Metal treatment – Stock – Magnetic
Reexamination Certificate
1999-02-02
2002-12-17
Sheehan, John (Department: 1742)
Metal treatment
Stock
Magnetic
C252S062540, C075S355000, C148S101000
Reexamination Certificate
active
06494968
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to lamellar rare earth-iron-boron-based magnet alloy particles, a process for producing the rare earth-iron-boron-based magnetic alloy particles and a bonded magnet produced from such rare earth-iron-boron-based magnet alloy particles, and more particularly, to lamellar rare earth-iron-boron-based magnet alloy particles which have a residual magnetic flux density (Br) as high as not less than 10 kG, an intrinsic coercive force (iHc) as large as not less than 3.5 kOe and a maximum energy product ((BH)
max
) as large as not less than 13 MGOe, and which are excellent in rust preventability and leafing effect, a process for producing the lamellar rare earth-iron-boron-based magnet alloy particles, and a bonded magnet produced from such lamellar rare earth-iron-boron-based magnet alloy particles.
Bonded magnets which are advantageous in that they can be produced in any shape and have a high dimensional accuracy, etc., have conventionally been used in various fields such as electric appliances and automobile parts. With a recent development of miniaturized and light-weight electric appliances and automobile parts, bonded magnets used therefor have been strongly required to be miniaturized.
For this purpose, magnets have been strongly required to show a high magnet performance, i.e., a high residual magnetic flux density (Br), a large intrinsic coercive force (iHc) and a large maximum energy product ((BH)
max
).
As is well known in the arts, a bonded magnet comprising magneto plumbite type ferrite such as barium ferrite or strontium ferrite (referred to as ‘ferrite bonded magnet’ hereinunder) and a binder resin has an excellent rust preventability because ferrite particles are an oxide. In addition, since the ferrite bonded magnets are produced from a cheap material such as oxides of barium and strontium and iron oxide, the ferrite bonded magnets are economical and are, therefore, widely used.
As to the magnetic characteristics of these ferrite bonded magnets, however, the residual magnetic flux density (Br) is about 2 to 3 kG, the intrinsic coercive force (iHc) is about 2 to 3 kOe, and the maximum energy product ((BH)max) is about 1.6 to 2.3 MGOe. Therefore, these bonded magnets are insufficient to accomplish the miniaturization and weight-reduction of apparatuses or equipments in which the bonded magnets are incorporated.
On the other hand, there is no end to a demand for a higher performance and a lower price of a magnet. Since Nd-iron-boron-based magnet alloys using Nd which is relatively low in price among rare earth elements, have been almost simultaneously developed in 1982 by Sumitomo Tokushu Kinzoku Co., Ltd. (Japan) and General Motors Corp. (USA), the magnet alloys have been used in extensive application fields, and it has also been attempted to apply the magnet alloy to the production of bonded magnets. To further improve the magnetic characteristics, rare earth-iron-boron-based alloys for exchange-spring magnets have been earnestly developed and some of them have already been put to practical use.
An exchange-spring magnet exhibits a magnetic spring phenomenon by the exchange interaction of iron (&agr;Fe) or an iron compound and an Nd
2
Fe
14
B
1
type tetragonal compound. Those magnets are characterized in a low rare earth element content and a high residual magnetic flux density (Br), and have a high possibility of being excellent on a cost/performance basis.
A rare earth-iron-boron-based alloy for exchange-spring magnets containing less than 10 atm % of a rare earth element such as Nd, has a high potential in magnetic characteristics as compared with a rare earth-iron-boron-based magnet alloy containing about 10 to 15 atm % of a rare earth element such as Nd which is in the vicinity of the stoichiometeric composition, e.g., commercially available “MQP” (trade name) developed by General Motors. Since it is possible to reduce the amount of expensive rare earth element used, this alloy is economically advantageous.
The rare earth-iron-boron-based alloy for exchange-spring magnets containing less than 10 atm % of a rare earth element such as Nd has a system containing &agr;Fe or a system containing Fe
3
B or Fe
2
B as the soft magnetic phase. The system containing &agr;Fe as the soft magnetic phase generally has a residual magnetic flux density (Br) as high as 10 to 13 kG, but the intrinsic coercive force (iHc) thereof is as low as less than 3.5 kOe at most. The system containing Fe
3
B or Fe
2
B as the soft magnetic phase generally has a comparatively high intrinsic coercive force (iHc) such as 3.5 to 7.7 kOe, but the residual magnetic flux density (Br) thereof is as low as less than 10 kG, and as a result, the bonded magnet produced from the system containing Fe
3
B or Fe
2
B as the soft magnetic phase has a higher residual magnetic flux density (Br) than that of “MQP”, but lower residual magnetic flux density (Br) than that composed of the system containing &agr;Fe as the soft magnetic phase.
In the field of small-sized motors for which bonded magnets produced from a rare earth-iron-boron-based magnet alloy is mainly used, bonded magnets are required to have well-balanced residual magnetic flux density (Br) and, intrinsic coercive force (iHc) from the point of view of miniaturization of motors and magnetic stability of the magnets used therefor. That is, bonded magnets are strongly required to have a residual magnetic flux density (Br) of not less than 10 kG and an intrinsic coercive force (iHc) of not less than 3.5 kOe.
On the other hand, a magnet alloy containing rare earth elements such as Nd is defective in that it is easily oxidized in the air and is likely to produce an oxide, so that the rust preventability is poor. Therefore, since bonded magnets produced from a magnet alloy containing a rare earth element such as Nd have a poor corrosion resistance, the bonded magnets are usually subjected to rust preventive coating-treatment such as dipping, spread coating or electro deposition using a resin and metal plating.
If the rust preventability of a magnet alloy containing a rare earth element such as Nd is enhanced, it may be possible to simplify or omit the rust preventive coating-treatment for the surfaces of bonded magnets even for the above-described use. In some uses, there is a possibility of omitting the rust preventive coating-treatment. Therefore, the enhancement of the rust preventability of a rare earth-iron-boron-based magnet alloy is strongly demanded.
The bonded magnets have also been produced usually by kneading magnet particles in a binder resin and forming the kneaded material into an appropriate shape. In this case, it is known that flake-like magnet particles are readily mechanically oriented, so that it is possible to enhance the packing density of these particles in the binder resin. However, in the case where the flake-like particles have curved surfaces, it becomes difficult to sufficiently enhance the packing density. In Japanese Patent Application Laid-open (KOKAI) No. 2-34706(1990), though the invention thereof relates to different application field from that of the present invention, it is described that “ . . . In general, as particles for paints, flake-like particles are preferred. That is, when such flake-like particles are mixed in a resin and the resultant paint is applied by a brush coating method or a spray coating method, these particles are deposited in parallel with the coating surface due to surface tension caused upon curing of the resin (called “leafing effect or phenomenon”), so that a continuous coating film composed of the particles is formed, thereby preventing the base material from coming into contact with outside air, and imparting a good corrosion resistance and weather resistance thereto . . . ”. Similarly, in the production of bonded magnets, when lamellar magnet alloy particles having no curved surfaces are used, the packing density of these particles in bonded magnets can be readily enhanced by the leafing effect thereof, whereby the residual magnetic flux d
Hamano Masaaki
Mizuguchi Hirotaka
Yamasaki Minoru
Sheehan John
Toda Kogyo Corporation
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