Metal treatment – Process of modifying or maintaining internal physical... – Magnetic materials
Utility Patent
1999-05-20
2001-01-02
Sheehan, John (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Magnetic materials
C164S463000, C164S475000, C164S477000
Utility Patent
active
06168673
ABSTRACT:
DESCRIPTION
Thin-Film Magnet Having a Fine Crystal Structure and Manufacturing Method Thereof; Manufacturing Method for Isotropic Permanent Magnet Powder
1. Technical Field
The present invention relates to a thin-film magnet optimal for use in various types of small motors, actuators, and magnetic circuits for magnetic sensors, and a method for its manufacture. More particularly, the present invention relates to a thin-film magnet having magnetic properties of iHc≧2 kOe and Br≧10 kG and having a crystal structure wherein substantially 90% or more of the crystal structure comprises FeB compounds coexisting with compound phases having &agr;—Fe and Nd
2
Fe
14
B crystal structures and which comprises a fine crystal structure having the mean crystal grain diameter of 10 nm-50 nm in each compound phase. The invention also relates to a method for manufacturing the thin-film magnet directly by continuous casting of thin sheets of alloy, with thicknesses of 70&mgr;m to 300&mgr;m, through the casting of an alloy melt with a specific composition, including 5 at % or less of rare earth elements and 15 to 30 at % of boron, in an inert gas atmosphere at a specific reduced pressure on rotating cooling rollers.
2. Background Art
Current requirements for household appliance, office equipment, and other electronics equipment include higher capacities, as well as smaller sizes and lighter weights. Designs for whole magnetic circuits using permanent magnets are being studied for the purpose of maximizing performance/weight ratio. In particular, permanent magnets with residual magnetic flux densities Br of 5 kG to 7 kG are optimal for use in the structure of dc brush motors, which make up more than half of current production figures. Such residual magnetic flux densities cannot be achieved with conventional hard ferrite magnets.
For example, Nd—Fe—B sintered magnets and Nd—Fe—B bonded magnets, which consist mainly of Nd
2
Fe
14
B, do have the requisite magnetic properties. However, these include 10 to 15 at % of Nd, for which the isolation and refining of the metal and reducing reactions require many steps and large scale facilities. These are therefore considerably more expensive than hard ferrite magnets and, in view of cost performance, only replace hard ferrite magnets in some instruments. Inexpensive permanent magnet materials having Br of 5 kG or more have yet to be discovered.
Also, small, thin magnetic circuits require that the permanent magnet itself be in the form of a thin sheet, 100 &mgr;m to 500 &mgr;m thick. With Nd—Fe—B sintered magnets, it is difficult to attain bulk material with thicknesses of 500 &mgr;m or less. These can therefore only be prepared by grinding, which greatly increases costs. Nd—Fe—B bonded magnets are made by bonding powder with thicknesses of 30 &mgr;m and diameters of 10 &mgr;m to 500 &mgr;m in resin, making it difficult to form magnets with thicknesses of 100 &mgr;m to 300 &mgr;m.
Meanwhile, magnetic materials comprising mainly Fe
3
B compounds in compositions approaching Nd
4
Fe
77
B
19
(at %) have been proposed as Nd—Fe—B magnets in recent years (R. Coehoorn et al., J. de Phys, C8, 1988, p. 669-670). The technical details thereof were disclosed in U.S. Pat. No. 4,935,074. Prior to that, Koon proposed in U.S. Pat. No. 4,402,770 a method for manufacturing permanent magnets comprising fine crystals by crystallizing heat treatment of La—R—B—Fe amorphous alloys, including La as an essential element.
More recently, as disclosed in European Patent No. 558691B1, Richter et al. reported that thin fragments having hard magnetic properties were attained by heat treatment at 700° C. of amorphous flakes attained by spraying an Nd—Fe—B—V—Si alloy bath, including 3.8 to 3.9 at % of Nd, on a rotating Cu roller. These permanent magnet materials are permanent magnet materials with metastable crystal structures including a blend of a soft magnetic Fe
3
B phase and a hard magnetic R
2
Fe
14
B phase, attained by crystallizing heat treatment of amorphous flakes with thicknesses of 20 &mgr;m to 60 &mgr;m.
Such permanent magnet materials have iHc of 2 kOe to 3 kOe and Br of 10 kG. Because the composition includes only 4 at % of expensive Nd, the raw materials are cheaper than those for an Nd—Fe—B magnet comprising mostly Nd
2
Fe
14
B. On the other hand, the liquid quenching conditions are restricted because the amorphous alloying of the raw materials is a necessary condition. Also the heat treatment conditions for attaining hard magnetic materials are narrowly restricted. This is therefore not practical in an industrial manufacturing setting and these magnets cannot be provided as inexpensive replacements for hard ferrite magnets. Because such permanent magnet materials are attained by crystallizing heat treatment of amorphous flakes with thicknesses of 20 &mgr;m to 60 &mgr;m, it is not possible to attain permanent magnets with the 70 &mgr;m to 300 &mgr;m thicknesses required of thin-film magnets.
On the other hand, for the quenched Nd—Fe—B magnetic materials in U.S. Pat. Nos. 4,802,931 and 5,056,585 and so forth, the crystal structures are attained directly by quenching methods, so crystallizing heat treatment is not necessary. However, the optimum peripheral speed of the rollers disclosed in those reports was 20 m/s; the thickness of the quenched alloy flakes attained under those conditions is only about 30 &mgr;m and these cannot be used as thin-film magnets. These are therefore pulverized to diameters of several tens to 500 &mgr;m and used in the bonded magnets discussed above.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a thin-film magnet and a method for the production thereof, such a thin-film magnet having a fine crystal structure with a thickness of 70 &mgr;m to 300 &mgr;m (thus contributing to the miniaturization of magnetic circuits), having a cost performance rivaling that of hard ferrite magnets and a residual magnetic flux density Br of 10 kG or more, and resolving the abovementioned problems with Nd—Fe—B magnets having fine crystal structures and containing 5 at % or less of rare earth elements.
The inventors performed various studies with the object of creating thin-film magnets of Nd—Fe—B permanent magnets with low concentrations of rare earth elements and wherein soft magnetic phases and hard magnetic phases are blended. As a result, they made the following discoveries and arrived at the present invention. They found that they could use continuous casting to attain thin-film magnet of continuously cast alloy with thicknesses of 70 &mgr;m to 300 &mgr;m and having fine crystal structures with the mean crystal grain diameter of each component phase being 50 nm or less, wherein 90% or more of the crystal structure comprises Fe
3
B compounds coexisting with compound phases having &agr; —Fe and Nd
2
Fe14B crystal structures. The method involves continuous casting of an alloy melt, having a specific composition with low content of rare earth elements, in an inert gas atmosphere with reduced pressures of 30 kPa or less on a single cooling roller or between a pair of rollers rotating at a specific peripheral speed. Furthermore, they found that magnetic properties of iHc≧2 kOe and Br≧10 kG were achieved when the mean crystal grain diameters of each phase composing the continuously cast alloy attained were 10 nm to 50 nm. Moreover, they found that, in the case of crystalline metal structures with mean crystal grain diameters of less than 10 nm, they could use a specific heat treatment with the object of grain growth to attain mean crystal grain diameters of 10 nm to 50 nm for each phase and magnetic properties of iHc≧2 kOe and Br≧10 kG.
Specifically, the present invention is a thin-film magnet having a fine crystal structure, magnetic properties of iHc≧2 kOe and Br≧10 kG, and thicknesses of 70 &mgr;m to 300 &mgr;m, comprising fine crystals with mean crystal grain diameters of 10 nm to 50 nm, wherein 90% or more of the crystal structure comprises Fe
3
B compounds coexisting with compound phases having &agr; —Fe and N
Hirosawa Satoshi
Kanekiyo Hirokazu
Dykema Gossett PLLC
Sheehan John
Sumitomo Special Metals Co. Ltd.
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