Metal treatment – Process of modifying or maintaining internal physical... – Magnetic materials
Reissue Patent
2001-11-02
2003-03-11
Sheehan, John P. (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Magnetic materials
C148S104000, C148S105000, C075S331000, C075S349000, C075S352000, C075S356000, C075S357000
Reissue Patent
active
RE038021
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a magnetically anisotropic magnetic powder composed of a rare earth element-iron-boron-gallium alloy powder, and a permanent magnet composed of such alloy powder dispersed in a resin, and more particularly to a resin-bonded permanent magnet having good thermal stability composed of a magnetically anisotropic rare earth element-iron-boron-gallium permanent magnet powder having fine crystal grains dispersed in a resin.
Typically conventional rare earth element permanent magnets are SmCo permanent magnets, and Sm
2
Co
17
permanent magnets. These samarium.cobalt magnets are prepared from ingots produced by melting samarium and cobalt in vacuum or in an inert gas atmosphere. These ingots are pulverized and the resulting powders are pressed in a magnetic field to form green bodies which are in turn sintered and heat-treated to provide permanent magnets.
The samarium.cobalt magnets are given magnetic anisotropy by pressing in a magnetic field as mentioned above. The magnetic anisotropy greatly increases the magnetic properties of the magnets On the other hand, magnetically anisotropic, resin-bonded samarium.cobalt permanent magnets are obtained by injection-molding a mixture of samarium-cobalt magnet powder produced from the sintered magnet provided with anisotropy and a resin in a magnetic field, or by compression-molding the above mixture in a die.
Thus, resin-bonded samarium.cobalt magnets can be obtained by preparing the sintered magnets having anisotropy, pulverizing them and then mixing them with resins as binders.
Recently, neodymium-iron-boron magnets have been proposed as new rare earth magnets surmounting the samarium cobalt magnets containing samarium which is not only expensive but also unstable in its supply. Japanese Patent Laid-Open Nos. 59-46008 and 59-64733 disclose permanent magnets obtained by forming ingots of neodymium-iron-boron alloys, pulverizing them to fine powders, pressing them in a magnetic field to provide green bodies which are sintered and then heat-treated, like the samarium.cobalt magnets. This production method is called a powder metallurgy method. Also, it was reported to obtain a resin-bonded magnet having magnetic anisotropy by pulverizing an ingot to 0.5-2 &mgr;m and then solidifying it with a wax (Appl. Phys. Lett. 48 (10), Mar. 1986, pp.670-672).
With respect to the Nd-Fe-B permanent magnet, GENERAL MOTORS has proposed an alternative method to the above-mentioned powder metallurgy method.
This method comprises melting a mixture of neodymium, iron and boron, rapidly quenching the melt by such a technique as melt spinning to provide fine flakes of the amorphous alloy, the heat-treating the flaky amorphous alloy to generate an Nd
2
Fe
14
B intermetallic compound. The fine flakes of this rapidly-quenched alloy is solidified with a resin binder (Japanese Patent Laid-Open No. 59-211549). However, the magnetic alloy thus prepared is magnetically isotropic. Then Japanese Patent Laid-Open No. 60-100402 discloses a technique of hot-pressing this isotropic magnetic alloy, and then applying high temperatures and high pressure thereto so that plastic flow takes place partially in the alloy thereby imparting magnetic anisotropy thereto.
The conventional Nd-Fe-B permanent magnets, however, have the following problems.
First, although the above powder metallurgy can provide magnetic anisotropy and magnetic properties of (BH)max=35-45MGOe, the resulting magnets essentially have low Curie temperature, large crystal grain size and poor thermal stability. Accordingly, they cannot be suitably used for motors, etc. which are likely to be used in a high-temperature environment.
Second, although molding is relatively easy by compression molding if rapidly-quenched powder is mixed with a resin, the resulting alloy is isotropic, so that its magnetic properties are inevitably low. For instance, the magnetic properties are (BH)max of 3-5MGOe for those obtained by injection molding and (BH)max of 8-10MGOe for those obtained by compression molding, and further the magnetic properties vary widely depending upon the strength of a magnetic field for magnetizing the alloy. To achieve (BH)max of 8MGOe, the magnetic field should be 50 kOe or so, and it is difficult to magnetize the alloy after assembling for various applications.
In addition, although hot pressing of the rapidly-quenched alloy powder-serves to increase the density of the alloy, eliminating pores from the pressed alloy powder to improve weathering properties thereof, the resulting alloy is
isotopic
isotropic
so that it is disadvantageous just like the permanent magnet prepared by mixing rapidly-quenched alloy powder with a resin. (BH)max of the resulting alloy is improved in proportion to the increase in the density, and it can reach 12 MGOe or so. However, it is still impossible to magnetize it after assembling.
By the method of hot-pressing rapidly-quenched alloy powder and then causing plastic flow therein, anisotropy can be achieved like the powder metallurgy method, providing (BH) max of 34-40 MGOe, but annular magnets, for instance, magnet rings of 30 mm in outer diameter, 25 mm in inner diameter and 20 mm in thickness cannot easily by formed because die upsetting should be utilized to provide anisotropy.
Finally, with respect to magnets prepared by pulverizing ingots and solidifying them with wax, powders used are so fine that they are likely to be burned, making it impossible to handle them in the atmosphere, Also since the magnets show a low squareness ratio in the magnetization curve, they cannot have high magnetic properties.
Incidentally, we tried to provide anisotropic resin-bonded magnets by pulverizing anisotropic sintered magnets prepared by the powder metallurgy method, mixing the pulverized particles with resins and molding them while applying a DC magnetic field, but high magnetic properties could not be achieved.
OBJECT AND SUMMARY OF THE INVENTION
Therefore, an object of the present invention is to solve the problems peculiar to the above conventional techniques, thereby providing an anisotropic resin-bonded magnet having good thermal stability and easily magnetizable after assembling, and magnetic powder usable therefor and a method of producing them.
To achieve the above object, the present invention comprises the following technical means.
That is, the object of the present invention has been achieve first by forming magnetically anisotropic magnetic powder having an average crystal grain size of 0.01-0.5 &mgr;m from an R-Fe-B-Ga alloy, wherein R represents one or more rare earth elements including Y, Fe may be partially substituted by Co to include an R-Fe-Co-B-Ga alloy, and one or more additional elements (M) selected from Nb, W, V, Ta, Mo, Si, Al, Zr, Hf, P, C and Zn may be contained to include an R-Fe-B-Ga-M alloy and an R-Fe-Co-B-Ga-M alloy, second by forming a pressed powder magnet therefrom, and third by forming a resin-bonded magnet from powder of the above alloy having an average particle size of 1-1000 &mgr;m.
The present invention is based on our finding that a thermally stable, anisotropic resin-bonded magnet can be obtained from magnetic powder of an average particle size of 1-1000 &mgr;m prepared by pulverizing a magnetically anisotropic R-Fe-B-Ga alloy having an average crystal grain size of 0.01-0.5 &mgr;m. It has been found that gallium (Ga) is highly effective to improve the thermal stability of the magnet.
Thus, the magnetically anisotropic magnetic powder according to the present invention has an average particle size of 1-1000 &mgr;m and is made from a magnetically anisotropic R-TM-B-Ga alloy having an average crystal grain size of 0.01-0.5 &mgr;m, wherein R represents one or more rare earth elements including Y, TM represents Fe which may be partially substituted by Co, B boron and Ga gallium.
The method of producing a magnetically anisotropic magnetic powder according to the present invention comprises the steps of rapidly quenching a melt of an R-TM-B-Ga alloy, wherein R represents one or more
Endoh Minoru
Tokunaga Masaaki
Finnegan Henderson Farabow Garrett & Dunner LLP
Hitachi Metals Ltd.
Sheehan John P.
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