Stock material or miscellaneous articles – Composite – Of metal
Reexamination Certificate
2002-06-14
2004-08-17
Sheehan, John P. (Department: 1742)
Stock material or miscellaneous articles
Composite
Of metal
C428S327000, C428S328000, C428S330000, C428S331000, C428S332000, C428S447000, C428S450000, C427S127000, C148S101000, C148S302000, C148S102000, C148S104000
Reexamination Certificate
active
06777097
ABSTRACT:
This invention relates to a corrosion resistant rare earth magnet and a method for preparing the same.
BACKGROUND OF THE INVENTION
Because of their excellent magnetic properties, rare earth permanent magnets are frequently used in a wide variety of applications such as electric apparatus and computer peripheral devices and are important electric and electronic materials. In particular, a family of Nd—Fe—B permanent magnets has lower starting material costs than Sm—Co permanent magnets because the key element neodymium exists in more plenty than samarium and the content of cobalt is low. This family of magnets also has much better magnetic properties than Sm—Co permanent magnets, making them excellent as permanent magnet materials. For this reason, the demand for Nd—Fe—B permanent magnets is recently increasing and the application thereof is spreading.
However, the Nd—Fe—B permanent magnets have the drawback that they are readily oxidized in humid air within a short time since they contain rare earth elements and iron as the main components. When Nd—Fe—B permanent magnets are incorporated in magnetic circuits, the oxidation phenomenon raises such problems as decreased outputs of magnetic circuits and contamination of the associated equipment with rust.
In the last decade, Nd—Fe—B permanent magnets find incipient use in motors such as automotive motors and elevator motors. The magnets are inevitably used in a hot humid environment. In some potential situations, the magnets are exposed to salt-containing moist air. It would be desirable if magnets are endowed with corrosion resistance at low cost. In the motors, the magnets can be heated at 300° C. or higher, though for a short time, in their manufacturing process. In this application, the magnets are also required to have heat resistance.
To improve the corrosion resistance of Nd—Fe—B permanent magnets, various surface treatments such as resin coating, aluminum ion plating and nickel plating are often implemented. It is difficult for these surface treatments of the state-of-the-art to accommodate the above-mentioned rigorous conditions. For example, resin coating provides insufficient corrosion resistance and lacks heat resistance. Nickel plating allows the underlying material to rust in salt-containing moist air because of the presence of some pinholes. The ion plating technique achieves generally satisfactory heat resistance and corrosion resistance, but needs a large size apparatus and is thus difficult to conduct at low cost.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an R—T—M—B rare earth permanent magnet such as a neodymium magnet which can withstand use under rigorous conditions as mentioned above, and more particularly, a corrosion resistant rare earth magnet which is arrived at by providing the magnet with a corrosion and heat-resistant coating. Another object is to provide a method for preparing the corrosion resistant rare earth magnet.
According to the invention, a rare earth permanent magnet represented by R—T—M—B wherein R, T and M are as defined below is treated on a surface thereof with a solution of a flake fine powder of a specific metal or alloy and a silicone resin by dipping the magnet in the solution or by coating the solution to the magnet. Subsequent heating forms on the magnet surface a composite coating in which the flake fine powder is bound with an oxidized product of the silicone resin such as silica. A highly corrosion resistant rare earth magnet is obtained in this way. The conditions necessary to achieve the object have been established.
In a first aspect, the present invention provides a corrosion resistant rare earth magnet comprising a rare earth permanent magnet represented by R—T—M—B wherein R is at least one rare earth element inclusive of yttrium, T is Fe or Fe and Co, M is at least one element selected from the group consisting of Ti, Nb, Al, V, Mn, Sn, Ca, Mg, Pb, Sb, Zn, Si, Zr, Cr, Ni, Cu, Ga, Mo, W, and Ta, and B is boron, the contents of the respective elements are 5 wt %≦R≦40 wt %, 50 wt %≦T≦90 wt %, 0 wt %≦M≦8 wt %, and 0.2 wt %≦B≦8 wt %, and a composite coating formed on a surface of the permanent magnet by treating the permanent magnet with a solution comprising at least one flake fine powder selected from the group consisting of Al, Mg, Ca, Zn, Si, Mn and alloys thereof and a silicone resin, followed by heating.
In a second aspect, the present invention provides a method for preparing a corrosion resistant rare earth magnet comprising the steps of providing a rare earth permanent magnet represented by R—T—M—B wherein R is at least one rare earth element inclusive of yttrium, T is Fe or Fe and Co, M is at least one element selected from the group consisting of Ti, Nb, Al, V, Mn, Sn, Ca, Mg, Pb, Sb, Zn, Si, Zr, Cr, Ni, Cu, Ga, Mo, W, and Ta, and B is boron, the contents of the respective elements are 5 wt %≦R≦40 wt %, 50 wt %≦T≦90 wt %, 0 wt %≦M≦8 wt %, and 0.2 wt %≦B≦8 wt %; treating a surface of the permanent magnet with a solution comprising at least one flake fine powder selected from the group consisting of Al, Mg, Ca, Zn, Si, Mn and alloys thereof and a silicone resin; and heating the treated permanent magnet to form a composite coating on the permanent magnet.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention starts with rare earth permanent magnets represented by R—T—M—B, such as Ne—Fe—B base permanent magnets. Herein R represents at least one rare earth element inclusive of yttrium, preferably Nd or a combination of major Nd with another rare earth element or elements. T represents Fe or a mixture of Fe and Co. M represents at least one element selected from among Ti, Nb, Al, V, Mn, Sn, Ca, Mg, Pb, Sb, Zn, Si, Zr, Cr, Ni, Cu, Ga, Mo, W, and Ta. B is boron. The contents of the respective elements are 5 wt %≦R≦40 wt %, 50 wt %≦T≦90 wt %, 0 wt %≦M≦8 wt %, and 0.2 wt %≦B≦8 wt %.
More particularly, R represents a rare earth element inclusive of yttrium, and specifically, at least one element selected from among Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. R should preferably include Nd. The content of R is 5% to 40% by weight and preferably 10 to 35% by weight of the magnet.
T represents Fe or a mixture of Fe and Co. The content of T is 50% to 90% by weight and preferably 55 to 80% by weight of the magnet.
M represents at least one element selected from among Ti, Nb, Al, V, Mn, Sn, Ca, Mg, Pb, Sb, Zn, Si, Zr, Cr, Ni, Cu, Ga, Mo, W, and Ta. The content of M is 0% to 8% by weight and preferably 0 to 5% by weight of the magnet.
The content of boron (B) is 0.2% to 8% by weight and preferably 0.5 to 5% by weight of the sintered magnet.
For the preparation of R—T—M—B permanent magnets such as Nd—Fe—B base permanent magnets, raw metal materials are first melted in vacuum or an atmosphere of an inert gas, preferably argon to form an ingot. Suitable raw metal materials used herein include pure rare earth elements, rare earth alloys, pure iron, ferroboron, and alloys thereof, which are understood to contain various impurities which incidentally occur in the industrial manufacture, typically C, N, O, H, P, S, etc. If necessary, solution treatment is carried out on the ingot because &agr;-Fe, R-rich and B-rich phases may sometimes be left in the alloy as well as the R
2
Fe
14
B phase. To this end, heat treatment may be carried out in vacuum or in an inert atmosphere of Ar or the like, at a temperature of 700 to 1,200° C. for a time of 1 hour or more.
The ingot thus obtained is crushed, then milled, preferably to an average particle size of 0.5 to 20 &mgr;m. Particles with an average particle size of less than 0.5 &mgr;m are rather vulnerable to oxidation and may lose magnetic properties. Particles with an average particle size of more than 20 &mgr;m may be less sinterable.
The powder is press molded in a magnetic field into a desired shape, which is then sintered. Sintering is generally conducted at a temperature in the range of 900 to 1,200° C. in vacuum o
Hamada Ryuji
Minowa Takehisa
Sheehan John P.
Shin-Etsu Chemical Co. , Ltd.
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