U.S. patent application number 12/749165 was filed with the patent office on 2010-09-30 for method of producing rare-earth magnet.
This patent application is currently assigned to TDK Corporation. Invention is credited to Noaki Mori, Hideki Nakamura, Hirofumi Nakano, Kouji Tanabe.
Application Number | 20100247367 12/749165 |
Document ID | / |
Family ID | 42174703 |
Filed Date | 2010-09-30 |
United States Patent
Application |
20100247367 |
Kind Code |
A1 |
Nakamura; Hideki ; et
al. |
September 30, 2010 |
METHOD OF PRODUCING RARE-EARTH MAGNET
Abstract
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 different from the first rare-earth element
includes: 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;
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.
Inventors: |
Nakamura; Hideki; (Tokyo,
JP) ; Mori; Noaki; (Tokyo, JP) ; Nakano;
Hirofumi; (Tokyo, JP) ; Tanabe; Kouji; (Tokyo,
JP) |
Correspondence
Address: |
DRINKER BIDDLE & REATH (DC)
1500 K STREET, N.W., SUITE 1100
WASHINGTON
DC
20005-1209
US
|
Assignee: |
TDK Corporation
Chuo-ku
JP
|
Family ID: |
42174703 |
Appl. No.: |
12/749165 |
Filed: |
March 29, 2010 |
Current U.S.
Class: |
419/27 ;
419/32 |
Current CPC
Class: |
H01F 1/09 20130101; H01F
1/0576 20130101; B22F 3/087 20130101; H01F 1/0558 20130101; H01F
1/083 20130101; H01F 1/08 20130101; B22F 2998/10 20130101; B22F
2998/10 20130101; H01F 41/0266 20130101; H01F 41/0293 20130101;
H01F 1/0556 20130101; B22F 3/26 20130101; B22F 1/0059 20130101;
B22F 3/087 20130101; B22F 2003/145 20130101; H01F 1/0578 20130101;
B22F 9/023 20130101; B22F 2998/10 20130101; H01F 1/0553 20130101;
H01F 41/0273 20130101; H01F 1/0573 20130101; B22F 9/023 20130101;
B22F 9/04 20130101; B22F 2003/145 20130101; B22F 3/087 20130101;
B22F 1/0003 20130101; B22F 3/02 20130101; B22F 1/0003 20130101;
B22F 3/087 20130101; B22F 3/02 20130101; B22F 2003/145 20130101;
B22F 2003/248 20130101 |
Class at
Publication: |
419/27 ;
419/32 |
International
Class: |
B22F 3/26 20060101
B22F003/26; B22F 3/12 20060101 B22F003/12 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 30, 2009 |
JP |
P2009-082275 |
Jan 8, 2010 |
JP |
P2010-002995 |
Claims
1. 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 different from the first rare-earth element,
comprising: 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;
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.
2. The method of producing the rare-earth magnet according to claim
1, further comprising an impregnating step of impregnating the
compact with resin and curing the resin to obtain the rare-earth
bonded magnet after the heating step.
3. The method of producing the rare-earth magnet according to claim
1, further comprising: a pulverizing step of pulverizing the
compact to prepare pulverized powder after the heating step; 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.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a method of producing a
rare-earth magnet.
[0003] 2. Related Background Art
[0004] 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.
[0005] 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.
[0006] 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.degree. 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
[0007] 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.
[0008] 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.
[0009] 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.
[0010] 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.
[0011] 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.
[0012] 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.
[0013] 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
[0014] 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.
[0015] 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
[0016] Hereinafter, exemplary embodiments of the invention will be
described with reference to the accompanying drawings.
[0017] 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.
[0018] 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.
[0019] 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 of Nd and Pr is used.
[0020] 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.
[0021] 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.
[0022] 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.
[0023] 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.degree. C. for 5 to 48 hours is performed.
[0024] 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.
[0025] 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.degree. C. for 0.5 to 2
hours. Accordingly, hydrogen is stored in a crystal lattice of the
raw material compound.
[0026] 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.degree.
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.
[0027] The decomposition product obtained by the hydrogenation
disproportionation includes hydride such as RH.sub.x and iron
compounds such as .alpha.-Fe and Fe.sub.2B. In this step, the
decomposition product has a minute matrix formed therein in the
order of 100 nm.
[0028] 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
.mu.m or less, more preferably 250 .mu.m or less, and even more
preferably 212 .mu.m or less. The lower limit of the particle size
of the rare-earth compound powder is not particularly limited, but
preferably, for example, 1 .mu.m or more.
[0029] 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.
[0030] 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.
[0031] 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.sub.2, DyF.sub.3, and TbH.sub.2.
[0032] 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.
[0033] An average grain diameter of the diffusion material is
preferably 100 nm to 30 .mu.m, more preferably 0.5 to 10 .mu.m, and
even more preferably 1 to 5 .mu.m. When the average grain diameter
of the diffusion material is more than 30 .mu.m, 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.
[0034] 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 %.
[0035] 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.
[0036] 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.
[0037] 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.
[0038] 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.
[0039] 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.degree. C., more preferably 700 to 950.degree. C.,
and even more preferably 800 to 900.degree. 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.
[0040] 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.
[0041] 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.
[0042] 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.
[0043] 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.
[0044] 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.
[0045] 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.degree. 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.
[0046] 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.
[0047] 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.
[0048] 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.
[0049] 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.
[0050] 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 .mu.m. The pulverizing method is not particularly
limited, and a jaw crusher or various kinds of the known
pulverizing means may be used.
[0051] 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 %.
[0052] 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.degree.
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.
[0053] 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.
[0054] 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
[0055] 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
[0056] A raw compound containing Nd.sub.2Fe.sub.14B as a main
component and having the following composition was prepared
according to a strip casting method.
[0057] Nd: 28.0 mass %
[0058] B: 1.1 mass %
[0059] Ga: 0.35 mass %
[0060] Nb: 0.30 mass %
[0061] Cu: 0.03 mass %
[0062] Co: 3.8 mass %
[0063] Fe and Inevitable Impurities: Remainder
[0064] 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.degree. C. for 24 hours
(homogenization heat treatment process). The product
(Nd.sub.2Fe.sub.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.
[0065] 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.
[0066] 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.degree. 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.degree. C. for 1.5 hours was performed.
[0067] Thereafter, a desorption recombination step was performed by
lowering the hydrogen pressure while the inside of the furnace was
kept at 850.degree. 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.sub.2Fe.sub.14B powder with a particle size of 300 .mu.m or
less.
[0068] Subsequently, a diffusion material was prepared as follows,
separately from the Nd.sub.2Fe.sub.14B powder. First, hydrogen was
stored in Dy powder in the atmosphere of hydrogen at 350.degree. C.
for 1 hour, and then the Dy powder was processed in the atmosphere
of Ar at 600.degree. C. for 1 hour, thereby obtaining Dy hydride.
The obtained Dy hydride was confirmed as DyH.sub.2 by X-ray
diffraction measurement. The obtained DyH.sub.2 powder was put into
an ethanol solution and was subjected to a ball mill pulverizing
process, and it was made into fine DyH.sub.2 powder with an average
particle size (d(50)) of 3 .mu.m.
[0069] The Nd.sub.2Fe.sub.14B powder obtained according to the
above-described method was mixed with the fine DyH.sub.2 powder as
the diffusion material using a V mixer, thereby preparing mixed
powder. A mixing ratio of the Nd.sub.2Fe.sub.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.
[0070] 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.sub.2Fe.sub.14B powder by a heat
treatment of heating the compact in the atmosphere of argon gas at
900.degree. C. for 30 minutes. A relative density of the compact
after the diffusion process was about 80%.
[0071] 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.
[0072] 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.
[0073] 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.degree. 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.
TABLE-US-00001 TABLE 1 Length of One Side (mm) Density Direction
(Note 1) a b c (g/cm.sup.3) Compact .alpha. before 11.102 11.367
10.114 5.900 Diffusion Process Rare-Earth Bonded Magnet .beta.
11.065 11.315 10.019 5.987 Variation in Size (.beta. -
.alpha.)/.beta. -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
[0074] <Measurement of Magnetic Characteristics>
[0075] 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 Elk on the basis of the following formula (1). The measurement
result is shown in Table 2.
[0076] 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.
Squareness Ratio (%)=Hk/HcJ.times.100 (1)
Example 2
[0077] 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.sub.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
[0078] 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
.mu.m 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.
[0079] The compact was kept in the thermostatic chamber at a
temperature of 150.degree. 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
[0080] A rare-earth bonded magnet was prepared and measured in the
same manner as Example 1, except for using fine DyH.sub.2 powder
with an average particle size of 1 .mu.m instead of the fine
DyH.sub.2 powder with an average particle size of 3 .mu.m. The
measurement result is shown in Table 2. The average particle size
of the fine DyH.sub.2 powder was controlled by changing the
conditions of the ball mill pulverization.
Example 5
[0081] 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 .mu.m instead of the fine DyH.sub.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 .mu.m.
Example 6
[0082] 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 .mu.m instead of the fine DyH.sub.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 .mu.m.
Example 7
[0083] A rare-earth bonded magnet was prepared and measured in the
same manner as Example 1, except for using fine DyF.sub.3 powder
with an average particle size of 3 .mu.M instead of the fine
DyH.sub.2 powder as the diffusion material. The measurement result
is shown in Table 2. The fine DyF.sub.3 powder was prepared by
pulverizing DyF.sub.3 powder made by Nihon Yttrium Co., Ltd. using
a ball mill in an ethanol solution at 100 rpm for 12 hours.
Example 8
[0084] 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.sub.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
[0085] Mixed powder of Nd.sub.2Fe.sub.14B powder and a diffusion
material (fine DyH.sub.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.sub.2Fe.sub.14B powder by a heat treatment of heating the mixed
powder in the atmosphere of argon gas at 900.degree. C. for 30
minutes.
[0086] 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.
[0087] The compact was kept in the thermostatic chamber at a
temperature of 150.degree. 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
[0088] 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.sub.2 powder from 3 mass % to 1 mass % in
the mixed powder. The measurement result is shown in Table 2.
Comparative Example 3
[0089] A rare-earth bonded magnet was produced and measured in the
same manner as Example 1 except for using only the
Nd.sub.2Fe.sub.14B powder instead of the mixed powder of the
Nd.sub.2Fe.sub.14B powder and diffusion material (fine DyH.sub.2
powder). The measurement result is shown in Table 2.
Comparative Example 4
[0090] Nd.sub.2Fe.sub.14B powder with a particle size of 300 .mu.m
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.sub.2Fe.sub.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.sub.2Fe.sub.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.
[0091] The compact was kept in the thermostatic chamber at a
temperature of 150.degree. 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
[0092] 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.sub.2Fe.sub.14B powder with a grain
diameter of 300 .mu.m or less. A rare-earth bonded magnet was
produced and measured in the same manner as Example 1 except for
using the Nd.sub.2Fe.sub.14B powder which was not subjected the
HDDR method instead of the Nd.sub.2Fe.sub.14B powder subjected to
the HDDR method. The measurement result is shown in Table 2.
TABLE-US-00002 TABLE 2 Diffusion Material Average Squareness
Particle Mixing Ratio Size Ratio Hcj Br Hk Hk/Hcj Density Kind
(.mu.m) (mass %) (kOe) (kG) (kOe) (%) (g/cm.sup.3) Ex. 1 DyH.sub.2
3 3 20.10 8.74 10.06 50.0 5.99 Ex. 2 DyH.sub.2 3 1 18.00 8.70 9.27
51.5 5.96 Ex. 3 DyH.sub.2 3 3 19.98 8.56 7.75 38.8 5.93 Ex. 4
DyH.sub.2 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 DyF.sub.3 3 3 19.00 8.78 9.42 49.6 5.97 Ex. 8 DyF.sub.3
3 1 17.05 8.91 8.15 47.8 5.94 Comp. 1 DyH.sub.2 3 3 18.60 8.52 5.21
28.0 5.96 Comp. 2 DyH.sub.2 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 DyH.sub.2 1 3 0.05 2.10 0.01 20.0 5.95
[0093] 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.sub.2Fe.sub.14B) which is not subjected to the
HDDR method is not anisotropic powder formed with a
microstructure.
[0094] 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.
* * * * *