Metal treatment – Process of modifying or maintaining internal physical... – Magnetic materials
Reexamination Certificate
2001-09-19
2003-10-28
Sheehan, John (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Magnetic materials
C148S101000, C148S253000, C252S062540, C241S017000, C241S023000
Reexamination Certificate
active
06638367
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a method of producing highly weather-resistant magnet powder and the product produced by the same method, more particularly a method of producing highly weather-resistant iron-based magnet powder containing a rare-earth element, highly weather-resistant magnet powder produced by the same method, resin composition containing the same powder for bonded magnets, and bonded magnet.
BACKGROUND OF THE INVENTION
The ferrite, Alnico and rare-earth magnets have been used for various purposes, e.g., motors. However, these magnets are mainly produced by the sintering method, and have various disadvantages. For example, they are generally fragile and difficult to form into thin or complex-shape products. In addition, they are low in dimensional precision, because of significant shrinkage of 15 to 20% during the sintering step, and need post-treatment, e.g., grinding, to improve their precision.
On the other hand, bonded magnets have been recently developed, in order to solve these disadvantages and, at the same time, to develop new applications. Bonded magnets are generally produced by filling them with a magnetic powder using a thermoplastic resin, e.g., polyamide or polyphenylene sulfide resin, as the binder.
Of these bonded magnets, those comprising iron-based magnetic powder, especially the ones containing a rare-earth element, tend to be rusted and lose their magnetic characteristics in a high temperature, humid atmosphere. To overcome these problems, the surface of the compact is coated with a film of, e.g., thermosetting resin or phosphate-containing coating material (as disclosed by Japanese Patent Laid-Open No.208321/2000), to prevent rusting. Nevertheless, however, they are still insufficient in rust-preventive effects and magnetic properties, e.g., coercive force.
It is necessary, when an iron-based magnet powder containing a rare-earth element is kneaded together with a resin for a bonded magnet, to crush the magnet alloy powder to several microns, in order to secure sufficient magnetic characteristics. The magnet alloy powder is normally crushed in an inert gas or solvent. However, finely crushing a magnet powder causes a problem. The finely crushed powder is so active that, when coming into contact with air before being coated, it will be rapidly rusted by oxidation to lose its magnetic characteristics.
Several attempts have been made to solve the above type of problems. For example, a magnet alloy powder is slowly oxidized, after it is crushed to several microns, with a very small quantity of oxygen introduced into the inert atmosphere. Another measure is coating the crushed magnet powder with a phosphate, as disclosed by Japanese Patent Laid-Open No.251124/1999.
However, the crushed magnetic particles agglomerate with each other by the magnetic force. Such a powder, although improved in resistance to weather in a dry atmosphere, is not satisfactorily improved in the practically important resistance in a humid atmosphere, even when the agglomerated particles are protected with the coating film, conceivably because of insufficient protection of the individual particles. Therefore, coating the powder still fails to solve the problem.
Under these circumstances, small-size motors, acoustic devices, OA devices or the like have been recently required to be still smaller, which requires the bonded magnets therefor to have still improved magnetic characteristics. However, the magnetic characteristics of the bonded magnet of the conventional iron-based magnet powder containing a rare-earth element are insufficient for the above purposes. Therefore, it is strongly desired to improve magnetic characteristics of bonded magnets in the early stage by improving resistance of the iron-based magnet powder containing a rare-earth element to weather.
It is an object of the present invention to provide a method of producing highly weather-resistant iron-based magnet powder containing a rare-earth element, particularly characterized by high coercive force in a practically important humid atmosphere, to solve the problems involved in the conventional techniques. It is another object of the present invention to provide a highly weather-resistant magnet powder produced by the same method. It is still another object of the present invention to provide a resin composition containing the same powder for bonded magnets. It is still another object of the present invention to provide a bonded magnet containing the same powder.
DISCLOSURE OF THE INVENTION
The inventors of the present invention have found, after having extensively studied to achieve the above objects, that a method of producing a magnet powder by crushing an iron-based magnet powder containing a rare-earth element in an organic solvent gives the desired magnet powder excellent in resistance to weather and controlled in decline of coercive force in a humid atmosphere, when phosphoric acid is added to the solvent in which the powder is crushed, reaching the present invention.
The first invention provides the method of producing a highly weather-resistant magnet powder by crushing an iron-based magnet powder containing a rare-earth element in an organic solvent, characterized by adding phosphoric acid to the solvent in which the powder is crushed.
The second invention provides the method of the first invention for producing a highly weather-resistant magnet powder, wherein phosphoric acid is added at 0.1 mols or more but less than 2 mols per kg of the magnet alloy powder.
The third invention provides the method of the first invention for producing a highly weather-resistant magnet powder, wherein the crushed magnet alloy powder is thermally treated at 100° C. or higher but lower than 400° C. in an inert or vacuum atmosphere.
The fourth invention provides a highly weather-resistant magnet powder produced by one of the first to third inventions.
The fifth invention provides a resin composition for bonded magnets, containing, as the main ingredient, the highly weather-resistant magnet powder of the fourth invention.
The sixth invention provides a bonded magnet produced by forming the resin composition of the fifth invention for bonded magnets.
PREFERRED EMBODIMENTS OF THE INVENTION
The present invention is described more concretely.
1. Magnet Alloy Powder
The magnet alloy powder for the present invention is not limited, so long as it is an iron-based magnet alloy powder at least containing a rare-earth element. Some of the examples include rare-earth/iron/boron-based and rare-earth/iron
itrogen-based magnetic powders normally used for bonded magnets. Of these, the more preferable ones include Nd—Fe—B-based alloy powder produced by a rapid quenching method, Sm—Fe—N-based alloy powder, Sm—Fe—N-based alloy powder coated with chemically reacted zinc, Nd—(Dy, Tb)—Fe—B-based alloy powder and Sm—Fe—Co—N-based alloy powder.
2. Method of Producing Highly Weather-Resistant Magnet Powder
The method of the present invention for producing a magnet powder includes crushing an iron-based magnet powder containing a rare-earth element in an organic solvent, wherein a given quantity of phosphoric acid is added to the solvent in which the powder is crushed.
Phosphoric acid useful for the present invention is not limited. Commercially available, normal phosphoric acid, e.g., 85% aqueous solution of phosphoric acid, may be used.
The method of adding phosphoric acid is not limited. For example, it may be added to the organic solvent in which the powder is crushed by an agitation mill. It may be added all at once before the crushing is started or little by little during the crushing process, in such a way to have a given content in the final stage. It is essential for phosphoric acid to be always present in the solvent to treat the new surfaces on the fractured particles immediately after they are produced by crushing. The organic solvent useful for the present invention is not limited. Some of the solvents normally used include alcohols, e.g., ethanol and isopropyl alcohol, ketones, lower hydrocarbons, aromatics and
Hashiguchi Kayo
Ohmori Kenji
Osako Toshiyuki
Yokosawa Kouichi
Armstrong Westerman & Hattori, LLP
Sheehan John
Sumitomo Metal & Mining Co., Ltd.
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