U.S. patent application number 12/081318 was filed with the patent office on 2008-08-21 for nd-fe-b type anisotropic exchange spring magnet and method of producing the same.
This patent application is currently assigned to NISSAN MOTOR CO., LTD.. Invention is credited to Makoto Kano, Hideaki Ono, Takae Ono, Munekatsu Shimada, Tetsurou Tayu.
Application Number | 20080199715 12/081318 |
Document ID | / |
Family ID | 32732666 |
Filed Date | 2008-08-21 |
United States Patent
Application |
20080199715 |
Kind Code |
A1 |
Shimada; Munekatsu ; et
al. |
August 21, 2008 |
Nd-Fe-B type anisotropic exchange spring magnet and method of
producing the same
Abstract
A Nd--Fe--B type anisotropic exchange spring magnet is produced
by a method of obtaining powder of a Nd--Fe--B type rare earth
magnet alloy which comprises hard magnetic phases and soft magnetic
phases wherein a minimum width of the soft magnetic phases is
smaller than or equal to 1 .mu.m and a minimum distance between the
soft magnetic phases is greater than or equal to 0.1 .mu.m,
obtaining a compressed powder body by compressing the powder, and
obtaining the Nd--Fe--B type anisotropic exchange spring magnet by
sintering the compressed powder body using a discharge plasma
sintering unit.
Inventors: |
Shimada; Munekatsu; (Tokyo,
JP) ; Ono; Hideaki; (Yokohama, JP) ; Ono;
Takae; (Yokohama, JP) ; Kano; Makoto;
(Yokohama, JP) ; Tayu; Tetsurou; (Kanagawa,
JP) |
Correspondence
Address: |
MCDERMOTT WILL & EMERY LLP
600 13TH STREET, N.W.
WASHINGTON
DC
20005-3096
US
|
Assignee: |
NISSAN MOTOR CO., LTD.
Kanagawa
JP
|
Family ID: |
32732666 |
Appl. No.: |
12/081318 |
Filed: |
April 15, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10705235 |
Nov 12, 2003 |
7371292 |
|
|
12081318 |
|
|
|
|
Current U.S.
Class: |
428/546 ;
241/170; 419/33; 419/38 |
Current CPC
Class: |
H01F 1/0579 20130101;
Y10T 428/12014 20150115; B82Y 25/00 20130101; H01F 41/0273
20130101 |
Class at
Publication: |
428/546 ;
241/170; 419/38; 419/33 |
International
Class: |
B22F 3/12 20060101
B22F003/12; B02C 17/00 20060101 B02C017/00; B22F 1/00 20060101
B22F001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 12, 2002 |
JP |
2002-328579 |
Claims
1-19. (canceled)
20. A method of producing powder of a Nd--Fe--B type rare earth
magnet alloy for a Nd--Fe--B type anisotropic exchange spring
magnet which comprises hard magnetic phases and soft magnetic
phases wherein a minimum width of the soft magnetic phases is
smaller than or equal to 1 .mu.m; a minimum distance between the
soft magnetic phases is greater than or equal to 0.1 .mu.m; and a
composition of the Nd--Fe--B type rare earth magnet alloy is
expressed by the following chemical formula (1)
Nd.sub.xFe.sub.100-x-y-zB.sub.yV.sub.z (1) where x is within a
range from 9 to 11; y is within a range from 5 to 8; z is within a
range of 0 to 2; and the chemical formula (1) optionally comprises
Co, and if Co is present in the alloy, 0.01 to 30 atom % of Fe is
replaced with Co, the method comprising: pulverizing the Nd--Fe--B
type rare earth magnet alloy by means of a ball mill using a
dispersant under a non-oxidation atmosphere.
21. The method according to claim 20, wherein the ball mill is of a
wet type.
22. The method according to claim 20, wherein the ball mill is of a
dry type.
23. A method of producing a Nd--Fe--B type anisotropic exchange
spring magnet, comprising: obtaining powder of a Nd--Fe--B type
rare earth magnet alloy which comprises hard magnetic phases and
soft magnetic phases wherein a minimum width of the soft magnetic
phases is smaller than or equal to 1 .mu.m; a minimum distance
between the soft magnetic phases is greater than or equal to 0.1
.mu.m; and a composition of the Nd--Fe--B type rare earth magnet
alloy is expressed by the following chemical formula (1)
Nd.sub.xFe.sub.100-x-y-zB.sub.yV.sub.z (1) where x is within a
range from 9 to 11; y is within a range from 5 to 8; z is within a
range of 0 to 2; and the chemical formula (1) optionally comprises
Co, and if Co is present in the alloy, 0.01 to 30 atom % of Fe is
replaced with Co; obtaining a compressed powder body by compressing
the powder at a compressing pressure ranging from 1 to 5
ton/cm.sup.2 in a magnetic field ranging from 15 to 25 kOe; and
obtaining a bulk magnet by sintering the compressed powder body at
a temperature ranging from 600 to 800.degree. C. and at a
compressing pressure ranging from 1 to 10 ton/cm.sup.2 in a
discharge plasma sintering unit.
24. The method according to claim 23, wherein the powder is
obtained by pulverizing the Nd--Fe--B type rare earth magnet alloy
by means of a ball mill.
25. A Nd--Fe--B type anisotropic exchange spring magnet produced by
a method of obtaining powder of a Nd--Fe--B type rare earth magnet
alloy which comprises hard magnetic phases and soft magnetic phases
wherein a minimum width of the soft magnetic phases is smaller than
or equal to 1 .mu.m; a minimum distance between the soft magnetic
phases is greater than or equal to 0.1 .mu.m; and a composition of
the Nd--Fe--B type rare earth magnet alloy is expressed by the
following chemical formula (1)
Nd.sub.xFe.sub.100-x-y-zB.sub.yV.sub.z (1) where x is within a
range from 9 to 11; y is within a range from 5 to 8; z is within a
range of 0 to 2; and the chemical formula (1) optionally comprises
Co, and if Co is present in the alloy, 0.01 to 30 atom % of Fe is
replaced with Co; obtaining a compressed powder body by compressing
the powder at a compressing pressure ranging from 1 to 5
ton/cm.sup.2 in a magnetic field ranging from 15 to 25 kOe; and
obtaining a bulk magnet by sintering the compressed powder body at
a temperature ranging from 600 to 800.degree. C. and at a
compressing pressure ranging from 1 to 10 ton/cm.sup.2 in a
discharge plasma sintering unit.
26. The Nd--Fe--B type anisotropic exchange spring magnet according
to claim 25, wherein a density of the anisotropic exchange spring
magnet is 95% of a true density of a magnet alloy having a
composition as same as that of the anisotropic exchange spring
magnet.
27. A motor comprising: a Nd--Fe--B type anisotropic exchange
spring magnet produced by a method of obtaining powder of a
Nd--Fe--B type rare earth magnet alloy which comprises hard
magnetic phases and soft magnetic phases wherein a minimum width of
the soft magnetic phases is smaller than or equal to 1 .mu.m; a
minimum distance between the soft magnetic phases is greater than
or equal to 0.1 .mu.m; and a composition of the Nd--Fe--B type rare
earth magnet alloy is expressed by the following chemical formula
(1) Nd.sub.xFe.sub.100-x-y-zB.sub.yV.sub.z (1) where x is within a
range from 9 to 11; y is within a range from 5 to 8; z is within a
range of 0 to 2; and the chemical formula (1) optionally comprises
Co, and if Co is present in the alloy, 0.01 to 30 atom % of Fe is
replaced with Co; obtaining a compressed powder body by compressing
the powder at a compressing pressure ranging from 1 to 5
ton/cm.sup.2 in a magnetic field ranging from 15 to 25 kOe; and
obtaining a bulk magnet by sintering the compressed powder body at
a temperature ranging from 600 to 800.degree. C. and at a
compressing pressure ranging from 1 to 10 ton/cm.sup.2 in a
discharge plasma sintering unit.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to a rare earth permanent
magnet having anisotropy and a method of producing the magnet.
[0002] Nd--Fe--B type permanent magnets have been widely used as a
magnet for a motor. Such magnets have been produced by means of a
melting method disclosed in M. Sagawa et al., Japanese Journal of
Applied Physics 26 (1987) 785 or a quenching method disclosed in R.
W. Lee, Applied Physics Letter 46 (1985) 790. Further, it is
possible to produce magnet powder having anisotropy by means of a
HDDR treatment disclosed in T. Takeshita et al., Proc. 10th Int.
Workshop on Rare Earth Magnets and Their Applications, Kyoto,
(1989) 511.
[0003] The magnetic properties of Nb--Fe--B type permanent magnets
have approached a theoretical limit, and therefore it is desired to
develop next-generation high-performance magnets. One of the
next-generation magnets is an exchange spring magnet, which is also
called nano-composite magnet, as disclosed in E. F. Kneller and R.
Hawig, IEEE Transaction Magnetics 27 (1991) 3588. Such an exchange
spring magnet has a structure where hard magnetic phases and soft
magnetic phases are finely dispersed at intervals of several tens
nm. The exchange spring magnet performs like as a unit hard
magnetic phase as a whole since the magnetization of the soft
magnetic phases is not easily reversed for the reason that the
magnetizations of the hard and soft magnetic phases are coupled by
the exchange interaction therebetween. Accordingly, it has been
evaluated that the nano-composite magnet has a possibility for
functioning as a very high-performance magnet. For example, it has
been reported in R. Skomski and J. M. D. Coey, Physical Review B48
(1993) 15812 such that if Sm.sub.2Fe.sub.17N.sub.3/Fe type alloy
can have a property of anisotropy, (BH)max=137 MGOe will be
theoretically obtained. A producing method of
Nd.sub.2Fe.sub.14B/Fe.sub.3B type exchange spring magnet has been
proposed in R. Coehoorn et al., Journal de Physique 49 (1988)
C8-669. Further, a producing method of Nd.sub.2Fe.sub.14B/Fe type
exchange spring magnet has been proposed in Japanese Patent
Provisional Publication Nos. 7-173501 and 7-176417 and in L.
Withanawasam et al., Journal of Applied Physics 76 (1994) 7065.
[0004] However, a melt spun method or mechanical alloying (MA)
method employed in the above magnet producing methods cannot
produce the magnet having the magnetic anisotropy, and therefore
the property of the obtained exchange spring magnet is not
sufficient as compared to the theoretical property.
[0005] Furthermore, various producing methods of an anisotropic
exchange spring magnet have been proposed. For example, Japanese
Patent Provisional Publication No. 11-8109 has disclosed a
producing method of crystallizing Nd--Fe--B amorphous alloy by
heating in a high magnetic field. Japanese Patent Provisional
Publication No. 11-97222 has disclosed a producing method of hot
working a quenched thin strip alloy so that hard and soft magnetic
phases are finely and dispersedly precipitated. Japanese Patent
Provisional Publication No. 2000-235909 has disclosed a method of
directly producing a magnet having the anisotropy by executing a
warm-working uniaxial-deformation under a liquid phase existing
condition of the raw material.
SUMMARY OF THE INVENTION
[0006] However, it is further required to improve the magnetic
property of an anisotropic exchange spring magnet and to develop a
method of easily producing such an improved anisotropic exchange
spring magnet.
[0007] It is therefore an object of the present invention to
provide a Nd--Fe--B type anisotropic exchange spring magnet having
a superior magnetic property and to provide a method of producing
the Nd--Fe--B type anisotropic exchange spring magnet.
[0008] It is another object of the present invention to provide a
magnet alloy and powder thereof which are used in producing the
exchange spring magnet.
[0009] It is a further object of the present invention to provide a
motor which comprises the Nd--Fe--B type anisotropic exchange
spring magnet.
[0010] An aspect of the present invention resides in a Nd--Fe--B
type rare earth magnet alloy which comprises hard magnetic phases
and soft magnetic phases, wherein a minimum width of the soft
magnetic phases is smaller than or equal to 1 .mu.m and a minimum
distance between the soft magnetic phases is greater than or equal
to 0.1 .mu.m.
[0011] Another aspect of the present invention resides in powder of
a Nd--Fe--B type rare earth magnet alloy which comprises hard
magnetic phases and soft magnetic phases, wherein a minimum width
of the soft magnetic phases is smaller than or equal to 1 .mu.m and
a minimum distance between the soft magnetic phases is greater than
or equal to 0.1 .mu.m.
[0012] A further aspect of the present invention resides in a
method of producing powder of a Nd--Fe--B type rare earth magnet
alloy which comprises hard magnetic phases and soft magnetic phases
wherein a minimum width of the soft magnetic phases is smaller than
or equal to 1 .mu.m and a minimum distance between the soft
magnetic phases is greater than or equal to 0.1 .mu.m. The method
comprises pulverizing the Nd--Fe--B type rare earth magnet alloy by
means of a ball mill using a dispersant under a non-oxidation
atmosphere.
[0013] A further aspect of the present invention resides in a
method of producing a Nd--Fe--B type anisotropic exchange spring
magnet, which method comprises obtaining powder of a Nd--Fe--B type
rare earth magnet alloy which comprises hard magnetic phases and
soft magnetic phases wherein a minimum width of the soft magnetic
phases is smaller than or equal to 1 .mu.m and a minimum distance
between the soft magnetic phases is greater than or equal to 0.1
.mu.m, obtaining a compressed powder body by compressing the powder
at a compressing pressure ranging from 1 to 5 ton/cm.sup.2 in a
magnetic field ranging from 15 to 25 kOe, and obtaining a bulk
magnet by sintering the compressed powder body at a temperature
ranging from 600 to 800.degree. C. and at a compressing pressure
ranging from 1 to 10 ton/cm.sup.2 in a discharge plasma sintering
unit.
[0014] A further aspect of the present invention resides in a
Nd--Fe--B type anisotropy exchange spring magnet produced by a
method of obtaining powder of a Nd--Fe--B type rare earth magnet
alloy which comprises hard magnetic phases and soft magnetic phases
wherein a minimum width of the soft magnetic phases is smaller than
or equal to 1 .mu.m and a minimum distance between the soft
magnetic phases is greater than or equal to 0.1 .mu.m; obtaining a
compressed powder body by compressing the powder at a compressing
pressure of 1 through 5 ton/cm.sup.2 in a magnetic field ranging
from 15 to 25 kOe; and obtaining a bulk magnet by sintering the
compressed powder body at a temperature ranging from 600 to
800.degree. C. and at a compressing pressure ranging from 1 to 10
ton/cm.sup.2 in a discharge plasma sintering unit.
[0015] A further aspect of the present invention resides in a motor
which comprises a magnet produced by a method of obtaining powder
of a Nd--Fe--B type rare earth magnet alloy which comprises hard
magnetic phases and soft magnetic phases wherein a minimum width of
the soft magnetic phases is smaller than or equal to 1 .mu.m and a
minimum distance between the soft magnetic phases is greater than
or equal to 0.1 .mu.m, obtaining a compressed powder body by
compressing the powder at a compressing pressure of 1 to 5
ton/cm.sup.2 in a magnetic field ranging from 15 to 25 kOe, and
obtaining a bulk magnet by sintering the compressed powder body at
a temperature ranging from 600 to 800.degree. C. and at a pressure
ranging from 1 to 10 ton/cm.sup.2 in a discharge plasma sintering
unit.
[0016] The other objects and features of this invention will become
understood from the following description with reference to the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 is a scanning electron microscope (SEM) photograph
showing a cross section of a rare earth magnet alloy (having a
composition of Nd.sub.9.1Fe.sub.75.8CO.sub.8B.sub.6.1V.sub.1) of
EXAMPLE I according to the present invention.
[0018] FIG. 2 is a graph showing magnetization curves of a VSM
sample produced from a thin strip alloy of EXAMPLE 1.
[0019] FIG. 3 is a graph showing magnetization curves measured for
checking a spring back phenomenon of the thin strip alloy of
EXAMPLE 1.
[0020] FIG. 4 is an enlarged view of the graph of FIG. 3 amplified
in the horizontal axis ten times.
[0021] FIG. 5 is a scanning electron microscope (SEM) photograph
showing powder of the rare earth magnet alloy (having a composition
of Nd.sub.9.1Fe.sub.75.8CO.sub.8B.sub.6.1V.sub.1) of EXAMPLE 1.
[0022] FIG. 6 is a graph showing magnetization curves of a VSM
sample produced from the as-milled magnet powder of EXAMPLE 1.
[0023] FIG. 7 is a graph showing magnetization curves for checking
the spring back phenomenon of FIG. 6 in the parallel direction.
[0024] FIG. 8 is a graph showing magnetization curves of a VSM
sample produced from heat-treated magnet powder of EXAMPLE 1.
[0025] FIGS. 9A and 9B are charts showing X-ray diffraction
analysis results of the as-milled powder (A) of EXAMPLE 1 and the
powder (B) which was obtained by heat treating the powder (A) at
612.degree. C.
[0026] FIG. 10 is a SEM photograph showing a cross section of the
rare earth magnet alloy (having a composition of
Nd.sub.11Fe.sub.72CO.sub.8B.sub.7.5V.sub.1.5) of EXAMPLE 4.
[0027] FIG. 11 is a graph showing magnetization curves of a VSM
sample produced from the as-milled magnet powder of EXAMPLE 4.
[0028] FIG. 12 is a graph showing magnetization curves of a VSM
sample produced from the heat-treated magnet powder of EXAMPLE
4.
[0029] FIG. 13 is a graph showing magnetization curves for checking
the spring back phenomenon of the heat-treated magnet powder of
EXAMPLE 4.
[0030] FIG. 14 is a cross sectional view showing a 1/4 part of a
concentrated winding and surface permanent magnet type motor
employing the exchange spring magnet.
DETAILED DESCRIPTION OF THE INVENTION
[0031] Inventors of the present invention found that it was
possible to obtain a Nd--Fe--B type anisotropic exchange spring
magnet having a superior magnetic property by using a rare earth
magnet alloy containing soft magnetic phases and hard magnetic
phases in a predetermined condition, as starting material. Further,
the inventors found that it was preferable to employ a ball-mill to
pulverize the starting material in producing the Nd--Fe--B type
anisotropic exchange spring magnet and further found a preferable
production condition for producing the Nd--Fe--B type anisotropic
exchange spring magnet.
[0032] First, a producing method of the anisotropic exchange spring
magnet is briefly discussed. By means of a strip casting method,
the rare earth magnet alloy according to the present invention is
obtained. FIG. 1 is a SEM photograph showing a cross-section of the
rare earth magnet alloy, whose composition is
Ni.sub.9.1Fe.sub.75.8CO.sub.8B.sub.6.1V.sub.1, according to the
present invention. In FIG. 1, whitish bubble-shaped parts and
whitish needle-shaped parts are soft magnetic phases. Throughout
the description of the present invention, "soft magnetic phases" is
a part recognized in the photograph having the scale factor as same
as that in FIG. 1. More specifically, the soft magnetic phase
having a size smaller than 0.1 .mu.m in FIG. 1 is excluded from
"soft magnetic phase" defined in the present invention. Further,
other darkish part except for "soft magnetic phase" is "hard
magnetic phase". The "hard magnetic phase" is a hard magnetic phase
region like as a mono-crystal. In pulverizing process of the magnet
alloy, a size of power becomes smaller than a size of the hard
magnetic phase region of the magnet alloy. More specifically, in
the pulverizing process, a crystalline grain size of the hard
magnetic phases, becomes fine by the repetition of cross-slip of
crystals. Further, cross-slip of the crystals including the soft
magnetic phases is also repeated in the pulverizing process.
Therefore, the powder obtained by the pulverizing process includes
the soft magnetic phases of fine size in addition to the hard
magnetic phases of fine size.
[0033] Even after the magnet alloy is pulverized into a size
smaller than a size of the hard magnetic phase region, each
fine-size hard magnetic phase in the obtained power maintains the
crystal orientation as same as that of the original hard magnetic
phase. The hard magnetic phases of the obtained powder contain
strains generated during pulverizing process using a ball mill. By
executing the heat treatment subsequently to the pulverizing
process, the hard magnetic phases of the obtained powder change
into the fine hard magnetic phases which have recovered the
magnetic property and in which the axes of the easy magnetization
are aligned. As discussed above, the powder obtained by pulverizing
the alloy using a ball mill also includes fine soft magnetic phases
in addition to the hard magnetic phases. Accordingly, the heat
treatment of the power provides a further superior anisotropic rare
earth magnet powder (anisotropic exchange spring magnet powder) in
which the soft magnetic phases and the hard magnetic phases whose
axes of easy magnetization are aligned are mixed.
[0034] Subsequently, there will be discussed methods of
conveniently checking that the rare earth magnet alloy comprises
the soft magnetic phases in addition to hard magnetic phases and
that the hard magnetic phase region is formed into like a
mono-crystal, that is, methods of conveniently checking the
alignment of the crystal orientation. One of the methods is a
method of checking the magnetic property of the exchange spring
magnet which was practically produced. First, a magnet to be
checked is obtained by pulverizing the magnet alloy using a ball
mill, by producing a compressed powder body by means of a
compressing process in a magnetic field, and by sintering the
compressed powder body under the temperature lower than 800.degree.
C. using an electro-discharge plasma apparatus so as to produce a
bulk magnet. Subsequently, the degree of the anisotropy of the bulk
magnet is checked by comparing a magnetization curve along the
direction parallel to the magnetic field direction applied during
the compressing process and a magnetization curve along the
direction perpendicular to the magnetic field direction during the
compressing process. When the anisotropy of the bulk magnet is
confirmed, it is determined that the hard magnetic phase regions
are put in an estimated condition.
[0035] Another method is a simple method. First, a sample for a
vibrating sample magnetometer (VSM) is produced by pulverizing thin
strips of a rare earth magnet alloy, by mixing the obtained powder
with epoxy resin functioning as adhesive and by hardening the
mixture poured in a VSM sample container while being put in a
magnetic field. Subsequently, there are measured a magnetization
curve of a parallel-set sample which is set in the VSM so that the
magnetic field applied to the sample by the VSM is parallel to the
direction of the magnetic field applied during the sample producing
process and the magnetization curve of the perpendicular-set sample
which is set in the VSM so that the magnetic field applied to the
sample by the VSM is perpendicular to the direction of the magnetic
field applied during the sample producing process. Finally, the
degree of mono-crystal like of the hard magnetic phase region,
which degree is indicative of the anisotropy of the produced
sample, is estimated from the ratio (Js ratio) between the
magnetizations (Js) of the parallel-set sample and the
perpendicular-set sample at the measured maximum magnetic field of
16 kOe. It is preferable that the thin strip of the rare earth
magnet alloy is further finely pulverized. However, in this
embodiment, the size of the obtained powder has been determined to
be smaller than or equal to 25 .mu.m for the reasons that this size
of 25 .mu.m is a limit which is capable of being manually produced
by pulverizing the material using a mortar and pestle and
classifying the obtained powder using sieves. It is assumed that
the crystal orientations of adjacent hard magnetic phase regions
are almost aligned, that is, they are textured.
[0036] The latter method is an effective means for the development
of the rare earth magnet alloys. As a result of actually producing
exchange spring magnets, when a rare earth magnet alloy having a Js
ratio greater than 1.3 was employed as starting material, it became
possible to produce an anisotropic exchange spring magnet
therefrom. As discussed above, the Js ratio is a ratio between the
magnetizations (Js) of the parallel-set sample and the
perpendicular-set sample at the measured maximum magnetic field.
Further, the Js ratio of a bulk magnet produced from the same alloy
was greater than the Js ratio of the VSM sample produced by this
method.
[0037] By compressing the rare earth magnet powder in a condition
of a predetermined magnetic field and a predetermined pressure, a
compressed powder body (green compact) having the anisotropic
property is obtained. Further, by sintering the green compact under
a relative low temperature so as not to grow crystals thereof, a
Nd--Fe--B type anisotropic exchange spring magnet having a superior
magnetic property is obtained. With the thus method, the aimed
magnet is easily obtained.
[0038] The inventors of the present invention found that a superior
anisotropic exchange spring magnet was obtained when Nd--Fe--B type
rare earth magnet alloy employed as starting material has a
structure wherein hard magnetic phases and soft magnetic phases are
mixed so as to satisfy a predetermined condition, as a result of
due consideration taking account of the properties and the
productivity of various anisotropic exchange spring magnets. That
is, the inventors reached a first aspect of the present invention
that a Nd--Fe--B type rare earth magnet alloy having a mixed
structure of the hard magnetic phases and soft magnetic phases is
characterized such that the minimum width of the soft magnetic
phase is substantially smaller than or equal to 1 .mu.l and the
minimum distance between the soft magnetic phases is substantially
greater than or equal to 0.1 .mu.m.
[0039] Herein, there are explained the minimum width of soft
magnetic phase and the minimum distance between soft magnetic
phases, with reference to FIG. 1. As already defined, "soft
magnetic phase" is a soft magnetic phase which is recognized in the
photograph having the scale factor in FIG. 1, and particularly has
a size greater than or equal to 0.1 .mu.m. "Minimum width of soft
magnetic phase" in the present invention is a minimum value of a
width of a soft magnetic phase detected when the rare earth magnet
alloy is observed. As to a bubble-shaped soft magnetic phase, a
length denoted by D1 in FIG. 1 corresponds to the minimum width in
this soft magnetic phase. As to a needle-shaped soft magnetic
phase, a width D2 at a portion except for end portions corresponds
to the minimum width in this soft magnetic phase. On the other
hand, "minimum distance between soft magnetic phases" is a distance
between the bubble-shaped soft magnetic phase and the soft magnetic
phase which is most adjacent to this bubble-shaped soft magnetic
phase. The distances denoted by D3 and D4 in FIG. 1 correspond to
the minimum distance between soft magnetic phases. As to the
needle-shaped soft magnetic phase, the minimum distance is also a
distance between the needle-shaped soft magnetic phase and the soft
magnetic phase which is most adjacent to this needle-shaped soft
magnetic phase.
[0040] Throughout the explanation of the present invention, a
sentence "the minimum width is substantially smaller than or equal
to 1 .mu.m" means that more than 90% of the minimum widths of the
respective soft magnetic phases are smaller than or equal to 1
.mu.m when the soft magnetic phases are selected at random in a SEM
photograph. For example, when 10 soft magnetic phases are selected
at random, the number of the soft magnetic phases having the
minimum width smaller than 1 .mu.m may be 9 or 10. It is preferable
that all of the soft magnetic phases have the minimum width smaller
than 1 .mu.m. Similarly, a sentence "minimum distance is
substantially greater than or equal to 0.1 .mu.m" means that more
than 90% of the minimum distances of the respective soft magnetic
phases are greater than or equal to 0.1 .mu.m when the soft
magnetic phases are selected at random in a SEM photograph. For
example, when 10 soft magnetic phases are selected at random, the
number of the soft magnetic phases having the minimum distance
greater than or equal to 0.1 .mu.m may be 9 or 10. It is preferable
that all of the soft magnetic phases have the minimum distance
greater than or equal to 0.1 .mu.m. The minimum width and the
minimum distance of the soft magnetic phases may be calculated by a
method of observing a cross section of the magnet alloy by means of
SEM (scanning electron microscope). The calculation procedure is
not limited to this method and may be executed by other method.
Although the recognition of the hard magnetic phases can be
executed using TEM (transmission electron microscopy), it may be
executed by means of a deductive reasoning as discussed above, as
far as no problem occurs.
[0041] When the minimum width of the soft magnetic phase and the
minimum distance between the soft magnetic phases satisfy the
above-discussed values, the anisotropic exchange spring magnet has
the superior properties. The reason thereof is that if the minimum
distance between the soft magnetic phases is too small, that is, if
the distance between the soft magnetic phases is too short, the
hard magnetic phase region becomes too small. Therefore, it will
become difficult to obtain the magnet powder wherein the hard
magnetic phases are fine, the axes of easy magnetization of the
hard magnetic phases are aligned, and the size of the magnet powder
is greater than 0.1 .mu.m. Further, if the minimum width of the
soft magnetic phase is too large, that is, if the soft magnetic
phase is too large, it will cause the problem that the pulverizing
of the starting material would increase a difficulty.
[0042] The preferable compositions of the magnet material for
obtaining the exchange spring magnet can be represented by the
following chemical formula (1).
Nd.sub.xFe.sub.100-x-y-zB.sub.yV.sub.z (1)
[0043] It was estimated from X-ray diffraction analysis and
measurement of the temperature dependency as to the magnetization
such that in these compositions the hard magnetic phase was
Nd.sub.2Fe.sub.14B, and the soft magnetic phase was .alpha.-Fe.
[0044] The compositions of Nd--Fe--Co--B type rare-earth magnet
alloy are, for example, Nd.sub.xFe.sub.85-xCo.sub.8B.sub.6V.sub.1
wherein x is within a range from 9 to 11. That is, by using the
rare-earth magnet ally having these compositions, an anisotropic
exchange spring magnet having a superior magnetic property is
obtained. It was estimated that the hard magnetic phase of the
magnet having these compositions was Nd.sub.2(Fe--Co).sub.14B, and
the soft magnetic phase was .alpha.-(Fe--Co), from the X-ray
diffraction analysis and the measurement of the temperature
dependency as to the magnetization of the produced magnet. However,
the composition is not limited to these compositions, and it is
preferable that x, y and z in the chemical formula (1) are
respectively within the ranges discussed hereinafter. Further, the
elements of the composition of Nd--Fe--Co--B rare-earth magnet
alloy may be replaced with the elements discussed later by a
predetermined quantity.
[0045] It is preferable that x in the chemical formula (1) ranges
from 9 to 11. If the quantity of Nd becomes greater than 11 atom %,
the ratio of the soft magnetic phase becomes smaller than 5%. This
lowers the properties of the exchange spring magnet. On the other
hand, if the quantity of Nd becomes smaller than 9 atom %, the
difficulty of producing the rare-earth magnet alloy increases.
[0046] It is preferable that y in the chemical formula (1) ranges
from 5 to 8. If the quantity of B becomes greater than 8 atom %,
there is a possibility that other phase except for the phase of
Nd.sub.2Fe.sub.14B and the phase of .alpha.-Fe is produced. On the
other hand, if the quantity of B becomes smaller than 5 atom %, the
difficulty of producing the rare-earth magnet alloy increases.
[0047] It is preferable that z in the chemical formula (1) ranges
from 0 to 2. V is added in the rare-earth magnet to improve the
fineness of crystals and the increase of coercivity. This addition
is not essential and may be cancelled. However, if this addition is
excessive, there is increased a possibility of lowering the
magnetic property.
[0048] Nd may be replaced with Pr within a range from 0.01 to 80
atom %. It is preferable that Nd is replaced with Pr within a range
from 20 to 60 atom %. When the quantity of the replacement with Pr
is within a range from 20 to 60 atom %, the residual flux density
is almost maintained, and the coercive force and the rectangular
characteristic in the magnetization curve are improved.
[0049] Nd may be replaced with Dy or Tb within a range from 0.01 to
10 atom %. When the quantity of this replacement with Dy or Tb is
within a range from 0.01 to 10 atom %, it is possible to improve
the coercive force and the thermal property without largely
lowering the residual flux density.
[0050] A part of Fe may be replaced with Co. When the part of Fe is
replaced with Co, it is preferable that the replacement percentage
is within a range from 0.01 to 30 atom %. This replacement within
this range improves the thermal properties without degrading the
coercive force and the residual flux density. In particular, when
the quantity of the replacement with Co is within 5 to 20 atom %,
it is possible to improve the residual flux density in addition to
the thermal properties. It was estimated that when Fe was replaced
with Co, the hard magnetic phase was Nd.sub.2(Fe--Co).sub.14B, and
the soft magnetic phase was .alpha.-(Fe--Co), from a result of the
X-ray diffraction analysis and the measurement of the temperature
dependency of the magnetization.
[0051] Fe or Co in Fe--Co may be replaced with a small quantity of
at least one of Al, Mo, Zr, Ti, Sn, Cu, Ga and Nb. This replacement
promotes a microstructure of the magnet alloy and increases the
coercive force. However, when the quantity of this replacement
becomes excessive, the magnetic property may be rather degraded.
From this viewpoint, it is preferable that the quantity of element
to be replaced is within a range from 0.1 to 3 atom % with respect
to the all composition.
[0052] It is impossible to completely eliminate a minute quantity
of impurities since the magnet according to the present invention
is alloy material. However, it is preferable that the quantity of
the impurities is as small as possible, and preferable that the
amount of the impurities is smaller than or equal to 1 weight
%.
[0053] The magnet alloy according to the present invention is
produced by the following procedures. First, metal elements for the
magnet alloy is mixed to achieve a desired composition. The
properly mixed metal elements are melted by a commonly known
melting method such as a high-frequency induction melting method
under a vacuum or argon atmosphere, and an ingot of the
desired-composition magnet alloy is obtained by solidifying the
melted alloy. Thereafter, the rare earth magnet alloy such as a
crystalline thin strip alloy according to the present invention is
obtained by a quenching method such as a strip casting method
wherein the melted magnet alloy is quenched and changes into
crystalline thin strip alloy. Such a strip casting method is
executed using a commonly known apparatus without specially
improving this apparatus. It is of course that the improvement of
the apparatus is preferable to further preferably obtain the
desired magnet alloy.
[0054] Generally, it is difficult to uniquely determine a producing
condition into one condition and it is necessary to properly set
each condition according to the employed apparatus and the kind of
alloys. In particular, it is important to finely control a cooling
speed in producing the magnet alloy according to the present
invention. If the cooling speed is out of the aimed cooling speed
range, there will degrade the properties of the magnet alloy such
that the size of crystals increases or the thin strip alloy
degrades in homogeneity. For example, if the cooling speed is too
high, the thickness of the thin strip alloy is decreased, and if
too slow, the thickness of the thin strip alloy is increased. Thus,
the cooling speed is closely related with the thickness of the thin
strip alloy produced by the strip casting method. In producing the
thin strip magnet alloy according to the present invention, it is
preferable that the cooling speed is controlled such that the
thickness of the thin strip alloy ranges from 30 to 300 .mu.m. For
example, it is preferable that the cooling speed is set at
1000.degree. C./sec or more.
[0055] Powder of the rare earth magnet alloy is obtained by
pulverizing the rare earth magnet alloy according to the present
invention into powder. It is preferable that the pulverizing of the
rare earth magnet alloy is executed using a ball mill. Another
aspect of the present invention, therefore, resides in the powder
of the rare earth magnet alloy which powder is obtained by
pulverizing the rare earth magnet alloy according to the first
aspect of the present invention using a ball mill. A further aspect
of the present invention resides in a producing method of the rare
earth magnet powder which is produced by pulverizing the rare earth
magnet alloy of the first aspect of the present invention using a
ball mill under a non-oxidizing atmosphere while adding dispersing
agent.
[0056] It is preferable that the rare earth magnet alloy is
pulverized into a size smaller than or equal to a size of the hard
magnetic phase region using a ball mill. For example, when the size
of the hard magnetic phase region is 0.5 .mu.m, it is preferable
that the rare earth magnet alloy is pulverized into a size smaller
or equal to 0.5 .mu.m. Usually, the magnet alloy is pulverized into
powder having a size of sub-micron size ranging from 0.1 to 1.0
.mu.m. In view of preferably realizing the magnetic field
orientation of the magnet powder, it is preferable that the magnet
alloy is pulverized into this sub-micron size. Although a lower
limit of the size of the magnet powder is not determined, if the
size of the magnet powder is too small, it becomes hard for the
magnet powder to exhibit the magnetic field orientation. Further,
the oxidation durability of the magnet powder degrades as the size
of the magnet powder becomes smaller. Consequently, it is
preferable that the size of the magnet powder is greater than or
equal to 0.1 .mu.m.
[0057] A type of the ball mill is not limited to a wet type or dry
type. It is preferable that the magnet alloy is pulverized under a
non-oxidizing atmosphere such as argon atmosphere or nitrogen
atmosphere, in order to prevent the magnetic property of the magnet
powder from being degraded by the oxidation of the magnet powder.
When a wet type ball mill is employed in pulverizing the material,
cyclohexane or the like is used. Further, in order to suppress the
aggregation of the obtained powder, dispersion agent is employed.
This also effectively functions in applying the magnetic field
orientation to the produced magnet powder. A typical dispersion
agent employed in a wet type ball mill is succinic acid, and a
typical dispersion agent employed in a dry type ball mill is
stearic acid. It was estimated on the basis of the magnetic
measurement and the X-ray diffraction analysis that the as-milled
powder was partially changed from crystalline to amorphous.
[0058] Further, by heat treating the powder obtained by pulverizing
the magnet ally, the power changes into further improved powder of
an anisotropic rare earth magnet alloy (exchange spring magnet)
where there are mixed the soft magnetic phases and the hard
magnetic phases whose axes of the easy magnetization are aligned.
For example, it is preferable that the temperature of the heat
treatment for the obtained powder is controlled within a range from
500 to 800.degree. C.
[0059] The heat-treated magnet powder is therefore put in a
condition that the hard magnetic phases and the soft magnetic
phases are finely mixed. Further, the easy magnetization axes of
the hard magnetic phases are aligned. As already explained, the
mono-crystal-like hard magnetic phases function to pulverize the
alloy into fine crystalline size hard magnetic phases during the
ball mill process and to disperse fine soft magnetic phases in the
hard magnetic phases.
[0060] Subsequently, the compressed powder body (green compact) is
obtained by compressing the obtained power in a die while applying
the magnetic field. The purpose of the magnetic field application
is the magnetic field orientation of the powder to be compressed.
The magnetic field orientation of the powder is basically completed
by the application of the magnetic field before starting an actual
compression of the powder. The device employed in the treatment for
the magnetic field orientation and in the compressing process is
not limited to a special device and may employ various
commonly-known treatment means. For example, this means may include
a means for compressing the magnet powder under a condition where
the axes of easy magnetization of the magnet powder are aligned in
the same direction by the application of the magnetic field. (Here,
the axis of easy magnetization of the magnet powder is parallel to
the axes of easy magnetization of the fine hard magnetic phases.)
When this means is employed, it is appropriate that the compressing
pressure is set within a range between from 1 to 5 tons/cm.sup.2
and the magnetic field is set within a range between from 15 to 25
kOe. Also, it is preferable that the compressed powder body is
produced using a die assembly for a discharge plasma sintering
unit. That is, after compressing the powder in the die assembly and
transferring to the discharge plasma sintering unit, the compressed
powder body in the assembly die is sintered by the discharge plasma
sintering unit while the compressing pressure is applied to the
compressed power body via the assembly die. This sintering process
is preferable in view of facilitating the workability of producing
the magnet.
[0061] A bulk of the anisotropic exchange spring magnet is obtained
by executing a sintering process of the obtained compressed powder
body under a compressed state in a discharge plasma sintering unit.
The use of the compressing and sintering technique in the discharge
plasma enables the compressed powder body to be sintered at a
relatively low temperature and thereby suppressing the formation of
the coarse crystals and ensuring a magnet having the superior
properties. Also, the discharge plasma compressing and sintering
technique may be executed using a general device, such as a device
"Model SPS-2040" made and sold by Izumitec Co. Ltd. and may be
executed with a suitably improved device depending on a produced
magnet to be obtained or a desired production line.
[0062] Accordingly, if the temperature of the discharge plasma
compressing and sintering process is too high, there may cause
forming the coarse crystals, degrading coercive force, and
weakening the exchange coupling. In order to prevent these
problems, it is preferable that the solidifying process is executed
at a temperature equal to or less than 800.degree. C., more
preferably equal to or less than 700.degree. C. In contrast, if the
temperature of the discharge plasma compressing and sintering
process is too low, the compressed powder body is insufficiently
densified, and therefore it is preferable that the temperature
during such process is maintained at a temperature equal to or
higher than 600.degree. C.
[0063] Further, it is preferable that such a compressing and
sintering technique is executed under a reduced pressure using a
rotary pump, and it is appropriate that the temperature is raised
at a temperature raising rate of approximately 15 to 25 K/min. It
is difficult to uniquely define a retaining time period of the
compressing and sintering technique since there is a need for
suitably changing the retaining time period according to various
factors such as the device, the operating temperature, and the size
of the compressed powder body. Generally, the retaining time period
is set at a value within approximately 0 to 10 min. After the
temperature is maintained for a predetermined time period, the
compressed powder body is cooled in the sintering apparatus at a
temperature lowering rate between approximately 10 to 30 K/min. In
this instance, the compressing pressure for the compressing and
sintering technique is appropriately selected from a range
approximately from 1 to 10 tons/cm.sup.2. The discharge plasma
compressing and sintering process may be executed in a condition
out of the above-discussed condition as far as the produced magnet
has a crystalline size within an allowable range and maintains the
desired magnetic property of the anisotropic rare earth magnet. The
use of such discharge plasma compressing and sintering technique
enables the bulk magnet with a low oxygen concentration to be
produced and thereby improving the magnetic property of the
produced magnet.
[0064] The magnitude of exchange-coupling is recognized from the
magnitude of the spring back behavior. The heat-treated magnet
powder exhibited the spring back phenomenon, although it is of
course possible to check the spring back phenomenon from the
finally obtained anisotropic exchange spring magnet. Further, as it
will be predicted from the above discussion, the powder obtained by
pulverizing the base material using a ball mill also exhibited the
spring back phenomenon in the second and third quadrants of the
magnetization curve.
[0065] The present invention provides a method of producing the
Nd--Fe--B anisotropic exchange spring magnet in such a manner as
described above. That is, a further aspect of the present invention
resides in a producing method of the Nd--Fe--B anisotropic exchange
spring magnet which method comprises a step of obtaining a
compressed powder body by compressing the rare earth magnet powder
according to the present invention at a compressing pressure
ranging from 1 to 5 ton/cm.sup.2 in a magnetic field ranging from
15 to 25 kOe, and a step of obtaining a bulk magnet by sintering
the compressed powder body in a discharge plasma apparatus under a
condition that a temperature is maintained within a range from 600
to 800.degree. C. and a compressing pressure is maintained within a
range from 1 to 10 ton/cm.sup.2.
[0066] It is preferable that the Nd--Fe--B anisotropic exchange
spring magnet according to this aspect of the present invention is
a rare earth magnet having a bulk density greater than 95% of the
true density of the rare earth magnet alloy. If the density of the
exchange spring magnet is greater than 95% of the true density, the
produced exchange spring magnet has a large energy product
(BHmax).
[0067] The anisotropic exchange spring magnet according to the
present invention is very variable as a use for various devices
requiring high magnetic properties. For example, the anisotropic
exchange spring magnet according to the present invention is
applicable to a motor. If the magnet is employed as a part of a
motor, it becomes possible to produce the motor into a further
compacted size since the magnet according to the present invention
has a magnetic flux larger than that of a conventional magnet.
Further, the magnet according to the present invention has a
superior thermal property. The magnet according to the present
invention is advantageously applicable to a drive motor for an
electric vehicle, a hybrid electric vehicle (HEV) and a fuel cell
vehicle (FCV), due to the superior properties of the magnet. The
reason that the magnetic flux of the magnet according to the
present invention is greater than that of a sintered magnet depends
on a fact that the exchange spring magnet according to the present
invention contains soft magnetic phases. Further, it is deemed that
the reason for performing the superior thermal property depends on
the coercive force mechanism. That is, it is estimated that the
coercive force mechanism of the magnet according to the present
invention is similar to a pinning type although that of a common
sintering magnet is a nucleation type. Due to this difference, it
is seemed that the temperature stability of the magnet according to
the present invention is improved.
[0068] Subsequently, there is discussed the present invention on
the basis of EXAMPLES although the invention is not limited to
these EXAMPLES.
EXAMPLE 1
[0069] A thin strip of the rare earth magnet alloy was produced
from an alloy ingot having a composition of
Nd.sub.9.1Fe.sub.75.8CO.sub.8B.sub.6.1V.sub.1 by means of the strip
casting method. A condition of the strip casting method was that a
cooling speed for cooling the melted alloy to a solidification
temperature of around 900.degree. C. was set at 2300.degree.
C./sec. By observing a cross-section of the thin strip alloy by
means of a SEM (scanning electron microscope), it was confirmed
that the thin strip alloy had a mixed structure of the hard
magnetic phases and the soft magnetic phases. Substantially the
minimum width of the soft magnetic phase was smaller than or equal
to 1 .mu.m, and the minimum distance between the soft magnetic
phases was greater than or equal to 0.1 .mu.m. The thin strip alloy
had a structure such that a surface of the thin strip facing a
roller of the strip casting method was more micro-structured than
that of the other surface. FIG. 1 is a SEM photograph showing a
center area of a cross section of the obtained thin strip alloy. As
discussed above, the whitish particles and the whitish needle like
portions were precipitates of .alpha.-Fe, more restrictedly
.alpha.-(Fe--Co). The other portion shown as a darkish region was
the hard magnetic phase region.
[0070] Powder of a sample for VSM (vibrating sample magnetometer)
was produced by pulverizing the thin strip alloy into a size
smaller than 25 .mu.m using a set of mortar and pestle. The VSM
sample was produced by mixing the obtained powder with epoxy
adhesive and by solidifying the mixture poured in a VSM sample
container in the magnetic field of 10 kOe. Magnetization curves of
the obtained VSM sample were measured using a VSM wherein the
maximum applied magnetic field of 16 kOe was applied to the VSM
sample. More specifically, the magnetization curves are
respectively measured as to a parallel direction setting in that a
direction of the magnetic field applied by the VSM is parallel to
the magnetic field direction applied during the sample producing
process and as to a perpendicular direction setting in that a
direction of the magnetic field applied by the VSM is perpendicular
to the magnetic field direction applied during the sample producing
process. Further, Js ratio (a ratio of the magnetization in the
parallel direction with respect to the magnetization in the
perpendicular direction) at the magnetic field of 16 kOe was
evaluated. The magnitude of the Js ratio reflects that the hard
magnetic phase region is large and the hard magnetic phase
constructs a texture. FIG. 2 shows an example of the measured
result wherein the Js ratio was 1.7.
[0071] Another VSM sample of the obtained thin strip alloy was
produced by means of the same method employed in the above
production, and the magnetization curve in the parallel direction
with respect to the direction of the magnetic field orientation of
the produced sample was measured as shown in FIGS. 3 and 4. FIG. 4
is a magnetization curve where the horizontal axis was elongated
ten times that of FIG. 3. As is recognized from FIGS. 3 and 4, the
VSM sample of the obtained thin strip alloy also exhibited the
spring back phenomenon. More specifically, when the decreased
magnetic field was inversely increased in the second and third
quadrants of the magnetization curve in the parallel direction, the
magnetization is increased as shown in FIG. 4. Thereafter, when the
measured magnetic field is decreased, the increased curve was
returned to a start point of the inverse increase of the measured
magnetic field. This behavior was like as a spring characteristic,
and thereby called the spring back phenomenon (exchange spring
behavior). In the normal magnetization, the magnetization does not
increase by inversely increasing the measured magnetic field in the
way of decreasing the measured magnetic field such in the second
and third quadrants of the magnetization curve in the parallel
direction. This spring back phenomenon exhibits that the hard
magnetic phase and the soft magnetic phase are adjacently located,
and that the interaction therebetween is strong.
[0072] A feature of the present invention is that a rare earth
magnet alloy is crystalline. In order to check this feature, a
differential thermal analysis (DTA) was executed. As a result of
this analysis, no clear thermal peak was found.
[0073] The obtained thin strip alloy was pulverized using a
wet-type ball mill wherein the milling operation was executed using
cyclohexane under argon atmosphere, and
polybutenyl-succinimide-tetraethylenepentamine (1300 molecular
weight) was used as dispersant. This dispersant included a small
quantity of mineral oil. The obtained powder (rare earth magnet
powder) was observed using the SEM. FIG. 5 shows a SEM photograph
of the obtained powder whose powder sizes were smaller than or
equal to 1 .mu.m and wherein there was found no powder larger than
1 .mu.m in powder size.
[0074] As to the as-milled powder, a VSM sample was produced in the
same manner as discussed above, and the magnetization curves
thereof were also measured. The measurement result of the
magnetization curves are shown in FIG. 6 wherein Js ratio thereof
was 1.5. This means that the obtained powder also exhibited the
anisotropy. As the coercive force thereof increased as compared
with that in FIG. 2, it is estimated that the hard magnetic phase
thereof was a fine grained structure. FIG. 7 distinctively shows
the spring back phenomenon represented in the magnetization curve
along the parallel direction in FIG. 6.
[0075] The magnet powder obtained by pulverizing the base material
was heat treated at 612.degree. C. Then, as to the VSM sample of
the obtained heat-treated magnet powder, the magnetization curves
thereof were also measured. The measurement result of the
magnetization curves are shown in FIG. 8 wherein the heat-treated
magnet powder exhibited the anisotropy and the Js ratio thereof was
around 1.4 which is a reference value. Since the heat-treated
powder was put in a flocculated state, the magnetic field
orientation in the VSM sample was insufficient and therefore the Js
ratio was around 1.4. By further pulverizing the heat-treated
powder by means of a set of mortar and pestle so as to release the
flocculated state in some degree, a VSM sample thereof was
produced, and the magnetization curves thereof were measured. As a
result, it was recognized that the Js ratio was largely improved.
The coercive force of this sample was largely increased as compared
with that shown in FIG. 6 and was 6.4 kOe. As a result of observing
the structure of this powder, the hard magnetic phases and the soft
magnetic phases were finely dispersed to form a mixed structure of
the hard magnetic phases and the soft magnetic phases.
[0076] FIGS. 9A and 9B show X-ray diffraction results of the
as-milled powder (a) and the powder (b) heat-treated at 612.degree.
C., respectively. As is apparent from FIGS. 9A and 9B, the
diffraction peaks of the powder (b) are sharpened. It is therefore
assumed that the as-milled powder (a) still remains relatively
large quantity of stains therein, or includes amorphous phases.
EXAMPLE 2
[0077] A thin strip of the rare earth magnet alloy was produced
from an alloy ingot having a composition of
Nd.sub.10Fe.sub.75CO.sub.8B.sub.6V.sub.1 by means of the strip
casting method and in the same manner as EXAMPLE 1. It was observed
that the thin strip alloy has had a mixed structure of the hard
magnetic phases and the soft magnetic phases, as a result of
observing a cross-section of the thin strip alloy by means of SEM.
Substantially the minimum width of the soft magnetic phases was
smaller than or equal to 1 .mu.m, and the minimum distance between
the soft magnetic phases was greater than or equal to 0.2 .mu.m.
Powder for a VSM sample was produced by pulverizing the thin strip
alloy into a size smaller than 25 .mu.m using a set of mortar and
pestle. The Js ratio of the VSM sample was 1.8.
EXAMPLE 3
[0078] A thin strip of the rare earth magnet alloy was produced
from an alloy ingot having a composition of
Nd.sub.11Fe.sub.74CO.sub.8B.sub.6V.sub.1 by means of the strip
casting method and in the same manner as EXAMPLE I. It was observed
that the thin strip alloy has had a mixed structure of the hard
magnetic phases and the soft magnetic phases, as a result of
observing a cross-section of the thin strip alloy by means of SEM.
Substantially the minimum width of the soft magnetic phase was
smaller than or equal to 1 .mu.m, and the minimum distance between
the soft magnetic phases was greater than or equal to 0.5 .mu.m.
Powder of a VSM sample was produced by pulverizing the thin strip
alloy into a size smaller than 25 .mu.m using a set of mortar and
pestle. The Js ratio of the VSM sample was 1.8.
COMPARATIVE EXAMPLE 1
[0079] A thin strip of the rare earth magnet alloy was produced
from an alloy ingot having a composition of
Nd.sub.8Fe.sub.77CO.sub.8B.sub.6V.sub.1 by means of the strip
casting method and in the same manner as EXAMPLE 1. It was observed
that the thin strip alloy has had a mixed structure of the hard
magnetic phases and the soft magnetic phases, as a result of
observing a cross-section of the thin strip alloy by means of SEM.
There were a lot of the soft magnetic phases whose minimum widths
were greater than 1 .mu.m and were substantially not smaller or
equal to 1 .mu.m. Further, there was a tendency that the minimum
distance between the soft magnetic phases was smaller than 0.1
.mu.m. Powder of a VSM sample was produced by pulverizing the thin
strip alloy into a size smaller than 25 .mu.m using a set of mortar
and pestle. The Js ratio of the VSM sample was 1.2.
EXAMPLE 4
[0080] A thin strip of the rare earth magnet alloy was produced
from an alloy ingot having a composition of
Nd.sub.11Fe.sub.72CO.sub.8B.sub.7.5V.sub.1.5 by means of the strip
casting method and in the same manner as EXAMPLE 1. A condition of
the strip casting method for producing EXAMPLE 4 was that a cooling
speed for cooling the material to the solidification temperature of
around 900.degree. C. was 2000.degree. C./sec. It was observed that
the thin strip alloy has had a mixed structure of the hard magnetic
phases and the soft magnetic phases, as a result of observing a
cross-section of the thin strip alloy by means of SEM.
Substantially the minimum width of the soft magnetic phase was
smaller than or equal to 1 .mu.m, and the minimum distance between
the soft magnetic phases was greater than or equal to 1 .mu.m. FIG.
10 is a SEM photograph showing a center area of a cross section of
the obtained thin strip alloy. Black stripes aligned along the
direction from a left upper side toward a right lower side are the
soft magnetic phases of .alpha.-(Fe--Co). The other regions are the
hard magnetic phase regions. That is, the hard magnetic phases are
separated by the soft magnetic phases of stripes.
[0081] As to the as-milled powder, a VSM sample was produced in the
same manner as EXAMPLE 1, and the magnetization curves thereof were
also measured. The measurement result of the magnetization curves
were shown in FIG. 11 wherein Js ratio thereof is a value slightly
greater than 1.5. It is assumed that even when the powder is
obtained by means of a ball mill, the same result with EXAMPLE 1 is
obtained. FIG. 12 shows the magnetization curves of the powder
heat-treated at 610.degree. C. The coercive force thereof was 9.0
kOe and was greater than that in FIG. 11. The reason for taking a
small Js ratio is the same as discussed above. That is, since the
heat-treated powder is put in flocculated state, the magnetic field
orientation in the VSM sample is not sufficiently achieved. FIG. 13
shows a magnetization curve of a sample produced in the same manner
as that in FIG. 12 in order to observe the spring back phenomenon.
As a result, this heat-treated magnet powder exhibited the spring
back phenomenon.
[0082] In order to find a preferred condition of the discharge
plasma sintering, the following experiments were carried out.
REFERENCE EXAMPLE 1
[0083] Rare earth magnet powder was obtained in the same manner as
EXAMPLE 1, that is, by pulverizing the thin strip of the rare earth
magnet alloy using a wet-type ball mill. A compressed powder body
was produced by compressing the rare earth magnet powder at a
pressure of 2 ton/cm.sup.2 while treating the magnetic field
orientation by applying the magnetic field of 20 kOe. A die
employed in this compressing process was a non-magnetic WC type.
The size of the compressed powder body was 10 mm.times.10
mm.times.7 mm.
[0084] A Nd--Fe--B type anisotropic exchange spring magnet was
obtained by forming a bulk from the compressed powder body by means
of the discharge plasma sintering unit under a condition that the
temperature is maintained at 650.degree. C. for 3 minutes while the
compressing pressure of 9 ton/cm.sup.2 is applied to the compressed
powder body. The density of the exchange spring magnet reached the
true density.
[0085] The obtained exchange spring magnet was observed using SEM
and TEM. As a result of these observations, the obtained magnet had
a mixed structure of hard magnetic phases and soft magnetic phases.
The crystalline grain size of the obtained magnet had a range from
15 to 40 nm. It was observed by TEM that the direction of the hard
magnetic phases was aligned.
REFERENCE EXAMPLE 2
[0086] A Nd--Fe--B type anisotropic exchange spring was obtained in
the same manner as REFERENCE EXAMPLE 1 except that the compressing
pressure during the discharge plasma sintering was 8 ton/cm.sup.2.
The exchange spring magnet had a density which is 95% of the true
density.
REFERENCE EXAMPLE 3
[0087] A Nd--Fe--B type anisotropic exchange spring was obtained in
the same manner as REFERENCE EXAMPLE 1 except that the compressing
pressure during the discharge plasma sintering was 7 ton/cm.sup.2.
The obtained exchange spring magnet had a density which is 90% of
the true density.
REFERENCE EXAMPLE 4
[0088] A Nd--Fe--B type anisotropic exchange spring was obtained in
the same manner as REFERENCE EXAMPLE 1 except that the temperature
during the discharge plasma sintering was 810.degree. C. The
obtained exchange spring magnet had the true density.
[0089] The magnetization curves of REFERENCE EXAMPLES 1 through 4
were measured using a direct-current BH curve tracer which is
capable of applying the maximum applied magnetic field of 20 kOe to
the samples. TABLE 1 shows the magnetization ratio (Js ratio 2)
between the magnetization in the direction of the magnetic field
orientation and the magnetization in the direction perpendicular to
the magnetic field orientation at the magnetic field of 20 kOe. The
Js ratio 2 of REFERENCE EXAMPLE 1 was 1.8 and was greater than Js
ratio (1.7) of EXAMPLE 1. Further, TABLE 1 shows energy product
(BHmax) obtained from each magnetization curve. As to the coercive
force, REFERENCE EXAMPLES 1 through 3 took values around 7.0 kOe,
and REFERENCE EXAMPLE 4 took 5.0 kOe. TABLE 1 clearly represents
that it is preferable that the density of the magnet is greater
than 95% of the true density and that the sintering temperature in
the discharge plasma apparatus is lower than or equal to
800.degree. C.
TABLE-US-00001 TABLE 1 COMPOSITION BHmax (atom %) Js ratio 2 (MGOe)
REF. EXAMPLE 1 Nd.sub.9.1Fe.sub.75.8Co.sub.8B.sub.6.1V.sub.1 1.8 25
REF. EXAMPLE 2 Nd.sub.9.1Fe.sub.75.8Co.sub.8B.sub.6.1V.sub.1 1.8 20
REF. EXAMPLE 3 Nd.sub.9.1Fe.sub.75.8Co.sub.8B.sub.6.1V.sub.1 1.8 18
REF. EXAMPLE 4 Nd.sub.9.1Fe.sub.75.8Co.sub.8B.sub.6.1V.sub.1 1.5
15
EXAMPLE 5
[0090] The exchange spring magnet of REFERENCE EXAMPLE 1 was
applied to a surface permanent magnet motor having a 12-pole stator
and 8-pole rotor. FIG. 14 is a cross section of 1/4 part of the
surface permanent magnet motor of a concentrated winding type to
which the exchange spring magnet is applied. An outer side of the
motor in FIG. 14 is an aluminum case 11, and an inner side thereof
is the stator 12 having 52 mm inner diameter and 108 mm outer
diameter. 1-2 is u-phase coil, 3-4 is v-phase coil, and 5-6 is
w-phase coil. Stator 12 is a laminated member of electrical steel
sheets. A plurality of magnets 13 are installed in a rotor core 14.
The maximum outer diameter of the rotor is 50.7 mm, and the total
thickness of the stator and the rotor was 79.8 mm.
[0091] The performance of the motor was that the maximum rated
output was 2 kW, the temperature limitation was 160.degree. C. The
coercive force of the exchange spring magnet was 7.2 kOe. On the
other hand, if a conventional Nd--Fe--B sintered magnet is employed
in a motor, the magnet is required to have the coercive force more
than 19.8 kOe in order to ensure the temperature limit as same as
that using the magnet of REFERENCE EXAMPLE 1. That is, the motor
employing the magnet according to the present invention gave a
superior performance in thermal design.
[0092] This application is based on prior Japanese Patent
Application No. 2002-328579. The entire contents of the Japanese
Patent Application No. 2002-328579 with a filing date of Nov. 12,
2002 are hereby incorporated by reference.
[0093] Although the invention has been described above by reference
to certain embodiments of the invention, the invention is not
limited to the embodiments described above. Modifications and
variations of the embodiments described above will occur to those
skilled in the art in light of the above teachings. The scope of
the invention is defined with reference to the following
claims.
* * * * *