Electricity: magnetically operated switches – magnets – and electr – Magnets and electromagnets – Magnet structure or material
Reexamination Certificate
1999-08-10
2001-12-11
Donovan, Lincoln (Department: 2832)
Electricity: magnetically operated switches, magnets, and electr
Magnets and electromagnets
Magnet structure or material
C148S302000
Reexamination Certificate
active
06329894
ABSTRACT:
TECHNICAL FIELD
The present invention pertains to a thin-plate magnet that is ideal for various types of magnetic circuits used in small motors, actuators, magnetic sensors, etc. The present invention is characterized in that a magnet with a fine crystalline structure is obtained by the method whereby a melt of a specific composition comprising 6 at % or less of a rare earth element and 15 to 30 at % boron is continuously cast on a cooling roller that is rotating in a specific inert gas atmosphere under reduced pressure, with the crystalline structure as cast (in the cast state) being essentially 90% or more of a mixture of an FE
3
B compound, &agr;-Fe and a coexisting compound phase having an Nd
2
Fe
14
B crystalline structure and the mean crystal grain diameter of each structural phase being 10 nm to 50 nm as cast (in the cast state). The present invention pertains to thin-plate magnets with magnetic properties of iHc≧2.5 kOe and Br≧9 kG and a fine crystalline structure with a thickness of 70 &mgr;m to 500 &mgr;m that are produced directly from an alloy melt.
BACKGROUND ART
Today home appliances, OA equipment, electrical fixtures, etc., that are even higher performance and smaller and lighter weight are in demand and designs for maximizing the performance-to-weight ratio of an entire magnetic circuit that uses permanent magnets are being studied. A permanent magnet with a residual magnetic flux density Br of 5 kG to 7 kG is ideal for direct-current motors with a brush attached, which account for more than half [of the motors] produced today, but these cannot be obtained by conventional hard ferrite magnets.
For instance, the abovementioned magnetic properties are satisfied with Nd—Fe—B sintered magnets and Nd—Fe—B bonded magnets that are mainly Nd
2
Fe
14
B. However, Nd—Fe—B magnets contain 10 to 15 at % Nd, which requires many processes and a large facility for separation and purification and reduction of the metal, and therefore, when compared to hard ferrite magnets, they are very expensive. Consequently, these magnets have been promoted as a substitute for hard ferrite magnets only in some types of equipment because of the performance-to-cost ratio. An inexpensive permanent magnet with a Br of 5 kG or higher has yet to be discovered.
Moreover, a thin-plate permanent magnet wherein thickness of the permanent magnet itself is 100 &mgr;m to 500 &mgr;m is needed in order to realize miniature and thin magnetic circuits. Since it is difficult to obtain a bulk material of 500 &mgr;m or less with Nd—Fe—B sintered magnets, thin-plate magnets can only be made by the method whereby sintered plates with a thickness of several mm are ground, or bulk material is sliced with a wire saw, etc., and therefore, there are problems in that finishing cost is high and the yield is low.
Nd—Fe—B bonded magnets are obtained by bonding powder with a thickness of approximately 30 &mgr;m and diameter of several 10 &mgr;m to 500 &mgr;m with resin and therefore, it is difficult to mold bonded magnets where the thin-plate thickness is 100 &mgr;m to 300 &mgr;m.
On the other hand, an Nd—Fe—B permanent magnet whose main phase is an Fe
3
B compound with an Nd
4
Fe
77
B
19
(at %) neighboring composition has recently been presented (R. Coehoorn et al., J. de Phys, C8, 1988, pages 669-670), and the details of this technology are disclosed in U.S. Pat. No. 4,935,074, etc.
Moreover, prior to this, Koon presented a method of producing permanent magnets consisting of fine crystals by performing crystallization heat treatment on an La—R—B—Fe amorphous alloy comprising La as the essential element in U.S. Pat. No. 4,402,770.
It recently has been reported that thin pieces with hard magnetic properties are obtained by spraying Nd—Fe—B—V—Si alloy melt containing 3.8 at % to 3.9 at % Nd onto a Cu roller that is rotating to make amorphous flakes and then heat treating these at 700° C., as disclosed in EP Patent Application 558691B1 by Richter et al. These permanent magnetic materials are obtained by crystallization heat treatment of amorphous flakes with a thickness of 20 &mgr;m to 60 &mgr;m and have a metastable structure with a crystal aggregate structure that is a mixture of an Fe
3
B phase with soft magnetism and an R
2
Fe
14
B phase with hard magnetism.
The abovementioned permanent magnetic material has a Br of 10 kG and an iHc of 2 kOe~3 kOe and has a low content of Nd, which is expensive, of 4 at % and therefore, the starting material cost is less expensive than with Nd—Fe—B magnets whose main phase is Nd
2
Fe
14
B. However, there are limits to the liquid solidification conditions, which are essential to making an amorphous alloy from the starting mixture, and, at the same time, there are limits to the heat treatment conditions for obtaining a material with hard magnetism. Therefore, [such magnets] are impractical in terms of industrial production and as a result, there is a problem in that they cannot be inexpensively presented as a substitute for hard ferrite magnets. Moreover, said permanent magnet materials are obtained by crystallization heat treatment of amorphous flakes with a thickness of 20 &mgr;m to 60 &mgr;m, and therefore, permanent magnets having a thickness of 70 &mgr;m to 500 &mgr;m as required for thin-plate magnets cannot be obtained.
On the other hand, U.S. Pat. No. 508,266, etc., disclose the fact that rapidly Nd—Fe—B magnetic materials consisting of a structure consisting of crystals with hard magnetic properties are directly obtained by rapidly solidifying an alloy melt on a roller at a circumferential speed of 20 m/s. However, since flake thickness of the rapidly solidified alloy obtained under these conditions is thin at approximately 30 &mgr;m, they can be crushed to a powder particle diameter of 10 &mgr;m to 500 &mgr;m and used as the abovementioned bonded magnets, but they cannot be used for thin-plate magnets.
DISCLOSURE OF THE INVENTION
An object of the present invention is to solve the above-mentioned problems with an Nd—Fe—B magnet comprising 6 at % or less of a rare earth element and having fine crystals, another object of the present invention is to obtain a magnet having an inherent coercive force iHc of 2.5 kOe or higher, residual magnetic flux density Br of 9 kG or higher, and a performance-to-cost ratio rivaling that of hard ferrite magnets by casting, and yet another object of the present invention is to present a thin-plate magnet that makes small, thin magnetic circuits possible by having a fine crystal structure that is 70 to 500 &mgr;m thick.
The inventors disclosed that a fine crystalline permanent magnet alloy with hard magnetic properties of iHc≧2 kOe and Br≧10 kG is directly obtained from an alloy melt by the production method whereby an alloy melt of a low-rare earth Nd—Fe—B ternary composition comprising 6 at % or less of Nd and 15 to 30 at % boron is continuously cast onto a cooling roller that is rotating at a roller circumferential speed of 2 m/s to 10 m/s in a specific inert gas atmosphere under reduced pressure. However, this method of producing an Nd—Fe—B ternary magnet has a problem in that the roller circumferential speed region within which hard magnetic properties are obtained is narrow. Furthermore, coercive force of only 2 kOe to 3 kOe is obtained with this Nd—Fe—B ternary magnet and therefore, thermal demagnetization is considerable, and it is necessary to raise the operating point of the magnet as much as possible in order to realize high magnetic flux density, leading to considerable limitation of the magnet shape and the environment in which it is used.
The inventors performed many tests of the problem points of methods of producing Nd—Fe—B fine crystal permanent magnets with low rare earths content that are mixtures of a soft magnetic phase and a hard magnetic phase. As a result, they discovered that the abovementioned problems can be solved by using an alloy melt to which specific elements have been added during the process previously presented by the inventors, whereby a fine crystalline permanent magnet alloy with
Hirosawa Satoshi
Kanekiyo Hirokazu
Donovan Lincoln
Dykema Gossett PLLC
Nguyen Tuyen T.
Sumitomo Special Metals Co. Ltd.
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