U.S. patent application number 12/132738 was filed with the patent office on 2008-11-06 for r-t-b based sintered magnet.
This patent application is currently assigned to HITACHI METALS, LTD.. Invention is credited to Hiroyuki TOMIZAWA.
Application Number | 20080274009 12/132738 |
Document ID | / |
Family ID | 39608107 |
Filed Date | 2008-11-06 |
United States Patent
Application |
20080274009 |
Kind Code |
A1 |
TOMIZAWA; Hiroyuki |
November 6, 2008 |
R-T-B BASED SINTERED MAGNET
Abstract
An R-T-B based sintered magnet according to the present
invention comprises: 12 at % to 17 at % of a rare-earth element R;
5.0 at % to 8.0 at % of boron B; 0.02 at % to 0.2 at % of Mn; and a
transition metal T as the balance. The rare-earth element R is at
least one element selected from the rare-earth elements, including
Y (yttrium), and includes 0.2 at % to 8 at % of Pr. And the
transition element T includes Fe as its main element.
Inventors: |
TOMIZAWA; Hiroyuki; (Osaka,
JP) |
Correspondence
Address: |
HITACHI METALS, LTD.;C/O KEATING & BENNETT, LLP
1800 Alexander Bell Drive, SUITE 200
Reston
VA
20191
US
|
Assignee: |
HITACHI METALS, LTD.
Tokyo
JP
|
Family ID: |
39608107 |
Appl. No.: |
12/132738 |
Filed: |
June 4, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
PCT/JP2007/059384 |
May 2, 2007 |
|
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12132738 |
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Current U.S.
Class: |
420/83 |
Current CPC
Class: |
C22C 38/005 20130101;
H01F 1/0577 20130101; C22C 38/06 20130101; C22C 38/04 20130101 |
Class at
Publication: |
420/83 |
International
Class: |
C22C 38/00 20060101
C22C038/00; C22C 38/10 20060101 C22C038/10 |
Claims
1. An R-T-B based sintered magnet having a composition comprising:
12 at % to 17 at % of a rare-earth element R; 5.0 at % to 8.0 at %
of boron B; 0.02 at % to less than 0.2 at % of Mn; and a transition
metal T as the balance, wherein the rare-earth element R is at
least one element selected from the rare-earth elements, including
Y (yttrium), and includes 0.2 at % to 8 at % of Pr, and wherein the
transition metal T includes Fe as its main element.
2. The R-T-B based sintered magnet of claim 1, wherein the magnet
includes at least one of Tb and Dy as the rare-earth element R.
3. The R-T-B based sintered magnet of claim 1, wherein the magnet
includes 20 at % or less of Co as the transition metal T.
4. An R-T-M-B based sintered magnet having a composition
comprising: 12 at % to 17 at % of a rare-earth element R; 5.0 at %
to 8.0 at % of boron B; 0.02 at % to less than 0.2 at % of Mn; more
than 0 at % to 5.0 at % of an additive element M; and a transition
metal T as the balance, wherein the rare-earth element R is at
least one element selected from the rare-earth elements, including
Y (yttrium), and includes 0.2 at % to 8 at % of Pr, and wherein the
transition metal T includes Fe as its main element, and wherein the
additive element M is at least one element selected from the group
consisting of Al, Ni, Cu, Zn, Ga, Ag, In, Sn, Bi, Ti, V, Cr, Zr,
Nb, Mo, Hf, Ta and W.
5. The R-T-M-B based sintered magnet of claim 4, wherein the magnet
includes at least one of Tb and Dy as the rare-earth element R.
6. The R-T-M-B based sintered magnet of claim 4, wherein the magnet
includes 20 at % or less of Co as the transition metal T.
Description
[0001] This is a continuation of International Application No.
PCT/JP2007/059384, with an international filing date of May 2,
2007.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to an R-T-B
(rare-earth-iron-boron) based sintered magnet.
[0004] 2. Description of the Related Art
[0005] R-T-B based sintered magnets have so good magnetic
properties as to find a wide variety of applications including
various types of motors and actuators and are now one of
indispensable materials for the electronics industry. Also, their
applications have been appreciably broadened to keep up with the
recent trend toward energy saving.
[0006] Lately, however, those motors and actuators are more and
more often required to exhibit much higher performance than
conventional ones in their rapidly expanding applications including
motors for driving, or generating electricity for, hybrid cars or
motors for hoisting elevators. And their requirements are becoming
increasingly severe nowadays.
[0007] One of the old drawbacks of R-T-B based magnets is their
relative low Curie temperature of approximately 300.degree. C., at
which their ferromagnetism is lost. And irreversible flux loss will
occur easily in R-T-B based magnets. To overcome such a problem,
various measures have been taken. For example, some people tried to
increase the coercivity of the R-T-B based magnets by adjusting the
combination of rare-earth elements to add. Other people attempted
to increase the Curie temperature by adding Co as disclosed in
Patent Document No. 1.
[0008] Several methods for increasing the coercivity have been
proposed so far.
[0009] One of those methods is disclosed in Patent Document No. 2,
in which heavy rare-earth elements such as Dy and Tb are included
in particular percentages in the rare-earth elements. In practice,
only Dy and Tb turned out to be effective enough. This method is
adopted in order to increase the coercivity of the magnet as a
whole, as well as the anisotropic magnetic field of its main phase
that determines its magnetic properties.
[0010] Another method is disclosed in Patent Documents Nos. 3 and
4, for example, in which the coercivity is increased by introducing
an additive element such as Al, Ga, Sn, Cu or Ag. It is not yet
quite clear exactly how these elements can increase the coercivity.
Nevertheless, it is at least known that the coercivity can be
increased by changing the physical properties of a grain boundary
phase (which is a so-called "R-rich phase") such as its wettability
with the main phase in a high temperature range and eventually
changing the microstructures with the addition of those
elements.
[0011] Furthermore, the additive elements such as Ti, V, Cr, Zr,
Nb, Mo, Hf and W disclosed in Patent Document No. 5, for example,
hinder the growth of crystal grains during the sintering process
and reduce the size of the resultant metallurgical structure of the
sintered body, thus contributing to increasing the coercivity.
[0012] As for the selection of rare-earth elements, Non-Patent
Document No. 1, for example, discloses magnetic properties that an
R.sub.2Fe.sub.14B compound would have with various rare-earth
elements, and the composition can be determined by reference to
such data. For example, the anisotropic magnetic field generated by
Pr has temperature dependence, which is heavier than that of Nd.
For that reason, even though Pr could increase the coercivity at
room temperature, the coercivity to be exhibited by the additive Pr
would rather be lower than that to be exhibited by Nd in a
temperature range exceeding 80.degree. C. That is why the addition
of Pr would be counteractive at least in terms of thermal
resistance.
[0013] Among these methods, the method that uses heavy rare-earth
elements is most effective because the decrease in magnetic flux
density is relatively small according to that method. According to
any of the other methods mentioned above, however, a significant
decrease in the magnetic flux density of the magnet is inevitable.
And those methods are applicable to only a narrow field. For that
reason, in making magnets actually, these techniques are used in an
appropriate combination. [0014] Patent Document No. 1: Japanese
Patent Application Laid-Open Publication No. 59-64733 [0015] Patent
Document No. 2: Japanese Patent Application Laid-Open Publication
No. 60-34005 [0016] Patent Document No. 3: Japanese Patent
Application Laid-Open Publication No. 59-89401 [0017] Patent
Document No. 4: Japanese Patent Application Laid-Open Publication
No. 64-7503 [0018] Patent Document No. 5: Japanese Patent
Application Laid-Open Publication No. 62-23960 [0019] Non-Patent
Document No. 1: S. Hirosawa et al., Magnetization and Magnetic
Anisotropy of Nd.sub.2Fe.sub.14B Measured on Single Crystals, J.
Appl. Phys., 59 (1986), pp. 873-879
[0020] However, those heavy rare-earth elements such as Dy and Tb
are among the rarest and expensive ones of all rare-earth elements.
For that reason, if a lot of such heavy rare-earth elements should
be used, then the price of the magnets would rise. In addition, as
the applications of such R-T-B based sintered magnet have been
rapidly expanding these days, resource-related restrictions on
those heavy rare-earth elements have become an issue these days
because those rare elements are available only in very limited
quantities and in very narrow areas.
[0021] Also, as mentioned above, none of those methods is so
effective by itself and each of them would generally result in a
significant decrease in the magnetic flux density of the magnet.
That is why it has been very difficult to increase the coercivity
without using any heavy rare-earth element.
[0022] Thus, an object of the present invention is to provide a
means for increasing the coercivity that would work independently
of the effects caused by a heavy rare-earth element such as Dy or
Tb.
SUMMARY OF THE INVENTION
[0023] An R-T-B based sintered magnet according to the present
invention comprises: 12 at % to 17 at % of a rare-earth element R;
5.0 at % to 8.0 at % of boron B; 0.02 at % to 0.2 at % of Mn; and a
transition metal T as the balance. The rare-earth element R is at
least one element selected from the rare-earth elements, including
Y (yttrium), and includes 0.2 at % to 8 at % of Pr. And the
transition element T includes Fe as its main element.
[0024] In one preferred embodiment, the magnet includes at least
one of Tb and Dy as the rare-earth element R.
[0025] In another preferred embodiment, the magnet includes 20 at %
or less of Co as the transition metal T.
[0026] An R-T-M-B based sintered magnet according to the present
invention comprises: 12 at % to 17 at % of a rare-earth element R;
5.0 at % to 8.0 at % of boron B; 0.02 at % to 0.2 at % of Mn; more
than 0 at % to 5.0 at % of an additive element M; and a transition
metal T as the balance. The rare-earth element R is at least one
element selected from the rare-earth elements, including Y
(yttrium), and includes 0.2 at % to 8 at % of Pr. The transition
element T includes Fe as its main element. The additive element M
is at least one element selected from the group consisting of Al,
Ni, Cu, Zn, Ga, Ag, In, Sn, Bi, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta and
W.
[0027] In one preferred embodiment, the magnet includes at least
one of Tb and Dy as the rare-earth element R.
[0028] In another preferred embodiment, the magnet includes 20 at %
or less of Co as the transition metal T.
[0029] If an R-T-B based sintered magnet includes Pr as an
essential element and an additive Mn in an amount that falls within
a predetermined range, its coercivity at around room temperature
can be increased and higher coercivity than conventional magnets'
can be achieved even at high temperatures of 80.degree. C. or more.
Also, by adding a predetermined amount of Mn, the sintering
reaction can be promoted during the manufacturing process of the
sintered magnet. As a result, the sintering process can be done
either at a lower temperature or in a shorter time and the sintered
magnet can have a homogenized structure. Consequently, the loop
squareness of the demagnetization curve can be improved as
well.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] FIG. 1 is a graph showing how the coercivity of an R-T-B
based sintered magnet changed with the temperature in a situation
where rare-earth elements R were added in various combinations.
[0031] FIG. 2A is a graph showing how the variation in the
coercivity of an R-T-B based sintered magnet with the temperature
changed according to the amount of Mn added in a situation where
R.dbd.Nd+Pr.
[0032] FIG. 2B is a partially enlarged one of the graph shown in
FIG. 2A.
[0033] FIG. 3 is a graph showing how the coercivity of an
Nd--Pr--Dy--Fe--Co--Al--Cu--Mn--B sintered magnet at room
temperature changed with the mole fraction x of Mn added.
[0034] FIG. 4 is a table showing compositions as specific examples
of the present invention.
[0035] FIG. 5 is a table showing compositions as specific examples
of the present invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0036] The present inventors discovered that by using Pr as one of
the rare-earth elements and adding Mn to the composition of a
magnet, its coercivity at room temperature could be increased and
the decrease in coercivity at high temperatures of 80.degree. C. or
more, which would otherwise be caused in a conventional magnet with
Pr, could be minimized.
[0037] An R-T-B based sintered magnet according to the present
invention has a composition including: 12 at % to 17 at % of a
rare-earth element R; 5.0 at % to 8.0 at % of boron B; 0.02 at % to
0.3 at % of Mn; and a transition metal T as the balance.
[0038] The rare-earth element R is at least one element selected
from the rare-earth elements, including Y (yttrium), and includes
0.2 at % to 10 at % of Pr. The transition element T includes Fe as
its main element.
[0039] Optionally, to achieve various effects, at least one element
selected from the group consisting of Ni, Cu, Zn, Ga, Ag, In, Sn,
Bi, Ti, V, Cr, Zr, Nb, Mo, Hf, Ta and W may be added as the
additive element M.
[0040] In the prior art, the effects caused by the addition of Mn
have been believed to be negative ones. That is to say, it has been
believed that the additive Mn would deteriorate all major magnetic
properties including the Curie temperature, anisotropic magnetic
field and magnetization. Meanwhile, the effects of Pr have been
researched from various angles in terms of the physical properties
of a Pr.sub.2Fe.sub.14B compound, for example. It is already well
known in the art that Pr generates a greater anisotropic magnetic
field than that of Nd at around room temperature, but that their
relation reverses in the vicinity of 80.degree. C., and that the
magnetization produced by Pr is somewhat smaller than that produced
by Nd.
[0041] However, it is virtually unknown what effects would be
caused if Pr and Mn are added in combination. That is to say, a
magnet including an Nd.sub.2Fe.sub.14B phase as a main phase would
have decreased coercivity and decreased magnetization if Fe were
partially replaced with Mn. However, the present inventors
discovered that by using the technique described above to
substitute Pr for a portion of Nd, the coercivity could be
increased. Also, this technique should work quite independently of
any known means for increasing the coercivity.
Composition
[0042] The element of rare-earth element(s) according to the
present invention is one of the most important factors to achieve
the effects of the present invention. In general, to achieve the
high performance of an R-T-B based sintered magnet, Nd is an
indispensable element. According to the present invention, R
includes Nd as an essential element and a predetermined amount of R
is added thereto in order to increase the coercivity.
[0043] If Pr were added in less than the predetermined amount, the
effects of the present invention could not be achieved. Besides,
the addition of Mn, which is another essential element according to
the present invention, would decrease both magnetization and
coercivity alike. However, if Pr were added in more than the
predetermined amount, the remanence would decrease more
significantly and the coercivity would decrease even more steeply
at temperatures higher than 100.degree. C. Also, if Pr added
exceeded the predetermined amount, a lot of Mn should be added to
increase the coercivity. However, the effect would be counteracted
because the addition of Mn itself would lead to a decrease in
coercivity.
[0044] The lower limit of a preferred composition range for Pr is
0.2 at %, more preferably 0.5 at %. The upper limit of the Pr range
is preferably 10 at %, more preferably 8.0 at %.
[0045] As long as it falls within the predetermined range to be
defined below, the higher the mole fraction of R, the higher the
coercivity and the smaller the remanence tend to be. Specifically,
if the mole fraction of R were less than 12 at %, the percentage of
the R.sub.2T.sub.14B compound as the main phase would decrease,
soft magnetic phases such as Fe would produce instead, and the
coercivity would decrease significantly. On the other hand, if the
mole fraction of R exceeded 17 at %, the percentage of the
R.sub.2T.sub.14B compound as the main phase would decrease and the
magnetization would drop. In addition, since excessive R would be
concentrated as metal elements in the grain boundary of the main
phase, and the anticorrosiveness might decrease significantly. For
these reasons, the mole fraction of R is preferably 12 at % to 17
at %, more preferably 12.5 at % to 15 at %.
[0046] One heavy rare-earth element or two such as Tb or Dy, which
would contribute to increasing the coercivity, may be added
depending on the required level of magnetic properties (or the
coercivity, among other things) because it would not counteract the
effect of the present invention. However, if the total mole
fraction of the substituent(s) Tb and/or Dy exceeded 6 at %, the
resultant remanence would be lower than 1.1 T. In addition,
considering its applications under high-temperature environments,
in particular, the performance of the R-T-B based sintered magnet
should be rather lower than that of an Sm--Co magnet. On top of
that, if a lot of Tb and/or Dy were used, then the material cost of
the magnet would be too high to maintain its advantage over the
Sm--Co magnet. In view of these considerations, the mole fraction
of Tb and/or Dy is preferably 6 at % or less to achieve good
industrial applicability. Meanwhile, the other rare-earth elements,
including Y, could also be included as inevitably contained
impurities, although they would not produce any benefits as far as
magnetic properties are concerned.
[0047] Boron is an essential element to make an R-T-B based
sintered magnet. The volume of the R.sub.2T.sub.14B compound as the
main phase is determined by that of boron. To achieve large
magnetization while holding sufficient coercivity for the sintered
magnet, the mole fraction of B is important. As long as it falls
within the predetermined range to be defined below, the greater the
mole fraction of B, the more easily sufficient coercivity could be
achieved. Also, if the mole fraction of B were small, the
coercivity would decrease steeply at a certain mole fraction of B.
For that reason, from an industrial standpoint, it is particularly
important to prevent the mole fraction of B from being short of
that certain mole fraction. The greater the mole fraction of B, the
lower the remanence. If the mole fraction of B were less than 5.0
at %, the percentage of the main phase would decrease and soft
magnetic compounds other than the main phase would be produced to
decrease the coercivity of the magnet eventually. However, if the
mole fraction of B were greater than 8.0 at %, the percentage of
the main phase would also decrease and the resultant magnet would
have decreased magnetization. For these reasons, the mole fraction
of B preferably falls within the range of 5.0 at % to 8.0 at %. To
obtain a high-performance magnet, the mole fraction of B is more
preferably 5.5 at % through 7.0 at %
[0048] The transition metal T includes Fe as its main element and
Mn as an essential element. In a magnetic alloy, Mn is present as a
solid solution in the main phase as a matter of principle. If
R.dbd.Nd, then the magnetization, anisotropic magnetic field and
Curie temperature of the main phase would all decrease
proportionally to the amount of Mn added and the performance of the
magnet would decline. For that reason, in the prior art, Mn was
included in as small an amount as possible. On the other hand, if
R.dbd.Pr, then there would be a composition range in which the
coercivity slightly increases and in which the amount of Mn added
is small. And if R.dbd.Pr+Nd, then the problem of low coercivity at
high temperatures, which would be produced when R.dbd.Pr, should be
resolved.
[0049] FIG. 1 is a graph showing how the coercivity of an R-T-B
based sintered magnet changed with the temperature in a situation
where the mole fraction of Mn added was fixed at 0.01 at % and
rare-earth elements were added in various combinations. In FIG. 1,
the curve #1 represents the characteristic of Sample #1 in which
R.dbd.Nd, and the (dashed) curve #4 represents the characteristic
of Sample #4 in which R.dbd.Nd+Dy. It can be seen that if a portion
of R.dbd.Nd was replaced with a heavy rare-earth element such as
Dy, the coercivity increased in the entire temperature range shown
in FIG. 1. On the other hand, curves #2 and #3 represent the
characteristics of Samples #2 and #3 in which R.dbd.Pr+Nd. The
coercivities of Samples #2 and #3 at room temperature were higher
than that of the sample in which R.dbd.Nd according to the amount
of Pr substituted. However, this tendency turned around at
80.degree. C. or higher temperatures. That is to say, considering
the application of the magnet at 80.degree. C. or even higher
temperatures, it would counteract the effects of the present
invention to substitute Pr. Also, the intersection between the
curve #1 and the curves #2 and #3 is not different between Samples
#2 and #3, including different mole fractions of Mn, and was
located at around 80.degree. C.
[0050] FIG. 2A is a graph schematically showing the effect of the
additive Mn on the coercivity of a magnet, and FIG. 2B is a
partially enlarged one of the graph shown in FIG. 2A. In FIGS. 2A
and 2B, the curves #1 and #3 are the same as the ones shown in FIG.
1. The curve #5 represents the characteristic of Sample #5, in
which the mole fraction of Mn of Sample #3 was changed into 0.15 at
%. Sample #5, to which a very small amount of Mn was added,
exhibited higher coercivity than Sample #3 in the entire
temperature range. As a result, the coercivity became higher than
that of Sample #1 at a higher temperature. The curve #6 represents
the characteristic of Sample #6 that was obtained by adding Mn to a
sample in which R.dbd.Nd. The coercivity decreased at any
temperature.
[0051] If the mole fraction of Mn exceeded 0.3 at %, then the
magnetization and coercivity would both decrease significantly. For
that reason, the mole fraction of Mn is preferably 0.3 at % or
less. More preferably, the mole fraction of Mn is less than 0.2 at
% because coercivity that is equal to or higher than the one
produced at room temperature by adding either no Mn at all or 0.01
at % or less of Mn would be achieved in that case.
[0052] Meanwhile, if the mole fraction of Mn were less than 0.02 at
%, then the effect of the present invention would not manifest
itself. That is why the lower limit of the preferred Mn mole
fraction range is 0.02 at %.
[0053] It is not yet quite clear exactly how effectively Mn works
in combination with Pr. But the mechanism could be understood in
following two ways. One of the two possibilities is that if
R.dbd.Pr, the anisotropic magnetic field of an R.sub.2Fe.sub.14B
compound could be increased by Pr at a particular mole fraction of
Mn. This type of function was reported as to a situation where
R.dbd.Y, for example. The other possibility is that whether it is
present in the main phase or not, Mn would contribute to an
interfacial reaction between the ferromagnetic main phase and the
paramagnetic grain boundary phase, thus increasing the wettability
or the degree of crystal matching. As this point in time, however,
we are not confident enough to determine which of these two
hypotheses is right or if there is any other factor.
[0054] The present inventors also discovered that even a very small
amount of Mn added promoted the sintering reaction, which is
another beneficial feature that contributes to producing a sintered
magnet efficiently. Specifically, by adding Mn, the density of the
magnet increased through the sintering reaction either at a lower
temperature or in a shorter time. As a result, a sufficient
sintered density could be achieved before the crystal grains grew
too much. In addition, the magnet could have a further homogenized
texture, and therefore, exhibited improved magnetic properties
including improved loop squareness in its demagnetization
curve.
[0055] To achieve the effect of improving the sintering behavior
with the addition of Mn, the additive Mn should account for at
least 0.02 at %, more preferably 0.05 at % or more.
[0056] The only cost-effective element that would achieve the
effect of improving the sinterability seems to be Mn. This is
probably because Mn should be the only element to produce a solid
solution substantially nowhere but in the main phase among various
useful elements. In the prior art, Al and Cu were considered
elements that would improve the sinterability. However, these
elements would achieve the effect of improving the physical
properties of the grain boundary phase but would act only
indirectly on the sintering reaction of the R.sub.2T.sub.14B phase
as the main phase. On the other hand, Mn will act directly on the
sintering reaction of the main phase.
[0057] In an R-T-B based sintered magnet, a portion of Fe may
sometimes be replaced with Co to improve the magnetic properties
and the anticorrosiveness. In carrying out the present invention,
the addition of Co would not counteract the effects of the present
invention but would achieve some effects of increasing the Curie
temperature and improving the anticorrosiveness. For that reason,
Co is preferably added. If the mole fraction of Co added exceeded
20 at %, the magnetization would decrease significantly and the
coercivity would drop steeply. That is why the upper limit of Co
added is preferably 20 at %.
[0058] According to their functions and effects, the additive
elements M can be classified into a first group consisting of Al,
Ni, Cu, Zn, Ga, Ag, In, Sn and Bi and a second group consisting of
Ti, V, Cr, Zr, Nb, Mo, Hf, Ta and W. An element in the first group
is mainly present in the grain boundary in the metallurgical
structure of the magnet and contributes to the interaction between
the grain boundary and main phases. More specifically, the element
will lower the melting point of the grain boundary phase to improve
the sintering behavior of the magnet or increase the wettability
between the main phase and the grain boundary phase, thereby
expanding the grain boundary phase into the interface with the main
phase more effectively and eventually increasing the coercivity of
the magnet. Among these elements, the most effective ones are Al
and Cu. On the other hand, any element in the second group will
make the sintered structure finer and increase the coercivity by
producing deposition with a high melting point, for example. No
element in the first and second groups function as a ferromagnetic
phase. For that reason, if a lot of such an element were added, the
magnetization of the magnet would decrease. That is why the maximum
mole fraction of these elements added is preferably 5 at % or less
in total, more preferably 2 at % or less.
[0059] Other elements are not defined in the present invention and
have nothing to do with the effect to be achieved by the present
invention. However, the presence of those other elements is not
necessarily ruled out according to the present invention. For
example, hydrogen, carbon, nitrogen and oxygen are inevitably
contained during the manufacturing process and are also detected
when analyzed in specific examples of the present invention. Among
other things, carbon and nitrogen may substitute for portions of B.
In that case, however, the magnetic properties will be affected
significantly (e.g., the coercivity of the magnet will decrease).
In a normal sintered magnet, carbon and nitrogen will react with
the rare-earth element just like oxygen to produce some carbide,
nitride or oxide and be present in some form that does not affect
the magnetic properties. Also, hydrogen and nitrogen are expected
to enter sites of the main phase between its lattices and would
increase the Curie temperature. However, this is not an effect to
be achieved by the present invention. F, Cl, Mg, Ca and other
elements may get included during the process step of refining a
rare-earth metal or may also stay in the composition of the magnet
as it is. P and S may be included in the Fe material. Also, Si and
Al may not only come from a ferroboron alloy, which is a material
source, but also get included as a crucible component while the
material alloy to make the magnet is being melted.
Manufacturing Process
[0060] No matter what method is adopted to make the R-T-B based
sintered magnet of the present invention, the effects of the
present invention will be achieved equally. That is to say, the
present invention is not limited to any specific manufacturing
process. However, an exemplary manufacturing process that can be
adopted will be described below.
Material Alloy
[0061] Material alloys may be prepared by any of various methods
and used in any of various forms. Typical examples of preferred
material alloys include an ingot alloy, a strip cast alloy, an
atomized powder, a powder obtained by a reduction diffusion process
and an alloy ribbon made by a rapid quenching process. Any of these
material alloys may be used by itself. Or multiple material alloys
of mutually different types may be used in combination as well.
Still alternatively, a so-called "two-alloy process" that uses two
alloys with different compositions in combination may also be
adopted. In that case, in order to not only increase the coercivity
but also improve the sinterability at high temperatures, Mn and Pr
may be included in one or both of the two alloys. In the former
case, Mn and Pr may be included in one of the two alloys, of which
the main phase has a composition closer to that of the magnetic
alloy. Furthermore, just to improve the sinterability, Mn and Pr
could be introduced into two different alloys and mixed together.
In that case, however, the coercivity could not be increased at
high temperatures so much as expected.
[0062] Optionally, the mother alloy may be subjected to a heat
treatment in order to improve the uniformity of its structure or
the distribution of elements or increase its homogeneity, for
example.
Pulverization
[0063] The pulverization process may also be carried out by any
arbitrary method. An appropriate method is adopted according to the
attribute of the start material. For example, if a strip cast alloy
is used as a start material, the alloy often needs to go through
the two pulverization process steps--a coarse pulverization process
step and a fine pulverization process step. In that case, the
coarse pulverization may be done by either a mechanical
pulverization process or a hydrogen decrepitation process, which
can be used effectively to pulverize a rare-earth alloy. As used
herein, the "hydrogen decrepitation process" refers to a process in
which a given alloy is enclosed along with hydrogen gas in a
vessel, the hydrogen gas is absorbed into the alloy, and the alloy
is pulverized by utilizing the strain to be caused by the variation
in the volume of the alloy. According to this method, a lot of
hydrogen will get included in the coarse powder. That is why the
excessive hydrogen can be released by heating the coarse powder if
necessary.
[0064] Optionally, after the alloy has been pulverized coarsely or
before the coarse powder is subjected to the fine pulverization
process step, the coarse powder may be classified with a sieve, for
example, such that all of its particle sizes are equal to or
smaller than a particular particle size.
[0065] The fine pulverization usually gets done by a jet milling
process that uses a jet flow. Alternatively, a mechanical fine
pulverization process or a wet ball milling process that uses a
dispersion medium may also be adopted. Also, before the
pulverization process is started, a pulverization assistant may be
added in advance. This is particularly useful to increase the
pulverization efficiency of the fine pulverization process
step.
[0066] As for how to handle the material alloy or the coarse
powder, it is important to handle them in an inert atmosphere to
make a high-performance magnet. As far as it is handled at ordinary
temperatures, it should be enough if the inert atmosphere is
nitrogen gas. However, if a heat treatment should be conducted at
300.degree. C. or even higher temperatures, helium gas or argon gas
needs to be used as the inert atmosphere.
[0067] The objective particle size of the pulverized powder is
determined by the intended performance of the magnet and various
restrictions to be imposed in the next compaction process step.
Normally, the objective particle size may be a D50 particle size of
3 .mu.m to 7 .mu.m according to the laser diffraction analysis
using the gas dispersion technique. This particle size falls within
such a particle size range that is easily achieved by a jet milling
process. The particle sizes of the fine powder are supposed to be
measured by the gas dispersion process because the fine powder is a
ferromagnetic that easily aggregates magnetically.
Compaction
[0068] To make an anisotropic sintered magnet, the fine powder is
compacted under a magnetic field and magnetic anisotropy is given
to the magnet. In general, the fine powder obtained by the
pulverization process is loaded into the die holes of a press
machine, a cavity is formed by upper and lower punches with a
magnetic field applied externally, and the fine powder is pressed
and compacted with the punches and then unloaded. In this process,
a lubricant may be added to the fine material powder to increase
the degree of alignment with the magnetic field applied or to
increase the lubricity of the die. The lubricant may be a solid one
or a liquid one, which may be determined with various factors into
consideration. Optionally, the fine powder may be granulated
appropriately to be loaded into the die holes more easily, for
example.
[0069] Also, as the aligning magnetic field, not only a static
magnetic field generated by a DC power supply but also a pulse
magnetic field generated by discharge of a capacitor or an AC
magnetic field may be used as well.
[0070] If the composition of the present invention is adopted, the
magnetic field applied preferably has a strength of 0.4 MA/m or
more usually, and more preferably has a strength of 0.8 MA/m or
more. After the compaction process, reverse magnetic field may be
applied to perform a demagnetizing process. By performing such a
demagnetizing process, the compact can be handled more easily after
that because the compact will have no remnant magnetization.
[0071] Optionally, if the directions of applying the magnetic field
during the compaction process are changed according to a special
pattern, a magnet with any of various aligned states can be made.
As for ring magnets, for example, the magnets may not only be
axially aligned but also radially aligned or anisotropically
aligned so as to have multiple magnetic poles.
[0072] The compaction process does not have to be performed using
the die and punches as described above. Alternatively, the
compaction process may also be performed using a rubber mold.
Optionally, the compaction and the application of the magnetic
field may be performed separately.
Sintering
[0073] The sintering process is carried out in either a vacuum or
an argon gas atmosphere. The pressure and other parameters of the
atmosphere may be determined arbitrarily. Optionally, the sintering
process may be carried out in a helium gas atmosphere. However, the
thermal efficiency of the sintering furnace could decrease due to
the good heat conduction of the helium gas.
[0074] The sintering process is usually carried out at a
temperature of 1,000.degree. C. to 1,100.degree. C. for 30 minutes
to 16 hours. In the composition range of the present invention, the
sintering process causes a liquid phase in the compact of the
present invention, and therefore, the temperature does not have to
be so high. If necessary, a number of sintering processes may be
performed either at the same temperature or multiple different
temperatures. As for the cooling process after the temperature has
been held, it is not always necessary to perform a rapid cooling
process or a gradual cooling process. Alternatively, various
conditions (including those of the heat treatment process to be
described below) may be combined appropriately.
[0075] After the sintering process, the magnet of the present
invention can have a specific gravity of at least 7.3, more
preferably 7.4 or more.
[0076] Optionally, any other sintering means for use in a powder
metallurgical process, such as a hot press in which the object is
heated while being subjected to an external pressure or an
electro-sintering process in which a given compact is supplied with
electricity and heated with Joule heat, may also be adopted. If any
of those alternative means is adopted, the sintering temperature
and process time do not have to be as described above.
Heat Treatment
[0077] To increase the coercivity, the sintered body may be
subjected to some heat treatment at a temperature that is equal to
or lower than the sintering temperature. Optionally, the heat
treatment may be conducted a number of times at either the same
temperature or multiple different temperatures. In performing the
heat treatment, various conditions may be set for the cooling
process.
[0078] It should be noted that if the as-sintered body already has
sufficient coercivity, there is no need to subject it to any heat
treatment.
Machining
[0079] The sintered body sometimes has a shape that is close to its
final one, but in most cases, is subjected to some machining
process such as cutting, polishing or grinding to have its shape
finished into a predetermined one. As long as it is done after the
sintering process, this machining process may be carried out either
before or after the heat treatment process or between multiple heat
treatment processes.
Surface Treatment
[0080] In a normal environment, a sintered magnet with a
composition according to the present invention would rust in the
long run. That is why the magnet should be subjected to some
surface coating treatment appropriately. Examples of preferred
surface treatments include resin coating, metal plating, and vapor
deposition of a film. Among these various surface treatments, an
appropriate one is selected with the application, required
performance and cost taken into consideration.
Magnetization
[0081] A magnet according to the present invention is usually
magnetized with a pulse magnetic field. This magnetization process
is often carried out after the magnet has been built in the product
for the convenience of the assembling process. However, it is
naturally possible to magnetize the magnet by itself and then build
the magnet into the product.
[0082] The magnetizing direction needs to be determined with the
aligning direction for the compaction process under the magnetic
field taken into consideration. Usually a high-performance magnet
cannot be obtained unless these two directions agree with each
other. Depending on the application, however, the aligning
direction for the compaction process does not have to agree with
the magnetizing direction.
EXAMPLES
Example 1
[0083] An alloy with an objective composition was prepared by
mixing together Pr and Nd with a purity of 99.5% or more, Tb and Dy
with a purity of 99.9% or more, electrolytic iron, and low-carbon
ferroboron alloy together with the other objective elements added
in the form of pure metals. The alloy was then melted and cast by a
strip casting process, thereby obtaining a plate-like alloy with a
thickness of 0.3 mm to 0.4 mm.
[0084] This material alloy was subjected to a hydrogen
decrepitation process within a hydrogen atmosphere with an
increased pressure, heated to 600.degree. C. in a vacuum, cooled
and then classified with a sieve, thereby obtaining a coarse alloy
powder with a particle size of 425 .mu.m or less. Then, zinc
stearate was added to, and mixed with, this coarse powder so as to
account for 0.05 mass % of the powder.
[0085] Next, the coarse alloy powder was subjected to a dry
pulverization process using a jet mill machine in a nitrogen gas
flow, thereby obtaining a fine powder with a particle size D50 of 4
.mu.m to 5 .mu.m. In this process, as for a sample that should have
an objective content of oxygen, the concentration of oxygen in the
pulverization gas was controlled to 50 ppm or less. This particle
size was obtained by the laser diffraction analysis using the gas
dispersion technique.
[0086] The fine powder thus obtained was compacted under a magnetic
field to make green compacts. In this process, a static magnetic
field of approximately 0.8 MA/m and a compacting pressure of 98 MPa
were applied. It should be noted that the direction in which the
magnetic field was applied and the direction in which the
compacting pressure was applied were orthogonal to each other.
Also, as for a sample that should have the objective oxygen
content, the sample was transported from the pulverizer into the
sintering furnace so as to be kept in a nitrogen atmosphere for as
much of the time as possible.
[0087] Next, those green compacts were sintered at a temperature of
1,020.degree. C. to 1,080.degree. C. for two hours in a vacuum. The
sintering temperature varied according to the composition. In any
case, the sintering process was carried out at as low a temperature
as possible as far as the sintered compacts could have a density of
7.5 Mg/m.sup.3.
[0088] The compositions of the sintered bodies thus obtained were
analyzed as shown in FIG. 4. And the results shown in FIG. 4 were
converted into the atomic percentages shown in FIG. 5. The analysis
was carried out using an ICP. However, the contents of oxygen,
nitrogen and carbon were obtained with a gas analyzer. Each of
these samples was subjected to a hydrogen analysis by a dissolution
technique. As a result, the contents of hydrogen in those samples
were in the range of 10 ppm to 20 ppm.
[0089] In addition to the elements shown in the table, not only
hydrogen but also Si, Ca, Cr, La, Ce and other elements could be
detected. In most cases, Si would come from the crucible along with
Al while the ferroboron material and the alloy were being melted,
and Ca, La and Ce would come from the rare-earth material. And Mn
and Cr could be included in iron. It is impossible to reduce all of
these impurities to absolutely zero. For example, although Sample
#1 was supposed to include no Al at all, actually Al was sill
detected from that sample.
[0090] The sintered bodies thus obtained were thermally treated at
various temperatures for an hour within an Ar atmosphere and then
cooled. The heat treatment was conducted with the temperatures
changed according to the composition. Also, some samples were
subjected to the heat treatment up to three times with the
temperatures changed. After those samples were machined, their
magnetic properties were measured at room temperature with a B--H
tracer. The magnetic properties of the same samples were measured
again at 140.degree. C. The results are shown in the following
Table 1:
TABLE-US-00001 TABLE 1 B.sub.r/T H.sub.cJ/MAm.sup.-1 No. 20.degree.
C. 20.degree. C. 140.degree. C. 1 1.380 0.853 0.240 2 1.392 0.954
0.271 3 1.394 0.988 0.281 4 1.391 1.006 0.285 5 1.388 0.992 0.291 6
1.410 0.998 0.294 7 1.424 1.016 0.314 8 1.402 0.996 0.298 9 1.274
1.684 0.584 10 1.268 1.683 0.584 11 1.374 1.251 0.411 12 1.280
1.712 0.632 13 1.372 0.827 0.226 14 1.394 0.992 0.274 15 1.389
0.975 0.274 16 1.278 1.655 0.552 17 1.376 1.230 0.392 18 1.282
1.683 0.607 19 1.410 0.880 0.248 20 1.398 0.956 0.228
[0091] It should be noted that among the samples that had been
thermally treated under various conditions, those exhibiting the
highest coercivity at room temperature were used as objects of
evaluation.
[0092] Samples #13 through #18 represent comparative examples
including less than 0.02 at % of Mn and had lower coercivity at
140.degree. C. than Samples #1 through #12 of the present invention
to which both Pr and Mn were added.
[0093] Samples #19 and #20 also represent comparative examples to
which either Pr or Nd was added as a rare-earth element. Compared
to Sample #4 representing a specific example of the present
invention (although the contents of the other elements are not
equal to each other between these two samples), Sample #19 had
lower coercivity at room temperature and Sample #20 had lower
coercivity at 140.degree. C.
Example 2
[0094] Magnets, of which the compositions were represented by
Nd.sub.13.5-.LAMBDA.Pr.sub..LAMBDA.Dy.sub.1.0Fe.sub.ba1.Co.sub.2.0Al.sub.-
0.5Cu.sub.0.1Mn.sub.xB.sub.6.0 (where subscripts are atomic
percentages), had their coercivity measured at room temperature
with the mole fraction A of Pr set to be 0, 2, 5, 8 and 11 (at %)
and with the mole fraction x of Mn varied. The results are shown in
FIG. 3. The magnets of this Example 2 were produced by the same
method as that adopted for Example 1.
[0095] As can be seen from FIG. 3, in a situation where A=0, as Mn
was added, the coercivity decreased monotonically. On the other
hand, if a portion of the rare-earth element was replaced with Pr,
the coercivity rather increased as long as the amount of Mn added
fell within a particular range.
[0096] However, in a situation where the mole fraction A of Pr was
11 at %, the coercivity did not increase appreciably even if Mn was
added.
Example 3
[0097] Sintered magnets, of which the compositions were represented
by
Nd.sub.11.5Pr.sub.1.0Dy.sub.1.2Fe.sub.ba1.Cu.sub.0.1Mn.sub.xB.sub.6.0
(where subscripts are atomic percentages), were made with the mole
fraction x varied and had their magnetic properties measured. The
results are shown in the following Table 2:
TABLE-US-00002 TABLE 2 Mole fraction Density Magnetic properties
No. x of Mn (at %) .rho./MGm.sup.-3 J.sub.r/T H.sub.cJ/kAm.sup.-1
H.sub.k/H.sub.cJ 21 0.01 7.34 1.337 1026 0.926 22 0.02 7.49 1.368
1122 0.971 23 0.05 7.51 1.372 1155 0.989 24 0.10 7.54 1.376 1134
0.987 25 0.15 7.53 1.372 1119 0.987 26 0.20 7.54 1.368 1105 0.988
27 0.25 7.54 1.363 1091 0.987 28 0.30 7.53 1.360 1074 0.988 29 0.40
7.54 1.351 1040 0.985 30 0.50 7.54 1.343 1008 0.988 31 0.60 7.54
1.335 981 0.983 32 0.80 7.53 1.316 908 0.978
[0098] The same manufacturing process as that adopted for Example 1
was also carried out. Every magnet with any of these compositions
was sintered at 1,020.degree. C. for two hours. The magnetic
properties were evaluated by calculating H.sub.k as an index and
figuring out H.sub.k/H.sub.cJ as an index to loop squareness. In
this case, H.sub.k represents a value of a demagnetization field
when the value of magnetization becomes 90% of Jr. The closer to
one the H.sub.k/H.sub.cJ ratio is, the better the loop squareness
and the more useful the given magnet should be.
[0099] As can be seen from Table 2, if the mole fraction of Mn
added was equal to or greater than 0.02 at %, the density of the
magnet increased compared to a magnet that was sintered under the
same condition. As a result, the remanence J.sub.r and the loop
squareness H.sub.k/H.sub.cJ of the demagnetization curve improved.
However, if the mole fraction of Mn exceeded 0.50 at %, the
magnetization of the main phase decreased with the addition of Mn.
Consequently, the remanence J.sub.r was lower than that of Sample
#21 including 0.01 at % of Mn. A preferred composition range for Mn
in which good magnetic properties were realized was 0.02 at %
through 0.30 at %.
[0100] According to the results of a gas analysis, 0.44 mass % to
0.49 mass % of oxygen, 0.035 mass % to 0.043 mass % of carbon,
0.010 mass % to 0.014 mass % of nitrogen, and less than 0.002 mass
% of hydrogen were included as inevitable impurities in the
sintered magnets. Also, according to the results of the ICP
analysis, at most 0.04 mass % of Si and 0.01 mass % or less of Cr,
Ce, Ca, etc. was detected.
Example 4
[0101] Sintered magnets with various compositions were obtained by
the same method as that adopted for Example 1. The mole fraction of
Mn added was fixed at 0.06 at %. As the additive elements M, Al, Cu
and Ga were selected from the first group and Mo was selected from
the second group. And the amounts of rare-earth elements, B and
additive elements M were changed into various values (including
zero). The compositions (analyzed values) of the magnets thus
obtained are shown in the following Table 3 and their magnetic
properties are shown in the following Table 4:
TABLE-US-00003 TABLE 3 Chemical symbols No. Nd Pr Tb Dy Fe Co Mn Al
Cu Ga Mo B O 33 11.0 1.0 81.5 0.06 0.04 0.10 5.83 0.51 34 12.0 0.5
80.8 0.06 0.04 0.10 5.86 0.68 35 12.0 3.0 76.7 0.06 0.04 0.10 6.06
2.05 36 7.0 10.0 74.5 0.06 0.04 0.10 5.97 2.31 37 12.8 4.2 75.2
0.06 0.04 0.10 5.06 2.49 38 7.0 7.0 77.9 0.06 0.04 0.10 0.05 5.51
2.34 39 13.0 0.2 77.8 0.06 0.04 0.10 0.05 7.00 1.79 40 12.0 2.0
76.0 0.06 0.04 0.10 7.98 1.80 41 9.0 3.5 1.30 78.0 0.06 0.24 0.10
5.98 1.78 42 5.2 7.0 1.60 72.5 3.00 0.06 0.04 0.10 1.50 7.00 1.96
43 12.0 0.2 1.60 66.4 5.00 0.06 0.48 0.10 4.00 8.00 2.11 44 9.0 3.4
1.30 72.8 5.30 0.06 0.24 0.10 0.05 5.91 1.86 45 9.1 3.5 1.30 68.2
9.50 0.06 0.24 0.10 0.05 5.92 1.98 46 9.1 3.4 1.30 58.0 20.00 0.06
0.24 0.10 5.97 1.86 47 3.6 8.0 2.10 75.7 2.10 0.06 0.48 5.97 1.94
48 8.2 1.0 4.50 75.8 2.10 0.06 0.48 0.05 5.85 1.93 49 12.5 0.5 0.25
0.55 75.7 2.10 0.06 0.48 5.92 1.94 50 12.7 0.5 0.60 75.8 2.10 0.06
0.48 5.92 1.82
TABLE-US-00004 TABLE 4 Magnetic properties No. J.sub.r/T
H.sub.cJ/kAm.sup.-1 33 1.457 712 34 1.459 705 35 1.351 885 36 1.275
912 37 1.206 622 38 1.349 770 39 1.401 916 40 1.332 971 41 1.377
1325 42 1.295 1726 43 1.167 2058 44 1.378 1312 45 1.372 1343 46
1.376 1308 47 1.311 1563 48 1.203 2314 49 1.384 1286 50 1.382
1298
[0102] With any of these compositions, the effects of the present
invention were achieved.
[0103] According to the results of a gas analysis, 0.032 mass % to
0.057 mass % of carbon, 0.010 mass % to 0.027 mass % of nitrogen,
and less than 0.002 mass % of hydrogen were included as inevitable
impurities in the sintered magnets. Also, according to the results
of the ICP analysis, at most 0.05 mass % of Si and 0.01 mass % or
less of Cr, Ce, Ca, etc. was detected.
[0104] A sintered magnet according to the present invention can be
used extensively in various applications that require
high-performance sintered magnets.
[0105] While the present invention has been described with respect
to preferred embodiments thereof, it will be apparent to those
skilled in the art that the disclosed invention may be modified in
numerous ways and may assume many embodiments other than those
specifically described above. Accordingly, it is intended by the
appended claims to cover all modifications of the invention that
fall within the true spirit and scope of the invention.
* * * * *