Metal treatment – Stock – Magnetic
Reexamination Certificate
2001-05-24
2004-03-16
Sheehan, John (Department: 1742)
Metal treatment
Stock
Magnetic
C252S062540, C420S083000, C420S121000
Reexamination Certificate
active
06706124
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to a method for producing a permanent magnet effectively applicable to motors and actuators of various types, and more particularly relates to a method for producing an iron-based rare earth alloy magnet including multiple ferromagnetic phases.
2. Description of the Related Art
Recently, it has become more and more necessary to farther enhance the performance of, and further reduce the size and weight of, consumer electronic appliances, office automation appliances and various other types of electric equipment. For these purposes, a permanent magnet for use in each of these appliances is required to maximize its performance to weight ratio when operated as a magnetic circuit. For example, a permanent magnet with a remanence B
r
of 0.5 T or more is now in high demand. Hard ferrite magnets have been used widely because magnets of this type are relatively inexpensive. However, the hard ferrite magnets cannot show that high remanence B
r
of 0.5 T or more.
An Sm—Co magnet, produced by a powder metallurgical process, is currently known as a typical permanent magnet with that high remanence B
r
of 0.5 T or more. Examples of other high-remanence magnets include Nd—Fe—B type magnets produced by a powder metallurgical or melt quenching process. An Nd—Fe—B type magnet of the former type is disclosed in Japanese Laid-Open Publication No. 59-46008, for example, and an Nd—Fe—B type magnet of the latter type is disclosed in Japanese Laid-Open Publication No. 60-9852, for instance.
However, the Sm—Co magnet is expensive, because Sm and Co are both expensive materials.
As for the Nd—Fe—B type magnet on the other hand, the magnet is mainly composed of relatively inexpensive Fe (typically accounting for 60 wt % to 70 wt % of the total quantity), and is much less expensive than the Sm—Co magnet. Nevertheless, it is still expensive to produce the Nd—Fe—B type magnet. This is partly because huge equipment and a great number of process steps are needed to separate and purify, or to obtain by reduction reaction, Nd, which usually accounts for 10 at % to 15 at % of the total quantity. Also, a powder metallurgical process normally requires a relatively large number of process steps by its nature.
Compared to an Nd—Fe—B type sintered magnet formed by a powder metallurgical process, an Nd—Fe—B type rapidly solidified magnet can be produced at a lower process cost by a melt quenching process. However, to obtain a permanent magnet in bulk by a melt quenching process, a bonded magnet should be formed by compounding a magnet powder, made from a rapidly solidified alloy, with a resin binder. Accordingly, the magnet powder normally accounts for at most about 80 volume % of the molded bonded magnet. Also, a rapidly solidified alloy, formed by a melt quenching process, is magnetically isotropic.
For these reasons, an Nd—Fe—B type rapidly solidified magnet produced by a melt quenching process has a remanence B
r
lower than that of a magnetically anisotropic Nd—Fe—B type sintered magnet produced by a powder metallurgical process.
As disclosed in Japanese Laid-Open Publication No. 1-7502, a technique of adding at least one element selected from the group consisting of Zr, Nb, Mo, Hf, Ta and W and at least one more element selected from the group consisting of Ti, V and Cr in combination effectively improves the magnetic properties of an Nd—Fe—B type rapidly solidified magnet. When these elements are added, the magnet can have its coercivity H
cJ
and anticorrosiveness increased. However, the only known effective technique of improving the remanence B
r
is increasing the density of a bonded magnet.
As for an Nd-Fe-B type magnet, an alternative magnet material was proposed by R. Coehoorn et al., in J. de Phys, C8, 1988, pp.669-670. The Coehoorn material has a composition including a rare earth element at a relatively low mole fraction (i.e., around Nd
3.8
Fe
77.2
B
19
, where the subscripts are indicated in atomic percentages) and an Fe
3
B primary phase. This permanent magnet material is obtained by heating and crystallizing an amorphous alloy that has been prepared by a melt quenching process. Also, the crystallized material has a metastable structure in which soft magnetic Fe
3
B and hard magnetic Nd
2
Fe
14
B phases coexist and in which crystal grains of very small sizes (i.e., on the order of several nanometers) are dispersed finely and uniformly as a composite of these two crystalline phases. For that reason, a magnet made from such a material is called a “nanocomposite magnet”. It was reported that a nanocomposite magnet like this has a remanence B
r
of as high as 1 T or more. But the coercivity H
CJ
thereof is relatively low, i.e., in the range from 160 kA/m to 240 kA/m. Accordingly, this permanent magnet is applicable only when the operating point of the magnet is 1 or more.
It has been proposed that various metal elements be added to the material alloy of a nanocomposite magnet to improve the magnetic properties thereof. See, for example, Japanese Laid-Open Publication No. 3-261104, U.S. Pat. No. 4,836,868, Japanese Laid-Open Publication No. 7-122412, PCT International Publication No. WO 00/03403 and W. C. Chan et. al., “The Effects of Refractory Metals on the Magnetic Properties of &agr;-Fe/R
2
Fe
14
B-type Nanocomposites”, IEEE Trans. Magn. No.5, INTER-MAG. 99, Kyongiu, Korea, pp.3265-3267, 1999. However, none of these proposed techniques can always obtain a sufficient “characteristic value per cost”.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method for producing an iron-based alloy permanent magnet, exhibiting excellent magnetic properties including a high coercivity H
cJ
of e.g., 480 kA/m or more and a high remanence B
r
of e.g., 0.85 T or more, at a low cost.
An iron-based rare earth alloy magnet according to the present invention has a composition represented by the general formula: (Fe
1-m
T
m
)
100-x-y-z
Q
x
R
y
M
z
, where T is at least one element selected from the group consisting of Co and Ni; Q is at least one element selected from the group consisting of B and C; R is at least one rare earth element substantially excluding La and Ce; and M is at least one metal element selected from the group consisting of Ti, Zr and Hf and always includes Ti. In this formula, the mole fractions x, y, z and m meet the inequalities of: 10 at %<x≦20 at %; 6 at %≦y<10 at %; 0.1 at %≦z≦12 at %; and 0≦m≦0.5, respectively. The magnet has two or more ferromagnetic crystalline phases including hard and soft magnetic phases. An average grain size of the hard magnetic phase is equal to or greater than 10 nm and equal to or less than 200 nm, while that of the soft magnetic phase is equal to or greater than 1 mn and equal to or less than 100 nm.
In one embodiment of the present invention, the mole fractions x, y and z preferably meet the inequalities of: 10 at %<x<17 at %; 8 at %≦y≦9.3 at %; and 0.5 at %≦z≦6 at %, respectively.
In another embodiment of the present invention, R
2
Fe
14
B phase, boride phase and &agr;-Fe phase may coexist in the same metal structure.
Specifically, an average crystal grain size of the &agr;-Fe and boride phases is preferably from 1 nm to 50 nm.
More specifically, the boride phase preferably includes an iron-based boride with ferromagnetic properties.
In this particular embodiment, the iron-based boride preferably includes Fe
3
B and/or Fe
23
B
6
.
In still another embodiment, the mole fractions x and z preferably meet the condition z/x≧0.1.
In yet another embodiment, the mole fraction y of the rare earth element(s) R may be 9.5 at % or less.
Alternatively, the mole fraction y of the rare earth element(s) R may also be 9.0 at % or less.
In yet another embodiment, the magnet may have been shaped in a thin strip with a thickness of 10 &mgr;m to 300 &mgr;m.
In yet another embodiment, the magnet may have been pulverized into powder particles.
Then, a mean particle size of the powder particles is preferably from 30 &m
Hirosawa Satoshi
Kanekiyo Hirokazu
Miyoshi Toshio
Shigemoto Yasutaka
Shioya Yusuke
Keating & Bennett LLP
Sheehan John
Sumitomo Special Metals Co. Ltd.
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