Claims
- 1. A rare earth-ferromagnetic metal alloy permanent magnet, produced by the method comprising the steps of:
- (a) mixing a particulate additive material selected from the group consisting of refractory oxides, carbides, and nitrides, in an amount which provides about 0.1 percent to about 2 percent by weight additive material in the mixture, with a major amount of a particulate rare earth-ferromagnetic metal alloy having an empirical formula corresponding approximately to RM.sub.5, wherein R is rare earth selected from the group consisting praseodymium and mixtures of praseodymium and samarium and M is ferromagnetic metal, and a minor amount of a particulate rate earth-ferromagnetic metal sintering aid alloy;
- (b) aligning magnetic domains of the mixture in a magnetic field;
- (c) compacting the aligned mixture to form a shape;
- (d) sintering the compacted shape; and wherein said method produces a magnet composition containing a major phase amount of said particulate rare earth-ferromagnetic metal alloy, a minor phase amount of said particulate rare earth-ferromagnetic metal sintering aid alloy, and added oxide, carbide or nitride from said particulate additive.
- 2. The magnet defined in claim 1 wherein all components of the mixture have been reduced to particle sizes less than about 10 microns.
- 3. The magnet defined in claim 1 wherein the sintering aid comprises up to about 15 percent by weight of the mixture.
- 4. The magnet defined in claim 3 wherein the sintering aid comprises about 10 percent to about 15 percent by weight.
- 5. The magnet defined in claim 1 wherein, during sintering, at least a portion of the sintering aid is liquid.
- 6. The magnet defined in claim 1 wherein R is praseodymium.
- 7. The magnet defined in claim 1 wherein M is cobalt.
- 8. The magnet defined in claim 1 wherein the sintering aid is an alloy containing an excess of rare earth over the amount required to form RM.sub.5, wherein R is rare earth and M is ferromagnetic metal.
- 9. The magnet defined in claim 1 wherein the sintering aid is an alloy of a ferromagnetic metal and a rare earth selected from the group consisting of praseodymium, samarium, and mixtures thereof.
- 10. The magnet defined in claim 1 wherein the sintering aid is an alloy of rare earth metal and cobalt.
- 11. The magnet defined in claim 1 wherein the additive material is an oxide.
- 12. The magnet defined in claim 1 wherein the additive material is an oxide of a metal selected from the group consisting of chromium, aluminum, and magnesium.
- 13. A praseodymium-cobalt based magnet, produced by the method comprising the steps of:
- (a) mixing together the components:
- (i) a particulate praseodymium-cobalt alloy, having an empirical formula corresponding approximately to PrCo.sub.5 ;
- (ii) a lesser amount of a particular sintering aid alloy selected from the group consisting of praseodymium-cobalt alloys, samarium-cobalt alloys, praseodymium-samarium-cobalt alloys, and mixtures thereof; and
- (iii) a particulate additive selected from the group consisting of refractory oxides, carbides, and nitrides, in an amount comprising about 0.1 to about 2 percent by weight of the mixture;
- (b) aligning magnetic domains of the mixture in a magnetic field;
- (c) compacting the aligned mixture to form a shape;
- (d) sintering the compacted shape at temperatures which cause at least a portion of the sintering aid to become liquid;
- and wherein said method produces a magnet composition containing a major phase amount of said particulate praseodymium-cobalt alloy, a minor phase amount of said particulate sintering aid alloy, and added oxide, carbide or nitride from said particulate additive.
- 14. The magnet defined in claim 13 wherein all components of step (a) have particle sizes less than about 10 microns.
- 15. The magnet defined in claim 13 wherein the sintering aid comprises about 10 to about 15 percent by weight of the mixture of step (a).
- 16. The magnet defined in claim 13 wherein the sintering aid alloy contains an excess of rare earth over an amount required to form RCo.sub.5, wherein R is praseodymium, samarium, or mixtures thereof.
- 17. The magnet defined in claim 13 wherein the additive is an oxide.
- 18. The magnet defined in claim 13 wherein the additive is an oxide of a metal selected from the group consisting of chromium, aluminum, and magnesium.
CROSS REFERENCE TO RELATED APPLICATION
This application is a continuation of application Ser. No. 856,701 filed Apr. 28, 1986 abandoned which is a division of U.S. patent application Ser. No. 595,290 filed Mar. 30, 1984, now U.S. Pat. No. 4,601,754.
This invention relates to rare earth ferromagnetic metal alloy compositions for producing rare earth-containing permanent magnets, and to magnet production methods utilizing the compositions.
Permanent magnets, defined as materials which exhibit permanent ferromagnetism (the ability to maintain magnetism following removal from a magnetizing field), have long been useful industrial materials, finding extensive applications in such devices as meters, loudspeakers, motors, and generators.
The more thoroughly developed permanent magnet compositions, for applications requiring the highest available residual magnetic strength, are alloys which contain rare earths and the ferromagnetic metals. Alloys of samarium and cobalt, sometimes containing minor amounts of other metals (such as iron, manganese, chromium, vanadium, aluminum, and copper--disclosed by Menth et al. in U.S. Pat. No. 4,131,495), have found considerable commercial success. A typical commercial samarium-cobalt magnet has the nominal empirical composition SmCo.sub.5, prepared by mixing powdered SmCo.sub.5 with a minor amount of samarium-cobalt alloy sintering aid which is richer in samarium than SmCo.sub.5, aligning the mixture in a magnetic field, pressing the mixture into a desired shape, and sintering the shape. During sintering, the sintering aid becomes at least partially liquid, permitting a large density increase in the shape. This general method is described in U.S. Pat. No. 3,655,464 to Benz.
Due to the relatively high cost and scarcity of samarium, it has been found desirable to replace as much of the metal as possible with the more abundant (and, consequently, less expensive) rare earths, such as praseodymium, lanthanum, cerium, and misch metal. The highest theoretical magnet strengths, for alloys having an atomic ratio of ferromagnetic metal to rare earth of about 5, are obtained with praseodymium-cobalt alloys, but these strengths have not yet been obtained in practice. Examples of magnet materials thus produced are shown in U.S. Pat. No. 3,682,714 to Martin, and in references made therein to other patent applications. The patent shows magnets in which praseodymium constitutes 75 percent of the total rare earth content.
J. Tsui and K. Strnat, Applied Physics Letters, Vol. 18, No. 4, pages 107-8 (1971), describe the preparation of PrCo.sub.5 magnets, using liquid-phase sintering aids containing either samarium and cobalt or praseodymium and cobalt.
Various methods have been used to prepare rare earth-containing magnets. Cech, in U.S. Pat. No. 3,625,779, mixes rare earth oxide and calcium hydride, then heats to reduce the oxide and form rare earth metal, which is melted with cobalt. The resulting alloy is then subjected to extensive treatments to remove even traces of formed calcium oxide, and used to produce magnets.
In general, it has been desirable to totally exclude oxygen from the rare earth-containing magnet production. U.S. Pat. No. 3,723,197 to Brischow et al. gives experimental evidence that Sm.sub.2 O.sub.3, formed during the production of SmCo.sub.5 magnets, is highly detrimental to the magnetic properties of the products. U.S. Pat. No. 4,043,845 to Dionne describes the use of carbon in mixtures of rare earth metal and cobalt, to prevent oxidation of rare earth-cobalt alloys.
Clegg, in U.S. Pat. No. 4,290,826, discloses a process for producing cobalt-rare earth alloys by mixing cobalt powder and refractory oxide powder, adding rare earth metal powder, and heating to form the alloy, without significant sintering. The avoidance of sintering is said to preserve the original small particle sizes, which improves the properties of magnets formed from the product powdered alloy.
Unsintered powders, however, must be bound together in resins, etc., to be useful as permanent magnets. The resulting low density of such magnets is reflected in the comparatively low magnetic strengths obtained. Further, the binders contribute to disadvantages such as the inability to use the magnets at elevated temperatures. In addition, sintered magnets have significantly greater mechanical strength.
Accordingly, it is an object of the present invention to provide compositions which form high strength rare earth-ferromagnetic metal permanent magnets.
It is a further object to provide compositions which can be sintered to form high strength rare earth-ferromagnetic metal permanent magnets.
A still further object is to provide a method for preparing sintered rare earth-ferromagnetic metal permanent magnets.
These, and other important objects, will become more apparent from consideration of the following description and the appended claims.
Compositions for the production of rare earth-ferromagnetic metal permanent magnets comprise (1) a major amount of a particulate rare earth-ferromagnetic metal alloy; (2) a minor amount of a particulate alloy sintering aid which contains rare earth and ferromagnetic metal; and (3) about 0.1 to about 2 percent by weight of an additive material selected from the group consisting of refractory oxides, carbides, and nitrides.
A preparation of permanent magnets comprises: (1) mixing the rare earth-ferromagnetic alloy with the sintering aid; (2) adding to the mixture the additive material; (3) aligning the magnetic domains of the mixture in a magnetic field; (4) compacting the aligned mixture to form a shape; and (5) sintering the compacted shape.
Use of the additive material yields sintered magnets having both improved coercivities and more square demagnetization curves.
US Referenced Citations (4)
Foreign Referenced Citations (2)
| Number |
Date |
Country |
| 0101552 |
Feb 1984 |
EPX |
| 56-44741 |
Apr 1981 |
JPX |
Divisions (1)
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Number |
Date |
Country |
| Parent |
595290 |
Mar 1984 |
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Continuations (1)
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Number |
Date |
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| Parent |
856701 |
Apr 1986 |
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Patent Grant
4891078
