Electricity: magnetically operated switches – magnets – and electr – Magnets and electromagnets – Magnet structure or material
Reexamination Certificate
2000-05-19
2001-11-13
Donovan, Lincoln (Department: 2832)
Electricity: magnetically operated switches, magnets, and electr
Magnets and electromagnets
Magnet structure or material
Reexamination Certificate
active
06317020
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to bond magnets used as high performance permanent magnets and manufacturing methods thereof, and actuators therewith such as motors.
2. Description of the Related Art
So far, as one kind of high performance permanent magnets rare earth system magnets such as Sm—Co system magnets and Nd—Fe—B system magnets are well known. A large amount of Fe and Co in these magnets contributes to increase saturation magnetic flux density. The rare earth elements such as Nd and Sm, due to a behavior of 4f electrons in a crystal field, cause very large magnetic anisotropy. Thereby, coercive force is increased.
The rare earth magnets of high performance are employed in electrical appliances mainly such as motors and measuring instruments. Recently, in order to cope with demands for smaller sizes and lower prices of various kinds of electrical appliances, permanent magnets of higher performance are in demand. In particular, in medium driving spindle motors used in hard disk drives (HDDs), floppy disk drives (FDDs), CD-ROM drives and DVD drives and actuators for driving light pick-ups used in CD-ROM drives and DVD-drives, in realizing smaller size and higher performance, further improved permanent magnet is in demand.
To the demands for permanent magnets such as described above, for instance R—Zr—(Fe, Co)—N system magnet material (R: rare earth elements) of which principal phase is TbCu
7
type crystal structure is proposed (cf. Japanese Patent Laid-Open Application (KOKAI) No. HEI 6-172936 and others). A permanent magnet using such magnet material is in general a bond magnet constituted of for instance a molded body (compression molded body) that is obtained by compression-molding a mixture of magnet powder and resin-based binder into a molded body. The mixture of the magnet powder and the resin-based binder is press-formed by use of a press machine of general use.
Magnetic performance of the bond magnet such as described above, residual magnetization and maximum magnetic energy product ((BH)
max
) in particular, is determined by magnetic properties of the magnet material being used and density of the molded body (compression molded body) constituting the bond magnet. That is, when the magnet material of equivalent performance being used, by increasing the density of the molded body, the bond magnet can be made higher performance. When the density of the molded body is increased to decrease voids, corrosion-resistance of the bond magnet is also expected to improve.
However, in a manufacturing method of the bond magnet therein an existing general press forming is applied, there is a limit in an improvement of the density of the compression molded body consisting of the mixture of the magnet powder and the binder. In particular, as in the magnet material of which principal phase is TbCu
7
type crystal phase, when flakes (or a ribbon) of magnet material obtained by quenching are used, during press forming, there occurs friction or spring back between magnet materials. Thereby, the compression-molded body is hindered from being densified. Accordingly, the bond magnet is in a situation difficult to further improve performance due to an improvement of the density of the molded body.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to provide a bond magnet in which a density of molded body of bond magnet is further increased to improve magnetic properties such as the maximum magnetic energy product, further corrosion-resistance. Here, in the bond magnet, magnet material of which principal phase is TbCu
7
crystal phase is used. Another object of the present invention is to provide a manufacturing method of bond magnets that enable to increase the density of the molded body with ease and reproducibility. Still another object of the present invention is to provide an actuator that is, by using bond magnet having the aforementioned characteristics, made smaller in size and higher in performance.
The present inventors studied hard to achieve the aforementioned objects. As a result, it was found that in manufacturing a molded body by compression-molding a mixture of magnet material and binder, by controlling conditions of press forming and shape of the molded body, the density of the molded body can be further increased.
That is, at first pressing, the mixture of the magnet material and the binder is low in fluidity to cause a large fluctuation in microscopic density. By contrast, when pressure is lowered once during compression molding, in the form of spring back the fluctuation of local residual stress is released. Simultaneously, microscopic movement of the magnet material broken during compression and the binder occurs. When the mixture in such a state is pressed again, the magnet material and the binder move so as to more homogenize internal stress to result in a decrease of microscopic fluctuation of the density within the molded body. Based on these, since press power during the compression molding can be more effectively utilized, the voids in the molded body (bond magnet) are decreased to enable to improve the density.
During compression molding, by applying pressure while rotating or reciprocating a forming mold such as a punch or a die, the density of the molded body also can be increased. Since rotary or reciprocating motion of the forming mold applies shearing stress or the like to the mixture, the mixture is more positively forced to flow. Accordingly, further densification of the bond magnet can be realized. Further, by curing the binder while applying pressure to the mixture, the bond magnet can be more densified.
Further, by optimizing the shape of the molded body, the density of the bond magnet can be easily increased. For instance, a ring of the bond magnet, by making appropriate a ratio of a thickness and a height of the ring portion or the specific value of a thickness, the density of the molded body can be further increased with ease. For the bond magnet formed in disk or a rectangular plate, a reduction of the thickness to less than an appropriate value is effective.
The present invention is accomplished based on such knowledge. The bond magnet of the present invention is one comprising a molded body in which the mixture containing magnet material of which main component is rare earth element-iron-nitrogen and a binder is formed into a magnet shape. The magnet material comprises TbCu
7
crystal phase as a principal phase and a shape of flake of a thickness of less than 200 &mgr;m, the molded body thereof having a density of 6×10
3
kg/m
3
(6g/cm
3
) or more. In the bond magnet of the present invention, the density of the molded body is preferable to exceeds 6.1×10
3
kg/m
3
(6.1 g/cm
3
).
The present invention, when as the flakes of magnet material one that is obtained by incorporating nitrogen in an alloy ribbon consisting of a quenched body of molten alloy is used and a thickness thereof is in the range of 5 to 50 &mgr;m, is particularly effective. The flakes of magnet material are preferable to comprise TbCu
7
crystal phase of which average grain diameter is in the range of 5 nm or more and less than 50 nm as a principal phase. Thus, by employing the flakes of magnet material which comprises fine TbCu
7
crystal phase as a principal phase, magnetic properties of the bond magnet can be further improved.
In the present bond magnet, the kind of the binder is not particularly restricted. However, the present invention is particularly effective for the bond magnet that uses resin-based binder. The content of the resin-based binder is preferable to be in the range of 0.5 to 5% by mass to magnet material. The present bond magnet comprises a compression-molded body of the mixture of for instance the flakes of magnet material and the binder.
A first method for manufacturing the bond magnet of the present invention is one for manufacturing the bond magnet that comprises a step of compression-molding a mixture of magnet material and binder into a desired magnet shape.
Arai Tomohisa
Kawashima Fumiyuki
Nakagawa Katsutoshi
Sakurada Shinya
Sawa Takao
Donovan Lincoln
Foley & Lardner
Kabushiki Kaisha Toshiba
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