FORM 2
THE PATENTS ACT, 1970
(39 of 1970)
&
THE PATENTS RULES, 2003
COMPLETE SPECIFICATION
[See section 10, Rule 13]
RARE EARTH SINTERED MAGNET, METHOD OF MANUFACTURING RARE
EARTH SINTERED MAGNET, ROTOR, AND ROTATING MACHINE;
MITSUBISHI ELECTRIC CORPORATION, A CORPORATION
ORGANISED AND EXISTING UNDER THE LAWS OF JAPAN, WHOSE ADDRESS
IS 7-3, MARUNOUCHI 2-CHOME, CHIYODA-KU, TOKYO 1008310, JAPAN
THE FOLLOWING SPECIFICATION PARTICULARLY DESCRIBES THE
INVENTION AND THE MANNER IN WHICH IT IS TO BE PERFORMED
2
DESCRIPTION
Technical Field
[0001]
5 This invention relates to a rare earth sintered magnet, which is a permanent
magnet made of sintered materials containing rare earth elements, a method of
manufacturing the rare earth sintered magnet, a rotor, and a rotating machine.
Background Technology
[0002]
10 R-T-B system rare earth sintered magnets are magnets whose main constituent
elements are a rare earth element R, a transition metal element T such as Fe (iron) or
Fe partially substituted with Co (cobalt), and B (boron). In particular, an Nd-Fe-B
system sintered magnet, in which the rare earth element R is Nd (neodymium), is
applied to various components because of its excellent magnetic properties. When the
15 R-Fe-B system sintered magnet is applied to industrial motors and the like, its
operating ambient temperature exceeds 100 deg C. Therefore, a heavy rare earth
element such as Dy (dysprosium) is added to the conventional R-T-B system rare earth
sintered magnet for high heat resistance. The addition of Dy, which has a higher
electrical resistivity than Nd, suppresses an eddy current loss generated in the
20 magnet.
This suppresses heat generation due to the eddy current loss to prevent the magnet to
become too hot. On the other hand, there are concerns about the supply of Nd and Dy
because their resources are unevenly distributed and their production is limited.
To reduce the amounts of Nd and Dy used in the conventional rare earth sintered
25 magnet, rare earth elements R other than Nd and Dy, such as Ce (cerium), La
(lanthanum), Sm (samarium), Sc (scandium), Gd (gadolinium), Y (yttrium), and Lu
(lutetium), are used. For example, Patent Document 1 discloses a permanent magnet
in which the amounts of Nd and Dy used are reduced by containing La and Sm as the
rare earth elements R.
30 Citation List
Patent Document
[0003]
Patent Document 1
WO 2019/111328
35 Summary of Invention
Technical Problem
[0004]
Patent Document 1 describes a permanent magnet containing Sm, which has a
higher electrical resistivity than Nd, but does not describe Sm in the magnet internal
40 structure and suppression of eddy current loss. In the permanent magnet of Patent
Document 1, La and Sm added to Nd2Fe14B are likely to be uniformly dispersed in the
permanent magnet. However, to suppress the eddy current loss, the Sm content in the
3
main phase, where eddy currents are generated, must be controlled to be higher. Thus,
simply including elements with high electrical resistivity is not enough to suppress the
heating of the magnet due to the eddy current loss.
[0005]
5 The present disclosure is made to solve the aforementioned problems and to
provide a rare earth sintered magnet that suppresses heat generation due to the eddy
current loss, a method of manufacturing the rare earth sintered magnet, a rotor
including the rare earth sintered magnet, and a rotating machine including the rare
earth sintered magnet.
10 Solution to Problem
[0006]
The rare earth sintered magnet according to the present disclosure is a rare earth
sintered magnet having a main phase and a grain boundary phase, wherein the main
phase has an R2Fe14B crystal structure and rare earth elements R include at least Nd
15 and Sm, and the content of Sm is higher in the main phase than in the grain boundary
phase.
Advantageous Effects of Invention
[0007]
According to the present disclosure, the higher content of Sm in the main phase
20 than in the grain boundary phase suppresses heat generation in the rare earth
sintered magnet due to the eddy current loss.
Brief Description of Drawings
[0008]
Fig. 1 is a schematic diagram of a part of a rare earth sintered magnet according to
25 Embodiment 1.
Fig. 2 is a schematic diagram of a part of the rare earth sintered magnet according to
Embodiment 1.
Fig. 3 is a schematic diagram of a part of the rare earth sintered magnet according to
Embodiment 1.
30 Fig. 4 is a schematic diagram of a part of the rare earth sintered magnet according to
Embodiment 1.
Fig. 5 is a diagram showing atomic sites in a tetragonal Nd2Fe14B crystal structure.
Fig. 6 is a flowchart showing a procedure of a method of manufacturing a rare earth
sintered magnet according to Embodiment 2.
35 Fig. 7 is a schematic diagram showing an operation of a raw alloy production process
according to Embodiment 2.
Fig. 8 is a schematic cross-sectional view of a rotor according to Embodiment 3.
Fig. 9 is a schematic cross-sectional view of a rotating machine according to
Embodiment 4.
40 Description of Embodiments
[0009]
Embodiment 1
4
A rare earth sintered magnet 1 according to Embodiment 1 is described with
reference to Fig. 1. Fig. 1 is a schematic diagram of a part of the rare earth sintered
magnet 1, schematically showing positions of Sm elements 4 by black dots. The rare
earth sintered magnet 1 includes a plurality of regions of a main phase 2 each having
5 an R2Fe14B crystal structure containing at least Nd and Sm as rare earth elements R,
and a grain boundary phase 3 formed among the plurality of regions of the main phase
2. The content of Sm is higher in the main phase 2 than in the grain boundary phase
3. Here, "the content of Sm is higher in the main phase 2 than in the grain boundary
phase 3" means that the detection intensity of Sm is higher on average in the main
10 phase 2 than in the grain boundary phase 3 in mapping analysis using an electron
probe micro analyzer (EPMA).
[0010]
The main phase 2 has the R2Fe14B crystal structure containing at least Nd and Sm
as the rare earth elements R. That is, the main phase 2 has an (Nd, Sm)2Fe14B crystal
15 structure formed by Sm substitution at some of Nd sites of an Nd2Fe14B crystal
structure. Further, La is preferably contained as a rare earth element R. When La is
contained, the crystal structure is (Nd, La, Sm)2Fe14B which is formed by substitution
made by La and Sm at some of the Nd sites of the Nd2Fe14B crystal structure. The
average size of crystal grains of the main phase 2 is, for example, less than 100 m,
20 preferably between 0.1 m and 50 m, to improve magnetic properties.
[0011]
The Sm content is higher in the main phase 2 than in the grain boundary phase 3. Sm
only needs to be present at a higher content on average in the main phase 2 than in
the grain boundary phase 3. That is, the Sm content does not need to be uniformly
25 high in the main phase 2 as shown in Fig. 1; for example, the Sm content in the main
phase 2 may have a distribution as shown in Fig. 2 to Fig. 4. Fig. 2 to Fig. 4 are
schematic diagrams of a part of the rare earth sintered magnet 1. In Fig. 2, the Sm
content differs depending on regions of the main phase 2. In Fig. 3, the Sm content
forms a core-shell structure in the main phase 2. The core-shell structure of the main
30 phase 2 is a structure in which the Sm content is different between a core 5, which is
the inner portion of a region of the main phase 2, and a shell 6, which is the outer
peripheral portion of the core 5. In the rare earth sintered magnet 1 in Fig. 3, the Sm
content is higher in the core 5 than in the shell 6. In Fig. 4, the Sm content forms the
core-shell structure in a region of the main phase 2, and the Sm content is higher in
35 the shell 6 than in the core 5. In the rare earth sintered magnet 1 shown in Fig. 1 to
Fig. 4, Sm is present in the main phase 2 at a higher content on average than in the
grain boundary phase 3.
[0012]
According to the Encyclopedic Dictionary of Chemistry published by Tokyo Kagaku
40 Doujin, the electrical resistivity of each element is as follows; Nd: 64 cm (25 deg C),
Sm: 92 cm (25 deg C), La: 59 cm (25 deg C), Dy: 91 cm (25 deg C).
[0013]
5
In the rare earth sintered magnet 1 according to the present embodiment, Sm,
which has a higher electrical resistivity than Nd, is present in the main phase 2 at a
higher content on average than in the grain boundary phase 3. This improves the
electrical resistivity of the main phase 2, which is responsible for the magnetic flux
5 generation, and reduces an eddy current loss. Therefore, heat generation in the rare
earth sintered magnet 1 due to the eddy current loss can be suppressed. In the case
where the Sm content in the main phase 2 is higher in the core 5 than in the shell 6 as
in the rare earth sintered magnet 1 shown in Fig. 3, the Sm substitution at the Nd
sites occurs more in the core 5 than in the shell 6. Therefore, contrary to the
10 distribution of Sm, Nd is distributed more in the shell 6 than in the core 5 in the main
phase 2. This results in a high content of Nd, which has higher magnetic anisotropy,
in the shell 6. The enhanced magnetic anisotropy in the shell 6 of the main phase 2
suppresses magnetization reversal.
[0014]
15 The grain boundary phase 3 is based on an oxide phase represented by (Nd, Sm)-O,
which is formed by the Sm substitution at some of the Nd sites of a crystalline NdO
phase. When the rare earth element R includes La, the crystalline grain boundary
phase 3 is based on (Nd, La, Sm)-O, which is formed by substitution made by La and
Sm at some of the Nd sites of the crystalline NdO phase. The content of La, which has
20 a lower electrical resistivity than Nd, is higher in the grain boundary phase 3 than in
the main phase 2. This prevents a decrease in the electrical resistivity of the main
phase 2 due to the addition of La, which has a lower electrical resistivity.
Experimental results also show that with the addition of La, Sm is present at a higher
content in the main phase 2 than in the grain boundary phase 3. Therefore, heat
25 generation in the rare earth sintered magnet 1 due to the eddy current loss can be
suppressed.
[0015]
The rare earth sintered magnet 1 according to Embodiment 1 may contain an
additive element M that improves magnetic properties. The additive element M is at
30 least one element selected from the group consisting of Al (aluminum), Cu (copper), Co,
Zr (zirconium), Ti (titanium), Ga (gallium), Pr (praseodymium), Nb (niobium), Dy, Tb
(terbium), Mn (manganese), Gd, and Ho (holmium).
[0016]
When the total of the elements contained in the rare earth sintered magnet 1
35 according to Embodiment 1 is 100 at% and the content ratios of Nd, La, Sm, Fe, B, and
the additive element M are a, b, c, d, e, and f, respectively, the following relational
expressions are desirably satisfied.
5  a  20
0  b + c  a
40 70  d  90
0.5  e  10
0  f  5
6
a + b + c + d + e + f = 100 at%
[0017]
Next, it is described at which atomic sites of the tetragonal R2Fe14B crystal
structure La and Sm make substitution. Fig. 5 is a diagram showing atomic sites in a
5 tetragonal Nd2Fe14B crystal structure (source: J. F. Herbst et al., PHYSICAL REVIEW
B, Vol. 29, No. 7, pp. 4176-4178, 1984). The site where the substitution is made was
determined by the numerical value of the stabilization energy due to substitution
obtained by band calculation and molecular field approximation of the Heisenberg
model.
10 [0018]
A method of calculating the stabilization energy of La is described. The
stabilization energy of La can be determined from the energy difference between
(Nd7La1)Fe56B4+Nd and Nd8(Fe55La1)B4+Fe using a Nd8Fe56B4 crystal cell. The
smaller the value of energy, the more stable it is when an atom is substituted at that
15 site. That is, La is likely to make substitution at an atomic site having the smallest
energy among the atomic sites. This calculation assumes that, when the original atom
is substituted with La, the difference in atomic radius does not change the lattice
constant of the tetragonal R2Fe14B crystal structure. Table 1 shows the stabilization
energies of La at each substitution site at different environmental temperatures.
20 [0019]
[Table 1]
[0020]
Table 1 shows that a stable La substitution site for temperatures of 1000 K or
25 higher is the Nd(f) site. It is considered that the La substitution is made
7
preferentially at the Nd (f) site, which is energetically stable; however, the
substitution may also be made at the Nd (g) site, which has a small energy difference
from the Nd (f) site among the La substitution sites. The Fe(c) site is a stable
substitution site at 293 K and 500 K. As described below, a method of manufacturing
5 the rare earth sintered magnet 1 includes sintering of a raw alloy at a temperature of
1000 K or higher in a sintering process 24. Then, the magnet is produced through a
cooling process 25 that holds the temperature between 500 K and 700 K for a certain
period of time. Therefore, in the sintering process, the substitution is made at the Nd
(f) site, which is the most stable substitution site, or at the Nd (g) site, which has a
10 small energy difference from the Nd (f) site. After that, the site of La substitution is
considered to be changed from the Nd (f) site or the Nd (g) site to the Fe (c) site in the
cooling process.
[0021]
A method of calculating the stabilization energy of Sm is described. The
15 stabilization energy of Sm can be determined from the energy difference between
(Nd7Sm1)Fe56B4+Nd and Nd8(Fe55Sm1)B4+Fe. Similar to the case of La, it is assumed
that the substitution of atoms does not change the lattice constant of the tetragonal
R2Fe14B crystal structure. Table 2 shows the stabilization energies of Sm at each
substitution site at different environmental temperatures.
20 [0022]
[Table 2]
[0023]
Table 2 shows that a stable substitution site for Sm is the Nd(g) site at any
25 temperature. It is considered that substitution is made preferentially at the Nd (g)
site, which is energetically stable; however, the substitution may also be made at the
8
Nd (f) site, which has a small energy difference from the Nd (g) site among the Sm
substitution sites.
[0024]
Further, a comparison between Table 1 and Table 2 shows that, in the rare earth
5 sintered magnet 1 manufactured by the manufacturing method described below, the
calculated stabilization energy of the Nd site is smaller and more stable for Sm than
for La. In other words, the substitution at the Nd site in the Nd2Fe14B crystal
structure of the main phase 2 is more likely to be made by Sm than by La. Therefore,
in the main phase 2, Sm is present at a high content, and La is present at a low
10 content.
[0025]
As described above, the rare earth sintered magnet 1 according to the present
embodiment includes the main phase 2 and the grain boundary phase 3; the main
phase 2 has an R2Fe14B crystal structure containing at least Nd and Sm as rare earth
15 elements R; the content of Sm, which has a higher electrical resistivity than Nd, is
higher in the main phase 2 than in the grain boundary phase 3. This improves the
electrical resistivity of the main phase 2, which is responsible for the magnetic flux
generation, and suppresses heat generation of the rare earth sintered magnet 1 due to
the eddy current loss. The Sm present in the main phase 2 couples in the same
20 magnetization direction as the ferromagnetic Fe and contributes to the improvement
of the residual magnetic flux density.
[0026]
La may be contained as a rare earth element R and may be present at a higher
content in the grain boundary phase 3 than in the main phase 2. La, which has a
25 lower electrical resistivity than Nd, is present at a higher content in the grain
boundary phase 3 than in the main phase 2. This prevents a decrease in the electrical
resistivity of the main phase 2 and suppresses heat generation of the rare earth
sintered magnet 1 due to the eddy current loss.
[0027]
30 In the cooling process 25, the site of La substitution changes from the Nd site,
which is a stable substitution site in the sintering process 24, to the Fe(c) site. The
stable site of Sm substitution is the Nd site at any temperature in the sintering
process 24 and the cooling process 25. Therefore, the inclusion of La promotes the Sm
substitution at the Nd site where the La substitution has been made in the sintering
35 process 24. This allows Sm to be present at a higher content in the main phase 2,
thereby suppressing the heat generation of the rare earth sintered magnet 1 due to
the eddy current loss.
[0028]
The rare earth sintered magnet 1 includes the crystalline grain boundary phase 3
40 based on an oxide phase represented by (Nd, Sm)-O, which is formed by the Sm
substitution at some of the Nd sites of the crystalline NdO phase. Thus, the presence
of Sm, which is a rare earth element R like Nd, in the grain boundary phase 3 allows
9
Nd to relatively diffuse into the main phase 2. This prevents the Nd in the main
phase 2 from being consumed in the grain boundary phase 3, and thus the magnetic
anisotropy constant and a saturated magnetic polarization are improved, enhancing
the magnetic properties.
5 [0029]
When La is contained as a rare earth element R, the grain boundary phase 3 is a
crystalline phase represented by (Nd, La, Sm)-O. Similar to Sm, the presence of La in
the grain boundary phase 3 allows Nd to relatively diffuse into the main phase 2. This
prevents Nd in the main phase 2 from being consumed in the grain boundary phase 3,
10 and thus the magnetic anisotropy constant and the saturated magnetic polarization
are improved, enhancing the magnetic properties.
[0030]
Sm may be added to a magnet containing Dy, which has a higher electrical
resistivity than Nd. The addition of Sm reduces the eddy current loss with a smaller
15 amount of Dy than usual. This can reduce the use of Dy, which is unstable in supply
due to its uneven distribution and limited production. La should be added to achieve a
well-balanced morphology of the magnet internal structure that enables both the
suppression of eddy current loss by increasing the electrical resistivity of the main
phase 2 and the magnetic properties with temperature rise.
20 [0031]
An excessive Sm content may lead to a relative decrease in the content of Nd,
which is an element having a high magnetic anisotropy constant and a high saturated
magnetic polarization, and degradation of the magnetic properties. Therefore, in the
rare earth sintered magnet 1, the composition ratio of Nd should be larger than that of
25 Sm. When La is contained as a rare earth element R, the composition ratio of Nd
should be larger than the sum of the composition ratio of La and that of Sm. In other
words, when rare earth elements R other than Nd are included, the total amount of
the rare earth elements R other than Nd should be less than the amount of Nd.
[0032]
30 Embodiment 2
The present embodiment relates to a method of manufacturing the rare earth
sintered magnet 1 according to Embodiment 1. The description thereof is made with
reference to Fig. 6 and Fig. 7. Fig. 6 is a flowchart showing a procedure of the method
of manufacturing the rare earth sintered magnet 1 according to the present
35 embodiment. Fig. 7 is a schematic diagram showing an operation of a raw alloy
production process 11. Hereinafter, the raw alloy production process 11 and a sintered
magnet production process 21 are described separately.
[0033]
[Raw alloy production process 11]
40 As shown in Fig. 6 and Fig. 7, the raw alloy production process 11 includes: a
melting process 12 in which a raw material of a rare earth magnet alloy 37 is heated
to a temperature of 1000 K or higher and melted; a primary cooling process 13 in
10
which the raw material in a molten state is cooled on a rotator 34 to produce a
solidified alloy 35; and a secondary cooling process 14 in which the solidified alloy 35 is
further cooled in a tray 36.
[0034]
5 In the melting process 12, the raw material of the rare earth magnet alloy 37 is
melted to produce a molten alloy 32. The raw material contains Nd, Fe, B, and Sm.
Other rare earth elements R may be contained, and preferably La is contained. As
additive elements, one or more elements selected from Al, Cu, Co, Zr, Ti, Ga, Pr, Nb,
Mn, Gd, and Ho may be contained. As exemplified in Fig. 7, in an atmosphere
10 containing an inert gas such as Ar or in a vacuum, the raw material of the rare earth
magnet alloy 37 is heated to a temperature of 1000 K or higher in a crucible 31 and
melted to produce the molten alloy 32.
[0035]
In the primary cooling process 13, as exemplified in Fig. 7, the molten alloy 32 is
15 poured into a tundish 33 and is rapidly cooled on the rotator 34, so that the solidified
alloy 35 thinner than an ingot alloy is produced from the molten alloy 32. In Fig. 7, a
single roll is exemplified as the rotator 34; however, twin rolls, a rotary disk, a rotary
cast cylinder, etc. may be used for rapid cooling by making contact therewith. For
efficient production of the thin solidified alloy 35, the cooling rate in the primary
cooling process 13 should be 10 to 107 deg C /sec, preferably 103 to 104 20 deg C /sec. The
thickness of the solidified alloy 35 is between 0.03 mm and 10 mm. The molten alloy
32 solidifies from the point where it contacts the rotator 34, and crystals grow in a
columnar or needle-like shape in the direction of thickness from the surface of contact
with the rotator 34.
25 [0036]
In the secondary cooling process 14, the solidified alloy 35 is cooled in the tray 36
as exemplified in Fig. 7. When entering the tray 36, the thin solidified alloy 35 is
broken into scale-like pieces of the rare earth magnet alloy 37 and cooled. Although
the scale-like pieces of the rare earth magnet alloy 37 are exemplified, ribbon-like
30 pieces of the rare earth magnet alloy 37 are produced depending on the cooling rate.
For producing the rare earth magnet alloy 37 with the optimum rare earth magnet
alloy 37 internal structure, the cooling rate in the secondary cooling process 14 should
be 0.01 to 105 deg C /sec, preferably 0.1 to 102 deg C /sec.
[0037]
35 Through the above-described raw alloy production process 11, the R-Fe-B system
rare earth magnet alloy 37 containing at least Nd and Sm as rare earth elements R is
produced.
[0038]
[Sintered magnet production process 21]
40 As shown in Fig. 6, the sintered magnet production process 21 includes: a
pulverization process 22 in which the rare earth magnet alloy 37 produced in the
above-described raw alloy production process 11 is pulverized; a molding process 23 in
11
which the pulverized rare earth magnet alloy 37 is molded to produce a compact; the
sintering process 24 in which the compact is sintered to produce a sintered compact;
and the cooling process 25 in which the sintered compact is cooled. The sintered
magnet production process 21 is not limited to this but may be performed, for example,
5 by hot working, in which the molding process 23 and the sintering process 24 are
performed at the same time.
[0039]
The pulverization process 22 is to pulverize the R-Fe-B system rare earth magnet
alloy 37, which contains Nd and Sm as rare earth elements R and is produced in the
10 above-mentioned raw alloy production process 11, and to produce a powder with a
grain diameter of no more than 200 m, preferably between 0.5 m and 100 m. The
rare earth magnet alloy 37 is pulverized by using, for example, an agate mortar, a
stamp mill, a jaw crusher, a jet mill, or the like. To obtain a powder with a small
particle diameter, the pulverization process 22 should be performed in an atmosphere
15 containing inert gas. The pulverization of the rare earth magnet alloy 37 in an
atmosphere containing inert gas can also prevent oxygen from entering the powder. If
the atmosphere in which the pulverization is performed does not affect the magnetic
properties of the magnet, the pulverization of the rare earth magnet alloy 37 may be
performed in the air.
20 [0040]
In the molding process 23, the powder of the rare earth magnet alloy 37 is molded
to produce a compact. For example, in the molding, only the powder of the rare earth
magnet alloy 37 may be press-molded, or a mixture of the powder of the rare earth
magnet alloy 37 and an organic binder may be press-molded. The molding may be
25 performed while applying a magnetic field. The magnetic field to be applied is 2 T, for
example.
[0041]
In the sintering process 24, the compact is heat-treated to produce a sintered
compact. The sintering is performed at a temperature between 600 deg C and 1300
30 deg C, for 0.1 hours to 10 hours. The sintering should be performed in an atmosphere
containing inert gas or in a vacuum to suppress oxidation. The sintering may be
performed while applying a magnetic field. A process may be added to allow
compounds containing Cu, Al, heavy rare earth elements, etc. to permeate the crystal
grain boundary, which is the boundary between the regions of the main phase 2.
35 [0042]
In the cooling process 25, the sintered compact sintered between 600 deg C and
1300 deg C is cooled. In the cooling process, the sintered compact is held at a
temperature between 227 deg C and 427 deg C (500 K and 700 K) for 0.1 hours to 5
hours. The sintered compact is then cooled to room temperature to complete the rare
40 earth sintered magnet 1.
[0043]
By controlling the temperatures and times of the sintering process 24 and cooling
12
process 25 described above, the magnet internal structure based on the calculated
stabilization energy described in Embodiment 1 can be produced. In other words, this
enables the production of the rare earth sintered magnet 1 in which Sm is present at a
higher content in the main phase 2 than in the grain boundary phase 3. The grain
5 boundary phase 3 has the (Nd, Sm)-O phase formed by the Sm substitution in the
crystalline NdO phase. This improves the electrical resistivity of the main phase 2,
which is responsible for the magnetic flux generation, and suppresses heat generation
of the rare earth sintered magnet 1 due to the eddy current loss.
[0044]
10 It is preferable to add La to the raw material of the rare earth magnet alloy 37. By
adding La and controlling the temperatures and times of the sintering process 24 and
the cooling process 25, Sm can be more stably present in the main phase 2. La is
present at a higher content in the grain boundary phase 3 than in the main phase 2,
but is also partially present in the main phase 2. Table 1 shows that the stable La
15 substitution site is the Nd(f) site at a temperature of 1000 K or higher, and is the Fe(c)
site at a temperature of 500 K or lower. In addition, experiments show that the La
substitution is likely to change from the Nd(f) site to the Fe(c) site at a temperature
between 500 K and 700 K. In contrast, table 2 shows that the stable substitution site
for Sm is the Nd(g) site at any temperature. It is considered that the substitution is
20 made preferentially at the Nd (g) site, which is energetically stable; however, the
substitution may also be made at the Nd (f) site, which has a small energy difference
from the Nd (g) site among the Sm substitution sites. These findings indicate that the
La substitution site in the main phase 2 changes from the Nd(f) site to the Fe(c) site
through the cooling process that holds the temperature between 227 deg C and 427
25 deg C (500 K and 700 K) for a certain period of time. This promotes, in the cooling
process 25, the Sm substitution at the Nd site where the La substitution has been
made in the sintering process 24, and allows the Sm content to be higher in the main
phase 2. Therefore, by controlling the temperatures and times of the sintering process
24 and cooling process 25, the rare earth sintered magnet 1 can be produced in which
30 the main phase 2 has the (Nd, La, Sm)2Fe14B crystal structure and the Sm content is
higher in the main phase 2 than in the grain boundary phase 3. The grain boundary
phase 3 has the (Nd, La, Sm)-O phase formed by the substitution made by La and Sm
in the crystalline NdO phase.
[0045]
35 Embodiment 3
The present embodiment relates to a rotor 41 that includes the rare earth sintered
magnet 1 according to Embodiment 1. The rotor 41 according to the present
embodiment is described with reference to Fig. 8. Fig. 8 is a schematic cross-sectional
view perpendicular to an axial direction of the rotor 41.
40 [0046]
The rotor 41 is rotatable about an axis of rotation 44. The rotor 41 includes a rotor
core 42 and a plurality of the rare earth sintered magnets 1 inserted into magnet
13
insertion holes 43 provided in the rotor core 42 along a circumferential direction of the
rotor 41. Fig. 8 shows an example including four magnet insertion holes 43 and four
rare earth sintered magnets 1; however, the number of magnet insertion holes 43 and
the number of rare earth sintered magnets 1 may be changed according to the design
5 of the rotor 41. The rotor core 42 is formed of a plurality of disk-shaped
electromagnetic steel plates stacked in the axial direction of the axis of rotation 44.
[0047]
The rare earth sintered magnets 1 are manufactured according to the
manufacturing method of Embodiment 2. The four rare earth sintered magnets 1 are
10 inserted into their respective magnet insertion holes 43. The four rare earth sintered
magnets 1 are magnetized in such a way that, on the radially outer side of the rotor 41,
each of the rare earth sintered magnets 1 has a polarity different from that of the
adjacent rare earth sintered magnets 1.
[0048]
15 A general rotor 41 becomes unstable in operation when the coercive forces of the
rare earth sintered magnets 1 decrease in a high-temperature environment. The rotor
41 in the present embodiment includes the rare earth sintered magnets 1
manufactured according to the manufacturing method described in Embodiment 2.
With the rare earth sintered magnets 1, the heat generation thereof due to the eddy
20 current loss can be suppressed. In addition, absolute values of temperature
coefficients of magnetic properties are small, as described later in the examples. This
suppresses the heat generation of the rare earth sintered magnets 1 and suppresses
the deterioration of the magnetic properties even in a high-temperature environment
such as 100 deg C or higher, thereby stabilizing the operation of the rotor 41.
25 [0049]
Embodiment 4
The present embodiment relates to a rotating machine 51 provided with the rotor
41 according to Embodiment 3. The rotating machine 51 according to the present
embodiment is described with reference to Fig. 9. Fig. 9 is a schematic cross-sectional
30 view perpendicular to an axial direction of the rotating machine 51.
[0050]
The rotating machine 51 includes the rotor 41 according to Embodiment 3 and an
annular stator 52 provided coaxially with the rotor 41 and disposed facing the rotor 41.
The stator 52 is formed of a plurality of electromagnetic steel plates stacked in the
35 axial direction of the axis of rotation 44. The configuration of the stator 52 is not
limited to this, and existing configurations may be employed. The stator 52 includes
teeth 53 protruding toward the rotor 41 along an inner surface of the stator 52. The
teeth 53 are provided with windings 54. The windings 54 may be wound in a
concentrated manner or distributed manner, for example. The number of magnetic
40 poles of the rotor 41 in the rotating machine 51 should be two or more; in other words,
the number of rare earth sintered magnets 1 should be two or more. Fig. 9 shows an
example of a magnet-embedded type rotor 41; however, a surface-magnet type rotor 41
14
having rare earth magnets fixed to the periphery with adhesive can also be used.
[0051]
The general rotating machine 51 becomes unstable in operation when the coercive
forces of the rare earth sintered magnets 1 decrease in a high-temperature
5 environment. The rotor 41 in the present embodiment includes the rare earth
sintered magnets 1 manufactured according to the manufacturing method described in
Embodiment 2. With the rare earth sintered magnets 1, the heat generation thereof
due to the eddy current loss can be suppressed. In addition, the absolute values of the
temperature coefficients of magnetic properties are small, as described later in the
10 examples. This suppresses the heat generation of the rare earth sintered magnets 1
and suppresses the deterioration of the magnetic properties even in a hightemperature environment such as 100 deg C or higher, thereby driving the rotor 41
stably and stabilizing the operation of the rotating machine 51.
[0052]
15 The configurations shown in the above-described embodiments are examples and
can be combined with another known technique. It is also possible to combine the
embodiments, and to omit or change a part of the configuration to the extent that it
does not depart from the gist.
[Examples]
20 [0053]
Evaluation results of the magnetic properties and eddy current losses of the rare
earth sintered magnets 1 produced by the manufacturing method of Embodiment 2 are
described with reference to Table 3. Table 3 is a summary of determination results of
the magnetic properties and eddy current losses of Examples 1 to 7 and Comparative
25 Examples 1 to 4, which are samples of the rare earth sintered magnets 1 having
different contents of Nd, La, and Sm.
[0054]
Table 3: Determination results of magnetic properties and eddy current losses of rare
earth sintered magnets 1
30
[0055]
15
The magnetic properties are determined by measuring the residual magnetic flux
density and coercive force of each sample using a pulse excitation type B-H curve
tracer. The maximum applied magnetic field by the B-H curve tracer is 6 T or higher,
at which the sample is completely magnetized. Instead of the pulse excitation type B5 H curve tracer, a DC recording magnetic flux meter, which is called a direct current
type B-H curve tracer, a vibrating sample magnetometer (VSM), a magnetic property
measurement system (MPMS), a physical property measurement system (PPMS), etc.
may be used if they can generate a maximum applied magnetic field of 6 T or more.
The measurements were performed in an atmosphere containing an inert gas such as
10 nitrogen, and evaluation was performed at room temperature. The magnetic
properties of each sample were measured at a first measurement temperature T1 and
a second measurement temperature T2 which are different from each other. A
temperature coefficient  [%/deg C] of the residual magnetic flux density is a value
obtained by calculating a ratio of a difference between the residual magnetic flux
15 density at the first measurement temperature T1 and the residual magnetic flux
density at the second measurement temperature T2 to the residual magnetic flux
density at the first measurement temperature T1 and by dividing the ratio by a
temperature difference (T2 - T1). A temperature coefficient  [%/deg C] of the coercive
force is a value obtained by calculating a ratio of a difference between the coercive
20 force at the first measurement temperature T1 and the coercive force at the second
measurement temperature T2 to the coercive force at the first measurement
temperature T1 and by dividing the ratio by the temperature difference (T2 - T1).
Therefore, the smaller the absolute values of the temperature coefficients  and of
the magnetic properties, the more the deterioration of the magnetic properties of the
25 magnet due to temperature rise is suppressed.
[0056]
The measurement conditions of the present example are described. Each sample
has a cube shape, and its length, width, and height are all 7 mm. In the
measurements of the temperature coefficient  of the residual magnetic flux density
30 and the temperature coefficient of the coercive force, the first measurement
temperature T1 is 23 deg C. and the second measurement temperature T2 is 200 deg
C. Here, 23 deg C is room temperature, and 200 deg C is a possible operating
environment temperature for automotive and industrial motors.
[0057]
35 The temperature coefficient of the residual magnetic flux density and the
temperature coefficient of the coercive force of samples of Examples 1 to 7 and
Comparative Examples 2 to 4 were determined in comparison with Comparative
Example 1. Table 3 shows the results of the comparison between the sample of
Comparative Example 1 and each of the other samples with respect to the absolute
40 value of the temperature coefficient  of the residual magnetic flux density and the
absolute value of the temperature coefficient  of the coercive force; when the value is
within 1%, which is considered to be a measurement error, the determination is
16
"Equivalent"; when the value is -1% or less, the determination is "Good"; when the
value is 1% or more, the determination is "Poor".
[0058]
The eddy current loss is determined using, for example, a DC magnetic property
5 test apparatus (magnetic flux integrator type) or an AC magnetic property test
apparatus (power meter method). The DC and AC magnetic properties of each sample
were evaluated by sandwiching the rare earth sintered magnet 1 between C-shaped
yokes, exciting the sample by AC with a primary winding inside the coil frame, and
detecting the induced voltage with a secondary winding. In the present example, the
10 number of turns of the primary winding was 200 and the number of turns of the
secondary winding was 100, but the number of turns may be changed depending on
the sample to be measured. In the present example, measurements were performed at
frequencies of 1 kHz, 2 kHz, and 3 kHz under the measurement conditions of magnetic
flux densities of 0.01 T and 0.1 T using the AC magnetic property. The eddy current
15 loss was calculated by subtracting the hysteresis loss from the obtained total iron loss.
The higher the electrical resistivity of the main phase 2 of the rare earth sintered
magnet 1 being evaluated, the smaller the eddy current loss. The smaller the eddy
current loss is, the less heat is generated by the eddy current loss in the sintered rare
earth magnet 1, which means that it is a rare earth sintered magnet 1 with
20 suppressed heat generation.
[0059]
The eddy current losses in samples of Examples 1 to 7 and Comparative Examples
2 to 4 were determined in comparison with Comparative Example 1. Table 3 shows
the results of the measurement at a residual magnetic flux density of 0.01 T and a
25 frequency of 3 kHz. When the value is within 3%, which is considered to be a
measurement error, the determination is "Equivalent"; when the value is -3% or less,
the determination is "Good"; when the value is 3% or more, the determination is
"Poor".
[0060]
30 Comparative Example 1 is a sample produced according to the manufacturing
method of Embodiment 2 using Nd, Fe, and B as raw materials of the rare earth
magnet alloy 37 so that the general formula will be Nd-Fe-B. The magnetic properties
and eddy current loss of this sample were determined by the above-mentioned method.
The temperature coefficient  of the residual magnetic flux density was 0.191%/deg C,
35 and the temperature coefficient  of the coercive force was 0.460%/deg C. The eddy
current loss was 2.98 W/kg. These values of Comparative Example 1 were used as
references.
[0061]
Comparative Example 2 is a sample produced according to the manufacturing
40 method of Embodiment 2 using Nd, Dy, Fe, and B as the raw materials of the rare
earth magnet alloy 37 so that the general formula will be (Nd, Dy)-Fe-B. The
magnetic properties and eddy current loss of this sample were determined by the
17
method described above; the temperature coefficient of residual magnetic flux density
was "Equivalent", the temperature property of coercive force was "Equivalent", and
the eddy current loss was "Good". This determination result indicates that
substitution made by Dy, which has a higher electrical resistivity than Nd, at some of
5 the Nd sites of the main phase 2 increased the electrical resistivity of the main phase
2 and reduced the eddy current loss.
[0062]
Comparative Example 3 and Comparative Example 4 are samples produced
according to the manufacturing method of Embodiment 2 using Nd, La, Fe, and B as
10 the raw materials of the rare earth magnet alloy 37 so that the general formula will be
(Nd, La)-Fe-B. The La contents (at%) in Comparative Example 3 and Comparative
Example 4 are 0.31 and 1.01, respectively. The magnetic properties and eddy current
losses of these samples were each determined by the method described above; the
temperature coefficient of residual magnetic flux density was "Poor", the temperature
15 property of coercive force was "Poor", and the eddy current loss was "Equivalent".
This result indicates that the addition of only La to Nd-Fe-B does not contribute to the
improvement of the magnetic properties. Comparative Example 3 and Comparative
Example 4 show that eddy current loss is "Equivalent" even when the content of La,
which has lower electrical resistivity than Nd, is increased. This means that because
20 the La content was higher in the grain boundary phase 3 than in the main phase 2,
the reduction of the electrical resistivity of the main phase 2, which is responsible for
the magnetic flux generation, was suppressed.
[0063]
Example 1 and Example 2 are samples produced according to the manufacturing
25 method of Embodiment 2 using Nd, Sm, Fe, and B as the raw materials of the rare
earth magnet alloy 37 so that the general formula will be (Nd, Sm)-Fe-B. The Sm
content (at%) in Example 1 and Example 2 are 0.29 and 1.01, respectively. The
magnetic properties and eddy current losses of these samples were each determined by
the method described above; the temperature coefficient of residual magnetic flux
30 density was "Poor", the temperature property of coercive force was "Poor", and the
eddy current loss was "Good".
[0064]
The samples of Example 1 and Example 2 are the rare earth sintered magnets 1 in
which the main phase 2 has the R2Fe14B crystal structure containing at least Nd and
35 Sm as the rare earth elements R, and the main phase 2 contains Sm at a higher
content than the grain boundary phase 3. Thus, the substitution made by Sm, which
has a high electrical resistivity, at some of the Nd sites of the main phase 2 increases
the electrical resistivity of the main phase 2 and reduces the eddy current loss. It was
also found that the addition of only Sm to Nd-Fe-B does not contribute to the
40 improvement of the magnetic properties.
[0065]
Examples 3 to 7 are samples produced according to the manufacturing method of
18
Embodiment 2 using Nd, La, Sm, Fe, and B as the raw materials of the rare earth
magnet alloy 37 so that the general formula will be (Nd, La, Sm)-Fe-B. The magnetic
properties and eddy current losses of these samples were each determined by the
method described above; the temperature coefficient of residual magnetic flux density
5 was "Good", the temperature property evaluation of coercive force was "Good", and the
eddy current loss was "Good".
[0066]
The samples of Examples 3 to 7 have an R2Fe14B crystal structure in which the
main phase 2 contains at least Nd, La, and Sm as the rare earth elements R. In the
10 rare earth sintered magnet 1, the content of Sm is higher in the main phase 2 than in
the grain boundary phase 3, and the content of La is higher in the grain boundary
phase 3 than in the main phase 2. In the cooling process 25, the inclusion of La
promotes the Sm substitution at the Nd site where the La substitution has been made
in the sintering process 24. This allows Sm to be present at a higher content in the
15 main phase 2, thereby suppressing the heat generation of the rare earth sintered
magnet 1 due to the eddy current loss.
[0067]
The rare earth sintered magnet 1 includes the grain boundary phase 3 based on
the oxide phase represented by (Nd, La, Sm)-O, which is formed by the substitution
20 made by La and Sm at some of the Nd sites of the crystalline NdO phase. Thus, the
presence of La and Sm in the grain boundary phase 3 allows Nd to relatively diffuse
into the main phase 2. This prevents Nd in the main phase 2 from being consumed in
the grain boundary phase 3, and thus the magnetic anisotropy constant and the
saturated magnetic polarization are improved, enhancing the magnetic properties.
25 [0068]
This also enables replacement of Nd and Dy, which are expensive, regionally
uneven, and risky in procurement, with inexpensive La and Sm. Furthermore, the
examples show that the rare earth sintered magnet 1 of the present disclosure
prevents heat generation due to the eddy current loss while suppressing the decrease
30 in magnetic properties as the temperature rises.
Reference Signs List
[0069]
1 rare earth sintered magnet
2 main phase
35 3 grain boundary phase
4 Sm element
5 core
6 shell
11 raw alloy production process
40 12 melting process
13 primary cooling process
14 secondary cooling process
19
21 sintered magnet production process
22 pulverization process
23 molding process 23
24 sintering process
5 25 cooling process
31 crucible
32 molten alloy
33 tundish
34 rotator
10 35 solidified alloy
36 tray
37 rare earth magnet alloy
41 rotor
42 rotor core
15 43 magnet insertion hole
44 axis of rotation
51 rotating machine
52 stator
53 teeth
20 54 winding
20
We Claim:

1. A rare earth sintered magnet comprising a main phase and a grain boundary
5 phase, wherein
the main phase has an R2Fe14B crystal structure,
rare earth elements R include at least Nd and Sm, and
a content of the Sm is higher in the main phase than in the grain boundary phase.
10 2. The rare earth sintered magnet according to claim 1, wherein the rare earth
elements R further include La, and a content of the La is higher in the grain boundary
phase than in the main phase.
3. The rare earth sintered magnet according to claim 1, wherein the grain boundary
15 phase has an (Nd, Sm)-O phase formed by substitution made by the Sm in a
crystalline NdO phase.
4. The rare earth sintered magnet according to any one of claims 1 to 3, wherein a
composition ratio of the Nd is larger than that of the Sm.
20
5. The rare earth sintered magnet according to claim 2, wherein the grain boundary
phase has an (Nd, La, Sm)-O phase formed by substitution made by the La and the Sm
in a crystalline NdO phase.
25 6. The rare earth sintered magnet according to claim 2 or claim 5, wherein a
composition ratio of the Nd is larger than a sum of a composition ratio of the La and
that of the Sm.
7. A method of producing a rare earth sintered magnet comprising:
30 a pulverization process of pulverizing an R-Fe-B system rare earth magnet alloy
containing at least Nd and Sm as rare earth elements R;
a molding process of molding a powder of the R-Fe-B system rare earth magnet
alloy to produce a compact;
a sintering process of sintering the compact between 600 deg C and 1300 deg C,
35 inclusive, to produce a sintered compact; and
a cooling process of holding the sintered compact at a temperature between 227
deg C and 427 deg C, inclusive, for 0.1 hours to 5 hours.
8. A rotor comprising:
40 a rotor core; and
the rare earth sintered magnet according to any one of claims 1 to 6 provided in
the rotor core.

9. A rotating machine comprising:
the rotor according to claim 8; and
an annular stator having windings provided on teeth, the teeth being on an inner
5 surface of a side where the rotor is disposed and protruding toward the rotor, the
stator being disposed facing the rotor.