U.S. patent application number 13/127402 was filed with the patent office on 2011-10-13 for method for producing sintered rare-earth magnet and powder-filling container for producing such magnet.
This patent application is currently assigned to INTERMETALLICS CO., LTD.. Invention is credited to Masato Sagawa.
Application Number | 20110250087 13/127402 |
Document ID | / |
Family ID | 42152675 |
Filed Date | 2011-10-13 |
United States Patent
Application |
20110250087 |
Kind Code |
A1 |
Sagawa; Masato |
October 13, 2011 |
METHOD FOR PRODUCING SINTERED RARE-EARTH MAGNET AND POWDER-FILLING
CONTAINER FOR PRODUCING SUCH MAGNET
Abstract
Provided is an easy and inexpensive method for producing a
sintered rare-earth magnet having cavities, such as slits, for
making the magnet less likely to be influenced from eddy currents
and/or performing a grain boundary diffusion process. The method
for producing a sintered rare-earth magnet includes performing the
following successive processes: a filling process ((a), (b)) for
filling a powder of rare-earth magnet alloy into a powder-filling
container together with a cavity-forming member; an aligning
process (b) for aligning the rare-earth magnet alloy powder in a
magnetic field; and a sintering process (e) for sintering the
rare-earth magnet alloy powder by heating the rare-earth magnet
alloy powder in a state of being held in the powder-filling
container, wherein (d) the cavity-forming member is removed after
the aligning process is completed and before the rare-earth magnet
alloy powder begins to be sintered.
Inventors: |
Sagawa; Masato; (Kyoto,
JP) |
Assignee: |
INTERMETALLICS CO., LTD.
Kyoto-shi, Kyoto
JP
|
Family ID: |
42152675 |
Appl. No.: |
13/127402 |
Filed: |
October 29, 2009 |
PCT Filed: |
October 29, 2009 |
PCT NO: |
PCT/JP2009/005726 |
371 Date: |
May 20, 2011 |
Current U.S.
Class: |
419/5 ;
425/78 |
Current CPC
Class: |
H01F 41/0273 20130101;
B22F 2005/103 20130101; B22F 2998/10 20130101; C22C 38/16 20130101;
C22C 38/06 20130101; B22F 2998/10 20130101; C22C 2202/02 20130101;
H01F 1/0577 20130101; B22F 5/10 20130101; B22F 2202/05 20130101;
B22F 2999/00 20130101; B22F 3/004 20130101; B22F 3/004 20130101;
B22F 3/10 20130101; B22F 2999/00 20130101; H01F 3/14 20130101; C22C
38/005 20130101; H01F 41/0293 20130101 |
Class at
Publication: |
419/5 ;
425/78 |
International
Class: |
B22F 5/10 20060101
B22F005/10; B22F 3/10 20060101 B22F003/10 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 6, 2008 |
JP |
2008-285007 |
Claims
1. A method for producing a sintered rare-earth magnet, wherein a
sintered rare-earth magnet having a cavity is produced by
performing following successive processes: a) a filling process for
filling a powder of rare-earth magnet alloy into a powder-filling
container together with a cavity-forming member; b) an aligning
process for aligning the rare-earth magnet alloy powder in a
magnetic field; and c) a sintering process for sintering the
rare-earth magnet alloy powder by heating the rare-earth magnet
alloy powder in a state of being held in the powder-filling
container, wherein d) the cavity-forming member is removed after
the aligning process is completed and before the rare-earth magnet
alloy powder begins to be sintered.
2. The method for producing a sintered rare-earth magnet according
to claim 1, wherein the removal of the cavity-forming member is
performed before the sintering process is initiated.
3. The method for producing a sintered rare-earth magnet according
to claim 1, wherein the cavity-forming member is a plate-shaped
member or a rod-shaped member.
4. The method for producing a sintered rare-earth magnet according
to claim 3, wherein the rare-earth magnet alloy powder is aligned
in a magnetic field parallel to the cavity-forming member in the
aligning process.
5. The method for producing a sintered rare-earth magnet according
to claim 1, wherein a binder is filled into the powder-filling
container together with the rare-earth magnet alloy powder in the
filling process.
6. The method for producing a sintered rare-earth magnet according
to claim 1, wherein an embedding member is filled into the cavity
after the removal of the cavity-forming member.
7. The method for producing a sintered rare-earth magnet according
to claim 1, wherein: the rare-earth magnet alloy is an Nd--Fe--B
magnet alloy; and a diffusing process for diffusing Dy and/or Tb
into the sintered compact is performed by injecting a substance
containing Dy and/or Tb into the cavity of the sintered compact
obtained by the sintering process.
8. The method for producing a sintered rare-earth magnet according
to claim 7, wherein an embedding member is filled into the cavity
after the diffusing process.
9. The method for producing a sintered rare-earth magnet according
to claim 6, wherein the embedding member is made of an insulating
material.
10. The method for producing a sintered rare-earth magnet according
to claim 1, wherein the filling process includes inserting the
cavity-forming member into the powder-filling container through an
insertion opening formed in either the powder-filling container or
a lid of the powder-filling container, and the removal process
includes pulling out the cavity-forming member from the insertion
opening.
11. A powder-filling container for producing a sintered rare-earth
magnet, comprising: a mold into which a powder of rare-earth magnet
alloy is to filled; a lid to be attached to the mold; and an
insertion opening, formed in either the mold or the lid, for
allowing insertion of a cavity-forming member.
12. A powder-filling container for producing a sintered rare-earth
magnet, comprising: a mold into which a powder of rare-earth magnet
alloy is to be filled; a lid to be attached to the mold; and a
cavity-forming member provided on either the mold or the lid.
13. The method for producing a sintered rare-earth magnet according
to claim 8, wherein the embedding member is made of an insulating
material.
Description
TECHNICAL FIELD
[0001] The present invention relates to a method for producing a
sintered rare-earth magnet, such as a sintered Nd--Fe--B magnet or
sintered Sm--Co magnet.
BACKGROUND ART
[0002] Sintered rare-earth magnets are commonly used as permanent
magnets capable of creating strong magnetic fields. In particular,
sintered Nd--Fe--B magnets are commonly used in motors for hybrid
cars or electric vehicles, compact motors for hard-disk drives,
large-sized industrial motors, power generators and other
applications.
[0003] In such motors or generators, a sintered rare-earth magnet
is used as the rotor and an electromagnet as the stator. The
electromagnet is operated to create a rotating magnetic field for
revolving the rotor. In this process, an eddy current is generated
in the sintered rare-earth magnet, causing a loss of energy or
overheating of the motor. A technique for solving this problem is
disclosed in Patent Document 1, in which a plurality of slits are
formed on the surface of the sintered rare-earth magnet to prevent
the generation of eddy currents.
[0004] In the case of producing a sintered Nd--Fe--B magnet
(neodymium magnet), an alloy powder having a portion of Nd replaced
by Dy and/or Tb is used to increase the coercive force of the
magnet. However, since Dy and Tb are both expensive and rare
elements, this technique increases the production cost and
negatively affects the stable supply of the magnet. Another
drawback of this technique is the decrease in the maximum energy
product. A conventional technique for solving these problems is the
grain boundary diffusion, which includes applying Dy and/or Tb to
the surface of a sintered compact of Nd--Fe--B alloy containing
neither Dy nor Tb and heating it to a temperature within a range
from 700 to 1000 degrees Celsius, whereby Dy and/or Tb is
transferred through the boundaries of alloy particles into deeper
regions of the sintered compact to create a product containing Dy
and/or Tb only in the vicinity of the surfaces of the alloy
particles. This technique has the effect of achieving a high
coercive force while preventing a significant decrease in the
maximum energy product as well as decreasing the usage of Dy and
Tb. Patent Document 2 discloses the technique of efficiently
injecting Dy and/or Tb into the vicinity of the surfaces of alloy
particles by forming slits on the surface of the sintered compact
of an Nd--Fe--B alloy and diffusing Dy and/or Tb from those slits
into the grain boundaries.
BACKGROUND ART DOCUMENT
Patent Document
[0005] Patent Document 1: JP-A 2000-295804 (Paragraphs
[0009]-[0011]) [0006] Patent Document 2: JP-A 2007-053351
(Paragraphs [0027]-[0028] and [0033]-[0035])
DISCLOSURE OF THE INVENTION
Problem to be Solved by the Invention
[0007] In any of the methods described in Patent Documents 1 and 2,
the slits are formed by a machining process using a cutter, wire
saw or similar tool. The use of a machining process inevitably
increases the production cost since it requires a considerable
amount of labor and time, with heavy consumption of the tool.
Furthermore, the slits created by such a machining process cannot
be very thin and hence considerably lower the ratio of the actual
volume of the magnet (i.e. the volume of the sintered portion) to
its outside volume. As a result, the performance of the product as
the magnet substantially deteriorates.
[0008] In the case where the slits are formed on a compressed
compact by machining before sintering, there will be another
problem that the alloy powder remaining in the slits cannot be
easily removed. If a compact with an alloy powder remaining in the
slits is heated for sintering, the alloy powder will partially clog
the slits, compromising the effect of preventing the generation of
eddy currents. Furthermore, Dy and/or Tb is prevented from
sufficiently reaching deep regions in the grain boundary diffusion
process.
[0009] Subjecting a compressed compact to machine work may also
cause the additional problems of chipping or cracking.
[0010] Thus, the problem to be solved by the present invention is
to provide an easy and inexpensive method for producing a sintered
rare-earth magnet having cavities (e.g. slits or holes) for making
the magnet less likely to be influenced from eddy currents and/or
for performing the grain boundary diffusion process.
Means for Solving the Problems
[0011] A method for producing a sintered rare-earth magnet
according to the present invention aimed at solving the
aforementioned problem is characterized in that a sintered
rare-earth magnet having a cavity is produced by performing the
following successive processes:
[0012] a) a filling process for filling a powder of rare-earth
magnet alloy into a powder-filling container together with a
cavity-forming member;
[0013] b) an aligning process for aligning the rare-earth magnet
alloy powder in a magnetic field; and
[0014] c) a sintering process for sintering the rare-earth magnet
alloy powder by heating the rare-earth magnet alloy powder in a
state of being held in the powder-filling container, wherein
[0015] d) the cavity-forming member is removed after the aligning
process is completed and before the rare-earth magnet alloy powder
begins to be sintered.
[0016] According to the present invention, a sintered rare-earth
magnet having a cavity can be easily produced by a simple method
including filling a powder of rare-earth magnet alloy into the
powder-filling container together with a cavity-forming member and
then removing the cavity-forming member before the rare-earth
magnet alloy begins to be sintered. Thus, in the present invention,
no machining is necessary to create the cavity and a sintered
rare-earth magnet having a cavity can be produced at a low
cost.
[0017] In most of the conventional methods for producing sintered
rare-earth magnets, the compression-molding and aligning of a
rare-earth magnet alloy powder is achieved by filling the powder
into a container and applying a magnetic field to the powder while
compressing it. By contrast, the inventor of the present patent
application discovered the fact that a sintered rare-earth magnet
could be created by filling a rare-earth magnet alloy powder into a
powder-filling container, aligning the rare-earth magnet alloy
powder without compression-molding this powder, and heating the
powder in a state of being held in the powder-filling container.
(This technique is called a press-less method. Refer to JP-A
2006-019521.) In the present invention, since the press-less method
is used, the cavity-forming member undergoes no pressure even if
this member is put in the powder-filling container together with
the rare-earth magnet alloy powder.
[0018] As a result of the aligning process in the magnetic field,
the particles of the rare-earth magnet alloy powder held in the
powder-filling container magnetically attract each other. In the
present invention, since the cavity-forming member is removed after
the aligning process, the cavity will not be destroyed when the
cavity-forming member is removed.
[0019] When the rare-earth magnet alloy powder is heated to higher
temperatures in the sintering process, the powder begins to be
sintered when its temperature exceeds a specific level (e.g.
approximately 600 degrees Celsius for a sintered Nd--Fe--B magnet),
after which the sintered compact shrinks as the sintering process
continues. To avoid impeding this shrinkage, the cavity-forming
member used in the present invention is removed before the
rare-earth magnet alloy powder begins to be sintered.
[0020] The removal of the cavity-forming member may be performed
before the sintering process is initiated. This is desirable in
that it eliminates the necessity of considering the heat resistance
of the cavity-forming member or the reactivity between the
cavity-forming member and the rare-earth magnet alloy powder.
[0021] It is possible to use a cavity-forming member that liquefies
or vaporizes at a temperature lower than the temperature at which
the sintering begins. In this case, the cavity-forming member will
be removed after the temperature begins to increase for the
sintering and before the sintering actually begins.
[0022] If the aforementioned rare-earth magnet alloy is an alloy of
a sintered Nd--Fe--B magnet, Dy and/or Tb can be diffused into the
sintered compact by injecting a substance containing Dy and/or Tb
into the cavity of the sintered compact obtained by the sintering
process.
[0023] If slits for preventing the influence of eddy currents need
to be formed on the sintered rare-earth magnet, a plate-shaped
member can be used as the cavity-forming member. If the grain
boundary diffusion is of primary importance, a rod-shaped member
may be used. In the latter case, a large number of rod-shaped
cavity-forming members may be arranged in the form of a matrix,
whereby Dy and/or Tb can be uniformly diffused from a large number
of holes. The cross-sectional shape of the rod-shaped
cavity-forming member is not specifically limited; for example, it
may be circular, quadrilateral or hexagonal.
[0024] If a plate-shaped or rod-shaped cavity-forming member is
used as the cavity-forming member, it is preferable to align the
rare-earth magnet alloy powder in a magnetic field parallel to the
cavity-forming member in the aligning process. The particles of the
rare-earth magnet alloy powder forms a chain-like structure
extending in the direction parallel to the cavity-forming member.
Therefore, even if the cavity-forming member is removed in this
state, the chain-like structure will not be broken off and the
cavity will remain undestroyed.
[0025] To assuredly prevent the destruction of the cavity, the
rare-earth magnet alloy powder may be mixed with a binder when it
is filled into the powder-filling container. Examples of the binder
include methyl cellulose, polyacrylamide, polyvinyl alcohol,
paraffin wax, polyethylene glycol, polyvinyl pyrrolidone,
hydroxypropyl cellulose, hydroxypropyl methylcellulose, ethyl
cellulose, acetyl cellulose, nitrocellulose, and polyvinyl acetate
resin. (Refer to JP-A 10-270278.)
[0026] When the rare-earth magnet alloy powder is filled into the
powder-filling container together with the cavity-forming member,
it is possible to simultaneously put both the powder of rare-earth
magnet alloy and the cavity-forming member into the powder-filling
container or to separately and sequentially fill them into the
container.
[0027] The cavity formed in the sintered compact by the production
method according to the present invention is mechanically weak and
rather fragile if left in its original state. Furthermore, the
cavity may retain moisture and cause corrosion or mechanical
destruction of the product. To avoid these problems, an embedding
member, such as epoxy resin, may be filled into the cavity to
increase its mechanical strength and prevent the retention of
moisture. The process of filling the embedding member is performed
after the removal of the cavity-forming member. If the embedding
member is an epoxy resin or similar material whose heat-resistant
temperature is lower than the sintering temperature of the
rare-earth magnet, the filling process is performed after the
sintering process. If the diffusion process is additionally
performed, the filling process is performed after the diffusion
process. The embedding member should desirably be made of an
insulating material to prevent the influence of eddy currents.
Effect of the Invention
[0028] With the present invention, a cavity can be formed by a
simple method including filling a powder of rare-earth magnet alloy
into a powder-filling container together with a cavity-forming
member, aligning the powder in a magnetic field, and removing the
cavity-forming member. By this method, a sintered rear-earth magnet
having a cavity can be easily produced at a low cost since no
machining is required.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] FIGS. 1A-1C show vertical-sectional views of the first
embodiment of the mold, a lid for this mold, and a cavity-forming
member used in a method for producing a sintered rare-earth magnet
according to the present invention, as well as a top view of the
aforementioned lid.
[0030] FIGS. 2A-2E are schematic diagrams showing the first
embodiment of the method for producing a sintered rare-earth magnet
according to the present invention.
[0031] FIGS. 3A-3C show vertical-sectional views of the second
embodiment of the mold, a lid for this mold, and a cavity-forming
member used in a method for producing a sintered rare-earth magnet
according to the present invention, as well as a bottom view of the
aforementioned mold.
[0032] FIGS. 4A-4E are schematic diagrams showing the second
embodiment of the method for producing a sintered rare-earth magnet
according to the present invention.
[0033] FIGS. 5A and 5B show vertical-sectional views of another
example of the mold and a lid for this mold in the present
invention.
[0034] FIG. 6 is a perspective view showing one embodiment of the
rod-shaped cavity-forming member.
[0035] FIG. 7 is a schematic diagram showing one embodiment of the
grain boundary diffusion process in the present invention.
[0036] FIG. 8 is a schematic diagram showing one embodiment of the
process for filling an embedding member into the cavities.
[0037] FIG. 9 is a perspective view of a sintered rare-earth magnet
created by the method according to Embodiment 1.
[0038] FIGS. 10A-10C show vertical-sectional views of the mold, a
lid for this mold, and a cavity-forming member used in Example 3-1,
as well as a top view of the aforementioned lid.
[0039] FIGS. 11A-11C show vertical-sectional views of the mold, a
lid for this mold, and a cavity-forming member used in Example 3-2,
as well as a top view of the aforementioned lid.
BEST MODES FOR CARRYING OUT THE INVENTION
[0040] Embodiments of the method for producing a sintered
rare-earth magnet according to the present invention are
hereinafter described by means of FIGS. 1A-11C.
[0041] FIGS. 1A-2E show the first embodiment of the present
invention. The method according to the first embodiment uses a mold
(powder-filling container) 10 and a cavity-forming member 14 shown
in FIGS. 1B and 1C. The mold 10, which is designed for creating a
plate-shaped magnet, has a rectangular-parallelepiped receiving
section 11, into which a powder of rare-earth magnet alloy is to be
filled. This receiving section 11 has an opening on its upper side,
thus allowing the filling of the rare-earth magnet alloy powder and
removal of a sintered rare-earth magnet after the sintering
process. A lid 13 for closing this opening is attached thereto.
Examples of the materials available for the mold 10 and the lid 13
include magnetic stainless steel, non-magnetic stainless steel, and
some types of carbon that are heat-resistant to temperatures equal
to or higher than the sintering temperature used for creating the
sintered rare-earth magnet. The lid 13 has two insertion openings
131 extending parallel to each other in the longitudinal direction
of the rectangular-parallelepiped receiving section 11. Each
insertion opening 131 allows the insertion of a plate-shaped
cavity-forming member 14, which is slightly smaller than the
insertion opening 131 in both width and length. Examples of the
materials available for the cavity-forming member 14 include
various kinds of metal, carbon and plastic (which do not need to be
heat-resistant in the present embodiment). There are two
cavity-forming members 14 standing on a plate-shaped attachment
base 15, with the same interval as the two insertion openings
131.
[0042] The method for producing a sintered rare-earth magnet
according to the present embodiment is hereinafter described by
means of FIGS. 2A-2E. Initially, a rare-earth magnet alloy powder
19 is filled in the receiving section 11 (FIG. 2A). In this step,
the rare-earth magnet alloy powder 19 in a pure form may be used,
or a binder may be mixed with the rare-earth magnet alloy powder
19. The filling density should preferably be within a range from 40
to 50% of the true density of the rare-earth magnet alloy powder.
Next, the lid 13 is attached to the mold 10, and the cavity-forming
members 14 are inserted through the insertion openings 131 into the
rare-earth magnet alloy powder 19 held in the receiving section 11
(FIG. 2B). Subsequently, the mold 10 is set into a magnetic-field
generation coil 17, and a pulsed magnetic field parallel to the
cavity-forming members 14 (and perpendicular to the lid 13) is
applied to align the rare-earth magnet alloy powder 19 (FIG. 2C).
The strength of this magnetic field should be within a range from 3
to 10 T, and more preferably from 4 to 8 T. While the magnetic
field is applied, the lid 13 should be securely pressed onto the
mold 10 to prevent the rare-earth magnet alloy powder 19 from
escaping. After the aligning process in the magnetic field, the
cavity-forming members 14 are pulled out from the rare-earth magnet
alloy powder 19 and the insertion openings 131 (FIG. 2D). Thus,
slit-shaped cavities 18 are formed in the compact of the rare-earth
magnet alloy powder 19. As a result of the aligning process in the
magnetic field, the fine particles of the powder magnetically
attract each other and hence will barely fall into the cavities 18.
Subsequently, the rare-earth magnet alloy powder 19 in a state of
being held in the receiving section 11 is heated (FIG. 2E). Thus, a
sintered rare-earth magnet having slit-shaped cavities is obtained.
During the sintering process, water and other substances that are
inevitably present in the rare-earth magnet alloy powder 19
vaporize, and the generated gas is discharged through the insertion
openings 131 to the outside of the mold.
[0043] By this method, the slits can be created at a much lower
cost than in the case of performing machine work using a wire saw
or similar tool after the sintering process. Furthermore, a narrow
slit that cannot be created by machining can be created. The
obtained slits are completely free from any unwanted matter (e.g.
residual powder inside the slits) which lowers the functionalities
of the slits. Thus, a high-quality slit can be obtained.
[0044] FIGS. 3A-4E show the second embodiment of the present
invention. The method according to the second embodiment uses a
mold 20 shown in FIGS. 3A and 3B and a cavity-forming member 24
shown in FIGS. 4A-4D. Similar to the mold 10 used in the first
embodiment, the mold 20 has a receiving section 21 to which a lid
23 can be attached. A difference from the first example exists in
that two insertion openings 221 are formed in the bottom of the
mold 20. No insertion opening is formed in the lid 23. Similar to
the first embodiment, the cavity-forming members 24 fixed to a
cavity-forming member attachment base 25 can be inserted into the
insertion openings 221.
[0045] The method for producing a sintered rare-earth magnet
according to the second embodiment is hereinafter described by
means of FIGS. 4A-4E. Initially, the cavity-forming members 24 are
inserted into the insertion openings 221 of the mold 20 (FIG. 4A).
Next, a rare-earth magnet alloy powder 29 is filled in the
receiving section 21, and the lid 23 is attached (FIG. 4B). Thus,
the insertion of the cavity-forming member and the filling of the
rare-earth magnet alloy powder are performed in reverse order as
compared to the first embodiment. Next, the mold 20 is set into a
magnetic-field generation coil 27, and a pulsed magnetic field
parallel to the cavity-forming members 24 (and perpendicular to the
lid 23) is applied to align the rare-earth magnet alloy powder 29
(FIG. 4C). Subsequently, the cavity-forming members 24 are pulled
out from the rare-earth magnet alloy powder 29 and the insertion
openings 221 to form cavities 28 (FIG. 4D), and the rare-earth
magnet alloy powder 29 in a state of being held in the receiving
section 21 is sintered by heat (FIG. 4E).
[0046] FIGS. 5A and 5B show another example of the mold. Unlike the
mold 10 shown in FIGS. 1B and 1C in which the cavity-forming
members 14 are fixed to the cavity-forming member attachment base
15 prepared separately from the lid 13, the cavity-forming members
14A in the present example are directly fixed to the lid 13A (FIG.
5A). If this lid 13A is used, the lid 13A is detached from the mold
after the aligning process in order to remove the cavity-forming
members 14.
[0047] The previous descriptions pertained to the cases where the
cavity-forming members are removed after the aligning process. On
the other hand, if the cavity-forming members are made of a
material that liquefies or vaporizes at a temperature lower than
the sintering temperature of the rare-earth magnet alloy powder, it
is possible to remove the cavity-forming members, without pulling
them out, by heating them together with the mold and rare-earth
magnet alloy powder. In this case, the cavity-forming members may
be attached to the inside of the receiving section. Specific
examples of the materials available for such a cavity-forming
member include polyvinyl alcohol or other plastic materials that
easily vaporize. FIG. 5B shows one example in which cavity-forming
members 14B stand at the bottom 12 of the receiving section 11.
[0048] The following description explains how to determine an
appropriate thickness and interval of the cavity-forming members as
well as an appropriate depth by which these members should be
inserted into the rare-earth magnet alloy powder (which is
hereinafter called the "insertion depth").
[0049] Initially, an appropriate width, insertion depth, number and
interval of the cavity-forming members will be explained for the
case where the primary purpose of the cavities is to prevent eddy
current during the usage of the sintered rare-earth magnet. In this
case, the intended objective, i.e. the prevention of the eddy
currents, can be achieved even if the slit is too narrow.
Therefore, in order to improve the inherent performance of the
magnet, the slits formed in the sintered compact should be as
narrow as possible. This means that the cavity-forming members
should be as thin as possible. For example, in the case of using a
member similar to a razor blade, which is a typical example of the
thin plate-shaped member, the lower limit of the thickness of the
cavity-forming member is approximately 0.05 mm. In this case, with
the sintering shrinkage taken into account, the width of the slit
to be eventually formed in the sintered compact will be
approximately 0.04 mm. With respect to the insertion depth, it is
preferable to increase this depth to improve the effect of reducing
the eddy currents. However, to ensure an adequate mechanical
strength of the sintered compact, the depth should be smaller than
the magnet's thickness in the direction of the insertion depth by 1
mm or more, and more preferably 2 mm or more.
[0050] If the cavity-forming member is excessively thick, the
volume ratio of the magnet (i.e. the ratio of the volume where the
magnet actually exists to the outside volume of the sintered
magnet) will be too low and the magnetic properties of the product
will deteriorate. Accordingly, the thickness of the cavity-forming
member should be appropriately determined so that the volume ratio
will be equal to or higher than 90%.
[0051] With respect to the interval of the slits, or the interval
of the cavity-forming members, it is preferable to reduce this
interval since the loss of energy due to the eddy currents
generated in the magnet is proportional to the second power of the
magnet size. However, increasing the number of slits reduces the
volume ratio of the magnet. Given these factors along with the
aforementioned conditions relating to the thickness and insertion
depth, the interval and number of the cavity-forming members should
be determined so that the volume ratio will exceed the level where
the required magnetic properties are obtained.
[0052] Next, an appropriate width, insertion depth, number and
interval of the cavity-forming members will be explained for the
case where the primary purpose of the cavities is to help the grain
boundary diffusion of Dy and/or Tb into the sintered compact. If
the cavity-forming member is too narrow, it is difficult to inject
a substance containing Dy and/or Tb into the slit formed in the
sintered compact. Therefore, it is preferable to form the slits
with a width equal to or larger than 0.1 mm. If the interval of the
slits is too large, the effect of grain boundary diffusion cannot
extend over the entirety of the sintered magnet, causing the
resulting product to have uneven magnetic properties. Accordingly,
the interval of the slits, or the interval of the cavity-forming
members, should preferably be equal to or smaller than 6 mm, and
more preferably equal to or smaller than 5 mm. With respect to the
insertion depth, the difference between this depth and the magnet's
thickness in the direction of the insertion depth should preferably
be equal to or smaller than 6 mm, and more preferably equal to or
smaller than 5 mm. However, to ensure an adequate mechanical
strength of the sintered compact, the difference should preferably
be equal to or larger than 1 mm, and more preferably equal to or
larger than 2 mm. Additionally, as in the previous case where the
primary purpose was to prevent the eddy current, the thickness,
insertion depth, number and interval of the cavity-forming members
should be determined so that the volume ratio of the product will
exceed the level where the required magnetic properties are
obtained.
[0053] The previous examples illustrated the case of using a
plate-shaped cavity-forming member. If the primary purpose is to
help the grain boundary diffusion, it is possible to use a
rod-shaped cavity-forming member. FIG. 6 shows one example, in
which a large number of rod-shaped cavity-forming members 34 are
arrayed in rows and columns in the form of a matrix on a
plate-shaped attachment base 35. The use of such a large number of
rod-shaped cavity-forming members 34 in the form of a matrix
results in a sintered compact having a large number of fine pores
(cavities). When a grain boundary diffusion process is performed to
create a sintered Nd--Fe--B magnet, Dy and/or Tb can be efficiently
diffused through these fine pores into the sintered compact.
[0054] To ensure the injection of a substance containing Dy and/or
Tb, the diameter of the fine pores formed in the sintered compact
should preferably be equal to or larger than 0.2 mm, and more
preferably equal to or larger than 0.3 mm. The interval of the
cavity-forming members 34 should preferably be equal to or smaller
than 6 mm, and more preferably equal to or smaller than 6 mm, to
diffuse Dy and/or Tb over the entirety of the sintered magnet. The
conditions to be considered for the insertion depth are the same as
in the case of the plate-shaped cavity-forming member.
[0055] The diffusion process includes filling a powder containing
Dy and/or Tb into the cavities 18 and then heating the filled
powder (FIG. 7). The heating temperature is typically within a
range from 700 to 1000 degrees Celsius. The Dy/Tb-containing
substance to be injected into the cavities may be a fluoride,
oxide, acid fluoride or hydride of Dy or Tb, an alloy of Dy or Tb
and another kind of metal, or a hydride of such an alloy. Examples
of the alloy of Dy or Tb and another kind of metal include alloys
of Ty or Tb and an iron group transition metal (e.g. Fe, Co or Ni),
B, Al or Cu. The grain boundary diffusion process can be
effectively performed by mixing the aforementioned substances in an
organic or similar solvent to prepare a slurry, injecting this
slurry into the cavities, and heating the slurry. This slurry may
be injected into the cavities only, or it may be additionally
applied to the surface of the sintered compact. In latter case, the
grain boundary diffusion takes place from both the cavities and the
surface of the sintered compact. After the slurry is injected into
the cavities of the sintered compact (and applied to its surface in
some cases), the grain boundary diffusion process is performed by
heating the sintered compact at 700 to 1000 degrees Celsius for one
to twenty hours under vacuum or in an inert-gas atmosphere. This
grain boundary diffusion process uses only a small amount of Dy
and/or Tb and yet can effectively increase the coercive force of
the sintered Nd--Fe--B magnet without significantly decreasing its
residual flux density even if the magnet has a substantially large
thickness of 5 mm or larger.
[0056] In the case where the cavities are formed for both purposes
of helping the grain boundary diffusion process and reducing the
loss of energy due to eddy currents, if the aforementioned slurry
is used in the grain boundary diffusion process, it is necessary to
control the amount of the slurry so that an electrically conductive
component of the injected slurry will not fill the cavity.
[0057] In any of the previously described embodiments, it is
possible to fill the cavities with an epoxy resin or similar
embedding member to prevent a decrease in the mechanical strength
of the product due to the presence of the cavities and the
corrosion or other problems due to retention of moisture in the
cavities. In this case, an epoxy resin in a liquid state is
injected into the cavities 18 and then cured at room temperature or
by heat (FIG. 8). For some type of material of the embedding
member, this embedding process can be performed before the
sintering process. In the case of using an epoxy resin or similar
adhesive resin, this process is performed after the sintering
process. If the diffusion process is additionally performed, the
embedding process is performed after the diffusion process.
Example 1
[0058] A strip-cast alloy of an Nd--Fe--B rare-earth magnet was
subjected to hydrogen pulverization and then a jet-mill process
using nitrogen gas, to obtain a rare-earth magnet powder with an
average particle size of 5 .mu.m. The composition of this
rare-earth magnet powder ratio was Nd: 25.8%, Pr: 4.3%, Dy: 2.5%,
Al: 0.23%, Cu: 0.1%, and Fe: the rest. The average particle size of
the rare-earth magnet powder was measured with a laser-type
particle-size analyzer.
[0059] This powder was filled into the mold 10 of the first
embodiment to an apparent density of 3.5 g/cm.sup.3, after which
the lid 13 was put on the mold 10. Subsequently, the cavity-forming
members 14 were inserted through the insertion openings 131. After
the mold 10 was fixed in a magnetic-field generation coil, a pulsed
magnetic field of 5 T was applied three times in the direction
parallel to the cavity-forming members 14 and perpendicular to the
bottom of the mold 10 so as to align the rare-earth magnet powder
in the magnetic field. Subsequently, the cavity-forming members 14
were pulled out from the mold 10, and then the mold 10 was put into
a sintering furnace. The entire process from the filling of the
powder to the putting of the mold into the furnace was carried out
in an argon-gas atmosphere. The sintering process was performed
under vacuum at 1010 degrees Celsius for two hours. In this
example, the mold 10 and the lid 13 were made of carbon and the
cavity-forming members 14 were made of non-magnetic stainless
steel. The thickness of the cavity-forming members 14 was 0.5
mm.
[0060] The sintered compact created by the previously described
process had a density of 7.56 g/cm.sup.3, which is as high as the
density of a sintered Nd--Fe--B magnet created by a normal pressing
method. The obtained sintered compact 31 (FIG. 9) had the shape of
a rectangular parallelepiped having a short-side length of 37 mm, a
long-side length of 39 mm and a height of 8.6 mm, with two slits 32
extending parallel to the shorter sides and at an interval of 12 mm
on the top face. No noticeable deformation in the outside shape of
the sintered compact or the slits 32 was recognized. The slits 32
had a width of approximately 0.4 mm and a depth of 6.2 mm. For
inspection, a metallic foil having a thickness of 0.3 mm was
inserted into each slit 32. The result confirmed that none of these
slits 32 was clogged or closed with foreign matter.
Example 2
[0061] Using the same powder as used in Example 1, a sintered
Nd--Fe--B magnet with slits was created by using the mold 20 and
the cavity-forming members 24 of the second embodiment. When the
mold 20 of the second embodiment is used, the powder needs to be
filled into the mold 20 with the cavity-forming members 24 attached
thereto. In filling the powder, it is necessary to carefully fill
it so that the powder will be uniformly put into the entire
receiving section 21. The filling density was 3.6 g/cm.sup.3. After
the powder was filled, the lid 23 was put on the mold 20.
Subsequently, the aligning process in the magnetic field and the
removal of the cavity-forming members 24 were performed under the
same conditions as in Example 1, and then the sintering process was
performed under the same conditions as in Example 1. After the
sintering process, the sintered compact was removed from the mold.
Similar to the product created in Example 1, the obtained sintered
compact had a high density and no deformation in its shape. The
slits were also found to be high-quality slits free from clogging
or closing. The outside shape of the sintered compact, the interval
of the slits, the width and other sizes of each slit were
approximately the same as those of Example 1.
Example 3
[0062] A sintered compact with cavities (slits or fine pores) was
created by using the molds and cavity-forming members shown in
FIGS. 10A-11C. The mold 40 shown in FIGS. 10B and 10C have a
rectangular-parallelepiped receiving section 41 having
square-shaped top and bottom sides. A lid 43 can be attached to the
top side. This lid 43 has two insertion openings 431 for allowing
the insertion of two plate-shaped cavity-forming members 44. The
mold used in the example shown in FIGS. 11B and 11C are the same as
this mold 40. A lid 53 to be attached to the mold 40 in the latter
example has four insertion openings 531 arranged in the form of a
square, thus allowing the insertion of four rod-shaped
cavity-forming members 54.
[0063] Using the same rare-earth magnet powder and method as used
in Example 1, a sintered compact with slits (Example 3-1) and a
sintered compact with fine pores (Example 3-2) were created by
using the cavity-forming members 44 and 54, respectively. Both
sintered compacts had a cubic outside shape with one side
approximately measuring 11 mm. The slits formed in the former
sintered compact had a width of 0.4 mm and a depth of 5.9 mm, and
were spaced by an interval of 3.3 mm. The fine pores formed in the
latter sintered compact had a diameter of 0.5 mm and a depth of 7.2
mm. For comparison, another sintered compact having a
rectangular-parallelepiped shape with neither slits nor fine pores
(Comparative Example 1) was also created under the same conditions
as used in the present Example (and Example 1) except that the
insertion and removal of the cavity-forming members 44 were
omitted. Each of the three types of sintered compacts was shaped
into a cube with one side accurately measuring 10 mm by using a
surface grinder. The obtained cubes were then subjected to alkaline
cleaning, acid cleaning and pure-water cleaning processes followed
by a drying process.
[0064] For these samples, a grain boundary diffusion process using
a Dy-containing alloy powder was performed as follows: Initially, a
Dy-containing alloy having a composition by atomic ratio of Dy:
80%, Ni: 14%, Al: 4%, and other kinds metals and impurities: 2% was
pulverized to an average particle size of 9 .mu.m with a jet mill
to obtain a Dy-containing alloy powder. Next, this powder was mixed
with ethanol by 50% by weight and stirred. The obtained mixture was
vacuum-impregnated into the slits of the sample of Example 3-1 and
the fine pores of the sample of Example 3-2, and then dried.
Subsequently, the Dy-containing powder was applied to the surface
of each of the magnets of Examples 3-1, 3-2 and Comparative
Example. These three types of sintered compacts were put into a
vacuum furnace and heated at 900 degrees Celsius for three hours.
After that, they were rapidly cooled to room temperature, then
heated to 500 degrees Celsius, and again rapidly cooled to room
temperature. The magnetic properties of the three samples created
in this manner are shown in Table 1. In this table, Comparative
Example 1-1 was obtained by performing the aforementioned grain
boundary diffusion process on the sintered compact of Comparative
Example 1. Comparative Example 1-2 was obtained by heating the
sintered compact of Comparative Example 1, without any
Dy-containing alloy powder applied to its surface, in the same
manner as in the grain boundary diffusion process.
TABLE-US-00001 TABLE 1 Maximum Residual Energy Ratio of Br Square-
Flux Coercive Product to Saturated ness Density Force (BH).sub.max
Magnetization H.sub.k/H.sub.cJ Br [kG] H.sub.cJ [kOe] [MGOe] Br/Js
[%] [%] Example 3-1 12.7 28.6 39.8 94.3 93.7 Example 3-2 12.8 28.2
40.0 94.5 95.2 Comparative 13.0 24.6 41.2 94.3 72.2 Example 1-1
Comparative 13.0 21.6 41.4 94.4 94.3 Example 1-2
[0065] As compared to the sample of Comparative Example 1-1, for
which the grain boundary diffusion process was performed with
neither slits nor fine pores, the samples of Examples 3-1 and 3-2
had higher coercive forces H.sub.cJ and higher squareness
H.sub.k/H.sub.cJ of magnetization curves. Their coercive forces
H.sub.cJ were also higher than that of the sample of Comparative
Example 1-2, for which no grain boundary diffusion process was
performed. These results demonstrate that the method according to
the present invention, which is an inexpensive method that does not
include the expensive machining process for forming slits after the
sintering process, is effective for enhancing the coercive force of
a sintered Nd--Fe--B magnet by grain boundary diffusion even in the
case where the magnet is large sized, like the 10-mm cube, for
which the grain boundary diffusion process has not been effective
before.
EXPLANATION OF NUMERALS
[0066] 10, 20, 40 . . . Mold (Powder-Filling Container) [0067] 11,
21, 41 . . . Receiving Section of Mold [0068] 12 . . . Bottom of
Mold [0069] 13, 23, 53 . . . Lid of Mold [0070] 131, 221, 431, 531
. . . Insertion Opening [0071] 14, 24, 34, 44, 54 . . .
Cavity-Forming Member [0072] 15, 25, 35 . . . Attachment Base for
Cavity-Forming Members [0073] 17, 27 . . . Magnetic-Field
Generation Coil [0074] 18, 28 . . . Cavity [0075] 19, 29 . . .
Rare-Earth Magnet Alloy Powder [0076] 31 . . . Sintered Compact
* * * * *