14B powder and a diffusion material (fine DyH
2 powder) was obtained in the same manner as Example 1. The mixed powder was subjected
to a diffusion process of diffusing the Dy included in the diffusion material into
the Nd
2Fe
14B powder by a heat treatment of heating the mixed powder in the atmosphere of argon
gas at 900°C for 30 minutes.
[0078] The mixed powder subjected to the diffusion process was mixed with the epoxy resin
solution in the same manner as Example 1, toluene was evaporated, the bonded magnet
compound formed of the magnetic powder and the resin is adjusted, and the compound
was shaped in the magnetic field under the conditions of a molding pressure of 980
MPa and an orientation magnetic field of 1.2 T, thereby obtaining a compact.
[0079] The compact was kept in the thermostatic chamber at a temperature of 150°C for 5
hours, and the epoxy resin in the compact was cured, thereby obtaining a rare-earth
bonded magnet. The obtained rare-earth bonded magnet was measured in the same manner
as Example 1. The measurement result is shown in Table 2.
(Comparative Example 2)
[0080] A rare-earth bonded magnet was produced and measured in the same manner as Comparative
Example 1, except for changing the content of the fine DyH
2 powder from 3 mass% to 1 mass% in the mixed powder. The measurement result is shown
in Table 2.
(Comparative Example 3)
[0081] A rare-earth bonded magnet was produced and measured in the same manner as Example
1 except for using only the Nd
2Fe
14B powder instead of the mixed powder of the Nd
2Fe
14B powder and diffusion material (fine DyH
2 powder). The measurement result is shown in Table 2.
(Comparative Example 4)
[0082] Nd
2Fe
14B powder with a particle size of 300 µm or less was obtained in the same manner as
Example 1. The epoxy resin solution prepared in the same manner as Example 1 was mixed
with the Nd
2Fe
14B powder at a ratio in which the content of the epoxy resin of the compact was 3 mass%,
toluene was evaporated, and a bonded magnet compound formed of magnetic powder (Nd
2Fe
14B powder) and resin was prepared. The bonded magnet compound was shaped in the magnetic
field under the conditions of a molding pressure of 980 MPa and an orientation magnetic
field of 1.2 T, and a rectangular parallelepiped compact was obtained.
[0083] The compact was kept in the thermostatic chamber at a temperature of 150°C for 5
hours, and the epoxy resin in the compact was cured, thereby obtaining a rare-earth
bonded magnet. The obtained rare-earth bonded magnet was measured in the same manner
as Example 1. The measurement result is shown in Table 2.
(Comparative Example 5)
[0084] An ingot with a predetermined composition was melted in a high-frequency melting
furnace, was pulverized using a stamp mill in the atmosphere of nitrogen gas, and
allowed to pass through a sieve, thereby obtaining Nd
2Fe
14B powder with a grain diameter of 300 µm or less. A rare-earth bonded magnet was produced
and measured in the same manner as Example 1 except for using the Nd
2Fe
14B powder which was not subjected the HDDR method instead of the Nd
2Fe
14B powder subjected to the HDDR method. The measurement result is shown in Table 2.
[0085]
[Table 2]
|
Diffusion Material |
Hcj (kOe) |
Br (kG) |
Hk (kOe) |
Squareness Ratio Hk/Hcj (%) |
Density (g/cm3) |
Kind |
Average Particle Size (µm) |
Mixing Ratio (mass%) |
Ex.1 |
DyH2 |
3 |
3 |
20.10 |
8.74 |
10.06 |
50.0 |
5.99 |
Ex.2 |
DyH2 |
3 |
1 |
18.00 |
8.70 |
9.27 |
51.5 |
5.96 |
Ex.3 |
DyH2 |
3 |
3 |
19.98 |
8.56 |
7.75 |
38.8 |
5.93 |
Ex.4 |
DyH2 |
1 |
3 |
20.50 |
8.69 |
10.97 |
53.5 |
5.95 |
Ex.5 |
80Dy-20Fe |
3 |
3 |
18.90 |
8.80 |
8.79 |
46.5 |
5.97 |
Ex.6 |
80Dy-10Fe-10Co |
3 |
3 |
19.30 |
8.78 |
9.55 |
49.5 |
5.98 |
Ex.7 |
DyF3 |
3 |
3 |
19.00 |
8.78 |
9.42 |
49.6 |
5.97 |
Ex.8 |
DyF3 |
3 |
1 |
17.05 |
8.91 |
8.15 |
47.8 |
5.94 |
Comp.1 |
DyH2 |
3 |
3 |
18.60 |
8.52 |
5.21 |
28.0 |
5.96 |
Comp.2 |
DyH2 |
3 |
1 |
16.90 |
8.45 |
4.95 |
29.3 |
5.92 |
Comp.3 |
- |
- |
0 |
15.40 |
8.94 |
4.17 |
27.1 |
5.82 |
Comp.4 |
- |
- |
0 |
16.00 |
8.67 |
5.23 |
32.7 |
5.70 |
Comp.5 |
DyH2 |
1 |
3 |
0.05 |
2.10 |
0.01 |
20.0 |
5.95 |
[0086] As shown in Table 2, in Examples 1 to 8 subjected to the diffusion process of the
diffusion material in the compact state, the magnetic characteristics were higher
than those of Comparative Examples 1 and 2 subjected to the diffusion process of the
diffusion material in the powder state. Particularly, the magnetic coercive force
and the squareness ratio were drastically improved. In Comparative Examples 3 and
4 which were not subjected to the diffusion process, the magnetic characteristics
were lower than those of Examples 1 to 8. In Comparative Example 5 using the rare-earth
compound powder which was not subjected to the HDDR method, the magnetic characteristics
were significantly lower than those of Examples 1 to 8. The reason seems to be that
the magnetic coercive force is not improved even when the diffusion material is diffused
into the rare-earth compound powder, since the rare-earth compound powder (Nd
2Fe
14B) which is not subjected to the HDDR method is not anisotropic powder formed with
a microstructure.
[0087] Fig. 2 is a diagram illustrating a magnetic hysteresis loop measured using a BH tracer.
In Fig. 2, a curve 1 is a magnetic hysteresis loop of the rare-earth bonded magnet
of Example 1, and a curve 2 is a magnetic hysteresis loop of the rare-earth bonded
magnet of Comparative Example 1. It was confirmed that the squareness property of
the rare-earth bonded magnet of Example 1 is more excellent than that of Comparative
Example 1.