Sintered rare earth magnet and making method

Details Sintered rare earth magnet and making method Sintered rare earth magnet and making method

2001-07-30

2003-09-23

Sheehan, John (Department: 1742)

Specialized metallurgical processes, compositions for use therei

Compositions

Consolidated metal powder compositions

C075S246000, C427S127000, C428S900000, C419S029000, C148S303000, C148S122000, C148S276000, C148S281000, C148S286000, C148S287000

Reexamination Certificate

active

06623541

ABSTRACT:

This invention relates to a Sm
2
Co
17
base magnet for use in motors intended for long-term exposure to a hydrogen atmosphere and a method for preparing the same.
BACKGROUND OF THE INVENTION
Metal compounds of rare earth elements and transition metals have the nature that hydrogen can penetrate between crystal lattices, that is, hydrogen is absorbed in and released from the alloy. This nature is utilized in a variety of applications. One example is a hydrogen battery based on a hydrogen storage alloy as typified by LaNi
5
. In connection with rare earth magnets, hydriding is utilized as means for pulverizing R
2
Fe
14
B base alloys and also in the manufacture of bonded R
2
Fe
14
B base magnets (HDDR method, see JP-A 3-129702).
However, hydrogen embrittlement is incurred when alloys or magnets are hydrided and dehydrided. When motors using rare earth magnets are used in a hydrogen atmosphere, there arises the problem that magnet blocks can be cracked, creviced and even pulverized.
Currently available sintered rare earth magnets include R
2
Fe
14
B, SmCo
5
, and Sm
2
Co
17
base magnets. In general, with respect to hydrogen, the 1-5 crystal structure has a lower plateau pressure than the 2-17 crystal structure, and the 2-7 crystal structure has a lower plateau pressure than the 1-5 crystal structure. That is, rare earth-rich (referred to as R-rich, hereinafter) alloys are more likely to absorb hydrogen and more susceptible to hydrogen embrittlement.
Often the R
2
Fe
14
B base magnet is surface treated as by plating or resin coating for the purpose of improving corrosion resistance although the surface treatment is not an effective means for preventing hydrogen embrittlement. As a solution to the problem of hydrogen embrittlement, it was proposed in JP-A 11-87119 to incorporate a hydrogen storage alloy into a surface treating coat on a R
2
Fe
14
B base magnet. The thus treated R
2
Fe
14
B base magnet does not undergo hydrogen embrittlement in a hydrogen atmosphere having a pressure of lower than 0.1 MPa, on account of an R-rich phase included therein. In a hydrogen atmosphere having a higher pressure, however, the magnet still undergoes hydrogen embrittlement and can thus be cracked, creviced and even pulverized.
Like the R
2
Fe
14
B base magnet, the SmCo
5
base magnet contains an R-rich phase and the SmCo
5
phase, the major phase has a plateau pressure of about 0.3 MPa. Then in a hydrogen atmosphere having a pressure in excess of 0.3 MPa, the SmCo
5
base magnet undergoes hydrogen embrittlement and can thus be cracked, creviced and even pulverized.
The Sm
2
Co
17
base magnet is less susceptible to hydrogen embrittlement since it has a major phase of 2-17 structure and is less R-rich than the R
2
Fe
14
B and SmCo
5
base magnets, and does not contain an R-rich phase. In a hydrogen atmosphere having a pressure in excess of 1 MPa, however, the Sm
2
Co
17
base magnet yet undergoes hydrogen embrittlement like other rare earth magnets, and can thus be cracked, creviced and even pulverized.
SUMMARY OF THE INVENTION
An object of the invention is to solve the above-described problems of prior art rare earth magnets that they, when exposed to a hydrogen atmosphere, undergo hydrogen embrittlement and can thus be cracked, creviced and even pulverized, and to provide a sintered Sm
2
Co
17
base magnet which has solved the problems and a method for preparing the same.
It has been found that by forming a composite layer containing Sm
2
O
3
and/or CoFe
2
O
4
in Co or Co and Fe on a surface of a sintered Sm
2
Co
17
base magnet, the sintered Sm
2
Co
17
base magnet becomes unsusceptible to hydrogen embrittlement even in a hydrogen atmosphere and thus suitable for use in motors or other equipment intended for long-term exposure to a hydrogen atmosphere. In the manufacture of a sintered Sm
2
Co
17
base magnet, by subjecting a sintered magnet after sintering and aging to machining and then optimum heat treatment, a hydrogen attack-resistant layer can be formed on the magnet surface at no sacrifice of magnetic properties.
The sintered Sm
2
Co
17
base magnet with the composite layer on the surface thereof is prone to chipping and thus requires careful handling during product assembly because the magnet can otherwise be chipped. A chip on the rare earth magnet does not affect its magnetic properties, but can substantially degrade hydrogen embrittlement resistance to the same level as in the absence of the surface layer. That is, the sintered Sm
2
Co
17
base magnet with the composite layer thereon, when held in a hydrogen atmosphere having a pressure in excess of 1 MPa, still has a likelihood that it undergoes hydrogen embrittlement and is cracked, creviced and even pulverized. It has been found that by applying a resin coating on the surface of the composite layer on the sintered Sm
2
Co
17
base magnet, an effect of preventing the magnet from chipping is achieved. The resin-coated, sintered Sm
2
Co
17
base magnet is thus best suited for use in motors or other equipment intended for long-term exposure to a hydrogen atmosphere.
In a first aspect, the invention provides a sintered rare earth magnet consisting essentially of 20 to 30% by weight of R wherein R is samarium or at least two rare earth elements containing at least 50% by weight of samarium, 10 to 45% by weight of iron, 1 to 10% by weight of copper, 0.5 to 5% by weight of zirconium, and the balance of cobalt and incidental impurities. The sintered rare earth magnet has on its surface a composite layer containing Sm
2
O
3
or CoFe
2
O
4
or both in Co or Co and Fe. In a preferred embodiment, the sintered rare earth magnet further has a resin coating on the composite layer.
In a second aspect, the invention provides a method for preparing a sintered rare earth magnet, comprising the steps of casting an alloy of the same composition as defined above; grinding the alloy, followed by comminution, compacting in a magnetic field, sintering and aging to form a sintered magnet; cutting and/or polishing the sintered magnet for surface finishing; and heat treating in an atmosphere having an oxygen partial pressure of 10
−6
to 152 torr for about 10 minutes to 20 hours. The method may further include the step of applying a resin coating on the surface of the sintered magnet after the heat treatment, typically by spray coating, electrodeposition, powder coating or dipping.


REFERENCES:
patent: 4284440 (1981-08-01), Tokunaga et al.
patent: 4902357 (1990-02-01), Imaizumi
patent: 5154978 (1992-10-01), Nakayama et al.
patent: 5244510 (1993-09-01), Bogatin
patent: 5781843 (1998-07-01), Anderson et al.
patent: 5840375 (1998-11-01), Katsumi et al.
patent: 6080498 (2000-06-01), Kikui et al.
patent: 56-81908 (1981-07-01), None
patent: 57-17109 (1982-01-01), None
patent: 58-48608 (1983-03-01), None
patent: 61-87310 (1986-05-01), None
patent: 61-148808 (1986-07-01), None
patent: 61-195964 (1986-08-01), None
patent: 3-129702 (1991-06-01), None
patent: 11-87119 (1999-03-01), None
Abstract of Japanese 3129702.
Abstract of Japanese 11087119.

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