Stock material or miscellaneous articles – Composite – Of metal
Reexamination Certificate
2000-08-29
2002-04-23
Koehler, Robert R. (Department: 1775)
Stock material or miscellaneous articles
Composite
Of metal
C427S127000, C427S372200, C427S397700, C428S446000, C428S447000, C428S450000, C428S900000, C428S926000, C428S928000
Reexamination Certificate
active
06376089
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for producing a rare earth metal-based permanent magnet having, on its surface, a thin and dense film having various characteristics required for use as a corrosion-resistant film.
2. Description of the Related Art
A rare earth metal-based permanent magnet such as an R—Fe—B based permanent magnet represented by an Nd—Fe—B based permanent magnet and an R—Fe—N based permanent magnet represented by an Sm—Fe—N based permanent magnet is made of a material rich in resource and inexpensive, and has a high magnetic characteristic, as compared with an Sm—Co based permanent magnet. Therefore, particularly, the R—Fe—B based permanent magnet is used at present in a variety of fields.
However, the rare earth metal-based permanent magnet is liable to be corroded by oxidation in the atmosphere, because it contains a highly reactive rare earth metal (R). When the rare earth metal-based permanent magnet is used without being subjected to any treatment, the corrosion of the magnet is advanced from its surface due to the presence of a small amount of acid, alkali and/or water to produce a rust, thereby bringing about the deterioration and dispersion of the magnetic characteristic. Further, when the magnet having the rust produced therein is incorporated into a device such as a magnetic circuit, there is a possibility that the rust is scattered to pollute surrounding parts or components.
With the forgoing in view, a process for forming a corrosion-resistant film on the surface of a rare earth metal-based permanent magnet has been examined hitherto. There are conventionally proposed processes, such as a process which involves applying a colloidal solution comprising water, an alcohol and inorganic fine particles (SiO
2
) to the surface of a magnet, and heating and solidifying the colloidal solution (see Japanese Patent Application Laid-open No.63-301506), a process which involves immersing a magnet into a treating solution comprising an aqueous solution of an alkali silicate containing ultra-fine silica particles dispersed therein, or applying such treating solution to the magnet, and then heating the magnet having the treating solution applied thereto (see Japanese Patent Application Laid-open No.9-63833), and a process which involves immersing a magnet into a treating solution comprising an aqueous solution of an alkali silicate containing fine metal particles dispersed therein, or applying such treating solution to the magnet, and then heating the magnet having the treating solution applied thereto (see Japanese Patent Application Laid-open No.2000-182813).
In recent years, a reduction in size of parts has been advanced in electronic and appliance industries in which a rare earth metal-based permanent magnet is used. In correspondence to this, it is required that the magnet itself is reduced in size and cost. From such a background, the surface treatment of the magnet must be carried out at higher dimensional accuracy (a reduction in thickness of a film and an increase in corrosion resistance of a thin film), with an increase in effective volume of the magnet and at a low cost. The following characteristics are required for the corrosion-resistant film.
First, the film must be dense. This is because if the film is not dense, it is impossible to prevent the corrosion of the magnet and to reduce the thickness of the film. Secondly, even if the film is dense, any physical defects such as cracks should not exist in the film. If a physical defect exists in the film, water enters the magnet through the defective portion and as a result, the corrosion begins from the surface of the magnet. Thirdly, the film itself must be excellent in corrosion resistance. If the film is liable to be corroded, it is impossible to prevent the corrosion of the magnet. Further, the film must be excellent in close adhesion to the magnet. Even if the film itself is excellent in corrosion resistance and the like, if the film is liable to be easily peeled off from the surface of the magnet, it is impossible to prevent the corrosion of the magnet. Finally, to form a film having a high dimensional accuracy, the film may be thin, but must have a uniform thickness and must satisfactorily exhibit the above-described characteristics.
The film produced by the process described in Japanese Patent Application Laid-open No.63-301506 is no more than a film formed with the inorganic fine particles merely bound together. Therefore, a void exists between the adjacent inorganic fine particles in the film. For this reason, the film lacks in denseness. The film is poor in reactivity with the surface of the magnet. For this reason, the film does not have an excellent close adhesion to the magnet. In the process described in Japanese Patent Application Laid-open Nos.9-63833 and 2000-182813, the content of alkali ions in the film can be reduced by dispersing ultra-fine silica particles or fine metal particles into the aqueous solution of alkali silicate, thereby enhancing the corrosion resistance of the film itself. However, if the content of alkali ions is over-reduced, cracks are generated. Therefore, it is difficult to simultaneously achieve the enhancement in the corrosion resistance of the film and the inhibition of the generation of physical defects, resulting in a problem that if any one of these characteristics is preferentially achieved, the other characteristic is poor.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a process for producing a rare earth metal-based permanent magnet having, on its surface, a thin and dense film having various characteristics required for use as a corrosion-resistant film.
The present inventors have made various studies with the above respect in view and as a result, they have found that in a heat treatment for forming a film by a hydrolyzing reaction and a thermally decomposing reaction of a silicon compound, followed by a polymerizing reaction, a stress is generated within the film by the shrinkage of the film, but such stress can be dispersed by dispersing the inorganic fine particles having a specific average particle size into the film phase formed from the silicon compound, thereby inhibiting the generation of the physical defects such as cracks. It has been also found that the voids between the adjacent inorganic fine particles are filled with the film phase formed from the silicon compound and hence, the film is dense; that the film itself is excellent in corrosion resistance, because no alkali ions are contained in the film; and that the excellent close adhesion of the formed film is achieved by the excellent reactivity with the surface of the magnet. Further, it has been found that the characteristics of the formed film are associated with the characteristics of the treating solution used for the formation of the film, and the excellent film can be formed, particularly, by controlling the viscosity of the treating solution.
It is described in Japanese Patent Application Laid-open No.7-230906 that a solid powder such as silica is added to a mixture of a silica precursor component such as tetraethyl orthosilicate and an organic precursor component such as vinyltriethoxysilane to form a film. However, the film described in this Patent is a film containing the silica precursor component and the organic precursor component as requisite components, and is different from the technical idea of the present invention. In addition, the relevant of the formed film to the particle size of the solid powder and the relevant of the formed film to the treating solution used for forming the film are not described in any way.
The present invention has been accomplished with the above knowledge in view, and to achieve the above object, according to a first aspect and feature of the present invention, there is provided a process for producing a rare earth metal-based permanent magnet having a corrosion-resistant film, comprising the steps of applying, to the surface of a magne
Kikugawa Atsushi
Kikui Fumiaki
Nishiuchi Takeshi
Armstrong Westerman & Hattori, LLP
Sumitomo Special Metals Co. Ltd.
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